Supramolecular Assembly via Hydrogen Bonds II (Structure and Bonding, Volume 111)

111 Structure and Bonding Series Editor: D. M. P. Mingos

Supramolecular Assembly via Hydrogen Bonds II Volume Editor: D. M. P. Mingos

Springer

Berlin Heidelberg New York

The series Structure and Bonding publishes critical reviews on topics of research concerned with chemical structure and bonding. The scope of the series spans the entire Periodic Table. It focuses attention on new and developing areas of modern structural and theoretical chemistry such as nanostructures, molecular electronics, designed molecular solids, surfaces, metal clusters and supramolecular structures. Physical and spectroscopic techniques used to determine, examine and model structures fall within the purview of Structure and Bonding to the extent that the focus is on the scientific results obtained and not on specialist information concerning the techniques themselves. Issues associated with the development of bonding models and generalizations that illuminate the reactivity pathways and rates of chemical processes are also relevant. As a rule, contributions are specially commissioned. The editors and publishers will, however, always be pleased to receive suggestions and supplementary information. Papers are accepted for Structure and Bonding in English. In references Structure and Bonding is abbreviated Struct Bond and is cited as a journal.

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ISSN 0081-5993 (Print) ISSN 1616-8550 (Online) ISBN-13 978-3-540-20086-4 DOI 10.1007/b13961 Springer-Verlag Berlin Heidelberg 2004 Printed in Germany

Series and Volume Editor Professor D. Michael P. Mingos Principal St. Edmund Hall Oxford OX1 4AR, UK E-mail: michael.mingos@st-edmund-hall. oxford.ac.uk

Editorial Board Prof. Allen J. Bard

Prof. James A. Ibers

Department of Chemistry and Biochemistry University of Texas 24th Street and Speedway Austin, Texas 78712, USA E-mail: [email protected]

Department of Chemistry North Western University 2145 Sheridan Road Evanston, Illinois 60208-3113, USA E-mail: [email protected]

Prof. Peter Day, FRS

Prof. Thomas J. Meyer

Director and Fullerian Professor of Chemistry The Royal Institution of Great Britain 21 Albemarle Street London WIX 4BS, UK E-mail: [email protected]

Associate Laboratory Director for Strategic and Supporting Research Los Alamos National Laboratory PO Box 1663 Mail Stop A 127 Los Alamos, NM 87545, USA E-mail: [email protected]

Prof. Jean-Pierre Sauvage Faculté de Chimie Laboratoires de Chimie Organo-Minérale Université Louis Pasteur 4, rue Blaise Pascal 67070 Strasbourg Cedex, France E-mail: [email protected]

Prof. Fred Wudl Department of Chemistry University of California Los Angeles, CA 90024-1569, USA E-mail: [email protected]

Prof. Herbert W. Roesky Institute for Inorganic Chemistry University of Göttingen Tammannstrasse 4 37077 Göttingen, Germany E-mail: [email protected]

Preface

During the last two centuries synthetic chemists have developed a remarkable degree of control over molecular architecture. Currently organic and inorganic chemists are able introduce a wide range of substituents in predictable positions on increasingly more complex molecular scaffolds and even control the three dimensional stereochemistries at particular chiral centres. Indeed only the skill and imagination of an individual chemist limits the range of molecules he is able to produce. This process has been accelerated by the synergic nature of synthetic chemistry and spectroscopic and structural techniques which have confirmed the three dimensional structures of molecules. A new frontier of chemistry has opened up in recent years which requires the development of analogous but new principles and methods which will enable chemists to predict how molecules interact with one another in the solid state. Indeed if we are to progress as “crystal engineers” as we have as “molecular engineers” we have to understand more predictively the factors which determine the three dimensional structures taken up by aggregates of molecules in the crystalline state. Therefore molecular recognition, material science, crystal engineering, nanotechnology, supramolecular chemistry the current goals of chemistry share the need to understand the very subtle factors which determine the way in which individual molecules come together in larger aggregates. In its most general form this is indeed a major problem because intermolecular forces are not very strong and are not very directional. However, this problem should be more amenable if there are groups on the surface of the molecules which are capable of hydrogen bonding. Not only are hydrogen bonds strong relative to other intermolecular forces but also they are more directional. Therefore, many groups have focussed their skills on the design of molecules with hydrogen bonding capabilities which can assemble in more predictable ways. These Volumes bring together recent results from a range of leading research laboratories and define the current advances in this area. We still have a long way to go for a complete understanding, but these Volumes demonstrate that rapid and exciting progress is being made. October 2003

D.M.P. Mingos

Contents

Hydrogen Bonding Interactions Between Ions: A Powerful Tool in Molecular Crystal Engineering D. Braga, L. Maini, M. Polito, F. Grepioni   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .

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Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Coordinated Guanidine Derivatives P. Hubberstey, U. Suksangpanya   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 33 Hydrogen-Bonding Templated Assemblies R. Vilar  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 85 Hydrogen Bonded Network Structures Constructed from Molecular Hosts M.J. Hardie   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 139 Author Index 101–111   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 175 Subject Index   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 179

Structure and Bonding, Vol. 111 (2004): 1–32 DOI 10.1007/b14139HAPTER 1

Hydrogen Bonding Interactions Between Ions: A Powerful Tool in Molecular Crystal Engineering Dario Braga1 · Lucia Maini1 · Marco Polito1 · Fabrizia Grepioni2 1

2

Dipartimento di Chimica G. Ciamician, Università degli Studi di Bologna, Via F. Selmi 2, 40126 Bologna, Italy E-mail: [email protected] Dipartimento di Chimica, Università degli Studi di Sassari, Via Vienna 2, 07100 Sassari, Italy E-mail: [email protected]

Abstract Hydrogen bonding interactions are the strongest of the non-covalent interactions and

are highly directional (hence transportable and reproducible).With respect to hydrogen bonds between neutral molecules the hydrogen bonding interactions between ions (inter-ionic hydrogen bonds) respond to additional energetic and topological constrains that depend on the convolution of the proton donor- proton acceptor interactions with the Coulombic field generated by the presence of ions. Directionality and strength are exploited in the design of molecular crystals, hence in molecular crystal engineering strategies. Molecular crystal engineering is the planning and utilisation of crystal-oriented syntheses for the bottom-up construction of functional molecular solids from molecules and ions. The success of crystal engineering strategies depends on the availability of robust and transferable interactions to glue together construction materials. This chapter is devoted to an important subset of non-covalent interactions, namely those involving hydrogen bonding and π-stacking interactions between ions. Some relevant analogies and differences between organic-type intermolecular interactions and those in which metal atoms are involved will be outlined. Selected examples of the utilization of inter-ionic hydrogen bonding interactions in crystal reactivity will also be described. Keywords Hydrogen bond · Ions · Molecular crystal engineering · Crystal synthesis · Gas-solid

reactions

1 Introduction  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .

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2 Charge-Assistance: Internal vs External   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .

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3 Data-Mining  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .

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4 How to Make Weak Hydrogen Bonds Less Weak   .  .  .  .  .  .  .  .  .  .  .  .

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5 O-H…O Interactions Between Polycarboxylic Acid Anions and Zwitterions   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 11 6 External Charge-Assistance to C-H…O Interactions and to π-Stacking

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7 How to Use Non-Covalent Interactions Between Ions  .  .  .  .  .  .  .  .  .  . 19 8 Hydrogen Bonded Networks Can React or Transform   .  .  .  .  .  .  .  .  . 22 9 Conclusions   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 28 10 References   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 29 © Springer-Verlag Berlin Heidelberg 2004

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1 Introduction The investigation of the bonds between molecules is one of the topical issues of our time [1]. It involves all areas of chemistry, in particular the thriving areas of supramolecular [2] and materials chemistry [3]. The motivation behind this broad interest is both scientific and utilitarian. Studies of intermolecular (or inter-ionic) bonds have great relevance for the fundamental sciences, but are also promising in terms of practical applications. It is recognized that an intelligent control of the recognition and assembly processes that lead from components to superstructures via tailoring of intermolecular interactions will allow us to obtain desired collective chemical and physical properties [4]. All these ideas also apply to crystal engineering [5], the area of supramolecular chemistry is devoted to the controlled design of molecular crystalline materials. The paradigm of molecular crystal engineering can be thus phrased: as non-covalent interactions are responsible for the existence and functioning of supermolecules, it is the convolution of crystal periodicity with intermolecular and/or interionic interactions that determines topology, energetics and properties of solid supermolecules. On this premise, those interactions that combine strength and directionality allow a better control of the aggregation process. Strength is synonym of cohesion and stability, while directionality implies topological control and selectivity. Directionality combined with strength are essential requisites to assemble building blocks in a desired and stable way. Directionality also implies reproducibility: only if the topological properties of a given interaction persist in different structural environments, i.e. on passing from one solid supermolecule to another, is the interaction useful in the construction of new solids [6]. The topological control can be reinforced by the use of multiple directional interactions within the same molecule [7]. The intermolecular interaction that best combines strength and directionality is the hydrogen bond (HB). The number of papers, reviews and books dealing with hydrogen bonds is countless. A recent ISI [8] search of the keywords “hydrogen bond” in abstracts and titles yielded a count of 15356 and 2794 occurrences in abstracts and titles, respectively, in the years 2000–2002. Some recent review articles or relevant books are listed in [9] but the reader is warned that new papers and new interesting findings are likely to appear in the literature by the time this contribution in Structure and Bonding is published. Because of the vastness of the subject matter, we shall focus our attention on hydrogen bonding interactions between ions and on the possibilities and limitations of their use in the design and construction of molecular materials of desired architectures and/or destined to predetermined functions. Obviously, the crystal engineer (or supramolecular chemist) needs to know the nature of the forces s/he is planning to master, since molecular and ionic crystals, even if constructed with similar building blocks, differ substantially in chemical and physical properties (solubility, melting points, conductivity, mechanical robustness, etc.). Since the identification of bona-fide hydrogen bonds in solid state studies is often a controversial issue, in particular in the cases involving weak donors [10]

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or ions [11], we will remain in the following with Linus Pauling’s definition: “There is a chemical bond between two atoms or groups of atoms in case that the forces acting between them are such as to lead to the formation of an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent molecular species” [12]. In Pauling’s approach the existence of a bond is linked to the energetic stability of the aggregate formed as consequence of the bond. This definition was adapted to intermolecular bonding by M. Etter [13]:“A hydrogen bond is an interaction that directs the association of a covalently bound hydrogen atom with one or more other atoms, groups of atoms, or molecules into an aggregate structure that is sufficiently stable to make it convenient for the chemist to consider it as an independent chemical species”. The focus is on the concept of “directed” association and of stability, and the existence of an intermolecular bond is conceptually associated to the energetic stability of the aggregate. In terms of energy, hydrogen bonding interactions span a large interval, ranging from tiny energies (few kJ/mol in the case of C-H…O or comparably weak interactions, see below) [10] to large values when the acceptor is an anion (more than a hundred kJ/mol in the case of O-H…O(–) or F-H…F(–) and similar interactions) [14]. Generally speaking, however, the HB interaction is generally stronger (when not much stronger) than the strongest van der Waals interaction. For this reason, within X-H…Y HB systems, H…Y and X…Y separations shorter than van der Waals contact distances and X-H…Y angles that tend to linearity are considered diagnostic of the presence of strong HB [9]. The X…Y distance criterion is, however, not sufficient when dealing with weak and very weak HB interactions [10]. It has been pointed out by Jeffrey and Saenger [9a] that”The use of a van der Waals distance cut-off criterion carries the wrong implication that hydrogen bonds become van der Waals interactions at longer distances” and overlooks the essentially electrostatic nature of the interaction. While van der Waals interactions fall off very rapidly (r–6), electrostatic interactions follow an r–1dependence (assuming primarily monopole-monopole and monopole-dipole interactions); thus HB can be stabilising at distances much greater than the sum of van der Waals radii. Furthermore, the distinction between strong and weak hydrogen bonds is, often, only conventional, and there is a difference between hydrogen bonding interactions involving ions and those involving neutral molecules in crystals because of the fundamentally different nature of the dominant forces, the differences in physical properties (solubility, melting point, behaviour under mechanical stress, etc.) arising from the presence of ions or neutral molecules.

2 Charge-Assistance: Internal vs External In this chapter, we shall focus on cases where the combination of ionic charges and non-covalent interactions, especially of the hydrogen bonding type, provides not only a simple means to devise stable architectures but also affords properties that are a convolution of those of molecular crystals and of molecular salts. In the case of hydrogen bonding it is, however, useful to recall in a schematic way how charges and location of donor-acceptor hydrogen bonding systems can be “com-

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Table 1 The possible combinations of neutral and ionic proton

donor/proton acceptor systems and the relationship between internal and external charge assistance Neutral HB X-H…Y

Internal charge assistance X-H…Y(–)

Requires external charge assistance (–)X-H…Y(–)

(+)X-H…Y

(+)X-H…Y(+)

(+)X-H…Y(–) (–)X-H…Y(+) X-H…Y(+)

bined”. This is summarised in Table 1 [15]. The ionic charge in brackets indicates the charge carried by the whole fragment carrying the HB donor or acceptor group. Leaving aside the “null option”, i.e. when both fragments are neutral, we distinguish between “internal” and “external” charge assistance to the hydrogen bonding interactions. This discrimination depends on whether the proton acceptor/proton donor systems carry charges of opposite sign, e.g. (+)X-H…Y(–), or one of them is neutral (middle column), or charges of the same sign, e.g. (–)X H…Y(–) and (+)X-H…Y(+) (right column). In the case of “external” charge assistance the stability of the hydrogen bonding aggregate towards dissociation will depend upon the presence of counterions. This is the case, for instance, of chains of cations or of chains of anions, which would be unstable towards dissociation in the absence of counterions [16] that are need to (over)compensate for the electrostatic repulsions. The implications are quite relevant: (i) even though the stabilising contribution of the HB interaction is small, the directionality is fully

a

b c

d

Fig. 1a–d Schematic representation of the relationship between neutral, internally chargeassisted and externally charge-assisted hydrogen bonds

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operative and (ii) the common assumption that the intermolecular separation between atoms or groups of atoms reflect the strength of the local interaction is not directly transferable from neutral to ionic environments [16]. The comparison between neutral HB and inter-ionic HB interactions is schematically represented in Fig. 1. “Internal” or “external” charge assistance can be successfully used to build periodical supermolecules based on HB. The utilization of ionic building blocks is, however, more common in inorganic crystal engineering, where metal atoms give easy access to charged species. Moreover, the variability of oxidation states makes possible the utilization of the same building block in both neutral and ionic environments.

3 Data-Mining The Cambridge structural database (CSD) [17] is a primary source of structural information validated statistically via the observation of recurring behaviours in large numerical sets of data. For this reason data-mining is yet another powerful tool available to the crystal engineer, mainly in the initial steps of project analysis and architecture design. The identification in a large number of different structural environments of the same interaction, or of the same packing motif associated with several interactions, guarantees that, when this motif is purposely encoded into a molecular or ionic building block, the chances that it will lead to the desired supramolecular arrangement are proportional to its frequency of occurrence in different crystal packings. This approach has led to the extension to the area of molecular crystal engineering of the concepts of retrosynthesis and supramolecular synthons [18] originally developed in the field of organic chemistry. Hydrogen bonding functional molecules are the synthons of choice in many crystal construction strategies [19]. By comparing organic and inorganic supramolecular synthons it has been shown that strong hydrogen bonding donor/acceptor groups, such as -COOH and -OH systems, as well as primary -CONH2 and secondary -CONHR amido groups, form essentially the same type of hydrogen bonding interactions whether as part of organic molecules or as metal co-ordinated ligands. This is not surprising, since hydrogen bonds formed by these groups are at least one order of magnitude stronger than most non-covalent interactions, and are most often already present in solution. In addition to these strong bonds and to the plethora of weaker (e.g. C-H…O, C-H…N, C-H…π etc.) ‘organic’-type hydrogen bonding interactions, the presence of metal atoms in molecular building blocks generates new types of interactions, which are characteristic of inorganic and organometallic systems. Several research groups are exploiting hydrogen bonded synthons to combine co-ordination chemistry and hydrogen bonding functionalities. Some examples of the utilization of the CSD in the evaluation of some cases of neutral vs ionic HB interactions will be discussed in the following section. The power of the CSD [17] and, of course, of the ICSD (although this latter database has yet to develop a user friendly interface for data mining) [20] – in the context of crystal engineering – lies in the statistical approach it permits in the

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analysis of crystal structures, which, in turns, allows the identification of recurring synthons. As the number of crystal structures in the databases have increased enormously in the recent past, intermolecular interactions have begun to be reliably examined. Quite apart from the fact that it is impossible today to exhaustively peruse the crystallographic literature manually, the sort of chemical conclusions that a CSD/ICSD analysis permits cannot be obtained from reading of the journals. Indeed, it is (conservatively) estimated that the number of entries in the CSD will increase to not less than 500,000 within the first decade of this century. Moreover, both CSD and ICSD can nowadays be utilized on conventional personal computers or on the web, with no need for expensive mainframe computers. Thanks to these factors new interactions are being discovered, or re-discovered, almost daily and data-mining is still one of the preliminary steps of any crystal engineering project [21]. For this reason some cautionary words may be in order.Very weak interactions, falling in the fluctuations of the crystal structure energetics – those due, for instance, to motions of atoms or atomic groups – may be useless in design strategies, because they are too feeble to control crystal construction. It is dangerous to focus exclusively on pairwise interactions, as one may forget that it is the overall balance of interactions, some acting at short range only, some acting at very long range, that accounts for cohesion in molecular crystals [22]. Only strong pairwise interactions (e.g. O-H…O, but also Cl…Cl, or Au…Au) may stand out above the noise level and act as true packing directors [23]. Preservation or preformation of robust intermolecular bonds often leads to molecular packings that do not correspond to the best van der Waals energy. This is, for instance, the case of water and accounts for the absorption of ca. 6 kJ mol–1 upon melting [24a]. This energy is required to break about 10% of the O-H…O bonds from the HB scaffolding of ice, hence determining the lower density of ice with respect to liquid water. One further point of concern arises from the customary ‘frozen’ picture of molecules in crystals, and from the consequent ‘frozen’ perception of the network of intermolecular interactions. When a non-rigid molecule or ion is taken from solution or gas phase into the solid state its geometry is distorted along soft deformational paths, and its rotations and vibrations, though restricted, often persist to a very large extent. Large amplitude oscillations and full-scale reorientational motions are often observed in crystals. The deformation on passing from vacuum to solid state is particularly dramatic in the case of supermolecules held together by intermolecular interactions; simple examples are the NH3:BH3 Lewis acid/base system or the acetic acid dimer CH3COOH…CH3COOH [24b], where the distinction between inter- and intramolecular structures is not so straightforward. In these cases the solid state structure of the molecular aggregate does not correspond to the vacuum or solution structure, because the supramolecular bonding energies are low enough to be significantly perturbed by intermolecular interactions. Distortions and dynamics are obviously significant in the case of flexible compounds: structural non-rigidity of the building blocks needs to be taken into account in evaluating the factors responsible for crystal stability, since molecular and crystal structure may affect each other in an often unpredictable manner.

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4 How to Make Weak Hydrogen Bonds Less Weak For the reasons given in the previous section, one can anticipate that database searches of intermolecular interactions that do no discriminate between ionic and neutral fragments may end up with unreliable (or only partially reliable) results. The situation of the prototype of strong hydrogen bonds, namely that between an O-H donor and an O acceptor in solid protonated or partially deprotonated polycarboxylic acids, provides an educative example. Figure 2 shows the result of an “acritical” CSD search of the O(H)…O distance distribution for all intermolecular interactions satisfying the criterion of O…O separations shorter than 2.80 Å. In order to avoid low quality X-ray structures, the R-factor was required to be <10%. One can see that the mean value for such an interaction (Fig. 2, top histogram) is at 2.6321 Å and that the lowest 10% percentile distances being shorter that 2.551 Å. When the presence of an ionic charge is taken into account and the distribution is split in COOH…COOH (Fig. 2, middle histogram), andCOOH…COO(–) (Fig. 2, bottom histogram) one can see that inter-neutral and inter-ionic distances follow different distributions, with shorter separations usually associated with the latter cases (mean O…O values 2.6501, 2.5333 Å and lowest 10% percentile 2.614, 2.462 Å, respectively). Clearly, neglect of the effect of charges can lead to untrustworthy conclusions. However, even this analysis is not entirely correct, as it does not take into account the possibility that the ionic charge could be localized on a given fragment and not on the entire ion. This is the case, for instance, of amino acid molecules in zwitterionic form and will be discussed later on within this chapter. The distinction between inter-neutral and inter-ionic interactions is not only important when dealing with strong interactions, those with a high directionality feature, but also when the interactions are weak or very weak. As an example, the role of ionic charges on C-H…O interactions between anions and cations has been investigated [25] by searching the CSD for inter-molecular and inter-ionic (C)H…O distances (H…O in the range 2.0–3.0 Å, C-H…O angles larger than 110°) for metal bound C-H systems, i.e. neutral systems containing (M)C-H…O interactions, and charged systems containing [(M)C-H]+…[O]–, respectively, where M=first row transition metal. The analysis is statistically sound because C-H…O hydrogen bonds, though weak, are very numerous in organometallic crystals as a consequence of the popularity of ligands such as arenes and cylopentadienyl ligands which carry a plethora of C-H units [26]. The analysis shows that interionic C-H…O hydrogen bond distances follow the order [(M)C-H]+…[O]–<(M)C-H…O (M=first row transition metal), which is the order of decreasing charge assistance to the weak hydrogen bonds (mean H…O values 2.62915, 2.74112 Å, and lowest deciles 2.347, 2.491 Å, respectively, see Fig. 3). Further insight on the effect of charges on weak bonds has been obtained by analysing C-H…O interactions involving the cations [PPh4]+and [PPN]+and the anion[BPh4]– and O-acceptors in anionic and cationic transition metal complexes [OM] (see Fig. 4).[PPh4]+,[PPN]+and [BPh4]– are amongst the most commonly used counterions for the crystallization of charged species and are very com-

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Fig. 2 O…O intermolecular/interionic interactions between COOH and COOH/COO(–)

groups in protonated or partially deprotonated polycarboxylic acids. Top histogram: the distribution of O…O interactions obtained without neutral/charge discrimination (mean value 2.6321 Å).When the presence of an ionic charge is taken into account the distributions of O…O distances are those in the middle [(n)O(H)…OCOOH(n) and (n)O(H)…OCOOH(–), mean value 2.6501 Å] and in the bottom [(–)O(H) …OCOO(–), mean value 2.5333 Å] histograms

monly used when X-ray suitable crystals of large coordination compounds are sought. The results are shown in Fig. 4.As in the previous case the analysis affords a rather consistent picture: the H…O distances, in both average and percentile values, follow the order [(PPh4)C-H]+…[OM]–<[(PPN)C-H]+…[OM]–<[(BPh4)CH]–…[OM]+ (mean H…O distance 2.7004, 2.7233 and 2.7569 Å, lowest deciles 2.428, 2.475 and 2.551 Å, respectively) which is the order of decreasing electro-

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Fig. 3 CSD intermolecular searches on H…O interactions in (M)C-H…O; and [(M)CH]+…[O]– systems [metal bound C-atoms with M=first row transition metal)

Fig. 4 CSD intermolecular searches on H…O interactions in [(PPh4)C-H]+…[OM]–, [(PPN)CH]+…[OM]-, [(BPh4)C-H]–…[OM]+ systems (OM indicates that the oxygen atom belongs to an organometallic or coordination complex)

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static reinforcement of the C-H…O interaction. The comparison between BPh4– and PPh4+ is particularly educative, since the [(BPh4)C-H]–…[OM]+ represents a situation of charge opposition to the C-H…O bonds and, in fact, this group of data is characterized by average distance values longer also with respect to the neutral sample. In terms of angularity, all these interactions follow the trend expected for hydrogen bonds, namely the C-H…O angle opens up as the distance between donor and acceptor increases. Another case which has given rise to controversial interpretations is that of the C-H…F interactions. The analysis of statistical data has led authors to conclude that, at least in the case of C-H…F contacts between neutral organic molecules, when fluorine is covalently bound to carbon it does not form hydrogen bonds with conventional hydrogen bond donors, including O-H, N-H and C-H groups [27]. The situation is different in crystals of coordination and organometallic cationic complexes crystallized with the “very popular” [PF6]– and [BF4]– anions. The CSD has been searched for intermolecular H-bonds of the (+)C-H…F(–) type involving the [PF6]– and the [BF4]– anions [28] (data have been updated for this work on the basis of the April 2002 version of the CSD). Since the number of compounds containing C-H groups is very large, two separate searches were carried out. In the first only C-H fragments for which the C atom is directly bound to the metal atom were considered (561 and 452 hits, for a total of 3972 and 2925 observations, for [PF6]– and [BF4]– compounds, respectively). In the second one no restrictions were applied to the C atom (1354 and 930 hits, for a total of 31,639 and 17,057 observations, for [PF6]– and [BF4]– compounds, respectively). Scattergrams of (Tr)C-H…F[PF6]– angles vs (Tr-C)H…F[PF6]– distances and of (Tr)CH…F[BF4]– angles vs (Tr-C)H…F[BF4]– distances, respectively, are shown in Fig. 5. Although the number of hits is different in the two cases, the two types of interactions follow the normal trend observed for hydrogen bonds, i.e. as the distance between hydrogen and acceptor atoms decreases, the C-H…F vectors becomes straighter and the bond approaches linearity. Beside this general behaviour, there are a large number of bonds that fall below the van der Waals cut-off distances and are clearly indicative of specific and directional interactions involving donors and acceptors. Comparison of the two types of search also show that there is no appreciable effect of the direct bonding to transition metal atoms on the geometry of the interaction, as the two populations have the same distributions. Interestingly, while 10% of the contacts fall in the range 2.000–2.391 Å in the case of (Tr-C)H…F[PF6]– interactions, the same percentage of contacts falls in the narrower range 2.000–2.322 on passing to (Tr-C)H…F[BF4]– interactions. This difference may be equally well explained with two hypotheses: (i) the presence of a large number of short contacts in the case of phosphorus bound to boron than in the case of phosphorus may reflect the higher polarity of the Fatom in the formed anion than in the latter (the anion charge is shared by four vs six fluorine atoms), and (ii) the smaller [BF4]– anion can approach the C-H donors more effectively than the bulkier [PF6]–. All examples discussed above, whether concerning very strong O-H…O hydrogen bonds or weak C-H…O and C-H…F interactions, clearly show that a donor-acceptor distance criterion is an unsafe instrument to check the relevance of hydrogen bonding interactions if the effect of ionic charges is neglected.

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Fig. 5 CSD (April 2002 version) intermolecular searches for intermolecular H…F of the (+)C-

H…F(–) type involving the [PF6]– and the [BF4]– anions: Scattergrams of (Tr)C-H…F[PF6]– angles vs (Tr-C)H…F[PF6]– distances (top) and of (Tr)C-H…F[BF4]– angles vs (Tr-C)H…F[BF4]– distances (bottom)

5 O-H…O Interactions Between Polycarboxylic Acid Anions and Zwitterions We mentioned above that the interaction between carboxylic and carboxylate groups constitutes a benchmark system for the investigation of the relationship between topology and charge carried by the components. In the case of inter-anion (–)O-H…O(–) hydrogen bonding interactions on mono-deprotonated polycarboxylic acids it has been shown [29] that deprotonated [COO–] groups are strongly polarised in the solid state and always behaving as -COO(–) groups,

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Table 2 Comparison of average C-O structural parameters (Å) for neutral and partially deprotonated polycarboxylic acids as extracted from the Cambridge Structural Database (CSD)

Interaction

I [COOH]N …[COOH]N N=Neutral system

II [COOH]A …[COOH]A A=Anionic system

III [COOH]A …[COO–]A A=Anionic system

IV [COOH]A …[COO–]A A=[HC2O4]–

O1…O3 C1O1 C1O2 C2O3 C2O4

2.6653 1.2981 1.2241 1.2251 1.2971

2.6448 1.3085 1.2154 1.2243 1.3005

2.5323 1.3011 1.2111 1.2611 1.2381

2.5456 1.2991 1.2081 1.2562 1.2342

irrespective of the ionic or neutral nature of the components. In contrast, protonated [COOH] groups, even when belonging to anions, behave exactly as in neutral molecules. The same behaviour is shown by the smallest dicarboxylic acid anion, the hydrogen oxalate anion [HC2O4]–. The study of zwitterionic amino acid molecules carrying more than one [COOH] group (e.g. aspartic and glutamic acids and derivatives) has been particularly informative, since these molecules carry both a protonated [COOH] and a deprotonated [COO–] group. The results of a CSD investigation are summarized in Table 2. Table 2 summarizes the numerical results on average intermolecular/interionic O…O and intramolecular/intra-ionic C-O structural parameters for the [COOH]N…[COOH]N, [COOH]A…[COOH]A and [COOH]A…[COO–]A intermolecular interactions, together with those obtained for the [COOH]A…[COO–]A sample in the case of the hydrogen oxalate anion. Data were retrieved from the CSD with a cut-off distance on O…O separations of 3.0 Å. A visual prospect of the data listed in column III is provided in Fig. 6, where histograms of intramolecular C-O distances within the protonated and deprotonated COOH/COO groups are presented. Though not very large, the samples represent the behaviour of a well defined class of interactions. One can see that O…O distances between protonated groups belonging to anions compare strictly with those between neutral molecules, whilst O…O distances between protonated and deprotonated groups are shorter than those between protonated groups belonging to anions or neutral molecules. Furthermore, in both neutral molecules and ions the C-O and C-O(H) distances within the COOH groups form two distinct sets of long and short ones (C1O1>C1-O2), while C-O distances within the COO– groups show a much smaller difference in length (C2-O3≅C2-O4). Hence, the metric analysis shows that (i) there is no appreciable structural consequence of the fact that the protonated COOH group belongs to an anion or to a neutral molecule, and (ii) proton removal from the COOH group leaves the extra electron essentially localised on the deprotonated [COO–] system. It has also been noted that the distribution of distances does not depend on the size of the fragment: with the small HC2O4– anion the protonated [COOH]A groups are geometrically similar to neutral [COOH]N groups and the extra electron remains ‘confined’ onto the deprotonated [COO-]A groups.

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Fig. 6 Histograms of C-O distances in the -COO(–) and -COOH groups belonging to hydrogen bonded systems of the type [COOH]A…[COO–]A (A=anionic system) [column III in Table 2]

Carboxylic zwitterions, i.e. those containing the deprotonated -COO(–) group, are useful for the present discussion because the molecules are formally neutral. Crystals of zwitterionic molecules are particularly relevant for the present discussion, because they are formally molecular in nature, i.e. they do not contain ions carrying charges of opposite sign. The number of zwitterionic molecules is quite large, but only 49 compounds, extracted from the CSD, show the simultaneous presence of a protonated -COOH and a deprotonated -COO(–) groups. Though small, the sample is amenable to some statistical considerations, mainly in the light of the observations made above. In terms of average values and lowest decile (10%) the distances are C1-O1 1.3072, 1.282; C1-O2 1.2112, 1.119; C2-O3 1.2602, 1.245; C2-O4 1.2422, 1.220 Å. From these figures it is easy to appreciate that, although formally neutral, the zwitterionic molecule behave, in HB formation, ex-

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a

Cationic chain

b

Neutral dimer

c

Anionic chain

d

Zwitterionic chain Fig. 7a–d A comparison of the geometries and O…O separations (in brackets) in a selection of neutral, charged and zwitterionic metallocene diacids: a the cationic chain in crystalline [(η5-C5H4COOH)2CoIII]+[PF6–]; b the neutral dimer in [(η5-C5H4COOH)2FeII], [O…O 2.600(2), and 2.606(2) Å, respectively]; c the anionic chain in [(η5-C5H4COO)(η5-C5H4COOH)FeII]–[(η5C5H5)2CoIII]+; d the neutral chain formed by the zwitterion in [(η5-C5H4COO)(η5C5H4COOH)CoIII] [O…O 2.453(3), and 2.456(2) Å, respectively]

actly as their anionic partners. This is also true when the average O…O separations, i.e. [COOH]ZW…[COO–]ZW are considered (the value of 2.5688 Å is comparable with the values of 2.5323 and 2.5456 reported in Table 2). This is a further indication that proton removal (or proton transfer) “leaves” the extra-charge essentially localized onto the -COO group. A small selection of specific cases can be used to illustrate the intriguing relationship between metrics and nature of the interactions. Figure 7 shows hydrogen bonding interaction geometries in a selection of neutral, cationic, anionic and zwitterionic chains of cobalt and iron dicarboxylic acid complexes. A comparison between (a) the cationic chain formed by [(η5-C5H4COOH)2CoIII]+ and (b) the neutral dimer present in crystalline [(η5-C5H4COOH)2FeII], and between the chains formed by (c) the anionic species [(η5-C5H4COO)(η5-C5H4COOH)FeII]– and (d) the neutral zwitterion [(η5-C5H4COO)(η5-C5H4COOH)CoIII], confirms that the difference in O…O separations (reported in the figure caption) between COOH…OC(OH) and COOH…(–)OOC interactions does not depend on the neutral or ionic nature of the complex. The presence of shorter O…O separations in

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cases (a) and (b) with respect to (c) and (d) suggests that, at least in the solid state, proton removal from COOH groups, whether belonging to a neutral or cationic acid,‘leaves’ the extra-electron localized on the deprotonated group. The electron localization on the acceptor group allows it to take full advantage of the stabilizing contribution arising from the electrostatic (δ+)-(δ–) component of the interaction even in the case of building blocks carrying the same charge. It may be argued that the stabilization cannot offset the dominating repulsive forces between like charges and is not sufficient per se to keep the anions together in the absence of counterions. In summary, COOH groups possess the same average metrics irrespective of the neutral or ionic nature of the system they belong to, and irrespective of the protonated or deprotonated nature of the acceptor site. On the other hand the deprotonated [COO–] groups appear to keep the negative charge after proton removal, irrespective of the size of the acid anion. At this stage, one may wonder if the same ‘conservative’ behaviour is shown, on passing from ionic to neutral crystals, by protonated and deprotonated groups. However, though interesting (perhaps) to theoreticians and crystallographers, the problem discussed in this section could be found of little relevance to the crystal engineers seeking reproducible and useful synthetic strategies.And yet it will be shown that the knowledge of the way zwitterions may react with acids and bases in proton transfer processes can be useful in devising gas-solid and solidsolid reactions [30, 31]. Furthermore, it should be recalled that superprotonic conductivity, an important process investigated for the development of fuel cells, is dependent on the possibility of proton jumps in chains of hydrogen phosphates and sulfates [32] associated with the onset of reorientational motions in the solid state. Some examples will be provided in the last section of this contribution.

6 External Charge-Assistance to C-H…O Interactions and to π-Stacking We have shown in the previous sections that the distance criterion is not always trustworthy when dealing with intermolecular interactions. There are several examples of close contacts that do not correspond to stable interactions or of cation assisted chemical bonds between ions [33a,b], as well as examples of short contacts that are not hydrogen bonds [33c]. For instance, in the cases of crystalline salts containing π-[TCNE] dimers (TCNE=tetracyanoethylene) the existence of exceptionally long (>2.9 Å) C-C bonding interactions involving π-electrons has been explained on the basis of the existence in the crystals of dimer dianions stabilized by cation…[TCNE](–) interactions, which provide the electrostatic stabilization necessary to overcome the intradimer electrostatic repulsion [33a]. The reasoning applied above to O…O interactions can be extended to weak C-H…O bonds. The presence of a short C-H…O distance is not necessarily indicative of an overall attractive interaction, if the C-H donor and the O acceptor groups belong to anions, as illustrated in the following example. The structure of the mixed-valent species [(η6-C6H6)2Cr]+[CrO3(OCH3)]– [34] is based on columns of methoxychromate anions and of columns of bis-benzene chromium

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a

b Fig. 8 a The crystal is formed of columns of methoxychromate anions and of columns of bis-

benzene chromium cations extending parallel to the c-axis. b The anions are apparently “linked” along the column via a short C-H…O interaction (H…O 2.381 Å, C-H…O 173°) between a methyl hydrogen and a chromate oxygen

cations (see Fig. 8a). The cations stack in piles (see Fig. 8b) with benzene-benzene distances of ca. 3.50 Å. While the interaction between cations and anions is based on “charge-assisted” C-H(+)…O(–) hydrogen bonds, the anions are apparently “linked” along the column via a short C-H…O interaction [H…C 2.381 Å, C-H…O angle 173°] between a methyl hydrogen and a chromate oxygen (see Fig. 8b). This [(Cr)C-H](–)…[O](–) interaction between anions along the chain has been investigated by means of theoretical ab initio calculations [34]. It has been shown

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Fig. 9 The ion organization in crystalline CsHC4O4 (stacking separation 3.32 Å]

that, in the absence of cations, the C-H…O interaction would be unable to hold together the anions in the chain because of the strong Coulombic repulsion between consecutive anions along the chain. The overall crystal stability is guaranteed by the balance between (–)…(+) attractive interactions and those of repulsive nature [(–)…(–) and (+)…(+)]. The [(Cr)C-H](–)…[O](–) interaction is stabilizing only on a relative scale; it contributes to crystal cohesion by decreasing the inter-anion repulsion and by conferring directionality to the linear arrangement, while the major cohesive contribution results from Coulombic interactions between the methoxychromate anions and the bis-benzene cations. Hence, the reasoning developed for negatively charged hydrogen bonding O-H(–)…O(–) interactions in chains formed by monodeprotonated dicarboxylic acids holds also in the case of weak C-H…O interactions. Another situation of external charge assistance to weak interactions has been observed when considering the occurrence of short interplanar stacking separations between flat systems containing π-electrons [35]. In the family of alkali hydrogen squarate salts MHC4O4 (M=Li, Na, K, Cs) the hydrogen squarate anions [HC4O4]– are connected via O-H(–)…O(–) interactions [O…O separations in the range 2.417–2.503 Å] [35], and the hydrogen squarates overlap to a various extent (see Fig. 9 for the packing in CsHC4O4); the stacking separation between squarate planes is considerably shorter than in neutral systems (range 3.13–3.32 vs 3.43–3.46 Å in organic crystals) and increases slightly on increasing the cation size, being apparently unrelated to the type of alkali cation (although the shortest value is with Li+). A theoretical approach analogous to that employed in the cases discussed above has demonstrated that the electrostatic component of the anion…anion interaction is repulsive in all regions relevant for intermolecular interactions [29]. Thus the short interplanar separation appears to originate from the electrostatic “compression” that brings the squarate planes at a distance shorter than between neutral systems. It is worth stressing that, even though the stabilizing contribution of inter-anionic π-interactions may be overridden by the presence of a strong electrostatic field, the directionality of the interactions is retained. Thus, while crystal cohesion is determined by the balance of attractive and repulsive

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ionic interactions (as in NaCl, for instance) the actual structure is controlled by the optimisation of inter-anionic π-stacking. The same reasoning applies to the family of the alkaline and ammonium croconate salts, i.e. containing the croconate dianion, C5O52– [36]. The dianions C5O52–organize themselves in columns, with interplanar separations falling in the narrow range 3.12–3.42 Å, i.e. comparable to that observed in the squarate family discussed above.As in these latter cases, the short interplanar separation does not necessarily reflect the presence of a π-stacking interaction. It has been argued that the close interplanar separation is the result of a compromise between packing of flat croconate units and the spherical cations together with the water molecules that fill the alkali metal coordination spheres. The croconate dianion, C5O52–, belongs, together with the rhodizonate C6O62–, the squarate C4O42– and the deltate C3O32– dianions to the family of oxocarbon dianions. The prototype of

a

b Fig. 10a,b The ion organization in: a crystalline Cs2C5O5 (the Rb+ salt is isomorphous); b crys-

talline Rb2C6O6

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these dianions is represented by the rhodizonate dianion C6O62–, which has attracted the interest of many researchers in view of the structural analogy with benzene, C6H6. The structure of rubidium rhodizonate has also been reported showing that similar charge-assisted π-stacking interactions are established [37]. The study of the salts of cyclic oxocarbon anions demonstrates that the packing problem is that of accommodating together spherical objects (the alkali cations) and flat discoidal units (the croconate and rhodizonate dianions, but also the squarates, the hydrogen croconates, etc.). The best compromise is often attained by placing the disks “flat-on-flat” in a stack and by placing the spherical cations around the stack, as shown in Fig. 10 for crystalline Cs2C5O5 (the Rb+ salt is isomorphous) and Rb2C6O6. If this is not sufficient to complete the coordination sphere of the smaller alkali cations, water molecules are brought in the crystal to take the place of the large dianions as coordination and space fillers. The fact that the discoidal dianions do not show a preferential relative orientation, and can slide on each other, indicates that the electronic gain from this type of weak non-covalent interaction is either null or too small to be of any relevance in the presence of the much stronger electrostatic interactions. Altogether, the short interatomic/interplanar separations in the oxocarbon salts appear to be the result of the “electrostatic compression” arising from the attractive M+…C5O52– interactions, that largely overcompensate for the repulsive M+…M+ and C5O52–…C5O52– interactions.

7 How to Use Non-Covalent Interactions Between Ions Several crystal engineering groups are exploiting the possibility of sustaining hydrogen bonding networks with ionic charges on the building blocks. The utilization of coordination complexes [38] allows the incorporation of transition metals into the design of hydrogen bonded crystalline solids. The metal centres are either used to provide a directing influence upon the hydrogen bonded links between neighbouring building blocks, e.g. perhalometallate systems and combined coordination chemistry-hydrogen bonded systems, or are appended as potentially functional groups to the parent hydrogen-bonded organic network, e.g. π-bonded organometallic systems. For instance, coordination of halide ligands to transition metals leads to good hydrogen bond acceptor capability. Methods have been developed to prepare salts of perhalometallate complexes ([MXn]m–, X=Cl, Br etc.; M=Pt, Zn, Mn, Pb etc.) with organic cations with N-H hydrogen bond donor functionality [39]. These systems are modular and robust and offer the opportunity to exploit shape, charge and functional groups of the ions in order to control the crystal structures and the hydrogen bond networks they form [40]. An alternative, widely used, approach to charge assisted networks is based on the exploitation of direct acid-base reactions. There are, broadly speaking, essentially two different means to obtain charge-assisted interactions, which depend on whether the network is constructed of ions of the same charge (homoionic hydrogen bonded networks) or of ions of opposite charge (hetero-ionic networks). These two limiting situations are shown in Fig. 1c,d. The utilization of

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multiple hydrogen bonding systems allows also intermediate situations, whereby homo-ionic chains are interlinked by hetero-ionic interactions. The most common situation is encountered when the proton acceptor is chosen such as to interact directly with the proton donor, e.g. assisted hydrogen bonding interactions of the (–)X…H-Y(+) types are formed. This is the case, for instance, of nitrogen containing bases, which are protonated upon reaction with polycarboxylic acid molecules, e.g. RCOOH–+NR3→RCOO(–)…(+)HNR3, leading to formation of strong (–)O…H-N(+) interactions, hence an anion-cation pairing in the solid state. The assembly can result in homo-ionic chains when the base cannot form competing hydrogen bonding interactions with the acid moiety. This is the case of the reaction between polycarboxylic acids with bases that do not carry strong acceptor/donor hydrogen bond groups. Partial deprotonation of the -COOH groups leads to self-assembly of acid anions via O-H…O(–) and (–)O-H…O(–) interactions [41]. By choosing the number of carboxylic groups (hence the number of potential donor/acceptor systems) and the stoichiometric ratios in the acid-base reactions, one can control the formation of O-H…O(–) and/or (–)O-H…O(–) interactions, hence homo-ionic self-assembly. There is a vast literature on the utilization of crystal engineering based on of inter-ionic hydrogen bonding interactions [42]; because of space limitations we shall be able to discuss only very few representative examples. The neutral dicarboxylic acid [Cr0(η6-C6H5COOH)2] and its oxidation product [CrI(η6-C6H5COOH)2]+ are isostructural, but not isoelectronic [43]. The two systems can be regarded as composed of a fundamental building block unit,“Cr(η6-C6H5COO)2”, which can participate in hydrogen bonding interactions as a mono-cation, a neutral species, a mono-anion or a di-anion, depending on the extent of protonation and on the metal atom oxidation state. Figure 11 shows a comparison of the neutral dimer formed by Cr0(η6-C6H5COO)2 molecules and the cationic chain formed by [CrI(η6-C6H5COOH)2]+ units in the [PF6]– salt. Soft molecular host networks based on layers of guanidinium cations spaced by pillars of sulfonate anions have been developed by Ward et al. (see Fig. 12) [44]. The (+)N-H…O(–) interactions between guanidinium cations and sulfonate anions render the superstructures at the same time robust and adaptable to the guest requirements, while the porosity can be tuned by changing the length of the pillars. These properties have been exploited in several applications, such as shapeselective separation of molecular isomers. Chiral crystals based on hydrogen l-malate anions have been assembled by Aakeröy et al. via (–)O-H…O(–) bridges in anionic layers. Since the two-dimensional network is highly reproducible, it can be transferred from crystal to crystal inducing non-centrosymmetry, a target on the route to materials for second harmonic generation. Analogous strategy has been used by us to self-assemble chiral frameworks around organometallic cations [45]. Lehn et al. have used inter-ionic hydrogen bridges to direct the recognition and self aggregation of metal complexes carrying terpyridine derived ligands joined by inter-cation (+)N-H…N(+) bridges [46]. The solid state arrangement of the Co(terpy)22+complex is highly dependent on the choice of counterion: in the [PF6]– salt a two-dimensional infinite network is formed via pairs of N-H…N in-

Hydrogen Bonding Interactions Between Ions: A Powerful Tool in Molecular Crystal Engineering

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a

b Fig. 11a,b The neutral dimer formed by: a two Cr(η6-C6H5COO)2 molecules is compared with; b the cationic chains formed by [CrI(η6-C6H5COOH)2]+ units in the [PF6](–) salt

Fig. 12 An example of guanidinium-sulfonate superstructures. The fundamental interaction responsible for both robustness and flexibility is the charge-assisted (+)N-H…O(–) hydrogen bonding between the guanidinium cations and the sulfonate anions, which can be varied in shape and length [44]

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Fig. 13 A two-dimensional infinite network is formed via pairs of (+)N-H…N(+) interactions in the [PF6]– salt of the cobalt complex Co(terpy)22+ (top), while a quarter of the potential HB interactions are not formed leading to a broken network in the case of the [BF4]– salt (bottom) [46]

teractions, while in the BF4– salt a broken network is observed (Fig. 13). This provides an example of competition between formation of N-H…N interactions and optimisation of the Coulombic interactions that, in turns, depends on the size of the ions.

8 Hydrogen Bonded Networks Can React or Transform The exploitation of the reactivity of molecular crystals lies close to the origins of crystal engineering and is at the heart of the pioneering work of Schmidt [47a]. The idea is that of organizing molecules in the solid state using the principles of molecular recognition and self-assembly. Successful results have been obtained with bimolecular reactions, particularly [2+2] photoreactivity and cyclisation [47b,c]. Another important area is that of host-guest chemistry.

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Diverse applications of inclusion compounds have been described by Toda, Kaupp and Nassimbeni [48] and others. Many reactions involve breaking and formation of hydrogen bonded networks. Another important application of hydrogen bond reactivity is in the investigation of reactions between engineered molecular solids and molecules in the vapour phase. Heterogeneous gas-solid reactions are well known in chemistry thanks to the pioneering work of Curtin and Paul [49]. A case study is the reaction of crystalline benzoic acid, which forms cyclic hydrogen bonded carboxylic rings in the solid state, with ammonia vapours. The reaction leads to quantitative formation of a 1:1 ammonium salt. Curtin and Paul were able to demonstrate that certain crystal faces are attacked preferentially by the ammonia vapour, and the resulting reaction front travels more rapidly through the crystals along directions corresponding to specific molecular arrangements [50, 51]. Crystalline p-chlorobenzoic anhydride reacts with gaseous ammonia to give the corresponding amide and ammonium salt [52]; similar reactions have been investigated in the case of optically active cyclopropane carboxylic acid crystals [53].All these reactions imply rupture of O-H…O hydrogen bonds between neutral molecules, and replacement with N-H…O bonds between cations and anions. One particularly useful combination is that between ionic charge and number of hydrogen bonding donor groups, when the ionic or neutral nature of the molecule can be tuned by means of acid-base reactions.An example is provided by the dicarboxylic acid [CoII(η5-C5H4COOH)2], which is readily oxidised in air to the cation [CoIII(η5-C5H4COOH)2]+ [54]. The zwitterion [CoIII(η5-C5H4COOH)(η5C5H4COO)] can be quantitatively prepared from the corresponding dicarboxylic cationic acid [CoIII(η5-C5H4COOH)2]+. The “cobaltocene diacid” family provides a grand total of six building blocks potentially useful in crystal engineering (see Fig. 14). By controlling the oxidation state and/or the pH, this relatively simple chemical system can afford complexes that can be diamagnetic or paramagnetic [Co(III), and Co(II)], neutral, cationic or anionic [from +1 to –2], hydrogen bonding donor or acceptor [-COOH, and -COO(–)], or a combination of these charac-

Fig. 14 Top: [(C5H4COOH)2CoII] and deprotonation derivatives. Bottom: [(C5H4COOH)2CoIII]+

and deprotonation derivatives

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Fig. 15 The solid zwitterion [CoIII(η5-C5H4COOH)(η5-C5H4COO)] reacts reversibly with acid

and base vapours

teristics. The zwitterion [CoIII(η5-C5H4COOH)(η5-C5H4COO)] possesses amphoteric behaviour because of the presence of one -COOH group, which can react with bases, and one -COO(–) group, which can react with acids. The zwitterion undergoes fully reversible heterogeneous reactions, exploiting the possibility of switching between neutral and charged hydrogen bonding interactions. These features can be exploited to prepare a reversible gas-trap system by reacting the zwitterionic species with hydrated vapours of a variety of acids (e.g. HCl, CF3COOH, HBF4, and HCOOH) and bases (e.g. NH3,NMe3,NH2Me) with formation of the corresponding salts [54]. The salts resulting from the heterogeneous reaction contain the organometallic moiety either in its fully protonated form [CoIII(η5C5H4COOH)2]+ (in the reaction with acids) or in its fully deprotonated form [CoIII(η5-C5H4COO)2]– (in the reaction with bases), as shown in Fig. 15. The two types of reactions imply the interconversion between neutral O-H…O hydrogen bonding interactions and (+)O-H…X(–) and (–)O…H-N(+) interactions, respectively. Contrary to all other reactions with acids the behaviour of the zwitterion towards vapours of formic acid is intriguing, since no proton transfer is observed. The formic acid vapour uptake generates a material composed of pairs of zwitterion molecules, linked by O-H…O bonds between the protonated -COOH and the deprotonated -COO(–) groups [O…O separation 2.526(4) Å] residing on the organometallic moiety. The zwitterion dimers interact with two formic acid molecules via O-H…O and C-H…O hydrogen bonds [O…O distance 2.541(4),

Hydrogen Bonding Interactions Between Ions: A Powerful Tool in Molecular Crystal Engineering

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(C)H…O distance 2.43(5) Å]. The intramolecular parameters are indicative of the presence of distinct -COOH and -COO– groups over the zwitterionic complex [C-O1 1.209(4), C-O2 1.306(4); C-O3 1.243(4), C-O4 1.247(4) Å]. On the other hand, the C-O distances within the HCOOH moiety [C-O5 1.305(5), C-O6 1.199(5) Å] indicate that the formic acid molecule retains its acidic hydrogen. This has also been confirmed by 13C CPMAS NMR spectroscopy [54c]. Since HCOOH is a weaker Brønsted acid than HCl, HBF4 and CF3COOH, the different behaviour has been attributed to the difference in relative acidity of the absorbed acid with respect to the zwitterion. Even though the weaker HCOOH acid does not protonate the zwitterion, it is still capable of association with the zwitterion via strong O-H…O hydrogen bonding interactions. On this premise, the reaction between the solid zwitterion and HCOOH(vapour) could be regarded as a special kind of solvation rather than as a heterogeneous acid-base reaction. In a sense, gas-solid reactions and gas-solid solvation differ only in the energetic ranking of the interactions (whether covalent or non-covalent) that are broken or formed through the processes. In the reversible supramolecular reaction of solid [CoIII(η5-C5H4COOH)(η5-C5H4COO)] with gaseous HCOOH, O-H…O and C-H…O bonds are disrupted and/or rearranged, while covalent bonds are not affected. The location of the hydrogen atoms in hydrogen bonded systems is often difficult to ascertain. When X-ray diffraction is used there is an experimental limitation to face, as it is usually difficult to locate the very light H-atom in Fourier maps and, even when this is possible, the technique can provide information on electron density centroids rather than on the position of the light nucleus. Neutron diffraction is required for an unambiguous location of the H-atom. In ionic hydrogen bonds the situation may occur where a knowledge of the proton position in a donor-acceptor system is necessary to know whether proton transfer, i.e. protonation of a suitable base, has occurred or not. The proton transfer process along an X-H…Y interaction, whether associated with a phase transition or not, may imply the transformation of a molecular crystal into a molecular salt.Wilson [55] has discussed, on the basis of an elegant neutron diffraction study, the migration of the proton along an O-H…O bond in a co-crystal urea-phosphoric acid (1:1), whereby the proton migrates towards the mid-point of the hydrogen bond as the temperature is increased, becoming essentially centred at T=335 K (see Fig. 16). Mootz and Wiechert [56], on the other hand, have isolated two crystalline materials composed of pyridine and formic acid of different composition. In the 1:1 co-crystal the formic acid molecule retains its proton and transfer to the basic N-atom on the pyridine does not take place (hence molecules are linked by neutral O-H…N interactions). In the 1:4 cocrystal, on the contrary, one formic acid molecules releases its proton to the pyridine molecule establishing N-H(+)…O interactions. In a sense this latter case could be regarded as an unusual case of solvate crystal (see Fig. 17). It is a useful notion to consider that the reaction of a molecular solid, whether formed of organic, organometallic molecules or coordination compounds, with a vapour is conceptually related to the supramolecular reaction of a crystalline material with a volatile solvent to form a new crystalline solid (Fig. 18). Indeed, the two processes, solid-gas reaction and solid-gas solvation, differ only in the

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Fig. 16 The migration of the proton along an O-H…O bond in a co-crystal urea-phosphoric acid (1:1), as the temperature is increased from 150 K (top) to 335 K (bottom), becoming essentially centred (neutron study) [55]

energetic ranking of the interactions that are broken or formed through the processes. In solvation-desolvation processes one is dealing mainly with non-covalent van der Waals or hydrogen bonding interactions, whilst in chemical reactions covalent bonds are broken or formed. This awareness is useful to the engineering of molecular materials whereby gas uptake is exploited not only to produce new crystalline forms of a given substance but also as a means to produce new materials in crystalline form. Clearly, the conceptual borderline between the two types of processes is very thin. One may purposefully plan to assemble molecules that are capable of absorbing molecules from the gas phase and, possibly, to react with them. The existence of different configurations of the proton atom location within O-H…O systems is at the basis of the possibility of constructing proton conducting materials. These materials, in particular those based on solid acids such as CsHSO4 [57a] (see Fig. 19), Rb3H(SeO4)2 etc., have applications in a number of devices such as H2 and H2O sensors, fuel and steam cells and high energy density batteries. The solid acids derived from polyprotic acids such as H3PO4 and H2SO4by partial deprotonation all show the presence of strong inter-anionic hydrogen bonding association in the solid state. These systems often undergo an order-disorder phase transition at high temperature, which is associated with a dramatic increase of the conductivity [57b,c]. Such behaviour is called superprotonic transition and has been recently begun to be exploited as solid fuel cell electrolytes because they have the advantage of anhydrous proton transport and are thermally stable (up to 500–550 K). Hybrid anionic systems, such as Cs2(HSeO4)(H2PO4), have also begun to be investigated [58]. The salt undergoes a gradual super-protonic transition over the temperature range 334–378 K, with

Hydrogen Bonding Interactions Between Ions: A Powerful Tool in Molecular Crystal Engineering

27

a

b Fig. 17a,b The two forms of the adduct between pyridine and formic acid of different com-

position: a in the 1:1 co-crystal a the formic acid molecules retains its proton and transfer to the basic N-atom on the pyridine does not take place (hence molecules are linked by neutral O-H…N interactions); b in the 1:4 co-crystal, in contrast, one formic acid molecules releases its proton to the pyridine molecule establishing N-H(+)…O interactions [56]

28

Dario Braga et al.

Fig. 18 The reaction of a molecular solid, whether formed of organic, organometallic molecules

or coordination compounds, with a vapour is conceptually related to the supramolecular reaction of a crystalline material with a volatile solvent to form a new crystalline solid

Fig. 19 The structure of crystalline CsHSO4 at 150 K (hydrogen atoms were not located) [57a]

an activation energy for proton conduction of 149.37 kJ mol–1. The phenomenon of superprotonic conductivity is explained with the possibility of proton jumps from one anion to the nearby one associated with the onset of the reorientational motion of the oxoacid anions.

9 Conclusions The principal non-covalent interaction in molecule-based crystal engineering is the hydrogen bond. The reason for this preference is simple, the hydrogen bond is the strongest of the non-covalent interactions and possesses a high degree of directionality. Strength and directionality (namely transferability and repro-

Hydrogen Bonding Interactions Between Ions: A Powerful Tool in Molecular Crystal Engineering

29

ducibility) are conditional for an interaction to be useful in the making and holding together superstructures, whether supermolecules or crystals. In this respect the combination of weak non-covalent interactions (hydrogen bonding and πstacking) with the Coulombic field generated by ions poses an intriguing problem: the architectural features are largely determined by the steric and energetic demand of the directional interactions, whilst much of the cohesion is provided – just as in any ionic crystal – by the balance of Coulombic attractions and repulsions. Hydrogen bonding interactions between ions need to occupy a special place in the library of non-covalent interactions of interest to supramolecular chemists and crystal engineers. We have argued in this chapter that the utilization of hydrogen bonding and π-stacking interactions between ions is a useful instrument to confer directionality to the Coulombic field generated by the ions. The (few) examples provided in the latter section demonstrate that the issue stated in the title is not academic only, but carries practical implications.As with the hydrogen bonds between molecules, the hydrogen bonding interactions between ions can be used to make aggregates, because they are directional, hence reproducible and transferable. The cohesion of the aggregate does of course depend on the Coulombic field generated by the ions. Consequently that the physical and chemical properties of the crystalline solid (solubility, melting, conductivity, etc.) will be those of ionic salts and not those of molecular crystals. An awareness of the importance of these factors is necessary when planning the construction or the exploitation of the resulting materials. When discussing the relative weights of the various types of interactions, simultaneously and not independently at work in a stable solid, it should be kept in mind that stabilizing interactions can be effective even in the presence of repulsive forces, as well as the opposite, namely destabilizing interactions may be observed in the presence of attractive forces. An appreciation of this conceptual distinction is crucial to the understanding of the effect of ionic charges on the nature of pair-wise non-covalent interactions. Particularly when the ions carry the same charge. We have shown that crystals held together by hydrogen bonding interactions between ions or molecular ions can be devised, designed and constructed by using the principles of molecular crystal engineering. These materials can then be reacted and/or transformed, for instance, by manipulating the hydrogen bond network. This is particularly relevant in the case of superprotonic conductivity and or gas-solid reactions discussed above.

10 References 1. (a) Lehn JM (1990) Angew Chem Int Ed Engl 29:1304; (b) Whitesides GM, Simanek EE, Mathias JP, Seto CT, Chin DN, Mammen M, Gordon DM (1995) Acc Chem Res 28:37; (c) Gans W, Boyens JCA (1997) Intermolecular interactions. Plenum, New York. (d) Braga D, Grepioni F (2000) Acc Chem Res 33:601 2. (a) Lehn, JM (1995) Supramolecular chemistry: concepts and perspectives.VCH,Weinheim; (b) Philp D, Stoddard JF (1996) Angew Chem Int Ed Engl 35:1154; (c) Steed JW, Atwood JL (2000) (eds) Supramolecular chemistry. Wiley, Chichester, UK; (d) Haiduc I, Edelmann FT

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19. (a) Braga D, Grepioni F (1996) J Chem Soc Chem Commun 571 20. Inorganic Crystal Structure Database (ICSD), Fachinformationszentrum (FIZ) Karlsruhe and Gmelin Institut 21. (a) Nangia A (2002) Cryst Eng Comm 4:93; (b) Steiner T (2001) Acta Cryst B57:103; (c) Ziao N, Laurence C, Le Questel JY (2002) Cryst Eng Comm 326; (d) Allen FH, Motherwell WDS (2002) Acta Cryst B58:407; (e) Steiner T, Schreurs AMM, Lutz M, Kroon J (2001) New J Chem 25:174; (f) Nangia A, Biradha K, Desiraju GR (1996) J Chem Soc Perkin Trans 2:943; (g) Takahashi O, Kohno Y, Iwasaki S, Saito K, Iwaoka M, Tomoda S, Umezawa Y, Tsuboyama S, Nishio M (2001) Bull Chem Soc Jpn 74:2421; (h) Suezawa H, Yoshida T, Umezawa Y, Tsuboyama S, Nishio M (2002) Eur J Inorg Chem 3148; (i) Ziao N, Graton J, Laurence C, Le Questel JY (2001) Acta Cryst B57:850; (i) Aakeroy CB, Evans TA, Seddon KR, Palinko I (1999) New J Chem 23:145 22. Dunitz JD, Gavezzotti A (1999) Acc Chem Res 32:677 23. (a) Pyykkö P (1997) Chem Rev 97:597; (b) Mathieson T, Schier A, Schmidbaur H (2000) J Chem Soc Dalton Trans 21:3881 24. (a) Ohmine I, Saito S (1999) Acc Chem Res 32:741; (b) Leopold KB, Canagartna M, Phillips JA (1997) Acc Chem Res 30:57 25. Braga D, Grepioni F (1998) New J Chem 1159 26. Braga D, Grepioni F (1997) Acc Chem Res 30:81 27. Dunitz JD, Taylor R (1997) Chem Eur J 3:89 28. Braga D, Scaccianoce L, Grepioni F, Draper SM (1996) Organometallics 15:4675 29. Braga D, Novoa JJ, Grepioni F (2001) New J Chem 25:226 30. (a) Toda F, Takumi H,Akehi M (1990) J Chem Soc Chem Commun 1270; (b) Toda F, Okuda K (1991) J Chem Soc Chem Commun 1212; (c) Tanaka K, Fujimoto D, Oeser T, Irngartinger H, Toda F (2000) Chem Commun 413 31. (a) Etter MC, Reutzel SM, Choo CG (1993) J Am Chem Soc 115:4411; (b) Rastogi R, Singh NB (1968) J Phys Chem 72:4446; (c) Caira MR, Nassimbeni LR, Wildervanck AF (1995) J Chem Soc Perkin Trans 2213; (d) Pedireddi VR, Jones W, Chorlton AP, Docherty R (1996) Chem Commun 987; (e) Nichols PJ, Raston CL, Steed JW (2001) Chem Commun 1062; (f) Fernandez-Bertran JF (1999) Pure Appl Chem 71:581; (g) Boldyrev VV (1993) Solid State Ionics 63/65:537; (h) Toda F, Miyamoto H (1995) Chem Letters 861; (i) Makhaev VD, Borisov AP, Petrova LA (1999) J Organomet Chem 590:222; (l) Cave GWV, Raston CL, Scott JL (2001) Chem Commun 2159 32. Norby T (2001) Nature 400:877 33. (a) Del Sesto RE, Miller JS, Lafuente P, Novoa JJ (2002) Chem Eur J 8:4894; (b) Novoa JJ, Lafuente P, Del Sesto RE, Miller JS (2002) Cryst Eng Comm 373; (c) Steiner T (1999) Chem Commun 313 34. Braga D, Grepioni F, Tagliavini E, Novoa JJ, Mota F (1998) New J Chem 755 35. Braga D, Bazzi C, Grepioni F, Novoa JJ (1999) New J Chem 23:577 36. Braga D, Maini L, Grepioni F (2002) Chem Eur J 8:1804 37. Dunitz JD, Seiler P, Czchtizky W (2001) Angew Chem Int Ed 40:1779 38. Braga D, Grepioni F, Desiraju GR (1998) Chem Rev 98:1375 39. (a) Aullon G, Bellamy D, Brammer L, Bruton EA, Orpen AG (1998) Chem Commun 653; (b) Gillon AL, Lewis GR, Orpen AG, Rotter S, Starbuck J, Wang XM, Rodriguez-Martin Y, Ruiz-Perez C (2000) J Chem Soc Dalton Trans 3897 40. Brammer L, Swearingen JK, Bruton EA, Sherwood P (2002) Proc Nat Acad Sci USA 99:4956 41. Braga D, Grepioni F (1999) J Chem Soc Dalton Trans 1 42. See for example: (a) Beatty AM (2001) Cryst Eng Comm 51; (b) Aakeroy CB, Beatty AM, Leinen DS (2002) Cryst Eng Comm 310; (c) Brammer L, Burgard MD, Eddleston MD, Rodger CS, Rath NP, Adams H (2002) Cryst Eng Comm 239; (d) Beauchamp DA, Loeb SJ (2002) Chem Eur J 8:5084; (e) Uemura K, Kitagawa S, Kondo M, Fukui K, Kitaura R, Chang HC, Mizutani T (2002) Chem Eur J 8:3586 43. Braga D, Maini L, Grepioni F, Elschenbroich C, Paganelli F, Schiemann O (2001) Organometallics 20:1875 44. Holman KT, Pivovar AM, Swift JA, Ward MD (2001) Acc Chem Res 34:107

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Structure and Bonding, Vol. 111 (2004): 33–83 DOI 10.1007/b14140HAPTER 1

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Coordinated Guanidine Derivatives Peter Hubberstey · Unchulee Suksangpanya School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK E-mail: [email protected]

Abstract The formation of hydrogen-bonded supramolecular 1-D chains and 2-D sheets by

planar metal-containing centres co-ordinated by members of the family of hydrogen-bonding ligands, biuret (bu), carbamoylguanidine (cg), biguanide (bg) and their substituted derivatives has been reviewed. Biurets co-ordinate transition metal centres through two carbonyl oxygens, carbamoylguanidines through one imino nitrogen and one carbonyl oxygen and biguanides through two imino nitrogens. Planar cations with the hydrogen-bonding potential to form 1-D chain or 2-D sheet structures are formed by bidentate ligands with square planar metal centres, especially Ni2+and Cu2+, when the metal:ligand ratio is 1:2 [ML2]n+ and by tetradentate ligands when the metal:ligand ratio is 1:1 [ML]n+. The majority of the literature abstracted for the review reports biguanide-type co-ordination. Only a limited number of authors have considered biuret-type co-ordination and even fewer have investigated carbamoylguanidine-type co-ordination. Where comparable data are available, trends can be discerned and common structural themes and supramolecular synthons identified. Thus [ML2]n+ species containing two bidentate biguanides often use their N-H donors to act as four-connecting cations forming hydrogen-bonded 2-D sheets with (4,4) topologies. Modification of the two-connecting anion-mediator leads to networks of different size and cohesion. Chloride and bromide form relatively small, fragile, frameworks whereas nitrate and tetrafluoroborate form larger, more robust, networks. Similarly [ML]n+ species containing a single tetradentate biguanide often use their N-H donors to form anion mediated 1-D chains. Depending on the relative orientations of the cations, the chains are linked by hydrogen-bonding contacts through anions and/or solvent molecules to form either 1-D ribbons or 2-D sheets. These [ML]n+ cations have also been shown to bind the pyrimidine bases cytosine and thymine in elegant hydrogen-bonded 2-D sheet architectures. Keywords Hydrogen bonding · Supramolecular chains · Supramolecular sheets · Coordination ·

Guanidine

1

Introduction   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 34

2

Structural Chemistry of the Free Ligands, Biuret, Biguanide and 1-Carbamoylguanidine   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 37

3

Biuret Complexes  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 41

3.1 3.2

[M(bu)2]2+-Containing Species   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 41 [M(bu-H)2]-Containing Species   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 45

4

Biguanide and Related Complexes   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 46

4.1 4.2

Biguanide Complexes   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 47 2-Chlorobiguanide   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 49 © Springer-Verlag Berlin Heidelberg 2004

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Peter Hubberstey et al.

4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.4 4.5

N,N-Dimethylbiguanide   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . [M(dmbg)2]2+ Containing Structures   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . [M(dmbg-H)2] Containing Structures   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . [(Hdmbg)MCl3] Containing Structures   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . [H2dmbg][MCl4] Containing Structures  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . N,N′-Diphenylbiguanide   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . Ethylenebis(biguanide) [1,2-Bis(biguanidinyl)ethane] Complexes  .  .

5

Carbamoylguanidine Complexes   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 59

6

Amidino-O-Alkylurea Complexes   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 60

6.1 6.2 6.2.1 6.2.2

Compounds of Bidentate Amidino-O-Alkylureas   .  .  .  .  .  .  .  .  .  .  . Compounds of Tetradentate Bis(Amidino-O-Alkylureas)   .  .  .  .  .  . Halide-Mediated 1-D Chain Formation   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . Alkylsulfate- and Dimethoxydifluoroborate-Mediated 1-D Chain Formation  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 6.2.3 In Situ Anion Generation  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 6.2.4 Binding of Pyrimidine Bases to [Cu(Lkl)]2+ Cations to Form 2-D Sheet Architectures   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .

50 51 53 55 55 56 56

61 71 71 74 77 78

7

Conclusions  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 81

8

References   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 82

1 Introduction Supramolecular architectures in which transition metal cationic centres are linked via hydrogen-bonded supramolecular synthons [1, 2] comprise an increasingly important class of inorganic co-ordination polymers [3], owing to their multi-dimensional, multi-functional network structures. A particularly significant collection of these materials, which forms the basis of this chapter, is that based on planar metal-containing centres co-ordinated by members of the family of hydrogen-bonding ligands, biuret (bu), carbamoylguanidine (cg), biguanide (bg), and their substituted derivatives. Schematic representations of the [ML2]2+ cations containing these ligands are shown in Scheme 1. While bu and bg co-ordinate the metal centre through two carbonyl oxygen atoms and two imino nitrogen atoms, respectively, cg acts as the intermediary using one imino nitrogen atom and one carbonyl oxygen atom. The most studied metal centre is square planar copper(II). There is also a significant number of compounds based on square planar nickel(II), palladium(II) and platinum(II).A limited number of compounds with trans-located auxiliary ligands in octahedral co-ordination spheres are also known for zinc(II). However, the majority of octahedral complexes containing biuret (bu), carbamoylguanidine (cg) or biguanide (bg) are [ML3]n+ species. These will not be covered in this chapter.

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

a

35

b

c

Scheme 1

The schematic representations of the [ML2]2+ cations with biuret (Scheme 1a; bu), 1-carbamoylguanidine (Scheme 1b; cg) and biguanide (Scheme 1c; bg) show the remarkable hydrogen-bonding potential that these cations have for generating 1-D chain and 2-D sheet supramolecular architectures, the total number of N-H donors available to form hydrogen-bonds increasing from 10 for [M(bu)2]n+ through 12 for [M(cg)2]n+ to 14 in [M(bg)2]n+. Derivatisation reduces the number of N-H donors but still leaves sufficient to form extended structures with interesting architectures. Commonly studied derivatives include N,N′-dimethylbiguanide (Scheme 2a; dmbg), an oral antihyperglycemic drug used in the management of non-insulin-dependent diabetes mellitus, N,N-diphenylbiguanide (Scheme 2a; dpbg) and ethylenebis(biguanide) (Scheme 2b; ebbg), which have 10 and 12 N-H donors, respectively. Our own efforts have concentrated on N-alkylamidino-O-alkylureas (Scheme 2c) and polymethylene bridged bis(amidino-Oalkylureas) (Scheme 2d), each of which has 8 N-H donors. In solution, the ligands exhibit acid-base behaviour forming cationic and anionic species: H O+

H O+

OH–

H O+

OH–

3 3 ––––––– ––––––– [H2bg]2+ ← [Hbg]+ ← bg ––––––→ [bg-H]– 3 ––––––– [Hcg]+ ← cg ––––––→ [cg-H]–

OH–

bu ––––––→ [bu-H]– The four species formed by biguanide are shown in Scheme 3. Although electrostatic repulsion is expected to prevent co-ordination by cationic species there

36

Peter Hubberstey et al.

Scheme 2

a

b

c

d

a

Scheme 3

c1

b2

b1

c2

d

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

37

are a limited number of examples in which [Hbg]+ acts as a monodentate ligand by adopting the conformation shown in Scheme 3 (b2). In this review, we will concentrate on the complexes formed by bidentate and tetradentate chelating neutral and anionic ligands.

2 Structural Chemistry of the Free Ligands, Biuret, Biguanide and 1-Carbamoylguanidine Structural data are available for biuret monohydrate [4, 5], for the 1:1 adduct of carbamoylguanidine and ethanol [6] and for biguanide [7]. All three molecules adopt molecular conformations containing intramolecular hydrogen-bonds as shown in Scheme 4, in which pertinent interatomic distances are also quoted. Although bu and cg adopt planar structures (Scheme 4a,b), which require the nitrogen atoms of the amino moieties to be sp2 hybridised with full pz orbitals, there is structural evidence that bg does not adopt the expected planar structure (Scheme 4c), one of the amino moieties forming a pyramidal arrangement. First,

Scheme 4

a

b

c

d

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Peter Hubberstey et al.

Fig. 1 A projection of the structure of biuret monohydrate onto the (1 0 0) plane, showing the honeycomb architecture of the 2-D sheet (atom identification: carbon – intermediate dark grey circles, nitrogen – intermediate black circles, oxygen – intermediate light grey circles; hydrogen – small light grey circles) [4, 5]

the range of angles at the nitrogen atom (104.5°–118.0°; average 109.4°) is typical of an sp3 hybridised system and, second, the longer C-NH2 interatomic distance in the bg framework compared with the comparable distances in the bu and cg frameworks suggests the presence of a directional lone pair on the nitrogen atom (Scheme 4d). This loss of planarity is attributed to the need for extra acceptor sites in bg. If all three molecules were totally planar, the number of N-H donors available for inter-molecular hydrogen-bond formation would decrease from 6 for bg, through 5 for clge to 4 for bu whilst the number of acceptor sites would increase from 1 for bg (one nitrogen lone pair) through 2 for clge (one nitrogen and one oxygen lone pair) to 3 for bu (three oxygen lone pairs). It follows that bu would have the greatest and bg the least potential for formation of 2-D sheet architectures. However, when bg adopts the structure shown in Scheme 4d it doubles the number of acceptor sites, markedly increasing its hydrogen-bonding capability. In biuret monohydrate [4, 5], the three oxygen lone pairs and three of the four N-H donors are used to form three double A·D…D·A [R22(8) motif] interactions (Fig. 1), which generate a honeycomb architecture of bu molecules. The (6,3) topology frameworks form slightly buckled 2-D sheets, which lie parallel to the (1 0 0) plane. The fourth N-H donor forms a weak N-H…O contact to the intramolecularly hydrogen-bonded oxygen which supports one of the R22(8) motifs by forming a R21(6) motif. The honeycomb cavities form channels running in the a direction, within which there are zig-zag chains of water molecules.

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

39

Fig. 2 A view of the 1-D ribbons which form the basis of the structure of the 1:1 adduct of

1-carbamoylguanidine and ethanol (atom identification as for Fig. 1) [6]

In the 1:1 adduct of carbamoylguanidine and ethanol [6], the two acceptor sites of clge form, with two of the five N-H donors, double A·D…D·A [R22(8) motif] interactions to generate zig-zag chains aligned along the a direction (Fig. 2). One of the three remaining N-H donors forms a N-H…O contact with the ethanol oxygen atom. The ethanol O-H donor forms a weak O-H…O interaction with the intramolecularly hydrogen-bonded oxygen similar to the comparable NH…O contact in biuret monohydrate. The N-H…O and O-H…O contacts allow the ethanol molecules to bridge pairs of clge molecules thus forming an R43(10) motif (Fig. 2). The resultant ribbons, rather than forming a 2-D sheet, align alternately parallel to the (0 1 1) and (0 –1 1) planes. The other two N-H donors are redundant. Owing to the paucity of acceptor sites in bg the acceptor site on the central nitrogen atom is shared by two N-H…N contacts and the nitrogen atom of one of the -NH2 groups loses its planarity and acts as an acceptor in a single N-H…N contact [7]. The former contacts form part of centrosymmetric R22(8) motifs which generate zigzag chains of bg molecules, which lie along the b direction (Fig. 3). The latter contact links the zig-zag chains of bg molecules to form a 2-D sheet construction parallel to the (1 0 0) plane (Fig. 4). Of the six bg N-H donors only three are utilised in hydrogen-bonds, the other three being redundant. Selected structural parameters for the hydrogen-bonding interactions in the structures of bu·H2O [4, 5], cg·EtOH [6] and bg [7] are collected in Table 1. Not unsurprisingly, the dimensions of the intra-molecular interactions are comparable, despite the acceptor being nitrogen in the case of bg and oxygen in the case of bu and cg. Also, in accord with normal observation [8], the intermolecular N-H…O contacts [N…O range=2.84–3.00 Å (average 2.95 Å); N-H…O range 165–170° (average 168°)] are shorter than the corresponding N-H…N contacts [N…N range=3.06–3.21 Å (average 3.15 Å); N-H…N range 168–174° (average 171°)] suggesting a stronger interaction [8]. The angles at the acceptor atoms are

40

Peter Hubberstey et al. a

0

b

c

Fig. 3 A view of the 1-D zig-zag chains which form the basis of the structure of biguanide

(atom identification as for Fig. 1) [7]

a

b

0

c

Fig. 4 A projection of the structure of biguanide onto the (0 1 0) plane, showing the N-H…NH2-

contacts, which generate the 2-D sheet architecture (atom identification as for Fig. 1) [7]

typical of sp2 hybridised oxygen and nitrogen atoms [108–126° (average 117°)] except for those at the non-planar nitrogen which are typical of an sp3 hybridised nitrogen atom [106–109° (average 108°)]. The low values of the torsion angles at the acceptor atoms for bu and cg confirm the planarity of their 2-D sheet architectures while the larger values observed for bg are typical of its more buckled construction.

41

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

Table 1 Comparison of the structural parameters of the hydrogen bonding interactions in bu·H2O [4, 5], cg·EtOH [6] and bg [7]. The sequence of N-H…X contacts considered is based on an anticlockwise consideration of the acceptor sites depicted in Scheme 4 starting from the intra molecular contact

N-H/Å

H…X/Å N…X/Å

N-H…X/Å H…X-C/Åd

Torsionc

Intra N-H…X bu (X=O) cg (X=O) bg (X=N)

0.92 0.95 0.90

2.14 1.97 1.98

2.72 2.64 2.64

120 125 129

– – –

8 9 20

Inter N-H…X bu (X=O) bu (X=O) bu (X=O) cg (X=O) cg (X=N) bg (X=N)a bg (X=N)b bg (X=N)b

0.92 0.79 0.93 0.95 0.87 1.06 0.92 0.93

1.94 2.22 2.05 2.06 2.34 2.01 2.29 2.21

2.84 3.00 2.96 2.99 3.19 3.06 3.21 3.13

165 170 168 167 168 174 172 170

116 119 119 126 108, 117 106, 108, 109 – –

10 12 12 6 40 88 28 48

Addl. Y-H…X N-H…O (bu…bu) O-H…O (EtOH…cg) N-H…O (cg…EtOH)

0.91 0.85 0.89

2.34 1.96 2.08

3.15 2.81 2.95

149 175 165

172 – –

18 89 –

a

sp3 hybridised N acceptor. Hybridised N acceptor. c These data are average torsion angles at the acceptor atom. d For the sp3 hybridised N acceptor two of these angles are H…N-H angles andone is an H…N-C angle. b

3 Biuret Complexes Biuret (bu) co-ordinates transition metals in both neutral and anionic forms. Compounds containing neutral bu include both bis- and tetrakis-complexes. The latter are limited to the larger cations of the group II {e.g., [Sr(bu)4]2+ [9]} and lanthanide groups {e.g., [Sm(bu)4]2+ [10, 11]}. Only the former are considered in detail in this review, together with bis-complexes containing anionic bu as they have the greater potential for formation of 1-D chains and 2-D sheets. 3.1 [M(bu)2]2+-Containing Species

When co-ordinating transition metal cations the neutral biuret molecule is versatile; it adopts diverse conformations to form [M(bu)2Cl2] (M=Cu [12], Zn [13], Cd [14] or Hg [15]) complexes. With cadmium(II) and mercury(II), bu adopts a similar conformation to that in the free ligand, retaining its short intramolecu-

42

Peter Hubberstey et al. a b 0

c

Fig. 5 A view of the structure of [Cd(bu)2(µ-Cl)2]∞ showing the [Cd(µ-Cl)2]∞ chains which align along the a axis (atom identification as for Fig. 1 plus cadmium large light grey circles; chlorine large dark grey circles) [14]

lar hydrogen-bond, leaving just one oxygen to bind to the metal centre and hence monodentate character.With copper(II) and zinc(II), however, bu acts as a bidentate chelating ligand, both oxygen atoms binding to the metal centre. The difference in behaviour has been attributed to the preferred formation, by cadmium(II) and mercury(II), of [MX2]∞ chains in which pairs of chloride anions link metal centres to give a square planar ‘MX4’ arrangement, the octahedral coordination sphere then being completed by two monodentate bu molecules; [MX2]∞ chain formation does not occur for copper(II) and zinc(II) and so the anions only occupy two of the six available co-ordination sites allowing bu to act as a bidentate chelating ligand. In [Cd(bu)2(µ-X)2]∞ [14], the [CdX2]∞ chains, which align along the a direction (Fig. 5), are linked by ribbons of hydrogen-bonded bu molecules to form 2-D sheets parallel to the (1 0 –2) plane (Fig. 6).

c a b 0

Fig. 6 A projection of the structure of [Cd(bu)2(µ-Cl)2]∞ onto the (1 0 –2) plane, showing the

2-D sheet architecture (atom identification as for Fig. 5) [14]

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

43

b

0 a

c

Fig. 7 A projection of the structure of [Zn(bu)2]Cl2 onto the (1 0 2) plane, showing the 2-D

sheet architecture (atom identification as for Fig. 1 plus zinc large light grey circles; chlorine large dark grey circles) [13]

The hydrogen-bonding interactions in the extended structure of [Zn(bu)2]Cl2 [13] lead to a 2-D sheet architecture parallel to the (1 0 2) plane (Fig. 7). One of the five N-H donors is involved in a centrosymmetric double A·D…D·A [R22(8) motif] interaction to an oxygen of an adjacent cation thus forming a ribbon of [Zn(bu)2]2+ cations. The other four N-H donors form N-H…Cl contacts to three separate chloride anions which bridge the ribbons to generate the 2-D extended structure. The sheets are linked by relatively long (2.53 Å) axial Zn…Cl coordinate bonds (Fig. 8). The situation is quite different in [Cu(bu)2X2] presumably owing to different structural parameters resulting from the weaker axial Cu…Cl (2.96 Å) and stronger equatorial (1.935 Å, c.f., Zn…O 2.027, 2.046 Å) interactions. In the copper(II) complex [12], a 2-D sheet is not formed, hydrogenbonding interactions linking isolated [Cu(bu)2X2] molecules to give a complex 3-D network. Lindoy et al. [16], have recently investigated the perturbation to the biuret lattice (Fig. 1) resulting from the introduction of [Cu(dmca)2]2+ cations [dmca= di(methoxycarbimido)amine], Br– anions, MeOH and MeCN molecules. An alternating layered structure is formed in which one layer [stacked parallel to (0 0 1) at z/c=0.00 and 0.50] comprises bu molecules and Br– anions (Fig. 9) and the other [stacked parallel to (0 0 1) at z/c=025 and 0.75] comprises [Cu(dmca)2]2+

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Peter Hubberstey et al.

c

a

0

b

Fig. 8 A view of the structure of [Zn(bu)2]Cl2 showing the axial Zn…Cl co-ordinate bonds which determine the stacking arrangements of the 2-D sheets (atom identification as for Fig. 7) [13]

a

c 0

b

Fig. 9 A projection of the structure of [Cu(dmca)2]Br2·2bu·2 MeOH·0.8 MeCN onto the (0 0 1)

plane showing the layers at z/c=0.00 or 0.50 comprising bu molecules and bromide anions (atom identification as for Fig. 1 plus copper large light grey circles; bromine large dark grey circles) [16]

cations, MeOH and MeCN molecules (Fig. 10). The principal links between layers are weak axial Cu…O interactions (2.977, 2.927 Å) involving the intramolecularly hydrogen-bonded bu carbonyl oxygen. The architecture of the bu containing layer (Fig. 9) is highly reminiscent of that of the sheets found in the structure of biuret monohydrate (Fig. 1). The honeycomb arrangement of bu molecules is exactly the same, the only real differences being the replacement of the water molecules present in the channels of the (6,3) topology network (Fig. 1)

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

45

a

c 0

b

Fig. 10 A projection of the structure of [Cu(dmca)2]Br2·2bu·2MeOH·0.8MeCN onto the (0 0 1)

plane showing the layers at z/c=0.25 or 0.75 comprising [Cu(dmca)2]2+ cations, MeOH molecules and MeCN molecules (atom identification as for Fig. 9) [16]

by bromide anions (Fig. 9) and a significantly increased corrugation of the sheet structure. Whereas the dihedral angle between the bu molecules and the stacking plane in biuret monohydrate is 7.3° those in the derivative structure are 20.1 and 25.2°. 3.2 [M(bu-H)2]-Containing Species

The structure of a deprotonated biuret derivative is typified by that of K2[Cu(bu)2]·4H2O (Fig. 11) [17]. Owing to the loss of two protons from the bu molecule, one from each NH2 group, the bu2– ligand binds the metal through nitrogen rather than oxygen atoms. Despite the reduced number of N-H donors, a 2-D structure, parallel to the (1 0 –3) plane, is formed. The N-H donors on the co-ordinated nitrogen atoms form a R21(6) motif with the acceptor oxygen of a lattice water molecule which, in turn uses one of its hydrogen atoms to form a OH…O hydrogen bond to a bu2– carbonyl oxygen. The 4,4 rhombic grid so formed provides space for two water molecules, each of which is hydrogen-bonded to a bu2– carbonyl oxygen, and two K+ cations (Fig. 11). The sheets are stacked such that deprotonated nitrogen atoms are located in the axial sites of copper(II) centres in adjacent planes (Cu…N=3.332 Å).

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Peter Hubberstey et al.

c

a 0

b

Fig. 11 A projection of the structure of K[Cu(bu)2]·4H2O onto the (1 0 –3) plane showing 2-D rhombic (4,4) grid formation (atom identification as for Fig. 1 plus copper large light grey circles; potassium large black circles) [17]

4 Biguanide and Related Complexes Although a significant number of transition metal complexes of biguanide and its derivatives, 2-chlorobiguanide, N,N-dimethylbiguanide, N,N′-diphenylbiguanide or ethylenebis(biguanide) have been structurally characterised, there is no systematic coverage. The studies are independent and trends have not been established. However, Lindoy et al. [18] have recently published an analysis of the conformations adopted by a number of square planar species containing pairs of biguanide-like bidentate ligands. The alkyl substituents on the ligands can adopt

Scheme 5

a

b

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

47

X,X′-substituted conformations in which they are either syn or anti to with respect to the atom to which they are bonded. A search of the CSD showed that neutral ligands were exclusively syn-syn (Scheme 5a) and anionic ligands were exclusively anti-anti (Scheme 5b) conformers [18]. 4.1 Biguanide Complexes

Depending on the co-ordination preference of the metal centre, both bis- and tris-complexes are well known. The tris-complexes are formed primarily by more highly charged octahedral metal cations, bg adopting both neutral {e.g., [Mn(bg)3]4+ [19]or [Co(bg)3]3+ [20–23]} and anionic forms {e.g., [Mn(bg-H)3]+ [24]or [Co(bg-H)3] [25]} The bis-complexes are formed primarily with less highly charged square planar metal cations within which the bg acts as a neutral ligand. It is the latter bis-complexes that are considered in this review as they have the greater potential to form 2-D extended architectures. As the majority of structurally characterised bis(biguanide) complexes, [Cu(bg)2][ClO4]2 [26], [Cu(bg)2]Cl2·2H2O [27, 28], [Ni(bg)2]F2·2H2O [29], [Ni(bg)2]Cl2 [30] and [Pd(bg)2][ClO4]2 [31], are considered in the early literature, atomic co-ordinates are not available. Such data are available, however, for three bis(biguanide)copper(II) complexes, [Cu(bg)2]Cl2·2H2O [28], [Cu(bg)2][CO3]·4H2O more recently reported [32] and [Cu(bg)2][Cu(acac)2Cl]2 [33].

b

c 0

a

Fig. 12 A projection of the structure of [Cu(bg)2][CO3]·4H2O onto the (0 0 1) plane showing the 2-D sheet architecture (atom identification as for Fig. 1 plus copper large light grey circles) [32]

48

Peter Hubberstey et al.

When co-ordinated in the square planar [Cu(bg)2]2+ cation, bg loses its acceptor site on the backbone nitrogen atom [Scheme 3(c2)]. Consequently, assuming a planar structure, all hydrogen-bonded contacts would be expected to involve acceptor sites on anions or solvent molecules. Although this is the case for [Cu(bg)2]Cl2·2H2O [28] and [Cu(bg)2][Cu(acac)2Cl]2 [33] there is, in [Cu(bg)2][CO3]·4H2O [32],a direct N-H…N contact analogous to that found in the structure of free biguanide (Fig. 4) [7] which involves an sp3 hybridised -NH2 moiety as illustrated by the average (108.7°) interatomic angle at that nitrogen atom. This contact generates a zig-zag chain of [Cu(bg)2]2+ cations aligned along the direction of the b-axis (Fig. 12). Of the remaining six bg N-H donors, five are used in the CO32–- and H2O-mediated hydrogen-bonded contacts which (i) support chain formation and (ii) generate an undulating 2-D sheet arrangement of [Cu(bg)2]2+ cations which lies parallel to the (0 0 1) plane (Fig. 12). The sixth N-H donor is involved in inter-sheet hydrogen-bonded interactions. The use of [Cu(acac)2Cl]– as anion in [Cu(bg)2][Cu(acac)2Cl]2 [33]provides the required hydrogen-bonding acceptor sites on the carbonyl oxygen and chlorine atoms. There are no direct hydrogen bonds between [Cu(bg)2]2+ cations. Six of the seven bg N-H donor sites are involved in three N-H…O and three N-H…Cl interactions as shown in Fig. 13. The hydrogen-bonding assembly so formed constructs an undulating 2-D sheet of [Cu(bg)2]2+ cations, which lies parallel to the (1 0 0) plane. Although there are no formal contacts between the parallel sheets, the acac ligands of the [Cu(acac)2Cl]– anions interdigitate in alternate sheets.

b

0

a

c

Fig. 13 A projection of the structure of [Cu(bg)2][Cu(acac)2Cl]2 onto the (1 0 0) plane show-

ing the undulating 2-D sheet architecture (atom identification as for Fig. 1 plus copper large light grey circles; chlorine large dark grey circles) [33]

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

49

c

a 0

b

Fig. 14 A projection of the structure of [Cu(bg)2]Cl2·2H2O onto the (0 0 1) plane showing the

1-D chain construction (atom identification as for Fig. 13) [28]

In [Cu(bg)2]Cl2·2H2O [28], the Cl– anions and H2O molecules perform the hydrogen-bonding acceptor function. The well separated [Cu(bg)2]2+ cations are linked into chains aligned along the direction of the b axis by relatively long N-H…O and N-H…Cl contacts (Fig. 14) using just two of the seven N-H donor sites of each bg ligand and one each of the Cl– anions and H2O molecules.A third N-H group acts as a donor in a N-H…O contact to the second H2O molecule which is hydrogen bonded in turn to an H2O molecule in the adjacent chain thus forming 2-D sheets, which are stacked through weak O-H…Cl hydrogen-bonds. The cations within the chains form a stepped arrangement with a perpendicular separation of 1.65 Å. To accommodate this arrangement the N-H groups tend to lie out of the plane of the [Cu(bg)2]2+ cation. 4.2 2-Chlorobiguanide

The structure of a single example of a chloro-substituted biguanide [Cu(clbg)2]Cl2·2H2O (clbg=2-chlorobiguanide), has been reported [34]. Chlorine substitution leads to a reduction in the number of N-H donors from seven to six. Five are involved in the construction of a 2-D architecture (Fig. 15). Two pairs of N-H donors form R21(4) assemblies with separate chlorine atoms; the fifth forms a N-H…O contact to the water molecule which also uses its O-H donors to link two chloride anions (Fig. 15). The resultant 2-D sheet architecture lies parallel to the (2 –1 1) plane. The sixth forms a N-H…Cl contact to a chloride anion in an adjacent sheet.

50

Peter Hubberstey et al.

b c a

0

Fig. 15 A projection of the structure of [Cu(clbg)2]Cl2·2H2O onto the (2 –1 1) plane showing the 2-D sheet architecture (atom identification as for Fig. 13) [34]

4.3 N,N-Dimethylbiguanide

Although there is limited structural data on bis(biguanide) complexes of transition metal centres (see above), the corresponding N,N-dimethylbiguanide (dmbg) derivatives have been more extensively studied. This is undoubtedly due to the fact that dmbg (trivial name Metformin; brand name Glucophage) is much used in the treatment of non-insulin-dependent diabetes mellitus (NIDDM). An oral medication designed to help control elevated blood sugar levels in NIDDM, it is believed to work by inhibiting hepatic glucose production and by increasing the sensitivity of peripheral tissues to insulin. Bis(N,N-dimethylbiguanide) complexes covered include not only neutral ligands (Scheme 3(c2)) {[Ni(dmbg)2]Cl(OH) [35], [Ni(dmbg)2][salicylate] [36], [Cu(dmbg)2][HCO3]2) [37], [Cu(dmbg)2]Cl2·2H2O [35], [TcN(dmbg)2(OH2)]·[38]} but also those containing anionic (Scheme 3d) {[Ni(dmbg-H)2] [43], [Cu(dmbgH)2]·8H2O [39], [TcO(dmbg-H)2]Cl [38]} ligands. Mono(N,N-dimethylbiguanide) complexes of transition metal centres have also been structurally characterised. Again, these include not only neutral ligands (Scheme 3(c2)) {[Cu(dmbg)Cl2] [35], [Pt(dmbg)Cl4]·2dmf (dmf=dimethylformamide) [40], [Pt(dmbg)(OSMe2)Cl]Cl} but also anionic (Scheme 3d) {[Au(dmbg-H)Cl2]·2dma (dma=dimethylacetamide) and cationic (Scheme 3(b2)) {[M(dmbgH)Cl3] (M=Co [35] or Zn [41])}ligands.With the exception of [M(dmbgH)Cl3], these will not be considered further.

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

51

Structurally characterised examples of the [dmbgH2]2+ cation (Scheme 3a) have also been reported (e.g. [dmbgH2][CuCl4]) [42]. 4.3.1 [M(dmbg)2]2+ Containing Structures

There is limited consistency in the networks involving [M(dmbg)2]2+ cations. For example, although the [M(dmbg)2]2+ cations in [Cu(dmbg)2]Cl2·2H2O [35] and [Ni(dmbg)2]Cl(OH) [35] act as four connecting centres to generate 2-D sheets of (4,4) topology, the hydrogen-bonding assemblies are quite different. That in [Cu(dmbg)2]Cl2·2H2O (Fig. 16) is mediated by both a Cl– anion and an H2O molecule. The Cl– anion forms part of (i) an R21(6) motif with two adjacent N-H donors and (ii) an R32(8) motif with a H2O molecule and two N-H donors. The H2O molecule also acts as an acceptor to the fifth (and final) N-H donor generating a secondary R21(6) motif. The effectively planar sheets in [Cu(dmbg)2]Cl2· 2H2O, which lie parallel to the (3 1 0) plane, stack to form a very long Cu…N (3.318 Å) co-ordinate bond and a short O-H…Cl hydrogen-bond (3.157 Ä). The two hydrogen-bonding assemblies in [Ni(dmbg)2]Cl(OH) [35] (Fig. 17) are much simpler. They are mediated separately by the Cl– and OH– anions, both of which are located on twofold symmetry axes. The Cl– anion forms two N-H…Cl interactions with symmetry related N-H donors of co-ordinated imines, while

c

a b

0

Fig. 16 A projection of the structure of [Cu(dmbg)2]Cl2·2H2O onto the (3 1 0) plane showing 2-D rhombic (4,4) grid formation (atom identification as for Fig. 13) [35]

52

Peter Hubberstey et al.

c

0b

a

Fig. 17 A projection of the structure of [Ni(dmbg)2]Cl(OH) onto the (0 1 0) plane showing the 2-D sheet architecture which has a slightly corrugated profile (atom identification as for Fig. 1 plus nickel large light grey circles; chlorine large dark grey circles) [35]

a

0 b

c

Fig. 18 A projection of the structure of [Cu(dmbg)2][HCO3]2 onto the (0 1 0) plane showing

1-D chain construction in the [1 2 2] and [0 0 1] directions (atom identification as for Fig. 1 plus copper large light grey circles) [37]

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

53

Fig. 19 A view of the structure of [TcN(dmbg)2(OH2)]Cl2 showing dimer formation (atom

identification as for Fig. 1 plus technetium large light grey circles; chlorine large dark grey circles) [38]

the OH– anion forms four N-H…O contacts to symmetry related N-H donors of non-co-ordinated imine/amine moieties, thereby generating two R12(6) motifs. The remaining three N-H donors form contacts with either the Cl– or OH– anion which are considered to be too long for viable N-H…Cl interactions [35]. The sheets, which lie parallel to the (0 1 0) plane, adopt a corrugated profile with the OH– hydrogen forming a short O-H…Cl contact (3.108 Ä) along the twofold symmetry axis. The intermolecular contacts in [Cu(dmbg)2][HCO3]2 [37], are quite different. The cations lie in sheets of (4,4) topology, which lie parallel to the (1 –2 0) plane (Fig. 18). In the [1 2 2] direction, the cations use two N-H donors to form skewed R22 (8) motifs, which has very short O-H…O contacts (2.593 Å), not shown in Fig. 18. Similarly, in the [0 0 1] direction, cations are bridged by pairs of anions but, in this case, the two N-H donors of each cation form contacts with separate HCO3– anions to give a pair of N-H…O…H-N assemblies. The structure of [TcN(dmbg)2(OH2)]Cl2 [38],is unusual in thatthe [TcN(dmbg)2 (OH2)]2+ cation is not centrosymmetric as found in the other dmbg compounds but has approximate mirror symmetry. This arrangement leads to the formation of a dinuclear unit (Fig. 19) in which the two [TcN(dmbg)2(OH2)]2+ moieties are linked through a bridge comprising two chloride anions and two water molecules. The bridging elements form an R42(8) motif, which is connected to the [TcN(dmbg)2(OH2)]2+ cations by R32(8) and R32(10) motifs. The dimers are linked into a complex hydrogen-bonded network via N-H…Cl contacts involving the remaining N-H donors on the uncoordinated amino groups. 4.3.2 [M(dmbg-H)2] Containing Structures

The recently reported extended structures of [Ni(bg-H)2] [43] and [Cu(bgH)2]·8H2O [39] are, not surprisingly, quite different.Whereas that of [Ni(bg-H)2] depends on the N-H…N contact to the deprotonated backbone nitrogen atom

54

Peter Hubberstey et al.

c

0 b

a

Fig. 20 A projection of the structure of [Cu(bg-H)2]·8H2O onto the (0 1 0) plane showing the

2-D sheet architecture, which has a slightly corrugated profile (atom identification as for Fig. 1 plus copper large light grey circles) [39]

(Scheme 3d), that of [Cu(bg-H)2]·8H2O is not similarly restricted owing to the large number of H2O acceptor sites. Thus in [Ni(bg-H)2] [43], each molecular unit is linked to four adjacent units to form a loosely connected 3-D network structure. In [Cu(bg-H)2]·8H2O [39], the molecular units lie in 2-D gently undulating sheets parallel to the (0 1 0) plane (Fig. 20). Owing to the complexity of the hydrogen-bonding assembly, the structure can be described in a number of different ways. It is important to remember, however, that the asymmetric unit contains only half a [Cu(bg-H)2] molecule and four waters. Our chosen method is to consider the [Cu(bg-H)2] units to form a series of parallel chains aligned in the [–1 0 1] direction (Fig. 20). Each pair of [Cu(bg-H)2] units is linked by a centrosymmetric set of six waters of which the four terminal molecules hydrogen-bond to the deprotonated nitrogen atom. The two central H2O molecules hydrogen-bond to symmetrically related examples of the fourth H2O molecule which are also attached to the [Cu(bg-H)2] units through a N-H…O contact. The centrosymmetric set of six H2O molecules also link the chains through two pairs of centrosymmetric N-H…O hydrogen-bonds; in so doing they also link the stacked sheets. The fourth N-H donor is not involved in the hydrogen bonded-network. An explanation for the totally different product formation from comparable experiments has not been proposed [39, 43].

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55

4.3.3 [(Hdmbg)MCl3] Containing Structures

The structures of two isostructural [(Hdmbg)MCl3] compounds (M=Co [35] or Zn [41]) have been reported. Despite being positively charged, the [Hdmbg]+ cation co-ordinates the metal centre binding through its sole nitrogen lone pair [Scheme 3(b2)].As the ligand is non-planar, an intra-molecular N-H…N contact similar to that in free biguanide [4] and in [Cu(bg)2][CO3]2·4H2O [32] is not formed. Instead, the [(Hdmbg)MCl3] molecules use all six N-H donor sites to form one intra- and five inter-molecular N-H…Cl interactions thereby generating 2-D bilayer sheets, which straddle the (1 0 0) plane (Fig. 21).

c

0

b

a

Fig. 21 A projection of the structure of [(Hdmbg)ZnCl3] onto the (1 0 0) plane showing the 2-D bilayer sheet construction (atom identification as for Fig. 1 plus zinc large light grey circles; chlorine large dark grey circles) [41]

4.3.4 [H2dmbg][MCl4] Containing Structures

The structure of [H2dmbg][CuCl4] has been reported recently [42]. Not unexpectedly, the [H2dmbg]2+ cation, unlike the [Hdmbg]+ cation, acts as a discrete cation. Like the [Hdmbg]+ cation, however, it is non-planar without an intra-molecular N-H…N contact. All seven N-H donor sites are utilised in cation-anion N-H…Cl interactions to form a 3-D network structure.

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4.4 N,N′-Diphenylbiguanide

Masuda et al. [44–46] have developed a strategy for the construction of organicinorganic hybridised molecular assemblies which involves the use of the neutral linkers [M(dpbg-H)2] (M=Cu or Ni). These linkers are based on square planar metal centres co-ordinated by two anionic dpbg-H ligands. Unlike dmbg and [dmgb-H]– where both methyl groups are on the same nitrogen atom, in [dpbg-H]– the phenyl groups are on different nitrogen atoms in a conformation analogous to that in Scheme 3d. One particularly interesting architecture is that formed between [Ni(dpbg-H)2] and [Rh2(O2CMe)4] [46]. It comprises an infinite 1-D chain of alternating [Ni(dpbg-H)2] and [Rh2(O2CMe)4] moieties linked by co-ordinate interactions between the rhodium centre and the backbone nitrogen (Rh-N=2.319 Å) supported by bifurcated hydrogen-bonding contacts between the -NHPh hydrogen atom and the carboxylate oxygen atoms (N…O range 2.85–3.08 Å; average 2.96 Å). The steric crowding caused by the presence of the phenyl groups prevents the N-H groups on the co-ordinated imino moieties from involvement in any hydrogen-bonded contacts. 4.5 Ethylenebis(biguanide) [1,2-Bis(biguanidinyl)ethane] Complexes

Unlike N,N-dimethylbiguanide (dmbg) which has been structurally characterised in a number of different states of ionisation ([dmbg-H]–, dmbg; [Hdmbg]+; [H2dmbg]2+), the only ethylenebis(biguanide) (ebbg) species to have been so studied are the [M(ebbg)2]2+ complexes. They include the isostructural pair of metal(II) complexes, [M(ebbg)]Cl2·H2O (M=Ni [47, 48] or Cu [49]) and the metal(III) complexes, [Mn(ebbg)(OH2)][NO3]3 [50, 51], [Ag(ebbg)][ClO4]3 [52, 53] and [Ag(ebbg)][SO4][HSO4]·H2O [54]. Their extended structures differ considerably, undoubtedly due to the differing anions (Cl–, NO3–, ClO4–, SO42–, HSO4–) and cation-anion stoichiometries. In [M(ebbg)]Cl2·H2O (M=Ni [47, 48] or Cu [49]), the cations are arranged in a gently undulating 2-D hexagonal net (Fig. 22) aligned parallel to (1 0 0). Ten of the 12 N-H donors are involved in a complex intra-sheet hydrogen-bonded assembly with either Cl– anions or H2O molecule acceptors (Fig. 22). Each cation is involved in three R21(6) motifs, two with Cl– acceptors the other with a H2O acceptor, one R32(8) motif involving both Cl– and H2O acceptors, two R42(8) motifs involving both Cl– and H2O acceptors, two R42(10) motifs involving Cl– acceptors only and two R53(18) motifs involving two Cl– and a single H2O acceptors. The remaining two N-H donors form inter-sheet contacts to Cl– anions in an adjacent sheet, thus forming a bilayer arrangement (Fig. 23). The bilayers are stacked via weak M…N co-ordinate interactions (Ni…N 3.268 Å; Cu…N 3.168 Å) and intersheet O-H…Cl contacts involving just one of the H2O hydrogen atoms (Fig. 23). The second H2O hydrogen atom is involved in the intra-sheet hydrogen-bonded assembly (Fig. 22). The principal difference in the two structures lies in the M…N equatorial (for M=Ni [48] range, 1.858–1.873 Å, average 1.865 Å; for M=Cu [49]

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

57

b

0 a

c

Fig. 22 A projection of the structure of [Ni(ebbg)]Cl2·H2O onto the (1 0 0) plane showing the gently undulating 2-D hexagonal net construction (atom identification as for Fig. 1 plus nickel large light grey circles; chlorine large dark grey circles) [47, 48]

a

b 0

c

Fig. 23 A projection of the structure of [Ni(ebbg)]Cl2·H2O onto the (0 1 0) plane showing the bilayer formation (atom identification as for Fig. 22) [47, 48]

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Peter Hubberstey et al.

a b

0

c

Fig. 24 A projection of the structure of [Ag(ebbg)][SO4][HSO4]·H2O onto the (1 –2 2) plane

showing the 2-D architecture (atom identification as for Fig. 1 plus silver large light grey circles; sulfur large dark grey circles) [54]

range, 1.933–1.989 Å, average 1.963 Å) and axial (for M=Ni [48], 3.268 Å; for M=Cu [49],3.168 Å) distances. The silver(II) complex, [Ag(ebbg)][SO4][HSO4]·H2O [54], also adopts a 2-D architecture aligned parallel to the (1 –2 2) plane. In this case, however, the cations are linked through a SO42–-HSO4– mediated hydrogen-bonded contact to form 1-D chains of alternating cations and anions aligned in the [0 1 1] direction (Fig. 24). Each cation uses its two sets of three co-parallel N-H donors to form R22(8) and R33(10) motifs with the SO42–-HSO4– pair, which has a very short O-H…O contact (O…O 2.51 Å; O-H…O 164°)[54]. Within each chain the cations are oriented in the same direction; adjacent chains face opposite directions (Fig. 24). The chains are linked into 2-D sheets either by a centrosymmetric R44(12) assembly involving cation and anions or by a centrosymmetric R42(16) assembly formed by two N-H…O…H-N H2O-mediated contacts. The sheets are stacked via relatively long Ag…O (SO42–; HSO4–) co-ordinate bonds (2.932, 2.944 Å) supported by hydrogenbonded contacts originating from the H2O molecule. Unfortunately, sufficient structural data are not available to allow comparison with the corresponding silver(III) perchlorate complex, [Ag(ebbg)][ClO4]3 [52, 53]. The extended structure of [Mn(ebbg)(OH2)][NO3]3 [50, 51], is difficult to ascertain, owing to the five co-ordinate Mn(III) geometry and the non co-planarity of the cations and anions. Approximate sheet architectures can be discerned in planes parallel to (–1 3 1) and (1 –2 –2). The latter is shown in Fig. 25. In this view, the cations form dimeric pairs, which are linked through NO3– anions and coordinated H2O molecules to form chains aligned in the [0 1 –1] direction. The

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

59

b

0

a

c

Fig. 25 A projection of the structure of [Mn(ebbg)(OH2)][NO3]3 onto the (1 –2 –2) plane showing the arrangement of hydrogen-bonded dimeric pairs in the construction of 2-D sheets (atom identification as for Fig. 1 plus silver large light grey circles; sulfur large dark grey circles) [50, 51]

resultant assembly uses nine of the 12 available N-H donors on the ligand and both O-H donors of the co-ordinated H2O molecule. The remaining three N-H donors are used to link cations in adjacent sheets.

5 Carbamoylguanidine Complexes The crystal structure of just one carbamoylguanidine co-ordination complex, [Cu(clge)2][NO3]2 has been reported [55],although the structure of [clgeH]2 [Cu(OH2)2Cl4], containing the mono-protonated carbamoylguanidine cation, has also been described [56]. The co-ordination properties of carbamoylguanidine lie between those of biguanides and biurets as it uses one imino nitrogen and one carbonyl oxygen to ligate the metal centre. The extended structure of [Cu(clge)2]]NO3]2 [55] consists of ribbons of coplanar [Cu(clge)2]2+ cations and [NO3]– anions linked through the anions and aligned within the (1 2 0) and (–1 2 0) planes.

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6 Amidino-O-Alkylurea Complexes The crystallographic study of the N-alkylamidino-O-alkylurea complexes of copper(II) by Suksangpanya et al. [57–62] forms a systematic attempt to analyse the influence of ligand substitution, ligand denticity, anion and solvent on the structural chemistry of complexes of this type. The ligands studied fall into two distinct types (Scheme 6). First, the classical bidentate amidino-O-alkylureas with differing N- and O-substitution; these ligands are designated Lij where i represents the N-alkyl substituent (H=hydrogen, m=methyl, e=ethyl and b=benzyl) and j represents the O-alkyl substituent (m=methyl, e=ethyl). Their co-ordina-

Scheme 6

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

61

tion properties are similar to those of biguanides (see above) as they use two imine donors as ligating centres. Second, tetradentate 1,2-ethane and 1,3-propane bridged bis(amidino-O-alkylureas); these ligands are designated Lkl where k represents the length of the linker (2=1,2-ethane, 3=1,3-propane) and l represents the O-alkylsubstituent (m=methyl, e=ethyl). Their co-ordination properties are similar to those of ethylenebis(guanide) (see above) as they use four imine donors as ligating centres. The complexes studied thus far have been limited to copper(II) species owing to the fact that copper(II) centres are particularly effective in promoting the solvolysis of the precursor ligand – a cyanoguanidine derivative – to the desired amidino-O-alkylurea (Scheme 6). Furthermore, all attempts to isolate the amidino-O-alkylurea from the copper(II) centre for treatment with other transition metal cations has been unsuccessful owing to ligand hydrolysis and loss. 6.1 Compounds of Bidentate Amidino-O-Alkylureas

The structures of 16 copper(II) complexes of bidentate amidino-O-alkylureas ([CuLij)2]X2) have been determined by single crystal methods [59–61, 63–66].The range of ligands and of copper(II) salts studied is summarised in Table 2. Although the N-alkylamidino-O-alkylurea derivatives form part of the systematic study by Suksangpanya et al. [59–61] [Cu(LHm)2]Cl2 [66], [Cu(LHm)2]Br2 [63], [Cu(LHe)2]Cl2 [64] and [Cu(LHe)2]Br2 [65] were prepared in independent studies. Table 2 Systems within which structural data are available.All structurally characterised com-

plexes have the formulation [Cu(Lij)2]X2 (X=anion) with the exception of [Cu(LHe)2]Cl2·2H2O, [Cu(Lmm)2]Br2·2MeOH, [Cu(Lme)2][EtOSO3]0.33[HOSO3]0.67 ·0.33Et2O, [Cu(Lee)2]Cl2·H2O, [Cu(Lee)2] Br2·MeOH, [Cu(Lbm)2]Cl2·2MeOH, [Cu(Lbm)2(ONO2)]·[Cu(Lbm)2(OHMe)]·3NO3·Et2O Lij

LHm LHe Lmm Lme Lem Lee Lbm

Anion Cl–

Br–

✓a,h,l ✓c,j ✓e,f,h,l

✓b,h,l ✓d,h,l ✓f,j ✓f,h,l

✓f,i ✓f,k

✓f,i

NO3–

✓g,h,l ✓g,h,m ✓g,h,m

BF4–

HOSO3–

✓g,h,l

✓g,h,l

✓g,h,m

a [66]; b [63]; c [64]; d [65]; e [59]; f [60]; g [61]. h

Anion-mediated hydrogen-bonded 2-D sheet (4,4) network structures. Anion-mediated hydrogen-bonded 1-D chain architectures. j Hydrogen-bonded 2-D sheet (4,4) network structures involving both anions and protic solvent. k Molecular. l Complementarity occurs between cavity size and accommodated groups. m Alkyl groups buckle out of the plane of the cavity to maintain 2-D sheet (4,4) network structure. i

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A remarkable facet of the extended structures of these species is the fact that an elegant 2-D sheet architecture featuring rhombic (4,4) networks is common to fourteen of the sixteen compounds (Table 2). The other two complexes, [Cu(Lee)2]Cl2·H2O and [Cu(Lee)2]Br2·MeOH, adopt totally different 1-D chain architectures. The rhombic (4,4) networks differ in the supramolecular interactions linking the cations and anions. For eleven (Table 2), alternating cations and anions are linked directly through hydrogen-bonded interactions. For two (Table 2) the supramolecular hydrogen-bonding assemblies incorporate protic solvents and for a single example (Table 2) there are no molecular interactions. A typical example of a hydrogen-bonded anion-mediated rhombic (4,4) network, that of [Cu(Lem)2][NO3]2, is shown in Fig. 26. These 2-D sheets generally adopt a corrugated profile with a small amplitude, quantified by the dihedral angle between the least squares mean planes of adjacent cations (ϕ; maximum value 33°) but there is a single example ([Cu(LHe)2]Br2) with a stepped arrangement. The 2-D sheets are stacked such that there is a very weak coordinative interaction between copper (II) centres and either axially located anions or axially located amino nitrogen atoms in adjacent sheets. A typical example of the stacking of the sheets, that in [Cu(LHm)2]Cl2 is shown in Fig. 27. The fourfold connectivity of the cation arises from the presence of the four pairs of N-H donors, as shown in Scheme 7, which form hydrogen-bonds to four

c a

0

b

Fig. 26 A typical example {[Cu(Lem)2][NO3]2} of a hydrogen-bonded anion-mediated 2-D

rhombic (4,4) network (atom identification as for Fig. 1 plus copper large light grey circles) [61]

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a c 0

b

Fig. 27 A typical example {[Cu(LHm)2]Cl2} of the stacking profile of hydrogen-bonded anion-

mediated 2-D rhombic (4,4) networks showing the weak axial Cu…N co-ordinate interactions linking sheets (atom identification as for Fig. 1 plus copper large light grey circles; chlorine large dark grey circles) [66]

anions (halide, nitrate, tetrafluoroborate or sulfate), each of which exhibits twofold connectivity. This arrangement generates the same type of supramolecular synthon (Scheme 8) in all four directions. For all anions except halide, the two N-H donor pairs of the non-co-ordinated imino nitrogen atoms hydrogenbond to anions in the form of an R22(8) motif (Fig. 26; Scheme 8), while the two N-H donor pairs of the co-ordinated imino nitrogen atoms hydrogen-bond to anions in the form of an R21(6) motif (Fig. 26; Scheme 8). For halide anions, all hydrogen-bonding assemblies necessarily adopt R21(6) motifs. Although the N-H…X (X=O or F) hydrogen-bonds of the R22(8) motifs are shorter than those of the R21(6) motifs, this may not reflect an intrinsic difference in the strength of R22(8) and R21(6) motifs. Consideration of the data for the halide mediated structures (Table 3), which contain only R21(6) motifs, shows that the N-H…X (X=Cl or Br) hydrogen-bonds involving the N-H donors from uncoordinated imino nitrogen atoms are shorter than those involving the N-H donors from co-ordinated imino nitrogen atoms. Since the R22(8) and R21(6) motifs of the

Scheme 7

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Peter Hubberstey et al.

Scheme 8

Table 3 Mean values and ranges (in parentheses) for the hydrogen-bonded contacts in the ex-

tended structures of the structurally characterised bis(N-alkylamidino-O-alkylurea)copper(II) nitrates, tetrafluoroborates, halides and sulfates N-H…O (nitrate) R22(8) motif R21(6) motif

N…O (Å) 2.875 (2.826–2.937) 3.024 (2.996–3.055)

H…O (Å) 2.017 (1.969–2.078) 2.190 (2.154–2.225)

N-H…O (°) 168.4(151–178) 159.0 (151–168)

N-H…O (sulfate) R22(8) motif R21(6) motif

N…O (Å) 2.839 (2.761–2.917) 2.963 (2.928–2.995)

H…O (Å)) 1.996 (1.893–2.114) 2.104 (2.063–2.136)

N-H…O (°) 161.5 (151–174) 165.5 (163–168)

N-H…F R22(8) motif R21(6) motif

N…F (Å) 2.904 (2.826–2.976) 3.171 (3.092–3.263)

H…F (Å)) 2.074 (1.955–2.210) 2.328 (2.226–2.456)

N-H…F (°) 161.5 (147–172) 162.8 (154–168)

N-H…Cla R21(6) motif (u) R21(6) motif (c)

N…Cl (Å) 3.187 (3.143–3.231) 3.421 (3.353–3.489)

H…Cl (Å)) 2.372 (2.298–2.445) 2.577 (2.488–2.665)

N-H…Cl (°) 155.0 (149–161) 162.0 (156–168)

N-H…Br a R21(6) motif (u) R21(6) motif (c)

N…Br (Å) 3.382 (3.272–3.503) 3.726 (3.633–3.850)

H…Br (Å) 2.536 (2.404–2.674) 2.886 (2.764–2.982)

N-H…Br (°) 162.1 (156–169) 162.6 (150–172)

a

u, N-H groups of uncoordinated imino nitrogen atoms; c, N-H groups of coordinated iminonitrogen atoms.

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

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nitrate and tetrafluoroborate mediated structures involve the N-H donors from uncoordinated and co-ordinated imino nitrogen atoms, respectively, the difference in their length (and, by implication, strength) may be attributed to greater steric crowding in the vicinity of the N-H donors from co-ordinated imino nitrogen atoms. The dimensions of the rhombic (4,4) networks formed by anion-mediated hydrogen-bonding assemblies are compared in Table 4. Owing to the corrugated nature of the sheets (quantified by the dihedral angle, φ; Table 4) and the asymmetry of the rhombic grids (quantified by the Cu…X…Cu angle and the major/ minor diagonal length ratio (ρ); Table 4) a direct comparison of the data is misleading when considering the space available within the cavity. This is particularly relevant for the more severely distorted halide-mediated rhombic (4,4) networks (see Table 4 for values of φ and Cu…X…Cu angles). To allow for this distortion, edge Cu…Cu distances have been estimated for planar rhombic 4,4 grids mediated by halide anions [58]. That for chloride (10.95 Å) is smaller than that formed by bromide (11.25 Å)which in turn is slightly smaller than the average values formed by tetrafluoroborate (12.05 Å), nitrate (12.09 Å) and hydrogen sulfate (12.14 Å). The cavities at the centres of the rhombic grids have to accommodate either two hydrogen atoms plus two alkyl groups (for amidino-O-alkylureas) or four alkyl groups (for N-alkylamidino-O-alkylureas) from separate cations (Fig. 26). The space available in the cavities formed by chloride-mediated (4,4) networks, is sufficient for either two hydrogen atoms and two methyl groups {in [Cu(LHm)2]Cl2} or four methyl groups {in [Cu(Lmm)2]Cl2}. For [Cu(LHe)2]Cl2·H2O, the hydrogen-bonding assemblies have to incorporate water molecules to maintain (4,4) grid formation (Scheme 9). For [Cu(Lee)2]Cl2·H2O, the four ethyl groups are too large to be accommodated in the chloride-mediated cavity and a totally different 1-D chain structure is adopted. Similarly, for [Cu(Lbm)2]Cl2·2MeOH, the two methyl and two benzyl groups are too large to be accommodated in the chloride-mediated cavity and a molecular system is formed with no inter-molecular hydrogen-bonding interactions.

Scheme 9

Lme/BF4– Lee/BF4– Lme/HOSO3–

Lem/NO3– Lee/NO3– Lbm/NO3–

LHm/Cl–a LHm/Br–a LHe/Br–a Lmm/Cl–a Lme/Br–a

Complexes

10.406 10.654 10.598 10.388 11.552, 11.391, 11.298, 11.262 12.063 12.053 12.130, 12.075, 12.031, 11.993 11.944 12.175, 12.022 12.194, 12.082

Edges

18.847 18.823 18.530

18.512 18.393 17.858

16.960 17.512 17.258 15.874 16.370

Major

Diagonals

Cu…Cu Separations (Å)

14.676 15.204 15.683

15.470 15.581 16.194

12.061 12.139 12.306 13.403 15.708

Minor

102.2, 78.5, 77.3 102.2, 102.2, 78.3, 77.3 100.1, 98.9, 80.5, 80.5

99.5, 80.6 95.8, 95.3, 84.4, 84.3 104.2, 75.8

109.2, 70.8 110.5, 69.5 109.8, 70.2 92.6, 91.5, 88.3, 86.4 103.2, 79.8

Cu…Cu…Cu Angles (°)

1.284 1.238 1.182

1.197 1.180 1.103

1.406 1.442 1.402 1.184 1.042

ρ

21.6 18.1 13.5

18.1 22.6 5.3

– 33.3 14.9

φ (°)

149.1 151.4, 155.8 151.8, 161.9

160.7 160.9 168.9, 170.6, 170.7, 171.9

147.3 145.1 142.7 149.8 161.0, 167.1, 167.9, 177.4

Cu…X…Cu Angles (º)

Table 4 Comparison of the dimensions [Cu…Cu separations (Å) and Cu…Cu…Cu angles (°)] of the rhombic grids formed by anion-mediated hydrogen-bonding assemblies

66 Peter Hubberstey et al.

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The space available in the cavities formed by bromide-mediated (4,4) networks, is suitable for either two hydrogen atoms and either two methyl or two ethyl groups {in [Cu(LHm)2]Br2, Cu(LHe)2]Br2}or two methyl and two ethyl groups{in [Cu(Lme)2]Br2}. For [Cu(Lmm)2]Br2·MeOH, the cavity is too large for the four methyl groups and the hydrogen-bonding assembly incorporates a methanol molecule to maintain (4,4) grid formation (Scheme 9). For [Cu(Lee)2]Br2· MeOH, the four ethyl groups are too large to be accommodated in the bromide-mediated cavity and a 1-D chain structure similar to that of [Cu(Lee)2]Cl2·H2O is adopted. The cavities formed by the nitrate-, tetrafluoroborate and sulfate-mediated hydrogen-bonding assemblies are similar in size. The space available is sufficient to allow two methyl and two ethyl groups to lie in the same plane as the ligating section of the cation in [Cu(Lme)2][BF4]2, [Cu(Lem)2][NO3]2 and [Cu(Lme)2] [EtOSO3]0.33[HOSO3]0.67·0.33Et2O (Fig. 28). For [Cu(Lee)2][NO3]2, [Cu(Lee)2][BF4]2 and [Cu(Lbm)2][NO3]2 there is insufficient room for the pendant alkyl groups. In all three cases the anion-mediated rhombic (4,4) network is retained but the pendant groups are forced to buckle out of the plane of the cavity. The buckling of the benzyl groups in [Cu(Lbm)2][NO3]2 is shown as a typical example in Fig. 29. Complementarity between the size of the cavity and the space required to accommodate the four juxtaposed alkyl groups without buckling occurs for [Cu(LHm)2]Cl2, [Cu(LHm)2]Br2 and [Cu(LHe)2]Br2 (two hydrogen atoms and either two methyl or two ethyl groups within a chloride- or bromide-bridged cavity), [Cu(Lmm)2]Cl2 (four juxtaposed methyl groups within a chloride-bridged cavity) and for [Cu(Lme)2]Br2, [Cu(Lem)2](NO3)2, [Cu(Lme)2](BF4)2 and [Cu(Lme)2] [EtOSO3]0.33[HOSO3]0.67·0.33Et2O (two methyl and two ethyl groups within a bromide-, nitrate-, tetrafluoroborate- or hydrogen sulfate-bridged cavity). It is worth noting that in order to accommodate the amidino-O-alkylureas in halide-mediated networks, the rhombic (4,4) grid is squashed to give a significantly larger major/minor diagonal ratio (1.402<ρ<1.442) than for the anion-mediated networks involving N-alkylamindino-O-alkylureas. When the hydrogen-bonding assemblies incorporate water {[Cu(LHe)2] Cl2·H2O} or methanol {[Cu(Lmm)2]Br2·MeOH} molecules to maintain (4,4) grid formation (Scheme 9) the 2-D sheet adopts a severely corrugated profile (φ=73.6° and 90°, respectively), Similarly, when a molecular structure is formed {[Cu(Lbm)2] Cl2}, the corrugated profile has a very large amplitude (φ=130.6°). Since buckling of the alkyl groups only occurs for nitrates, tetrafluoroborates and sulfates it would appear that the hydrogen-bonded rhombic (4,4) grid of alternating cations and anions is much more robust when nitrate-, tetrafluoroborate- or sulfate-mediated than when based on chloride or bromide. Mean values for the structural parameters of the hydrogen-bonded contacts responsible for rhombic (4,4) grid formation are collected in Table 3. They follow well recognised behaviour patterns increasing in length from N-H…O(NO2) [or NH…O(SO3)] through N-H…FBF3 and N-H…Cl to N-H…Br reflecting their decreasing strength [8] and thus rationalising the more robust nature of the nitrate-, sulfate- and tetrafluoroborate-mediated rhombic grids, compared to the halide-mediated ones. This feature is illustrated most effectively by the structures

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a

b

Scheme 10

c

Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Co-Ordinated Guanidine

69

Fig. 28 The hydrogen-bonded tetrafluoroborate-mediated rhombic (4,4) grid in the structure

of [Cu(Lme)2][BF4]2 showing the complementarity between cavity size and alkyl substituents (atom identification as for Fig. 1 plus copper large light grey circles; boron intermediate black circles; fluorine intermediate light grey circles) [61]

of the Lbm-containing complexes; whereas the nitrate-mediated supra-molecular synthon (Scheme 8) is strong enough to force the pendant phenyl moiety of the benzyl group out of the plane of the sheet and hence permit rhombic (4,4) grid formation (Fig. 29), the corresponding chloride-mediated supra-molecular synthon is too weak and a molecular structure is formed. As noted earlier, the extended structures of [Cu(Lee)2]Cl2·H2O and [Cu(Lee)2] Br2·MeOH are quite different. The principal building block is a 1-D chain (Fig. 30) in which [Cu(Lee)2]2+ cations use the N-H donors of the uncoordinated imino groups to link through chloride anions, giving the hydrogen-bonding assemblies shown in Scheme 10. The structures differ solely in the location of the second anion and of the solvent molecule, both of which are hydrogenbonded to the N-H donors of the co-ordinated imino groups. Whereas the water molecules in [Cu(Lee)2]Cl2·H2O are on alternating sides of the chain (Fig. 30) the methanol molecules in [Cu(Lee)2]Br2·MeOH are on the same side of the chain (Fig. 31). The chains align to form an almost planar 2-D sheet structure. The chains are cross-linked into a 3-D network structure by a hydrogen-bonding interaction between the protic solvent molecules and the halide anions not involved in chain formation. The sheets are stacked such that the amino nitrogen atoms of cations from adjacent sheets are located along the axial

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Fig. 29 The hydrogen-bonded nitrate-mediated rhombic (4,4) grid in the structure of

[Cu(Lbm)2][NO3]2 showing the buckling of the benzyl groups out of the plane of the cavity (atom identification as for Fig. 1 plus copper large light grey circles) [61]

Fig. 30 The hydrogen-bonded chloride-mediated 1-D chain in the structure of [Cu(Lee)2]Cl2·H2O

showing the alternating arrangement of [Cu(Lee)2]2+ cations (atom identification as for Fig. 1 plus copper large light grey circles; chlorine large dark grey circles) [61]

Fig. 31 The hydrogen-bonded bromide-mediated 1-D chain in the structure of [Cu(Lee)2]

Br2·MeOH showing the parallel arrangement of [Cu(Lee)2]2+ cations (atom identification as for Fig. 1 plus copper large light grey circles; bromine large dark grey circles) [61]

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directions of the copper(II) centre giving it a tetragonally elongated octahedral co-ordination geometry. 6.2 Compounds of Tetradentate Bis(Amidino-O-Alkylureas)

Suksangpanya et al. have recently published the results of their extensive study of the structural chemistry of nine copper(II) complexes of tetradentate bis(amidino-O-alkylureas [57, 58, 62]. Those compounds for which structural data are available are listed in Table 5 as a function of ligand and copper(II) salt.As for the compounds formed by the bidentate N-alkylamidino-O-alkylureas, these materials have several recurrent structural features, the most significant of which is the formation of a hydrogen-bonded 1-D chain of alternating cations and anions. The second is the linking of the chains into ribbons and then 2-D sheets by hydrogenbonding contacts between cations, anions and solvent molecules (water and/or methanol) and the third is the formation of a 3-D matrix by extremely weak axial interactions at the copper(II) centres. The structures of these compounds are best reviewed on the basis of the anion mediating the 1-D chain formation. 6.2.1 Halide-Mediated 1-D Chain Formation

A supramolecular synthon effectively identical to that found in [Cu(Lee)2] Br2·MeOH (Scheme 10b) is conserved in five of the six halide-containing compounds. It serves to generate a chain of alternating [CuLkl]2+ cations and anions. The construction of a typical chain, that in [CuL3m]Cl2·MeOH·0.5H2O, is shown in Fig. 32. Four of the eight N-H donors on each cation (Fig. 32), two on each side, generate with the intermediate anion four N-H…X (X=Cl or Br) hydrogen-bonds in the form of two R21(6) motifs. The only difference between the chloride- and bromide-mediated chains is the Cu…Cu separation, that for the bromide Table 5 Compounds for which structural data are available Anion

Cl– Cl–; [CuCl4]2– Cl–;(EtO)SO3– Br– (MeO)SO3– (EtO)SO3– BF4– (MeO)2BF2– a b c

[58] [57] [62]

Lkl L2m

L2e

L3m

[CuL2m]Cl2·2H2Oa [CuL2m]4[CuCl4]Cl6·5H2Oa

[CuL2e]Cl2·MeOH·3H2Oa

[CuL3m]Cl2·MeOH·0.5H2Oa

[CuL2e][EtOSO3]Cl·2H2Ob [CuL3m]Br2·MeOH·0.3H2Oa [CuL2m][MeOSO3]2b [CuL2e][EtOSO3]2b [CuL2m][BF4]2c [CuL2e][(MeO)2BF2][BF4]c

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Table 6 Cu…Cu intrachain distances (Å) in halide mediated

1-D chains Compound

Intrachain Cu…Cu/Å

[CuL2m]Cl2·2H2O [CuL2e]Cl2·MeOH·3H2O [CuL3m]Cl2·MeOH·0.5H2O [CuL3m]Br2·MeOH·0.3H2O [CuL2m]4[CuCl4]Cl6·5H2O [CuL2e][EtOSO3]Cl·2H2O

13.267 12.677 12.656 12.971 12.625 12.910

a

b

c Fig. 32a–c The hydrogen-bonded anion-mediated 1-D chains in the structures of: a [CuL3m]

Cl2·MeOH·0.5H2O; b [CuL2m][MeOSO3]2; c [CuL2e][(MeO)2BF2][BF4] (atom identification as for Fig. 1 plus copper large light grey circles; chlorine large dark grey circles; boron intermediate black circles; fluorine intermediate light grey circles; sulfur large black circles) [57, 58, 62]

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Fig. 33 The 2-D sheet construction in the structure of [CuL2m]Cl2·2H2O showing the parallel

arrangement of the hydrogen-bonded anion and water-mediated 1-D chains (atom identification as for Fig. 1 plus copper large light grey circles; chlorine large dark grey circles) [58]

(12.971 Å) being ca. 0.3 Å [i.e., 2(rBr–-rCl–)=2(1.96–1.81) Å] longer than those for the chlorides (12.625–12.677 Å; Table 6). The chain in the sixth compound, [CuL2m]Cl2·2H2O, is marginally different, a water molecule being inserted in a N-H…Cl contact to give a N-H…O-H…Cl arrangement (Scheme 10c), which converts one of the R21(6) motifs to a R32(8) motif. The construction of this chain is shown in Fig. 33. The resultant intrachain Cu…Cu separation is somewhat longer (13.267 Å) than those mediated solely by chloride anions (12.625–12.910 Å; Table 6). The principal differences between the six halide-containing structures lie in the way that the extended architectures are generated. The chains align either facing in the same direction (parallel) to form a 2-D sheet ([CuL2m]Cl2·2H2O and [CuL2e][EtOSO3]Cl·2H2O) or facing opposite directions (anti-parallel) to form ribbons. To generate the 2-D sheets, cations from adjacent chains utilise all four N-H donors remaining after chain formation. They are taken from both faces of the cations generating a parallel alignment of chains (Fig. 33) with intermediate anions and solvent molecules. The linking of pairs of chains to give the ribbons involves just two of the four N-H donors remaining after chain formation. As these are taken from the same face of the cations, the chains are aligned anti-parallel. Again anions and solvent molecules are involved in the hydrogen-bonding assembly as exemplified for [CuL2e]Cl2·MeOH·3H2O in Fig. 34. The other two N-H donors, the co-ordinated imino moieties, hydrogen-bond to oxygen atoms of solvate molecules located on the edges of the ribbons. The ribbons align to

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Fig. 34 The 1-D ribbon construction in the structure of [CuL2e]Cl2·MeOH·3H2O showing the anti-parallel arrangement of the hydrogen-bonded anion-mediated 1-D chains (atom identification as for Fig. 1 plus copper large light grey circles; chlorine large dark grey circles) [58]

generate a 2-D sheet architecture. As all the N-H donors of the [CuLkl]2+ cations are involved in ribbon construction, the only hydrogen-bonding contacts between the ribbons involve the solvent molecules. 6.2.2 Alkylsulfate- and Dimethoxydifluoroborate-Mediated 1-D Chain Formation

A chain of alternating [CuLkl]2+ cations and anions, similar to that found in the analogous halides, is the principal architectural feature of the two alkylsulfate{[CuL2m][MeOSO3]2 and[CuL2e][EtOSO3]2} and sole dimethoxydifluoroborate-containing compound {[CuL2e][(MeO)2BF2][BF4]}. The structures of these three chains are compared with that of a chloride-mediated chain in Fig. 32 and the corresponding hydrogen-bonded supramolecular synthons are compared in Scheme 11. For the alkylsulfates, four of the eight N-H donors on each cation, two on each side, generate with the intermediate anion four N-H…O hydrogen-bonds in the form of one R21(6) motif and one R22(8) motifs. For the dimethoxydifluoroborate, the same four N-H donors generate with the intermediate anion, two N-H…O and two N-H…F hydrogen-bonds in the form of two R22(8) motifs. The intrachain Cu…Cu separations are similar but considerably longer than those for the halides, those for the alkylsulfates (13.851 and 14.157 Å) and dimethoxydifluoroborate (13.842 Å) being over ca. 1.3 Å longerthan those for the chlorides (12.625–12.910 Å; Table 6).

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a

b

Scheme 11

c

The extended architectures of [CuL2m][MeOSO3]2 and [CuL2e][(MeO)2BF2][BF4] are generated by the chains facing in the same direction (parallel) to form a 2-D sheet as depicted in Fig. 35. To generate the 2-D sheets, cations from adjacent chains utilise three of the four N-H donors remaining after chain formation. They are taken from both faces of the cations generating a parallel alignment of chains. Whereas for ([CuL2m][MeOSO3]2, this hydrogen bonding assembly involves intermediate MeOSO3– anions (Fig. 35a), for [CuL2e][(MeO)2BF2][BF4] it involves BF4– anions (Fig. 35b). In both cases the remaining N-H donor hydrogen bonds to an [MeOSO3]– or [(MeO)2BF2]– anion of an adjacent sheet. The sheets are stacked such that weak Cu…O contacts are formed between the copper(II) centre and oxygen atoms of the [MeOSO3]– or [(MeO)2BF2]– anion giving the former a tetragonally elongated octahedral co-ordination geometry. The framework structure of [CuL2e][EtOSO3]2 is quite different owing to the fact that its 1-D chain has a sinusoidal alignment rather than the near linear arrangements of all the other 1-D chains formed by [CuLkl]2+ cations. The chains are stacked directly on top of each other. A second set of chains is linked to the first set to form a bilayered arrangement in which the two sets of chains, which face opposite directions, are connected through two N-H…O hydrogen-bonds at their peaks and troughs.

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a

b Fig. 35a,b Construction of the 2-D sheet architectures in the structures of: a [CuL2m]

[MeOSO3]2; b [CuL2e][(MeO)2BF2][BF4] by linking the hydrogen-bonded anion-mediated 1-D chains through [MeOSO3]– anions and [BF4]– anions, respectively (atom identification as for Fig. 1 plus copper large light grey circles; boron intermediate black circles; fluorine intermediate light grey circles; sulfur large black circles) [57, 62]

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6.2.3 In Situ Anion Generation

A remarkable feature of the alkylsulfate and dimethoxydifluoroborate compounds (Table 5) is the fact that the anions involved in chain formation are generated in situ; dimethoxydifluoroborate from tetrafluoroborate during recrystallisation from methanol and alkylsulfate from sulfate during solvolysis of the precursor cyanoguanidine ligand. To rationalise the formation of [CuL2e] [(MeO)2BF2][BF4], it is necessary to assume that an equilibrium involving [BF4]–, [MeOBF3]– and [(MeO)2BF2]– (Eq. 1) exists in all MeOH solutions containing copper(II) tetrafluoroborate: + MeOH

+ MeOH

– HF

– HF

[BF4]– U u [MeOBF3]– U u [(MeO)2BF2]– 0 0 0 0

(1)

The products crystallising from solution will depend on the concentrations of, and the solubilities of the various products that can be formed from, all species in solution.As the methanolysis products (Eq. 1) will be present in extremely low concentrations, the compounds normally crystallised from MeOH will be [BF4]– salts. In this particular case, however, crystallisation of the difluorodimethoxyborate-containing product dominates. Despite its very low concentration, a significant amount of [(MeO)2BF2]– anion can be produced over the extended recrystallisation process (ca. 14 days), owing to the equilibrium (Eq. 1) being continually disturbed by removal of [(MeO)2BF2]– anion from solution. A similar process involving the esterification of sulfate to form alkylsulfate (Eq. 2) has been proposed to rationalise the formation of [CuL2l][ROSO3]2 (2l=2m, R=Me; 2l=2e, R=Et). The favoured crystallisation of [CuL2e][(MeO)2BF2] [BF4] and [CuL2l][ROSO3]2 is attributed to the stability of the hydrogen-bonding supramolecular synthons (Scheme 11b,c) associated with the 1-D chain of alternating [Cu(L2i)]2+ cations and either [(MeO)2BF2]– or [ROSO3]– anions. + ROH

u [ROSO3]– [SO4]2– U 0 0 – OH–

(2)

Recrystallisation of [CuL2m][BF4]2 from methanol leaves it unaltered. Structural analysis revealed a 2-D sheet structure but with a totally different hydrogenbonding network (Fig. 36); tetrafluoroborate does not form a similar supramolecular synthon to halide, alkylsulfate or difluorodimethoxyborate. The basic building block of [CuL2m][BF4]2 comprises a zig-zag chain of square planar [CuL2m]2+ cations bridged through hydrogen-bonded [BF4]– anions, which form an R22(8) ring to one cation and a single N-H…F contact to the other. Chain formation is supported by an anion-mediated hydrogen-bonding assembly which links every second cation through an R21(6) ring and a single N-H…F contact (Fig. 36). The chains, which adopt the appearance of ribbons, align next to each other to form 2-D sheets.Although there are no interactions between ribbons on the same level, there are interactions, mediated by [BF4]– anions, between cations on adjacent layers. The failure of [BF4]– to form, with [CuL2m)]2+, a supramolec-

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Fig. 36 The hydrogen-bonded 2-D sheet construction in the structure of [CuL2m][BF4]2 (atom

identification as for Fig. 1 plus copper large light grey circles; boron intermediate black circles; fluorine intermediate light grey circles) [62]

ular synthon comparable to those found in the halide-, alkylsulfate- and difluorodimethoxyborate-mediated 1-D chains (Fig. 32; Scheme 11) was unexpected as tetrafluoroborate has a similar arrangement of hydrogen-bond acceptor groups to [(MeO)2BF2]– and [ROSO3]–. In [CuL2m][BF4]2 and [CuL2e][(MeO)2BF2][BF4] it is interesting to note that the [BF4]– anion behaves in a similar fashion acting as an acceptor to three N-H donors. It does not act as an acceptor to four N-H donors as required to generate the 1-D chain of alternating [CuLkl]2+ cations and anions. This is surprising as it does act in a similar manner to the [(MeO)2BF2]– and [ROSO3]– anions acting as an acceptor to four N-H donors in [Cu(Lij)2]·2BF4 (see above). 6.2.4 Binding of Pyrimidine Bases to [Cu(Lkl)]2+ Cations to Form 2-D Sheet Architectures

In a recent series of experiments, Suksangpanya et al. [67] have investigated the interactions between pyrimidine bases and [Cu(Lkl)]2+ cations.Amidino-O-alkylureas contain an A·D·D sequence, similar to that of guanine. The only difference lies in the oxygen acceptor, which is part of an ether linkage in the amidino-Oalkylurea but part of a carbonyl functionality in guanine. Despite this difference, the formation of a triple complementary interaction between amidino-O-alkylurea and cytosine, which has a D·A·A sequence was thought to be feasible. Complementary triple hydrogen-bond formation involving transition metal complexes has been little studied. Mingos et al. [68] have investigated the cocrystallisation of platinum(II) complexes of the uracil derivative orotic acid (2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid), which generally coordinates as the dianion, and 2,6-diaminopyridine (2,6-dap), in which complementary A·D·A…D·A·D triple hydrogen bonds are formed between the orotate

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79

Scheme 12

ligand and 2,6-dap. In addition, Mingos et al. [69] have studied the formation of complementary intermolecular hydrogen bonds between uracil moieties attached to bifunctional, chelating ligands based on ethylenedinitrilotetraacetic acid. The potential triple A·D·D…D·A·A interaction between cytosine and bis(amidino-O-methylurea)ethane, when co-ordinated to copper(II), is shown in Scheme 12 (l.h.s.).Alternative complementary interactions between amidino-Oalkylureas and DNA pyrimidine bases include the double D·A…A·D interaction with thymine shown in Scheme 12 (r.h.s.). Co-crystallisation of cytosine or thymine with [Cu(L2m)][BF4]2 {the tetrafluoroborate was chosen owing to the fact that [BF4]– anion forms, relative to nitrate anions, considerably weaker hydrogen-bonding interactions} [67] led to [Cu(L2m)][BF4]2·2cyt and [Cu(L2m)][BF4]2·thy·H2O, both of which adopt 2-D sheet architectures. The layered structure of [Cu(L2m)][BF4]2·2cyt is shown as a typical example in Fig. 37. In both structures, the sheets are linked by anion-mediated hydrogen-bonding interactions. Cytosine does hydrogen-bond to the copper(II) cation, but not with the anticipated A·A·D…D·D·A interaction. Instead two different constructions are formed (Fig. 37). The simpler construction involves an R22(8) motif based on a double A·A…D·D interaction. The donors are N-H groups on the cation while the acceptors are a carbonyl oxygen and a heterocyclic nitrogen on a cytosine molecule (Fig. 37). The more complex construction can be considered to be an R21(6) motif supported by an N-H…O contact which forms a secondary R22(8) ring. The R21(6) motif is based on two N-H donors from the cation and a single carbonyl oxygen acceptor from the cytosine molecule. The supporting N-H…O contact, which arises from an N-H donor on the cytosine and the ether oxygen from the cation, results in a skewed R21(6) motif. The hydrogen-bonding assembly is completed by a R22(8) motif based on a double A·D…D·A interaction between cytosine molecules. The donors are N-H groups, while the acceptors are a carbonyl oxygen and a heterocyclic nitrogen (Fig. 37). Thymine hydrogen-bonds to the copper(II) cation, but not with the anticipated A·D…D·A interaction. Instead, an R21(6) motif based on two N-H…O

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Fig. 37 The hydrogen-bonded 2-D sheet architecture of [CuL2m][BF4]2·2cytosine (atom iden-

tification as for Fig. 1 plus copper large light grey circles; boron intermediate black circles; fluorine intermediate light grey circles) [67]

Fig. 38 The hydrogen-bonded 2-D sheet architecture of [CuL2m][BF4]2·thymine (atom identi-

fication as for Fig. 37) [67]

interactions is formed (Fig. 38). The donors are N-H groups on the cation while the acceptor is a carbonyl oxygen on a thymine molecule. Two thymine molecules are used to link the cations. Unlike the corresponding assembly in the cytosine complex (Fig. 37), this construction is centrosymmetric (Fig. 38). It is an R22(8) motif based on a pair of self-complementary N-H…O contacts involving a heterocyclic N-H group and a carbonyl oxygen.

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The assembly discussed above for [Cu(L2m)][BF4]2·2cyt forms part of a linear chain in which pairs of cytosine molecules alternate with [Cu(L2m)]2+ cations (Fig. 37). Parallel chains are linked through tetrafluoroborate-mediated hydrogen-bonded contacts to form a 2-D sheet. The hydrogen-bonding assembly involves two ring motifs. In the first, a cation and anion are linked through an R21(6) motif typical of cation-[BF4]– contacts (see above). In the second, the anion is linked to a cation and a cytosine molecule of a parallel chain through an unusual R33(10) motif. Only one anion is involved in sheet construction, the other being simply attached to the cation through a single N-H…F contact (Fig. 37). The structural fragment described above for [Cu(L2m)][BF4]2·thy·H2O is the basic building block of a linear chain, in which fragments are linked through water and tetrafluoroborate-mediated centrosymmetric R66(20) assemblies (Fig. 38). Each half of the assembly involves a direct cation – anion link through a R21(6) motif, typical of cation-[BF4]– contacts (see above), and a water-mediated cationanion link involving an R21(6) motif, typical of cation-water contacts and a water[BF4]– O-H…F contact (Fig. 38). Parallel chains are linked through tetrafluoroborate-mediated hydrogen-bonded contacts to form a 2-D sheet (Fig. 38). The anion is linked to a cation and a thymine molecule of one chain through separate N-H…F contacts, which form part of a R44(14) motif involving the cation, anion and two thymine molecules, and to a cation of an adjacent chain through a single N-H…F contact, which forms, with a N-H…O contact, a bifurcated hydrogen-bond. Owing to the bifurcated nature of this contact, the inter-chain link is relatively weak. Stacking of the sheets depends on weak axial co-ordinate interactions between copper(II) centre and cytosine carbonyl oxygen and on N-H…F and O-H…F hydrogen bond interactions between cation, anion and solvent molecules.

7 Conclusions When co-ordinated to transition metals, members of the family of ligands based on bu, cg and bg are highly valuable in promoting the construction of hydrogenbonded supra-molecular 1-D and 2-D networks. Square planar [ML2]n+ cations, where L is a bidentate ligand, and [ML]n+ cations, where L is a tetradentate ligand, are particularly effective in acting as hydrogen-bonding donors to form, with anions acting as hydrogen-bonding acceptors, chain and sheet networks. The 2-D sheets, including elegant rhombic (4,4) grids are generally formed by [ML2]n+ cations using all their hydrogen-bonding capacity to act as four-connecting nodes. The 1-D chains are normally constructed by [ML]n+ cations acting as twoconnecting nodes; their remaining hydrogen-bonding capacity is used to extend the dimensionality of the structure either by ribbon or sheet formation. When comparable data are available for a significant number of analogous structures, it is clear that the supra-molecular synthons responsible for these architectures differ little with anion (Scheme 8), despite the considerable differences in the molecular geometries of the anions. They do differ, however, in robustness, the halide mediated ones being somewhat more fragile than those mediated by nitrate or tetrafluoroborate.

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30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69.

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Creitz TC, Gsell R, Wampler DL (1969) J Chem Soc D 1371 Nag DK, Guha S (1971) Ind J Phys 45:139 Coghi L, Lanfranchi M, Pelizzi G, Taraconi P (1978) Transition Met Chem 3:69 Hota SK, Saha CR, Pritzkow H (1984) J Coord Chem 13:131 Saha CR, Sen D, Guha S (1975) J Chem Soc Dalton Trans 1701 Lemoine P, Chiadmi M, Bissery V, Tomas A, Viossat B (1996) Acta Crystallogr C52:1430 Lemoine P, Tomas A, Viosset B, Prange T (1999) Z Krist New Cryst Struct 214:369 Viosset B, Tomas A, Dung N-H (1995) Acta Crystallogr C51:213 Marchi A, Marvelli L, Cattabriga M, Rossi R, Neves M, Bertalosi V, Ferretti V (1999) J Chem Soc Dalton Trans 1937 Zhu M, Lu L, Yang P, Jin X (2002) Acta Crystallogr E58:m217 Tomas A, Morgant G, Nguyen-Huy D, Lemoine P, Voissat B (2002) Z Krist 217:131 Zhu M-L, Lu L-P, Jin X-L, Yang P (2002) Acta Crystallogr C58:m158 Lemoine P, Tomas A, Viosset B, Dung N-H (1994) Acta Crystallogr C50:1437 Zhu M, Lu L, Yang P, Jin X(2002) Acta Crystallogr E58:m272 Kitamura H, Ozawa T, Jitsukawa K, Masuda H, Einaga H (2000) Molec Cryst Liq Cryst Sci Technol Sect A 342:69 Kitamura H, Ozawa T, Jitsukawa K, Masuda H, Einaga H (1996) Molec Cryst Liq Cryst Sci Technol Sect A 285:281 Kitamura H, Ozawa T, Jitsukawa K, Masuda H, Einaga H (1999) Chem Lett 1225 Ward DL, Caughlan CN, Smith GD (1971) Acta Crystallogr B27:1541 Holian BL, Marsh RE (1970) Acta Crystallogr B26:1049 Kunchur NR, Matthew M (1966) Chem Commun 86; Matthew M, Kunchur NR (1970) Acta Crystallogr C26:2054 De A (1990) Acta Crystallogr C46:1004 De A (1990) J Crystallogr Spectrosc Res 20:279 Simms ML, Atwood JL, Zatko DA (1973) J Chem Soc Chem Commun 46 De A (1990) J Crystallogr Spectrosc Res 20:279 Coghi L, Pelizzi G (1975) Acta Crystallogr B31:131 Begley MJ, Hubberstey P, Moore CHM (1986) J Chem Res (S) 120–121 Begley MJ, Hubberstey P, Martindale SP, Moore CHM, Price NS (1988) J Chem Res (S) 2 Blake AJ, Hubberstey P, Suksangpanya U, Wilson C (2000) J Chem Soc Dalton Trans 3873 Suksangpanya U, Blake AJ, Hubberstey P, Wilson C (2002) Cryst Eng Comm 4:552 Hubberstey P, Suksangpanya U, Wilson C (2000) Cryst Eng Comm 2:141 Suksangpanya U, Blake AJ, Hubberstey P, Parker D, Teat SJ, Wilson C (2003) Cryst Eng Comm 5:10 Suksangpanya U, Blake AJ, Hubberstey P, Parker D, Teat SJ, Wilson C (2003) Cryst Eng Comm 5:23 Suksangpanya U, Blake AJ, Hubberstey P, Wilson C (2002) Cryst Eng Comm 4:638 Pickardt J, Kuehn B (1996) Z Krist 211:728 Begley MJ, Hubberstey P, Moore CHM (1986) J Chem Res (S) 172 Begley MJ, Hubberstey P, Moore CHM (1991) J Chem Res (S) 334 Das BC, Dey I, Biswas G, Banerjee R, Iitaka Y, Banerjee A (1993) J Crystallogr Spectrosc Res 23:509 Suksangpanya U, Blake AJ, Hubberstey P, Wilson C (unpublished results) Xu X, James SL, Mingos DMP,White AJP,Williams DJ (2000) J Chem Soc Dalton Trans 3783 Ulvenlund S, Georgopoulou AS, Mingos DMP, Baxter I, Lawrence SE, White AJP, Williams DJ (1998) J Chem Soc Dalton Trans 1869

Structure and Bonding, Vol. 111 (2004): 85–137 DOI 10.1007/b14141HAPTER 1

Hydrogen-Bonding Templated Assemblies Ramón Vilar Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, UK E-mail: [email protected]

Abstract The use of templates in synthesis is now a well-established approach for the prepa-

ration of a wide range of molecular and supramolecular species. Templating agents can organize an assembly of building blocks to achieve a particular linking of atoms. By doing so it is possible to synthesise assemblies with complex structures and topologies that could not be prepared otherwise. This review aims to discuss the importance of hydrogen bonding in templated processes. More specifically, the use of hydrogen bonding templates in the synthesis of macrocycles, cages, interlocked species and for the photodimerisation of olefins is herein presented. Furthermore, the use of hydrogen-bonding templates in dynamic combinatorial libraries and in self-replicating systems is discussed. Keywords Templates · Hydrogen Bonding · Self-assembly · Cages · Macrocycles · Catenanes · Rotaxanes · Dynamic Libraries · Self-replication · Photodimerisation

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

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Introduction

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Hydrogen Bonding Properties of Templates

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Templated Synthesis of Macrocycles and Cages . . . . . . . . . . . 87

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Interlocked Structures: Catenanes, Rotaxanes and Pseudorotaxanes 93

4.1 4.2 4.2.1 4.2.2 4.2.3

Catenanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rotaxanes and Pseudorotaxanes . . . . . . . . . . . . . . . . . . Ammonium Salts as Templates . . . . . . . . . . . . . . . . . . . Templated Synthesis by Hydrogen Bonding to Amides . . . . . . Assembly of Rotaxanes by an “Independent” Hydrogen-Bonding Template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen Bonding vs p-p Stacking in Templated Synthesis of Interlocked Species . . . . . . . . . . . . . . . . . . . . . . .

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. 108 . 114

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Stranded Helicates . . . . . . . . . . . . . . . . . . . . . . . . . . 118

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Hydrogen-Bonding Templates in Dynamic Combinatorial Libraries 120

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Hydrogen Bonding Templates in the Photodimerisation of Olefins 129

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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

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© Springer-Verlag Berlin Heidelberg 2004

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1 Introduction The past two decades have seen a remarkable development in the synthesis of supramolecular assemblies. A wide range of complex architectures such as cages, helicates, rotaxanes and catenanes have been prepared by self-assembly processes using a combination of non-covalent interactions. In such processes the synthesis of the desired product quite often requires the presence of a templating (or directing) agent. As defined by Busch “a chemical template organizes an assembly of atoms, with respect to one or more geometric loci, in order to achieve a particular linking of atoms” [1]. The template organises a collection of building blocks in defined spatial arrangements favouring the formation of a specific product in high yields. This provides an efficient route to the preparation of a particular assembly of atoms when several others can be potentially formed [2]. Ideally the templating agent should be removed from the final product once the reaction has finished. However, as will be shown throughout this review, templates very often form an integral part of the final product and hence they cannot always be removed from it. The ability of a chemical to act as a template is frequently attributed to a combination of thermodynamic and kinetic factors.As has been defined by Busch [3] a thermodynamic template binds more strongly to one of the products present in an equilibrium (i.e. a mixture under thermodynamic control) shifting the reaction towards the formation of this specific product which is then obtained in higher yields. In contrast, kinetic templates operate under irreversible conditions by stabilising the transition state leading to the final product. Templated processes can employ covalent or non-covalent bonding forces, depending on the interactions established between the directing group and the building blocks [4, 5]. The latter rely on a wide range of supramolecular interactions such as electrostatic forces, hydrogen bonding, p-p stacking and hydrophobic effects. From these, hydrogen bonding interactions are particularly important since they are relatively strong and are more directional. Several templated processes in nature make use of hydrogen bonding interactions, one of the most elegant examples being the replication and transcription of nucleic acids. Inspired by nature, synthetic chemists have developed templated processes where the hydrogen bond donor and acceptor properties of a template have been used to direct the formation of a specific molecular assembly. This review aims to discuss the importance of hydrogen bonding in templated reactions (i.e. where there are direct hydrogen bonding interactions between the templating agent and the building blocks that give rise to the final assembly). Due to the very large potential scope of such a topic, only those examples where hydrogen bonding interactions play a major part in on the templation process and where such interactions have been clearly identified will be discussed. The review will also be restricted in terms of the size of the assemblies formed, concentrating only on those examples that generate finite species (where usually the process of templation can be identified much more clearly). Consequently systems such as molecular imprinted polymers [6–9], template effects in coordination polymers and networks [10–12], templated synthesis of polymers [13] and synthesis

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of zeolites and other inorganic solids by templation [14–16] will not be covered here. Excellent reviews on these topics can be found elsewhere (see reference above). Finally, due to the large amount of information available, templated processes in biology (e.g. nucleic acids transcription and replication, protein folding, etc.) are not discussed in this review.

2 Hydrogen Bonding Properties of Templates Hydrogen bonds are usually established between hydrogen and two atoms that are more electronegative than hydrogen, for example C, N, O, F, P, S, Cl, Se, Br and I. The X-H group is usually known as the hydrogen bond donor while the atom (Y) that interacts with the hydrogen of this group via a lone pair (X-H…Y) is referred as the hydrogen bond acceptor. The larger the dipole moment of the X-H bond, the stronger the hydrogen bond interaction to Y will be. Consequently the strongest hydrogen bonds occur when highly electronegative atoms such as N, O and F act as X and Y. Templates can be hydrogen bond donors, acceptors or a combination of both, which obviously dictates the type of reaction or assembly they are capable of templating [17]. Although there are several reactions templated by simple hydrogen bond acceptors (such as mono-atomic anions) or donors (such as quaternary alkyl-ammonium salts), the potential amount of information that a template can contain by combining donor and acceptor groups is very large. Templates with a large set of hydrogen bond donor/acceptor groups will be much more specific for a particular process; one of the best examples of this being the complementarity that exists between two strands of DNA. Consequently, when designing hydrogen-bonding templates the careful choice of their donor/acceptor properties to make them complementary to the process to be templated is highly important. In the following sections, examples of hydrogen-bonding templates for the synthesis of macrocycles, cages, interlocked species, helicates and for the photochemical reaction of olefins will be discussed. The use of hydrogen-bonding templates in dynamic combinatorial libraries will also be presented.

3 Templated Synthesis of Macrocycles and Cages Since Pedersen’s original work on the use of cations to template the formation of crown ethers [18–20], a large number of different templating agents for macrocyclization reactions have been reported.While the initial work concentrated on the use of metal cations, further developments demonstrated that species with hydrogen bonding donor or acceptor properties could be equally useful to template the synthesis of macrocyclic molecules. Cram reported in 1977 the use of guanidinium (a hydrogen bond-donor template) for the formation of benzo-27-crown-9 (1 in Scheme 1) [21, 22]. Although guanidinium was not as good a template as the K+ cation, it increased the yield of macrocylisation by tenfold in comparison to the yield obtained when the

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1 Scheme 1 Guanidinium-templated synthesis of macrocycle 1

reaction was carried out in the presence of tetramethylguanidine (which is larger and does not have the ability to hydrogen bond to the oxygen atoms of the ether). As will be discussed later, ammonium salts have also been used as hydrogen bond-donor templates to favour the formation of macrocycles (yielding pseudorotaxanes and rotaxanes). Hydrogen bond-acceptor templates (such as anions) have also demonstrated to be very useful in the directed synthesis of macrocycles.Although the use of anions as templates (whether by hydrogen bonding or electrostatic interactions) was not exploited until relatively recently, there are now several elegant examples that demonstrate their utility as directing agents (for recent reviews see [23–25]). Wright, for example, has recently reported the anion-templated synthesis of a series of macrocycles with general formula [{P(m-Nt-Bu)2}(m-NH)]n where hydrogen-bonding interactions play an important role in determining the size of the macrocycle [26]. These species can be prepared by reacting [ClP(m-Nt-Bu)]2 with [NH2P(m-Nt-Bu)]2 in the presence of a base. The major product is the tetrameric compound [{P(m-Nt-Bu)2}(m-NH)]4 (2) when the reaction is carried out in THF/NEt3. Interestingly, when the same reaction is carried out in the presence of an excess of LiCl tetramer formation is suppressed while the synthesis of the pentamer [{P(m-Nt-Bu)2}(m-NH)]5·(HCl) (3) is amplified (see Scheme 2). Structural characterisation of this pentamer has demonstrated that the chloride (a hydrogen bond acceptor) is positioned at the centre of the macrocycle forming five H-bonds with the NH groups of the ring with an average N-H…Cl distance of 2.54(1) Å (comparable to previously reported N-H…Cl interactions). Mechanistic studies on this system have demonstrated that the reaction does not follow a simple stepwise condensation pathway as first assumed. Instead, the authors have proposed a divergent mechanism by which the production of 2 and 3 are linked by a common intermediate (see Scheme 3). In such a mechanism, hydrogen-bonding interactions between the donor molecular moieties and the templating halide (acceptor) play an important role. Another series of macrocycles prepared by hydrogen bonding to anion templates are the [14]imidazoliophanes recently reported by Alcalde [27]. The preparation of such macrocycles is based on a 3+1 convergent macrocyclization reaction which is highly dependent on the presence of specific anions (see Scheme 4). The yields of this reaction vary between 42%, when no anion is pre-

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3

Scheme 2 The presence of chloride favours the formation of the pentagon 3

3

2

Scheme 3 Wright has proposed the presence of a common intermediate in the formation of

3 and 4. The presence of NEt3 is essential for the high-yield formation of the tetramer while excess of chloride favours the formation of the pentamer

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4 Scheme 4 Alcalde has reported that the synthesis of [I4]imidazolliophanes such as 4 is templated by anionic species

sent, to 82% in the presence of chloride. The templating role of the halides has been attributed to the formation of an intermediate in which C-H…Cl hydrogen bonds are formed providing the optimal conformation for the cyclisation step. In a recent study, some mechanistic aspects of this templated process have been determined quantitatively [28]. Using UV-Vis spectroscopy to monitor the kinetics of the macrocyclization reaction, it has been established that the rate of ring closure of the cationic precursor to the [14]imidazoliophane (4) is increased up to ten times in the presence of 0.04 mol/l solution of a chloride source. The chloride stabilizes the transition state (i.e. a kinetic template) favouring the macrocyclization through hydrogen bonding. The examples discussed above make use of spherically symmetric hydrogen bond acceptors (i.e. the halide anions); however, polynuclear anions with more complex geometries have also been employed as templates for the synthesis of organic macrocycles. An early example of the use of nitrate as a directing agent is the macrocyclisation reaction reported by Sessler [29]. In this work, it was demonstrated that the acid catalysed synthesis of the oligopyrrolic macrocycle 5 (see Scheme 5) requires HNO3, rather than other acids such as HCl, to take place in high yields. Under these conditions the nitrate salt of the protonated macrocycle precipitated out of the reaction mixture leading the authors to suggest a

5 Scheme 5 One of the first anion-templated reactions was the macrocyclisation of 5 reported

by Sessler

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templating effect exerted by the anionic nitrate (where hydrogen bonding plays an important role). Kim has reported the directing role of the pseudo-octahedral anion [CaCl3(DMAc)3]– (DMAc=dimethyl acetamide) in the synthesis of cyclic aromatic amides (see Scheme 6) [30]. This anion, formed in the course of the reaction from CaCl2 and free chloride, has an influence on the size of the macrocycle formed when reacting isophthalic acid chloride with m-phenylenediamine (the 3+3 macrocycle 6 being the most abundant product). In the absence of the anion, this macrocycle is not obtained as the major product and the formation of oligomers and macrocycles of different sizes is observed. Structural characterization of the favoured 3+3 cycle has revealed the presence of several N-H…Cl interactions between the amide protons of the macrocycle and the chlorides of the central (templating) anion. This strongly suggests that hydrogen bonding is responsible for directing the selective synthesis of the 3+3 macrocycle.

6 Scheme 6 Kim has employed anionic species to control the size of the rings formed when re-

acting isophthalic acid chloride with m-phenylenediamine. Hydrogen bonding interactions play an essential role in this process

Metal-organic macrocycles can also be prepared employing a combination of hydrogen bonding interactions together with metal-to-anion Lewis acid/base interactions. A series of metalla-macrocycles and cages based on the amidinothiourea ligand [atu=H2NC(=S)N(H)C(=NH)NH2], nickel and palladium have been reported by Mingos [31, 32] and Vilar [33]. By reacting Ni(atu)2 with [Pd(PPh3)2]2+ in the presence of selected anions (which act as hydrogen bond acceptors) the boxes [Pd2Ni2(atu)4(PPh3)4X]3+ (X=Cl, 7a; Br, 7b; I, 7c; atu=amidinothiourea) can be readily synthesized in yields ranging between 54 and 74% (see Scheme 7). However, in the presence of other anions such as triflate, nitrate or acetate the formation of the metalla-macrocycles is not observed and instead monometallic species are formed (which, upon addition of stoichiometric amounts of halide convert to the corresponding metalla-assembly confirming the templating role of the halides). The spherical metalla-cages [M2Ni4(atu)8X]3+ (M=Ni, 8a,b; Pd, 9a,b; X=Cl, Br) have also been prepared by an anion templated process. These species can be obtained by reacting Ni(atu)2 with NiX2 and [PdX2(PhCN)2] respectively. Once again the formation of these metalla-assemblies is highly dependent on the pres-

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Scheme 7 The syntheses of the metalla-cages and metalla-macrocycles shown in this scheme

is highly dependant on the nature of the anions present in solution. Hydrogen bonding interactions with the templating halides play an essential part in the templated process

Fig. 1 Molecular structure of 8a showing the encapsulated chloride anion at the centre of the

cage. The anion interacts with cage via eight N-H…Cl hydrogen bonds and two Ni…Cl Lewis acid-base interactions

Hydrogen-Bonding Templated Assemblies

93

ence of the appropriate anion (bromide and chloride in this case). The crystallographic characterisation of these boxes and cages (see Fig. 1) demonstrated that the halides which are encapsulated at their centre are establishing hydrogen bonds with the NH groups from the surrounding atu ligands. The hydrogen bonding geometry around the encapsulated halide can be seen as a square planar arrangement for the macrocycles and cubic for the cages (i.e. the central halide forms four and eight hydrogen bonds respectively with the NH groups positioned in the vertices).

4 Interlocked Structures: Catenanes, Rotaxanes and Pseudorotaxanes Structures such as rotaxanes (where one or more macrocycles encircled a linear axle) and catenanes (where two or more macrocycles are mechanically interlocked) are supramolecular assemblies with uncommon topologies.Although examples of such structures are found in nature (for example DNA strands can form catenanes) the synthesis of interlocked assemblies was not achieved until the early 1960s and several years had to pass before the original procedures were improved to become reliable and systematic. These assemblies were originally considered chemical curiosities; however their recent application as molecular switches and motors [34–39] has stimulated the field and there is increasing interest in finding efficient and high-yielding routes for their preparation. The first interlocked structures were prepared by statistical synthetic methods. However, these approaches are very inefficient and give low yields. As supramolecular chemistry developed and non-covalent interactions became better understood, novel synthetic strategies for the preparation of interlocked species were established. Particularly, the use of templates led the way to their rational and high-yielding synthesis [40–44].A template can be used to position the components of the final assembly in the appropriate spatial arrangement so that further transformations yield the interlocked structures. Several templated approaches for the synthesis of rotaxanes and catenanes have been developed over the last two decades. Sauvage introduced in 1983 a strategy that relies upon a transition metal template that organises the building blocks by coordination [45–47]. Some years later, Stoddart [42, 48] developed templated methodologies that use, as the main supramolecular interaction, aromatic stacking forces (although, as will be discussed later, hydrogen bonding also plays an important role in this approach). More recently, hydrogen-bonding interactions (to neutral or charged templates) have been used for the templated synthesis of these interlocked structures. The following two sections will discuss selected examples of catenanes and rotaxanes synthesised using hydrogen-bonding templates. 4.1 Catenanes

In the early 1990s Hunter [49] and Vögtle [50, 51] discovered independently the possibility of using hydrogen bonding interactions displayed by amide groups to template the formation of [2]catenanes.While studying the reaction between the

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10 +

11 13

12 14 Scheme 8 Hunter and Vögtle reported independently the templated synthesis of catenanes 12

and 14

diamine 10 and the bis(acid chloride) 11 to prepare a 2+2 macrocycle Hunter obtained, besides the expected macrocycle (in 51% yield), the [2]catenane 12 in 34% yield (see Scheme 8). A similar experiment was reported by Vögtle in which the reaction between the diamine 11 and the bis(acid chloride) 13 in high dilution conditions led to the direct formation of catenane 14 in 8.4% yield [52]. In both these reactions the complementary hydrogen bonding between the NH and CO groups of the amides position the two components in the appropriate spatial arrangement for the successful formation of the catenane (see Scheme 9). More recently Vögtle has used this templated approach for the synthesis of [3]catenanes (though in low yields) and molecular knots [53]. The template in this case not only acts as directing agent but also is a component of the final assembly. By reacting covalently with the bis(acid chloride), the

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Scheme 9 Hydrogen bonding interactions are responsible for the appropriate orientation of the components that lead to the formation of catenanes 12 and 14

incorporation of the template in the catenane is irreversible (this is in contrast to the catenanes based on Sauvage’s approach where the templating transition metal can be removed after the catenation has taken place). A similar approach was employed by Leigh [54] to prepare catenane 15 in one step by reacting bis(acid chloride) 11 with bis-amine 16 under high dilution conditions (see Scheme 10).

11 +

16 15 Scheme 10 Leigh has reported the hydrogen-bonding templated synthesis of catenane 15

Each catenane consists of two identical, interlocked 26-membered rings with a relatively small internal cavity (with dimensions of 4¥6 Å). This interlocked species was the first amide-catenane to be structurally characterised (although Hunter’s and Vögtle’s catenanes were reported earlier). The structure supported the proposal that the driving force for catenane formation is hydrogen bonding between the newly formed 1,3-diamine units and carbonyl groups of the acid

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chlorides. The stacking of the electron-rich xylylene and electron-poor isophthaloyl rings may also play an important role in the assembly process. In a subsequent communication Leigh demonstrated that his approach was not an isolated case but quite a general procedure to prepare a wide range of catenanes (based on larger macrocycles). More recently Leigh [55] has reported the formation of [2]catenanes by exploiting the hydrogen bond-mediated assembly of self-complementary macrocycles and the reversible ring opening-closing metathesis reaction (RORCM). In this system, while the hydrogen-bonding motif of the species acts as a template (in a similar fashion to the previous examples) the RORCM catalyst allows for the reaction to be thermodynamically controlled (see Scheme 11). Monitoring the reaction by HPLC demonstrated that once the equilibrium between the macrocycle 17 and the catenane 18 was reached, the latter was present in yields higher than 95%.

17

18

Scheme 11 Hydrogen-bonding templated preparation of catenane 18 under thermodynamic

control

The distribution of products is dependant on the concentrations demonstrating that this is a truly thermodynamically controlled reaction. 4.2 Rotaxanes and Pseudorotaxanes

Rotaxanes may be prepared using several different synthetic strategies which are summarized in Scheme 12. In the clipping method (b) a linear axle templates the formation of a macrocycle around itself. In the threading-followed-by-capping method (a), the wheel can be considered as a concave template that positions the axle in the appropriate geometry for the formation of the rotaxanes (a similar process is observed in the snapping mechanism). Over the past two decades several hydrogen-bonding species, such as amides and ammonium salts, have been employed as templates for the synthesis of rotaxanes. In the following sections the most relevant examples are presented and discussed.

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a

b

c

d

Scheme 12a–d There are several general methods to prepare rotaxanes: a threading followed

by capping; b clipping; c slippage; d snapping

4.2.1 Ammonium Salts as Templates

The strong hydrogen bonding interactions observed between the oxygen atoms of crown ethers and the N-H groups of ammonium groups can be successfully employed to prepare pseudorotaxanes and rotaxanes by templated processes. This approach has been extensively utilised by Stoddart, Busch and others to obtain a wide range of interlocked species. One of the several ammonium-templated rotaxanes reported by Stoddart [56] is based on a threadlike ammonium species with terminal azido-groups (19). Upon mixing this axle with crown ether 20, the [2]pseudorotaxane 21 was formed (see Scheme 13).

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20

21 22 19 Scheme 13 Templated synthesis of rotaxane 22 employing the threading-followed-by-capping methodology

The pseudorotaxane 21 was then converted into the [2]rotaxane 22 (in 30% yield) after the covalent attachment, under kinetic control, of bulky groups at both ends of the axle. The same methodology can be employed to prepare [3]rotaxanes such as 23 shown in Scheme 14 [57]. Busch has employed an analogous templated approach to prepare the [2]rotaxane 24 (see Scheme 15) [58]. In this case the axle 25 contains a secondary amine and a terminal ammonium group. Upon mixing this species with crown ether 20 (which can be considered a hydrogen bond-acceptor concave template) a strong hydrogen bonded host-guest complex is formed. A shift of the proton from the terminal ammonium group to the secondary amine induces threading

23 Scheme 14 [3]Rotaxane 23 can be prepared by hydrogen-bonding templated synthesis employ-

ing the threading-followed-by-capping methodology

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25 26 20

24 Scheme 15 Templated synthesis of rotaxane 24 employing the threading-followed-by-capping methodology. The shift of the proton from the terminal amine to the secondary amine allows for the threading to occur yielding the pseudorotaxane 26

of the axle to form [2]pseudorotaxane 26, which upon acylation of the primary amine, yields the [2]rotaxane 24.Again this approach can be taken further to prepare [3]rotaxanes (see 27 in Scheme 16) [59]. The threading-followed-by-capping method has been recently employed by Stoddart to prepare a [2]rotaxane under thermodynamic control [60]. In this approach, the dibenzylammonium ion 28 – which is terminated by an aldehyde function – is mixed with the dibenzo[24]crown-8 ether (20) to form a threaded species. Upon addition of a bulky amine, the aldehyde-terminated template can be converted into an imine in a reversible reaction establishing a dynamic equilibrium (see 29 and 30 in Scheme 17). The dynamic nature of the system offers the crown ether access to the ammonium centre allowing self-assembly of the corresponding “dynamic” [2]rotaxanes 31 to occur.“Fixing” of the interlocked assembly can be achieved by reducing the imine groups in 31 to the corresponding amine so that a kinetically inert [2]rotaxane forms.

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20

27 Scheme 16 Synthesis of [3]rotaxane 27 by hydrogen-bonding templated synthesis

Another example where a rotaxane is synthesised under thermodynamic control was reported in 2000 by Takata [61]. In this system, the threadlike molecule 32 with disulfide linkages and two ammonium ion centres for hydrogenbonding interactions was prepared. When such species was mixed with the dibenzo[24]crown-8 macrocycle (20), no threading was detected due to the large capping groups present in the potential axle. However, when catalytic amounts of PhSH were added to the mixture, formation of both [2] and [3]rotaxanes was observed upon prolonged periods of time. The system reached stationary state after 30 days. From this mixture [2]rotaxane 33 and [3]rotaxane 34 were isolated by preparative GPC in 8% and 54% yields respectively. This observation is attributed to the reversible reaction between 32 and PhSH to generate the new thiol 35 and disulfide 36 (see Scheme 18) both of which are now capable of threading through the macrocycle (which acts as hydrogen bonding template). Since the reaction between disulfides and thiols is reversible, a complex dynamic equilibrium is established which eventually leads to the formation of the [2] and [3]rotaxanes under thermodynamic control. The rotaxanes presented so far in this section have been prepared by the threading-followed-by-capping methodology. However, hydrogen bond-donor templates

Scheme 17 Synthesis of [2]rotaxane 31 under thermodynamic control

28 29

31

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Hydrogen-Bonding Templated Assemblies

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34 + 35

33

35

33

32 33 + 35

36

34

Scheme 18 The reversibility of the disulfide bonds allows for the formation of [2]- and [3]rotaxanes (33 and 34) under thermodynamic control

32

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Hydrogen-Bonding Templated Assemblies

can also be employed to prepare rotaxanes by the clipping method. Stoddart has recently reported the templated synthesis of a [2]rotaxane by a clipping approach that employs the formation of reversible imine bonds (see Scheme 19) [62]. 2,6-Pyridinedicarboxaldehyde (37) and tetraethyleneglycol bis(2-aminophenyl)ether (38) react to form macrocycle 39. 1H NMR studies indicate that the three components in this mixture are in equilibrium since the imino groups, water and aldehydo/amino groups are constantly interconverting. Addition of bis(3,5-dimethoxybenzyl)ammonium hexafluorophosphate (40) species has a

+

37

39

38 40

41

42

Scheme 19 Stoddart has employed the clipping method to prepare [2]rotaxane 42. Reversible imine bonds are used to generate a dynamic mixture containing the threaded and un-threaded components

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dramatic effect on the composition of the equilibrated mixture. Within minutes equilibrium is established and the formation of the [2]rotaxane 41 is observed (the signals in the 1H NMR spectrum becoming much sharper than before the addition of 40) suggesting that this is a thermodynamically stable product. The added stability of this assembly relative to that of the unthreaded species has been attributed to the strong [N+-H…X] (where X=O, N) hydrogen bonds formed between the protons of the secondary ammonium group and the macrocycle oxygen atoms. Subsequent reduction of the imino groups affords the kinetically stable [2]rotaxane 42 in practically quantitative yields. 4.2.2 Templated Synthesis by Hydrogen Bonding to Amides

The hydrogen bonding templated methodology introduced by Hunter [49] and Vögtle [50, 51] to prepare catenanes (see above) can also be employed to prepare rotaxanes.Vögtle, for example, has used hydrogen bond-donor templates to prepare [2]rotaxanes by a three-component threading-followed-by-capping method (see Scheme 20) [63, 64].

11

43

44

45 Scheme 20 Synthesis of [2]rotaxane 45 by a three-component threading-followed-by-capping mechanism

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In this approach the wheel 43 acts as a concave template which interacts by hydrogen bonding with the bis(acid chloride) forming the pseudorotaxane (44). This host-guest complex is converted into a [2]rotaxane (45) by attaching bulky stopper groups to the ends of the axle (which prevent it from de-threading). As for the catenanes described in the previous section, here the template becomes an integral part of the final interlocked assembly. Vögtle has developed this approach further and employed a series of anionic templates to prepare rotaxanes (instead of the neutral template in the above reaction) [65–67]. In this approach a phenolate, thiophenolate or sulfonamide anion is non-covalently bound to the tetralactam macrocycle (46) forming a hostguest complex via hydrogen bonding (see Scheme 21). In this assembly the anion is positioned at the centre of the ring (see Scheme 22) to react further with a second component, such as an alkyl bromide or acyl chloride, producing the final rotaxane in good yields. The anion-templated synthesis of rotaxanes first introduced by Vögtle has now been employed by others to produce other interlocked assemblies. Schalley for example [68], has reported an improved yield for the synthesis of rotaxanes by

46 Scheme 21 Anion-templated synthesis of [2]rotaxanes first introduced by Vögtle

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Scheme 22 The reacting phenolate interacts with the wheel of the rotaxane by hydrogen-

bonding

separating the functional groups that interact with the wheel from the sites of “stopper” attachment. In Vögtle’s approach to the synthesis of rotaxanes, the phenolate oxygen is buried inside of the wheel’s cavity (see Scheme 22) and hence is protected against attack by the semi-axle to form the rotaxane. In order to avoid this steric problem Schalley has prepared centrepieces (47) which bear a phenolate – that hydrogen-bonds to the templating wheel (46) – and two remote reactive sites for “stopper” attachment (see Scheme 23). With this separation of the hydrogen bond-acceptor template and the sites for the “stopper” attachment, the steric effects due to shielding by the wheel are avoided making the synthesis of the rotaxane 48 more efficient (in some of the systems the yields of rotaxane formation increase from 5 to 30% when the functionalised centre-piece is added). The templated syntheses of amide-based rotaxanes discussed until now have made use of the threading-followed-by-capping method. However there are also examples in which the clipping approach has been employed. Leigh, for example, has used a five-component clipping method to prepare [2]rotaxanes. Isophthalamide and peptide-based threads were shown to template the formation of benzylic amide macrocycles about them in non-polar solvents [69, 70]. When the peptide-based threads (49) contain bulky stoppers at their ends, the [2]rotaxanes (50) can be prepared in high yields (see Scheme 24) [71]. In the absence of a suitable template, species such as 51 are long lived in solution (due to their preference for a linear conformation) reacting intermolecularly to form larger macrocycles in preference to intramolecular ring closure. The template, on the other hand, provides the hydrogen bonding acceptor groups to

107

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47 46

48 Scheme 23 Synthesis of [2]rotaxane 48 employing centrepieces bearing a phenolate and to re-

mote sites for stopper attachment

promote a conformational change in 51 leading to intramolecular cyclisation. In order to improve the efficiency of this reaction even further, Leigh reported the use of rigid templates (52 and 53) which allow for a higher degree of pre-organisation increasing the yields of [2]rotaxane formation to 97%. These interlocked species have been fully characterised in solution and the solid state demonstrating the important role played by the hydrogen bonding acceptor groups (the C=O of the amides) present in the templating species. Leigh also reported the use of esters (where the NH groups of the amides are substituted by oxygen atoms) as templates for the synthesis of [2]rotaxanes. Once again the hydrogen bond acceptor properties of the templating esters play an essential role in the formation of the final interlocked assemblies.

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+

50

51

Scheme 24 Templated synthesis of [2]rotaxane 50 by the clipping mechanism

Hunter has recently reported a three-molecule approach to noncovalent [2]rotaxane assemblies [72]. In this approach a combination of hydrogen bonding and metal-ligand coordination has been employed to prepare the rotaxanes (see Scheme 25). The substituted terephthalic acid 54 (with bulky porphyrin groups as stoppers) acts as a hydrogen bond-acceptor template bringing together two units of the zinc complex 55. By doing so, a macrocycle is formed around the central templating axle and the [2]rotaxanes 56 is formed. In contrast to the examples discussed so far, in this case the rotaxane is held together by noncovalent interactions only (i.e. hydrogen bonding and metal coordination). 4.2.3 Assembly of Rotaxanes by an “Independent” Hydrogen-Bonding Template

All the examples of hydrogen-bonding templated rotaxanes and catenanes discussed up to this point involve templates that become one of the components (either the axle or macrocycle) of the final interlocked assembly; furthermore, in several of the examples the template is incorporated by covalent (irreversible) transformations. Synthetic procedures where the templating agent is not one of the components of the interlocked assembly but an “independent” template are fewer. This might be partly due to the difficulties in identifying the templating species if they are not kept as part of the final assembly. As the following examples illustrate, most of these “independent” templates are anionic species acting as hydrogen bond acceptors. Stoddart and Williams have reported the syntheses of a series of pseudorotaxanes where the templating agent employed (an anion), although part of the

54

Scheme 25 Three-molecule approach to the synthesis of the non-covalent [2]rotaxane 56

+

56

55

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57 Scheme 26 Anion directed synthesis of [5]pseudorotaxane 57. Structural characterisation of

this species has demonstrated that the PF6– anion is located at its centre forming multiple hydrogen bonds to 57’s components

final assembly, is not one of the components of the pseudorotaxane (i.e. the wheel or the axle) and is not covalently modified in the process [73]. Mixing four equivalents of [NH2(CH2Ph)2][PF6] with one equivalent of the large macrocycle tetrakis-p-phenylene[68]crown-20 produced the quadruple-stranded pseudorotaxane 57 (see Scheme 26). This product of supramolecular assembly was structurally characterized and revealed the presence of a PF6– anion at its centre forming multiple C-H…F hydrogen bonds with the hydroquinone methine and the benzylic methylene hydrogen atoms, which strongly suggests that the PF6– anion plays a templating role in the assembly process (besides the hydrogen bonding interactions present between the ammonium axle and the ether oxygen atoms of the wheel). Further studies by the same authors have demonstrated that PF6– acts a hydrogen bond-acceptor template in the assembly of several other interwoven structures. In an extensive study aimed at using a combination of hydrogenbonding motifs to self-assemble pseudorotaxanes into more complex structures it was discovered that PF6– assists on the organization of the components that yield the final superstructure [74]. Particularly, it was found that this anion dictates the orientation of the two carboxylic acid groups of the [3]pseudorotaxanes 58 and 59 (see Schemes 27 and 28); when these groups are co-directional with respect to each other the formation of discrete hydrogen-bonded dimers is observed. Structural characterisation of 58 and 59 have demonstrated that the PF6– anion is located in the cleft between the two dialkylammonium cations forming hydrogen-bonds with the benzylic hydrogen atoms of one of the cations and with one of the hydrogen atoms of a hydroquinone ring. In contrast, a polymeric as-

Hydrogen-Bonding Templated Assemblies

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Scheme 27 Thread-like molecules with carboxylic acid groups

sembly is obtained when the analogous isophthalic acid-substituted cation is used to form the [3]pseudorotaxane (see (c) in Scheme 28). The formation of an extended structure instead of dimeric assemblies is, to a certain extent, controlled by the interactions of the assembly’s components with the PF6– anion. More recently, Beer has reported an example of halide-templated synthesis of [2]pseudorotaxanes [75, 76]. The approach used in this process involves a cationic thread (60) designed to hydrogen bond to a central chloride anion; the templating halide remains coordinatively unsaturated until the second component 61 of the pseudorotaxane (62) is added (see Scheme 29). The chloride acts as a template since it organizes the two components (the macrocycle and the linear species) in an orthogonal fashion in relation to each other by means of hydrogen-bonding. In contrast to the efficient templating role played by the chloride, other anions such as Br–, I– and PF6– proved to be poor templates for this process. Beer has recently extended this approach to prepare [2]rotaxanes [77]. In this case, two acyclic species are orthogonally positioned in respect to each other via hydrogen bonding to a templating hydrogen bond acceptor chloride (see Scheme 30). One of the acyclic components (63) contains terminal olefins which, upon ring-closing metathesis using Grubbs’ catalyst, yields the [2]rotaxane 65. To avoid de-threading once the ring is closed the second component of the assem-

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b

58

59

c

Scheme 28 The templating anion determines the orientation of the threads in the pseudorotaxane, which has important consequences for the solid-state structure of these assemblies

61

62

60 Scheme 29 Anion-templated synthesis of [2]pseudorotaxane 62

63

Scheme 30 Chloride templated synthesis of [2]rotaxane 65

64

65

Hydrogen-Bonding Templated Assemblies

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bly contains bulky terminal groups. Once the rotaxane is formed, the templating chloride can be removed and the anion-binding properties of the template-free species studied. The crystallographic characterization of rotaxane 65 confirmed the structure and revealed that the chloride anion is tightly bound via several NH…Cl and C-H…Cl hydrogen bonding interactions. This elegant synthesis has demonstrated the possibility of using a hydrogen bond-acceptor template to prepare template-free interlocked species (in a similar fashion to Sauvage’s catenanes syntheses which make use of transition metals as templates). 4.3 p Stacking in Templated Synthesis of Interlocked Species Hydrogen Bonding vs p -p

The templates employed in the synthesis of the interlocked assemblies presented throughout the previous sections utilize hydrogen bonding as the main driving supramolecular interaction.Another approach to the synthesis of catenanes and rotaxanes pioneered and developed by Stoddart relies on p-p stacking interactions between electron rich and electron poor aromatic systems as the main driving force (see Scheme 31). This synthetic approach has been employed extensively and several reviews have covered the topic and highlighted the importance of the p-stacking interactions in the assembly process [40, 42, 48]. Besides these p-p interactions, several studies (in solution and solid state) by Stoddart and Williams have demonstrated that C-H…O and C-H…p hydrogen bonding interactions also play an important role in the assembly process that leads to the formation of interlocked species [78–81]. For example, in the [2]pseudorotaxane 66 (which can be seen as a precursor to the syntheses of catenanes and rotaxanes) shown in Scheme 31, p-p interactions are established between the pyridinium groups and the di-substituted benzene rings of the pseudorotaxanes components. Furthermore, C-H…O hydrogen bonds are formed between some of the polyether oxygen atoms and the a-bipyridinium protons.A second type of hydrogen bonding interaction, C-H…p, is observed between the 1,4-dioxybenzene protons and the p-phenylene spacers. In most systems it is very difficult to measure their individual strength and importance in the assembly process. However, it is usually assumed that p-stacking between complementary aromatic species is the main supramolecular interaction in these systems. Houk, Stoddart and Williams have carried out a detailed investigation (involving experimental and theoretical studies) to quantify the strength of C-H…O hydrogen bonds in the formation of catenanes [82]. While the formation of the [3]catenane 67 was successful, its conversion to the [4]catenane 68 could not be achieved (see Scheme 32). This is surprising since [3]catenane 67 has ideal pockets for binding the bipyridinium cation which should lead to the formation of the [4]catenane 68. A thorough investigation of the structural data and an extensive theoretical analysis of model compounds, demonstrated that the absence of C-H…O hydrogen bonding interactions is responsible for the instability of this structure. Consequently, the targeted [4]catenane cannot be prepared using this approach.

precursor for the formation of [2]rotaxanes and catenanes

Scheme 31 [2]Pseudorotaxane 66 is formed by a combination of p-p stacking and C-H…O hydrogen bonding interactions. It is a good

66

Hydrogen-Bonding Templated Assemblies

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67

68 Scheme 32 Schematic representation of [3]catenane 67 and [4]catenane 68

Further theoretical studies by Stoddart and Houk have estimated the strength of these hydrogen-bonding interactions demonstrating that they contribute greatly to the stability of the host-guest complexes [83]. These studies demonstrate that in the formation of catenanes and rotaxanes using p-stacking interactions, hydrogen bonding should not be ignored. Loeb has reported a series of pseudorotaxanes [84, 85] and rotaxanes [86, 87] where C-H…O hydrogen bonding interactions (together with N+…O attractive forces) play an important contribution in templating the formation of the interlocked species. In particular, the formation of a pseudorotaxane was observed when equimolar amounts of [pyCH2CH2py]2+ and the crown ether 20 were mixed. The structural characterization of the resulting host-guest complex

Hydrogen-Bonding Templated Assemblies

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20

Scheme 33 Loeb has reported the templated syntheses of a series of pseudorotaxanes and rotaxanes which is based on C-H…O hydrogen bonding interactions as the main directing force

demonstrated it to be a [2]pseudorotaxane. This interlocked species is stabilized by eight C-H…O hydrogen bonding interactions between the oxygen atoms in the wheel (20) and: the four C-H groups in the ethane of the axle, and the four apyridinium hydrogen atoms (see Scheme 33). Further studies by the same authors have led to the formation of [2]rotaxanes, [3]rotaxanes and pseudo-polyrotaxanes [85–87]. In all these interlocked species, in spite of the presence of aromatic rings in the axle and wheel, p-p interactions do not seem to play a role in the templating process. This highlights once again the importance of C-H…O hydrogen bonding in the assembly of interlocked species.

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5 Stranded Helicates The helical motif – which is a common geometrical arrangement in nature – has been actively sought by synthetic chemists. Several successful templated syntheses of multiply stranded helicates have now been reported; in most of them metal centres are used as templates organizing the strands into the helical structures [88–90]. In contrast, there are only few examples of helicates which have been prepared by hydrogen-bonding templated processes (although nature’s helical structures tend to be hydrogen bonded). De Mendoza reported the first example of anion-directed helix formation in 1996 [91]. The assembly of this helical structure relies, not only on electrostatic interactions between the anionic template and the positively charged strands, but also on hydrogen bonding. The tetraguanidinium strand 69 (see Scheme 34) selfassembles around a sulfate anion via hydrogen bonding to produce a double helical structure. The formation of this assembly and its anion-dependence was proposed on the basis of NMR and CD spectroscopic studies.

69 Scheme 34 The tetraguanidinium strand 69 form helical structures in the presence of sulfates

Kral has recently reported a related system in which porphyrins bearing bicyclic guanidine substituents (such as the tetra-substituted species 70 shown is Scheme 35) form highly ordered chiral assemblies in aqueous solution [92]. Circular dichroism and UV/Vis spectroscopy indicated that the aggregation and chirality of the supramolecular assemblies is controlled by anionic species such as di-carboxylates. Although no structural information is yet available, hydrogen-bonding interactions between the di-carboxylate anions and the positively charged bicyclic guanidine groups are likely to be directing the formation of the specific assemblies. Using a different set of hydrogen bonding fragments Kruger and Martin have reported the formation and structural characterization of the double helicate 71 [93]. This helical structure can be prepared by assembling 72 diammonium-bispyridinium salt around two chloride anions (see Scheme 36). The structural characterization of this assembly has revealed that chloride coordination (via hydrogen bonding to the protonated pyridyl groups of the strands) induce the strands to adopt a double-helical structure in the solid state.

Scheme 35 The substituted porphyrin 70 forms helical assemblies in the presence of certain d-carboxylic acids

70

Hydrogen-Bonding Templated Assemblies

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71

72 Scheme 36 Two strands of 72 come together in the presence of chloride to form a helical structure in the solid state

6 Hydrogen-Bonding Templates in Dynamic Combinatorial Libraries Chemical templates are being increasingly employed for the development of dynamic combinatorial libraries (DCL) [94–98]. These (virtual) libraries of compounds are produced from all the possible combinations of a set of basic components that can reversibly react with each other with the consequent potential to generate a large pool of compounds. Because of the dynamic equilibria established in a DCL, the stabilization of any given compound by molecular recognition will amplify its formation. Hence the addition of a template to the library usually leads to the isolation of the compound that forms the thermodynamically more stable host-guest complex (see Scheme 37). These libraries may be applied either to aid the discovery of a substrate for a particular receptor or the construction of a particular receptor for a given guest. Depending on whether the receptor or the substrate act as a target-template, Lehn has defined two possible processes for the generation of DCLs: casting, which involves the receptor acting as a template to amplify the formation of a substrate; and moulding, when a receptor is obtained for a given guest template [99, 100]. Both these approaches to DCLs rely not only on the reversible formation of bonds between the different basic components of the mixture, but also on their ability to display non-covalent interactions with the template. Few examples of covalent and non-covalent DCLs have been reported over the past few years, with only a small number of them making use of hydrogen-bonding templates. One of such examples is the barbiturate receptor 73 reported by

Hydrogen-Bonding Templated Assemblies

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Scheme 37 The equilibrium established in a mixture of receptors (Rn) can be modified by the presence of a good template (guest for the receptor, T). This shifts the equilibrium amplifying the formation of the receptor that interacts more strongly with the template

Lehn, which was obtained from a dynamic library of hydrogen-bonding building blocks [101]. This system involved the equilibration between different products resulting from the condensation of 5,5-dimethyl-1,3-cyclohexanedione (74) and 2-hydrazinopyridine (75) (see Scheme 38). In a 1:2 ratio these compounds yield a mixture of (Z/Z), (E/E) and (E/Z) isomers plus a substantial amount of the (E) and (Z) isomers of the monohydrazone monoketone. The complexity of the mixture (due to several equilibria that are established between its components) was illustrated by its complex 1H NMR spectrum. However, addition of one equivalent of barbiturate to the mixture led to a dramatic simplification of the NMR spectrum with the disappearance of the different products and the emergence of a single new species. This remarkable simplification of the spectrum is a consequence of the templating effect played by the barbiturate that amplifies the formation of only one of the receptors – in this case the Z,Z isomer. Interestingly, the addition of other species to the mixture such as acetate or 1-benzyluracil (with similar pKa values to barbiturate but different hydrogen bonding patterns) did not lead to the amplification of any specific species. Lehn has also reported the hydrogen-bonding templated assembly of receptors based on bipyridine copper and palladium complexes [102]. A mixture of substituted bipyridines (76, 77) (see Scheme 39) with copper(I) triflate generates a mixture of tetrahedral complexes and uncoordinated ligands. The complexes formed have a tetrahedral geometry generating, in the case of the hydrogen bonding ligands, cavities where guests of specific size can bind. The equilibrium between the different complexes in this DCL can be modified upon addition of the hydrogen-bonding substrate 78 which acts as a template to amplify the formation of complex 79.Although the amplification of 79 is modest (2.3 times) this example demonstrates the possibility of using a combination of

75

Scheme 38 The formation of barbiturate receptor 73 is amplified in the presence of a barbiturate template

74

73

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76

77

78

complex

Scheme 39 Addition of 78 to a mixture of metal, 76 and 77 shifts the equilibrium to favour the formation of the most stable host-guest

79

Hydrogen-Bonding Templated Assemblies

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hydrogen bonding and metal-ligand coordination for the generation of selective receptors. Similar studies where performed with palladium(II) compounds, although the amplification was not as good as that with copper(I). In 1998 Still [103] reported a hydrogen-bonding directed DCL for selecting and amplifying receptors suitable for the detection of the tri-peptide (D)Pro(L)Val(D)Val. These authors based their library on previous findings which had demonstrated that linked oligomers of isophthalic acid and trans-1,2diamines were highly selective for the detection of specific peptide sequences. In this work, two thiol-containing compounds, 80-SH and 81-SH, were prepared (see Scheme 40). The disulfide 80-SS-81 was formed from these two thiols and demonstrated to bind very poorly to the polymer-supported (D)Pro(L)Val(D)Val tripeptide.

80-SH

81-SH

80-S-S-80 Scheme 40 Synthesis of receptor 80-S-S-80 from a dynamic combinatorial library based on 80-SH and 81-SH. The receptor is amplified in the presence of a tripeptide template

Addition of 81-SH to 80-SS-81 led to formation of the homodisulfide compounds and an equilibrium, with an exchange constant of 1.8, was established. The presence of the templating (D)Pro(L)Val(D)Val tri-peptide in this mixture, shifted the equilibrium dramatically and the formation of the homodisulfide 80SS-80 was amplified with a Keq=32. Since the templating tri-peptide was supported on polymer beads, the isolation of receptor 80-SS-80 (in 97% purity) was achieved easily by extraction of the beads. The formation of multiple hydrogen bonds between the template and the components of the DCL, led to the isolation of the best possible receptor available from the building blocks present in the equilibrated mixture.

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The examples discussed in this section provide encouraging leads for the future development of DCLs. Since a large number of biologically relevant substrates (and receptors) involve hydrogen bonding, it is certain that more templates using such interactions to amplify the formation of specific compounds from a dynamic library, will be developed in the future.

7 Hydrogen-Bonding Templates in Self-Replicating Systems Templating effects are important in the context of supramolecular catalysis where, by analogy with enzymatic processes, the reactants are organized by the template so that the formation of a particular linking of atoms is accelerated. An early example of this approach reported by Kelly is shown in Scheme 41 [104].

Scheme 41 The formation of a ternary complex via hydrogen-bonding accelerates the nucle-

ophilic attack that leads to the final product

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The catalysts (or template) is able to bind two reactants forming a ternary complex via multiple hydrogen bonds. In this complex, the reactants are brought in close proximity accelerating the nucleophilic attack that yields the final product. In the previous example the template is unrelated to the starting materials and to the final product. However, if the product formed by templation is the actual template, a self-replicating system emerges (see Scheme 42). Some excellent reviews on this topic have already appeared over the last few years, hence only some selected examples will be presented herein [105–108]. In 1986 von Kiedrowski reported the first example of a non-enzymatic selfreplicating system [109]. The self-complementary trinucleotides d(Me-CCGp) (5¢ terminus protected as its methyl ether) and d(oCGG-PhCl) (3¢ terminus protected with an o-chlorophenyl group) were reacted to yield, among other products, the hexanucleotide d(Me-CCGpoCGG-PhCl).Addition of the template product increased the rate of formation of the complementary hexanucleotide following the empirical square-root law expected for a templated autocatalysis. This reaction was further studied by introducing variations in the sequence of the oligonucleotides which showed that the self-complementary products were formed faster than any other sequences [110, 111]. This strongly indicated that the autocatalysis is result of a template effect.

Scheme 42 Schematic representation of a self-replicating system

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Hydrogen-Bonding Templated Assemblies

In 1990 Rebek reported an interesting self-replicating systems based on the Kemp’s triacid derivative 82 (see Scheme 43) [112–114]. By a combination of hydrogen bonding and p-p stacking interactions, diamine 83 forms a binary complex with 82. In such a complex, the amine is ideally positioned to react with the activated ester in 82 yielding the cis-amide complex 84. The strained cis-amide complex undergoes a fast isomerisation to the transamide species 85 which exposes two hydrogen-bonding binding sites suitable to interact with two new molecules of reactants 82 and 83. Such molecules can then

83

84

82

85 Scheme 43 Rebek’s self-replicating system based on Kemp’s triacid derivative 82

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86

88 87 Scheme 44 Self-replicating system based on the condensation of aldehyde 86 and amine 87

form a new trans-amide complex 85 and hence initiate the autocatalytic templated process. A tenfold increase in the rate of reaction of the amine 83 and activated ester 82 is observed in comparison to the non-autocatalytic system. Von Kiedrowski reported a simpler self-replicating system based on the condensation of aldehyde 86 with amine 87 (see Scheme 44) [115]. The condensation reaction yields an imine 88 with the appropriate set of hydrogen bond donor/acceptor groups to template its own formation via a ternary complex (involving the product and the two reactants). Closer inspection to this reaction has revealed that the tertiary complex is actually more stable (in some of the reaction studied) than the duplex formed between the template and the product. Consequently, once the templation has taken place, the duplex is separated and both the product and original template are ready to accelerate the reaction of the two reactants. Since the number of templates has now doubled, the enhancement of the reaction could in principle follow an exponential rate. These examples have demonstrated that it is possible to use the templating properties of a compound to accelerate its own formation. This is a potentially very attractive approach for the production of large quantities of a specific product with high selectivity. Furthermore, synthetic self-replicating systems also provide interesting models for their biological counterparts, which in turn could provide important clues to understand chemical evolution and indeed the origin of life itself. In spite of their attractive features, one of the problems usually encountered in self-replicating systems is the formation of very stable dimmers between the template and the complementary product formed. This obviously imposes important limitations to the use of self-replicating processes for the formation of large quantities of a specific product.

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8 Hydrogen Bonding Templates in the Photodimerisation of Olefins The solid state [2+2] photochemical reaction of olefins is an attractive transformation for the generation of C-C bonds. However, this type of reaction can only take place when the olefins to be dimerized crystallise in the appropriate relative orientation. For several decades chemists have strived to design molecules that will predictably crystallize in such orientations. In spite of these efforts, to date there are not general and reliable methods to align olefins in the solid state so that the photodimerisation reactions can take place. In this context, an approach that has started to emerge as a potentially useful alternative to orient olefins in the solid state is by templated processes. Some examples where hydrogen-bonding templates have been used in the photodimerisation of olefins have appeared and are discussed herein. Stoddart has employed the anion-directed [3]pseudorotaxane assemblies described above to control the outcome of solid-state photodimerisation reactions of olefins [116]. A combination of supramolecular interactions (one of them being hydrogen-bonding to PF6–) has been employed to pre-organize bis(dialkylammonium) salts containing trans-stilbenoid units (89) into the [3]pseudorotaxane assembly 90 shown in Scheme 45. A crystalline sample of this supramolecular assembly was irradiated with UV light and the formation of the corresponding cyclobutane 91 with syn-anti-syn stereochemistry was observed. In contrast, the photodimerisation of transstilbenoid-bis(dialkylammonium) salts does not take place in the absence of the macrocycle, indicating the importance of pre-organizing the stilbenoid units (which requires the presence of the anion) for this solid-state reaction to occur. Teramae has recently reported another example where hydrogen bonding to an anionic species directs the photodimerisation of olefins [117]. The thyminefunctionalised isothiouronium 92 (see Scheme 46) forms a photodimer at the thymine moiety in methanol upon UV irradiation. The products resulting from this reaction can be syn or anti. In the absence of a templating anion, the anti conformation is preferred (93). However, when the photodimerisation reaction is carried out in the presence of pyrophosphate the preferential formation of the syn dimer (94, 95) is observed. This anion has the appropriate geometry and hydrogen-bonding pattern to interact complementary with the isothiouronium groups of the substrate directing the course of this reaction. Linear templates containing hydrogen bond donor groups have been successfully employed by MacGillivray to direct the photodimerisation of olefins [118]. Specifically, trans-1,2-bis(4-pyridyl)ethylene (96) crystallises forming a layered structure with the olefins in neighbouring layers oriented in an orthogonal fashion. Consequently, the crystalline sample of the olefin does not adhere to the topochemical principle and is photostable. However, by cocrystallising it with linear templates such as 1,3-dihydroxybenzene (97) or 1,8-naphthalenedicarboxylic acid (98) the olefins orient in the solid state lying parallel to each other and hence become photoactive (see Scheme 47) [119, 120].

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89

90

91 Scheme 45 Templated photodimerisation of [3]pseudorotaxane 90

When the co-crystallised samples were irradiated with UV light for several hours, the stereospecific (100%) formation of rctt-tetrakis(4-pyridyl)cyclobutane (99) was observed. This was confirmed by spectroscopic, analytical and structural methods. The latter showed that, once irradiated, the new co-crystallized materials (99)·2(97) and (99)·2(98) are formed respectively. In both these structures the molecular components are held together by hydrogen bonding interactions. This type of hydrogen bond-donor template has been recently employed by the same authors to direct the assembly of a hexa-copper cage [121]. The procedure involves two steps: in the first one, the resorcinol-based linear template 100 is co-crystallised with trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (101) and irradiated with UV light to yield rctt-1,2-bis(2-pyridyl)-3,4-bis(4-pyridyl)cyclobutane (102) (see Scheme 48). In the second step the product resulting from the photochemical reaction is mixed in solution with Cu(ClO4)2 in a 1:1 ratio to yield the novel cage compound [Cu6(2,4-tpcb)6(H2O)6][ClO4]12 (103). This is a

131

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93 92

94

95 Scheme 46 The presence of pyrophosphate directs the formation of dimers 94 and 95 in preference to 93

good illustration of how a linear hydrogen bond-donor template can induce the formation of a specific ligand that in turn favours the assembly of a metal cage in relatively high yields. Bassani has reported the substrate-templated photochemical synthesis of barbituric acid receptors by irradiating a solution of olefin 104 in the presence of template 105 (see Scheme 49) [122]. In the absence of such template, the major cycloadducts formed are the head-to-head (106) and head-to-tail dimers (107) (see Scheme 49). However, in the presence of barbituric acid, the distribution of products changes and the head-to-head cycloadduct is formed in higher proportions than the rest. This selectivity can be attributed to the presence of the templating agent which, by hydrogen bonding, pre-organises the olefins before irradiation.

97

97

Scheme 47 Linear templates direct the phodimerisation of olefin 96

99

96

96

99 98

98

132 Ramón Vilar

133

Hydrogen-Bonding Templated Assemblies

102

100

101

101

Scheme 48 The hydrogen-bonded directed photodimerisation of 100 yields 102 which is a good

ligand for the formation of an hexa-copper cage compound

105

104

106

107

108

109

Scheme 49 Substrate-templated photochemical synthesis of receptors for 105

107

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Ramón Vilar

9 Conclusions The last 20 years have seen an increasing use of hydrogen bonding templates to direct the course of reactions or assembly processes. This is not surprising if we keep in mind that nature employs this type of non-covalent interaction in a wide range of processes. Hydrogen bonding species are ideally suited to act as templates. This is partly due to the directional nature of hydrogen bonds and the fact that they are relatively strong (in comparison to other non-covalent interactions). Moreover, the possibility of having several combinations of hydrogen bonding donor/acceptor functionalities within the same templating molecule, provides the potential to codify a large amount of information required for very selective templated processes. The examples presented in this review give a clear indication of the important role played by hydrogen bonding in templated processes. Macrocycles, cages and species with unusual topologies, such as catenanes and rotaxanes, can be efficiently and selectively prepared in the presence of the appropriate templates (i.e. with the right donor/acceptor complementarity, size and charge). Hydrogen bonding templates have also provided a degree of control in the photodimerisation of olefins by assembling the olefin-containing monomers in the appropriate orientations. More recently, templation by hydrogen bonding has been employed to amplify the formation of specific products from dynamic combinatorial libraries. Furthermore, although not discussed in this review, hydrogen-bonding templates have also demonstrated their potential in the design of coordination polymers and networks, the preparation of molecularly imprinted polymers and the sequence-specific synthesis of polymeric materials. The successful use of hydrogen-bonding templates in all these systems is a clear indication of the great potential these species have for the formation of more selective receptors, preparation of assemblies with complex topologies and control over reactivity. Acknowledgments I would like to thank Dr. Joachim Steinke for valuable discussions and useful comments to this review. EPSRC (The Engineering and Physical Sciences Research Council) is acknowledged for financial support.

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Structure and Bonding, Vol. 111 (2004): 139–174 DOI 10.1007/b14142HAPTER 1

Hydrogen Bonded Network Structures Constructed from Molecular Hosts Michaele J. Hardie Department of Chemistry, University of Leeds, Leeds, LS2 9JT, UK E-mail: [email protected]

Abstract Molecular hosts are molecules that show intrinsic inclusion behaviour, usually involving guest molecules occupying cavities within the molecular framework of the host. Molecular hosts such as crown ethers, functionalised calixarenes, cyclotriveratrylene, cyclodextrins and cucurbituril have hydrogen bond donor or acceptor functionality. This can be used to incorporate molecular hosts into crystalline materials as components of 1D, 2D or 3D network structures. This may have several interesting consequences, including altering the host-guest characteristics of the host molecule, and creating crystalline materials that show multiple inclusion behaviour such as lattice inclusion and site specific host-guest interactions. Furthermore a number of hitherto elusive species have been isolated as components of such networks. Keywords Network structures · Molecular hosts · Inclusion chemistry

1

Introduction   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 140

2

Crown Ethers   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 141

3

Calixarenes and Related Hosts   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 146

3.1 Calixarenes   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 146 3.2 Sulfonated Calixarenes   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 147 3.3 C-Methylcalix[4]resorcinarene   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 152 4

Cyclotriveratrylene   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 159

5

Cyclodextrins  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 167

6

Cucurbiturils   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 169

7

Conclusions   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 171

8

References   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 172

© Springer-Verlag Berlin Heidelberg 2004

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List of Abbreviations bipy bpe CD Cp CTC CTV DMF PyH

4,4′-Bipyridine trans-Bis(4-pyridyl)ethylene Cyclodextrin Cyclopentadienyl Cyclotricatechylene Cyclotriveratrylene Dimethylformamide Pyridinium

1 Introduction Crystal engineering concerns controlling and exploiting the aggregation of molecules in crystals, or more simply crystalline phase supramolecular chemistry [1, 2]. One of the aims of crystal engineering is to construct crystalline molecular or polymeric materials with highly ordered networks or infinite framework structures. Infinite framework structures consist of molecular components organised into 1D chain or ladder, 2D grid or 3D network structures, (Fig. 1). Molecular components are organised using pre-defined geometric parameters coupled with a linking mechanism such as a hydrogen bonding motif or coordinate interactions. For instance the tetrahedral molecular components tetra(cyanophenyl)methane and Cu(I) may be expected to self-assemble into a diamondrelated network, which is indeed the case [3]. The network structure thus formed is porous, with large cavities and channels filled with counter-ions and solvent molecules acting as lattice type guests. Many of the proposed applications of these types of materials rely on their inclusion behaviour and a zeolitic ability to withstand sorption and desorption of solvent and counter-ions from the pores. Unfortunately there are few examples of coordination or hydrogen bonded network structures that withstand de-solvation [4–6]. Most network structures are effective host structures for small guest molecules, often the solvent. Exceptions arise when there is a high degree of interpenetration, i.e. where two or more networks are entangled [7]. This type of hostguest behaviour is not intrinsic to the molecular components themselves, but occurs in cavities or clefts created by the assembly of the network structure. Mol-

Fig. 1 Examples of types of network structures

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ecular hosts are single molecules, rather than assemblies of molecules, that are able to form host-guest complexes. Most molecular hosts are macrocyclic, and guest molecules sit inside or perch above molecular cavities or inside the macrocycle. Some common molecular hosts include crown ethers such as [18] crown6 1, calixarenes 2, cyclotriveratrylene 3, cyclodextrins 4 and cucurbituril 5.

1

2

3

4 5

This chapter will survey examples where such molecular hosts form part of an infinite network structure. This can involve incorporating known host molecules into hydrogen bonded network structures or coordination polymers, or using host-guest interactions to build up the infinite structural motif. This may have several interesting consequences, including altering the host-guest characteristics of the host molecule without the need for extensive covalent synthesis, and creating crystalline materials that show multiple inclusion behaviour such as lattice inclusion and site specific host-guest interactions. Furthermore a number of hitherto elusive species have been isolated as components of such networks. The network structures to be discussed will all involved hydrogen bonding as the supramolecular synthon. It should be noted however that other interactions such as coordinate bonds and host-guest interactions may also organise host molecules into network structures. Coordination polymers constructed from molecular hosts may involve functionalised calixarenes [8–11], cyclotriveratrylene [12], or cucurbituril [13]. Calixarenes have also been used to build up network structures via host-guest interactions [14, 15]. It is also notable that volatile species may be trapped within the solid state lattice of calix[4]arene with a structure entirely composed of van der Waals’ interactions [16].

2 Crown Ethers Crown ethers are cyclic polyethers designated [n]crown-m where n is the ring size and m the number of oxygen atoms, for instance [18]crown-6 1. They show a high affinity for cationic guest molecules, especially alkali metal cations, where the cation is commonly complexed within the cavity of macrocycle or sand-

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wiched between two hosts. The presence of polyether linkages means that crown ethers are effective second sphere ligands for transition and lanthanide metal complexes, often with hydrogen bonding interactions between the primary ligands and crown ether, or coordinate interactions with a metal-crown complex [17]. The types of hydrogen bonded network structures formed by crown ethers have been well reviewed and discussed by Steed et al. in recent years hence only a few representative examples will be highlighted here [17, 18]. Most network structures involving crown ethers are simple hydrogen bonded chains where the crown forms second sphere coordination interactions with a complex ion. These are known for [18]crown-6, [15]crown-5 and [12]crown-4 hosts with a variety of metal complexes [17–25]. For instance when combined with the small [12]crown-4, the perchlorate salts of Mn(II), Ni(II) and Zn(II) form polymeric chain structures with alternating crown ethers and [M(H2O)6]2+ cations [19]. Similarly the larger [18]crown-6 forms simple linear chains with metal complexes and cations such as trans-[Pt(NH3)2Cl2] [20], [Cu(NH3)4(H2O)]2+ (Fig. 2) [21], and [Mg(H2O)5(NO3)]+ [22]. [15]Crown-5 also forms simple linear structures, such as those with [NdCl2(H2O)6]+ [23], and the dinuclear cation [Ni2Cl2(H2O)8]2+ [18], though may also form more complicated structures. Other types of chain are know such as the helical hydrogen bonded chain structure of the 1:1 [La(NO3)3(H2O)2(1,10phenanthroline)] and [15]crown-5 complex, Fig. 3 [24].A more complicated four component hydrogen bonded chain is found in the complex [Fe(H2O)6]2[Fe2(µ-O) (H2O)10](NO3)10.4([15]crown-5).6H2O 6 [25]. A hydrogen bonded chain is formed between [15]crown-5, water, [Fe(H2O)6]3+ cation and the linearly oxo bridged [Fe2(µ-O)(H2O)10]4+ cation. The [Fe2(µ-O)(H2O)10]4+ cation in complex 6 is an unusual linear oxo bridged species that was first isolated and structurally characterised using [18]crown-6 as a second sphere ligand [26]. The isolation and subsequent characterisation of otherwise elusive cations by embedding them within a solid state lattice with host molecules has also been successfully utilised with calixarene and cucurbituril hosts (see later).

Fig. 2 Linear hydrogen bonded chain structure formed between [18]crown-6 and [Cu(NH3)4

(H2O)]2+ [21]

Hydrogen Bonded Network Structures Constructed from Molecular Hosts

143

Fig. 3 Helical hydrogen bonded chain structure formed between [15]crown-5 and [La(NO3)3

(H2O)2(1,10-phenanthroline)].[24]

Crown ethers may be employed to isolate oxonium cations from solution. The H7O3+ oxonium ion is isolated in the complex (H7O3)[AuCl4].[15]crown-5 7 and the structure was determined by neutron diffraction allowing for unambiguous assignment of hydrogen positions [27]. The H7O3+ ion in 7 has five OH protons that are not involved in interactions within the oxonium cation and four of these form hydrogen bonds to two crown ethers, with these O-H…O hydrogen bonds propagating a linear chain. Within the crystal structure of 7 the oxonium ion also forms an unusual bifurcated OH…Cl-Au interaction to the square planar [AuCl4]– anion. There are close Au…O contacts between adjacent chains. The larger oxonium ion H13O6+ has been isolated in the complex [(H13O6)[PtCl5(H4O2)].2[18]crown-6 8 [28]. The cation in 8 is an octahedral Pt complex with five chloride ligands and an aquo ligand with close contact to a water and is thus given the formulation [PtCl5(H4O2)]–. This hydrogen bonds to one [18]crown-6 through the H4O2 moiety. Each H13O6+ oxonium ion is surrounded by three [18]crown-6 hosts leading to a hydrogen bonded chain. In some instances linear hydrogen bonded chains are cross-linked into 2D grid structures by additional hydrogen bonding interactions. An example is [Er(NO3)3(H2O)3]4.[15]crown-5.(H2O)4 where a linear chain is formed by hydrogen bonding interactions between approximately trans aquo ligands of [Er(NO3)3(H2O)3] and [15]crown-5, and adjacent chains are cross-linked by hydrogen bonds between uncoordinated O atoms of nitrate ligands and water molecules [18]. Crown ethers are excellent host molecules as well as second sphere ligands and crown ether host-guest complexes may also take part in hydrogen bonding networks (see also section 3.2). The complex [([18]crown-6.NH4)2][SiF6].4H2O [29] has a typical ([18]crown-6.NH4)+ host-guest complex with three N-H…O hydrogen bonds between the ammonium guest and crown host. This leaves one N-H available for hydrogen bonding and this is a hydrogen bond donor to the octahedral SiF62–. The SiF62– also accepts hydrogen bonds from four water molecules while these waters hydrogen bond to a crown ether or to another SiF62– anion via another water. The overall network is a 1D ladder type structure.

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The ability of crown ethers to host cationic ammonium guests with stabilisation from N-H…O hydrogen bonds has been exploited to great effect in creating threaded supermolecules [30]. Pseudorotaxanes, for instance, are supermolecules where a linear molecule is threaded through a macrocyclic molecule, a simple example is the pseudorotaxane 9 formed by dibenzylammonium and dibenzo[24]crown-8. By imparting additional hydrogen bonding functionality on the dibenzylammonium guest, pseudorotaxanes may self-assemble into secondary structures through guest…guest hydrogen bonding interactions. Typically the carboxylic acid dimer is utilised as a supramolecular synthon. Such selfassembly has been termed supramolecular weaving, and may produce chain structures (dubbed daisy-chains) or 2D networks [30, 31]. While the host molecules hydrogen bond to the guest molecules, it is guest…guest interactions that produce the network structure. This means that in terms of network formation the host molecule is topologically irrelevant, and these assemblies are not strictly relevant to this article. An example where the carboxylic acid dimer does not form and a chain structure is propagated through guest…host interactions between adjacent pseudorotaxanes is found in the pseudorotaxane 10 of dibenzo[24]crown-8 and para-carboxylic acid substituted dibenzylammonium. In 10 the 1:1 pseudorotaxanes are linked into chains by a bifurcated N-H…O interaction between adjacent pseudorotaxanes, as shown in Fig. 4 [31]. While dibenzo[24]crown-8 shows a strong affinity with dibenzylammonium cations to form pseudorotaxanes, the same cannot be said of tetrabenzo[24]crown8 11 which shows negligible affinity for dibenzylammonium in solution [32]. In this case, however, the presence of stabilising hydrogen bonding interactions within a network structure affects the functionality of the host, and in the solid state a pseudorotaxane is formed with PF6– counteranions. The pseudorotaxane is stabilised by N-H…O and C-H…O hydrogen bonding within the pseudorotaxane as well as C-H…F hydrogen bonds between the pseudorotaxane and PF6–. These interactions build up the layered structure shown in Fig. 5 [32]. Interest-

9 10

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Fig. 4 The pseudorotaxane 10 of dibenzo[24]crown-8 and para-carboxylic acid substituted

dibenzylammonium, linked to form a hydrogen bonded chain. Only hydrogen atoms involved in hydrogen bonding are shown [31]

Fig. 5 Layer structure of (tetrabenzo[24]crown-8)(dibenzylammonium)(PF6) held together by

weak C-H…F hydrogen bonding between the pseudorotaxane and PF6–. Only N-H hydrogens shown [32]

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ingly the solid state structure of 11 itself is strongly dependent on the crystallisation solvent. When recrystallised from CH3CN, the crown ether adopts a tennis ball conformation with two molecular cavities, both containing a guest CH3CN molecule. A chain structure is propagated through C-H…π hydrogen bonds between adjacent 11 molecules. When recrystallised from ether-chloroform crystals of solvent free 11 result, where the crown ether is in a flattened conformation and multiple intermolecular C-H…π hydrogen bonds create a 2D layered structure.

3 Calixarenes and Related Hosts 3.1 Calixarenes

Calix[n]arenes 2 and related hosts are some of the most commonly studied preformed molecular hosts. The small calix[4,5]arenes often adopt a bowl-conformation with an inherent molecular cavity which may contain neutral, or charged guest molecules [33]. Despite the presence of acidic hydroxyl groups there are no structurally authenticated examples of calixarenes being incorporated into hydrogen bonded network structures through the lower rim hydroxyl group. This can be easily rationalised by considering the strong intramolecular hydrogen bonds formed by calixarenes, especially when in either bowl or cone conformations [33]. Perhaps the closest example of a network structure is a series of macrocyclic-based gelators synthesised by Shinkai and coworkers of the general form 12. Here the formation of inter-calixarene OH…O=C hydrogen bonding is thought responsible for the gelling properties when mixed with various organic solvents [34]. In examples of hydrogen bonded network structures involving calixarenes, the calixarenes have additional potential hydrogen bonding functionality. The most common are sulfonated calixarenes and C-methylcalix[4]resorcinarene which are dealt within sections 3.2 and 3.3 respectively. Other examples are detailed below. The p-hydroxyl calixarene octa(p-hydroxyl)octakis(propyl)calix[8]arene 13 forms O-H…X hydrogen bonds to pyridine or water in the complex (octa(p-hydroxyl)octakis(propyl)calix[8]arene).9(pyridine).2(H2O) [35]. The calix[8]arene is in a chair conformation and adjacent calix[8]arene molecules are linked together via water molecules to form a hydrogen bonded chain structure. The tetraurea calix[4]arene 14 has two intramolecular N-H…O=C hydrogen bonds, leaving two other N-H groups free to form intermolecular interactions [36]. An infinite chain structure is formed through N-H…O=C hydrogen bonds between adjacent 14 molecules. In solution 14 forms capsule-like dimeric structures. Likewise 15 forms a hydrogen bonded chain through N-H…O=C interactions [36]. The calix[4]-nucleoside 16 forms a 2D network structure in the solid state through eight N-H…O hydrogen bonds between thymine base groups and the amide linkage [37]. The overall packing of the networks is a hydrophobic-hydrophilic bi-layer structure. Molecules related to calixarenes may also form hydrogen bonded network structures. The heterocalixarene (calix[1]benzimidazol-2-one[2]arene) 17 forms

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12

16

13

15

14

17

18

a complicated hydrogen bonded network structure with water [38]. Another example is the calix[4]resorcinarene cavitand with tetracarboxylic acid functionality 18 which forms a hydrogen bonded chain structure in the complex 18. p-xylene.1.5(DMF), where DMF=dimethylformamide [39]. The carboxylic acid groups of adjacent 18 molecules form a single hydrogen bond, while two carboxylic acid groups also act as hydrogen bond donors to DMF. The resultant structure is a wave-like ribbon or chain where the cavitands have alternating orientations, Fig. 6. Each cavitand includes a molecule of p-xylene. The wave-like chains pack to form box-like cavities which contain additional DMF and p-xylene host molecules. 3.2 Sulfonated Calixarenes

Calixarenes may be sulfonated in the para position to give sulfonic acid or anionic sulfonate derivatives. The most commonly studied sulfonated calixarene is the tetra-anionic p-sulfonatocalix[4]arene 19, although the 5- anion with one deprotonated phenol is also important. The most common structure for 19 is a bilayer arrangement [40, 41], where the p-sulfonatocalix[4]arene anions pack in an up-down arrangement through π-π interactions. The bi-layer structure has well defined hydrophilic and hydrophobic regions, reminiscent of clays. The host 19 may act as a second sphere ligand for a number of metal cations, or may directly coordinate to metal centres through sulfonate groups. The complexes thus formed have been recently reviewed and will not be discussed further here [42].

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Fig. 6 Wave-like ribbon structure of complex 18.p-xylene.1.5(DMF), where DMF=dimethyl-

formamide. Guest p-xylene occupies the cavitand molecular cavity, while disordered DMF (only one position shown) occupies the cage created by pendant arms at the lower rim [39]

More complicated supramolecular assemblies of p-sulfonatocalix[4]arene including giant spheres and tubular assemblies have been established through upup, rather than up-down, associations [43]. A simple example of a hydrogen bonded network structure involving 19 are the complexes [M(H2O)8][19+H].16H2O, where M=Tb, Gd, Tm, which assume a tetragonal structure [44]. The octa-aquolanthanide ion has a square antiprism geometry, and the four aquo ligands that make up a square face all form hydrogen bonding interactions to a sulfonate group of two p-sulfonatocalix[4]arene molecules. Each p-sulfonatocalix[4]arene accepts hydrogen bonds through two O atoms of each sulfonate group and binds to four [M(H2O)8]3+ cations. Hence a tetragonal 2D network is formed. Networks pack with the familiar bi-layer arrangement with the hydrophilic layers augmented by the other aquo ligands and waters of crystallisation. The ability of 19 to engage in second-sphere coordination of metals is well illustrated by a series of “Russian Doll” complexes [44–47]. In the Russian Dolls a {Na(H2O)2([18]crown-6)}+ host-guest complex is encapsulated by two p-sulfonatocalix[4]arene hosts which are arranged in a head-to-head fashion to form the Russian Doll super-anion or capsule [{Na(H2O)2([18]crown-6)}(19)2]7– 20. The super-anion is formed through hydrogen bonding interactions between the trans coordinated aquo ligands and sulfonate groups augmented by a complementar-

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ity of curvature between the [18]crown-6 guest and calix[4]arene host. The metal centre is usually Na+, but there is also one report of a Ce3+ Russian Doll [44]. The actual charge of the super-anion is variable and protonation of some sulfonate groups may occur. The Russian Doll super-anion has been extremely successful in isolating cationic species that have otherwise proved to be difficult to crystallise. The structure of the hydrolytic Cr(III) tetra-nuclear cation [Cr4(OH)6(H2O)12]6+ was elucidated by diffraction methods for the first time when it was crystallised in the Russian Doll complex [Cr4(OH)6(H2O)12](20+H).15.5H2O [45]. Two [Cr4(OH)6(H2O)12]6+ cations hydrogen bond to the sulfonate groups of the 19 through hydroxyl and aquo ligand donors, with each cation hydrogen bonding to two super-anions to propagate a hydrogen bonded chain structure, Fig. 7. Adjacent chains are linked into layers by additional hydrogen bonding interactions between the [Cr4(OH)6(H2O)12]6+ cations and water molecules. Hydrogen bonded 2D layers pack within the crystal lattice by the familiar bi-layer packing of p-sulfonatocalix[4]arenes. The smaller hydrolytic Cr(III) cations [Cr3(OH)4(H2O)10]5+ and [Cr2(OH)2(H2O)8]4+ were also structurally authenticated for the first time by crystallisation with the Russian Doll super-anion, however the structures were severely disordered so that a meaningful analysis of hydrogen bonding patterns cannot be made [45].

19

22 20

21

Similarly, analogous Rh(III) hydrolytic cations can be isolated by crystallisation with the super-anion, however these complexes show quite different hydrogen bonding networks.A second type of Russian Doll super-anion is found in the complex {[Rh4(OH)6(H2O)12](20.21+8H)}.33H2O [46]. The dehydrated Russian Doll {Na([18]crown-6)(19)2}7– 21 is skewed by comparison with the capsule structure of 20. The two trans aquo ligands are not present in the dehydrated super-anion, and the axial Na coordination positions are occupied by oxygens

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Fig. 7 Hydrogen bonding interactions between the Russian Doll super-anions 20 and

[Cr4(OH)6(H2O)12]6+ cations in the complex [Cr4(OH)6(H2O)12](20+H).15.5H2O [45]

Fig. 8 A tetra-nuclear [Rh4(OH)6(H2O)12]6+ cation hydrogen bonding to the sulfonate groups of four super-anions in the complex {[Rh4(OH)6(H2O)12](20.21+8H)}.33H2O.Each super-anion hydrogens bonds to two tetra-nuclear cations, extending into a 2D rectangular grid [46]

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of sulfonate groups from each of the two calixarenes moieties that form the shell of the super-anion. Each tetra-nuclear [Rh4(OH)6(H2O)12]6+ cation hydrogen bonds with the sulfonate groups of four super-anions, Fig. 8; two 20 capsules positioned opposite each other across the cation, and two 21 superanions. Each super-anion hydrogen bonds to two tetra-nuclear cations, extending into a two dimensional rectangular grid.A different hydrogen bonded 2D assembly is found in {[Rh2(OH)2(H2O)8](20+3H).11.5H2O [46]. There are two types of cation with both positioned at the periphery of the hydrophilic sulfonate equator of the globular super-anion. One cation forms O-H…O-S hydrogen bonds to two super-anions, while the other type of cation hydrogen bonds to four super-anions. Each super-anion is connected to three [Rh2(OH)2(H2O)8]4+ cations and the array of hydrogen bonding interactions form an infinite two dimensional network, a section of which is shown in Fig. 9. The topology of this network is an unusual flat 3,4-connected net forming 4 and 6-gons. The 2D networks stack in layers throughout the crystal lattice. A 2D rectangular hydrogen bonded grid is formed between the super-anion and Keggin ion [Al13O4(OH)24(H2O)12]7+ in the complex [Al13O4(OH)24(H2O)12] (20+H)0.5(19).29H2O [47]. The Keggin ion consists of twelve AlO6 octahedra around a central AlO4 tetrahedron and has numerous outer hydroxide and aquo groups capable of being both hydrogen bond donors and acceptors. The Keggin ions form hydrogen bonded chains which are cross-linked into a sheet structure by hydrogen bonds to super-anions.Another molecule of p-sulfonatocalix[4]arene that is not involved in a super-anion fills the cavities created by this network.

Fig. 9 The 2D hydrogen bonded net found in {[Rh2(OH)2(H2O)8](20+3H).11.5H2O, with hydrogen bonds between two types of [Rh2(OH)2(H2O)8]4+ cation and the super-anion 20 giving a 3,4-connected net [46]

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A “ferris wheel” assembly involving a 1:1 complex of 19 and metallated [18]crown-6 is found in the cationic supermolecule {[La(H2O)3([18]crown-6)] (19+2H)}+ [48]. The lanthanum ion is coordinated by one calixarene sulfonate group, the [18]crown-6 and three aquo ligands, and the metallated crown sits inside the calixarene cavity. A helical hydrogen bonded chain structure is formed between the cationic assembly, water and chloride ions. The ferris wheel structural motif is also found in Ce3+ complex which simultaneously contains a Russian Doll assembly [44]. Russian Doll type capsules have also been reported with un-metallated small host molecules, such as [18]crown-6 and cyclam 22, encapsulated by two headto-head p-sulfonatocalix[4]arene hosts [49, 50]. In the neutral assembly {([18]crown-6)[Y(H2O)7]1.33(19)2} the crown ether is encapsulated by two 19 hosts [49]. Each 19 host binds a [Y(H2O)7]3+ complex via a sulfonate group and the aquo complexes form multiple hydrogen bonding interactions to the encapsulated crown ether. A second, partially occupied and disordered [Y(H2O)7]3+ links the capsules into linear chains through coordinate and hydrogen bonding interactions. The tetraprotonated-cyclam containing capsule {(22+4H)(19)2}4– 23 crystallises as a salt of [Cr2(OH)2(H2O)8]4+ [50]. Unlike in the case of [Cr2(OH)2(H2O)8]4+ with the original 20 Russian Doll capsule, the dimeric cation is well ordered and forms a hydrogen bonded 2D grid structure with 23. 3.3 C-Methylcalix[4]resorcinarene

Calix[4]resorcinarenes are similar to calix[4]arenes however with resorcinol rather than phenol functional groups. Like calix[4]arenes they may adopt a number of conformations in solution and the solid state, including bowl, boat and chair conformations. Bowl conformation calix[4]resorcinarenes show similar host-guest properties to those of bowl conformation calix[4]arene [51]. In recent years C-methylcalix[4]resorcinarene 24 has been reported to form hydrogen bonded network structures with linear hydrogen bond acceptors such as 4,4′bipyridine (=bipy) 25 [52–63], bis(4-pyridyl)ethylene 26 [62, 63], and 4,4′trimethylenedipyridine 27 [64]. Network structures of 24 with water acting as a hydrogen bond donor and acceptor are also known [64–67]. Notably water also plays an important role in the self-assembly of discrete snub cube assemblies of 24 [68].

24

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The combination of 24 and 4,4′-bipyridine (=bipy) yields numerous distinct structural types, including discrete assemblies, ribbon and 2D and 3D networks. What determines which type of structure is obtained from a crystallisation experiment is not entirely clear, however crystallisation conditions, including the cooling rate, are certainly a factor as well as the stoichiometry and the nature of any guest molecules present. Not all hydrogen bonding interactions lead to network structures, and discrete capsules are found for the complexes [C-methylcalix[4]resorcinarene.2(4,4′-bipyridine)].X, where X=4-nitrobenzene [69], benzophenone [60]. In these capsule assemblies molecules of 24 have a bowl conformation, and two are arranged in a head-to-head fashion bridged by four bipy molecules. The upper-rim hydrogen bonding pattern usually found in bowl conformation 24 has C2v molecular symmetry with four OH hydrogen atoms pointing upwards, and alternate resorcinol groups with two intramolecular hydrogen bond donors [70]. This is maintained within the capsule and the OH groups available for intermolecular bonding form hydrogen bonds to the bipy. Guest molecules occupy the capsule cavity. Bowl conformation C-methylcalix[4]resorcinarene has given rise to two types of network structure when combined with bipy. The first of these to be reported was the wave-like hydrogen bonded chain or ribbon structure observed in (Cmethylcalix[4]resorcinarene).2(4,4′-bipyridine)(CH3CN) [52].As for the capsule structures, the C2v pattern of intramolecular O-H…O hydrogen bonding is observed, leaving four OH groups available for intermolecular hydrogen bonding. These donate to 4,4-bipyridine molecules, which in turn bridge to another molecule of 24 which is in an inverted orientation from the first, but displaced in the b direction. Hence the wave-like ribbon structure is propagated and shown in Fig. 10, and is similar to that of complex 18 described above. Each 24 hosts a molecule of CH3CN, which interacts with the host via C-H…π interactions. The wave-like ribbons pack such that the C-methylcalix[4]resorcinarene molecules are stacked in the same orientation. The wave-like ribbon structure is observed

Fig. 10 Wave-like ribbon structure of (C-methylcalix[4]resorcinarene).2(4,4′-bipyridine)

(CH3CN) with C-methylcalix[4]resorcinarene in a bowl conformation [52]

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Fig. 11 Linear 1D hydrogen bonded chain structure of (C-methylcalix[4]resorcinarene).2(4,4′bipyridine)(4-bromobiphenyl) [57]

for a number of different guest molecules including ferrocene, acetyl ferrocene [53], chlorotoluene, adamantanone, [2.2]paracyclophane [54], nitromethane [55], tetrahydrofuran, and a mixed tetrahydrofuran, acetonitrile guest system [56]. The complex (C-methylcalix[4]resorcinarene).2(4,4′-bipyridine)(4-bromobiphenyl) also features a 1D hydrogen bonded chain structure that is an architectural isomer of the wave-like 1D ribbon structure described above [57]. Architectural isomers occur where two or more framework structures have identical composition and supramolecular connectivity yet have different topology [71]. As in the wave-like structure, the C-methylcalix[4]resorcinarene host is in a bowl conformation with a C2v arrangement of intramolecular hydrogen bonds. The bipy molecules form bridging hydrogen bond interactions between the 24 molecules, however each bipy is subtended at an angle of approximately 90° to the principal rotation axis of 24. Each molecule of 24 within the chain has the same orientation, giving the relatively straight chain structure shown in Fig. 11. Adjacent chains form pairwise associations through self-inclusion where the bipy molecules of one chain are perched above the 24 molecular cavity of an adjacent chain. The pairs of self-included chains pack in a tail-to-tail fashion which creates elliptical cavities within the structure. Each cavity is occupied by two molecules of 4-bromobiphenyl.A very similar 1D chain structure is observed for (C-methylcalix[4]resorcinarene).2(4,4′-trimethyldipyridine).0.5(CH3OH) [64], where the flexible, linear pyridyl donor 4,4′-trimethyldipyridine is utilised in place of 4,4′-bipyridine. When C-methylcalix[4]resorcinarene adopts a different conformation entirely different hydrogen bonded network structures result. The boat or flattened cone conformation is adopted in the complex (C-methylcalix[4]resorcinarene).2(4,4′bipyridine).0.5(decamethylruthenocene).2(ethanol) [58]. Flattened cone conformation 24 is essentially T-shaped with no intramolecular hydrogen bonding, leaving all eight OH groups capable of forming intermolecular interactions. Four O-H…O hydrogen bonds link neighbouring molecules of 24 into straight chains. Adjacent 24 molecules within the chains have inverted orientations. The four remaining OH groups of each 24 act as hydrogen bond donors to four 4,4′-bipyridine molecules, which bridge across parallel hydrogen bonded chains of Cmethylcalix[4]resorcinarene, rather like four struts. The overall network is 2D

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Fig. 12 Brick-wall 2D network of complex (C-methylcalix[4]resorcinarene).2(4,4′-bipyridine).0.5(decamethylruthenocene).2(ethanol).The 24 host molecule is in a flattened cone conformation. Disordered guest molecules are not shown and occupy the apparent cavities [58]

with a skewed brick-wall structure, Fig. 12. There are large hydrophobic cavities within the network where guest decamethylruthenocene and ethanol molecules reside. This network structure has been observed for other hydrogen bonded complexes involving flattened cone conformation 24 and 4,4′-bipyridine with guest molecules including m-xylene [59], and mixed benzophenone and water [60]. In some examples the guest molecules are organised within the hydrophobic cavities. (C-Methylcalix[4]resorcinarene)2.(4,4′-bipyridine).2(m-xylene), for example, has four m-xylene guests per cavity which form edge-to-face π-π interactions with each other [59]. Other distorted brick-wall motifs are also known where there are fewer than four bipy struts linking the hydrogen bonded chains of C-methylcalix[4]resorcinarene together. In these examples other hydrogen bond acceptor molecules such as ethanol interact with the OH groups of 24 and there may be only two or three bipy struts, and the wall pattern may be asymmetric [61].A novel brick-wall 2D sheet structure is seen in (C-methylcalix[4]resorcinarene).2(4,4′-bipyridine).2(H2O) where the bipy spacers are in an equatorial connection mode [61]. The flattened cone conformation 24 may also form a complicated 3D hydrogen bonded network structure with 4,4′-bipyridine found in (C-methylcalix[4]resorcinarene).1.5(4,4′-bipyridine).5(H2O) 28. In 28 columns of hydrogen bonded C-methylcalix[4]resorcinarene molecules are formed, as shown in Fig. 13. Nonintersecting molecular columns are connected into a 3D net by hydrogen bonds between them, and via the bipy molecules [61].

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Fig. 13 3D hydrogen bonded network structure of (C-methylcalix[4]resorcinarene).1.5(4,4′-

bipyridine).5(H2O) 28 [61]

C-Methylcalix[4]resorcinarene has also been reported to adopt a C2h chair conformation in hydrogen bonded network structures synthesised by hydrothermal methods with bipy or trans-bis(4-pyridyl)ethylene (=bpe) [62]. Chair conformation C-methylcalix[4]resorcinarene does not feature any intramolecular hydrogen bonding. In (C-methylcalix[4]resorcinarene).3(4,4′bipyridine) 29 adjacent 24 molecules are connected into infinite chains through intermolecular O-H…O hydrogen bonding, and the chains are linked into a stepped sheet by O-H…N hydrogen bonds to bipy molecules. The sheets are connected together by a second type of bipy molecule to form a 3D network structure. There are no additional guest molecules in 29.An isostructural network is obtained when bpe is used in place of bipy. A similar network structure is found in (C-methylcalix[4]resorcinarene).3(4,4′-bipyridine).2(H2O)(benzophenone), however with water molecules interspersed within the chains of adjacent 24 molecules. This additional spacing is sufficient to form cavities within the structure which are occupied by benzophenone guest molecules. Much of the impetus for the study of hydrogen bonded network structures of 24 and bipy or other linear N-donor ligands has come from the desire to perform time-resolved crystallographic studies of photoactive guest species embedded within the network as a guest [58, 62, 63]. Embedding a photoactive species within a network structure effectively dilutes it in the solid state. The advantages of this include improved uniformity of illumination of the crystal, less photons are

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Fig. 14 The infinite 2D network formed by water and 24 in [Na+[2.2.2]cryptate][C-methylcalix[4]resorcinarene-H]–.4(CH3CN).3(H2O) 30 [66]

needed to achieve the required percentage conversion to the excited state, and the distance between excited molecules is increased which decreases the likelihood of exciton-exciton annihilation. Photoactive guest molecules such as benzophenone, benzil and decamethylruthenocene are being investigated in this regard [63]. Hydrogen bond network structures are also formed by 24 and water. The host C-methylcalix[4]resorcinarene usually adopts a bowl conformation with four intramolecular O-H…O hydrogen bonds. [Na+[2.2.2]cryptate][C-methylcalix[4]resorcinarene-H]–.4(CH3CN).3(H2O) 30 forms an infinite 2D hydrogen bonded sheet structure between 24 and water molecules [66]. Four of the 24 hydroxyl groups are intramolecular hydrogen bond donors. There are four intermolecular hydrogen bond donors, two on the same resorcinol group. These form hydrogen bonds to water or another molecule of 24. Note that charge balance requires the 24 to be mono-deprotonated and the averaged structure has half a proton sited on centres of inversion. Each of the water molecules within the network accepts a hydrogen bonding proton from one 24 hydroxyl group and donates to two adjacent 24 molecules. A section of the network is shown in Fig. 14. Each 24 host complexes a [Na+[2.2.2]cryptate] guest cation.

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Fig. 15 2D network structure of (C-methylcalix[4]resorcinarene).2(H2O).2(CH3CN) 31 where, as for 30 there is a 2D hydrogen bonded network between water and 24 [67]

Likewise the complex (C-methylcalix[4]resorcinarene).2(H2O).2(CH3CN) 31 has a 2D hydrogen bonded network between water and 24, Fig. 15 [67]. The intramolecular hydrogen bonding pattern for 24 is the same as seen in the complex 30 where the C-methylcalix[4]resorcinarene was mono-deprotonated. In 31 there are two distinct water molecules. Both water molecules accept hydrogen bonds from a molecule of 24 however one water molecule acts as a hydrogen bond donor to two molecules of 24 whilst the other is a hydrogen bond donor for one molecule of 24 and one molecule of CH3CN.A second molecule of CH3CN is contained within the molecular cavity of 24. The side-on profile of the 2D networks is wave-like and the host molecules of adjacent networks stack on top of one another in the same orientation. The doubly solvated 1:1 adduct of 24 and 2,2′-bipyridine, (C-methylcalix[4]resorcinarene)(2,2′-bipyridine)(H2O)(CH3OH) 32, also features a 2D hydrogen bonded network structure of C-methylcalix[4]resorcinarene and water [64]. There are four intermolecular hydrogen bonds arranged differently to those seen

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in 30 and 31. Here the resorcinol ring with two intramolecular hydrogen bonds is adjacent rather than opposite a ring that forms only intermolecular hydrogen bonds. Each molecule of 24 forms hydrogen bonds to one methanol, one 2,2′bipyridine molecule, one other molecule of 24 and a water molecule. The water molecule accepts hydrogen bonds from one C-methylcalix[4]resorcinarene and donates to two C-methylcalix[4]resorcinarene molecules, to give a 2D rhomboidal network structure. Within the overall structure the networks pack such that pairs of calixarenes from adjacent nets have opposite orientation and generate large cavities which encapsulate two molecules of 2,2′-bipyridine. An unusual scoop conformation of 24 has been characterised in the hydrate (C-methylcalix[4]resorcinarene)(H2O) [65]. The scoop conformation is a hybrid of the bowl and flattened cone conformations and there are two intramolecular O-H…O hydrogen bonds from adjacent resorcinol units. Adjacent 24 molecules form O-H…O hydrogen bonds and there are additional links via water molecules resulting a 2D sheet structure. The sheets pack in a bi-layer arrangement.

4 Cyclotriveratrylene Cyclotriveratrylene (=CTV) 3 has a relatively rigid bowl-shape and shows quite different host-guest chemistry to that described above for the calixarenes[72]. Until the mid-1990s examples of CTV inclusion chemistry were restricted to crystalline clathrate materials, with two main phases being identified [73, 74]. In both phases, the CTV molecules stack on top of one another and the guest molecules are contained in channels created by the packing of CTV molecules. One example of a γ-phase has been recently reported, where acetone is complexed between two CTV molecules forming a capsule-like dimer [75]. CTV has, however, proved to be a valuable building-block for the synthesis of more sophisticated host molecules such as cryptophanes and extended arm cavitands [76]. More recently it was shown that CTV is a good molecular host for large guest molecules such as fullerene-C60 [77]. A little exploited attribute of CTV is the trigonal arrangement of dimethoxy moieties. These may act as hydrogen bond acceptors, as has been seen in α-phase CTV clathrates, and as potential ligation sites. In terms of hydrogen bonded network structures CTV is usually a 3-connecting centre, although not usually a trigonal connector due to its rigid bowl-shape. For example, in the tris-Pt(II) complex of a CTV analogue angles between dimethoxy groups range from 87 to 89° [78]. This leads to up-down isomerism possibilities that do not occur with flat 3-connecting centres. Two early examples of CTV being incorporated into network structures involves CTV acting as a hydrogen bond acceptor when the hydrogen bond donor is the acidic C-H groups of o-carborane [79]. Carboranes form C-H hydrogen bonds of this type in a number of other instances [80], and may also act as spherical guest molecules for CTV [77]. In the crystal structure of (o-carborane)2(CTV) 33 carborane molecules act in both capacities [79]. One type of carborane is included as an intracavity guest molecule to form a ball-and-socket supermolecule which has been previously ob-

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Fig. 16 Puckered hexagonal 2D hydrogen bonded network of (o-carborane)2(CTV) 33 where the carborane molecule is disordered with three icosohedral vertices having mixed C and B character. The other type of carborane (not shown) is an intracavity guest for the CTV [79]

served in (o-carborane)2(CTV) [81, 82]. The other carborane molecule is disordered with three icosohedral vertices on one face having mixed C and B character. Each of these C/B-H sites forms a bifurcated hydrogen bond to the methoxy groups of a CTV molecule, to give a puckered hexagonal 2D hydrogen bonded network, Fig. 16. The hydrogen bond interaction is distinctly out-of-plane, allowing the CTV to be a strict trigonal connecting centre in this network. The hydrogen bonded carborane is more tightly bound within the crystal lattice than is the guest carborane, as indicated by its considerably smaller atomic displacement parameters. Networks pack together in a space filling manner. The complex (C70)(o-carborane)(CTV)(1,2-dichlorobenzene) 34 also features a hydrogen bonding network between CTV and o-carborane, however in this instance the carborane is ordered and interacts with two symmetry equivalent CTV molecules [79]. Each CTV connects to two carboranes forming an infinite helical chain, Fig. 17. The crystals are chiral despite being composed of weakly interacting achiral molecular components. Each CTV receptor site within the helices binds a molecule of C70. The C70 molecule shows π-π stacking with the CTV host molecule. The dichlorobenzene molecules within the crystal lattice fit snugly into the grooves of each helix, with plane of the solvent lying normal to the direction of the helices. An isostructural toluene solvate is also known. A notable

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Fig. 17 Section of the helical chain structure found in (C70)(o-carborane)(CTV)(1,2-dichloro-

benzene) 34 with C70 guest molecules in each CTV molecular cavity [79]

feature of this material is that crystalline complexes do not result from simple mixtures of CTV and fullerene-C70, unlike the case with fullerene-C60 [77]. Although no covalent chemistry has been performed, the normal inclusion characteristics of CTV have been altered. Initial indications are that the higher fullerene C76 may likewise be complexed within this helical structure [83]. Most other examples of CTV combining with hydrogen bond donors to produce network structures involve water as the hydrogen bond donor. In the complex [(DMF)(CTV)]2(H2O)4(o-carborane)] 35 the 2D hydrogen bonded network structure shown in Fig. 18 is formed [82]. A tetramer of hydrogen bonded water molecules form hydrogen bonds to six CTV molecules. Each CTV molecule hydrogen bonds to three water tetramers to create a two-tiered 2D network. Networks pack within the crystal lattice via π stacking between aromatic rings of the CTVs with carborane guest molecules contained in hydrophobic channels. The host-guest behaviour is notable, with DMF perched within the CTV cavity. A series of complexes with identical topology and very similar geometric features to 35 are known which feature a combination of coordination interactions to the dimethoxy groups of the CTV and hydrogen bonding interactions with water or hydroxide as the hydrogen bond donor [82, 84]. These complexes may be formed with either o-carborane or the similar icosohedral anion [CB11H12]– as steric templating molecules. In the latter case, charge balance is achieved with some of the hydroxide ligands being replaced by water.An example is the complex [K(OH)(CTV)(DMF)]2(C2B10H12) 36 [82], for which the network structure is

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Fig. 18 2D hydrogen bonded network structure of [(DMF)(CTV)]2(H2O)4(o-carborane)] 35

with water tetramers hydrogen bonding to CTV molecules [82]

shown in Fig. 19. In 36, K+ ions are 6-coordinate with two chelating CTV ligands and two cis hydroxide ligands. All ligands are bridging and a -K-µ-(OH)2-K-µ(CTV)2-K-µ-(OH)2-K-coordinate chain is observed. Adjacent coordinate chains are hydrogen bonded via the hydroxide protons forming a bifurcated hydrogen bond to the dimethoxy unit of CTV not involved in coordinate bonding. The overall topology and crystal packing is the same as that for 35. The Na+ analogue achieves essentially the same result in a slightly different manner. In [Na(CTV)2(H2O)3(DMF)2](CB11H12) 37 a coordination chain is not formed [84]. Instead each Na+ is 6-coordinate with two chelating CTV ligands and two cis aquo ligands. The aquo ligands form hydrogen bonds to a water molecules and both the aquo ligands and this water form hydrogen bonds to adjacent CTV molecules to form the same network topology as for 35 and 36. Similar complexes with Cs+ and Rb+ are also known [84]. In all these cases, DMF molecules act as intracavity guest molecules for the CTV hosts, and pseudopolymorphic complexes with trifluoroethanol or acetonitrile guest molecules in the CTV cavity have also been isolated. That essentially the same structure can be obtained from a disparate range of coordination and hydrogen bonding interactions appears to indicate that this is an example of a preferred crystal packing mode for the major components of within these systems – namely CTV and o-carborane or isostructural [CB11H12]–. The complex [Sr(H2O)8][(CH3CN)(CTV)]4(H2O)4[Co(C2B9H11)2]2 38 has a more complicated 3D hydrogen bonded network [85].As before, the hydrogen bonding interactions that make up the network are between CTV and water, however in this instance, the water hydrogen bond donors are both uncomplexed water and aquo

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Fig. 19 Two -K-µ-(OH)2-K-coordinate chains (one highlighted in dark) hydrogen bond to-

gether to form a 2D network in [K(OH)(CTV)(DMF)]2(C2B10H12) 36, that is topologically the same as that of 35 [82]

ligands. The [Sr(H2O)8]2+ complex ion hydrogen bonds to four water molecules to give a {[Sr(H2O)8]2+/(H2O)4} assembly. This assembly hydrogen bonds to twelve CTV molecules. Each CTV molecule hydrogen bonds to three {[Sr(H2O)8]2+/(H2O)4} assemblies to form a highly unusual 3,12-connected network shown schematically in Fig. 20.A further feature of the structure is the backto-back stacking of CTV molecules into tetrameric clusters with approximately tetrahedral geometries. This packing arrangement of CTV is notably similar to the back-to-back packing of twelve p-sulfonatocalix[4]arene molecules to produce a polyhedral cluster [86]. Within these tetramers the dimethoxy groups of adjacent CTV molecules are roughly aligned, so that a single {[Sr(H2O)8]2+/(H2O)4} assembly hydrogen bonds to two CTV molecules at these points. For each [CTV]4 cluster there are 6 such {[Sr(H2O)8]2+/(H2O)4} species arranged in a near perfect octahedron, Fig. 20. The overall arrangement of the Sr6[CTV]4 assembly is a deformed adamantoid cage, and it is apparent that the overall network topology shown in Fig. 20 is of vertex sharing adamantoid cages, where six cages radiate from each vertex. Other examples of vertice-sharing adamantoid network structures involve sharing between only two adamantoid cages [87]. A considerably simpler network structure can be conceptualised by considering the tetrameric cluster of back-to-back CTV molecules as a single six-connecting centre. As each {[Sr(H2O)8]2+/(H2O)4} hydrogen bonds to six [CTV]4 clusters, they are also sixconnecting centres. Hence the simplified view of the network is of α-Po topology with alternating [CTV]4 and Sr centres. The rectangular channels expected for an α-Po type structure are filled by [Co(C2B9H11)2]– anions. Furthermore each CTV hosts a molecule of acetonitrile as a guest species. Complexes that are isostructural with 38 may be obtained with other 2+ metal ions including other group 2 metals and Fe2+ [88]. The Fe(II) complex can be

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a

b Fig. 20a,b. The 3D hydrogen bonding structure of [Sr(H2O)8][(CH3CN)(CTV)]4(H2O)4

[Co(C2B9H11)2]2 38: a back-to-back tetramer of CTV molecules accepting hydrogen bonds from six {[Sr(H2O)8]2+/(H2O)4} assemblies to form an adamantoid cage; b schematic of the unusual 3,12-connected network CTV centres and Sr cations (spheres) shown. One adamantoid unit has been highlighted [85]

obtained in high yield with composition [Fe(H2O)6][(CH3CN)(CTV)]4(H2O)4 [Co(C2B9H11)2]2 39. The hexaaquoiron(II) complex in 39 has the same charge as the octa-aquostrontium(II) ion in 38, however with a completely different geometry.Within the crystal structure the octahedral [Fe(H2O)6]2+ is disordered across a fourfold inversion centre to give a complex ion disordered over two sites to effectively mimic the geometry of the [Sr(H2O)8]2+ ion of 38, and a similar pattern of hydrogen bonds and host-guest interactions are observed.

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Metal aquo ions with higher charge and coordination number lead to more complicated hydrogen bonding network structures as can be seen with the complex [Eu(H2O)9]1.5(CTV)6(CH3CN)5.5(H2O)7.5[Co(C2B9H11)2]4.5 40 [89]. The structure of 40 features two types of [Eu(H2O)9]3+ cation in 2:1 proportions, each in distorted capped triangular dodecahedral geometry. Four of the aquo ligands of each [Eu(H2O)9]3+ cation are hydrogen bonded to additional water molecules. The [Eu(H2O)9](H2O)4 sub-units act as hydrogen bond donors to CTV molecules, with each type interacting with eight CTV molecules.Aquo ligands also hydrogen bond to guest acetonitrile molecules. One of the nine aquo ligands of one [Eu(H2O)9], and four from the same face of the triangular dodecahedron of the other [Eu(H2O)9], are not involved in any hydrogen bonding interactions. CTV molecules stack together as back-to-back tetramers with two structurally distinct types. One has an approximately tetrahedral arrangement of the CTV molecules as was seen in 38 and 39. In the other type, the [CTV]4 unit is less regular having been pushed apart by the insertion of an acetonitrile molecule. The two types of [CTV]4 assemblies exist in 2:1 proportions throughout the structure. Each CTV molecule also hosts an acetonitrile guest. The dimethoxy groups of the CTV molecules accept hydrogen bonds from either free or coordinated water. In addition to hydrogen bonding interactions between the [CTV]4 and [Eu(H2O)9](H2O)4, the two types of [CTV]4 also hydrogen bond to a water cluster acting as a 3-connecting centre. A section of the overall hydrogen bonding network is shown in Fig. 21. The large [Co(C2B9H12)]– anions occupy spaces within the hydrogen bonded structure and are in close proximity.

Fig. 21 Section of the highly complicated 3D hydrogen bonding network formed by [Eu(H2O)9]3+

cations, CTV and water in [Eu(H2O)9]1.5(CTV)6(CH3CN)5.5(H2O)7.5[Co(C2B9H11)2]4.5 40 [89]

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Fig. 22 Hydrogen bonded ladder structure of ((NH2)3C)(CH3CN ∩ CTV)[Co(C2B9H11)2] 41 with N-H…OMe hydrogen bonding between guanidinium cations and CTV [90]

The complex ((NH2)3C)(CH3CN ∩ CTV)[Co(C2B9H11)2] 41 is a rare example of a CTV containing hydrogen bonded network with N-H…OMe hydrogen bonding [90]. Each of the three NH2 groups of the guanidinium cation forms a single hydrogen bond through one N-H group to a methoxy group of a CTV. Each CTV accepts hydrogen bonds from two guanidinium cations. The infinite hydrogen bonded network formed is a 1D ladder structure, shown in Fig. 22. CTV host molecules within the ladder are oriented so that their molecular cavities face inwards. Each CTV cavity acts as a host for a molecule of CH3CN. The ladder structure effectively creates channels containing guest CH3CN molecules running in the b direction. The ladders pack together in the ab plane through π–π stacking between the two of the arene rings on each CTV. Layers of the hydrogen bonded ladders are separated in the c direction by layers of [Co(C2B9H12)2]– anions. Each anion forms a weak nonclassical C-H…π hydrogen bond to one of the arene rings of a CTV molecule. CTV may be demethylated to give the tris-catechol derivative cyclotricatechylene (=CTC). Whereas CTV has only hydrogen bond acceptor capabilities, CTC is both a strong hydrogen bond donor and acceptor and would be expected to form network structures in the solid state. This is borne out by the structure of (CTC).2(i-propanol) 42 where the guest molecule is also both a strong hydrogen bond donor and acceptor [74]. Complex 42 has a 2D hydrogen bonded grid structure, Fig. 23. Each hydroxyl group of the CTC acts as an intermolecular hydrogen bond donor, forming hydrogen bonds with adjacent CTC hosts or ipropanol molecules. Two of the CTC hydroxyl protons form bifurcated hydrogen bonds, also forming intramolecular O-H…O interactions. There are two types of i-propanol molecules, both of which span between two CTC molecules via hydrogen bonds, while one also occupies the molecular cavity of CTC. The CTC host molecules within each hydrogen bonded layer are not coplanar, falling into two tiers and the overall net is a simple corrugated 4-connected grid. Layers pack together in a bi-layer arrangement reminiscent of that of p-sulfonatocalixarene (see above) however without obvious π–π interactions. The related complex (CTC).2(DMF).2(H2O) has a similar structure [73].

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Fig. 23 Hydrogen bonding network formed between CTC and i-propanol in (CTC).2(ipropanol) 42 [74]

5 Cyclodextrins Cyclodextrins (=CD) are cyclic oligosaccharides 4. The three main forms are α-CD with n=6, β-CD with n=7 and γ-CD with n=8. The host-guest chemistry of cyclodextrins is extensive, with inclusion complexes formed with a wide range of guest molecules or ions including aliphatic and aromatic organic compounds, organic and inorganic ions, organometallic complexes and noble gases [91]. Cyclodextrins find applications in petrochemical and fine chemical, pharmaceutical, cosmetic and food industries, with significant uses including chromatography and drug transport and stabilisation [92]. Cyclodextrins have one primary hydroxyl group and two secondary hydroxyl groups per saccharide and are thus capable of forming multiple hydrogen bonds both as donors and acceptors. They have a strong tendency to form intramolecular hydrogen bonds which may preclude the formation of well ordered network structures. When intermolecular hydrogen bonds are formed with guest molecules it is usually through the primary hydroxyl group [93]. CD dimers are a common structural motif, with dimerization usually occurring in a head-to-head fashion [94]. When cyclodextrins are crystallised from water they form hydrates with large numbers of water molecules, and 2D networks of CD and water result [93, 94]. These networks fall into two main categories: channel structures and cages. The type of structure that occurs is usually dependent on the size of the

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Fig. 24 The Si12O3012– anion intercalated between a pair of α-CD molecules in K12Si12O30.2αCD.36H2O 43. Hydrogen bonding between the secondary hydroxyl groups of each α-CD and water propagate a 3D structure with sheets of α-CD linked by the silicate guest molecules [95]

CD and the type of guest molecule. In channel structures the CD molecules line up so that a 2D network of alternating CD and water layers result. In cage structures CD dimers and water clusters form a chessboard arrangement where dimers of adjacent layers are related by a two-fold axis, giving CD and water regions alternating in two directions. Intermediate structural types have also been established, where there is misalignment of channels or herringbone motifs. The inorganic-organic composite material K12Si12O30.2α-CD.36H2O 43 is an unusual example of a hydrogen bonded network structure involving a CD where the intramolecular hydrogen bonding of the secondary hydroxyl groups has been broken in favour of intermolecular interactions [95]. In the structure a prismatic dicyclohexasilicate ion Si12O3012– is intercalated between a pair of α-CD molecules, Fig. 24. Interestingly the prismatic dodecasilicate isolated here is not a significant species in aqueous alkaline solutions [96], and its enrichment has been attributed to the maximizing of entropy resulting from leaving numerous water donors unbound. The main hydrogen bonding interactions are between the silicate ion and α-CD molecules. Both α-CD molecules sandwiching the silicate ion are oriented with their primary hydroxyl groups (i.e. the upper, wider rim of the CD macrocycle) pointing towards the silicate. Si12O3012– ion, which has six terminal silicate O– groups on each hexagonal face of the silicate prism. Each of these acts as bifurcated hydrogen bond acceptor, interacting with two primary hydroxyl groups from adjacent saccharide groups within the CD. The secondary hydroxyl groups of each α-CD are on the outside of this assembly and form hydrogen bonding interactions with water molecules, which in turn form hydrogen bonding interactions to the secondary hydroxyl groups of other [Si12O30.2α-CD] assemblies propagating a 3D structure with sheets of α-CD linked by the silicate guest molecules. The K+ counter-cations are mainly located between the [Si12O30.2α-CD] assemblies with average K+ ion coordination of 3.5 water molecules and 3.5 O donors of α-CD molecules. Two K+ ions are coordinated by a hexagonal face of each silicate ion and project into the α-CD cavity.

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6 Cucurbiturils Cucurbituril 5 is a rigid macrocyclic cavitand with a barrel shape and two identical portals lined by carbonyl groups. The carbonyl groups of cucurbituril are ideal hydrogen bond acceptor groups, and may also be used to complex metal centres, with direct coordination of group 1 and 2 metal ions established [13, 97], and more recently lanthanide ion coordination has been reported [98]. Fedin and co-workers have synthesised numerous cucurbituril complexes with a range of metal complexes and complex metal clusters, many of which form infinite hydrogen bonded networks [98–105]. The previously unreported complex ions [In(H2O)6]3+, trans-[InCl2(H2O)4]+ and trans-[InCl4(H2O)2]– have been isolated and characterised by forming supramolecular adducts with 5 [99]. A similar supramolecular approach to isolating hydrated complex ions was described in section 3.2 with the Russian Doll structures. In the complex (H3O)3[InCl4(H2O)2]3.25.17H2O there are both cis- and trans-[InCl2(H2O)4]+ cations. The cis-[InCl2(H2O)4]+ cation forms O-H…O hydrogen bonds to a 5 carbonyl group via one aquo ligand while the other aquo ligand forms O-H…O hydrogen bonds to a water molecule (which subsequently hydrogen bonds to a 5 molecule) and O-H…Cl hydrogen bonds to an adjacent cis-[InCl2(H2O)4]+ cation. The trans-[InCl2(H2O)4]+ cation forms O-H…O hydrogen bonds to water molecules which subsequently hydrogen bond to 5 hosts. A 3D network structure is thus produced.Additional water or H3O+ molecules are guests for the 5 hosts. The first structurally characterised hexaaquoindium(III) ion is found in [In(H2O)6](NO3)3.5.9H2O [99]. Water and 5 molecules and [In(H2O)6]3+ cations form hydrogen bonded chains.A similar hydrogen bonding motif is found in the related Al complex [Al(H2O)6]Cl3.5.18H2O, where there are additional interactions to the chloride counter-anions [99]. The tetrahedral ions [GaCl4]–, and [FeCl4]– form isostructural complexes of composition (H7O3)4[MCl4]2Cl2.5.2H2O, M=Ga [100], or Fe [101]. The centres of the 5 molecules are arranged with body-centred packing with [MCl4]– anions in spaces between them. A complicated hydrogen bonded network joins the carbonyl O atoms of 5 with water, the complex anions and chloride within the structure. Lanthanide ions may be coordinated by the hard carbonyl O atoms of 5 in a multi-dentate fashion. In the isostructural series of complexes {[Ln(NO3)(H2O)4] (5)}(NO3)2.nH2O 44, where Ln=Gd, Ho or Yb, the 5 host is a bidentate ligand coordinating to the lanthanide ion through two adjacent carbonyl groups [98]. The lanthanide is has a distorted triangular dodecahedral geometry with a chelating nitrate ligand and four aquo ligands in addition to the bidentate 5. The aquo ligands form hydrogen bonding interactions to solvent water molecules or to carbonyl groups of the 5 host. The O-H…O=C interactions are either intramolecular or intermolecular, with intermolecular interactions generating hydrogen bonded chains. The crystal lattice of 44 has the centres of the host molecules forming a body-centred motif and all the hydrogen bonded chains run in the same direction, hence the complex is polar. The host molecules contain a water guest molecule and additional water and nitrate ions are located between the

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hydrogen bonded chains.An isostructural Gd complex with an ethanol ligand in place of one aquo has also been characterised [98]. The complex {[La(H2O)6(SO4)](5)}(NO3).12H2O 45 has a nine coordinate La3+ ion with mono-capped square antiprism geometry [98]. As for 44 the La3+ ions are coordinated by the 5 host in a bidentate fashion, and the coordination sphere is completed by six aquo ligands and a monodentate sulphate anion. Three of the aquo ligands form intramolecular O-H…O=C hydrogen bonds, and an aquo ligand hydrogen bonds to a guest water molecule contained within the 5 cavity. Neighbouring {[La(H2O)6(SO4)](5)}+ complexes are connected by intermolecular hydrogen bonds between aquo ligands and 5 carbonyl groups and C-H…O=C hydrogen bonds between adjacent molecules of 5. Unlike 44, adjacent host molecules are not aligned in the same orientation, but are at approximately right angles to each other. Channels between corrugated layers of {[La(H2O)6(SO4)](5)}+ complexes are filled with water and nitrate anions which form a complicated network of hydrogen bonds. A series of complexes of host 5 and M3E4Clx(H2O)9–x clusters where M=Mo,W; E=S, Se, and related clusters, have been reported, in some cases the mixed aquo and chloro clusters were isolated for the first time using the supramolecular approach [102–105]. The trinuclear M3E4 core of the clusters have an incomplete cuboidal shape with a triangle of M atoms capped by a single µ3-E atom and three coplanar µ2-E atoms. In the parent [M3E4(H2O)9]4+ cluster the six aquo ligands cis to the µ3-E atom were identified as complementary to the six carbonyl groups adorning one portal of 5. Indeed in the complex [{W3Se4(H2O)8Cl}2](5).6Cl.12H2O 46 each end of the 5 host is capped by a [W3Se4(H2O)8Cl]3+ cluster through six O-H…O=C hydrogen bonds, effectively forming a second coordination sphere for the cluster [102]. Furthermore the [W3Se4(H2O)8Cl]3+ clusters self-associate through Se…Se interactions and a supramolecular chain structure is found. A similar chain structure is propagated through hydrogen bonds and S…S interactions in the mixed cationic supramolecular complex {[Mo3S4(H2O)6Cl3][Mo3S4 (H2O)7Cl2](PyH)(5)}4+ 47 [103]. In 47 each 5 molecule is a host for a pyridinium cation, PyH+.When a Hg bridged double-cuboidal cluster [{M3E4(H2O)7Cl2}2Hg]4+ is used an isostructural chain structure is formed, assembled entirely by hydrogen bonds. In the series of complexes [{M3E4(H2O)7Cl2}2Hg](5)Cl4.14H2O 48 (M=Mo; E=S, Se and M=W; E=Se) the [{M3E4(H2O)7Cl2}2Hg]4+ clusters are sandwiched between host 5 molecules. An infinite hydrogen bonded chain is formed with the 5 molecules aligned within the chain, Fig. 25 [102]. The related cluster compound [Mo3NiS4(H2O)8Cl2]2+ forms hydrogen bonds to 5 in the complex {[Mo3NiS4(H2O)8Cl2](PyH)(5)}Cl3.14.5H2O 49 in a mono-capped arrangement [104]. A trans aquo ligand bound to one of the Mo centres forms a strong O-H…O=C hydrogen bond to the carbonyl groups of the uncapped portal of an adjacent 5, forming a hydrogen bonded chain. In the chain structures of 46–49 the cucurbituril molecules are aligned or nearly aligned along the direction of the approximately straight chain. Zig-zag chains may also be formed by side-on interactions between aquo ligands of the M3E4Clx(H2O)9–x cluster and adjacent 5 molecules. In {[W3S4(H2O)7Cl2](5)} Cl2.10H2O B8 each cucurbituril molecule is mono-capped by a [W3S4(H2O)7Cl2] cluster, which also forms O-H…O=C hydrogen bonds to the carbonyl groups of

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Fig. 25 The hydrogen bonded chain structure in complex [{Mo3S4(H2O)7Cl2}2Hg](5)Cl4.14H2O where [{Mo3S4(H2O)7Cl2}2Hg]4+ clusters are sandwiched between host 5 molecules [102]

Fig. 26 The zig-zag hydrogen bonded chain of {[W3S4(H2O)7Cl2](5)}Cl2.10H2O B8 [103]

the uncapped portal of an adjacent cucurbituril giving a zig-zag chain, Fig. 26 [103]. Such side-on hydrogen bonding interactions are also found in the complex {[W3S4(H2O)8Cl](PyH)(5)}.Cl4.15.5H2O, where there are two types of 5 host, one doubly capped by hydrogen bonding [W3S4(H2O)8Cl] clusters, and one uncapped [105].

7 Conclusions A variety of molecular hosts, namely crown ethers, calixarenes, cyclotriveratrylene, cyclodextrins and cucurbituril, may be incorporated into network structures through hydrogen bonding interactions. Examples of chain, ladder, 2D grid and 3D networks have all been characterised. Interestingly, there are currently no known examples of interpenetrating structures involving molecular hosts.As yet there have been few attempts to systematically study the types of network structures that molecular hosts may form, and examples of genuinely designed or engineered network structures involving hosts are rare. The crystalline materials formed may show useful or unusual properties, such as multiple inclusion behaviour by including one type of guest in their molecu-

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lar cavity and another type as a lattice type guest. The inclusion behaviour of the host may be altered from that of the lone host molecules as is seen with cyclotriveratrylene. There are also advantages in embedding photoactive guest molecules into such networks for time-resolved crystallographic studies. Of particular significance is that elusive or unusual species may be isolated and crystallised as components of network structures with molecular hosts. This has been observed with crown ether, calixarene, cyclodextrin and cucurbituril host molecules for species such as Si12O3012–, [Fe2(µ-O)(H2O)10]4+, and [Cr4(OH)6(H2O)12]6+. Given the range of host molecules that may be that be put into network structures and strong current interest in the inclusion properties of crystalline network materials, embedding molecular hosts into hydrogen bonded network structures will continue to be a fruitful and exciting area of inclusion and structural chemistry.

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Subject Index

Adamantoid cages 163 Aggregation process 2 Amides, cyclic aromatic 91 Amidino-O-alkylurea complexes 60 Amino acids 12 Barbiturate receptor 121, 122 Biguanide 33, 34 – complexes 46–49 4,4′-Bipyridine 152–156 Bis(N-alkylamidino-O-alkylurea)copper(II) nitrates 64 Bis-benzene chromium dicarboxylic acid 20 Bis(4-pyridyl)ethylene 152, 156 Biuret 33, 34 – complexes 41 – monohydrate 44 Cadmium(II) 41 Cages 85 Calix[1]benzimidazol-2-one[2]arene 146, 147 Calix[4]nucleoside 146 Calix[4]resorcinarene 153–159 Cambridge structural database (CSD) 5–12 Capsule-like dimers 146, 153, 159 Carbamoylguanidine 33, 34, 59 o-Carborane 159–161 Carboxylate/carboxylic group 7, 11 Casting 120 Catalyst 96, 111, 125, 126 Catenanes 93–96 2-Chlorobiguanide 49 Cobalt(II) 47, 50, 55 Cobaltocene diacid 23 Cobaltocene dicarboxylic acid 14, 23 Compression, electrostatic 19 Coordination 33, 34 – polymers 141 Copper(II) 34, 41–61, 71, 79 Cr(III) cations, hydrolytic 149 Croconic acid 18

[12]Crown-4 142 [15]Crown-5 complexes 142, 143 [18]Crown-6 complexes 141–143 –, in Russian doll complexes 148–152 Crown ethers 87, 98, 99, 116 Cryptate 157 Crystal engineering 2, 5, 15, 20, 22, 140 Crystal structure 6 Cucurbiturils 169–171 Cyclam 152 Cyclodextrin hydrates 167, 168 Cyclodextrins/dicyclohexasilicate 168 Cyclotricatechylene 166, 167 Cyclotriveratrylene 159–165 Cytosine 78 Data-mining 5 2,6-Diaminopyridine 78 Dibenzo[24]crown-8 144 Dicyclohexasilicate 168 N,N-Dimethylbiguanide 50 N,N-Diphenylbiguanide 56 Disulfide 100, 102, 124 Dynamic libraries 85 Esterification 77 Ethylenebis(biguanide)

56

Ferrocene dicarboxylic acid Fuel cells 26 Fullerene 159–161

14

Gas-solid reactions 15, 23, 25 Gas-trap system, reversible 24 Gelators, macrocyclic-based 146 Glucophage 50 Gold(III) 50 Guanidine 33, 87 Guanidinium-sulfonate superstructure 21 Guanidinium-templated synthesis 88 Guanine 78 Halides 64 Helicates 118–120

180 Honeycomb 38, 44 Hydrogen bond 2 – –, charge-assisted 3–7, 15, 16 – –, distance distribution 7–11 – –, homo-ionic/hetero-ionic 19, 20 – –, organometallic systems 10, 14, 19 – –, reactivity 23, 24 ICSD 5, 6 Imidazoliophanes 88 Indium complexes 169 Intermolecular bonding 3, 6 Ion interactions, non-covalent 19 Keggin ion [Al13O4(OH)24(H2O)12]7+ 151 Lanthanides 142, 148, 152, 165, 169, 170 Libraries, dynamic combinatorial 120–124 Macrocycle, oligopyrrolic 90 Macrocycles 85 Manganese(II)/(III) 47, 56 Mercury(II) 41 Metalla-cages 91, 92, 130 Metalla-macrocycles 91, 92 Metathesis, ring opening-closing 96, 111 Metformin 50 Methanolysis 77 C-Methylcalix[4]resorcinarene 142, 148, 152–159 Moulding 120 Network, one-dimensional 140–143, 153, 154 –, three-dimensional 140 Nickel(II) 34, 47, 50–56 Octa(p-hydroxyl)octakis(propyl)calix[8]arene 146 Oligopyrrolic macromycle 90 Organometallic systems 5, 10, 14, 19 Organometallic zwitterions 24 Orotic acid 78 Oxalic acid 12 Oxocarbon 19 Oxonium cations 143 Packing 1, 2, 19 Palladium(II) 34, 47 Peptide receptor 124 Phenolate, rotaxane 106 Photoactive guests 156, 157 Photodimerisation 129–133 Pi-interactions 17 Pi-pi stacking interactions 86, 114–117, 127

Subject Index Platinum(II) 34, 50, 78 Porphyrins 108, 109, 118, 119 Proton acceptor/donor 4 Proton transfer 25 Proton-conducting material 26 Pseudorotaxanes 96–114, 144, 145 Pyridine/formic acid 27 2,6-Pyridinedicarboxaldehyde 103 Recognition 2 Recrystallization 77 Repulsion, electrostatic 15–17 Rhodium(II) 56 Rhodium(III) cations, hydrolytic 149–151 Rhodizonic acid 19 Rhombic architecture 45, 62, 81 Ring opening-closing metathesis 96 Rotaxanes 96–114 –, ammonium salts 96, 97 Russian doll complexes 149 – – – and Keggin ions 151 – – – and polynuclear Rh cations 150, 151 – – – of p-sulfonatocalixarene 148–152 Second sphere ligands, crown ethers 142 – – –, p-sulfonatocalix[4]arene 147 Self-replicating systems 125–128 Silver(III) 56 Squaric acid 17 Sulfates 64 p-Sulfonatocalix[4]arene 147, 148 – and lanthanide complexes 148, 152 Superprotonic transition 26 Supramolecular reaction 25 Synthons 5, 6 Technetium(III/V) 50 Templates, anions 88–91, 105–111, 118, 119 –, guanidinium 87–88 –, hydrogen-bonding properties 87 –, metal cations 87 Tetrabenzo[24]crown-8 144, 146 Tetracarboxylic acid calix[4]resorcinarene 147, 148 Tetracyanoethylene (TCNE) 15 Tetrafluoroborates 64 Tetraguanidinium, sulfates 118 Tetraurea calix[4]arene 146 Thermodynamic control 96, 99, 100, 120 Thymine 79 Topological control 2 4,4′-Trimethylenedipyridine 152, 154 Two-dimensional network structures 140 Zinc(II) 34, 41, 50, 55 Zwitterions 12–15, 24