Hydrogen-bonded supramolecular synthons

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. 

Figure 17 The hydrogen-bonded supramolecular synthon present in all helical tubulate inclusion compounds formed by diol 11 (and its family of hosts). The molecules hydrogen bond together O-H O-H O-H to form the threefold screw axis structure illustrated. Identical arrangements subtended by the second diol hydroxy groups give rise to the chiral tubular lattice structure shown in Figure 16. Hydrogen atoms are omitted for clarity.

Figure 18 The hydrogen-bonded supramolecular synthon present in the helical tubulate compound (12)2-(13)-(toluene). Hydrogen atoms are omitted for clarity.

Wuest has demonstrated that the pyridone moiety also generates a hydrogen-bonded supramolecular synthon that is suitable for building extended arrays.67 Remarkably, methanetetra(6-phenylethynyl-2-pyridone) exhibits a diamondoid network, sevenfold interpenetration and cavities large enough to enclathrate butyric or valeric acid.27 Wuest introduced the concept of tectons to describe molecules that inherently possess the molecular structure and intermolecular recognition features to predictably self-assemble into crystalline networks. He followed this study with several other examples of diamondoid networks sustained by the pyridone moiety 27c d... 

K. Maruyoshi, D. luga, O.N. Antzutkin, A. Alhalaweh, S.P. Velaga, S.P. Brown, Identifying the intermolecular hydrogen-bonding supramolecular synthons in an indomethacin-nicotinamide cocrystal by sohd-state NMR, Chem. Commun. 48 (88) (2012) 10844-10846. 

Finally, in order to find a way of fine-tuning melting point and aqueous solubility of a representative. A, of a family of anticancer compounds, Aakeroy and coworkers reported the synthesis of five cocrystals with aliphatic, even-chained, dicarboxylic acids. The supramolecular synthesis was driven by the well-known COOH- py hydrogen-bond-based synthon, and in each reported case, infinite APT diacid- API- diacid chains were obtained, and these were subsequently arranged into 2D layers driven by API-based self-complementary amide- amide hydrogen bonds (Scheme 23). 

The triclinic form [29] was obtained by crystallization from ethanol or by reflux in ethanol with the addition of copper(II) chloride in a 1 IM ratio. The sulfasalazine in this form (QIJZOY) is present as an amide tautomer forming the supramolecular dimer, stabilized by two R2 (8) hydrogen bonds, namely synthon R between the carboxylic part of one molecule and pyridine N and amide N-H groups from the other molecule (Scheme 13.12). The contacts with other dimers are realized mostly through weaker hydrogen bond C-H- -O and C-H- -N... 

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]. 

Cocrystal systems are assembled through the association of individual molecules into fundamental building block units that are known as supramolecular synthons [13]. For example, one such synthon would be formed by hydrogen-bond interactions between a phenyl-carboxylic acid and a phenyl-amide, with the molecules being linked into a dimeric species through 0 H-N and O-H O hydrogen bonds [14], This mode of interaction can be illustrated using the synthon that would result from the dimerization of benzoic acid and benzamide ... 

Two different of supramolecular synthons have been detected in the two polymorphs of the carbamazepine-saccharin cocrystal system [45]. In the Form-I structure, the carbamazepine molecules formed a homo-synthon, with the saccharine molecules also forming a hydrogen-bonded homodimer. The interaction of these two synthons resulted in formation of a one-dimensional array of molecules in a crinkled tape motif. In the Form-II structure, a heterosynthon is formed by the interaction of a carbamazepine and a saccharin molecule. This latter synthon packs in one-dimensional chains that extended along the crystallographic c-axis. 

The amide-carboxylic acid interaction forms one of the most studied supramolecular synthons. For example, the binary synthon formed by benzamide and benzoic acid arises by the hydrogen-bond interactions that cause the molecules to become linked into a dimeric species through O -- H-N and O-H O hydrogen bonds ... 

The structures of three cocrystals of caffeine having a 1 1 stoichiometry with various hydroxy-2-naphthoic acids have been reported [62], The anticipated imidazole-carboxylic acid supramolecular synthon was observed in caffeine cocrystals containing l-hydroxy-2-naphthoic acid and 3-hydroxy-2-naphthoic acid, while a hydrogen-bonded carboxylic acid dimer (and no hydroxyl-caffeine heterosynthon) was observed in the caffeine cocrystal with 6-hydroxy-2-naphthoic acid. 

T.R. Shattock, K.K. Arora, P. Vishweshwar, M.J. Zaworotko, Hierarchy of supramolecular synthons persistent carboxylic acid pyridine hydrogen bonds in cocrystals that also contain a hydroxyl moiety, Cryst. Growth Des. 8 (2008) 4533M545. 

Figure 8.15 Centrosymmetric 8 membered hydrogen bonded amide ring supramolecular synthon found in the crystal structure of saccharin.

Hydrogen bonded rings are among the most beautiful and most versatile supramolecular synthons in crystal engineering. A systematic survey of the CSD was undertaken in 1999 to determine the tendency of particular functional groups to form hydrogen bonded ring dimer synthons as opposed to other motifs... 

Figure 8.57 Supramolecular synthons 8.44 and 8.45 formed to halometallate complexes along with the tape structure formed by 4,4 bipyridinium tetrachloroplatinate(II) via an Rf (4) hydrogen bonded...

Another case of major enantiomer separation occurs when helical tubuland diols (Section 3.2.1) are crystallised with small phenol molecules and intimately hydrogen bonded co-crystals are produced. A typical example is (11 ) ( p-chloro-phenol) [33], The major supramolecular synthon is H-0 H-0 H-0 hydrogen bonding with eclipsed stacks of the participating molecules surrounding a pseudo-threefold screw axis (Figure 13). This chiral motif involves molecules of p-chloroplienol and only one of the enantiomers of 11. 

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