Supramolecular synthons solid-state structure

Recently, we have been investigating the chemistry of a series of racemic diquinoline compounds that form lattice inclusion hosts. The solid-state structures of these involve only rather weak intermolecular attractions. A number of these hosts assemble by means of centrosymmetric supramolecular synthons, and therefore their enantiomer separation is limited in the solid state. 

A, while that between the pairs is 3.383(6) A. The 7r-stacked arrays of cations are linked by centrosymmetric pairs of Se(2)—N(l ) contacts, thereby generating ribbons running along the y-direction. These four-center Se—N supramolecular synthons, which are common in diselenadiazolylium salts and diselenadiazolyl radicals, are likely to dominate the solid-state structures of reduced diselenazolyl materials. 

Complex supramolecular structures involving intermo-lecular Sn S, S S secondary bonds and N-H . S hydrogen bonds were observed in organotin derivatives of substituted 5-mercapto-l,3,4-thiadiazoline-2-thione. The simultaneous presence of S-Sb CP secondary bond synthons and CP H-N hydrogen bonds in the solid-state structure of dichlorobis(trimethylenethiour-ea)antimony(III) chloride also leads to an interesting supramolecular self-organization. The cooperation of secondary bonds and Te ti aryl interactions also produces some spectacular supramolecular architectures. 

In this extensive study of thiourea synthesis by coupling isothiocyanates with amines under mechanochemical conditions of neat and LAG, a total of 49 different thiourea derivatives were prepared. Since the supramolecular chemistry of simple synunetrical and nonsymmetrical diarylthiouieas had not previously been studied in detail, the information from the PXRD patterns of the milling products in this work also allowed for a systematic analysis of structural features of diarylthio-ureas. The three main self-assembly motifs were identified. A characteristic supramolecular synthon based on bifurcated N—H - S hydrogen bonds was found in most cases, whereby the individual thiourea molecules were assembled into chains aligned in a head-to-head (parallel molecular dipoles) or head-to-tail (antiparallel molecular dipoles) manner. In the case of slerically hindered thioureas, such as A -(2,6-dimethylphenyl)-A -phenylthiourea, the third motif arising from discrete cen-trosymmetric dimers based on rI(S) supramolecular synthon was found. A detailed look at the measured PXRD patterns for aU diarylthioureas revealed that solid-state structures of these compounds could be classified into two structural families, denoted as family I and family II (Fig. 1.11). 

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]. 

With the recognition that many substances may cocrystallize in a single continuous lattice structure, scientists have more recently initiated intense studies of the mixed molecular crystal systems that have become known as cocrystals [10]. This particular area of solid-state research has led pharmaceutical scientists into the areas of crystal engineering and assembly of appropriate supramolecular synthons, with particular emphasis on understanding the origins of the molecular self-assembly that takes place in the formation of cocrystal systems. 

To continue, the crystal structure of fefraA i -(4-iodophenyl)methane (16) is isomorphous with 17 having an I---I distance of 3.95 and 4.16 A in the corresponding 14-synthon. In this family of halo-substituted tetraphenylmethanes with S4- or pseudo-S4-symmetry, the molecules exhibit similar colunmar packing in the solid state [14]. In summary, if a supramolecular strucUire is defined as a network and analysed retrosynthetically, general connectivity strategies can be systematically derived. 

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