Halogen-bonded co-crystals

At this point we are essentially left with two families of compounds binary charge-transfer complexes and hydrogen-bonded or halogen-bonded co-crystals. This section will focus almost exclusively upon representatives from the latter categories [67]. The terms binary/ternary supermolecule indicates a discrete species with predictable and desirable connectivity, constructed from two /three different molecular species and assembled via directional non-covalent forces [68-78]. 

Fig. 3 Double-stranded chain structure in the halogen-bonded co-crystal 5 between 2-mercapto-l-methylimidazole (1) and 1,4-diiodotetrafluorobenzene (2). Halogen-bonding distances S-. 1=3.314(1) A. 3.342(1) A. Colors are as follows black 70%—nitrogen black 50%—iodine black 30%—fluorine and hydrogen black 10%—sulfur and carbon. Dashed lines represent intermolecular halogen and hydrogen bondings. (View this art in color at www.dekker.com)...

Scheme 2.5.12 The primary halogen bonds in co-crystals of dicyanoalkyls and diiodoperfluoroalkanes.

The stepwise mechanism of co-crystal formation has been interpreted in terms of kinetic and thermodynamic co-crystallisation products, wherein the finite assemblies are the kinetic product whose formation is driven by the formation of stronger N I halogen bonds. The formation of the thermodynamic product is subsequently followed by the formation of enthalpically less favourable S I halogen bonds. 

This stepwise mechanism is most likely the consequence of the hierarchy of strong and weak halogen bonds in the reaction mixture. The stepwise mechanism is likely to be a general characteristic of co-crystal formation by grinding reactant molecules that have significantly different halogen- or... 

Figure 8.15 (a) The stepwise formation of linear infinite halogen-bonded chains of the co-crystal of 1,4-thiomorpholine and tetrafluoro-l,4-diiodobenzene, via a finite assembly intermediate and (b) stepwise formation of infinite zig zag halogen-bonded chains of the co-crystal of 1,4-thiomorpholine and tetrafluoro-l,2-diiodobenzene. ... 

C.M. Widdifield, G. Cavallo, G.A. Facey, T. Pilati, J. Lin, P. Metrangolo, G. Resnati, D.L. Bryce, Multinuclear solid-state magnetic resonance as a sensitive probe of structural changes upon the occurrence of halogen bonding in co-crystals, Chem. Eur. J. 19 (2013) 11949-11962. 

By using co-crystallization via halogen bonding and other assisting interactions, ten cocrystals between PAHs and their heterocyclic analogues and haloperfluoro-benzenes were successfully assembled. The structures and cocrystal components are shown in Scheme 1 and Table 1. 

Gao HY, Shen QJ, Zhao XR, Yan XQ, Pang X, Jin WJ (2012) Phosphorescent co-crystal assembled by 1,4-diiodotetrafluorobenzene with carbazole based on C-I---3I halogen bonding. J Mater Chem 22 5336-5343... 

The first halogen-bonded photomechanical cocrystals were published in 2014, employing materials shown in Fig. 15 [68]. They comprise compounds 7 and 8, also used in Bushuyev et al. [65] and Saccone et al. [67], which function as halogen-bond donors, and the dipyridine derivatives (9, 10, and 11), one containing the trans-azo link, and the other two having the trans and cis C=C links, respectively. Co-crystals were obtained by slow evaporation of a concentrated 1 1 solution of the halogen-bond donor and acceptor, and the stmctural characterization revealed some of the features already observed in the co-crystals of 8 with other halogen-bond acceptors [67] (Fig. 16). 

X-Ray structural analysis of the co-crystal of p-34 with tri(dimethylamino) cyclopropenium chloride showed several halogen bonds to the halide. Most importantly, a bidentate coordination to chloride is observed for (me side of the halogen-bond donor. As this induces a slight distortion of the central aiyl backbone, the distance between the iodine atoms on the other side of the XB donor is elcmgated, and two monodentate halogen-bonds to chloride are found on this side (Fig. 13). 

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