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Isomorphous Crystals by Chloro–Methyl Exchange in Polymorphic Fuchsones

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posted on 07.11.2012, 00:00 by Naba K. Nath, Ashwini Nangia
The X-ray crystal structures of four fuchsone derivatives in which a chloro group is replaced by methyl were analyzed to understand isostructurality upon Cl–Me exchange in a polymorphic family of molecules. The four methyl groups in tetramethyl fuchsone (TMF, 2,6-dimethyl and α,α-di-p-tolyl) were substituted with chlorine pairwise to give dichloro dimethyl (CMF, 2,6-dichloro and α,α-di-p-tolyl), dimethyl dichloro (MCF, 2,6-dimethyl and α,α-di-p-chlorophenyl), and tetrachloro derivatives (TCF, 2,6-dichloro and α,α-di-p-chlorophenyl). The first three compounds are polymorphic, whereas TCF afforded one crystal modification only. TMF, CMF, and MCF are isostructural and isomorphous crystals in the category of polymorphs, solid solutions, and solvates. The first case of color polymorphism in fuchsone dyes is reported for CMF dimorphs. The formation of solid solution is one of the most stringent tests of isostructutality, which was observed for TMF, CMF, and MCF but not for TCF. Crystal packing in TCF is dominated by short Cl···Cl interactions, and consequently this crystal structure is different from the first three members which are largely a result of space filling. Crystal structures were analyzed using the XPac program to calculate the dissimilarity index of supramolecular constructs and Hirshfeld fingerprint plots to quantify the contribution of Cl···Cl interactions. Isostructurality was observed up to 50% exchange of Cl with Me, but after that point the structure deviates to another packing motif.