Isomorphous Crystals by Chloro–Methyl Exchange in Polymorphic Fuchsones

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.