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.

为探究分子多晶型家族中氯（Cl）-甲基（Me）交换过程中的同构性（isostructurality），本研究对4种将氯原子替换为甲基的品酮（fuchsone）衍生物的X射线晶体结构（X-ray crystal structures）开展了分析。四甲基品酮（tetramethyl fuchsone, TMF，2,6-二甲基且α,α-二对甲苯基）上的四个甲基被成对替换为氯原子，分别得到二氯二甲基品酮（dichloro dimethyl fuchsone, CMF，2,6-二氯且α,α-二对甲苯基）、二甲基二氯品酮（dimethyl dichloro fuchsone, MCF，2,6-二甲基且α,α-二对氯苯基）以及四氯品酮衍生物（tetrachloro derivative, TCF，2,6-二氯且α,α-二对氯苯基）。前三种化合物存在多晶型现象，而TCF仅有一种晶体晶型。TMF、CMF与MCF均属于同构且同晶的晶体，涵盖多晶型、固溶体（solid solutions）以及溶剂化物（solvates）范畴。本研究首次报道了品酮染料中的颜色多晶型现象，对应CMF的两种晶型。固溶体的形成是检验同构性最严苛的标准之一，该现象在TMF、CMF与MCF中均被观测到，但未在TCF中出现。TCF的晶体堆积主要由短程氯-氯（Cl···Cl）相互作用主导，因此其晶体结构与前三种化合物截然不同，后者的晶体堆积主要源于空间填充效应。本研究通过XPac程序分析晶体结构，以计算超分子构造（supramolecular constructs）的差异指数，并通过希尔德费尔德指纹图（Hirshfeld fingerprint plots）量化氯-氯相互作用的贡献占比。当氯原子与甲基的交换比例达到50%时仍可观测到同构性，但超过该比例后，晶体结构会转变为另一种堆积模体（packing motif）。