Non-coplanar aromatic carboxylic acids: Unusual conformation-dependent self-assembly and pseudopolymorphism of di(3-carboxymesityl)methane

The dicarboxylic acids 3 and 4, i.e., di(3-carboxy-2,4,6-trimethylphenyl)methane and di(3-carboxyphenyl)methane, are created by a methylene tethering of mesitoic acid and benzoic acid, respectively. These diacids may explore two low energy conformations, viz., syn and anti. Whereas the syn-diacid 3 is found to undergo self-assembly in the solid state via a very rare tetrameric motif, the anti-diacid 3 is found to exhibit pseudopolymorphism with guest molecules such as DMSO and PhOH–H₂O. The unusual patterns of assembly and the occurrence of pseudopolymorphism for the syn and anti conformers, respectively, appear to emanate from an unique structural feature that emerges as a consequence of tethering. It appears that the molecules that contain strongly interacting functional groups in non-coplanar aryl rings will suffer from packing problems, which manifests itself in a new mode of packing and the phenomenon of pseudopolymorphism.