Shape and Size Effects in the Crystal Structures of Complexes of 1,3,5-Trinitrobenzene with some Trigonal Donors: The Benzene–Thiophene Exchange Rule

The crystal chemistry of molecular complexes of several trigonal donor molecules with the trigonal acceptor 1,3,5-trinitrobenzene, TNB, is reported. Generally, replacement of a moiety by another of similar shape and size does not change the overall packing. The 1:1:1 (triphenylisocyanurate)·(TNB)·(benzene) solvate is isostructural to the corresponding 1:1:1 thiophene solvate, confirming the so-called benzene–thiophene exchange rule. The 1:1 complex of tris-2,4,6-(4-methylphenyl)-1,3,5-triazine and TNB is layered and the layers have quasi-trigonal symmetry. The triazine ring may be replaced by a phenyl ring without any change in the crystal structure. Thus, 1,3,5-tris(4-methylphenyl)benzene and TNB form an isostructural 1:1 complex. Such shape/size exchange may be further explored in the 1:1 complex of 1,3,5-tris[5-(2-chlorothienyl)]benzene and TNB. Here both phenyl–thienyl and chloro–methyl exchanges are simultaneously possible and yet another isostructural complex is obtained. Finally, the 1:1 complex of 1,3,5-tris(2-thienyl)benzene and TNB is also found to have a very similar structure. However, when 1,3,5-triphenylbenzene and TNB are taken in 1:1 ratio in solution, the result is a 1:3 molecular complex. This is unexpected in view of the phenyl–thienyl exchange rule, and some rationalisation is provided for the unusual formation of this 1:3 complex. Many of these structures are pertinent from the viewpoint of carry-over of trigonal molecular symmetry into the crystal, a contemporary theme in the engineering of crystal structures for octupolar non-linear optical applications, while ready access to the 1,3,5-trisubstituted benzenes for this study was made possible by a general and efficient protocol for the synthesis of these compounds from the corresponding acetyl aromatics.