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 mole cules with the trigonal acceptor 1,3,5-trinitrobenzene, TNB, is reported. G enerally, replacement of a moiety by another of similar shape and size does not change the overall packing. The 1:1:1 (triphenyl isocyanurate) (TNB) ( benzene) solvate is isostructural to the corresponding 1:1:I thiophene solv ate, confirming the so-called benzene-thiophene exchange rule. The 1:1 comp lex of tris-2,4,6-(4-methylphenyl)-1,3,5-triazine and TNB is layered and th e 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-tri s(4-methylphenyl)benzene and TNB form an isostructural 1:1 complex. Such sh ape/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-me thyl exchanges are simultaneously possible and yet another isostructural co mplex 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 i s a 1:3 molecular complex. This is unexpected in view of the phenyl-thienyl exchange rule, and some rationalisation is provided for the unusual format ion of this 1:3 complex. Many of these structures are pertinent from the vi ewpoint of carry-over of trigonal molecular symmetry into the crystal, a co ntemporary theme in the engineering of crystal structures for octupolar non -linear optical applications, while ready access to the 1,3,5-trisubstitute d benzenes for this study was made possible by a general and efficient prot ocol for the synthesis of these compounds from the corresponding acetyl aro matics. (C) 2000 Elsevier Science Ltd. All rights reserved.