Crystallographic and molecular mechanics investigation of an order--disorder transition and dimorphism in 5it H,10it H-dithiolo[2,3-it b]-2,5-benzodithiocine-2-thione

Single-crystal X-ray structures are presented for three forms of 5it H,10it H-dithiolo[2,3-it b]-2,5-benzodithiocine-2-thione. The $ (at 150K) and $$^ (at ambient) forms are very similar and differ only in the presence of crystallographic it m symmetry in the molecules of $$^, which is absent in the case of $. This pair is related by an order--disorder transition. The $ phase (also determined at 150K) has a different structure in terms of the molecular packing from either of the other two and therefore constitutes a true polymorph. Molecular mechanics calculations indicated that the most stable CHCl$$-solvated conformations for the title compound were a pair of twisted it U-shaped enantiomers, sb R$ and sb L$, it i.e. similar to the arrangements found in the $ and $ phases, with the low-lying saddle point between them corresponding to the situation in the $$^ phase. These calculations also indicated that the most stable CHCl$$-solvated conformation for the related dibromo-5it H,10it H-dithiolo[2,3-it b]-2,5-benzodithiocine-2-thione was it Z-shaped, in agreement with the crystal structure determined earlier for its DMSO solvate [Wang it et al. (1998). it Synthesis, pp. 1615--1618].