Structure of metallathiacycles: Planar vs nonplanar geometries. A theoretical and experimental investigation

Since the previously reported results of Fenske-Hall molecular orbital calculations suggested that the bent metallacycle geometries observed in transition-metal-inserted thiophene complexes do not result from electronic properties, molecular mechanics calculations were employed to explore how steric factors could influence ring geometries. These calculations have identified steric factors responsible for the observed ring deformations in (C5Me5)Rh(PMe3)(eta(2)-C,S-2,5-Me2C4H2S) and benzothiophene and dibenzothiophene analogs, The results of the molecular mechanics calculations have been supported by X-ray structural characterization of the products formed via insertion of the reactive 16-electron metal fragment [(C5Me5)Rh(PMe3)] into the C-S bonds of unsubstituted thiophene, benzothiophene, and dibenzothiophene. The parent thiophene complex, unlike the previously reported ring-substituted analog (and as predicted by the molecular mechanics calculations) is found to adopt a planar six-membered ring structure, The benzothiophene insertion product shows a moderate degree of bending of the metallathiacycle, whereas the dibenzothiophene molecule is strongly bent.