The role of bimetallic thiophene-bridged complexes in homogeneous desulfurization reactions

A detailed study of bimetallic thiophene-bridged complexes, formed by coordination of a second metal fragment to a transition-metal-inserted thiophene complex, is reported. The bonding interactions between mononuclear metallathiacycles and a second metal center, the effects of coordination of a second metal center on the M-C and M-S bonds of the heterocycle, the stability of bimetallic thiophene-bridged complexes, and their possible role in homogeneous desulfurization reactions are addressed. Fenske-Hall molecular orbital calculations on the square-planar, transition-metal-inserted benzothiophene (BT) complex (dippe)Ni(eta(2)-S,C-BT) (6) (dippe = (i-Pr2PCH2)(2)) and the bimetallic bridged adduct [(dippe)Ni](2)(eta-BT) (7) show that the [(dippe)Ni] fragment coordinates to 6 via a sigma interaction to the sulfur atom and a pi-bonding/back-bonding interaction to the C-C double bond of the heterocycle. Both interactions are weak, and coordination of the second [(dippe)Ni] fragment has little effect on the electronic structure of 6, suggesting that coordination, by itself, does not lead to bond activation. The thermodynamic stability of bimetallic thiophene-bridged adducts is primarily determined by the strength of the interaction between the HOMO of the coordinating metal fragment and the LUMO of the metal-inserted thiophene. Calculations indicate that this interaction is favored in complexes containing earlier, first-row transition metals and T or BT. Our calculational results, along with supporting literature reports, suggest that bimetallic thiophene-bridged complexes may lead to formation of a trimetallic mu(3)-coordinated-sulfur intermediate which could be responsible for the S-C bond activation observed in desulfurization reactions of several bimetallic thiophene-bridged complexes.