Models for Deep Hydrodesulfurization of Alkylated Benzothiophenes. Reductive Cleavage of C−S Bonds Mediated by Precoordination of Manganese Tricarbonyl to the Carbocyclic Ring

The reduction of (η6-benzothiophene)Mn(CO)3 + with cobaltocene under CO leads to insertion of the Mn(CO)4 - fragment into the C(aryl)−S bond to afford a neutral bimetallic complex. When there are methyl substituents at the 2,3-, 2,7-, or 3,7-positions, the regioselectivity and product distribution in the reductive insertion reactions are significantly affected, and several new types of complexes are formed. Low-temperature chemical reduction of (η6-benzothiophene)Mn(CO)3 + complexes affords unstable η4-species, (η4-BT)Mn(CO)3 -, which do not react with the η6-cation to give insertion products. Similarly, electrochemical reduction of (η6-benzothiophene)Mn(CO)3 + changes from a one-electron chemically irreversible process at room temperature to a two-electron chemically reversible but electrochemically irreversible process at −90 °C. It is concluded from these results that the room-temperature reductive cleavage of a C−S bond in (η6-benzothiophene)Mn(CO)3 + occurs by a radical mechanism. Crystallographic data and density function theory (DFT) calculations indicate that insertion into the C(aryl)−S and not the C(vinyl)−S bond of (η6-benzothiophene)Mn(CO)3 + is favored. A redetermination of the crystal structure of the η1-S complex (η1-3-MeBT)Re(Cp*)(CO)2 revealed that the C(aryl)−S and C(vinyl)−S bonds are of similar lengths, suggesting that an η1-S intermediate is not predisposed to insert into the latter bond, as was previously thought. DFT calculations of (η6-BT)Mn(CO)3 + bonded in an η1-S fashion to Mn(CO)4 - indicated that an intermediate of this sort is viable in the C−S insertion reactions.