Sulfur dioxide activation: a theoretical investigation into dual S═O bond cleavage by three-coordinate molybdenum(III) complexes.

Cummins et al. have observed that 3 equiv of Mo(N[R]Ar)3 (R = C(CD3)2CH3, Ar = 3,5-C6H3Me2) are required for dual S═O bond cleavage within a SO2 molecule. Using density functional theory calculations, this theoretical study investigates a mechanism for this SO2 cleavage reaction that is mediated by MoL3, where L = NH2 or N[(t)Bu]Ph. Our results indicate that an electron transfers into the SO2 ligand, which leads to Mo oxidation and initiates SO2 coordination along the quartet surface. The antiferromagnetic (AF) nature of the (NH2)3Mo-SO2 adduct accelerates intersystem crossing onto the doublet surface. The first S═O bond cleavage occurs from the resulting doublet adduct and leads to formation of L3Mo═O and SO. Afterward, the released SO molecule is cleaved by the two remaining MoL3, resulting in formation of L3Mo═S and an additional L3Mo═O. This mononuclear mechanism is calculated to be strongly exothermic and proceeds via a small activation barrier, which is in accordance with experimental results. An additional investigation into a binuclear process for this SO2 cleavage reaction was also evaluated. Our results show that the binuclear mechanism is less favorable than that of the mononuclear mechanism.