A density functional study of oxygen atom transfer reactions between biological oxygen atom donors and molybdenum(IV) bis(dithiolene) complexes

Density functional calculations have been used to investigate oxygen atom transfer reactions from the biological oxygen atom donors trimethylamine N-oxide (MeNO) and dimethyl sulfoxide (DMSO) to the molybdenum(IV) complexes [MoO(mnt)] and [Mo(OCH)(mnt)] (mnt = maleonitrile-1,2-dithiolate), which may serve as models for mononuclear molybdenum enzymes of the DMSO reductase family. The reaction between [MoO(mnt)] and trimethylamine N-oxide was found to have an activation energy of 72 kJ/mol and proceed via a transition state (TS) with distorted octahedral geometry, where the MeNO is bound through the oxygen to the molybdenum atom and the N-O bond is considerably weakened. The computational modeling of the reactions between dimethyl sulfoxide (DMSO) and [MoO(mnt)] or [Mo(OCH)(mnt)] indicated that the former is energetically unfavorable while the latter was found to be favorable. The addition of a methyl group to [MoO(mnt)] to form the corresponding des-oxo complex not only lowers the relative energy of the products but also lowers the activation energy. In addition, the reaction with [Mo(OCH)(mnt)] proceeds via a TS with trigonal prismatic geometry instead of the distorted octahedral TS geometry modeled for the reaction between [MoO(mnt)] and MeNO.