Does a multiply bonded oxo ligand directly participate in B-H bond activation by a high-valent di-oxo-molybdenum(VI) complex? A density functional theory study

The reduction of organic substrates using high-valent oxo-transition metal complexes represents a new catalytic activity. In this study, we theoretically investigated the mechanism of catalytic reduction of amides, amines, nitriles and sulfoxides with boranes by the high-valent di-oxo-molybdenum(VI) complex MoO2Cl2. Our computational results reveal that reduction of sulfoxides with boranes catalyzed by MoO2Cl2 proceeds via a [2 + 2] addition pathway involving the B-H bond of borane adding across the Mo=O bond to form a metal hydride intermediate, followed by the elimination of the new species HOBcat, accompanied by the loss of the sulfide. The activation free energy of the turnover-limiting step is calculated to be 24.0 kcal mol(-1). By contrast, borane additions to either amide, amine or nitrile proceed through an ionic outersphere mechanism, in which the substrates attack the boron center to prompt the heterolytic cleavage of the B-H bond, generating an anionic molybdenum(VI) hydride paired with a borylated amide/amine/nitrile cation. Then, the activated organic substrates abstract a hydride from the molybdenum(VI) center to complete the catalytic cycle. The activation free energies of the turnover-limiting step along the ionic outersphere pathway are calculated to be similar to 22.7, 19.7 and 30.6 kcal mol(-1) for benzamide, N-(iphenylmethylene) benzenamine, and benzonitrile, respectively. These values are energetically more favorable (similar to 3-8.0 kcal mol(-1)) than those via the [2 + 2] addition pathway. Along the ionic outer-sphere pathway, the multiply bonded oxo ligand does not participate in the activation of the B-H bond. The ionic outer-sphere mechanism suggests that the high-valent di-oxo-molybdenum(VI) complex MoO2Cl2 acts as a Lewis acid in catalyzing the reduction reaction and activation of B-H bonds.