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Why vanadium complexes perform poorly in comparison to related molybdenum complexes in the catalytic reduction of dinitrogen to ammonia (Schrock cycle): a theoretical study.

Energetics and mechanistic details for the conversion of dinitrogen to ammonia mediated by vanadium triamidoamine has been studied theoretically involving various mechanistic possibilities. For most of the cases, protonation at the amido nitrogen atom is more favorable compared to the terminal one. Further, the most important steps of the mechanism were compared with the well established chemistry of nitrogen fixation mediated by molybdenum. Such a comparison helps in understanding why vanadium triamidoamine complex performs poorly compared to molybdenum. The main factors responsible for the poor performance of the vanadium complex toward NH(3) production are identified as low exergonic cleavage of the N-N bond and limitation of the ligand exchange step via a dissociative mechanism at the end of the cycle to only one possible pathway. A major aspect of the failure of the vanadium complex to mediate the reduction of N(2) to ammonia is the fact that the protonation steps involve major barriers, which cannot be surmounted thermally. Moreover, unlike molybdenum, the associative mechanism with vanadium triamidoamine complex is not likely to operate during the NH(3)/N(2) exchange step.