N Reduction versus H Evolution in a Molybdenum- or Tungsten-Based Small-Molecule Model System of Nitrogenase.

Molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase. In comparison, analogous tungsten complexes have in most cases found to be catalytically inactive. Herein, a tungsten complex was shown to be supported by a pentadentate tetrapodal (pentaPod) phosphine ligand, under conditions of N fixation, primarily catalyzes the hydrogen evolution reaction (HER), in contrast to its Mo analogue, which catalytically mediates the nitrogen-reduction reaction (N RR). DFT calculations were employed to evaluate possible mechanisms and identify the most likely pathways of N RR and HER activities exhibited by Mo- and W-pentaPod complexes. Two mechanisms for N RR by PCET are considered, starting from neutral (M(0) cycle) and cationic (M(I) cycle) dinitrogen complexes (M=Mo, W). The latter was found to be energetically more favorable. For HER three scenarios are treated; that is, through bimolecular reactions of early M-N H intermediates, pure hydride intermediates or mixed M(H)(N H ) species.