N<sub>2</sub> Reduction versus H<sub>2</sub> Evolution in a Molybdenum‐ or Tungsten‐Based Small‐Molecule Model System of Nitrogenase
Jannik Junge, Tobias A. Engesser, Felix Tuczek
Abstract
Abstract 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 2 fixation, primarily catalyzes the hydrogen evolution reaction (HER), in contrast to its Mo analogue, which catalytically mediates the nitrogen‐reduction reaction (N 2 RR). DFT calculations were employed to evaluate possible mechanisms and identify the most likely pathways of N 2 RR and HER activities exhibited by Mo‐ and W‐pentaPod complexes. Two mechanisms for N 2 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 x H y intermediates, pure hydride intermediates or mixed M(H)(N x H y ) species.