Relating N-H Bond Strengths to the Overpotential for Catalytic Nitrogen Fixation.

Nitrogen (N) fixation to produce bio-available ammonia (NH) is essential to all life but is a challenging transformation to catalyse owing to the chemical inertness of N. Transition metals can, however, bind N and activate it for functionalization. Significant opportunities remain in developing robust and efficient transition metal catalysts for the N reduction reaction (NRR). One opportunity to target in catalyst design concerns the stabilization of transition metal diazenido species (M-NNH) that result from the first N functionalization step. Well-characterized M-NNH species remain very rare, likely a consequence of their low N-H bond dissociation free energies (BDFEs). In this essay, we discuss the relationship between the BDFE of a given M-NNH species to the observed overpotential and selectivity for NRR catalysis with that catalyst system. We note that developing strategies to either increase the N-H BDFEs of M-NNH species, or to avoid M-NNH intermediates altogether, are potential routes to improved NRR efficiency.