Abstract
Nitrogen (N(2)) fixation to produce bio-available ammonia (NH(3)) is essential to all life but is a challenging transformation to catalyse owing to the chemical inertness of N(2). Transition metals can, however, bind N(2) and activate it for functionalization. Significant opportunities remain in developing robust and efficient transition metal catalysts for the N(2) reduction reaction (N(2)RR). One opportunity to target in catalyst design concerns the stabilization of transition metal diazenido species (M-NNH) that result from the first N(2) 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(N-H) of a given M-NNH species to the observed overpotential and selectivity for N(2)RR 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 N(2)RR efficiency.