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

Nitrogen (N2) fixation to produce bio-available ammonia (NH3) is essential to all life but is a challenging transformation to catalyse owing to the chemical inertness of N2. Transition metals can, however, bind N2 and activate it for functionalization. Significant opportunities remain in developing robust and efficient transition metal catalysts for the N2 reduction reaction (N2RR). One opportunity to target in catalyst design concerns the stabilization of transition metal diazenido species (M-NNH) that result from the first N2 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 BDFEN-H of a given M-NNH species to the observed overpotential and selectivity for N2RR 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 N2RR efficiency.