Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring p K Effects and Demonstrating Electrocatalysis.

Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N, CO, and O is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, PFe, for catalyzing the nitrogen reduction reaction (NRR, N-to-NH conversion) versus HER changes as a function of acid p K. We find that there is a strong correlation between p K and NRR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N reduction (e.g., Fe(NNH) or Fe(NNH)) in the presence of the metallocene reductant CpCo. This suggests that the interaction of acid and reductant is playing a critical role in N-H bond-forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N-H bonds critical for NRR. Furthermore, DFT studies also suggest that the observed p K effect on NRR efficiency is attributable to the rate and thermodynamics of CpCo protonation by the different anilinium acids. Inclusion of Cp*Co as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv of NH per Fe at -2.1 V vs Fc).