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

Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N 2, CO 2, and O 2 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, P 3 B Fe +, for catalyzing the nitrogen reduction reaction (N 2 RR, N 2 -to-NH 3 conversion) versus HER changes as a function of acid p K a . We find that there is a strong correlation between p K a and N 2 RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N 2 reduction (e.g., Fe(NNH) or Fe(NNH 2 )) in the presence of the metallocene reductant Cp* 2 Co. 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 N 2 RR. Furthermore, DFT studies also suggest that the observed p K a effect on N 2 RR efficiency is attributable to the rate and thermodynamics of Cp* 2 Co protonation by the different anilinium acids. Inclusion of Cp* 2 Co + as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH 3 . 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 3 per Fe at −2.1 V vs Fc +/0 ).