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 N₂, CO₂, and O₂ 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, P₃^BFe⁺, for catalyzing the nitrogen reduction reaction (N₂RR, N₂-to-NH₃ conversion) versus HER changes as a function of acid pK_a. We find that there is a strong correlation between pK_a and N₂RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N₂ reduction (e.g., Fe(NNH) or Fe(NNH₂)) in the presence of the metallocene reductant Cp∗₂Co. 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 N₂RR. Furthermore, DFT studies also suggest that the observed pK_a effect on N₂RR efficiency is attributable to the rate and thermodynamics of Cp∗₂Co protonation by the different anilinium acids. Inclusion of Cp∗₂Co⁺ as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH₃. 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 NH₃ per Fe at -2.1 V vs Fc^+/0).