Abstract
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)).