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

Substrate selectivity in reductive multi-electron/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. One common strategy to achieve selectivity is to limit the direct reaction between acid and reductant with the intent of slowing background HER. 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. Experimental support for the hypothesis that Cp* 2 Co plays a critical role in P 3 B Fe-catalyzed N 2 RR comes from electrochemical studies. Inclusion of Cp* 2 Co + as a co-catalyst 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 NH 3 per Fe at −2.1 V vs Fc +/0 ). While the electrocatalysis is modest in terms of turnover, the comparatively favorable Faradaic efficiencies for NH 3 (up to 31%) highlight the value of studying molecular N 2 RR catalysts to define design criteria for selective N 2 RR electrocatalysis. Our collective results contribute to a growing body of evidence that metallocenes may play multiple roles during reductive catalysis. While they can behave as single electron transfer (SET) reagents in the reductive protonation of small molecule substrates, ring-functionalized metallocenes, previously considered