We have recently reported on several Fe catalysts for N_2-to-NH_3 conversion that operate at low temperature (−78 °C) and atmospheric pressure while relying on a very strong reductant (KC_8) and acid ([H(OEt_2)_2][BArF_4]). Here we show that our original catalyst system, P_3^BFe, achieves both significantly improved efficiency for NH_3 formation (up to 72% for e^– delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH_3 per Fe site), when employing a significantly weaker combination of reductant (Cp*_2Co) and acid ([Ph_2NH_2][OTf] or [PhNH_3][OTf]). Relative to the previously reported catalysis, freeze-quench Mössbauer spectroscopy under turnover conditions suggests a change in the rate of key elementary steps; formation of a previously characterized off-path borohydrido–hydrido resting state is also suppressed. Theoretical and experimental studies are presented that highlight the possibility of protonated metallocenes as discrete PCET reagents under the present (and related) catalytic conditions, offering a plausible rationale for the increased efficiency at reduced driving force of this Fe catalyst system
