Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

have recently reported on several Fe catalysts for N-2-to-NH3 conversion that operate at low temperature (-78 degrees C) and atmospheric pressure.. while relying on a very strong reductant (KC8) and acid ([H(OEt2)(2)][BAr4F]). Here we show that our original catalyst system, (P3Fe)-Fe-B, achieves both significantly improved efficiency for NH3 formation (up to 72% fore delivery) and a comparativelyof NH3 per Fe site), when employing a significantly weaker combination of I high turnover number for a synthetic molecular Fe catalyst (84 equiv reductant (Cp*Co-2) and acid ([Ph2NH2] [OTf] or [PhNH3][OTf]). Relative to the previously reported catalysis, freeze-quench Miissbauer 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.