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

 http://pubs.acs.org/doi/full/10.1021/acscentsci.7b00014

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

Matthew J. Chalkley†, Trevor J. Del Castillo†, Benjamin D. Matson†, Joseph P. Roddy, and Jonas C. Peters^*

Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech), Pasadena, California 91125, United States

ACS Cent. Sci., Article ASAP

DOI: 10.1021/acscentsci.7b00014

Publication Date (Web): February 14, 2017

A synthetic Fe complex catalyzes nitrogen fixation at lower driving force increasing its relevance as a functional model of nitrogenase. Theory and experiment lead to the proposal that protonated cobaltocenes may play a role as PCET reagents.

• 
  

Abstract

We have recently reported on several Fe catalysts for N₂-to-NH₃ conversion that operate at low temperature (−78 °C) and atmospheric pressure while relying on a very strong reductant (KC₈) and acid ([H(OEt₂)₂][BAr^F₄]). Here we show that our original catalyst system, P₃^BFe, achieves both significantly improved efficiency for NH₃ formation (up to 72% for e^– delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH₃ per Fe site), when employing a significantly weaker combination of reductant (Cp*₂Co) and acid ([Ph₂NH₂][OTf] or [PhNH₃][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.