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Published March 22, 2017 | Supplemental Material + Published
Journal Article Open

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


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][BArF₄]). 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.

Additional Information

© 2017 American Chemical Society. ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Received: January 9, 2017. Publication Date (Web): February 14, 2017. This work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant No. ACI-1053575. M.J.C., T.J.D.C., and B.D.M. acknowledge the support of the NSF for Graduate Fellowships (GRFP). Author Contributions: M.J.C., T.J.D.C., and B.D.M. contributed equally to this work. The authors declare no competing financial interest.

Attached Files

Published - acscentsci.7b00014.pdf

Supplemental Material - oc7b00014_si_001.pdf

Supplemental Material - oc7b00014_si_002.mol


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