Catalytic NH₃ Synthesis using N₂/H₂ at Molecular Transition Metal Complexes: Concepts for Lead Structure Determination using Computational Chemistry

Authors

Dr. Markus Hölscher^* and Prof. Dr. Walter Leitner

DOI: 10.1002/chem.201604612

http://onlinelibrary.wiley.com/wol1/doi/10.1002/chem.201604612/abstract

Abstract

While industrial NH₃ synthesis based on the Haber–Bosch-process was invented more than a century ago, there is still no molecular catalyst available which reduces N₂ in the reaction system N₂/H₂ to NH₃. As the many efforts of experimentally working research groups to develop a molecular catalyst for NH₃ synthesis from N₂/H₂ have led to a variety of stoichiometric reductions it seems justified to undertake the attempt of systematizing the various approaches of how the N₂ molecule might be reduced to NH₃ with H₂ at a transition metal complex. In this contribution therefore a variety of intuition-based concepts are presented with the intention to show how the problem can be approached. While no claim for completeness is made, these concepts intend to generate a working plan for future research. Beyond this, it is suggested that these concepts should be evaluated with regard to experimental feasibility by checking barrier heights of single reaction steps and also by computation of whole catalytic cycles employing density functional theory (DFT) calculations. This serves as a tool which extends the empirically driven search process and expands it by computed insights which can be used to rationalize the various challenges which must be met.

Monday, February 27, 2017

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.