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Dinitrogen Reduction to Ammonium at Rhenium Utilizing Light and Proton-Coupled Electron Transfer

preprint
submitted on 13.09.2019, 03:04 and posted on 16.09.2019, 15:26 by Quinton J. Bruch, Gannon Connor, Chun-Hsing Chen, Patrick L. Holland, James Mayer, Faraj Hasanayn, Alexander Miller

The direct scission of the triple bond of dinitrogen (N2) by a metal complex is an alluring entry point into the transformation of N2 to ammonia (NH3) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N2 to NH3 via a well-defined reaction sequence involving reductive formation of a bridging N2 complex, photolytic N2 splitting, and proton-coupled electron transfer (PCET) reduction of the metal-nitride bond. The new complex (PONOP)ReCl3 (PONOP = 2,6- bis(diisopropylphosphinito)pyridine) is reduced under N2 to afford the trans,trans-isomer of the bimetallic complex [(PONOP)ReCl2]2(μ-N2) as an isolable kinetic product that isomerizes sequentially upon heating into the trans,cis and cis,cis isomers. All isomers are inert to thermal N2 scission, and thetrans,trans-isomer is also inert to photolytic N2 cleavage. In striking contrast, illumination of the trans,cisand cis,cis-isomers with blue light affords the octahedral nitride complex cis-(PONOP)Re(N)Cl2 in 47% spectroscopic yield and 11% quantum efficiency. The photon energy drives an N2 splitting reaction that is thermodynamically unfavorable under standard conditions, producing a nitrido complex that reacts with SmI2/H2O to produce a rhenium tetrahydride complex and furnish ammonia in 74% yield.

Funding

Collaborative Research: INFEWS N/P/H2O: Electrochemical Approaches to Sustainable Dinitrogen Fixation

Directorate for Mathematical & Physical Sciences

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Collaborative Research: INFEWS N/P/H2O: Electrochemical Approaches to Sustainable Dinitrogen Fixation

Directorate for Mathematical & Physical Sciences

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Graduate research Fellowship Program (GRFP)

Directorate for Education & Human Resources

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MRI: Acquisition of a Mass Spectrometer

Directorate for Mathematical & Physical Sciences

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Alliance for Molecular PhotoElectrode Design for Solar Fuels (AMPED)

Basic Energy Sciences

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History

Email Address of Submitting Author

ajmm@email.unc.edu

Institution

University of North Carolina at Chapel Hill

Country

United States

ORCID For Submitting Author

0000-0001-9390-3951

Declaration of Conflict of Interest

No conflict of interest

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