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

• Published in: Journal of the American Chemical Society Vol. 141; no. 51; pp. 20198 - 20208
• Main Authors: Bruch, Quinton J, Connor, Gannon P, Chen, Chun-Hsing, Holland, Patrick L, Mayer, James M, Hasanayn, Faraj, Miller, Alexander J. M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 26.12.2019; American Chemical Society (ACS)
• Subjects: ammonia; ammonium; blue light; catalytic activity; chemical bonding; cleavage (chemistry); crystal cleavage; electron transfer; energy; heat; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; isomers; ligands; lighting; molecular structure; nitrides; nitrogen; photons; pyridines; redox reactions; rhenium; spectroscopy
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.9b10031

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 the trans,trans-isomer is also inert to photolytic N2 cleavage. In striking contrast, illumination of the trans,cis and cis,cis-isomers with blue light (405 nm) affords the octahedral nitride complex cis-(PONOP)­Re­(N)­Cl2 in 47% spectroscopic yield and 11% quantum yield. 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 (38% yield) and furnish ammonia in 74% yield.