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

The direct scission of the triple bond of dinitrogen (N 2 ) by a metal complex is an alluring entry point into the transformation of N 2 to ammonia (NH 3 ) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N 2 to NH 3 via a well-defined reaction sequence involving reductive formation of a bridging N 2 complex, photolytic N 2 splitting, and proton-coupled electron transfer (PCET) reduction of the metal–nitride bond. The new complex (PONOP)ReCl 3 (PONOP = 2,6-bis(diisopropylphosphinito)pyridine) is reduced under N 2 to afford the trans,trans -isomer of the bimetallic complex [(PONOP)ReCl 2 ] 2 (μ-N 2 ) as an isolable kinetic product that isomerizes sequentially upon heating into the trans,cis and cis,cis isomers. All isomers are inert to thermal N 2 scission, and the trans,trans -isomer is also inert to photolytic N 2 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)Cl 2 in 47% spectroscopic yield and 11% quantum yield. The photon energy drives an N 2 splitting reaction that is thermodynamically unfavorable under standard conditions, producing a nitrido complex that reacts with SmI 2 /H 2 O to produce a rhenium tetrahydride complex (38% yield) and furnish ammonia in 74% yield.