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

The direct scission of the triple bond of dinitrogen (N) by a metal complex is an alluring entry point into the transformation of N to ammonia (NH) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N to NH via a well-defined reaction sequence involving reductive formation of a bridging N complex, photolytic N splitting, and proton-coupled electron transfer (PCET) reduction of the metal-nitride bond. The new complex (PONOP)ReCl (PONOP = 2,6-bis(diisopropylphosphinito)pyridine) is reduced under N to afford the -isomer of the bimetallic complex [(PONOP)ReCl](μ-N) as an isolable kinetic product that isomerizes sequentially upon heating into the and isomers. All isomers are inert to thermal N scission, and the -isomer is also inert to photolytic N cleavage. In striking contrast, illumination of the , and -isomers with blue light (405 nm) affords the octahedral nitride complex -(PONOP)Re(N)Cl in 47% spectroscopic yield and 11% quantum yield. The photon energy drives an N splitting reaction that is thermodynamically unfavorable under standard conditions, producing a nitrido complex that reacts with SmI/HO to produce a rhenium tetrahydride complex (38% yield) and furnish ammonia in 74% yield.