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 (NH3) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N-2 to NH3 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)ReCl3 (PONOP = 2,6-bis(diisopropylphosphinito)pyridine) is reduced under N-2 to afford the trans,trans-isomer of the bimetallic complex RPONOP)ReCl2](2)(mu-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 SmI2/H2O to produce a rhenium tetrahydride complex (38% yield) and furnish ammonia in 74% yield.