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

Abstract

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 <i>trans,trans</i>-isomer of the bimetallic complex [(PONOP)ReCl₂]₂(μ-N₂) as an isolable kinetic product that isomerizes sequentially upon heating into the <i>trans,cis</i> and <i>cis,cis</i> isomers. All isomers are inert to thermal N₂ scission, and the <i>trans,trans</i>-isomer is also inert to photolytic N₂ cleavage. In striking contrast, illumination of the <i>trans</i>,<i>cis</i> and <i>cis,cis</i>-isomers with blue light (405 nm) affords the octahedral nitride complex <i>cis</i>-(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.