Ammonium Synthesis

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.

In work published in the Journal of American Chemical Society, Quinton Bruch of the Miller Group, led a collaborative team of colleagues from Yale University and the American University in Beirut, Lebanon, who report a pincer-ligated rhenium system that, for the first time, 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.