Evaluation of excited state bond weakening for ammonia synthesis from a manganese nitride: Stepwise proton coupled electron transfer is preferred over hydrogen atom transfer

Concepts for the thermodynamically challenging synthesis of weak N-H bonds by photoinduced proton coupled electron transfer are explored. Upon irradiation with blue light, ammonia synthesis was achieved from the manganese nitride (^tBuSalen)MnN (^tBuSalen = (S,S)-(+)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine) in the presence of 9,10-dihydroacridine and a ruthenium photocatalyst in ⁱPrOH solution. Although in one case the ruthenium complex bears a remote N-H bond that weakens to 41 kcal mol^-1 upon irradiation, control experiments with the N-methylated analog demonstrate the ruthenium complex serves as a photoreductant rather than hydrogen-atom transfer catalyst in aprotic solvents. Luminescence quenching experiments support a ruthenium(ii)/(iii) cycle rather than a ruthenium(i)/(ii) alternative. Identification of the manganese complex following ammonia synthesis was also accomplished.