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, N0-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 hydrogenatom transfer catalyst in aprotic solvents. Luminescence quenching experiments support a ruthenium(II)/(III) cycle rather than a ruthenium(I)/(II) alternative. Identification of themanganese complex following ammonia synthesis was also accomplished.