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 irridiation 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-cyclohexane¬di-amine) 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 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.