Photodriven Ammonia Synthesis from Manganese Nitrides: Photophysics and Mechanistic Investigations

Ammonia synthesis from N,N,O,O-supported manganese(V) nitrides and 9,10-dihydroacridine using proton-coupled electron transfer and visible light irradiation in the absence of precious metal photocatalysts is described. While the reactivity of the nitride correlated with increased absorption of blue light, excited-state lifetimes determined by transient absorption were on the order of picoseconds. This eliminated excited-state manganese nitrides as responsible for bimolecular N-H bond formation. Spectroscopic measurements on the hydrogen source, dihydroacridine, demonstrated that photooxidation of 9,10-dihydroacridine was necessary for productive ammonia synthesis. Transient absorption and pulse radiolysis data for dihydroacridine provided evidence for the presence of intermediates with weak E-H bonds, including the dihydroacridinium radical cation and both isomers of the monohydroacridine radical, but notably these intermediates were unreactive toward hydrogen atom transfer and net N-H bond formation. Additional optimization of the reaction conditions using higher photon flux resulted in higher rates of the ammonia production from the manganese(V) nitrides due to increased activation of the dihydroacridine.