Photodriven Ammonia Synthesis from Manganese Nitrides: Photo-physics 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 a precious metal photocatalyst 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. The transient absorption and pulse radiolysis data for dihy-droacridine provided evidence for the presence of intermediates with weak E–H bonds, including the dihydroacridinium radical cation and both isomers of monohydroacridine radical. Additional optimization of the reaction conditions using catalytic amounts of acridine, resulted in higher rates of the ammonia production from the manganese(V) nitrides due to introduction of a photooxidant.