Photodriven Ammonia\nSynthesis from Manganese Nitrides:\nPhotophysics and Mechanistic Investigations

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