Catalytic Hydrogenation of a Manganese(V) Nitride to Ammonia

The catalytic hydrogenation of a metal nitride to produce free ammonia using a rhodium hydride catalyst that promotes H₂ activation and hydrogen-atom transfer is described. The phenyl­imine-substituted rhodium complex (η⁵-C₅Me₅)­Rh­(^MePhI)H (^MePhI = N-methyl-1-phenyl­ethan-1-imine) exhibited higher thermal stability compared to the previously reported (η⁵-C₅Me₅)­Rh­(ppy)H (ppy = 2-phenyl­pyridine). DFT calculations established that the two rhodium complexes have comparable Rh–H bond dissociation free energies of 51.8 kcal mol^–1 for (η⁵-C₅Me₅)­Rh­(^MePhI)H and 51.1 kcal mol^–1 for (η⁵-C₅Me₅)­Rh­(ppy)­H. In the presence of 10 mol% of the phenyl­imine rhodium precatalyst and 4 atm of H₂ in THF, the manganese nitride (^tBuSalen)­MnN underwent hydrogenation to liberate free ammonia with up to 6 total turnovers of NH₃ or 18 turnovers of H^• transfer. The phenyl­pyridine analogue proved inactive for ammonia synthesis under identical conditions owing to competing deleterious hydride transfer chemistry. Subsequent studies showed that the use of a non-polar solvent such as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthesis by proton-coupled electron transfer.