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 H-2 activation and hydrogen-atom transfer is described. The phenylimine-substituted rhodium complex (eta(5)-C5Me5)Rh-((PhI)-Ph-Me)H ((PhI)-Ph-Me = N-methyl-l-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (eta(5)-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 (eta(5) C5Me5)Rh((PhI)-Ph-Me)H and 51.1 kcal mol(-1) for (eta(5)-C5Me5)Rh(ppy)H. In the presence of 10 mol% of the phenylimine rhodium precatalyst and 4 atm of H-2 in THF, the manganese nitride ((tBu)Salen)Mn N underwent hydrogenation to liberate free ammonia with up to 6 total turnovers of NH3 or 18 turnovers of H-center dot 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.