posted on 2020-05-11, 16:34authored bySangmin Kim, Hongyu Zhong, Yoonsu Park, Florian Loose, Paul J. Chirik
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)MnN 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.