Hydration of Aliphatic Nitriles Catalyzed by an Osmium Polyhydride: Evidence for an Alternative Mechanism

[Image: see text] The hexahydride OsH(6)(P(i)Pr(3))(2) competently catalyzes the hydration of aliphatic nitriles to amides. The main metal species under the catalytic conditions are the trihydride osmium(IV) amidate derivatives OsH(3){κ(2)-N,O-[HNC(O)R]}(P(i)Pr(3))(2), which have been isolated and fully characterized for R = (i)Pr and (t)Bu. The rate of hydration is proportional to the concentrations of the catalyst precursor, nitrile, and water. When these experimental findings and density functional theory calculations are combined, the mechanism of catalysis has been established. Complexes OsH(3){κ(2)-N,O-[HNC(O)R]}(P(i)Pr(3))(2) dissociate the carbonyl group of the chelate to afford κ(1)-N-amidate derivatives, which coordinate the nitrile. The subsequent attack of an external water molecule to both the C(sp) atom of the nitrile and the N atom of the amidate affords the amide and regenerates the κ(1)-N-amidate catalysts. The attack is concerted and takes place through a cyclic six-membered transition state, which involves C(nitrile)···O–H···N(amidate) interactions. Before the attack, the free carbonyl group of the κ(1)-N-amidate ligand fixes the water molecule in the vicinity of the C(sp) atom of the nitrile.