Stoichiometric and Catalytic Deuteration of Pyridine and Methylpyridines by H/D Exchange with Benzene-<i>d</i>₆ Promoted by an Unsaturated Osmium Tetrahydride Species

Treatment of toluene solutions of the hexahydride complex OsH6(PiPr3)2 (1) with pyridine, 3-methylpyridine, and 4-methylpyridine affords the tetrahydride derivatives OsH4(Rpy)(PiPr3)2 (Rpy = py (3), 3-methylpyridine (4), and 4-methylpyridine (5)). In benzene-d6, these compounds release the heterocycles, and the resulting unsaturated tetrahydride OsH4(PiPr3)2 (2) promotes the stoichiometric and catalytic deuteration of pyridine, 3-methylpyridine, and 4-methylpyridine by means of H/D exchanges between the heterocycles and the solvent. The deuteration rates of the pyridinic C−H bonds depend upon their positions in the heterocycles. For pyridine, they increase as the C−H bonds are separated from the heteroatom. A methyl substituent has a marked negative effect on the deuteration of its adjacent C−H bonds. The kinetic analysis of the deuteration reveals that the rate-determining step for the H/D exchanges is the C−H activation of the bond that is deuterated. DFT calculations show that this step is formed by two elemental stages: the direct coordination of the C−H bond and its subsequent rupture.