Coordination and Rupture of Methyl C(sp³)−H Bonds in Osmium−Polyhydride Complexes with δ Agostic Interaction

Abstract

The hexahydride complex OsH6(PiPr3)2 (1) reacts with 8-methylquinoline and 2-(dimethylamino)pyridine to give (2) and (3), respectively, as a result of the release of two hydrogen molecules of 1 and the C(sp3)−H bond activation of a methyl group of the organic substrates. In solution the hydride ligands and the hydrogen atoms of the methylene group of 2 exchange their positions. The activation parameters for the process are ΔH⧧ = 18.9 ± 0.1 kcal·mol-1 and ΔS⧧ = 3 ± 2 eu. Treatment of 2 with HBF4 affords the hydride-elongated dihydrogen derivative (6), with the methyl group of the quinoline ligand coordinated in a η3-H2C fashion. The X-ray structure of 6 and the DFT optimization of the structure of the model cation [OsH(η2-H2)(CH4)(NH3)(PMe3)2]+ prove that the methyl coordination in the δ agostic complex is similar to the methane coordination in the model compound. The reaction of 3 with HBF4 leads to the cyclic carbene derivative (7), as a result of the release of a hydrogen molecule and a C(sp3)−H bond activation on the methylene group of 3. The formation of 2, 3, 6, and 7 has been analyzed by DFT calculations.