Synthesis, characterization, and theoretical study of stable hydride-azavinylidene osmium(IV) complexes

The dihydride-dichloro complex OsH2Cl2((PPr3)-Pr-i)(2) (1) reacts with cyclohexanone oxime in toluene under reflux to give after 12 h OsHCl2{N=C(CH2)(4)CH2 }((PPT3)-P-i)(2) (2), which can be also obtained by reaction of the oximate compound, OsH2Cl{kappa-N,kappa-O[ON=C(CH2)(4)CH2]}-((PP3)-P-i)(2) (3) with HCl. Complex OsHCl2{N=C(CH3)(2)}((PPr3)-Pr-i)(2) (4) has been similarly prepared by treatment of compound OsH2Cl{kappa-N,kappa-O[ON=C(CH3)(2)]}((PPr3)-Pr-i)(2) (5) with HCl. When the reaction of I and cyclohexanone oxime, in toluene under reflux, was quenched after 1 h, a mixture of 1, 2, 3, and the trichloroazavinylidene OsCl3{N=C(CH2)(4)CH2}((PPr3)-Pr-i)(2) (6) was obtained. The structures in the solid state of 2 and 6 have been determined by X-ray diffraction studies. In both cases, the geometry around the metal center can be described as a distorted octahedron with the phosphorus atoms of the phosphines occupying trans positions and the C=N group and the carbon atoms bonded to this group lying in a plane that is parallel to the Cl-Os-Cl plane. CCSD(T)//B3LYP calculations on the model complexes OsXCl2(N=CH2)(PH3)(2) [X = H (2t), X = Cl (6t)] state that the above-mentioned conformation is 15.3 (2t) or 12.1 (6t) kcal mol(-1) more stable than that with the azavinylidene ligand parallel to the P-Os-P plane. In solution the azavinylidene ligands of 2 and 4 rotate around the Os-N-C axis. The activation parameters of the process are Delta H double dagger = 14.4 +/- 0.8 kcal mol(-1) and Delta S double dagger = -1.1 +/- 1.3 cal mol(-1) K-1 for 2 and Delta H double dagger = 13.1 +/- 0.8 kcal mol(-1) and Delta S double dagger = 0.0 +/- 2.8 cal mol(-1) K-1 for 4.