Synthesis and characterization of OsX{NH=C(Ph)C6H4}H2(PiPr3)2 (X = H, Cl, Br, I):: Nature of the H2 unit and its behavior in solution

The hexahydride OsH6((PPr3)-Pr-i)(2) (1) reacts with benzophenone imine to give the trihydride derivative OsH3{NH=C(Ph)C6H4}((PPr3)-Pr-i)(2) (2) The three hydride ligands and the bidentate group of 2 are situated in the equatorial plane of a pentagonal-bipyramidal arrangement of ligands around the metallic center. In solution, two thermally activated exchange processes take place between these hydride ligands, one of them faster than the other one. The reaction of 2 with HCl leads to OsH3Cl(NH=CPh2)((PPr3)-Pr-i)(2) (3), which evolves in solution into the elongated dihydrogen compound OsCl{NH=C(Ph)C6H4}(eta(2)-H-2)((PPr3)-Pr-i)(2) (4). Complex 4 and the related compounds OsX{NH=C(Ph)C6H4}(eta(2)-H-2)((PPr3)-Pr-i)(2) (X = Br (5), I (6)) can be also prepared by protonation of 2 with HBF4. OEt2 in dichloromethane and subsequent treatment with NaX (X = Cl, Br, I). The structure of 4 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted octahedron, with the triisopropylphosphine ligands occupying two relative trans positions. The remaining perpendicular plane is formed by the mutually cis disposed chloro and dihydrogen ligands and the metalated benzophenone imine group, which has a bite angle of 75.1(1)degrees. The H-2 unit of 4-6 shows a restricted rotational motion in solution. Thus, the H-1 NMR spectra in the high-field region are a function of the temperature. Lowering the sample temperature leads to a broadening of the dihydrogen resonances. At 213 K, decoalescence occurs, and at 193 K, two signals are clearly observed. Theoretical calculations suggest that the transition states for the hydrogen exchanges in 2 and 4-6 present dihydrogen-like nature.