SYNTHESIS AND CHARACTERIZATION OF OSX(NH=C(PH)C6H4)H-2((PPR3)-PR-I)(2) (X = H, CL, BR, I) - NATURE OF THE H-2 UNIT AND ITS BEHAVIOR IN SOLUTION

The hexahydride OsH6((PPr3)-Pr-i)(2) (1) reacts with benzophenone imin e 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 situat ed in the equatorial plane of a pentagonal-bipyramidal arrangement of ligands around the metallic center. In solution, two thermally activat ed exchange processes take place between these hydride ligands, one of them faster than the other one. The reaction of 2 with HCl leads to O sH3Cl(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)-P r-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 p rotonation of 2 with HBF4. OEt2 in dichloromethane and subsequent trea tment 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 desc ribed as a distorted octahedron, with the triisopropylphosphine ligand s occupying two relative trans positions. The remaining perpendicular plane is formed by the mutually cis disposed chloro and dihydrogen lig ands and the metalated benzophenone imine group, which has a bite angl e 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 regio n are a function of the temperature. Lowering the sample temperature l eads to a broadening of the dihydrogen resonances. At 213 K, decoalesc ence occurs, and at 193 K, two signals are clearly observed. Theoretic al calculations suggest that the transition states for the hydrogen ex changes in 2 and 4-6 present dihydrogen-like nature.