Mer, fac, and Bidentate Coordination of an Alkyl-POP Ligand in the Chemistry of Nonclassical Osmium Hydrides.

Abstract

Nonclassical and classical osmium polyhydrides containing the diphosphine 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(PiPr2)2), coordinated in κ3-mer, κ3-fac, and κ2-P,P fashions, have been isolated during the cyclic formation of H2 by means of the sequential addition of H+ and H- or H- and H+ to the classical trihydride OsH3Cl{xant(PiPr2)2} (1). This complex adds H+ to form the compressed dihydride dihydrogen complex [OsCl(H···H)(η2-H2){xant(PiPr2)2}]+ (2). Under argon, cation 2 loses H2 and the resulting unsaturated fragment dimerizes to give [(Os(H···H){xant(PiPr2)2})2(μ-Cl)2]2+ (3). During the transformation the phosphine changes its coordination mode from mer to fac. The benzofuran counterpart of 1, OsH3Cl{dbf(PiPr2)2} (4; dbf(PiPr2)2 = 4,6-bis(diisopropylphosphino)dibenzofuran), also adds H+ to afford the benzofuran counterpart of 2, [OsCl(H···H)(η2-H2){xant(PiPr2)2}]+ (5), which in contrast to the latter is stable and does not dimerize. Acetonitrile breaks the chloride bridge of 3 to form the dihydrogen [OsCl(η2-H2)(CH3CN){xant(PiPr2)2}]+ (6), regenerating the mer coordination of the diphosphine. The hydride ion also breaks the chloride bridge of 3. The addition of KH to 3 leads to 1, closing a cycle for the formation of H2. Complex 1 reacts with a second hydride ion to give OsH4{xant(PiPr2)2} (7) as consequence of the displacement of the chloride. Similarly to the latter, the oxygen atom of the mer-coordinated diphosphine of 7 has a tendency to be displaced by the hydride ion. Thus, the addition of KH to 7 yields [OsH5{xant(PiPr2)2}]- (8), containing a κ2-P,P-diphosphine. Complex 8 is easily protonated to afford OsH6{xant(PiPr2)2} (9), which releases H2 to regenerate 7, closing a second cycle for the formation of molecular hydrogen.