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

Nonclassical and classical osmium polyhydrides containing the diphosphine 9,9-dimethyl-4,5-bis-(diisopropylphosphino)xanthene (xant((PPr2)-Pr-i)(2)), coordinated in kappa(3)-mer, kappa(3)-fac, and kappa(2)-P,P fashions, have been isolated during the cyclic formation of H-2 by means of the sequential addition of H+ and H or H and H+ to the classical trihydride OsH3Cl{xant((PPr2)-Pr-i)(2)} (1). This complex adds H+ to form the compressed dihydride dihydrogen complex [OsCl(HH)(eta(2)-H-2){xant((PPr2)-Pr-i)(2)}](+) (2). Under argon, cation 2 loses H-2 and the resulting unsaturated fragment dimerizes to give [(Os(HH){xant((PPr2)-Pr-i)(2)})(2)(mu-Cl)(2)](2+) (3). During the transformation the phosphine changes its coordination mode from mer to fac. The benzofuran counterpart of 1, OsH3Cl{dbf((PPr2)-Pr-i)(2)} (4; dbf((PPr2)-Pr-i)(2) = 4,6-bis(diisopropylphosphino)dibenzofuran), also adds H+ to afford the benzofuran counterpart of 2, [OsCl(HH)(eta(2)-H-2){xant((PPr2)-Pr-i)(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(eta(2)-H-2)(CH3CN){xant((PPr2)-Pr-i)(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 H-2. Complex 1 reacts with a second hydride ion to give OsH4{xant((PPr2)-Pr-i)(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((PPr2)-Pr-i)(2)}](-) (8), containing a kappa(2)-P,P-diphosphine. Complex 8 is easily protonated to afford OsH6{xant((PPr2)-Pr-i)(2)} (9), which releases H-2 to regenerate 7, closing a second cycle for the formation of molecular hydrogen.