SYNTHESIS, REACTIVITY, MOLECULAR-STRUCTURE, AND CATALYTIC ACTIVITY OF THE NOVEL DICHLORODIHYDRIDOOSMIUM(IV) COMPLEXES OSH2CL2(P-I-PR3) OSH2CL2(PME-TERT-BU2)

The six-coordinate diamagnetic osmium(IV) complexes OsH2Cl2(PR3)2[PR3 = P-i-Pr3 (2), PMe-t-Bu2(3)] are prepared from OsCl3.xH2O and PR3 in boiling 2-propanol in ca. 80% yield. Treatment of 3 with CO leads to reductive elimination of H-2 and formation of all,trans-OsCl2(CO)2(PMe-t-Bu2)2 (4). Likewise, 3 reacts with excess PMe3 to give cis-OsCl2(PMe3)4 (5). The crystal and molecular structure of 2 has been determined. Crystal data for 2: monoclinic, P2(1)/c, a = 12.2791 (6) angstrom, b = 8.5700 (3) angstrom, c = 23.5061 (9) angstrom, beta = 103.526 (4)-degrees, Z = 4, R = 0.022, R(w) = 0.024 based on 3705 observed unique reflections. The coordination polyhedron around the six-coordinate osmium atom is described as a somewhat distorted variant of the D(4d) square antiprism with two vacant coordination sites in alternate positions at one square base of this polyhedron. The reaction of 4 with LiAlH4 in THF leads to the formation of all,trans-OsH2(CO)2(PMe-t-Bu2)2 (6), whereas treatment of 2 and 3 with NaBH4 in methanol/benzene affords the hexahydrido complexes OsH6(PR3)2 in ca. 60% yield. These hexahydrides are probably catalyst precursors for hydrogen transfer from 2-propanol to ketones such as cyclohexanone, methylcyclohexanones, and acetophenone catalyzed by 2 and 3 in presence of NaBH4. Compound 2 catalyzes the hydrogenation of olefins and dienes such as 1,5- and 1,3-C8H12 at considerable initial rates. Benzylideneacetone and benzylideneacetophenone are also reduced with 2 as catalyst to the saturated ketones with high selectivity.