SYNTHESIS, MOLECULAR-STRUCTURE, AND REACTIVITY OF OCTAHEDRAL ALKYLHYDRIDOOSMIUM(II) COMPLEXES [OSH(R)(CO)2(PR'3)2]

In contrast to the reaction of [OsH(eta-1-BH4)(CO)(PR'3)2] (2a,b) with methanol under reflux, which gives the dihydrides cis,cis,trans-[OsH2(CO)2(PR'3)2] (4a,b), the corresponding reaction with ethanol or 2-methoxyethanol under the same conditions leads to the formation of the alkylhydridoosmium(II) complexes [OsH(R)(CO)2(PR'3)2] (5a,b (R = CH3), 6a,b (R = MeOCH2)) in good yields. The X-ray structural analysis of 5a reveals an octahedral coordination sphere around the osmium center with the CO ligands in cis and the phosphines in trans positions. Reactions of 5a,b with electrophiles preferentially leads to cleavage of the Os-CH3 bond; thus, on treatment with HX (X = Cl, CH3CO2, CF3CO2) the monohydrides [OsHX(CO)2(PR'3)2] (11, 12, 13a,b) are formed. Protonation of 5a with HBF4 in ether/acetone yields quantitatively the cationic hydrido complex [OsH(acetone)(CO)2(PiPr3)2]BF4 (15) whereas from 5a,b and HBF4 in the presence of water the compounds [OsH(H2O)(CO)2(PR'3)2]BF4 (16a,b) are obtained. Complex 15 reacts with acetonitrile, trimethyl phosphite, or pyrazole by displacement of the acetone ligand to give the compounds [OsH(L)(CO)2(PiPr3)2]BF4 (17-19). Subsequent reaction of 19 (L = pyrazole) with the dimers [M(mu-OMe) (diolefin) ] 2 (20, 22, M = Rh; 2 1, M = Ir) produces the heterobinuclear complexes 23-25; in these the metal centers (Os and Rh or Ir) are bridged by a hydride and a pyrazolyl group. Treatment of 15 with methyl vinyl ketone and CO2Me-substituted alkynes RC=CO2Me gives cationic four- and five-membered metallacycles 26-29 which are formed by a Markovnikov or an anti-Markovnikov type of insertion of the unsaturated substrate into the Os-H bond.