Dehydrogenation of a Coordinated Alkylphosphine as a Method to Prepare Cyclopentadienyl-α- alkenylphosphine-osmium Complexes

Abstract

The π-alkyne complex Os(η5-C5H5)Cl(η2-PhC⋮CPh)(PiPr3) (2) has been prepared by a two-step procedure involving the oxidative addition of H2 to Os(η5-C5H5)Cl(PiPr3)2 (1) and the subsequent reaction of the resulting dihydride with diphenylacetylene. In methanol, complex 2 evolves to give the isopropenyldi(isopropyl)phosphine derivative Os(η5-C5H5)Cl{[η2-CH2C(CH3)]PiPr2} (4) and Z-stilbene, by hydrogen transfer from one isopropyl group of the triisopropylphosphine to diphenylacetylene. When the hydrogen transfer reaction is carried out in the presence of KPF6, the cationic Z-stilbene compound [Os(η5-C5H5){η2-(Z)-PhCHCHPh}{[η2-CH2C(CH3)]PiPr2}]PF6 (5) is formed. The isopropenyldi(isopropyl)phosphine ligand of 4 and 5 shows hemilabile properties. The hemilabile character of the isopropenyl substituent of the phosphine of 4 is revealed by the reaction of this complex with H2, which yields an equilibrium mixture of the starting compound and the transoid-dihydride OsH2(η5-C5H5)Cl{PiPr2[C(CH3)CH2]} (6). The hemilabile character of the phosphine of 5 is shown in the reaction of this compound with MeLi, which affords OsH(η5-C5H4CH3){η2-(Z)-PhCHCHPh}{PiPr2[C(CH3)CH2]} (7). In the solid state and in solution the Z-stilbene ligand of 7 isomerizes to give OsH(η5-C5H4CH3){η2-(E)-PhCHCHPh}{PiPr2[C(CH3)CH2]} (8). Complexes 5 and 8 have been characterized by X-ray diffraction analysis.