Dehydrogenation of a coordinated alkylphosphine as a method to prepare cyclopentadienyl-α-alkenylphosphine-osmium complexes

The pi-alkyne complex Os(eta(5)-C5H5)Cl(eta(2)-PhCdropCPh)((PPr3)-Pr-i) (2) has been prepared by a two-step procedure involving the oxidative addition of H-2 to Os(eta(5)-C5H5)Cl((PPr3)-Pr-i)(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(eta(5)-C5H5)Cl{[eta(2)-CH2=C(CH3)](PPr2)-Pr-i} (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(eta(5)-C5H5){eta(2)-(Z)-PhCH=CHPh}{[eta(2)-CH2=C(CH3)](PPr2)-Pr-i}]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 H-2, which yields an equilibrium mixture of the starting compound and the transoid-dihydride OsH2(eta(5)-C5H5)Cl{(PPr2)-Pr-i[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(eta(5)-C5H4CH3){eta(2)-(Z)-PhCH=CHPh}{(PPr2)-Pr-i[C(CH3)=CH2]} (7). In the solid state and in solution the Z-stilbene ligand of 7 isomerizes to give OsH(eta(5)-C5H4CH3){eta(2)-(E)-PhCH=CHPh}{(PPr2)-Pr-i[C(CH3)=CH2]} (8). Complexes 5 and 8 have been characterized by X-ray diffraction analysis.