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

The π-alkyne complex Os(η 5 -C 5 H 5 )Cl(η 2 -PhC⋮CPh)(P i Pr 3 ) ( 2 ) has been prepared by a two-step procedure involving the oxidative addition of H 2 to Os(η 5 -C 5 H 5 )Cl(P i Pr 3 ) 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(η 5 -C 5 H 5 )Cl{[η 2 -CH 2 C(CH 3 )]P i Pr 2 } ( 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 KPF 6, the cationic Z -stilbene compound [Os(η 5 -C 5 H 5 ){η 2 -( Z )-PhCH CHPh}{[η 2 -CH 2 C(CH 3 )]P i Pr 2 }]PF 6 ( 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 OsH 2 (η 5 -C 5 H 5 )Cl{P i Pr 2 [C(CH 3 ) CH 2 ]} ( 6 ). The hemilabile character of the phosphine of 5 is shown in the reaction of this compound with MeLi, which affords OsH(η 5 -C 5 H 4 CH 3 ){η 2 -( Z )-PhCH CHPh}{P i Pr 2 [C(CH 3 ) CH 2 ]} ( 7 ). In the solid state and in solution the Z -stilbene ligand of 7 isomerizes to give OsH(η 5 -C 5 H 4 CH 3 ){η 2 -( E )-PhCH CHPh}{P i Pr 2 [C(CH 3 ) CH 2 ]} ( 8 ). Complexes 5 and 8 have been characterized by X-ray diffraction analysis.