Preparation of Half-Sandwich Osmium-Allyl Complexes by Consecutive C−C Bond Formation and C−H Bond Activation Reactions

The vinylidene complex Os(η5-C5H5)Cl(CCHPh)(PiPr3) (1) reacts with MeMgCl to give the
osmaindene (2), which isomerizes into the exo-allyl compound
Os(η5-C5H5)(η3-CH2CHCHPh)(PiPr3) (3) in refluxing toluene. Treatment of 3 with HBF4·OEt2 leads to
the endo-d4-allyl derivative [OsH(η5-C5H5){η3-CH2CHCHPh}(PiPr3)]BF4 (4), which can be also obtained
by addition of HBF4·OEt2 to 2. Complex 4 contains the terminal CHPh group of the allyl cisoid disposed
to the phosphine. In dichloromethane at 40 °C, it isomerizes into an endo-allyl isomer 5 with the terminal
CHPh group cisoid disposed to the hydride. Complex 1 also reacts with EtMgCl. The reaction affords
(6), which in toluene at 70 °C is converted into the exo-allyl complex Os(η5-C5H5){η3-CH(CH3)CHCHPh}(PiPr3) (7). Treatment of 7 with HBF4·OEt2 leads to
an equilibrium mixture of exo-d4-allyl derivatives of formula [OsH(η5-C5H5){η3-CH(CH3)CHCHPh}(PiPr3)]BF4 (8 and 9). The addition of HBF4·OEt2 to 6 gives an endo-d4-allyl isomer, 10, which is transformed
into the equilibrium mixture of 8 and 9 after 7 days in dichloromethane at 40 °C. Treatment of 1 with
PhMgCl gives rise to (11), which reacts with HBF4·OEt2 in the
presence of acetonitrile to afford stilbene and the solvento complex [Os(η5-C5H5)(CH3CN)2(PiPr3)]BF4
(12). The X-ray structures of 3, 5, and 8 are also reported.