C−H Bond Activation Reactions in π-Allene−Osmium−Triisopropylphosphine Complexes with Cyclopentadienyl or Hydridotris(pyrazolyl)borate Ligands: Formation of Isopropenyldiisopropylphosphine versus Hydride−Alkenylcarbyne Derivatives

The reactions of the bis(solvento) complexes [OsCp(MeCN)2(PiPr3)]PF6 (1; Cp = cyclopentadienyl) and [OsTp(κ1-OCMe2)2(PiPr3)]BF4 (1a; Tp = hydridotris(pyrazolyl)borate) with allenes have been investigated. Complex 1 reacts with 1-methyl-1-(trimethylsilyl)allene and 1,1-dimethylallene to give the π-allene derivatives [OsCp(η2-CH2CCRMe)(MeCN)(PiPr3)]PF6 (R = SiMe3 (2), Me (3)). In fluorobenzene at 80 °C, complexes 2 and 3 are moderately stable and evolve into the isopropenyldiisopropylphosphine derivative [OsCp{κ3P,C,C-PiPr2[C(Me)CH2]}(MeCN)]PF6 (4) by hydrogen transfer from an isopropyl substituent of the phosphine to the coordinated double bond of the allene. Under an ethylene atmosphere the acetonitrile ligand of 4 is displaced by the olefin. The resulting π-ethylene derivative [OsCp(η2-CH2CH2){κ3P,C,C-PiPr2[C(Me)CH2]}]PF6 (5) is obtained through a one-pot synthesis procedure by the stirring of 3 in fluorobenzene at 80 °C under 2 atm of ethylene. Treatment of 2 and 3 with dimethyl acetylenedicarboxylate gives [OsCp{η2-CH(CO2Me)CH(CO2Me)}{κ3P,C,C-PiPr2[C(Me)CH2]PF6 (6). The reaction of 1a with 1,1-dimethylallene leads to [OsTp(η2-CH2CCMe2)(κ1-OCMe2)(PiPr3)]BF4 (7). In contrast to its Cp counterpart, complex 7 evolves into the hydride−alkenylcarbyne derivative [OsHTp(CCHCMe2)(PiPr3)]BF4 (8), by means of a double migration of the hydrogen atoms of the terminal CH2 group of the allene. One of them migrates to the central carbon atom of the allene, and the other one goes to the metal center. The alkenylcarbyne group of 8 is selectively deprotonated in the presence of the hydride ligand to afford the hydride−alkenylvinylidene OsHTp{CCHC(Me)CH2}(PiPr3) (9). The X-ray structures of 2, 5, 8, and 9 are also reported.