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)((PPr3)-Pr-i)]PF6 (1; Cp = cyclopentadienyl) and [OsTp(kappa(1)-OCMe2)(2)((PPr3)-Pr-i)]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 pi-allene derivatives [OsCp(eta(2)-CH2=C=CRMe)(MeCN)((PPr3)-Pr-i)]PF6 (R = SiMe3 (2), Me (3)). In fluorobenzene at 80 degrees C, complexes 2 and 3 are moderately stable and evolve into the isopropenyldiisopropylphosphine derivative[OsCp{kappa P-3,C,C-(PPr2)-Pr-i[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 pi-ethylene derivative [OsCp(eta(2)-CH2=CH2){kappa P-3,C,C-(PPr2)-Pr-i[C(Me)=CH2]}]PF6 (5) is obtained through a one-pot synthesis procedure by the stirring of 3 in fluorobenzene at 80 degrees C under 2 atm of ethylene. Treatment of 2 and 3 with dimethyl acetylenedicarboxylate gives [OsCp{eta(2)-CH(CO2Me)=CH(CO2Me)}{(KP)-P-3,C,C-(PPr2)-Pr-i[C(Me)=CH2]PF6 (6). The reaction of la with 1,1-dimethylallene leads to [OsTp(eta(2)-CH2=C=CMe2)(kappa(1)-OCMe2)((PPr3)-Pr-i)]BF4 (7). In contrast to its Cp counterpart, complex 7 evolves into the hydride-alkenylcarbyne derivative [OsHTp(equivalent to CCH=CMe2)((PPr3)-Pr-i)BF4 (8), by means of a double migration of the hydrogen atoms of the terminal CH, 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}((PPr3)-Pr-i) (9). The X-ray structures of 2, 5, 8, and 9 are also reported.