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

Abstract

The reactions of the bis(solvento) complexes [OsCp(MeCN) 2 (P i Pr 3 )]PF 6 ( 1; Cp = cyclopentadienyl) and [OsTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 ( 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(η 2 -CH 2 ═C═CRMe)(MeCN)(P i Pr 3 )]PF 6 (R = SiMe 3 ( 2 ), Me ( 3 )). In fluorobenzene at 80 °C, complexes 2 and 3 are moderately stable and evolve into the isopropenyldiisopropylphosphine derivative [OsCp{κ 3 P, C, C -P i Pr 2 [C(Me)═CH 2 ]}(MeCN)]PF 6 ( 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(η 2 -CH 2 ═CH 2 ){κ 3 P, C, C -P i Pr 2 [C(Me)═CH 2 ]}]PF 6 ( 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{η 2 -CH(CO 2 Me)═CH(CO 2 Me)}{κ 3 P, C, C -P i Pr 2 [C(Me)═CH 2 ]PF 6 ( 6 ). The reaction of 1a with 1,1-dimethylallene leads to [OsTp(η 2 -CH 2 ═C═CMe 2 )(κ 1 -OCMe 2 )(P i Pr 3 )]BF 4 ( 7 ). In contrast to its Cp counterpart, complex 7 evolves into the hydride−alkenylcarbyne derivative [OsHTp(≡CCH═CMe 2 )(P i Pr 3 )]BF 4 ( 8 ), by means of a double migration of the hydrogen atoms of the terminal CH 2 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)═CH 2 }(P i Pr 3 ) ( 9 ). The X-ray structures of 2, 5, 8, and 9 are also reported.