Synthesis, spectroscopic characterization, and reactivity of the unusual five-coordinate hydrido-vinylidene complex OsHCl(C=CHPh)(PiPr3)2: Precursor for dioxygen activation

The hydrido−carbyne complex OsHCl2(CCH2Ph)(PiPr3)2 (1) reacts with sodium methoxide in 1:1 molar ratio to give the five-coordinate hydrido−vinylidene OsHCl(CCHPh)(PiPr3)2 (2), which affords [OsHCl(CCH2Ph)(H2O)(PiPr3)2]BF4 (3) by reaction with HBF4·H2O. The spectroscopic data obtained for 2 indicate that in solution, it is a mixture of two conformers in equilibrium. The thermodynamic magnitudes involved in the equilibrium as well as the activation parameters for the conversion between them were determined by 1H NMR spectroscopy in toluene-d8. The values obtained were ΔH° = −0.7 (±0.1) kcal mol-1, ΔS° = −2.3 (±0.6) cal K-1 mol-1 , ΔH⧧ = 11.0 (±0.2) kcal mol-1 and ΔS⧧ = 3.2 (±0.6) cal K-1 mol-1. At room temperature under argon, complex 2 is stable in the solid state and in solution. However in the presence of air, it activates molecular oxygen to give the dioxo−styryl compound OsCl{(E)-CHCHPh}(O)2(PiPr3)2 (4), which has been characterized by X-ray diffraction analysis. The geometry around of the osmium atom can be described as a distorted octahedron with the two oxygen atoms occupying two relative trans positions (O(1)−Os−O(2) = 179.5(5)°). Complex 2 also reacts with trimethylphosphite, sodium acetylacetonato (acetylacetonato = acac), and sodium acetato to give the six-coordinate hydrido−vinylidene derivatives OsHCl(CCHPh){(P(OMe)3}(PiPr3)2 (8), OsH(acac)(CCHPh)(PiPr3)2 (9), and OsH(η2-O2CCH3)(CCHPh)(PiPr3)2 (10), respectively. Complex 10 can also be prepared by treatment of OsH3(η2-O2CCH3)(PiPr3)2 (11) with phenylacetylene. Similarly, the reactions of 11 with 1,1-diphenyl-2-propyn-1-ol and 1-ethynyl-1-cyclohexanol lead to OsH (η2-O2CCH3){CCHC(OH)Ph2}(PiPr3)2 (12) and OsH(η2-O2CCH3){CCHCCH (CH2)3CH2}(PiPr3)2 (13), respectively.We thank the D. G. I. C. Y. T. (Project PB95-0806, Programa de Promoción General del Conocimiento) and E. U. (Project: Selective Precesses and Catalysis Involving Small Molecules) for financial support. M.B. thanks the E. U. for a postdoctoral grant.Peer reviewe