Kinetic and mechanistic investigation of the sequential hydrogenation of phenylacetylene catalyzed by OsHCl(CO)(PR3)2 [PR3 = PMe-tert-Bu2 and P-i-Pr3]

The reactivities of the hydrido carbonyl complexes OsHCl(CO)(PR3)2 (PR3 = PMe-/-Bu2 (1), P-i-Pr3 (2)) toward hydrogen, alkynes, and oxygen have been studied. The solutions of 1 and 2 are rapidly decolorized upon contact with H2 under ambient conditions; the decolorized solution of 2 shows in benzene-d6 a NMR spectrum that is consistent with the formation of the dihydrogen compound OsHC1(?/-H2) (CO) (P-z-Pr3)2 (3). The reactivity of 1 and 2 toward alkynes depends on the type of alkyne used. The title complexes react with acetylene, propyne, and phenylacetylene by insertion to give the five-coordinate vinylosmium compounds Os(CH=CHR)Cl(CO)(PR3)2 (7 and 8); the same starting materials in the presence of /-BuC=CH and PhC=CPh are completely inert. Treatment of 2 with Me02CC=CC02Me leads to the compound Os[C-(=CHC02Me)C(0Me)=0]Cl(C0)(P-i-Pr3)2; the trans- and c/5-alkyne hydrido intermediates have been observed. The hydridoosmium(II) complexes 1 and 2 also react with 02 to form the dioxygen adducts 0sHCl(t;2-02)(C0)(PR3)2 (11 and 12). The complexes 1 and 2 catalyze the sequential hydrogenation of phenylacetylene in 2-propanol solution at 60 °C. Selectivities close to 100% are achieved for the hydrogenation of the alkyne to the alkene. The kinetic investigation of this reaction provides evidence that indicates that the formation of styryl derivatives is the step that determines the selectivity for the hydrogenation to the alkene.