Square-Planar Alkylidyne-Osmium and Five-Coordinate Alkylidene-Osmium Complexes: Controlling the Transformation from Hydride-Alkylidyne to Alkylidene.

Square-planar alkylidyne and five-coordinate alkylidene mixed (i)Pr3P-Os-IPr (IPr = 1,3-bis(diisopropylphenyl)imidazolylidene) complexes have been discovered and characterized, and their formation has been rationalized. The cationic five-coordinate hydride-alkylidyne compounds [OsHX(≡CPh)(IPr)(P(i)Pr3)]OTf (X = Cl (1), F (4); OTf = CF3SO3) undergo deprotonation with KO(t)Bu to afford the trans-halide-alkylidyne square-planar derivatives OsX(≡CPh)(IPr)(P(i)Pr3) (X = Cl (2), F (5)). Oxidative addition of the C(sp)-H bond of phenylacetylene and methyl propiolate along the Cl-Os-CPh axis of 2 with the hydrogen atom directed to the alkylidyne leads to alkynyl-cis-hydride-alkylidyne intermediates, which rapidly evolve into the five-coordinate alkylidene complexes Os(C≡CR)Cl(═CHPh)(IPr)(P(i)Pr3) (R = Ph (6), CO2Me (7)) as a consequence of the migration of the hydride from the metal center to the Cα atom of the alkylidyne. Oxidative addition of the C(sp)-H bond of methyl propiolate along the X-Os-CPh axis of 2 and 5 with the hydrogen atom directed to the halide gives the alkynyl-trans-hydride-alkylidyne derivatives OsH(C≡CCO2Me)X(≡CPh)(IPr)(P(i)Pr3) (X = Cl (8), F (9)). Complex 8 evolves into 7. However, complex 9 containing the stronger π-donor fluoride is stable. The oxidative addition of HCl to 2 selectively yields the cis-hydride-alkylidyne compound OsHCl2(≡CPh)(IPr)(P(i)Pr3) (10), which is also stable.