Alkyne Adducts of [W2(OCH2tBu)8]. Cases of Perpendicular and Skewed Bridges in Equilibrium with Terminal Bonded Isomers

Abstract

Alkynes react with a suspension of [W 2 (OCH 2 tBU) 8 ](M=M) in hydrocarbon solvents to give alkyne adducts W 2 (μ-RCCR')(OCH 2 tBu) 8 , where R = H and R' = Ph, Me, Me 3 Si and R = Me and R' = Me and Ph. For PhC≡CH, there is a μ-perpendicular mode of alkyne bonding, θ = 84°, but for MeC≡CH and MeC≡CMe, the bridging alkyne is distinctly skewed with respect to the W-W axis, 0 = 64° and 67°, respectively. In contrast, the PhC≡CMe adduct has the alkyne bonded to only one tungsten, η 2 -PhC≡CMe, with the alkyne C≡C axis being 30° from the W-W axis. In all of these structures, there are alkoxide bridges, and the dinuclear unit may be viewed as a confacial bioctahedron where either one alkyne and two alkoxides or three alkoxides occupy the bridging face. In toluene-d 8 , NMR spectroscopic studies indicate that certain of these alkyne adducts (MeC≡CH, MeC≡CMe, PhC≡CMe, and Me 3 SiC≡CH) exist as a mixture of alkyne-bridged and η 2 -alkyne-bonded isomers. The isomers are fluxional on the NMR time scale by rapid exchange of terminal and bridging alkoxide groups. However, μ-η 2 alkyne exchange is slow on the NMR time scale. The structural data are compared with electronic structure calculations employing density functional theory on model compounds of formula W 2 (alkyne)(OMe) 8 . The calculations suggest that a μ-parallel mode of bonding is energetically favorable relative to u-perpendicular and, further, that the energy difference between bridge and terminal alkyne adducts is, in all cases, less than 5 kcal mol - 1 .