Alkyne Adducts of Ditungsten Tetrapivalate: [W2(κ2-O2CtBu)4(μ-RCCR‘)2], Where R = R‘ = Me, Et, Ph and R = Me, R‘ = Ph

Abstract

Aromatic hydrocarbon solutions of [W 2 (O 2 C t Bu) 4 ] react with alkynes RC≡CR' to give adducts of the formula [W 2 (κ 2 -O 2 C t Bu) 4 (μ-RCCR') 2 ], where R = R' = Me, Et, Ph and R = Me, R' = Ph, as orange to purple air-stable,hydrocarbon-soluble, crystalline solids which have been characterized by infrared, NMR spectroscopy, and mass spectrometry. The single-crystal X-ray structures have also been obtained for R = R' = Me and R = Me, R' = Ph. In both structures, there are a pair of perpendicularly bridged alkyne ligands with central C-C distances of ca. 1.34 A and a W-W bond of ca. 2.5 A. The crystallographically characterized molecules have virtual C 2 h symmetry, and in solution, the NMR data are consistent with the presence of this isomer and a second having D 2 symmetry. The two isomers can be related to meso and Δ/Λ isomers of an edge-bridged octahedral unit containing four chelates: [(chelate) 2 M(μ-X) 2 M(chelate) 2 ], where X represents the central position of the μ-perpendicular alkyne. The isomerization in solution was found to be intramolecular and solvent-assisted, with THF-d 8 > benzene-d 6 toluene-d 8 > CD 2 Cl 2 , and can be catalyzed by the addition of t BuCO 2 H. The electronic structure and bonding in these compounds have been investigated using density functional theory calculations on the model compounds D 2 and C 2 h [W 2 (κ 2 -O 2 -CH) 4 (μ-HCCH) 2 ]. These calculational results lend credence to the notion that there is a W-W bond of order 2.