Alkyne adducts of ditungsten tetrapivalate:: [W2(κ2-O2CtBu)4(μ-RCCR′)2], where R = R′ = Me, Et, Ph and R = Me, R′ = Ph

Aromatic hydrocarbon solutions of [W-2((O2CBu)-Bu-t)(4)] react with alkynes RC=CR' to give adducts of the formula [W-2(kappa(2)-(O2CBu)-Bu-t)(4)(mu-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 Angstrom and a W-W bond of ca. 2.5 Angstrom. The crystallographically characterized molecules have virtual C-2h 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 Delta/Lambda isomers of an edge-bridged octahedral unit containing four chelates: [(chelate)(2)M(mu-X)(2)M(chelate)(2)], where X represents the central position of the mu-perpendicular alkyne. The isomerization in solution was found to be intramolecular and solvent-assisted, with THF-d(8) > benzene-d(6) approximate to toluene-d(8) > CD2Cl2, and can be catalyzed by the addition of (BuCO2H)-Bu-t. The electronic structure and bonding in these compounds have been investigated using density functional theory calculations on the model compounds D-2 and C-2h [W-2(kappa(2)-O2CH)(4)(mu-HCCH)(2)]. These calculational results lend credence to the notion that there is a W-W bond of order 2.