Alkyne to carbyne coupling reactions of the unsaturated methoxycarbyne-bridged complex [Mo2(η5-C5H5)2(μ-COMe)(μ-PCy2)(CO)2]

The 32-electron complex [Mo2Cp2(μ-COMe)(μ-PCy2)(CO)2] (Cp = η5-C5H5) reacts at 333 K with terminal alkynes HCtriple bond; length of mdashCR to give the corresponding derivatives [Mo2Cp2{μ-κ2:η3-CRCHC(OMe)}(μ-PCy2)(CO)2], (R = p-tol, CO2Me), with good yields. These compounds are both obtained selectively as trans-dicarbonyl isomers and display a propenyl-ylidene type ligand bridging the dimetal centre, derived from the specific coupling of the alkyne to the carbyne ligand through the terminal carbon atom of the alkyne. The reaction with the internal alkyne (MeO2C)Ctriple bond; length of mdashC(CO2Me) gives the related derivative [Mo2Cp2{μ-κ2:η3-C(CO2Me)C(CO2Me)C(OMe)}(μ-PCy2)(CO)2], but now this product is obtained as an equimolar mixture of the corresponding trans- and cis-dicarbonyl isomers. The structures of the new compounds are discussed on the basis of the available spectroscopic data in solution and an X-ray diffraction analysis of the product derived from p-tolylacetylene (Mo−Mo = 2.8646(8) Å).