Cluster-mediated alkenyl isomerism and carbon–carbon bond formation: The reaction of the unsaturated benzothiazole cluster [Os3(CO)9(μ3-C7H4NS)(μ-H)] with dimethyl acetylenedicarboxylate

Upon mild heating (60–65 °C), electron-deficient [Os3(CO)9(μ3-C7H4NS)(μ-H)] (1) readily adds to the activated alkyne, RCtriple bond; length of mdashCR (R = CO2Me), to give two isomers of the alkenyl complexes, [Os3(CO)9(μ-C7H4NS)(μ3-RCCHR)] (2 and 3), differing in the orientation of the benzothiazole ligand. In both the alkenyl ligand acts as a five-electron donor binding through both carbons and one of the oxygen atoms of the substituents, which adopt a relative cis disposition. Heating 2 cleanly affords 3 suggesting that the former is the kinetic product and the latter is thermodynamically favoured. At higher temperatures (110 °C), 3 rearranges to a third isomer [Os3(CO)9(μ-C7H4NS)(μ-RCCHR)] (4) which differs in the binding mode of the alkenyl ligand and where the alkyne substituents are in a relative trans disposition. A second product of this reaction is [Os3(CO)8(μ-OCH3){μ3-C7H4NSC(R)C}(μ-H)] (5) which results from the loss of two moles of CO, carbon–carbon coupling between hydrocarbyl and benzothiazole ligands and carbon–hydrogen addition to the triosmium centre. All new clusters are characterized by a total of 50 valence electrons and contain two metal–metal bonds as shown by single crystal X-ray diffraction studies.