Cluster-mediated alkenyl isomerism and carbon–carbon bond formation: The reaction of the unsaturated benzothiazole cluster [Os 3(CO) 9(μ 3-C 7H 4NS)(μ-H)] with dimethyl acetylenedicarboxylate

Abstract

Upon mild heating (60–65 °C), electron-deficient [Os 3(CO) 9(μ 3-C 7H 4NS)(μ-H)] ( 1) readily adds to the activated alkyne, RC CR (R = CO 2Me), to give two isomers of the alkenyl complexes, [Os 3(CO) 9(μ-C 7H 4NS)(μ 3-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 [Os 3(CO) 9(μ-C 7H 4NS)(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 [Os 3(CO) 8(μ-OCH 3){μ 3-C 7H 4NSC(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.