Organic chemistry of dinuclear metal centres. Part 1. Combination of alkynes with carbon monoxide at di-iron and diruthenium centres: crystal structure of [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2Ph2}(η-C5H5)2]
Abstract
Under u.v. radiation a variety of alkynes (HC2H, MeC2Me, PhC2Ph, MeO2CC2CO2Me, MeC2H, PhC2H, and PhC2Me) reacts with [Fe2(CO)4(η-C5H5)2] to form complexes [Fe2(CO)(µ-CO){µ-σ : η3-C(O)C2R2}(η-C5H5)2] in 10–90% yields. Only PhC2Ph produces an analogous complex with [Ru2(CO)4(η-C5H5)2], but [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2Ph2}(η-C5H5)2] undergoes alkyne exchange on heating in toluene with HC2H, MeC2Me, MeC2H, PhC2H, or PhC2Me to afford the appropriate [Ru2(CO)(µ-CO){µ-σ:η3-C(O)C2R2}(η-C5H5)2] in near quantitative yield. The linking of alkyne and CO to produce a dimetallacyclopentenone ring was established through an X-ray diffraction study of the title compound. Crystals are orthorhombic, space group Pbca, with Z= 8 in a unit cell of dimensions a= 14.797,(3), b= 17.805(8), and c= 16.739(8)Å. The structure was solved by heavy-atom methods and refined by least squares to R 0.033 for 3 726 diffractometer-measured reflection intensities. The molecule contains a dimetallacyclopentenone ring in which the ethylenic bond is η2-bound to ruthenium, so that the bridging C(O)C(Ph)C(Ph) ligand is σ-co-ordinated to one ruthenium and η3-co-ordinated to the other. Compounds [M2(CO)(µ-CO){µ-σ : η3-C(O)C(R′)C(R2)}(η-C5H5)2](M = Fe or Ru) in which R1≠ R2 exist as isomers as a result of linking of either end of the alkyne with CO. Steric factors appear to determine the relative stability of the isomers when one of R is H, but electronic factors are influential when neither is H. The dimetallacyclopentenones are fluxional, undergoing synchronous carbonyl ‘insertion’ into, and elimination from, the dimetallacycle. Free energies of activation appear dependent upon the size of the ‘alkyne’ substituents. In boiling toluene, carbonyl elimination from the dimetallacycle in [Fe2(CO)(µ-CO){µ-σ : η3-C(O)C2(CO2Me)2}(η-C5H5)2] becomes irreversible, and the dimetallacyclobutene complex [Fe2(CO)2(µ-CO){µ-C2(CO2Me)2}(η-C5H5)2] is formed quantitatively as cis and trans isomers. Only cis-[Ru2(CO)2(µ-CO){µ-C2(CO2Me)2}(η-C5H5)2] is generated when [Ru2(CO)(µ-CO)(µ-σ : η3-C(O)C2Ph2}(η-C5H5)2] is heated with MeO2CC2CO2Me. The ease with which carbon–carbon bond-making and -breaking occurs at the di-iron and diruthenium centres is recognised.
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