Preparation and Fluxional Behavior of μ-Acetylene-Hexakis(trifluorophosphine)Dirhodium Complexes

A. Bennett, R. N. Johnson, T. W. Turney

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Acetylenes react with octakis(trifluorophosphine)dirhodium, Rh2(PF3)8, to give a series of binuclear complexes Rh2-(PF3)6(RC2R′) which have been characterized by elemental analysis, 1H and 19F NMR spectroscopy, and mass spectrometry. Like the well-known Co2(CO)6(RC2R′) complexes, the rhodium compounds contain a bridging acetylene ligand lying over and perpendicular to a metal-metal bond. The phenylacetylene derivative (R = C6H5, R′ = H) catalyzes the linear polymerization of phenylacetylene, unlike the corresponding cobalt carbonyl complex, which catalyzes cyclotrimerization. Variable-temperature 19F NMR studies show that the PF3 groups of Rh2(PF3)6(RC2R′) undergo intramolecular exchange leading to NMR equivalence at room temperature. At low temperatures the 19F resonances appear in a 2:1 ratio when R = R′ and a 1:1:1 ratio when R≠R′, 31P noise decoupling has been used to simplify the spectra, and the apparatus for removing the large 31P-19F coupling (∼1400 Hz) is described. The free energy of activation (ΔG2021;) is of the order 6-10 kcal/mol, being higher for electron-withdrawing and for sterically hindered acetylenes. Possible mechanisms for the exchange process are discussed.

Original languageEnglish
Pages (from-to)90-97
Number of pages8
JournalInorganic Chemistry
Issue number1
Publication statusPublished - 1 Jan 1976
Externally publishedYes

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