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

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 (ΔG^2021;) 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.