Stereochemically Nonrigid Organometallic Molecules. XXIV. Preparation and Nuclear Magnetic Resonance Study of Some Fluxional Cyclooctatetraene Derivatives of Ruthenium Carbonyl

The preparation and characterization of four fluxional organometallic molecules derived from the reaction of Ru₃(CO)₁₂ with cyclooctatetraene are reported. The molecules are C₈H₈Ru(CO)₃, C₈H₈Ru₂(CO)₆, C₈H₈-Ru₂(CO)₅, and (C₈H₈)₂Ru3(CO)₄. The limiting low-temperature pmr spectrum of C₈H₈Ru(CO)₃ has been observed at about –140° and shows that the instantaneous structure is of the 1,2,3,4-tetrahapto type. The qualitative nature of the line-shape changes at intermediate temperatures shows that rearrangement takes place mainly by 1,2 shifts. Detailed analysis using computer-simulated spectra confirms this and enables us to set an upper limit of about 30% on the occurrence of 1,3 or other shift processes. The Arrhenius activation energy is 9.4 ± 0.5 kcal/mol. For C₈H₈Ru₂(CO)₆ the pmr spectrum has been recorded down to –120°. The limiting low-temperature spectrum confirms that the dissymmetric structure previously observed (Cotton and Edwards) in the crystalline substance persists as the instantaneous structure in solution. Spin-decoupling experiments on the room-temperature pmr spectrum show that a time-average mirror plane exists. These results, along with the changes observed in the spectra at intermediate temperatures, are interpreted in terms of an oscillation of the (OC)₃Ru-Ru(CO)₃ group relative to the C₈H₈ ring such that the enantiomers are rapidly interconverted at room temperature. C₈H₈Ru₂-(CO)₆ readily loses CO at higher temperatures giving C₈H₈Ru₂(CO)₅, This is a fluxional molecule (single, sharp proton line at 25°) presumably isostructural with C₈H₈Fe₂(CO)₅. Because of its very low solubility at lower temperatures efforts to observe a limiting low-temperature spectrum have failed. The preparation and characterization of (C₈H₈)₂Ru₃(CO)₄ are fully described. This fluxional molecule has also failed to yield a low-temperature spectrum because of a severe solubility problem.