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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry