The mechanism of fluxionality of [Ru(η6-C8H8)(η4-C7H8)], [Ru(η4-C8H8)(η4-C7H8)L][L = PEt3 or P(OMe)3], and [Os(η6-C8H8)(η4-C8H12)](C8H8= cyclo-octatetraene, C7H8= norborna-2,5-diene, and C8H12= cyclo-octa-1,5-diene)

Abstract

Magnetization transfer measurements have been applied to determining the mechanism of fluxionality of [Ru(η⁶-C₈H₈)(η⁴-C₇H₈)](C₈H₈= cyclo-octatetraene, C₇H₈= norborna-2,5-diene) and [Os(η⁶-C₈H₈)(η⁴-C₈H₁₂)](C₈H₁₂= cyclo-octa-1,5-diene) in [²H₈] toluene. Contrary to a previous report on the ruthenium complex, which was based on lineshape analysis, the dominant mechanism of cyclo-octatetraene fluxionality for both complexes is not a random shift, or a [1,3]-shift, but is a [1,5]-shift, with [1,3]-shifts occurring at a slower rate. In addition, the osmium complex shows two lower-energy processes consistent with a staggered relative arrangement of the two polyene ligands. The lowest energy process is an oscillation of the two ligands producing an apparent plane of symmetry, and the higher-energy process is the complete rotation of the two ligands. It appears that the ruthenium complex behaves similarly, but the activation energy for rotation is lower, and only signal broadening is observed at low temperature. It is also shown that the metal shifts in [Ru(η⁴-C₈H₈)(η⁴-C₇H₈)L][L = PEt₃ or P(OMe)₃] are [1,2]-shifts.