Synthesis, Structure, and Fluxional Behavior of Bis(cyclooctatetraene)ruthenium(0) and Its Derivatives

The complex Ru((1−6η)-C₈H₈)((1−4-η)-C₈H₈) (2), which is made in two steps from Ru(acac)₃, shows three fluxional processes:  (1) equilibration of the protons and carbon atoms of the (1−4-η)-C₈H₈ ring, which appear as a singlet even at −100 °C; (2) role reversal (exchange of hapticity) of the two rings; (3) a 1,5-shift within the (1−6-η)-C₈H₈ ring. Processes 2 and 3 cause the ¹H and ¹³C resonances of C₈H₈ to appear as a broad singlet at room temperature, though the (1−6-η)-ring is static at ca. −60 °C. The free energies of activation ΔG^⧧₂₃₅ for processes 2 and 3 have been estimated from magnetization transfer experiments to be 12.8 ± 0.2 and 13.5 ± 0.2 kcal mol^-1, respectively. Complex 2 reacts with ligands L to give adducts Ru(L)((1−4-η)-C₈H₈)((1,2,5,6-η)-C₈H₈) (L = CO (4), CN-t-Bu (5), P(OMe)₃ (6), PMe₃ (7), and PEt₃ (8)). The (1,2,5,6-η)-rings in these compounds are static at room temperature and do not exchange with the fluxional (1−4-η)-C₈H₈ rings. The (1−4-η)-C₈H₈ ring in 4 undergoes the usual 1,2-shift, the free energy of activation ΔG^⧧₂₅₉ of 13.7 ± 0.1 kcal mol^-1 being considerably higher than that of 2 or of Ru(CO)₃((1−4-η)-C₈H₈). The X-ray structures of 2, 4, 5, and 7 are reported.