Synthesis, structure, and fluxional behavior of bis(cyclooctatetraene)ruthenium(0) and its derivatives

The complex Ru((1-6 eta)-C8H8)((1-4-eta)-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-eta)-C8H8 ring, which appear as a singlet even at -100 degrees C; (2) role reversal (exchange of hapticity) of the two rings; (3) a 1,5-shift within the (1-6-eta)-C8H8 ring. Processes 2 and 3 cause the H-1 and C-13 resonances of C8H8 to appear as a broad singlet at room temperature, though the (1-6-eta)-ring is static at ca. -60 degrees C. The free energies of activation Delta G(235)(double dagger) 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-eta)-C8H8)((1,2,5,6-eta)-C8H8) (L = CO (4), CN-t-Bu (5), P(OMe)(3) (6), PMe3 (7), and PEt3 (8)). The (1,2,5,6-eta)-rings in these compounds are static at room temperature and do not exchange with the fluxional (1-4-eta)-C8H8 rings. The (1-4-eta)-C8H8 ring in 4 undergoes the usual 1,2-shift, the free energy of activation Delta G(259)(double dagger) of 13.7 +/- 0.1 kcal mol(-1) being considerably higher than that of 2 or of Ru(CO)(3)((1-4-eta)-C8H8). The X-ray structures of 2, 4, 5, and 7 are reported.