Fluxional η3-allyl derivatives of ansa-scandocenes and an ansa-yttrocene.: Measurements of the barriers for the η3 to η1 process as an indicator of olefin binding energy to d0 metallocenes

Variable-temperature H-1 NMR spectroscopy indicates fluxional behavior for a number of group 3 metallocene allyl complexes. Spectral simulations and line shape analyses for the variable-temperature spectra indicate an allyl rearrangement mechanism involving rate-determining carbon-carbon double-bond dissociation from the metal center, i.e. an eta(3) to eta(1) change in coordination. Activation barriers to olefin dissociation have been determined for (eta(5)-C5Me5)(2)Sc(eta(3)-C3H5), meso-Me2Si(eta(5)-3-CMe3-C5H3)(2)Sc(eta(3)-C3H5), meso-Me2Si[eta(5)-2,4-(CHMe2)(2)C5H2](2)Sc(eta(3)-C3H5), meso-Me2Si{eta(5)-3-[2-(2-Me)-adamantyl]-C5H3}(2)Sc(eta(3)-C3H5), meso-Me2Si{eta(5)-3-[2-(2-Me)-adamantyl]-C5H3}(2)Y(eta(3)-C3H5), rac-Me2Si[eta(5)-2,4-(CHMe2)(2)-C5H2](2)Sc(eta(3)-C3H5)), and R-(C20H12O2)Si(eta(5)-2-SiMe3-4-CMe3-C5H2)(2)Sc(eta(3)-C3H5): Delta G double dagger = 11-16 kcal mol(-1) at ca. 300-350 K. Donor solvents do not significantly affect the rate of olefin dissociation. A second rearrangement mechanism that involves 180 degrees rotation of the eta(3)-C3H5 moiety has been found to operate in those metallocenes whose ancillary ligand arrays adopt rigid meso geometries. Line shape analysis indicates that the rate of eta(3)-C3H5 rotation is generally,more than 1 order of magnitude faster than olefin dissociation for a given meso metallocene. The data do not allow unambiguous assessments of the mechanism(s) for the fluxional behavior for the allyl derivatives of the racemic metallocenes. An X-ray structure determination for rac-Me2Si[eta(5)-C5H2-2,4-(CHMe2)(2)](2)Sc(eta(3)-C3H5 has been carried out.