Fluxional η^3-Allyl Derivatives of ansa-Scandocenes and an ansa-Yttrocene. Measurements of the Barriers for the η^3 to η^1 Process as an Indicator of Olefin Binding Energy to d^0 Metallocenes

Variable-temperature ^1H 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 η^3 to η^1 change in coordination. Activation barriers to olefin dissociation have been determined for (η^5-C_5Me_5)_2Sc(η^3-C^3H^5), meso-Me^2Si(η^5-3-CMe^3-C_5H_3)_2Sc(η^3-C_3H_5), meso-Me_2Si[η^5-2,4-(CHMe_2)_2-C_5H_2]_2Sc(η^3-C_3H_5), meso-Me_2Si{η^5-3-[2-(2-Me)-adamantyl]-C_5H_3}_2Sc(η^3-C_3H_5), meso-Me_2Si{η^5-3-[2-(2-Me)-adamantyl]-C_5H_3}_2Y(η^3-C_3H_5), rac-Me_2Si[η^5-2,4-(CHMe_2)_2-C_5H_2]_2Sc(η^3-C_3H_5)), and R-(C_(20)H_(12)O_2)Si(η^5-2-SiMe_3-4-CMe_3-C_5H_2)_2Sc(η^3-C_3H_5):  ΔG^⧧ = 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° rotation of the η^3-C_3H_5 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 η^3-C_3H_5 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-Me_2Si[η^5-C_5H_2-2,4-(CHMe_2)_2]_2Sc(η^3-C_3H_5) has been carried out.