Portal de Periódicos da CAPES

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

1999; American Chemical Society; Volume: 18; Issue: 8 Linguagem: Inglês

10.1021/om980893s

1520-6041

Michael B. Abrams, Jeffrey C. Yoder, Cyrille Loeber, Michael W. Day, John E. Bercaw,

Organoboron and organosilicon chemistry

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 η3 to η1 change in coordination. Activation barriers to olefin dissociation have been determined for (η5-C5Me5)2Sc(η3-C3H5), meso-Me2Si(η5-3-CMe3-C5H3)2Sc(η3-C3H5), meso-Me2Si[η5-2,4-(CHMe2)2-C5H2]2Sc(η3-C3H5), meso-Me2Si{η5-3-[2-(2-Me)-adamantyl]-C5H3}2Sc(η3-C3H5), meso-Me2Si{η5-3-[2-(2-Me)-adamantyl]-C5H3}2Y(η3-C3H5), rac-Me2Si[η5-2,4-(CHMe2)2-C5H2]2Sc(η3-C3H5)), and R-(C20H12O2)Si(η5-2-SiMe3-4-CMe3-C5H2)2Sc(η3-C3H5): 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 η3-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 η3-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[η5-C5H2-2,4-(CHMe2)2]2Sc(η3-C3H5) has been carried out.