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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

Abrams, Michael B. and Yoder, Jeffrey C. and Loeber, Cyrille and Day, Michael W. and Bercaw, John E. (1999) 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. Organometallics, 18 (8). pp. 1389-1401. ISSN 0276-7333. http://resolver.caltech.edu/CaltechAUTHORS:20180518-145758180

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Abstract

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.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://dx.doi.org/10.1021/om980893sDOIArticle
https://pubs.acs.org/doi/suppl/10.1021/om980893sPublisherSupporting Information
Alternate Title: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
Additional Information:© 1999 American Chemical Society. Received October 30, 1998. This work has been supported by the USDOE Office of Basic Energy Sciences (Grant No. DE-FG03-85ER13431) and by Exxon Chemicals America.
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG03-85ER13431
Exxon Chemicals AmericaUNSPECIFIED
Record Number:CaltechAUTHORS:20180518-145758180
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20180518-145758180
Official Citation: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 Michael B. Abrams, Jeffrey C. Yoder, Cyrille Loeber, Michael W. Day, and John E. Bercaw Organometallics 1999 18 (8), 1389-1401 DOI: 10.1021/om980893s
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:86467
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:21 May 2018 15:38
Last Modified:21 May 2018 15:38

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