Zirconocene ketimides: Synthesis, structural characterization, ethylene polymerization activity, and ab initio computational studies

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Armstrong, D.R. and Henderson, K.W. and Little, I. and Jenny, C. and Kennedy, A.R. and McKeown, A.E. and Mulvey, R.E. (2000) Zirconocene ketimides: Synthesis, structural characterization, ethylene polymerization activity, and ab initio computational studies. Organometallics, 19 (21). pp. 4369-4375. ISSN 0276-7333 (http://dx.doi.org/10.1021/om000451s)

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Abstract

The metathetical reaction between the lithium 1-azaallyl compound [{HMPA·LiN(H)C(t-Bu)CH2}2] (1) with zirconocene dichloride (2) results in the formation of the zirconocene ketimide [Cp2Zr(Cl)N=C(t-Bu)CH3] (3; Cp = cyclopentadienyl) and lithium chloride. After it is transferred to the transition metal, the azaallyl ligand isomerizes to a ketimido variation. An energetic preference of 11.9 kcal mol-1 in favor of the zirconocene ketimide over its theoretical azaallyl isomer was determined by ab initio molecular orbital calculations (at the HF/LanL2DZ level). These studies, in combination with an X-ray diffraction analysis of 3, suggest that the preference for the ketimide isomer is due to the presence of a heteroallenic (Zr-N-C) interaction. This bonding mode is consistent with the short Zr-N bond length of 2.007(2) î.. found in the crystal structure of 3. Treatment of 3 with MeLi affords the corresponding methyl derivative [Cp2Zr(Me)N=C(t-Bu)CH3] (4). 1H NMR spectroscopic experiments reveal that addition of the Lewis acid B(C6F5)3 to 4 results in methide abstraction, with retention of the ketimide unit on the cationic zirconocene. Ab initio molecular orbital calculations confirmed the energetic preference for the 1-ketimide cation [Cp2ZrN=C(t-Bu)CH3]+ over either the 1-azaallyl isomer [Cp2Zr{1-N(H)C(t-Bu)CH2}]+ or the 3-azaallyl isomer [Cp2Zr{3-N(H)C(t-Bu)CH2}]+ (by 17.9 and 1.2 kcal mol-1, respectively). Both 3 and 4 have been shown to be active catalysts for the polymerization of ethylene in combination with a MAO cocatalyst. In addition, 4 polymerizes ethylene in combination with a mixed B(C6F5)3/i-Bu3Al cocatalyst.

ORCID iDs

Armstrong, D.R., Henderson, K.W., Little, I., Jenny, C., Kennedy, A.R. ORCID logoORCID: https://orcid.org/0000-0003-3652-6015, McKeown, A.E. and Mulvey, R.E. ORCID logoORCID: https://orcid.org/0000-0002-1015-2564;
  • Item type:Article
    ID code:1277
    Dates:
    Date
    Event
    20 September 2000
    Published
    Subjects:Science > Chemistry
    Department:Faculty of Science > Pure and Applied Chemistry
    Depositing user:Users 45 not found.
    Date deposited:23 Jun 2006
    Last modified:11 Nov 2024 08:15
    URI:https://strathprints.strath.ac.uk/id/eprint/1277
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