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

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

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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.