Zirconocene Ketimides: Synthesis, Structural Characterization, Ethylene Polymerization Activity, and ab Initio Computational Studies

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) Å 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 η1-ketimide cation
[Cp2ZrNC(t-Bu)CH3]+ over either the η1-azaallyl isomer [Cp2Zr{η1-N(H)C(t-Bu)CH2}]+ or
the η3-azaallyl isomer [Cp2Zr{η3-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.