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 fo rmation of the zirconocene ketimide [Cp2Zr(Cl)N=C(t-Bu)CH3] (3; Cp= cyclope ntadienyl) 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 orbit al calculations (at the HF/LanL2DZ level). These studies, in combination wi th 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)interacti on. This bonding mode is consistent with the short Zr-N bond length of 2.00 7(2) Angstrom 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). H -1 NMR spectroscopic experiments reveal that addition of the Lewis acid B(C 6F5)(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 eta (1)-ketimide cation [Cp2ZrN =C(t-Bu)CH3](+) over either the eta (1)-azaallyl isomer [Cp2Zr{eta (1)-N(H) C(t-Bu)CH2}](+) or the eta (3)-azaallyl isomer [Cp2Zr{eta (3)-N(H)C(t-Bu)CH 2}](+) (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 combina tion with a MAO cocatalyst. In addition, 4 polymerizes ethylene in combinat ion with a mixed B(C6F5)(3)/i-Bu3Al cocatalyst.