Synthesis and structure of zirconium(IV) alkyl complexes with bi-, tri-, tetra- and penta-dentate amido ligands

Abstract

The reaction of C₆H₁₀(NLiSiMe₃)₂-1,2 (Li₂L¹) with [ZrCl₄(thf )₂] (thf = tetrahydrofuran) afforded the tetraamide [ZrL¹₂] 1. Similarly treatment of (RHNSiMe₂)₂O (H₂L², R = Bu^t; H₂L³, R = cyclohexyl) with LiBuⁿ followed by [ZrCl₄(thf )₂] led to [ZrL²₂] 2 and [ZrL³₂] 3, respectively. Reaction of Zr(CH₂Ph)₄ with H₂L³ gave the pale yellow zirconium dibenzyl compound [Zr(CH₂Ph)₂L³] 4, while the analogous reaction with H₂L⁴ (R = quinolin-8-yl) led to ruby-red [Zr(CH₂Ph)₂L⁴] 5. In addition the bis(pyrrole) [(2-C₄H₃NH)CH=NCH₂]₂ (H₂L⁵) reacted with Zr(CH₂Ph)₄ giving the complex [Zr(CH₂Ph)₂L⁵] 6. The crystal structures of 1, 4 and 5 have been determined. Compound 1 has a distorted tetrahedral structure. In 4 and 5 all available donor atoms co-ordinate to zirconium, including the silyl ether moiety, leading to a distorted trigonal bipyramidal structure for 4 and an approximately pentagonal bipyramidal geometry for 5. Although L³ and L⁴ have flexible frameworks, in both 4 and 5 the heteroatom donors and the metal form an essentially coplanar arrangement. The zirconium–amido nitrogen distances proved to be highly variable, depending on the degree of electron deficiency and the co-ordination of the metal centres, and range from an average of 2.056 Å in 1 and 2.096 Å in 4 to 2.169 Å in 5. Complexes 1 and 4 activated with methylaluminoxane gave high molecular weight polyethylene with moderate activity.

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