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

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Abstract
The reaction of C 6 H 10 (NLiSiMe 3 ) 2 -1,2 (Li 2 L 1 ) with [ZrCl 4 (thf) 2 ] (thf = tetrahydrofuran) afforded the tetraamide [ZrL 1 2 ] 1. Similarly treatment of (RHNSiMe 2 ) 2 O (H 2 L 2 , R = Bu t ; H 2 L 3 , R = cyclohexyl) with LiBu n followed by [ZrCl 4 (thf) 2 ] led to [ZrL 2 2 ] 2 and [ZrL 3 2 ] 3, respectively. Reaction of Zr(CH 2 Ph) 4 with H 2 L 3 gave the pale yellow zirconium dibenzyl compound [Zr(CH 2 Ph) 2 L 3 ] 4, while the analogous reaction with H 2 L 4 (R = quinolin-8-yl) led to ruby-red [Zr(CH 2 Ph) 2 L 4 ] 5. In addition the bis(pyrrole) [(2-C 4 H 3 NH)CHNCH 2 ] 2 (H 2 L 5 ) reacted with Zr(CH 2 Ph) 4 giving the complex [Zr(CH 2 Ph) 2 L 5 ] 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 L 3 and L 4 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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