No abstract

Octahedral titanium and zirconium complexes based on salicyloxazoline ligands with sterically demanding ortho-substituents provide a new family of extremely active ethene polymerization catalysts [up to 10(8) g PE (mol bar h)(-1)] which are in some cases "single site".

The structural properties of three new classes of titanium and zirconium complex bearing tetradentate salicylaldiminato proligands are elucidated using X-ray diffraction and NMR spectroscopy. On activation with MAO co-catalyst, their behaviour in ethene polymerization depends strongly on the nature of the structure and the substitution pattern. One titanium complex based on a 2-aminobenzylamine (C3-chain) backbone has a trans arrangement of the co-ligand sites and, unsurprisingly, does not polymerize ethene. The 1,8-diaminonaphthalene (C3-chain) backbone gives a rather ring-strained cis complex, but was also unproductive. A range of cis complexes of zirconium with the 2,2'-diaminobibenzyl (C6-chain) backbone give low to moderate productivities of multimodal poly(ethene), while in contrast the structurally analogous titanium compounds provide highly active, single site catalysts. Thermal degradation of these catalysts is slowed significantly by a substitution pattern on the phenolate unit which sterically protects the imine donor unit; a phenomenon which has been previously observed in much lower activity catalysts based on 2,2'-diaminobiphenyl (C4-chain) but which does not improve the stability of the very highly active unbridged systems.

DFT calculations indicate that contrary to a prior report, N-heterocycle-augmented constrained geometry ligands e.g. Cp'-SiMe(2)-NR (Cp' = Me(4)C(5), R = 2-pyridine) should be capable of binding both atoms of the diazaallyl fragment at zirconium. This was confirmed by a molecular structure of a previously reported complex. Similar R = 2-oxazoline complexes were also shown to be feasible, although an additional N,O binding mode was accessible. The proligands HCp'-SiMe(2)-NHR (R = chiral non-racemic 2-oxazoline) were readily synthesised in high yield via base mediated reaction of 2-aminooxazolines and Cp'-SiMe(2)Cl. Subsequent reaction with Zr(NMe(2))(4) gave, rather than the desired complexes, configurationally stable chiral-at-zirconum guanidinate/alkoxide chelate products; the aminooxazolinate units had undergone ring-opening and migratory insertion of -NMe(2). Trends in the level diastereoselection follow the steric demand of the oxazoline substituent, with the larger groups ((t)Bu, (i)Pr) giving single diastereomers. The modest performance of these guanidinate compounds in enantioslective catalytic cyclohydroamination of aminoalkenes follows the expected trends for metal accessibility in a sigma-amido insertative mechanism.