Coles, Stuart R., Clarkson, Guy J., Gott, Andrew L., Munslow, Ian J., Spitzmesser, Stefan K. and Scott, Peter. (2006) Half-sandwich group 4 salicyloxazoline catalysts. ORGANOMETALLICS, 25 (26). pp. 6019-6029. ISSN 0276-7333

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Abstract

A new class of zirconium and hafnium half-sandwich complexes bearing Cp* and salicyloxazolinato ( L) ligands has been prepared by salt elimination and protonolysis routes. The analogous Cp and indenyl compounds are generally inaccessible, as are the titanium compounds. The molecular structures of four examples [Cp*MLX2] (variously M = Zr, Hf; X = Cl, Me) reveal chiral-at-metal structures which persist in solution, according to variable-temperature NMR studies; Delta G(298)(double dagger) for the racemization process was found to be ca. 75 kJ mol(-1). Treatment of these compounds with MAO, [Ph3C][B(C6F5)(4)], or [PhNMe2H][B(C6F5)(4)] leads to catalysts for alkene polymerization, the nature of which depends on the cocatalyst chosen. The anilinium salt smoothly produces a single chiral species, [Cp*ZrLMe][B(C6F5)(4)], detected also by H-1 NMR spectroscopy, which is a highly active single-site catalyst for polymerization of ethene ( and less active for copolymerization of ethene/hexene). The trityl activator produces the same catalyst and at least one other catalytically competent species, as evidenced by NMR spectroscopy and polymer modality. The use of MAO leads to a less well-defined catalyst system. The steric demand of the salicyloxazoline ligand affects the catalyst performance significantly, and computational studies show that access of ethene to either of two inequivalent coordination sites is restricted. This stability of the species [Cp*ZrLMe](+) with respect to addition appears to be the limiting factor for catalytic activity. Catalyst stability is addressed, and the steric and electronic factors affecting this are consistent with a mechanism of catalyst death by salicyloxazoline ligand loss.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Journal or Publication Title:	ORGANOMETALLICS
Publisher:	AMER CHEMICAL SOC
ISSN:	0276-7333
Date:	18 December 2006
Volume:	25
Number:	26
Number of Pages:	11
Page Range:	pp. 6019-6029
Identification Number:	10.1021/om060687h
Publication Status:	Published
URI:	http://wrap.warwick.ac.uk/id/eprint/32607

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