Half-Sandwich Group 4 Salicyloxazoline Catalysts

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; ΔG⧧298 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 1H 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 a...