Half-Sandwich Group 4 Salicyloxazoline Catalysts

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 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.