Formation and Activation of Zr/Hf Bis(phenolate-ether) Precatalysts

Zr and Hf complexes of bis(phenolate-ether) (“O4”) ligands feature high activity, stereoselectivity and molecular weight capability for propene polymerization at high temperature. Here we report a simplified ligand synthesis and several new examples of O4 ligands. The formation of precatalysts LMR2 (M = Zr, Hf; R = Bn, Me) from LH2 and MR4 was found to be accompanied in some cases by the formation of dimers (μ-L)2[MR2]2, and X-ray structures of two such dimers have been determined. Treatment of LMMe2 with [Ph3C]+[B(C6F5)4]– produces fairly clean cationic species LMMe+ which were studied by 1H NMR. 2D ROESY data, in particular, suggest that for “smaller” O4 ligands the LMMe+ cation reversibly rearranges from the active (fac/fac) form to a presumably inactive fac/mer or mer/mer form; more bulky substituents appear to suppress this rearrangement. Implications for polymerization catalysis are discussed.