Group 4 Metal Complexes Supported by [ONNO]-Type Bis(<i>o</i>-aminophenolato) Ligands: Synthesis, Structure, and α-Olefin Polymerization Activity

Reaction of 1,4-diazabutanediyl-bridged bis(phenols) [(HOC6H2-4,6-tBu2)2NR(CH2)2NR] (R = H, 1; Me, 2) with the appropriate metal precursors MX4 (M = Ti, Zr, Hf; X = Cl, OiPr) gave a series of group 4 metal dichloro and di(isopropoxy) complexes [M{2,2′-(OC6H2-4,6-tBu2)2NR(CH2)2NR}X2] (R = H, X = Cl, M = Ti, 3; R = Me, X = OiPr, M = Ti, 4; R = Me, X = Cl, M = Ti, 5; Zr, 6; Hf, 7). While for complexes 3, 4, 6, and 7 single isomers were observed, titanium complex 5 formed as a mixture of cis-α (5a) and cis-β (5b) isomers. Variable-temperature NMR spectroscopy shows that titanium complexes 3 and 5b are unstable and favor a cis-α ligand configuration at elevated temperatures, whereas 4 and 5a are configurationally stable. Complexes 6 and 7 show rigid C2-symmetric structures in solution up to 100 °C. Upon activation with methylaluminoxane, 3−7 show good (M = Ti) to excellent (M = Zr, Hf) activity in the polymerization of 1-hexene, giving low molecular weight atactic polymers. Styrene is polymerized with modest activity.