Synthesis, structure and α-olefin polymerization activity of group 4 metal complexes with [OSSO]-type bis(phenolate) ligands

The tetradentate [OSSO]-type bis(phenol) ligands, [{2,2′-(HOC6H2-4,6-R2)2CH2SCH2CH2SCH2}] (R = tBu, 2; Br, 3) react with MBz4 (M = Zr, Hf) to yield the corresponding dibenzyl complexes, [M{2,2′-(OC6H2-4,6-R2)2CH2SCH2CH2SCH2}Bz2] (R = Br, M = Zr, 4Br; Hf, 5Br; R = tBu, M = Hf, 5) in a good to very good yield. Zirconium diamido complexes, [Zr{2,2′-(OC6H2-4,6-R2)2CH2SCH2CH2SCH2}(NMe2)2] (R = tBu, 6; Br, 6Br) were prepared in a reaction of the corresponding disodium salt of 2 or 3 generated in situ with ZrCl2(NMe2)2(THF)2. Heating of 6 with TMSCl at 35 °C afforded zirconium dichloro complex, [Zr{2,2′-(OC6H2-4,6-tBu2)2CH2SCH2CH2SCH2}Cl2] (7), whereas the titanium analog 8 was prepared in a direct reaction with TiCl4. While for complexes 4Br, 5, 5Br, 6, 6Br and 7 single C2-symmetric isomers were observed in solution at room temperature, as revealed by the NMR spectroscopic data, titanium complex 8 formed as a mixture of cis-α (8a) and cis-β (8b) isomers in a ratio of approx. 20:80% (measured in CD2Cl2). The VT NMR studies revealed a reversible conversion of 8a into 8b above 60 °C. The X-ray crystal structure determination of complexes 4Br, 5Br and 7 confirmed their C2-symmetrical configuration in the solid state with cis-arranged benzyl/chloro groups and the trans-coordination of two bulky phenolato moieties. The zirconium dibenzyl complexes exhibit good catalytic activities in homopolymerization of 1-hexene (atactic poly(1-hexene), PDI = 1.5–1.7) and vinylcyclohexane (isotactic poly(vinylcyclohexane), PDI = 1.2–1.8) upon activation with a co-catalyst. In both polymerizations no increase of activity was observed for the complex 4Br with electron-withdrawing substituents on phenolate rings. Moreover, polymerization of liquid propylene catalyzed by the titanium dichloro isomeric mixture 8 afforded at 5 °C ultrahigh molecular weight atactic/isotactic polypropylene mixtures.