Monoanionic bidentate phenoxy–ether [O−,O] and phenoxy–thioether [O−,S] ligands were synthesized and used to prepare octahedral group 4 metal complexes with general formula [O−,E]2ML2 (complex 1: E = S, M = Hf, L = Bn; complex 2: E = O, M = Ti, L = NMe2; complex 3: E = S, M = Ti, L = NMe2; complex 4: E = S, M = Ti, L = Cl). Variable-temperature 1H NMR studies were performed on all the complexes 1–4 in the range of +20 to −70 °C. For the hafnium complex 1 a dynamic interchange of the Δ and Λ enantiomers for a C2-symmetric isomer was revealed. Eyring analysis indicated that rearrangement of the ligands occurs via a non-dissociative mechanism. In the range of the explored temperatures, a broadening of the signals was observed for complexes 2–3. Finally, a reversible fluxional process between several isomers was revealed for complex 4. Complexes 1–4 were tested as pre-catalysts for ethylene and propene polymerization in combination with different activators and under variable conditions. Complex 1 was found to be inactive whereas complexes 2–4 showed moderate activities in the polymerization of ethylene and propene. A mixture of polymers with different microstructure was obtained in all cases, coherently with the fluxional behaviour of the pre-catalysts observed in solution.
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