Hafnium complexes supported by CNN-tridentate ligands containing an sp-C donor: synthesis, activation and unexpected catalysis in olefin polymerization

Abstract

This study presents rare examples of hafnium complexes (Hf1–Hf4) bearing CNN-tridentate ligands with an sp3-C donor. Complexes Hf1–Hf4 were fully characterized by 1H and 13C NMR spectroscopy as well as single-crystal X-ray diffraction analysis, revealing the unusual activation of C(sp3)–H and the formation of Hf–C(sp3). These complexes showed high activity in ethylene homopolymerization and ethylene/α-olefin copolymerization (up to 33 750 kg(polymer) mol−1(Hf) h−1) and produced polymers with impressive molecular weights of up to 146.3 × 104 g mol−1. In terms of molecular weight distribution, Hf1 and Hf2 afforded polymers with bimodal distributions, while Hf3 and Hf4 produced polymers with unimodal distributions. For comparison, Hf5 with an sp2-C donor was also synthesized. The most significant difference between Hf1–Hf4 with an sp3-C donor and Hf5 with an sp2-C donor is their copolymerization capability, for which the latter synthesized copolymers with much higher 1-octene incorporation than the former under otherwise identical conditions. The activation of Hf complexes and the resulting active species was further studied by NMR and MS spectroscopy and DFT calculations to elucidate the underlying mechanism for these distinctive catalytic properties.