Imino-Amido Hf and Zr Complexes: Synthesis, Isomerization, and Olefin Polymerization

Abstract

New hafnium and zirconium imino-amido complexes containing trimethylethylidene bridging groups are reported. Investigation into the thermal stability of these complexes revealed that they undergo facile rearrangement of the ligand backbone at 80 °C to produce isomeric imino-amido complexes. Mechanistic experiments and DFT calculations indicate that this isomerization reaction proceeds via a direct 1,2-methyl shift rather than a multistep mechanism involving the metal center. The initial imino-amido complexes (pre isomerization) exhibit high ethylene/1-octene copolymerization activities and generate polymers with very high molecular weights and relatively low comonomer contents. On the other hand, the isomerized complexes display significantly lower polymerization activities and give polymers with lower molecular weights and broad molecular weight distributions, indicative of multisite behavior.

Synthesis of Ethylene/1-Octene Copolymers with Ultrahigh Molecular Weights by Zr and Hf Complexes Bearing Bidentate NN Ligands with the Camphyl Linker

“…In contrast to previous reports that ene-diamido complexes usually formed polymers with low molecular weights and broad distributions [28,29], 1-Zr and 2-Zr in this study displayed remarkable molecular weight capability and produced ultrahigh molecular weight polyethylene (UHMWPE). A polymer sample with molecular weight as high as 337 ×10 4 g•mol −1 was synthesized by 2-Zr/[Ph3C][B(C6F5)4] (Figure S9), which is much higher than previous reports [28,29,[46][47][48][49]. The polymer prepared by 1-Zr/[Ph3C][B(C6F5)4] possessed an even higher molecular weight, which should be over 600 × 10 4 g•mol −1 but could not be accurately calibrated because of the limitation of GPC equipment.…”

“…In 2007, Klosin and coworkers reported imino-amido Zr and Hf tribenzyl complexes, which were synthesized by the reaction of α-diimine ligand with M(CH2Ph)4 (M = Zr or Hf) and showed good activity toward ethylene/1-octene copolymerizations [28]. However, using imino-amido Zr and Hf tribenzyl complexes as catalysts for olefin polymerizations is limited, because they are usually unstable at high temperature and would go through dibenzyl elimination to form ene-diamido complexes, which exhibit relatively lower activity and form polymers with lower molecular weights and broad distributions [29]. Therefore, increasing the thermal stability of α-diimine based metal complexes is highly desired for developing new olefin polymerization catalysts.…”

Post‐Metallocenes in the Industrial Production of Polyolefins

“…For the zirconium analogue the activity is somewhat lower (3.7×10 8 g(polymer)mol(Zr) −1 h −1 ), but the molecular weight is higher ( M w =7.8×10 5 g mol −1 , M w / M n =2.8). The polymerization activity of 20 was about 20 times lower in both cases and the catalysts exhibited multisite behavior resulting in multimodal molecular‐weight distributions 115…”

Imidazole‐Bridged Tetrameric Group(IV) Heteroleptic Complexes from the Spontaneous Metal‐Ligand Assembly of a Potentially N4‐Tetradentate Ligand

“…To the best of our knowledge, this is the first example of a structurally characterized hafnium(IV) imidazole complex to date. The main structural metrics [ d (Hf‐N im ) = 2.37 Å; d (Hf–N anilido ) = 2.12 Å; d (Hf–NMe 2 ) = 2.08 Å; α(N im –Hf–N anilido ) = 79.8°] fall in the typical range observed for similar complexes in the CSD containing anilido ligands bound to hafnium(IV) [ d (Hf–N anilido ) = 2.11 Å] . Intramolecular C‐H ··· N interactions are present in the molecule between C‐H bonds from methylene or dimethylamido groups and N‐atoms from the same ‐NMe 2 ligands or from the uncoordinated pyridine [d(C ··· NMe 2 ) = 3.662(7) Å; d(C ··· N py ) = 3.483(12) Å].…”