Neutral and Cationic N‑Heterocyclic Carbene Zirconium and Hafnium Benzyl Complexes: Highly Regioselective Oligomerization of 1‑Hexene with a Preference for Trimer Formation

Various zirconium and hafnium amido, chloro, and benzyl complexes supported by a tridentate N-heterocyclic carbene bis-phenolate dianionic ligand ((OCO)2–) have been synthesized and structurally characterized. The alcohol elimination reaction of the protio ligand N,N′-bis­(2-hydroxy-3,5-di-tert-butylphenyl)-4,5-dihydroimidazolium chloride (1) and the metal alkoxide precursors M­(OiPr)4(HOiPr) (M = Zr, Hf) and a subsequent alkoxide/chloride exchange reaction (upon addition of trimethylsilyl chloride, TMSCl) afforded the corresponding Zr and Hf carbene dichloro complexes as THF adducts: (OCO)­MCl2(THF) (2a-THF, M = Zr; 2b-THF, M = Hf). As determined by single-crystal X-ray crystallographic studies, the molecular structure of the Hf derivative 2b-THF confirmed the proposed formulation and the effective formation of a (OCO)­Hf chelate. In the case of Zr, an amine elimination reaction between protio ligand 1 and Zr­(NMe2)4 yielded the corresponding Zr amido THF adduct (OCO)­Zr­(NMe2)­(Cl)­(THF) (3a-THF) when carried in THF as a solvent, while the Zr–NHMe2 adduct (OCO)­Zr­(NMe2)­(NHMe2)­(THF) (3a-NHMe2) was isolated using CH2Cl2 as the reaction solvent. 3a-THF may be readily and quantitatively converted to the dichloro derivative 2a-THF upon addition of TMSCl. The toluene elimination reaction of protio ligand 1 and M­(CH2Ph)4 (M = Zr, Hf) followed by a salt metathesis with 1 equiv of PhCH2MgCl afforded the corresponding Zr and Hf carbene dibenzyl complexes (OCO)­M­(CH2Ph)2 (4a, M = Zr; 4b, M = Hf), whose solid-state structures were confirmed by X-ray crystallography. 4a and 4b each feature a five-coordinate metal center with both benzyl moieties binding in a η2 fashion. The protonolysis reaction between species 4a (or 4b) and [HNMe2Ph]­[B­(C6F5)4] afforded the clean and quantitative formation of the corresponding Zr (or Hf) anilinium benzyl cation 5a+ (or 5b+). Remarkably, the cation 5a+ catalyzes the highly regioselective oligomerization of 1-hexene with a marked preference for trimer formation.