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.