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

Abstract

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(O i Pr) 4 (HO i Pr) (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)MCl 2 (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(NMe 2 ) 4 yielded the corresponding Zr amido THF adduct (OCO)Zr(NMe 2 )(Cl)(THF) (3a-THF) when carried in THF as a solvent, while the Zr−NHMe 2 adduct (OCO)Zr(NMe 2 )(NHMe 2 )(THF) (3a-NHMe 2 ) was isolated using CH 2 Cl 2 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(CH 2 Ph) 4 (M = Zr, Hf) followed by a salt metathesis with 1 equiv of PhCH 2 MgCl afforded the corresponding Zr and Hf carbene dibenzyl complexes (OCO)M(CH 2 Ph) 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 η 2 fashion. The protonolysis reaction between species 4a (or 4b) and [HNMe 2 Ph][B(C 6 F 5 ) 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.

Unusual Benzyl Migration Reactivity in NHC-Bearing Group 4 Metal Chelates: Synthesis, Characterization, and Mechanistic Investigations

“…9−11 Some of the derived NHC group 4 metal chelates of the type (OCO)MX 2 (X = halide, alkoxide, alkyl) were successfully employed in lactide ROP catalysis and as 1-hexene oligomerization precatalysts. 10,11 In addition, as part of these studies, we earlier disclosed preliminary results on an unprecedented NHC-involving rearrangement arising from the reaction of the Zr-NHC benzyl derivative 2 Zr with THF (acting as a Lewis base) to yield the formation of the heptacoordinate species 4 Zr -THF (Scheme 2). 12,13 Remarkably, such a Lewis base assisted intramolecular nucleophilic attack of a Zr-Bn group onto the NHC C carbene atom clearly evidences a Fischer-type carbene reactivity (i.e., electrophilic C carbene center), which is unusual in NHC coordination chemistry.…”

Mechanistic Insights on the Controlled Switch from Oligomerization to Polymerization of 1-Hexene Catalyzed by an NHC-Zirconium Complex

“…Presumably their larger size results in still further structural distortion, resulting in a site that is too crowded to accommodate incoming monomer. It is noteworthy that a close analogue of 2, with a saturated central NHC ring and no fused benzo group, does catalyze 1-hexene polymerization when activated by the anilinium salt; 11 for that structure there is no large group that can be pushed over into the vacant site by the large coordinated ligand, and so inhibition is not observed.…”

Redox and Luminescent Properties of Robust and Air-Stable N-Heterocyclic Carbene Group 4 Metal Complexes

“…These complexes exhibit up to four reversible redox signals, and, interestingly, the generated radical cations (arising from oxidation) are thermally stable under argon. Given the potential utility of OCO−M-type species (M = group 4 metal) in catalysis, 15 Further studies in the area will focus on the use of such redoxactive NHC complexes in catalysis as well as on ligand modulations and computational studies for a thorough understanding of these systems.…”

“…Aiming at robust NHC-supported high-oxidation-state metal complexes, we have developed tridentate bis(aryloxide) NHC chelating ligands (A) that form robust and stable earlytransition-metal NHC-based chelate complexes, 14−17 with successful applications in polymerization catalysis. 15,17 In addition to their excellent chelating properties, tridentate ligands of type A may exhibit interesting redox properties because they contain electron-rich aryloxide groups typically known as redox "noninnocent" ligands. 18−22 In this regard, a few early-transition and high-oxidation-state metal complexes containing aryloxide moieties have been shown to be redoxactive metal complexes, 23−26 leading to the mediation of various bond-activation processes 27,28 and group-transfer reactions.…”