Coordination behavior of bis-phenolate saturated and unsaturated N-heterocyclic carbene ligands to zirconium: reactivity and activity in the copolymerization of cyclohexene oxide with CO2

Abstract

Tetravalent zirconium complexes supported by tridentate bis-phenolate imidazolidin-2-ylidene (L1), imidazol-2-ylidene (L2) and benzimidazol-2-ylidene (L3) NHC ligands were synthesized and evaluated as precursors for the copolymerization of cyclohexene oxide (CHO) with CO₂. While the reactivity of the imidazolidinium [H₃L1] chloride salt with Zr(OiPr)₄(HOiPr), and subsequent ligand exchanges with either (CH₃)₃SiCl or LiOiPr lead to a series of heteroleptic compounds (κ³-O,C,O-L1)Zr(X)₂(THF) (X = Cl, OiPr), both imidazolium [H₃L2] and benzimidazolium [H₃L3] chloride salts give a mixture of homoleptic (κ³-O,C,O-NHC)₂Zr and zwitterionic (κ²-O,O-HL)ZrCl₂(OiPr) compounds along with traces or the absence of the heteroleptic intermediate (κ³-O,C,O-NHC)Zr(Cl)(OiPr)(THF). Such dissimilar reactivity between the unsaturated and saturated NHC ligands is predominantly ascribed to the increased acidity of azolium salts along with the π-donor strength of the C_carbene in L2 and L3–Zr moieties. The reactivity with the more acidic azolium salts (H₃L2/3) and the destabilized Zr–X_trans to NHC_carbene bond results in a significant increase in the amount of homoleptic compounds generating HCl. The released HCl reacts preferentially with the heteroleptic intermediates having non-planar NHC ligands (i.e. L2/3) promoting the formation of zwitterionic complexes. The in situ deprotonation of the isolated zwitterionic (κ²-O,O-HL3)ZrCl₂(OiPr) compound by using Ag₂O gives the homoleptic complex as the major component along with a bimetallic hydroxo-bridged [(κ³-O,C,O-L3)Zr(μ-OH)(OiPr)]₂ compound. Of particular interest is that only the heteroleptic NHC–Zr(IV) complexes were identified to be active and highly selective towards the copolymerization of CHO with CO₂ independently of the co-catalysts used (both anionic and neutral) under mild conditions (P_CO2 < 1 bar, T = 60 °C), and gave atactic and completely alternating copolymers in a controlled manner (M_w/M_n ≈ 1.3–1.8). In contrast, the isolated homoleptic, zwitterionic and bimetallic zirconium species were found to be inactive under similar reaction conditions. Although the activity found for NHC–Zr(IV) complexes is nearly of the same order of magnitude as that of the NHC–Ti(IV) analogues, these results are the first examples of tetravalent zirconium complexes achieving high selectivity (99% in PCHC) in the catalyzed copolymerization of CHO with CO₂.