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.