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 Ccarbene in L2 and L3-Zr moieties. The reactivity with the more acidic azolium salts (H3L2/3) and the destabilized Zr-Xtrans to NHCcarbene 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 (PCO2 < 1 bar, T = 60 °C), and gave atactic and completely alternating copolymers in a controlled manner (Mw/Mn ≈ 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.