Groups 1, 2 and Zn(II) Heterodinuclear Catalysts for Epoxide/CO₂ Ring-Opening Copolymerization

Abstract

A series of heterodinuclear complexes are reported where both Zn(II) and a metal from Group 1 or 2 are chelated by a macrocyclic diphenolate-tetra-amine ligand. The complexes are characterized in the solid state, where relevant by single crystal X-ray crystallography and elemental analysis, and in solution, using NMR spectroscopy and mass spectrometry. The complex 2 synthesis is achieved by reaction of the ligand with diethyl zinc, to form the mono-zinc complex, in situ, followed by subsequent coordination of the second metal; this method enables heterodinuclear conversions >90 % as determined by NMR spectroscopy. Alternatively, the same heterodinuclear complexes are accessed by reaction between the two homodinuclear complexes, at elevated temperatures for extended periods. These findings suggest that most of the heterodinuclear complexes are the thermodynamic products; the only exception is the Na(I)/Zn(II) complex which is unstable with respect to the homodinuclear counterparts. The catalytic activities and selectivity of the stable heterodinuclear complexes are compared, against each other and the relevant homodinuclear analogues, for the ring opening copolymerization (ROCOP) of CO2 and CHO. Nearly all the heterodinuclear complexes are less active than the di-zinc analogues, but the Mg(II)/Zn(II) catalyst is more active. The co-ligand influences the product selectivity, with iodide ligands resulting in cyclic carbonate formation and carboxylate ligands in a high selectivity for polycarbonate.

Heterodinuclear complexes featuring Zn(ii) and M = Al(iii), Ga(iii) or In(iii) for cyclohexene oxide and CO2 copolymerisation

“…For 1, the Zn-Zn separation is 3.04(1) Å, consistent with previous di-zinc complexes coordinated by this ligand and with the proposed distance for effective catalysis. 20,[44][45][46] For 2-4, Zn-M(III) separations increase with the increasing ionic radius of the Group 13 element and are 3.02(3) Å, 3.12(3) Å and 3.15(8) Å for complexes 2, 3 and 4, respectively (Fig. 3).…”

“…The 1 H and 13 C NMR spectra are in-line with related macrocycle di-zinc complexes with coligands such as acetate, phenyl, benzoate and iodide. 13,20,43,44 The MALDI-ToF mass spectrum displays a peak at 717 m/z, corresponding to the molecular cation, [LZn 2 (Cl)] + .…”

“…Within the series of complexes, only the Zn(II)/Mg(II) combination showed synergy and all others were less active than the Zn(II)/Zn(II) homodinuclear complex. 20 In this work, we investigate a series of complexes featuring Zn(II) and Group 13 elements. The metal selection is motivated by some precedent for Al(III) complexes in this catalysis, for example Al(III) salen, 21-23 porphyrin [24][25][26][27][28] or trisphenolate 29 complexes are all active as part of bicomponent systems (i.e.…”

“…When targeting new catalysts, it is proposed that both metals should have sufficiently small ionic radii to enable inplane coordination in the macrocycle cavity. 20…”

A Rational Investigation of the Lewis Acid-Promoted Coupling of Carbon Dioxide with Cyclohexene Oxide: Towards CO2-Sourced Polycyclohexene Carbonate under Solvent- and Cocatalyst-Free Conditions

“…Thus, for the selective and efficient synthesis of polycarbonates (PCs) from the copolymerization of CO 2 and epoxides, many Lewis acid catalysts such as organometallic complexes usually used in combination with a nucleophile have been developed. In particular, the important characteristics of metal-based catalysts (zinc [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37], aluminum [38,39], chromium [40][41][42][43], cobalt [44][45][46][47], magnesium [48,49], iron [19,[50][51][52][53][54], titanium [55,56], copper [57], ytterbium [58], . .…”