Heterodinuclear zinc and magnesium catalysts for epoxide/CO₂ ring opening copolymerizations

Abstract

Eight heterodinuclear catalysts comprising Zn(ii) and Mg(ii) metal centres show high activity and selectivity for CO2/epoxide ring-opening copolymerization. Detailed kinetic analyses are reported and a chain-shuttling mechanistic hypothesis proposed.

Waste not, want not: CO2 (re)cycling into block polymers

“…6,7,17 Indeed, some of us have already reported high activity dinuclear ROCOP catalysts which are effective at low pressures (1 bar) of CO 2 . [18][19][20][21] Here, a di-zinc catalyst is targeted that is straightforward to prepare and which shows a zeroth order in CO 2 pressure over the range 1-40 bar. 22,23 In pioneering earlier work, Thomas and co-workers demonstrated that N-carboxyanhydride ROP could be used to produce polypeptides and the released CO 2 was subsequently used to make cyclic carbonates via tandem catalysis.…”

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

“…53 A recent investigation into analogous ZnMg catalysts showed that by applying carboxylate ligands, in place of halides, it was possible to significantly enhance turn-over-frequency values. 18 Nonetheless, in this series of catalysts complex 6, featuring a p-CF3 benzoate ligand, showed no discernible difference in rate or selectivity, but a slight decrease in ether linkages, compared with 4. Attempts to further substitute the two remaining chloride ligands in 6 with p-CF3 benzoate groups were unsuccessful and resulted in broad, complex NMR spectra.…”

“…Overall, it is quite clear that the activity values for these complexes are very low and particularly compared to leading catalysts in this field. 18,19,54,55 The Zn(II)Mg(II) catalyst shows a TOF of ∼78 h −1 and quantitative selectivity for carbonate linkages, under the same conditions (Table 1). Nonetheless, it is worth noting that the activities are in-line with other ( previously reported) Group 13 catalysts (Table 1).…”

“…A major and minor set of resonances are observed for the pCF3 benzoate protons which is attributed to both κ 1 vs. κ 2 coordination modes, as was previously observed for an analogous Mg(II)/Zn(II) complex. 18 Single crystals, suitable for X-ray crystallography, were isolated by diffusion of pentane into a saturated solution of the complexes in acetonitrile (1, 2 and 3) or THF (4 and 6), at −40°C in a glovebox (Fig. S30-S34 and Tables S1-S5 †).…”

“…17 Recently we investigated a series of these Zn(II)/Mg(II) heterocatalysts and proposed a chain shuttling mechanism whereby magnesium coordinates and activates the epoxide whilst zinc provides the nucleophilic carbonate group. 18 In 2018, Okuda and Mashima reported a series of hetero-tetranuclear catalysts of the form Zn 3 Ln which were also highly active. 19 Given the promise of these mixed metal catalysts it is important to understand which metal combinations are most effective and how synergy functions.…”