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

Abstract

The ring opening copolymerisation of CO₂ and epoxides is a useful means to valorise waste emissions and to reduce pollution in polymer manufacturing. Heterodinuclear catalysts, particularly those of Zn(II)/Mg(II), have shown better performances than homodinuclear analogues in this reaction. As part of on-going efforts to better understand the catalytic synergy, this work describes a series of heterodinuclear complexes, combining Zn(II) with a metal from Group 13 (M = Al(III), Ga(III) or In(III)). The complexes are synthesised from a symmetrical macrocyclic ligand in high yields via sequential metalation steps and are the thermodynamic reaction products. The Zn(II)/Group 13 complexes are effective homogeneous catalysts for the ring opening copolymerisation (ROCOP) of cyclohexene oxide at 1 bar pressure of carbon dioxide, but all show inferior performances compared to the di-zinc analogue. The CO₂ uptake into the polymer increases in the order Al(III) < Ga(III) < In(III) which is attributed to lower Lewis acidity heavier Group 13 homologues showing a reduced tendency to form ether linkages. Concurrently, polycarbonate activity increases down the Group 13 series consistent with weaker metal–oxygen bonds which show enhanced lability to insertion reactions.