Ring-opening copolymerization of CO2 with epoxides catalyzed by binary catalysts containing half salen aluminum compounds and quaternary phosphonium salt

The synthesis and characterization of two different categories of Al(III) compounds supported by unsymmetrical phenoxy-imine ligands with furan-2-ylmethanimine and thiophen-2-ylmethanimine side arms is described. Reactions of the ligands with Al(CH3)3 in a 1:1 and 2:1 stoichiometric ratio resulted in the formation of the corresponding monosubstituted and disubstituted Al(III) compounds in quantitative yields. These compounds were fully characterized using various spectroscopic techniques and elemental analysis. Single-crystal X-ray diffraction studies revealed that the Al(III) center in the monosubstituted compounds adopt a distorted tetrahedral geometry while those in the disubstituted compounds adopt a distorted trigonal bipyramidal geometry with a geometric index τ = 0.72. These compounds along with tetraphenylphosphonium chloride (TPPCl) form a binary catalyst system for the ring-opening copolymerization (ROCOP) of CO2 with cyclohexene oxide (CHO) and propylene oxide (PO). They show notable activity producing alternating polycarbonate with TOF up to 107 h−1 and high selectivity (carbonate linkages > 80%). The catalytic activities of the compounds have been investigated by varying the substituent in the phenolate core and in the imine backbone.