Ring-opening copolymerization of CO<inf>2</inf> 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.