Rare earth–zinc heterobimetallic complexes are more active than monometallic rare earth complexes in the cycloaddition of CO2 and epoxides under ambient conditions.
A series of bifunctional rare-earth metal complexes bearing zwitterionic amino-bis(phenolato) ligands were synthesized and structurally characterized. These complexes were applied as catalysts for the synthesis of cyclic carbonates from epoxides and CO 2 . Lanthanum complex 1 La was most active, converting terminal epoxides into the cyclic carbonates in moderate to excellent yields at 40 °C in the presence of 1 bar CO 2 . The catalyst was also recycled five runs without loss of catalytic activity. The kinetics of the cycloaddition of CO 2 and epichlorohydrin (ECH) catalyzed by complex 1 La were investigated. According to the Arrhenius equation, the activation barrier for cyclic carbonate formation is determined to be 8.29 � 0.68 kcal/mol.
A series of Nd−Zn heterometallic complexes bearing o-phenylenediamine-bridged tris(phenolato) ligands (L) were synthesized and characterized. By tuning the backbones of ancillary tris(phenolato) ligands and initiating benzyloxy groups, a Nd−Zn heterometallic complex 12 ( Cl LNdZnOBn CF3 ) was found to be highly active for the copolymerization of CO 2 and cyclohexene oxide (CHO) to produce perfect alternating poly-(cyclohexene carbonate) with a high turnover frequency up to 5640 h −1 under the polymerization of 90 °C and 20 bar CO 2 pressure. The kinetics study showed that CO 2 /CHO copolymerization catalyzed by 12 was the first order dependence of 12 and CHO concentration and the zero-order dependence of CO 2 pressure. The reaction of 12 with CO 2 generated a carbonate-coordinated [NdZnNd] trinuclear complex 13, which was believed to be the key intermediate to initiate CO 2 /CHO copolymerization. On the basis of some experiments, a plausible synergistic polymerization mechanism was proposed.
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