Heterobimetallic rare earth metal-zinc catalysts for reactions of epoxides and CO₂ under ambient conditions

Four homodinuclear rare earth metal (RE) complexes 1-4 bearing a multidentate diglycolamine-bridged bis(phenolate) ligand were synthesized. In addition, seven heterobimetallic RE-Zn complexes 5-11 were prepared through a one-pot strategy. In these heterobimetallic complexes, two RE centers are bridged by either Zn(OAc)(2) or Zn(OBn)(2) moieties. All complexes were characterized by single crystal X-ray diffraction, elemental analysis, IR spectroscopy, and multinuclear NMR spectroscopy (in the case of diamagnetic complexes 1, 4, 7 and 11). Moreover, the multi-nuclear structures of complexes 4 and 11 in solution were also studied by H-1 DOSY spectroscopy. These complexes were applied in catalyzing the coupling reaction of carbon dioxide (CO2) with epoxides. Zn(OAc)(2)- and Zn(OBn)(2)-bridged heterobimetallic complexes showed comparable catalytic activities under ambient conditions and were more active than monometallic RE complexes. Significant synergistic effect in heterobimetallic complexes is observed. Mono-substituted epoxides were converted into cyclic carbonates under 1 atm CO2 at 25 degrees C in 88-96% yields, whereas di-substituted epoxides reacted under 1 atm CO2 at higher temperatures in 40-80% yields.