Cycloaddition of epoxides and CO2 catalyzed by bisimidazole-functionalized porphyrin cobalt(iii) complexes†
A series of innovative bisimidazole-functionalized porphyrin cobalt(III) complexes have been devised, synthesized and characterized using NMR, MS and elemental analysis. These homogeneous catalysts were applied to the cycloaddition of epoxides and carbon dioxide without organic solvent and co-catalyst. It was found that the performance of the catalysts deeply relies on their structural features. The alkoxyl chain length of the linkage and the imidazole position relative to the phenyl rings of porphyrin evidently affects the catalyst activities. [5,15-Di(3-((8-imidazolyloctyl)oxy)phenyl)porphyrin] cobalt(III) chloride (J-m8) and [5,15-di(2-((6-imidazolylhexyl)oxy)phenyl)porphyrin] cobalt(III) chloride (J-o6) demonstrated excellent activity under optimal reaction conditions. Synchronously, a preliminary kinetic investigation of this reaction was carried out using three catalysts and illustrated the activation energies of cyclic carbonate formation. Furthermore, a tri-synergistic catalytic mechanism has been carefully proposed in light of the features of the new catalysts and experimental results.