Halide-Free and Bifunctional One-Component Catalysts for the Coupling of Carbon Dioxide and Epoxides

In this paper, we first report a new class of halide-free and bifunctional one-component catalysts for the coupling of CO₂ with epoxides. The catalysts do not need halide-based additives or tethered salts attached to the ligand when used for this coupling reaction. As the halide-free and bifunctional one-component catalysts, we chose nonionic and monomeric tetracarbonylchromium(0), tetracarbonylmolybdenum(0), and tetracarbonyltungsten(0) complexes chelated by modified ethylenediamines, namely N, N-dimethylethylenediamine, N, N'-dimethylethylenediamine, N, N, N'-trimethylethylenediamine, and N, N, N', N'-tetramethylethylenediamine. A simple mixture of M(CO)₆ (M = Cr, Mo, and W) with the modified ethylenediamines shows only one-third of the activity achieved with the tetracarbonyl metal complexes precoordinated to the corresponding modified ethylenediamines. Increasing the number of methyl substituents on the nitrogen atoms of the ethylenediamine derivatives as well as the chromium metal center in the metal carbonyl complex significantly enhanced the catalytic activity. Thus, among the 12 catalysts tested, tetracarbonyl(tetramethylethylenediamine)chromium(0) exhibited the best catalytic activity under the same reaction conditions. Various terminal and internal epoxides were easily converted into the corresponding cyclic carbonates using this chromium system. Calculations based on density functional theory were also carried out to elucidate the mechanism of the coupling reaction.