Lanthanide Tetrabromoterephthalates: Promotion of Cycloaddition Reactions of Carbon Dioxide without Preactivation, Solvent, and Co-Catalyst at Ambient Pressure through Structural Design.

To catalyze the cycloaddition reactions of carbon dioxide and epoxides without additional solvent or cocatalyst under ambient pressure, [LnIII2(tbta)2(COO)2(DMF)4] (LnIII = SmIII (ISm), EuIII (IEu), and GdIII (IGd); H2tbta = tetrabromoterephthalic acid; DMF = dimethylformamide) were synthesized and characterized. Their crystal structures were elucidated. LnIII were selected because of their hard acidity and tendency to function without preactivation. H2tbta was chosen because of its abundant bromide substituents and its potential acidic and basic sites. [EuIII2(tbta)2(COO)2(H2O)6]·5H2O (IIEu) and [GdIII(tbta)1.5(DMF)(H2O)4]·H2O (IIIGd) were additionally prepared through the transformation of IEu and IGd. The catalytic activities of ISm, IEu, and IGd were explored using a range of epoxides under ambient pressure without additional solvent and cocatalyst. They showed selective activities toward epichlorohydrin. The best performances, in terms of turnover number and turnover frequency, were obtained at 90 °C and 12 h (ISm: 307 and 26 h-1; IEu: 327 and 27 h-1; IGd: 340 and 28 h-1). Unfortunately, they were unstable after catalysis due to the loss of bromide, which is their essential limitation. However, they demonstrated the possibility of using an organic linker to activate the epoxide and the potential for solid cocatalysts. Computational studies of structural transformation and the feasibility of substituting DMF with ECH were also conducted.