Lanthanide Coordination Polymers through Design for Exceptional Catalytic Performances in CO₂ Cycloaddition Reactions

Abstract

New lanthanide coordination polymers, (I) [LnIII(Habtc)(H2O)2]·2H2O (Ln = SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, ErIII, and TmIII), (II) [LnIII(Habtc)(H2O)4]·3H2O (LnIII = EuIII, GdIII, TbIII, DyIII, HoIII, TmIII, and YbIII), and (III) [EuIII2(Habtc)2(H2O)6]·2.75H2O (H4abtc = 3,3′,5,5′-azobenzenetetracarboxylic acid), were designed and synthesized for use as catalysts. The framework structures, phase formation, and purity were characterized. The catalytic performances of [EuIII(Habtc)(H2O)2]·2H2O (Ib) and [EuIII(Habtc)(H2O)4]·3H2O (IIb) were evaluated based on CO2 cycloaddition with epichlorohydrin under ambient CO2 pressure and solvent-free conditions. Discussion has been made with respect to the multiple Lewis and Brønsted acidic and Lewis basic sites coresiding in the frameworks, which were revealed by single-crystal X-ray diffraction and measurements of acidity and basicity. Depending on the reaction conditions, excellent turnover numbers and turnover frequencies (2778 and 694 h–1 for Ib and 2870 and 718 h–1 for IIb) with ≥99% conversion and ≥87% selectivity could be achieved. The robustness of Ib and IIb over 10 cycles of catalysis and the makeup process was disclosed. The potential of Ib and IIb as promising catalysts for other epoxides was also examined.