Lanthanide Azobenzenetetracarboxylates in Synergy with Mixed Solvents for Catalysis of Knoevenagel Reactions.

To obtain high-performance heterogeneous catalysts for green manufacturing of Knoevenagel reactions at room temperature, [LnIII(Habtc)(DMSO)(H2O)2] (LnIII = GdIII (I), TbIII (II), and DyIII (III); H4abtc = 3,3',5,5'-azobenzenetetracarboxylic acid, DMSO = dimethyl sulfoxide) were prepared and characterized, including their single-crystal structures. The thermal and chemical robustness and the surface acidity and basicity were examined. Employing the reactions between benzaldehyde and malononitrile as a model reaction, catalytic capabilities of I-III were evaluated. While DMSO was revealed to be critical for the catalysts to perform, water promoted their performance. The catalysts were shown to be robust and reusable with high catalytic performance in the presence of an optimal solvent mixture, from which their synergy was discussed. Using DMSO and water as a mixed solvent, the highest yield of 99(±1)% could be achieved in 8 min, with the corresponding turnover number and turnover frequency of 582 and 4,376 h-1, respectively. Density Functional Theory (DFT) calculations were additionally performed to study the influences of hydrogen bonding interactions at different plausible catalytic sites. Based on the elucidated single-crystal structures, experimental investigation, and the DFT calculations, the plausible catalytic mechanism has been proposed.