Comparison of the alkalinity of hydroxypyridine anion-based protic ionic liquids and their catalytic performance for Knoevenagel reaction: The effect of the type of cation and the position of nitrogen atom of anion

The design of acidity and alkalinity is an important characteristic of ionic liquids, and it also affects their application. For this reason, the alkalinity of six hydroxypyridine anion-based protic ionic liquids (PILs) was determined by potentiometric titration, and the effect of the type of cation and the position of nitrogen atom of anion on the alkalinity was discussed. Furthermore, six hydroxypyridine anion-based PILs were used as catalysts for Knoevenagel reaction, and the relationship between the catalytic performance and the alkalinity was investigated. The results indicate that the type of cation has little effect on the alkalinity of the studied PILs, while the position of nitrogen atom of anion has a significant influence on the alkalinity, and the order of the alkalinity is as follows: [2‑Op]− > [4‑Op]− > [3‑Op]−, resulting from the positive cooperative interaction between electronegative nitrogen and oxygen atoms, which is confirmed by DFT calculation. Consequently, the PILs with [2‑Op]− anion showing stronger alkalinity exhibits better catalytic performance for Knoevenagel reaction.