Quantitative determination of the Lewis acidity of phenylboronic catechol esters — Promising anion receptors for polymer electrolytes

A series of catechol boronates were synthesized by esterification of the corresponding boronic acids. Lewis acidity of the obtained compounds was quantitatively evaluated by means of a modified Gutmann method. Introduction of the fluorine substituent into the phenyl ring resulted in increased Lewis acidity of the catechol esters. The highest effect was observed for the 2-fluoro derivative. No correlation of the AN was observed for the further increasing number of fluorine atoms, however the pentafluoro-substituted derivative was the strongest Lewis acids among studied. Its acidity is comparable to tris(pentafluorophenyl)borane, being considered one of the strongest organoboron acids. NMR-DFT calculations confirmed the relative trends in the Lewis acidity of compounds under study. It was found that the investigated complexation is reversible, which is crucial in both quantitative acceptor number determination as well as future applications of catechol boronates.