Influence of Fluorination and Boronic Group Synergy on the Acidity and Structural Behavior of o‑Phenylenediboronic Acids

The solid-state and solution structural properties and acidity of a series of fluorinated 1,2-phenylenediboronic acids were investigated. Solution NMR studies indicate that these compounds equilibrate with their dehydrated forms, in the simplest case presumably possessing the cyclic benzoxadiborole structure. Ab initio calculations showed that the stability of such cyclic semianhydrides is improved by fluorination of the aromatic ring and complexation of one of the boron centers with water. This was demonstrated by the crystal structure determination of tetrafluoro-1,2-phenylenediboronic acid. The coordinated water molecule participates in very strong intermolecular hydrogen bonding with the OH group bonded to the four-coordinate boron center (dO···O = 2.423(2) Å, Eint = −87 kJ mol–1). This indicates that in fact this compound is an oxonium, i.e., Brønsted acid, which is exceptional for boronic acids. Under different crystallization conditions, tetrafluoro-1,2-phenylenediboronic acid dimerizes by aggregation of boronic groups, which leads to the formation of an uncommon eight-membered B4O4 ring. Such a coordination dimer exists as the boat conformer, featuring π–π interactions of fluorinated aromatic rings. The enhanced acidity of 1,2-phenylenediboronic acids can be rationalized in terms of a synergic effect of two adjacent boronic groups and is manifested by relatively low pKa values ranging from 6.0 (1,2-phenylenediboronic acid) to only 3.0 for the perfluorinated derivative.