Xiaopei Li, Yongjie Zhang, Zhenqiang Shi, Dongdong Wang, Hang Yang, Yahui Zhang, Haijuan Qin, Wenqi Lu, Junjun Chen, Yan Li, and Guangyan Qing*
Nat. Commun. 2024, 15, 835
https://www.nature.com/articles/s41467-024-45464-z
https://www.nature.com/articles/s42004-024-01136-z
Boroxane hexacycles are a very important class of molecular structures that are widely used in the preparation of covalent organic framework materials, healing/reprocessable polymeric materials and flame retardants. However, boroxane hexacyclic structures have an Achilles' heel, i.e., hydrolytic instability, which stems from the fact that boroxane hexacyclic structures are prepared by boric acid dehydration under anhydrous conditions, and their rapid hydrolysis to boric acid structures upon exposure to water leads to loss of function (Fig. 1, left). Much work has been done to enhance the water stability of boroxane hexacyclic structures, with the main strategy being to reduce the electrophilicity of the boron Lewis acid site by introducing electron-donating groups or/and bulky groups capable of sterically protecting the boron atoms in the structure, or/and forming adducts with N-donor ligands. Although these approaches have led to an increase in the resistance of the boroxane hexacyclic structure to hydrolysis, they have not fundamentally addressed the challenge of its hydrolytic instability.
The group found that o-hydroxyphenylboronic acid similarly dehydrates in its natural environment to form a unique dimeric structure that combines AIEE with dynamic covalent chemical effects (Figure 1, right). However, unlike o-aminophenylboronic acid dimer (which is stable in water), upon exposure to water, o-hydroxyphenylboronic acid dimer is rapidly converted to o-hydroxyphenylboronic acid trimer with a water-stabilized boron oxyhexacyclic structure in the natural environment. Single-crystal culture and characterization, solid and liquid nuclear magnetic resonance spectroscopy, fluorescence spectroscopy, mass spectroscopy, and Raman spectroscopy confirmed that o-hydroxyphenylboronic acid does not exist as a monomer in the natural environment either. In the solid state, o-hydroxyphenylboronic acid dehydrates to form a dimer. In the presence of water, o-hydroxyphenylboronic acid dimer rapidly transforms into a trimer with a water-stabilized boroxane hexacyclic structure. The trimer exhibits superb binding ability for fluoride ions in the aqueous phase, and the further developed polymer gel materials based on this trimer have high acid-base stability and reversible gel-sol transition ability.
The discovery of the water-stabilized boroxane hexacyclic structure fundamentally solves this problem and opens a new chapter in the study of boroxane hexacyclic structures. The research results were highly evaluated by the reviewers: “This discovery has far-reaching implications for materials science and chemistry, offering solutions to the challenges posed by boroxine hydrolysis and enabling innovative applications in aqueous environments”，(Nat. Commun., 2024, 15, 1207). This paper has been selected as the "Feature Paper" by Nat. Commun. and highlighted by the editor in Commun. Chem. 2024, 7, 50.
