A mechanistic study of Lewis acid-catalyzed covalent organic framework formation

Abstract

Three boronate ester-linked covalent organic frameworks (COFs) were synthesized using a new approach that employs polyfunctional boronic acid and acetonide-protected catechol reactants in the presence of the Lewis acid catalyst BF₃·OEt₂. This transformation avoids the use of unstable and insoluble polyfunctional catechols. The COF-5 and COF-10 hexagonal lattices were obtained from a triphenylene tris(acetonide) and the appropriate diboronic acid linker, whereas a square Ni phthalocyanine COF was prepared from the appropriate Ni phthalocyanine tetra(acetonide). The powder X-ray diffraction, infrared spectra, and measured surface areas of these materials matched or exceeded previously reported values. A mechanistic study of this transformation revealed that the dehydrative trimerization of boronic acids to boroxines and the formation of a nonproductive aryl boronic acid–BF₃ complex strongly affect the rate of boronate ester formation. Crossover experiments employing substituted boronate ester derivatives suggest that esterhydrolysis is the most likely exchange mechanism during COF formation under BF₃·OEt₂-catalyzed conditions.