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

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 ester hydrolysis is the most likely exchange mechanism during COF formation under BF₃·OEt₂-catalyzed conditions.