Trimethylboroxine-free Mechanochemical Formation of Porous COF-1 from a Preconditioned Boronic-acid Precursor

Abstract

Boroxine-linked covalent organic frameworks are difficult to form mechanochemically because dehydrative condensation generates water that can hydrolyse newly formed B₃O₃ linkages. This study examines whether deliberate preconditioning of 1,4-benzenediboronic acid (BDBA) can enable trimethylboroxine-free formation of porous COF-1. BDBA was sequentially vacuum-dried, manually ground, continuously sonicated in anhydrous 1,4-dioxane, isolated, thermally dried, and then subjected to liquid-assisted planetary ball milling with a 1:1 1,4-dioxane/mesitylene mixture. The preconditioned precursor showed changes in the O–H envelope and low-frequency boron–oxygen region relative to pristine BDBA; these changes were interpreted qualitatively because pristine BDBA also contains spectral structure near ~704 cm⁻¹. Milling at 1100 rpm for 40 min with 70 µL of liquid produced a material with FTIR, PXRD, ¹³C CP/MAS NMR, and ¹¹B MAS NMR signatures consistent with COF-1. A matched direct-milling control prepared from pristine BDBA remained spectroscopically less converted after identical work-up, indicating that precursor history affects the extent of conversion under the conditions examined. Nitrogen adsorption at 77 K gave an apparent BET surface area of 711 m² g⁻¹. Across four mechanochemical runs, the isolated product mass recovery was 60.0 ± 0.7%. These findings establish a trimethylboroxine-free route to porous COF-1 under the investigated conditions while supporting further quantitative investigation of precursor-conditioning effects.