Construction of Crystalline 2D Covalent Organic Frameworks with Remarkable Chemical (Acid/Base) Stability via a Combined Reversible and Irreversible Route

TLDR

The COFs were synthesized via Schiff base reactions between 1,3,5‑triformylphloroglucinol and p‑phenylenediamine or 2,5‑dimethyl‑p‑phenylenediamine in a 1:1 mesitylene/dioxane mixture. The resulting TpPa‑1 and TpPa‑2 COFs exhibit exceptional acid and base stability, resisting 9 N HCl, boiling water, and for TpPa‑2 also 9 N NaOH, due to irreversible proton tautomerism yielding a keto‑enamine structure.

Abstract

Two new chemically stable [acid and base] 2D crystalline covalent organic frameworks (COFs) (TpPa-1 and TpPa-2) were synthesized using combined reversible and irreversible organic reactions. Syntheses of these COFs were done by the Schiff base reactions of 1,3,5-triformylphloroglucinol (Tp) with p-phenylenediamine (Pa-1) and 2,5-dimethyl-p-phenylenediamine (Pa-2), respectively, in 1:1 mesitylene/dioxane. The expected enol-imine (OH) form underwent irreversible proton tautomerism, and only the keto-enamine form was observed. Because of the irreversible nature of the total reaction and the absence of an imine bond in the system, TpPa-1 and TpPa-2 showed strong resistance toward acid (9 N HCl) and boiling water. Moreover, TpPa-2 showed exceptional stability in base (9 N NaOH) as well.