Direct and Linker-Exchange Alcohol-Assisted Hydrothermal Synthesis of Imide-Linked Covalent Organic Frameworks.

Johannes Maschita,Bettina V Lotsch,Tanmay Banerjee

doi:10.1021/acs.chemmater.1c04051

Abstract

Covalent organic frameworks (COFs) are an extensively studied class of porous materials, which distinguish themselves from other porous polymers in their crystallinity and high degree of modularity, enabling a wide range of applications. However, the established synthetic protocols for the synthesis of stable and crystalline COFs, such as imide-linked COFs, often requires the use of high boiling solvents and toxic catalysts, making their synthesis expensive and environmentally harmful. Herein, we report a new environmentally friendly strategy—an alcohol-assisted hydrothermal polymerization approach (aaHTP) for the synthesis of a wide range of crystalline and porous imide-linked COFs. This method allows us to gain access to new COFs and to avoid toxic solvents by up to 90% through substituting commonly used organic solvent mixtures with water and small amounts of n-alcohols without being restricted to water-soluble linker molecules. Additionally, we use the aaHTP to demonstrate an eco-friendly COF-to-COF transformation of an imine-linked COF into a novel imide-linked COF via linkage replacement, inaccessible using published reaction conditions.

Highlights

We started our research with the literature-known TAPAPMDA-covalent organic frameworks (COFs) and TAPB-PMDA-COF, which are synthesized from the precursor molecules tris(4-aminophenyl)amine (TAPA), 1,3,5-tris(4-aminophenyl)benzene (TAPB), and pyromellitic dianhydride (PMDA; Scheme 1).[13]
We report an environmentally friendly alcohol-assisted hydrothermal polymerization approach for the synthesis of imide-linked COFs
TAPE-PMDA-COF, which crystallizes in a kagome-type structure, could only be synthesized using the alcoholassisted hydrothermal polymerization approach (aaHTP) protocol, showing the importance of such complementary procedures for the synthesis of COFs

Summary

Introduction

COF synthesis is typically conducted according to the principles of dynamic covalent chemistry: the COF formation reaction has to be reversible to enable crystal defect correction during the polymerization process, carried out under precise thermodynamic and kinetic control.[1,6] bond breaking being as crucial as bond formation in dynamic covalent synthesis stability is achieved at the expense of crystallinity, frequently leading to poorly crystalline products, rendering synthesis of stable and crystalline COFs challenging.[7] To achieve sufficient reversibility of the COF formation reaction, toxic and high boiling solvents like mesitylene, 1,2-dioxane, o-dichlorobenzene, or N-methyl-2-

Methods

Results

Conclusion