Mixed-Linker Chiral 2D Covalent Organic Frameworks with Controlled Layer Stacking for Electrochemical Asymmetric Catalysis

Abstract

Covalent organic frameworks (COFs) have undergone extensive research as heterogeneous catalysts for a wide range of significant reactions, but they have not yet been investigated in the realm of electrochemical asymmetric catalysis, despite their recognition as an economical and sustainable strategy for producing enantiopure compounds. Here, we report a mixedlinker strategy to design multicomponent two-dimensional (2D) chiral COFs with tunable layer stacking for highly enantioselective electrocatalysis. By crystallizing mixtures of triamines with and without the MacMillan imidazolidinone catalyst or aryl substituent (ethyl and isopropyl) and a dialdehyde derivative of thieno-[3,2b]thiophene, we synthesized and structurally characterized a series of three-component homochiral 2D COFs featuring either AA or ABC stacking. The stacking modes that can be synthetically controlled through steric tuning using different aryl substituents affect their chemical stability and electrochemical performance. With the MacMillan catalyst periodically appended on their channels, all three COFs with conductive thiophene moieties can be highly enantioselective and recyclable electrocatalysts for the asymmetric αarylation of aldehydes, affording alkylated anilines with up to 97% enantiomeric excess by an anodic oxidation/organocatalytic protocol. Presumably due to their higher charge transfer ability, the ABC stacking COFs exhibit improved reactivity compared to the AA stacking analogue. This work therefore advances COFs as electrocatalysts for asymmetric catalysis and may facilitate the design of more redox-active crystalline organic polymers for electrochemical enantioselective processes.

Controlling the Degree of Interpenetration in Chiral Three-Dimensional Covalent Organic Frameworks via Steric Tuning

“…Only the positions and brightness of the diffraction spots for 22-OEt closely match those in the calculated SAED pattern, suggesting that the simulated structure is reasonable (Figure S12). − Despite significant efforts, we were unable to acquire high-quality data suitable for structural analysis of the other CCOFs by SAED due to the severe aggregation tendency of CCOF particles and the very small domain size of the CCOF crystals …”

Designed Synthesis of Imine-Linked 2D Covalent Organic Frameworks with Enhanced Stability and Functionality

“…These features make COFs widely applicable in adsorption, − catalysis, − energy storage, − and other fields. − In applications, both crystallinity and stability are key factors influencing material performance. , However, for most COFs formed by dynamic covalent bonds, crystallinity, stability, and functionality are often mutually exclusive. − While reversible bonds enhance the crystallinity of materials, they concurrently compromise their stability. Additionally, functional sites, usually possessing large sizes or nonsymmetric structures, might impede the crystallization of COFs. , Postsynthetic modification of the imine bonds, such as oxidation, reduction, and cyclization, is demonstrated to be a general method to construct robust and functional COFs. − However, this strategy often encounters problems related to reduced crystallinity and porosity. − Therefore, achieving a balance among the crystallinity, stability, and functionality in COFs is an important research topic …”

Supramolecular Assembly Frameworks (SAFs): Shaping the Future of Functional Materials

“…Over the last two decades, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have emerged as pivotal materials in the advancement of various scientific and technological domains, owing to their distinct porosity and crystallinity. These characteristics have rendered them indispensable in applications spanning gas storage, [1][2][3][4] separation, [5,6] water harvesting, [7][8][9] and catalysis, [10][11][12] among others. [13][14][15] MOFs are synthesized from metal ions or clusters connected by organic linkers through coordination bonds, forming structures that can be one-, two-, or three-dimensional.…”