Single-Crystal X-ray Structures of Homochiral Brønsted Acidic Covalent Organic Frameworks

Abstract

Determining the crystal structures of covalent organic frameworks (COFs) with atomic precision is pivotal for uncovering their properties and optimizing functionalities. However, the synthesis of high-quality single crystals of COFs suitable for X-ray diffraction analysis, especially chiral COFs (CCOFs), remains a formidable challenge. In this work, we report two three-dimensional (3D) CCOFs synthesized via imine condensation of tetrahedral tetraamine and tetraaldehydes derived from optically active 1,1'-biphenol phosphoryl chloride or thiophosphoryl chloride. Single crystals of varying sizes are obtained through either a low-temperature modulation strategy, yielding large crystals up to 100 μm, or a solvothermal method. The large single crystals are structurally characterized by single-crystal X-ray diffraction, achieving a resolution of 0.90 Å. These two CCOFs are isostructural and each features a 4-fold interpenetrated diamondoid open framework with all phosphoric acid groups periodically aligned within tubular helical channels, displaying enhanced Brønsted acidity compared to non-immobilized acids. The frameworks exhibit permanent porosity, chemical resistance in boiling water, 14 M NaOH, and 0.1 M HCl, and thermal stability up to 400 °C. Notably, these CCOFs serve as efficient and recyclable heterogeneous Brønsted acid catalysts in the asymmetric addition to aromatic aldehydes, enantioselective transfer hydrogenation of ketimines, and three-component direct asymmetric Mannich reactions involving aldimines and cyclic ketones, achieving good to high enantioselectivities (up to 99.5% ee) that surpass those obtained in analogous systems with homogeneous catalysts. This work represents the first successful demonstration of single-crystal structures of homochiral COFs, paving the way for in-depth investigations into structure-property relationships in enantioselective processes and facilitating the design of novel functional chiral organic materials.