7. Rapid, Ambient Temperature Synthesis of Imine Covalent Organic Frameworks Catalyzed by Transition Metal Nitrates

Covalent organic frameworks (COFs) are crystalline, porous organic materials that are promising for applications including catalysis, energy storage, electronics, gas storage, water treatment, and drug delivery. Conventional solvothermal synthesis approaches require elevated temperatures, inert environments, and long reaction times. Herein, we show that transition-metal nitrates can catalyze the rapid synthesis of imine COFs under ambient conditions. We first tested a series of transition metals for the synthesis of a model COF and found that all transition-metal nitrates tested produced crystalline COF products even in the presence of oxygen. Fe(NO3)3·9H2O was found to produce the most crystalline product, and crystalline COFs could be produced within 10 min by optimizing the catalyst loading. Fe(NO3)3·9H2O was further tested as a catalyst for six different COF targets varying in linker lengths, substituents, and stabilities, and it effectively catalyzed the synthesis of all imine COFs tested. This catalyst was also successful in the synthesis of 2D imine COFs with different geometries, 3D COFs, and azine-linked COFs. This work demonstrates a simple, low-cost approach for the synthesis of imine COFs and will significantly lower the barrier for the development of imine COFs for applications.