Regioselective One-Step Cyclization and Aromatization towards Directly Amino-Functionalized Covalent Organic Framework with Stable Benzodiimidazole Linkage

Synthesis of Carboxyl-Functionalized COFs with Alternate Stable β-Ketoenamine and Benzimidazole Linkages: Unraveling Exceptional Solvent Effects for Efficient Uranium Separation

The prevalent π-π interactions in 2D covalent organic frameworks (COFs) impart a certain flexibility to the structures, making the stacking of COF layers susceptible to external stimuli and introducing some structural disorder. Recent research indicates that the flexibility between COF layers and the associated disorder significantly influence their selective adsorption performance toward gas molecules. However, the adsorption process in a solution environment is more complex compared to gas-phase adsorption, involving interactions between adsorbents and adsorbates, as well as the solvation effects of flexible 2D COFs. Therefore, the inherent flexibility and disorder in 2D COFs under solution conditions and their impact on the adsorption performance of metal ions have not been observed yet. Herein, the synthesis of a novel carboxyl-functionalized COF featuring stable β-ketoenamine and benzimidazole linkages, named DMTP-COOH, is presented. DMTP-COOH exhibits excellent selective adsorption capability for uranium, with significantly different adsorption capacities observed after treatment with different solvents. This notable difference in adsorption capacity is observed under varying pH, concentration, time, and even in the presence of multiple competing ions. This work represents the first observation of the significant impact of solvent soaking treatment on the selective adsorption performance of COFs for uranium under liquid conditions.

Rational Synthesis of Functionalized Covalent Organic Frameworks via Four-Component Reaction

The construction of function-oriented covalent organic frameworks (COFs) remains a challenge as it requires simultaneous consideration of diversified structures, robust linkage, and tailorable functionalities. Herein, we report the rational synthesis of functionalized COFs via a four-component reaction strategy. Through the four-component Debus-Radziszewski reaction, 11 N-substituted imidazole-based COFs with diversified structures were facilely constructed from readily available building blocks. By forming the N-substituted imidazole linkage, these synthesized COFs displayed ultrastability toward strong acids and base. Moreover, the four components reaction allows the rational synthesis of COFs with tailorable functionalities. As an example, the phosphonate-functionalized COF (LZU-530) was rationally constructed for the efficient adsorption of uranium(VI). The uranium(VI) uptake of LZU-530 reaches up to 95 mg·g-1 in 2 M HNO3, which is the highest uptake of the existing organic porous materials under such harsh conditions. Our results highlight the use of multicomponent reaction for the rational synthesis of robust and functionalized COFs toward targeted applications.

Branched-Chain-Induced Host-Guest Assembly in Covalent-Organic Frameworks for Efficient Separation of No-Carrier-Added 177Lu

No-carrier-added (NCA) 177Lu is one of the most interesting nuclides for endoradiotherapy. With the dramatically rapid development of radiopharmaceutical and nuclear medicine, there is a sharp increase in the radionuclide supply of NCA 177Lu, which has formed a great challenge to current radiochemical separation constituted on classical materials. Hence, it is of vital importance to design and prepare new functional materials able of recovering 177Lu from an irradiated target with excellent efficacy. In this work, we proposed to apply noncovalent interactions to regulate the porous properties of covalent organic frameworks (COFs) by tuning the branched chain, rendering related covalent hosts different encapsulation abilities toward a flexible guest, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (P507). More interestingly, we found that the noncovalent interaction has a great effect on the host-guest complexes, which can achieve efficient NCA 177Lu separation with high recovery (95.97%). A systematic mechanism combined with experimental and theoretical investigations has confirmed that the noncovalent interactions between COFs and P507 play a preeminent role in adjusting the macroscopic properties of the host-guest complexes. This work not only uncovers that noncovalent interactions can affect the basic properties of covalent organic bonded materials but also provides a strategy for the design and preparation of other new moieties with specific functionalities.