Chemical Conversion and Locking of the Imine Linkage: Enhancing the Functionality of Covalent Organic Frameworks

Abstract

Imine‐based covalent organic frameworks (COFs) are a widely studied class of functional, crystalline, and porous nanostructures which combine a relatively facile crystallization with tuneable compositions and porosities. However, the imine linkage constitutes an intrinsic limitation due to its reduced stability in harsh chemical conditions and its unsuitability for in‐plane π‐conjugation in COFs. Urgent solutions are therefore required in order to exploit the full potential of these materials, thereby enabling their technological application in electronics, sensing, and energy storage devices. In this context, the advent of a new generation of linkages derived from the chemical conversion and locking of the imine bond represents a cornerstone for the synthesis of new COFs. A marked increase in the framework robustness is in fact often combined with the incorporation of novel functionalities including, for some of these reactions, an extension of the in‐plane π‐conjugation. This Minireview describes the most enlightening examples of one‐pot reactions and post‐synthetic modifications towards the chemical locking of the imine bond in COFs.

Hydrazide- and Diazole-Linked Covalent Organic Frameworks and Their Water Harvesting Properties

“…FT-IR spectroscopic traces of the product indicated attenuation of the imine band at 1625 cm -1 corresponding to the hydrazine linkages and an emerging band at 1674 cm -1 , which was assigned to the carbonyl stretching vibration of the hydrazide moiety (see SI, Section S3). The solid-state 15 N multi-CP-MAS NMR spectroscopy 18 was performed on a 50% 15 N-labeled sample and revealed the partial conversion of the hydrazine to the hydrazide (see SI, Section S4). Here, the resonances corresponding to the functionalities were fit to Gaussian line shapes and their relative areas were compared.…”

“…PXRD analysis of the microcrystalline powder confirmed retention of crystallinity after oxidation and did not reveal additional diffraction peaks associated with hydrolyzed starting materials or additives (see SI, Section S6). The PXRD pattern of AB-COF shows high-intensity peaks at 6.8°, 11.8°, and 26.4° 2q values, associated with the (100), (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), and (002) lattice planes. After oxidation, the broad peak indexed as the lattice plane (002) shifts to a higher 2q value (26.6° 2q), resulting from changes in the interlayer stacking of the structure.…”

“…Similarly, we observed attenuation of the characteristic imine band at 1625 cm -1 in the FT-IR spectroscopic traces and appearance of the carbonyl band at 1674 cm -1 . The solid-state 15 N multi-CP-MAS NMR spectroscopy on a 50% 15 N-labeled sample indicated a partial conversion of the hydrazine-to the hydrazide-linkage (see SI, Section S4). We then subjected COF-482-hydrazide to POCl3 (neat) at 100 °C and P2S5 (7 equiv.)…”

“…5,8,10,11 This has been demonstrated for a number of imine-linked COFs, but synthetically more challenging moieties, such as heterocycles, are still rare. [13][14][15] Here, we developed diazole-linked COFs following a retrosynthetic approach. This provided us with the synthetic challenge to explore disconnection sequences leading to an initial reversible assembly of the framework from organic building units as illustrated in Scheme 1 for transformation of irreversible diazoles to irreversible hydrazides to reversible hydrazines.…”

“…In total, ten COFs were synthesized, 8 of which are new COFs with hydrazine, hydrazide, and diazole linkages; the latter two are new linkages in COF chemistry. The COFs were analyzed by 15 N multiple cross-polarization magic-angle spinning spectroscopy (multi-CP-MAS NMR), 18 giving a quantitative readout of the chemical transformations. Fourier-transform infrared (FT-IR), solid-and solution-state nuclear magnetic resonance (NMR) spectroscopies, elemental analysis (EA), thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and nitrogen sorption substantiated the structural and compositional integrity of the frameworks.…”