Owing to the unique structural features and facile tunability of the subcomponents and channels, chiral COFs show great potential in heterogeneous catalysis, enantioselective separation, and recognition.
While crystalline covalent organic frameworks (COFs) linked by C–C bonds are highly desired in synthetic chemistry, it remains a formidable challenge to synthesize. Efforts to generate C–C single bonds in COFs via de novo synthesis usually afford amorphous structures rather than crystalline phases. We demonstrate here that C–C single bond-based COFs can be prepared by direct reduction of CC bond-linked frameworks via crystal-to-crystal transformation. By Knoevenagel polycondensation of chiral tetrabenzaldehyde of dibinaphthyl-22-crown-6 with 1,4-phenylenediacetonitrile or 4,4′-biphenyldiacetonitrile, two olefin-linked chiral COFs with 2D layered tetragonal structure are prepared. Reduction of olefin linkages of the as-prepared CCOFs produces two C–C single bond linked frameworks, which retains high crystallinity and porosity as well as high chemical stability in both strong acids and bases. The quantitative reduction is confirmed by Fourier transform infrared and cross-polarization magic angle spinning 13C NMR spectroscopy. Compared to the pristine structures, the reduced CCOFs display blue-shifted emission with enhanced quantum yields and fluorescence lifetimes, while the parent CCOFs exhibit higher enantioselectivity than the reduced analogs when be used as fluorescent sensors to detect chiral amino alcohols via supramolecular interactions with the built-in crown ether moieties. This work provides an attractive strategy for making chemically stable functionalized COFs with new linkages that are otherwise hard to produce.
Porosity and surface area analysis play a prominent role in modern materials science. At the heart of this sits the Brunauer–Emmett–Teller (BET) theory, which has been a remarkably successful contribution to the field of materials science. The BET method was developed in the 1930s for open surfaces but is now the most widely used metric for the estimation of surface areas of micro‐ and mesoporous materials. Despite its widespread use, the calculation of BET surface areas causes a spread in reported areas, resulting in reproducibility problems in both academia and industry. To prove this, for this analysis, 18 already‐measured raw adsorption isotherms were provided to sixty‐one labs, who were asked to calculate the corresponding BET areas. This round‐robin exercise resulted in a wide range of values. Here, the reproducibility of BET area determination from identical isotherms is demonstrated to be a largely ignored issue, raising critical concerns over the reliability of reported BET areas. To solve this major issue, a new computational approach to accurately and systematically determine the BET area of nanoporous materials is developed. The software, called “BET surface identification” (BETSI), expands on the well‐known Rouquerol criteria and makes an unambiguous BET area assignment possible.
A key challenge in heterogeneous catalysis is the design and synthesis of heterogeneous catalysts featuring high catalytic activity, selectivity, and recyclability. Here we demonstrate that high-performance heterogeneous asymmetric catalysts can be engineered from a metal-organic framework (MOF) platform by using a ligand design strategy. Three porous chiral MOFs with the framework formula [MnL(HO)] are prepared from enantiopure phosphono-carboxylate ligands of 1,1'-biphenol that are functionalized with 3,5-bis(trifluoromethyl)-, bismethyl-, and bisfluoro-phenyl substituents at the 3,3'-position. For the first time, we show that not only chemical stability but also catalytic activity and stereoselectivity of the MOFs can be tuned by modifying the ligand structures. Particularly, the MOF incorporated with -CF groups on the pore walls exhibits enhanced tolerance to water, weak acid, and base compared with the MOFs with -F and -Me groups. Under both batch and flow reaction systems, the CF-containing MOF demonstrated excellent reactivity, selectivity, and recyclability, affording high yields and enantioselectivities for alkylations of indoles and pyrrole with a range of ketoesters or nitroalkenes. In contrast, the corresponding homogeneous catalysts gave low enantioselectivity in catalyzing the tested reactions.
3D covalent organic frameworks (COFs) with well‐defined porous channels are shown to be capable of inducing chiral molecular catalysts from non‐enantioselective to highly enantioselective in catalyzing organic transformations. By condensations of a tetrahedral tetraamine and two linear dialdehydes derived from enantiopure 1,1′‐binaphthol (BINOL), two chiral 3D COFs with a 9‐fold or 11‐fold interpenetrated diamondoid framework are prepared. Enhanced Brønsted acidity was observed for the chiral BINOL units that are uniformly distributed within the tubular channels compared to the non‐immobilized acids. This facilitates the Brønsted acid catalysis of cyclocondensation of aldehydes and anthranilamides to produce 2,3‐dihydroquinazolinones. DFT calculations show the COF catalyst provides preferential secondary interactions between the substrate and framework to induce enantioselectivities that are not achievable in homogeneous systems.
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