Effect of building block transformation in covalent triazine‐based frameworks for enhanced CO2 uptake and metal‐free heterogeneous catalysis

Invited for the cover of this issue is the group of Pascal Van Der Voort at the University of Ghent and colleagues at Technische Universitat Berlin. The image depicts the covalent triazine frameworks reported in the manuscript for the sorption of CO2 and also in metal-free catalysis. Read the full text of the article at 10.1002/chem.201903926

Metalporphyrin on PMO catalysts for the conversion of CO2 to cyclic carbonates

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Functionalized periodic mesoporous organosilicas : from metal free catalysis to sensing

In this work a widely usable post-modification route for periodic mesoporous organosilicas (PMOs) was developed. Using the developed method, two diverse ligands, picolinic acid (Pic) and 4,4 ',4 '',4 '''-porphyrin-5,10,15,20-tetrayltetrabenzoic acid (Porph), were successfully covalently coupled onto the PMO material and well-characterized. Both obtained materials show high BET surface areas (565 m(2) g(-1) for Pic@PMO, 483 m(2) g(-1) for Porph@PMO and 548 m(2) g(-1) for the unmodified PMO) and pore sizes (5.1 nm). The materials were subsequently tested for their catalytic activity in the cycloaddition of epoxide and CO2, a frequently studied carbon capture and utilization reaction. Interestingly, both materials showed very good reactivity (with conversions of up to 90%) as metal free heterogeneous catalysts and proved to be perfectly stable in recyclability and aging tests. Moreover, by (co-)grafting Eu3+ and Tb3+ ions onto Pic@PMO and Yb3+ ions onto Porph@PMO a strong emission was observed in the visible and near-infrared (NIR) range, respectively. Eu,Tb@Pic@PMO showed potential for use as a temperature sensor in the physiological range (a maximum S-r value of 2.11 %K-1 was obtained at 273 K), while Yb@Porph@PMO could efficiently be excited within the human tissue penetrating window showing characteristic Yb3+ luminescence (with decay times of around 10 mu s). These findings prove that simple modifications of this PMO can provide smart materials for very diverse applications

L-proline functionalized modualted Zr-MOFs for the aldol reaction

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Triggering white-light emission in a 2D imine covalent organic framework through lanthanide augmentation

Recently, covalent organic frameworks (COFs) have emerged as an interesting class of porous materials, featuring tunable porosity and fluorescence properties based on reticular construction principles. Some COFs display highly emissive monocolored luminescence, but attaining white-light emission from COFs is difficult as it must account for a wide wavelength range. White-light emission is highly desired for solid-state lighting applications, and obtaining it usually demands the combination of red-, green-, and blue-light components. Hence, to achieve the targeted white-light emission, we report for the first time grafting of lanthanides (Eu3+/Tb3+) on a two-dimensional imine COF (TTA-DFP-COF). We studied the luminescence properties of the hybrid materials prepared by anchoring Eu3+ (red light) and Tb3+ (green light) beta-diketonate complexes onto the TTA-DFP-COF. Reticular construction is exploited to design strong coordination of Eu3+ and Tb3+ ions into nitrogen-rich pockets of the imine COF. Mixed Eu3+/Tb3+ materials are then prepared to incorporate red and green components along with the inherent blue light from the organic moieties of the COF to produce white-light emission. We show that COFs have the potential for hosting Eu3+ and Tb3+ complexes, which can be tuned to obtain desired excitations for applications in the field of optoelectronics, microscopy, optical sensing, and bioassay

Structural versatility of the quasi-aromatic Möbius type zinc(II)-pseudohalide complexes : experimental and theoretical investigations

In this contribution we report for the first time fabrication, isolation, structural and theoretical characterization of the quasi-aromatic Mobius complexes [Zn(NCS)(2)L-I] (1), [Zn-2(mu(1,1)-N-3)(2)(L-I)(2)][ZnCl3(MeOH)](2)center dot 6MeOH (2) and [Zn(NCS)L-II](2)[Zn(NCS)(4)]center dot MeOH (3), constructed from 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene)hydrazono)ethane (L-I) or benzilbis(acetylpyridin-2-yl)methylidenehydrazone (L-II), respectively, and ZnCl2 mixed with NH4NCS or NaN3. Structures 1-3 are dictated by both the bulkiness of the organic ligand and the nature of the inorganic counter ion. As evidenced from single crystal X-ray diffraction data species 1 has a neutral discrete heteroleptic mononuclear structure, whereas, complexes 2 and 3 exhibit a salt-like structure. Each structure contains a Zn-II atom chelated by one tetradentate twisted ligand L-I creating the unusual Mobius type topology. Theoretical investigations based on the EDDB method allowed us to determine that it constitutes the quasi-aromatic Mobius motif where a metal only induces the pi-delocalization solely within the ligand part: 2.44|e| in 3, 3.14|e| in 2 and 3.44|e| in 1. It is found, that the degree of quasi-aromatic pi-delocalization in the case of zinc species is significantly weaker (by similar to 50%) than the corresponding estimations for cadmium systems - it is associated with the Zn-N bonds being more polar than the related Cd-N connections. The ETS-NOCV showed, that the monomers in 1 are bonded primarily through London dispersion forces, whereas long-range electrostatic stabilization is crucial in 2 and 3. A number of non-covalent interactions are additionally identified in the lattices of 1-3

Engineering a highly defective stable UiO-66 with tunable Lewis-Brønsted acidity : the role of the hemilabile linker

The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to a maximum of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects but also as a coligand that enhances the stability of the resulting defective framework. Furthermore, upon a postsynthetic treatment in H2SO4, the average number of defects increases to the optimum of six missing BDC linkers per cluster (three per formula unit), leaving the Zr-nodes on average sixfold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of a-pinene oxide. This reaction is particularly sensitive to the Bronsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Bronsted acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD3CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Bronsted to Lewis acid ratio in the materials and thus their catalytic activity and selectivity