Yolk-Shell and Hollow Zr/Ce-UiO-66 for Manipulating Selectivity in Tandem Reactions and Photoreactions.

One of the hallmarks of multicomponent metal-organic frameworks (MOFs) is to finely tune their active centers to achieve product selectivity. In particular, obtaining bimetallic MOF hollow structures with precisely tailored redox centers under the same topology is still challenging despite a recent surge of such efforts. Herein, we present an engineering strategy named "cluster labilization" to generate hierarchically porous MOF composites with hollow structures and tunable active centers. By partially replacing zirconium with cerium in the hexanuclear clusters of UiO-66, unevenly distributed yolk-shell structures (YSS) were formed. Through acid treatment or annealing of the YSS precursor, single-shell hollow structures (SSHS) or double-shell hollow structures (DSHS) can be obtained, respectively. The active centers in SSHS and DSHS differ in their species, valence, and spatial locations. More importantly, YSS, SSHS, and DSHS with distinct active centers and microenvironments exhibit tunable catalytic activity, reversed selectivity, and high stability in the tandem reaction and the photoreaction.

Modeling the Enzyme Specificity by Molecular Cages through Regulating Reactive Oxygen Species Evolution.

Mimicking the active site and the substrate binding cavity of the enzyme to achieve specificity in catalytic reactions is an essential challenge. Herein, porous coordination cages (PCCs) with intrinsic cavities and tunable metal centers have proved the regulation of reactive oxygen species (ROS) generating pathways as evidenced by multiple photo-induced oxidations. Remarkably, in the presence of the Zn4 -µ4 -O center, PCC converted dioxygen molecules from triplet to singlet excitons, whereas the Ni4 -µ4 -O center promoted the efficient dissociation of electrons and holes to conduct electron transfer towards substrates. Accordingly, the distinct ROS generation behavior of PCC-6-Zn and PCC-6-Ni enables the conversion of O2 to 1 O2 and O2 ⋅- , respectively. In contrast, the Co4 -µ4 -O center combined the 1 O2 and O2 ⋅- together to generate carbonyl radicals, which in turn reacted with the oxygen molecules. Harnessing the three oxygen activation pathways, PCC-6-M (M=Zn/Ni/Co) display specific catalytic activities in thioanisole oxidation (PCC-6-Zn), benzylamine coupling (PCC-6-Ni), and aldehyde autoxidation (PCC-6-Co). This work not only provides fundamental insights into the regulation of ROS generation by a supramolecular catalyst but also demonstrates a rare example of achieving reaction specificity through mimicking natural enzymes by PCCs.

Iron(II) Immobilized within a Metal-Organic Framework Mixed-Matrix Membrane as a H2O2 Turn-On Sensor.

H2O2 detection is closely relevant to human health; however, most of the H2O2 probes suffer from low accuracy and sensitivity because of the aggregating nature of solid sensors. In contrast, a mixed-matrix membrane (MMM) with high processability and flexibility is a suitable H2O2 probe to overcome these drawbacks. Herein, we fabricated MOF-based MMMs by using a robust UiO-66-(COOH)2 with carboxylate-chelating moieties, which were utilized for binding Fe (II) metal centers. The Fe (II)-immobilized MOF-MMM involved in a Fenton reaction when treated with H2O2, exhibiting a fluorescence turn-on property. Compared to the bulk-state MOF powder, the MOF-MMM sensor showed much-improved sensitivity (detection limit down to 0.0215 µM) because of the uniform dispersion of the probe and a sufficient contact with the analyte. This MOF-MMM sensor combinedly exhibited a turn-on fluorescence response and outstanding sensing properties with flexibility and processability, providing a novel platform suitable for practical sensing applications.

Iron Nitrate-Mediated Selective Synthesis of 3-Acyl-1,2,4-oxadiazoles from Alkynes and Nitriles: The Dual Roles of Iron Nitrate.

A direct strategy for the selective synthesis of 3-acyl-1,2,4-oxadiazoles from alkynes and nitriles has been developed under iron(III) nitrate-mediated conditions. The mechanism includes three sequential procedures: iron(III) nitrate-mediated nitration of alkynes leads to α-nitroketones, dehydration of α-nitroketones provides the nitrile oxides, and 1,3-dipolar cycloaddition of nitrile oxides with nitriles produces 3-acyl-1,2,4-oxadiazoles under iron-mediated conditions. Iron(III) nitrate plays dual roles in the nitration of alkynes and the activation of nitriles, while the formation of pyrimidine/isoxazole byproducts can be efficiently inhibited.