Incorporating highly basic polyoxometalate anions comprising Nb or Ta into nanoscale reaction fields of porous ionic crystals

Water-Tolerant Superbase Polyoxometalate [H2(Nb6O19)]6- for Homogeneous Catalysis

Here, doubly protonated Lindqvist-type niobium oxide cluster [H2(Nb6O19)]6-, fabricated by microwave-assisted hydrothermal synthesis, exhibited superbase catalysis for Knoevenagel and crossed aldol condensation reactions accompanied by activating C-H bond with pKa >26 and proton abstraction from a base indicator with pKa=26.5. Surprisingly, [H2(Nb6O19)]6- exhibited water-tolerant superbase properties for Knoevenagel and crossed aldol condensation reactions in the presence of water, although it is well known that the strong basicity of metal oxides and organic superbase is typically lost by the adsorption of water. Density functional theory calculation revealed that the basic surface oxygens that share the corner of NbO6 units in [H2(Nb6O19)]8- maintained the negative charges even after proton adsorption. This proton capacity and the presence of un-protonated basic sites led to the water tolerance of the superbase catalysis.

Self-assembled mononuclear complexes: open metal sites and inverse dimension-dependent catalytic activity for the Knoevenagel condensation and CO2 cycloaddition

To lessen the greenhouse effect, measures such as improving the recovery of crude oil and converting carbon dioxide (CO₂) into valuable chemicals are necessary to create a sustainable low-carbon future. To this end, the development of efficient new oil-displacing agents and CO₂ conversion has aroused great interest in both academia and industry. The Knoevenagel condensation and CO₂ cycloaddition are the key reactions to solve the above problems. Four Cu- or Zn-based molecular complexes built from different ligands possessing hydrophilic-hydrophobic layers and different dimensionalities were chosen as solid catalysts for this study. Structural analysis revealed the presence of hydrophilic-hydrophobic layers and open metal sites in the low-dimensional complexes. To obtain deep insight into the reaction mechanism, first-principles density functional theory (DFT) calculations were carried out. These calculations confirmed that in the Knoevenagel condensation reaction, the final formation of benzylidenemalononitrile is the rate-determining step (an energy barrier (ΔE) value of 73.2 kJ mol^-1). The zero-dimensional (0D) Cu molecular complex with unsaturated metal centers, hydrophilic and hydrophobic layers, exhibited higher catalytic activity (yield: 100%, temperature: room temperature, and time: 2 h) compared with one- and two-dimensional Cu complexes. In the presence of a 0D Zn complex co-catalyzed with Br^- in the CO₂ cycloaddition reaction, the ΔE value reduces to 35.5 kJ mol^-1 for the ring opening of styrene oxide (SO), which is significantly lower than Br^- catalyzed (80.9 kJ mol^-1) reactions. The roles of unsaturated metal centers, hydrophilic-hydrophobic layers and dimensionality in the Knoevenagel condensation and CO₂ cycloaddition were explained in the results of structure-activity relationships.

Single-ion chelation strategy for synthesis of monodisperse Pd nanoparticles anchored in MOF-808 for highly efficient hydrogenation and cascade reactions

Ultrafine Pd nanoparticles are prepared using a single-ion precursor on a MOF-808 carrier. The ligand 2,3-pyrazinedicarboxylic acid (Pza) is dispersed in porous MOF-808 via grafting on formic acid sites, and thus Pd²⁺ ions are chelated by Pza to form a new single-ion precursor Pd@MOF-808-Pza. Then a Pd-nano@MOF-808-Pza catalyst is prepared by direct reduction of this precursor using NaBH₄. Material characterization reveals the homogeneous dispersion of 3-6 nm Pd nanoparticles within the MOF-808 matrix. Pd-nano@MOF-808-Pza exhibits excellent catalytic activity in the hydrogenation of unsaturated nitrogen-containing compounds, and other typical reactions, such as the Knoevenagel condensation, Suzuki/Heck cross-coupling, and hydrogen tandem reactions. In addition, density functional theory (DFT) calculations are carried out to elucidate the chelation of Pd²⁺ ions by Pza on MOF-808 and propose mechanisms of hydrogenation reactions. This work provides an effective reduction catalyst, and more importantly, a single-ion chelation strategy for design and synthesis of metal supported catalysts.

Three rare-earth incorporating 6-peroxotantalo-4-selenates and catalytic activities for imidation reaction

Functionalization with belt lanthanide groups allows for the crystallization and structural characterization of homometallic selenotantalates, CsK[Ln(H2O)6Se4(TaO2)6(OH)3O18]·nH2O (Ln = Eu/Gd, n = 14 (STD-Eu, STD-Gd) and Ln = Lu, n = 12 (STD-Lu)). The basket-shaped {Se4(TaO2)6} archetype is assembled in a simple one-pot reaction of Na2SeO3 and K8[Ta6O19]·17H2O in acidic aqueous medium (pH 2) and in the presence of hydrogen peroxide. This unit has been proven to be an effective precursor for the preparation of a range of new POMs containing the {Se4(TaO2)6} unit. The lanthanide derivatives STD-Eu, STD-Gd and STD-Lu have been fully characterized with single-crystal X-ray diffraction, IR spectroscopy, TG analysis and PXRD in the solid state. The photoluminescence and lifetime decay behaviours of STD-Eu have been studied at room temperature, and the photoluminescence spectrum displays the characteristic emission of the Eu3+ cation. In addition, the catalytic activities of STD-Eu, STD-Gd and STD-Lu on the reaction of phthalic anhydride with phenylamines have been investigated. STD-Eu shows good catalytic activities for imidation reactions.

Basicity of Isostructural Porous Ionic Crystals Composed of Nb/Ta-Substituted Keggin-Type Polyoxotungstates

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW12O40]5– (BW12), [SiW11NbO40]5– (SiW11Nb), and [SiW11TaO40]5– (SiW11Ta)) are synthesized. Experimental and theoretical characterizations...

POM-based porous supramolecular framework for the efficient sulfide-sulfoxide transformation with low molar O/S ratio

The selective oxidation of organic sulfides is a pivotal avenue to prepare sulfoxides that can act as synthetic intermediates of fine chemicals, bioactive molecules, and asymmetric catalysis ligands. To construct...