Crystalline Molecular Assemblies of Complexes Showing Eightfold Coordinated Niobium(IV) Dodecahedral Geometry in the Pyridine-Dicarboxylic Acid System

The reactivity of 2,3-pyridine-dicarboxylic (known as quinolinic or H2qui) acid and 2,5-pyridine-dicarboxylic (known as isocinchomeronic or H2icc) acid has been investigated as a complexing agent toward the niobium(IV) tetrachloride precursor (NbCl4·2THF) in different organic solvent mixtures. It resulted in the isolation of four crystalline assemblies of mononuclear coordination complexes 1–4 [Nb(HL)4·solvent], where HL is the monoprotonated quinolinate (Hqui) ligand (complexes 1–3) or the monoprotonated isocinchomeronate ligand (complex 4). For each complex, the discrete niobium(IV) center is eightfold coordinated to four oxygen atoms from the deprotonated carboxylate arm and four nitrogen atoms from the pyridine part of the dicarboxyl ligand with a dodecahedral environment [NbO4N4]. The remaining carboxyl arm (either in 3 or in 5 position) remained under its protonated form, leading to neutral [Nb(HL)4] moieties for compounds 1, 2, and 4, or the anionic [Nb(qui)(Hqui)3]− moiety for compound 3. The complexes are observed in various molecular arrangements, involving intercalated solvent molecules such as acetonitrile in compound 1 ([Nb(Hqui)4·0.8(CH3CN)], obtained at room temperature), a mixture of acetonitrile and pyridine in compound 2 ([Nb(Hqui)4·0.7CH3CN·2PYR], obtained via the solvothermal reaction at 80 °C), a mixture of pyridine and triethylamine, in addition with water and chloride species, in compound 3 ([Nb(qui)(Hqui)3·Cl·HPYR·HTEA·1.5H2O], obtained via solvothermal reaction at 80 °C), and N,N-dimethylformamide in compound 4 ([Nb(Hicc)4·6DMF], obtained at room temperature). The d1 configuration expected for the niobium(IV) centers has been analyzed by magnetic measurements, as well as by EPR and XPS. An anti-ferromagnetism transition has been observed at very low temperatures for complexes 1 (3.6 K) and 4 (3.3 K), for which the shortest Nb···Nb interatomic lengths occur

Wafer-Scale Performance Mapping of Magnetron-Sputtered Ternary Vanadium Tungsten Nitride for Microsupercapacitors

To power Internet-of-Things applications, new materials are currently being investigated as efficient electrodes for microsupercapacitors. In recent years, multicationic materials were demonstrated to be an attractive new class of materials for electrodes. In this study, we deposited vanadium tungsten nitride by cosputtering. Our film shows excellent electrochemical performance, a capacitance of 700 F·cm–3, and no loss in capacitance retention after 5000 cycles. In addition, the properties of the film were investigated in many aspects using advanced characterization and mapping techniques. Our approach opens new perspectives and provides a powerful characterization tool for electrochemical materials

Water-Based Synthesis of Zr₆‑Based Metal–Organic Framework Nanocrystals with Sulfonate Functions: Structural Features and Application to Fructose Dehydration

A series of zirconium-based metal–organic framework (MOF) nanocrystals (95–211 nm) displaying sulfonate functions (UiO-66-SO3H) was prepared in N,N-dimethylformamide (DMF)the conventional solventand water, and their physicochemical properties were thoroughly investigated. In particular, X-ray diffraction results suggest that upon replacing DMF with water, the resulting MOF crystal structure presents a highly defective structure belonging to the space group Im3̅ instead of typical Fm3̅m. The acid catalysts were applied to the fructose dehydration into 5-hydroxymethylfurfural (5-HMF). Complete conversion of fructose over UiO-66-SO3H prepared in water was reached after only 30 min at 100 °C, in line with its stronger Brønsted acidity. In comparison, its counterpart prepared in DMF showed only 30% fructose conversion. Moreover, the intrinsic catalytic effect at 80 °C was only observed with the water-based UiO-66-SO3H. Without reactivation of the catalyst, recycling tests demonstrated the preservation of its structural integrity upon nine consecutive cycles, while a gradual loss of the catalyst activity was attributed to the humin adsorption on the MOFs