Base Catalysis by Mono- and Polyoxometalates

Facile synthesis of polyoxometalate supported on magnetic graphene oxide as a hybrid catalyst for efficient oxidation of aldehydes

In the present study, Anderson-type polyoxometalate [N(C₄H₉)₄] [FeMo₆O₁₈(OH)₆] (FeMo₆) was immobilized on amino-modified magnetic graphene oxide and employed as a new hybrid catalyst in oxidation of aldehydes to carboxylic acids. The synthesized hybrid catalyst Fe₃O₄/GO/[FeMo₆] was characterized using thermogravimetric analysis (TGA), scanning electron microscopies (SEM), Fourier transform infrared (FT-IR), vibrating sample magnetometry (VSM), energy-dispersive X-ray analysis (EDX), Raman spectroscopy and inductively coupled plasma atomic emission spectroscopy (ICP-OES). The results indicated that our catalyst was quite active in oxidizing the aldehydes to their corresponding carboxylic acids in the presence of hydrogen peroxide. The synthesized catalyst can be easily separated from the reaction medium and reused for six consecutive runs without a significant reduction in reaction efficiency.

Incorporating heterogeneous lacunary Keggin anions as efficient catalysts for solvent-free cyanosilylation of aldehydes and ketones

Polyoxometalates (POMs) as efficient catalysts can be used a wide range of chemical transformations due to their tunable Brønsted/Lewis-acidity and redox properties. Herein, we reported two hybrid and heterogeneous lacunary Keggin catalysts: (TBA)₇[PW₁₁O₃₉] (TBA-PW₁₁) and (TBA)₈[SiW₁₁O₃₉]·4H₂O (TBA-SiW₁₁) (TBA⁺: tetrabutylammonium) in which [XW₁₁O₃₉]ⁿ⁻ anions were coated by TBA⁺ cations. In this form, TBA⁺ can easily trap reactants on the surface of the catalysts and increase the catalytic reaction. Therefore, the catalytic performance of both POMs was tested in cyanosilylation of numerous compounds bearing-carbonyl group and trimethylsilyl cyanide under solvent-free conditions. TBA-PW₁₁ is more effective than TBA-SiW₁₁, conceivably due to the higher Lewis acidity of the P than the Si center and to the higher accessibility of the metal centers in the framework structure. Noteworthy, the recyclability and heterogeneity of both POMs catalysts were also examined, and the results confirmed that they remain active at least after three recycling procedures.

Improvement of the Hydrolytic Stability of the Keggin Molybdo- and Tungsto-Phosphate Anions by Cyclodextrins

Keggin-type molybdo- and tungsto-phosphate polyoxoanions are among the most popular polyoxometalates (POMs) but suffer from their limited stability at low pH in aqueous solution. Their superchaotropic properties generate strong supramolecular complexes with cyclodextrins (CDs), which significantly affect the hydrolytic stability of POM. This chaotropically driven stabilization effect was systematically monitored by ³¹P NMR spectroscopy covering a wide range of pH (from 0 to 8) and varying the nature of the CD (α-, β-, and γ-form). A shift of ca. two pH units of the stability domains of these POMs was found in the presence of two equivalents of γ-CD compared to pure water, leading to keep intact the PW₁₂O₄₀^3- anion without any decomposition up to pH 3.5 (versus 1.5 in pure water) and pH 2.5 for PMo₁₂O₄₀^3-, which begins to decompose even at pH 0 in pure water. The effect of the smaller CDs (α- and β-form) is much less pronounced (only 0.5 pH units shift of the stability domain) confirming the importance of host-guest size matching to form a sandwich-type inclusion complex and thus protect the POM structure against basic hydrolysis. Such complexation was further supported by ¹⁸³W and ¹H NMR spectroscopy. Finally, using quantitative ³¹P NMR analyses, the new speciation and formation constants of phospho-molybdates and phospho-tungstates in the presence of cyclodextrins are determined and compared to those previously reported in pure water or in the 50:50 water/1,4-dioxane mixture.

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...

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

Polyoxometalates (POMs) are oxide cluster anions composed of high-valence early transition metals and are widely used as catalysts. Yet base catalysis of POMs remains an ongoing challenge; group V (V, Nb, and Ta) elements form more negatively charged POMs than group VI (Mo and W) elements, and in particular, polyoxoniobates and polyoxotantalates are known to show strong basicity in solution due to the highly negative surface oxygen atoms. Herein, we report for the first time porous ionic crystals (PICs) comprising Nb or Ta. The PICs are composed of Dawson-type Nb/W or Ta/W mixed-addenda POMs with oxo-centered trinuclear Cr^III carboxylates and potassium ions as counter cations to control the crystal structure. Among the PICs, those with Nb or Ta tri-substituted POMs exhibit the highest yield (78-82%) and selectivity (99%) towards the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate (353 K, 6 h), which is a typical base-catalyzed reaction, as reusable solid catalysts, and they can also catalyze the reaction of other active methylene compounds. A detailed investigation into the crystal structures together with DFT calculations and in situ IR spectroscopy with methanol as a basic probe molecule shows that the exposure of [Nb₃O₁₃] or [Ta₃O₁₃] units with highly negative surface oxygen atoms to the pore surface of PICs is crucial to the catalytic performance. These findings based on the composition-structure-function relationships show that Nb- and Ta-containing PICs can serve as platforms for rational designing of heterogeneous base catalysts.

Synthesis of cyanooxovanadate and cyanosilylation of ketones

The cyanosilylation was performed by using metavanadate catalysts, and in situ measurements revealed the formation of [VO₂(CN)₃]²⁻ and [VO₄TMS₂]⁻ under reaction conditions. The reaction of [VO₂(CN)₃]²⁻, trimethylsilyl cyanide (TMSCN), and water afforded [VO₄TMS₂]⁻ and CN⁻, which reacted with ketones to yield the corresponding cyanohydrin trimethylsilyl ethers over [VO₂(CN)₃]²⁻. Compound [VO₂(CN)₃]²⁻ showed high catalytic performance for cyanosilylation of various carbonyl compounds. In the case of n-hexanal, turnover frequency reached up to 250 s⁻¹.

Two key catalytic vanadium species involved in cyanosilylation of ketones were observed by in situ measurement.

Valorisation of plastic waste via metal-catalysed depolymerisation

Metal-catalysed depolymerisation of plastics to reusable building blocks, including monomers, oligomers or added-value chemicals, is an attractive tool for the recycling and valorisation of these materials. The present manuscript shortly reviews the most significant contributions that appeared in the field within the period January 2010–January 2020 describing selective depolymerisation methods of plastics. Achievements are broken down according to the plastic material, namely polyolefins, polyesters, polycarbonates and polyamides. The focus is on recent advancements targeting sustainable and environmentally friendly processes. Biocatalytic or unselective processes, acid–base treatments as well as the production of fuels are not discussed, nor are the methods for the further upgrade of the depolymerisation products.

Inhibition of Mitochondrial ATP Synthesis and Regulation of Oxidative Stress Based on {SbW8 O30 } Determined by Single-Cell Proteomics Analysis

The 10-nuclear heteroatom cluster modified {SbW₈ O₃₀ } was successfully synthesized and exhibited inhibitory activity (IC₅₀ =0.29 μM). Based on proteomics analysis, Na₄ Ni₂ Sb₂ W₂ -SbW₈ inhibited ATP production by affecting the expression of 16 related proteins, hindering metabolic functions in vivo and cell proliferation due to reactive oxygen species (ROS) stress. In particular, the low expression of FAD/FMN-binding redox enzymes (relative expression ratio of the experimental group to the control=0.43843) could be attributed to the redox mechanism of Na₄ Ni₂ Sb₂ W₂ -SbW₈ , which was consistent with the effect of polyoxometalates (POMs) and FMN-binding proteins on ATP formation. An electrochemical study showed that Na₄ Ni₂ Sb₂ W₂ -SbW₈ combined with FMN to form Na₄ Ni₂ Sb₂ W₂ -SbW₈ -2FMN complex through a one-electron process of the W atoms. Na₄ Ni₂ Sb₂ W₂ -SbW₈ acted as catalase and glutathione peroxidase to protect the cell from ROS stress, and the inhibition rates were 63.3 % at 1.77 μM of NADPH and 86.06 % at 10.62 μM of 2-hydroxyterephthalic acid. Overall, our results showed that POMs can be specific oxidative/antioxidant regulatory agents.

Magneto-structural correlations of cyclo-tetravanadates functionalized with mixed-ligand copper(ii) complexes

Bimetallic materials based on tetravanadate anions and mixed ligand copper(ii) complexes were readily synthesized under non-hydrothermal conditions. The compounds show interesting structural and magnetic diversity mediated by copper symmetry.

Heterogenisation of polyoxometalates and other metal-based complexes in metal–organic frameworks: from synthesis to characterisation and applications in catalysis

This review provides a thorough overview of composites with molecular catalysts (polyoxometalates, or organometallic or coordination complexes) immobilised into MOFs via non-covalent interactions.

Hofmeister effect in the Keggin-type polyoxotungstate series

The chaotropic character of Keggin-type polyoxotungstate anions was evaluated with respect to their ability to bind to γ-cyclodextrin (γ-CD) by varying the global charge density of the nanometer-sized polyanion.

Imidazole-induced self-assembly of polyoxovanadate cluster organic framework for efficient Knoevenagel condensation under mild conditions

Three new polyoxovanadates have been made, one of them displays highly efficient heterogeneous solvent-free catalytic activity and excellent recyclability in the Knoevenagel condensation at room temperature.

Polyoxometalate-Based Catalysts for CO2 Conversion

Polyoxometalates (POMs) are a diverse class of anionic metal-oxo clusters with intriguing chemical and physical properties. Owing to unrivaled versatility and structural variation, POMs have been extensively utilized for catalysis for a plethora of reactions. In this focused review, the applications of POMs as promising catalysts or co-catalysts for CO₂ conversion, including CO₂ photo/electro reduction and CO₂ as a carbonyl source for the carbonylation process are summarized. A brief perspective on the potentiality in this field is proposed.

Cycloaddition of CO2 with epoxides and esterification reactions using the porous redox catalyst Co-POM@MIL-101(Cr)

The catalytic activity of the Co-POM@MIL-101(Cr) composite in solvent-free cycloaddition of CO₂ to epoxides and esterification of acetic acid with alcohols is due to an outer-sphere electron transfer mechanism using the Co(iii)/Co(ii) redox pair.

Calix[4]arene-based polyoxometalate organic–inorganic hybrid and coordination polymer as heterogeneous catalysts for azide–alkyne cycloaddition and Knoevenagel condensation reaction

One POM-based Cu(i)-hybrid and one Cd(ii) compound have been achieved by a calix[4]arene ligand. They exhibit efficient catalytic abilities for azide–alkyne cycloaddition and Knoevenagel condensation reactions, respectively.

Toward Exploiting the Behavior of Niobium-Containing Mesoporous Silicates vs. Polyoxometalates in Catalysis

Classification of polyoxometalates (POMs) is based on their chemical composition, basically represented by two general formulae: a) [MmOy]p- b) [XxMmOy]q-, where M is the main transition metal, O is the oxygen atom and X can be a non-metal atom such as Si. Additionally, in the most cases, the structure of the polyoxometalates is derived from a combination of octahedral units MO6 with a central metal atom M and the oxygen atoms placed at their corners. In such octahedra, oxygen atoms allow the condensation between two octahedral units, while one oxygen atom (or max. two atoms) makes double bond with the central metal atom and is not shared with other metal atoms within the complex (terminal oxygens). On the other hand, niobium-containing mesoporous silicates contain mainly MO4 tetrahedra and reveal superior activity in heterogeneous catalysis. Thus, the proper coordination of niobium is crucial for the catalytic activity and will be deeply discussed. The similarity in the catalytic behavior of niobium-polyoxometalates and heterogeneous niobium single-site catalysts in selective oxidations will be demonstrated.