Efficient Catalytic Oxidative Desulfurization toward Thioether and Sulfur Mustard Stimulant by Polyoxomolybdate–Resorcin[4]arene-Based Metal–Organic Materials

Research Progress in the Degradation of Chemical Warfare Agent Simulants Using Metal-Organic Frameworks

Chemical warfare agents primarily comprise organophosphorus nerve agents, saliva alkaloids, cyanides, and mustard gas. Exposure to these agents can result in severe respiratory effects, including spasms, edema, and increased secretions leading to breathing difficulties and suffocation. Protecting public safety and national security from such threats has become an urgent priority. Porous metal-organic framework (MOF) materials have emerged as promising candidates for the degradation of chemical warfare agents due to their large surface area, tunable pore size distribution, and excellent catalytic performance. Furthermore, combining MOFs with polymers can enhance their elasticity and processability and improve their degradation performance. In this review, we summarize the literature of the past five years on MOF-based composite materials and their effectiveness in degrading chemical warfare agents. Moreover, we discuss key factors influencing their degradation efficiency, such as MOF structure, pore size, and functionalization strategies. Furthermore, we highlight recent developments in the design of MOF-polymer composites, which offer enhanced degradation performance and stability for practical applications in CWA degradation. These composite materials exhibit good performance in degrading chemical warfare agents, playing a crucial role in protecting public safety and maintaining national security. We can expect to see more breakthroughs in the application of metal-organic framework porous materials for degrading chemical warfare agents. It is hoped that these innovative materials will play a positive role in achieving social stability and security.

Cavity-Directed Synthesis of Labile Polyoxometalates for Catalysis in Confined Spaces

The artificial microenvironments inside coordination cages have gained significant attention for performing enzyme-like catalytic reactions by facilitating the formation of labile and complex molecules through a "ship-in-a-bottle" approach. Despite many fascinating examples, this approach remains scarcely explored in the context of synthesizing metallic clusters such as polyoxometalates (POMs). The development of innovative approaches to control and influence the speciation of POMs in aqueous solutions would greatly advance their applicability and could ultimately lead to the formation of elusive clusters that cannot be synthesized by using traditional methods. In this study, we employ host-guest stabilization within a coordination cage to enable a novel cavity-directed synthesis of labile POMs in aqueous solutions under mild conditions. The elusive Lindqvist [M6O19]2- (M=Mo or W) POMs were successfully synthesized at room temperature via the condensation of molybdate or tungstate building blocks within the confined cavity of a robust and water-soluble Pt6L4(NO3)12 coordination cage. Importantly, the encapsulation of these POMs enhances their stability in water, rendering them efficient catalysts for environmentally friendly and selective sulfoxidation reactions using H2O2 as a green oxidant in a pure aqueous medium. The approach developed in this paper offers a means to synthesize and stabilize the otherwise unstable metal-oxo clusters in water, which can broaden the scope of their applications.

Design and oxidative desulfurization of Ag/Ti heterometallic clusters based on Hard-Soft Acid-Base principle

Hard-Soft Acid-Base (HSAB) principle plays an important guiding role in the design and synthesis of novel clusters and coordination compounds, in which "soft acids prefer to react with soft bases, while hard acids have an affinity for hard bases". Based on HSAB principle, four Ag/Ti heterometallic clusters, including Ag2Ti10, Ag2Ti11 with "Ti-encapsulated Ag" configurations, and two "Ag-encapsulated Ti" structures Ag2Ti2 and Ag2Ti12, were synthesized under solvothermal conditions. In addition, Ag2Ti12 exhibited an efficient and stable catalytic activity for sulfide oxidation. This work provides not only a new structural model for the modulation of the catalytic oxidative desulfurization properties of Ag/Ti heterometallic clusters but also a new insight of the utilization of phosphine-containing ligands to regulate the structure of Ag/Ti heterometallic clusters.

Defective MOF-74 with ancillary open metal sites for the enhanced adsorption of chemical warfare agent simulants

The development of effective porous adsorbents plays a vital role in eliminating hazardous substances from the environment. Toxic chemicals, including chemical warfare agents (CWAs), pose significant risks to both humans and ecosystems, highlighting the urgency to create efficient porous adsorbents. Therefore, substantial attention has been directed towards advancing adsorption techniques for the successful eradication of CWAs from the environment. Herein, we demonstrate a rational approach for enhancing the adsorption capability of a porous metal-organic framework (MOF) by employing ancillary open metal sites within the MOF structure. To generate defective MOF-74 (D-MOF-74) with ancillary open metal sites, some of the 2,5-dihydroxy-1,4-bezenedicarboxylic acid (DHBDC) linkers originally present in the MOF-74 structure were replaced with 1,4-benzenedicarboxylic acid (BDC) linkers. The absence of hydroxyl groups in the BDC linkers compared to the original DHBDC linkers creates ancillary open metal sites, which enhance the adsorption ability of D-MOF-74 for CWA simulants such as dimethyl methyl phosphonate, 2-chloroethyl ethyl sulfide, and methyl salicylate by providing effective interaction sites for the targeted molecules. However, excessive creation of open metal sites causes the collapse of the originally well-developed MOF-74 structure, resulting in a substantial reduction in its empty space and a subsequent decline in adsorption efficiency. Thus, to produce a defective MOF with the best performance, it is necessary to replace an appropriate amount of organic linker and create suitable open metal sites. Moreover, D-MOF-74 displays excellent recyclability during consecutive adsorption cycles without losing its original structure and morphology, suggesting that D-MOF-74 is an effective and stable material for the removal of CWA simulants.

High Oxidation Desulfurization of Fuels Catalyzed by Vanadium-Substituted Phosphomolybdate@Polyaniline@Chitosan as an Inorganic-Organic Hybrid Nanocatalyst

From the environmental protection and human health perspectives, the design and synthesis of efficient and reusable oxidative desulfurization nanocatalysts has always been sought after by scientists and industries. In this regard, a new heterogeneous nanocatalyst (V-SPM@PANI@CH) was synthesized by immobilizing Keggin-type vanadium-substituted phosphomolybdate ([PVMo₁₁O₃₉]^4-) (named V-SPM) clusters on the surface of polyaniline (PANI) and chitosan (CH) polymers. The features of the assembled nanocatalyst were detected by Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray spectroscopy techniques in detail. The XRD studies indicated that the average crystallite size of V-SPM@PANI@CH was estimated to be about 36 nm. The catalytic performance of V-SPM@PANI@CH was investigated in the extractive and catalytic oxidation desulfurization (ECOD) procedure of real and thiophenic model gasoline by H₂O₂/AcOH (volume proportion of 2:1) as an oxidizing system. The optimal desulfurization conditions for ECOD reactions were as follows: 50 mL of model/real gasoline, 0.1 g of V-SPM@PANI@CH, reaction time of 60 min, and reaction temperature of 35 °C. Under the experimental conditions outlined above and the designed ECOD system, the content of sulfur in real gasoline could decline from 0.4985 to 0.0193 wt %, which corresponds to an efficiency of 96%. Moreover, the removal percentage of aromatic hydrocarbons, including thiophene (Th), benzothiophene (BT), and di-benzothiophene (DBT) as model fuels decreases in the order of DBT ≥ BT > Th under identical operating conditions. High catalytic activity was maintained with only a slight loss during five cycles. This work offers the ECOD system (V-SPM@PANI@CH/AcOH/H₂O₂) for the desulfurization of liquid fuels, which had a great repercussion on the ECOD efficiency.

A Zr-added Dawson-type poly(polyoxometalate)

A novel Zr-added trimer, [H₂N(CH₃)₂]₁₀H₁₄[(Zr₂P₂W₁₆O₆₁)₃]·7H₂O (1), has been made under hydrothermal conditions, and contains the highest number of Zr centers in known Dawson-type poly(POM)s. A remarkable feature of this study is the first discovery of a new type of divacant [α-5,10-P₂W₁₆O₆₀]^14- fragment, which assembles with Zr⁴⁺ ions to form a cyclic trimer. Furthermore, 1 as a heterogeneous catalyst exhibits high activity for the selective oxidative degradation of a sulfur mustard simulant CEES.

New insights into the reactivity of aminomethylene derivatives of resorc[4]arene: amine group transfer, conformational analysis, reaction mechanism

Using the example of aminomethylene derivatives of resorc[4]arene and their Michael reaction with 4-hydroxycoumarin, the possibility of transferring an amine molecule from substrate to product is demonstrated. The conformation of the aminocoumarin derivatives of resorc[4]arene formed is controlled by the polarity of the solvent. For one of the products, conformational analysis was performed by kinetic sampling using metadynamics (MTD). The energies of the final set of conformers were calculated by DFT (r2scan-3c). A reaction mechanism based on multiscale (ONIOM) Nudged Elastic Band (NEB-TS) reaction profile calculations is discussed.

Using the example of aminomethylene derivatives of resorc[4]arene and their Michael reaction with 4-hydroxycoumarin, the possibility of transferring an amine molecule from substrate to product is demonstrated.

Two polymolybdate-directed Zn(ii) complexes tuned by a new bis-pyridine-bis-amide ligand with a diphenylketone spacer for efficient ampere sensing and dye adsorption

Two polymolybdate-based Zn(ii) complexes were constructed from a new bis-pyridine-bis-amide ligand, which can be used as electrocatalysts and electrochemical sensors for Cr(vi), KBrO3, H2O2 and AA, and exhibit selective adsorption of CV and MB.

Two new calix[4]resorcinarene-based coordination cages adjusted by metal ions for the Knoevenagel condensation reaction

Two new calix[4]resorcinarene-based coordination cages, namely, [Zn4(TPC4R)(PDC)4]·2DMF·6H2O (1-Zn) and [In11(TPC4R)2(PDC)16(μ2-OH)2(H2O)2]·[(CH3)2NH2]·8DMF·20H2O·EtOH (2-In), have been synthesized via solvothermal reactions (TPC4R = tetra(2-(4H-pyrazol-3-yl)pyridine)calix[4]resorcinarene, H2PDC = 3,5-pyridinedicarboxylic acid, DMF = N,N'-dimethylformamide). By carefully tuning different metal ions, two structurally different cages 1-Zn and 2-In were achieved. The former shows a bowl-shaped structure, while the latter features a dumbbell-like structure. After activation, they exhibited unsaturated Zn(ii) or In(iii) Lewis acid sites and the free nitrogen Lewis base sites of the PDC2-. Therefore, they were employed as catalysts for the Knoevenagel condensation reaction in the absence of a solvent. Particularly, 1-Zn featured high structural stability and enhanced the catalytic activity.

Reduced polyoxomolybdate immobilized on reduced graphene oxide for rapid catalytic decontamination of a sulfur mustard simulant

Keggin-type polyoxometalates (POMs) were immobilized on poly(diallyldimethylammonium chloride) (PDDA) functionalized reduced graphene oxide (rGO) by a facile and broad-spectrum hydrothermal method. The prepared POMs@PDDA-rGO composites (POM = H3PMo12O40, H3PW12O40, H5PMo10V2O40) have been thoroughly characterized using a series of techniques. The three composites can catalyze the oxidative decontamination of a sulfur mustard simulant, 2-chloroethyl ethyl sulfide (CEES) in the order of PMo12@PDDA-rGO > PMo10V2@PDDA-rGO > PW12@PDDA-rGO. Notably, under ambient conditions PMo12@PDDA-rGO can convert 99% of CEES within 30 min in the presence of nearly stoichiometric aqueous H2O2 (3 wt%) and its catalytic activity is significantly higher than that of homogeneous H3PMo12O40. XPS spectral analysis and control experiments indicate that the Mo center of POM is reduced from +6 to +5 during the hydrothermal process, and the excellent catalytic performance is related to the reduction of Mo. Moreover, the PMo12@PDDA-rGO composite is stable during the decontamination process and it can be used for at least five cycles without loss of activity.

Self-Assembly of Polyoxometalate-Resorcin[4]arene-Based Inorganic-Organic Complexes: Metal Ion Effects on the Electrochemical Performance of Lithium Ion Batteries

With their adjustable structures and diverse functions, polyoxometalate (POM)-resorcin[4]arene-based inorganic-organic complexes are a kind of potential multifunctional material. They have potential applications for lithium ion batteries (LIBs). However, the relationship between different coordinated metal ions and electrochemical performance has rarely been investigated. Here, three functionalized POM-resorcin[4]arene-based inorganic-organic materials, [Co₂ (TMR4 A)₂ (H₂ O)₁₀ ][SiW₁₂ O₄₀ ]⋅2 EtOH⋅4.5 H₂ O (1), [Ni₂ (TMR4 A)₂ (H₂ O)₁₀ ][SiW₁₂ O₄₀ ]⋅4 EtOH⋅13 H₂ O (2), and [Zn₂ (TMR4 A)₂ (H₂ O)₁₀ ][SiW₁₂ O₄₀ ]⋅2 EtOH⋅2 H₂ O (3), have been synthesized. Furthermore, to enhance the conductivities of these compounds, 1-3 were doped with reduced graphene oxide (RGO) to give composites 1@RGO-3@RGO, respectively. As anode materials for LIBs, 1@RGO-3@RGO can deliver very high discharge capacities (1445.9, 1285.0 and 1095.3 mAh g^-1 , respectively) in the initial run, and show discharge capacities of 898, 665 and 651 mAh g^-1 , respectively, at a current density of 0.1 A g^-1 over 100 runs. More importantly, the discharge capacities of 319, 283 and 329 mAh g^-1 were maintained for 1@RGO-3@RGO even after 400 cycles at large current density (1 A g^-1 ).

Three Resorcin[4]arene-Based Two-Dimensional Zn(II) Supramolecular Isomers Synthesized via a Structure-Directing Strategy for Knoevenagel Condensation

Herein, in the presence of three structure-directing agents (SDAs), a family of imidazole-functionalized resorcin[4]arene-based coordination polymers (CPs), [Zn(TIC4R)(HCOO)]·HCOO·0.5DMF·1.5H₂O (1), [Zn(TIC4R)(CN)]·HCOO·DMF·2.5H₂O (2), and [Zn(TIC4R)(H₂O)]·2HCOO·2H₂O (3), were assembled under solvothermal conditions [TIC4R = tetra(imidazole) resorcin[4]arene]. 1 exhibits a double-layer structure with rectangle windows, and 2 and 3 display monolayer structures. The layers of CPs 2 and 3 are slides with different offsets along the a-axis. In addition, three CPs were used as catalysts to catalyze Knoevenagel condensations. Strikingly, all CPs exhibit remarkable catalytic performance for several substrates. To the best of our knowledge, this is the first time that a small organic acid as SDA was used in the syntheses of resorcin[4]arene-based supramolecular isomers.

Polyoxometalate-Templated Cobalt-Resorcin[4]arene Frameworks: Tunable Structure and Lithium-Ion Battery Performance

By employing a bowl-like tetra(benzimidazole)resorcin[4]arene (TBR4A) ligand, two new polyoxometalate-templated metal-organic frameworks (POMOFs), [Co₈Cl₁₄(TBR4A)₆]·3[H_3.3SiW₁₂O₄₀]·10DMF·11EtOH·20H₂O (1) and [Co₃Cl₂(TBR4A)₂(DMF)₄]·[SiW₁₂O₄₀]·2EtOH·3H₂O (2), have been prepared under solvothermal conditions (DMF = N,N'-dimethylformamide). 1 shows a 2D cationic layer, whereas 2 exhibits a 3D framework. Remarkably, the Keggin POMs in 1 and 2 were located in the cavities formed by two bowl-like resorcin[4]arenes in sandwich fashions. Their framework structures were highly dependent on the coordination modes of the TBR4A ligands. To increase the conductivity of POMOFs, the samples of 1 and 2 were loaded on the conductive polypyrrole-reduced graphene oxide (PPy-RGO) via ball milling (1@PG and 2@PG). Then, the obtained composites experienced calcination at a proper temperature to produce 1@PG-A and 2@PG-A. The resulting 1@PG-A and 2@PG-A composites, with improved conductivities, uniform sizes and micropores, exhibited promising electrochemical performance for lithium-ion batteries. We herein proposed a size-controlled route for the rational fabrication of functional POMOFs and their usage in energy fields.

Assembly of polyoxometalate-thiacalix[4]arene-based inorganic-organic hybrids as efficient catalytic oxidation desulfurization catalysts

Self-assembly of polyoxometalates, Ni(ii)/Ag(i) cations and tetra-[5-(mercapto)-1-methyltetrazole]-thiacalix[4]arene (L) yielded three inorganic-organic hybrids, namely, [Ni₃L₂(CH₃OH)₆(H₂O)₄][PMo₁₂O₄₀]₂·3CH₃OH·2H₂O (1), [Ni₃L₂(CH₃OH)₆(H₂O)₄][PW₁₂O₄₀]₂·3CH₃OH·2H₂O (2) and [Ag₃L(PMo₁₂O₄₀)] (3). In hybrids (1) and (2), Ni(ii) cations are linked by L ligands to produce layered frameworks, and H bonds among the [PMo₁₂O₄₀]^3-/[PW₁₂O₄₀]^3- anions and L ligands lengthen the structures to form 3D supramolecular architectures. Hybrid (3) exhibits a 3D architecture, of which Ag(i) cations not only coordinated with the N and O atoms of L ligands and [PMo₁₂O₄₀]^3- anions simultaneously, but also connected each other by Ag-Ag interactions. It is worth mentioning that 1 and 3 as recyclable catalysts show excellent heterogeneous catalytic activity in oxidation desulfurization reactions.

Crystal structure and photophysical properties of a novel polyoxomolybdate porphyrin

A novel polyoxomolybdate with a diprotonated porphyrin as counter-cation, namely, 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,22H,23H,24H-porphine(2+) hexamolybdate(VI) pentahydrate, (C₄₈H₃₂N₄O₈)[Mo₆O₁₉]·5H₂O or (H₂TCPP)[Mo₆O₁₉]·5H₂O, I, was prepared via the hydrothermal reaction of MoCl₅, 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,23H-porphine (TCPP) and distilled water. The crystal structure of hydrated polyoxometalate (POM) salt I was characterized by single-crystal X-ray diffraction. The compound is characterized by an isolated (zero-dimensional, 0D) structure, because it cannot extend via covalent bonds. The structure contains one [Mo₆O₁₉]^2- anion, one (H₂TCPP)²⁺ cation and five lattice water molecules. Each of the Mo⁶⁺ ions is six-coordinated and displays a distorted octahedral motif. The (H₂TCPP)²⁺ cation displays a distorted saddle motif. A three-dimensional (3D) supramolecular framework is formed via hydrogen-bonding interactions. The compound shows a red photoluminescence emission.

Multi-functional photoelectric sensors based on a series of isopolymolybdate-based compounds for detecting different ions

Five molybdate-based compounds can be used as multi-functional photoelectric sensors. They act as electrochemical sensors for sensing NO₂⁻ and Cr(vi). Compound 3 also can be used as a fluorescent sensor for detecting Hg²⁺.