Synthesis and Characterization of 6-Ti-Substituted Polyoxomolybdate with High Catalytic Activity for Sulfide Oxidation

A 6-Ti-substituted polyoxometalate, (NH4)5Cs7Na3H2[Cs@(Ti2GeMo10O39)3]·34H2O (1), was synthesized by reacting (NH4)6Mo7O24·4H2O, GeO2, and TiOSO4 through the conventional aqueous method. Polyanion 1a is composed of three {Ti2GeMo10} segments linked by Ti-O-Ti linkages and shows a trefoil-shaped structure. Furthermore, one Cs+ cation is encapsulated in the cavity of 1a. Notably, it possesses the highest number of Ti centers among the reported polyoxomolybdates. In addition, serving as a high-efficiency heterogeneous catalyst, 1 enables the conversion of methyl phenyl sulfide within 20 min, yielding 96.4% of the corresponding sulfoxide with good recyclability.

Stepwise and Controllable Synthesis of Mesoporous Heterotrimetallic Catalysts Based on Predesigned Al4 Ln4 Metallocycles

The motivation for making heterometallic compounds stemmed from their emergent synergistic properties and enhanced capabilities for applications. However, the atomically precisely controlled synthesis of heterometallic compounds remains a daunting challenge of the complications that arise when applying several metals and linkers. Herein, a stepwise and controlled method is reported for the accurate addition of second and third metals to homometallic aluminum macrocycles based on the synergistic coordination and hard-soft acid-base theory. These heterometallic compounds showed a good Lewis acid catalytic effect, and the addition of hetero-metals significantly improved the catalytic effect and rate, among that the conversion rate of compound AlOC-133 reached 99.9% within half an hour. This method combines both the independent controllability of stepwise assembly with the universality of one-step methods. Based on the large family of clusters, the establishment of this method paves the way for the controllable and customized molecular-level synthesis of heterometallic materials and creates materials customized for preferential application.

Synthesis and Characterisation of Monolacunary Keggin Monovanado-deca-tungstophosphate and Its Complexes with Transition Metal Cations

Five new complexes with metal cations (Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, and Cu²⁺) of monolacunary Keggin monovanado-deca-tungstophosphate, K₈[PVW₁₀O₃₉]·15H₂O, have been synthesised. The molar ratio of the combination between metal cations and K₈[PVW₁₀O₃₉]·15H₂O has been established to be 1:1, and its general molecular formulas were found to be: K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O (n = 5 for M = Fe³⁺ and n = 6 for M = Mn²⁺, Co²⁺, Ni²⁺, and Cu²⁺). Optimal conditions for complexes' synthesis (pH, temperature, and reaction time) have been determined. The characterisation of K₈[PVW₁₀O₃₉]·15H₂O and of its compounds K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O have been performed using AAS, TG-DTA-DTG, UV-VIS, IR, Raman, and powder XRD methods. In UV spectra, two maximums of absorption were obtained, at 200 and 250 nm, characteristic of Keggin polycondensate compounds. The coordination of cations Ni²⁺, Co²⁺, and Cu²⁺ through oxygen atoms from K₈[PVW₁₀O₃₉]·15H₂O in an octahedron system has been reflected with VIS spectroscopy. All these methods have proved the compositions and structures of K₈[PVW₁₀O₃₉]·15H₂O and K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O, their similarity with other vanadotungstophosphates, and their achievements in the Keggin class. Additionally, all analysis methods have shown an increase in the degree of structural symmetry and the thermal stability of a polyoxoanion complex after attaching metal cations compared to the monolacunary, K₈[PVW₁₀O₃₉]·15H₂O.

Recent Advances of Ti/Zr-Substituted Polyoxometalates: From Structural Diversity to Functional Applications

Polyoxometalates (POMs), a large family of anionic polynuclear metal-oxo clusters, have received considerable research attention due to their structural versatility and diverse physicochemical properties. Lacunary POMs are key building blocks for the syntheses of functional POMs due to their highly active multidentate O-donor sites. In this review, we have addressed the structural diversities of Ti/Zr-substituted POMs based on the polymerization number of POM building blocks and the number of Ti and Zr centers. The synthetic strategies and relevant catalytic applications of some representative Ti/Zr-substituted POMs have been discussed in detail. Finally, the outlook on the future development of this area is also prospected.

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.

Metal-Organic Frameworks Based on Group 3 and 4 Metals

Metal-organic frameworks (MOFs) based on group 3 and 4 metals are considered as the most promising MOFs for varying practical applications including water adsorption, carbon conversion, and biomedical applications. The relatively strong coordination bonds and versatile coordination modes within these MOFs endow the framework with high chemical stability, diverse structures and topologies, and interesting properties and functions. Herein, the significant progress made on this series of MOFs since 2018 is summarized and an update on the current status and future trends on the structural design of robust MOFs with high connectivity is provided. Cluster chemistry involving Y, lanthanides (Ln, from La to Lu), actinides (An, from Ac to Lr), Ti, and Zr is initially introduced. This is followed by a review of recently developed MOFs based on group 3 and 4 metals with their structures discussed based on the types of inorganic or organic building blocks. The novel properties and arising applications of these MOFs in catalysis, adsorption and separation, delivery, and sensing are highlighted. Overall, this review is expected to provide a timely summary on MOFs based on group 3 and 4 metals, which shall guide the future discovery and development of stable and functional MOFs for practical applications.

A micro-environment tuning approach for enhancing the catalytic capabilities of lanthanide containing polyoxometalate in the cyanosilylation of ketones

The as-synthesized (TBA)8H5[Nd(SiW11O39)2] manifested high catalytic activity for cyanosilylation of ketones, and its catalytic activity could be improved further through rational design of the reaction micro-environment beyond the molecular level, and the corresponding mechanism has been systematically studied.

H-shaped oxalate-bridging lanthanoid-incorporated arsenotungstates

A series of oxalate-bridging di-Ln³⁺-incorporated H-shaped polyoxometalates were synthesized and their stability in solution was evidenced by Raman spectroscopy.

Double-Oxalate-Bridging Tetralanthanide Containing Divacant Lindqvist Isopolytungstates with an Energy Transfer Mechanism and Luminous Color Adjustablility Through Eu3+/Tb3+ Codoping

A double-oxalate-bridging tetra-Gd³⁺ containing divacant Lindqvist dimeric isopolytungtate Na₁₀[Gd₂(C₂O₄)(H₂O)₄(OH)W₄O₁₆]₂·30H₂O (Gd₄W₈) was obtained based on the reaction of Na₂WO₄·2H₂O, H₂C₂O₄, and GdCl₃ in aqueous solution. Its dimeric polyoxoanion is established by two divacant Lindqvist [W₄O₁₆]^8- segments connected by a rectangular tetra-nuclearity [Gd₄(C₂O₄)₂(H₂O)₈(OH)₂]⁶⁺ cluster. Notably, neighboring trinuclear [Na₃O₄(H₂O)₁₁]^5- clusters are interconnected to construct a picturesque 1-D sinusoidal Na-O cluster chain. The most outstanding characteristic is that 1-D sinusoidal Na-O cluster chains combine [Gd₂(C₂O₄)(H₂O)₄(OH)W₄O₁₆]₂^10- polyoxoanions together, giving rise to an intriguing 3-D extended porous framework. The red emitter Eu³⁺ ions and green emitter Tb³⁺ ions are first codoped into Gd₄W₈ to substitute Gd³⁺ ions for the exploration of the energy transfer (ET) mechanism between Eu³⁺ and Tb³⁺ ions and the color-tunable PL property in the isopolytungtate system. The PL emission spectra and decay lifetime measurements of the Eu³⁺/Tb³⁺ codoped Gd₄W₈ system illustrate that under excitation at 370 nm, Tb³⁺ ions can transfer energy to Eu³⁺ ions. When the molar concentration of Tb³⁺ ions is fixed at 0.9 and that of the Eu³⁺ ions gradually increases from 0.01 to 0.08, the calculated ET efficiency (η_ET) from Tb³⁺ to Eu³⁺ ions increases from 7.9% for Gd_0.36Tb_3.6Eu_0.04W₈ to 67.3% for Gd_0.08Tb_3.6Eu_0.32W₈. The energy transfer mechanism (Tb³⁺ → Eu³⁺) is a nonradiative dipole-dipole interaction. Furthermore, upon excitation at 370 nm, Eu₄W₈ and Tb₄W₈ show visible red- and green-emitting lights, respectively. When codoping trace amounts of Eu³⁺ ions in Tb₄W₈, under excitation at 370 nm, Tb_3.92Eu_0.08W₈ displays near white-light emission.

Discovery of a New Family of Polyoxometalate-Based Hybrids with Improved Catalytic Performances for Selective Sulfoxidation: The Synergy between Classic Heptamolybdate Anions and Complex Cations

A series of functional cation-regulated isopolymolybdate-based organic-inorganic hybrid compounds, Na₂H₂[Mo₄O₁₂(C₈H₁₇O₅N)₂]·10H₂O (1), Na₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·10H₂O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris = 2,2-Bis(hydroxymethyl)-2,2',2″-nitrilotriethanol], and (NH₄)₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·6H₂O (M = Zn, 6; Cu, 7), were synthesized and characterized toward advanced molecular catalyst design. Compound 1 is a covalently bonded adduct, and its self-assembly process can be probed by electrospray ionization mass spectrometry (ESI-MS). Compounds 2-7 are polyoxometalate (POM)-based hybrids containing classic heptamolybdate anions and complex cations with Bis-tris ligands. All of these compounds showed remarkable catalytic effects for selective sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other transition-metal-containing compounds and the compound constructed by Na⁺ has higher catalytic activity than that constructed by NH₄⁺. The mechanism studies show that the improvements of the catalytic performance are caused by the synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV-vis spectra revealed that the POM anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate thioanisole with complex cations was characterized by NMR experiments and UV-vis spectra. Thus, a new synergistic mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.

Novel primary amide-based cationic metal complexes: green synthesis, crystal structures, Hirshfeld surface analysis and solvent-free cyanosilylation reaction

A new symmetrical and flexible primary amide functionalized ligand, 2,2'-(ethane-1,2-diylbis((pyridin-2-ylmethyl)azanediyl))diacetamide (2-BPEG), has been synthesized and structurally characterized. Using this multidentate ligand, four novel metal complexes, namely [Cu(2-BPEG)](ClO4)2·0.5H2O (1), [Zn(2-BPEG)](ClO4)2 (2), [Zn(2-BPEG)](ZnCl4)·H2O (3) and [Cd(2-BPEG)(H2O)](ClO4)2·H2O (4), have been synthesized under ambient conditions and characterized by elemental, spectroscopic and thermal analysis, and single and powder X-ray diffraction. Complexes 1-3 are hexacoordinated with an N4O2 donor set (provided by the hexadentate 2-BPEG ligand), while complex 4 is heptacoordinated with an additional coordinated water molecule. In all cases, the 2-BPEG ligand acts as a hexadentate ligand. A change in the starting metal salt has resulted in the formation of 2 and 3 with different tetrahedral anions, ClO4- and ZnCl4-, respectively. This has provided an opportunity to showcase anion-directed supramolecular networks for these compounds. Compounds 1, 2 and 4 with perchlorate anions show similar and comparable intermolecular interactions in their 3D networks. On the other hand, the supramolecular self-assembly of 3 is dominated by a variety of intermolecular interactions such as C-HCl, N-HCl, O-HCl and C-HO due to the presence of a tetrachlorozincate(ii) ion. Moreover, the role of weak intermolecular interactions in the crystal packing has been analysed and quantified using Hirshfeld surface analysis. Furthermore, compound 4 exhibiting an open Lewis acid site has been found to be a very efficient and recyclable heterogeneous catalyst for the solvent-free cyanosilylation of various aldehydes with trimethylsilyl cyanide (TMSCN) producing the corresponding trimethylsilyl ether in high yields.

A silicotungstate-based copper–viologen hybrid photocatalytic compound for efficient degradation of organic dyes under visible light

An efficient visible light responsive compound [Cu(PBPY)]₂[SiW₁₂O₄₀] (1) has been constructed by integrating a silicotungstate cluster into the copper–viologen framework. It shows high photocatalytic efficiency for the degradation of dye pollutants.