Polyoxometalate functionalized architectures

Bis(β-diketone)-based metallacycles with haloalkane-induced fluorescence enhancement

A series of metallarectangles 1-5 were synthesized by the selective combination of (p-cymene)Ru-corner, bis(β-diketone) arms and bifunctional pyridyl linkers. They exhibited a very rare phenomenon of haloalkane-induced fluorescence enhancement.

Organo-functionalized polyoxovanadates: crystal architecture and property aspects

Polyoxovanadates (POVs), as one of the most prominent members of polyoxometalates (POMs), have been subject to extensive studies by virtue of their aesthetically intriguing structures and potential applications in catalysis, magnetism, and optics, among others. In recent years, organo-functionalized POVs have received considerable attention due to the combination of the advantages of POVs with the importance of organic species. In this review, the key developments of polyoxovanadates and, particularly, the achievements that are related to polyoxovanadates modified with organic ligands and transition metal-organic ligand are summarized. Herein, we systematically introduce the structural features of organo-functionalized POVs and their main applications involved in the magnetism and catalysis aspects. Finally, the current challenges and future prospects in the design, synthesis, and property investigation of polyoxovanadates are also discussed.

Bifunctional Noble-Metal-Free Catalyst for the Selective Aerobic Oxidation-Knoevenagel One-Pot Reaction: Encapsulation of Polyoxometalates into an Alkylamine-Modified MIL-101 Framework

One-pot reactions offer economic and environmental advantages. Therefore, the design and synthesis of multifunctional catalysts capable of catalyzing multistep organic transformations are highly important. Herein, an effective bifunctional heterogeneous catalyst is presented. For the first time, the encapsulation of H₅PMo₁₀V₂O₄₀ (PMoV₂) polyoxometalate into the cages of an alkylamine-modified MIL-101 using an optimized double-solvent method is reported. The obtained PMoV₂@DETA-MIL-101 material displays a great catalytic performance (99% conversion of alcohols) for the selective aerobic oxidation-Knoevenagel one-pot reaction. To the best of our knowledge, this is one of the first reports on the usage of noble-metal-free catalysts for the aerobic oxidation-Knoevenagel one-pot reaction without the addition of additives. The catalyst is very stable and can be used for at least five cycles with no leaching of the active sites.

Tricarboxylic-Ligand-Decorated Lanthanoid-Inserted Heteropolyoxometalates Built by Mixed-Heteroatom-Directing Polyoxotungstate Units: Syntheses, Structures, and Electrochemical Sensing for 17β-Estradiol

Organic-inorganic hybrid metal-oxide clusters have been pursued for many years, benefiting from their abundant structures and prominent performances. Upon our exploration, a family of unusual mixed-heteroatom (Sb^III, P^III)-directing lanthanoid (Ln)-inserted heteropolyoxotungstates (Ln-HPOTs), [(CH₃)₂NH₂]₂Na₇H₃[Ln₄(HP^III)W₈(H₂O)₁₂(H₂ptca)₂O₂₈][Sb^IIIW₉O₃₃]₂·27H₂O [Ln = Ce³⁺ (1), La³⁺ (2), Pr³⁺ (3)], functionalized by 1,2,3-propanetricarboxylic acid (H₃ptca) was achieved. The intriguing trimeric [Ln₄(HP^III)W₈(H₂O)₁₂(H₂ptca)₂O₂₈][Sb^IIIW₉O₃₃]₂^12- polyanion was established by two trivacant [B-α-Sb^IIIW₉O₃₃]^9- segments mounted on both sides and one rare [HP^IIIW₄O₁₈]^8- segment at the bottom, which are bridged via an organic-inorganic hybrid [W₄Ln₄(H₂O)₁₂O₁₀(H₂ptca)₂]¹⁴⁺ central moiety. Such Ln-HPOTs involving dual-heteroatom-directing mixed building blocks, and even simultaneously modified by tricarboxylic ligands, are rather unseen in polyoxometalate chemistry. Moreover, the detection of 17β-estradiol through a 1-based electrochemical biosensor has been explored, demonstrating a low detection limit (7.08 × 10^-14 M) and considerable stability.

Constructing chiral polyoxometalate assemblies via supramolecular approaches

Polyoxometalates (POMs), as a typical class of discrete metal oxide clusters that are known in inorganic and structural chemistry since long, have displayed more and more interesting applications over recent years. However, in comparison to the chemical synthesis, the photochemical, electrochemical, and magnetic properties, the structural asymmetry, and relative characteristic investigations arising therefrom are far behind even if they are very important for functional materials, especially in solution systems. One of the main reasons is that it is hard to control and maintain a stable chiral state of POMs to carry out further corresponding performances. Aiming to overcome these disadvantages, the main concerns of this review are to discuss the generation of the chirality for discrete metal oxide clusters, chirality transfer via a supramolecular approach, chirality amplification in self-assemblies, and the related functional properties such as photochromism, catalysis, and bioactivities in solutions. Considering that some previous reviews dealt with chiral structures and packing architectures in the crystalline solids of POMs, this article only concentrates on the induced chirality and material properties in solution systems, which have been more active recently but no review article has been involved in this interesting area.

Thiacalix[4]arene-supported molecular clusters for catalytic applications

Thiacalixarenes are intriguing ligands that have attracted sustained interest because of their changeable conformations and excellent coordination ability. Thiacalix[4]arene analogues, which can bind metal ions to form modular second building units, are capable of constructing molecular-based functional materials with defined structures and various applications via directional coordination assembly. Due to rich metal-sulfur bonds, thiacalix[4]arene-based molecular clusters also exhibit diverse properties compared to other clusters. In particular, the combination of thiacalixarenes with currently popular molecular architectures, such as high-nuclearity clusters and coordination cages, has shown special catalytic performances. In this perspective, the latest advances in catalytic applications of thiacalix[4]arene-based molecular clusters, including molecular clusters themselves as catalysts and coordination cages serving as reaction vessels encapsulating metal nano-components for catalysis, are highlighted.

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.

Atomically precise vanadium-oxide clusters

Polyoxovanadate (POV) clusters are an important subclass of polyoxometalates with a broad range of molecular compositions and physicochemical properties. One relatively underdeveloped application of these polynuclear assemblies involves their use as atomically precise, homogenous molecular models for bulk metal oxides. Given the structural and electronic similarities of POVs and extended vanadium oxide materials, as well as the relative ease of modifying the homogenous congeners, investigation of the chemical and physical properties of pristine and modified cluster complexes presents a method toward understanding the influence of structural modifications (e.g. crystal structure/phase, chemical makeup of surface ligands, elemental dopants) on the properties of extended solids. This review summarises recent advances in the use of POV clusters as atomically precise models for bulk metal oxides, with particular focus on the assembly of vanadium oxide clusters and the consequences of altering the molecular composition of the assembly via organofunctionalization and the incorporation of elemental "dopants".

A New Family of Polyoxometalates: Tris-functionalized Lindqvist-Type Hexatungstovanadates

We successfully designed and obtained a new family of polyoxometalates (POMs) containing mixed-metal elements and a trialkoxyl (TRIS) ligand via a very simple one-pot process under mild condition. Single-crystal X-ray diffraction revealed that this family belongs to compact Lindqvist-type hexatungstovanadates. In particular, the hydroxyl-containing product can be further functionalized through esterification. Not only does this work open a broad door for unusual POM clusters involving vanadium and tungsten atoms in the future, but also the design concept of this work also provides new insight for the synthesis and further exploration of POMs.

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.

Polyoxometalate-based metal–organic frameworks for heterogeneous catalysis

POM-based MOFs simultaneously possessing the virtues of POMs and MOFs exhibit excellent heterogeneous catalytic properties.

Tandem-like vanadium cluster chains in a polyoxovanadate-based metal–organic framework for efficient catalytic oxidation of sulfides

The vanadium cluster chain in V-Ni-MOF can efficiently catalyze the oxidation of sulfides with hydrogen peroxide as the oxidant, achieving the complete conversion from sulfides to sulfones within 1 hour at 40 °C.

Self-assembly of Keggin-type U(vi)-containing tungstophosphates with a sandwich structure: an efficient catalyst for the synthesis of sulfonyl pyrazoles

Two Keggin-type U(vi)-containing tungstophosphates with sandwich structure were synthesized and characterized. Compound 1 presents excellent catalytic activity towards the condensation cyclization of sulfonyl hydrazines with 1,3-diketones to synthesize sulfonyl pyrazoles.

Ruthenium(ii)-supported phosphovanadomolybdates [Ru(dmso)3PMo6V3O32]6− and [Ru(PMo6V3O32)2]14−, and their use as heterogeneous catalysts for oxidation of alcohols

Two stable ruthenium(ii)-substituted phosphovanadomolybdates [Ru^II(dmso)₃PMo^VI₆V^V₃O₃₂]⁶⁻ and [Ru^II(PMo^VI₆V^V₃O₃₂)₂]¹⁴⁻ are synthesized. They could catalyze oxidation of alcohols with good conversion and selectivity.

COF-inspired fabrication of two-dimensional polyoxometalate based open frameworks for biomimetic catalysis

The development of highly efficient and robust biomimetic catalysts is an essential and feasible strategy to overcome the intrinsic drawbacks of natural enzymes. Inspired by the synthetic strategy of covalent organic frameworks, we adopted a covalent-bond-driven strategy to prepare polyoxometalate (POM) based open frameworks (NKPOM-OFs = Nankai University POM-OFs) with abundant Mo[double bond, length as m-dash]O groups that can mimic the active center of sulfite oxidase. Four 2-dimensional (2D) NKPOM-OFs were designed and synthesized via the condensation reaction of linear amino-containing POMs with planar tetra-aldehyde monomers. Benefitting from the high crystallinity, the structures of 2D POM-OFs can be successfully determined from structural simulations. The results unveiled that NKPOM-OFs possessed 2D staggered stacking layered structures with the sql topology. All these NKPOM-OFs exhibited high crystallinity and stability and demonstrated outstanding performance to serve as biomimetic catalysts of sulfite oxidase with good recyclability. Notably, exfoliation of NKPOM-OFs under ultrasonic treatment can significantly boost the catalytic activity with almost two times faster reaction rates. This study not only enriches the facile and versatile synthesis strategy for POM-OFs but also provides new biomimetic platforms for biocatalysis.

POM@MOF Hybrids: Synthesis and Applications

The hybrid materials that are created by supporting or incorporating polyoxometalates (POMs) into/onto metal–organic frameworks (MOFs) have a unique set of properties. They combine the strong acidity, oxygen-rich surface, and redox capability of POMs, while overcoming their drawbacks, such as difficult handling, a low surface area, and a high solubility. MOFs are ideal hosts because of their high surface area, long-range ordered structure, and high tunability in terms of the pore size and channels. In some cases, MOFs add an extra dimension to the functionality of hybrids. This review summarizes the recent developments in the field of POM@MOF hybrids. The most common applied synthesis strategies are discussed, together with major applications, such as their use in catalysis (organocatalysis, electrocatalysis, and photocatalysis). The more than 100 papers on this topic have been systematically summarized in a handy table, which covers almost all of the work conducted in this field up to now.

Inorganic–organic hybrid polyoxovanadates based on [V4O12]4− or [VO3]22− clusters: controllable synthesis, crystal structures and catalytic properties in selective oxidation of sulfides

Three inorganic–organic hybrid polyoxovanadates have been synthesized. Among them, complex [Ni₂(1-vIM)₇H₂O][V₄O₁₂]·H₂O (1) exhibits extraordinary heterogeneous catalytic performance in the selective oxidation of sulfides using H₂O₂ as the oxidant.