Anderson-like alkoxo-polyoxovanadate clusters serving as unprecedented second building units to construct metal-organic polyhedra

Synthesis and Structure of High-Nuclearity Carboxylate-Modified Heteropolyoxovanadate Serving as a Heterogeneous Catalyst for Selective Oxidation of Alkylbenzenes

A high-nuclearity carboxylic-modified heteropolyoxovanadate, Na2K10H15[P8VIV24(tart)15(H2O)15(OH)O51]·58H2O [1, tart = C4H2O6], has been successfully synthesized by a conventional aqueous method under mild conditions. The crystallographic study reveals that compound 1 crystallizes in the tetragonal I41/a space group and is composed by a trilayer saddle-like polyoxoanion {P8V24}. Two {V3(tart)(H2O)O11} as linking units bridge the top {P4VIV9(tart)7(H2O)4(OH)O23} and the bottom {P4VIV9(tart)6(H2O)9O22} layers via tartrate ligands and {PO4} tetrahedra, resulting in a 24-nuclearity POV skeleton structure. More interestingly, compound 1 serves as a heterogeneous catalyst for the selective oxidation of diphenylmethanes with 96.2% conversion and 93.6% selectivity under the optimized conditions.

A d-Methionine-Bonded Nanosized Heteropolyoxotungstate with Photocoloration

A d-methionine-bonded nanosized arsenotungstate, Ba9K10H6[{As2W19O67(H2O)}2{AsW9O33}2{W3O6(H2O)(d-Met)}2{W2O4(OH)(d-Met)}]·60H2O [1; d-Met = d-methionine (C5H11NO2S)], is constructed without the use of lanthanide ions. The polyanion of 1 contains two {As2W19O67(H2O)}14- building blocks and two {B-β-AsW9O33}9- subunits, integrated together with a {W2O4(OH)(d-Met)}2+ and two {W3O6(H2O)(d-Met)}5+ subclusters. Interestingly, 1 displays a highly reversible photocoloration property with a half-life (t1/2) time measured as about 0.793 min.

Timing matters: pre-assembly versus post-assembly functionalization of a polyoxovanadate-organic cuboid

The pre-assembly and post-assembly approaches in the functionalization of a polyoxovanadate-organic cuboid, [{V6S}8(QPTC)8{V3}2]10-, are discussed. We have shown that the two pathways have led to distinctly different systems, with either an expanded or contracted interior void space, when phenylphosphonate is introduced at different stages of the self-assembly. One leaves the cuboid framework largely intact, whereas the other results in a compact, twisted cuboid. Kinetic factors will have to be considered in the equilibrium of these complex processes. Furthermore, the exceptional stability of these polyoxometalate-organic systems facilitates mass spectrometric characterization, which confirms the composition of the complexes and also indicates that the methoxide groups on the vanadium cluster nodes are labile. The results will help deepen the mechanistic understanding of the formation mechanisms of polyoxovanadate-based metal-organic cages and other functionalized polyoxovanadate clusters in general.

Face-Directed Construction of a Metal-Organic Isohedral Tetrahedron for the Highly Efficient Capture of Environmentally Toxic Oxoanions and Iodine

Geometric analysis has been guiding the design and construction of metal-organic polyhedra. Here, a series of isohedral tetrahedra ZrIT-1 and -2 and VIT-1 and -2 were synthesized by a one-pot method relying on trivalent molecular building blocks. Structural analysis shows that the isohedral tetrahedra constructed with {V₆(SO₄)(CO₂)₃} have three different sets of prism lengths, while those constructed with {Zr₃O(CO₂)₃} have two different sets of prism lengths. Comparison of two types of polyhedra reveals that the different sizes and coordination flexibilities of the two MBBs result in different cavity volumes. The environmentally toxic oxoanion trapping ability of ZrIT-1 was explored due to its structural stability and cation cage properties. The results show that ZrIT-1 can capture permanganate and dichromate anions in water with high efficiency and selectivity. Notably, the permanganate adsorption capacity can reach ∼276.6 mg/g, which exceeds those of most metal-organic framework materials. In addition, the adsorption and desorption of iodine showed that ZrIT-1 has a reversible adsorption capacity for iodine.

Simple preparation of Ag-BTC-modified Co3Mo7O24 mesoporous material for capacitance and H2O2-sensing performances

{Co3Mo7O24}@Ag-BTC-2 was synthesized by a grinding method, and it showed excellent performance in a supercapacitor and H2O2 sensing.

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.

How to not build a cage: endohedral functionalization of polyoxometalate-based metal-organic polyhedra

Introducing functionalities into the interior of metal-organic cage complexes can confer properties and utilities (e.g. catalysis, separation, drug delivery, and guest recognition) that are distinct from those of unfunctionalized cages. Endohedral functionalization of such cage molecules, for decades, has largely relied on modifying their organic linkers to covalently append targeted functional groups to the interior surface. We herein introduce an effective coordination method to bring in functionalities at the metal sites instead, for a set of polyhedral cages where the nodes are in situ formed polyoxovanadate clusters, [VIV 6O6(OCH3)9(μ6-SO4)(COO)3]2-. Replacing the central sulfates of these hexavanadate clusters with more strongly coordinating phosphonate groups allows the installation of functionalities within the cage cavities. Organophosphonates with phenyl, biphenyl, and terphenyl tails were examined for internalization. Depending on the size/shape of the cavities, small phosphonates can fit into the molecular containers whereas larger ones inhibit or transform the framework architecture, whereby the first non-cage complex was isolated from a reaction that otherwise would lead to entropically favored regular polyhedra cages. The results highlight the complex and dynamic nature of the self-assembly process involving polyoxometalates and the scope of molecular variety accessible by the introduction of endo functional groups.

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".

Coordination-Assembled Molecular Cages with Metal Cluster Nodes

Molecular cages have attracted great attention because of their fascinating topological structures and well-defined functional cavities. These discrete cages were usually fabricated by coordination assembly approach, a process employing directional metal-ligand coordination bonds due to the nature of the divinable coordination geometry and the required lability to encode dynamic equilibrium/error-correction. Compared to these coordination molecular cages with mononulcear metal-nodes, an increasing number of molecular cages featuring dinuclear and then polynuclear metal-cluster nodes have been synthesized. These metal-cluster-based coordination cages (MCCCs) combine the merits of both metal clusters and the cage structure, and exhibit excellent performances in catalysis, separation, host-guest chemistry and so on. In this review, we highlight the syntheses of MCCCs and their potential functions that is donated by the metal-cluster nodes.

Monocarboxylate-driven structural growth in Calix[n]arene-polyoxotitanate hybrid systems: utility in hydrogen production from water

A carboxylate-driven assembly strategy has been developed for the first time to build calix[n]arene-based polyoxotitanate clusters with tuneable nuclearity and structures. Photocatalytic studies revealed that these clusters exhibit structural-dependent H₂ evolution ability with a maximum rate up to 415.11 μmol h^-1 g^-1, which is almost the highest recorded in polyoxotitanate clusters.

The rise of metal-organic polyhedra

Metal-organic polyhedra are a member of metal-organic materials, and are together with metal-organic frameworks utilized as emerging porous platforms for numerous applications in energy- and bio-related sciences. However, metal-organic polyhedra have been significantly underexplored, unlike their metal-organic framework counterparts. In this review, we will cover the topologies and the classification of metal-organic polyhedra and share several suggestions, which might be useful to synthetic chemists regarding the future directions in this rapid-growing field.

Face-Directed Assembly of Molecular Cubes: In Situ Substitution of a Predetermined Concave Cluster

Systematic design and self-assembly of metal-organic polyhedra with predictable configurations has been a long-standing challenge in crystal engineering. Herein a concave polyoxovanadate cluster, [V₆ O₆ (OCH₃ )₉ (SO₄ )₄ ]^5- , which can be generated in situ under specific reaction conditions, is reported. Based on this cluster, a potential trivalent molecular building block, [V₆ O₆ (OCH₃ )₉ (SO₄ )(CO₂ )₃ ]^2- , can be obtained by the bridging-ligand-substitution strategy and it possesses appropriate angle information for the design of molecular cubes. Utilizing the face-directed assembly of the trivalent molecular building block and a diverse set of tetratopic carboxylate linkers, a series of metal-organic cubes (VMOC-1-VMOC-5) with the same topology but different functionalities and dimensions were designed and constructed. An inclusion study using VMOC-3 shows that they are potential molecular receptors for selective capture of size-matching polycyclic aromatic hydrocarbon guest molecules.

A Rare 3D Porous Inorganic-Organic Hybrid Polyoxometalate Framework Based on a Cubic Polyoxoniobate-Cupric-Complex Cage with a High Water Vapor Adsorption Capacity

A rare 3D porous inorganic-organic polyoxoniobate framework based on the cubic polyoxoniobate-cupric-complex cage {[Cu(en)₂]@{[Cu₂(en)₂(trz)₂]₆(Nb₆₈O₁₈₈)}} (1a), has been successfully synthesized by a hydrothermal method. The cubic cages 1a are connected with 4-(tetrazol-5-yl)pyridine to form a 1D pillar-like chain structure, and every 1D pillar-like chain is further linked with four adjacent pillar-like chains by the [Cu(en)₂]²⁺ complex to form a 3D porous inorganic-organic polyoxoniobate framework with 4-connected CdSO₄-type topology. To our knowledge, it is the first time that three different types of organic ligands are simultaneously introduced into one polyoxoniobate. This material also exhibits a high vapor adsorption capacity and good ionic conductivity properties.

Two polyoxovanadate-based metal-organic polyhedra with undiscovered "near-miss Johnson solid" geometry

Newfangled frangipani-like [MV₅O₆(μ₃-O)₅(SO₄)(COO)₅] (M = Nb/W) polyanions served as 5-connected molecular building blocks (MBBs) that simultaneously assembled with 4-connected [V₅O₉Cl] MBBs and tricarboxylate ligands (H₃BTC) to form two new polyoxovanadate-based metal-organic polyhedra {[MV₅O₆(μ₃-O)₅(SO₄)]₄[V₅O₉Cl]₄(BTC)₁₂} with undiscovered "near-miss Johnson solid" geometry. Moreover, the variable-temperature magnetic susceptibilities were investigated.

Nickel(II) cluster-based mixed-cation coordination polymer synthesized from 2-mercaptobenzoic acid and its application

High-nuclearity metal clusters have received considerable attention not only because of their diverse architectures and topologies, but also because of their potential applications as functional materials in many fields. To explore new types of clusters and their potential applications, a new nickel(II) cluster-based mixed-cation coordination polymer, namely poly[hexakis[μ₄-(2-carboxylatophenyl)sulfanido]di-μ₃-chlorido-tri-μ₂-hydroxido-octanickel(II)sodium(I)], [Ni₈NaCl₂(OH)₃(C₇H₄O₂S)₆]_n, 1, was synthesized using nickel chloride hexahydrate and mercaptobenzoic acid (H₂mba) as starting reactants under hydrothermal conditions. The material was characterized by single-crystal X-ray diffraction (SCXRD), Fourier transform IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction and X-ray photoelectron spectroscopy analysis. SCXRD shows that 1 consists of a hexanuclear nickel(II) [Ni₆] cluster, dinuclear Ni^II nodes and a mononuclear Na^I node, resulting in the formation of a complex covalent three-dimensional network. In addition, a tightly packed NiO/C&S nanocomposite is fabricated by sintering the coordination precursor at 400 °C. The uniform nanocomposite consists of NiO nanoparticles, incompletely carbonized carbon and incompletely vulcanized sulfur. When used as a supercapacitor electrode, the synthesized composite shows an extra-long cycling stability (>5000 cycles) during the charge/discharge process.

From Octahedral to Icosahedral Metal-Organic Polyhedra Assembled from Two Types of Polyoxovanadate Clusters

Design and synthesis of metal-organic polyhedra (MOPs) with targeted geometries from predetermined secondary building units (SBUs) is a long-standing challenge in chemistry and material science. Theoretical prediction shows that there are 6 possible polyhedra from the 3-coordinated, 4-coordinated octahedron ((3,4)-c octahedron) to (3,5)-c icosahedron with minimal transitivity (simplest possible). Except for one missing polyhedron (mtr) due to the unfavorable angles, we report five MOPs based on these structures, including an octahedral (3,4)-c VMOP-21 (rdo), an icosahedral (3,5)-c VMOP-25 (trc), and three intermediate derived trinodal (3,4,5)-c VMOP-22-24 (ghm, hmg, xum). Remarkably, all these MOPs obey the minimal transitivity principle and are consistent with geometrical predictions.

Photopolymerization of metal–organic polyhedra: an efficient approach to improve the hydrostability, dispersity, and processability

Metal–organic polyhedra are covalently linked by flexible polymer chains through photopolymerization, endowing the materials with enhanced processability, dispersity, and hydrostability.

Surfactant-assisted synthesis and electrochemical properties of an unprecedented polyoxometalate-based metal–organic nanocaged framework

The first polyoxometalate-based nanocaged three-dimensional metal–organic framework was synthesized by a surfactant-assisted hydrothermal method.

Interconvertible vanadium-seamed hexameric pyrogallol[4]arene nanocapsules

Research into stimuli-responsive controlled self-assembly and reversible transformation of molecular architectures has received much attention recently, because it is important to understand and reproduce this natural self-assembly behavior. Here, we report two coordination nanocapsules with variable cavities: a contracted octahedral V₂₄ capsule and an expanded ball-shaped V₂₄ capsule, both of which are constructed from the same number of subcomponents. The assemblies of these two V₂₄ capsules are solvent-controlled, and capable of reversible conversion between contracted and expanded forms via control of the geometries of the metal centers by association and dissociation with axial water molecules. Following such structural interconversions, the magnetic properties are significantly changed. This work not only provides a strategy for the design and preparation of coordination nanocapsules with adaptable cavities, but also a unique example with which to understand the transformation process and their structure-property relationships.

Adapting the cavity of a coordination capsule generally involves the addition or removal of subcomponents. Here, the authors report two vanadium-organic coordination nanocapsules with the same number of components but variable cavity sizes—an expanded ball and contracted octahedron—whose solvent-controlled interconversion is attributed to the versatile coordination geometry of the vanadium centers.

Self-Assembly of Goldberg Polyhedra from a Concave [WV5O11(RCO2)5(SO4)]3- Building Block with 5-Fold Symmetry

Nanoscale regular polyhedra with icosahedral symmetry exist naturally as exemplified by virus capsids and fullerenes. Nevertheless, their generation by supramolecular chemistry through the linking of 5-fold symmetry vertices remains unmet because of the absence of 5-fold symmetry building blocks with the requisite geometric features. This situation contrasts with that of tetrahedral and octahedral symmetry metal-organic polyhedra (MOPs), for which appropriate triangular and square molecular building blocks (MBBs) that can serve as vertices or faces are readily available. Herein, we report isolation of a pentagonal [WV₅O₁₁(SO₄)₆]^8- cluster and reveal its utility to afford the first four examples of nanoscale Goldberg MOPs, based upon 5-fold MBBs. Two 32-faced G _v(1,1) MOPs and two 42-faced G _v(2,0) MOPs were formed using linear or triangular organic ligands, respectively. The largest Goldberg MOP-4, exhibits a diameter of 4.3 nm, can trap fullerene C₆₀ molecules in its interstitial cavities.

A polyoxometalate-based metal–organic polyhedron constructed from a {V5O9Cl} building unit with rhombicuboctahedral geometry

The design and construction of metal–organic polyhedra has received much attention by chemists due to the intriguing diversity of architectures and topologies that can be achieved. There are several crucial factors which should be considered for the construction of metal–organic polyhedra, such as the starting materials, reaction time and temperature, solvent and suitable organic ligands. Recently, polyoxometalates (POMs), serving as secondary building units to construct POM-based metal–organic polyhedra, have been the subject of much interest. The title compound, dodecakis(dimethylammonium) octakis(μ-benzene-1,3,5-tricarboxylato)hexa-μ-chlorido-tetracosa-μ-oxido-triacontaoxidotriacontavanadium, (NH2Me2)12[(V5O9Cl)6(C9H3O6)8], was synthesized successfully by self-assembly of VOCl3 and benzene-1,3,5-tricarboxylic acid under solvothermal conditions. The title polyhedron has an rdo topology when the {V5O9Cl} building unit and the benzene-1,3,5-tricarboxylate (BTC3−) ligand were simplified into 4-connected and 3-connected vertices. Interestingly, when the {V5O9Cl} building unit and the BTC3− ligand are considered as quadrangular and triangular faces, the structure displays rhombicuboctahedral geometry with an outer diameter of 21.88 Å. The packing of the polyhedra produces a circular channel with a diameter of 8.2 Å. The title compound was characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, thermogravimetric analysis and powder X-ray diffraction.

Functionalized polyoxometalate-based metal-organic cuboctahedra for selective adsorption toward cationic dyes in aqueous solution

Two functionalized polyoxovanadate-based metal-organic polyhedra with heterocube formations are synthesized under solvothermal conditions. The structures of VMOP-18 and VMOP-19 display similar cuboctahedral geometries when the polyoxovanadate {V6O6(OCH3)9X(COO)3}n- (X = VO4, n = 1; SO4, n = 2) building units and organic ligands are considered as triangular faces of the polyhedra. Each cuboctahedron was surrounded by eight neighbouring cuboctahedra via strong C-Hπ interactions, leading to a 3D open supramolecular structure. Furthermore, the absorption ability toward the ionic dyes of VMOP-18 was investigated. Only cationic dyes can be absorbed into the cavity of VMOP-18, which indicates that the cationic dye absorption process is an ion-exchange process.