Extended Architectures Constructed from Sandwich Tetra‐Metal‐Substituted Polyoxotungstates and Transition‐Metal Complexes

Three unprecedented 2D architectures made up of sandwich‐type tetra‐metal‐substituted polyoxotungstates and transition‐metal complexes, [Cu(dien)(H₂O)]₂{[Cu(dien)(H₂O)]₂‐[Cu(dien)(H₂O)₂]₂[Cu₄(SiW₉O₃₄)₂]}? 5H₂O ( 1 ; dien=diethylenetriamine), [Zn(enMe)₂(H₂O)]₂{[Zn(enMe)₂]₂[Zn₄‐ (HenMe)₂(PW₉O₃₄)₂]}?8H₂O ( 2 ; enMe =1,2‐diaminopropane), and [Zn(enMe)₂‐(H₂O)]₄[Zn(enMe)₂]₂{(enMe)₂{[Zn‐ (enMe)₂]₂[Zn₄(HSiW₉O₃₄)₂]}{[Zn‐ (enMe)₂(H₂O)]₂[Zn₄(HSiW₉O₃₄)₂]}}? 13H₂O ( 3 ) were hydrothermally synthesized and structurally characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and single‐crystal X‐ray diffraction. Compound 1 consists of anions [Cu₄(SiW₉O₃₄)₂]^12? linked by copper complexes into a 2D structure, whereas 2 is constructed from novel inorganic–organic hybrid anions [Zn₄(HenMe)₂(PW₉O₃₄)₂]^8? linked by zinc complexes into a 2D structure. The most interesting is the unique 2D network 3 , which consists of anions [Zn₄(PW₉O₃₄)₂]^10? with two types of bridging groups: zinc complexes and enMe ligands.     

Heterodinuclear Lanthanoid‐Containing Polyoxometalates: Stepwise Synthesis and Single‐Molecule Magnet Behavior

Polyoxometalates (POMs) with heterodinuclear lanthanoid cores, TBA₈H₄[{Ln(μ₂‐OH)₂Ln′}(γ‐SiW₁₀O₃₆)₂] ( LnLn′ ; Ln=Gd, Dy; Ln′=Eu, Yb, Lu; TBA=tetra‐n‐butylammonium), were successfully synthesized through the stepwise incorporation of two types of lanthanoid cations into the vacant sites of lacunary [γ‐SiW₁₀O₃₆]^8? units without the use of templating cations. The incorporation of a Ln³⁺ ion into the vacant site between two [γ‐SiW₁₀O₃₆]^8? units afforded mononuclear Ln³⁺‐containing sandwich‐type POMs with vacant sites ( Ln1 ; TBA₈H₅[{Ln(H₂O)₄}(γ‐SiW₁₀O₃₆)₂]; Ln=Dy, Gd, La). The vacant sites in Ln1 were surrounded by coordinating W? O and Ln? O oxygen atoms. On the addition of one equivalent of [Ln′(acac)₃] to solutions of Dy1 or Gd1 in 1,2‐dichloroethane (DCE), heterodinuclear lanthanoid cores with bis(μ₂‐OH) bridging ligands, [Dy(μ₂‐OH)₂Ln′]⁴⁺, were selectively synthesized ( LnLn′ ; Ln=Dy, Gd; Ln′=Eu, Yb, Lu). On the other hand, La1 , which contained the largest lanthanoid cation, could not accommodate a second Ln′³⁺ ion. DyLn′ showed single‐molecule magnet behavior and their energy barriers for magnetization reversal (ΔE/k_B) could be manipulated by adjusting the coordination geometry and anisotropy of the Dy³⁺ ion by tuning the adjacent Ln′³⁺ ion in the heterodinuclear [Dy(μ₂‐OH)₂Ln′]⁴⁺ cores. The energy barriers increased in the order: DyLu (ΔE/k_B=48 K)< DyYb (53 K)< DyDy (66 K)< DyEu (73 K), with an increase in the ionic radii of Ln′³⁺; DyEu showed the highest energy barrier.     

An inorganic–organic hybrid supramolecular framework based on the γ‐[Mo8O26]4− cluster and cobalt complex of aspartic acid: X‐ray structure and DFT study

A new inorganic–organic hybrid based on an aspartate functionalized polyoxomolybdate, [pentaaquacobalt(II)]‐μ‐aspartate‐[γ‐octamolybdate]‐μ‐aspartate‐[pentaaquacobalt(II)] tetrahydrate, [Co₂(C₄H₆NO₄)₂(γ‐Mo₈O₂₆)(H₂O)₁₀]·4H₂O ( 1 ), has been synthesized under hydrothermal conditions from the reaction of an Evans–Showell‐type polyoxometalate, (NH₄)₆[Co₂Mo₁₀H₄O₃₈], and l ‐aspartic acid. The complex exhibits a supramolecular three‐dimensional framework structure in the crystal lattice. Compound 1 was structurally characterized by elemental analyses, IR and UV–Vis (diffuse reflectance) spectroscopy and single‐crystal X‐ray diffraction. In this compound, aspartic acid acts as a bridge between the two Co atoms and the Mo centres, with the –CH₂COOH side chain directly linked to the Mo centre in γ‐[Mo₈O₂₆]^4? and the α‐carboxylate side chain bound to the Co centre. Commonly, the binding of transition‐metal complexes to POMs involves coordination of the metal to a terminal O atom of the POM so that 1 , with a bridging ligand between Mo and Co atoms, belongs to a separate class of hybrid materials. While the starting materials are both chiral and one might expect them to form a chiral hybrid, the decomposition of the chiral Evans–Showell‐type POM and its conversion to the centrosymmetric γ‐octamolybdate POM, plus the presence of two aspartate ligands centrosymmetrically placed on either side of the POM, leads to the formation of an achiral hybrid. We have studied energetically by means of density functional theory (DFT) calculations and using the Bader's `atoms‐in‐molecules' analysis the electrostatically enhanced hydrogen bonds (EEHBs) observed in the solid state of 1 , which are crucial for the formation of one‐dimensional supramolecular assemblies.     

Synthesis and Disassembly/Reassembly of Giant Ring‐Shaped Polyoxotungstate Oligomers

The disassembly and reassembly of giant molecules are essential processes in controlling the structure and function of biological and artificial systems. In this work, the disassembly and reassembly of a giant ring‐shaped polyoxometalate (POM) without isomerization of the monomeric units is reported. The reaction of a hexavacant lacunary POM that is soluble in organic solvents, [P₂W₁₂O₄₈]^14?, with manganese cations gave the giant ring‐shaped POM [{γ‐P₂W₁₂O₄₈Mn₄(C₅H₇O₂)₂(CH₃CO₂)}₆]^42?. This POM is a hexamer of manganese‐substituted {P₂W₁₂O₄₈Mn₄} units, and its inner cavity was larger than any of those previously reported for ring‐shaped polyoxotungstates. It was disassembled into monomeric units in acetonitrile, and the removal of the capping organic ligands on the manganese cations led to reassembly into a tetrameric ring‐shaped POM, [{γ‐P₂W₁₂O₄₈Mn₄(H₂O)₆}₄(H₂O)₄]^24?.     

Mechanism of Thioether Oxidation over Di‐ and Tetrameric Ti Centres: Kinetic and DFT Studies Based on Model Ti‐Containing Polyoxometalates

The oxidation of thioethers by the green oxidant aqueous H₂O₂ catalysed by the tetratitanium‐substituted Polyoxometalate (POM) (Bu₄N)₈[{γ‐SiTi₂W₁₀O₃₆(OH)₂}₂(μ‐O)₂], as a model catalyst comprising tetrameric titanium centres, was investigated by kinetic modelling and DFT calculations. Several mechanisms of sulfoxidation were evaluated by using methyl phenyl sulfide (PhSMe) as a model substrate in the experiments and dimethyl sulfide in the calculations. The first mechanism assumes that the active hydroperoxo species forms directly through interaction of the Ti₂(μ‐OH)₂ group in [{γ‐SiTi₂W₁₀O₃₆(OH)₂}₂(μ‐O)₂]^8? ( 1 D ) with H₂O₂. The second mechanism includes hydrolysis of Ti‐O‐Ti bonds linking two γ‐Keggin units in structure 1 D to produce the monomer [(γ‐SiW₁₀Ti₂O₃₈H₂)(OH)₂]^4? ( 1 M ), followed by the formation of an active hydroperoxo species upon interaction of the Ti hydroxo group with H₂O₂. Both kinetic modelling and DFT calculations support the mechanism through the monomeric species that involves the hydrolysis step. According to the DFT studies the activation of H₂O₂ by compound 1 M is preferred by 6.5 kcal mol^?1 with respect to anion 1 D due to the more flexible Ti environment of the terminal Ti hydroxo group in 1 M . The calculations also indicate that for the ?monomeric“ mechanism two pathways are operative: the mono‐ and the multinuclear pathway. In the mononuclear mechanism, the active group is the terminal Ti?OH group, whereas in the multinuclear path the active group is the bridging Ti₂(μ‐OH) moiety. Moreover, unlike previous studies, the sulfoxidation is preferred through a β‐oxygen atom transfer from the Ti hydroperoxo group because the α‐oxygen atom transfer leads to an unfavourable seven‐fold coordinated Ti environment in the transition state. Finally, we have generalised these results to other Ti‐containing POMs: the Ti‐monosubstituted α‐Keggin ion [α‐PTi(OH)W₁₁O₃₉]^4? and the dititanium‐substituted sandwich‐type ion [Ti₂(OH)₂As₂W₁₉O₆₇]^8?.     

Poly[[trans‐diaquabis(μ2‐biphenyl‐2,2′‐dicarboxylato)bis(μ2‐4,4′‐bipyridine)dicobalt(II)] biphenyl‐2,2′‐dicarboxylic acid disolvate]

In the title compound, {[Co₂(C₁₄H₈O₄)₂(C₁₀H₈N₂)₂(H₂O)₂]·2C₁₄H₁₀O₄}_n, each Co^II ion is six‐coordinate in a slightly distorted octahedral geometry. Both Co^II ions are located on twofold axes. One is surrounded by two O atoms from two biphenyl‐2,2′‐dicarboxylate (dpa) dianions, two N atoms from two 4,4′‐bipyridine (bpy) ligands and two water molecules, while the second is surrounded by four O atoms from two dpa dianions and two N atoms from two bpy ligands. The coordinated dpa dianion functions as a κ³‐bridge between the two Co^II ions. One carboxylate group of a dpa dianion bridges two adjacent Co^II ions, and one O atom of the other carboxylate group also chelates to a Co^II ion. The Co^II ions are bridged by dpa dianions and bpy ligands to form a chiral sheet. There are several strong intermolecular hydrogen bonds between the H₂dpa solvent molecule and the chiral sheet, which result in a sandwich structure.     

Chirality‐ and pH‐Controlled Supramolecular Isomerism in Cobalt Phosphonates and Its Impact on the Magnetic Behavior

Two pairs of enantiomeric compounds with formulas (S)‐ or (R)‐Co₃(ppap)₂(4,4′‐bpy)₂(H₂O)₂ ? 4 H₂O [(S)‐ 1 or (R)‐ 1 ], (S)‐ or (R)‐Co₃(ppap)₂(4,4′‐bpy)₂(H₂O)₂ ? 3 H₂O [(S)‐ or (R)‐ 2 ), and related racemic compound Co₃(ppap)₂(4,4′‐bpy)₂(H₂O)₂ ? 4 H₂O (rac‐ 3 ; 4,4′‐bpy=4,4′‐bipyridine, H₃ppap=3‐phenyl‐2‐[(phosphonomethyl)amino]propanoic acid) are reported. Compounds 1 and rac‐ 3 show identical three‐dimensional framework structures, whereas compounds 2 have two‐dimensional layer structures. Compounds 1 and 2 are catenation isomers, formation of which is controlled solely by the pH of the reaction mixtures, whereas the formation of isomeric compounds 1 and rac‐ 3 is controlled purely by the chirality of the phosphonate ligand. The magnetic properties of fully dehydrated (S)‐ 1 , (S)‐ 2 , and rac‐ 3 are highly dependent on both structure and chirality.     

Breathing Effect in a Cobalt Phosphonate upon Dehydration/Rehydration: A Single‐Crystal‐to‐Single‐Crystal Study

Two cobalt phosphonates, [Co₂(2,2′‐bpy)₂(H₂O)(pbtcH)] ( 1 ) and [Co₂(H₂O)(pbtcH)(phen)₂] ( 2 ; pbtcH₅=5‐phosphonatophenyl‐1,2,4‐tricarboxylic acid, 2,2′‐bpy=2,2′‐bipyridine, phen=1,10‐phenanthroline), with layer structures are reported. Compound 1 contains O‐C‐O and O‐P‐O bridged tetramers of Co₄, which are further connected by pbtcH^4? units to form a layer. In compound 2 , the cobalt tetramers made up of water‐bridged Co₂ dimers and O‐P‐O linkages are connected into a layer by pbtcH^4? units. Upon dehydration, compounds 1 and 2 experience single‐crystal‐to‐single‐crystal (SC–SC) structural transformations to form [Co₂(2,2′‐bpy)₂(pbtcH)] ( 1 a ) and [Co₂(pbtcH)(phen)₂] ( 2 a ), respectively. The process is reversible in each case. Notably, a breathing effect is observed for 1 , accompanied by pore opening and closing due to the reorientation of the coordinated 2,2′‐bpy molecules. The transformation was also monitored by in situ IR measurements. Magnetic studies reveal that antiferromagnetic interactions are mediated between the magnetic centers in compounds 1 and 1 a , whereas ferromagnetic interactions are dominant in compound 2 .     

A New Polynuclear Copper‐Complex‐Substituted Tungstoarsenate(V)

A new polynuclear copper‐complex‐substituted dimeric tungstoarsenate(V), H₂[{Cu(2,2′‐bpy)}₈(H₂O)₂(AsW₉O₃₄)₂] · 12H₂O ( 1 ) (2,2′‐bpy = 2,2′‐bipydine), was synthesized hydrothermally and its structure was determined by single‐crystal X‐ray diffraction. The title compound has Ci symmetry and consists of two trilacunary Keggin anions [α‐AsW₉O₃₄]^9– supported by eight copper complex cations. The compound was also characterized by IR and fluorescence spectroscopy, TG analysis, and magnetic measurements. The emission spectrum of the compound in solid‐state exhibits a redshift relative to those of Na₈[A‐HAsW₉O₃₄] · 11H₂O and the free ligand 2,2′‐bpy. Magnetic measurements of the compound indicate competing ferro‐ and antiferromagnetic intramolecular coupling among the Cu^II atoms in the cluster anion.     

Visible‐Light‐Induced Photoredox Catalysis with a Tetracerium‐Containing Silicotungstate

The development of visible‐light‐induced photocatalysts for chemoselective functional group transformations has received considerable attention. Polyoxometalates (POMs) are potential materials for efficient photocatalysts because their properties can be precisely tuned by changing their constituent elements and structures and by the introduction of additional metal cations. Furthermore, they are thermally and oxidatively more stable than the frequently utilized organometallic complexes. The visible‐light‐responsive tetranuclear cerium(III)‐containing silicotungstate TBA₆[{Ce(H₂O)}₂{Ce(CH₃CN)}₂(μ₄‐O)(γ‐SiW₁₀O₃₆)₂] (CePOM; TBA=tetra‐n‐butylammonium) has now been synthesized; when CePOM was irradiated with visible light (λ>400 nm), a unique intramolecular Ce^III‐to‐POM(W^VI) charge transfer was observed. With CePOM, the photocatalytic oxidative dehydrogenation of primary and secondary amines as well as the α‐cyanation of tertiary amines smoothly proceeded in the presence of O₂ (1 atm) as the sole oxidant.     

catena‐Poly[bis[tetraaquacobalt(II)‐μ‐4,4′‐bipyridine‐κ2N:N′] benzene‐1,2,4,5‐tetracarboxylate(4−) dihydrate]

The title compound, [Co(C₁₀H₈N₂)(H₂O)₄]₂(C₁₀H₂O₈)·2H₂O, consists of two crystallographically independent Co^II atoms linked by 4,4′‐bi­pyridine ligands into one‐dimensional chains, which are further connected into a three‐dimensional framework linked by [C₆H₂(COO)₄]^4? anions and water mol­ecules, achieved through complex hydrogen bonding.     

1D Polyoxometalate‐based Inorganic‐Organic Hybrid Derived from ß‐Octamolybdate — Synthesis and Crystal Structure of [CoII(2, 2′‐bipy)2]2[Mo8O26]

A polyoxometalate‐based inorganic–organic hybrid compound [Co^II(2, 2′‐bpy)₂]₂[Mo₈O₂₆] ( 1 ) was synthesized by hydrothermal methods and structurally characterized by IR spectrum, TG analysis and X‐ray diffraction. The compound crystallizes in the monoclinic system, space group P2₁/n, a = 10.0681(2), b = 16.4467(2), c = 15.7838(3) Å, β = 100.046(1)°, V = 2573.52(8) ų, Z = 2. The structure of 1 is built up from β‐[Mo₈O₂₆]^4? subunits covalently linked via [Co^II(2, 2′‐bpy)₂]²⁺ fragments into a infinite 1D {[Co^II(2, 2′‐bpy)₂]₂[Mo₈O₂₆]}_∞ polymer.     

Syntheses and Structural Characterization of Two Nitrilotriacetate Cobalt Complexes: [{CoK2(NTA)(Hmta)(H2O)3}NO3]n and [{Co(4,4′‐bpy)2(H2O)4}{Co2(NTA)2(4,4′‐bpy)(H2O)2}]

Two nitrilotriacetate cobalt complexes {[CoK₂(NTA)(Hmta)(H₂O)₃]NO₃}_n ( 1 ) and [{Co(4,4′‐bpy)₂(H₂O)₄}{Co₂(NTA)₂(4,4′‐bpy)(H₂O)₂}] ( 2 ) (NTA = nitrilotriacetate anion, Hmta = hexamethylenetetramine and 4,4′‐bpy = 4,4′‐bipyridine) were prepared and characterized by IR, elemental analysis and single crystal X‐ray diffraction study. The influence of the neutral ancillary ligands on the formation of the complexes with different structures in the Co‐NTA system was discussed. The coordination of NTA and Hmta to Co²⁺ ions only resulted in the formation of mononuclear [Co(NTA)(Hmta)]^? ions which are further connected by K⁺ ions and water molecules to form a three‐dimensional network. The use of 4,4′‐bpy as ancillary ligand in 2 led to the formation of separate mononuclear [Co(4,4′‐bpy)₂(H₂O)₄]²⁺ and dinuclear [Co₂(NTA)₂(4,4′‐bpy)(H₂O)₂]^2? which are further connected by hydrogen bonds to form a supramolecular three‐dimensional network. In these cases it seems to suggest that the addition of neutral ancillary ligand into the Co‐NTA system leads to the formation of lower dimensional structures when the contribution of alkali ions to the structural dimensionality is neglected.     

Tetradecacobalt(II)‐Containing 36‐Niobate [Co14(OH)16(H2O)8Nb36O106]20− and Its Photocatalytic H2 Evolution Activity

A gigantic Co₁₄‐containing 36‐niobate, Na₁₂K₈[Co₁₄(OH)₁₆(H₂O)₈Nb₃₆O₁₀₆] ? 71H₂O ( 1 ), has been prepared by the hydrothermal method and structurally characterized. Polyanion [Co₁₄(OH)₁₆(H₂O)₈Nb₃₆O₁₀₆]^20? ( 1 a ) comprises a central Co₇ core, surrounded by another seven isolated Co²⁺ ions and six Lindqvist‐type (Nb₆O₁₉) hexaniobate fragments. This is the first example of a high‐nuclear cobalt‐cluster‐containing polyoxoniobate. The photocatalytic H₂ evolution activity of Pt‐loaded 1 was observed in methanol solution under irradiation using a 300 W Xe lamp.     

catena‐Poly­[bis­[aqua(2,2‐bi­pyridine‐κ2N,N′)­cobalt(II)]‐μ‐1,2,4,5‐benzene­tetra­carboxyl­ato‐κ4O1:O2:O3:O4]

In the title complex, [Co₂(C₁₀H₂O₈)(C₁₀H₈N₂)₂(H₂O)₂], the four carboxyl­ate groups are fully deprotonated and coordinate to four Co^II cations in a monodentate fashion, forming a one‐dimensional ribbon‐like double‐chain structure, with centrosymmetric [Co₂(C₁₀H₂O₈)(C₁₀H₈N₂)₂(H₂O)₂] repeating units and a cavity of approximately 6.8 × 6.6 Å. Moreover, a three‐dimensional supramolecular structure is formed by face‐to‐face π–π interactions between the aromatic rings of the 2,2′‐bi­pyridine moieties of two adjacent chains, and by hydrogen‐bonding interactions between the coordinated aqua O atom and the coordinated carboxyl O atom from different chains.     

Solvent-tuned polyoxometalate-based supramolecular hybrids constructed from different metal-organic motifs: Various structures and adsorption properties for dyes

Three polyoxometalate-based supramolecular hybrids with different metal-organic motifs have been synthesized by tuning the solvents, which show various adsorption activities for different organic dyes.     

Poly­[[di­aqua­cobalt(II)]‐μ‐4,4′‐bi­pyridine‐κ2N:N′‐μ‐p‐phenyl­enebis­(oxy­acetato)‐κ2O:O′]

The title compound, [Co^II(C₁₀H₈O₆)(C₁₀H₈N₂)(H₂O)₂]_n, was obtained by the hydro­thermal reaction of CoSO₄ with benzene‐1,4‐dioxy­di­acetate [systematic name: p‐phenyl­ene­bis­(oxy­acetate)] and 4,4′‐bi­pyridine (4,4′‐bpy). The Co atom lies at an inversion center and the benzene‐1,4‐dioxydiacetate and 4,4′‐bipyridine moieties lie about other inversion centers. The benzene‐1,4‐dioxydiacetate ligands bridge the octahedral Co^II coordination centers, forming a one‐dimensional zigzag chain. The chains are further bridged by 4,4′‐bpy ligands, forming a novel two‐dimensional supramolecular architecture. Hydro­gen‐bonding interactions between the coordinated water mol­ecules and the carboxyl­ate O atoms lead to the formation of a three‐dimensional network structure.     

Two CoII coordination polymers of biphenyl‐2,2′,5,5′‐tetracarboxylic acid with flexible N‐donor ligands: syntheses,structures and magnetic properties

Two Co^II‐based coordination polymers, namely poly[(μ₄‐biphenyl‐2,2′,5,5′‐tetracarboxylato){μ₂‐1,3‐bis[(1H‐imidazol‐1‐yl)methyl]benzene}dicobalt(II)], [Co₂(C₁₆H₆O₈)(C₁₄H₁₄N₄)₂]_n or [Co₂(o,m‐bpta)(1,3‐bimb)₂]_n ( I ), and poly[[aqua(μ₄‐biphenyl‐2,2′,5,5′‐tetracarboxylato){1,4‐bis[(1H‐imidazol‐1‐yl)methyl]benzene}dicobalt(II)] dihydrate], {[Co₂(C₁₆H₆O₈)(C₁₄H₁₄N₄)₂(H₂O)₂]·4H₂O}_n or {[Co₂(o,m‐bpta)(1,4‐bimb)₂(H₂O)₂]·4H₂O}_n ( II ), were synthesized from a mixture of biphenyl‐2,2′,5,5′‐tetracarboxylic acid, i.e. [H₄(o,m‐bpta)], CoCl₂·6H₂O and N‐donor ligands under solvothermal conditions. The complexes were characterized by IR spectroscopy, elemental analysis, single‐crystal X‐ray diffraction and powder X‐ray diffraction analysis. The bridging (o,m‐bpta)^4? ligands combine with Co^II ions in different μ₄‐coordination modes, leading to the formation of one‐dimensional chains. The central Co^II atoms display tetrahedral [CoN₂O₂] and octahedral [CoN₂O₄] geometries in I and II , respectively. The bis[(1H‐imidazol‐1‐yl)methyl]benzene (bimb) ligands adopt trans or cis conformations to connect Co^II ions, thus forming two three‐dimensional (3D) networks. Complex I shows a (2,4)‐connected 3D network with left‐ and right‐handed helical chains constructed by (o,m‐bpta)^4? ligands. Complex II is a (4,4)‐connected 3D novel network with ribbon‐like chains formed by (o,m‐bpta)^4? linkers. Magnetic studies indicate an orbital contribution to the magnetic moment of I and II due to the longer Co…Co distances. An attempt has been made to fit the χ_MT results to the magnetic formulae for mononuclear Co^II complexes, the fitting indicating the presence of weak antiferromagnetic interactions between the Co^II ions.     

Aluminum‐ and Gallium‐Containing Open‐Dawson Polyoxometalates

Al‐ and Ga‐containing open‐Dawson polyoxometalates (POMs), K₁₀[{Al₄(μ‐OH)₆}{α,α‐Si₂W₁₈O₆₆}] · 28.5H₂O ( Al₄ ‐ open ) and K₁₀[{Ga₄(μ‐OH)₆}(α,α‐Si₂W₁₈O₆₆)] · 25H₂O ( Ga₄ ‐ open ) were synthesized by the reaction of trilacunary Keggin POM, [A‐α‐SiW₉O₃₄]^10–, with Al(NO₃)₃ · 9H₂O or Ga(NO₃)₃ · nH₂O, and unequivocally characterized by single‐crystal X‐ray analysis, ²⁹Si and ¹⁸³W NMR, and FT‐IR spectroscopy as well as elemental analysis and TG/DTA. Single‐crystal X‐ray analysis revealed that the {M₄(μ‐OH)₆}⁶⁺ (M = Al, Ga) clusters were included in an open pocket of the open‐Dawson polyanion, [α,α‐Si₂W₁₈O₆₆]^16–, which was constituted by the fusion of two trilacunary Keggin POMs via two W–O–W bonds. These two open‐Dawson structural POMs showed clear difference of the bite angles depending on the size of ionic radii. In cases of both compounds, the solution ²⁹Si and ¹⁸³W NMR spectra in D₂O showed only one signal and five signals, respectively. These spectra were consistent with the molecular structures of Al₄ ‐ and Ga₄ ‐ open , suggesting that these polyoxoanions were obtained as single species and maintained their molecular structures in solution.     

Modified polyoxometalate: a novel monocapped bi‐supporting and reduced α‐Keggin structure {PMo12O40[Cu(2,2′‐bpy)]}[Cu(2,2′‐bpy)(en)(H2O)]2

A novel modified polyoxometalate, {PMo₁₂O₄₀[Cu(2,2′‐bpy)]}[Cu(2,2′‐bpy)(en)(H₂O)]₂ [2,2′‐bpy is 2,2′‐bipyridyl (C₁₀H₈N₂) and en is ethylenediamine (C₂H₈N₂)], has been synthesized hydrothermally and structurally characterized by elemental analysis, TG, IR, XPS and single‐crystal X‐ray diffraction. The structural analysis reveals that the compound contains the reduced Keggin polyanion [PMo₁₂O₄₀]^6? as the parent unit, which is monocapped by [Cu(2,2′‐bpy)]²⁺ fragments via four bridging O atoms on an {Mo₄O₄} pit and bi‐supported by two [Cu(2,2′‐bpy)(en)(H₂O)]²⁺ coordination cations simultaneously. There exist strong intramolecular π–π stacking between the capping and supporting units, which play a stabilizing role during the crystallization of the compound. Adjacent POM clusters are further aggregated to form a three‐dimensional supramolecular network through noncovalent forces, hydrogen bonding and π–π stacking interactions. In addition, the photocatalytic properties were investigated in detail, and the results indicated that the compound can be used as a photocatalyst towards the decomposition of the organic pollutant methylene blue (MB).