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.     

A {Co4O4} Cubane Incorporated within a Polyoxoniobate Cluster

A novel octacobalt‐containing polyoxoniobate, Na₆K₁₂[H₂Co₈O₄(Nb₆O₁₉)₄]?39 H₂O, has been prepared by a combination of hydrothermal and diffusion methods. The polyanion [H₂Co₈O₄(Nb₆O₁₉)₄]^18? incorporates a tetrameric assembly of Lindqvist‐type [Nb₆O₁₉]^8? fragments trapping a {Co^II₄Co^III₄} cluster which comprises a central {Co^III₄O₄} cubane core, surrounded by another four Co^II ions linkers. Furthermore, magnetic measurements show that the compound exhibits antiferromagnetic interactions.     

High‐Field Pulsed EPR Spectroscopy for the Speciation of the Reduced [PV2Mo10O40]6− Polyoxometalate Catalyst Used in Electron‐Transfer Oxidations

An in‐depth spectroscopic EPR investigation of a key intermediate, formally notated as [PV^IVV^VMo₁₀O₄₀]^6? and formed in known electron‐transfer and electron‐transfer/oxygen‐transfer reactions catalyzed by H₅PV₂Mo₁₀O₄₀, has been carried out. Pulsed EPR spectroscopy have been utilized: specifically, W‐band electron–electron double resonance (ELDOR)‐detected NMR and two‐dimensional (2D) hyperfine sub‐level correlation (HYSCORE) measurements, which resolved ⁹⁵Mo and ¹⁷O hyperfine interactions, and electron–nuclear double resonance (ENDOR), which gave the weak ⁵¹V and ³¹P interactions. In this way, two paramagnetic species related to [PV^IVV^VMo₁₀O₄₀]^6? were identified. The first species (30–35 %) has a vanadyl (VO²⁺)‐like EPR spectrum and is not situated within the polyoxometalate cluster. Here the VO²⁺ was suggested to be supported on the Keggin cluster and can be represented as an ion pair, [PV^VMo₁₀O₃₉]^8?[V^IVO²⁺]. This species originates from the parent H₅PV₂Mo₁₀O₄₀ in which the vanadium atoms are nearest neighbors and it is suggested that this isomer is more likely to be reactive in electron‐transfer/oxygen‐transfer reaction oxidation reactions. In the second (70–65 %) species, the V^IV remains embedded within the polyoxometalate framework and originates from reduction of distal H₅PV₂Mo₁₀O₄₀ isomers to yield an intact cluster, [PV^IVV^VMo₁₀O₄₀]^6?.     

{Nb288O768(OH)48(CO3)12}: A Macromolecular Polyoxometalate with Close to 300 Niobium Atoms

A protein‐sized (ca. 4.2×4.2×3.6 nm³) non‐biologically derived molecule {Nb₂₈₈O₇₆₈(OH)₄₈(CO₃)₁₂} ( Nb₂₈₈ ) containing up to 288 niobium atoms has been obtained, which is by far the largest and the highest nuclearity polyoxoniobate (PONb). Particularly, in terms of metal nuclearity number, Nb₂₈₈ is the second largest cluster so far reported in classic polyoxometalate chemistry (V, Mo, W, Nb, and Ta). Nb₂₈₈ can be described as a giant windmill‐like cluster aggregate of six nanoscale high‐nuclearity PONb units {Nb₄₇O₁₂₈(OH)₆(CO₃)₂} ( Nb₄₇ ) joined together by six additional Nb ions. Interestingly, the 47‐nuclearity Nb₄₇ units generated in situ can be isolated and bridged by copper complexes to form an inorganic–organic hybrid three‐dimensional PONb framework, which exhibits effective catalytic activity for hydrolyzing nerve agent simulant of dimethyl methylphosphonate. The unique Nb₄₇ cluster also provides a new type of topology to very limited family of Nb‐O clusters.     

Self‐Assembly of a Phosphate‐Centered Polyoxo‐Titanium Cluster: Discovery of the Heteroatom Keggin Family

Over the past 200 years, the most famous and important heteroatom Keggin architecture in polyoxometalates has only been synthesized with Mo, W, V, or Nb. Now, the self‐assembly of two phosphate (PO₄^3?)‐centered polyoxo‐titanium clusters (PTCs) is presented, PTi₁₆ and PTi₁₂, which display classic heteroatom Keggin and its trivacant structures, respectively. Because Ti^IV has lower oxidate state and larger ionic radius than Mo^VI, W^VI, V^V, and Nb^V, additional Ti^IV centres in these PTCs are used to stabilize the resultant heteroatom Keggin structures, as demonstrated by the cooresponding theoretical calculation results. These photoactive PTCs can be utilized as efficient photocatalysts for highly selective CO₂‐to‐HCOOH conversion. This new discovery indicates that the classic heteroatom Keggin family can be assembled with Ti, thus opening a research avenue for the development of PTC chemistry.     

New process for preparing aqueous solutions of Mo-V-phosphoric heteropoly acids

A new ecofriendly process is proposed for the synthesis of aqueous solutions of Mo-V-phosphoric Keggin heteropoly acids H_{3 + x} PV _x Mo_{12 ? x} O₄₀(HPA_?x ). First, V₂O₅ is dissolved in cooled H₂O₂ to form peroxyvanadium compounds, which then spontaneously decompose to yield the H₆V₁₀O₂₈ solution. The latter is stabilized by the addition of H₃PO₄ to yield an H₉PV₁₄O₄₂ solution. This solution is gradually added to a boiling H₃PO₄ + MoO₃ aqueous suspension. This suspension is gradually evaporated that is followed by dissolution of MoO₃ to produce an HPA_?x solution. This process is reliable and almost non-waste and is promising for preparing HPA_?x solutions with x = 2–6 on pilot and large scales.     

High‐dimensional Polyoxoniobates Constructed from Lanthanide‐incorporated High‐nuclear {[Ln(H2O)4]3[Nb24O69(H2O)3]2} Secondary Building Units

In this work, two rare high‐dimensional polyoxoniobates with formulas of H₉[Cu(en)(H₂O)₂][Cu(en)₂]₈[Dy(H₂O)₄]₃[Nb₂₄‐O₆₉(H₂O)₃]₂ ? 36H₂O ( 1 ) and H₉K[Cu(en)₂(H₂O)]₅[Cu(en)₂]₄‐[Eu(H₂O)₄]₃[Nb₂₄O₆₉(H₂O)₃]₂ ? 2en ? 45H₂O ( 2 ) have been obtained under hydrothermal conditions. These extended materials are constructed from lanthanide‐incorporated triangular‐prism‐like polyoxoniobate secondary building units (SBUs) {[Ln(H₂O)₄]₃[Nb₂₄O₆₉(H₂O)₃]₂} (Ln=Dy, Eu). 1 and 2 represent the first examples of high‐dimensional polyoxometalate materials based on such lanthanide‐incorporated triangular‐prism‐like polyoxoniobate SBUs. Furthermore, the proton conduction property and the luminescent emission of these materials were evaluated.     

Record High‐Nuclearity Polyoxoniobates: Discrete Nanoclusters {Nb114}, {Nb81}, and {Nb52}, and Extended Frameworks Based on {Cu3Nb78} and {Cu4Nb78}

A series containing the highest nuclearity polyoxoniobate (PONb) nanoclusters, ranging from dimers to tetramers, has been obtained. They include one 114‐nuclear {Li₈⊂Nb₁₁₄O₃₁₆}, one 81‐nuclear {Li₃K⊂Nb₈₁O₂₂₅}, and one 52‐nuclear {H₄Nb₅₂O₁₅₀}. The Nb nuclearity of these PONbs is remarkably larger than those of all known high‐nuclearity PONbs (≤32). Furthermore, the introduction of 3d Cu²⁺ ions can lead to the generation of extended inorganic–organic hybrid frameworks built from novel, high‐nuclearity, nanoscale heterometallic PONb building blocks {H₃Cu₃Nb₇₈O₂₂₂} or {H₃Cu₄(en)Nb₇₈O₂₂₂}. These building blocks also contain the largest number of Nb centers of any heterometallic PONbs reported to date. The synthesis of new‐type PONbs has long been a challenging subject in PONb chemistry.     

Theoretical studies on redox properties,protonation sites,and electronic spectrum of a new type of polyoxometalate [Ti12Nb6O44]10− by DFT

The electronic structure of a new type of polyoxometalate [Ti₁₂Nb₆O₄₄]^10? has been investigated using density functional theory (DFT). The calculations represent that the LUMO in fully oxidized [Ti₁₂Nb₆O₄₄]^10? delocalizes among the titanium (Ti) and niobium (Nb). Therefore, both Ti and Nb have the probability to accept extra electron when [Ti₁₂Nb₆O₄₄]^10? as catalyst is reduced, which has been reinforced by the spin density for the monoreduced specie [Ti₁₂Nb₆O₄₄]^11?. Three kinds of possible protonated isomers [HTi₁₂Nb₆O₄₄]^9? are discussed. The results reveal that the preferred protonation sites correspond to bridging oxygens Nb? O? Ti. In addition, the calculation of electronic spectrum shows that there is an obvious intramolecular charge transfer from oxygen to metal. The solvent effects were also considered in the calculations by using a conductor‐like screening model (COSMO) of solvation with the solvent‐excluding surface. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A {Nb6P2W12}‐Based Hexameric Manganese Cluster with Single‐Molecule Magnet Properties

By deliberately using a metastable polyanion [(NbO₂)₆P₂W₁₂O₅₆]^12? ( 1 ), which was formed in situ, we have discovered the unprecedented hexameric cluster {Mn₁₅(Nb₆P₂W₁₂O₆₂)₆} ( 2 ), in which the six polyanions [Nb₆P₂W₁₂O₆₁]^10? are alternately connected by four intriguing trinuclear {Mn^III₃} moieties and four {Mn^II} linkers. This discovery is the first in which the phosphoniobotungstate has been made accessible by using transition‐metal ions; furthermore, polyanion 2 represents the largest niobotungstate cluster reported to date. Analysis by means of electrospray ionization mass spectrometry (ESI‐MS ) provides insight into the self‐assembly process, and the peaks observed relate to the different charge states of the parent cluster, thus confirming the stability of 2 . In addition, magnetic‐susceptibility measurements reveal that each {Mn^III₃} subunit is a separate single‐molecule magnet (SMM). This discovery results from the exploration of the reverse effect of metastable polyanion 1 possessing high reactivity, thereby turning a disadvantage into an advantage. This finding could define a new synthetic strategy for the design and synthesis of magnetic polyoxometalate (POM) clusters.     

Acid‐Stable Peroxoniobophosphate Clusters To Make Patterned Films

Two new peroxoniobophosphate clusters were isolated as tetramethylammonium (TMA) salts having the stoichiometries: TMA₅[HNb₄P₂O₁₄(O₂)₄]?9 H₂O and TMA₃[H₇Nb₆P₄O₂₄(O₂)₆]?7 H₂O. The former is stable over the pH range: 3<pH<12 and the latter is stable only below pH 3. These two molecules interconvert as a function of solution pH. The [H₇Nb₆P₄O₂₄(O₂)₆]^3? cluster can be used to fabricate patterned niobium phosphate films by electron‐beam lithography after solution deposition.     

Aggregation of Giant Cerium–Bismuth Tungstate Clusters into a 3D Porous Framework with High Proton Conductivity

The exploration of high nuclearity molecular metal oxide clusters and their reactivity is a challenge for chemistry and materials science. Herein, we report an unprecedented giant molecular cerium–bismuth tungstate superstructure formed by self‐assembly from simple metal oxide precursors in aqueous solution. The compound, {[W₁₄Ce^IV₆O₆₁]([W₃Bi₆Ce^III₃(H₂O)₃O₁₄][B‐α‐BiW₉O₃₃]₃)₂}^34? was identified by single‐crystal X‐ray diffraction and features 104 metal centers, a relative molar mass of ca. 24 000 and is ca. 3.0×2.0×1.7 nm³ in size. The cluster anion is assembled around a central {Ce₆} octahedron which is stabilized by several molecular metal oxide shells. Six trilacunary Keggin anions ([B‐α‐BiW₉O₃₃]^9?) cap the superstructure and limit its growth. In the crystal lattice, water‐filled channels with diameters of ca. 0.5 nm are observed, and electrochemical impedance spectroscopy shows pronounced proton conductivity even at low temperature.     

Synthesis, structure, and molecular modeling of a titanoniobate isopolyanion

Polyoxoniobate chemistry, both in the solid state and in solution is dominated by [Nb₆O₁₉]⁸⁻, the Lindquist ion. Recently, we have expanded this chemistry through use of hydrothermal synthesis. The current publication illustrates how use of heteroatoms is another means of diversifying polyoxoniobate chemistry. Here we report the synthesis of Na₈[Nb₈Ti₂O₂₈]·34H₂O and its structural characterization from single-crystal X-ray data. This salt crystallizes in the P-1 space group (a=11.829(4) Å, b=12.205(4) Å, c=12.532(4) Å, α=97.666(5)°, β=113.840(4)°, γ=110.809(4)°), and the decameric anionic cluster [Nb₈Ti₂O₂₈]⁸⁻ has the same cluster geometry as the previously reported [Nb₁₀O₂₈]⁶⁻ and [V₁₀O₂₈]⁶⁻. Molecular modeling studies of [Nb₁₀O₂₈]⁶⁻ and all possible isomers of [Nb₈Ti₂O₂₈]⁸⁻ suggest that this cluster geometry is stabilized by incorporating the Ti⁴⁺ into cluster positions in which edge-sharing is maximized. In this manner, the overall repulsion between edge-sharing octahedra within the cluster is minimized, as Ti⁴⁺ is both slightly smaller and of lower charge than Nb⁵⁺. Synthetic studies also show that while the [Nb₁₀O₂₈]⁶⁻ cluster is difficult to obtain, the [Nb₈Ti₂O₂₈]⁸⁻ cluster can be synthesized reproducibly and is stable in neutral to basic solutions, as well.     

Formation of Silicon-Containing Polyoxoniobates from Hexaniobate Under High Temperature Conditions

Hydrothermal reaction of K₇H[Nb₆O₁₉]·13H₂O with Na₂SiO₃·9H₂O (220 °C, 24 h) produces a lacunary siliconiobate [Si₄Nb₁₆O₅₆]^16?, which was isolated as mixed salt NaK₈H₆[Na@Si₄Nb₁₆O₅₆]·26H₂O (1). Changing the silicon source to Ph₂Si(OH)₂ under the same conditions slightly improves the yield of [Si₄Nb₁₆O₅₆]^16?, which was isolated as K₁₄H[K@Si₄Nb₁₆O₅₆]·26H₂O (2). Extending the reaction time leads to rearrangement of [Si₄Nb₁₆O₅₆]^16? into Keggin-type silicododecaniobate [SiNb₁₂O₄₀]^16?, which was isolated and characterized as K₈H₂(Nb₂O₂)[SiNb₁₂O₄₀]·20H₂O (3). The complexes were characterized by X-ray single crystal analysis, elemental analysis, thermogravimetry, ²⁹Si NMR.     

A Bicapped α‐Keggin Unit‐Supported Transition Metal Complex: Hydrothermal Synthesis and Characterization of [Cu(en)2(H2O)]2[Cu(en)2]0.5[Mo8V7O42{Cu(en)2}]

A new metal‐oxo cluster supported transition metal complex, [Cu(en)₂(H₂O)]₂[Cu(en)₂]_0.5[Mo^VI₈V^IV₆V^VO₄₂{Cu(en)₂}], has been synthesized under hydrothermal conditions. Its structure was determined by single‐crystal X‐ray diffraction. The compound crystallizes in the triclinic system, space group (No. 2), a = 12.245(5), b = 12.669(5), c = 20.949(8) Å, α = 77.120(13), β = 78.107(17), γ = 65.560(14)°, V = 2860(2) ų, Z = 2. The metal‐oxo cluster contains a novel bicapped a‐Keggin structure unit and a [Cu(en)₂]²⁺ unit covalently bonded to the [Mo₈V₇O₄₂]^7? cluster.     

Giant Hollow Heterometallic Polyoxoniobates with Sodalite‐Type Lanthanide–Tungsten–Oxide Cages: Discrete Nanoclusters and Extended Frameworks

The first series of niobium–tungsten–lanthanide (Nb‐W‐Ln) heterometallic polyoxometalates {Ln₁₂W₁₂O₃₆(H₂O)₂₄(Nb₆O₁₉)₁₂} (Ln=Y, La, Sm, Eu, Yb) have been obtained, which are comprised of giant cluster‐in‐cluster‐like ({Ln₁₂W₁₂}‐in‐{Nb₇₂}) structures built from 12 hexaniobate {Nb₆O₁₉} clusters gathered together by a rare 24‐nuclearity sodalite‐type heterometal–oxide cage {Ln₁₂W₁₂O₃₆(H₂O)₂₄}. The Nb‐W‐Ln clusters present the largest multi‐metal polyoxoniobates and a series of rare high‐nuclearity 4d‐5d‐4f multicomponent clusters. Furthermore, the giant Nb‐W‐Ln clusters may be isolated as discrete inorganic alkali salts and can be used as building blocks to form high‐dimensional inorganic–organic hybrid frameworks.     

Following the Reaction of Heteroanions inside a {W18O56} Polyoxometalate Nanocage by NMR Spectroscopy and Mass Spectrometry

By incorporating phosphorus(III)‐based anions into a polyoxometalate cage, a new type of tungsten‐based unconventional Dawson‐like cluster, [W₁₈O₅₆(HP^IIIO₃)₂(H₂O)₂]^8?, was isolated, in which the reaction of the two phosphite anions [HPO₃]^2? within the {W₁₈O₅₆} cage could be followed spectroscopically. As well as full X‐ray crystallographic analysis, we studied the reactivity of the cluster using both solution‐state NMR spectroscopy and mass spectrometry. These techniques show that the cluster undergoes a structural rearrangement in solution whereby the {HPO₃} moieties dimerize to form a weakly interacting (O₃PH???HPO₃) moiety. In the crystalline state the cluster exhibits a thermally triggered oxidation of the two P^III template moieties to form P^V centers (phosphite to phosphate), commensurate with the transformation of the cage into a Wells–Dawson {W₁₈O₅₄} cluster.     

Host–Guest Chemistry: Oxoanion Recognition Based on Combined Charge‐Assisted C−H or Halogen‐Bonding Interactions and Anion⋅⋅⋅Anion Interactions Mediated by Hydrogen Bonds

Several bis‐triazolium‐based receptors have been synthesized and their anion‐recognition capabilities have been studied. The central chiral 1,1′‐bi‐2‐naphthol (BINOL) core features either two aryl or ferrocenyl end‐capped side arms with central halogen‐ or hydrogen‐bonding triazolium receptors. NMR spectroscopic data indicate the simultaneous occurrence of several charge‐assisted aliphatic and heteroaromatic C?H noncovalent interactions and combinations of C?H hydrogen and halogen bonding. The receptors are able to selectively interact with HP₂O₇^3?, H₂PO₄^?, and SO₄^2? anions, and the value of the association constant follows the sequence: HP₂O₇^3?>SO₄^2?>H₂PO₄^?. The ferrocenyl end‐capped 7²⁺?2 BF₄ ^? receptor allows recognition and differentiation of H₂PO₄^? and HP₂O₇^3? anions by using different channels: H₂PO₄^? is selectively detected through absorption and emission methods and HP₂O₇^3? by using electrochemical techniques. Significant structural results are the observation of an anion???anion interaction in the solid state (2:2 complex, 6²⁺? [ H₂P₂O₇ ] ^2? ), and a short C?I???O contact is observed in the structure of the complex [ 8 ²⁺][SO₄]_0.5[BF₄].     

[Ho5(H2O)16(OH)2As6W64O220]25−, ein großes neuartiges Polyoxoanion aus trivakanten Keggin‐Fragmenten

[Ho₅(H₂O)₁₆(OH)₂As₆W₆₄O₂₂₀]^25?, a Large Novel Polyoxoanion from Trivacant Keggin Fragments The novel polyoxotungstate Na₇K₁₈[Ho₅(H₂O)₁₆(OH)₂As₆W₆₄O₂₂₀] · 56 H₂O ( 1 ) was synthesized in aqueous solution and characterized by X‐ray structure analysis, elemental analysis and IR spectroscopy. The anion in 1 represents one of the largest polyoxoanions known yet and exhibits an unusual arrangement of six Keggin units. It consists of six trivacant lacunary α‐B‐(AsW₉O₃₃)^9? Keggin fragments which are connected by a bridging [Ho₅W₁₀(H₂O)₁₆(OH)₂O₂₂]²⁹⁺ unit. The five Ho^III atoms are coordinated by eight oxygen atoms, forming a square‐antiprism.     

Synthesis of α‐Dawson‐Type Silicotungstate [α‐Si2W18O62]8− and Protonation and Deprotonation Inside the Aperture through Intramolecular Hydrogen Bonds

The design of structurally well‐defined anionic molecular metal–oxygen clusters, polyoxometalates (POMs), leads to inorganic receptors with unique and tunable properties. Herein, an α‐Dawson‐type silicotungstate, TBA₈[α‐Si₂W₁₈O₆₂] ? 3 H₂O ( II ) that possesses a ?8 charge was successfully synthesized by dimerization of a trivacant lacunary α‐Keggin‐type silicotungstate TBA₄H₆[α‐SiW₉O₃₄] ? 2 H₂O ( I ) in an organic solvent. POM II could be reversibly protonated (in the presence of acid) and deprotonated (in the presence of base) inside the aperture by means of intramolecular hydrogen bonds with retention of the POM structure. In contrast, the aperture of phosphorus‐centered POM TBA₆[α‐P₂W₁₈O₆₂]?H₂O ( III ) was not protonated inside the aperture. The density functional theory (DFT) calculations revealed that the basicities and charges of internal μ₃‐oxygen atoms were increased by changing the central heteroatoms from P⁵⁺ to Si⁴⁺, thereby supporting the protonation of II . Additionally, II showed much higher catalytic performance for the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde than I and III .