Giant polyniobate clusters based on [Nb7O22]9- units derived from a Nb6O19 precursor

Rational self-assembly of hexaniobate Lindqvist-type precursor [HNb6O19]7- with soluble Cu2+ salts utilizing different strategies produces a series of giant polyniobate clusters, namely, (H2en)1.25[Cu(en)2(H2O)]2Cl4[Nb24O72H21.5]7 H2O (1; en: ethylenediamine), [Cu(en)2]3[Cu(en)2(H2O)]9[{H2Nb6O19} subset{[({KNb24O72H10.25}{Cu(en)2})2{Cu3(en)3(H2O)3}{Na1.5Cu1.5(H2O)8}{Cu(en)2}4]6}]144 H2O (2), K12Na4[H23NaO8Cu24(Nb7O22)8]106 H2O (3), and K16Na12[H9Cu25.5O8(Nb7O22)8] 73.5 H2O (4). Their structures were determined and further characterized by single-crystal X-ray diffraction analysis, IR and Raman spectroscopy, thermogravimetric analysis (TGA), and elemental analysis. Structural analyses reveal that compound 2 comprises a giant capsule anion based on a wheel-shaped cluster encapsulating a Lindqvist diprotonated cluster [H2Nb6O19]6- unit, and forms a honeycomb-like structure with the inclusion of Lindqvist-type anions [H2Nb6O19]6- in the holes, whereas 3 and 4 represent an unprecedented giant cube-shaped framework. All the compounds are built from [Nb7O22]9- fundamental building blocks. Solution Raman spectroscopy studies of 2 and 3 reveal that the solid-state structures of these polyniobate cluster anions disassemble and exist in the form of the [Nb6O19]8- unit in solution. Magnetic susceptibility measurement of 3 shows antiferromagnetic coupling interactions between CuII ions with the spin-canting phenomenon.     

Electroactive Benzothiazole Hydrazones and Their [Mo6O19]2− Derivatives: Promising Building Blocks for Conducting Molecular Materials

The electroactive benzothiazole hydrazone AMBTH–H₂, a new member of the 2,2′‐azino‐bis(N‐alkylbenzothiazole) family, was synthesised in a five‐step procedure and characterised by using X‐ray diffraction along with two intermediates and the N‐methylbenzothiazole hydrazone MBTH–H₂. Both AMBTH–H₂ and MBTH–H₂ were coupled to [Mo₆O₁₉]^2? in acetonitrile in the presence of dicyclohexylcarbodiimide and dimethylaminopyridine to give two new diazoalkane–hexamolybdates, which were isolated as tetrabutylammonium salts and characterised by using IR, UV/Vis and NMR spectroscopies, cyclic voltammetry and, for one of them, X‐ray diffraction. The packing arrangement molecules in crystals of AMBTH–H₂, the redox features of the AMBTH–hexamolybdate hybrid together with a good electronic communication between the organic π system and the molybdenum centres make these compounds very promising blocks for the synthesis of conducting molecular materials.     

The Mixed Gold–Palladium Polyoxo‐Noble‐Metalate [NaAuIII4PdII8O8(AsO4)8]11−

The first fully inorganic, discrete gold–palladium–oxo complex [NaAu^III₄Pd^II₈O₈(AsO₄)₈]^11? has been synthesized in aqueous medium. The combination of single‐crystal XRD, elemental analysis, mass spectrometry, and DFT calculations allowed establishing the structure and composition of the novel polyanion, and UV/Vis studies suggest that it is stable in neutral aqueous media.     

Self‐Assembly of [B‐SbW9O33]9− Subunit with Transition Metal Ions (Mn2+, Cu2+, Co2+) in Aqueous Solution: Syntheses,Structures and Magnetic Properties of Sandwich Type Polyoxometalates with Subvalent SbIII Heteroatom

Rational self‐assembly of Sb₂O₃ and Na₂WO₄, or (NH₄)₁₈[NaSb₉W₂₁O₈₆] with transition‐metal ions (Mn²⁺, Cu²⁺, Co²⁺), in aqueous solution under controlled conditions yield a series of sandwich type complexes, namely, Na₂H₂[Mn_2.5W_1.5(H₂O)₈(B‐β‐SbW₉O₃₃)₂]?32 H₂O (1) , Na₄H₇[Na₃(H₂O)₆Mn₃(μ‐OAc)₂(B‐α‐SbW₉O₃₃)₂]?20 H₂O (OAc=acetate anion) (2) , NaH₈[Na₂Cu₄Cl(B‐α‐SbW₉O₃₃)₂]?21 H₂O (3) , Na₈K[Na₂K(H₂O)₂{Co(H₂O)}₃(B‐α‐SbW₉O₃₃)₂]? 10 H₂O (4) , and Na₅H[{Co(H₂O)₂}₃W(H₂O)₂(B‐β‐SbW₉O₃₃)₂]?11.5 H₂O (5) . These structures are determined by using the X‐ray diffraction technique and further characterized by obtaining IR spectra and performing elemental analysis. Structure analysis reveals that polyoxoanions in 1 and 5 comprise of two [B‐β‐SbW₉O₃₃]^9? building units, whereas 2 , 3 , and 4 consist of two isomerous [B‐α‐SbW₉O₃₃]^9? building blocks, which are all linked by different transition‐metal ions (Mn²⁺, Cu²⁺, or Co²⁺) with different quantitative nuclearity. It should be noted that compound 2 represents the first one‐dimensional sinusoidal chain based on sandwich like tungstoantimonate building blocks through the carboxylate‐bridging ligands. Additionally, 3 is constructed from sandwiched anions [Na₂Cu₄Cl(B‐α‐SbW₉O₃₃)₂]^9? linked to each other to form an infinitely extended 2D network, whereas 5 shows an interesting 3D framework built up from offset sandwich type polyoxoanion [{Co(H₂O)₂}₃W(H₂O)₂(B‐β‐SbW₉O₃₃)₂]^6? linked by Co²⁺ and Na⁺ ions. EPR studies performed at 110 K and room temperature reveal that the metal cations (Mn²⁺, Cu²⁺, Co²⁺) reside in a square‐pyramidal geometry in 2 , 3 , and 4 . The magnetic behavior of 1 – 4 suggests the presence of weak antiferromagnetic coupling interactions between magnetic metal centers with the exchange integral J=?0.552 cm^?1 in 2 .     

Titanium‐Substituted Polyoxotantalate Clusters Exhibiting Wide pH Stabilities: [Ti2Ta8O28]8− and [Ti12Ta6O44]10−

Two new substituted polyoxotantalate clusters, [Ti₂Ta₈O₂₈]^8? and [Ti₁₂Ta₆O₄₄]^10?, considerably expand the pH range where tantalates persist in aqueous solution. The structures of [Ti₂Ta₈O₂₈]^8? and [Ti₁₂Ta₆O₄₄]^10? are reported as tetramethylammonium salts after synthesis at hydrothermal conditions in aqueous solution. These Ti‐substituted polyoxotantalate clusters have analogues among recently discovered niobates, but are slightly larger and more persistent in solution. Most importantly, they exhibit a much wider range of pH stability than the familiar hexatantalate cluster, which is the only other tantalate known to be stable at highly basic pH conditions. These molecules are kinetically stable to near‐neutral pH, making them excellent synthons for further development into materials and catalysts, and an significant advance in adapting tantalates for use in aqueous solutions.     

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?.     

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.