Structural Variety of Niobium(V) Polyoxo Clusters Obtained from the Reaction with Aromatic Monocarboxylic Acids: Isolation of {Nb2O}, {Nb4O4} and {Nb8O12} Cores

The reactivity of aryl monocarboxylic acids (benzoic, 1- or 2-naphtoic, 4’-methylbiphenyl-4-carboxylic, and anthracene-9-carboxylic acids) as complexing agents for the ethoxide niobium(V) (Nb(OEt)₅ precursor has been investigated. A total of eight coordination complexes were isolated with distinct niobium(V) nuclearities as well as carboxylate complexation states. The use of benzoic acid gives a tetranuclear core Nb₄(μ₂-O)₄(L)₄(OEt)₈] (L=benzoate ( 1 )) with four Nb−(μ₂-O)−Nb linkages in a square plane configuration. A similar tetramer, 7 , was obtained with 2-naphtoic acid by using a 55 % humid atmosphere synthetic route. Two types of dinuclear brick were identified with one central Nb−(μ₂-O)−Nb linkage; they differ in their complexation state, with one bridging carboxylate ([Nb₂(μ₂-O)(μ₂-OEt)(L)(OEt)₆], with L=1-naphtoate ( 3 ) or anthracene-9-carboxylate ( 5 )) or two bridging carboxylate groups ([Nb₂(μ₂-O)(L)₂(OEt)₆], with L=4’-methylbiphenyl-4-carboxylic ( 4 ) or anthracene-9-carboxylate ( 6 )). An octanuclear moiety [Nb₈(μ₂-O)₁₂(L)₈(η₁-L)_4−x(OEt)_4+x] (with L=2-naphtoate, x=0 or 2; 8 ) was obtained by using a solvothermal route in acetonitrile; it has a cubic configuration with niobium centers at each node, linked by 12 μ₂-O groups. The formation of the niobium oxo clusters was characterized by infrared and liquid ¹H NMR spectroscopy in order to analyze the esterification reaction, which induces the release of water molecules that further react through oxolation with niobium atoms, in different {Nb₂O}, {Nb₄O₄} and {Nb₈O₁₂} nuclearities.     

[{Ni4(OH)3AsO4}4(B‐α‐PW9O34)4]28−: A New Polyoxometalate Structural Family with Catalytic Hydrogen Evolution Activity

A new structural polyoxometalate motif, [{Ni₄(OH)₃AsO₄}₄(B‐α‐PW₉O₃₄)₄]^28?, which contains the highest nuclearity structurally characterized multi‐nickel‐containing polyanion to date, has been synthesized and characterized by single‐crystal X‐ray diffraction, temperature‐dependent magnetism and several other techniques. The unique central {Ni₁₆(OH)₁₂O₄(AsO₄)₄} core shows dominant ferromagnetic exchange interactions, with maximum χ_mT of 69.21 cm³ K mol^?1 at 3.4 K. Significantly, this structurally unprecedented complex is an efficient, water‐compatible, noble‐metal‐free catalyst for H₂ production upon visible light irradiation (photosensitizer=[Ir(ppy)₂(dtbbpy)][PF₆]; sacrificial electron donor=triethylamine or triethanolamine). The highest turnover number of approximately 580, corresponding to a best quantum yield of approximately 4.07 %, is achieved when using triethylamine as electron donor in the presence of water. The mechanism of this photodriven process has been probed by time‐solved luminescence and by static emission quenching.     

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.     

K3Na5(H2NCH2CH2NH3)2{(VO)12O6[B3O6(OH)]6}(H2O)•12H2O的合成与晶体结构

利用水热合成技术成功制备出一种新型多钒硼氧化合物， 用X射线单晶衍射分析技术对其晶体结构和分子结构进行了确定。结果表明在该化合物中多钒硼氧阴离子具有一个新颖的三明治结构。上下两个结构单元都是由六个VO₅四角锥交替地通过顺式和反式共边的方式连接起来构成的一个钒氧三角形结构。中间的结构单元是由BO₃平面三角形和BO₄四面体以共角的方式相互连接形成的一个折叠型的B₁₈O₃₆(OH)₆环。三明治结构中层与层之间通过桥氧相连。一个水分子处于它的核心位置上，与每个VO₅四角锥中的钒原子都保持几乎相等的距离。该化合物及其晶体中存在着丰富的化学结构和成键信息，同时也有作为氧化还原反应催化剂的潜能。     

Isolation of a Dodecanuclear Polyoxo Cluster {Nb12O21} Showing a Rare Case of Five-fold Coordinated Niobium(V) Centers with a Square Pyramidal Geometry

The reactivity of the complexing anthracene-9-carboxylate ligand has been investigated with a niobium(IV) tetrachloride precursor (NbCl₄ ⋅ 2THF) in isopropanol solvent. This resulted in the crystallization of a molecular assembly containing two distinct {Nb₁₂O₂₁} cores surrounded by multiple isopropanolate and anthracenoate ligands. The compound is formulated [Nb₁₂₍(μ₃-O)₃(μ-O)₁₈(C₁₅H₉O₂)₈(OⁱPr)₁₀] ⋅ [Nb₁₂(μ₃-O)₂(μ-O)₁₉(C₁₅H₉O₂)₈(OⁱPr)₁₀] illustrating the two different dodecameric oxo-clusters, for which the niobium(IV) precursor was oxidized in the niobium(V) state during the reactional process. The two distinct {Nb₁₂O₂₁} units mainly differs by the environment of the niobium centers, which exhibits unexpected five-fold coordination (square pyramid) for some of them, together with the classical six-fold coordination (octahedron) as usually found for niobium(V). In the crystallization process, the. IR spectroscopy was used to analyze the esterification reaction occurring between the anthracene acid an isopropanolate ligands responsible of the production of water used in the oxo-condensation of the niobium centers. ⁹³Nb Solid state NMR was tentatively used to assess the occurrence of the different niobium environments.     

Vicinal Dinitridoruthenium‐Substituted Polyoxometalates γ‐[XW10O38{RuN}2]6− (X=Si or Ge)

Reaction of the divacant polyoxometalate K₈[γ‐XW₁₀O₃₆] (X=Si, Ge) with two equivalents of the metal‐nitrido precursor Cs₂[Ru^VINCl₅], at room temperature in water, produces K₂(Me₂NH₂)₂H₂[γ‐XW₁₀O₃₈{RuN}₂], X=Si ( DMA ‐ 1 a ) or Ge ( DMA ‐ 1 b ). The X‐ray crystal structures of both complexes show monomeric complexes with highly unusual vicinal terminal metal‐nitrido units. The Ru?N bond lengths are 1.594(10) and 1.612(11) Å in 1 a and 1 b , respectively. EXAFS studies confirmed the key structural assignments from X‐ray crystallography. The XANES spectrum of DMA‐1 a , diamagnetism, NMR (²⁹Si and ¹⁸³W) chemical shifts, voltammetric behavior, reductive titrations with [PW₁₂O₄₀]^4?, and computational data are all consistent with d² Ru^VI centers in these complexes. The FT‐IR and Raman spectra show the expected vibrational modes of the {γ‐XW₁₀} unit and the Ru?N stretch at 1080 cm^?1, respectively. Interestingly, reduction of DMA‐1 a by 4 equivalents of [PW₁₂O₄₀]^4? produces NH₃ in nearly quantitative yield. Cyclic voltammetry versus pH and calculations provide the energetics for the possible two‐electron reduction and two‐proton addition processes in this reaction.     

Three Cobalt(II)‐Linked {P8W48} Network Assemblies: Syntheses,Structures, and Magnetic and Photocatalysis Properties

Three cobalt(II)‐containing tungstophosphate compounds, Na₈Li₈Co₅[Co_5.5(H₂O)₁₉P₈W_48.5O₁₈₄] ? 60 H₂O ( 1 ), K₂Na₄Li₁₁Co₅[Co₇(H₂O)₂₈P₈W₄₈O₁₈₄]Cl ? 59 H₂O ( 2 ), and K₂Na₄LiCo₁₁[Co₈(H₂O)₃₂P₈W₄₈O₁₈₄](CH₃COO)₄Cl ? 47 H₂O ( 3 ), have been synthesized and characterized by IR spectroscopy, thermogravimetric analysis, elemental analyses, and magnetic measurements. The pH value impacts the formation of distinct cobalt‐linked frameworks. The cyclic cavity of the polyanion accommodates 5.5, 7, and 8 cobalt ions in 1 , 2 , and 3 , respectively. In compounds 1 and 2 , each {Co_5.5P₈W₄₈} and {Co₇P₈W₄₈} fragment links to four others through multiple {Co‐O‐W} coordination bonds to generate a two‐dimensional network. Compound 3 can be considered as a 3D network based on the {Co‐O‐W} coordination bonds and the {Co₃(CH₃COO)₂(H₂O)₁₀} linkers between the {P₈W₄₈} fragments. Interestingly, acetate ligands have been employed to form the {Co₃(CH₃COO)₂(H₂O)₁₀} unit, thereby inducing the construction of a 12‐connected framework. To the best of our knowledge, compound 3 contains the largest‐ever number of cobalt ions in a {P₈W₄₈}‐based polyoxometalate when counterions are taken into account and the {P₈W₄₈} unit shows the highest number of connections thanks to the carboxyl bridges. The UV/Vis diffuse reflectance spectra of these powder samples indicate that the corresponding well‐defined optical absorption associated with E_g can be assessed at 2.58, 2.48, and 2.73 eV and reveal the presence of an optical band gap. The photocatalytic H₂ evolution activities of these {P₈W₄₈}‐based compounds are evaluated.     

STM/STS observation of polyoxoanions on HOPG surfaces: the wheel-shaped [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- and the ball-shaped [{Sn(CH3)2(H2O)}24{Sn(CH3)2}12(A-PW9O34)12]36-

A combination of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) techniques have been performed on the wheel-shaped [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- and the ball-shaped [{Sn(CH3)2(H2O)}24{Sn(CH3)2}12(A-PW9O34)12]36- deposited on highly oriented pyrolytic graphite surfaces. Small, regular molecule clusters, as well as separated single molecules, were observed. The size of the molecules is in agreement with the data determined by X-ray crystallography. In STS measurements, we found a rather large contrast at the expected location of the Cu metal centers in our molecules, i.e., the location of the individual Cu ions in their organic matrix is directly addressable by STS.