Cation-Dependent Self-assembly of Vanadium Polyoxoniobates

An Organodiphosphate-Containing Polyoxoniobate Ring and Its Assembly into a Three-Dimensional Framework through Hydrogen Bonding

In this work, a novel organodiphosphate-containing inorganic-organic hybrid polyoxoniobate (PONb) ring {(PO3CH2CH2PO3H)4Nb8O16}4- (Nb8P8) has been achieved by a one-pot hydrothermal method. The ring is constructed from a tetragonal {Nb8O36} motif and four {PO3CH2CH2PO3H} ligands. Interestingly, Nb8P8 can be joined together via K-H2O clusters {K2(H2O)4(OH)2} to form one-dimensional chains {[K2(H2O)4(OH)2]Nb8P8}n and further linked by {Cu(en)2}2+ (en = ethylenediamine) complexes, resulting in a three-dimensional supramolecular framework {[Cu(en)2]2[K2(H2O)4(OH)2]Nb8P8}·3en·H2O (1). 1 exhibits good chemical and thermal stability and has a high water vapor adsorption capacity of ≤224 cm3 g-1 (22.71 mol·mol-1) at 298 K, outperforming most of the known polyoxometalate-based materials. Impedance measurements prove that 1 can transfer protons with moderate conductivity. This study not only contributes to the structural diversity of organodiphosphate-containing PONbs and PONb rings but also provides a reference for the development of PONb-based materials with unique performance.

Fullerene-like Niobovanadate Cage Built from {(Nb)V5 } Pentagon

The striking aesthetic appeal of fullerene-like clusters has captured the interest of researchers. Nevertheless, the assembly of fullerene-like polyoxovadanadate (POV) cages remains a significant challenge due to the scarcity of suitable pentagonal motif. Herein, we have successfully synthesized the first fullerene-like all-inorganic POV cage, {(V2 O)V30 Nb12 O102 (H2 O)12 } (V30 Nb12 ), by introducing Nb into the POVs. V30 Nb12 is assembled by 12 heterometallic {(Nb)V5 } pentagons through sharing V centers with Ih symmetry, reminiscent of C60 . To our knowledge, the fullerene-like V30 Nb12 not only represents the highest-nuclearity POV cage but also stands as the first niobovanadate cluster. Notably, V30 Nb12 exhibits excellent solution stability, as confirmed by ESI-MS, FT-IR and UV/Vis spectra. As there is no protection organic ligand on its outer surface, V30 Nb12 can be further modified with Cu-complexes to form a fullerene-like cluster based zigzag chain (Cu-V30 Nb12 ).

A Rhombus-Like Tetrameric Vanadoniobate Containing Pseudo-Sandwich-Type {Li ⊂ V2O8(Nb5O14)2} and Its Electrocatalytic Activity for the Selective Oxidation of Benzyl Alcohol

Ongoing research on V-containing polyoxoniobates (PONbs) is driven by their diverse structures and potential applications. Although Lindqvist-type {Nb6O19} is a widely used building block in PONbs, vanadoniobates based on {Nb6O19} and/or its derivatives are still very limited. Herein, a discrete vanadoniobate, LiNa14K11[Li2 ⊂ VIV8Nb32O110]·45H2O (1), has been synthesized by a hydrothermal method, which shows a rhombus-like tetrameric structure composed of two {V2O6(Nb6O19)} and two {Li ⊂ V2O8(Nb5O14)2} subunits derived from {Nb6O19}. Notably, the {Li ⊂ V2O8(Nb5O14)2} subunit has an interesting pseudo-sandwich-type structure, where a {LiV2O8} belt is coordinated by two monolacunary {Nb5O14} molecules and the central site of the cluster is occupied by Li+. Considering that 1 has both basic hexaniobates and redox active V centers, 1 was used as a noble metal-free electrocatalyst for the selective oxidation of benzyl alcohol to benzaldehyde, achieving complete conversion of benzyl alcohol with 94% selectivity for benzaldehyde in 3 h under ambient conditions without using any alkaline additives. Moreover, the catalytic performance of 1 remained largely unchanged after four cycles.

Not So Similar: Different Ways of Nb(V) and Ta(V) Catecholate Complexation

The reactions between catechol (H₂cat) and niobium(V) or tantalum(V) precursors in basic aqueous solutions lead to the formation of catecholate complexes of different natures. The following complexes were isolated and characterized by single-crystal X-ray diffraction (SCXRD): (1) (NH₄)₃[NbO(cat)₃]∙4H₂O; (2) K₂[Nb(cat)₃(Hcat)]·2H₂cat·2H₂O; (3) Cs₃[NbO(cat)₃]·H₂O; (4) (NH₄)₄[Ta₂O(cat)₆]·3H₂O; (5) Cs₂[Ta(cat)₃(Hcat)]·H₂cat; (6) Cs₄[Ta₂O(cat)₆]·7H₂O. The isolated crystalline products were characterized by elemental analysis, X-ray powder diffraction (XRPD), FTIR, and TGA. The structural features of these complexes, such as {Ta₂O} unit geometry, Cs-π interactions, and crystal packing effects, are discussed.

Controllable Assembly of Vanadium-Containing Polyoxoniobate-Based Materials and Their Electrocatalytic Activity for Selective Benzyl Alcohol Oxidation

During the controllable synthesis of two vanadium-containing Keggin-type polyoxoniobates (PONbs), [Ni(en)₂]₅[PNb₁₂O₄₀(VO)₅](OH)₅·18H₂O (1) and [Ni(en)₃]₅[PNb₁₂O₄₀(VO)₂]∙17H₂O (2, en = ethylenediamine) are realized by changing the vanadium source and hydrothermal temperature. Compounds 1 and 2 have been thoroughly characterized by single-crystal X-ray diffraction analysis, FT-IR spectra, X-ray photoelectron spectrum (XPS), powder X-ray diffraction (PXRD), etc. Compound 1 contains a penta-capped Keggin-type polyoxoniobate {PNb₁₂O₄₀(VO)₅}, which is connected by adjacent [Ni(en)₂]²⁺ units into a three-dimensional (3D) organic-inorganic framework, representing the first nickel complexes connected vanadoniobate-based 3D material. Compound 2 is a discrete di-capped Keggin-type polyoxoniobate {PNb₁₂O₄₀(VO)₂} with [Ni(en)₃]²⁺ units as counter cations. Compounds 1 and 2 have poor solubility in common solvents and can keep stable in the pH range of 4 to 14. Notably, both 1 and 2 as electrode materials are active for the selective oxidation of benzyl alcohol to benzaldehyde. Under ambient conditions without adding an alkaline additive, compound 1 as a noble metal free electrocatalyst can achieve 92% conversion of benzyl alcohol, giving a Faraday efficiency of 93%; comparatively, 2 converted 79% of the substrate with a Faraday efficiency of 84%. The control experiments indicate that both the alkaline polyoxoniobate cluster and the capped vanadium atoms play an important role during the electrocatalytic oxidation process.

Synthesis, structures and stability of three V-substituted polyoxoniobate clusters based on [TeNb9O33]17- units

Three structurally intriguing polyoxoniobates (PONbs) based on the trivacant B-type α-Keggin ion {TeNb9O33}, H4K(CN3H6)2{[Cu4(2,2'-bipy)4(H2O)2][TeNb9V2O37]}·29H2O (1, 2,2'-bipy = 2,2'-bipyridine), H0.5K5Na2.5{[Cu(en)H2O]3[TeNb9V3O39]}·10H2O (2, en = ethylenediamine), and K3Na5{[Cu(1,3-dap)H2O]3[TeNb9V3O39]}·11H2O (3, 1,3-dap = 1,3-diaminopropane), are assembled by the conventional aqueous solution methods using a series of N-containing organic ligands. In 1, each of the two {VO4} units is attached to two coplanar NbO6 octahedra on the {Nb3O13} cluster of the {TeNb9O33} unit. Differently, three {VO4} units in 2 and 3 are linked to two edge-sharing NbO6 octahedra, respectively. Compounds 1-3 represent the first oxo NbTeV clusters and also the first vanadoniobates based on the trivacant Keggin PONb units. All three compounds were characterised by single-crystal X-ray structural analysis, TGA and IR, ESI-MS and 51V NMR spectroscopy. Furthermore, the magnetic properties of compounds 1 and 2 were also studied.

A large copper-niobate cluster with the pagoda-shaped subunit {Nb20O59}

A 72-nuclearity niobium cluster was synthesized, in which two {CuNb26O76} clusters and one {Nb20O59} cluster are fused in a triangular fashion, resulting in a {Nb12} cavity. Further, the simple nature of the species allowed its investigation by ESI-MS analysis, yielding two subunits with time.

Tubular polyoxoanion [(SeMo6O21)2(C2O4)3]10− and its transformations

Tubular [(SeMo₆O₂₁)₂(C₂O₄)₃]¹⁰⁻ polyoxoanion can be assembled from Na₂MoO₄/SeO₂/H₂C₂O₄ system. TBABr precipitates complexes of [Mo₆O₁₉]²⁻ and [β-Mo₈O₂₆Na₂(NO₃)]³⁻ from its aqueous solution. Speciation of [β-Mo₈O₂₆]⁴⁻ and [Mo₆O₁₉]²⁻ has been studied.

Coordination capacity of Keggin anions as polytopic ligands: case study of [VNb12O40]15

Reaction of Na9H4[VNb12O40{NbO(CO3)}2] with [(C6H6)RuCl2]2 (molar ratio {VNb12} : {(C6H6)Ru} = 1 : 4) in aqueous solution gives a mixture of [α-{(C6H6)Ru}4VNb12O40]7- and [α-{(C6H6)Ru}3VNb12O40]9-. Direct acetone diffusion into mother liquor leads to crystallization of Na6H[α-{(C6H6)Ru}4VNb12O40]·41.25H2O (1), characterized by single crystal X-ray diffraction (SCXRD). This anion has four organometallic fragments coordinated to the α-Keggin type [VNb12O40]15- backbone in different manner. Three {(C6H6)Ru}2+ groups cap triangular faces and one group a rectangular face of [VNb12O40]15-. Equilibrated mixture of [α-{(C6H6)Ru}4VNb12O40]7- and [α-{(C6H6)Ru}3VNb12O40]9- was studied by 1H DOSY NMR, HPLC-ICP-AES and HPLC-ESI-MS combined techniques. Direct chromatographic separation of these complexes results in unexpected transformation of both species into [α-{(C6H6)Ru}5VNb12O40]5-, isolated and characterized as Na5[α-{(C6H6)Ru}5VNb12O40]·16H2O (2). This anion contains five coordinated organometallic groups occupying both triangular and rectangular faces.

Hexaniobate anions connected by [Ni(cyclam)]2+ complexes yield two interpenetrating three-dimensional networks

Syntheses were performed at room temperature using Ni(NO3)2·6H2O, cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) and the precursors Li8[Nb6O19]· ≈22H2O or Na7[HNb6O19]·15H2O in a DMSO-H2O mixture. Yellow crystals of the new compound {[Ni(cyclam)]2H4Nb6O19}·12H2O could be obtained after one week applying the Li+ or Na+ salt as starting materials. The crystal structure is unique in polyoxoniobate (PONb) chemistry and displays two interpenetrating three-dimensional (3D) networks. The [Nb6O19]8– anion is expanded by four Ni2+ centered complexes via Ni–O bonds to terminal O2− anions of the hexaniobate anion. The 3D networks are generated by further Ni–O bond formation between neighboring [Nb6O19]8− anions. The remaining void space is occupied by H2O molecules which form a water cluster.

Self-assembly of polyoxovanadate-capped polyoxoniobates and their catalytic decontamination of sulfur mustard simulants

Two novel polyoxovanadate-capped polyoxoniobates with non-classical structural motifs have been successfully assembled, which are active heterogeneous catalysts for the oxidative detoxification of sulfur mustard simulants.

New pronounced progress in the synthesis of group 5 polyoxometalates

This highlight summarizes new developments made in group 5 polyoxometalate science of high nuclearity clusters with focus on synthetic approaches.

How Does the Redox State of Polyoxovanadates Influence the Collective Behavior in Solution? A Case Study with [I@V18O42] q- ( q = 3, 5, 7, 11, and 13)

A series of stable reduction-oxidation states of the cagelike [I@V^IV _xV^V_{18- x}O₄₂]^{5- x} polyoxovanadate (POV) with x = 8, 10, 12, 16, and 18 were studied with density functional theory and molecular dynamics to gain insight into the structural and electron distribution characteristics of these metal-oxo clusters and to analyze the charge/redox-dependent assemblage processes in water and acetonitrile (MeCN) solutions. The calculations show that the interplay between the POV redox state (molecular charge) and the solvent polarity, countercation size, and hydrophilicity (or hydrophobicity) controls the POV agglomeration phenomena, which substantially differ between aqueous and MeCN media. In MeCN, agglomeration is more pronounced for intermediate-charged POVs, whereas in water, the lowest-charged POVs and organic countercations tend to agglomerate into a microphase. Tests made on wet MeCN show diminished agglomeration with respect to pure MeCN. Simulations with alkali countercations in water show that only the highest-charged POV can form agglomerates. The herein presented theoretical investigation aims to support experimental studies of POVs in the field of functional nanomaterials and surfaces, where controlled molecular deposition from the liquid phase onto solid substrates requires knowledge about the features of these metal-oxo clusters in discrete solutions.

Niobium uptake by a [P8W48O184]40−macrocyclic polyanion

Incorporation of Nb into the [P₈W₄₈O₁₈₄]⁴⁰⁻anionic macrocyclic cavitand leads to formation of new Nb–W POMs. Inclusion of up to five {NbO(H₂O)}³⁺groups was observed. Solution speciation of the Nb-encapsulating macrocycles was studied by HPLC-ICP-AES and SAXS.

On the influence of the titanium source on the composition and structure of novel titanoniobates

Systematic variation of the titanium source and the reaction temperature applied during hydrothermal synthesis led to crystallization of four new titanoniobates: {[Ni(cyclam)]₄[Ti₂Nb₈O₂₈]}_n·∼28nH₂O (I), K[Ni(cyclam)]₃[TiNb₉O₂₈]·xH₂O; x = 18 (II), x = 14 (III) and x ∼ 10(IV).