A 3D heteropolyoxoniobate framework based on heart-shaped {Te2Nb19O60} clusters with proton conductivity property

A 3D tellurium-substituted heteropolyoxoniobate framework H5K3Na[Cu(en)2]2[Cu(en)0.75(H2O)2.5]{[(Te2Nb19O58)(μ3-OH)2]}·24H2O (1, en = ethylenediamine) with a 6-connected pcu topology is built from heart-shaped {Te2Nb19O60} clusters and copper complexes. The {Te2Nb19O60} cluster represents the new tellurniobate structure type with a 19-nuclearity Nb cluster. It consists of two new monovacant Lindqvist {Nb5O19} clusters, one boat-shaped {Nb9O32} cluster and two TeO32- anions. The {Te2Nb19O60} polyanions are interlinked by [Cu(en)2]2+ complexes into a 2D (4, 4) grid-like layer containing rhombic sheets. The Cu2+ supports the adjacent layers through Te-O-Cu-O-Te- bonds to form a three-dimensional heteropolyoxoniobate framework with 1D channels. This compound exhibits good chemical and solvent stability and proton conductivity, with a conductivity of 7.9 × 10-3 S cm-1 at 85 °C under 98% RH.

Tuning the Inorganic Core of a reduced Ni2Ge2 Cluster

Tetranuclear cores (M-E)2 of transition metals (M) and tetrylenes (EII=Si, Ge, Sn) are key motifs in homogeneous and heterogeneous catalysis. They exhibit a continuum of M-M and E-E bonding within the inorganic core that leads to a variety of structures for which there are no specific synthetic methods. Herein, we report a series of highly reduced [Ni0GeII]2 squares solely stabilized by bulky terphenyl (C6H3-2,6-Ar2) ligands, for which we provide complementary and high-yielding syntheses. Reactivity studies with common Lewis bases (carbene and CO) evince that the structure of the (M-E)2 core can be transformed. We have investigated this core modification by computational means, offering a rationale to better understand the continuum of bonding across these clusters.

Two Unprecedented Germanoniobate Frameworks Based on High-Nuclearity Peanut-Shaped {Ge12Nb38} Clusters

By variation of the amount of GeO2, two organic-inorganic hybrid germanoniobate frameworks with 6-connected pcu and 10-connected bct topologies were constructed from peanut-shaped {α-Ge12Nb38} and {β-Ge12Nb38} clusters, respectively. The {α-Ge12Nb38} and {β-Ge12Nb38} clusters contain the most Ge centers of germanoniobates reported so far. The compounds exhibit proton conduction properties with a conductivity of 3.04 × 10-4 S·cm-3 for 1 and 1.62 × 10-4 S·cm-3 for 2 at 85 °C and 98% RH. The water vapor adsorption capacities for 1 and 2 are 5.86 and 4.40 mmol·g-1, respectively.

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.

A 1D Helical Chain Heterpolyniobate Templated by AsO33

A novel polyoxoniobate cluster directed by AsO₃^3-, {[Cu(phen)]₅[Nb₆O₁₉][As₂Nb₆O₂₂]}₂ (1a), has been successfully synthesized by a "two-pot" strategy, namely, the conventional aqueous solution and diffusional methods. It incorporates two {[Cu(phen)]₂Nb₆O₁₉} and two {[Cu(phen)]₃As₂Nb₆O₂₂} subunits. Surprisingly, by changing the reaction conditions, one of the subunits {[Cu(phen)]₂Nb₆O₁₉} was isolated as a guanidinium salt. These architectures were structurally characterized by single crystal X-ray diffraction, TGA, IR, elemental analysis and PXRD, etc. Furthermore, the behavior of 1 in aqueous solution was measured by electrospray ionization mass spectrometry, and the magnetic property in the low temperature range indicates the presence of antiferromagnetic interactions.

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.

A novel peroxopolyoxoniobate incorporating mixed heteroatoms: [P2Se2Nb6(O2)6O22]8-

A novel hetero selenato-phosphato-peroxopolyoxoniobate with the formula Cs₄H₄[P₂Se₂Nb₆(O₂)₆O₂₂]·10H₂O has been successfully isolated in an acidified aqueous hydrogen peroxide solution. The synthesized cluster represents the first example of a selenium-containing polyoxoniobate. Furthermore, the ESI-MS spectra show that the polyoxoanion structural unit [P₂Se₂Nb₆(O₂)₆O₂₂]^8- remains intact in aqueous solution.

Toward Exploiting the Behavior of Niobium-Containing Mesoporous Silicates vs. Polyoxometalates in Catalysis

Classification of polyoxometalates (POMs) is based on their chemical composition, basically represented by two general formulae: a) [MmOy]p- b) [XxMmOy]q-, where M is the main transition metal, O is the oxygen atom and X can be a non-metal atom such as Si. Additionally, in the most cases, the structure of the polyoxometalates is derived from a combination of octahedral units MO6 with a central metal atom M and the oxygen atoms placed at their corners. In such octahedra, oxygen atoms allow the condensation between two octahedral units, while one oxygen atom (or max. two atoms) makes double bond with the central metal atom and is not shared with other metal atoms within the complex (terminal oxygens). On the other hand, niobium-containing mesoporous silicates contain mainly MO4 tetrahedra and reveal superior activity in heterogeneous catalysis. Thus, the proper coordination of niobium is crucial for the catalytic activity and will be deeply discussed. The similarity in the catalytic behavior of niobium-polyoxometalates and heterogeneous niobium single-site catalysts in selective oxidations will be demonstrated.

Two nanoscale Nb containing polyoxometalates based on {P2W15Nb3O62} clusters and chromium cations

We have introduced mixed-addenda Nb/W polyoxometalates as a new class of materials for lithium-ion batteries and demonstrated their capability as anode materials with good cycling stability.

Unprecedented high-nuclear transition-metal-cluster-substituted heteropolyoxoniobates: synthesis by {V8 } ring insertion into the POM matrix and antitumor activities

Reactions of hexaniobate with vanadate in the presence of Ni(2+) , Zn(2+) , or Cu(2+) have furnished three high-nuclear vanadium cluster-substituted heteropolyoxoniobates (HPNs): {Ni(en)3 }5 H{V(V) Nb8 V(IV) 8 O44 }⋅9 H2 O (1), (H2 en)Na2 [{Zn(en)2 (Hen)}{Zn(en)2 (H2 O)}2 {PNb8 V(IV) 8 O44 }]⋅11 H2 O (2), and Na{Cu(en)2 }3 {[Cu(en)2 ]2 [PNb8 V(IV) 8 O44 ]}⋅11 H2 O (3) (en=1,2-diaminoethane). Their structures have been determined and characterized by single-crystal X-ray diffraction analysis, thermogravimetric analysis (TGA), and elemental analysis. Structural analysis has revealed that compounds 1-3 contain similar {V8 }-substituted [X(V) Nb8 V(IV) 8 O44 ](11-) (X=P, V) clusters, obtained by inserting a {V8 } ring into tetravacant HPN [XNb8 O36 ](27-) . To the best of our knowledge, compounds 1-3 represent the first high-nuclear vanadium cluster-substituted HPNs, and compound 1 is the largest vanadoniobate cluster yet obtained in HPN chemistry. Nickel and zinc cations have been introduced into HPNs for the first time, which might promise a more diverse set of structures in this family. Antitumor studies have indicated that compounds 1 and 2 exhibit high activity against human gastric cancer SGC-7901 cells, SC-1680 cells, and MG-63 cells.

Transformation of potassium Lindquist hexaniobate to various potassium niobates: solvothermal synthesis and structural evolution mechanism

This paper introduces the formation reactions and reaction mechanisms of a series of potassium niobates from a potassium salt of the Lindquist hexaniobate [Nb6O19](8-) ion under solvothermal conditions. The structure and particle morphology of the potassium niobate product can be controlled easily with the reaction solution alkalinity using this solvothermal process. KNb3O8 with a plate-like morphology, K4Nb6O17·4.5H2O with a plate-like morphology, a new phase of K2Nb2O6·H2O with fibrous morphology, KNbO3 perovskites with cubic morphology are obtained at pH = 5.5, and in 0.3, 0.5, 1.0 mol L(-1) KOH solutions at 230 °C, respectively. The reaction conditions are much milder than those in the normal hydrothermal process. Furthermore, the K2Nb2O6·H2O fibers can be topotactically transformed into KNbO3 fibers, Nb2O5 fibers after H(+)-exchange-treatment, and LiNbO3 fibers after Li(+)-exchange-treatment by heat-treatments at 730, 560, and 520 °C, respectively. The formation reaction and structure of these potassium niobates were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), energy-dispersive spectroscopy (EDS), Raman spectra and TG-DTA. The formation mechanism of this series of potassium niobates from the [Nb6O19](8-) precursor is systematically explained via the correlation between the octahedrons [NbO6] sharing forms in the precursor structure and in the product structures.