{Nb 288 O 768 (OH) 48 (CO 3 ) 12 }: A Macromolecular Polyoxometalate with Close to 300 Niobium Atoms

Giant Polyoxoniobate-Based Inorganic Molecular Tweezers: Metal Recognitions, Ion-Exchange Interactions and Mechanism Studies

This work reports the interesting and unique cation-exchange behaviors of the first indium-bridged purely inorganic 3D framework based on high-nuclearity polyoxoniobates as building units. Each nanoscale polyoxoniobate features a fascinating near-icosahedral core-shell structure with six pairs of unique inorganic "molecular tweezers" that have changeable openings for binding different metal cations via ion-exchanges and exhibit unusual selective metal-uptake behaviors. Further, the material has high chemical stability so that can undergo single-crystal-to-single-crystal metal-exchange processes to produce a dozen new crystals with high crystallinity. Based on these crystals and time-dependent metal-exchange experiments, we can visually reveal the detailed metal-exchange interactions and mechanisms of the material at the atomic precision level. This work demonstrates a rare systematic and atomic-level study on the ion-exchange properties of nanoclusters, which is of significance for the exploration of cluster-based ion-exchange materials that are still to be developed.

Structural Variety of Niobium(V) Polyoxo Clusters Obtained from the Reaction with Aromatic Monocarboxylic Acids: Isolation of {Nb2 O}, {Nb4 O4 } and {Nb8 O12 } 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 ¹ H 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 {Nb₂ O}, {Nb₄ O₄ } and {Nb₈ O₁₂ } nuclearities.

Deliberate Construction of Polyoxoniobates Exploiting the Carbonate Ligand

Polyoxometalates (POMs, metals=V^V , Nb^V , Ta^V , Mo^VI , W^VI ) are molecular metal oxides that can be isolated without capping ligands. The high negative charge of polyoxoniobates (PONb) provides strong interactions with heterocations, advantageous for electrostatic assembly of modular materials. In four single-crystal X-ray structures, we demonstrate that carbonate combined with the very reactive decaniobate [Nb₁₀ O₂₈ ]^6- reassembles into a new decaniobate, [Nb₁₀ O₂₅ (CO₃ )₆ ]^12- , featuring three carbonate-ligated Nb-polyhedra. These Nb-sites can be replaced by heterometals (lanthanides), and the tridentate carbonate can serve as an anchor point to build niobate-frameworks. Small-angle X-ray scattering and two additional X-ray structures reveal that the reaction pathway proceeds through a Nb₂₄ -PONb intermediate, and the obtained PONb (with or without carbonate) is counterion, temperature, and solvent-dependent (water or mixed water-methanol). This provides an uncommon level of control for PONb chemistry.

An Unprecedented Bird Nest Molybdenum(V) Cobalto-Phosphate Nanosized Wheel Constructed from the [H55 (Mo24 O48 )(Co4 O)2 Co16 (PO4 )42 (py)6 (EtOH)2 (H2 O)11 ]3- Anion

An unprecedented bird-nest high-nuclear molybdenum(V) cobalto-phosphate nanosized wheel modified by imidazole (im) and pyridine (py), {[H₅₅ (Mo₂₄ O₄₈ )(Co₄ O)₂ Co₁₆ (PO₄ )₄₂ (py)₆ (EtOH)₂ (H₂ O)₁₁ ]- @[(Him)₂ (Hpy)]}(N-Et-py)(H₂ PO₄ )(py)₇ (EtOH)⋅12 H₂ O (1), has been successfully synthesized by self-assembly. The anionic huge wheel consists of two rare {Co₄ O} squares, four {Co₄ } tetramers, four {Mo₄ } tetramers and four {Mo₂ } dimers, linked by bridging oxygen atoms and [PO₄ ] groups and encloses two imidazolium cations and a protonated pyridium for charge balance. Surprisingly, 1 represents the first twisted wheel-shaped cluster with a record high-nuclear molybdenum(V) cobalto-phosphate. The delocalized electron effects of the cluster are enhanced with the help of coordinated py ligands, which endows 1 with an excellent third-order nonlinear optical (NLO) response. Additionally, 1 also shows a better photocatalytic water oxidation activity than Co(NO₃ )₂ with the O₂ production of 205 μmol during 6 h in the absence of the [Ru(bpy)₃ ]²⁺ photosensitizer.

A large molecular cluster with high proton release capacity

We present a single molecular polyoxometalate cluster (K₄₁[(P₂W₁₂Nb₆O₆₂)₆{Mn₃(OH)₃(H₂O)₆}₄{Mn₃Na(H₂O)₁₆}]·26H₂O) with controllable release of a large number of protons (∼40 per molecule) in its aqueous solution upon addition of a base. The deprotonation/protonation process is reversible with the clusters remaining intact. This molecule can also absorb up to 11 protons per cluster when an acid, HCl, was added to its original aqueous solution. To the best of our knowledge, such large proton absorption/release capacity along with excellent stability is unprecedented.

Inorganic-Organic Hybrid Polyoxoniobates: Polyoxoniobate Metal Complex Cage and Cage Framework

The combination of polyoxoniobates (PONbs) with 3d metal ions, azoles, and organoamines is a general synthetic procedure for making unprecedented PONb metal complex cage materials, including discrete molecular cages and extended cage frameworks. By this method, the first two PONb metal complex cages K₄ @{[Cu₂₉ (OH)₇ (H₂ O)₂ (en)₈ (trz)₂₁ ][Nb₂₄ O₆₇ (OH)₂ (H₂ O)₃ ]₄ } and [Cu(en)₂ ]@{[Cu₂ (en)₂ (trz)₂ ]₆ (Nb₆₈ O₁₈₈ )} have been made. The former exhibits a huge tetrahedral cage with more than 120 metal centers, which is the largest inorganic-organic hybrid PONb known to date. The later shows a large cubic cage, which can act as building blocks for cage-based extended assembly to form a 3D cage framework {[Cu(en)₂ ]@{[Cu₂ (trz)₂ (en)₂ ]₆ [H₁₀ Nb₆₈ O₁₈₈ ]}}. These materials exhibit visible-light-driven photocatalytic H₂ evolution activity and high vapor adsorption capacity. The results hold promise for developing both novel cage materials and largely unexplored inorganic-organic hybrid PONb chemistry.

Soluble Hetero-Polyoxovanadates and Their Solution Chemistry Analyzed by Electrospray Ionization Mass Spectrometry

Polyoxometalates (POMs) are an intriguing class of compounds due to their tremendous structural variety and the wide spectrum of resulting properties, which make them interesting for applications in fields such as catalysis, material science or nanotechnology. Their ability to form large supramolecular architectures by self-assembly offers an entry to complex, functional systems. After an introduction into the structure and synthesis of POMs of the early transition metals, recently discovered water-soluble antimonato polyoxovanadates (Sb-POVs) and the investigation of their chemical reactivity are discussed. Electrospray ionization mass spectrometry (ESI-MS) is presented as an analytical technique suitable to investigate the structure of complex POM assemblies in solution and to probe the underlying reactivity and formation mechanisms. This Minireview highlights the first studies on the soluble Sb-POVs and how the knowledge of their reactivity obtained by ESI-MS has fostered the syntheses of numerous novel Sb-POV compounds.

All-Inorganic Ionic Porous Material Based on Giant Spherical Polyoxometalates Containing Core-Shell K6 @K36 -Water Cage

This work demonstrates that the use of high-negative and high-symmetry lacunary polyoxometalates (POMs) for the clustering of alkali metal ions is a feasible strategy not only for the formation of rare high-nuclearity alkali-metal clusters but also for the construction of new-type all-inorganic ionic porous materials. By the strategy, an unprecedented high-nuclearity K-H₂ O cluster {K₄₂ (H₂ O)₆₀ } with core-shell K₆ @K₃₆ configuration is stabilized by 8 C_3v -symmetry trivacant POMs [GeW₉ O₃₄ ]^10- , forming a novel giant ionic alkali-metal-POM composite cluster {K₄₂ Ge₈ W₇₂ O₂₇₂ (H₂ O)₆₀ } with more than 100 metal centers. The incorporated 42-nuclearity K-H₂ O cluster {K₄₂ (H₂ O)₆₀ } exhibits the highest-nuclearity alkali-metal-water cluster known to date in POM chemistry. Further, the giant {K₄₂ Ge₈ W₇₂ O₂₇₂ (H₂ O)₆₀ } clusters can be linked by another kind of alkali metal ions Na⁺ to generate a fascinating three-dimensional all-inorganic ionic porous framework with high chemical stability, proton conductivity, and water vapor adsorption.