Exploring the Molecular Growth of Two Gigantic Half‐Closed Polyoxometalate Clusters {Mo180} and {Mo130Ce6}

Understanding the process of the self‐assembly of gigantic polyoxometalates and their subsequent molecular growth, by the addition of capping moieties onto the oxo‐frameworks, is critical for the development of the designed assembly of complex high‐nuclearity cluster species, yet such processes remain far from being understood. Herein we describe the molecular growth from {Mo₁₅₀} and {Mo₁₂₀Ce₆} to afford two half‐closed gigantic molybdenum blue clusters {Mo₁₈₀} ( 1 ) and {Mo₁₃₀Ce₆} ( 2 ), respectively. Compound 1 features a hat‐shaped structure with the parent wheel‐shaped {Mo₁₅₀} being capped by a {Mo₃₀} unit on one side. Similarly, 2 exhibits an elliptical lanthanide‐doped wheel {Mo₁₂₀Ce₆} that is sealed by a {Mo₁₀} unit on one side. Moreover, the observation of the parent uncapped {Mo₁₅₀} and {Mo₁₂₀Ce₆} clusters as minor products during the synthesis of 1 and 2 strongly suggests that the molecular growth process can be initialized from {Mo₁₅₀} and {Mo₁₂₀Ce₆} in solution, respectively.     

Assembly of Tungsten‐Oxide‐Based Pentagonal Motifs in Solution Leads to Nanoscale {W48}, {W56}, and {W92} Polyoxometalate Clusters

We report an approach to synthesize molecular tungsten‐oxide‐based pentagonal building blocks, in a new {W₂₁O₇₂} unit, and show how this leads to a family of gigantic molecular architectures including [H₁₂W₄₈O₁₆₄]^28? {W₄₈}, [H₂₀W₅₆O₁₉₀]^24? {W₅₆}, and [H₁₂W₉₂O₃₁₁]^58? {W₉₂}. The {W₄₈} and {W₅₆} clusters are both dimeric species incorporating two {W₂₁} units and the {W₅₆} species is the first example of a molecular metal oxide cluster containing a chiral “double‐stranded” motif which is stable in solution as confirmed by mass spectrometry. The {W₉₂} anion having four {W₂₁} units is one of the largest transition metal substituted isopolyoxotungstates known.     

Self‐Templating and In Situ Assembly of a Cubic Cluster‐of‐Clusters Architecture Based on a {Mo24Fe12} Inorganic Macrocycle

Engineering self‐templating inorganic architectures is critical for the development of bottom‐up approaches to nanoscience, but systems with a hierarchy of templates are elusive. Herein we describe that the cluster‐anion‐templated (CAT) assembly of a {CAT}?{Mo₂₄Fe₁₂} macrocycle forms a giant ca. 220 nm³ unit cell containing 16 macrocycles clustered into eight face‐shared tetrahedral cluster‐of‐clusters assemblies. We show that {CAT}?{Mo₂₄Fe₁₂} with different CATs gives the compounds 1 – 4 for CAT=Anderson {FeMo₆} ( 1 ), Keggin {PMo₁₂} ( 2 ), Dawson {P₂W₁₈} ( 3 ), and {Mo₁₂O₃₆(HPO₃)₂} ( 4 ) polyoxometalates. “Template‐free” assembly can be achieved, whereby the macrocycle components can also form a template in situ allowing template to macrocycle to superstructure formation and the ability to exchange the templates. Furthermore, the transformation of template clusters within the inorganic macrocycle {Mo₂₄Fe₁₂} allows the self‐generation of an uncapped {Mo₁₂O₃₆(HPO₃)₂} in compound 4 .     

Guest‐Directed Supramolecular Architectures of {W36} Polyoxometalate Crowns

The {W₃₆} isopolyoxotungstate cluster provides a stable inorganic molecular platform for the binding of inorganic and organic guest molecules. This is achieved by a binding pocket formed by six terminal oxo ligands located in the central cavity of the all‐inorganic cation binding host. Previously it was shown that the cluster can specifically bind primary amines and importantly, functionalized diamines through a combination of electrostatic and hydrogen bonding interactions. Here we transform this assembly strategy to utilize the binding of long‐chain alkyldiammonium guest cations to physically define the supramolecular structure of the clusters with respect to each other and demonstrate the structure direction as a function of alkyl chain length. The systematic variation of the chain length gives access to five supramolecular assemblies which were all fully characterized using single crystal XRD, TGA, ¹H NMR, and elemental analysis. In compound 1 , diprotonated 1,8‐diaminooctane molecules link the {W₃₆} clusters into infinite 1D zigzag chains, whereas compounds 2 and 3 feature trimeric {W₃₆} assemblies directly connected through protonated 1,9‐diaminononane ( 2 ) or 1,10‐diaminodecane ( 3 ) linkers . Compound 4 contains dumb‐bell shaped dimeric units as a result of direct center‐to‐center linkages between the {W₃₆} clusters formed by protonated 1,12‐diaminododecane. In compound 5 , triply protonated bis(hexamethylene)triamine was employed to obtain linear 1D chains of directly connected {W₃₆} cluster units.     

Isotope and Hydrogen-Bond Effects on the Self-Assembly of Macroions in Dilute Solution

We report on the disparity in the assembly behavior of four types of nano-sized macroions induced by isotopic substitution of protium (H) to deuterium (D) in solvents. Macroions with modest charge density can self-assemble into single-layer, hollow, spherical “blackberry”-type structures, with larger assembly sizes representing stronger attractions among the macroions. Kinetically, all assembly processes become slower in D₂O than in H₂O. Thermodynamically, the polyoxometalate {SrPd₁₂}, the uranium cage {U₆₀} with alkali metal counterions, and the metal–organic cationic cage {Pd₁₂L₂₄} demonstrate similar assembly sizes in both H₂O and D₂O, whereas the metal oxide cluster {Mo₇₂Fe₃₀} as a weak acid shows an unusually large assembly size in H₂O—suggesting a stronger contribution from the hydrogen bonding in the last case.     

Time‐Resolved Assembly of Cluster‐in‐Cluster {Ag12}‐in‐{W76} Polyoxometalates under Supramolecular Control

We report the time‐resolved supramolecular assembly of a series of nanoscale polyoxometalate clusters (from the same one‐pot reaction) of the form: [H_(10+m)Ag₁₈Cl(Te₃W₃₈O₁₃₄)₂]_n, where n=1 and m=0 for compound 1 (after 4 days), n=2 and m=3 for compound 2 (after 10 days), and n=∞ and m=5 for compound 3 (after 14 days). The reaction is based upon the self‐organization of two {Te₃W₃₈} units around a single chloride template and the formation of a {Ag₁₂} cluster, giving a {Ag₁₂}‐in‐{W₇₆} cluster‐in‐cluster in compound 1 , which further aggregates to cluster compounds 2 and 3 by supramolecular Ag‐POM interactions. The proposed mechanism for the formation of the clusters has been studied by ESI‐MS. Further, control experiments demonstrate the crucial role that TeO₃^2?, Cl^?, and Ag⁺ play in the self‐assembly of compounds 1 – 3 .     

Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U38

The ex‐situ qualitative study of the kinetic formation of the poly‐oxo cluster U₃₈, has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X‐ray absorption near edge structure (XANES), extended X‐ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO₂ from t=3 h, after the formation of unknown solid phases at an early stage. The crystallization of U₃₈ occurred from t=4 h at the expense of UO₂, and is completed after t=8 h. Starting from pure uranium(IV) species in solution (t=0–1 h), oxidation reactions are observed with a U^IV/U^VI ratio of 70:30 for t=1–3 h. Then, the ratio is inversed with a U^IV/U^VI ratio of 25/75, when the precipitation of UO₂ occurs. Thorough SEM observations of the U₃₈ crystallites showed that the UO₂ aggregates are embedded within. This may indicate that UO₂ acts as reservoir of uranium(IV), for the formation of U₃₈, stabilized by benzoate and THF ligands. During the early stages of the U₃₈ crystallization, a transient crystallized phase appeared at t=4 h. Its crystal structure revealed a new dodecanuclear moiety (U₁₂), based on the inner hexanuclear core of {U₆O₈} type, decorated by three additional pairs of dinuclear U₂ units. The U₁₂ motif is stabilized by benzoate, oxalates, and glycolate ligands.     

Synthesis,Structure and Light-Driven H2 Production of a {Ti21} Titanium-Oxo Cluster Featuring Pentagonal {Ti(Ti5)} Motifs

Pentagonal {Ti(Ti₅)} motifs play a crucial role during the assembly of novel titanium-oxide clusters (TOCs), whose development is still in its infancy. In this work, an unprecedented TOC [(CH₃)₂NH₂]₂[Ti₂₁O₂₉(OiPr)₁₂(DMF)₆(SO₄)₈] ({Ti₂₁}) based on {Ti(Ti₅)} motifs had been obtained under hydrothermal conditions. It possesses ‘fused’ dimeric {Ti₁₁} as well as trimeric {Ti₁₂} subunits featuring unexpected bonding modes based on pentagonal {Ti(Ti₅)} motifs. Furthermore, {Ti₂₁} exhibits good water and solvent tolerance. As an atomically precise TOCs-based semiconductor photocatalyst, {Ti₂₁} exhibits appreciable photocatalytic hydrogen evolution activity with an optimal hydrogen generation rate of 191.94 μmol/g/h under UV/Vis light. This work provides significant advances and favorable models for the further synthesis of TOCs with unique structure and functionality.     

Anion‐Templated Assembly and Magnetocaloric Properties of a Nanoscale {Gd38} Cage versus a {Gd48} Barrel

The comprehensive study reported herein provides compelling evidence that anion templates are the main driving force in the formation of two novel nanoscale lanthanide hydroxide clusters, {Gd₃₈(ClO₄)₆} ( 1 ) and {Gd₄₈Cl₂(NO₃)} ( 2 ), characterized by single‐crystal X‐ray crystallography, infrared spectroscopy, and magnetic measurements. {Gd₃₈(ClO₄)₆}, encapsulating six ClO₄^? ions, features a cage core composed of twelve vertex‐sharing {Gd₄} tetrahedrons and one Gd???Gd pillar. When Cl^? and NO₃^? were incorporated in the reaction instead of ClO₄^?, {Gd₄₈Cl₂(NO₃)} is obtained with a barrel shape constituted by twelve vertex‐sharing {Gd₄} tetrahedrons and six {Gd₅} pyramids. What is more, the cage‐like {Gd₃₈} can be dynamically converted into the barrel‐shaped {Gd₄₈} upon Cl^? and NO₃^? stimulus. To our knowledge, it is the first time that the linear M‐O‐M′ fashion and the unique μ₈‐ClO₄^? mode have been crystallized in pure lanthanide complex, and complex 2 represents the largest gadolinium cluster. Both of the complexes display large magnetocaloric effect in units of J kg^?1 K^?1 and mJ cm^?3 K^?1 on account of the weak antiferromagnetic exchange, the high N_Gd/M_W ratio (magnetic density), and the relatively compact crystal lattice (mass density).     

A (3,6)-connected metal-organic framework consisting of chair-like {Fe6} clusters and BTC linkers

A 2-D framework composed of a chair-like {Fe₆} cluster and BTC linkers, [Fe₆(BTC)₂(HCOO)₆(DMF)₆] (1, BTC?=?1,3,5-benzenetricarboxylate), has been synthesized under solvothermal conditions and characterized by elemental analysis, infrared spectroscopy, thermal gravimetric analysis, and single-crystal X-ray diffraction analysis. The {Fe₆} cluster contains six iron ions and six formate ligands, with each formate coordinating with three iron ions to construct the hexa-nuclear iron wheel. The {Fe₆} wheels are further connected via BTC ligands resulting in the first example of a 2-D framework based on {Fe₆} clusters and BTC linkers. A magnetic study indicates that intramolecular antiferromagnetic interactions exist in the hexa-nuclear iron cluster.     

Water-soluble Self-assembled {Pd84}Ac Polyoxopalladate Nano-wheel as a Supramolecular Host

Polyoxopalladates (POPs) are a class of self-assembling palladium-oxide clusters that span a variety of sizes, shapes and compositions. The largest of this family, {Pd₈₄}^Ac, is constructed from 14 building units of {Pd₆} and lined on the inner and outer torus by 28 acetate ligands. Due to its high water solubility, large hydrophobic cavity and distinct ¹H NMR fingerprint {Pd₈₄}^Ac is an ideal molecule for exploring supramolecular behaviour with small organic molecules in aqueous media. Molecular visualisation studies highlighted potential binding sites between {Pd₈₄}^Ac and these species. Nuclear Magnetic Resonance (NMR) techniques, including ¹H NMR, ¹H Diffusion Ordered Spectroscopy (DOSY) and Nuclear Overhauser Spectroscopy (NOESY), were employed to study the supramolecular chemistry of this system. Here, we provide conclusive evidence that {Pd₈₄}^Ac forms a 1 : 7 host-guest complex with benzyl viologen (BV²⁺) in aqueous solution.     

Redox behavior of $${\text{VO}}^{2 + } / {\text{VO}}_{2}^{ + }$$ as a simulant of $${\text{NpO}}_{2}^{ + } / {\text{NpO}}_{2}^{2 + }$$NpO2+/NpO22+ in boiling nitric acid solution

To evaluate the redox behavior of \({\text{VO}}^{2 + } / {\text{VO}}_{2}^{ + }\) as a simulant of \({\text{NpO}}_{2}^{ + } / {\text{NpO}}_{2}^{2 + }\) in boiling nitric acid solution, i.e., typical operating conditions for nuclear fuel reprocessing plants, oxidation rate measurements for VO²⁺ in boiling and non-boiling nitric acid solutions, thermodynamic calculations, and kinetic calculations were performed. The results indicated that the apparent oxidation rate of VO²⁺ to \({\text{VO}}_{2}^{ + }\) is accelerated by a decrease in \({\text{NO}}_{2}^{ - }\) and HNO₂ concentrations owing to the boiling phenomena of nitric acid solution.     

Surfactant-Induced Trans-Interface Transportation and Complex Formation of Giant Polyoxomolybdate-Based Clusters

The interaction and complex formation between cationic surfactants dimethyldioctadecylammonium Bromide (DODA-Br) and a polyoxomolybdate (POM)-based giant cluster {Mo₇₂Fe₃₀}, in its both single cluster (in aqueous solution, these clusters exist as anions) format and supramolecular format in aqueous solution, are studies by using laser light scattering (LLS) techniques. DODA/{Mo₇₂Fe₃₀} complexes containing basically single {Mo₇₂Fe₃₀} clusters are observed when the {Mo₇₂Fe₃₀} aqueous solution is freshly prepared and contains mainly unimer or oligmer {Mo₇₂Fe₃₀} anions. The {Mo₇₂Fe₃₀} clusters tend to form supramolecular vesicle structures slowly in solution. At high surfactant concentrations, the DODA cationic surfactants can break the vesicle structure and form single {Mo₇₂Fe₃₀}/DODA complexes. At low surfactant concentrations, complexes containing the whole vesicles coated by a layer of DODA is formed and transferred into the organic phase. For the surfactant concentrations in between, the vesicles are partially destroyed, leading to the formation of complexes with large size distribution. Studying the behaviors of the interaction between DODA and {Mo₇₂Fe₃₀} anionic structures will help to further explore the complicated mechanism of the POM vesicle formation, which was recently discovered but still not fully understood. Such unique complex structures may also have potential applications as nanoreactors or nanocontainers.     

Solid-state Structures and Solution Behavior of Alkali Salts of the [\hbox{Nb}_{6}\hbox{O}_{19}]^{8-} Lindqvist Ion

The hexaniobate Lindqvist ion has long been known as the dominant specie in alkaline niobium oxide solutions. Recent advances in heteropolyniobate chemistry continue to be greatly aided by use of alkali salts as soluble precursors; in particular, potassium, sodium and lithium hexaniobate salts. We report here the solid-state characterization and solution behavior of Li, K, Rb and Cs Lindqvist salts. Synthesis and single-crystal X-ray diffraction data is reported for nine new hexaniobate salts. These structures differ in the number of charge-balancing alkali cations, protonation of the clusters, relative arrangement of the clusters and alkali metal cations, amount of lattice water and its mode of interaction with other lattice species. Trends of alkali-cluster bonding are observed as a function of alkali radius. Protonation of the clusters in the solid-state is influenced by the method of crystallization of the salt. Lability of the cluster oxygens is observed by solution ¹⁷O NMR experiments. Rates of isotopic enrichment of the bridging oxygen, terminal oxygen and bridging hydroxyl cluster sites are compared for aqueous solutions of Li, K, Rb and Cs hexaniobate salts. Parameters influencing the oxo-ligand exchange rates of the salts are discussed relative to their use as heteropolyniobate precursors.This paper is dedicated to Professor Michael T. Pope on the event of his retirement to acknowledge his fruitful career in polyoxometalate chemistry.     

Tuning the Surface Hydrophobicity of Keplerate {Mo<Subscript>72</Subscript>Fe<Subscript>30</Subscript>} Porous Molecular Capsules by Surface Ligand-Replacement Process

A spontaneous ligand-replacement process at the external surface of Keplerate polyoxometalate molecular nanocapsules {Mo₇₂Fe₃₀} occurs when dissolving {Mo₇₂Fe₃₀} in pure methanol, with the water ligands coordinated to the Fe(III) centers of {Mo₇₂Fe₃₀} gradually being replaced by CH₃OH ligands, resulting in the formation of partially hydrophobic nanocapsules. The new nanocapsules have negligible deprotonation capability and different solution behaviors. The methanol ligands can be easily exchanged back to water ligands when a small amount of water is added into the system. The exchange of water ligands dramatically changed the self-assembly behaviors of the {Mo₇₂Fe₃₀} clusters in solution.     

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.     

Bandgap Engineering of Titanium–Oxo Clusters: Labile Surface Sites Used for Ligand Substitution and Metal Incorporation

Through the labile coordination sites of a robust phosphonate‐stabilized titanium–oxo cluster, 14 O‐donor ligands have been successfully introduced without changing the cluster core. The increasing electron‐withdrawing effect of the organic species allows the gradual reduction of the bandgaps of the {Ti₆} complexes. Transition‐metal ions are then incorporated by the use of bifunctional O/N‐donor ligands, organizing these {Ti₆} clusters into polymeric structures. The coordination environments of the applied metal ions show significant influence on their visible‐light adsorption. Both the above structural functionalizations also tune the photocatalytic H₂ production activities of these clusters. This work provides a systematic bandgap engineering study of titanium–oxo clusters, which is important not only for their future photocatalytic applications, also for the better understanding of the structure–property relationships.     

Overcoming the Crystallization Bottleneck: A Family of Gigantic Inorganic {Pdx}L (x=84, 72) Palladium Macrocycles Discovered using Solution Techniques

The {Pd₈₄}^Ac wheel, initially discovered serendipitously, is the only reported giant palladium macrocycle—a unique structure that spontaneously assembles from small building blocks. Analogues of this structure are elusive. A new modular route to {Pd₈₄}^Ac is described, allowing incorporation of other ligands, and a new screening approach to cluster discovery. Structural assignments were made of new species from solution experiments, overcoming the need for crystallographic analysis. As a result, two new palladium macrocycles were discovered: a structural analogue of the existing {Pd₈₄}^Ac wheel with glycolate ligands, {Pd₈₄}^Gly , and the next in a magic number series for this cluster family—a new {Pd₇₂}^Prop wheel decorated with propionate ligands. These findings confirm predictions of a magic number rule for the family of {Pd_x} macrocycles. Furthermore, structures with variable fractions of functional ligands were obtained. Together these discoveries establish palladium clusters as a new class of tunable nanostructures. In facilitating the discovery of species that would not have been discovered by orthodox crystallization approaches, this work also demonstrates the value of solution‐based screening and characterization in cluster chemistry, as a means to decouple cluster formation, discovery, and isolation.     

Applications of NMR spectroscopy of chiral association complexes 5. Stereodynamics and diastereotopism in chiral 1,3-dienes \documentclass{article}\pagestyle{empty}\begin{document}${\rm HOCMe}_{\rm 2} \rlap{--} ({\rm CCl =\!= CCl\rlap{--})}_{\rm 2} {\rm X}$\end{document}

The barriers to partial rotation around the central single bond in chiral dienes \documentclass{article}\pagestyle{empty}\begin{document}${\rm HOCMe}_{\rm 2} \rlap{--} ({\rm CCl =\!= CCl\rlap{--})}_{\rm 2} {\rm X}$\end{document} have been determined by coalescence of either ¹H NMR signals (X = CH₂OCH₃) or ¹³C NMR signals (X = H). In the presence of the optically active shift reagent (+) ? Eu(hfbc)₃ all ¹H signals were split at temperatures where the interconversion of enantiomers is slow. The temperature dependence of these spectra also yielded free activation enthalpies for the enantiomerizations which were in agreement with the ones obtained without Eu(hfbc)₃. The assignment of the four methyl resonances appearing in the presence of (+) ? Eu(hfbc)₃ at low temperature was possible by gradually increasing the rate of enantiomerization or gradually replacing the optically active auxiliary compound by the racemic one.     

Record High‐Nuclearity Polyoxoniobates: Discrete Nanoclusters {Nb114}, {Nb81}, and {Nb52}, and Extended Frameworks Based on {Cu3Nb78} and {Cu4Nb78}

A series containing the highest nuclearity polyoxoniobate (PONb) nanoclusters, ranging from dimers to tetramers, has been obtained. They include one 114‐nuclear {Li₈⊂Nb₁₁₄O₃₁₆}, one 81‐nuclear {Li₃K⊂Nb₈₁O₂₂₅}, and one 52‐nuclear {H₄Nb₅₂O₁₅₀}. The Nb nuclearity of these PONbs is remarkably larger than those of all known high‐nuclearity PONbs (≤32). Furthermore, the introduction of 3d Cu²⁺ ions can lead to the generation of extended inorganic–organic hybrid frameworks built from novel, high‐nuclearity, nanoscale heterometallic PONb building blocks {H₃Cu₃Nb₇₈O₂₂₂} or {H₃Cu₄(en)Nb₇₈O₂₂₂}. These building blocks also contain the largest number of Nb centers of any heterometallic PONbs reported to date. The synthesis of new‐type PONbs has long been a challenging subject in PONb chemistry.