Probing Dynamic Library of Metal-Oxo Building Blocks with γ-Cyclodextrin

Reactions of [Ru(NO)Cl5]2- with pseudotrilacunary {XW9O33}9- (X = AsIII, SbIII) anions

Reactions of [Ru(NO)Cl₅]^2- with pseudotrivacant B-α-[XW₉O₃₃]^9- (X = As^III, Sb^III) at 160 °C result in the rearrangement of polyoxometalate backbones into {XM₁₈} structures. In the case of arsenic, oxidation of As^III to As^V takes place with the formation of a mixture of plenary and monosubstituted Dawson [As₂W₁₈O₆₂]^6- and [As₂W₁₇Ru(NO)O₆₁]^7- anions, of which the latter was isolated as Me₂NH₂⁺ (DMA-1a and DMA-1b) and Bu₄N⁺ (Bu₄N-1) salts and fully characterized. Both α₁ and α₂ isomers of [As₂W₁₇Ru(NO)O₆₁]^7- were present in the reaction mixture; pure [α₂-As₂W₁₇Ru(NO)O₆₁]^7- was isolated as the Bu₄N⁺ salt. In the case of antimony, [SbW₉O₃₃]^9- is converted into a mixture of [SbW₁₈O₆₀]^9- and [SbW₁₇{Ru(NO)}O₅₉]^10-. The formation of trisubstituted [SbW₁₅{Ru(NO)}₃O₅₇]^12- as a minor byproduct was detected by HPLC-ICP-AES. The monosubstituted [SbW₁₇{Ru(NO)}O₅₉]^10- anion was isolated as DMAH⁺ (DMA-2) and mixed inorganic cation (CsKNa-2) salts and characterized by XRD, HPLC-ICP-AES, EA and TGA techniques. X-ray analysis shows the presence of the {Ru(NO)}-group in the 6-membered ("equatorial") belt of the Sb-free hemisphere. The experimental findings were confirmed and interpreted by means of quantum chemical calculations.

Discovery of a New Family of Polyoxometalate-Based Hybrids with Improved Catalytic Performances for Selective Sulfoxidation: The Synergy between Classic Heptamolybdate Anions and Complex Cations

A series of functional cation-regulated isopolymolybdate-based organic-inorganic hybrid compounds, Na₂H₂[Mo₄O₁₂(C₈H₁₇O₅N)₂]·10H₂O (1), Na₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·10H₂O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris = 2,2-Bis(hydroxymethyl)-2,2',2″-nitrilotriethanol], and (NH₄)₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·6H₂O (M = Zn, 6; Cu, 7), were synthesized and characterized toward advanced molecular catalyst design. Compound 1 is a covalently bonded adduct, and its self-assembly process can be probed by electrospray ionization mass spectrometry (ESI-MS). Compounds 2-7 are polyoxometalate (POM)-based hybrids containing classic heptamolybdate anions and complex cations with Bis-tris ligands. All of these compounds showed remarkable catalytic effects for selective sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other transition-metal-containing compounds and the compound constructed by Na⁺ has higher catalytic activity than that constructed by NH₄⁺. The mechanism studies show that the improvements of the catalytic performance are caused by the synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV-vis spectra revealed that the POM anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate thioanisole with complex cations was characterized by NMR experiments and UV-vis spectra. Thus, a new synergistic mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.

Gas-Phase Fragmentation of Host-Guest Complexes of Cyclodextrins and Polyoxometalates

Gas-phase fragmentation pathways of host-guest complexes of cyclodextrins (CDs) and polyoxometalates (POMs) were examined using collision-induced dissociation (CID). The host-guest complexes studied here were composed of two different classes of POMs-Keggin (PW₁₂O₄₀^3-) and Lindqvist (M₆O₁₉^2-, M = Mo, W)-and three types of CDs (α-, β-, and γ-CD) differing in the diameter of the inner cavity. The CD-POM complexes were generated either by mixing methanol solutions of POM and CD or through a one-step acidic condensation of tetraoxometalates MO₄^2- (M = Mo, W) with CDs for complexes with Keggin and Lindqvist anions, respectively, and introduced into the gas phase using electrospray ionization (ESI). We observe distinct differences in fragmentation pathways of the complexes of Keggin and Lindqvist POMs under high- and low-energy CID conditions. Specifically, direct dissociation and proton transfer from CD to POM accompanied by the separation of fragments is observed in CID of Keggin CD-POM complexes. In contrast, dissociation of CD complexes with Lindqvist POMs is dominated by the simultaneous loss of multiple water molecules. This unusual fragmentation channel is attributed to dissociation of the POM cluster inside the CD cavity accompanied by covalent bond formation between the fragments and CD and elimination of multiple water molecules. The observed covalent coupling of metal oxide clusters opens up opportunities for derivatization of macrocyclic host molecules using collisional excitation of gaseous non-covalent complexes.

Pathways towards true catalysts: computational modelling and structural transformations of Zn-polyoxotungstates

Current catalysis undergoes a paradigm shift from molecular and heterogeneous realms towards new dynamic catalyst concepts. This calls for innovative strategies to understand the essential catalytic motifs and true catalysts emerging from oxidative transformation processes. Polyoxometalate (POM) clusters offer an inexhaustible reservoir for new noble metal-free catalysts and excellent model systems whose structure-activity relationships and mechanisms remain to be explored. Here, we first introduce a new {Zn_nNa_6-n(B-α-SbW₉O₃₃)₂} (n = 3-6) catalyst family with remarkable tuning options of the Zn-based core structure and high activity in H₂O₂-assisted catalytic alcohol oxidation as a representative reaction. Next, high level solution-based computational modelling of the intermediates and transition states was carried out for [Zn₆Cl₆(SbW₉O₃₃)₂]^12- as a representative well-defined case. The results indicate a radical-based oxidation process with the involvement of tungsten and adjacent zinc metal centers. The {Zn_nNa_6-n(B-α-SbW₉O₃₃)₂} series indeed efficiently catalyses alcohol oxidation via peroxotungstate intermediates, in agreement with strong spectroscopic support and other experimental evidence for the radical mechanism. Finally, the high performance of [Zn₆Cl₆(SbW₉O₃₃)₂]^12- was traced back to its transformation into a highly active and robust disordered Zn/W-POM catalyst. The atomic short-range structure of this resting pre-catalyst was elucidated by RMC modelling of the experimental W-L₃ and Zn-K edge EXAFS spectra and supported with further analytical methods. We demonstrate that computational identification of the reactive sites combined with the analytical tracking of their dynamic transformations provides essential input to expedite cluster-based molecular catalyst design.

Self-Assembly of Nanoscale Lanthanoid-Containing Selenotungstates: Synthesis, Structures, and Magnetic Studies

The reaction of mid-lanthanide (Ln) ions with the preformed {Se₆W₃₉} precursor under reasonably acidic aqueous conditions in the presence of organic amine cations results in an unprecedented nanoscale lanthanide-functionalized polyoxotungstate family, which are rare examples of mid-lanthanide-containing selenotungstates. (C₄H₁₀NO)₉Na₃[Dy₃Se_3.5W₃₀O_107.5(H₂O)₁₀]·22H₂O (1) and (NH₄)₃(C₂H₈N)Na₂[Dy₄Se₆W₃₈O₁₃₂(H₂O)₂₆(OH)₆]·18H₂O (2) reveal a trimeric Keggin assembly and a cyclic {Se₆W₃₈}-based chain, respectively, whereas (NH₄)₄Na₈[Gd₄Se₆W₄₈O₁₆₆(H₂O)₂₀(OH)₄]·21H₂O (3) and (NH₄)₉(C₂H₈N)₄Na₅[Ln₆Se₆W₅₈O₂₀₂(H₂O)₂₀(OH)₄]·58H₂O (4; Ln = Gd, Tb, or Dy) are a few examples of polyoxometalates consisting of both classical Keggin and Wells-Dawson building blocks, and (NH₄)₄(C₂H₈N)₅Na₁₃[Ln₄Se₈W₅₆O₁₉₆(H₂O)_x(OH)₁₀]·40H₂O (5; Ln = Gd, Tb, or Dy; x = 12 for Gd and Tb and 10 for Dy) features the largest "pure" Wells-Dawson selenotungstate {Se₈W₅₆} bearing a length of 3.73 nm. A library of Se-templated species involving the first reported Keggin {α-SeW₈} and Wells-Dawson {α-Se₂W₁₆} building blocks as well as some decisive assembly factors during the synthesis is responsible for these architectures. All of the compounds were structurally characterized in the solid and solution by single-crystal X-ray diffraction, IR, thermogravimetric-differential thermal analysis, and electrospray ionization mass spectrometry. Magnetic properties indicate that 1 and 4-Dy show probable single-molecule-magnet behavior with obvious frequency dependence, whereas 3 and 4-Gd present the antiferromagnetic interactions between the Gd^III centers.

Polyoxometalates as components of supramolecular assemblies

The non-covalent interaction of polyoxometalates (POMs) with inorganic- or organic-based moieties affords hybrid assemblies with specific physicochemical properties that are of high interest for both fundamental and applied studies, including the discovery of conceptually new compounds and unveiling the impact of their intra-supramolecular relationships on the fields of catalysis, molecular electronics, energy storage and medicine. This minireview summarises the recent advances in the synthetic strategies towards the formation of such non-covalent POM-loaded assemblies, shedding light on their key properties and the currently investigated applications. Four main emerging categories according to the nature of the conjugate are described: (i) POMs in metal-organic frameworks, (ii) POMs merged with cationic metal complexes, (iii) architectures generated with solely POM units and (iv) POMs assembled with organic molecular networks.

Polyoxometalate-Cyclodextrin Metal-Organic Frameworks: From Tunable Structure to Customized Storage Functionality

Self-assembly allows structures to organize themselves into regular patterns by using local forces to find the lowest-energy configuration. However, assembling organic and inorganic building blocks in an ordered framework remains challenging due to  difficulties in rationally interfacing two dissimilar materials. Herein, the ensemble of polyoxometalates (POMs) and cyclodextrins (CDs) as molecular building blocks (MBBs) has yielded two unprecedented POM-CD-MOFs, namely [PW₁₂O₄₀]^3- and α-CD MOF (POT-CD) as well as [P₁₀Pd_15.5O₅₀]^19- and γ-CD MOF (POP-CD), with distinct properties not shared by their isolated parent MBBs. Markedly, the POT-CD features a nontraditional enhanced Li storage behavior by virtue of a unique "amorphization and pulverization" process. This opens the door to a new generation of hybrid materials with tuned structures and customized functionalities.

Preparation, characterization and electrocatalysis performance of a trimeric ruthenium-substituted isopolytungstate

The ruthenium-containing isopolytungstate Rb₁₀K₃H₆[SeO₃(H₉Ru_5.5W_30.5O₁₁₄)]Cl₃·48H₂O was isolated and then served as a catalyst, showing electrochemical catalytic activity towards the oxidation reaction of nitrite.

From achiral to helical bilayer self-assemblies of a 1,3,5-triazine-2,4,6-triphenol-grafted polyanionic cluster: countercation and solvent modulation

An organic-component grafted polyanionic cluster performs assembly structures from regular head to tail bilayer to inverse helical packing upon solvent polarity and counterions.

Tuning the chaotropic effect as an assembly motif through one-electron transfer in a rhenium cluster

As small change as one electron transfer within the hydrophilic rhenium cluster [{Re₆Se₈}(CN)₆]^4−/3− induces dramatic alteration in supramolecular self-assembly properties with γ-cyclodextrin as a result of chaotropic effect driven process.

Cyclodextrin-Driven Formation of Double Six-Ring (D6R) Silicate Cage: NMR Spectroscopic Characterization from Solution to Crystals

Identification and isolation of secondary building units (SBUs) from synthesis media of zeolites still represent a challenging task for chemists. The cage structure anion Si12O3012− known as the double six-ring (D6R) was synthesized from α-cyclodextrin (α-CD) mediated alkaline silicate solutions and conditions of its stability and reactivity in aqueous solution were studied by using nuclear magnetic resonance (NMR) spectroscopy. A single crystal X-ray diffraction (XRD) analysis revealed a novel polymorph of the hybrid complex K12Si12O30·2α-CD·nD2O (n ≈ 30–40), which crystallizes in the orthorhombic C2221 space group symmetry with a = 14.841(4) Å, b = 25.855(6) Å, and c = 41.91(1) Å. The supramolecular adduct of the silicate anion sandwiched by two α-CDs forms a perfect symmetry matching the H-bonding donor-acceptor system between the organic macrocycle and the D6R unit. The driving force of such a hybrid assembly has found to be strongly dependent on the nature of the cation present as large alkali counter ions (K+, Rb+ and Cs+), which stabilize the D6R structure acting as templates. Lastly, we provided the first 29Si MAS NMR measurement of 3Q Si in an isolated D6R unit that allows the verification of the linear correlation between the chemical shift and <SiOSi> bond angle for 3Q Si species in DnR cages (n = 3, 4, 6).