Electronic properties of polyoxometalates: a DFT study of alpha/beta-[XM(12)O(40)](n-) relative stability (M=W, Mo and X a main group element)

Factors governing the protonation of Keggin-type polyoxometalates: influence of the core structure in clusters

Atomic substitution is a promising approach for controlling structures and properties for developing clusters with desired responses. Although many possible coordination candidates could be deduced for substitution, not all can be prepared. Therefore, predicting the correlation between structures and physical properties is important prior to synthesis. In this study, regarding Keggin-type polyoxometalates (POMs) as a model cluster, the dominant factors affecting the protonation were investigated by atomic substitutions and geometry changes. The valence of Keggin-type POMs and the constituent elements of the cluster shell structure affect the charge and potential distribution, which change the protonation sites. Furthermore, the valence of Keggin-type POMs and the bond length between the core and shell structure determine the protonation energy. These factors also affect the HOMO-LUMO gap, which governs photochemical and redox reactions. These governing factors derived from actual parameters of the α-isomer of Keggin-type POMs enabled us to deduce the protonation energy of the β-isomer, which is more difficult to prepare and isolate than the α-isomer.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

Modulation of the Band Gap and Redox Properties by Mixed Addenda in Sandwich Polyoxometalates

Due to the wide range of applications of band-gap engineering in optoelectronics and photocatalysis, the rational design of polyoxometalate (POM) frameworks is highly desired. Here, we have successfully synthesized a series of mixed addenda (Mo and W) sandwich POMs by systematically varying pH, concentrations of salts, and counterions in Weakley-type sandwich POMs by incorporating Mo into the framework of tetrasubstituted sandwich POMs. Crystallographic analysis reveals the centrosymmetric structure; with variation in the Mo to W ratio, Mo preferentially binds to μ2 oxygen connected to transition metals in the sandwich position. UV-visible spectroscopy, electrochemical, and theoretical modeling rationalize the band-gap modulations. Theoretical studies and cyclic voltammograms indicate that during the reduction, the incoming electrons preferentially go to substituted transition metals followed by Mo. Flat band potential calculated from the Mott-Schottky enables tuning of the electronic properties of composites based on these sandwich POMs. Moreover, the dioxygen binding and activation studies of these polyoxometalates have been highlighted.

Reducing Systematic Uncertainty in Computed Redox Potentials for Aqueous Transition-Metal-Substituted Polyoxotungstates

Polyoxometalates have attracted significant interest owing to their structural diversity, redox stability, and functionality at the nanoscale. In this work, density functional theory calculations have been employed to systematically study the accuracy of various exchange-correlation functionals in reproducing experimental redox potentials, U0Red in [PW11M(H2O)O39]q- M = Mn(III/II), Fe(III/II), Co(III/II), and Ru(III/II). U0Red calculations for [PW11M(H2O)O39]q- were calculated using a conductor-like screening model to neutralize the charge in the cluster. We explicitly located K+ counterions which induced positive shifting of potentials by > 500 mV. This approximation improved the reproduction of redox potentials for Kx[XW11M(H2O)O39]q-x M = Mn(III/II)/Co(III/II). However, uncertainties in U0Red for Kx[PW11M(H2O)O39]q-x M = Fe(III/II)/Ru(III/II) were observed because of the over-stabilization of the ion-pairs. Hybrid functionals exceeding 25% Hartree-Fock exchange are not recommended because of large uncertainties in ΔU0Red attributed to exaggerated proximity of the ion-pairs. Our results emphasize that understanding the nature of the electrode and electrolyte environment is essential to obtain a reasonable agreement between theoretical and experimental results.

Influence of surface and intermolecular interactions on the properties of supported polyoxometalates

Polyoxometalates (POMs) with localized radical or open-shell metal sites have the potential to be used as transformative electronic spin based molecular qubits (MQs) for quantum computing (QC). For practical applications, MQs have to be immobilized in electronically or optically addressable arrays which introduces interactions with supports as well as neighboring POMs. Herein, we synthesized Keggin POMs with both tungsten (W) and vanadium (V) addenda atoms. Ion soft landing, a highly-controlled surface modification technique, was used to deliver mass-selected V-doped POMs to different self-assembled monolayer surfaces on gold (SAMs) without the solvent, counterions, and contaminants that normally accompany deposition from solution. Alkylthiol, perfluorinated, and carboxylic-acid terminated monolayers were employed as representative model supports on which different POM-surface and POM-POM interactions were characterized. We obtained insights into the vibrational properties of supported V-doped POMs and how they are perturbed by interactions with specific surface functional groups using infrared reflection absorption and scattering-type scanning near-field optical microscopy, as well as tip enhanced Raman spectroscopy. Different functional groups on SAMs and nanoscale heterogeneity are both shown to modulate the observed spectroscopic signatures. Spectral shifts are also found to be dependent on POM-POM interactions. The electronic structure of the V-doped POMs was determined in the gas phase using negative ion photoelectron spectroscopy and on surfaces with scanning Kelvin probe microscopy. The chemical functionality and charge transfer properties of the SAMs are demonstrated to exert an influence on the charge state and electronic configuration of supported V-doped POMs. The geometric and electronic structure of the POMs were also calculated using density functional theory. Our joint experimental and theoretical findings provide insight into how V substitution as well as POM-surface and POM-POM interactions influence the vibrational properties of POMs.

Supramolecular multi-electron redox photosensitisers comprising a ring-shaped Re(i) tetranuclear complex and a polyoxometalate

Redox photosensitisers (PSs) play essential roles in various photocatalytic reactions. Herein, we synthesised new redox PSs of 1 : 1 supramolecules that comprise a ring-shaped Re(i) tetranuclear complex with 4+ charges and a Keggin-type heteropolyoxometalate with 4- charges. These PSs photochemically accumulate multi-electrons in one molecule (three or four electrons) in the presence of an electron donor and can supply electrons with different reduction potentials. PSs were successfully applied in the photocatalytic reduction of CO₂ using catalysts (Ru(ii) and Re(i) complexes) and triethanolamine as a reductant. In photocatalytic reactions, these supramolecular PSs supply a different number of electrons to the catalyst depending on the redox potential of the intermediate, which is made from the one-electron-reduced species of the catalyst and CO₂. Based on these data, information on the reduction potentials of the intermediates was obtained.

Silver-Templated γ-Keggin Alkyltin-Oxo Cluster: Electronic Structure and Optical Limiting Effect

As one of the most representative polyoxometalate (POM) structures, Keggin clusters have attracted considerable attention. Nevertheless, the noble-metal-templated Keggin structure has not been reported to date. In this work, for the first time, a Ag atom was successfully incorporated to template the formation of a γ-Keggin alkytin-oxo cluster. Moreover, the central Ag atom has brought a significant heavy atom effect, showing the important influence on the electronic structure and optical properties. Theoretical calculations demonstrate that the Ag atom affects the frontier molecular orbitals and excited states of the AgSn₁₂ cluster, and also the process of electron transfer. The solid structure of the AgSn₁₂ cluster exhibits a significant third-order nonlinear optical (NLO) response, and an excellent optical limiting effect has been experimentally verified. The success of this work opens the way for the construction and optical properties modulation of noble metal templated Keggin structures.

New phosphotetradecavanadate hybrids: crystal structure, DFT analysis, stability and binding interactions with bio-macromolecules

Two novel bicapped Keggin polyoxidovanadates with organic cations, (C₆H₈N)₅[H₄PV₁₄O₄₂]·5H₂O (1) and (C₆H₁₄N₄)₂(NH₄)[H₄PV₁₄O₄₂]·11H₂O (2), (PV₁₄O₄₂^6- = PV14, C₆H₇N = 3-picoline and C₆H₁₂N₄ = methenamine) were synthesized. These compounds were isolated and characterized in the solid state and in solution by elemental analysis, powder X-ray diffraction, FTIR, UV-vis, ⁵¹V, ³¹P, ¹³C and ¹H NMR, and fluorescence spectroscopy. Further confirmation of the PV14 structures was obtained by single-crystal X-ray diffraction studies of 1 and 2. The Hirshfeld surface analysis was performed to confirm that within the intermolecular interactions occurring in the two crystals, the O⋯H/H⋯O, O⋯O and H⋯H interactions dominate. The protonation and one-electron reduction of the PV14 moiety were also analysed by means of DFT calculations; besides confirming the protonation sites and correctly predicting the pK_a values, the DFT results also indicate that molecular reduction is energetically more favourable in protonated PV14 anions. Upon the addition of PV14 anions to bovine serum albumin (BSA) up to a ratio of 1 : 1, the fluorescence decreased by 45% for both 1 and 2, indicating that the interaction of vanadium-containing species with this protein takes place; log(K_SV) values of ca. 5.5 were obtained in both systems. Upon the addition of 1 or 2 to solutions of calf-thymus DNA (ctDNA), changes were observed in the UV-vis absorption and circular dichroism spectra. The significance of the changes observed is discussed considering the several V-containing species that form in the solution.

Layer-by-layer assembly of multilayered double hydroxides/polyoxometalate-coated magnetic nanoparticles for highly efficient phosphopeptide enrichment

A layer-by-layer (LbL) assembly strategy was developed to prepare multilayered double hydroxide/polyoxometalate shell-coated magnetic nanoparticles. The introduction of functional shells not only offered abundant affinity sites of metal oxide and metal ions but also increased the surface area for the contact with targets. By combining the enrichment strategies of immobilized metal ion affinity chromatography and metal oxide affinity chromatography, the nanomaterial can capture phosphopeptides via a synergistic effect. The method presented a low detection limit of 0.1 fmol in combination with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) analysis. The nanomaterial showed satisfactory selectivity (1:1:5000 M ratio of α-/β-casein/bovine serum albumin), good recovery (92.07%), high adsorption capacity (117.6 mg g^-1), and ten times reusability for capturing phosphopeptides.

Recent advancements in understanding the self-assembly of macroions in solution via molecular modeling

Macroions fill the gap between simple ions and colloids in size but display a completely different self-assembly behavior in solution.

Aqueous Stability of Zirconium Clusters, Including the Zr(IV) Hexanuclear Hydrolysis Complex [Zr6O4(OH)4(H2O)24]12+, from Density Functional Theory

Framework materials constitute a broad family of solids that range from zeolites and metal-organic frameworks (MOFs) to coordination polymers. The synthesis of such network structures typically rely on precursor molecular building blocks. As an example, the UiO-66 MOF series is constructed of hexanuclear [Zr₆O₄(OH)₄(CO₂)₁₂] cluster nodes and linear carboxylate linkers. Unfortunately, these Zr MOF cluster nodes cannot currently be manufactured in a sustainable way, motivating a search for "green" alternative synthesis methods. Stabilizing the hexanuclear Zr(IV) cluster (i.e., the hexamer, {Zr₆¹²⁺}) without the use of organic ligation would enable the use of environmentally friendly solvents such as water. The Zr(IV) tetranuclear cluster (i.e., the tetramer, {Zr₄⁸⁺}) can be stabilized in solution with or without organic ligands, yet the hexamer has yet to be synthesized without supporting ligands. The reasons why certain zirconium clusters are favored in aqueous solution over others are not well understood. This study reports the relative thermodynamic instability of the hypothetical hexamer {Zr₆¹²⁺} compared to the ubiquitous {Zr₄⁸⁺} tetramer. Density functional theory calculations were performed to obtain the hydrolysis Gibbs free energy of these species and used to construct Zr Pourbaix diagrams that illustrate the effects of electrochemical potential, pH, and Zr(IV) concentration. It was found that the aqueous {Zr₆¹²⁺} hexamer is ∼17.8 kcal/mol less stable than the aqueous {Zr₄⁸⁺} tetramer at pH = 0, V = 0, and [Zr(IV)] = 1 M, which is an energy difference on the order of counterion interactions. Electronic structure analyses were used to explore trends in the highest occupied molecular orbital-lowest unoccupied molecular orbital gap, frontier molecular orbitals, and electrostatic potential distribution of these clusters. The evidence suggests that the aqueous {Zr₆¹²⁺} hexamer may be promoted with more strategic syntheses incorporating minimal ligands and counterions.

A lacunary tungstomolybdophosphate as an electronic pendulum: The "blue" electron under examination

The photoreduction of a Keggin type lacunary tungstomolybdophosphate, α-(Bu₄N)₄[H₃PW₉Mo₂O₃₉], in acetonitrile, led to the formation of a monoreduced lacunary heteropoly anion, or a one electron reduced "heteropoly blue" species, whereby the added "blue" electron was captured by the molybdenum atoms. The magnetic properties and behavior of the "blue" electron were studied by a modified Evans nuclear magnetic resonance method (small downshift of the ³¹P signal) and variable-temperature electron paramagnetic resonance (g = 1.936 for Mo^V). The intermolecular exchange of the "blue" electron was limited by a geometrical factor, which requires the contact between Mo caps to exchange it between the heteropoly couple. The intramolecular exchange of the "blue" electron between Mo atoms was rather fast (5.3 × 10⁹ s^-1), with a rate of more than six orders of magnitude larger than the intermolecular exchange rate. Density functional theory was used to determine the most prevalent protonation sites in the mixed lacunary isomers with the aim of studying the intramolecular electron transfer pathway in the isolated [H₄PW₉Mo₂O₃₉]^4- species. The singly occupied molecular orbital (SOMO) is essentially localized in one of the two nonequivalent molybdenum sites. The kinetics of the intramolecular electron exchange equilibrium Mo^V + Mo^VI → Mo^VI + Mo^V between the two molybdenum atoms bridged by an oxygen atom was found to be fast in agreement with the experimental result. The transition state is of mixed-valence type, with the SOMO delocalized over the Mo-O-Mo group. Spectroscopic parameters were found to be in fair agreement with experimental results.

Hybrid Hairy Platelets with Tunable Structures by Inclusion Crystallization of Polyferrocene-Containing Block Copolymers and Silicotungstic Acid

Herein, we report hybrid hairy platelets formed by block copolymers containing a poly(ethylene oxide) (PEO) midblock and polyferrocene end-blocks with silicotungstic acid (STA). As inclusion crystallization of PEO midblocks and STA clusters lead to crystalline lamellae in thin films at room temperature, the polyferrocene end-blocks that are excluded from the lattice will graft on the surface of the lamellae, giving rise to hybrid hairy platelets. The dependence of crystallographic structures and morphologies of platelets on the length of end-blocks is investigated. The presence of relatively long end-blocks induces the formation of an inclusion crystal with a tetragonal cell (termed as β-form), whereas an inclusion crystal with an orthorhombic cell (termed as α-form) can be formed with block copolymers with relatively shorter end-blocks.

Ion-pairing in polyoxometalate chemistry: impact of fully hydrated alkali metal cations on properties of the keggin [PW12O40]3- anion

The counterions of polyoxometalates (POMs) impact properties and applications of this growing class of inorganic clusters. Here, we used density functional theory (DFT) to elucidate the impact of fully hydrated alkali metal cations on the geometry, electronic structure, and chemical properties of the polyoxotungstate anion [PW₁₂O₄₀]^3-. The calculations show that the HOMO of the free anion [PW₁₂O₄₀]^3- is a linear combination of the 2p AOs of the bridging oxygens, and the first few LUMOs are the 5d orbitals of the tungsten atoms. The S₀→ S₁ electron excitation, near 3 eV, is associated with the O(2p) → W(5d) transition. Anion/cation complexation leads to formation of [PW₁₂O₄₀]^3-[M⁺(H₂O)₁₆]₃ ion-pair complexes, where with the increase of atomic number of M, the M⁺(H₂O)₁₆ cluster releases several water molecules and interacts strongly with the polyoxometalate anion. For M = Li, Na and K, [PW₁₂O₄₀]^3-[M⁺(H₂O)₁₆]₃ is characterized as a "hydrated" ion-pair complex. However, for M = Rb and Cs, it is a "contact" ion-pair complex, where the strong anion-cation interaction makes it a better electron acceptor than the "hydrated" ion-pair complexes. Remarkably, the electronic excitations in the visible part of the absorption spectrum of these complexes are predominantly solvent-to-POM charge transfer transitions (i.e. intermolecular CT). The ratio of the number of intermolecular charge transfer transitions to the number of O(2p)-to-W(5d) valence (i.e. intramolecular) transitions increases with the increasing atomic number of the alkali metals.

Fe-Doped Polyoxometalate as Acid-Aggregated Nanoplatform for NIR-II Photothermal-Enhanced Chemodynamic Therapy

The combination of reactive oxygen species-involved chemodynamic therapy (CDT) and photothermal therapy (PTT) holds great promise in enhancing anticancer effects. Herein, a multifunctional Fe-doped polyoxometalate (Fe-POM) cluster is fabricated via a simple method. The Fe-POM can not only be utilized as PTT agents to generate a hyperthermia effect for cancer cell killing under near-infrared (NIR) II laser (1060 nm) irradiation, but also can be used as CDT agents to convert endogenous less-reactive H₂ O₂ into harmful ·OH and simultaneously deplete glutathione for an amplified CDT effect. Notably, the hyperthermia induced by PTT can further enhance the CDT effect, achieving a synergistic PTT/CDT effect. Owing to the self-assembling properties at lowered pH values, the Fe-POM exhibits high tumor accumulation as revealed by photoacoustic imaging. More importantly, Fe-POM enables effective destruction of tumors without inducing noticeable damage to normal tissues under 1060 nm laser irradiation. The work presents a new type of multifunctional agent with high PTT/CDT efficacy, providing promising methods for PTT-enhanced CDT in a NIR-II biowindow.

Butyltin Keggin Ion with a Rare Four-Coordinate Ca Center

Alkyltin clusters are exploited in nanolithography for the fabrication of microelectronics. The alkyltin Keggin family is unique among Keggin clusters across the periodic table; its members appear to favor the lower-symmetry β and γ isomers rather than the highly symmetrical α and ε isomers. Therefore, the alkyltin Keggin family may provide important fundamental information about the formation and isomerization of Keggin clusters. We have synthesized and structurally characterized a new butyltin Keggin cluster with a tetrahedral Ca²⁺ center, fully formulated [(BuSn)₁₂(CaO₄)(OCH₃)₁₂(O)₄(OH)₈]²⁺ (β-CaSn₁₂). The synthesis is a simple one-step process. Extensive solution characterization including electrospray ionization mass spectrometry, small-angle X-ray scattering, and multinuclear (¹H, ¹³C, and ¹¹⁹Sn) nuclear magnetic resonance shows β-CaSn₁₂ is essentially phase-pure and stable. This differs from the previously reported Na-centered analogues that always form a mixture of β and γ isomers, with facile interconversion. Therefore, this study has clarified prior confusion over complex spectroscopic and crystallographic characterization of the Na-centered analogues. Density functional theory calculations showed the following stability order: γ-CaSn₁₂ < γ-NaSn₁₂ < β-CaSn₁₂ < β-NaSn₁₂. The β analogue is always more stable than the γ analogue, consistent with experiment. Notable outcomes of this study include a rare tetrahedral Ca coordination, a Na-free alkyltin cluster (important for microelectronics manufacturing), and a better understanding of Keggin families built of different metal cations.

Polyoxometalate-based room-temperature phosphorescent materials induced by anion–π interactions

A series of polyoxometalate-based host–guest materials emit strong red room-temperature phosphorescence attributed to intermolecular charge-transfer states which was caused by unorthodox anion–π interactions.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

Ligand-Directed Template Assembly for the Construction of Gigantic Molybdenum Blue Wheels

Template-mediated synthesis is a powerful approach to build a variety of functional materials and complex supramolecular systems. However, the systematic study of how templates structurally evolve from basic building blocks, and then affect the templated self-assembly, is critical to understanding and utilizing the underlying mechanism, to work towards designed assembly. Here we describe the templated self-assembly of a series of gigantic Mo Blue (MB) clusters 1-4 using l-ornithine as a structure-directing ligand. We show that by using l-ornithine as a structure director, we can form new template⊂host assemblies. Based on the structural relationship between encapsulated templates of {Mo8 } (1), {Mo17 } (2) and {Mo36 } (4), a pathway of the structural evolution of templates is proposed. This provides insight into how gigantic Mo Blue cluster rings form and could lead to full control over the designed assembly of gigantic Mo-blue rings.

Controlling the Activity and Stability of Electrochemical Interfaces Using Atom-by-Atom Metal Substitution of Redox Species

Understanding the molecular-level properties of electrochemically active ions at operating electrode-electrolyte interfaces (EEI) is key to the rational development of high-performance nanostructured surfaces for applications in energy technology. Herein, an electrochemical cell coupled with ion soft landing is employed to examine the effect of "atom-by-atom" metal substitution on the activity and stability of well-defined redox-active anions, PMo _xW_{12- x}O₄₀^3- ( x = 0, 1, 2, 3, 6, 9, or 12) at nanostructured ionic liquid EEI. A striking observation made by in situ electrochemical measurements and further supported by theoretical calculations is that the substitution of only one to three tungsten atoms by molybdenum atoms in the PW₁₂O₄₀^3- anions results in a substantial spike in their first reduction potential. Specifically, PMo₃W₉O₄₀^3- showed the highest redox activity in both in situ electrochemical measurements and as part of a functional redox supercapacitor device, making it a "super-active redox anion" compared with all other PMo _xW_{12- x}O₄₀^3- species. Electronic structure calculations showed that metal substitution in PMo _xW_{12- x}O₄₀^3- causes the lowest unoccupied molecular orbital (LUMO) to protrude locally, making it the "active site" for reduction of the anion. Several critical factors contribute to the observed trend in redox activity including (i) multiple isomeric structures populated at room temperature, which affect the experimentally determined reduction potential; (ii) substantial decrease of the LUMO energy upon replacement of W atoms with more-electronegative Mo atoms; and (iii) structural relaxation of the reduced species produced after the first reduction step. Our results illustrate a path to achieving superior performance of technologically relevant EEIs in functional nanoscale devices through understanding of the molecular-level electronic properties of specific electroactive species with "atom-by-atom" precision.

Combination-Responsive MoO3- x-Hybridized Hyaluronic Acid Hollow Nanospheres for Cancer Phototheranostics

It is of extreme importance to reduce side effects resulting from the nonspecific uptake of phototherapeutic agents by normal tissues. Currently, the single responsive strategy still cannot entirely satisfy the requirements of practical applications. In this study, we developed one kind of combination-responsive phototherapeutic nanoplatforms, where oxygen-deficient molybdenum oxide (MoO_{3- x}) hybridized hyaluronic acid (HA) hollow nanospheres, namely, MoO_{3- x}@HA HNSs, were constructed via a facile one-step method. In MoO_{3- x}@HA HNSs, the reasonable combination of actively targeted specificity endowed by the HA component and tumor microenvironment-responsive phototherapy activity induced by the MoO_{3- x} component can effectively improve the precision of phototherapy. The in vitro and in vivo experimental results confirm that MoO_{3- x}@HA HNSs can selectively kill CD44-overexpressing cancer cells and inhibit tumor growth under an 808 nm laser irradiation, revealing their remarkable synergistic photothermal therapy/photodynamic therapy effect with CD44 receptor-targeted specificity and pH responsiveness in treating cancer. We also prove that MoO_{3- x}@HA HNSs can serve as one kind of contrast agent to achieve the computed tomography/photoacoustic imaging. Encouraged by these results, it is anticipated that the reasonable combination of active targeting and tumor microenvironment responsiveness can be a promising strategy to develop phototherapeutic nanoplatforms for precise multimodality cancer theranostics.

Alkyltin clusters: the less symmetric Keggin isomers

The Keggin structure is prevalent in nature and synthesis, self-assembled from many metals across the periodic table as both isolated clusters and building blocks of condensed framework oxides. Here we present a one-step synthesis to obtain the sodium-centered butyltin Keggin ion in high yield and high purity, important for mechanistic nanolithography studies. Extensive solution characterization (small-angle X-ray scattering, 1H, 13C and 119Sn nuclear magnetic resonance, electrospray mass spectrometry) also confirms solutions contain only the Na-centered dodecamers. We report three butyltin Keggin structures: the β-isomer (β-NaSn12), the γ-isomer (γ-NaSn12), and a γ-isomer capped with an additional butyltin (γ-NaSn13). All Keggin ions presented here have the general formula [NaO4BuSn12(OCH3)12(O,OH)12] (Bu = butyl), and are of neutral charge. The lack of counterions (OH-) facilitates mechanistic lithographic studies without inference from hydrolysis chemistry. The methanol reaction media enables solubility and ligates the cluster, both important to obtain high purity materials. Despite the monospecific nature of the NaSn12 solutions, NMR reveals both isomer interconversion and ligand exchange. DFT computational comparisons of our three isolated structures, the capped β-isomer (β-NaSn13), along with hypothetical α-isomers (α-NaSn12 and α-NaSn13), showed that the stability ranks β-NaSn12 > γ-NaSn12 > α-NaSn12, consistent with experimental observation. The uncapped isomers were computationally determined to be more stable than the respective capped analogues. These clusters provide a unique opportunity to investigate the lower-symmetry Keggin isomers, and to determine structural factors that control isomer selectivity as well as isomer labilization.

Directed Self-Assembly, Symmetry Breaking, and Electronic Modulation of Metal Oxide Clusters by Pyramidal Heteroanions

Mixed valence/metal polyoxometalate (POM) clusters are one of the most interesting host species because they show the ability to incorporate a wide range of heteroatoms of various charges and geometries. We report herein the incorporation of pyramidal EO₃^2- heteroanions (E=PH, S, Se, Te) that are responsible not only for directing the templated assembly of a family of mixed-metal POMs but also for the symmetry-breaking of the traditional Dawson architecture and modulation of the electronic characteristics of the cluster's shell. The isolated family of POMs consists of four members: (Me₂ NH₂ )₅ Na₂ [Mo₁₁ V₇ O₅₂ (HPO₃ )]⋅MeOH⋅5 H₂ O (1), (NH₄ )₇ [Mo₁₁ V₇ O₅₂ (SO₃ )]⋅12 H₂ O (2), K₇ [Mo₁₁ V₇ O₅₂ (SeO₃ )] ⋅31 H₂ O (3), and (Me₂ NH₂ )₆ Na[Mo₁₁ V₇ O₅₂ (TeO₃ )]⋅15 H₂ O (4), and were characterized by X-ray structural analysis, electrospray ionization mass spectrometry (ESI-MS), thermogravimetric analysis (TGA), UV/Vis, FTIR, elemental analysis, flame atomic absorption spectroscopy (FAAS), and inductively coupled plasma optical emission spectroscopy (ICP-OES). Cyclic voltammetry (CV) and electron paramagnetic resonance (EPR) spectroscopic studies in concert with density functional theoretical (DFT) calculations have been used to elucidate the effect of the heteroatom on the electronic properties of the cluster.

Unveiling the relative stability and proton binding of non-classical Wells-Dawson isomers of [(NaF6)W18O54(OH)2]7- and [(SbO6)W18O54(OH)2]9-: a DFT study

Density functional theory calculations combined with the energy and building-block decomposition analyses have been carried out to investigate the structures, stability orders, redox potentials and proton binding of the six Baker-Figgis isomers (α, β, γ, α*, β* and γ*) of [(SbO₆)W₁₈O₅₄(OH)₂]^9- {H₂SbW₁₈} and [(NaF₆)W₁₈O₅₄(OH)₂]^7- {H₂NaW₁₈} anions at the level of PBEsol-D3/TZP. Both bonding energy and Gibbs free energy analyses exhibit that the two non-classical Wells-Dawson (WD) species behave quite differently from each other. The pyroanimonate {H₂SbW₁₈}, with a stability order of γ* > β* > α > α* > β > γ, is a non-classical WD species, while the hexafluoride {H₂NaW₁₈} (α > β > γ > γ* > β* > α*) is a transition intermediate between classical and non-classical WD types, possessing both non-classical ([XW₁₈O₆₀(OH)₂]^n-, X = I, Te and W) and classical [Si₂W₁₈O₆₂]^8- properties. Energy decomposition analyses (EDA) reveal that spatial arrangement (E_host), host-guest fragment interaction energy (FIE), and structural distortion energy (DE) are three key factors governing the relative stability of isomers; among these, DE is always dominant, while FIE and E_host are subordinated but are still important. Building-block decomposition analyses (BDA) disclose that the octahedral {MO₆} units of the equatorial belt, particularly the staggered belt, are always more distorted than those of the two polar caps inside each structure. The theoretical redox potentials demonstrate that the oxidizing power increases with a trend of α < β < γ and α* < β* < γ* for both species, and the first redox potential is closely related to the energy level of the LUMO of each anion. Evaluation of the proton inclusion energies suggests that {H₂NaW₁₈} can only embed two protons, while {H₂SbW₁₈} may encapsulate four; the number of embedded protons is controlled by both the charge of the heteroatom X and the volume of the tetrahedral {O₄}/{OF₃} cavity.

Effect of protonation, composition and isomerism on the redox properties and electron (de)localization of classical polyoxometalates

This publication reviews some relevant features related with the redox activity of two inorganic compounds: [XM12O40]q- (Keggin structure) and [X2M18O62]q- (Wells-Dawson structure). These are two well-known specimens of the vast Polyoxometalate (POM) family, which has been the subject of extensive experimental and theoretical research owing to their unmatched properties. In particular, their redox activity focus a great deal of attention from scientists due to their prospective related applications. POMs are habitually seen as ‘electron sponges’ since many of them accept several electrons without losing their chemical identity. This makes them excellent models to study mechanisms of electrochemical nature. Their redox properties depend on: (i) the type and number of transition metal atoms in the structure, (ii) the basicity of the first reduced species and, occasionally, of the fully oxidized species; (iii) the size of the molecule, (iv) the overall negative charge of the POM, and (v) the size of the central heteroatom. In the last years, important collaboration between the experimental and theoretical areas has been usual on the development of POM science. In the present chapter three of these synergies are highlighted: the influence of the internal heteroatom upon the redox potentials of Keggin anions; the dependence of the redox waves of Fe-substituted Wells-Dawson compounds with pH; and the role of electron delocalization and pairing in mixed-metal Mo/W Wells-Dawson compounds in their ability to accept electrons. In these three cases, a complete understanding of the problem would not have been possible without the mutual benefit of experimental and computational data.

Charge retention of soft-landed phosphotungstate Keggin anions on self-assembled monolayers

Preferential immobilization of the 2− charge state observed for polyoxotungstate Keggin anions soft-landed onto self-assembled monolayer surfaces.

A highly aluminum-doped β-isomer Keggin-type tungstoaluminate

A highly Al-doped β-isomer Keggin-type cluster was synthesized and characterized.

Disorder and polymorphism in Cu(ii)-polyoxometalate complexes: [Cu1.5(H2O)7.5PW12O40]·4.75H2O, cis- & trans-[Cu2(H2O)10SiW12O40]·6H2O

Three highly crystalline Cu(ii)-polyoxometalate (POM) complexes, [Cu_1.5(H₂O)_7.5PW₁₂O₄₀]·4.75H₂O and cis- and trans-[Cu₂(H₂O)₁₀SiW₁₂O₄₀]·6H₂O, were successfully synthesized and characterized by single crystal and powder X-ray diffraction as well as thermogravimetric analysis.

Isolation and characterization of the [Ga2Al18O8(OH)36(H2O)12](8+) cluster: cationic variations on the Wells-Dawson topology

The structural chemistry of Group 13 polyoxometalates lags far behind related negatively charged transition metal species and limits the development of advanced materials. A novel heterometallic cluster [Ga2Al18O8(OH)36(H2O)12](8+) (Ga2Al18) has been isolated using a supramolecular approach and structurally characterized using single-crystal X-ray diffraction. Ga2Al18 represents the Wells-Dawson structure polycations and variations in the structural topology may be related to the initial stabilization of the Keggin isomer. DFT calculations on the related ε-Keggins (GaAl12 and Al13), Ga2Al18, and theoretical Al2Al18 clusters reveal similar features of electronic structure, suggesting additional heteroatom substitution in other isostructural clusters should be possible.

The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box

The molybdenum blue reaction, used predominantly for the determination of orthophosphate in environmental waters, has been perpetually modified and re-optimised over the years, but this important reaction in analytical chemistry is usually treated as something of a 'black box' in the analytical literature. A large number of papers describe a wide variety of reaction conditions and apparently different products (as determined by UV-visible spectroscopy) but a discussion of the chemistry underlying this behaviour is often addressed superficially or not at all. This review aims to rationalise the findings of the many 'optimised' molybdenum blue methods in the literature, mainly for environmental waters, in terms of the underlying polyoxometallate chemistry and offers suggestions for the further enhancement of this time-honoured analytical reaction.

Gas-Phase Fragmentation Pathways of Mixed Addenda Keggin Anions: PMo12-nW nO 40 3- (n = 0-12)

We report a collision-induced dissociation (CID) investigation of the mixed addenda polyoxometalate (POM) anions, PMo(12-n)W(n)O(40)(3-) (n = 0-12). The anions were generated in solution using a straightforward single-step synthesis approach and introduced into the gas phase by electrospray ionization (ESI). Distinct differences in fragmentation patterns were observed for the range of mixed addenda POMs examined in this study. CID of molybdenum-rich anions, PMo(12-n)W(n)O(40)(3-) (n = 0-2), generates an abundant doubly charged fragment containing seven metal atoms (M) and 22 oxygen atoms (M(7)O(22)(2-)) and its complementary singly charged PM(5)O(18)(-) ion. In comparison, the doubly charged Lindqvist anion, (M(6)O(19)(2-)) and its complementary singly charged PM(6)O(21)(-) ion are the dominant fragments of Keggin POMs containing more than two tungsten atoms, PMo(12-n)W(n)O(40)(3-) (n = 3-12). The observed transition in the dissociation pathways with an increase in the number of W atoms in the POM may be attributed to the higher barrier of tungsten-rich anions towards isomerization. We present evidence that the observed distribution of Mo and W atoms in the major M(6)O(19)(2-) and M(7)O(22)(2-) fragment ions is different from that predicted by a random distribution, indicating substantial segregation of the addenda metal atoms in the POMs. Charge reduction of the triply charged precursor anion resulting in formation of doubly charged anions is also observed. This is a dominant pathway for mixed POMs having a majority (8-11) of W atoms and a minor channel for other precursors indicating a close competition between fragmentation and charge loss pathways in CID of POM anions.