Polyoxometalates In Solution Research Articles

Nitrite Electroreduction Enhanced by Hybrid Compounds of Keggin Polyoxometalates and 1‐Butyl‐3‐Vinylimidazolium

AbstractWe describe here an immobilization method of four Keggin‐type polyoxometalates (POMs) ([H2W12O40]6−, [BW12O40]5− [SiW12O40]4−, [PW12O40]3−) by using the reaction with an ionic liquid, 1‐butyl‐3‐vinylimidazolium (BVIM) bromide. The reaction yields a hybrid material (BVIM‐POM) as a water‐insoluble salt. The chemical structure of both compounds is preserved, as indicated by infrared spectroscopy (FT‐IR), although with a reduced crystallinity (shown by X‐ray diffraction analysis) due to a decrease of water content (shown by thermogravimetric analysis). Cross polarization 1H‐31P NMR evidenced the presence of BVIM in the structure of (BVIM)3[PW12O40]. The salt is mixed with carbon powder and Nafion to prepare an ink and casted on glassy carbon electrodes. The electrochemical behavior of immobilized POMs material is preserved. The electrochemical activity for nitrite reduction is measured by cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). It was observed that the reduction current of 10 mM HNO2 at pH 1 in 0.5 M Na2SO4 is enhanced in the presence of these hybrid materials. DEMS has evidenced the formation of nitrous oxide (N2O) at potentials more positive compared to the use of parent POMs in solution.

Three in One: Three Different Molybdates Trapped in a Thiacalix[4]arene Protected Ag72 Nanocluster for Structural Transformation and Photothermal Conversion.

Polyoxometalates (POMs) represent crucial intermediates in the formation of insoluble metal oxides from soluble metal ions, however, the rapid hydrolysis-condensation kinetics of MoVI or WVI makes the direct characterization of coexisted molecular species in a given medium extremely difficult. Silver nanoclusters have shown versatile capacity to encapsulate diverse POMs, which provides an alternative scene to appreciate landscape of POMs in atomic precision. Here, we report a thiacalix[4]arene protected silver nanocluster (Ag72b) that simultaneously encapsulates three kinds of molybdates (MoO4 2- , Mo6 O22 8- and Mo7 O25 8- ) in situ transformed from classic Lindqvist Mo6 O19 2- , providing more deep understanding on the structural diversity and condensation growth route of POMs in solution. Ag72b is the first silver nanocluster trapping so many kinds of molybdates, which in turn exert collective template effect to aggregate silver atoms into a nanocluster. The post-reaction of Ag72b with AgOAc or PhCOOAg produces a discrete Ag24 nanocluster (Ag24a) or an Ag28 nanocluster based 1D chain structure (Ag28a), respectively. Moreover, the post-synthesized Ag28a can be utilized as potential ignition material for further application. This work not only provides an important model for unlocking dynamic features of POMs at atom-precise level but also pioneers a promising approach to synthesize silver nanoclusters from known to unknown.

Spectroscopic, Crystallographic, and Electrochemical Study of Different Manganese(II)‐Substituted Keggin‐Type Phosphomolybdates

Adjusting the RedOx activity of polyoxometalate catalysts is a key challenge for the catalysis of selective oxidation reactions. For this purpose, the possibility of influencing the RedOx potential by the introduction of an additional RedOx‐active element was investigated. Thereby, Keggin‐type polyoxometalates (POMs) with up to three different elements in the metal framework were created. An advanced and reproducible synthetic procedure to incorporate MnII and additionally VV into Keggin‐type heteropolyacids alongside comprehensive characterization of the new molecules is presented. The success of our syntheses was confirmed by vibrational spectroscopy (IR and Raman) and elemental analysis. Furthermore, the new compounds were analyzed by NMR spectroscopy to investigate the characteristics of the POMs in solution. The structures of successfully crystalized compounds were determined by single‐crystal X‐ray diffraction. Moreover, all synthesized compounds were characterized using UV/Vis spectroscopy and electrochemical analysis to get further insights into the electronic transfer processes and redox potentials.

Small-angle X-ray scattering studies of emergent polyoxometalates in solution

The characterization of polyoxometalates (POMs) structures in solutions is challenging although X-ray diffraction is widely used to solve the POMs’ crystalline structures. Here, we introduce small-angle X-ray scattering (SAXS) to explore the solutions of several emergent POMs mainly from Prof. Enbo Wang’s group. Direct evidence of their crystal structure retained in solution is obtained by fitting their SAXS data with appropriate crystal structure models, ensuring credibility of their applications for catalysis and magnetism. The capability of distinguishing dimer and monomer structures of {Ce20W100} in solution is of special interest, which is difficult for other characterization techniques.

Polyoxometalates in solution: speciation under spotlight.

Polyoxometalates (POMs) are a large group of anionic polynuclear metal-oxo clusters with discrete and chemically modifiable structures. In most aqueous POM solutions, numerous, and often highly negatively charged, species of different nuclearities are formed. It is rather difficult to determine the dominant POM species or their combination, which is responsible for the specific POM activity, during a particular application. Thus, the identification of all individual speciation profiles is essential for the successful implementation of POMs in solution applications. This review article summarizes species that are present in isopoly- and heteropolyvanadates, -niobates, -molybdates and -tungstates aqueous solutions and covers their stability and transformations. The ion-distribution diagrams over a wide pH range are presented in a comprehensive manner. These diagrams are intended for the targeted use of POMs, and in a clear form shows species that are in equilibrium at the given pH value. Thus, the data accumulated in this review can serve as both a starting point and a complete reference material for determining the composition of POM solutions. Some examples are highlighted where the POM speciation studies led to a detailed understanding of their role in applications. In doing so, we aim to motivate the POM community for more speciation studies and to make the subject more comprehensible, both for synthetic POM chemists and for scientists with different backgrounds interested in applying POMs in biological, medical, electrochemical, supramolecular and nanochemistry fields, or as homogeneous catalysts and other water-soluble materials.

The application of scattering technology in the study of polyoxometalate solutions

Polyoxometalates(POMs), a large group of well-defined nanoclusters, are formed by linking early-transition-metal oxide polyhedrons through shared corners, edges and planes. POMs are widely used in various fields, such as catalysis, single molecular magnets, photoelectric materials, proton conductors, magnetic materials, and biomaterials, due to their abundant compositions and structures. However, how to design and synthesize POMs with specific structure and function remains a challenge for researchers. In-depth studies of POMs solution behavior are required to solve this problem. Scattering techniques, using microwaves, (near)infrared, visible light, ultraviolet light, X-rays, and neutrons as probe, are employed to investigate the structure and dynamics of materials. By detecting the interactions between the probe and the particles, physical properties, such as particle size, shape and internal structure, can be determined. This article focuses on the application of laser light scattering (LLS), small angle X-ray scattering (SAXS), and small angle neutron scattering (SANS) in the study of polyoxometalate solutions. By LLS, researchers discovered the self-assembly of POM macroanions, for example, researchers find the supramolecular blackberry structure formed by {Mo154} macroions in aqueous solution. Meanwhile the self-assembly processes and the self-recognition behaviors were determined, in mixed dilute aqueous solutions, the clusters {Mo72Fe30} and {Mo72Cr30} self-assemble into different blackberry structures of the Cr30 and Fe30 type. SAXS are employed to study POMs morphology and solution behavior, determine the counterion distribution around POMs in solutions, and probe the interactions among POMs in solutions. The effect of Rb+ on the assembly process, and the effect of solvent polarity on the assembly process, all of these can determined by SAXS. Moreover, the kinetic behaviors of confined hydrogen atoms in POMs and the morphology of POMs in hybrid materials can be obtained through SANS. Researchers study the difference between the mean square displacement measured in fully hydrogenated and partially deuterated {Mo72V30} by SANS. These studies are instructive to the design of POMs structure and function. However, there are still many basic problems of POM need to be solved. For example, the correlation between structure and properties of POM. And in the preparation of POM, how do the reducing agent, pH and catalyst work? Solving these basic issues requires numerous chemists effort. Scattering techniques play a key role in the study of polyoxometalate solutions since the structure and morphology information of nanoscale molecules can be obtained. With its unique advantages, scattering techniques will promote the development of POM.

The applications of small-angle X-ray scattering in studying nano-scaled polyoxometalate clusters in solutions

Nano-scaled polyoxometalates (POMs) clusters with sizes ranging from 1 to 10 nm attract tremendous attention and have been extensively studied due to POMs’ fascinating structural characteristics and prospects for wide-ranging applications. As a unique class of nanoparticles with well-defined structural topologies and monodispersed masses, the structures and properties of POMs in both bulk state and solutions have been explored with several well-developed protocols. Small-angle X-ray scattering (SAXS) technique, as a powerful tool for studying polymers and nanoparticles, has been recently extended to the investigating of solution behaviors of POMs. In this mini-review, the general principle and typical experimental procedures of SAXS are illustrated first. The applications of SAXS in characterizing POMs’ morphology, counterion distribution around POMs, and short-range interactions among POMs in solutions are highlighted.

A General Approach to Access Morphologies of Polyoxometalates in Solution by Using SAXS: An Ab Initio Modeling Protocol.

Herein, we reported a general protocol for an ab initio modeling approach to deduce structure information of polyoxometalates (POMs) in solutions from scattering data collected by the small-angle X-ray scattering (SAXS) technique. To validate the protocol, the morphologies of a serious of known POMs in either aqueous or organic solvents were analyzed. The obtained particle morphologies were compared and confirmed with previous reported crystal structures. To extend the feasibility of the protocol to an unknown system of aqueous solutions of Na2 MoO4 with the pH ranging from -1 to 8.35, the formation of {Mo36 } clusters was probed, identified, and confirmed by SAXS. The approach was further optimized with a multi-processing capability to achieve fast analysis of experimental data, thereby, facilitating in situ studies of formations of POMs in solutions. The advantage of this approach is to generate intuitive 3D models of POMs in solutions without confining information such as symmetries and possible sizes.

Hydrophobic effect as a driving force for host-guest chemistry of a multi-receptor Keplerate-type capsule.

The effectiveness of the interactions between various alkylammonium cations and the well-defined spherical Keplerate-type {Mo132} capsule has been tracked by (1)H DOSY NMR methodology, revealing a strong dependence on the self-diffusion coefficient of the cationic guests balanced between the solvated and the plugging situations. Analysis of the data is fully consistent with a two-site exchange regime involving the 20 independent {Mo9O9} receptors of the capsule. Furthermore, quantitative analysis allowed us to determine the stability constants associated with the plugging process of the pores. Surprisingly, the affinity of the capsule for a series of cationic guests increases continuously with its apolar character, as shown by the significant change of the stability constant from 370 to 6500 for NH4(+) and NEt4(+), respectively. Such observations, supported by the thermodynamic parameters, evidence that the major factor dictating selectivity in the trapping process is the so-called "hydrophobic effect". Computational studies, using molecular dynamics simulations, have been carried out in conjunction with the experiments. Analysis of the radial distribution functions g(r) reveals that NH4(+) and NMe4(+) ions behave differently in the vicinity of the capsule. The NH4(+) ions do not exhibit well-defined distributions when in close vicinity. In contrast, the NMe4(+) ions displayed sharp distributions related to different scenarios, such as firmly trapped or labile guest facing the {Mo9O9} pores. Together, these experimental and theoretical insights should aid in the exploitation of these giant polyoxometalates in solution for various applications.

Electrografting of Diazonium‐Functionalized Polyoxometalates: Synthesis, Immobilisation and Electron‐Transfer Characterisation from Glassy Carbon

Polyoxometalates (POMs) are attractive candidates for the rational design of multi-level charge-storage materials because they display reversible multi-step reduction processes in a narrow range of potentials. The functionalization of POMs allows for their integration in hybrid complementary metal oxide semiconductor (CMOS)/molecular devices, provided that fine control of their immobilisation on various substrates can be achieved. Owing to the wide applicability of the diazonium route to surface modification, a functionalized Keggin-type POM [PW11 O39 {Ge(p-C6 H4 -CC-C6 H4 -${{\rm N}{{+\hfill \atop 2\hfill}}}$)}](3-) bearing a pending diazonium group was prepared and subsequently covalently anchored onto a glassy carbon electrode. Electron transfer with the immobilised POM was thoroughly investigated and compared to that of the free POM in solution.

Influence of heteroanion and ammonium cation size on the composition and gas-phase fragmentation of polyoxovanadates

This paper describes the results of a systematic experimental investigation of the influence of different size alkyl ammonium cations and heteroanions on the composition, ionic charge state and gas-phase fragmentation pathways of anionic polyoxovanadates synthesized in solution. Four separate solutions of polyoxometalates (POMs) were prepared using all possible combinations of the tetraethylammonium [(C2H5)4N+] cation, chloride (Cl−) heteroanion, tetrabutylammonium [(C4H9)4N+] cation and acetate (CH3CO2−) heteroanion. Employing electrospray ionization combined with high-resolution mass spectrometry (ESI-MS) we demonstrate that POM solutions synthesized using the small [(C2H5)4N+] cation and Cl− heteroanion are composed predominately of large doubly and triply charged chlorine containing species with a size distribution centered at 14 vanadium atoms. POM solutions prepared using the Cl− anion and a larger [(C4H9)4N+] cation are shown to contain slightly larger species with 15 and 16 vanadium atoms, thereby indicating that the size of the ammonium cation exerts only a weak influence on the size of polyoxovanadates formed in solution. POM solutions prepared using (C2H5)4NCl and (C4H9)4NCl also produced peaks consistent with the attachment of one and two ammonium cations to the larger vanadium oxide species. Solutions prepared using the large CH3CO2− heteroanion, in contrast, contain much smaller singly and doubly charged species with a size distribution centered at six vanadium atoms. In addition, while incorporation of one and two ammonium cations into the smaller vanadium oxide species was observed, no POMs containing the CH3CO2− heteroanion were identified. The gas-phase fragmentation pathways of representative POMs containing one and two ammonium cations were examined using collision induced dissociation (CID) and mass spectrometry. Similar primary fragmentation pathways involving partial loss of the ammonium cation were observed for species containing both one and two ammonium cations largely independent of the size, composition and charge state of the precursor ion. The [(C4H9)4N+] was found to exhibit stronger interactions with the core of the POMs resulting in higher abundance of fragment ions containing (C4H9) units compared to (C2H5) units originating from [(C2H5)4N+]. These results provide fundamental insight into the interactions between anionic metal oxides, heteroanions and ammonium cations that are responsible for the size and composition-controlled synthesis of POMs in solution.

Towards a computational treatment of polyoxometalates in solution using QM methods and explicit solvent molecules

This study is aimed at developing an explicit solvent model for highly charged species such as polyoxometalates. The model includes solvent molecules in the first solvation shell explicitly, and long-range bulk effects and counter-ions as a set of single point charges. The model strongly stabilizes the electronic structure of the Keggin anion. The energies of the Kohn–Sham orbitals obtained using our model lie very close to those computed using the COSMO continuum solvent model; moreover, the total solvation energy evaluated with our model compares well to the value calculated by COSMO.

Unravelling the Complexities of Polyoxometalates in Solution Using Mass Spectrometry: Protonation versus Heteroatom Inclusion

A route to unravel and the complexities of polyoxometalates in solution using electrospray mass spectrometry is presented. This reveals the limited speciation of the clusters in organic solvent compared to that in aqueous solution and allows the unambiguous assignment of the protonation states of the cluster as a function of heteroatom inclusion.