Introduction for the Emergent Polyoxometalates and Soft-oxometalates thematic issue

As a cradle of new chemistry, New Journal of Chemistry presents a thematic issue showcasing an emergent discipline, that of soft-oxometalates (SOMs) combined with novel polyoxometalates (POMs). The contributions in this issue are based on polyoxometalate, soft-oxometalate and cluster research that can lead to emergent phenomena and futuristic materials; their wide range of applications defines the content of this volume. This thematic issue contains contributions that range from the conception and characterization of the POMs/SOMs and related clusters with descriptions of their fundamental properties, soft matter properties and biological effects, to their applications, which include, but are not limited to, active matter, water oxidation, catalysis and biology. New POMs continue to be prepared either as inorganic materials or more complex materials. This issue reports studies of new POMs, but also highlights POMs that upon treatment under various conditions can form different soft structural states and assemblies; such states of POMs are now referred to as SOMs. The papers are presented here in the topic order of synthesis, characterization and applications, with the Perspective reviews and communications (NJC Letters) placed at the front of the issue.

The issue commences with two Perspective reviews encompassing the synthesis, structural characterization and reactivity of the clusters, and representative applications. The first review by Wang and coworkers describes polymer–POM hybrids (DOI: 10.1039/C5NJ01257K). This group has reported new materials by integrating a series of POMs within a polymer through covalent linking of the POM to the polymer. These materials provide a solid foundation upon which specific photochromic or electrochromic properties can be developed. Different W-based Keggin, Dawson and related lacunary POMs are used to facilitate the handling, allowing the POM to be covalently coordinated to the polymer. The Perspective provides a focused review of the synthesis of these POM–polymer conjugates based on covalent modifications, their self-assembled structures and potential applications.

The second Perspective by Casey and Rustad describes mechanistic investigations at the interface of geochemistry and POM chemistry via oxygen exchange chemistry (DOI: 10.1039/C5NJ00985E). Using ¹⁷O NMR spectroscopy, the dissolution of oxide minerals was investigated using oxygen isotopic exchange rates, along with molecular dynamics and electronic structure calculations. Several steps for oxygen and metal ion exchange were identified as important, and pathways were proposed for these processes. Metal ions play a crucial role because a coordinatively unsaturated metal ion can associate with oxygen atoms present in water, hydronium ions or hydroxide ions, and thus facilitate protonation and hydrolysis, which leads to metal ion release. These processes take place for metal ions near the surface and result in formation of metastable structures, some of which are in a long-lived state and are important in the conversion to more stable forms. The composition and symmetry of the starting geometry of the oxide mineral determines the pathways through which each surface species can convert. These restrictions make it difficult to simulate the reactions at the surfaces.

Communications are also included in this volume; the one from the Yin and Wei groups describes the functionalization of POMs using nucleophilic substitution reactions of a hexamolybdate-based organic–inorganic hybrid POM (DOI: 10.1039/C5NJ01090J). The POM is functionalized by a chloralkane fragment, and the chlorine atom is reactive enough to be substituted by iodine and nitrate through nucleophilic substitution reactions. These high yielding reactions provide examples of post-functionalization protocols that serve as representative reactions for modifying polyoxometalate-based hybrid materials under mild conditions.

The contribution from the Dalal, Pope and Kortz groups explores the feasibility of applying high-resolution solid-state ¹⁹⁵Pt MAS-NMR spectroscopy to several Pt^IV-containing polyoxotungstates (DOI: 10.1039/C5NJ01242B). These studies were supplemented by solid-state ¹⁹⁵Pt NMR as well as ⁵¹V NMR to describe a Pt^IV-containing polyoxovanadate. This method is particularly useful for polyanions, which are unstable in solution and/or poorly soluble, as well as for systems exhibiting crystallographic disorder of the Pt and W sites. The authors also report solution ¹⁹⁵Pt and ¹⁸³W NMR spectra for the hexatungstoplatinate(IV) [H₃PtW₆O₂₄]⁵⁻.

The Nyman group investigated the effects of cesium on W- and Mo-based POMs and compared these to the effects of cesium on Nb and Ta POM salts with regard to their solubility in water and their ion association in aqueous solution (DOI: 10.1039/C5NJ02914G). Mixed clusters allowed the opportunity to investigate trend reversals. Crystallographic data show that increasing the Cs content in the clusters increases the Nb content, and the perturbations are clearly evidenced by changes in bond lengths in the clusters and their UV-Vis absorbance spectra. DFT calculations show that the HOMO–LUMO energy gap widens with increasing Nb content, presumably because of the overall poorer mixing of the Nb_4dversus W_5d atomic orbitals with the O_2p atomic orbitals. These results suggest an emerging framework upon which the properties of these materials can be categorized and used for development of new materials.

In the article by Sokolov and coworkers, the conversions of the sandwich complex (NMe₄)_4.83[(Se^IVW^VI_4.57V^V_4.43O₃₃)₂(W^VI(O)(H₂O))(VVO)_2.6]·10.57H₂O are described (DOI: 10.1039/C5NJ00858A). By slowly losing Se, the POM converts into (NMe₄)₂Na₂[W^VI₄V^V₂O₁₉]·8H₂O and, after continued disproportionation reactions, into the Lindquist-type structure [VW₅O₁₉]³⁻. These transformations were monitored with electrospray mass spectrometry, capillary electrophoresis and ⁵¹V and ⁷⁷Se NMR spectroscopy.

Nogueira and Cavaleiro and their coworkers describe the synthesis and characterization of new metal-substituted tetraalkylphosphonium POM ionic liquids (DOI: 10.1039/C5NJ01093D). The hybrids were prepared from Keggin structures with a Fe, Co or Mn center. These new materials are all colored liquids at ambient temperature; solidification takes place at −25 °C for the Fe and Co materials. They are therefore true ionic compounds, in contrast to most of the analogous systems that were reported previously. Both the POM and the ionic liquid cation are essentially unchanged in the material, which is important for developing materials with specific properties. These types of materials are likely to have interesting catalytic properties, and thus extend the properties of the POMs.

A second communication included in this volume describes a polymer comprising inorganic clusters (DOI: 10.1039/C5NJ01326G). In this study, reported by the Peng and Liu groups, a polymeric material containing molecular POM clusters was prepared and its properties described. This material was found to retain a single-chain, extended rod-like conformation in a solvating solvent such as DMF. Changing the DMF solvent by titration with water results in a system that does not solvate the polymer, such that a hollow cylindrical supramolecular structure is formed. Variation of the cations shows that the self-assembly process is driven by counterion-mediated attraction, and the corresponding interactions mimic the counterion association behavior of small inorganic macro ions. These systems help us to understand the polyelectrolyte behavior of POM-containing rigid polymers.

The Wang group report self-assembly of POM–organic conjugates that are sensitive to temperature and demonstrate a delicate control of the molecular interactions (DOI: 10.1039/C5NJ02271A). The conjugate is comprised of an organically modified Anderson-type POM cluster with two cholesterol moieties, demonstrating that the symmetrical and rigid structure of the Anderson-type POM cluster is a building block for constructing functional supramolecular assemblies. Cholesterol is well known to associate in a wide range of assemblies and, because of its rigid structure together with strong van der Waals interactions, it is a unique building block. In a mixed DMF/toluene (v/v = 1 : 9) solvent system the conjugate assembles into crystalline rods at 40.0 °C, and into a gel at 20.0 °C. When assembled into crystalline rods, the material consists of ultra long and rigid microrods. In the gel, the conjugate is immobilized by massive short and elongated nanoribbons. This temperature-mediated change in material consistency provides a feasible method for the fabrication of nanostructured materials.

In a study from the groups of Levinger, Alvarez and Crans, a combination of ⁵¹V NMR spectroscopy, dynamic light scattering and continuous shape analysis was used to probe the solvation of two polyoxometalates, a decavanadate and a molybdenum-containing decavanadate, encapsulated in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)–isooctane reverse micelles (DOI: 10.1039/C5NJ01788B). Because size and shape could both be critical for the interaction with interfaces, two differently charged oxometalates were placed in a nanosized water droplet to investigate how they are solvated. The decametalates require at least 2–3 layers of water to stabilize them in the reverse micelles, and these studies show that this is regardless of whether the oxometalate has a charge of −5 or −6. Continuous shape analysis was used to investigate the changes in structure upon protonation of decavanadate and the replacement of one vanadium atom with platinum. The changes observed in the ⁵¹V NMR spectra correlate to differences in the protonation state, and no differences are attributed to the solvation of the molybdenum-substituted decavanadate compared to decavanadate in the reverse micelle. Because no solvation differences were observed between the decavanadates, the authors conclude that shape is most important for these decavanadates and that changes in charge have a smaller impact on the system under these conditions.

The Parac-Vogt group showed that a Zr(IV)-substituted Keggin POM is able to cleave 18 different dipeptides that contain the X-Ser, Gly-X, or X-Gly sequences (DOI: 10.1039/C5NJ00561B). The POM isolated was a dimer, (Et₂NH₂)₈[α-PW₁₁O₃₉Zr(μ-OH)(H₂O)₂]·7H₂O, and the hydrolysis reactions of ten peptides were studied by kinetic experiments and ¹H/¹³C NMR spectroscopy. The observed rate constants are highly dependent on the bulkiness and chemical nature of the X amino acid side chain. For example, peptides with a positive charge have higher rate constants, and for some dipeptides deamination reactions take place. This study is an important stepping-stone towards investigations showing cleavage specificity.

Another communication describes the superior catalytic properties of POMs. In this work by Lan and coworkers, the Mo-containing POM-assisted fabrication of a Pd nanoparticle–reduced graphene oxide nanocomposite is reported (DOI: 10.1039/C5NJ01659B). Because of the expense of Pd nanoparticles, other materials that would decrease the cost while maintaining the high catalytic properties were investigated for preparing the sample. The hybrid exhibits enhanced catalytic activity and has excellent methanol tolerance compared to its counterparts, due to the synergistic effect of Pd, POM, and reduced graphene oxide.

Bi and coworkers describe a series of polyoxometalate-stabilized gold nanoparticles (Au NPs) (DOI: 10.1039/C5NJ01983D). These nanoparticles were prepared by a one-pot procedure at room temperature in water using a tungstophosphate Pressler-type POM, (NH₄)₁₄[NaP₅W₃₀O₁₁₀]·31H₂O (abbreviated P₅W₃₀), as both reducing agent and for stabilization of the NPs. Composite films of this material in combination with polydimethyldiallyl ammonium chloride (PDDA) were constructed on ITO-coated glass electrodes and quartz substrates using layer-by-layer assembly. The results indicate that the tri-component composite film (PDDA/Au@P₅W₃₀/PDDA/graphene oxide)₈ shows enhanced electrocatalytic activity towards the reduction of H₂O₂. This is attributed to the synergistic effect between the proximity of the three components (Au NPs, P₅W₃₀ and graphene oxide), the large active sites and the electron conductivity of graphene oxide and Au NPs.

A contribution from the Roy group describes a soft-oxometalate-based photocatalytic system that readily oxidizes water to oxygen (DOI: 10.1039/C5NJ01099C). The activity of a double sandwich-type manganese-based molecular polyoxometalate, Na₁₇[Mn₆P₃W₂₄O₉₄(H₂O)₂]·43H₂O, as a water oxidation catalyst was investigated when it was placed directly on a graphene oxide matrix and when it was incorporated into a new SOM, Na₁₇[Mn₆P₃W₂₄O₉₄(H₂O)₂]·43H₂O@graphene oxide. The efficiency of the photocatalytic water oxidation by the SOM is almost double that of the photocatalytic water oxidation by Na₁₇[Mn₆P₃W₂₄O₉₄(H₂O)₂]·43H₂O alone. The enhanced reactivity is attributed to the presence of electron-accepting graphene sheets. Moreover, the immobilization of the POMs to make SOMs facilitates the water oxidation reaction and consequently increases the oxygen yield.

In the contribution by Murugavel and coworkers, sterically encumbered arylimido hexamolybdates were prepared and then applied in organic synthesis reactions (DOI: 10.1039/C5NJ02330K). Facile synthesis of mono- and di-functionalized arylimido hexamolybdates, (nBu₄N)₂[Mo₆O₁₈(L1)], (nBu₄N)₂[Mo₆O₁₇(L1)₂] and (nBu₄N)₂[Mo₆O₁₈(L2)], where L1 is 4-bromo-2,6-diisopropylaniline and L2 is 2,2′,6,6′-tetraisopropylbenzidine, was accomplished from the parent (nBu₄N)₂[Mo₆O₁₉]. The arylimido derivatives were characterized by multiple methods. Steric hindrance at the ortho positions of the aryl amine effectively protects the newly formed Mo≡N bonds. This work demonstrates that an aryl amine, when added to the surface of the hexamolybdate, transforms this otherwise inactive parent POM into a useful catalyst. The POM was found to effectively catalyze the oxidation of cyclohexene to cyclohexene epoxide, and that of benzyl alcohol to benzaldehyde and benzoic acid.

Mizuno and coworkers report a divacant lacunary silicotungstate, TBA₄H₄[γ-SiW₁₀O₃₆] (SiW₁₀), that can catalyze reactions upon irradiation with visible light (DOI: 10.1039/C5NJ01045D). The photoredox catalyst facilitates functional group transformations of sulfur-containing compounds, including aerobic oxygenation of sulfides to sulfoxides and deoxygenation of sulfoxides to sulfides. In the presence of an electron-transfer mediator for oxygenation (Ce³⁺) and an electron and proton donor for deoxygenation (an alcohol), SiW₁₀ exhibits visible light-induced charge transfer reactions. The highest occupied molecular orbitals of the adduct of Ce³⁺ and/or alcohol at the vacant site of SiW₁₀ allow for selective oxygenation of sulfides and deoxygenation of sulfoxides after irradiation with visible light (λ > 400 nm). The SiW₁₀ photocatalyst can be readily recovered and reused for these high yielding transformations.

In the communication by Polarz and coworkers, a new class of surfactants containing a head group composed of an inorganic material is described (DOI: 10.1039/C5NJ01544H). The POM surfactant with the composition H₃[(C₁₆H₃₃Si)₂PW₁₁O₃₉], denoted I-SURFs, is found to simultaneously support four different types of effects, in contrast to organic surfactants that only support emulsification. The secondary effects are manifested by the polymerization of the monomeric units, which is catalyzed in a protic environment, leading to spherical PDMS/POM surfactant particles. Upon removal of the solvent, assembly of a nanoporous network takes place. The third effect that these materials exhibit is conductivity of protons inside the channels. This occurs because the POMs are placed on the surface of the channel network. Fourthly, the POM head group can be used as a precursor for WO₃, leading to nanostructured silica–WO₃ hybrid materials.

A series of polyoxometalate-based stable polymeric hybrids were synthesized by Leng and coworkers to serve as epoxidation catalysts (DOI: 10.1039/C5NJ01541C). These were prepared using polyhedral oligomeric vinylsilsesquioxanes (POSS) and ionic liquids bearing hydrophobic alkyl chains as the building blocks, which conveniently added to Keggin-type phosphotungstic acid (PW) in a non-covalent fashion by ion exchange. The POSS–IL_x–PW hybrids were demonstrated to be mesostructured and amphiphilic materials with good thermal stability. Catalytic tests for H₂O₂-based epoxidation of cyclooctene showed that these newly designed catalysts exhibit extraordinary catalytic activity, versatility in different solvents, catalytic rate, stability and reusability. The catalysts’ excellent performance in epoxidation reactions with H₂O₂ is attributed to the amphiphilic nature and the mesoporous structure.

A quantum mechanical modeling study accompanied by experimental work was carried out by the Mitchell and Poblet groups to describe gold nanoparticle nucleation (DOI: 10.1039/C5NJ02773J). Specifically, during the reduction of [Au(III)Cl₄]⁻ by the Kabanos-type POM, proton-coupled electron transfer (PCET) greatly promotes the reduction process. These results reveal that this particular POM is a multi-electron donor and thermodynamically it is sufficiently powerful to reduce Au^III to Au^I. The intermediary stages, including the non-classical reduced POM, were explored using computational methods. The theoretical analysis suggests that protons facilitate the final Au^III to Au^I reduction step that ultimately leads to nanoparticle formation.

A companion paper from the Mitchell and Moros groups describes studies with a Kabanos-type POM, [Na(Mo₂VO₄)₃(μ₂O)₃(μ₂SO₃)₃(μ₆SO₃)₂]¹⁵⁻, and the corresponding AuNPs@POM (DOI: 10.1039/C5NJ02775F). The Au nanoparticles have a diameter of approximately 29 nm. The authors chose this particular POM because of its redox properties; it contains two layers of hexanuclear polyoxomolybdenum(V) sulfite anions with all the Mo atoms in a one-electron reduced oxidation state, Mo(V). These AuNPs@POM were then incubated in fetal bovine serum for adsorption of proteins. Adding these AuNPs@POM to kidney epithelial (Vero) and skin melanoma cells (B16) showed that, in both cases, the AuNPs@POM internalize and localize in the endosomes. The AuNPs@POM have greater anti-proliferative action on the B16 tumor cells than on the Vero cells.

In the contribution by Banerjee and coworkers, the SOM-mediated phase explosion of water at the solid–water interface is explored using laser irradiation (DOI: 10.1039/C5NJ01856K). A tightly focused CW laser beam induces the phase explosion in a microscopic domain inside a thin film of water in contact with a SOM-coated glass surface. The high absorptivity of the SOM transmits the intensity of the laser beam to induce a nucleated micro-bubble at the water–SOM-coated glass interface. Changes in laser power result in changes in bubble size due to convective effects. In a simulation using the heat equation it was possible to correlate the size of the bubble at the glass substrate–water interface with the phase explosions in superheated water. At certain laser powers the bubble size increases dramatically, and simulations demonstrate that this is when the water is approaching its critical point. At lower laser powers the microscopic flows of water around the bubble can be changed, and this suggests that these micro-bubble SOM structures may be used in the context of controlled lithography.

The final paper of this thematic issue is a contribution by Roy and coworkers describing their efforts to explore SOMs as active matter (DOI: 10.1039/C5NJ01097G). A metal oxide system has been designed to respond to a chemical stimulus, such as that provided by the reducing agent dithionite. Specifically, the authors investigated the movement of molybdenum oxide rods of about 200 nm in length, which were characterized by SEM and DLS. The displacement of the SOM rods stems from the evolution of SO₂ gas arising from the oxidation of dithionite to SO₂ with concomitant reduction of Mo₇ SOMs to molybdenum blue SOMs. The reaction creates an osmotic boundary between the newly generated SOM and gas products and the unreacted SOM surface. The chemical potential gradient between the SOM surface and the reactive osmotic interface gives rise to the movement of the SOMs. The velocity of the SOMs can be increased until the SOM surface is saturated with evolved gases, as seen from the adsorption isotherm. This proof-of-concept shows that by using a simple redox system it is possible to construct SOM micro motors and pave the way for developing other systems and fine-tuning their motion. This contribution highlights the special properties of the soft state of POMs and the applications of SOMs as active matter.

This thematic issue demonstrates that POMs continue to hold a lot of promise for the future. Amid the structural richness of POMs there also exists the soft state of POMS, the SOMs, which are currently being characterized. The structural organization of SOMs shows high potential for applications ranging from phase explosion of water, to water oxidation catalysis, to active matter. With the ability to modify the higher structural order by combining POM–SOM chemistry, the ability to develop emergent phenomena in materials chemistry is enhanced dramatically.

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