Journal of Molecular and Engineering Materials

In this paper, a critical perspective on the state-of-the-art, current developments and future applications of polyoxometalate-ionic liquids (POM-ILs) is presented. This paper is focused on recent developments for true polyoxometalate ionic liquids, where the POM cluster acts as the anionic component in an ionic liquid. A brief overview of the initial development of POM-ILs is given and key features of the materials such as viscosity, conductivity and thermal stability are compared. Current applications of POM-ILs are exemplified and the advantages as well as limitations of POM-ILs for usage as catalysts, sensors and electrochemically active materials are discussed. Potential future areas of application are described and initial studies in these areas are highlighted.

In this paper, a green and facile synthetic protocol for the preparation of photoactive gold nanoparticle-soft oxometalates (AuNP-SOM) using a unique keplarate type oxomolybdate cluster viz. {Mo₁₃₂}, is reported. The as-synthesized AuNP-SOMs are fully characterized by electronic absorption spectroscopy, Raman spectroscopy and high resolution transmission electron microscopy (HR-TEM). We further demonstrate that by merely tuning the ratio of the precursors in solution it is possible to control the morphology of AuNP-SOM nanostructures. Moreover, these photoactive AuNP-SOMs are employed as photocatalysts for the photopolymerization of styrene to generate colloidal monodisperse polystyrene microspheres in a controllable way in the absence of any external co-initiator or inert atmosphere. Effect of styrene concentration on the size of microspheres is studied using DLS and optical microscopy. Finally, the role of AuNP-SOMs as efficient photocatalyst is established through several control experiments and a reaction pathway is proposed.

Four new metal diphosphonates Zn(pyHhedpH)⋅H₂O(1), Cd(pyHhedpH)⋅H₂O(2), Pb(pyHhedpH)⋅H₂O(3) and Pb₂(pyHedpH)₂(4) (pyHhedpH₃ = 2-(4-pyridyl)-1-hydroxyl-1,1-ethylidenediphosphonic acid, pyedpH₄ = (2-(pyridin-4-yl)ethene-1,1-diyl)diphosphonic acid) were synthesized under hydrothermal conditions. Compounds 1 and 2 are isostructural and have ladder-like chain structures in which the {M₂((μ - O)₂)}(M = Zn, Cd) dimer units are connected by O-P-O bridges. Compound 3 shows a unique type of double chain structure, in which the adjacent {Pb₂(μ - O)₂} dimers are bridged by O-P-O and O-P-C-O groups into a chain. Compound 4 has a 2D structure, where the vertex-shared {PbO₅} polyhedra are connected by {CPO₃} tetrahedra through corner-sharing. The photoluminescence properties of compounds 1-4 were studied. Their potential as soft-oxometalate precursors is also discussed.

Many interesting properties of polyoxometalates (POMs) are related to their ability to reversibly accept and release electrons. The correct computational description of metal electrons in POMs is central to fully understand the implications and mechanisms related to reduced structures. We performed DFT calculations on fully oxidized [P₂W₁₈O₆₂]^6- and α₁/α₂-[P₂MW₁₇O₆₂]^n-(M = V, Mo) anions, and their one-electron reduced partners. Atomic spin populations of the reduced forms reveal that pure GGA functionals, which overdelocalize the electron density by nature, cannot correctly describe the extra electron especially when competition between the localized and delocalized situations is very close in energy, namely for α₂-[P₂MoW₁₇O₆₂]^7-. On the other hand, hybrid functionals like B3LYP or M05 can localize the electron at the correct site. Within these "well-behaved" density functionals, only the B3LYP gives the correct ordering and relative reduction energies (REs) with respect to electrochemical measurements, although the absolute values are always less exothermic than expected. At present, this is the best density functional for computing redox properties of POMs.

In this paper, we present direct evidences for two-stage mechanism of crystallization of patchy colloids from replica exchange Monte Carlo simulations. The patchy model colloid mimics the structure and interactions of a certain class of polyoxometalates (POM). We find that individual colloids self-assemble into two-dimensional sheets in hexagonal close-packed structure, and these sheets themselves stack to form crystals. The simulation explains the formation of hollow shell-like objects in POM solution [T. Liu, B. Imber, E. Diemann, G. Liu, K. Cokleski, H. Li, Z. Chen and A. Muller, J. Am. Chem. Soc.128, 15914 (2006)]. Simulation also predicted the formation of pentagonal caps that are essential for the formation of hollow, closed shells of POMs. Similar two-step crystallization of apoferritin protein was earlier found in experiments [S. T. Yau and P. G. Vekilov, Nature406, 494 (2000)]. The simulation study suggests nonclassical route to crystallization in patchy colloids.

An important challenge in the field of materials design and synthesis is to deliberately design mesoscopic objects starting from well-defined precursors and inducing directed movements in them to emulate biological processes. Recently, mesoscopic metal-oxide-based soft oxometalates (SOMs) have been synthesized from well-defined molecular precursors transcending the regime of translational periodicity. Here, we show that it is actually possible to controllably move such an asymmetric SOM, with the shape of a "peapod" along complex paths using tailor-made sophisticated optical potentials created by spin–orbit interaction of light due to a tightly focused linearly polarized Gaussian beam propagating through a stratified medium in an optical trap. We demonstrate motion of individual trapped SOMs along circular paths of more than 15 μm in a perfectly controlled manner by simply varying the input polarization of the trapping laser. Such controlled motion can have a wide range of applications starting from catalysis to the construction of dynamic mesoscopic architectures.

Decavanadate is a polyoxometalate consisting of 10 octahedral vanadium centers, which has been found to exert biological effects and has been observed in vivo. Biological activity implies that a material is taken up into a cell or that the material interacts with membrane receptors. Because of the large size and the high molecular charge, it is nontrivial to anticipate how such a large anion interacts with membranes and whether it will be taken up by cells. Therefore, it becomes important to investigate how the anion interacts with membranes and membrane model systems. Since ion pairing is important for the interaction of this large complex with any membrane interface system, we investigate both the nature of Coulombic and neutral noncovalent interactions with membrane model interface systems and cellular systems. Specifically, we used microemulsions as model systems, and in the specific phase diagram regime where reverse micelles form. We find that, there is a large difference in the interaction with different interfaces, and that charge can have an important role. The negatively charged interface repels the anion, whereas a positive interface attracts the anion. However, the interface with neutral surfactant head groups also is found to repel the decavanadate. This result demonstrates that the discrete charge Coulombic interactions are not the only forces in effect, and that the interactions are at least to a first approximation dictated by the interface charge and not by the counterions in the system. Alternative forces include van der Waals attraction, pH of the water pool, and field and surface effects. Because biological membranes have differently charged ligands, it is not clear which interface systems provide the best analogy with cell surfaces. However, surface charge may affect the compounds and facilitate the interactions that could be important. For example, a positively charged surface could potentially facilitate hydrolysis and sequential abstraction of one or two vanadium atoms at a time from decavanadate. Recently, decavanadate was used as a structural model for the V₂O₅ material. Negatively charged interfaces have also been found to accelerate compound hydrolysis or in other ways alter reactions in compounds near the interface. Lipid-like interfaces potentially contribute to processing of coordination compounds. Decavanadate has been found to interact with proteins and insulin enhancing effects have been reported. Interactions with coordination compounds and the mechanisms of interactions should continue to be investigated because such systems may reveal the mode of interaction of these compounds.