ELECTROACTIVE COLLOIDAL MATERIALS DERIVED FROM POLYOXOMETALTES

Polyoxometalates (POMs) are transition metal-oxide clusters formed by the early transition metals V, Mo, and W. As a subclass of metal oxides, POMs have a number of prototypical physical and chemical properties: solubility; thermal and solvent stability; Brønsted acidity; chemical and photochemical redox activity; photo- and electrochromism; and magnetic properties. POMs have been applied to a variety of technical areas, including catalysis, photocatalysis, biomedicine, and magnetism. As discreet molecular units having well-defined chemical properties, POMs are attractive building blocks for new materials whose properties could be rationalized in terms of their constituents at the molecular level. Towards that end, we have synthesized and characterized a series of new extended structure materials containing the organofunctionalized polyoxocationic species, [NaVIV 6O6{(OCH2CH2)2N(CH2CH2OH)}6]+ (NaV6 +). Three of the new materials are classified as POM ionic crystals and contain NaV6 + and Keggin polyoxoanions [(PW12O40)3-, (SiW12O40)4- or (SiMo12O40)4-] as molecular constituents: [NaVIV 6O6{(OCH2CH2)2N(CH2CH2OH)}6]3(PW12O40)·12H2O G1, [NaVIV 6O6{(OCH2CH2)2N(CH2CH2OH)}6]4(SiW12O40)·20H2O G2, and [NaVIV 6O6{(OCH2CH2)2N(CH2CH2OH)}6]3(HSiMo12O40)·13H2O G3. The final two materials (hereafter C1 and C2) are obtained as redistributable colloids derived from NaV6 + and the mixed-addenda Keggin structure polyoxoanion (PVW11O40)4- (VW11 4-). C1 contains the single polyoxocation constituent NaV6 +, while C2 contains both NaV6 + and VW11 4-. We have characterized the materials using PXRD, FTIR and UV-vis spectroscopy, TGA, SEM/EDX, elemental analysis, and cyclic voltammetry. G1, G2, and G3 are nanocrystalline and composite in nature, containing intact oxometallic cores of their respective polyoxoanion and polyoxocation building block units. Intercluster Coulombic interactions are the driving force of composite formation. These are the first materials derived from Coulombic interactions of polyoxoanions with covalently organo-functionalized polyoxocations. Cl and C2 are colloidal in nature and obtained in the form of micrometer-scale monoliths. They are the first POM-derived colloidal materials that are readily synthesized, isolated, and redistributed in practical quantities. The monoliths are insoluble in a variety of organic media including lower aliphatic alcohols. They readily form thin-films (δ < 100 um) by solvent casting. Owing to their extended nature, redistributable and isolable properties, the materials C1 and C2 may considered a new form of POM heterogenation. We use cyclic voltammetry (CV) to show that the constituents of the new materials are electrochemically addressable; a finding which opens the door to possible applications in sensing, energy storage and (electro-)chemical catalysis. With an eye towards technological applications, we demonstrate the systematic deposition of C2 onto indium tin oxide (ITO) using electrophoretic deposition, and show (CV) that we can access the electrochemical redox centers of its constituents following deposition. These materials demonstrate the activity of NaV6 + as a soft-chemical constituent partner to similar scale POM counter-anions, suggesting further synthetic investigations using this unique species. Owing to the redox properties of their constituents, the new materials present hybrid surfaces that may be considered intriguing prospective (multifunctional) heterogeneous oxidation catalysts. The electroactive nature of the materials also suggests prospective applications in energy storage and conversion, for example, as the active components of ion-insertion type capacitors and batteries. Electrocatalysis and sensing are also prospective application areas. The extended, as opposed to molecular, structure of the materials, suggests possible advantages when forming polymeric composites with conducting and nonconducting polymers, such as: an increased resistance to leeching, the emergence of favourable polymeric film mechanical properties, and the emergence of favorable electron and counterion tranport properties.Ph.D. in Chemistry, May 201