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An approach for the modification of electrode surfaces with thin films composed of polyoxometallate anions and large water-soluble cationic species is described. In the procedure, a ca. monolayer of the iso-or heteropolyanion is first adsorbed onto the electrode surface. By immersing the resulting system into a solution containing a large monovalent, multivalent, or polyvalent cation, a composite layer is formed due to the interaction between the adsorbed polyanion and the solution cation. After rinsing, this electrode is reimmersed into the solution of the polyanion, and immobilization of an additional quantity of the polyanion takes place. By the repeated and alternate immersions into the anionic and cationic modification solutions, the amount of material on the electrode can be increased systematically in a controlled fashion leading to stable three-dimensional multilayered molecular assemblies. The immobilized polyanions (isopolymolybdate, phosphotungstate, or silicotungstate) have redox characteristics similar to those of their solution counterparts. The precipitate-forming cationic species include tetrabutylammonium and tris(1,10-phenanthroline)-iron(II) ions, as well as protonated poly(4vinylpyridine). Composite films of heteropoly-12-tungstate anions with protonated poly(4-vinylpyridine) are the most robust. The approach permits introduction of multiple redox centers into the thin films on electrodes, formation of bilayer-type systems, and, in some cases, even insulating coatings. Details of the preparation and physicochemical, particularly electrochemical, properties of the produced systems are described.

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We have investigated inorganic cluster-surfactant materials to better understand the structural evolution of these phases as the surfactant:cluster ratio increases above 4:1 and the cluster charge increases beyond -4. Our studies suggest that both ordering of the surfactant molecules into bilayers as well as the cluster charge are the primary influences on the hybrid cluster-surfactant phase structure. However, cluster geometry, inclusion of solvent molecules, surfactant tail length, cation head size, etc. also influence the self-assembly of these materials. We present the synthesis, characterization, and single-crystal X-ray structure of [SiMo 12 O 40 ][C 16 H 33 N(CH 3 ) 3 ] 4 (monoclinic P2 1 /c, a ) 13.136(1) Å, b ) 20.139(2) Å, c ) 41.030(3) Å, β ) 93.443(1)°, and V ) 10834.7(17) Å 3 ) and the synthesis and characterization of a related phase that forms upon chemical reduction of the silicomolybdate anion. Structural and chemical comparisons are made between these two compounds, as well as other phases formed from polyoxometalates (with charges ranging from -3 to -16) and surfactants. The structure of [SiMo 12 O 40 ][C 16 H 33 N-(CH 3 ) 3 ] 4 is compared to the structures of other reported cluster-surfactant phases that also have a 1:4 cluster:surfactant ratio. Analyses of these phases provide some insight as to why it is thus far only this 1:4 ratio that provides crystals suitable for single-crystal diffraction studies and how the structure of the cluster-surfactant phases evolves as the surfactant/cluster ratio is increased.