Synthesis, characterization and fabrication on a glassy carbon electrode of a tetra-iron substituted sandwich-type pentadecatungstodiarsenate heteropolyanion

Abstract

A novel sandwich-type compound, Na₁₂[Fe₄(H₂O)₂(As₂W₁₅O₅₆)₂]·41H₂O, has been synthesized. The compound was well-characterized by means of IR, UV-vis, ¹⁸³W NMR and elemental analyses. The compound crystallizes in the triclinic, Pī symmetry group. The structure of the compound is similar to that of Na₁₆[M₄(H₂O)₂(As₂W₁₅O₅₆)₂]·nH₂O (M = Cu, Zn, Co, Ni, Mn, Cd), and consists of an oxo-aqua tetranuclear iron core, [Fe^III₄O₁₄(H₂O)₂], sandwiched by two trivacant α-Wells-Dawson structural moieties, α-[As₂W₁₅O₅₆]. Redox electrochemistry of the compound has been studied in buffer solutions at pH = 4.7 using polarography and cyclic voltammetry (CV). The compound exhibited four one-electron couples associated with the Fe(III) center followed by three four-electron redox processes attributed to the tungsten-oxo framework. The compound-containing monolayer and multilayer films have been fabricated on a 4-aminobenzoic acid modified glassy carbon electrode surface by alternating deposition with a quaternized poly(4-vinylpyridine) partially complexed with [Os(bpy)₂Cl]^2+/−. CV, X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy and atomic force microscopy (AFM) have been used to characterize the multilayer films. It is proved that the multilayer films are uniform and stable. The electrocatalytic activities of the multilayer films have been investigated on the reduction of two substrates of important analytical interest, NO₂⁻ and H₂O₂.