Multi-iron tungstodiarsenates.: Synthesis, characterization, and electrocatalytic studies of αββα-(FeIIIOH2)2FeIII2(AS2W15O56)2 12-

Reaction of the trivacant lacunary complex, alpha-Na-12[AS(2)W(15)O(56)], with an aqueous solution of Fe(NO3)(3)(.)9H(2)O yields the sandwich-type polyoxometalate, alphabetaalpha-Na-12((FeOH2)-O-III)(2)Fe-2(III)(AS(2)W(15)O(56))(2) (Na1). The structure of this complex, determined by single-crystal X-ray crystallography (a = 13.434(1) Angstrom, b = 13.763(1) Angstrom, c = 22.999(2) Angstrom, alpha = 90.246(2)degrees, beta = 102.887(2)degrees, gamma = 116.972(1)degrees, triclinic, P1, R1 = 5.5%, based on 25342 independent reflections), consists of an Fe-4(III) unit sandwiched between two trivacant alpha-As2W15O5612- moieties. UV-vis, infrared, cyclic voltammetry, and elemental analysis data are all consistent with the structure determined from X-ray analysis. Magnetization studies confirm that the four Fe((III)) centers are antiferromagnetically coupled. A cyclic voltammogram of Na1 reveals that a three-wave W(VI) system replaces the two-wave W(VI) system found in the precursor alpha-As2W15O5612- complex. The observed modifications in the CV patterns of Na1 and alpha-As2W15O5612- are most likely due to subsequent changes in the acid-base properties of two reduced POMs that occur as a result of Fe(III) incorporation. Na1 is shown to be more efficient than the monosubstituted complex alpha2-As-2((FeOH2)-O-III)W17O617- in the electrocatalytic reduction of dioxygen. This is attributed to cooperativity effects among the adjacent Fe(III) centers in Na1.