Electrochemical behaviour of first row transition metal substituted polyoxotungstates:: A comparative study in acetonitrile

A comparative study of the electrochemical behaviour of several transition metal substituted polyoxotungstates in acetonitrile solution with tetra-n-butylammonium perchlorate as a supporting electrolyte is presented. Tetra-n-butylammonium salts of Keggin-type polyoxoanions, alpha-[(C4H9)(4)N](4)H-x[XW11M(H2O)O-39].nH(2)O, X = P, Si; M-II = Co, Ni, M-III = Fe, Mn, x = 0-2, n = 0-2, and sandwich-type polyoxoanions, alpha-B-[(C4H9)(4)N](7)H-3[M-4(H2O)(2)(PW9O34)(2)], M-II = Co, Mn, have been studied by cyclic voltammetry and spectroelectrochemistry. Under the conditions used, the tungsten-oxo framework of the metal substituted Keggin anions could accept up to three electrons in quasi-reversible one-electron consecutive processes. Comparison with the corresponding lacunary anions is presented. Redox processes at the metal centre have been detected for all metals M except Ni. For the anions alpha-[XW11M(H2O)O-39](m-), the co-existence of species with a different degree of protonation was deduced from cyclic voltammetry results whenever protons were included in the molecular formula. The influence of the solids' composition on the electrochemical results is stressed. For the sandwich anions, cyclic voltammetry revealed redox processes occurring on the W atoms and also on the metallic belt. Two metal atoms in the belt of the alpha-B-[CO4II(H2O)(2)(PW9O34)(2)](10-) anion could be electrochemically oxidised to Co-III in two separate steps. A similar result was observed for the Mn-II analogue, but the corresponding first step was not as well defined. Re-oxidation was only detected for the Co anion. This work is the first comparative study of the electrochemical behaviour of such transition metal substituted polyoxotungstates in an organic medium. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).