Electrochemical and catalytic behaviour of Dawson-type complexes derived from [(1),2.3-P2Mo2W15O61](10-) and first transition metal ions

The acid-base and electrochemical behaviour of the lacunary Dawson-type het eropolyanion alpha-[(1)-2,3-P2Mo2W15O61](10-) and of Its derivatives with V -IV, Mn-II, Fe-III, Co-II, Ni-II, Zn-II, and Cu-II has been studied as a fu nction of pH. Essentially two groups can he distinguished. In the first gro up, constituted by the V-IV, Mn-II, Co-II, Ni-II, Zn-II-substituted complex es, the redox activity, if any, of the heterometallic cations occurs far fr om the potential location of that of the molybdenum moieties. At pH 3, the molybdenum waves are very close to each other and to that of the precursor lacunary complex. For higher pH values the apparent slowness observed for t he first wave of the lacunary compound becomes less pronounced in the subst ituted complexes, but is modulated differently by each substituent. For the Fe-III and Cu-II substituted complexes, the redox activity of these cation s can mix with that of the molybdenum centres, depending on the pH of the s olution. Spectroelectrochemical experiments have helped to clarify the cond itions of the separation of redox activities in the case of the Fe-III-subs tituted complex. It has been possible to obtain an overall three electron p rocess on the first wave of this compound. In contrast, copper deposition i s observed with the Cu-II substituted complex In appropriate conditions. Al l the complexes show electrocatalytic properties on their respective first wave for the reduction of NO. Owing to the very high stability of the V-IV- substituted complex throughout the whole pH domain, it has been possible to study the electrocatalytic oxidation of NADH to NAD(+) at pH 8. (C) 1999 E lsevier Science S.A. All rights reserved.