Electrochemical behaviour of mixed d metal-iron containing Wells-Dawson sandwich-type complexes: [(FeOH2)2M2(X2W15O56)2]n− and [(MOH2)2Fe2(X2W15O56)2]n− (M = CrIII, MnIII, MnII, CoII, NiII, CuII, ZnII, X = AsV or PV and n = 12 or 14)

Abstract

An innovative study on the electrochemical behaviour of mixed d metal-iron containing Wells-Dawson sandwich-type complexes [(FeOH2)2M2(X2W15O56)2]14− and [(MOH2)2Fe2(X2W15O56)2]14− (with M=CrIII, MnII, MnIII, CoII, NiII, CuII or ZnII and X=PV or AsV) was carried out. These complexes have a four-centre equatorial metal cluster constituted of two Fe atoms and two atoms of another metal. The FeIII centres are either in an external position, [(FeOH2)2M2], or in an internal position, [M2(OH2)2Fe2]. Experimental methods (cyclic voltammetry and controlled potential coulometry) and theoretical calculations (density functional theory) allowed us to determine and analyse the redox potential values associated with the reduction of the FeIII centres in these species. The influence of the position of the FeIII centres, the nature of the metal centre M and the electron density distribution in the tetranuclear cluster (either [(FeOH2)2M2] or [(MOH2)2Fe2]) have been studied and rationalised in order to account for the observed behaviours. The data suggest that the most stable isomers are those where FeIII centres are internally-located, [(MOH2)2Fe2]. Consequently, their reduction is more difficult than those having externally-located FeIII isomers, [(FeOH2)2M2]. Some experimental results revealed a few exceptions to this rule which have not been rationalised yet.