An electrochemical study of thiohydroxamate and hydroxamate complexes of iron(III)

Abstract

Direct-current polarographic, cyclic voltammetric, and constant-potential coulometric measurements have been made on acetone solutions of tris(thiohydroxamato)- and tris(hydroxamato)-complexes of iron(III), [Fe{ONR′CR(X)}₃](X = S or O; R = H, Ph, C₆H₄Me-p, or CH₂Ph; R′= H, Me, Ph, or cyclohexyl). In all cases rapid one-electron reductions to Fe^II are observed with the reduction potential for the thiohydroxamate chelates being less cathodic than those of the hydroxamates, consistent with the softer S-donor ligand stabilizing the iron(II) state. A notable feature is the reversible electron transfer which occurs when the ligand nitrogen is substituted with an alkyl or aryl group. In contrast, when N is bonded to an hydrogen atom an irreversible Fe^III–Fe^II electron transfer is observed, probably because of decomposition of the iron(II) complex. The relation of the present results to iron-release mechanisms in microbial iron-transport compounds (siderophores) is briefly discussed.