Stable solutions of NiIII complexes of the 14membered macrocyclic ligands (I), (11), and (111) were obtained by electrochemical oxidation on gold and platinum electrodes of the corresponding Nix1 complexes in mild acidic aqueous solutions in the presence of sulphate; the NiIII complexes formed represent a new kind of strong, easily accessible, single-electron , oxidizing agent with E" ca. 1 V vs. standard hydrogen electrode (S.H.E.) in aqueous media.

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Electrochemical and pulse-radiolytic oxidation of [ N i ( c y ~I a m ) ] ~+ and [Cu(cyclam)12+ (cyclam = 1,4,8,11tetra-azacyclotetradecane) have been studied in aqueous acidic perchlorate and sulphate media. As with other nickel macrocyclic systems, [ N i ( c y ~I a m ) ] ~+ is oxidized to Ni*'I, the stability of which is enhanced by sulphate complexation. In contrast, [Cu(~yclam)]~+ is much less stable than the nickel analogue and does not form a stable sulphate complex. Electrochemical oxidation of [Cu(cyclam)12+ at pH 1.6 is a multistep process which is catalysed by sulphate ion. The mechanism appears to involve the transfer of four electrons resulting in a copper(t1) macrocyclic diene complex.