Properties of the Nickel(III) Complex with 1,4,8,11-Tetraazacyclotetradecane-1,4,8,11-tetraacetate in Aqueous Solution

Oxidation of trans-I-Ni^IIL, or Ni^IIL_out, by ^•OH radicals was carried out by irradiating N₂O saturated solutions containing the complex, prepared by mixing equimolar concentrations of the metal ion and the ligand at pH 3.50. The intermediate thus formed has a strong absorption band at 310 nm and is stable for hours. That this unstable complex is a nickel(III) species was verified by EPR spectroscopy and its chemical reduction with ascorbic acid, which was found to be instantaneous. The most striking feature of the oxidation by ^•OH radicals, which are powerful single electron oxidizing agents, is that it produces a nickel(III) species which is different from the electrochemically produced one as manifested by their absorption spectra and EPR signals. The radiolytically produced nickel(III) species undergoes decarboxylation leading to the production of CO₂, CH₂O, Ni^IIL′ and Ni^IIL (L′ = the new ligand produced after oxidative degradation of one of the N-acetate groups of the ligand). The electrochemical oxidation of trans-I-Ni^IIL, or Ni^IIoutL, proceeds via the degradation of two of the acetate arms on average. The yield of CH₂O is ∼2.0 mol per mole of the nickel(II) complex in electrochemical oxidations. These observations point out that the two techniques produce different nickel(III) complexes which do not interconvert. The mechanisms leading to the production of Ni^IIL′ (oxidation by ^•OH radicals, radiolytically) and Ni^IIL″ (electrochemically) differ from that proposed in the literature.