Redox characteristics of Schiff base manganese and cobalt complexes related to water-oxidizing complex of photosynthesis

In an effort to obtain synthetic analogues of water-oxidizing complex (WOC) of photosystem II (PS II) of plant photosynthesis, a Schiff base manganese and a cobalt complex, employing Niten, a SALEN type ligand, have been prep ared. Cyclic and square wave voltammetric measurements have been performed to assess their redox characteristics. Both complexes undergo several reduc tion processes in cathodic negative potential region at more or less simila r potentials. In view of these reductions being independent of the nature o f the metal, they are thought to be ligand-localized. Although similar in n egative region, a marked difference in the behavior of the complexes is obs erved in anodic region. While the cobalt complex is electrochemically inact ive in the positive potentials up to +1.0 V vs. Ag/AgCl, the manganese comp lex displays two oxidation waves at +0.25 and +0.5 V vs. Ag/AgCl. The prese nce of oxidation wave in manganese complex at +0.5 V vs. Ag/AgCl or +0.7 V vs. NHE suggests that this complex can catalyze the oxidation of water and can, thus, simulate the WOC of PS II. (C) 1999 Elsevier Science S.A. All ri ghts reserved.