Effects of chloride ions on electrochemical reactions of manganese(III) complexes

Electrochemical reactions of manganese(III) complexes, Mn-III(L)X (L; salen, salpn, 5-NO2-salen or 5-NO2-salpn, X; Cl, Br or NO2) and Mn-III(L')(2)X (L': N-Bu-sal, N-Oct-sal, N-Oct-5-Br-sal or N-Oct-5-NO2-sal, X; Cl or Br), were investigated by voltammetry at a glassy carbon electrode in the absence/presence of Cl- in acetonitrile solution. By the addition of Cl-, oxidation processes of Mn-III(L)X and Mn-III(L')(2)X have been found to be improved from quasi-reversible to reversible, and their oxidation products, [Mn-IV(L)X](+) and [Mn-IV(L')(2)X](+), were stabilized by the combination with Cl- resulting in [Mn-IV(L)Cl-2] and [Mn-IV(L')(2)Cl-2], respectively. On the other hand, the reduction processes of Mn-III(L)X and Mn-III(L')(2)Cl were not so significantly affected by Cl- as those observed for their oxidation. Other types of manganese(III) complexes and iron(III) complex were also investigated. The present Study may clarify the role of Cl- being involved in OEC (oxygen-evolving center) in photosystem II. (C) 2008 Elsevier Ltd. All rights reserved.