Oxidation Chemistry of Metal(II) Salen-Type Complexes

Abstract

The oxidation chemistry of metal complexes has been widely developed in recent years, affording deep insights into the reaction mechanisms for many useful homogeneous catalytic reactions and enzymatic reactions at the active site of metalloenzymes [1]. In the course of the studies, a large number of novel complexes have been synthesized and well characterized [2-18], and especially high valent metal complexes formed as a result of redox reactions have become important in catalytic and biological systems. In general high valent metal complexes have been meant to show the complexes oxidized at the metal center, and the formal oxidation state is identical with the oxidation state of the central metal ion reported in [19-23]. For example, one-electron oxidation of potassium hexacyanoferrate(II), K4[Fe(CN)6], gives potassium hexacyanoferrate(III), K3[Fe(CN)6], whose valence state of the central iron ion is +III and agrees with the experimental valence state of the ion. In contrast to this, the formal oxidation number of the central metal ion of the complexes of iminophenolate dianion, (LAP)2is not always identical with the experimental valence state reported in references [24,25]. In the case of [Ni(LAP)2]0, the formal oxidation state of the central nickel ion is +IV, but the experimental valence state of nickel can be assigned to be +II, and two iminophenolate dianions are oxidized to iminosemiquinonate radical anions (LSQ)(Figure 1).