Abstract
An “experimental” valence state of metal complexes is sometime different from the “formal” oxidation state, especially in the species having redox active ligands. This difference can be seen in biological system, such as iron(IV)-porphyrin π-cation radical in some heme proteins and copper(II)-phenoxyl radical in galactose oxidase (GO). Although structural characterizations of these species by X-ray diffraction methods have been rare due to their stability, some artificial metal-phenoxyl radical complexes have been synthesized and successfully characterized by X-ray crystal structure. In this review, syntheses and X-ray crystal structures of the one-electron oxidized metal-phenolate complexes, metal- phenoxyl radical, and high-valent metal phenolate species are discussed.
Highlights
The CrIII-phenoxyl radical complex having a three phenolate moieties connected with the triazacyclononane backbone is the first example of the metal-monodentate phenoxyl radical complex revealed by the X-ray crystal structure analysis (Figure 3) [38]
X-ray crystal structures of the one-electron oxidized species of metal-phenolate complexes and metal(II) salentype complexes are discussed in this review with reference to the relationship between geometrical and electronic structures
Significant structural changes from the precursor complexes are not detected in these radical complexes, since the phenoxyl radical complexes are generated as the simple electron transfer reaction products
Summary
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 reactions at the active site of metalloenzymes [1]. The valence state difference as expected for one-electron oxidized Cu(II)-phenolate species can be detected in the X-ray crystal structure. The X-ray structure of this complex revealed that the electronic structure of the ligand is different from that of the free ligand, H2LAP; the C-O bond in complexes is shorter and the ortho-C-C bonds are longer in comparison to H2LAP, which is assignable to the semiquinonate monoradical Findings on such an oxidation state difference were reported early in 2000, though some analogues of this complex had previously been reported [15]. The X-ray diffraction method is sometimes difficult for full understanding of the experimental valence state and the detailed electronic structure, while the recent high resolution analysis possibly reveals the electronic structures, since the small geometrical change upon oxidation can be detected. This review mainly focuses on the relationship between geometrical and electronic structures of one-electron oxidized metal-phenolate complexes
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