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.
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 [
Some oxidative reaction intermediates have been successfully characterized by various methods [2-11], and especially X-ray crystal structure analysis of some intermediates revealed the geometrical change in the reaction and the oxidant binding structure of metal complexes [5-12]. On the other hand, the oxidation state of the metal ions in the active species has not been fully understood, because the oxidation locus on oxidized metal complexes is often different from the “formal” oxidation site [13-15]. The “formal” and “experimental” oxidation numbers are frequently used as synonyms, since the term of the physical or experimental oxidation state has not been accepted in some areas of chemistry.
The valence state difference began to attract attention in early 1990’s from the studies on galactose oxidase (GO) [16-18]. GO is a single copper oxidase, which catalyzes a two-electron oxidation of a primary alcohol to the corresponding aldehyde. The active site structure of the inactive form of GO is shown in
ties of tyrosine residues, and an acetate ion are coordinated to the copper(II) ion [
Conversion to the active form of GO occurs upon one-electron oxidation, since this active form should act as a two-electron oxidant [16-19]. The formal oxidation state of the active form of GO can be described as a Cu(III)-phenolate species. Actually, the active form of GO had been considered to be a copper(III) species [20, 21], but various spectroscopic studies of the active form of GO revealed the formation of the phenoxyl radical species and the Cu(II)-phenoxyl radical bond [
The valence state difference as expected for one-electron oxidized Cu(II)-phenolate species can be detected in the X-ray crystal structure. For example, the formal oxidation number of the central metal ion of the complexes of iminophenolate dianion, (LAP)2− is not always identical with the experimental valence state [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 +II, and two iminophenolate dianions are oxidized to iminosemiquinonate radical anions (LSQ)− (
This review deals with recent advances in the chemistry related with the oxidation behavior of some metalphenolate complexes and their properties, with emphasis on X-ray crystal structures of metal-phenoxyl radical complexes together with high valent metal-phenolate complexes. In general, 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.
Due to the low stability of the uncoordinated free phenoxyl radical, the metal-phenoxyl radical complexes were prepared by one-electron oxidation of the metal-phenolato complexes. The first successful formation of the phenoxyl radical complexes was achieved by chemical and photochemical oxidations of the iron(III)-phenolato complexes (Scheme 1) [
This iron-phenoxyl radical complex was stable for more than one year under the dry air at room temperature, and therefore this complex could be isolated as a powder. The oxidants used for generation of the metal-phenoxyl radical species were unstable monoradical species, such as SO4•–. Use of more stable chemical reagents and electrochemical oxidation later became popular partly due to easy stochiometric handling. For these methods, determination of the redox potential of the radical formation is important. The tyrosyl radical formation potential by one-electron oxidation of the phenol moiety was reported to be 0.94 V vs. NHE [
Scheme 1. The iron(III)-phenoxyl radical complex generated by photochemical and chemical oxidations from the iron(III)-phenolate complex.
Scheme 2. The copper(II)-phenoxyl radical complex generated by addition of chemical oxidant (NH4)2Ce(NO3)6 (CAN) from the copper(II)-phenolate complex.
CAN is one of the strong oxidants with the CeIV/CeIII potential higher than 1.7 V, which indicates that it can fully oxidize metal-phenolato complexes by one electron to form the corresponding higher valent species [
Elelctrochemical oxidation of the metal-phenolate precursors gives the metal-phenoxyl radical complexes [
Free phenoxyl radicals are electron deficient and therefore highly reactive [
Difference between metal-centered oxidation and metalphenoxyl radical species can be detected in X-ray crystal structures. The metal-oxygen bond lengths of high-valent metal-phenolate complexes are shorter than those before oxidation, which is different from the characteristic of meta-phenoxyl radical complexes. The high-valent metalphenolate complexes should have “phenolate” moieties, so that the structural change of the phenolate moiety is rather small in comparison to the phenoxyl radical complexes [
Recently, X-ray crystal structures of some of the metalphenoxyl radical species have been reported, while the examples are still limited [
Scheme 3. Canonical forms of phenoxyl radical.
netic and redox innocent in the classical potential range. 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 (
The first example of X-ray crystal structural analysis of the CuII-phenoxyl radical complex shown in
The X-ray crystal structure analyses of one-electron oxidized group 10 metal salen-type complexes are shown in
However, a close look into the details of the structures
reveals that there are subtle differences between them, and especially the oxidized Pd(II) complexes are different from the other complexes [
aThe bond lengths are described for one of the two salen units of the molecule.
one-electron oxidized Pd(II) complexes can be assigned to relatively localized PdII(phenoxyl)(phenolate) complexes.
However, a close look into the details of the structures reveals that there are subtle differences between them, and especially the oxidized Pd(II) complexes are different from the other complexes [
The Ni and Pt 5-membered dinitrogen chelate complexes also exhibited a clear symmetrical coordination sphere contraction in both the two M-O and two M-N bond lengths (ca. 0.02 Å) upon oxidation, and the C-O bond distances of these complexes are also shorter than the same bonds before oxidation. These observations suggest that the complexes have the phenoxyl radical characteristics and that the radical electron is delocalized on the two phenolate moieties. Indeed, the XPS and K-edge XANES of an oxidized Ni complex showed the same binding energies and pre-edge peak of the nickel ion as those of the complex before oxidation [42,43,45]. These results supported that the valence state of the nickel ion is +II. On the other hand, NiIII complexes of salen-type ligands can be generated by the axial ligand coordination to the oxidized nickel complex. The X-ray crystal structure of the NiIII-salen-type complex, [Ni (salen)]2O, was reported in 2000 by Mitra et al. (
a(A) and (B) indicates the bond lengths of the two independent molecules (A) and (B) in the unit cell.
However, a close look into the details of the crystal structures reveals that there are subtle differences between them, and especially the oxidized Pd(II) complexes are different from the other complexes [42,43]. Comparison of the 5-membered dinitrogen chelate backbones of the Salcn and Salen complexes indicates that upon oxidation the Ni and Pt complexes exhibited a clear coordination sphere contraction due to shortening of the M-O and M-N bond lengths. On the other hand, the Pd complex showed an unsymmetrical contraction [
In the case of Pt complexes, the XPS of the oxidized complex was slightly different from that before oxidation. The binding energies of the Pt ion in the oxidized complex were +0.2 eV higher, and LIII-edge XANES exhibited an increasing white line. Such spectral features suggest that the oxidation state of the Pt ion in the oxidized complex is higher than +II but that the differences are rather small [
The six-membered NiII and PtII Salpn chelate complexes are slightly different from the 5-membered dinitrogen chelate Salcn and Salen complexes [
The electronic structure determinations of the Cu complexes are clearly made by X-ray structure analyses. Structures of four one-electron oxidized CuII salen-type complexes are shown in