Recent advances in metal-phenoxyl radical chemistrynopr.niscair.res.in/bitstream/123456789/11231/1... · SHIMAZAKI & YAMAUCHI: RECENT ADVANCES IN METAL-PHENOXYL RADICAL CHEMISTRY

Indian Journal of Chemistry Vol. 50A, March-April 2011, pp. 383-394

Recent advances in metal-phenoxyl radical chemistry

Yuichi Shimazakia, * & Osamu Yamauchib, * aCollege of Science, Ibaraki University, Mito 310-8512, Japan

Email: [email protected] bFaculty of Chemistry, Materials, and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan

Email: [email protected]

Received 19 August 2010; accepted 16 December 2010

The phenolate ligand is well-known as one of the non-innocent ligands. It is typically shown in biological systems as the ligand of a single copper enzyme galactose oxidase, forming a relatively stable Cu(II)-phenoxyl radical intermediate. On the other hand, some of the one-electron oxidized metal-phenolate complexes are revealed to be high valent metal-phenolate complexes, which are isoelectronic with the metal-phenoxyl radical complexes. Therefore, characterization of the one-electron oxidized metal-phenolate complexes has recently been the subject of intense studies aiming at understanding their oxidation states under the given conditions. This review focuses on metal-phenolate complexes and their one-electron oxidized species and compares the properties of the metal-phenoxyl and high-valent metal-phenolate complexes.

Keywords: Bioinorganic chemistry, Metal-phenoxyl radicals, Phenoxyl radicals, Non-innocent ligands

The redox 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 several studies, a large number of novel complexes have been synthesized and well characterized, such as iron(IV)-porphyrin-π-cation radical,2 high valent non-heme oxo-iron(IV),3 nitrido-iron(V),4 and bis(µ-oxo)dicopper(III) complexes,5 and so on.6 Species formed as a result of redox reactions such as organic and other radicals are important in biological systems. Radicals of amino acid residues, such as tyrosyl, cysteinyl, glycyl and tryptophyl radicals, are known to be formed in the course of enzymatic reactions,7,8 while Cu,Zn- and other superoxide dismutases quench the radical anion superoxide by converting it to dioxygen and hydrogen peroxide.9

The phenoxyl radical from the tyrosyl residue has been well established in metalloenzymes such as class I ribonucleotide reductase and prostaglandin endoperoxide synthase.7,8 It was discovered that the phenoxyl radical can bind with a metal ion as an open-shell ligand funtioning as an organic radical cofactor in a metalloenzyme, galactose oxidase (GO), which is a single copper oxidase catalyzing two-electron oxidation of the primary alcohol to the

aldehyde (Fig. 1).10 Until the discovery of the radical coordination in the active center, the active form of GO had been considered to be the copper(III) species,11 but early in 1990’s, the resonance Raman spectra of the active form of GO revealed the formation of the phenoxyl radical species and the Cu(II)-phenoxyl radical bond.10,12 The proposed reaction mechanism of GO is that the primary alcohol is coordinated to the Cu(II)-phenoxyl radical species generated by molecular oxygen and oxidized by an intramolecular two-electron redox reaction with the hydrogen atom scission from the alcohol moiety to give the aldehyde and the Cu(I)-phenol species

Fig. 1 − Structure of the active site of galactose oxidase (based on ref. 10).

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(Scheme 1).10 The Cu(I)-phenol complex is oxidized by molecular oxygen to regenerate the Cu(II)-phenoxyl radical species. For understanding the detailed mechanism of GO and the properties of the metal complexes with the coordinated phenoxyl radical, many metal-phenolate complexes have been synthesized and characterized.13,14 While studies on oxidative reaction intermediates are in progress, the oxidation state of the metal ions in the active species has not been fully understood until now. Detailed descriptions of the oxidation state of the intermediate are sometimes complicated, because the oxidation locus on oxidized metal complexes is often different from the “formal” oxidation site.15 Although “formal” and “experimental” oxidation numbers are identical in many cases, they are often used as synonyms, since the term of the physical or experimental oxidation state has not been accepted in some areas of chemistry. This practice leads to considerable confusion and presents an obstacle to the progress of the experimental determination of the intermediate. The confusion is easily understandable from the following example. When an open shell compound, that is, a radical ligand is coordinated to the metal ion, we may expect that there would be at

least two possibilities, a metal-open shell ligand complex and a metal-closed shell ligand complex formed as a result of an intramolecular electron transfer between the metal ion and the radical ligand (Scheme 2). However, when the metal-ligand complex is oxidized by one-electron, the metal center oxidation is normally presumed, and the possibility of the ligand-centered oxidation is not always taken into consideration. In this review, we will deal with recent advances in the chemistry related with the synthesis, characterization, and reactivity of some metal-phenoxyl radical complexes, focusing on comparison of their properties with those of the high valent metal-phenolate complexes which are iso-electronic with the phenoxyl radical complexes. Emphasis is laid also on the fact that the electronic structure difference between the metal-centered and phenolate-centered oxidation species leads to the different reactivity toward organic substrates. Generating the metal-phenoxyl radical species:

Oxidation chemistry of the metal-phenolate

complexes

The metal-phenoxyl radical complexes were generated by one-electron oxidation of the metal-phenolate complexes, since the stability of the uncoordinated phenoxyl radical ligands are generally low. The first discovery of the formation of the phenoxyl radical complexes was made by chemical and photochemical oxidations of the iron(III)-phenolate complexes by Wieghardt et al.

16 (Scheme 3). They used as oxidizing agents monoradical species, such as SO4

•–, HPO4•–, and O2

•+, which are not stable. In contrast, the iron-phenoxyl radical complexes could be isolated as a powder, which was stable for more than one year under dry air at room temperature. Use of more stable chemical reagents and electrochemical oxidation later became popular for generation of the phenoxyl radical complexes, because the metal-

Difference between “formal” and “experimental” oxidation numbers

Scheme 2

Proposed mechanism of galactose oxidase (adapted from ref.10)

Scheme 1

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phenoxyl radical complexes were known to be relatively stable. For these methods, determination of the redox potential of the radical formation is important. The one-electron oxidation potential, E, of the tyrosine phenol moiety was reported to 0.94 V vs NHE,17 which is relatively high as compared with the stable metal-centered oxidation potentials for Fe(II)/Fe(III), Co(II)/Co(III), etc.18 The oxidation potentials of the metal coordinated phenolates so far reported are generally lower than those of free phenols, and the potential for generation of the active form of GO is as low as 0.41 V.11b This may be due to the coordination effect of the metal ion. In 1996, the Cu(II) and Zn-phenoxyl radical complexes with similar ligands were reported by Tolman et al.,13,19 who obtained Cu(II)- and Zn(II)-phenoxyl radical complexes by electrochemical oxidation and chemical oxidation using (NH4)2Ce(NO3)6 (usually described as CAN) (Scheme 4). CAN is one of the strong oxidants with the Ce(IV)/Ce(III) potential18 higher than 1.7 V, which indicates that it can fully oxidize metal-phenolate complexes by one electron to form the corresponding higher valent species. Due to its stability this oxidant can be used stoichiometrically, and two-electron oxidized species can be obtained by using two equivalents of CAN. On the other hand, CAN is soluble only in relatively polar solvents such

as acetone and CH3CN, and it cannot be used for oxidation of metal-phenolate complexes dissolved in CH2Cl2. In such cases stoichiometric addition of CAN is attained by adding a small volume of an over 10 times more concentrated solution of CAN in CH3CN to the CH2Cl2 solution of metal-phenoxyl radical complexes. Stack et al.

20,21 reported formation of the radical species by stoichiometric addition of AgSbF6. This oxidant is soluble in CH2Cl2, forming Ag(0) in the course of oxidation. The oxidation potential of Ag(I)/Ag(0) is 0.799 V vs. NHE,18,22 which is higher than the potentials for many metal-coordinated radicals. After the reaction, this oxidant can be removed from the reaction mixture by filtration and is therefore useful for in situ crystallization of the metal-phenoxyl radical species. Phenoxyl radical species were found to be produced from some Cu(II)-phenolate complexes by disproportionation.23,24 The reaction was possible with some limited Cu(II) complexes with an appropriate ligand structure and coordination geometry and was dependent on counter anions, solvents, etc. For example, addition of Cu(ClO4)2•6H2O in CH3CN to a CH2Cl2 solution of the 2N2O-tripodal ligand, H2tbuL2, having two phenol moieties gave the relatively unstable phenoxyl radical species rapidly at low temperature via disproportionation (Scheme 5)23.

The iron(III)-phenoxyl radical generated by photochemical and chemical oxidations (adapted from ref.16)

Scheme 3

Cu(II)-phenoxyl radical generated by addition of 1 eq. of (NH4)2Ce(NO3)6 (adapted from ref.19)

Scheme 4


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This reaction depended on the counter anion; addition of Cu(CH3COO)2 to the ligand afforded the stable Cu(II)(phenolate)(phenol) species, while CuCl2•2H2O used in place of Cu(CH3COO)2 gave the decomposition product of the phenoxyl radical complex. On the other hand, with a 2N2O-tripodal ligand having an ortho-substituted phenolate moiety, a relatively stable Cu(II)-phenoxyl radical complex was formed. Actually, a methoxy substituted phenoxyl radical-Cu(II) complex has been reported to show the properties of catalytic benzyl alcohol oxidation25, the maximum turnover number being ~300. We believe that formation of the stable phenoxyl radical by the Cu(II) ion is prerequisite for the alcohol oxidation mechanism for GO. The metal-phenoxyl radical complexes generated by O2 is important for the understanding of the mechanism of Cu(II)-phenoxyl radical in GO. The activated oxygen species can act as the oxidant for the formation of the phenoxyl radical, and indeed some studies on metal-superoxo intermediate have shown the activity of phenol oxidation.26 In the case of the

metal coordinated phenolate species, the phenoxyl radical formation by O2 may be difficult. Although reactivities of some Cu(I)-phenolate complexes with O2 have been reported,27 generation of the stable metal-phenoxyl radical species by O2 had not been reported until recently. Very recently, however, Van Doorslear et al.

28 reported formation of the Co(III)-phenoxyl radical species from a Co(II)-salen-type complex by aerobic oxidation in the presence of acetic acid (Scheme 6).28 When the acetate ligated Co(II)-salen-type complex was reacted with O2, the stable Co(III)-phenoxyl radical species and hydrogen peroxide were formed. The reaction was proposed to proceed via the mechanism involving a short-lived superoxo intermediate.

General properties of the metal-phenoxyl radical

species Oxidation of metal-phenolate complexes may give the metal phenoxyl radical species or the metal-centered high valent metal-phenolate species depending on various conditions. For example,

Reaction of a 2N2O ligand with Cu(II), and, formation of Cu(II)-phenoxyl radical via disproportionation (based on ref. 23)

Scheme 5

Formation of the Co(III)-phenoxyl radical species by O2 (adapted from ref. 28)

Scheme 6


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oxidation of the V(IV) complex of a diphenolate ligand connected with the triazacyclononane moiety gave the metal-center-oxidized V(V)-diphenolate complex, while oxidation of the Co(III) complex of the same ligand afforded the Co(III)-phenoxyl radical.29 The properties of the metal-centered and ligand-centered oxidation products are quite different. Metal-centered oxidation means that one electron assigned to the metal d electron is removed, so that the spectral properties of the resulting species are based on the high-valent metal complex with some of the phenolate vibrations maintained. On the other hand, one electron removal from the phenolate ligand causes the spectral changes that are different from those due to the metal-centered oxidation. The coordinated phenoxyl radical exhibits absorption peaks at ca. 400 and 600 nm, which are assigned to the π-π* transition of the phenoxyl radical.30 The resonance Raman bands of the phenoxyl radical are different from those of the phenolate moiety, the characteristic bands of the coordinated phenoxyl radical being observed12-14,31,32 at ~1500 cm-1 and ~1600 cm-1. These two bands were assigned to the ν7a and ν8a modes of the phenoxyl radical, respectively, which have some contribution of C–O stretching in C–C vibration of the phenoxyl ring. Since the phenoxyl radical has an open-shell configuration, it exhibits a characteristic radical EPR signal at g = 2.00 when bound to a closed shell metal ion. A clear example of the phenoxyl radical formation can be found in the one-electron oxidized Zn(II)-phenolate complexes, because metal-centered oxidation of Zn(II) complexes does not occur under normal conditions. Zn(II)-phenolate complexes are colorless with no characteristic band in the visible region. Upon one-electron oxidation, they show the intense phenoxyl radical EPR signal at g = 2.00, and their color becomes bright green32 (Scheme 7). Absorption spectra of the Zn(II)-phenoxyl radical complexes

were reported to exhibit the characteristic π-π* transitions of the phenoxyl radical as an intense band around 400 nm and a broad band around 600 nm.31,33

Magnetic properties of the phenoxyl radical bound

to the metal ion with an open shell configuration The behavior of the phenoxyl radical bound to metal ions with an open-shell configuration such as Cu(II) having the d

9 configuration is different from that bound to Zn(II). In general, Cu(II) complexes have some tendency to be oxidized at the metal center to give the Cu(III) complexes, though formation of Cu(III) complexes may be normally rather difficult. Cu(III) complexes have a square-planar geometry and are diamagnetic and EPR silent due to the low-spin d8 configuration. The Cu(II)-phenoxyl radical complex, on the other hand, has two electron spins on different nuclei, and therefore the spin-spin interaction should be considered. The spin-spin interaction between a radical and a 3d-electron in Cu(II) is classified into the ferromagnetic (S = 1) and antiferromagnetic (S = 0) interactions. The magnetic properties depend on the exchange coupling between two unpaired spins, and the super-exchange coupling constant J based on the spin Hamiltonian H = JS1•S2 is important.34 The description that the oxidized Cu(II)-phenolate complex was EPR silent at 77 K is misleading and sometimes causes considerable confusion. The expression “EPR silent at 77 K” does not specify the detailed electronic structure of the oxidized Cu(II)-phenolate complex, since it could refer to any of the cases, antiferromagnetism, ferromagnetism, or diamagnetism. In the case of the relatively weak ferromagnetism, the EPR spectrum at temperatures below 10 K shows characteristic signals around g = 4 and g = 2. The �Ms = 1 transition corresponding to signals around g = 2 is due to the zero-magnetic splitting, and the �Ms = 2 transition around g = 4

One-electron oxidation of a Zn(II)-phenolate complex (based on ref. 32)

Scheme 7


388

means the existence of two unpaired electrons (Fig. 2).35,36 Theoretically these signals can also be observed at 77 K or higher temperatures in the case of a relatively weak antiferromagnetism resulting from Cu(II)-phenoxyl radical complex formation.37 However, such a weak antiferromagnetic interaction between the Cu(II) and phenoxyl radical unpaired electrons has not been reported. Octahedral or trigonal bipyramidal structures of the high-spin d

8 Ni(II) complexes show different EPR signals. The spectra of Ni(III) complexes have in general the anisotropic signals around g =2.2 and 2.0 in frozen solution, due to the separation between the z axis and the x and y components, while the spectra of high spin d8 Ni(II)-phenoxyl radical complexes have been reported to give the isotropic signal at g = 2.2. This result suggests a relatively strong antiferromagnetic spin-spin exchange coupled with the z axis.38 Other metal ions having unpaired electrons have basically magnetic exchange coupling properties in the metal-phenoxyl radical complexes, which leads to characteristic EPR signals.39

X-ray diffraction and absorption of the metal-

phenoxyl radical complexes

Recently, X-ray crystal structures of a number of metal-phenoxyl radical species have been reported, though the examples are still limited. They revealed that the C–C and C–O bond lengths of the phenoxyl moiety are different from the corresponding values of the phenolate complexes.40,41 A Cu(II)-phenoxyl radical complex, shown in Fig. 3, has a relatively localized structure with one of the two phenolate moieties oxidized to the phenoxyl radical.40 The two C–C bonds involving the C–O moiety in the radical are 0.03-0.04 Å longer and the C–O bond is ca. 0.06 Å shorter than the corresponding bonds in the phenolate moiety. Such a trend is also observed in [Pd(tbu-salcn)]SbF6 (Fig. 4), which shows the two C–C bonds involving the C–O moiety that are 0.03-0.04 Å longer, and, the C–O bond that is 0.05 Å shorter than the corresponding bonds in the phenolate complex, [Pd(tbu-salcn)].41 The metal–O bond length of the phenoxyl radical species becomes longer than that of phenolate complex, indicating that the charge density on the oxygen atom decreases in the phenoxyl

Fig. 2 − Temperature-dependent ESR spectral change of the one-electron oxidized [Cu(tbuLmepy)Cl]. (Reproduced from ref. 35 with permission from Japanese Chemical Society, Japan).

Fig. 3 − The first example of crystal structure of the Cu(II)-phenoxyl radical complex.. [(A) Systematic scheme of the delocalization of the radical electron based on the bond lengths; (B) Bond lengths of the Cu(II)-phenolate moiety in the Cu(II)-phenoxyl radical complex; (C) Bond lengths of the Cu(II)-phenoxyl radical moiety in the Cu(II)-phenoxyl radical complex] (adapted from ref. 40).


389

radical. The C–C bonds in the radical complex are distorted, which is similar to the situation in the quinoid form. These characteristics observed for the radical complexes become more pronounced by the p-methoxy substitution in the phenol ring, due to the stable conjugated system involving this group.42,43 The spin density of the p-methoxy substituted phenoxyl radical species can be more localized on the phenoxyl ring as compared with that in the tert-butyl

substituted radical species. Recent results reported by Thomas et al.

43 revealed that the p-methoxy substituted phenoxyl radical Cu(II) and Ni(II) complexes can be described in their canonical forms with different properties (Fig. 5) in contrast to the tert-butyl substituted phenoxyl radical complexes (vide supra). These observations clearly indicate that the structural features of the metal-phenoxyl radical and the metal-phenolate complexes are very different from each other.

Fig. 4 − X-ray crystal structure of [Pd(tbu-salcn)]SbF6. [(A) Full view; (B) Comparison of the bond lengths between the phenolate (black) and phenoxy (red) moieties] (adapted from ref. 41).

Fig. 5 − X-ray crystal structure analysis of Cu(II)-p-methoxyphenoxyl radical complex. [(A) Formation of the Cu(II)-p-methoxyphenoxyl radical complex; (B) Comparison of the bond lengths between the phenolate (black) and phenoxy (red) moieties; (C) The canonical forms of the p-methoxyphenoxyl radical] (adapted from ref. 43).


390

The XAFS experiments of one-electron oxidized metal complexes give useful information on the valence state of the metal ion. In general, the edge and pre-edge peaks can be assigned to the 1s to 3d orbital and 1s to 4p orbital transitions in the metal ion, respectively.44 The energy of transition of the 1s electron increases with the increasing formal oxidation state. However, it is important to note that the 1s, 3d, and 4p orbitals refer to the atomic orbitals, while observation of these transitions is based on the molecular orbitals, that is, the energies depend on their ligand fields.45,46 Such a tendency can be seen also in X-ray photoelectron spectroscopy, but the recent development of the time-dependent density functional theory (TD-DFT) may provide a more detailed electronic structure.47 Recent problems of metal-phenoxyl radical species

Localization and delocalization

The metal-phenoxyl radical complexes are sometimes stabilized by delocalization of the radical electron in the widely spread π-conjugated system. Such delocalization is seen in the salen-type complexes of group 10 metals21,41,48 like Ni(II), Pd(II), and Pt(II) (Fig. 6). Salen-type ligands prefer the square-planar environment, since they have two phenolate moieties connected by the C=N double bonds. All of the group 10 metal-salen-type ligand complexes have been shown to have the d8 low-spin configuration and are therefore diamagnetic before one-electron oxidation. While these complexes have similar spectroscopic properties, the one-electron oxidized species have different properties depending on the metal ion.41,48 The EPR spectra of the oxidized complexes are different; the oxidized [Ni(tbu-salcn)] and [Pd(tbu-salcn)] complexes showed an isotropic signal at g = ca. 2.0. On the other hand, the spectra of Ni(III) and Pd(III) complexes exhibit three parameters with a large g tensor anisotropy (in the case of Ni(III) gav = ca. 2.2) indicating a rhombic symmetry,49,50 which is different from one-electron oxidized

[Ni(tbu-salcn)] and [Pd(tbu-salcn)]. Therefore, the oxidized Ni and Pd complexes are concluded to have a radical electron. These are supported by the XAFS and XPS studies, which indicate that the energy of the pre-edge peak and the binding energies of the Ni and Pd ions in the complexes remained the same after one-electron oxidation. The EPR spectrum of oxidized [Pt(tbu-salen)], on the other hand, showed a large anisotropy due to the effect of the Pt ion. This is in line with the slightly higher binding energies of the Pt ion and the increased white line of the XAS of the oxidized Pt complex and indicates that the valence state of the Pt ion is higher than +2.

The X-ray structures of these complexes revealed the different characters in the solid state41 (Scheme 8). The oxidized Pd complex has a structure characteristic of a relatively localized phenoxyl radical complex; the C–O bond of one of the phenolate moieties is longer than that of the other phenolate moiety, and the phenyl ring involving it is more distorted than the other, indicating a quinoid structure formation. In contrast, the oxidized Ni and Pt complexes are different from the Pd complex and other localized phenoxyl radical complexes. The C–O bond distances of the two phenolate moieties in these complexes are very similar and are slightly longer than the distances before oxidation. The C–C bond distances of the two phenyl rings are similar, and no significant difference in the distances were detected before and after oxidation. These results suggested that the radical electron spin in the oxidized complexes is delocalized between the two phenolate moieties and can be exchanged via the central metal ion. Especially, in the case of the oxidized Pt complex, the slightly higher valence state of the Pt ion after one-electron oxidation may support this spin exchange pathway.

The near infra-red (NIR) transition can be used for consideration of the detailed mechanism of the delocalization. The above mentioned oxidized salen-type complexes show an intense band at ~5000 cm-1, which is assigned to the phenolate to phenoxyl radical (ligand-to-ligand) charge transfer (LLCT) band by TD-DFT analysis. The degree of delocalization can be estimated by Robin-Day classification for understanding the mixed valence system. The classification is categorized in three systems as follows: (1) a fully localized system (class I), (2) a fully delocalized system (class III), and, (3) a moderately coupled system (class II).51 In the case of

Fig. 6 − Structures of M(II) complexes of tbu-salen and tbu-salcn ligands.


391

the fully localized system, there is no characteristic LLCT band in the NIR region, while the fully delocalized system shows an intense NIR LLCT band. On the other hand, the moderate coupling system exhibits a less intense NIR band, which depends on small perturbations such as solvent polarity. The NIR spectrum of the oxidized Pd complex showed a less intense band than the bands of the oxidized Ni and Pt complexes, and this band was the most solvent dependent. These results indicate that the oxidized Pd complex may be slightly closer to the class II moderately coupled system among the oxidized group 10 metal-salen complexes, that is, the oxidized Pd complex is a more localized system. On the other hand, the NIR band of the Pt complex was the highest in intensity and less solvent dependent. These results strongly support the conclusion that Pt complex belongs to the class III delocalization system, which is also supported by all other experimental results. It is therefore obvious that there is a clear difference between the delocalized and localized systems of the phenoxyl radical species.

Metal-phenoxyl radical complexes vs higher valent metal-

phenolate complexes

Although the one-electron oxidized group 10 metal-salen complexes are assigned to the phenoxyl radical complexes, the one-electron oxidized Cu(II)-salen complexes behave differently. The one with the same salen-type ligand as that used for the group 10 metal complexes, [Cu(tbu-salcn)] (Fig. 6), can be assigned to the Cu(III)-phenolate ground state52 (Scheme 9). The X-ray crystal structure analysis revealed that the Cu–O and Cu–N bonds are contracted by one-electron oxidation in contrast to those shown in Fig. 3, while the changes of the C–C and C–O bonds in the phenolate moieties before and after oxidation are rather small. The XPS and XAFS data of this complex were quite different from those observed for the oxidized group 10 metal-salen type complexes. The pre-edge in K-edge XANES of the oxidized complex was more than one electron volt higher, and the binding energies in XPS became higher after oxidation. These results clearly indicate that the oxidized Cu-salen-type complex is mainly a

Properties of oxidized group 10 metal-salen type complexes (adapted from refs 41 & 48)

Scheme 8


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metal-centered oxidation complex, which was concluded to be diamagnetic on the basis of the temperature-dependent magnetic susceptibility measurement. Although EXAFS of the frozen solution of the oxidized Cu(II) complex, [Cu(tbu-salcn)]SbF6, suggested no large structural changes in the solid state, the solution of this oxidized complex showed a slightly different character. The UV-vis-NIR spectrum of the oxidized complex exhibited two characteristic intense bands at 18000 cm-1 (ε = 6500 M-1 cm-1) and at 5700 cm-1 (ε = 2900 M-1 cm-1), which can be assigned to the phenolate-to-Cu(III) charge transfer (LMCT) from the TD-DFT calculation. It was noticed that these LMCT band intensities depend on the temperature reversibly, the intensity decreasing with the temperature increase. Further, the temperature dependent magnetic susceptibility also changes; the number of the spin increasing with the temperature increase. These properties reveal that the one-electron oxidized Cu(II)-salen type complex undergoes a temperature dependent valence tautomerism between the Cu(III)-phenolate and Cu(II)-phenoxyl radical species. The Cu(III)-phenolate:Cu(II)-phenoxyl ratio was estimated to be ca.1:1 at 300 K. The reactivity difference between the phenoxyl radical and higher valent phenolate complexes had not been clear especially when similar ligand systems were used. However, [Cu(tbu-salcn)]SbF6 showed a very low reactivity toward primary alcohol oxidation as compared with the oxidized Cu(II) complex of an unsymmetric salen-type ligand having one of the C=N bonds reduced to a secondary amine –CH2NH–20. Oxidation of this unsymmetric Cu(II) complex generated the temperature independent Cu(II)-phenoxyl radical species. The reaction rate analysis of these two one-electron oxidized complexes revealed

that Cu(II)-phenoxyl radical species initially formed an alcohol adduct in the course of the oxidation. This alcohol adduct intermediate may play an important role in effective catalysis.20 The above mentioned properties of the Cu(II) complexes can be changed by introducing substituents into the phenolate moieties. A localized phenoxyl radical–Cu(II) complex having a p-methoxyphenolate group in the same salen skeleton has been reported very recently.43 The X-ray structure revealed a large difference in the structures of the phenolate moieties, where the methoxy substituted phenolate moiety was considerably distorted, with the C–O bond shorter than that of the other phenolate moiety and that observed before oxidation. This complex was shown to have a weak ferromagnetic interaction between Cu(II) and the phenoxyl radical unpaired electron by the EPR experiments. It maintained the electronic structure in solution, since it exhibited an absorption spectrum with the temperature independent phenoxyl radical π-π* transition at ~400 nm, which is distinct from the spectrum of the valence tautomeric Cu complex. These results suggest that the p-methoxy-substituted phenolate moiety prefers a more localized electronic structure probably due to the conjugated system involving this substituent. As described above, the one-electron oxidized Ni-salen type complex showed the formation of the delocalized Ni(II)-phenoxyl radical species. However, addition of exogenous ligands, such as pyridine, to the solution of the Ni-salen type complexes caused the color change from green to purple.49,53 The equilibrium measurement indicated that two pyridine molecules coordinated to the Ni center to give an octahedral six-coordinated structure with two pyridines as axial ligands. The purple species exhibited a different EPR spectrum with anisotropic

Structure of a Cu(III)-phenolate complex, [Cu(tbu-salcn)]SbF6, and, its properties in CH2Cl2 solution (adapted from ref. 52)

Scheme 9


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signals at g = 2.2 and 2.0, indicating a Ni(III) complex formation.49,50 The pre-edge peak of the frozen solution of the pyridine adduct shifted to a higher energy by one electron volt in XANES, indicating the metal-centered oxidation. In the UV-vis-NIR spectrum, the π-π* band around 400 nm and the LLCT band in the NIR region disappeared, and a new band at 550 nm assigned to the phenolate-to-Ni(III) LMCT appeared. Thus, coordination of exogenous ligands induced conversion of the Ni(II)-phenoxyl radical complex to the Ni(III)-phenolate complex. These observations indicate that coordination environment change is an important factor for determination of the electronic structure. With the complexes of the other group 10 metals, no such change was observed upon addition of exogenous ligands, which may be due to the fact that the M(III) state is an unstable oxidation state.

Conclusions

The chemistry of phenoxyl radical complexes has been widely developed with the detailed determination of the valence state in recent years. Metal-phenoxyl radical complexes showed experimental properties different from those of their higher valent metal-phenolate complexes with the same formal oxidation number. The UV-vis spectrum of the phenoxyl radical species exhibits the characteristic phenoxyl radical π-π* bands around 400 and 600 nm. The resonance Raman spectra of the metal-phenoxyl radical complexes clearly indicate some characteristic vibrations of the metal-phenoxyl radical species. The X-ray crystal structure analysis supported the conclusions from the Raman spectra, indicating that the phenoxyl radical moiety is more distorted in comparison with the phenolate moieties in metal-phenolate complexes. The metal ions having unpaired electrons undergo magnetic coupling with the radical electron of the phenoxyl radical complex, and therefore the EPR spectra and the temperature-dependent magnetic susceptibilities are distinct from those of the higher valent metal-phenolate complexes. The phenoxyl radical complexes are stabilized by a relatively large π-conjugated system especially in some salen-type complexes, and the radical electron is delocalized over the two-phenolate moieties via the central metal ion as seen in the Ni and Pt complexes. Recently, a Cu(III)-phenolate complex in the ground state has been reported; the difference between Cu(II)-phenoxyl radical and Cu(III)-phenolate complexes has been made clearer, showing that many

physical properties are different. Understanding the detailed electronic structure of the oxidized phenolate moiety may give further insights into the reactivity difference among the metal-phenoxyl radical complexes. In this review we focused on recent studies on one-electron oxidized metal-phenolate complexes and comparatively described their properties including their possible catalysis and biological relevance. These findings suggest their structural and functional possibilities in coordination chemistry yet to be explored.

Acknowledgement

The authors are grateful to Prof. Fumito Tani, Kyushu University, Japan, Prof. Tim Storr, Simon Fraser University, Canada, Prof. T Daniel P Stack, Stanford University, USA, and, Prof. Yasuo Nakabayashi and Prof. Tatsuo Yajima, Kansai University, Japan, for helpful comments and suggestions during preparation of this manuscript.

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