Metal–Oxyl Species and Their Possible Roles in Chemical Oxidations

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Metal−Oxyl Species and Their Possible Roles in Chemical Oxidations Yoshihiro Shimoyama†,‡ and Takahiko Kojima*,† †

Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan

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‡

ABSTRACT: Metal−oxyl (Mn+-O•) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal−oxo (M(n+1)+O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of Mn+-O• species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of Mn+-O• complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the Mn+-O• species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.

1. INTRODUCTION In biological and chemical oxidation reactions, high-valent metal−oxo (MO) complexes have been known to act as reactive species.1−12 So far, the first-, second-, and third-row transitionmetal complexes with oxo ligands have been extensively studied with strong concerns on the structures,13−20 electronic structures,21−25 and reactivity in oxidation reactions of not only organic substrates26−34 but also inorganic substrates including water.35−39 As previously described by Winkler and Gray, the “oxo wall” has been known to lie between the group 8 and 9 elements in the Periodic Table.40 As for a metal−oxo complex in C4v symmetry and the d4 electronic configuration, the complex can form a metal−oxo double bond; however, in the same symmetry, dn (n ≥ 5) metal ions cannot afford metal−oxo species with double bonds because of occupation of the π* orbitals of the MO bonds. Although some d5 and d6 metal−oxo complexes are in C3v, C2v, and C∞v symmetry, these can form a metal−oxo double bond without obeying the “oxo wall” principle.40b In light of this concept, the elements earlier than group 8 in the tetravalent state can form double or triple bonds with an oxo (O2−) ligand.41 On the contrary, the elements later than group 9 in the tetravalent state cannot form multiple bonds and just form a single bond to afford metal−oxyl (M−O•) complexes because of occupation of the π* orbital of the M−O bond.42,43 As for the metal−oxo complexes, the structures, properties, and reactivities in substrate oxidation reactions have been intensively investigated. For example, FeIV−oxo complexes bearing porphyrin π-radical cations as ligands have been investigated in light of modeling reactive species in oxidation reactions catalyzed by heme enzymes such as horseradish peroxidase (HRP)44 and cytochrome P-450.45 In addition, nonheme iron enzymes such as α-ketoglutarate-dependent taurine dioxygenase (TauD) have also been reported to generate FeIV−oxo intermediates as responsible species in oxidation reactions.46 After a breakthrough report by Nam, Que, and their co-workers on the crystallographic characterization of a FeIV−oxo complex with a cyclic tetramine ligand,47 a number of FeIV−oxo complexes have © XXXX American Chemical Society

been prepared and characterized to shed light on the characteristics and reactivities of those complexes.48 Not only FeIV−oxo complexes but also other metal−oxo complexes including CrV− oxo49 and MnV−oxo50 complexes have been studied as well. Other than the first-row transition-metal ions, high-valent Ru−oxo complexes have been studied intensively. Major reasons for the investigation on Ru−oxo complexes should be the stability of the complexes to study the structures and reactivities in detail.51 After the first report on the formation of a RuIV−oxo complex through proton-coupled electron-transfer (PCET) oxidation of the corresponding RuII−aqua precursor complex in water by Meyer and Moyer,52 the procedure has been widely applied to various metal−oxo complexes including iron and manganese complexes. Formal hydrogen-atom transfer (HAT) can be classified mechanistically as mentioned below into genuine HAT in a radical reaction and PCET.53 PCET reactions can be elucidated in accordance with a thermochemical square scheme, as shown in Figure 1a:54 PCET processes involve proton transfer followed by electron transfer, electron transfer followed by proton transfer, and concerted proton−electron transfer. Genuine HAT involves a single atomic site acting as a hydrogen-atom acceptor from a hydrogen-atom donor (Figure 1b). In PCET, however, the proton and electron are transferred to different atoms to accomplish a formal HAT reaction (Figure 1c). Therefore, in hydrogen-atom abstraction by the metal−oxo species, the proton-acceptor site is the oxo ligand that is a formal O2− ion and the electron-acceptor site is the metal center in affording the corresponding one-electron-reduced metal− hydroxo species, which can be classified as the PCET mechanism. On the other hand, hydrogen-atom abstraction by reactive species having spin density at the terminal oxygen atoms such as the metal−oxyl species can be classified as the HAT reaction because of the consistency of the acceptor site of the proton and electron. Received: December 12, 2018

A

DOI: 10.1021/acs.inorgchem.8b03459 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Thermochemical PCET square scheme for Y and X−H. The horizontal arrows represent electron-transfer (ET) processes, and the vertical ones represent proton-transfer (PT) processes. (b) Schematic representation of genuine HAT from H−X to Y•. (c) Schematic representation of PCET from H−X to M−L.

In terms of metal−oxyl complexes, however, there has been no rigorous definition for classification of the HAT/PCET mechanism. It should be noted that, concerning the reactivity of hydrogen-atom-accepting entities in HAT such as metal−oxo species, it has been commented that spin states of hydrogenatom abstractors and the amount of the unpaired spin density on the abstracting atom do not affect the reactivity of the active species in formal HAT.27b,55 In a very narrower definition of HAT, which is not separated as H+ and e− from the “same bond” of a hydrogen-atom donor to an abstractor such as oxyl radicals including •OH and •OR, there should be a significant influence of the spin density at the oxygen atom upon a HAT process, as mentioned by Mayer and Saouma.27b Besides a vast number of papers on high-valent metal−oxo complexes that have been published, metal−oxyl complexes, which are electronically equivalent species of the corresponding metal−oxo complexes with one-electron-reduced metal centers, have not been reported as often. Thus, examples of metal−oxyl complexes are limited; however, the species should exhibit different reactivities in substrate oxidation reactions compared to the metal−oxo complexes. We will provide a landscape of the transition-metal−oxyl complexes as potential reactive species with specific reactivity in oxidation reactions. As for main-group elements, an oxygen ligand binds to the metal center in the highest valence states (AlIII, MgII, CaII, etc.); fulfilling the octet rule required in electronic configurations of the corresponding noble gases, the valency of the metal center does not change any more in most cases. Therefore, oneelectron oxidation of main-group metal oxide species is expected to afford metal−oxyl species having oxygen-centered radicals, which enables one to discuss the reactivity of metal−oxyl species.56 These characteristics allow easy access to information about HAT reactions as mentioned above. In this Viewpoint, however, we will focus on transition-metal−oxyl species and will not mention main-group metal−oxyl species.

Figure 2. Schematic descriptions of the resonance structures of metal− oxo and metal−oxyl species with their corresponding Lewis structures. A red circle in part a represents a hole on the oxygen ligand.57

high-valent Ru−oxo complexes,51 elongation of the M−O bond may be expected for the induction of metal−oxyl character to reduce π-bonding interactions (Figure 2b).

3. EXPERIMENTALLY CONFIRMED METAL−OXYL SPECIES Several metal−oxyl species have been satisfactorily characterized by various spectroscopic methods and supported by density functional theory (DFT) calculations. The following species can be recognized as satisfactorily characterized metal−oxyl species. The reactivities of the complexes will be also mentioned. 3.1. Zinc−Oxyl Species. Recently, Oda and co-workers have reported the formation of a ZnII−oxyl (ZnII−O•) species stabilized in the MFI-type zeolite (Figure 3).58a As the precursor of the ZnII−O• species, a ZnII-η2-ozonido (ZnII−η2-O3•−) species has been formed by the reaction of dioxygen with a ZnII− hydrido species, which is prepared by the ZnII-exchanged MFI with dihydrogen via the H−H bond heterolysis, under UV irradiation. The ZnII−ozonido species can be converted to the ZnII−O• species by removing dioxygen under vacuum at room temperature. The ZnII−O• species also reacts with dioxygen to form the ZnII−ozonido species reversibly. The ZnII−O• species has been characterized by various spectroscopic methods and DFT calculations. As shown in Figure 4, the species shows vibronic absorption bands around 12000 cm−1 (∼830 nm) in the UV−vis−near-IR (NIR) spectrum. The weaker second band assigned to the E0−1 vibronic transition shows an isotropic shift upon 18O labeling, and the separation changes from 605 to 575 cm−1; the isotropic ratio is 1.05, consistent with the value calculated for ν(Zn−16O)/ ν(Zn−18O) = {μ(Zn−18O)/μ(Zn−16O)}1/2 = 1.05. This agreement supports the assignment of the absorption bands to the vibrational Franck−Condon progression in the Zn−O stretching mode. DFT calculations has been applied to optimize the ground state of ZnII−O• species in the MFI models, which consist of [Si91Al1O151H66]− (Figure 5, left) and [Si2Al1O4H8]−

2. DEFINITION OF A METAL−OXYL SPECIES As a contributing structure of a metal−oxo species having a double bond between the metal center and oxygen ligand, a metal−oxyl species should have nearly one unpaired electron or hole on the oxygen ligand and a metal center in a nearly oneelectron-reduced oxidation state compared to the counterpart, as shown in Figure 2.57 In the case of the oxyl ligand, the oxygen atom bound to the metal center does not satisfy the octet rule to possess formally seven electrons, as shown in Figure 2a. In this case, the π-bonding character of the metal−oxygen bond is reduced to single-bond character. It should be mentioned that the ratio of the contribution of each resonance structure influences the degree of oxyl character of the oxo ligand. Although the oxidation state of the metal center in the metal−oxygen species is not directly related to the M−O bond length, as observed for B


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Figure 3. Schematic description of the formation of a ZnII−oxyl species from a ZnII−η2-O3•− species.58a

Figure 4. UV−vis−NIR vibronic absorption spectra of the ZnII−O• complex formed in the MFI-type zeolite framework. (a) NIR regions for the Zn−16O• (top) and Zn−18O• species (bottom). (b) Schematic description of vibronic transitions of the ZnII−O• species. The dotted lines in part a represent the individual Gaussian contribution of the corresponding transition. This figure has been provided through a courtesy of Dr. A. Oda [PREST (JST)/Okayama University].

Figure 5. Optimized structures of ZnII−oxyl species in the MFI models: in the [Si91Al1O151H66]− (a) and [Si2Al1O4H8]− (b) frameworks. This figure has been provided through a courtesy of Dr. A. Oda [PREST (JST)/Okayama University].

(Figure 5, right). The ZnII−O• moiety is coordinated by μ-oxo ligands bridging silicon and aluminum centers, as shown in Figure 5. The Zn−O bond lengths have been calculated to be 1.85 Å for part a and 1.86 Å for part b, respectively. Because the zinc(II) center in the d10 electronic configuration does not allow the formation of a ZnIIO double bond, the spin distribution is completely localized on the oxygen ligand, suggesting oxyl

character. Recently, Oda and co-workers have also reported on noncatalytic methane oxidation using the ZnII−O• species to form methanol in 94% selectivity at room temperature.58b 3.2. Ruthenium−Oxyl Complexes. Tanaka and co-workers have reported the first and structurally characterized example of metal-bound oxyl radical species, RuII−oxyl (RuII−O•) complexes formed from ruthenium complexes having polypyridyl C


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Figure 6. Deprotonation of the RuIII−OH2 complexes to generate the corresponding RuII−O• species.59

and semiquinonato ligands, [RuIII(terpy)(4ClSQ)(OH2)] (RuIIIClSQ−OH2; terpy = 2,2′:6′,2″-terpyridine and 4ClSQ = 4-chloro-1,2-benzosemiquinonato) and [RuIII(terpy)(DBSQ)(OH2)] (RuIIIDBSQ−OH2; DBSQ = 3,5-di-tert-1,2-benzosemiquinonato) (Figure 6).59 Upon the addition of a strong base such as KOtBu into a CH2Cl2 solution of RuIIIClSQ−OH2 or RuIIIDBSQ−OH2, UV−vis spectral changes due to deprotonation from the aqua ligand of the corresponding Ru III −OH 2 complexes have been observed. On the basis of electron paramagnetic resonance (EPR), resonance Raman (rR), and X-ray photoelectron spectroscopy (XPS) analyses, the deprotonated products have been revealed to be [RuII(terpy)(DBSQ)(O•)] (RuIIDBSQ−O•) and [RuII(terpy)(4ClSQ)(O•)] (RuIIClSQ− O•). The RuII−O• complexes can be reversibly protonated with the addition of 2.0 equiv of HClO4 to recover the corresponding RuIII−OH2 complexes. The EPR spectrum of RuIIDBSQ−O• at 3.9 K exhibited an isotropic broad signal at g = 2.054 assigned to the Δms = 1 transition with a fine structure of |D| = 0.020 cm−1 and |E| = 0.005 cm−1 and an isotropic signal at g = 4.18 due to the forbidden Δms = 2 transition, indicating the triplet state of a biradical compound. On the basis of the |D| and g values, the spin−spin distance has been estimated to be 5.09 Å, which is consistent with the distance between the oxyl and DBSQ ligands. In the rR spectrum of RuIIDBSQ−O• in CH2Cl2 upon photoexcitation at 704.3 nm, four peaks correlated with the ν(Ru−O) bands, which are coupled with the ν(C−C) bands of the DBSQ ligand, can be observed at 503, 521, 556, and 590 cm−1. The RuII−O• species RuIIDBSQ-O• has been reported to be stable enough to form a single crystal suitable for X-ray crystallography. In the crystal structure of RuIIDBSQ−O• (Figure 7), the Ru−O• bond length was 2.043(7) Å, indicating a single-bond character of the Ru−O bond.59a The reaction of RuIIDBSQ−O• with a spin-trapping reagent, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), affords a spin adduct showing an EPR signal with hyperfine splitting (hfs) derived from nitrogen and hydrogen nuclei at g = 2.006 at 193 K in CH2Cl2. This result supports the strong radical character of the RuII−O• complex.59b The RuII−O• species have been applied to water oxidation60 and organic substrate oxidation by a dinucleating strategy to connect the two RuII−O• complexes mentioned above in a close

Figure 7. ORTEP drawing of the RuII−O• species RuIIDBSQ−O•.59a All hydrogen atoms are omitted for clarity. This figure has been provided through a courtesy of Prof. K. Tanaka and Dr. K. Kobayashi [Kyoto University].

distance.61 The reaction mechanisms have been investigated experimentally or theoretically. In the water oxidation, Tanaka and co-workers have used quinone or 2,2′-bipyridine as bidentate ligands for a comparison of the water oxidation ability.60b Cyclic voltammograms of a semiquinonato complex, [RuII2(btpyan)(Bu2Sq)2(OH)2]2+ [Ru2SQ; btpyan = 1,8-bis(2,2′:6′,2″terpyridyl)anthracene and Bu2Sq = 3,6-di-tert-butyl-1,2-benzosemiquinonato], and a 2,2′-bipyridine complex, [RuII2(btpyan)(bpy)2(OH)2]2+ (Ru2bpy), in the presence of water (10% v/v) in a CF3CH2OH/ether (1/1, v/v) solution showing large catalytic currents at potentials more positive than +1.0 V have been observed in both cases. In particular, the controlled-potential electrolysis of Ru2SQ at +1.70 V evolved 0.69 mL of dioxygen in 91% current efficiency. Water oxidation using indium−tin oxide (ITO) electrodes modified with Ru2SQ or Ru2bpy showed a sharp contrast to each other in their catalytic reactivities. The controlled-potential electrolysis of Ru2SQ supported on an ITO electrode at +1.70 V in acidic water, 1.5 mL of dioxygen with the current efficiency of 95%, and a turnover number of 2400 has been formed after 27.5 C passed in the electrolysis, whereas electrolysis of Ru2bpy showed only a negligible amount of dioxygen evolution. Although it has been concluded that the distinct difference in the water oxidation abilities is associated with the function of the redox-noninnocent character of the quinone D


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Figure 8. Plausible mechanism of O−O bond formation in Ru2SQ proposed by Tanaka and co-workers.60e

Figure 9. Oxidation of hydrocarbons by di- and mononuclear RuIII−hydroxo−quinone complexes (Ru2Q and RuQ) in the presence of AgClO4 and tBuOK.61

ligand, the significance of an oxygen-centered radical has been investigated by theoretical analyses as described below. It is easily considered that two RuII−O• species located closely in the ligand framework will enable radical coupling between these two oxyl radicals to form an O−O bond. However, upon the addition of 2 equiv of tBuOK to deprotonate from hydroxo ligands in Ru2SQ, a decrease of absorption at 576 nm and the growth of a new band at 850 nm, which are indicative of the generation of RuII−O• species, are observed. This result indicates that a tetraradical complex, [RuII2(btpyan)(Bu2Sq)2(O•)2], still remains as a stable compound without undergoing O−O bond formation despite its remarkable catalytic activity toward water oxidation.60f The most plausible mechanism of O−O bond formation is considered to be the radical coupling between two oxyl radical species induced by spin inversion (SI) at the ruthenium(III) centers based on theoretical studies, as shown in Figure 8. Because the tetraradical species are in a local triplet diradical (LTD) state, facile O−O bond formation is suppressed, as explained by Hund’s rule. Then, further two-electron oxidation of [RuII2(btpyan)(Bu2Sq)2(O•)2] will afford the open-shell ruthenium(III) complex [RuIII2(btpyan)(Bu2Sq)2(O•) 2]2+ having six unpaired electrons. The spins on these ruthenium(III) centers will undergo facile SI because of the heavy-atom effect, which resulted in the formation of a local singlet diradical (LSD) state that proceeds radical coupling between the two oxyl radical species to form the O−O bond (Figure 8).60e

The reactivities in substrate oxidation have been compared between dinuclear [RuII2(OH)2(Q)(btpyan)]2+ [Ru2Q; Q = 3,6di-tert-butyl-1,2-benzoquinone and btpyan = 1,8-bis(2,2′:6′,2″terpyridyl)anthracene] and mononuclear [RuII(OH2)(Q)(Phterpy)]2+ (RuQ; Ph-terpy = 4′-phenyl-2,2′:6′,2″-terpyridine).61 The RuII−O• complexes as reactive species can be formed from both complexes by using Ag+ as a mild oxidant and t-BuOK as a base, as shown in Figure 9. Ru2Q could oxidize 1,3-cyclohexadiene and 1,2-dihydronaphthalene to afford benzene and naphthalene within 1 min, whereas RuQ could oxidize these substrates in low yields. On the other hand, Ru2Q cannot oxidize 9,10dihydroanthracene (DHA) at all, whereas RuQ shows relatively high reactivity in the DHA oxidation to afford anthracene in 42% yield. The inability of Ru2Q to oxidize DHA apparently results from steric hindrance in the approach of the substrate to the oxo group in the cavity of the dimeric linkage. Recently, Kojima and co-workers have reported a novel RuIII−oxyl (RuIII−O•) complex, an electronically equivalent structure of the corresponding RuIV−oxo (RuIVO) complex, formed via PCET oxidation of a RuII−aqua (RuII−OH2) precursor complex having an N-heterocyclic carbene (NHC) ligand, 1,3-bis(2-pyrdylmethyl)imidazolin-2-ylidene (BPIm), in acidic water (Figure 10).62 Generally, in light of the “oxo wall” paradigm, it has been recognized that a double bond should be formed between the d4 ruthenium(IV) center in the octahedral geometry and the oxo ligand.51 The key point of formation of the RuIII−O• complex is the strong trans E


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Figure 10. PCET oxidation of the RuII−OH2 complex to afford a RuIII−O• species.62

influence of the NHC ligand bound to the position trans to the oxo ligand because it has been widely known that a strongly σ-donating ligand such as NHC makes coordination of a ligand at the trans position of the NHC moiety weaker because of its trans influence.63 This concept has been applied to elongate the Ru−O double bond in a RuIVO complex to generate the RuIII−O• complex. Characterization of the RuIII−O• complex has been conducted by several spectroscopic analyses. In the rR spectra, a Raman band derived from a stretching vibration of the Ru−O bond is observed at 732 cm−1, which is shifted at 696 cm−1 in 18 O-labeled water. The value of 732 cm−1 is the lowest value in comparison with those of any other RuIVO complexes reported so far, which indicates a significantly weaker Ru−O bond in the RuIII−O• species. In X-ray absorption spectroscopy, an energy shift of a half-height energy of the Ru−K absorption edge has been reported to be 1.5 eV from the corresponding RuII−OH2 to the RuIII−OH2 complex. On the other hand, the energy shift from RuIII−OH2 to RuIII−O• is only 0.5 eV. These results indicate that the valency of the ruthenium center is almost 3+ rather than 4+. The characteristics of the RuIII−O• complex could be revealed by DFT calculations at the UB3LYP/SDD (ruthenium) and D95** (carbon, hydrogen, nitrogen, and oxygen) levels of theory. In the optimized structure of RuIII−O• species in the most stable triplet state, the spin density has been estimated to be 0.96 at the ruthenium atom and 1.03 at the oxygen atom; thus, the species can be described as a RuIII−O• biradical. In addition, Mayer bond order (MBO) analysis has allowed us to estimate the bond order of the Ru−O bond as 1.3, supporting the single-bond character of the Ru−O bond. The radical character of RuIII−O• species also reflected the catalytic oxidation of organic substrates in acidic water. The obvious difference of the reactivity was observed in benzaldehyde oxidation in water. Almost all RuIVO complexes having polypyridyl ligands could not oxidize benzaldehyde and its derivatives.64 However, the RuII(NHC)−aqua complex can catalyze oxidation of benzaldehydes effectively using (NH4)2[CeIV(NO3)6] (CAN) as a sacrificial oxidant in acidic water (pH ∼0.6) at 278 K (Figure 11).61 The results indicate the high reactivity of the RuIII−O•

Figure 12. Hammett plots for the oxidation of benzaldehyde derivatives: the RuIII−O• complex in water (blue line)62 and [RuIV(O)(bpy)2(py)]2+ in CH3CN (black line).65

substituents is observed on the initial rates of catalytic oxidation of para-substituted benzaldehydes (blue line in Figure 12). In contrast, a relatively negative slope (ρ = −0.65) has been observed for the oxidation of benzaldehydes by [RuIV(O)(bpy)2(py)]2+ as a reactive species in acetonitrile (MeCN; black line in Figure 12).62,65 These results indicate that the catalytic benzaldehyde oxidation proceeds not via nucleophilic attack of an oxyl ligand to the formyl group (ρ > 0), not via a polarized transition state (TS; ρ < 0), but via a nonpolarized radical mechanism. In addition, the RuIII−O• species has been revealed to show unique reactivity, which is totally different from that of RuIVO complexes, that is, the oxidative cracking of aromatic rings. In acidic water (pH 1.6 or pD 1.2) at 283 K, benzene has been catalytically converted to formic acid (HCOOH) and CO2 in the presence of the RuII−OH2 complex shown in Figure 13 as a

Figure 13. Catalytic oxidative benzene cracking in water using the RuII(NHC)−aqua complex (Figure 10) as a catalyst and CAN as an oxidant at 283 K. Figure 11. Oxidation of benzaldehydes by the RuII(NHC)−aqua complex using CAN as an oxidant.

catalyst and CAN as an oxidant.66 The origin of HCOOH has been confirmed to be benzene by 2H NMR spectroscopy using C6D6 as a substrate to observe DCOOH under the same conditions. HCOOH formed in the reaction has been used as a substrate to generate dihydrogen from the reaction mixture using [RhIII(Cp*)(bpy)(H2O)]2+ (Cp* = pentamethylcyclopentadienyl)67 as a catalyst after simple pH adjustment to pH 3.3 by adding an aqueous NaOH solution. The scope and limitation of aromatic substrates have been examined. Benzene derivatives such as toluene, biphenyl, naphthalene, and anthracene can be easily converted to HCOOH. It should be noted that monosubstituted benzene derivatives having benzylic

complex in the oxidation of organic substrates compared to RuIVO species. The catalytic oxidation proceeds via HAT from the C−H bond of the formyl group to the RuIII−O• moiety, which should be involved in the rate-determining step (RDS), as indicated by the kinetic isotope effect (KIE) on the oxidation of benzaldehyde-d1 (kcatH/kcatD = 8.0). In addition, the electronic effect on the oxidation of para-substituted benzaldehyde by RuIII−O• showed a sharp contrast to that by RuIVO in the Hammett plot (Figure 12). In the case of RuIII−O•, no electronic effect (ρ = −0.07) of the F


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Inorganic Chemistry C−H bonds have been oxidized to afford the corresponding carboxylic acids holding substituents without oxidation of the benzylic positions (Figure 14). In the case of benzenesulfonate

Figure 14. Change of the selectivity in benzene derivatives by the RuII−OH2 complex in Figure 10 in water using CAN as an oxidant.66 Figure 16. (a) Water-adsorbed n-SrTiO3 surface (Ow, oxygen of adsorbed water; Oh, hydroxide). (b) Surface after photoexcitation. Changes of the electron density are described in yellow for a decrease and in cyan for an increase. Reprinted with permission from 68b. Copyright 2016 Springer Nature Publishing.

derivatives, because of the strong electron-withdrawing effect of the sulfonate group to make the aromatic ring electron-deficient, oxidation is switched to occur at the benzylic position (Figure 14).62,66 Kinetic analysis on the oxidative benzene cracking has allowed us to conclude that the reactive species, the RuIII−O• complex, exhibits strong electrophilicity toward aromatic rings rather than the hydrogen-atom abstraction from benzylic positions or ring C−H bonds. The electrophilicity has been supported by a Hammett plot showing a ρ value of −1.41 for the oxidation of monosubstituted benzene derivatives.66 The reaction mechanism of the oxidative benzene cracking has been proposed, as shown in Figure 15, on the basis of experimental results.66 The RuIII−O• species attacks the aromatic ring to form benzene oxide, which undergoes rearrangement to afford oxepin. As in the NMR spectrum of the reaction mixture, 1,4-benzoquinone and muconic acid have been observed as intermediates to generate HCOOH. This unique reactivity of the Ru−O• species may contribute to the development of a method to produce an energy source from environmentally harmful aromatic wastes in water.

a TiIII−oxyl species (TiIII−O•) has been detected by ultrafast in situ attentuated-total-reflectance IR spectroscopy at an interface with water (Figure 16b).68 Upon photoexcitation at 296 nm by a p-polarized (perpendicular to the surface) amplified Ti:sapphire laser beam, a signal was observed at 795 cm−1, which was assigned to the Ti−O• bond vibration on the basis of the known range of Ti−O stretches. The formation of the transient Ti−O• species is derived from photoinduced charge separation. The species survives for >1 ns; however, the lifetime is shortened by increasing the methanol concentration. Also, the Ti−O• species having a hole is indicated to be active in the water oxidation to evolve dioxygen. DFT calculations on the Ti−O• species at the water interface has been applied to elucidate the characteristics of the Ti−O• formed, indicating that the hole is located at Ox (see Figure 16b68b) in the lowest energy by 0.46 eV. 4.2. Nickel−Oxyl Complexes. Because nickel belongs to group 10 in the Periodic Table, in light of “the oxo wall”, Ni−oxyl complexes should be preferably formed rather than Ni−oxo complexes. Recently, the preparation and spectroscopic characterization of NiIII−oxyl (NiIII−O•) complexes have been reported. Company and co-workers have reported on the formation of a

4. PARTIALLY CHARACTERIZED METAL−OXYL SPECIES The following metal−oxyl species have been proposed on the basis of some nondefinitive spectroscopic evidence. Mainly, the arguments have been made on the basis of DFT calculations. 4.1. Titanium−Oxyl Species. In the course of water oxidation by 0.1% niobium-doped perovskite n-SrTiO3 (Figure 16a),

Figure 15. Proposed mechanism of oxidative cracking of benzene by the RuIII−O• complex.66 G


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generated by the electron spray ionization of CuII(ClO3)2 in CH3CN, using infrared photodissociation (IRPD) spectroscopy and its Franck−Condon analysis.70 In the IRPD spectrum of the species at 5 K, the Cu−O stretching band can be observed at 693 cm−1, which shifts to 672 cm−1 upon 18O labeling at the ground state. DFT calculations have been applied to optimize the structure at the B3LYP/6-311+G** level and to elucidate the electronic structure using the multistate restricted active space second-order perturbation theory (MS-RASPT2). DFT analysis indicates that [(CH3CN)CuO]+ involves a contribution from 56% of a triplet biradical made of a copper(I) center having the triplet oxygen atom and 36% of the CuII−O• configuration. 4.4. Ruthenium−Oxyl Complexes. Kojima and co-workers reported a series of ruthenium(III) complexes having tris(2pyridylmethyl)amine (TPA) derivatives and the catalytic reactivity toward cyclohexane by using HmCPBA as a terminal oxidant.71 All complexes catalyzed cyclohexane oxidation to afford cyclohexanol (CyOH) and cyclohexanone (CyO). Attention has been focused on the reactions of RuIII−TPA with unsubstituted TPA and RuIII−TPA2COOEt with TPA having two electronwithdrawing ethoxycarbonyl (COOEt) groups (Figure 18). These complexes show redox potentials of −0.26 V (vs Fc/Fc+, RuII/RuIII) and 1.34 V (RuIII/RuIV) for RuIII−TPA and −0.05 V (RuII/RuIII) and 1.53 V (RuIII/RuIV) for RuIII−TPA2COOEt, respectively, which reflects that the electron density at the ruthenium center is governed by substituents on the pyridine rings. The KIE values of CyOH formation by RuIII−TPA and RuIII−TPA2COOEt were determined to be 3.1 and 4.3, respectively. These KIE values are as low as those of hydrogen-atom abstraction by free radicals (5.6−6.3),72 which indicates strong oxidation by RuIII−TPA, the production of CyOH and CyO showed induction periods, and both products were formed at comparable rates. On the other hand, the reaction catalyzed by RuIII−TPA2COOEt proceeded efficiently without any induction period, in sharp contrast to that of RuIII−TPA. In the isotopelabeling experiment in the presence of H218O, only 9% of 18 O was incorporated into CyOH in the case of RuIII−TPA, whereas 100% 18O incorporation was observed in the case of RuIII−TPA2COOEt. These results indicated that the reaction of RuIII−TPA gave free-radical species derived from HmCPBA, followed by autoxidation of cyclohexane, whereas that of RuIII− TPA2COOEt gave radical character of the reactive species.73 During catalysis, cyclohexane with HmCPBA affords a Ru−oxo species as a reactive species in cyclohexane oxidation. This reactive species generated by the reaction of RuIII−TPA2COOEt with HmCPBA in CH3CN was characterized by rR spectroscopy. A Raman band derived from the Ru−O stretching frequency was observed at 752 cm−1 in the presence of H216O, which completely shifted to 708 cm−1 in the presence of H218O. This value of 752 cm−1 was relatively weak compared to those reported for RuIVO or RuVO complexes, indicating a lower Ru−O bond energy. Such a high reactivity with a KIE value of 4.3, the feasibility of quantitative isotopic exchange, and the lower energy of Ru−O

Figure 17. Reaction of a nickel(II) complex with mCPBA to form a NiIII−O• complex.69

NiIII−O• complex in the reaction of a nickel(II) precursor with HmCPBA in CH3CN at −30 °C, as depicted in Figure 17.69 In the course of the reaction, the nickel(II)-bound mCPBA− ligand undergoes heterolytic O−O bond cleavage to generate an oxidatively active species, which is proposed to be formulated as a NiIII−O• complex. The NiIII−O• complex has been characterized by X-ray absorption, rR, and EPR spectroscopies. Note that, however, the putative NiIII−O• complex is not selectively formed (35% yield) and thus not completely characterized. In particular, X-ray absorption spectroscopies should not be conducted on a complex mixture of nickel species to discuss the valency of the metal center and M−O bond distances. The dark-yellow NiIII−O• complex survives for 4.5 h at −30 °C while showing an absorption maximum at 420 nm and a shoulder at 520 nm; however, the reaction mixture contains other nickel(III) species to hamper the convincing characterization of the NiIII−O• complex. The Ni−O distance has been estimated to be 2.12 Å using extended X-ray absorption fine structure (EXAFS) analysis. DFT calculations have suggested that the Ni−O bond length of the NiIII−O• species is 1.95 Å and the MBO is 0.65 for the Ni−O bond. The reactivity of the NiIII−O• species has been examined using para-substituted thioanisoles, para-substituted styrenes, and aromatic compounds having benzylic C−H bonds as substrates in CH3CN. In the oxidation of para-substituted anisoles and styrenes, products are the corresponding sulfoxides and epoxides, respectively. The second-order rate constants have been plotted against the Hammett parameters, affording Hammett plots showing a negative ρ value of −0.86 in both cases. This indicates the electrophilic character of the NIII−O• species. Furthermore, NiIII−O• species could perform hydrogen-atom abstraction from C−H bonds of activated methylene moieties of organic compounds. As for the C−H oxidation of fluorine, 1,4-cyclohexadiene, DHA, and xanthene, a Bell−Evans−Polanyi plot, in which the logarithms of the normalized second-order rate constants are plotted against the bond-dissociation-energy (BDE) values of C−H bonds to be cleaved, affords a linear relationship with a slope of −0.23. These results strongly suggest that hydrogen-atom abstraction is involved in the RDS in the substrate oxidation reactions. 4.3. Copper−Oxyl Complex. Roithová and co-workers have reported the electronic structure of gas-phase [(CH3CN)CuO]+

Figure 18. RuIII−O• complex in a larger contribution in the resonance structures. H


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Inorganic Chemistry

Figure 19. Structures of [4,5]4+ (left), [3,4]4+ (center), and [3,4]4+-prime (right).

Figure 20. MnIV−oxyl intermediate in water oxidation by [MnII(Py2NR2)(H2O)2]2+.79

stretching vibration suggest a large contribution of RuIII−O• character for the resonance structures of the RuIVO intermediate (Figure 18), which could have resulted from the higher redox potential of the RuIII/RuIV redox couple of RuIII−TPA2COOEt.71 Pushkar and co-workers reported the experimental evidence of radicaloid character of RuVO intermediates in a diruthenium complex, [4,5]4+ (Figure 19), based on EPR measurements in 17O-enriched water.74 [4,5]4+ also has been considered as a reactive intermediate in water oxidation by a “blue dimer” catalyst.75 The EPR spectrum of the blue dimer BD [4,5]4+ in H216O showed an EPR signal derived from the S = 1/2 spin state with g tensors of gxx = 2.03, gyy = 1.98, and gzz = 1.87. The simulated EPR spectrum clarified the ruthenium hfs of Ayy = 40 ± 5 G and Azz = 25 ± 5 G, reflecting the isotopic composition for 99, 101 Ru (I = 5/2), which was consistent with the reported apparent 99, 101Ru hfs on the order of 40 G. In 50−60% 17 O-enriched water, the BD [4,5]4+ intermediate having terminal Ru−O groups labeled with 17O was extracted by subtracting the contribution of [3,4]4+-prime, the spectrum showed a significant broadening of the EPR signals, and hfs due to 17O nuclei (I = 5/2) was clearly observed on the low field side of the signal at g ∼ 2.1 with a splitting of 60 G. These experiments showed a high spin density on the oxygen in the d3 RuVO moiety of the BD [4,5]4+ intermediate, which was considered to be a reactive species of water oxidation. The RuVO unit in BD [4,5]4+ is the first to show large radicaloid character and is associated with large 17O hfs.

an oxo bridging ligand of a Mn−O−Ca moiety including the external manganese center attached to the Mn3CaO4 cubane cluster in the oxyl−oxo coupling to form the O−O bond for dioxygen evolution.57,77 Batista and co-workers have also proposed a reaction mechanism of the O−O bond formation, in which the oxyl ligand on the manganese(IV) center undergoes the nucleophilic attack of a water molecule initially coordinated to the calcium(II) ion, on the basis of quantum mechanics/ molecular mechanics (QM/MM) calculations, X-ray diffraction, and EXAFS analysis.78 Theoretical investigations on mononuclear Mn−oxo complexes also suggest Mn−oxyl character for a reactive species derived from [MnII(Py2NR2)(H2O)2]2+ via PCET oxidation. As reported by Baik and co-workers, the reactive species in the triplet spin state bears oxyl character on an oxo ligand, as shown in Figure 20.79,80 The species is activated to form a quintet transition state (5TS) involving two oxyl ligands bound to the manganese(III) center (Figure 20, center) to form the O−O bond, affording a MnIII−peroxo intermediate, which undergoes further oxidation to afford dioxygen. Jackson and co-workers have described the electronic structure of [MnIV(O)(N4Py)]2+ for both the ground state and a 4E excited state derived from a dxz, dyz → dx2−y2 transition to elucidate the reactivity in HAT from a C−H bond of a substrate to the complex. The low-lying 4E excited state has been demonstrated to bear greater oxyl character (21%) with a longer Mn−O bond length, lowering the barrier for HAT.81 Thus, MnIII−O• character can better facilitate HAT processes. In this excited state, the species has a hole in the lower-energy oxo orbital rather than a hole in the dxz or dyz orbital to accept an electron from the C−H bond of a substrate to generate a highspin (HS) MnIII−hydroxo complex as a product of HAT. On the contrary, Borovik and co-workers have reported that MnV−oxo complexes having a tripodal triamido ligand (Figure 21) do not bear MnIV−oxyl character in light of EPR spectral data involving 17O-labeling experiments and DFT calculations to indicate that the Mn−oxo moiety has strong covalent character as an oxo ligand rather than an oxyl ligand.82 5.2. Iron−Oxyl Complexes. Iron−oxyl complexes have yet to be formed and characterized. However, the occurrence of oxyl

5. PROPOSED METAL−OXYL SPECIES BASED ON DFT CALCULATIONS Other than the above-mentioned metal−oxyl species, metal− oxyl species have been proposed to be formed and argued on the basis of DFT calculations, as described in the following sections. 5.1. Manganese−Oxyl Complexes. From the viewpoint of theoretical chemistry, the formation of Mn−O• species has been reported to be relavant to water oxidation by the oxygenevolving complex in the photosystem II for natural photosynthesis. Such Mn−O• species have been proposed to be involved in the O−O bond formation in the course of water oxidation to afford dioxygen.76 Siegbahn and co-workers have reported on the involvement of MnIV−O• species in the S4 state to react with I


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Inorganic Chemistry

covalency of the Fe−O π bond, leading to FeIII−O• character involving a hole in the oxygen pz orbital. In the case where a CH4 molecule encloses to the Fe−O moiety from an equatorial (horizontal) direction, the Fe−O bond is significantly elongated in the TS. This elongation of Fe−O bonding involves two different electronic states. First, the valency of the iron center is trivalent, and there is a hole in the pz orbital of the oxygen atom (Fe−O: 1.86 Å). These electronic structures of iron−oxo species should reflect the feature of FeIII−O• character and can be described as FeIII−O• species. This switching of a hole in the p orbital of [FeO]+ in HS state in the TS causes the mixing of σ89 and π89 pathways and results in the proposal of a “hybrid pathway”.88 5.3. Cobalt−Oxyl Complexes. A cobalt(IV)−oxo complex should be difficult to form in a tetragonal geometry in light of the “oxo wall” paradigm because cobalt resides in group 9. A dinuclear CoIII−bis(μ-hydroxo) complex, [{CoIII(μ-OH)(TPA)}2]4+, has been proposed to generate a dinuclear CoIII− bis(μ-oxyl) complex through PCET oxidation with CAN or [RuIII(bpy)3]3+ in water as a reactive species to perform O−O bond formation in the course of water oxidation (Figure 22).90 Although any direct evidence has yet to be obtained, DFT calculations on the dinuclear CoIII−bis(μ-oxyl) complex have suggested that the unpaired electrons lie on the two oxygen bridges with a spin density of 0.830, while that on the cobalt center is 0.044 to support the formulation. The two oxyl ligands couple together in an intramolecular manner to afford a dinuclear CoIII−μ-peroxo complex as an intermediate, followed by twoelectron oxidation of the dinuclear CoIII−μ-peroxo intermediate by CAN to afford dioxygen. Intramolecular O−O bond formation has been confirmed on the basis of 18O-labeling experiments for demonstrating the 18O distribution in the formed dioxygen. Co−oxo clusters and Co−oxides have been intensively investigated as effective water oxidation catalysts. In the course of water oxidation by Co−oxo clusters, it has been proposed that CoIV−oxyl species act as reactive entities to perform O−O bond formation toward the generation of peroxo intermediates.91 In addition, Dismukes and co-workers have reported on mechanistic insights into water oxidation by a cubane-type Co4O4 cluster on the basis of electrochemical and kinetic analyses.92 Terminal CoII−oxyl (CoIII−oxo) or CoIII−oxyl (CoIV−oxo) complexes have been proposed as active intermediates in intramolecular C−H abstraction or substrate oxidation. The CoII− oxyl complex was generated from O−O bond homolysis of CoII−μ-peroxo complexes having a Tp′ ligand [Tp′ = hydridotris(3-tert-butyl-5-methylpyrazolyl)borate], as shown in Figure 23.93 When toluene-d8 is used as a solvent, the incorporation of deuterium into the hydroxo ligand of Tp′CoIIOH could not be observed. This indicates that the HAT reaction occurred not via intermolecular reaction but via intramolecular reaction such as ligand oxidation by a CoII−O• species, as described in Figure 23. Theopold and co-workers have reported a follow-up study on these CoII−oxyl complexes.94 They successfully trapped a dinuclear peroxo-bridged CoII complex (Tp″CoII(O2)CoIITp″) at −78 °C having hydrotris (3-isopropyl-5-methylpyrazolyl)borate (Tp″) as a precursor of a transient CoII−oxyl complex. On the basis of a kinetic study by 1 H NMR spectroscopy, the absence of a significant solvent effect on the intramolecular HAT militates against a heterolytic cleavage of the O−O bond. In addition, a KIE value of 22 has been determined for the intramolecular HAT reaction in the use of Tp″-d30 at

Figure 21. Schematic description of a MnV−oxo complex reported by Borovik and co-workers.82

character in Fe−oxo complexes has been proposed and rationalized on the basis of theoretical calculations to explain the reactivity in C−H oxidation reactions. An α-ketoglutarate-dependent mononuclear nonheme iron enzyme, syringomycin halogenase, can catalyze halogenation and hydroxylation of unreactive C−H bond through dioxygen activation. Solomon and co-workers have reported that an FeIV− oxo intermediate formed in the catalytic cycle bears 40% of FeIII−oxyl character in the ground state and up to 65% in the TS of hydrogen abstraction, as indicated by variable-temperature magnetic circular dichroism spectroscopy and DFT calculations.83 Another nonheme iron enzyme, taurine/TauD, hydroxylates taurine to afford amino acetaldehyde and sulfite. Concerning the reactive species in the catalytic cycle of TauD, a HS (S = 2) FeIVO species called intermediate J has been observed and characterized spectroscopically.84 Concerning the reaction of intermediate J with the substrate, the FeIVO species has been proposed to be converted to a FeIII−oxyl species en route to a 5TS on the basis of DFT calculations.85 In the 5TS of HAT, the Fe−O bond is elongated from 1.62 to 1.76 Å and is interpreted as a HS iron(III) center (S = 5/2) antiferromagnetically coupled with a three-centered C−H−O radical (S = 1/2). Gunnoe and co-workers have reported on the formation of an FeIII−oxyl complex in the course of oxygen migration from coordinated pyridine N-oxide (PyO) to the iron(II) center in [FeII(Cp*)(Ph)(CO)(PyO)] (Cp* = pentamethylcyclopentadienyl) on the basis of DFT calculations. The FeIII−oxyl complex, [FeIII(Cp*)(O•)(Ph)(CO)], bears a large spin density (0.9 e−) at the oxyl ligand together with 1.3 e− at the iron center, as calculated at the B3LYP/CEP-31G(d) level of theory.86 Borovik and co-workers have reported on the electronic structure of an FeIV−oxo complex in the quintet (S = 2) spin state with the same ligand as that used for the MnV−oxo complex in Figure 21. The 17O-labeled FeIV−oxo complex has been scrutinized by EPR spectroscopy to reveal that the spin population on the oxygen ligand is 0.56, indicating a strong covalency in the Fe−oxo double bond without Fe−oxyl character.87 Note that Pushkar’s report on the ruthenium catalyst as mentioned above also showed spin population at the terminal oxygen atom based on 17O-labeled EPR measurements.74 In the report, the spin density at the terminal oxygen ligand was calculated to be 1.07 by DFT calculations. On the contrary, Borovik and co-workers have the spin density at the oxo ligand as only 0.56 based on electron-nuclear double resonance (ENDOR) measurements. Thus, the spin density at the oxygen ligand can also be a useful parameter for assuring metal−oxyl species by DFT calculations or EPR measurements. On the other hand, Li and co-workers have reported a lowering of the activation energy of C−H activation of CH4 by HS [FeIV(O)(F)5]3− on the basis of DFT analysis.88 They propose that the strong π-donating equatorial ligands reduce the J


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Inorganic Chemistry

Figure 22. Water oxidation by a CoIII−TPA complex via the formation of a dinuclear CoIII−bis(μ-oxyl) intermediate.90a

Figure 23. Proposed mechanism of intramolecular HAT by the transient CoII−oxyl (CoII−O•) species itself.93

toluene in the presence of a proton source. Because the reaction proceeds via formation of the CoIV−O• intermediate in resonance with the CoVO complex, it is suggested that the CoIV−O• complex may not possess very high reactivity in HAT. In addition, under the same conditions, the CoIII2(μ-O22−) complex can oxygenate triphenylphosphine (PPh3) to afford OPPh3 and the CoIII−OH2 complex, together with an OPPh3bound CoIII complex.95 5.4. Nickel−Oxyl Complex. In the reaction of a NiII−alkyl metallacycle complex, [Ni(κ2-C,C-CH2CH2CH2CH2)(bpy)], with N2O in benzene, oxygen-atom insertion into the Ni−C bond in the metallacycle is made to form an κ2-C,O-alkoxo metallacycle, as reported by Hillhouse and co-workers (Figure 25).96 On the basis of DFT calculations to elucidate the reaction mechanism, a square-pyramidal NiIII−oxyl complex has been postulated as an intermediate of the reaction through oxygenatom transfer from N2O to the nickel(II) center (Figure 25).97

281 K. Such a large KIE value suggests that the HAT reaction by the CoII−oxyl species proceeds via a tunneling mechanism. Very recently, Maron and co-workers have proposed the formation of a CoIV−O• complex by O−O bond cleavage of a dinuclear CoIII2(μ-O22−) complex, as shown in Figure 24.95 The CoIV−O• complex, [CoIV(O•)(B2Pz4Py)]+, has also been reported to form through the oxidation of a CoIII−hydroxo complex, [CoIII(OH)(B2Pz4Py)], by Magic Green [tris(2,4dibromophenyl)aminium hexafluoroantimonate] as an oxidant in benzene. DFT calculations on the CoIV−O• complex have indicated that the species should be in the triplet ground state with a Co−O bond length of 1.671 Å and bears spin density at both the oxygen ligand and cobalt center. The CoIV−O• complexes react with each other to afford dioxygen and a CoIII species as products.95 The dinuclear CoIII2(μ-O22−) complex does not react with toluene; however, it reacts with cyclohexadiene to produce benzene and a CoIII−OH2 complex in K


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Inorganic Chemistry

Figure 24. Formation of a CoIV−O• complex having pentadentate B2Pz4Py2− as a ligand [Ar = p-methylphenyl (p-tolyl)].95

Figure 25. Oxygen-atom insertion into a nickel(II) metallacycle complex to form a NiII−alkoxo metallacycle.96,97

5.5. Copper−Oxyl Complexes. Copper−oxygen species as reactive intermediates are highly important in substrate oxidation in vivo.98 Although there are plenty of reports on reactive intermediates such as [Cu2(O2)]2+,99 [Cu2(μ-O)2]2+,100 and [CuOOH]+,101 mononuclear Cu−oxyl species (CuII−O• or CuIII−O•) have yet to be well-understood. Computational study101 and synthetic copper(II) complexes102 showed that the activation energies of substrate oxidation by CuII−oxyl species are predicted to be much lower than those of CuII−OOH or Cu−O2 species as reactive intermediates. Monomeric Cu−oxyl species have been proposed as reactive intermediates in substrate oxidation reactions, catalyzed by copper-containing metalloenzymes such as dopamine β-monooxygenase,103 peptidylglycine α-hydroxylating monooxygenase,104 and lytic polysaccharide monooxygenases (LPMOs)105 on the basis of theoretical study. Schwarz reported the synthetic formation of [CuO]+ species and its reactivity toward methane in the gas phase, which showed high reactivity.106 Triplet-ground-state [CuO]+ has a configuration of (1σ) 2 (2σ) 2 (1π x ) 2 (1π y ) 2 (1δ) 4 (3σ*) 2 (2πx*)1(2πy*)1(4σ*)0, which is comparable to the 3Σg− ground state of molecular oxygen. Despite the fact that the electronic configuration of [CuO]+ in a triplet state should represent biradical character, DFT calculations estimate the spin density of 1.68 at oxygen and that of only 0.32 at the copper center.106 This large population of spin densities at oxygen indicates the significantly large contribution of radicaloid character at the terminal oxygen atom. In addition, unless there is a much more strongly bonding interaction between the iron center and oxygen atom involved in the FeIVO unit, the Cu−O bond in [CuO]+ is quite weaker, which is reflected in the low BDEs determined to be 31.1 kcal/mol (1.35 eV) experimentally107 and estimated to be 25.3 kcal/mol based on a theoretical study at the CCSD(T) level.108 LPMOs utilize molecular oxygen and an electron donor to catalyze the oxidative cleavage of insoluble polysaccharides.109 The active site of LPMO contains a mononuclear copper center ligated by two histidine residues. The reactive species is considered to be a CuII−O• species, not a CuII−O2− one, based on

theoretical studies. This LPMO catalysis can be activated also with the use of hydrogen peroxide (H2O2) as an oxidant in the presence of a substoichiometric amount of an external reductant. The CuI−H2O2 adduct in LPMO has a singlet ground state, and it undergoes homolytic O−O bond cleavage to form of a CuII−OH species and HO• radical (Figure 26a) with an activation barrier of 5.8 kcal/mol.109 Two possible pathways are considered for substrate oxidation in LPMO: HAT from the anomeric carbon of an adjacent sugar molecule by the trapped HO• radical (1TS2 in Figure 26b) and HAT from CuII−OH to HO• to afford CuII−O• species as the reactive species for hydrogen-atom abstraction from the anomeric carbon (1TS3 in Figure 26b). The former pathway has relatively a large activation barrier compared to the latter pathway by 4.9 kcal/mol. It is suggested that the relatively lower activation barrier is derived from fixation of the HO• radical by a hydrogen-bonding network in the active site (Figure 26a) to direct the oxygen-centered radical in HO• to a C−H bond of the C4 position of a substrate (Figure 26a). Furthermore, this hydrogen-bonding network is assumed to prevent the HO• radical from moving outside the hydrophobic site around the reaction center. The CuII−O• formed has a high spin density of 0.83 on the oxygen atom with a Cu−O bond of 1.89 Å. Reflecting the strong radical character of the CuII−O• species, HAT from the C4 position of the sugar requires it to overcome a small activation barrier of only 5.5 kcal/mol. This result suggests that CuII−O• can be a highly reactive species for C−H activation. Tolman et al. have reported the formation of Cu−oxyl (Cu−O•) species with dioxygen as an oxidant.110 In this case, an α-keto acid (α-ketophenylacetic acid) has been used as a ligand for the formation of Cu−O• by reductive oxygen activation, followed by the loss of CO2 and O−O bond homolysis of a peracid intermediate inspired by β-ketoglutarate-dependent nonheme iron enzymes. Exposing solutions of copper complexes LHCu(O2CC(O)Ph) (CuLH) or Lm‑OMeCu(O2CC(O)Ph) (CuLOMe) to dioxygen in acetone at −80 °C affords brown solutions within several hours. After warming and removal of the copper ion by L


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Inorganic Chemistry

Figure 26. (a) Hydrogen-bonding network around the HO• radical in an intermediate (1IC1 in part a) formed in the LPMO active site. (b) Relative energies calculated by the QM/MM methods (UB3LYP/B2, kcal/mol) for the reaction profile of the CuI−H2O2 intermediate formed in LPMO in the presence of polysaccharide. Reprinted with permission from 109. Copyright 2018 American Chemical Society.

Figure 27. Calculated mechanism for arene substituent hydroxylation of the model.110b

the addition of a base, a mixture of ligands (LH or Lm‑OMe) and hydroxylated ligands (LOH or Lm‑OMe−OH) was identified by several spectroscopic analyses. In the case of using 18O2, one 18 O atom was incorporated into the hydroxylated ligand and the other into the benzoate based on electrospray ionization mass spectrometry, indicating that O−O bond cleavage occurs in the course of the reaction. The “peracid” as one of the possible intermediates is assumed to undergo two plausible reaction pathways based on DFT calculations (Figure 27).110b The first possible pathway goes through a singlet transition state (1TS) structure involving oxidation of the ligand by the peracid itself, which is defined as “TS-Peracid” (Figure 27, top). Because the bond length of Cu−O in the structure of “TS-Peracid” is 1.72 Å, it seems to bear a strong “oxo-like” character. The second pathway involves a square-planar intermediate named “Oxo (sp)” followed by a triplet-transition-state structure “TS-Oxo” having a Cu−O bond length of 1.84 Å, which is described as a triplet CuII−oxyl species (Figure 27, bottom). Because these pathways have activation free energies of 17.8 and 10.4 kcal/mol

relative to the peracid intermediate, respectively, the latter pathway is predicted to be more accessible than the former. Therefore, these theoretical calculations have yielded intriguing mechanistic notions for the process, implicating hydroxylation pathways that involve [CuI−OOC(O)R] and [CuII−O•] species. Kodera and co-workers have recently reported selective hydroxylation of benzene by a dicopper(II) complex, having an ethylene-bridged polypyridyl ligand, [Cu2(μ-OH)(6-hpa)](ClO4)3 (Cu2(6-hpa); 6-hpa = 1,2-bis[2-[bis(2-pyridylmethyl)aminomethyl]-6-pyridyl]ethane), with H2O2 as a sacrificial oxidant (Figure 28).111 The main product from the reaction was identified as phenol based on gas chromatography and 1H NMR spectroscopy analysis. A time profile of phenol production catalyzed by Cu2(6-hpa) revealed a high turnover frequency (TOF) of 1010 h−1 and a turnover number of 12000 after 40 h. When a 1:1 mixture of C6H6 and C6D6 is used as a substrate, the KIE has been determined to be 1.04, indicating that C−H bond cleavage of benzene is not involved in the rate-limiting step. In addition, the active species of benzene hydroxylation M


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Inorganic Chemistry

Figure 28. Proposed mechanism of H2O2 activation and benzene hydroxylation by Cu2(6-hpa).111

Figure 29. Schematic representation of benzene oxidation to phenol catalyzed by TpXCuI(NCMe).115

values of 43 and 53 cm−1, respectively, when H218O2 is used. These Raman bands are attributed to O−O stretching bands on the basis of those reported so far.114 Although no direct evidence has been provided to support the formation of CuII−O• or CuII−OO• species, the reaction mechanism of benzene hydroxylation has been proposed, as shown in Figure 28. Catalytic benzene hydroxylation reactions to afford phenol have also been reported by Pérez and co-workers, using TpXCuI(NCMe) (TpX = hydrotrispyrazolylborate) complexes as catalysts and H2O2 as an oxidant in MeCN.115 The copper(I) catalysts react with H2O2 to generate monomeric CuII−O• species presumably via heterolysis of the O−O bond of H2O2 (Figure 29). The catalytic systems have provided 92% selectivity for phenol and benzoquinone in 8% yield. The KIE value of benzene hydroxylation has been determined to be 1.12 ± 0.01 with the use of an equimolar mixture of C6H6 and C6D6 as the substrate. As mentioned above, the KIE value is also out of the range (1.7−1.8) of those reported for the Fenton-type hydroxylation of benzene.112 In this case, the formation of a bis(aryl) derivative, derived from the homocoupling of aryl radicals, as one of the typical products was not observed. Furthermore, benzene conversion into phenol is not significantly affected even in the presence of radical-trapping reagents such as CCl4 or CBrCl3, although chloro- or bromobenzene can be obtained in very low yields (ca. 0.5%). These results indicate that the main pathway in the benzene hydroxylation reaction should not involve the generation of phenyl radicals. In addition, electronic substituent effects on the benzene ring have been surveyed for benzene hydroxylation, and negative

obtained from Cu2(6-hpa) is probably not be a hydroxyl radical (•OH) because the KIE values reported for a Fenton-type reaction are in the range of 1.7−1.8.112 In the case of nitrobenzene, toluene, and phenol as substrates, the initial TOFs are in the following order: phenol ∼ toluene ≫ nitrobenzene. This indicates that the higher electron density of the aromatic ring in the substrate gives a higher initial TOF value. Therefore, the reactive species formed from the catalyst should bear electrophilic character in benzene hydroxylation. The regioselectivities of the oxidation sites for the substrates have been revealed to be the ortho and para positions for toluene, the ortho position for nitrobenzene, and the para position for phenol, which also implies that the active species generated from the catalyst bears electrophilic radical character, i.e., a metal-bound oxyl radical, as proposed in the reactions of nickel and copper complexes.69,113 Upon the addition of 1 equiv of H2O2 to Cu2(6-hpa) in MeCN containing triethylamine (5 equiv) at −40 °C, an end-on trans-peroxodicopper(II) (Cu2O2) was obtained (Figure 28, middle). On the other hand, upon the addition of an excess amount of H2O2 to Cu2(6-hpa), the Cu2O2 complex rapidly reacted to afford hydroperoxocopper(II) (CuO2H; Figure 28). Because the reaction rate of this conversion process showed saturation behavior against the H2O2 concentration, an equilibrium involving the Cu2O2 complex and H2O2 should be operative. In addition, the two intermediates, Cu2O2 and CuOOH, have been characterized by rR spectroscopy. Cu2O2 and CuO2H show Raman bands at 821 and 846 cm−1, respectively, and then these bands are isotopically shifted by N


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Inorganic Chemistry slopes were observed in the Hammett plots: ρ = −0.75 for Tp*,BrCu(NCMe) and −1.1 for TpBr3Cu(NCMe). The negative slopes indicate electrophilic character of the Cu−O• complexes as the reactive species. In the course of electrocatalytic water oxidation by a CuII− aqua complex having triglycinato (TGG4−) as a pentadentate ligand at pH 11 in 0.25 M phosphate buffer at 1.30 V (vs NHE), the CuII−aqua complex undergoes PCET oxidation to generate a putative CuIII−O• complex (Figure 30a), which reacts with

intermediates have two radicaloid oxyl moieties generated in situ that promote water oxidation at room temperature. There are two possible mechanisms of water oxidation by diruthenium catalysts having radicaloid intermediates that have been considered in the past (Figure 31).118 Pathway A indicates O−O bond formation between a metal-bound oxygen atom and a water molecule as a solvent, which affords a RuIII− hydroperoxo intermediate. The direct coupling of the terminal oxo groups has been proposed to form a dinuclear (μ-oxo)(μperoxo)RuIV2 complex, shown as pathway B in Figure 31.118 Although the latter pathway is intuitively reasonable, it has been ruled out on the basis of experimental results.119 In the energetic analysis on the reactant [5,5]4+ ion, the most stable state has been proposed to be an antiferromagnetically coupled low-spin (LS) state in a staggered geometry. To propose the catalytic mechanism of water oxidation, a free water molecule was added to serve as the reference for all relative energies. It has been found that the formation of [4,4]*, modeled by adding one explicit water molecule to [5,5]4+, is an endothermic process with a relative energy of 20.2 kcal/mol (Figure 32, middle). [4,4]* is in an eclipsed geometry and is the antiferromagnetically coupled HS complex, where intramolecular electron transfer affords oxyl radical intermediates. Then the radicaloid oxyl moiety of [4,4]* attacks water in an electrophilic fashion. Spin densities of ±1.46 at ruthenium and ±1.07 at the terminal oxo ligands indicated that there is only a small electronic perturbation by the weak attachment of a water molecule to the eclipsed HS [5,5]4+. The first step of catalytic water oxidation involves O−H bond cleavage of water and the formation of hydroperoxo intermediate [3′,3]2+ from the reactant [4,4]*. This reaction has been considered to be RDS, having a free energy of activation of 25.9 kcal/mol in the solution phase (Figure 32). They concluded that the fundamental basis for the catalytic activity lied in a highly spin-polarized RuVO core structure, which underwent intramolecular electron transfer to afford a RuIV−O• species as a strong oxidant having an electrophilic reactivity toward a water molecule. 5.7. Rhenium−Oxyl Complexes. Strategies to obtain metal−oxyl species have been considered to require singleelectron occupancy of a metal−oxo π*-symmetry antibonding orbital120 or to remove an electron from a metal−oxo π -bonding orbital. The latter is applied by Soper and co-workers to a novel five-coordinate ReVI−oxo complex, bearing 2,4-di-tert-butyl-6(phenylamido)phenolate ([apPh]2−) and 2,4-di-tert-butyl-6(phenylamido)semiquinolate ([isqPh]−) as the one-electronoxidized species of [apPh]2− (Figure 33).121 The unique properties of ReVI(O)(apPh)(isqPh)Cl (ReVI(O)) are derived from symmetry-allowed mixing of a Re−O π bond with an orbital of the semiquinone-type ligand [isqPh]•−. The mixing of a Re−O bond gives oxyl radical character of the oxo ligand. Intramolecular charge transfer at closed-shell oxo ligands is a new strategy for the formation of metal−oxyl species, showing unique reactivity with a lower activation barrier. Qualitative π-orbital interactions in ReVI(O) are shown in Figure 34. As shown in Figure 34, the metal-based unpaired electron (d1) does not satisfy single-electron occupancy of metal−oxo π*-antibonding orbital because of the orthogonality of the dxy orbital to the Re−Ooxo π-bonding orbital. However, the [isqPh]•− ligand at the trans position to the oxo ligand has its unpaired electron in a π-symmetry orbital, which overlaps with the dxy orbital of the rhenium center and contributes to overlap with the Re−O π bonds (Figure 34b). Such symmetry-allowed mixing of the [isqPh]•− ligand with a Re−O π bond provides an

Figure 30. Putative CuIII−oxyl intermediates formed in electrocatalytic water oxidation: (a) [CuIII(O•)(TGG)]2−;116 (b) [CuIII(O•)(pyalk)2].117b

a water molecule to form an O−O bond toward dioxygen evolution.116 In addition, electrocatalytic water oxidation by a copper(II) complex having 2-pyridyl-2-propanolato (pyalk−) as a ligand, [CuII(pyalk)2], has been developed,117a and the reaction mechanism has been investigated on the basis of DFT calculations.117b In the proposed mechanism, a five-coordinated CuIII−O• complex (Figure 30b) has been formed through PCET oxidation of a CuIII−OH intermediate and reacts with a water molecule as well, forming a CuII−OOH complex, followed by a further PCET oxidation to generate dioxygen.117b 5.6. Ruthenium−Oxyl Complexes. Ruthenium-bound oxyl radical intermediates are highly important not only in catalytic substrate oxidation but also in water oxidation. The blue diruthenium complex cis,cis-[(bpy)2Ru(OH2)]2O4+ (bpy = 2,2′-bipyridine), which is called as the “blue dimer”, is one of the very few structurally well-defined molecular catalysts in water oxidation to afford molecular dioxygen.75 Although the reactive species have been experimentally proposed as cis,cis[(bpy)2RuVO]2O4+ ([5,5]4+ in Figure 31) for several decades,

Figure 31. Diruthenium complexes [3,3]4+ and [5,5]4+ (top) and two possible mechanisms of O−O bond formation (bottom).

key plausible intermediates have recently been proposed as formally cis,cis-[(bpy)2RuIVO•]2O4+ based on theoretical analysis.118 The O


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Inorganic Chemistry

Figure 32. Optimized structures of diruthenium complex [5,5]4+ (antiferromagnetically coupled spin state) in staggered (a) and eclipsed (b) geometry.118 (c) Formation of a precursor complex having two RuIV−O• moieties triggered by a water molecule.

Figure 33. Reaction between ReVI−oxo species ReVI(O) and a trityl radical.121

Figure 34. Qualitative π-orbital interactions in ReVI(O). Reprinted with permission from 121. Copyright 2011 American Chemical Society.

overlapping of the [isqPh]•− singly occupied molecular orbital (MO) with the Re−Ooxo π-bonding orbital. These mixings of a π orbital of [isqPh]•− and Re−Ooxo π-bonding orbitals cause a decrease of the bond order for the Re−Ooxo bond from 3.0 to 2.5. However, the oxidation potentials of [apPh]2− and O2− imply that intramolecular electron transfer from O2− to [isqPh]•− has a

large energy barrier, and thus it is not appropriate to describe the electronic structure of ReVI(O) at the ground state as ReVI(O•)(apPh)2Cl, which absolutely has an oxyl radical ligand.121 The contribution of oxyl radical character to the electronic structure of ReVI(O) was reflected in a very slow reaction with the trityl radical Ph3C• to afford Ph3COH and a deoxygenated P


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Inorganic Chemistry metal complex at 25 °C. Two possible reaction mechanisms were proposed for formation of the alkoxide complex, i.e., initial outer-sphere electron transfer, as the first step to give ReV(O) and the direct Ph3C• addition to ReVI(O). However, the former mechanism is concluded as unlikely because the one-electronreduced product of Re VI (O), [Re V (O)(ap Ph )(isq Ph )Cl] − (ReV(O)), performs rapid formation of a bis(μ-oxo) dimer, ReV2(μ-O)2(apPh)2(isqPh)2 (ReV2(μ-O)2), which cannot react with Ph3C+ to give Ph3COH. Thus, the latter mechanism seems more reasonable. It is noteworthy that all of the closed-shell Re− oxo complexes reported so far, including complexes capable of oxo transfer reactions to other substrates, have been Ph3C•.41,122 Consequently, ReVI(O) unexpectedly reacted with Ph3C•, in spite of the fact that ReVI(O) was neither a strong oxidant nor a ground-state oxyl radical. In addition, the reduction potential is lower by ca. 550 mV than that of ferrocenium, and thus ReVI(O) cannot oxidize even PPh3. 5.8. Rhodium−Oxyl Complexes. One example has been reported for a Rh−O• complex so far.123 A reaction of [RhIIIH{tBu2PCH2SiMe2CH2PtBu(CMe2CH2)}] (Rh−H) and N2O in the 1:1 mole ratio at −78 °C in toluene-d8 shows complete consumption of Rh−H accompanied by the production of [Rh(N2)(PNP)] and the C2v-symmetric, paramagnetic coproduct Rh−O, as evidenced by 1H NMR spectroscopy analysis (Figure 35). Experimental observations such as no

Figure 36. Water (a) or alcohol (b) oxidation by WV−O• species generated from WVIO under photoirradiation.

WV−O• species formed can abstract a hydrogen atom from an interacting water molecule homolytically to afford a hydroxyl radical. Various experiments have supported that such a strong oxidant acts as a reactive species in substrate oxidation in aqueous media. On the other hand, in the absence of associated water molecules, the substrates can be directly oxidized by the photoexcited POM cluster having WV−O• moieties. The characterization of such a transient short-lived LMCT excited state and reactive charge-transfer intermediates has been conducted by picosecond flash excitation in MeCN.127 After 355 nm pulse irradiation of the MeCN solution of [W10O32]4−, a rise of the initial transient at 390 nm was observed with a lifetime of ca. 30 ps, assigned to an LMCT excited state. In the transient absorbance spectra measured at 0.5 or 14.5 ns delay, identical absorption at 780 nm and arelatively broad band from 400 to 560 nm were observed and decayed slowly with τ ≥ 80 ns. The latter species showing the transient broad absorption band with a long lifetime affords growth of the transient absorption band derived from the reduced species [W10O32]5− in the presence of 4 M 2-butanol or 0.5 M cyclohexene.126

Figure 35. Reaction of a RhIII−H (Rh−H) species with N2O in toluene-d8.123

6. HOW TO STABILIZE METAL−OXYL SPECIES As in the case of stable Ru−O• complexes that are fully characterized, π-accepting ligands such as bpy and semiquinonato derivatives coordinate to the ruthenium centers. MOs of [RuIII(O•)(BPIm)(bpy)]2+ (Figure 10) related to the Ru−O• bond are shown in Figure 37. As can be seen, frontier orbitals of the Ru−O• bond consist of filled σ-bonding [93 and 94 (α-spin) and 96 (β-spin)] and π-bonding [113 (β-spin)] and π-antibonding orbitals (119). In the π-bonding orbitals (103 and 105 of the β-spin orbitals), delocalization of the MOs can be seen in the π* orbitals of the bpy ligand. Thus, electron delocalization from the Ru−O moiety to the bpy ligand should be indispensable for stabilization of the oxyl species. Also, the contribution of the NHC moiety to the π-antibonding orbital (119) of the Ru−O bond reduces the bond order of the Ru−O bond to 1.3. Thus, in [RuIII(O•)(BPIm)(bpy)]2+, strong electron donation from the NHC moiety coordinated at the trans position to the oxygen ligand is also important to generate the oxyl species. In the cases of RuIIDBSQ-O• and RuIIClSQ-O• in Figures 6 and 7, the complexes have two π-accepting ligands, semiquinonato and terpy. Although MO analysis on the complexes has yet to be reported, delocalization of the π-bonding orbitals involving the Ru−O bond can also be expected for the two complexes. The terpy ligand should have lower-energy π* orbitals compared to the bpy ligand because of the expansion of π conjugation. In addition, the semiquinonato ligand also has a hole in the π-bonding orbital to accept electron density

31

P NMR signals and paramagnetically shifted 1H NMR signals suggest that Rh−O should be a paramagnetic species. DFT calculations of Rh−O show that Rh−O in the triplet state is more stable by 15.3 kcal/mol compared to that in the singlet state. The distribution of the spin density of Rh−O in the triplet state is 0.92 on the rhodium center, 0.08 on nitrogen atom, and 0.96 on the oxygen atom, indicating triplet biradical character as a formally RhII−O• complex. 5.9. Tungsten−Oxyl Complexes. Tungstate−oxyl species (W(n−1)+−O•) have been intensively investigated as the main players in the photoredox chemistry of polyoxometalates (POMs) for several decades.124 POMs have been known as one of the metal oxide assemblies that have prominent subclasses of group 5 or 6 metal oxoanions such as tungstates, niobates, vanadates, molybdates, and tantalates.125,126 In particular, heteropolyoxotungstates, [XxWyOz]n− (X = Si, P, S), have been widely investigated because they often exhibit high thermodynamic stability and high reactivity in the oxidation of water and organic substrates. The reactivity is derived from photoexcitation of the WO moiety to produce W−O• species, as shown in Figure 36. POMs exhibit high photoactivity under near-visible or UV-light irradiation because of the high molecular absorption coefficients (ε > 1 × 104 M−1 cm−1) derived from O → W ligand-to-metal charge-transfer (LMCT) bands; photoexcitation affords W(n−1)+−O• as reactive species in the substrate oxidation reactions (Figure 36).124 In the presence of water, it is widely considered that a lot of water molecules interact with the cluster shell by hydrogen bonding. Upon photoexcitation of the cluster, Q


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Inorganic Chemistry

Figure 37. Selected MOs of [RuIII(O•)(BPIm)(bpy)]2+ (Figure 10) corresponding to the bonding and antibonding orbitals of the Ru−O bond. Values in parentheses are the MBO contribution from the corresponding MOs. Reprinted with permission from 62. Copyright 2016 Wiley-VCH.

7. SPECTROSCOPIC CRITERIA FOR EXPERIMENTAL CHARACTERIZATION OF METAL−OXYL SPECIES

from the ruthenium center, as discussed for the aforementioned the Re−O• complex (Figure 34) having an iminosemiquinonato ligand. Note that RuIIDBSQ-O• and RuIIClSQ-O• also have π-donating semiquinonato ligands at the positions trans to the oxygen ligands. It can be concluded that stabilization of the metal−oxyl complexes may require the introduction of a π-accepting ligand to reduce the electron density on a MO bond and a strong donating ligand at the trans position to the oxygen ligand. With regard to the metal center, ruthenium has been found to be one of the good candidates to generate oxyl complexes. The reason why ruthenium can stabilize the Ru−oxyl complexes may be the strong π-back-bonding ability to heteroaromatic ligands including π-conjugated polypyridyl ligands such as bpy and terpy and may be also semiquinonato ligands. As mentioned above, the π-bonding orbitals involving the RuO bond can delocalize into the bpy ligand in [RuIII(O•)(BPIm)(bpy)]2+ to reduce electron population of the RuO π bonding. In order to delocalize the π-bonding electrons of the MO moiety, the second- and third-row metals may be appropriate because of the larger lobes of dπ orbitals to facilitate the π-bonding interaction with π* orbitals of the π-conjugated ligand to reduce the bond order (Figure 38). The reason why such M−O• species are difficult to stabilize for the first-row transition-metal complexes is the smaller lobes of the 3dπ orbitals than the 4dπ and 5dπ orbitals.

Figure 39. Spectroscopic and structural features and experimental methodologies to prove metal−oxyl species. Some methodologies in parentheses are applicable to the cases where they are effective.

Figure 38. Electron flow through MO formation to stabilize a metal−oxyl species: (a) strong σ donation from a ligand binding at the trans position to the oxyl ligand; (b) strong π-back bonding to a π-accepting ligand.

Figure 40. Proposed and determined structures of spectroscopically and crystallographically characterized metal−oxyl complexes.

In this section, we provide some information on the methodologies for the characterization of metal−oxyl species. At the very least, metal−oxyl species should show spectroscopic characteristics: lower valency of the metal center of Mn+−O• than that of M(n+1)+O species based on XPS or XANES and lower frequency of the M−O vibration than MO species based on IR or rR spectroscopy (Figure 39). In addition, longer M−O distances than the corresponding MO bonds based on X-ray crystallography or EXAFS studies may be suggestive of M−O• character. However, careful analysis on the oxidation state should be made using other spectroscopic methods such as XANES because the

R


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Inorganic Chemistry

Despite indirect assignments, it is informative to use spin-trapping reagents, such as DMPO or (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, that afford stable products to be analyzed. In the cases where Mössbauer or EPR measurements are applicable, these spectroscopies should be effective in arguing the oxidation states of the metal centers in metal−oxyl species. In addition, computational studies on metal−oxyl species often show a large spin population on its oxygen atom (∼1.0) to support these experimental analyses based on DFT calculations. Finally, schematic descriptions of the proposed and determined structures of M−O• species are summarized in Figures 40 and 41.

8. SUMMARY In this Viewpoint, we surveyed M−O• species as a new category of possible reactive species in oxidation reactions as an extreme resonance structure of the corresponding MO species. Structural and spectroscopic parameters of selected M−O• species are summarized in Table 1. Formation of the M−O• species can be achieved through PCET oxidation of the corresponding lower-valent metal−aqua precursor complexes as observed for the Cu−O• and Ru−O• complexes, O−O bond cleavage of metal-bound peroxides as seen for the Cu−O• and Ni−O• complexes, and photoactivation and photoexcitation of metal−oxo complexes as observed for terminal oxo groups in POMs such as heteropolyoxotungstates. In addition, M−O• species have been proposed en route to the TSs of C−H oxidation by

Figure 41. Proposed structures of partially characterized metal−oxyl species. bond length is not always relevant to the oxidation state of the metal center.51 The spin density at the oxygen ligand can be verified by 17 O-enriched EPR and ENDOR measurements in possible cases.

Table 1. Summary of Parameters Observed and Estimated for Selected Metal−Oxyl Species by Experimental and Theoretical Methodsa experimental methods nomenclature [RuIII(O•)(BPIm)(bpy)]2+ [RuII(DBSQ)(O•)(terpy)] RuIII(O•)(TPA2COOEt) [{RuV(O)(bpy)2}(μ-O) {RuIV(OH)(bpy)2}]4+ [{RuV(O)(bpy)2}2(μ-O)]4+ (eclipsed) [MnIV(O•)(O)(Py2NR2)]2+

M−O bond, Å (method) 1.77(1) (XAS) 2.043(7) (XC)

1.71 (XAS)

νM−O, cm−1 732 (rR) 503, 521, 556, 590 (rR) 752 (rR) 816−818 (rR)

theoretical methods g values in EPR

761

1.76

B3LYP/dgdzvp

71 74

1.919

B3LYP/LACVP, 6-31G**

118

1.620 (Mn−O•), 1.591 (MnO) 1.818 >1.85 1.76

M06/LACVP, 6-31G**

79

TD-DFT, CASSCF hybrid B3LYP/TZVP, SV(P) B3LYP/CEP-31G(d) B3LYP/Wachters−Hay, D95**e B3PW91/Stuttgart−Köln ECP, 6-31G** B3LYP/TZVP CASPT2/M06L

81 84

1.671 450, 477 (rR)

1.95 1.84 1.89 1.7

693 (IRPD)

ZnII−O• in MFI zeolite

605 (UV−vis− NIR)

433

2.37, 1.98

1.858

643

B3LYP/SDD, D95**

ref

2.03, 1.98, 1.87

1.673 1.843

2.12 (XAS)

method

1.801

[CoIV(O•)(B2Pz2Py)]+

[ReVI(O)(apPh)(isqPh)Cl

νM−O, cm−1

4.31 4.18, 2.054

[MnIII(O•)2(Py2NR2)]2+ [MnIII(O•)(N4Py)]2+ FeIII−O• model of TauD (intermediate J) [FeIII(Cp*)(O•)(Ph)(CO)] [CoIII2(μ-O•)2(TPA)2]4+ b

[NiIII(L)(O•)]c LHCuII(O•)(OC(O)Ph) (TSOxo) CuII−O• moiety in LPMO [(CH3CN)CuO]+

M−O bond, Å

QM/MM (B3LYP/B2) B3LYP/6-311+G**MSRASPT2 B3LYP/LanL2DZ, 6-31G(d,p), 3-21G

1.7064(17) (XC)

62 59a

86 90 95 69 110b 109 70 58a 121

[RhII(O•)(PNP)]

1.814

B3LYP/LACVP, 6-31G**

123

Abbreviations used in the “nomenclature” of metal−oxyl species and their structures in this table have been described in main text. rR = resonance Raman spectroscopy. XAS = X-ray absorption spectroscopy (EXAFS in this table). XC = X-ray crystallography. IRPD = infrared photodissociation spectroscopy. bThe O• is not terminal but a bridging atom. cL = tetradentate dianionic macrocyclic ligand with two amidate, one pyridine, and one aliphatic amine groups. dMössbauer spectral analysis is conducted instead of EPR spectroscopy, in which ΔEQ = 0.60 mm/s and δ = 0.13 mm/s are obtained. eThe (14s9p5d)/[9s5p3d] primitive set of Wachters−Hay with one polarization f function (α = 1.117) and D95** were used. a

S


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Inorganic Chemistry the Fe−oxo complexes. As can be seen for the RuII−O• and Re−O• complexes, the use of π-accepting ligands such as bpy, terpy, and semiquinonato ligands can facilitate the formation of M−O• complexes because of the delocalization of unpaired electron density. The introduction of strongly σ-donating (such as NHC and semiquinonato) ligands at the position trans to the oxo ligand is also effective for the M−O• complexes, as seen in the RuII−O• and RuIII−O• complexes. Thus, it is expected that the 4d and 5d transition metals should be preferred to the 3d counterparts because of the more facile formation of π-type interactions with π-accepting ligands. As can be seen in the RuIII−O• complex, M−O• complexes can exhibit unique reactivity such as oxidative aromatic-ring cracking through the electrophilic attack on aromatic rings. Although the number of species observed is still limited, their pursuit is very attractive in terms of their characteristics and reactivity. M−O• species show unique reactivity in the oxidation of organic substrates and water. Especially, a RuIII−O• complex has been demonstrated to perform unique oxidation reactions of aromatic compounds, and a Zn−O• species has been reported to enable methane oxidation. Increasing numbers of proposals relevant to M−O• species as reactive species and also their involvement in the TSs of oxidation reactions have been reported. Thus, a reliable methodology for the formation of M−O• species and their characterization should become more important in oxidation chemistry on the basis of not only metal complexes but also solid catalysts. It can be expected that the reactivity of M−O• complexes will provide a way of achieving potentially difficult oxidative conversion of substrates including methane oxidation.

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Prof. Takahiko Kojima graduated from the Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, in 1986 and obtained his doctor degree in engineering from Graduate School of Engineering, The University of Tokyo, in 1991 under the supervision of Prof. Masanobu Hidai. After working as a postdoctoral associate in the group of Prof. Lawrence Que, Jr., at University of Minnesota, he joined the Department of Chemistry, Kyushu University, as an assistant professor in 1994. In 2005, he moved to the Department of Materials and Life Sciences, Osaka University, as an associate professor in the group led by Prof. Shunichi Fukuzumi. Since 2008, he has been a professor in the Department of Chemistry, University of Tsukuba. He obtained the Award for Creative Work from Japan Society of Coordination Chemistry in 2018. His research interests include the development of functionality of transition-metal complexes and porphyrin derivatives (especially nonplanar porphyrins) based on redox and photochemical reactions, including PCET and artificial photosynthesis.

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AUTHOR INFORMATION

ACKNOWLEDGMENTS This work has been supported by JST CREST (Grant JPMJCR16P1) and Grants-in-Aid 15H00915, 17H03027, and 18K19089 from the Japan Society of Promotion of Science of Japan (JSPS). T.K. also is grateful for financial support from the Yazaki Memorial Foundation for Science and Technology. Y.S. is thankful for support from a Research Fellowship for Young Scientists provided by JSPS (Grant 18J12050).

Corresponding Author

*E-mail: [email protected]. ORCID

Takahiko Kojima: 0000-0001-9941-8375 Notes

The authors declare no competing financial interest.

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Biographies

REFERENCES

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Dr. Yoshihiro Shimoyama received his Ph.D. degree in 2019 from the Department of Chemistry at University of Tsukuba, Ibaraki, Japan, under the supervision of Prof. Kojima. He is currently a postdoctorial researcher at the Interdisciplinary Research Center for Catalytic Chemistry, AIST. His current research interests lie in the development of innovative catalytic substrate oxidation and reduction systems to afford useful materials in water. T


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