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The Lewis-acid-adduct AgF promotes both reactivity and selectivity in high-valence metal-halogen chemistry.

Metalloporphyrins, metallophthalocyanines, and analogs are known to act as efficient catalysts for the reduction of dioxygen in the presence of an acid in solution. Whether dioxygen is reduced to hydrogen peroxide (H2O2: two-electron reduced species of O2) or water (H2O: four-electron reduced species of O2) is changed depending on metals and ligands. This mini-review focuses on the catalytic production of hydrogen peroxide by a combination of 2e−O2 reduction and 2e−or 4e− H2O oxidation with the use of metalloporphyrins and metallophthalocyanines as photocatalysts. H2O2generated by the photocatalytic H2O oxidation with O2 can be used as a fuel in hydrogen peroxide fuel cells in which metalloporphyrins, metallophthalocyanines, and analogs are employed as the cathode catalysts. Metalloporphyrins are also employed as efficient electrocatalysts to exhibit significantly improved performance in a Zn-air battery that has a much higher energy density than that of lithium-ion batteries.

This perspective article highlights redox catalysis of organic molecules via photoinduced electron transfer, which is well exploited for the important photoredox reactions including hydrogen evolution, water oxidation and synthetic applications.

There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [Ni

Disproportionation of Cpd II models depends on the electron-richness of the porphyrin ligand; Cpd II with an electron-deficient ligand is difficult to disproportionate, whereas Cpd II with an electron-rich ligand readily disproportionates to form Cpd I as a true oxidant.

A coordination polymer composed of cationic [CuIICl2(Hdatrz)2]2+ and anionic [CuI2Cl4]2− substructures was obtained from the reaction of 3,5-diamino-1,2,4-triazole (datrz) with CuICl and CuIICl2 in the presence of HCl.

This study provides valuable insights into the oxidative vs. basic asynchronous CPET mechanisms of HAT reactions to exhibit reversed electronic substitution effects on HAT reactions of Mn(iii)-hydroxo and Mn(iii)-aqua complexes.

Introduction of iodosylarnes into biomimetic nonheme chemistry has made great achievement on identification of the subtle metal-oxygen reaction intermediates. However, after more than three decades of experimental and theoretical efforts the nature of the metal-iodosylarene adducts and the related dichotomous one-oxidant/multiple-oxident controversy have remained a matter of speculation. Herein, we report a theoretical study of the structure-activity relationship of the noted iron(III)-iodsylarene complex, FeIII(PhIO)(OTf)3 (1), in oxygenation of cyclohexene. The calculated results revealed that 1 behaves like a chameleon by adapting its roles as a 2e-oxidant or an oxygen donor, as a response to the regioselective attack of the C–H bond and the C=C bond. The oxidative C–H bond activation by 1 was found, for the first time, to proceed via a novel hydride transfer process to form a cyclohexene carbonium intermediate, such non-rebound step triggers the Ritter reaction to uptake an acetonitrile molecule to form the amide product, or proceeds with the rebound of the hydroxyl group return to the solvent cage to form the hydroxylated product. While in the C=C bond activation, 1 is a normal oxygen donor and shows two-state reactivity to present the epoxide product via a direct oxygen atom transfer mechanism. These mechanistic findings fit and explain the famous Valentine's experiments and enrich the non-rebound scenario in bioinorganic chemistry.

Binding of Lewis acidic metal ions and Bronsted acid at the metal-oxo group of high-valent metal-oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal-oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)-oxo complex binding triflic acid, {[(dpaq)MnIV(O)]-HOTf}+ (1-HOTf). First, 1-HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn-O stretching frequencies of 1-HOTf (e.g., νMn-O at ∼793 cm-1) and 1-Mn+ (Mn+ = Ca2+, Zn2+, Lu3+, Al3+, or Sc3+) and the Lewis acidities of H+ and Mn+ ions, suggesting that H+ and Mn+ bind at the metal-oxo moiety of [(dpaq)MnIV(O)]+. Interestingly, a single-crystal structure of 1-HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)-oxo complex but a Mn(IV)-hydroxide complex, [(dpaq)MnIV(OH)](OTf)2 (4), with a Mn-O bond distance of 1.8043(19) A and a Mn-O stretch at 660 cm-1. More interestingly, 4 reverted to 1-HOTf upon dissolution, demonstrating that 1-HOTf and 4 are interconvertible depending on the physical states, such as 1-HOTf in solution and 4 in isolated solid. The reactivity of 1-HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1-Mn+ complexes; an interesting correlation between the Mn-O stretching frequencies of 1-HOTf and 1-Mn+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

Water nucleophilic attack (WNA) on high-valent terminal Mn-oxo species is proposed for O-O bond formation in natural and artificial water oxidation. Herein, we report an electrocatalytic water oxidation reaction with MnIII tris(pentafluorophenyl)corrole (1) in propylene carbonate (PC). O2 was generated at the MnV/IV potential with hydroxide, but a more anodic potential was required to evolve O2 with only water. With a synthetic MnV(O) complex of 1, a second-order rate constant, k2(OH-), of 7.4 × 103 M-1 s-1 was determined in the reaction of the MnV(O) complex of 1 with hydroxide, whereas its reaction with water occurred much more slowly with a k2(H2O) value of 4.4 × 10-3 M-1 s-1. This large reactivity difference of MnV(O) with hydroxide and water is consistent with different electrocatalytic behaviors of 1 with these two substrates. Significantly, during the electrolysis of 1 with water, a MnIV-peroxo species was identified with various spectroscopic methods, including UV-vis, electron paramagnetic resonance, and infrared spectroscopy. Isotope-labeling experiments confirmed that both O atoms of this peroxo species are derived from water, suggesting the involvement of the WNA mechanism in water oxidation by a Mn complex. Density functional theory calculations suggested that the nucleophilic attack of hydroxide on MnV(O) and also WNA to 1e--oxidized MnV(O) are feasibly involved in the catalytic cycles but that direct WNA to MnV(O) is not likely to be the main O-O bond formation pathway in the electrocatalytic water oxidation by 1.

Hydrogen peroxide has attracted increasing interest as an environmentally benign and green oxidant that can also be used as a solar fuel in fuel cells. This review focuses on recent progress in production of hydrogen peroxide by solar-light-driven oxidation of water by dioxygen and its usage as a green oxidant and fuel. The photocatalytic production of hydrogen peroxide is made possible by combining the 2e− and 4e− oxidation of water with the 2e− reduction of dioxygen using solar energy. The catalytic control of the selectivity of the 2e− vs. 4e− oxidation of water is discussed together with the selectivity of the 2e− vs. 4e− reduction of dioxygen. The combination of the photocatalytic 2e− oxidation of water and the 2e− reduction of dioxygen provides the best efficiency because both processes afford hydrogen peroxide. The solar-light-driven hydrogen peroxide production by oxidation of water and by reduction of dioxygen is combined with the catalytic oxidation of substrates with hydrogen peroxides, in which dioxygen is used as the greenest oxidant.

A cobalt porphyrin complex with a pendant imidazole base ([(L

Selective and efficient catalytic two-electron two-proton (2e–/2H+) reduction of dioxygen (O2) to hydrogen peroxide (H2O2) has been achieved less successfully than four-electron four-proton (4e–/4H...

We report the synthesis, characterization, and electron-transfer (ET) oxidation reactivity of a chromium(V)–oxo TAML cation radical complex binding Sc3+ ion, {[CrV(O)(TAML•+)]-Sc3+}3+ (2-Sc). Its p...

Metal-oxygen complexes, such as metal-oxo [M(O2-)], -hydroxo [M(OH-)], -peroxo [M(O22-)], -hydroperoxo [M(OOH-)], and -superoxo [M(O2•-)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenylphosphine (Ph3P). However, OAT of metal-aqua complexes, [M(OH2)]n+, has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(III)-aqua complex, [(dpaq)MnIII(OH2)]2+ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), to PhSMe and Ph3P derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(III)-hydroxo complex, [(dpaq)MnIII(OH)]+ (2), to the substrates. Mechanistic studies reveal that OAT reaction of 1 occurs via electron transfer from 4-methoxythioanisole to 1 to produce the 4-methoxythioanisole radical cation and [(dpaq)MnII(OH2)]+, followed by nucleophilic attack of H2O in [(dpaq)MnII(OH2)]+ to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)2C6H3S•(OH)Me, which disproportionates or undergoes electron transfer to 1 to yield methyl 4-methoxyphenyl sulfoxide. Formation of the thioanisole radical cation derivatives is detected by the stopped-flow transient absorption measurements in OAT from 1 to 2,4-dimethoxythioanisole and 3,4-dimethoxythioanisole, being compared with that in the photoinduced electron transfer oxidation of PhSMe derivatives, which are detected by laser-induced transient absorption measurements. Similarly, OAT from 1 to Ph3P occurs via electron transfer from Ph3P to 1, and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and Ph3P derivatives, to 1 are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.

Multifrequency, multitechnique pulse EPR spectroscopy was employed to unravel the spin Hamiltonian parameters of 17O in the [FeVO] moiety with two different tetraamido macrocyclic ligands (TAMLs), [FeV(O)(TAML-1)]− (1, H4(TAML-1) = 3,4,8,9-tetrahydro-3,3,6,6,9-hexamethyl-1H-1,4,8,11-benzotetraazocyclotridecane-2,5,7,10-(6H,11H)-tetrone) and [FeV(O)(TAML-2)]− (2, H4(TAML-2) = H4[(Me2CNCOCMe2NCO)2CMe2]), to investigate the electronic structure of FeV-oxo species. Although rigorous computational studies on high-valent iron-oxo species have been reported recently, experimental evidence to explicate the electronic structure of FeV-oxo species is sparse. In particular, a complete hyperfine tensor of 17O can hardly be detected. Herein, we successfully probed the hyperfine tensor of 17O of the FeV-oxo moiety using ENDOR spectroscopy. Hence, the EPR spectroscopic results reported here provide a conclusive experimental basis for elucidating the electronic structure of the FeV-oxo complex. Moreover, the reactivity of the two different complexes is very distinct, and our results may provide insight into how their electronic structure contributes to their reactivity.

Acids are known to enhance the reactivities of metal-oxygen intermediates, such as metal-oxo, -hydroperoxo, -peroxo, and -superoxo complexes, in biomimetic oxidation reactions. Although metal-aqua (and metal-hydroxo) complexes have been shown to be potent oxidants in oxidation reactions, acid effects on the reactivities of metal-aqua complexes have never been investigated previously. In this study, a mononuclear manganese(III)-aqua complex, [(dpaq

High-valent metal–oxo species with multiply-bonded M–O groups have been proposed as key intermediates in many biological and abiological catalytic oxidation reactions. These intermediates are implicated as active oxidants in alkane hydroxylation, olefin epoxidation and other oxidation reactions. For example, [FeivO(porphyrinato•−)]+ cofactors bearing π-radical porphyrinato•− ligands oxidize organic substrates in cytochrome P450 enzymes, which are common to many life forms. Likewise, high-valent Mn–oxo species are active for H2O oxidation in photosystem II. The chemistry of these native reactive species has inspired chemists to prepare highly oxidized transition-metal complexes as functional mimics. Although many synthetic Fe–O and Mn–O complexes now exist, the analogous oxo complexes of the late transition metals (groups 9–11) are rare. Indeed, late-transition-metal–oxo complexes of tetragonal (fourfold) symmetry should be electronically unstable, a rule commonly referred to as the ‘oxo wall’. A few late metal–oxos have been prepared by targeting other symmetries or unusual spin states. These complexes have been studied using spectroscopic and theoretical methods. This Review describes mononuclear non-haem Fe–O and Mn–O species, the nature of the oxo wall and recent advances in the preparation of oxo complexes of Co, Ni and Cu beyond the oxo wall. High-valent metal–oxo species are implicated in biological and industrially important oxidations. This Review describes mononuclear iron and manganese oxos, the nature of the ‘oxo wall’ and recent advances in late-transition-metal–oxo complexes.

A mononuclear nonheme cobalt(III) iodosylbenzene complex, [CoIII (TQA)(OIPh)(OH)]2+ (1), is synthesized and characterized structurally and spectroscopically. While 1 is a sluggish oxidant in oxidation reactions, it becomes a competent oxidant in oxygen atom transfer reactions, such as olefin epoxidation, in the presence of a small amount of proton. More interestingly, 1 shows a nucleophilic reactivity in aldehyde deformylation reaction, demonstrating that 1 has an amphoteric reactivity. Another interesting observation is that 1 can be used as an oxygen atom donor in the generation of high-valent metal-oxo complexes. To our knowledge, we present the first crystal structure of a CoIII iodosylbenzene complex and the unprecedented reactivity of metal-iodosylarene adduct.

There are mechanistic dichotomies with regard to the formation, electronic structures and reaction mechanisms of metal-oxygen intermediates, since these metal-oxygen species could be composed of different resonance structures or canonical structures of the oxidation states of metals and ligands, which may undergo different reaction pathways. Even the same metal-oxygen intermediates, such as metal-oxo species, may undergo an electron-transfer pathway or a direct hydrogen or oxygen atom transfer pathway depending on the one-electron redox potentials of metal-oxo species and substrates. Electron-transfer pathways are also classified into two mechanisms, such as outer-sphere and inner-sphere pathways. The one-electron redox potentials of metal-oxygen species and substrates are also shifted because of the binding of acids, which can result from either hydrogen bonding or protonation. There are a rebound pathway and a non-rebound pathway following the initial electron transfer or hydrogen atom transfer step to produce hydroxylated products, depending on the one-electron redox potentials of metal-oxo species and substrates. Nucleophilic reactions can be switched to electrophilic pathways, depending on reaction conditions such as reaction temperature. Spin states of metal-oxygen intermediates are also an important factor that controls the redox reactivity of oxidants in oxidation reactions. Here, we review such various mechanistic dichotomies in redox reactions of metal-oxygen intermediates with the emphasis on understanding and controlling the redox reactivity of metal-oxygen intermediates from experimental and theoretical points of view.

The present study reports the first example of a proton-promoted disproportionation reaction of a non-heme iron(v)-imido TAML (1) complex to give an iron(v)-imido TAML cation radical (2) and an iron(iv) TAML (3) upon addition of acids. Detailed mechanistic investigations revealed that two molecules of 1 react with one proton to yield 2 and [FeIV(NHTs)(TAML)]- (4), followed by the reaction of 4 with another proton to afford 3 and NH2Ts.

Metalloporphyrinoids are utilized as efficient sensitizers and catalysts in photosynthesis and the reverse reaction that is respiration. Because metalloporphyrinoids show strong absorption in the visible region and redox active, metalloporphyrinoids are also suited as photoredox catalysts for photo-driven redox reactions using solar energy. In particular, metalloporphyrins are utilized as pivotal components to mimic the structure and function of the photosynthetic reaction center. Metalloporphyrins are used as photoredox catalysts for hydrogen evolution from electron and proton sources combining hydrogen evolution catalysts. Metalloporphyrins also act as thermal redox catalysts for photocatalytic reduction of CO2 with photoredox catalysts. Metalloporphyrins are also used as dual catalysts for a photoredox catalyst for oxygenation of substrates with H2O and a redox catalyst for O2 reduction when dioxygen is used as a two-electron oxidant and H2O as an oxygen source, both of which are the greenest reactants. Free base porphyrins can also be employed as promising photoredox catalysts for C–C bond formation reactions.

C–H bond activation in which reaction rates are correlated with the pKa of C–H bonds proceeds via a PCET(PT) mechanism.

A mononuclear nonheme cobalt(II) complex, [(TMG(3)tren)Co(II)(OTf)](OTf) (1), activates dioxygen in the presence of hydrogen atom donor substrates, such as tetrahydrofuran and cyclohexene, resulting in the generation of a cobalt(II)-alkylperoxide intermediate (2), which then converts to the previously reported cobalt(IV)-oxo complex, [(TMG(3)tren)Co(IV)(O)](2+)-(Sc(OTf)(3))(n)] (3), in >90% yield upon addition of a redox-inactive metal ion, Sc(OTf)(3). Intermediates 2 and 3 represent the cobalt analogues of the proposed iron(II)-alkylperoxide precursor that converts to an iron(IV)-oxo intermediate via O-O bond heterolysis in pterin-dependent nonheme iron oxygenases. In reactivity studies, 2 shows an amphoteric reactivity in electrophilic and nucleophilic reactions, whereas 3 is an electrophilic oxidant. To the best of our knowledge, the present study reports the first example showing the generation of cobalt-oxygen intermediates by activating dioxygen at a cobalt(II) center and the reactivities of the cobalt-oxygen intermediates in oxidation reaction.