Proton-Induced, Reversible Interconversion of a μ-1,2-Peroxo and a μ-1,1-Hydroperoxo Dicopper(II) Complex

Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties

Copper ion is a versatile and ubiquitous facilitator of redox chemical and biochemical processes. These include the binding of molecular oxygen to copper(I) complexes where it undergoes stepwise reduction-protonation. A detailed understanding of thermodynamic relationships between such reduced/protonated states is key to elucidate the fundamentals of the chemical/biochemical processes involved. The dicopper(I) complex [CuI2(BPMPO-)]1+ {BPMPOH = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper(II) peroxo [CuII2(BPMPO-)(O22-)]1+, hydroperoxo [CuII2(BPMPO-)(-OOH)]2+, and superoxo [CuII2(BPMPO-)(O2•-)]2+ species, characterized by UV-vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [CuII2(BPMPO-)(O2•-)]2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [CuII2(BPMPO-)(O22-)]1+ ⇄ [CuII2(BPMPO-)(O2•-)]2+, is established utilizing Me8Fc+/Cr(η6-C6H6)2, allowing for [CuII2(BPMPO-)(O2•-)]2+/[CuII2(BPMPO-)(O22-)]1+ reduction potential calculation, E°' = -0.44 ± 0.01 V vs Fc+/0, also confirmed by cryoelectrochemical measurements (E°' = -0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [CuII2(BPMPO-)(O22-)]1+ produces [CuII2(BPMPO-)(-OOH)]2+; using a phosphazene base, an acid-base equilibrium was achieved, pKa = 22.3 ± 0.7 for [CuII2(BPMPO-)(-OOH)]2+. The BDFEOO-H = 80.3 ± 1.2 kcal/mol, as calculated for [CuII2(BPMPO-)(-OOH)]2+; this is further substantiated by H atom abstraction from O-H substrates by [CuII2(BPMPO-)(O2•-)]2+ forming [CuII2(BPMPO-)(-OOH)]2+. In comparison to known analogues, the thermodynamic and spectroscopic properties of [CuII2(BPMPO-)] O2-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO- framework and the resulting enhanced CuII-ion Lewis acidity.

Selective Four-Electron Reduction of Oxygen by a Nonheme Heterobimetallic CuFe Complex

We report herein the first nonheme CuFe oxygen reduction catalyst ([CuII (bpbp)(μ-OAc)2 FeIII ]2+ , CuFe-OAc), which serves as a functional model of cytochrome c oxidase and can catalyze oxygen reduction to water with a turnover frequency of 2.4×103  s-1 and selectivity of 96.0 % in the presence of Et3 NH+ . This performance significantly outcompetes its homobimetallic analogues (2.7 s-1 of CuCu-OAc with %H2 O2 selectivity of 98.9 %, and inactive of FeFe-OAc) under the same conditions. Structure-activity relationship studies, in combination with density functional theory calculation, show that the CuFe center efficiently mediates O-O bond cleavage via a CuII (μ-η1  : η2 -O2 )FeIII peroxo intermediate in which the peroxo ligand possesses distinctive coordinating and electronic character. Our work sheds light on the nature of Cu/Fe heterobimetallic cooperation in oxygen reduction catalysis and demonstrates the potential of this synergistic effect in the design of nonheme oxygen reduction catalysts.

Jellyfish-type Dinuclear Hafnium Azido Complexes: Synthesis and Reactivity

Di- and multinuclear hafnium complexes bridged by ligands have been rarely reported. In this article, a novel 3,5-disubstituted pyrazolate-bridged ligand LH5 with two [N2 N]2- -type chelating side arms was designed and synthesized, which supported a series of dinuclear hafnium complexes. Dinuclear hafnium azides [LHf2 (μ-1,1-N3 )2 (N3 )2 ][Na(THF)4 ] 3 and [LHf2 (μ-1,1-N3 )2 (N3 )2 ][Na(2,2,2-Kryptofix)] 4 were further synthesized and structurally characterized, featuring two sets of terminal and bridging azido ligands like jellyfishes. The reactivity of 3 under reduction conditions was conducted, leading to a formation of a tetranuclear hafnium imido complex [L1 Hf2 (μ1 -NH)(N3 ){μ2 -K}]2 5. DFT calculations revealed that the mixed imido azide 5 was generated via an intramolecular C-H insertion from a putative dinuclear HfIV -nitridyl intermediate.

Calcium-Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores

Coupled dinuclear copper oxygen cores (Cu₂ O₂ ) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O₂ transport and substrate oxidation in many organisms. μ-1,2-cis peroxido dicopper cores (^C P) have been proposed as key structures in the early stages of O₂ binding in these proteins; their reversible isomerization to other Cu₂ O₂ cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic ^C P cores in biological and synthetic systems, the properties and reactivity of ^C P Cu₂ O₂ species remain largely unexplored. Herein, we report the reversible interconversion of μ-1,2-trans peroxido (^T P) and ^C P dicopper cores. Ca^II mediates this process by reversible binding at the Cu₂ O₂ core, highlighting the unique capability for metal-ion binding events to stabilize novel reactive fragments and control O₂ activation in biomimetic systems.

Structurally Characterized μ-1,2-Peroxo/Superoxo Dicopper(II) Pair

Superoxo complexes of copper are primary adducts in several O₂-activating Cu-containing metalloenzymes as well as in other Cu-mediated oxidation and oxygenation reactions. Because of their intrinsically high reactivity, however, isolation of Cu_x(O₂^•-) species is challenging. Recent work (J. Am. Chem. Soc. 2017, 139, 9831; 2019, 141, 12682) established fundamental thermochemical data for the H atom abstraction reactivity of dicopper(II) superoxo complexes, but structural characterization of these important intermediates was so far lacking. Here we report the first crystallographic structure determination of a superoxo dicopper(II) species (3) together with the structure of its 1e^- reduced peroxo congener (2; a rare cis-μ-1,2-peroxo dicopper(II) complex). Interconversion of 2 and 3 occurs at low potential (-0.58 V vs Fc/Fc⁺) and is reversible both chemically and electrochemically. Comparison of metric parameters (d(O-O) = 1.441(2) Å for 2 vs 1.329(7) Å for 3) and of spectroscopic signatures (ν̃(¹⁶O-¹⁶O) = 793 cm^-1 for 2 vs 1073 cm^-1 for 3) reflects that the redox process occurs at the bridging O₂-derived unit. The Cu^II-O₂^•--Cu^II complex has an S = 1/2 spin ground state according to magnetic and EPR data, in agreement with density functional theory calculations. Computations further show that the potential associated with changes of the Cu-O-O-Cu dihedral angle is shallow for both 2 and 3. These findings provide a structural basis for the low reorganization energy of the kinetically facile 1e^- interconversion of μ-1,2-superoxo/peroxo dicopper(II) couples, and they open the door for comprehensive studies of these key intermediates in Cu_x/O₂ chemistry.

Dicopper(II) Complexes of p-Cresol-2,6-Bis(dpa) Amide-Tether Ligands: Large Enhancement of Oxidative DNA Cleavage, Cytotoxicity, and Mechanistic Insight by Intracellular Visualization

Dicopper complexes of a new p-cresol-2,6-bis(dpa) amide-tether ligand (HL1), [Cu₂(μ-OH₂)(μ-1,3-OAc)(L1)](ClO₄)₂ (1) and [Cu₂(μ-1,1-OAc)(μ-1,3-OAc)(L1)]X (X = ClO₄ (2a), OAc (2b)) were synthesized and structurally characterized. 2b rapidly cleaves supercoiled plasmid DNA by activating H₂O₂ at neutral pH to a linear DNA and shows remarkable cytotoxicity in comparison with related complexes. As 2b is more cytotoxic than HL1, the dicopper core is kept in the cell. A boron dipyrromethene (Bodipy)-modified complex of the p-cresol-2,6-bis(dpa) amide-tether ligand having a Bodipy pendant (HL2), [Cu₂(μ-OAc)₂(L2)](OAc) (3), was synthesized to visualize intracellular behavior, suggesting that 2b attacks the nucleolus and mitochondria. A comet assay clearly shows that 2b does not cleave nuclear DNA. The apoptotic cell death is evidenced from flow cytometry.

Oxidative DNA Cleavage, Formation of μ-1,1-Hydroperoxo Species, and Cytotoxicity of Dicopper(II) Complex Supported by a p-Cresol-Derived Amide-Tether Ligand

Metal complexes to promote oxidative DNA cleavage by H₂O₂ are desirable as anticancer drugs. A dicopper(II) complex of known p-cresol-derived methylene-tether ligand Hbcc [Cu₂(bcc)]³⁺ did not promote DNA cleavage by H₂O₂. Here, we synthesized a new p-cresol-derived amide-tether one, 2,6-bis(1,4,7,10-tetrazacyclododecyl-1-carboxyamide)-p-cresol (Hbcamide). A dicopper(II) complex of the new ligand [Cu₂(μ-OH)(bcamide)]²⁺ was structurally characterized. This complex promoted the oxidative cleavage of supercoiled plasmid pUC19 DNA (Form I) with H₂O₂ at pH 6.0-8.2 to give Forms II and III. The reaction was largely accelerated in a high pH region. A μ-1,1-hydroperoxo species was formed as the active species and spectroscopically identified. The amide-tether complex is more effective in cytotoxicity against HeLa cells than the methylene-tether one.

Ligand Identity-Induced Generation of Enhanced Oxidative Hydrogen Atom Transfer Reactivity for a CuII2(O2•-) Complex Driven by Formation of a CuII2(-OOH) Compound with a Strong O-H Bond

A superoxide-bridged dicopper(II) complex, [Cu^II₂(XYLO)(O₂^•-)]²⁺ (1) (XYLO = binucleating m-xylyl derivative with a bridging phenolate ligand donor and two bis(2-{2-pyridyl}ethyl)amine arms), was generated from chemical oxidation of the peroxide-bridged dicopper(II) complex [Cu^II₂(XYLO)(O₂^2-)]⁺ (2), using ferrocenium (Fc⁺) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 °C. Using Me₁₀Fc⁺, a 1 ⇆ 2 equilibrium was established, allowing for calculation of the reduction potential of 1 as -0.525 ± 0.01 V vs Fc^+/0. Addition of 1 equiv of strong acid to 2 afforded the hydroperoxide-bridged dicopper(II) species [Cu^II₂(XYLO)(OOH)]²⁺ (3). An acid-base equilibrium between 3 and 2 was achieved through spectral titrations using a derivatized phosphazene base. The pK_a of 3 was thus determined to be 24 ± 0.6 in MeTHF at -125 °C. Using a thermodynamic square scheme and the Bordwell relationship, the hydroperoxo complex (3) O-H bond dissociation free energy (BDFE) was calculated as 81.8 ± 1.5 (BDE = 86.8) kcal/mol. The observed oxidizing capability of [Cu^II₂(XYLO)(O₂^•-)]²⁺ (1), as demonstrated in H atom abstraction reactions with certain phenolic ArO-H and hydrocarbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reaction of TEMPO-H with 1 in MeTHF, with k (-100 °C) = 5.6 M^-1 s^-1. Density functional theory (DFT) calculations reveal how the structure of 1 may minimize stabilization of the superoxide moiety, resulting in its enhanced reactivity. The thermodynamic insights obtained herein highlight the importance of the interplay between ligand design and the generation and properties of copper (or other metal ion) bound O₂-derived reduced species, such as pK_a, reduction potential, and BDFE; these may be relevant to the capabilities (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.

Dioxygen/Hydrogen Peroxide Interconversion Using Redox Couples of Saddle-Distorted Porphyrins and Isophlorins

Interconversion between dioxygen (O₂) and hydrogen peroxide (H₂O₂) has attracted much interest because of the growing importance of H₂O₂ as an energy source. There are many reports on O₂ conversions to H₂O₂; however, no example has been reported on O₂/H₂O₂ interconversion. Herein, we describe successful achievement of a reversible O₂/H₂O₂ conversion based on an N21, N23-dimethylated saddle-distorted porphyrin and the corresponding two-electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O₂ to afford the corresponding porphyrin and H₂O₂; conversely, the porphyrin also reacted with excess H₂O₂ to reproduce the corresponding isophlorin and O₂. The isophlorin-O₂/porphyrin-H₂O₂ interconversion was repeatedly proceeded by alternate bubbling of Ar or O₂, although no reversible conversion was observed in the case of an N21, N22-dimethylated porphyrin as a structural isomer. Such a drastic change of the reversibility was derived from the directions of inner N H protons in hydrogen-bond formation of the isophlorin core with O₂ as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H₂O₂. The intriguing isophlorin-O₂/porphyrin-H₂O₂ interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O₂/porphyrin-H₂O₂ states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O₂/porphyrin-H₂O₂ interconversion using ¹H NMR and UV-vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N21, N23-dimethylated porphyrin and H₂O₂ as an intermediate.

Stoichiometric and electrocatalytic production of hydrogen peroxide driven by a water-soluble copper(ii) complex

Herein, a water-soluble molecular copper complex was investigated as a catalyst for O₂ reduction in both water and an organic solvent.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O₂ to H₂O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O₂ to water via Co(III)-O-O-Co(III) peroxo intermediates.

Dioxygen Activation by Nonheme Diiron Enzymes: Diverse Dioxygen Adducts, High-Valent Intermediates, and Related Model Complexes

A growing subset of metalloenzymes activates dioxygen with nonheme diiron active sites to effect substrate oxidations that range from the hydroxylation of methane and the desaturation of fatty acids to the deformylation of fatty aldehydes to produce alkanes and the six-electron oxidation of aminoarenes to nitroarenes in the biosynthesis of antibiotics. A common feature of their reaction mechanisms is the formation of O2 adducts that evolve into more reactive derivatives such as diiron(II,III)-superoxo, diiron(III)-peroxo, diiron(III,IV)-oxo, and diiron(IV)-oxo species, which carry out particular substrate oxidation tasks. In this review, we survey the various enzymes belonging to this unique subset and the mechanisms by which substrate oxidation is carried out. We examine the nature of the reactive intermediates, as revealed by X-ray crystallography and the application of various spectroscopic methods and their associated reactivity. We also discuss the structural and electronic properties of the model complexes that have been found to mimic salient aspects of these enzyme active sites. Much has been learned in the past 25 years, but key questions remain to be answered.

Unprecedented (μ-1,1-Peroxo)diferric Structure for the Ambiphilic Orange Peroxo Intermediate of the Nonheme N-Oxygenase CmlI

The final step in the biosynthesis of the antibiotic chloramphenicol is the oxidation of an aryl-amine substrate to an aryl-nitro product catalyzed by the N-oxygenase CmlI in three two-electron steps. The CmlI active site contains a diiron cluster ligated by three histidine and four glutamate residues and activates dioxygen to perform its role in the biosynthetic pathway. It was previously shown that the active oxidant used by CmlI to facilitate this chemistry is a peroxo-diferric intermediate (CmlIP). Spectroscopic characterization demonstrated that the peroxo binding geometry of CmlIP is not consistent with the μ-1,2 mode commonly observed in nonheme diiron systems. Its geometry was tentatively assigned as μ-η2:η1 based on comparison with resonance Raman (rR) features of mixed-metal model complexes in the absence of appropriate diiron models. Here, X-ray absorption spectroscopy (XAS) and rR studies have been used to establish a refined structure for the diferric cluster of CmlIP. The rR experiments carried out with isotopically labeled water identified the symmetric and asymmetric vibrations of an Fe-O-Fe unit in the active site at 485 and 780 cm-1, respectively, which was confirmed by the 1.83 Å Fe-O bond observed by XAS. In addition, a unique Fe···O scatterer at 2.82 Å observed from XAS analysis is assigned as arising from the distal O atom of a μ-1,1-peroxo ligand that is bound symmetrically between the irons. The (μ-oxo)(μ-1,1-peroxo)diferric core structure associated with CmlIP is unprecedented among diiron cluster-containing enzymes and corresponding biomimetic complexes. Importantly, it allows the peroxo-diferric intermediate to be ambiphilic, acting as an electrophilic oxidant in the initial N-hydroxylation of an arylamine and then becoming a nucleophilic oxidant in the final oxidation of an aryl-nitroso intermediate to the aryl-nitro product.

Hydrogen Atom Abstraction Thermodynamics of a μ-1,2-Superoxo Dicopper(II) Complex

Pyrazolate-based μ-1,2-peroxo dicopper(II) complex 1 undergoes clean 1e^- oxidation at low potential (-0.59 V vs Fc/Fc⁺) to yield the rather stable μ-1,2-superoxo dicopper(II) complex 3, which was characterized by spectroscopic methods (ν̃(O-O) = 1070 cm^-1, Δ(¹⁸O-¹⁶O) = -59 cm^-1) and analyzed by DFT calculations. 3 is also formed via H-atom abstraction from the corresponding μ-1,1-hydroperoxo dicopper(II) complex 2, while 3 itself is able to abstract H-atoms from weaker X-H bonds such as TEMPO-H to re-form 2. Kinetic and thermodynamic analyses evidence a concerted proton-electron transfer pathway for these processes. The thermodynamic square scheme reveals a bond dissociation free energy of 71.7 ± 1.1 kcal mol^-1 for the hydroperoxo OO-H bond of 2.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013-1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.

Reversible Oxygenation of 2,4-Diaminobutanoic Acid-Co(II) Complexes

This paper introduces the structural characterization and studies on reversible oxygenation behavior of a new oxygen carrier Co(II)-2,4-diaminobutanoic acid (DABA) complex in aqueous solution. The composition of the oxygenated complex was determined by gas volumetric method, molar ratio method, and mass spectrometry, and the formula of the oxygenated complex was determined to be [Co(DABA)₂O₂]. In aqueous solution, the complex can continuously uptake and release dioxygen and exhibit excellent reversibility of oxygenation and deoxygenation ability. This complex can maintain 50% of its original oxygenation capacity after 30 cycles in 24 h and retain 5% of the original oxygenation capacity after more than 260 cycles after 72 h. When a ligand analogue was linked to histidine (His), the new complex exhibited as excellent reversible oxygenation property as His-Co(II) complex. Insight into the relationship between structural detail and oxygenation properties will provide valuable suggestion for a new family of oxygen carriers.

Electron transfer mechanism of catalytic superoxide dismutation via Cu(ii/i) complexes: evidence of cupric-superoxo/-hydroperoxo species

To understand the electron transfer mechanisms (outer versus inner sphere) of catalytic superoxide dismutation via a Cu(ii/i) redox couple such as occur in the enzyme copper-zinc superoxide dismutase, the Cu(ii/i) complexes [(L1)2Cu](ClO4)2·CH3CN, (1·CH3CN) and [(L1)2Cu](ClO4), (2) supported by a bis-N2Sthioether ligand, 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine (L1) have been synthesized and structurally characterised. Both 1 and 2 display the same cyclic voltammogram (CV) featuring a quasireversible response at E1/2 = +0.33 V vs. SCE that falls in the SOD potential window of -0.04 V to +0.99 V. These complexes catalytically dismutate superoxide radicals at 298 K in aqueous medium (the IC50 for 1 is 2.15 μM). Electronic absorption spectra (233 K and 298 K), FTIR, ESI mass spectra, CV (233 K and 298 K) and DFT calculations collectively indicate formation of [(L1)2Cu(O2˙(-))](+), [(L1)2Cu(O2(2-))] and [(L1)2Cu(OOH(-))](+) species and help to elucidate the electron transfer mechanism for the SOD function of 1 and 2. Once O2˙(-) binds to Cu(II) (evident at 233 K), the first step of the catalytic cycle (Cu(II) + O2˙(-)→ Cu(I) + O2) does not follow but the second step (Cu(I) + O2˙(-) + 2H(+)→ H2O2 + Cu(II)) does follow. Therefore, the catalytic disproportionation of superoxide radicals via1 and 2 at 298 K indicates that the first and second steps of the catalytic cycle proceed through outer and inner sphere electron transfer mechanisms, respectively. Feasibility of the first step to occur in pure aprotic solvent (where 18-crown-6-ether is used to solubilise KO2) was tested and also supports the same notion of the electron transfer mechanisms as stated above.

Reversible Oxygenation of α-Amino Acid-Cobalt(II) Complexes

We systematically investigated the reversibility, time lapse, and oxygenation-deoxygenation properties of 15 natural α-amino acid-Co(II) complexes through UV-vis spectrophotometer, polarographic oxygen electrode, and DFT calculations, respectively, to explore the relationship between the coordinating structure and reversible oxygenation of α-amino acid-Co(II) complexes. Results revealed that the α-amino acid structure plays a key role in the reversible oxygenation properties of these complexes. The specific configuration of the α-amino acid group affects the eg (1) electron of Co(II) transfer to the π (⁎) orbit of O2; this phenomenon also favors the reversible formation and dissociation of Co-O2 bond when O2 coordinates with Co(II) complexes. Therefore, the co-coordination of amino and carboxyl groups is a determinant of Co complexes to absorb O2 reversibly. The group adjacent to the α-amino acid unit evidently influences the dioxygen affinity and antioxidation ability of the complexes. The presence of amino (or imino) and hydroxy groups adjacent to the α-amino acid group increases the oxygenation-deoxygenation rate and the number of reversible cycles. Our findings demonstrate a new mechanism to develop reversible oxygenation complexes and to reveal the oxygenation of oxygen carriers.