Proton-coupled Electron Transfer Reduction Research Articles

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis.

Invited for this month's cover are the groups of Menny Shalom at the Ben-Gurion University of the Negev, Israel and Dr. Biswajit Mondal at Indian Institute of Technology Gandhinagar, India. The image shows the connection between two half-cells: an electron transfer-mediated [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl] (TEMPO)-catalyzed benzylamine oxidation at the anode and a proton-coupled electron transfer at the cathode for hydrogen generation. The difference in pH dependence of the anodic and cathodic processes enables hybrid water electrolysis at low cell potential (∼1.0 V) by adjusting only the pH value of the electrolytic medium. The Research Article itself is available at 10.1002/cssc.202202271.

Front Cover: The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis (ChemSusChem 10/2023)

The Front Cover shows the combination of an electron transfer mediated anode for [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl] (TEMPO)-catalysed benzylamine oxidation to valuable chemicals and proton-coupled electron transfer process at the cathode for hydrogen generation from water. The difference in pH dependence of the anodic and cathodic processes enables hybrid water electrolysis at low cell potential (∼1.0 V) by adjusting only the pH value of the electrolytic medium. Product selectivity and rate of the anodic reaction can also be controlled by adjusting the pH, keeping the amount of hydrogen production unaffected. More information can be found in the Research Article by B. Mondal et al.

QM Calculations Revealed that Outer-Sphere Electron Transfer Boosted O-O Bond Cleavage in the Multiheme-Dependent Cytochrome bd Oxygen Reductase.

The cytochrome bd oxygen reductase catalyzes the four-electron reduction of dioxygen to two water molecules. The structure of this enzyme reveals three heme molecules in the active site, which differs from that of heme-copper cytochrome c oxidase. The quantum chemical cluster approach was used to uncover the reaction mechanism of this intriguing metalloenzyme. The calculations suggested that a proton-coupled electron transfer reduction occurs first to generate a ferrous heme b595. This is followed by the dioxygen binding at the heme d center coupled with an outer-sphere electron transfer from the ferrous heme b595 to the dioxygen moiety, affording a ferric ion superoxide intermediate. A second proton-coupled electron transfer produces a heme d ferric hydroperoxide, which undergoes efficient O-O bond cleavage facilitated by an outer-sphere electron transfer from the ferrous heme b595 to the O-O σ* orbital and an inner-sphere proton transfer from the heme d hydroxyl group to the leaving hydroxide. The synergistic benefits of the two types of hemes rationalize the highly efficient oxygen reduction repertoire for the multi-heme-dependent cytochrome bd oxygen reductase family.

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis.

Electrolysis of water is a sustainable route to produce clean hydrogen. Full water-splitting requires a high applied potential, in part because of the pH-dependency of the H2 and O2 evolution reactions (HER and OER), which are proton-coupled electron transfer (PCET) reactions. Therefore, the minimum required potential will not change at different pHs. TEMPO [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl], a stable free-radical that undergoes fast electro-oxidation by a single-electron transfer (ET) process, is pH-independent. Here, we show that the combination of PCET and ET processes enables hydrogen production from water at low cell potentials below the theoretical value for full water-splitting by simple pH adjustment. As a case study, we combined the HER with the oxidation of benzylamine by anodically oxidized TEMPO. The pH-independent electrocatalytic oxidation of TEMPO permits the operation of a hybrid water-splitting cell that shows promise to perform at a low cell potential (≈1 V) and neutral pH conditions.

Proton-Coupled Electron-Transfer Reduction of Dioxygen: The Importance of Precursor Complex Formation between Electron Donor and Proton Donor.

The proton-coupled electron transfer (PCET) reaction has drawn extensive attention for its widespread occurrence in chemistry, biology, and materials science. The mechanistic studies via model systems such as tyrosine and phenol oxidation have gradually deepened the understanding of PCET reactions, which was widely accepted and applied to bond activation and transformation. However, direct PCET activation of nonpolar bonds such as the C-H bond, O2, and N2 has yet to be explored. Herein, we report that the interaction between electron donor and proton donor could overcome the barrier of direct O2 activation via a concerted electron-proton transfer mechanism. This work provides a new strategy for developing direct PCET activation of nonpolar bonds.

Antiferromagnetically coupled [Fe8S9] cluster catalyzed acetylene reduction in a nitrogenase-like enzyme DCCPCh: Insights from QM/MM calculations

QM/MM calculations have been used to elucidate the reaction mechanism of the reduction of acetylene to ethylene catalyzed by a nitrogenase-like enzyme DCCPCh with an unusual [Fe8S9] double-cubane cluster. Various plausible reaction pathways, involving different oxidation states and protonation states of the iron-sulfur cluster, have been analyzed to find the most favorable one. The acetylene substrate bridges to the [Fe8S8] cluster via its triple bond in a μ-(bis-η2) coordination mode. After a proton-coupled electron transfer reduction of the reactant complex, the acetylene reduction may be described to proceed via a special “double-electron transfer induced proton transfer” mechanism, deduced from the principal interacting orbital analysis. The anti-ferromagnetically coupled [Fe8S8] cluster delivers a pair of α- and β-electrons to the substrate, concertedly but asynchronously with a proton transfer mediated by the second-shell Lys147 residue to one of the acetylene carbon atoms. Subsequently, the second proton-coupled electron transfer proceeds, followed by protonation of the substrate to afford the ethylene product. During the two-electron reaction, the oxidation states of the [Fe8S8] cluster cycle from [FeII6FeIII2], to [FeII7FeIII], to [FeII5FeIII3], and back to [FeII6FeIII2].

A Mechanistic Dichotomy in Two‐Electron Reduction of Dioxygen Catalyzed by N,N’‐Dimethylated Porphyrin Isomers

Selective two-electron reduction of dioxygen (O2 ) to hydrogen peroxide (H2 O2 ) has been achieved by two saddle-distorted N,N'-dimethylated porphyrin isomers, an N21,N'22-dimethylated porphyrin (anti-Me2 P) and an N21,N'23-dimethylated porphyrin (syn-Me2 P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti-Me2 P with higher turnover number (TON=250 for 30 min) than that by syn-Me2 P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me2 Iph or syn-Me2 Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O2 with isophlorins based on kinetic analysis. The ORR rate by anti-Me2 Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn-Me2 Iph by external protons was observed. The different mechanisms in the O2 reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O2 with inner NH protons of the isophlorins.

CO2 Reduction: From Homogeneous to Heterogeneous Electrocatalysis.

Due to increasing worldwide fossil fuel consumption, carbon dioxide levels have increased in the atmosphere with increasingly important impacts on the environment. Renewable and clean sources of energy have been proposed, including wind and solar, but they are intermittent and require efficient and scalable energy storage technologies. Electrochemical CO2 reduction reaction (CO2RR) provides a valuable approach in this area. It combines solar- or wind-generated electrical production with energy storage in the chemical bonds of carbon-based fuels. It can provide ways to integrate carbon capture, utilization, and storage in energy cycles while maintaining controlled levels of atmospheric CO2. Electrochemistry allows for the utilization of an electrical input to drive chemical reactions. Because CO2 is kinetically inert, highly active catalysts are required to decrease reaction barriers sufficiently so that reaction rates can be achieved that are sufficient for electrochemical CO2 reduction. Given the reaction barriers associated with multiple electron-proton reduction of CO2 to CO, formaldehyde (HC(O)H), formic acid, or formate (HC(O)OH, HC(O)O-), or more highly reduced forms of carbon, there is also a demand for high selectivity in catalysis. Catalysts that have been explored include homogeneous catalysts in solution, catalysts immobilized on surfaces, and heterogeneous catalysts. In homogeneous catalysis, reduction occurs following diffusion of the catalyst to an electrode where multiple proton coupled electron transfer reduction occurs. Useful catalysts in this area are typically transition-metal complexes with organic ligands and electron transfer properties that utilize combinations of metal and ligand redox levels. As a way to limit the amount of catalyst, in device-like configurations, catalysts are added to the surfaces of conductive substrates by surface binding, in polymeric films, or on carbon electrode surfaces with molecular structures and electronic configurations related to catalysts in solution. Immobilized, homogeneous catalysts can suffer from performance losses and even decomposition during long-term CO2 reduction cycles, but they are amenable to detailed mechanistic investigations. In parallel efforts, heterogeneous nanocatalysts have been explored in detail with the development of facile synthetic procedures that can offer highly active catalytic surface areas. Their high activity and stability have attracted a significant level of investigation, including possible exploitation for large-scale applications. However, translation of catalytic reactivity to the surface creates a new reactivity environment and complicates the elucidation of mechanistic details and identification of the active site in exploring reaction pathways. Here, the results of previous studies based on transition-metal complex catalysts for CO2 electroreduction are summarized. Early studies showed that transition-metal complexes of Ru, Ir, Rh, and Os, with well-defined structures, are all capable of catalyzing CO2 reduction to CO or formate. Derivatives of the complexes were surface attached to conducting electrodes by chemical bonding, noncovalent bonding, or polymerization. The concept of surface binding has also been extended to the preparation of surface area electrodes by the chemically controlled deposition of nanostructured catalysts such as nano tin, nano copper, and nano carbon, all of which have been shown to have high selectivities and activities toward CO2 reduction. In our presentation, we end this Account with recent advances and a perspective about the application of electrocatalysis in carbon dioxide reduction.

A self-gating proton-coupled electron transfer reduction of hexavalent chromium by core-shell SBA-Dithiocarbamate chitosan composite

We have proposed a novel strategy for the reduction plus adsorption process for hexavalent chromium elimination by thiol functional hybrid materials through a self-gating process. Namely, we exploit that coating dithiocarbamate chitosan at the surface of SBA-15 affords a core-shell composite that undergoes reversible shape transformations while thiol functional groups acted as proton-coupled electron donor for [Cr2O7]2−. The reduction of [Cr2O7]2− to Cr3+ was highly efficient and exceptionally rapid, occurred within 5 min with the reduction amount of 899.66 mg of [Cr2O7]2− / 1 g of nanocomposite as a record high value. During the reduction of [Cr2O7]2−, thiol functional groups (-SH) were oxidized into disulfide linkages (SS), and simultaneously chitosan matrix turned into shrunken structure because of the consuming of protons, preventing any release of Cr3+. Disulfides can also be reversely reduced to thiols by thiosulphates (S2O32−), which was attractive for regeneration and recyclability of the nanocomposite. Moreover, the [Cr2O7]2− elimination through self-gating process was highly selective against a huge concentration of background electrolytes. This alternative strategy ensures the outstanding and stable performance in applied fields, and could be conducted in various pollution control techniques like permeable reactive barriers.

Ligand‐Based PCET Reduction in a Heteroleptic Ni(bpy)(dithiolene) Electrocatalyst Giving Rise to Higher Metal Basicity Required for Hydrogen Evolution

AbstractProton abstraction leading to the formation of a hydride species required to evolve H2 largely relies on the basicity of d orbital of the metal responsible for this action. Here we report that a square‐planar NiII(bpy)(dcbdt) hydrogen evolution catalyst shows substantial acceleration in the proton abstraction rate due to the increased basicity at the filled Ni dz2 orbital after formation of [NiI(bpy−.)(dcbdt)]2− via consecutive two one‐electron reductions (bpy=2,2′‐bipyridine; dcbdt=4,5‐dicyanobenzene‐1,2‐dithiolate). The catalyst is likely to adopt the EECC′ mechanism in which the rate of the first protonation step is by far higher than that of the second step, even though an alternative path requiring another reduction (i. e., ECEC′) remains unexcluded. Our DFT calculations reveal that the first and second reductions are correlated with the electron injection into the metal‐ligand anti‐bonding and π*(bpy) orbitals, respectively, where the latter orbital shows non‐negligible hybridization with the nickel d orbital. In addition, a homoleptic catalyst [NiII(dcbdt)2]2− is shown to adopt the EC′EC mechanism with the rate‐determing step being a hydride forming step, consistent with the largely delocalized nature of the injected electron over the two dcbdt ligands (π*(dcbdt) orbital). This work demonstrates the importance of raising the basicity of the metal d orbital, relevant to promote the proton‐coupled electron transfer (PCET).

Efficient photocatalytic proton-coupled electron-transfer reduction of O2 using a saddle-distorted porphyrin as a photocatalyst.

Photocatalytic O2 reduction reactions proceeded to produce H2O2 using a diprotonated saddle-distorted dodecaphenylporphyrin as a photocatalyst. The quantum yield (12%), the turnover number (3000 for 6 h), and the turnover frequency (500 h-1) are achieved in photocatalytic systems based on free-base porphyrins for the first time. The photocatalytic reaction mechanism has been revealed by ns-laser flash photolysis and kinetic analysis.

Solvation and Proton-Coupled Electron Transfer Reduction Potential of 2NO• to 1HNO in Aqueous Solution: A Theoretical Investigation.

In this work, quantum mechanical calculations and Monte Carlo statistical mechanical simulations were carried out to investigate the solvation properties of HNO in aqueous solution and to evaluate the proton-coupled one electron reduction potential of 2NO to 1HNO, which is essential missing information to understand the fate of 2NO in the biological medium. Our results showed that the 1HNO molecule acts mainly as a hydrogen bond donor in aqueous solution with an average energy of -5.5 ± 1.3 kcal/mol. The solvation free energy of 1HNO in aqueous solution, computed using three approaches based on the linear response theory, revealed that the current prediction of the hydration free energy of HNO is, at least, 2 times underestimated. We proposed two pathways for the production of HNO through reduction of NO. The first pathway is the direct reduction of NO through proton-coupled electron transfer to produce HNO, and the second path is the reduction of the radical anion HONO•-, which is involved in equilibrium with NO in aqueous solution. We have shown that both pathways are viable processes under physiological conditions, having reduction potentials of E°' = -0.161 V and E°' ≈ 1 V for the first and second pathways, respectively. The results shows that both processes can be promoted by well-known biological reductants such as NADH, ascorbate, vitamin E (tocopherol), cysteine, and glutathione, for which the reduction potential at physiological pH is around -0.3 to -0.5 V. The computed reduction potential of NO through the radical anion HONO•- can also explain the recent experimental findings on the formation of HNO through the reduction of NO, promoted by H2S, vitamin C, and aromatic alcohols. Therefore, these results contribute to shed some light into the question of whether and how HNO is produced in vivo and also for the understanding of the biochemical and physiological effects of NO.

Proton-Coupled Electron Transfer Reduction of a Quinone by an Oxide-Bound Riboflavin Derivative

The redox properties of a surface-bound phosphate flavin derivative (flavin mononucleotide, FMN) have been investigated on planar-FTO and nanoITO electrodes under acidic conditions in 1:1 CH3CN/H2O (V:V). On FTO, reversible 2e-/2H+ reduction of FTO|-FMN to FTO|-FMNH2 occurs with the pH and scan rate dependence expected for a 2e-/2H+ surface-bound couple. Addition of tetramethylbenzoquinone (Me4Q) results in rapid electrocatalyzed reduction to the hydroquinone by a pathway first order in quinone and first order in acid with kH = (2.6 ± 0.2) × 106 M–1 s–1. Electrocatalytic reduction of the quinone also occurs on derivatized, high surface area nanoITO electrodes with evidence for competitive rate-limiting diffusion of the quinone into the mesoporous nanostructure.

Highly Efficient and Selective Two-Electron Reduction of Dioxygen Catalyzed By Cobalt Chlorin Complexes to Produce Hydrogen Peroxide

Hydrogen peroxide (H2O2) is an ideal energy carrier alternative to hydrogen, because H2O2 can be produced by photocatalytic oxidation of water by O2 using solar energy,1 and H2O2 can be used as a fuel in a one-compartment H2O2 fuel cell with low cost material.2-4 H2O2 can also be produced by electrochemical two-electron reduction of O2 using solar cells.5 Thus, it is highly desired to develop efficient catalysts for selective two-electron reduction of O2 to produce H2O2. We report herein that cobalt chlorin complexes in Chart 1 act as highly efficient and selective catalysts for two-electron reduction of O2 to H2O2. The detailed study based on the kinetics and thermodynamics have provided valuable insight into the development of the more efficient electrocatalyst for two-electron re-duction of O2 to H2O2, which is essential to realize H2O2-energy society. of O2 by 1,1’-dimethylferrocene (Me2Fc) efficiently and selectively to produce H2O2 in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K (eq 1). The rate of formation of Me2Fc+was 2Me2Fc + O2 + 2H+ --> 2Me2Fc+ + H2O2 (1) CoII(Ch n ) proportinal to concentrations of CoII(Ch n ) and O2, whereas the rate was independent of concentraton of Me2Fc. On the other hand, the catalytic rate increased with increasing concentraiton of HClO4 to reach a constant value. Such kinetic results indicate that the rate-determining step in the catalytic cycle (Scheme 1) is the proton-coupled electron transfer reduction of O2 by CoII(Ch n ), followed by fast electron transfer from Me2Fc to [CoII(Ch n )]+. The one-electron reduction potential of [CoIII(Ch 1 )]+ was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [CoIII(Ch 1 )]+.6 The introduction of electron-withdrawing groups on the chlorin ligand in CoII(Ch n ) (n = 2, 3) resulted in the positive shift of the redox potential for Co(III/II) from 0.37 V to 0.45 and 0.40 V, respectively. In the presence of HClO4, however, no positive shifts of the redox potential for [CoIII(Ch n )]+/CoII(Ch n ) (n = 2, 3) were observed. As a result, CoII(Ch 3 ), which has the lowest one-electron oxidation potential in the presence of HClO4 among the cobalt chlorin complexes in Chart 1, exhibited the highest catalytic reactivity for the two-electron recution of O2 by Me2Fc in the presence of HClO4in PhCN. The electrocatalytic two-electron reduction of O2 was also examined by using a CoIICh 1 -modified carbon paper electrode with multiwalled carbon nanotubes (MWCNTs) in an aqueous solution (pH = 1). A catalytic current of two-electron reduction of O2 was observed at a small overpotential. The electrocatalytic two-electron reduction of O2 was combined with the photocatalytic oxidation of water using semiconductor photocatalysts to produce H2O2 from H2O and O2using solar energy (eq 2). hν 2H2O + O2 --> 2H2O2 (2) catalysts References [1] Kato, S.; Jung, J.; Suenobu, T.; Fukuzumi, S. Energy Environ. Sci. 2013, 6, 3756. [2] Yamada, Y.; Yoshida, S.; Honda, T.; Fukuzumi, S. Energy Environ. Sci. 2011, 4, 2822. [3] Yamada, Y.; Yoneda, M.; Fukuzumi, S. Chem.–Eur. J. 2013, 19, 11733. [4] Yamada, Y.; Yoneda, M.; Fukuzumi, S. Inorg. Chem. 2014, 53, 1272. [5] Yamada, Y.; Fukunishi, Y.; Yamazaki, S.; Fukuzumi, S. Chem. Commun. 2010, 46, 7334. [6] Mase, K.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 2800. Figure 1

Much enhanced catalytic reactivity of cobalt chlorin derivatives on two-electron reduction of dioxygen to produce hydrogen peroxide.

Effects of changes in the redox potential or configuration of cobalt chlorin derivatives (Co(II)(Chn) (n = 1-3)) on the catalytic mechanism and the activity of two-electron reduction of dioxygen (O2) were investigated based on the detailed kinetic study by spectroscopic and electrochemical measurements. Nonsubstituted cobalt chlorin complex (Co(II)(Ch1)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O2) by a one-electron reductant (1,1'-dimethylferrocene) to produce hydrogen peroxide (H2O2) in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K. The detailed kinetic studies have revealed that the rate-determining step in the catalytic cycle is the proton-coupled electron transfer reduction of O2 with the protonated Co(II)(Ch1) complex ([Co(II)(Ch1H)](+)), where one-electron reduction potential of [Co(III)(Ch1)](+) was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [Co(III)(Ch1)](+). The introduction of electron-withdrawing aldehyde group (position C-3) (Co(II)(Ch3)) and both methoxycarbonyl group (position C-13(2)) and aldehyde group (position C-3) (Co(II)(Ch2)) on the chlorin ligand resulted in the positive shifts of redox potential for Co(III/II) from 0.37 V to 0.45 and 0.40 V, respectively, whereas, in the presence of HClO4, no positive shifts of those redox potentials for [Co(III)(Chn)](+)/Co(II)(Chn) (n = 2, 3) were observed due to lower acceptability of protonation. As a result, such a change in redox property resulted in the enhancement of the catalytic reactivity, where the observed rate constant (kobs) value of Co(II)(Ch3) was 36-fold larger than that of Co(II)(Ch1).

High-valent chromium-oxo complex acting as an efficient catalyst precursor for selective two-electron reduction of dioxygen by a ferrocene derivative.

Efficient catalytic two-electron reduction of dioxygen (O2) by octamethylferrocene (Me8Fc) produced hydrogen peroxide (H2O2) using a high-valent chromium(V)-oxo corrole complex, [(tpfc)Cr(V)(O)] (tpfc = tris(pentafluorophenyl)corrole) as a catalyst precursor in the presence of trifluoroacetic acid (TFA) in acetonitrile (MeCN). The facile two-electron reduction of [(tpfc)Cr(V)(O)] by 2 equiv of Me8Fc in the presence of excess TFA produced the corresponding chromium(III) corrole [(tpfc)Cr(III)(OH2)] via fast electron transfer from Me8Fc to [(tpfc)Cr(V)(O)] followed by double protonation of [(tpfc)Cr(IV)(O)](-) and facile second-electron transfer from Me8Fc. The rate-determining step in the catalytic two-electron reduction of O2 by Me8Fc in the presence of excess TFA is inner-sphere electron transfer from [(tpfc)Cr(III)(OH2)] to O2 to produce the chromium(IV) superoxo species [(tpfc)Cr(IV)(O2(•-))], followed by fast proton-coupled electron transfer reduction of [(tpfc)Cr(IV)(O2(•-))] by Me8Fc to yield H2O2, accompanied by regeneration of [(tpfc)Cr(III)(OH2)]. Thus, although the catalytic two-electron reduction of O2 by Me8Fc was started by [(tpfc)Cr(V)(O)], no regeneration of [(tpfc)Cr(V)(O)] was observed in the presence of excess TFA, regardless of the tetragonal chromium complex being to the left of the oxo wall. In the presence of a stoichiometric amount of TFA, however, disproportionation of [(tfpc)Cr(IV)(O)](-) occurred via the protonated species [(tpfc)Cr(IV)(OH)] to produce [(tpfc)Cr(III)(OH2)] and [(tpfc)Cr(V)(O)].

Efficient Two-Electron Reduction of Dioxygen to Hydrogen Peroxide with One-Electron Reductants with a Small Overpotential Catalyzed by a Cobalt Chlorin Complex

A cobalt chlorin complex (Co(II)(Ch)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O(2)) by one-electron reductants (ferrocene derivatives) to produce hydrogen peroxide (H(2)O(2)) in the presence of perchloric acid (HClO(4)) in benzonitrile (PhCN) at 298 K. The catalytic reactivity of Co(II)(Ch) was much higher than that of a cobalt porphyrin complex (Co(II)(OEP), OEP(2-) = octaethylporphyrin dianion), which is a typical porphyrinoid complex. The two-electron reduction of O(2) by 1,1'-dibromoferrocene (Br(2)Fc) was catalyzed by Co(II)(Ch), whereas virtually no reduction of O(2) occurred with Co(II)(OEP). In addition, Co(II)(Ch) is more stable than Co(II)(OEP), where the catalytic turnover number (TON) of the two-electron reduction of O(2) catalyzed by Co(II)(Ch) exceeded 30000. The detailed kinetic studies have revealed that the rate-determining step in the catalytic cycle is the proton-coupled electron transfer reduction of O(2) with the protonated Co(II)(Ch) ([Co(II)(ChH)](+)) that is produced by facile electron-transfer reduction of [Co(III)(ChH)](2+) by ferrocene derivative in the presence of HClO(4). The one-electron-reduction potential of [Co(III)(Ch)](+) was positively shifted from 0.37 V (vs SCE) to 0.48 V by the addition of HClO(4) due to the protonation of [Co(III)(Ch)](+). Such a positive shift of [Co(III)(Ch)](+) by protonation resulted in enhancement of the catalytic reactivity of [Co(III)(ChH)](2+) for the two-electron reduction of O(2) with a lower overpotential as compared with that of [Co(III)(OEP)](+).

Fluorinated-cobalt phthalocyanine catalyzed oxygen reduction at liquid/liquid interfaces

Cobalt fluoro-phthalocyanine (CoFPc) can efficiently catalyze the reduction of oxygen (O2) by a weak electron donor, namely tetrathiafulvalene (TTF), at the polarized water/1,2-dichloroethane (DCE) interface. Two-phase shake flask experiments and ion transfer voltammetry results suggest that the catalytic reaction proceeds as a proton-coupled electron transfer reduction of oxygen to mainly hydrogen peroxide, where a proton transfer from water to DCE is occurring concomitantly with the electron transfer reaction between TTF and oxygen.

Proton-Coupled Electron-Transfer Reduction of Dioxygen Catalyzed by a Saddle-Distorted Cobalt Phthalocyanine

Proton-coupled electron-transfer reduction of dioxygen (O(2)) to afford hydrogen peroxide (H(2)O(2)) was investigated by using ferrocene derivatives as reductants and saddle-distorted (α-octaphenylphthalocyaninato)cobalt(II) (Co(II)(Ph(8)Pc)) as a catalyst under acidic conditions. The selective two-electron reduction of O(2) by dimethylferrocene (Me(2)Fc) and decamethylferrocene (Me(10)Fc) occurs to yield H(2)O(2) and the corresponding ferrocenium ions (Me(2)Fc(+) and Me(10)Fc(+), respectively). Mechanisms of the catalytic reduction of O(2) are discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The active species to react with O(2) in the catalytic reaction is switched from Co(II)(Ph(8)Pc) to protonated Co(I)(Ph(8)PcH), depending on the reducing ability of ferrocene derivatives employed. The protonation of Co(II)(Ph(8)Pc) inhibits the direct reduction of O(2); however, the proton-coupled electron transfer from Me(10)Fc to Co(II)(Ph(8)Pc) and the protonated [Co(II)(Ph(8)PcH)](+) occurs to produce Co(I)(Ph(8)PcH) and [Co(I)(Ph(8)PcH(2))](+), respectively, which react immediately with O(2). The rate-determining step is a proton-coupled electron-transfer reduction of O(2) by Co(II)(Ph(8)Pc) in the Co(II)(Ph(8)Pc)-catalyzed cycle with Me(2)Fc, whereas it is changed to the electron-transfer reduction of [Co(II)(Ph(8)PcH)](+) by Me(10)Fc in the Co(I)(Ph(8)PcH)-catalyzed cycle with Me(10)Fc. A single crystal of monoprotonated [Co(III)(Ph(8)Pc)](+), [Co(III)Cl(2)(Ph(8)PcH)], produced by the proton-coupled electron-transfer reduction of O(2) by Co(II)(Ph(8)Pc) with HCl, was obtained, and the crystal structure was determined in comparison with that of Co(II)(Ph(8)Pc).

Competing Pathways in the photo-Proton-Coupled Electron Transfer Reduction of fac-[Re(bpy)(CO)3(4,4′-bpy]+* by Hydroquinone

The emitting metal-to-ligand charge transfer (MLCT) excited state of fac-[ReI(bpy)(CO)3(4,4′-bpy)]+ (1) (bpy is 2,2′-bipyridine, 4,4′-bpy is 4,4′-bipyridine), [ReII(bpy–•)(CO)3(4,4′-bpy)]+*, is reductively quenched by 1,4-hydroquinone (H2Q) in CH3CN at 23 ± 2 °C by competing pathways to give a common electron–proton-transfer intermediate. In one pathway, electron transfer (ET) quenching occurs to give ReI(bpy–•)(CO)3(4,4′-bpy)]0 with k = (1.8 ± 0.2) × 109 M–1 s–1, followed by proton transfer from H2Q to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+. Protonation triggers intramolecular bpy•– → 4,4′-bpyH+ electron transfer. In the second pathway, preassociation occurs between the ground state and H2Q at high concentrations. Subsequent Re → bpy MLCT excitation of the adduct is followed by electron–proton transfer from H2Q in concert with intramolecular bpy•– → 4,4′-bpyH+ electron transfer to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+ with k = (1.0 ± 0.4) × 109 s–1 in 3:1 CH3CN/H2O.