Proton-coupled Redox Research Articles

Electric-field-assisted proton coupling enhanced oxygen evolution reaction.

The discovery of Mn-Ca complex in photosystem II stimulates research of manganese-based catalysts for oxygen evolution reaction (OER). However, conventional chemical strategies face challenges in regulating the four electron-proton processes of OER. Herein, we investigate alpha-manganese dioxide (α-MnO2) with typical MnIV-O-MnIII-HxO motifs as a model for adjusting proton coupling. We reveal that pre-equilibrium proton-coupled redox transition provides an adjustable energy profile for OER, paving the way for in-situ enhancing proton coupling through a new "reagent"- external electric field. Based on the α-MnO2 single-nanowire device, gate voltage induces a 4-fold increase in OER current density at 1.7 V versus reversible hydrogen electrode. Moreover, the proof-of-principle external electric field-assisted flow cell for water splitting demonstrates a 34% increase in current density and a 44.7 mW/cm² increase in net output power. These findings indicate an in-depth understanding of the role of proton-incorporated redox transition and develop practical approach for high-efficiency electrocatalysis.

Template Assisted Formation of 32 and 34π Octaphyrins Embedded with Dithienopyrrole Cores: A New Scaffold to Unravel Proton Coupled Redox Switching and (Anti)Aromaticity.

Herein, we report two distinct octaphyrins obtained by the condensation of new dithieno[3,2-b:2',3'-d]pyrrole based tetrapyrrane under two different acidic conditions. Fourfold meso-substituted octaphyrin was the major product when the reaction was performed in the presence of an aryl aldehyde using trifluoroacetic acid. Whereas, the sixfold meso-substituted octaphyrin was obtained when the precursor was condensed with pentafluorobenzaldehyde using para-toluenesulfonic acid. Such a template effect of aryl aldehydes in oxidative coupling reactions is realized for the first time in literature. Experimental and theoretical studies suggested that the oxidized form of fourfold meso-substituted octaphyrin is 32π antiaromatic and undergoes proton-coupled electron transfer (PCET) to the protonated form of 34π aromatic congener upon treatment with protic acids. Furthermore, small organic molecules such as alcohols and amines were also found to promote chemical reduction. Single crystal X-ray structure revealed that the aromatic counterpart is highly planar and stabilized by several intermolecular H-bonding and F-F interactions, leading to a large 3D supramolecular arrangement and exhibited colorimetric sensing for fluoride and hydroxide anions. On the other hand, sixfold meso-substituted octaphyrin did not show (anti)aromatic features, PCET or anion sensing, but its intriguing absorption features associated with protonation could make it an ideal candidate for pH-dependent bioimaging.

Correlation between Electronic Descriptor and Proton-Coupled Electron Transfer Thermodynamics in Doped Graphite-Conjugated Catalysts.

Graphite-conjugated catalysts (GCCs) provide a powerful framework for investigating correlations between electronic structure features and chemical reactivity of single-site heterogeneous catalysts. GCC-phenazine undergoes proton-coupled electron transfer (PCET) involving protonation of phenazine at its two nitrogen atoms with the addition of two electrons. Herein, this PCET reaction is investigated in the presence of defects, such as heteroatom dopants, in the graphitic surface. The proton-coupled redox potentials, EPCET, are computed using a constant potential periodic density functional theory (DFT) strategy. The electronic states directly involved in PCET for GCC-phenazine exhibit the same nitrogen orbital character as those for molecular phenazine. The energy εLUS of this phenazine-related lowest unoccupied electronic state in GCC-phenazine is identified as a descriptor for changes in PCET thermodynamics. Importantly, εLUS is obtained from only a single DFT calculation but can predict EPCET, which requires many such calculations. Similar electronic features may be useful descriptors for thermodynamic properties of other single-site catalysts.

Distinctive Aspects in Aquation, Proton-Coupled Redox, and Photoisomerization Reactions between Geometric Isomers of Mononuclear Ruthenium Complexes with a Large-π-Conjugated Tetradentate Ligand.

Geometric isomers of mononuclear ruthenium(II) complexes, distal-/proximal-[Ru(tpy)(dpda)Cl]+ (d-/p-RuCl, tpy = 2,2':6',2″-terpyridine, dpda = 2,7-bis(2-pyridyl)-1,8-diazaanthracene), were newly synthesized to comprehensively investigate the geometric and electronic structures and distinctive aspects in various reactions between isomers. The ultraviolet (UV)-visible absorption spectra of d-/p-RuCl isomers show intense bands for metal-to-ligand charge transfer (MLCT) at close wavelengths of 576 and 573 nm, respectively. However, time-dependent density functional theory (TD-DFT) calculations suggest that the MLCT transition of d-RuCl involves mainly single transitions to the π* orbital of the dpda ligand in contrast to mixing of the π* orbitals of the dpda and tpy ligands for p-RuCl. The aquation reaction (1.5 × 10-3 s-1) of p-RuCl to yield proximal-[Ru(tpy)(dpda)(OH2)]2+ (p-RuH2O) is faster than that (5.3 × 10-6 s-1) of d-RuCl in D2O/CD3OD (4:1 v/v) by three orders of magnitude, which resulted from the longer Ru-Cl bond by 0.017 Å and the distorted angle (100.2(3)°) of Cl-Ru-N (a nitrogen of dpda, being on a tpy plane) due to the steric repulsion between Cl and dpda for p-RuCl. Electrochemical measurements showed that d-RuH2O undergoes a 2-step oxidation reaction of 1H+-coupled 1e- processes of RuII-OH2/RuIII-OH and RuIII-OH/RuIV═O at pH 1-9, whereas p-RuH2O undergoes a 1-step oxidation reaction of a 2H+-coupled 2e- process of RuII-OH2/RuIV═O in the pH range of pH 1-10. The irreversible photoisomerization from d-RuH2O to p-RuH2O was observed in aqueous solution with an internal quantum yield (Φ) of 5.4 × 10-3% at 520 nm, which is lower compared with Φ = 1.1-2.1% of mononuclear Ru(II) aquo complexes with similar bidentate ligands instead of dpda by three orders of magnitude. This is possibly ascribed to the faster nonradiative decay rate from the excited 3MLCT state to the ground state for d-RuH2O due to the lower π* level of dpda ligands according to the energy-gap law: the rate decreases exponentially with the increasing energy gap.

Theoretical Modeling of Electrochemical Proton-Coupled Electron Transfer.

Proton-coupled electron transfer (PCET) plays an essential role in a wide range of electrocatalytic processes. A vast array of theoretical and computational methods have been developed to study electrochemical PCET. These methods can be used to calculate redox potentials and pKa values for molecular electrocatalysts, proton-coupled redox potentials and bond dissociation free energies for PCET at metal and semiconductor interfaces, and reorganization energies associated with electrochemical PCET. Periodic density functional theory can also be used to compute PCET activation energies and perform molecular dynamics simulations of electrochemical interfaces. Various approaches for maintaining a constant electrode potential in electronic structure calculations and modeling complex interactions in the electric double layer (EDL) have been developed. Theoretical formulations for both homogeneous and heterogeneous electrochemical PCET spanning the adiabatic, nonadiabatic, and solvent-controlled regimes have been developed and provide analytical expressions for the rate constants and current densities as functions of applied potential. The quantum mechanical treatment of the proton and inclusion of excited vibronic states have been shown to be critical for describing experimental data, such as Tafel slopes and potential-dependent kinetic isotope effects. The calculated rate constants can be used as input to microkinetic models and voltammogram simulations to elucidate complex electrocatalytic processes.

Multicapacitor Approach to Interfacial Proton-Coupled Electron Transfer Thermodynamics at Constant Potential

Theoretical calculations of interfacial thermodynamics at constant potential enhance understanding of heterogeneous electrocatalytic reactions. Herein, a strategy is devised for computing reaction thermodynamics for electrochemical proton-coupled electron transfer (PCET), a key elementary step in a wide range of electrocatalytic processes. In this approach, Gibbs free energies obtained from constant charge periodic density functional theory calculations are transformed to grand potentials by using a grid-based mapping procedure in conjunction with a multicapacitor model that accounts for constantly charged surface adsorbates. The energetic contribution of the adsorbate capacitor to the grand potential is found to be essential for computing proton-coupled redox potentials at constant potential. This strategy is applied to graphite-conjugated catalysts, wherein organic acids are attached to carbon surfaces through a conductive phenazine bridge. In these systems, PCET occurs at both the phenazine bridges and the organic acids, which can be negatively or positively charged. For a given graphite-conjugated catalyst active site, the charge of the organic acid adsorbate remains virtually constant for the relevant range of electrode potentials. Moreover, the potential-dependent graphite surface charge density, which excludes this adsorbate charge, is consistent across all systems studied. Within this framework, the potential-dependent PCET reaction free energies are independent of cell size, thereby avoiding the need for computationally expensive cell size extrapolation techniques. The proton-coupled redox potentials computed with this strategy are in agreement with experimental data. This computational strategy, as well as the conceptual insights about the impact of charged adsorbates on electrochemical interfaces, is applicable to other materials and processes.

Highly selective electrocatalysis for carbon dioxide reduction to formic acid by a Co(II) complex with an equatorial N4 ligand

A Co complex, [CoII(bibpy)L] (1-L, L is an axial ligand) with an equatorial N4 ligand of bibpy2− (H2bibpy = 6,6’-bis-(1H-benzimidazol-2-yl)-2,2’-bipyridine) has been newly synthesized to study electrocatalytic CO2 reduction in a homogeneous DMF solution. Cyclic voltammogram (CV) of 1-L in DMF under Ar exhibited two reversible redox responses at -1.42 V (1st step) and -2.32 V (2nd step) vs. the ferrocene/ferrocenium (Fc/Fc+) redox potential, which could be assigned to 1-L/1− (with release of L) and 1−/12− pairs, respectively. The CV measurement with adding 4.0% water suggests that water replaces the axial L ligand on 1-L, but no longer interacts with 1− and 12− because of requiring no axial ligand. In CV of 1-L in the CO2-saturated DMF solution, a relatively weak reduction wave appeared at -1.70 V of the peak reduction potential (Ep1’) in the 1’st step, which is attributed to the reduction of 1-L with the concerted oxidative coordination of CO2 onto the axial site to form [1-CO2]−. CV of 1-L exhibited the significant catalytic current below the onset potential (Eon) of -1.9 V for CO2 reduction. The electrocatalytic CO2 reduction proceeded stably in the presence of 2.0% water as a proton source. The Eon value increased linearly from -1.98 to -1.89 V with the water content increase from 0 to 4.0%. The hypothetical [1-CO2]−/[1-CO2]2− reduction (2’nd step) involved in the catalytic CO2 reduction might be proton-coupled redox process of [1-CO2]−/[1-CO2H]−. The bulk electrolysis by 1-L in the 2.0% water-containing DMF solution at -2.32 V afforded H2 with 3.0% Faraday efficiency (FE) and HCO2H with 68% FE. The selectivity (RHCO2H) for HCO2H production was 96%, defined as the molar fraction of HCO2H in all the products. The overpotential (η) for CO2 reduction to HCO2H was 0.62 V (at -2.07 V vs Fc/Fc+). These performances of 1-L are among those of the hitherto-reported state-of-the-art Co-complexes for electrocatalytic CO2 reduction to HCO2H in homogenous solutions.

Computing Proton-Coupled Redox Potentials of Fluorotyrosines in a Protein Environment.

The oxidation of tyrosine to form the neutral tyrosine radical via proton-coupled electron transfer is essential for a wide range of biological processes. The precise measurement of the proton-coupled redox potentials of tyrosine (Y) in complex protein environments is challenging mainly because of the highly oxidizing and reactive nature of the radical state. Herein, a computational strategy is presented for predicting proton-coupled redox potentials in a protein environment. In this strategy, both the reduced Y-OH and oxidized Y-O• forms of tyrosine are sampled with molecular dynamics using a molecular mechanical force field. For a large number of conformations, a quantum mechanical/molecular mechanical (QM/MM) electrostatic embedding scheme is used to compute the free-energy differences between the reduced and oxidized forms, including the zero-point energy and entropic contributions as well as the impact of the protein electrostatic environment. This strategy is applied to a series of fluorinated tyrosine derivatives embedded in a de novo α-helical protein denoted as α3Y. The force fields for both the reduced and oxidized forms of these noncanonical fluorinated tyrosine residues are parameterized for general use. The calculated relative proton-coupled redox potentials agree with experimentally measured values with a mean unsigned error of 24 mV. Analysis of the simulations illustrates that hydrogen-bonding interactions between tyrosine and water increase the redox potentials by ∼100-250 mV, with significant variations because of the fluctuating protein environment. This QM/MM approach enables the calculation of proton-coupled redox potentials of tyrosine and other residues such as tryptophan in a variety of protein systems.

Interfacial Field-Driven Proton-Coupled Electron Transfer at Graphite-Conjugated Organic Acids.

Interfacial proton-coupled electron transfer (PCET) reactions are central to the operation of a wide array of energy conversion technologies, but molecular-level insights into interfacial PCET are limited. At carbon surfaces, designer sites for interfacial PCET can be incorporated by conjugating organic acid functional groups to graphite edges though aromatic phenazine linkages. At these graphite-conjugated catalysts (GCCs) bearing organic acid moieties, PCET is driven by complex interfacial electrostatic and field gradients that are difficult to probe experimentally. Herein, the spatially inhomogeneous interfacial electrostatic potentials and electric fields of GCC organic acids are computed as functions of applied potential. The calculated proton-coupled redox potentials for the PCET reactions at the GCC phenazine bridges and organic acid sites are in agreement with cyclic voltammetry measurements for a series of GCC acids. The trends in these redox potentials are explained in terms of the acidity of the molecular analogues and continuous conjugation between the acid and the graphite surface. The calculations illustrate that this conjugation is interrupted in a GCC acetic acid system, providing an explanation for the absence of a cyclic voltammetry peak corresponding to PCET at this acid site. This combined theoretical and experimental study demonstrates the critical role of continuous conjugation and strong electronic coupling between the GCC acid site and the graphite to enable interfacial field-driven PCET at the acid site. Understanding the connection between the atomic structure of the surface and the interfacial electrostatic potentials and fields that govern PCET thermochemistry may guide heterogeneous catalyst design.

Concerted Proton-Electron Transfer Reactivity at a Multimetallic Co4O4 Cubane Cluster.

High-valent oxocobalt(IV) species have been invoked as key intermediates in oxidative catalysis, but investigations into the chemistry of proton-coupled redox reactions of such species have been limited. Herein, the reactivity of an established water oxidation catalyst, [Co4O4(OAc)4(py)4][PF6], toward H-atom abstraction reactions is described. Mechanistic analyses and density functional theory (DFT) calculations support a concerted proton-electron transfer (CPET) pathway in which the high energy intermediates formed in stepwise pathways are bypassed. Natural bond orbital (NBO) calculations point to cooperative donor-acceptor σ interactions at the transition state, whereby the H-atom of the substrate is transferred to an orbital delocalized over a Co3(μ3-O) fragment. The mechanistic insights provide design principles for the development of catalytic C-H activation processes mediated by a multimetallic oxo metal cluster.

Proton-coupled redox properties and water oxidation catalysis of an aqua-coordinated (µ-oxo)diruthenium(III) complex

The electronic absorption and redox properties as well as the catalysis for water oxidation reaction (WOR) of an aqua-coordinated oxo-bridged diruthenium(III) complex trans(µ-O,OH2)-[Ru2(µ-O)(µ-CH3COO)2(bpy)2(H2O)2](PF6)2·3H2O ([1](PF6)2·3H2O; bpy = 2,2′-bipyridine) in aqueous media are reported. [1]2+ displays a visible absorption band at 566 nm ascribed to metal-to-metal charge transition (MMCT) within the Ru2O core. This visible band redshifts due to the deprotonation of the terminal aqua ligands of [1]2+ in alkaline solutions. [1]2+ shows proton-coupled electron transfer behaviors in aqueous solutions at pH 0–14. The oxidation of RuIIIRuIII in [1]2+ has successfully reached the higher oxidation states including RuIVRuIV and RuVRuV which are stabilized by the deprotonation of the terminal aqua ligands and are important for catalytic water oxidation. WOR catalyzed by [1]2+ in acidic solutions was studied using electrochemical and chemical oxidation. In the solution of [1]2+ at pH 1.0, the overpotential for WOR was ~0.38 V. A turnover number of 23 was obtained by using (NH4)2Ce(NO3)6 as the oxidant. A nucleophilic-attack catalytic mechanism was proposed.

Catalytically important damage-free structures of a copper nitrite reductase obtained by femtosecond X-ray laser and room-temperature neutron crystallography.

Copper-containing nitrite reductases (CuNiRs) that convert NO2 - to NO via a CuCAT-His-Cys-CuET proton-coupled redox system are of central importance in nitrogen-based energy metabolism. These metalloenzymes, like all redox enzymes, are very susceptible to radiation damage from the intense synchrotron-radiation X-rays that are used to obtain structures at high resolution. Understanding the chemistry that underpins the enzyme mechanisms in these systems requires resolutions of better than 2 Å. Here, for the first time, the damage-free structure of the resting state of one of the most studied CuNiRs was obtained by combining X-ray free-electron laser (XFEL) and neutron crystallography. This represents the first direct comparison of neutron and XFEL structural data for any protein. In addition, damage-free structures of the reduced and nitrite-bound forms have been obtained to high resolution from cryogenically maintained crystals by XFEL crystallography. It is demonstrated that AspCAT and HisCAT are deprotonated in the resting state of CuNiRs at pH values close to the optimum for activity. A bridging neutral water (D2O) is positioned with one deuteron directed towards AspCAT Oδ1 and one towards HisCAT N∊2. The catalytic T2Cu-ligated water (W1) can clearly be modelled as a neutral D2O molecule as opposed to D3O+ or OD-, which have previously been suggested as possible alternatives. The bridging water restricts the movement of the unprotonated AspCAT and is too distant to form a hydrogen bond to the O atom of the bound nitrite that interacts with AspCAT. Upon the binding of NO2 - a proton is transferred from the bridging water to the Oδ2 atom of AspCAT, prompting electron transfer from T1Cu to T2Cu and reducing the catalytic redox centre. This triggers the transfer of a proton from AspCAT to the bound nitrite, enabling the reaction to proceed.

Proton-Coupled Reversible Redox Reaction of Zinc-Terephthalate Metal-Organic Framework

Redox properties of metal-organic frameworks (MOFs) between Zn2+ ion and terephthalic acid (TPA) in compositions as Zn3(OH)4(TPA)・6H2O, Zn4(OH)6(TPA) and Zn2(OH)2(TPA)・H2O were examined. Among them, Zn4(OH)6(TPA) exhibited a remarkable reversible redox centered at around -1 V vs. Ag/AgCl in a neutral KCl aqueous solution. While cathodic reduction was slow resulting in a broad peak in its cyclic voltammogram, the oxidation was relatively fast, both showing diffusion-limited behavior. Slight acidification of the electrolyte resulted in a significant acceleration of the redox, while the redox potential was independent on pH. Peak splitting systematically increased on increasing the scan rate. Thus, proton exchange by TPA was concluded, for its field driven migration to limit the reaction rates. Potentiostatic charging / discharging experiments confirmed coulombic reversibility over 92% and revealed an active fraction of 17.7% per coated molar amount, so that the proton-coupled redox was achieved within the MOF bulk.

Redox-targeted catalysis for vanadium redox-flow batteries

Vanadium redox-flow battery (VRB) as a promising electrochemical power source for large-scale energy storage, suffers from various polarization losses despite that it has been extensively studied in the past decades. Among these losses, the sluggish interfacial charge transfer of the vanadium species on the respective electrode renders large overpotentials giving rise to inevitable hydrogen and oxygen evolutions during the charging process. In this study, we report an unprecedented method based on the redox targeting concept to tackle the above issues. Prussian blue (PB) and a Prussian blue analogues (PBA) with identical redox potentials to VO2+/VO2+ and V2+/V3+ are grafted on cathode and anode, respectively. Upon operation, the reversible proton-coupled redox targeting reactions between PB and VO2+/VO2+ on cathode, PBA and V2+/V3+ on anode facilitate the interfacial charge transfer of the vanadium species and concomitantly inhibit the hydrogen and oxygen evolutions, which improves the selectivity of the redox reactions and considerably enhances the round-trip energy efficiency and cycling performance of VRB in a wide range of current densities. The above redox-assisted catalytic reactions were scrutinized and the mechanisms are unequivocally manifested with various electrochemical and spectroscopic measurements. We anticipate the surface immobilized redox catalysis approach demonstrated here would generically provide a paradigm for improving the sluggish kinetic processes in a variety of electrochemical devices.

Conduction and Reactivity in Heterogeneous-Molecular Catalysis: New Insights in Water Oxidation Catalysis by Phosphate Cobalt Oxide Films.

Cyclic voltammetry of phosphate cobalt oxide (CoPi) films catalyzing O2-evolution from water oxidation as a function of scan rate, phosphate concentration and film thickness allowed for new insights into the coupling between charge transport and catalysis. At pH = 7 and low buffer concentrations, the film is insulating below 0.8 (V vs SHE) but becomes conductive above 0.9 (V vs SHE). Between 1.0 to 1.3 (V vs SHE), the mesoporous structure of the film gives rise to a large thickness-dependent capacitance. At higher buffer concentrations, two reversible proton-coupled redox couples appear over the capacitive response with 0.94 and 1.19 (V vs SHE) pH = 7 standard potentials. The latter is, at most, very weakly catalytic and not responsible for the large catalytic current observed at higher potentials. CV-response analysis showed that the amount of redox-active cobalt-species in the film is small, less than 10% of total. The catalytic process involves a further proton-coupled-electron-transfer and is so fast that it is controlled by diffusion of phosphate, the catalyst cofactor. CV-analysis with newly derived relationships led to a combination of the catalyst standard potential with the catalytic rate constant and a lower-limit estimation of these parameters. The large currents resulting from the fast catalytic reaction result in significant potential losses related to charge transport through the film. CoPi films appear to combine molecular catalysis with semiconductor-type charge transport. This mode of heterogeneous molecular catalysis is likely to occur in many other catalytic films.

Probing redox reaction of azurin protein immobilized on hydroxyl-terminated self-assembled monolayers with different lengths

Immobilization of proteins with controlled electron transfer properties is an important task for future biomedical applications. In this work, immobilization of azurin on gold electrodes modified with 6-mercaptohexanol (MCH), and 11-mercaptoundecanol (MUD) self-assembled monolayers (SAMs) was studied by electrochemical techniques. The immobilized azurin showed a quasi-reversible redox response on MCH/Au and MUD/Au electrodes, with E1/2=0.11V vs. Ag/AgCl and ∆Ep=10mV and 40mV, respectively. Cyclic voltammograms (CVs) of azurin at different pHs revealed that proton-coupled redox reaction of azurin is a one-electron, three-proton process. The tunneling electron transfer rate constant (k0) of immobilized azurin on MCH/Au electrode was estimated 1.35 (±0.05)×103s−1 using a fast scan CV method, and that on MUD/Au electrode was measured as 120±10s−1 by scanning electrochemical microscopy (SECM). In SECM studies, the approach curves were recorded at different substrate overpotentials with different surface coverages of azurin. For extraction of kinetic parameters, azurin was reduced on MUD/Au electrode via tunneling electron transfer across the SAM, and oxidized back via solution-phase tip-generated ferrocyanide. The bimolecular electron transfer rate constant (kBI) between immobilized azurin and [Fe(CN)6]4− mediator was estimated as 3×108mol−1cm3s−1. Based on distribution of hydrophilic and hydrophobic amino acids on azurin structure, and the distinct voltammetric behavior of azurin on MCH and MUD SAMs as hydroxyl-terminated SAMs vs. methyl- and thiol-terminated SAMs, possible orientations of azurin on different SAMs were also assessed.

Tunable Electrochemical and Catalytic Features of BIAN- and BIAO-Derived Ruthenium Complexes.

This article deals with a class of ruthenium-BIAN-derived complexes, [Ru(II)(tpm)(R-BIAN)Cl]ClO4 (tpm = tris(1-pyrazolyl)methane, R-BIAN = bis(arylimino)acenaphthene, R = 4-OMe ([1a]ClO4), 4-F ([1b]ClO4), 4-Cl ([1c]ClO4), 4-NO2 ([1d]ClO4)) and [Ru(II)(tpm)(OMe-BIAN)H2O](2+) ([3a](ClO4)2). The R-BIAN framework with R = H, however, leads to the selective formation of partially hydrolyzed BIAO ([N-(phenyl)imino]acenapthenone)-derived complex [Ru(II)(tpm)(BIAO)Cl]ClO4 ([2]ClO4). The redox-sensitive bond parameters involving -N═C-C═N- or -N═C-C═O of BIAN or BIAO in the crystals of representative [1a]ClO4, [3a](PF6)2, or [2]ClO4 establish its unreduced form. The chloro derivatives 1a(+)-1d(+) and 2(+) exhibit one oxidation and successive reduction processes in CH3CN within the potential limit of ±2.0 V versus SCE, and the redox potentials follow the order 1a(+) < 1b(+) < 1c(+) < 1d(+) ≈ 2(+). The electronic structural aspects of 1a(n)-1d(n) and 2(n) (n = +2, +1, 0, -1, -2, -3) have been assessed by UV-vis and EPR spectroelectrochemistry, DFT-calculated MO compositions, and Mulliken spin density distributions in paramagnetic intermediate states which reveal metal-based (Ru(II) → Ru(III)) oxidation and primarily BIAN- or BIAO-based successive reduction processes. The aqua complex 3a(2+) undergoes two proton-coupled redox processes at 0.56 and 0.85 V versus SCE in phosphate buffer (pH 7) corresponding to {Ru(II)-H2O}/{Ru(III)-OH} and {Ru(III)-OH}/{Ru(IV)═O}, respectively. The chloro (1a(+)-1d(+)) and aqua (3a(2+)) derivatives are found to be equally active in functioning as efficient precatalysts toward the epoxidation of a wide variety of alkenes in the presence of PhI(OAc)2 as oxidant in CH2Cl2 at 298 K, though the analogous 2(+) remains virtually inactive. The detailed experimental analysis with the representative precatalyst 1a(+) suggests the involvement of the active {Ru(IV)═O} species in the catalytic cycle, and the reaction proceeds through the radical mechanism, as also supported by the DFT calculations.

ChemInform Abstract: Catalysis Application of Metal (Rh, Ir) Quinonoid Complexes

AbstractReview: 45 refs.

Design of synthetic molecular units including quinones towards the construction of artificial photosynthesis

Quinones are essential components in many biological systems, notably in photosynthesis. This is largely due to the characteristic proton-coupled redox chemistry of quinones. This review article overviews the use of quinones in studies on artificial photosynthesis, as one-electron electron acceptors, reversible proton/electron carriers, and replacements for sacrificial oxidant and reductants in photosynthetic chemical conversion. Topics included are the early attempts on intramolecular photoinduced electron transfer involving quinones, subsequent reactions after photoinduced electron transfer between pigments and quinones, photochemistry in molecular assemblies containing quinones, and photochemical quinone/hydroquinone interconversion.

PH-induced photocurrent switching based on a highly stable drop-casting film of imidazole moiety-containing dinuclear Ru(II) Complex

A new dinuclear Ru(II) complex of [(H2L1)Ru(H2L2)Ru(H2L1)](ClO4)4 {H2L1 = 2,6-bis(2-benzimidazolyl)pyridine; H2L2=2,6-bis(4-([2,2′:6′,2″-terpyridin]-4′-yl)phenyl)-1,5- dihydrobenzo[1,2-d:4,5-d’]diimidazole} is synthesized and characterized. The Ru(II) complex modified indium-tin oxide electrode prepared using a drop-casting method, exhibited a couple of stable surface-confined Ru(III)/Ru(II)-based redox waves centered at +0.65V vs saturated calomel electrode that were almost unchanged after 50 consecutive cyclic voltammetry scanning. The modified electrode showed pH-dependent redox behaviors with the formal potential being decreased by 430mV due to the occurrance of the proton-coupled redox reactions. The cathodic photocurrent generation of the modified electrode was also found to be highly pH-dependent, switching from an “off” state at pH ∼11.0 to an “on” state at pH=2.20 with an enhancement factor of 18. The modified electrode was shown to have promising applications as photoelectrochemical pH sensing and switching devices.