Remote Oxidative Activation of a [Cp*Rh] Monohydride**

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Herein, the bis(diphenylphosphino)ferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e⁻. Chemical and electrochemical studies show that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible Rh^II/I process at −0.96 V vs. ferrocenium/ferrocene (Fc^+/0). This redox manifold was confirmed by isolation of an uncommon Rh^II species, [Cp*Rh(dppf)]⁺, that was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of Cp*Rh(dppf) with anilinium triflate yielded an isolable and inert monohydride, [Cp*Rh(dppf)H]⁺, and this species was found to undergo a quasireversible electrochemical oxidation at +0.41 V vs. Fc^+/0 that corresponds to iron-centered oxidation in the dppf backbone. Thermochemical analysis predicts that this dppf-centered oxidation drives a dramatic increase in acidity of the Rh−H moiety by 23 pK_a units, a reactivity pattern confirmed by in situ ¹H NMR studies. Taken together, these results show that remote oxidation can effectively induce M−H activation and suggest that ligand-centered redox activity could be an attractive feature for the design of new systems relying on hydride intermediates.     

Surface-enhanced Raman scattering (SERS) on silver electrodes as a technical tool in the study of the electrochemical reduction of cyanopyridines and in quantitative analysis

The SER spectra of 4-, 3- and 2-cyanopyridines adsorbed on a silver electrode are presented. The results show that cyanopyridines may adsorb in two different orientations, end-on (with the N atom of the Py ring bound to the surface) and flat, and that for potentials more negative than −1.1 V (SCE), the cyanopyridine radical anions can also be detected. The SERS intensity vs. potential curves show more than one potential of maximum SERS intensity which are assigned to the existence of more than one species on the electrode surface. The analytical potentially of SERS on electrodes has also been investigated. It is shown that the relative SERS intensity (ν_CN of the 4-CNPy (2120 cm⁻)/breathing mode of Py (1008 cm⁻¹)), at a fixed Py bulk concentration and at a fixed potential and exciting radiation, depends linearly on the 4-CNPy bulk concentration in the range 10⁻⁷-10⁻⁵M. The selectivity of the technique has also been investigated by studying the SER spectrum and the SERS intensity vs. potential curves for a mixture of 10⁻³M 4-CNPy, pyridine (Py) and 4-methylpyridine(4-MePy) in 0.1 M KCl aqueous solution.     

Electrochemical behavior and parameters of hydrofullerene C60H36

Electrochemistry of hydrofullerene C₆₀H₃₆ was studied by cyclic voltammetry in THF and CH₂Cl₂ in the −47–14 °C temperature range. Hydrofullerene undergoes reversible one-electron reduction to form a radical anion in THF (E ⁰=−3.18 V (Fc⁰/Fc⁺), Fc=ferrocene) and irreversible one-electron oxidation in CH₂Cl₂ (E _p ^a =1.22 V (Fc⁰/Fc⁺)). The reduction potential was used to estimate electron affinity of hydrofullerene as EA=−0.33 eV. It was suggested that C₆₀H₃₆ is an isomer withT-symmetry in which 12 double bonds form four isolated benzenoid rings located in vertices of an imaginary inscribed tetrahedron on the molecular surface. For hydrofullerene, the “electrochemical gap” is an analog of the energy gap (HOMO−LUMO), equal to (E ^Ox−E ^Red)=4.4 V, and indicates that C₆₀H₃₆ is a sufficiently “hard” molecule with a low reactivity in redox reactions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2083–2087, November, 1999.     

Electroreductive Radical Borylation of Unactivated (Hetero)Aryl Chlorides Without Light by Using Cumulene-Based Redox Mediators**

Single-electron transfer (SET) plays a critical role in many chemical processes, from organic synthesis to environmental remediation. However, the selective reduction of inert substrates (E_p/2<−2 V vs Fc/Fc⁺), such as ubiquitous electron-neutral and electron-rich (hetero)aryl chlorides, remains a major challenge. Current approaches largely rely on catalyst photoexcitation to reach the necessary deeply reducing potentials or suffer from limited substrate scopes. Herein, we demonstrate that cumulenes–organic molecules with multiple consecutive double bonds–can function as catalytic redox mediators for the electroreductive radical borylation of (hetero)aryl chlorides at relatively mild cathodic potentials (approximately −1.9 V vs. Ag/AgCl) without the need for photoirradiation. Electrochemical, spectroscopic, and computational studies support that step-wise electron transfer from reduced cumulenes to electron-neutral chloroarenes is followed by thermodynamically favorable mesolytic cleavage of the aryl radical anion to generate the desired aryl radical intermediate. Our findings will guide the development of other sustainable, purely electroreductive radical transformations of inert molecules using organic redox mediators.     

Electrochemical and Spectroscopic Study of EuIII and EuII Coordination in the 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid

Separation processes based on room temperature ionic liquids (RTIL) and electrochemical refining are promising strategies for the recovery of lanthanides from primary ores and electronic waste. However, they require the speciation of dissolved elements to be known with accuracy. In the present study, Eu coordination and Eu^III/Eu^II electrochemical behavior as a function of water content in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf₂]) was investigated using UV–visible spectrophotometry, time-resolved laser fluorescence spectroscopy, electrochemistry, and X-ray absorption spectroscopy. In situ measurements were performed in spectroelectrochemical cells. Under anhydrous conditions, Eu^III and Eu^II were complexed by NTf₂, forming Eu−O and Eu−(N,O) bonds with the anion sulfoxide function and N atoms, respectively. This complexation resulted in a greater stability of Eu^II, and in quasi-reversible oxidation–reduction with an E₀’ potential of 0.18 V versus the ferrocenium/ferrocene (Fc⁺/Fc) couple. Upon increasing water content, progressive incorporation of water in the Eu^III coordination sphere occurred. This led to reversible oxidation–reduction reactions, but also to a decrease in stability of the +II oxidation state (E₀’=−0.45 V vs. Fc⁺/Fc in RTIL containing 1300 mm water).     

Electrocatalytic H2 Generation from Water Relying on Cooperative Ligand Electron Transfer in “PN3P” Pincer-Supported NiII Complexes

Water is the most sustainable source for H₂ production, and the efficient electrocatalytic production of H₂ from mixed water/acetonitrile solutions by using two new air-stable nickel(II) pincer complexes, [Ni(κ³-2,6-{Ph₂PNR}₂(NC₅H₃)Br₂] (R=H I , Me II ) is reported. Hydrogen generation from H₂O/CH₃CN solutions is initiated at −2 V against Fc^+/0, and bulk electrocatalysis studies showed that the catalyst functions with an excellent Faradaic efficiency and a turnover frequency of 160 s⁻¹. A DFT computational investigation of the reduction behavior of I and II revealed a correlation of H₂ formation with charge donation from electrons originating in a reduced ligand-localized orbital. As a result, these catalysts are proposed to proceed by a novel mechanism involving electron/proton transfer between a Ni^0I species bonded to an anionic PN³P ligand (“L⁻/Ni^0I”) and a Ni^I-hydride (“Ni−H”). Furthermore, these catalysts are able to reduce phenol and acetic acid, more active proton sources, at lower potentials that correlate with the substrate pK_a.     

Cobalt Carbonate as an Electrocatalyst for Water Oxidation

Co^II salts in the presence of HCO₃⁻/CO₃²⁻ in aqueous solutions act as electrocatalysts for water oxidation. It comprises of several key steps: (i) A relatively small wave at E_pa≈0.71 V (vs. Ag/AgCl) owing to the Co^III/II redox couple. (ii) A second wave is observed at E_pa≈1.10 V with a considerably larger current. In which the Co^III undergoes oxidation to form a Co^IV species. The large current is attributed to catalytic oxidation of HCO₃⁻/CO₃²⁻ to HCO₄⁻. (iii) A process with very large currents at >1.2 V owing to the formation of Co^V(CO₃)₃⁻, which oxidizes both water and HCO₃⁻/CO₃²⁻. These processes depend on [Co^II], [NaHCO₃], and pH. Chronoamperometry at 1.3 V gives a green deposit. It acts as a heterogeneous catalyst for water oxidation. DFT calculations point out that Coⁿ(CO₃)₃ⁿ⁻⁶, n=4, 5 are attainable at potentials similar to those experimentally observed.     

An Adaptable N-Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

A neutral hybrid macrocycle with two trans-positioned N-heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I–+III) in a series of structurally characterized complexes ( 1 – 3 ). Redox interconversion of [LCu]^+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E_1/2=−0.45 V and +0.82 V (vs. Fc/Fc⁺)). A linear C^NHC-Cu-C^NHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of Cu^I versus Cu^II/Cu^III. Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high-energy-resolution fluorescence detection X-ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1 – 3 are textbook examples for Cu^I, Cu^II, and Cu^III species. Facile 2-electron interconversion combined with the exposure of two basic pyridine N sites in the reduced Cu^I form suggest that [LCu]^+/2+/3+ may operate in catalysis via coupled 2 e⁻/2 H⁺ transfer.     

Mediated bioelectrocatalytic O2 reduction to water at highly positive electrode potentials near neutral pH

Combinations of bilirubin oxidase and metal complexes: [W(CN)₈]^3−/4−, [Os(CN)₆]^3−/4− and [Mo(CN)₈]^3−/4− (the formal potentials, E^0′(M), being 0.320, 0.448, and 0.584 V vs. Ag|AgCl, respectively, at pH 7.0), allowed bioelectrocatalytic reduction of O₂ to water at their formal potentials near neutral pH. The O₂ reduction current appeared even at the standard potential of the O₂/H₂O redox couple, E^0′(O₂/H₂O), when [Mo(CN)₈]^3−/4− was used at pH 7.4, though the magnitude was small. The magnitude of the bioelectrocatalytic current systematically decreased with the decrease in the potential difference between E^0′(O₂/H₂O) and E^0′(M). A limiting current as large as 17 mA/cm² of a projected electrode surface area was obtained at 0.25 V (−0.37 V vs. E^0′(O₂/H₂O)) for the O₂ reduction at pH 7.0 with a carbon felt electrode modified with electrostatically entrapped bilirubin oxidase and [W(CN)₈]^3−/4− at the electrode rotation rate of 4000 rpm.     

Synthesis of an elusive,stable 2-azaallyl radical guided by electrochemical and reactivity studies of 2-azaallyl anions

The super electron donor (SED) ability of 2-azaallyl anions has recently been discovered and applied to diverse reactivity, including transition metal-free cross-coupling and dehydrogenative cross-coupling processes. Surprisingly, the redox properties of 2-azaallyl anions and radicals have been rarely studied. Understanding the chemistry of elusive species is the key to further development. Electrochemical analysis of phenyl substituted 2-azaallyl anions revealed an oxidation wave at E_1/2 or E_pa = −1.6 V versus Fc/Fc⁺, which is ∼800 mV less than the reduction potential predicted (E_pa = −2.4 V vs. Fc/Fc⁺) based on reactivity studies. Investigation of the kinetics of electron transfer revealed reorganization energies an order of magnitude lower than commonly employed SEDs. The electrochemical study enabled the synthetic design of the first stable, acyclic 2-azaallyl radical. These results indicate that the reorganization energy should be an important design consideration for the development of more potent organic reductants.

The super electron donor (SED) capabilities of 2-azaallyl anions has recently been discovered and applied to diverse reactivity; their structures and electron transfer characteristics are reported herein.     

Molecular electrochemistry of hydrofullerenes C70H36—46

Electrochemistry of a mixture of hydrofullerenes C₇₀H_36—46 composed of C₇₀H₃₆, C₇₀H₃₈, C₇₀H₄₄, and C₇₀H₄₆ (50, 20, 14, and 15%, respectively) was studied by cyclic voltammetry in THF and CH₂Cl₂ in the –43—–13 °C temperature range. Two cathodic peaks, namely, one-electron reversible (E° = –3.16 V (Fc^0/+), Fc is ferrocene) and irreversible (E _p = –3.37 V (Fc^0/+)) were observed for this mixture in THF. The irreversible broad oxidation peak (E _p = 1.22 V (Fc^0/+)) was observed in CH₂Cl₂. The reversible reduction peak (E° = –3.16 V) and irreversible oxidation peak (E _p = 1.22 V) were attributed to the most stable hydrofullerene C₇₀H₃₆. The irreversible reduction (E _p = –3.37 V) and oxidation (E _p = 1.22 V) peaks were attributed to hydrofullerenes C₇₀H_44—46 with a higher degree of hydrogenation. The values of an electrochemical gap, which is an analog of the energy gap (HOMO—LUMO), are 4.38 and 4.59 V for C₇₀H₃₆ and C₇₀H_44—46, respectively, and indicate that these hydrofullerenes are sufficiently hard molecules with low reactivity in redox reactions.     

Acidité dans le nitrométhane: III. Détermination électrochimique de la basicité du nitrométhane — Corrélation Dn/Eo(Hs+/H2/

The use of the complex acid HAlCl₄ (HCl+AlCl₃) permits the detemrination of the standard potential of the hydrogen electrode in nitromethane. The result (E₀(H_s⁺/H₂)=0.5 V vs. Fc/Fc⁺, Fc=ferrocene) shows that nitromethane is very weakly basic. This measurement is confirmed by showing that the standard potential of the hydrogen electrode in various solvents is linked to Gutmann's donor numbers of these solvents. The E₀(H_s⁺/H₂) value obtained in nitromethane belongs to the correlation line.     

An Adaptable N‐Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

A neutral hybrid macrocycle with two trans‐positioned N‐heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I–+III) in a series of structurally characterized complexes ( 1 – 3 ). Redox interconversion of [LCu]^+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E_1/2=?0.45 V and +0.82 V (vs. Fc/Fc⁺)). A linear C^NHC‐Cu‐C^NHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of Cu^I versus Cu^II/Cu^III. Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high‐energy‐resolution fluorescence detection X‐ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1 – 3 are textbook examples for Cu^I, Cu^II, and Cu^III species. Facile 2‐electron interconversion combined with the exposure of two basic pyridine N sites in the reduced Cu^I form suggest that [LCu]^+/2+/3+ may operate in catalysis via coupled 2 e^?/2 H⁺ transfer.     

Electrochemical Evaluation of Diketopyrrolopyrrole Derivatives for Nonaqueous Redox Flow Batteries

Redox flow batteries (RFBs) employing nonaqueous electrolytes could potentially operate at much higher cell voltages, and therefore afford higher energy and power densities, than RFBs employing aqueous electrolytes. The development of such high-voltage nonaqueous RFBs requires anolytes that are electrochemically stable, especially in the presence of traces of oxygen and/or moisture. The inherent atmospheric reactivity of anolytes mandates judicious molecular design with high electron affinity and electrochemical stability. In this study, diketopyrrolopyrrole (DPP)-based TDPP-Hex-CN₄ is proposed as a stable redox-active molecule for anolytes in nonaqueous organic RFBs. We demonstrate organic RFBs using TDPP-Hex-CN₄ as anolyte with unisol blue (UB) 1,4-bis(isopropylamino)anthraquinone and 1,4-di-tert-butyl-2,5-bis(2-methoxyethoxy)benzene (DBBB) as catholytes. Cyclic voltammetry measurements with scans repeated over 200 cycles were performed to establish the electrochemical stability of the redox pairs. Symmetric flow-cell studies show that TDPP-Hex-CN₄ exhibits stable capacity up to 700 cycles. Redox flow cells employing TDPP-Hex-CN₄ /UB and TDPP-Hex-CN₄/DBBB as redox pairs demonstrate that DPP derivatives are propitious materials for anolytes in all organic nonaqueous RFBs.     

Electrochemical formation of stable ferrocene anion and the formal rate constant of the ferrocene0/− electrode

A reversible cyclic voltammogram for the one-electron reduction of ferrocene in 1,2-dimethoxyethane is recorded under experimental conditions that enable the ferrocene anion to exist for a few minutes. The formal rate constant of the ferrocene^0/? electrode, determined by cyclic voltammetry at ?45°C, ca. 10^?3 cm s^?1, is in striking contrast with that of ferrocene^+/0, > 10^?1 cm s^?1. The distortion of the ferrocene molecule caused by reduction may be a reason for this difference in electron-transfer rate.     

Predicting reduction potentials of 1,3,6‐triphenyl fulvenes using molecular electrostatic potential analysis of substituent effects

The influence of mono‐ and multiple substituent effect on the reduction potential (E⁰) of 1,3,6‐triphenyl fulvenes is investigated using B3LYP‐SMD/6‐311+G(d,p) level density functional theory. The molecular electrostatic potential (MESP) minimum at the fulvene π‐system (V_min) and the change in MESP at any of the fulvene carbon atoms (ΔV_C) for both neutral and reduced forms are used as excellent measures of substituent effect from the para and meta positions of the 1,3 and 6‐phenyl moieties. Substitution at 6‐phenyl para position has led to significant change in E⁰ than any other positions. By applying the additivity rule of substituent effects, an equation in ΔV_C is derived to predict E⁰ for multiply substituted fulvenes. Further, E⁰ is predicted for a set of 2000 hexa‐substituted fulvene derivatives where the substituents and their positions in the system are chosen in a random way. The calculated E⁰ agreed very well with the experimental E⁰ reported by Godman et al. Predicting E⁰ solely by substituent effect offers a simple and powerful way to select suitable combinations of substituents on fulvene system for light harvesting applications. © 2018 Wiley Periodicals, Inc.     

Spectroelectrochemical titrations and cyclic voltammetry of methyl pheophorbide in acid: Possible role of pheophytin enol iminium in the primary process of PS II

In this paper we examine methyl pheophorbide (MP), a model of pheophytin_l, with UV-visible spectroelectrochemistry and cyclic votammetry in aprotic solvent with varying molar equivalents of acid. Our results provide evidence for the formation of the monocation of MP and its electrochemically generated radical. The spectroelectrochemical experiments furnished information about the products at equilibrium at each applied potential. In addition, when only one equivalent of acid was used, an isosbestic point was observed from the spectra taken at different potentials, thus supporting the interconversion of MP monocation and the radical. While the E°′'s for the two redox couples of MP in acid free solution were −0.66 V and 0.88 V vs. the acetonitrile/calomel reference, after one mole of nonaqueous acid was added, the first E°′, shifted from −0.66 to −0.55 V. Protonation of MP, apparently to yield the monocation, facilitates reduction by 0.11 V. It is known that the chlorophyll pair in the excited state effects electron transfer to pheophytin_L which is then followed by electron transfer to a quinone. In contrast, pheophytin_M is apparently not involved in electron transfer. One key difference between these pheophytins is that the former may convert to the iminium form during enolization. In addition to a discussion of the electrochemical results, a hypothesis is advanced for a role of pheophytin enol iminium in the photosynthetic primary process.     

Photoluminescence of Seven-Coordinate Zirconium and Hafnium Complexes with 2,2′-Pyridylpyrrolide Ligands

Luminescent seven-coordinated zirconium and hafnium complexes bearing three mono-anionic 2,2′-pyridylpyrrolide ligands and one chloride were synthesized. Solid-state structures and the dynamic behaviors in solution were probed by X-ray crystallography and variable temperature ¹H NMR experiments, respectively. Absorption spectroscopy and time-dependent density functional theory (TD-DFT) calculations supported a hybrid of ligand-to-metal charge transfer (LMCT)/ligand-to-ligand charge transfer (LLCT) for the visible light absorption band. The complexes (^MePMP^Me)₃MCl (M=Zr, Hf, ^MePMP^Me=3,5-dimethyl-2-(2-pyridyl)pyrrolide) are emissive in solution at room temperature upon irradiation with visible light due to a combination of phosphorescence and fluorescence characterized by excited state lifetimes in the μs and low to sub-ns timescale, respectively. Electrochemical experiments revealed that the zirconium complex possesses a reversible redox event under highly reducing condition (−2.29 V vs. Fc^+/0).     

Sensitized photo-redox reaction: VII. The Synthesis,FAB mass spectrum,stationary absorption,emission, transient absorption spectra,redox potential of dihydroxy-tin (IV)-mesoporphyrin dimethyl ester and its sensitized photo-reduction of methyl viologen

In DMSO/water (4:1), photolysis of the dihydroxy-Sn (IV)-mesoporphyrin dimethyl ester (SnP)/methyl viologen (MV²⁺)/ethylene diamine tetraacetic acid (EDTA) ternary system produces methyl viologen cation radical with a quantum yield of 0.67, much higher than that of systems with other metal complexes of mesoporphyrin dimethyl ester. Neither EDTA nor MV²⁺ quenches the stationary fluorescence of SnP, implying that the reaction does not take place at the singlet state. With flash photolysis we obtain the T-T absorption spectrum of SnP (λ_max 440 nm). By following the decay of this absorption, the triplet life time of SnP is estimated to be 41 μs. The life time is related to the concentration of either MV²⁺ or EDTA. Good linear relationships are obtained by plotting τ₀τ vs. the concentration of MV²⁺ or EDTA (Stern-Volmer plot), from which we determine the quenching constants: _kq(MV²⁺) =5.5 × 10⁷ mol^?1, s^?1; k_q (EDTA) =2.7 × 10⁷ mol^?1, s^?1. The data suggests that upon photolysis of the above ternary system, both oxidative quenching and reductive quenching of the triplet state of the sensitizer are occurring. From the measured phosphorescence spectrum (λ_max 704 nm) and the ground state redox, potentials (E^red_1/2?-0.84V, E^ox_1/2?+1.43 V, vs. Ag/AgCl, KCl (sat.)), we obtain the redox potential of triplet SnP to be E(P⁺/P*_T)?-0.33 V, E(P*_T+/P^?)?+0.92 V. Matching this data with the redox potential of MV²⁺ and EDTA, we establish the fact that during the photolysis of the SnP/MV²⁺/EDTA ternary system, both oxidative and reductive quenching are thermodynamically favorable processes. This is also the reason why the SnP sensitized reaction is much more efficient relative to other mesoporphyrin derivatives.     

Facile Synthesis of MOF-derived Mn3O4@N-doped Carbon with Efficient Oxygen Reduction

Integration of MnO_x into the carbon matrix proves a viable strategy to improve the electrochemical performance of MnO_x materials. Mn₃O₄ nanoparticle-decorated N-doped carbon composites (Mn₃O₄@N-doped carbon) were facilely prepared from a non-porous eight-fold interpenetrated Zn^II-based MOF, which involves first synthesis of bimetallic Mn/Zn-MOF in one-pot reaction followed by direct pyrolysis at 1000 °C. In 0.1 m KOH solution, the optimal Mn₃O₄@N-doped carbon exhibits decent oxygen reduction activity with the onset potential (E_onset) of 0.94 V (vs. RHE) and half-wave potential (E_1/2) of 0.81 V (vs. RHE), excellent methanol tolerance as well as good durability.