Multi-electron Oxidation Research Articles

(Invited) Electrochemical Multi-Electron Oxidation of Thiophene-Derivatives Promoted by Coordination of Electrolytes

Supporting electrolyte is an essential component of the electrochemical systems, although its effect on the reactivity of electrochemically generated cation/anion species has been rarely discussed.[1][2] Our recent study on the electrochemical behavior of tellurium-containing π-conjugated molecule revealed that the coordination of anion species toward the electrochemically generated radical cation species determines the number of electrons, where 1e/2e oxidation were switched depending on the donor number of electrolyte-derived anion species as well as solvents.[3] In the present study, we expanded this idea to thiophene-derivatives to explore the synthetic applicability of electrolyte-coordination. As expected, electrolyte-dependent oxidation behavior of π-expanded thiophene derivatives was confirmed. Especially, the use of coordinating anion promoted multi-electron oxidation of thiophene derivatives, which enabled facile C-C coupling chemistry and oxidative intramolecular cyclization to give rare thiophenium motif in a single step. This study implies the importance of a rational design of the electrolyte to develop novel electrosynthetic reactions.[1] N. Shida, Electrochemistry, 2022, 90, 101004[2] S. Yoshinaga, M. Atobe, N. Shida, Electrochemistry, in press (DOI: 10.5796/electrochemistry.23-67013)[3] N. Shida, H. Nishiyama, F. Zheng, S. Ye, D. S. Seferos, I. Tomita, S. Inagi, Commun. Chem. 2019, 2, 124. Figure 1

Simultaneous Harvesting of Multiple Hot Holes via Visible‐Light Excitation of Plasmonic Gold Nanospheres for Selective Oxidative Bond Scission of Olefins to Carbonyls

AbstractUsing visible photoexcitation of gold nanospheres we successfully demonstrate the simultaneous harvesting of plasmon‐induced multiple hot holes in the complete oxidative scission of the C=C bond in styrene at room temperature to selectively form benzaldehyde and formaldehyde, which is a reaction that requires activation of multiple substrates. Our results reveal that, while extraction of hot holes becomes efficient for interband excitation, harvesting of multiple hot holes from the excited Au nanospheres becomes prevalent only beyond a threshold light intensity. We show that the alkene oxidation proceeded via a sequence of two consecutive elementary steps; namely, a binding step and a cyclic oxometallate transition state as the rate‐determining step. This demonstration of plasmon‐excitation‐mediated harvesting of multiple hot holes without the use of an extra hole transport media opens exciting possibilities, notably for difficult catalytic transformations involving multielectron oxidation processes.

Simultaneous Harvesting of Multiple Hot Holes via Visible-Light Excitation of Plasmonic Gold Nanospheres for Selective Oxidative Bond Scission of Olefins to Carbonyls.

Using visible photoexcitation of gold nanospheres we successfully demonstrate the simultaneous harvesting of plasmon-induced multiple hot holes in the complete oxidative scission of the C=C bond in styrene at room temperature to selectively form benzaldehyde and formaldehyde, which is a reaction that requires activation of multiple substrates. Our results reveal that, while extraction of hot holes becomes efficient for interband excitation, harvesting of multiple hot holes from the excited Au nanospheres becomes prevalent only beyond a threshold light intensity. We show that the alkene oxidation proceeded via a sequence of two consecutive elementary steps; namely, a binding step and a cyclic oxometallate transition state as the rate-determining step. This demonstration of plasmon-excitation-mediated harvesting of multiple hot holes without the use of an extra hole transport media opens exciting possibilities, notably for difficult catalytic transformations involving multielectron oxidation processes.

Reversible phosphorus-based five-electron transfer reaction for aluminium–phosphorus batteries

The use of multi-electron redox materials has been proved as an effective strategy to increase the energy density of batteries. Herein, we report a new reversible phosphorus-based five-electron transfer reaction (P(0) ⇆ P(+5)) in chloroaluminate ionic liquids (CAM-ILs), which represents a new reaction mechanism offering one of the theoretically highest specific capacities of 4325 mAh g−1 among all secondary batteries. Theoretical calculations further confirm the phosphorus electrochemical oxidation pathway with a potential oxidation voltage of ∼2.0 V. Detailed characterisation reveals fresh insight of the role of CAM-ILs, which can catalyze the electrochemical oxidation process of phosphorus by forming electron-enriched state of phosphorus. As a result, a conceptual aluminium-phosphorus battery was assembled, which deliver a high performance of 1512 mAh g−1 and 1176 Wh kg−1, outperforming many similar metal ion battery chemistry. This study sheds light on phosphorus oxidation process in CAM-ILs and presents a new pathway using non-metallic element based multi-electron oxidation to achieve high performance batteries.

Chiral Induced Spin Selectivity Gives a New Twist on Spin-Control in Chemistry.

ConspectusThe electron’s spin, its intrinsic angular momentum, is a quantum property that plays a critical role in determining the electronic structure of molecules. Despite its importance, it is not used often for controlling chemical processes, photochemistry excluded. The reason is that many organic molecules have a total spin zero, namely, all the electrons are paired. Even for molecules with high spin multiplicity, the spin orientation is usually only weakly coupled to the molecular frame of nuclei and hence to the molecular orientation. Therefore, controlling the spin orientation usually does not provide a handle on controlling the geometry of the molecular species during a reaction. About two decades ago, however, a new phenomenon was discovered that relates the electron’s spin to the handedness of chiral molecules and is now known as the chiral induced spin selectivity (CISS) effect. It was established that the efficiency of electron transport through chiral molecules depends on the electron spin and that it changes with the enantiomeric form of a molecule and the direction of the electron’s linear momentum. This property means that, for chiral molecules, the electron spin is strongly coupled to the molecular frame. Over the past few years, we and others have shown that this feature can be used to provide spin-control over chemical reactions and to perform enantioseparations with magnetic surfaces.In this Account, we describe the CISS effect and demonstrate spin polarization effects on chemical reactions. Explicitly, we describe a number of processes that can be controlled by the electron’s spin, among them the interaction of chiral molecules with ferromagnetic surfaces, the multielectron oxidation of water, and enantiospecific electrochemistry. Interestingly, it has been shown that the effect also takes place in inorganic chiral oxides like copper oxide, aluminum oxide, and cobalt oxide. The CISS effect results from the coupling between the electron linear momentum and its spin in a chiral system. Understanding the implications of this interaction promises to reveal a previously unappreciated role for chirality in biology, where chiral molecules are ubiquitous, and opens a new avenue into spin-controlled processes in chemistry.

Geometrical Isomerism Directed Electrochemical Sensing.

We report the independent role of isomerism of secondary sphere substituents over their nature, a factor often overlooked in molecular electrocatalysis pertaining to electrochemical sensing, by establishing that isomerism redefines the electronic structure at the catalytic reaction center via geometrical factors. UV-vis spectroscopy and X-ray photoelectron spectroscopy suggest that a substituent's isomerism in molecular catalysts conjoins molecular planarity and catalytic activation through competing field effects and resonance effects. As a classical example, we demonstrate the influence of isomerism of the -NO2 substituents for the electrocatalytic multi electron oxidation of As(III), a potentially important electrochemical pathway for water remediation and arsenic detection. The isomerism dependent oxidative activation of catalytic center leads to a nonprecious molecular catalyst capable for direct As(III) oxidation with an experimental detection limit close to WHO guidelines. This work opens up an unusual approach in analytical chemistry for developing various sensing platforms for challenging chemical and electrochemical reactions.

Fuel cell and electrolyzer using plastic waste directly as fuel

The effective utilization of plastic waste, including its use as an energy or chemical resource, has attracted much attention. Nevertheless, energy recovery from plastics via incineration generates air pollutants and toxic compounds, while chemical conversion requires significant energy inputs, especially in the case of gasification. Herein, we report the electrochemical conversion of plastics into electricity or hydrogen without the use of special procedures. When a mixture of plastic solid combined with an acidic solution was fed into an electrochemical cell, the solid was found to dissolve in the solution at 100 °C or higher, followed by the release of protons from the anode to the cathode according to a multi-electron oxidation reaction. This oxidation reaction required an anode that was sufficiently porous so as to allow transport of the reactants. Taking the sponge sample as an example, the dissolved polyurethane had a molecular weight of 2000 or higher, the transport of which was facilitated using a carbon support with a pore diameter of approximately 10 nm. In addition, carbon black having an ordered porous structure exhibited better reagent transport compared to a disordered porous carbon black with similar pore diameters. As a consequence, this cell continuously provided power densities on the order of mW cm−2 in the fuel cell mode and generated hydrogen at a low cell voltage of 0.55 V in the electrolyzer mode, using plastics as fuels at an operational temperature of 200 °C.

Influences of the heme-lysine crosslink in cytochrome P460 over redox catalysis and nitric oxide sensitivity.

Ammonia (NH3)-oxidizing bacteria (AOB) derive total energy for life from the multi-electron oxidation of NH3 to nitrite (NO2-). One obligate intermediate of this metabolism is hydroxylamine (NH2OH), which can be oxidized to the potent greenhouse agent nitrous oxide (N2O) by the AOB enzyme cytochrome (cyt) P460. We have now spectroscopically characterized a 6-coordinate (6c) {FeNO}7 intermediate on the NH2OH oxidation pathway of cyt P460. This species has two fates: it can either be oxidized to the {FeNO}6 that then undergoes attack by NH2OH to ultimately generate N2O, or it can lose its axial His ligand, thus generating a stable, off-pathway 5-coordinate (5c) {FeNO}7 species. We show that the wild type (WT) cyt P460 exhibits a slow nitric oxide (NO)-independent conversion (kHis-off = 2.90 × 10-3 s-1), whereas a cross-link-deficient Lys70Tyr cyt P460 mutant protein underwent His dissociation via both a NO-independent (kHis-off = 3.8 × 10-4 s-1) and a NO-dependent pathway [kHis-off(NO) = 790 M-1 s-1]. Eyring analyses of the NO-independent pathways for these two proteins revealed a significantly larger (ca. 27 cal mol-1 K-1) activation entropy (ΔS‡) in the cross-link-deficient mutant. Our results suggest that the Lys-heme cross-link confers rigidity to the positioning of the heme P460 cofactor to avoid the fast NO-dependent His dissociation pathway and subsequent formation of the off-pathway 5c {FeNO}7 species. The relevance of these findings to NO signaling proteins such as heme-nitric oxide/oxygen binding (H-NOX) is also discussed.

Uranium doubles up as a catalyst

Reversible, two-electron redox processes in which a substrate is added and subsequently eliminated from a transition metal are a defining feature of most catalytic cycles. This behavior is typically unheard of, though, when it comes to lanthanide and actinide metals. The f-block metals are used as catalysts, but they tend to exhibit irreversible one- or multielectron oxidation or reduction steps, and the complete redox sequence hasn’t been observed in one reaction system. Researchers led by Stephen T. Liddle of the University of Manchester and Laurent Maron of the University of Toulouse have now found evidence that a uranium complex can satisfy all the criteria of classical single-metal, two-electron oxidative addition-reductive elimination, and they make a case that uranium can mimic traditional transition-metal catalysis (Nat. Commun. 2017, DOI: 10.1038/s41467-017-01363-0). The researchers added azobenzene to a uranium(III) triamide complex they previously reported to form a dimeric uranium(V) imido complex, which readily expels

Evidence for single metal two electron oxidative addition and reductive elimination at uranium

Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.

A Chromium(III)‐Superoxo Complex as a Three‐Electron Oxidant with a Large Tunneling Effect in Multi‐Electron Oxidation of NADH Analogues

Metal-superoxo species are involved in a variety of enzymatic oxidation reactions, and multi-electron oxidation of substrates is frequently observed in those enzymatic reactions. A CrIII -superoxo complex, [CrIII (O2 )(TMC)(Cl)]+ (1; TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), is described that acts as a novel three-electron oxidant in the oxidation of dihydronicotinamide adenine dinucleotide (NADH) analogues. In the reactions of 1 with NADH analogues, a CrIV -oxo complex, [CrIV (O)(TMC)(Cl)]+ (2), is formed by a heterolytic O-O bond cleavage of a putative CrII -hydroperoxo complex, [CrII (OOH)(TMC)(Cl)], which is generated by hydride transfer from NADH analogues to 1. The comparison of the reactivity of NADH analogues with 1 and p-chloranil (Cl4 Q) indicates that oxidation of NADH analogues by 1 proceeds by proton-coupled electron transfer with a very large tunneling effect (for example, with a kinetic isotope effect of 470 at 233 K), followed by rapid electron transfer.

Electrochemical oxidation of glucose and gluconate by an electrode modified with a carbon-supported Rh phthalocyanine

A carbon-supported Rh phthalocyanine catalyzed the electro-oxidation of glucose in basic solution. The overpotentials for the oxidation are lower than those with other conventional Co phthalocyanine-based catalysts. This electrocatalytic oxidation is coupled with the redox potential of Rh phthalocyanine. The introduction of an electron-donating group causes negative shift in the redox potential, and decreases the onset potential for the oxidation of glucose. The Rh phthalocyanine catalyst needs activation before the catalytic oxidation of glucose. This activation is achieved by the exposure of the catalyst to negative potentials. This catalyst also oxidizes gluconate, which is a possible 2-electron oxidation product of glucose. Electrolysis of glucose solution shows that glucose undergoes multi-electron (more than 2) oxidation by the Rh phthalocyanine.

A New Guide for Searching High-Performance Catalysts for Direct Alcohol Fuel Cells

Direct alcohol fuel cells (DAFCs) are important power sources for distributed energy applications, such as automobiles. They are superior to combustion engines in lower operation temperatures and to hydrogen fuel cells in higher energy density and ease of fuel-handling. Two of major issues are the elucidation of the multi-electron oxidation mechanism and the development of high-performance (activity, selectivity, stability) electrocatalysts. For over two decades, cyclic voltammetry has been used as a convenient and powerful tool to assess catalysts’ performance. The oxidation peak in the backward scan is assigned to the oxidation of intermediate carbonaceous compounds (mainly CO) that are formed in the proceeding forward scan. Thus, the ratio of peak intensity in the forward and backward scans (If/Ib) is believed to indicate the tolerance of CO poisoning; the higher the If/Ib, the better the tolerance. This figure-of-merit has been applied ever since for searching high-performance catalysts, including the Pt-based materials for acidic cells and Pd-based materials for alkaline cells. However, to date, catalysts that are practically superior to commercial counterparts have not been identified yet. In this presentation, we will provide direct and unambiguous evidence to show that 1) the backward oxidation peak is not due to the oxidation of intermediate carbonaceous compounds that are formed in the proceeding forward scan; 2) the ratio If/Ib has nothing to do with the CO oxidation and a higher Ib/If value can indicate superior capability in surface regeneration. These findings are applicable to Pt in acidic cells, Pd in alkaline cells, and various alcohols. Our discovery corrects a long-term misunderstanding on the reaction mechanism of catalytic electroxidation of alcohols, provides a new guide for searching high-performance catalysts, and has the potential to be generalized to other fields of electrocatalysis.

Shaping the beating heart of artificial photosynthesis: oxygenic metal oxide nano-clusters

Water oxidation is the crucial stage in the chemical and molecular sequence of photosynthesis, designed by Nature to convert solar light into chemical energy. The artificial “off-leaf” transposition is a major goal of energy research, aiming at the continuous production of hydrogen as a solar fuel, through the photo-catalytic splitting of water. Success in this task primarily depends on the interplay of light-activated multi-electron oxidation and reduction cycles and on the invention of stable and robust water oxidation catalysts, liberating oxygen with fast rates, high quantum yield, and long-term activity. A promising perspective is herein envisaged in the molecular design of functional metal-oxide cores and composite nano-materials.

A mononuclear ruthenium complex showing multiple proton-coupled electron transfer toward multi-electron transfer reactions

Two new ruthenium(II) complexes bearing dissociable protons, [Ru(trpy)(H(2)bim)Cl]PF(6) (1) and [Ru(trpy)(H(2)bim)(OH(2))](PF(6))(2) (2) (H(2)bim = 2,2'-biimidazole and trpy = 2,2':6',2''-terpyridine), were synthesized and characterized, where the H(2)bim and M-OH(2) moieties are expected to serve as proton-dissociation sites. Single crystal X-ray diffraction analyses revealed that the H(2)bim and M-OH(2) moieties act as proton donors in intermolecular hydrogen bonds. Two pK(a) values of 2 (pK(a1) = 9.0 and pK(a2) = 11.3) were spectrophotometrically determined, where the first proton dissociation is assigned to that from H(2)bim and the second is from M-OH(2). This assignment was supported by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations for two sets of conjugated bases, [Ru(trpy)(Hbim)(OH(2))](+) and [Ru(trpy)(H(2)bim)(OH)](+) for the first proton dissociation, and [Ru(trpy)(Hbim)(OH)](+) and [Ru(trpy)(bim)(OH(2))](+) for the second dissociation. Electrochemical studies in aqueous solutions under various pH conditions afforded the Pourbaix diagram (potential versus pH diagram) of 2, where the pK(a) values found from the diagram agree well with those determined spectrophotometrically. It was also found that demonstrates four-step proton-coupled electron transfer (PCET) reactions to give the four-electron oxidized species, [Ru(IV)(trpy)(bim)(O)](2+), without electrostatic charge buildup during the reactions. The multiple PCET ability of 2 would be applicable to various multi-electron oxidation reactions. Catalysis of electrochemical water oxidation was indeed evaluated in the initial attempt to demonstrate multi-electron oxidation reactions, revealing that the water oxidation potential for 2 is lower than that for other ruthenium catalysts, [Ru(trpy)(bpy)(OH(2))](2+), [Ru(trpy)(bpm)(OH(2))](2+) and [Ru(tmtacn)(bpy)(OH(2))](2+) (bpy = 2,2'-bipyridine, bpm = 2,2'-bipyrimidine, and tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), which are known to be active catalysts for water oxidation.

Functional chemical models of multielectron water oxidation in photosynthesis

The study of water oxidation by Mn n IV clusters, which are functional chemical models of the manganese cofactor (enzyme that oxidizes water in photosystem II of natural photosynthesis) has demonstrated that a Mn 2 IV cluster oxidizes two water molecules to form one oxygen molecule, Mn 4 IV oxidizes four water molecules to form two O2 molecules, and Mn 8 IV oxidizes eight water molecules to form four O2 molecules. A Mn 6 IV cluster oxidizes six water molecules to two ozone molecules, whereas Mn 12 IV oxidizes twelve water molecules to four ozone molecules. The six-electron oxidation of water to ozone is also observed in photosynthesis in red and brown sea algae under conditions of water deficit. It is hypothesized that, in the case of water deficit in algae, the manganese cofactor with four manganese ions turns into the cofactor with six manganese ions.

Structures and Properties of Spherical 90‐Vertex Fullerene‐Like Nanoballs

By applying the proper stoichiometry of 1:2 to [Cp(R)Fe(eta(5)-P(5))] and CuX (X=Cl, Br) and dilution conditions in mixtures of CH(3)CN and solvents like CH(2)Cl(2), 1,2-Cl(2)C(6)H(4), toluene, and THF, nine spherical giant molecules having the simplified general formula [Cp(R)Fe(eta(5)-P(5))]@[{Cp(R)Fe(eta(5)-P(5))}(12){CuX}(25)(CH(3)CN)(10)] (Cp(R)=eta(5)-C(5)Me(5) (Cp*); eta(5)-C(5)Me(4)Et (Cp(Et)); X=Cl, Br) have been synthesized and structurally characterized. The products consist of 90-vertex frameworks consisting of non-carbon atoms and forming fullerene-like structural motifs. Besides the mostly neutral products, some charged derivatives have been isolated. These spherical giant molecules show an outer diameter of 2.24 (X=Cl) to 2.26 nm (X=Br) and have inner cavities of 1.28 (X=Cl) and 1.20 nm (X=Br) in size. In most instances the inner voids of these nanoballs encapsulate one molecule of [Cp*Fe(eta(5)-P(5))], which reveals preferred orientations of pi-pi stacking between the cyclo-P(5) rings of the guest and those of the host molecules. Moreover, pi-pi and sigma-pi interactions are also found in the packing motifs of the balls in the crystal lattice. Electrochemical investigations of these soluble molecules reveal one irreversible multi-electron oxidation at E(p)=0.615 V and two reduction steps (-1.10 and -2.0 V), the first of which corresponds to about 12 electrons. Density functional calculations reveal that during oxidation and reduction the electrons are withdrawn or added to the surface of the spherical nanomolecules, and no Cu(2+) species are involved.

Multielectron oxidation of water on nanoelectrodes in photocatalytic systems

Multielectron oxidation of water on nanoelectrodes in photocatalytic systems

Diaryl Diselenides and Related Compounds as Oxygen-Transfer Agents

The progress in application of diaryl diselenides and related compounds as transfer agents is presented. The review is limited to the diaryl diselenides having electron-withdrawing substituents, such as nitro, trifluoromethyl, carbamoylphenyl, and polymeric diselenides PADS and PPDS. The methods of their synthesis are described. These compounds are used as effective catalyst for a simple and multi-electron hydroperoxide oxidation of different organic compounds. Transformation of functional groups, cycloalkanones ring contraction, cyclocondensation and degradation of electron–rich arenes is described. Mechanisms of the reactions involved Se(II) and Se(IV) intermediates are discussed.

Microwave activation of electrochemical processes: High temperature phenol and triclosan electro-oxidation at carbon and diamond electrodes

The electrochemical oxidation of phenolic compounds in aqueous media is known to be affected by the formation of electro-polymerized organic layers which lead to partial or complete electrode blocking. In this study the effect of high intensity microwave radiation applied locally at the electrode surface is investigated for the oxidation of phenol and triclosan in alkaline solution at a 500 μm diameter glassy carbon or at a 500 μm × 500 μm boron-doped diamond electrode. The temperature at the electrode surface and mass transport enhancement are determined by calibration with the Fe(CN) 6 3−/4− redox system in aqueous 0.3 M NaOH and 0.2 NaCl (pH 12) solution. The calibration shows that strong thermal and mass transport effects occur at both glassy carbon and boron-doped diamond electrodes. The average electrode temperature reaches up to 390 K and mass transport enhancements of more than 20-fold are possible. For the phenol electro-oxidation at glassy carbon electrodes and at a concentration below 2 mM a multi-electron oxidation (ca. 4 electrons) occurs in the presence of microwave radiation. For the electro-oxidation of the more hydrophobic triclosan only the one-electron oxidation occurs. Although currents are enhanced in presence of microwave radiation, rapid blocking of the electrode surface in particular at high phenol concentrations still occurs.