Two-electron Oxidation Research Articles

Weakened Interfacial Hydrogen Bond Connectivity Drives Selective Photocatalytic Water Oxidation toward H2O2 at Water/Brookite-TiO2 Interface.

The formation of H2O2 through the two-electron photocatalytic water oxidation reaction (WOR) is significant but encounters the competition with the four-electron O2 evolution reaction. Recent studies showed a crystal-phase dependence in H2O2 selectivity, where high purity brookite TiO2 (b-TiO2) exhibits remarkable H2O2 selectivity in contrast to the common rutile phase TiO2 (r-TiO2). However, the origin of such a structure-induced selectivity preference remains elusive, primarily due to the complexities associated with the solid-liquid interface system and excited-state chemistry. Herein, we conducted a comprehensive investigation into the selectivity mechanism of WOR at the water/b-TiO2(210) and water/r-TiO2(110) interfaces, employing first-principles molecular dynamics simulations and microkinetic analyses. Intriguingly, our results reveal that the intrinsic catalytic ability of the b-TiO2(210) itself does not enhance H2O2 selectivity compared to r-TiO2(110). Instead, it is the weakened interfacial hydrogen bond connectivity, modulated by the herringbone-like local atomic structure of the b-TiO2(210) surface, that determines the selectivity. Specifically, this weakened H-bond connectivity (i.e., local low water density) at the interface, owing to the strong water adsorption and distinct adsorption orientation, can stabilize the OH• radical and inhibit its deprotonation, leading to an improved H2O2 selectivity. By contrast, the relatively strong interface H-bond connectivity established over r-TiO2(110) accelerates the deprotonation of OH•, with the OH• coverage being 3 orders of magnitude lower than at the water/b-TiO2(210) interface. This study quantitatively demonstrates that the local H-bond structure (water density) at the liquid/solid interface significantly influences photocatalytic selectivity, and this insight may offer a rational approach to enhance the H2O2 selectivity.

A novel alkane monooxygenase evolved from a broken piece of ribonucleotide reductase in Geobacillus kaustophilus HTA426 isolated from Mariana Trench

We have accidentally found that a thermophilic Geobacillus kaustophilus HTA426 is capable of degrading alkanes although it has no alkane oxygenating enzyme genes. Our experimental results revealed that a putative ribonucleotide reductase small subunit GkR2loxI (GK2771) gene encodes a novel heterodinuclear Mn–Fe alkane monooxygenase/hydroxylase. GkR2loxI protein can perform two-electron oxidations similar to homonuclear diiron bacterial multicomponent soluble methane monooxygenases. This finding not only answers a long-standing question about the substrate of the R2lox protein clade, but also expands our understanding of the vast diversity and new evolutionary lineage of the bacterial alkane monooxygenase/hydroxylase family.

Synthesis and Properties of Novel Alkyl-Substituted Hexaazacyclophanes and Their Diradical Dications.

Radicals based on arylamine cyclophanes can be used as functional materials and show application potential in fields such as synthetic chemistry, molecular electronic components, organic light-emitting diodes, and catalytic chemistry. Using a Buchwald-Hartwig palladium-catalyzed aryl halide amination method, we synthesized a series of neutral hexaazacyclophane compounds 1-3 with different substituents in the meta-meta-meta positions of the phenyl rings. Three characteristic high-spin hexaazacyclophane diradical dications were obtained by two-electron oxidation using AgSbF6: 12·+•2[SbF6]-, 22·+•2[SbF6]-, and 32·+•2[SbF6]-. The electronic structures and physical properties of these compounds were then investigated by 1H and 13C nuclear magnetic resonance spectroscopy, cyclic voltammetry, electron paramagnetic resonance spectroscopy, superconducting quantum interferometry, ultraviolet-visible spectroscopy, and density functional theory calculations. The findings provide new ideas for designing radical species with novel physical properties and electronic structures. Importantly, the obtained radical species are not sensitive to air, making them valuable functional materials for practical applications.

Radical Cations of Bilayer Nanographenes.

Chemical redox reactions of bilayer nanographene complexes, (C96H24Ar6)2 (Ar = 2,6-dimethylphenyl) (12) and (C42H12R6)2 (R = tBu) (22), were investigated. Upon two-electron oxidation reactions, 12 and 22 were transformed to radical cations 122•+ and 222•+, respectively. SQUID and EPR measurements on 122•+ and 222•+ indicate that they possess an open-shell singlet ground state with antiferromagnetic interactions between two layers. The shortest separation distance between bilayers in 222•+ (3.30 Å) is shorter than that in 22 (3.44 Å) and 22•+ (3.40 Å), illustrating stronger interaction upon loss of electrons.

Metallacyclobuta-(2,3)-diene: A Bidentate Ligand for Stream-line Synthesis of First Row Transition Metal Catalysts for Cyclic Polymerization of Phenylacetylene.

Described here is a direct entry to two examples of 3d transition metal catalysts that are active for the cyclic polymerization of phenylacetylene, namely, [(BDI)M{κ2 -C,C-(Me3 SiC3 SiMe3 )}] (2-M) (BDI=[ArNC(CH3 )]2 CH- , Ar=2,6-i Pr2 C6 H3 ; M=Ti, V). Catalysts are prepared in one step by the treatment of [(BDI)MCl2 ] (1-M, M=Ti, V) with 1,3-dilithioallene [Li2 (Me3 SiC3 SiMe3 )]. Complexes 2-M have been spectroscopically and structurally characterized and the polymers that are catalytically formed from phenylacetylene were verified to have a cyclic topology based on a combination of size-exclusion chromatography (SEC) and intrinsic viscosity studies. Two-electron oxidation of 2-V with nitrous oxide (N2 O) cleanly yields a [VV ] alkylidene-alkynyl oxo complex [(BDI)V(=O){κ1 -C-(=C(SiMe3 )CC(SiMe3 ))}] (3), which lends support for how this scaffold in 2-M might be operating in the polymerization of the terminal alkyne. This work demonstrates how alkylidynes can be circumvented using 1,3-dianionic allene as a segue into M-C multiple bonds.

Efficient Photosynthesis of Hydrogen Peroxide by Cyano-Containing Covalent Organic Frameworks from Water, Air and Sunlight.

The insufficient exciton (e- -h+ pair) separation/transfer and sluggish two-electron water oxidation are two main factors limiting the H2 O2 photosynthetic efficiency of covalent organic frameworks (COFs) photocatalysts. Herein, we present an alternative strategy to simultaneously facilitate exciton separation/transfer and reduce the energy barrier of two-electron water oxidation in COFs via a dicyano functionalization. The in situ characterization and theoretical calculations reveal that the dicyano functionalization improves the amount of charge transfer channels between donor and acceptor units from two in COF-0CN without cyano functionalization to three in COF-1CN with mono-cyano functionalization and four in COF-2CN with dicyano functionalization, leading to the highest separation/transfer efficiency in COF-2CN. More importantly, the dicyano group activates the neighbouring C atom to produce the key *OH intermediate for effectively reducing the energy barrier of rate-determining two-electron water oxidation in H2 O2 photosynthesis. The simultaneously enhanced exciton separation/transfer and two-electron water oxidation in COF-2CN result in high H2 O2 yield (1601 μmol g-1 h-1 ) from water and oxygen without using sacrificial reagent under visible-light irradiation. COF-2CN can effectively yield H2 O2 in water with wide pH range, in different real water samples, in scaled-up reactor under natural sunlight irradiation, and in continuous-flow reactor for consecutively producing H2 O2 solution for water decontamination.

A triradical-containing trinuclear Pd(II) complex: spin-polarized electronic transmission, analog resistive switching and neuromorphic advancements.

Neuromorphic computation has emerged as a potential alternative to subvert the von Neumann bottleneck issue in conventional computing. In this context, the development of resistive switching-based memristor devices mimicking various synaptic functionalities has engendered paramount attention. Here, we report a triradical-containing trinuclear Pd(II) cluster with a cyclohexane-like framework constituted by the Pd-Se coordination motif displaying facile memristor property with neuromorphic functionality as a thin-film device. The metal-ligand complex (complex 1) possessed an St = 1/2 ground state by experiencing a spin-frustrated-type magnetic coupling phenomenon amongst the three ligand-based organic radicals (SR = 1/2), coordinated to the Pd(II) ions. Three reversible one-electron reduction waves countered with a one-electron and one two-electron reversible oxidation waves were noticed in the cyclic voltammogram of the complex, confirming electrons accepting and releasing capacity of the complex at low potentials, i.e., within +0.2 V to -1.1 V. Employing the radical-containing complex 1 as the active thin-film sandwiched between two orthogonal electrodes, resistive switching based memristor property with biological synaptic actions were successfully emulated. Intriguingly, the artificial neural network (ANN) simulated efficient pattern recognition demonstrated using the recorded potentiation and depression curves from the device, which is a step ahead for the hardware realization of neuromorphic computing. The performance of the ANN on MNIST data with reduced image resolution has further been evaluated. Density functional theory (DFT)-based theoretical calculation predicted that the spin-polarized electronic transmission substantiated the memristive property in the neutral complex 1.

Effective electrochemical water oxidation to H2O2 based on a bimetallic Fe/Co metal–organic framework

A bimetallic Fe/Co metal–organic framework exhibits high-efficiency catalysis for electrochemical two-electron water oxidation reaction to produce hydrogen peroxide.

Open shell (4n + 2)π and closed shell 4nπ planar core-modified decaphyrins.

Planar 44π and 46π core-modified decaphyrins with ten thiophene units have been synthesized from short thiophene oligomers. They have been structurally characterized by single crystal X-ray diffraction with further support from spectroscopic analysis and quantum chemical calculations. Our analysis revealed diradicaloid characteristics for 46π species in contrast to the closed shell property of the 44π congener. Further, 44π and 46π undergo reversible two-electron chemical oxidation, as observed by spectro-electrochemical measurements.

Accessing Ni(0) to Ni(IV) via nickel-carbon-phosphorus bond reorganization.

Two-electron oxidation of a NiIIPh(PCP) pincer complex initiates phosphine ligand insertion, generating an η6-arylphosphonium moiety coordinated to NiII. The reaction is fully reversible under reducing conditions. X-ray crystallography, NMR/EPR spectroscopy, electrochemistry, and DFT calculations support the proposed Ni-C-P bond reorganization mechanisms, which access oxidation states from Ni0 to NiIV.

Hydrogen atom abstraction as a synthetic route to a square planar CoII complex with a redox-active tetradentate PNNP ligand.

Redox-active ligands improve the reactivity of transition metal complexes by facilitating redox processes independent of the transition metal center. A tetradentate square planar (PNCH2CH2NP)CoII (1) complex was synthesized and the ethylene backbone was dehydrogenated through hydrogen atom abstraction to afford (PNCHCHNP)CoII (2), which now contains a redox-active ligand. The ligand backbone of 2 can be readily hydrogenated with H2 to regenerate 1. Reduction of 1 and 2 with KC8 in the presence of 18-crown-6 results in cobalt-based reductions to afford [(PNCH2CH2NP)CoI][K(18-crown-6)] (3) and [(PNCHCHNP)CoI][K(18-crown-6)] (4), respectively. Cyclic voltammetry revealed two reversible oxidation processes for 2, presumed to be ligand-based. Following treatment of 2 with one equivalent of FcPF6, the one-electron oxidation product {[(PNCHCHNP)CoII(THF)][PF6]}·THF (5) was obtained. Treating 5 with an additional equivalent of FcPF6 affords the two-electron oxidation product [(PNCHCHNP)CoII][PF6]2 (6). Addition of PMe3 to 5 produced [(PNCHCHNP)CoII(PMe3)][PF6] (7). A host of characterization methods including nuclear magnetic resonance (NMR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, cyclic voltammetry, magnetic susceptibility measurements using SQUID magnetometry, single-crystal X-ray diffraction, and density functional theory calculations were used to assign 5 and 6 as ligand-based oxidation products of 2.

Passivation protein-adhesion platform promoting stent reendothelialization using two-electron-assisted oxidation of polyphenols

Superhydrophilic surfaces play an important role in nature. Inspired by this, scientists have designed various superhydrophilic materials that are widely used in the field of biomaterials, such as PEG molecular brushes and zwitterionic materials. However, superhydrophilic coatings with only anti-fouling properties do not satisfy the requirements for rapid reendothelialization of cardiovascular stent surfaces. Herein, a novel polyphenol superhydrophilic surface with passivated protein-adsorption properties was developed using two-electron oxidation of dopamine and polyphenols. This coating has a multiscale effects: 1) macroscopically: anti-fouling properties of superhydrophilic; 2) microscopically: protein adhesion properties of active groups (quinone-, amino-, hydroxyphenyl groups and aromatic ring). Polyphenols not only enhance the ability of coating to passivate protein-adsorption, but also make the coating have polyphenol-related biological functions. Therefore, the polyphenol and passivated protein-adsorption platform together maintain the stability of the scaffold microenvironment. This, in turn, provides favorable conditions for the growth of endothelial cells on the scaffold surface. In vivo implantation of the coated stents into the abdominal aorta resulted in uniform and dense endothelial cells covering the surface of the neointima. Moreover, new endothelial cells secreted large amounts of functional endothelial nitric oxide synthase like healthy endothelial cells. These results indicate that the polyphenol superhydrophilic coating potentially resists intra-stent restenosis and promotes surface reendothelialization. Hence, polyphenol superhydrophilic coatings with passivated protein-adsorption properties constructed by two-electron-assisted oxidation are a highly effective and versatile surface-modification strategy for implantable cardiovascular devices.

The Nexus of Innovation: Electrochemically Synthesizing H2O2 and Its Integration with Downstream Reactions.

Hydrogen peroxide (H2O2) represents a chemically significant oxidant that is prized for its diverse applicability across various industrial domains. Recent innovations have shed light on the electrosynthesis of H2O2 through two-electron oxygen reduction reactions (2e- ORR) or two-electron water oxidation reactions (2e- WOR), processes that underscore the attractive possibility for the on-site production of this indispensable oxidizing agent. However, the translation of these methods into practical utilization within chemical manufacturing industries remains an aspiration rather than a realized goal. This Perspective intends to furnish a comprehensive overview of the latest advancements in the domain of coupled chemical reactions with H2O2, critically examining emergent strategies that may pave the way for the development of new reaction pathways. These pathways could enable applications that hinge on the availability and reactivity of H2O2, including, but not limited to the chemical synthesis coupled with H2O2 and waste water treatment byFenton-like reactions. Concurrently, the Perspective acknowledges and elucidates some of the salient challenges and opportunities inherent in the coupling of electrochemically generated H2O2, thereby providing a scholarly analysis that might guide future research.

Flower-like WO2@C prepared from the pyrolyzation of ionic liquid for effective elctrocatalytic two-electron water oxidation

Flower-like tungsten dioxide@carbon (WO2@C) is prepared through the pyrolysis of 1-octyl-3-methylimidazolium tungstate ionic liquids. The resultant WO2 particles are surrounded by a few of carbon fibers, displaying the unique flower-like morphology. The WO2@C testified efficient electrocatalytic activity for two-electron water oxidation reaction (2e- WOR) to H2O2. The electrocatalytic H2O2 production rate reaches about 6.55 μmol min−1 cm−2, over 2 times higher than WO2. This may be ascribed to the enhanced electron transferring capacities and abundant defects of WO2@C. This work introduces a special ionic liquid-derived metal oxide@C with the improved electrocatalytic activity for 2e- WOR.

Synthesis, characterization, electrochemistry and spectroelectrochemistry of new non-peripherally substituted lutetium bis phthalocyanines

The synthesis, characterization, spectral, and electrochemical properties of bis-(phthalocyanine)lutetium (III) (LuPc2) bearing halogenated (F (F-LuPc2), Cl (Cl-LuPc2) and Br (Br-LuPc2)) phenoxy groups were reported for the first time. To explore the possible application of the LuPc2 complexes, their detailed redox mechanisms were determined by voltammetric and spectroelectrochemical characterizations. One of the Pc rings exhibits radical characteristics as a result of the sandwich structures of the complexes, which led to the typical one electron reduction process between 0.20 and 0.40 V and the typical one electron oxidation process between 0.60 and 0.80 V.In addition to the characteristic Pc.-Lu3+Pc2-/Pc2-Lu3+Pc2-1- reduction and Pc.-Lu3+Pc2-/Pc.-Lu3+Pc.-1+ oxidation couple of the complexes, further two-electron reduction and oxidation processes were also observed due to the simultaneous electron transfer reactions of each Pc ring at very close potentials. Moreover, at more negative potentials sequential one electron reduction waves for each Pc ring were also observed. While the Cl-LuPc2 reduced at less positive potential and more negative potential, changing Br with Cl and F alters the ease of the electron transfer reactions of the complexes. In situ spectroelectrochemical results supported these voltammetric assignments. Moreover, distinct color changes of LuPc2 in the solution observed with in situ spectroelectrochemical measurements indicated their functionality as electrochromic materials.

Kinetics and mechanism of the oxidation of hydrazine with [RuIII(pic)3] in aqueous solution

The kinetics and mechanism of the oxidation of hydrazine by [RuIII(pic)3] (pic- = picolinate anion) were studied spectrophotometrically as a function of hydrazine concentrations at a constant pH of 5.5. The rate of the reaction was followed by monitoring the appearance of the MLCT band of [RuII(pic)3]- at 466 nm. The reaction rate was found to be first-order for Ru(III)-complex concentration. The observed rate constant (k obs) values increased linearly with increase in the hydrazine concentration. The oxidation reaction was studied at varied temperature (20–40 °C). The ΔH≠ (73 ± 3 kJ mol−1 kJ·mol−1) and the negative ΔS≠ (34 ± 7 J·deg−1mol−1) values are consistent with the proposed outer-sphere electron transfer process operative in the oxidation of hydrazine with [RuIII(pic)3]. A working mechanism involving the formation of an active diazene (HN = NH) intermediate followed by its rapid two-electron oxidation by [RuIII(pic)3] in a kinetically inconsequential step producing N2 as sole product is proposed for an overall four-electron oxidation of hydrazine to dinitrogen.

Electrochemical Behavior and Determination of Tolvaptan Using Glassy Carbon and Boron-Doped Diamond Electrodes

Tolvaptan (TOL), a vasopressin V2 receptor antagonists drug, is used to treat hyponatremia in people with heart failure. Herein, we report for the first time the electrochemical behavior of TOL and its oxidation metabolite TOL-oxo on glassy carbon and boron-doped diamond electrodes (GCE, BDDE) by cyclic voltammetry (CV). For both molecules, two-electron irreversible oxidation was observed in 0.5 M H2SO4, while single-electron irreversible oxidation occurred at pH 7.0. A sensitive and rapid electroanalytical technique for the detection of TOL has been developed, based on differential pulse voltammetry (DPV). Linearity was obtained with GCE within a concentration range between 1–1000 μM, using anodic currents appearing around 1397 mV. For BDDE, the concentration range was 8–1000 μM using anodic currents around 1490 mV.

Photocatalytic degradation of chlorinated organic pollutants by ZnS@ZIF-8 composite through hydrogen peroxide generation by activating dioxygen under simulated sunlight irradiation

Photocatalytic hydrogen peroxide (H2O2) production and its application in advanced oxidation processes (AOPs) are regarded as low-cost and environmentally friendly wastewater treatment processes. Herein, by modifying a small amount of sulphide on the zeolitic imidazolate framework-8 (ZIF-8), a ZnS@ZIF-8 composite and used for photocatalytic H2O2 production to degrade chlorinated organic pollutants under simulated sunlight (SSL). ZnS@ZIF-8 composite could enhance the separation of photo-induced charge carriers, promote electron transfer from zinc sulphide (ZnS) to ZIF-8, which exhibited good selectivity for the two-electron oxygen reduction reaction (2e--ORR) and two-electron water oxidation (2e--WOR) pathways. Based on oxygen (O2) activation, the developed ZnS@ZIF-8/O2/SSL system could achieve 6.43 mmol/L H2O2 production within 150 min, which was approximately 8.66 and 10.36 times higher than that of the ZnS/O2/SSL and ZIF-8/O2/SSL systems, respectively. In the ZnS@ZIF-8/O2/SSL system, the ORR, WOR and H2O2 photolysis led to the generation of hydroxyl radical (•OH), while the photochemical behavior of ZnS in ZnS@ZIF-8 composite resulted in the generation of active hydrogen (*H). Benefitting from the high concentration of H2O2 and the coexistence of redox species in the ZnS@ZIF-8/O2/SSL system, various chlorinated organic pollutants could be dechlorinated and mineralized. In addition, a possible mechanism for photocatalytic H2O2 production was also proposed. Importantly, the proposed process did not involve an additional sacrificial agent or Fenton-like catalysts. This work provides insights into the potential application of ZnS@ZIF-8 composite for H2O2 production and wastewater treatment.

Decoupling of the Confused Complex in Oxidation of 3,3',5,5'-Tetramethylbenzidine for the Reliable Chromogenic Bioassay.

Regulation of the reaction pathways is a perennial theme in the field of chemistry. As a typical chromogenic substrate, 3,3',5,5'-tetramethylbenzidine (TMB) generally undertakes one-electron oxidation, but the product (TMBox1) is essentially a confused complex and is unstable, which significantly hampers the clinic chromogenic bioassays for more than 50 years. Herein, we report that sodium dodecyl sulfate (SDS)-based micelles could drive the direct two-electron oxidation of TMB to the final stable TMBox2. Rather than activation of H2O2 oxidant in the one-electron TMB oxidation by common natural peroxidase, activation of the TMB substrate by SDS micelles decoupled the thermodynamically favorable complex between TMBox2 with unreacted TMB, leading to an unusual direct two-electron oxidation pathway. Mechanism studies demonstrated that the complementary spatial and electrostatic isolation effects, caused by the confined hydrophobic cavities and negatively charged outer surfaces of SDS micelles, were crucial. Further cascading with glucose oxidase, as a proof-of-concept application, allowed glucose to be more reliably measured, even in a broader range of concentrations without any conventional strong acid termination.

A general strategy to enhance hydrogen peroxide generation via two-electron water oxidation by antimony modification for removal of triethyl phosphate and hexavalent chromium

Two-electron water oxidation has attracted more and more attention for H2O2 production. This strategy mainly uses water and does not require bubbling of oxygen gas, which can be widely used in oxygen-deficient environments. Currently, the most important research for two-electron water oxidation is the design of high-efficiency anode materials. Herein, a general method is developed to enhance the activity of two-electron water oxidation for H2O2 production via Sb2O3 modification. This strategy is suitable for a series of electrodes, including carbon fiber paper, graphite plate, WO3, CuWO4 and WO3&CuWO4 composite. The mechanism for Sb2O3 modification toward enhanced H2O2 generation is clearly demonstrated. It should be due to three factors: 1) enhance the reaction kinetic, 2) enlarge the specific surface area and 3) suppress H2O2 decomposition. According to density function calculation, it is firstly found that the Sb optimizes the Gibbs free energy of adsorbed *OH intermediate of its nearby atom, making its adjacent atom (such as W) the active site. Finally, these Sb-modified electrodes are firstly used to produce H2O2 for high-efficiency removal of triethyl phosphate, an environmental pollutant that are difficult to be degraded. It can also be used for the removal of hexavalent chromium, one kind of heavy metal.