Single-electron Reduction Research Articles

Mechanistic Insights into Enantioselective C(sp3 )-H Acylation to Construct α-Amino Ketones via Photoredox and Ni(II) Dual Catalysis: A DFT Study.

The development of the merger of a Ni(II) catalyst with an appropriate photocatalyst under visible-light irradiation provides a new strategy for realizing direct functionalization of C(sp3 )-H bonds. Mechanistically, whether the reduction of Ni catalyst to form a Ni(0) species is necessary in the dual catalysis still remains under debate. Herein, DFT calculations were carried out to gain a mechanistic insight into the enantioselective acylation of α-amino C(sp3 )-H bonds to furnish α-amino ketones via photoredox and Ni dual catalysis. A feasible mechanistic pathway for the Ni catalysis via the Ni(I)-Ni(III)-Ni(II)-Ni(III)-Ni(I) cycle is suggested with the sequential elementary steps of oxidative addition, single electron reduction, radical addition, and reductive elimination in leading to the final product, whereas a nickel catalytic cycle, Ni(I)-Ni(0)-Ni(II)-Ni(III)-Ni(I), might not be feasible for the photoredox and Ni dual-catalyzed acylation of α-amino C(sp3 )-H bonds. The origin of the stereoselectivity for this reaction is also discussed, which could be attributed to the minimization of the steric hindrance between the alkyl moiety of radical part and phenyl group of the chiral ligand.

Thiazolothiazole-Containing Conjugated Polymer with Electrochromism and Electrofluorochromism-Based Dual Performance for a Flip-Flop Molecular Logic Gate

Dual-performance devices based on electrochromism and electrofluorochromism have recently gained much attention in display technology. However, getting electrochromism and electrofluorochromism from the same material is very challenging. Herein, we have synthesized a low-band-gap donor–acceptor conjugated thiazolothiazole-containing organic polymer, P-TzTz, that exhibited a quasi-reversible single-electron reduction to demonstrate a pale yellow-to-black (NIR region) electrochromism and a red-to-quenched electrofluorochromism at a low potential window of −1.7 to +0.6 V with suitable electrochromic/electrofluorochromic parameters and stability. Possible redox-induced modulation of π–π* and intramolecular charge transfer in the P-TzTz polymer, which was validated through density functional theory calculations, interplayed the key role of tunable absorption from vis to NIR and on–off fluorescence switching. Finally, mimicking a flip-flop logic gate model with optical memory functions was also demonstrated by taking the potentials as inputs and electrochromic and electrofluorochromic dual performance signals as outputs.

Evidence for Porphyrin-Mediated Electron Transfer in the Radical SAM Enzyme HutW.

Bacteria that infect the human gut must compete for essential nutrients, including iron, under a variety of different metabolic conditions. Several enteric pathogens, including Vibrio cholerae and Escherichia coli O157:H7, have evolved mechanisms to obtain iron from heme in an anaerobic environment. Our laboratory has demonstrated that a radical S-adenosylmethionine (SAM) methyltransferase is responsible for the opening of the heme porphyrin ring and release of iron under anaerobic conditions. Furthermore, the enzyme in V. cholerae, HutW, has recently been shown to accept electrons from NADPH directly when SAM is utilized to initiate the reaction. However, how NADPH, a hydride donor, catalyzes the single electron reduction of a [4Fe-4S] cluster, and/or subsequent electron/proton transfer reactions, was not addressed. In this work, we provide evidence that the substrate, in this case, heme, facilitates electron transfer from NADPH to the [4Fe-4S] cluster. This study uncovers a new electron transfer pathway adopted by radical SAM enzymes and further expands our understanding of these enzymes in bacterial pathogens.

Modified quantum mechanical approach for the estimation of single-electron reduction potential of nitroaromatic compounds in aqueous medium

The midpoint single-electron reduction potential of nitroaromatic compounds in aqueous medium at pH 7.0 (potential of ArNO2/ArNO2 · – couple, E m7) frequently determines their therapeutic and/or toxic properties. However, its estimation remains a complex problem. We propose a modified method of E m7 estimation by quantum mechanical calculations, based on the use of the dielectric continuum model together with a certain number of H2O molecules at the vicinity of nitro group. The optimal number of H2O molecules corresponds to a minimal difference between the experimentally determined and calculated values of E m7, and/or the most negative value of calculated E m7. This enabled us to calculate the E m7 values for a number of ArNO2 (n = 19) with the average deviation of 0.027 V from the experimentally determined ones. Apart from nitrobenzene derivatives, the application of this approach for the representatives of nitropyridines, nitrofurans, nitrothiophenes, and nitrothiazoles was demonstrated. In this case, nitroimidazole derivatives are an exception, evidently due to a strong proton accepting properties of N3 atom of their free radicals.

Surface Ru-H Bipyridine Complexes-Grafted TiO2 Nanohybrids for Efficient Photocatalytic CO2 Methanation.

A series of novel surface Ru-H bipyridine complexes-grafted TiO2 nanohybrids were for the first time prepared by a combined procedure of surface organometallic chemistry with post-synthetic ligand exchange for photocatalytic conversion of CO2 to CH4 with H2 as electron and proton donors under visible light irradiation. The selectivity toward CH4 increased to 93.4% by the ligand exchange of 4,4'-dimethyl-2,2'-bipyridine (4,4'-bpy) with the surface cyclopentadienyl (Cp)-RuH complex and the CO2 methanation activity was enhanced by 4.4-fold. An impressive rate of 241.2 μL·g-1·h-1 for CH4 production was achieved over the optimal photocatalyst. The femtosecond transient IR absorption results demonstrated that the hot electrons were fast injected in 0.9 ps from the photoexcited surface 4,4'-bpy-RuH complex into the conduction band of TiO2 nanoparticles to form a charge-separated state with an average lifetime of ca. 50.0 ns responsible for the CO2 methanation. The spectral characterizations indicated clearly that the formation of CO2•- radicals by single electron reduction of CO2 molecules adsorbed on surface oxygen vacancies of TiO2 nanoparticles was the most critical step for the methanation. Such radical intermediates were inserted into the explored Ru-H bond to generate Ru-OOCH species and finally CH4 and H2O in the presence of H2.

Synthesis of Multifluoromethylated γ-Sultines by a Photoinduced Radical Addition-Polar Cyclization.

Despite the significance of sultines in synthesis, medicine, and materials science, the chemistry of sultines has remained unexplored due to their inaccessibility. Herein, we demonstrate the development of a photoredox-catalyzed multifluoromethyl radical addition/SO2 incorporation/polar cyclization cascade approach to multifluoromethylated γ-sultines. The reactions proceed by single electron transfer induced multifluoromethyl radical addition to an alkene followed by SO2 incorporation, and single-electron reduction for polar 5-exo-tet cyclization. Key to the success of the protocol is the use of easily oxidizable multifluoroalkanesulfinates as bifunctional reagents. The reactions proceed with excellent functional-group tolerance to deliver γ-sultines in moderate to excellent yields.

Effect of Group-10 Element M (Ni, Pd, Pt) on Electronic Structure of Icosahedral M@Au12 Cores of MAu24L18 (L = Alkynyl, Thiolate)

Heterometal doping into chemically modified gold clusters is a straightforward method to modulate their electronic structure. Although examples exist of alloy clusters containing a series of heterometeal dopants from different groups of the periodic table, it is rare for a complete set of alloy clusters to contain dopants from the same group. In this work, we newly synthesize Ni-doped gold clusters with NiAu24L18 [L = ArFC≡C– (ArF = 3,5-(CF3)2C6H3), PhC2H4S–] by using hydrogen-containing phosphine-protected NiAu clusters as precursors. The synthesis of the Ni-doped counterpart allows us to compare the geometric and electronic structures of prototypical alloy clusters with a M@Au12 core (M = Ni, Pd, Pt). Single-crystal X-ray diffraction analysis indicates that due to the small atomic radius of Ni, NiAu24L18 has a slightly contracted core than clusters with M = Pd or Pt. Voltammetry and optical absorption spectroscopy suggest that the modified jellium model cannot solely explain the period-dependent doping effect because the Ni@Au12 core contracts. MAu24L18 clusters (L = PhC2H4S–) are paramagnetized by single electron reduction, and their magnetic properties are investigated by using electron spin resonance spectroscopy. This work thus provides a guide for designing gold clusters by heterometal doping within a given group of the periodic table.

5-(Trifluorovinyl)dibenzothiophenium Triflate: Introducing the 1,1,2-Trifluoroethylene Tether in Drug-Like Structures.

This manuscript reports the synthesis and structure of an unprecedented sulfonium salt, 5-(trifluorovinyl)dibenzothiophenium triflate, and its use as a versatile reagent for the introduction of the bioisosteric 1,1,2-trifluoroethylene linker in drug-like structures. The protocol developed consists of the reaction of this compound with alcohols and phenols to deliver a complete set of 1,2,2-trifluoro-2-(alkoxy-/aryloxy)ethyl sulfonium salts, which have been purified by column chromatography and fully characterized. Subsequent single electron reduction under mild photochemical conditions efficiently affords the corresponding fluoroalkyl radicals that are trapped either intra- or intermolecularly through their reaction with (hetero)arenes. Theoretical calculations are used to evaluate the conformational consequences derived from the presence of the CF2 -CHF tether.

Unveiling the latent reactivity of imines on pyridine-functionalized covalent organic frameworks for H2O2 photosynthesis

Unveiling the active site for the oxygen reduction reaction (ORR) holds the key to understanding and improving the photocatalytic activity of covalent organic frameworks (COFs) for H2O2 evolution. However, for imine-linked COFs, the role of the imine group is often overlooked in photosynthesis compared with other groups with electrophilicity or light-harvesting capabilities. Herein, a strategy is presented for eliciting the latent photoreactivity of imines by introducing an electron-acceptor structure (pyridine unit) near the imine bonds to enhance the kinetic and thermodynamic advantages of imines for the photocatalytic ORR to H2O2. Experiments and theoretical simulations indicate that the hindered visible light absorption and charge carrier separation caused by the weak electron delocalization can be substantially improved by introducing pyridinic N, leading to full solar spectrum absorption. Meanwhile, the pyridinic N can act with the N atom of the imine as an enhanced site for O2 adsorption and activation, and the strong electron transfers from COFs to O2 and ORR intermediates enable a two-step single-electron reduction route of O2 in pyridine-functionalized COFs for a more feasible H2O2 generation (706.2 µmol g−1 h−1) than original imine-linked COF (372.7 µmol g−1 h−1) under visible light irradiation. This work provides a new idea for designing and modifying imine-linked COFs in advanced photocatalytic applications.

Rücktitelbild: Visible‐Light‐Induced N‐Heterocyclic Carbene‐Catalyzed Single Electron Reduction of Mono‐Fluoroarenes (Angew. Chem. 12/2023)

Rücktitelbild: Visible‐Light‐Induced N‐Heterocyclic Carbene‐Catalyzed Single Electron Reduction of Mono‐Fluoroarenes (Angew. Chem. 12/2023)

Back Cover: Visible‐Light‐Induced N‐Heterocyclic Carbene‐Catalyzed Single Electron Reduction of Mono‐Fluoroarenes (Angew. Chem. Int. Ed. 12/2023)

Back Cover: Visible‐Light‐Induced N‐Heterocyclic Carbene‐Catalyzed Single Electron Reduction of Mono‐Fluoroarenes (Angew. Chem. Int. Ed. 12/2023)

Visible‐Light‐Induced N‐Heterocyclic Carbene‐Catalyzed Single Electron Reduction of Mono‐Fluoroarenes

AbstractFluoroarenes are abundant and readily available feedstocks. However, due to the high reduction potentials of mono‐fluoroarenes, their photoreduction remains a continuing challenge, motivating the development of efficient activation modes to address this issue. This report presents the blue light‐induced N‐heterocyclic carbene (NHC)‐catalyzed single electron reduction of mono‐fluoroarenes for biaryl cross‐couplings. We discovered that under blue light irradiation, NHC/tBuOK combination could construct powerful photoactive architectures to promote single electron transfer for Caryl−F bond reduction via forming highly reducing NHC radical anion. Notably, the strategy was also successful to reduce Caryl−O, Caryl−N, and Caryl−S bonds for biaryl cross‐couplings.

Visible-Light-Induced N-Heterocyclic Carbene-Catalyzed Single Electron Reduction of Mono-Fluoroarenes.

Fluoroarenes are abundant and readily available feedstocks. However, due to the high reduction potentials of mono-fluoroarenes, their photoreduction remains a continuing challenge, motivating the development of efficient activation modes to address this issue. This report presents the blue light-induced N-heterocyclic carbene (NHC)-catalyzed single electron reduction of mono-fluoroarenes for biaryl cross-couplings. We discovered that under blue light irradiation, NHC/tBuOK combination could construct powerful photoactive architectures to promote single electron transfer for Caryl -F bond reduction via forming highly reducing NHC radical anion. Notably, the strategy was also successful to reduce Caryl -O, Caryl -N, and Caryl -S bonds for biaryl cross-couplings.

FeNi3 foam as cathode catalyst in electrocatalytic peroxydisulfate system for enhanced Hg0 removal from simulated flue gas

The feasibility of elemental mercury (Hg0) removal from simulated flue gas in electrocatalytic peroxydisulfate (PDS) system employing FeNi3 foam as cathode catalyst was studied for the first time. The electrocatalytic (EC) performance of FeNi3 foam activating PDS process was evaluated to remove Hg0 using Pt anode, FeNi3 cathode, and Na2SO4 electrolyte at room temperature. The results showed that the FeNi3 foam/PDS/EC system was superior to the other removal processes. According to the influence of experimental parameters, it was found that about 94% of Hg0 was removed within 20 min under the conditions of voltage = 2 V, PDS = 1 mM, Na2SO4 = 0.05 M. Theoretical calculations showed that the Ni atom of the FeNi3 cluster would improve its reactivity. The chemical adsorption and hybridization between PDS and FeNi3 had contributed to an efficient production of SO4•− free radical. The high removal efficiency of Hg0 was due to the following assistances: (i) the O2 gas adsorbed on the FeNi3 foam cathode surface was able to generate radicals •O2– and •OH by single electron reduction reaction, (ii) FeNi3 foam catalyst could afford Fe(II) and Ni(II) ions via two electron transfer, activating PDS to produce SO4•−, and (iii) the copresence of Fe and Ni was useful to accelerate the ion cycles of Fe(III)/Fe(II) and Ni(II)/Ni(I), which played a crucial role in FeNi3 foam/PDS/EC system. The novel proposed FeNi3 foam cathode electro-catalyzing PDS system provided a promising prospect in the pollutant control field of Hg0 removal.

One Carbon Ring Expansion of Bipyrrole to Bipyridine Enables Access to a π-Extended, Non-innocent, Corrole-like Ligand.

A π-extended, diaza-triphenylene embedded, mono-anionic corrole analogue and its NiII complex were synthesized from a diaza-triphenylene precursor, which was obtained from a double one-carbon insertion into a naphthobipyrrole diester. Following conversion to the corresponding activated diol and acid-catalyzed condensation with pyrrole, subsequent reaction with pentafluorobenzaldehyde afforded mono-anionic, π-extended bipyricorrole-like macrocycle. Attempted NiII insertion with Ni(OAc)2 ⋅ 4H2 O resulted an ESR active, NiII bipyricorrole radical complex, which was converted to a stable cationic NiII complex upon treatment with [(Et3 O)+ (SbCl6 )- ]. Both complexes were characterized by 1 H and 13 C NMR, UV/Vis spectroscopy and single crystal X-ray diffraction analysis. The NiII bipyricorrole radical complex is converted to a cationic NiII complex by single-electron reduction using cobaltocene. Both the cationic NiII complex and the radical NiII complex exhibited ligand-centered redox behavior, whereas the NiII remains in the +2 oxidation state.

Facile synthesis of O-doped carbon nitride nanofibers for two-step single-electron oxygen reduction to H2O2

O doping is an effective and attractive method for improving the photocatalytic oxygen reduction to H2O2 generation performance.

Compositionally engineered Cd-Mo-Se alloyed QDs toward photocatalytic H2O2 production and Cr(VI) reduction with a detailed mechanism and influencing parameters.

With the exceptional advantages of safety, greenness, and low cost, photocatalytic H2O2 generation has kindled a wonderful spark, although being severely hampered by the terrible photoinduced exciton recombination, migration, and surface decomposition. Here, employing reflux method, the Cd-Mo-Se quantum dots of varying molar ratios of Cd and Mo were synthesized using thioglycolic acid as the capping ligand to regulate their growth. This type of metal alloying promotes rapid charge migration, improves light harvesting, and reduces the rate of charge recombination. The improved optoelectronic properties and boosted activity of Cd-rich ternary CMSe-1 QDs led to the observed exceptional photocatalytic H2O2 yield of 1403.5 μmol g-1 h-1 (solar to chemical conversion efficiency, 0.27%) under visible light, outperforming the other ternary and Se-based QD photocatalysts. Additionally, CMSe-1 shows 93.6% (2 h) hazardous Cr(VI) photoreduction. The enhanced catalytic performance of CMSe-1 corresponds to effective charge carrier separation and transfer efficiency, well supported by PL, TRPL, and electrochemical measurements. Photocatalytic H2O2 production was also studied under varying experimental conditions and the scavenger test suggests a superoxide radical intermediate 2-step single electron reduction pathway. The catalyst-assisted Cr(VI) reduction is substantiated by the zero-order kinetics as well as the determination of the pHPZC value. The catalyst can be employed for a maximum of four times while retaining its activity, according to the photostability and reusability test outcomes. This research presents interesting approaches for producing ternary QDs and modified systems for efficient photocatalytic H2O2 production and Cr(VI) reduction.

Bifunctionality of Zn dust in Ullmann C–C cross-coupling by Ni/Pd dual catalysis: theoretical insight

Zinc powder presents bifunctionality for heterogeneous single-electron reduction and homogeneous transmelation in Ullmann C–C bond cross-coupling by Ni/Pd dual catalysis.

Photochemical Organocatalytic Functionalization of Pyridines via Pyridinyl Radicals.

We report a photochemical method for the functionalization of pyridines with radicals derived from allylic C-H bonds. Overall, two substrates undergo C-H functionalization to form a new C(sp2)-C(sp3) bond. The chemistry harnesses the unique reactivity of pyridinyl radicals, generated upon single-electron reduction of pyridinium ions, which undergo effective coupling with allylic radicals. This novel mechanism enables distinct positional selectivity for pyridine functionalization that diverges from classical Minisci chemistry. Crucial was the identification of a dithiophosphoric acid that masters three catalytic tasks, sequentially acting as a Brønsted acid for pyridine protonation, a single electron transfer (SET) reductant for pyridinium ion reduction, and a hydrogen atom abstractor for the activation of allylic C(sp3)-H bonds. The resulting pyridinyl and allylic radicals then couple with high regioselectivity.

Dual H2O2 production paths over chemically etched MoS2/FeS2 heterojunction: Maximizing self-sufficient heterogeneous Fenton reaction rate under the neutral condition

Sufficient generation of H2O2 played an essential role in boosting self-sufficient heterogeneous Fenton reaction. Herein, the dual H2O2 production paths were designed to maximize the yield over H2O2-etched MoS2/FeS2 Z-scheme heterojunction, whose surface was modified with abundant –OH groups and sulfur vacancies (SVs). Mechanism studies revealed that H2O2 was generated via two-step single-electron reduction by electrons on conduction band, where H2O2 production was significantly improved by the internal hole-scavenging effect from –OH groups. Additionally, electrons surrounding SVs served as another sites for H2O2 production via one-step two-electron reduction even in darkness. H2O2 yield in neutral aqueous solution reached 1.5 mM/g/h without external hole scavenger. Tetracycline and rhodamine B were effectively degraded under light or in darkness. The degradation performance was even comparable to the heterogeneous Fenton reaction. This work provides new insights for the design of self-sufficient heterogeneous Fenton system with exceptional degradation performance.