Showing papers on "Radical ion published in 2022"

Electro-/photocatalytic alkene-derived radical cation chemistry: recent advances in synthetic applications.

Abstract: Alkene-derived radical cations are versatile reactive intermediates and have been widely applied in the construction of complex functionalized molecules and cyclic systems for chemical synthesis. Therefore, the synthetic application of these alkene-derived radical cations represents a powerful and green tool that can be used to achieve the functionalization of alkenes partially because the necessity of stoichiometric external chemical oxidants and/or hazardous reaction conditions is eliminated. This review summarizes the recent advances in the synthetic applications of the electro-/photochemical alkene-derived radical cations, emphasizing the key single-electron oxidation steps of the alkenes, the scope and limitations of the substrates, and the related reaction mechanisms. Using electrocatalysis and/or photocatalysis, single electron transfer (SET) oxidation of the CC bonds in the alkenes occurs, generating the alkene-derived radical cations, which sequentially enables the functionalization of translocated radical cations to occur in two ways: the first involves direct reaction with a nucleophile/radical or two molecules of nucleophiles to realize hydrofunctionalization, difunctionalization and cyclization; and the second involves the transformation of the alkene-derived radical cations into carbon-centered radicals using a base followed by radical coupling or oxidative nucleophilic coupling.

Photoredox-Catalyzed and Copper(II) Salt-Assisted Radical Addition/Hydroxylation Reaction of Alkenes, Sulfur Ylides, and Water

Abstract: A visible light-driven photoredox-catalyzed and copper(II)-assisted three-component radical addition/hydroxylation reaction of alkenes, sulfur ylides, and water is reported. This process shows broad substrate scope and high functional group tolerance, with respect to both readily available sulfur ylides and alkenes, providing high-yielding and practical access to valuable γ-hydroxy carbonyl compounds. Key to the success of the reaction is the controlled generation of α-carbonyl carbon radicals from sulfur ylides via sulfonium salts by a visible-light-driven proton-coupled electron transfer (PCET) strategy in a mixture of 2,2,2-trifluoroethanol/CH2Cl2. Addition of Cu(TFA)2·H2O helps to accelerate the radical-cation crossover to improve the reaction efficiency. Mechanistic studies suggest that the hydroxy moiety in the products stems from water. This study also builds up a platform for further investigation into the radical synthetic chemistry of sulfur ylides.

Highly Stable Neutral Corrole Radical: Amphoteric Aromatic-Antiaromatic Switching and Efficient Photothermal Conversion.

Abstract: The preparation of novel stable radical systems that survive and may be manipulated under harsh conditions is essential for their practical applications, such as energy storage and conversion materials. Here, we present a facile synthesis of an electrically neutral benzo-fused nickel corrole radical that shows remarkable photo- and thermal stability. The carbon-based organic radical character was confirmed using electron spin resonance and spin population analyses. This radical may be reversibly converted to its aromatic or antiaromatic ion via a one-electron redox process, as indicated by nuclear magnetic resonance chemical shifts and theoretical calculations. Notably, the antiaromatic state is stable, showing intense ring currents with complex pathways. The spectroscopic characteristics and calculated molecular orbitals of the corrole radical exhibit a combination of aromatic and antiaromatic features. On the basis of the aromatic light-harvesting property and antiaromatic emission-free character, the corrole radical exhibits highly robust, efficient photothermal energy conversion in water after encapsulation within nanoparticles, with the unpaired spin simultaneously retained. These results provide a fundamental understanding of the relationship between the (anti)aromaticity and photophysical properties of a porphyrinoid radical and a promising platform for the design of radical-based functional materials.

Substituent-dependent absorption and fluorescence properties of perylene bisimide radical anions and dianions

Abstract: Perylene-3,4:9,10-bis(dicarboximides) (PBIs) rank among the most important functional dyes and organic semiconductors, but only recently have their radical anions and dianions attracted interest for a variety of applications. Here, we systematically elucidate the functional properties (redox, absorption, and emission) of five PBI anions and dianions bearing different bay-substituents attached to the chromophore core. Cyclic voltammetry measurements reveal the influence of the substituents ranging from electron-withdrawing cyano to electron-donating phenoxy groups on the oxidation and reduction potentials that relate to the HOMO and LUMO levels ranging from -7.07 eV to -6.05 eV and -5.01 eV to -4.05 eV, respectively. Spectroelectrochemical studies reveal a significant number of intense absorption bands in the NIR-spectral range (750-1400 nm) for the radical anions, whereas the dianionic species are characterized by similar spectra to those for the neutral dyes, however being bathochromically shifted and with increased molar extinction coefficients of approximately 100 000 M-1 cm-1. The increase of the transition dipole moment is up to 56% and accompanied by an almost cyanine-like red-shifted (by 300 nm) absorption spectrum for the most electron-poor tetracyanotetrachloro PBI. Whilst the outstanding fluorescence properties of the neutral PBIs are lost for the radical anions, an appreciable near-infrared (NIR) fluorescence with a quantum yield of up to 18% is revealed for the dianions by utilizing a custom-built flow-cell spectroelectrochemistry setup. Time-dependent density functional theory calculations help to assign the absorption bands to the respective electronic transitions.

Substituent-dependent absorption and fluorescence properties of perylene bisimide radical anions and dianions.

Abstract: Perylene-3,4:9,10-bis(dicarboximides) (PBIs) rank among the most important functional dyes and organic semiconductors, but only recently have their radical anions and dianions attracted interest for a variety of applications. Here, we systematically elucidate the functional properties (redox, absorption, and emission) of five PBI anions and dianions bearing different bay-substituents attached to the chromophore core. Cyclic voltammetry measurements reveal the influence of the substituents ranging from electron-withdrawing cyano to electron-donating phenoxy groups on the oxidation and reduction potentials that relate to the HOMO and LUMO levels ranging from −7.07 eV to −6.05 eV and −5.01 eV to −4.05 eV, respectively. Spectroelectrochemical studies reveal a significant number of intense absorption bands in the NIR-spectral range (750–1400 nm) for the radical anions, whereas the dianionic species are characterized by similar spectra to those for the neutral dyes, however being bathochromically shifted and with increased molar extinction coefficients of approximately 100 000 M−1 cm−1. The increase of the transition dipole moment is up to 56% and accompanied by an almost cyanine-like red-shifted (by 300 nm) absorption spectrum for the most electron-poor tetracyanotetrachloro PBI. Whilst the outstanding fluorescence properties of the neutral PBIs are lost for the radical anions, an appreciable near-infrared (NIR) fluorescence with a quantum yield of up to 18% is revealed for the dianions by utilizing a custom-built flow-cell spectroelectrochemistry setup. Time-dependent density functional theory calculations help to assign the absorption bands to the respective electronic transitions.

Simultaneous and Spontaneous Oxidation and Reduction in Microdroplets by the Water Radical Cation/Anion Pair

Abstract: Abstract Microdroplets show unique chemistry, especially in their intrinsic redox properties, and to this we here add a case of simultaneous and spontaneous oxidation and reduction. We report the concurrent conversions of several phosphonates to phosphonic acids by reduction (R−P → H−P) and to pentavalent phosphoric acids by oxidation. The experimental results suggest that the active reagent is the water radical cation/anion pair. The water radical cation is observed directly as the ionized water dimer while the water radical anion is only seen indirectly though the spontaneous reduction of carbon dioxide to formate. The coexistence of oxidative and reductive species in turn supports the proposal of a double‐layer structure at the microdroplet surface, where the water radical cation and radical anion are separated and accumulated.

Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

Abstract: Spontaneous oxidation of compounds containing diverse X=Y moieties (e.g., sulfonamides, ketones, esters, sulfones) occurs readily in organic-solvent microdroplets. This surprising phenomenon is proposed to be driven by the generation of an intermediate species [M+H2O]+·: a covalent adduct of water radical cation (H2O +· ) with the reactant molecule (M). The adduct is observed in the positive ion mass spectrum while its formation in the interfacial region of the microdroplet (i.e., at the air-droplet interface) is indicated by the strong dependence of the oxidation product formation on the spray distance (which reflects the droplet size and consequently the surface-to-volume ratio) and the solvent composition. Importantly, based on the screening of a ca. 21,000-compound library and the detailed consideration of six functional groups, the formation of a molecular adduct with the water radical cation is a significant route to ionization in positive ion mode electrospray, where it is favored in those compounds with X=Y moieties which lack basic groups. A set of model monofunctional systems was studied and in one case, benzyl benzoate, evidence was found for oxidation driven by hydroxyl radical adduct formation followed by protonation in addition to the dominant water radical cation addition process. Significant implications of molecular ionization by water radical cations for oxidation processes in atmospheric aerosols, analytical mass spectrometry and small-scale synthesis are noted.

Evidence and Governing Factors of the Radical-Ion Photoredox Catalysis

Abstract: Photoinduced electron transfer of a radical species provides a promising strategy for expanding the scope of photoredox catalysis. However, the mechanism underlying the radical-ion photoredox catalysis has yet been fully established. In the present research, we investigated the generation and annihilation of the key catalytic intermediate in the borylation reaction of aryl halides. To disentangle the intricate catalysis cycle, we conducted experiments using electrochemical, transient absorption and photoluminescence spectroscopy and quantum chemical calculation techniques. Our mechanistic studies provided convincing evidence that the active catalyst species is the excited-state radical anion. We also found that the catalyst activation is retarded by multiple annihilative processes, among which the intrinsic relaxation of the excited-state radical anion of the catalyst is the most destructive. We expect these mechanistic findings will guide the future development of photoredox catalysis of radical species.

The excited-state dynamics of the radical anions of cyanoanthracenes

Abstract: The radical anion of 9,10-dicyanoanthracene (DCA) has been suggested to be a promising chromophore for photoredox chemistry, due to its nanosecond excited-state lifetime determined from indirect measurements. Here, we investigate the excited-state dynamics of the radical anion of three cyanoanthracenes, including DCA˙-, produced by photoinduced electron transfer in liquid using both pump-probe and pump-pump probe transient electronic absorption spectroscopy. All three excited radical ions are characterised by a 3-5 ps lifetime, due to efficient non-radiative deactivation to the ground state. The decay pathway most probably involves D1/D0 conical intersection(s), whose presence is favoured by the enhanced flexibility of the radical anions relative to their neutral counterparts. The origin of the discrepancy with the nanosecond lifetime of DCA˙-* reported previously is discussed. These very short lifetimes limit, but do not preclude, photochemical applications of the cyanoanthracene anions.

Charge-Transfer Cocrystal via a Persistent Radical Cation Acceptor for Efficient Solar-Thermal Conversion.

Abstract: Designing organic charge-transfer (CT) cocrystals for efficient solar-thermal conversion is a long-sought goal but remains challenging. Here we construct a unique CT cocrystal by using persistent 2,2'-azino-bis (3 ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS +• ) as the electron acceptor. The strong persistency and electron affinity of ABTS +• endow a high degree of electron delocalization between ABTS +• and 3,3',5,5'-tetramethylbenzidine donor. Together with the intrinsic long-wavelength absorption of ABTS +• , the synthesized cocrystal can effectively capture the full solar spectrum and show distinguished photothermal efficiency. Such cocrystal is further used for solar-driven interfacial evaporation, and a high evaporation rate of 1.407 kg m -2 h -1 and a remarkable solar-to-vapor efficiency of 97.0% have been achieved upon 1 sun irradiation. This work indicates the enormous prospects for charge transfer-based functional materials through rational radical cations engineering.

Two-Carbon Ring Expansion of Cyclobutanols to Cyclohexenones Enabled by Indole Radical Cation Intermediate: Development and Application to a Total Synthesis of Uleine

Abstract: A single-electron transfer (SET) oxidation of indole or benzo[b]thiophene to a radical cation reverses the intrinsic polarity of these π-excessive bicyclic heteroarenes. Here we report an oxidative two-carbon homologation of cyclobutanols to cyclohexenones under a visible-light photoredox catalysis. 1-(Indol-2-yl)cyclobutan-1-ols are converted to 2,3,4,9-tetrahydro-1H-carbazol-1-ones, important structural motifs found in alkaloids and pharmaceuticals, with a broad substrate scope. A mechanistic study suggests that the reaction is initiated by an SET from an indole to an excited acridinium salt to generate the radical cation, which is followed by two consecutive 1,2-alkyl migrations and a rearomatization. Benzo[b]thiophene-substituted cyclobutanols are similarly converted to 2,3-dihydrodibenzo[b,d]thiophen-4(1H)-ones. A total synthesis of (±)-uleine featuring this ring-expansion process is documented.

Visualizing an Electrochemically Induced Radical Cation of Bipyridine at Au(111)/Ionic Liquid Interfaces toward a Single-Molecule Switch.

Abstract: Room-temperature ionic liquids (RTILs) emerged as ideal solvents, and bipyridine as one of the most used ligands have been widely employed in surface science, catalysis, and molecular electronics. Herein, in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and STM break junction (STM-BJ) technique has been employed to probe the electrochemical process of bipyridine at Au(111)/IL interfaces. It is interestingly found that these molecules undertake a redox process with a pair of well-defined reversible peaks in cyclic voltammograms (CVs). The spectroscopic evidence shows a radical cation generated with rising new Raman peaks related to parallel CC stretching of a positively charged pyridyl ring. Furthermore, these electrochemically charged bipyridine is also confirmed by electrochemical STM-BJ at the single-molecule level, which displays a binary conductance switch ratio of about 400% at the redox potentials. This present work offers a molecular-level insight into the pyridine-mediated reaction process and electron transport in RTILs.

Concerted and Stepwise Proton-Coupled Electron Transfer for Tryptophan-Derivative Oxidation with Water as the Primary Proton Acceptor: Clarifying a Controversy

Abstract: Concerted electron-proton transfer (CEPT) reactions avoid charged intermediates and may be energetically favorable for redox and radical-transfer reactions in natural and synthetic systems. Tryptophan (W) often partakes in radical-transfer chains in nature but has been proposed to only undergo sequential electron transfer followed by proton transfer when water is the primary proton acceptor. Nevertheless, our group has shown that oxidation of freely solvated tyrosine and W often exhibit weakly pH-dependent proton-coupled electron transfer (PCET) rate constants with moderate kinetic isotope effects (KIE ≈ 2–5), which could be associated with a CEPT mechanism. These results and conclusions have been questioned. Here, we present PCET rate constants for W derivatives with oxidized Ru- and Zn-porphyrin photosensitizers, extracted from laser flash-quench studies. Alternative quenching/photo-oxidation methods were used to avoid complications of previous studies, and both the amine and carboxylic acid groups of W were protected to make the indole the only deprotonable group. With a suitably tuned oxidant strength, we found an ET-limited reaction at pH < 4 and weakly pH-dependent rates at pH > ∼5 that are intrinsic to the PCET of the indole group with water (H2O) as the proton acceptor. The observed rate constants are up to more than 100 times higher than those measured for initial electron transfer, excluding the electron-first mechanism. Instead, the reaction can be attributed to CEPT. These conclusions are important for our view of CEPT in water and of PCET-mediated radical reactions with solvent-exposed tryptophan in natural systems.

Kinetic model for reversible radical transfer in ribonucleotide reductase

Abstract: Significance All living organisms require a balance of deoxyribonucleotides in the cell to synthesize DNA. Ribonucleotide reductase (RNR) is an enzyme crucial to this process, converting ribonucleotides to deoxyribonucleotides. To initiate this reaction, RNR transfers an electron over long distances using specific amino acids as stepping stones. Detecting this moving electron is challenging, but has been achieved by activating the process with light for modified RNRs. We use experimental data and computer simulations to build a mathematical model that describes the time evolution of electron movement along the pathway. This model provides timescales of key steps in the pathway and illustrates the effects of slowing down or speeding up each step, thereby enhancing understanding of this biochemically and pharmacologically significant enzyme.

Simultaneous and Spontaneous Oxidation and Reduction in Microdroplets by the Water Radical Cation/Anion Pair

Abstract: Microdroplets show unique chemistry, especially in their intrinsic redox properties, and to this we here add a case of simultaneous and spontaneous oxidation and reduction. We report the concurrent conversions of several phosphonates to phosphonic acids by reduction (R−P → H−P) and to pentavalent phosphoric acids by oxidation. The experimental results suggest that the active reagent is the water radical cation/anion pair. The water radical cation is observed directly as the ionized water dimer while the water radical anion is only seen indirectly though the spontaneous reduction of carbon dioxide to formate. The coexistence of oxidative and reductive species in turn supports the proposal of a double-layer structure at the microdroplet surface, where the water radical cation and radical anion are separated and accumulated.

Mechanistic Investigations into Amination of Unactivated Arenes via Cation Radical Accelerated Nucleophilic Aromatic Substitution.

Abstract: A mechanistic investigation into the amination of electron-neutral and electron-rich arenes using organic photoredox catalysis is presented. Kinetic and computational data support rate-limiting nucleophilic addition into an arene cation radical using both azole and primary amine nucleophiles. This finding is consistent with both fluoride and alkoxide nucleofuges, supporting a unified mechanistic picture using cation radical accelerated nucleophilic aromatic substitution (CRA-SNAr). Electrochemistry and time-resolved fluorescence spectroscopy confirm the key role solvents play in enabling selective arene oxidation in the presence of amines. The synthetic limitations of xanthylium salts are elucidated via photophysical studies. An alternative catalyst scaffold with improved turnover numbers is presented.

Radical cation Diels–Alder reactions of arylidene cycloalkanes

Abstract: TiO2 photoelectrochemical and electrochemical radical cation Diels–Alder reactions of arylidene cycloalkanes are described, leading to the construction of spiro ring systems. Although the mechanism remains an open question, arylidene cyclobutanes are found to be much more effective in the reaction than other cycloalkanes. Since the reaction is completed with a substoichiometric amount of electricity, a radical cation chain pathway is likely to be involved.

Opening of the Diamondoid Cage upon Ionization Probed by Infrared Spectra of the Amantadine Cation Solvated by Ar, N2, and H2O

Abstract: Abstract Radical cations of diamondoids, a fundamental class of very stable cyclic hydrocarbon molecules, play an important role in their functionalization reactions and the chemistry of the interstellar medium. Herein, we characterize the structure, energy, and intermolecular interaction of clusters of the amantadine radical cation (Ama+, 1‐aminoadamantane) with solvent molecules of different interaction strength by infrared photodissociation (IRPD) spectroscopy of mass‐selected Ama+L n clusters, with L=Ar (n≤3) and L=N2 and H2O (n=1), and dispersion‐corrected density functional theory calculations (B3LYP−D3/cc‐pVTZ). Three isomers of Ama+ generated by electron ionization are identified by the vibrational properties of their rather different NH2 groups. The ligands bind preferentially to the acidic NH2 protons, and the strength of the NH…L ionic H‐bonds are probed by the solvation‐induced red‐shifts in the NH stretch modes. The three Ama+ isomers include the most abundant canonical cage isomer (I) produced by vertical ionization, which is separated by appreciable barriers from two bicyclic distonic iminium ions obtained from cage‐opening (primary radical II) and subsequent 1,2 H‐shift (tertiary radical III), the latter of which is the global minimum on the Ama+ potential energy surface. The effect of solvation on the energetics of the potential energy profile revealed by the calculations is consistent with the observed relative abundance of the three isomers. Comparison to the adamantane cation indicates that substitution of H by the electron‐donating NH2 group substantially lowers the barriers for the isomerization reaction.

Isolation and Structure Analysis of a Ni(II) Norcorrole Radical Anion

Abstract: A radical anion of Ni(II) meso-dimesitylnorcorrole was prepared and isolated upon chemical reduction of the corresponding neutral species with cobaltocene (CoIICp2). The radical anion was substantially stable under degassed solution conditions. X-ray crystal analysis of the radical anion revealed that the structural change by one-electron reduction is marginal. DFT calculations demonstrated that the reorganization energy of Ni(II) norcorrole upon one-electron reduction is smaller than those of Zn(II) porphyrin and C60 fullerenes.

Synthesis of a Bench-Stable Manganese(III) Chloride Compound: Coordination Chemistry and Alkene Dichlorination.

Abstract: The complex [MnCl3(OPPh3)2] (1) is a bench-stable and easily prepared source of MnCl3. It is prepared by treating acetonitrile solvated MnCl3 (2) with Ph3PO and collecting the resulting blue precipitate. 1 is useful in coordination reactions by virtue of the labile Ph3PO ligands, and this is demonstrated through the synthesis of {Tpm*}MnCl3 (3). In addition, methodologies in synthesis that rely on difficult or cumbersome to prepare solutions of reactive MnCl3 can be accomplished using 1 instead. This is demonstrated through alkene dichlorinations in a wide range of solvents, open to air, and with good substrate scope. Light-accelerated halogenation and radical sensitive experiments support a radical mechanism involving stepwise Cl-atom transfer(s) from 1.

A Persistent Phosphanyl‐Substituted Thioketyl Radical Anion

Abstract: Alkali metal salts, M + [Ter( i Pr)P-C(=S)-P( i Pr) 2 S] •− (M = Na, K; 2_M; Ter = 2,6-bis-(2,4,6-trimethyl-phenyl)-phenyl) containing a room-temperature-stable thioketyl radical anion were obtained by reduction of the thioketone precursor, Ter( i Pr)P-C(=S)-P( i Pr) 2 S ( 1 ), with alkali metals (Na, K). Single crystal X-ray studies as well as EPR spectroscopy revealed the unequivocal existence of a thioketyl radical anion in the solid state and in solution, respectively. The computed Mulliken spin density within 2_M is mainly located at the sulfur (49%) and the carbonyl carbon (33%) atoms. Upon adding [2.2.2]-cryptand to the radical species 2_K to minimize the interionic interaction, an activation reaction was observed, yielding a potassium salt with a phosphanyl thioether based anion, [K(crypt)] + [Ter( i Pr)P-C(–S- i Pr)-P( i Pr) 2 S] − ( 3 ) as the product of an intermolecular shift of an i Pr group from a second anion. The products were fully characterized and application of the radical anion as a reducing agent was demonstrated.

π-Stacked Ion Pairs: Tightly Associated Charged Porphyrins in Ordered Arrangement Enabling Radical-Pair Formation.

Abstract: π-Electronic ion pairs are of interest for fabricating electronic materials that use intermolecular interactions based on electrostatic and dispersion forces, defined as iπ-iπ interactions, to provide dimension-controlled assemblies. Porphyrin ions, whose charge is delocalized in the core units, are suitable for ordered arrangement and assemblies by ion pairing. Herein, charged porphyrins were found to form solid-state assemblies and solution-state stacked ion pairs according to the peripheral electron-donating groups (EDGs) and electron-withdrawing groups (EWGs). The concentration-dependent 1H NMR signal shifts of a porphyrin ion pair, comprising a meso-EWG cation and a meso-EWG anion, provided a hetero-dimerization constant of 2.8 × 105 M-1 in CD2Cl2 at 20 °C. In the ion pair of a meso-EWG cation and a meso-EDG anion, the electron transfer in the steady and excited states according to solvent polarity and photoexcitation, respectively, produced the radical pairs. The electron spin resonance analysis in frozen toluene revealed the formation of a heterodiradical in a closely stacked structure by the antiferromagnetic dipolar interaction and temperature-dependent spin transfer behavior.

Impact of substitution on reactions and stability of one-electron oxidised phenyl sulfonates in aqueous solution

Abstract: Highly reactive aromatic cation radicals have been invoked lately in synthetic routes and in the degradation pathways of hydrocarbon-based polymers. Changes in the electron density of aromatic compounds are expected to alter the reaction pathway following one electron oxidation through altering the pKa of the formed intermediate cation radical. Electron-donating groups increase its stability, however, little experimental data are known. While, in theory, the cation radical can be repaired by simple electron transfer, electron transfer to or from its deprotonated form, the hydroxycyclohexadienyl radical, will cause permanent modification or degradation. Time-resolved absorption spectroscopy indicates a pKa ≈ 2-3 for the 4-(tert-butyl)-2-methoxyphenylsulfonate (BMPS) radical cation, while its parent compound 4-(tert-butyl) phenylsulfonate (BPS) is much more acidic. The stability of both compounds towards oxidation by HO˙ was evaluated under air at pH 5 and pH 0. At pH 5, both BMPS and BPS are unstable, and superstoichiometric degradation was observed. Degradation was slightly reduced for BPS at pH 0. In contrast, the more electron rich BMPS showed 80% lower degradation. We unambigously showed that in the presence of Ce(III) and H2O2 at pH 0 both BMPS and BPS could be catalytically repaired via one electron reduction, resulting in further damage moderation.