Carbon Dioxide Radical Anion Research Articles

Taming CO2•- via Synergistic Triple Catalysis in Anti-Markovnikov Hydrocarboxylation of Alkenes.

The direct utilization of carbon dioxide as an ideal one-carbon source in value-added chemical synthesis has garnered significant attention from the standpoint of global sustainability. In this regard, the photo/electrochemical reduction of CO2 into useful fuels and chemical feedstocks could offer a great promise for the transition to a carbon-neutral economy. However, challenges in product selectivity continue to limit the practical application of these systems. A robust and general method for the conversion of CO2 to the polarity-reversed carbon dioxide radical anion, a C1 synthon, is critical for the successful valorization of CO2 to selective carboxylation reactions. We demonstrate herein a hydride and hydrogen atom transfer synergy driven general catalytic platform involving CO2•- for highly selective anti-Markovnikov hydrocarboxylation of alkenes via triple photoredox, hydride, and hydrogen atom transfer catalysis. Mechanistic studies suggest that the synergistic operation of the triple catalytic cycle ensures a low-steady-state concentration of CO2•- in the reaction medium. This method using a renewable light energy source is mild, robust, selective, and capable of accommodating a wide range of activated and unactivated alkenes. The highly selective nature of the transformation has been revealed through the synthesis of hydrocarboxylic acids from the substrates bearing a hydrogen atom available for intramolecular 1,n-HAT process as well as diastereoselective synthesis. This technology represents a general strategy for the merger of in situ formate generation with a synergistic photoredox and HAA catalytic cycle to provide CO2•- for selective chemical transformations.

Visible-Light-Promoted Reduction of Nitroarenes with Formate Salts as Reductants.

A visible-light-promoted reduction of nitrobenzenes using formate salts as the reductant was developed. A wide range of nitrobenzenes can be converted into aniline products in a transition metal free fashion. Mechanistic studies revealed that radical species (carbon dioxide radical anion and thiol radical) are key intermediates for the transformation. We anticipate that this method will provide a valuable and green strategy for the reduction of nitrobenzenes.

Radiocatalytic Synthesis of Acetic Acid from CH4 and CO2.

The C-C coupling of methane (CH4) and carbon dioxide (CO2) to generate acetic acid (CH3COOH) represents a highly atom-efficient chemical conversion, fostering the comprehensive utilization of greenhouse gases. However, the inherent thermodynamic stability and kinetic inertness of CH4 and CO2 present obstacles to achieving efficient and selective conversion at room temperature. Our study reveals that hydroxyl radicals (⋅OH) and hydrated electrons (eaq -) produced by water radiolysis can effectively activate CH4 and CO2, yielding methyl radicals (⋅CH3) and carbon dioxide radical anions(⋅CO2 -) that facilitate the production of CH3COOH at ambient temperature. The introduction of radiation-synthesized CuO-anchored TiO2 bifunctional catalyst could further enhance reaction efficiency and selectivity remarkably by boosting radiation absorption and radical stability, resulting in a concentration of 7.1 mmol ⋅ L-1 of CH3COOH with near-unity selectivity (>95 %). These findings offer valuable insights for catalyst design and implementation in radiation-induced chemical conversion.

Radiocatalytic Synthesis of Acetic Acid from CH4 and CO2

AbstractThe C−C coupling of methane (CH4) and carbon dioxide (CO2) to generate acetic acid (CH3COOH) represents a highly atom‐efficient chemical conversion, fostering the comprehensive utilization of greenhouse gases. However, the inherent thermodynamic stability and kinetic inertness of CH4 and CO2 present obstacles to achieving efficient and selective conversion at room temperature. Our study reveals that hydroxyl radicals (⋅OH) and hydrated electrons (eaq−) produced by water radiolysis can effectively activate CH4 and CO2, yielding methyl radicals (⋅CH3) and carbon dioxide radical anions(⋅CO2−) that facilitate the production of CH3COOH at ambient temperature. The introduction of radiation‐synthesized CuO‐anchored TiO2 bifunctional catalyst could further enhance reaction efficiency and selectivity remarkably by boosting radiation absorption and radical stability, resulting in a concentration of 7.1 mmol ⋅ L−1 of CH3COOH with near‐unity selectivity (>95 %). These findings offer valuable insights for catalyst design and implementation in radiation‐induced chemical conversion.

Near UV and Visible Light Photodegradation in Solid Formulations: Generation of Carbon Dioxide Radical Anions from Citrate Buffer and Fe(III).

Near UV and visible light photodegradation can target therapeutic proteins during manufacturing and storage. While the underlying photodegradation pathways are frequently not well-understood, one important aspect of consideration is the formulation, specifically the formulation buffer. Citrate is a common buffer for biopharmaceutical formulations, which can complex with transition metals, such as Fe(III). In an aqueous solution, the exposure of such complexes to light leads to the formation of the carbon dioxide radical anion (•CO2-), a powerful reductant. However, few studies have characterized such processes in solid formulations. Here, we show that solid citrate formulations containing Fe(III) lead to the photochemical formation of •CO2-, identified through DMPO spin trapping and HPLC-MS/MS analysis. Factors such as buffers, the availability of oxygen, excipients, and manufacturing processes of solid formulations were evaluated for their effect on the formation of •CO2- and other radicals such as •OH.

Trans-selective carboxylative cyclization of 1,6-dienes using the CO2 radical anion

Abstract The reactivity of the carbon dioxide radical anion (CO2•−) toward alkenes has garnered remarkable attention owing to its potential to synthesize valuable organic compounds. Herein, we report the reactions between CO2•− and 1,6-dienes, yielding 5-membered lactam derivatives via highly trans-selective 5-exo-trig cyclization. Both carboxylative cyclization and hydrocyclization occur with up to 74% combined yield. Computational studies show that the transition state during cyclization favors the trans-configuration over the cis-configuration. This work offers new insights into the synthesis of N-heterocycles.

Formate and CO2 Enable Reductive Carboxylation of Imines: Synthesis of Unnatural α-Amino Acids.

Herein, a photocatalytic umpolung strategy for reductive carboxylation of imines for the synthesis of α-amino acids was disclosed. Carbon dioxide radical anion (CO2•-) generated from formate is the key single electron reductant in the reactions. An unprecedentedly broad substrate scope of imines with excellent reaction yields was obtained with carbon dioxide (CO2) and formate salt as carbon sources.

Near UV Light Photo-Degradation of Lactate and Related α-Hydroxycarboxylates in the Presence of Fe(III): Formation of Carbon Dioxide Radical Anion

In recent studies we have reported on the near-UV light-induced degradation of iron complexes of various pharmaceutical excipients, such as Fe(III)-citrate and Fe(III)-amino acid complexes. Mechanistic studies revealed a common photo-degradation pattern, i.e. the formation of carbon dioxide radical anion, a potent reducing agent, via an alkoxyl/amino radical intermediate generated by light-induced ligand-to-metal charge transfer (LMCT) involving α-hydroxycarboxylates or amino acids. Herein, we confirm the proposed general photo-degradation pathways through the study of the iron complexes of other α-hydroxycarboxylates that may be present in protein formulations, such as lactate and glycolate. The results indicate that lactate generates even higher yields of •CO2− as compared to citrate, suggesting a significant potential of lactate for the promotion of photo-degradation in pharmaceutical formulations.

Photoinduced Remote Selective C(sp3)−H Alkylation Mediated by Cesium Formate

AbstractHerein, we developed a photoinduced remote selective C(sp3)−H alkylation of protected amines in the presence of cesium formate. The protected amines were synthesized from commercially available inexpensive 2‐iodo benzoyl chloride. Under mild reaction conditions, the in situ generated aryl radical is converted to α‐amidoalkyl radical via [1,5]‐HAT process, which combines with different Michael acceptors and affords corresponding alkylation products in good yields. Preliminary mechanistic studies revealed that formate anion acts as a source of carbon dioxide radical anion (CO2⋅−) and hydrogen atom donor was directly confirmed by isotope‐labeling studies.

Carbon dioxide radical anion mediated dehalogenation kinetics and mechanisms of halogenated alkanes

Carbon dioxide radical anion (CO2•−) recently becomes appreciated in halogenated contaminants elimination; nevertheless, its application has been restricted by insufficient mechanistic understanding. Herein, we provided a quantitative insight into the kinetics and mechanisms of CO2•− mediated dehalogenation of halogenated alkanes. A CO2•− dominated UV254/H2O2/HCOO− system has been successfully established and demonstrated for effective elimination of 7 kinds of halogenated alkanes (71.3 % to 100 % of removal). Using a laser flash photolysis technology, the second-order rate constants of CO2•− (kCO2•−″) reacting with CCl4, CHCl3 and CH2Cl2 were firstly reported, to be 2.5 × 108, 6.2 × 107 and 5.8 × 106 M−1s−1, respectively. kCO2•−″ presented a significant negative correlation with the lowest unoccupied molecular orbital energy (ELUMO) of chlorinated alkanes, proving that the enhanced dehalogenation of CO2•− was attributed by direct electron transfer mechanism. A fitting model was developed accordingly for kCO2•−″ prediction. This study also demonstrated that the CO2•− mediated ARP effectively removed halogenated alkanes regardless of pH condition (6.0∼9.0) and bicarbonate concentrations. These findings are significant in advancing the scientific understanding of CO2•− mediated ARP. This reductive process a promising control strategy for halogenated contaminants, such as polyfluoroalkyl substances (PFAS) and halogenated pharmaceuticals.

Siderite with chelator as high-efficiency Fenton reagent to degrade naphthalene via ferrous liberation and carbon dioxide radical anion-mediated iron redox cycle

Siderite (FeCO3) has been verified to be an effective iron material in Fenton-like process, but large dose of FeCO3 and oxidants limits its application. Here, chelators were successfully used to enhance the degradation of naphthalene (NAP). When 1.0 mM of hydrogen peroxide (H2O2) and 1.0 g L−1 of FeCO3 were applied, in the presence of 0.2 mM oxalic acid (OA), 0.1 mM nitrilotriacetic acid (NTA), or 0.1 mM DL-tartaric acid (DL-TA), 99.3 %, 100 %, or 96.7 % of NAP was degraded within 24 h, respectively. The different roles of chelators were confirmed. For OA and DL-TA, due to weak complexation with iron ions, acid effect and generation of carbon dioxide radical anion (CO2−) contributed to high concentration of Fe2+, while for NTA, chelation and CO2− were two major factors. In addition, the excellent potential application of H2O2/FeCO3/chelator was determined because of (1) lower environmental risk of chemicals; (2) strong tolerance to water matrixes; (3) the excellent stability of FeCO3 confirmed by surface characterizations and cyclic experiment, as NAP was completely degraded after the 6th cycle. At last, H2O2/FeCO3/chelator had superiority over other reduced iron minerals due to lower running cost in lab study, better reusability, and environmental friendliness. Therefore, it is expected that H2O2/FeCO3/chelator technique can provide an effective alternative to Fenton-like process in remediation of NAP contaminated sites.

Simultaneous Degradation, Dehalogenation, and Detoxification of Halogenated Antibiotics by Carbon Dioxide Radical Anions

Despite the extensive application of advanced oxidation processes (AOPs) in water treatment, the efficiency of AOPs in eliminating various emerging contaminants such as halogenated antibiotics is constrained by a number of factors. Halogen moieties exhibit strong resistance to oxidative radicals, affecting the dehalogenation and detoxification efficiencies. To address these limitations of AOPs, advanced reduction processes (ARPs) have been proposed. Herein, a novel nucleophilic reductant—namely, the carbon dioxide radical anion (CO2−)—is introduced for the simultaneous degradation, dehalogenation, and detoxification of florfenicol (FF), a typical halogenated antibiotic. The results demonstrate that FF is completely eliminated by CO2−, with approximately 100% of Cl− and 46% of F− released after 120 min of treatment. Simultaneous detoxification is observed, which exhibits a linear response to the release of free inorganic halogen ions (R2 = 0.97, p < 0.01). The formation of halogen-free products is the primary reason for the superior detoxification performance of this method, in comparison with conventional hydroxyl-radical-based AOPs. Products identification and density functional theory (DFT) calculations reveal the underlying dehalogenation mechanism, in which the chlorine moiety of FF is more susceptible than other moieties to nucleophilic attack by CO2−. Moreover, CO2−-based ARPs exhibit superior dehalogenation efficiencies (> 75%) in degrading a series of halogenated antibiotics, including chloramphenicol (CAP), thiamphenicol (THA), diclofenac (DLF), triclosan (TCS), and ciprofloxacin (CIP). The system shows high tolerance to the pH of the solution and the presence of natural water constituents, and demonstrates an excellent degradation performance in actual groundwater, indicating the strong application potential of CO2−-based ARPs in real life. Overall, this study elucidates the feasibility of CO2− for the simultaneous degradation, dehalogenation, and detoxification of halogenated antibiotics and provides a promising method for their regulation during water or wastewater treatment.

Near UV Photodegradation Mechanisms of Amino Acid Excipients: Formation of the Carbon Dioxide Radical Anion from Aspartate and Fe(III).

Carbon dioxide radical anion (•CO2-) is a powerful reducing agent that can reduce protein disulfide bonds and convert molecular oxygen to superoxide. Therefore, the generation of •CO2- can be detrimental to pharmaceutical formulations. Iron is among the most prevalent impurities in formulations, where Fe(III) chelates of histidine (His) can produce •CO2- upon exposure to near-UV light (Zhang and Schöneich, Eur. J. Pharm. Biopharm. 2023, 190, 231-241). Here, we monitor by spin-trapping in combination with electron paramagnetic resonance spectroscopy and/or high-performance liquid chromatography-mass spectrometry analysis the photochemical formation of •CO2- for a series of common amino acid excipients, including arginine (Arg), methionine (Met), proline (Pro), glutamic acid (Glu), glycine (Gly), aspartic acid (Asp), and lysine (Lys). Our results indicate that in the presence of Fe(III), Asp, and Glu produce significant yields of •CO2- under photoirradiation with near-UV light. Notably, Asp demonstrates the highest efficiency of •CO2- generation compared with that of the other amino acid excipients. Stable isotope labeling indicates that •CO2- exclusively originates from the α-carboxyl group of Asp. Mechanistic studies reveal two possible pathways for •CO2- formation, which involve either a β-carboxyl radical or an amino radical cation intermediate.

Electrochemical Redox Conversion of Formate to CO via Coupling Fe-Co Layered Double Hydroxides and Au Catalysts.

Formate has been considered an inactive molecule and thus cannot be further reduced under CO2 reduction conditions, which limits its widespread application as feedstock. Here we present an electrochemical redox conversion of formate to CO through the potential-dependent generation of carbon dioxide radical anions (CO2 ⋅- ) on Fe-Co layered double hydroxides (Fe-Co LDHs) and the subsequent reduction of CO2 ⋅- to CO on Au catalysts. We present an electrodeposition protocol for the synthesis of Fe-Co LDHs with precise composition control and find that Fe1 Co4 exhibits a promising potential window for CO2 ⋅- formation between 1.14 and 1.4 V and an optimized potential at 1.24 V at a neutral pH condition. We further determined the formation of CO2 ⋅- at 1.24 V via electron paramagnetic resonance and CO2 at >1.4 V through differential electrochemical mass spectrometry. This work provides a redox chemistry route for converting formate into CO through a coupled slit parallel-plate electrode system.

Single electron reduction of NHC-CO2-borane compounds.

The carbon dioxide radical anion [CO2˙-] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO2 to generate [CO2˙-] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC-CO2-BR3 species - generated from the Frustrated Lewis Pair (FLP)-type activation of CO2 by N-heterocyclic carbenes (NHCs) and boranes (BR3) - undergo single electron reduction at a less negative potential than free CO2. A net gain of more than one volt was notably measured with a CAAC-CO2-B(C6F5)3 adduct, which was chemically reduced to afford [CAAC-CO2-B(C6F5)3˙-]. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO2 moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO2˙-] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO2˙-]-like species.

Engineering a pyrene MOF composite photocatalyst toward the formation of carbon dioxide radical anions through regulating the charge transfer from type II to Z scheme via a chemical bond-modulated strategy

A new approach was reported for regulating the charge transfer mechanism in a pyrene-based MOF composite, transitioning from a type-II to a Z-scheme configuration.

Enhancement in the Rate of CO2 Photoreduction Using Divalent Strontium Cation (Sr2+) Doped TiO2 Nanotube Arrays

The sunlight-assisted conversion of CO2 into portable value-added products has the potential to address both the energy and environmental challenges faced by the world. Therefore, we currently witness intense research activity and substantial progress in the field of CO2 photoreduction. Anatase TiO2 nanotube arrays (TNTAs) formed by electrochemical anodization constitute a well studied semiconductor photocatalyst with excellent optoelectronic properties and photocatalytic behavior [1, 2]. Yet, many aspects related to structure, surface composition, adsorbates, reaction intermediates, rate of reaction, multi-step electron transfer steps, etc. remain poorly understood even for TiO2. This led to our interest in forming surface coatings and bulk nanocomposites of TNTAs with metal oxides of Sr, Mg, Ca, Zn etc, which provide basic sites on the surface of the catalyst for the chemisorption of CO2 molecules. The higher basicity of hydroxyl groups associated with alkaline earth metals generate carbonate and bicarbonate adsorbates which enhance the reactivity of CO2 [3]. Divalent metal cations are also known to influence and modify the crystallographic texture of TiO2 during the annealing treatment step at elevated temperature [4].Here we performed divalent strontium cation (Sr2+) doping of anodically grown TNTAs using two different techniques – an electrochemical cathodic treatment in Sr2+-containing electrolytes and a wet impregnation technique using an aqueoussolution of strontium salts. The X-ray diffraction pattern and Raman spectra indicate the formation of Sr compounds at the TNTA surface. Sr-doping results in modification of the crystallographic texture of anatase phase TNTAs, increased lattice strain, reduced crystal size, and phonon confinement. Subtle changes are observed in the infrared spectra of the CO2 adsorbed on the Sr-doped TNTAs specifying a larger prevalence of monodentate and bidentate carbonate species on the surface. This attributes to the higher alkalinity of surface hydroxyls bound to Sr in comparison to the Ti-bound hydroxyls. In addition to linearly adsorbed CO2, a smaller population of bent CO2 molecules on the surface is observed, which suggests an enhanced stabilization of the carbon dioxide anion radical on the Sr-doped TNTA surfaces. Finally, under AM1.5G one sun illumination, the cathodized TNTAs produce 25 µmolg-1hr-1 whilethe wet impregnated Sr-doped TNTAs produce 17 µmolg-1hr-1 of CO which are nearly 3 and 2 times the amount of CO generated by bare TNTAs (7.7 µmolg-1hr-1) respectively.REFERENCES: Vahidzadeh, Ehsan, et al. "Asymmetric multipole plasmon-mediated catalysis shifts the product selectivity of CO2 photoreduction toward C2+ products." ACS Applied Materials & Interfaces13.6 (2021): 7248-7258.Zeng, Sheng, et al. "Optical control of selectivity of high rate CO2 photoreduction via interband-or hot electron Z-scheme reaction pathways in Au-TiO2 plasmonic photonic crystal photocatalyst." Applied Catalysis B: Environmental267 (2020): 118644.Kwon, Stephanie, et al. "Alkaline-earth metal-oxide overlayers on TiO 2: application toward CO 2 photoreduction." Catalysis Science & Technology6.21 (2016): 7885-7895.Kisslinger, Ryan, et al. "Preferentially oriented TiO2 nanotube arrays on non-native substrates and their improved performance as electron transporting layer in halide perovskite solar cells." Nanotechnology30.20 (2019): 204003.

Recent Advances in Reactions Involving Carbon Dioxide Radical Anion

Recent Advances in Reactions Involving Carbon Dioxide Radical Anion

Carbon Dioxide Anion Radicals Assisted Highly Efficient Photocatalytic H2O2 Production over Bi(C2O4)OH.

Carbon dioxide anion radical (CO2•-) can act as a versatile single electron reductant, but its generation pathways are quite limited. Herein, we demonstrate that oxalic acid (OA) could be effectively and continuously utilized to produce CO2•- over Bi(C2O4)OH, a novel photocatalyst, under light irradiation. Bi(C2O4)OH would proceed with self-redox reactions under the light irradiation producing CO2•-, through the oxidation of C2O42-. OA in the solution could recoordinate with Bi3+, thus maintaining the structure of the photocatalysts and the stability of the reactions. Benefiting from the fast reaction between CO2•- and O2 in forming •O2-, hydrogen peroxide (H2O2) would be efficiently produced (219.0 μmol/h). This study proposes a novel approach for harnessing OA containing wastewater and explores its potential application in the efficient production of H2O2.

Exploring the applications of carbon dioxide radical anion in organic synthesis

Exploring the applications of carbon dioxide radical anion in organic synthesis