Articles ChemElectroChem doi.org/10.1002/celc.202100329 Electrochemical Reduction of 4-Nitrobenzyl Phenyl Thioether for Activation and Capture of CO2 Silvia Mena, Cyril Louault, Verónica Mesa, Iluminada Gallardo,* and Gonzalo Guirado*[a] In this work, a new simple molecule, 4-nitrobenzyl phenyl redox mediator that allows the reduction of CO2 at ca. 1.2 V thioether (1), is prepared and used for controlling and tuning vs. SCE. The electrochemical generation of 12 at more negative CO2 reactivity in function of the electrode potential. The first potential values leads to a C S bond cleavage reaction that part of the study is devoted to determining the electrochemical yields the corresponding nitrobenzyl and thiosulfate anions, reduction mechanism of 1 in N,N-dimethylformamide under which react with CO2. The nitro aromatic anion derivative makes nitrogen. The compound shows a first reversible one-electron it possible to obtain electrocarboxylated derivatives, whereas transfer process, whereas the reaction cleavage of the C S the thiophenolate anion captures CO2 reversibly. Hence, this bond takes place after a second electron transfer process research opens a new way of tuning and controlling the through a stepwise mechanism (thermodynamic and kinetic reaction processes associated with CO2 from homogenous parameters are conveniently determined). In the second part of catalysis at low negative potentials, to electrocarboxylation the study, the inert atmosphere was replaced by a CO2 processes passing to CO2 reversible electrochemically triggered atmosphere. At low potential values, compound 1 acts as a adsorption processes. 1. Introduction (stepwise process) or in a one-step (concerted process) pathway (Scheme 1). There has been growing interest in activation and cleavage of The strength of the carbon halogen bond, the leaving the carbon sulfur bond due to its role in the field of fuel, coal group ability of the resultant anion and the reaction medium industry, bioorganic and synthesis for decades.[1–3] There are play important roles in determining the preferred process.[12] many ways to activate and cleave C heteroatom bonds, such as Similarly, different bond cleavages have been researched, with transition metals.[4,5] However, this chemistry requires high notably the C O and C S bond cleavages in ether and loading of toxic and air-sensitive transition metals and thioether.[1,13–17] These bond cleavages have a particular interest extremely high temperatures and is only feasible with more in different biological phenomenon, such as in the DNA stand reactive methyl and ethyl esters.[6] Electrochemistry has been break of the C3’ O3’ bond,[18] or for photocleavage of the widely used in the activation and cleavage of C heteroatom benzyl sulfide bond in a ribonucleoside, which is interesting for bonds. The electron-transfer reaction may lead to chemically its successful photoaffinity labelling of a membrane transport stable species and to bond cleavage or bond formation. General protein.[19] Focusing on the C S bond cleavage reaction, Singh reactivity models have been constructed for this kind of et al.[20] recently found that it is possible to generate benzylth- reaction. There are extensive reports on the mechanisms of iolate anion moieties (PhCH2S ) through an electrochemical carbon halogen bond cleavage.[7–11] The general mechanism reduction process of benzyldisulfide (PhCH2 S S CH2Ph) of consists in the electrochemical activation of organic molecules high-negative potentials, and these benzylthiolate anions are (A–B) by producing a radical anion after the electron transfer, able to reversibly capture CO2 (Scheme 2). which cleaves to produce a radical and an anion in two steps Electrochemistry methods can also be used to perform carboxylation reactions, and have been widely studied as a “greener” alternative to chemical methods.[21–23] Hence, the use of CO2 as a feedstock is a promising strategy for obtaining [a] Dr. S. Mena, Dr. C. Louault, V. Mesa, Prof. I. Gallardo, Dr. G. Guirado Departament de Química Universitat Autònoma de Barcelona Campus UAB, 08193-Bellaterra, Barcelona, Spain E-mail: iluminada.gallardo@uab.cat gonzalo.guirado@uab.cat Supporting information for this article is available on the WWW under https://doi.org/10.1002/celc.202100329 An invited contribution to a joint Special Collection in memory of Prof. Jean- Michel Savéant © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited Scheme 1. Electrochemically triggered dissociative electron transfer path- and is not used for commercial purposes. ways. ChemElectroChem 2021, 8, 2649 – 2661 2649 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 carboxylic acids via electrochemical fixation of CO2. Moreover, catalysts in the electrolysis. In terms of green and sustainable CO2 can be easily incorporated into the organic skeleton under chemistry, this approach is essentially environmentally benign mild conditions by using suitable electrodes, electrolytes and and carbon dioxide is an eco-friendly C1 source.[24–27] Usually, electrocarboxylation reactions of organic compounds using CO2 are carried out through two different reaction pathways. The first one involves the electrochemical generation of organic anionic intermediates on the passage of two electrons, which undergo a nucleophilic attack on carbon dioxide to yield the corresponding carboxylic acids [Scheme 3, Eqs. (1)–(4)]. This pathway takes place when the reduction potentials of the organic substrates are less negative than that of carbon dioxide (Ereduction, CO2 = 2.2 V vs SCE). The second approach is based on the reactivity of the CO2 radical anion and can take place when Scheme 2. Electrochemical cycle that enables capture and release of carbon the reduction potentials of the organic substrates are more dioxide with the use of benzylthiolate as capture agent. negative than the carbon dioxide reduction potential. This mechanism describes the electrogeneration of the radical anion of carbon dioxide,[28] which can be generated directly on the surface of the cathodic electrode [Scheme 3, Eq. (5)], or through a homogeneous catalysis in a bulk solution with an organic mediator [Scheme 3, Eq. (5’)]. This radical anion reacts with organic substrates leading to the corresponding carboxylic acids [Scheme 3, Eqs. (6)–(8)]. It is important to remember that the use of electrochemical methods also makes electro-reducing carbon dioxide possible, but with a homogeneous catalyst based on organic molecules (A). Radical anion of aromatic nitriles,[26,29] esters,[29] and recently electrochemically-generated nitrocompounds[30] have the re- markable property of reducing CO2 to its radical anion specie, CO2 . This radical anion would eventually evolve leading to * different CO2 reduction products depending on the experimen- tal conditions (Scheme 4). Hence, the aim of this work is to design a new molecule for controlling and tuning the CO2 reactivity in function of the electrode potential. Therefore, it seems to be possible to use electrochemical control for activating, capturing and valorizing CO2 with the 4-nitrobenzyl phenyl thioether (1) (Figure 1), an Scheme 3. Electrocarboxylation reactions of organic compounds using CO2. Scheme 4. Electroreduction of CO2 with the organic molecule as the homogeneous catalyst. ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2650 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 GC-MS (70 eV): m/z (%) 245 (100) [M + ], 199 (19) [M + NO2], 136 (49) [C7H6NO2 · ], 120 (13), 109 (14) [C6H5S · ], 106 (23), 90 (21) [C7H62 · ], 89 (20), 78 (24). Figure 1. 4-Nitrobenzyl phenyl thioeter, 1. Instruments and procedures 1 H NMR spectra were recorded on a Brucker DPX250 spectrometer operating at 250,13 MHz (Billerica, MA, USA). Proton chemical shifts were reported in ppm (δ) (CD3CN, δ = 1.98). The J values were aryl thioether compound with a nitro group substituent. reported in Hz. Introducing a nitro substituent into the aromatic ring makes it Gas phase chromatography were performed with a Clarus 500 from possible to obtain a stable radical anion, which can potentially Perkin Elmer (vector gas: Helium, column: Elite 5, l = 30 m, d = act as a redox mediator for CO2 activation. Moreover, in a 0.25 mm). A Hewlett Packard 6890 apparatus was used for gas second electron transfer process the dianion, 12 , can occur phase chromatography coupled with mass spectrometry. The flame through the C S bond cleavage reaction, leading to the ionization detector (FID) was a Hewlett Packard 5973. corresponding nitrobenzyl and thiosulfate anions. The electro- Products obtained after electrolysis were analyzed by the Servei chemical generation of the nitrobenzyl anion may react with d’Analisis Quimica (SAQ) of the Autonomous University of Barcelona CO2 to yield an electrocarboxylated product, whereas the (UAB) by GC-MS (gas phase chromatography GC 6890, Agilent thiophenolate anion could also reversibly capture CO2. Technologies, with mass spectroscopy Hewlett-Packard 5973). Electrochemical procedures Experimental Section Electrochemical measurements were performed in a one-compart- ment cell using the three-electrode configuration. The working Chemicals electrode was a 1 mm diameter glassy carbon electrode disk carefully polished with 1 μm diamond paste (DP-Paste, P) and Reagents rinsed with ethanol. The counter electrode was a 2 mm platinum electrode disk and the reference electrode a Saturated Calomel 4-nitrobenzyl bromide and sodium thiophenolate were obtained Electrode (SCE) separated from the solution by a sintered-glass disk from Sigma-Aldrich and were the highest purity available. (salt-bridge). Therefore, all potentials were reported compared to this reference. All experiments were carried out at 15 °C, and the double wall jacketed cell was thermostated by circulation of Reactant Gases methanol. Carbon dioxide (CO2) and nitrogen and argon were purchased from The solution to be studied was prepared by dissolving the substrate Carburos Metalicos S.A. with a purity of 99.9999 %. in a solution containing the solvent (DMF) and 0.10 mol · L 1 of support electrolyte (TBABF4), except for fast cyclic (scan rate (v) greater than 100 V · s 1) for which the concentration of the support Solvent and supporting electrolyte electrolyte is 0.60 mol · L 1. The substrate concentration was a few Dichloromethane, methanol, ethanol, toluene and pentane were millimoles per liter and the volume of the solution was 10 mL. obtained from SDS. All chemicals were used as received. Between taking records, the solution was stirred by a current of argon in order to homogenize the solution and remove the oxygen. N,N’-dimethylformamide, DMF anhydrous, 99.8 % (SigmaAldrich) and the supporting electrolyte, tetrabutylammonium tetrafluorobo- Cyclic voltammetry was recorded using a potentiostat equipped rate, TBABF4, (Aldrich) were used as received without further with positive feedback compensation and current measurer. The purification. triangular signal is delivered by a TACUSSEL generator type GSTP 4. An electronic system compensates the resistance (Ru) between the working electrode and the reference electrode. The voltammo- 4-nitrobenzyl phenyl thioether, 1 grams are first recorded on a TEKTRONIX 2212 digital oscilloscope and then transcribed to paper using a HEWLETT PACKARD 7475 A Compound 1 was prepared by the reaction between 4-nitro- plotter. benzylbromide (0.176 g, 0.82 mmol) and sodium thiophenolate (1.078 g, 8.16 mmol) in dichloromethane under an inert atmos- Controlled potential electrolysis was carried out using a potentio- phere. The mixture was stirred for 48 h at room temperature. The stat EG&G PRINCETON APPLIED RESEARCH model 273 A. The same solution obtained was poured into water and extracted three times electrochemical cell, as it was previously described for cyclic with dichloromethane. Organic layers were reunited and dried over voltammetry, has been used for controlled potential electrolysis sodium sulfate anhydrous. The residue after solvent evaporation experiments. However, in this case the working electrode was a was purified using a silica gel chromatography column with a n- graphite rod (area: 8.1 cm2); the counter electrode a platinum wire pentane/dichloromethane (3 : 2) mixture and produced the desired and the reference electrode a SCE electrode. Counter and reference product as a light yellow solid (60 % yield). electrodes were separated from the solution by a salt bridge. This 1 salt-bridge ends in a frit, which is made of a ceramic material, H-NMR (CDCl3): δ (ppm) 8.12 (d, J = 8.6 Hz, 2H), 7.38 (d, J = 8.6 Hz, avoiding appreciable contamination. 2H), 7.27 (m, 5H), 4.14 (s, 2H). Controlled potential electrolysis and electrocarboxylation reactions were performed under either nitrogen or argon and CO2 atmos- phere, respectively. For electrocarboxylation reactions a Mass Flow ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2651 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 Meter was used to control the CO2 concentrations and flows. All experiments were also monitored using Cyclic Voltammetry. After electrolysis, water was added to the solution. The aqueous phase was extracted three times with toluene. The combined organic phases were dried with anhydrous sulfate sodium. The solvent was removed under a high vacuum. The residue was dissolved in dichloromethane and characterized by gas chromatography-mass spectrometry (GC-MS) and Proton Nuclear Magnetic Resonance (1H- RMN). Thiophenol: 1H-NMR (CDCl3): δ (ppm) 7.35–7.31 (m, 4H), 7.20 (t, J = 6.9 Hz, 1H), 3.50 (s, 1H). GC-MS (70 eV): m/z (%) 110 (100) [M + ], 109 (24.1) [M + H], 77 (12.3) [M + SH], 66 (31), 65 (10.7). 4-nitrotoluene: 1H-NMR (CDCl3): δ (ppm) 8.10 (d, J = 8.43 Hz, 2H), 7.31 (d, J = 8.43 Hz, 2H), 2.46 (s, 3H). GC-MS (70 eV): m/z (%) 137 (75.7) [M + ], 91 (100) [M + NO2], 77 (11.8) [C6H5 + ], 65 (57.8), 63 (13.7), 39 (21.2). Diphenyl disulfide (2): 1H-NMR (CDCl3): δ (ppm) 7.52 (d, J = 7.52 Hz, 4H), 7.30 (t, J = 7.52 Hz, 4H), 7.24 (t, J = 7.21, 2H). GC-MS (70 eV): m/z (%) 218 (100) [M + ], 184.9 (12.1), 140 (5.8) [M + C6H5 · ], 109 (92.2) [M + C6H5S · ], 65 (27.7), 51 (8.7). 4-nitrobenzaldehyde (p-NO2 Ph CHO): 1H-NMR (CDCl3): δ (ppm) 10.16 (s, 1H), 8.39 (d, J = 8.4 Hz, 2H), 8.07 (d, J = 8.4 Hz, 2H). GC-MS (70 eV): m/z (%) 151 (100) [M + ], 150 (86.9) [M + H · ], 105 (27.2), 104 (19.4), 77 (68.1) [C6H5 + ], 65 (11.1), 51 (63.4), 50 (28.3). Figure 2. Cyclic voltammetry of 5.01 mM of 1 in DMF/0.1 M TBABF4 at 15 °C, 4,4’-dinitrobibenzyl, D1: 1H-NMR (CDCl3): δ (ppm) 8.08 (d, J = 8.5 Hz, on a glassy carbon disk, under an inert atmosphere. Scan rate: 0.5 V · s 1. 4H), 7.22 (d, J = 12.7 Hz, 4H), 3.6 (s, 4H). GC-MS (70 eV): m/z (%) 272 Scan range: 0.0/–1.4/1.5/0.0 V (top), and Scan range: 0.0/–2.7/1.5/0.0 V third (46.9) [M + ], 226 (10.8) [M + NO2], 178 (4.8), 136 (100) [C7H6NO22 · ], electron transfer (bottom). 106 (24.5), 90 (10.9) [C7H62 · ]. 4,4’-dinitrostilbene, D2: 1H-NMR (CDCl3): δ (ppm) 8.29 (d, J = 8.9 Hz, 4H), 7.72 (d, J = 8.9 Hz, 4H), 7.33 (s, 2H). GC-MS (70 eV): m/z (%) 270 transfer, with Epc,1 = 1.12 V (vs SCE), followed by a second (100) [M + ], 178 (25.9) [M + 2NO2], 165 (21.1), 76 (9.2) [C6H42 · ]. irreversible electron transfer, Epc,2 = 2.0 V (vs SCE), and a third 4-aminobenzeneacetic acid (3): 1H-NMR (CDCl3): δ (ppm) 6.89 (d, J = multi-electron irreversible cathodic wave at Epc,3 = 2.36 V (vs 7.6 Hz, 2H), 6.50 (d, J = 7.6 Hz, 2H), 3.33 (s, 2H). GC-MS (70 eV): m/z SCE), where the reduction of the nitroso group takes place. It is (%) 151 (32) [M + ], 106 (100) [M + COOH · ], 77 (8.3) [C6H5 + ], 18 (6.6). possible to detect the formation of nitroso derivative after the 4-[(phenylthio)methyl]benzenamine (4): 1H-NMR (CDCl3): δ (ppm) 7.34 third electron transfer, since appears new oxidation waves at (m, 4H), 7.20 (m, 3H), 6.69 (d, J = 7.8 Hz, 2H), 4.28 (s, 2H), 4.13 (s, Epa = 0.54 V (vs SCE) and Epa = 0.21 V in the anodic counter 2H). GC-MS (70 eV): m/z (%) 215 (40) [M + ], 106 (100) [M + C6H5S · ], scan. These oxidation waves correspond to the oxidation of the 77 (84), 65 (30), 51 (19). nitroso and hidroxylamine derivatives radical anions to its neutral derivatives after the third reduction wave (Scheme 5). Determination of the CO2 concentration in the solvent However, the current study will focus only on the first two A thermal flow meter of molecular construction with “laboratory electron transfers. style” pc-board housing (EL-FLOW® Mass Flow Meter/Controller, Bronkhorst Hi-Tec) was employed for monitoring the CO2 concen- tration in the solution. Control valves were included to measure 2.1.1. Electrochemical Study of 1 Under an Inert Atmosphere at and control the flow from the lowest range of 0.2–1 mL/min. the First Electron Transfer The first wave corresponds to the reversible reduction of 1, so 2. Results and Discussion the compound takes a first electron reversibly, which is localized on the nitro group according to the value of the 2.1. Electrochemical Study of 4-Nitrobenzyl Phenyl Thioether standard potential (E0 = 1.16 V vs SCE) to form a radical anion, * Under Inert Atmosphere 1 (Figure 2). No oxidation products were detected after the reduction scan (Figure 2, blue scan). Compound 1 was electrochemically studied by cyclic voltam- metry (CV) under an inert atmosphere. The typical CV of a 5 mM solution of compound 1 in DMF/0.1 M TBABF4 is depicted in Figure 2 at different scan ranges. A cathodic scan, up to 2.7 V, shows three different electron transfers. A first monoelectronic fast reversible electron ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2652 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 Scheme 5. Reduction mechanism of nitroaromatic derivates associated with the third reduction wave (where SH = solvent). 2.1.2. Electrochemical Study of 1 Under an Inert Atmosphere at the Second Electron Transfer in an anodic scan after reduction at 0.33 V (Epa,A ) and 0.09 V (Epa,B ) vs. SCE, respectively. Figure 3, shows a second quasi-reversible monoelectronic wave In order to obtain the value of the electrochemical at 2.00 V vs. SCE. The electrochemical behavior related to this parameters of the second transfer of compound 1, CVs at second electron transfer is analyzed at different scan rates. Slow different scan rates were recorded in the same conditions. The scan rates give an irreversible peak (Figure 3a), but when the reversibility of the second reduction wave is achieved at high scan rate was increased it became reversible (Figure 3b). More- scan rates (700 V · s 1), hence it is possible to determine the over, when the scan rate is slow, oxidation peaks are produced standard potential value of this second reduction process (E0 = ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2653 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 Figure 3. Cyclic voltammetry of 5.22 mM of 1 under an inert atmosphere at 15 °C. a) glassy carbon disk, Scan rate: 0.5 V · s 1, in DMF/0.1 M TBABF4 b) glassy carbon disk ultra-micro-electrode (Φ = 9.7 μm). Scan rate: 700 V · s 1, in DMF/0.6 M TBABF4. 1.63 V vs SCE). Note that the oxidation peaks related to the second electron transfer coupled reaction have disappeared (Figure 3b). Thus, we can propose that a chemical reaction is produced after the second electron transfer. The electrochem- ical parameters obtained for the second electron transfer were E02 = 2.63 V, ksap = 9 · 10 4 cm · s 1, α = 0.34, and k = 3 · 104 s 1 and were determined experimentally and confirmed by simu- lation of the experimental curves using the DIGISIM® software (Supporting information, Scheme S1 and Table S1). A control potential electrolysis at Eap = 2.1 V (vs SCE) was performed to determine the nature of the products obtained after the second electron transfer. After 2 F of charge and chemical treatment only, 16 % of the reagent remained. The products obtained are summarized in Table 1. Figure 4. Cyclic voltammetry in DMF/0.1 M TBABF4 at 15 °C, on a glassy Taking a close look at Table 1, the products obtained carbon disk, under an inert atmosphere. Scan rate: 0.5 V · s 1. Solution of confirmed that the C S cleavage is produced efficiently after 5 mM of NaSPh (black line) and 5 mM of Ph S S Ph (blue line). passing the charge corresponding to two electrons. The main products obtained are 4-nitrotoluene and diphenyl disulfide, Ph S S Ph. Moreover, aldehyde and dimer derivatives like 4,4’- Moreover, sodium thiophenolate electrochemical character- dintrobibenzyl (D1) and 4,4’- dinitrostilbene (D2) were also ization showed two oxidation peaks (Epa,1 = 0.09 V vs SCE and detected. Epa,2 = + 0.55 V vs SCE) and a reduction wave at Epc = 1.64 V (vs Considering the products obtained after the electrolysis, it is SCE). The first oxidation wave corresponds to the oxidation of possible to relate the oxidation peaks obtained at slow scan sodium thiophenolate to its radical form, which could evolve rates after the second electron transfer to the oxidation of following two different reaction pathways. It can either abstract nitrobenzyl anion[31] (Epa,A = 0.33 V (vs SCE)), and thiophenolate a hydrogen atom leading to benzenethiol, which would be anion oxidation[32] (Epa,B = 0.09 V (vs SCE)). The second product oxidized at Epa = + 0.55 V (vs SCE), or would evolve to dimerize was determined by comparison between it and CV obtained by into diphenyl disulfide, Ph S S Ph, which has the reduction commercial analogue, sodium thiophenolate, NaSPh (Figure 4, wave at Epc = 1.64 V (vs SCE), Figure 5, blue line. This second black line). pathway seems to be favored in the electrolysis of 1 because Table 1. Results of electrochemical reduction of 1 under an inert atmosphere. Entries Electrolysis parameters % Yield Product Eap [V] Charge 1 Thiophenol 4-nitrotoluene Ph S S Ph p-NO2 Ph CHO D1 D2 (Q = nzF) 1 1.2 1F 100 – – – – 2 2.1 2F 16 11 34 36 3 18 4 ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2654 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 Figure 5. Cyclic voltammetry in DMF/0.1 M TBABF4 at 15 °C on a glassy carbon disk. Scan rate: 0.5 V · s 1. Solution of 5 mM a) Compound 1 in saturated CO2 atmosphere b) TBA2Ox. much more diphenyl disulfide (36 % yield) is obtained than obtained from the first order constant, where the pre- thiophenol (11 % yield) with the passage of 2 F. exponential factor A is taken to be equal to 5 · 1012 s 1 [Eq. (2)]: In the CV and electrolysis experiments, it is possible to � � propose a mechanism for the electrochemical reduction of 1: an Dr G6¼ k ¼ Aexp (2) EEC mechanism (Scheme 6), in which compound 1 is reduced RT to obtain its radical anion, 1 · , with an electronic transfer (E), which is followed by another one (E), obtaining the dianion 12 , The activation free energy, ΔrG ¼6, is related quadratically to that is broken through a chemical reaction (C) in which there is the standard free energy of the cleavage reaction [Eq. (3)]: C S bond cleavage. This gives 4-nitrobenzyl and thiophenolate � � anions as products, which would eventually lead to nitrotoluene 4Dr G06¼ D G0 2 Dr G6¼ ¼ 1 þ r 06¼ (3) and thiophenol through a protonation reaction. Besides, PhS 4 4Dr G and nitrotoluene anion can reduce nitrotoluene by formation of a charge transfer complex during the electrolysis, which So, the standard activation free energy, ΔrG0 ¼6, is [Eq. (4)]: explains the formation of explains the presence of PhS SPh, D1 rffi�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi and D2 as electrolysis products. A detailed description of the Dr G0 Dr G0 2 ðDr G0 Þ2 6¼ electrochemical reduction dimerization mechanism of nitro- Dr G þ Dr G6¼ 2 2 4 (4) toluene has been previously published by some of us.[33] Dr G06¼ ¼ 2 The results obtained using equation 1, confirms the exer- 2.1.3. Thermodynamics and Kinetics of C S Bond Cleavage gonic character of the reaction cleavage at the dianion level (ΔrG° = 55.7 kJ·mol 1) for compound 1. Being the activation The experimental data (standard potentials E01 and E02, hetero- free energy, ΔrG ¼6, and standard activation free energy, ΔrG0 ¼6, geneous electron transfer rate constants, ks, and first order 59.3 and 84.9 kJ · mol 1, respectively. Besides, as it is expected, a constant, k) are treated following the theoretical model of similar analysis at radical anion level reveals that the reaction dissociative electron transfer developed by Savéant.[34] cleavage is not thermodynamically favorable since the standard According to the thermodynamic cycle presented in free energy (ΔrG° > 0, Equation 5), which is in agreement with Scheme 7, the standard free energy of the cleavage reaction is the above-mentioned electrochemical data [Eq. (5)]. [Eq. (1)]: � � 0 Dr G0 ¼ DNO2 PhCH2 SPh þ F E01 E0ðSPh� Þ=ðSPhÞ TDr S0 (5) Dr G ¼ DNO2 PhCH2 SPh þ F � � (1) E01 þ E02 E0ðSPh� Þ=ðSPhÞ E0NO2 PhCH2 � =NO2 PhCH2 TDr S0 At this point, it is possible to envisage the electrochemical control of CO2 reactivity by tuning the electrochemical reduc- The D (bond dissociation energy) value is 218 kJ mol 1, tion process of 1 in function of the applied potential. As the whereas the E0 values are the previously determined of the first electron transfer yields a stable radical anion, it is possible subscript species, respectively. ΔrS0 is approximately 1 meV · K 1 to use this reactant as a homogenous catalyst for CO2 reduction. (96.5 J·K 1 · mol 1). The effect of the solvent reorganization is However, the second electron transfer yields carbon and sulfur considered negligible. The activation free energy ΔrG ¼6 is anions, which can potentially react with CO2 to yield carbox- ylate products (Scheme 8). ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2655 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 Scheme 6. Proposed EEC mechanism for the electrochemical reduction of 1. 2.2. Electrochemical Study of 4-Nitrobenzyl Phenyl Thioether current values increase significantly when the solution is Under a CO2 Saturated Atmosphere saturated with CO2 (Figure 5a). These electrochemical features are characteristic of an electrocatalytic process in which The electrochemical behavior of 4-thionitrobenzyl phenyl compound 1 acts as a redox mediator (organic catalysts) in a thioether under a CO2 atmosphere was completely different homogeneous catalysis leading to a CO2 reduction. It is from its behavior under an inert atmosphere, and it could be important to note that, as far as we are aware, this is the lowest divided into two reduction electron transfer processes. reduction potential value reported for reducing CO2 using an organic compound as a mediator. The reduction potential value of CO2 decreases to almost 1.0 V. On the other hand, the 2.2.1. The First Reduction Electron Transfer Process reduction potential of CO2 in aprotic media is 2.2 V (vs SCE) in inert electrodes, which is very negative potential,[29] and also is Figure 5 shows the cyclic voltammetry response of 1 under a reduced the potential in which CO2 is directly reduced into its CO2 saturated atmosphere. The first wave, which was reversible radical anion form, in almost 1 V. under an inert atmosphere, loses its reversibility, and its peak ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2656 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 Scheme 7. Thermodynamic cycle for dianion cleavage. Scheme 8. Reactivity of 1 and CO2 depending on the potential applied. The new peak at Epa = + 0.26 V (vs SCE) is related to the In order to determine the nature of the products obtained oxidation of oxalate in aprotic media, which is in agreement after the catalytic process, a control potential electrolysis was with that of tetrabutylammonium oxalate, TBA2Ox (Figure 5b), performed at Eap = 1.2 V (vs SCE). The CO2 reduction products and in the literature.[35–37] were characterized in situ by Fourier transform infrared – attenuated total reflectance (FTIR-ATR) spectroscopy (Figure 6a) Figure 6. Characterization of compound 1 electrolysis a) IR (ATR) of formate pattern and electrolysis product of 1 b) Cyclic voltammogram of 1 (5.1 mM) in DMF/0.1 M TBABF4 at 15 °C under an inert atmosphere. Glassy carbon disk. Scan rate: 0.5 V · s 1. ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2657 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 Scheme 9. Proposed mechanism for reducing compound 1 after the first electron transfer, under a CO2 saturated atmosphere. after the passage of 1 F. The IR data show new peaks at frequencies υ = 1594 cm 1, υ = 1380 cm 1, and υ = 1337 cm 1, which would correspond with the formation of formate, HCOO .[38–40] The reduction process was monitored by cyclic voltammetry (Figure 6b). The cyclic voltammogram recorded at the end of the electrolysis showed two new peaks at anodic scan and cathodic scan related to the oxalate oxidation (Epa = + 0.25 V vs SCE), and a formate radical reduction formed on the oxalate oxidation (Epc = 0.89 V vs SCE), respectively. It is important to note that 90 % of compound 1 is recovered at the end of the process as a redox organic mediator. Therefore, compound 1 is reduced with an electron transfer giving a radical anion species that, through a homogeneous catalysis, would reduce CO2 into a radical anion. This evolves into a Figure 7. Cyclic voltammogram of 1 (5.1 mM) in DMF/0.1 M TBABF4 at 15 °C, under inert atmosphere and 0.06 M CO2 concentration. Glassy carbon disk. formate anion and recovers the catalyst (Scheme 9). The Scan rate: 0.5 V · s 1. production of formate after the control potential electrolysis seems to indicate that the presence of CO2 in solution acidifies the reaction media.[41,42] Hence, two reduction electrochemical reduction mechanism are taking place at the same time. The related to the second electron transfer gets wider. However, the generated CO2 anion radical, after a one electron-transfer anodic peaks related to the C S cleavage products change. The reduction at the electrode surface CO2, can either dimerize peak of nitrobenzyl anion oxidation, Epa,A = 0.33 V (vs SCE), leading to oxalate or be protonated yielding to formate disappears as the CO2 concentration increases in the solution. (Scheme 9). Moreover, the oxidation peak related to thiophenolate moves into more positive potentials and it may overlap with the oxalate oxidation peak. 2.2.2. The Second Reduction Electron Transfer Process Figure 8 shows a CV of sodium thiophenolate under a nitrogen and CO2 atmosphere. It can be seen that the peak After the second reduction wave of 1, a completely different potential value related to its oxidation increases, as was behavior is seen under a CO2 atmosphere (Figure 7a). The previously described by Singh et al. for similar thiophenolate cathodic scan shows that as the amount of CO2 rises in the derivatives.[20] This can be explained by the formation of the solution, the peak related to the first electron transfer loses its corresponding carboxylate derivative ((phenylsulfanyl)formate, reversibility and increases its intensity, acting as an organic (Scheme 10)). Moreover, the oxidation of (phenylsulfanyl) mediator in a catalysis process. At the same time, the peak formate makes it possible to deliver and recover the CO2 ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2658 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 determined by nuclear magnetic resonance (1H-NMR) and gas chromatography coupled with mass spectrometry (GC-MS). At this point, it is possible to propose three different processes that occur at the same time. First, a homogeneous catalysis (radical anion level), which gives rise to a formate anion and the recovery of the reagent in a 40–50 % yield, as explained in Scheme 9. When a controlled potential electrolysis is performed at 2.3 V (vs SCE) (Table 2), C S cleavage reaction takes place, leading to the corresponding carboxylated products, such as 3, due to a nucleophile electrophile reaction between CO2 and the formed aromatic and sulfur anion moieties. Note that the nitro group is reduced to its amine analog, 3. According to the results in the literature, this may be due to an acidification of the electrolyte solvent due to CO2 bubbling.[41,42] To analyze this effect, a controlled potential Figure 8. Cyclic voltammogram of thiophenolate, NaPhS, (5 mM) in DMF/ electrolysis of 1 was performed under the conditions depicted 0.1 M TBABF4 at 15 °C. Glassy carbon disk. Scan rate: 0.5 V · s 1. in Table 2, but adding one equivalent of acid (HClO4) instead of CO2, obtaining compound 4 with 10 % yield after 2 F (Table 2, entry 1). previously captured. Thus, the thiophenolate can act as a On the other hand, the dimer, diphenyl disulfide is obtained reversible CO2 capture system that is electrochemically trig- by radical-radical coupling of two units of thiophenyl radicals. gered. These radicals are generated as a result of the oxidation of the A second electron transfer and a controlled potential (phenylsulfanyl)methanoate anion, which leads to the loss of electrolysis under a CO2 atmosphere were performed to CO2 and the thiophenyl radical. This oxidation process could determine the nature of the products formed on the reduction take place in the reaction with the oxygen in the air, probably obtained by 1. Note that, at this point, it is expected to obtain during chemical treatment of the sample. Therefore, the products coming from the homogeneous catalysis, which is (phenylsulfanyl)methanoate anion is not stable enough under produced after the first electron transfer, as well as products these experimental conditions (Scheme 11). arising from the electrocarboxylation through C S cleavage, which occurs after the second electron transfer. Table 2 summarizes the products related to C S cleavage reaction, the 3. Conclusions electrocarboxylate aromatic compound (4) was obtained in a 10 % yield after the passage of 3F. Besides, in the electrolyzed The electrochemical reduction mechanism of 4-nitrobenzyl solution formate was obtained in a 22 mM concentration. The phenyl thioether, 1, has been studied using electrochemical yielding of this electrocatalytic CO2 product involved a 5 % of techniques, including cyclic voltammetry and control potential charge consumption. The nature of the products obtained was electrolysis under nitrogen and carbon dioxide atmospheres, Scheme 10. Proposed mechanism for (phenylsulfanyl)formate oxidation. Table 2. Results of electrochemical carboxylation of compound 1. Entries Electrochemical parameters Yield [%] Eap Charge (Q = znF) 1 2 4 Carboxylation product (V vs SCE) 3 1 N2 1.9[a] 2F 75 5 20 – 2 CO2 2.3 3F 48.7 41.3 – 10 [a] Addion of 1 equivalent of HClO4 to the acetonitrile solution ChemElectroChem 2021, 8, 2649 – 2661 www.chemelectrochem.org 2659 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH Articles ChemElectroChem doi.org/10.1002/celc.202100329 Scheme 11. Proposed mechanism for the electrochemical reduction of 1 under CO2 at 2.3 V (vs SCE). Keywords: carbon sulfur cleavage · electrochemistry · carbon respectively. Its electrochemical reduction mechanism has been dioxide · electrocarboxylation · homogeneous catalysis · CO2 fully disclosed, and the thermodynamic and kinetics parameters capture determined. 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