INTRODUCTION

Nowadays, keen attention is drawn to the selective and ecologically safe methods of functionalization of various organic compounds using electrocatalytic processes [1–3]. The recent achievements showed that the bonds carbon–carbon and carbon–heteroatom (C–S, C–N, etc.) can be formed by electrochemical cross dehydrogenation [4–6]. From the standpoint of the ecologically-friendly strategy, the use of electrocatalytic processes in the modern organic synthesis is a reliable approach that allows synthesizing compounds unavailable by chemical methods and without using toxic oxidants or reducers [7, 8].

Furthermore, of special interest are the electrode processes on platinum (Pt), because the latter is a unique low-temperature electrocatalyst [9–11] that exhibits anodic stability and corrosion resistance to oxidants and also can selectively polarize, oxidize or reduce functional groups in organic molecules leaving untouched the remaining part [12–14].

In [15], the method of voltammetry of immobilized microparticles (VIMP) was proposed as a diagnostic criterion of the electrochemical oxidation/reduction of organosulfur compounds which allowed analyzing the vast analytical information on poorly soluble organosulfur compounds contained in the asphalt-concrete road covering. The method is based on measuring voltammetric responses of a solid that is attached mechanically to the surface of a porous inert electrode (usually graphite electrode impregnated with kerosene) in contact with a suitable electrolyte.

The study object, dimethyl sulfone (DMSO2), due to its high permittivity and electrochemical stability (in nonaqueous media) can dissolve sufficiently well salts of many metals by almost ionizing them [16, 17] and, hence, is widely used in different electrochemical applications [18–21].

In [22–24], for synthesizing thiosulfonates and aromatic sulfones, a series of methods of electrochemical C‒S coupling were considered, in which halides of metals and ammonium were used as the supporting electrolytes and redox catalysts.

The authors of [25] reported on the selective oxidation of diarylsulfides and arylalkylsulfides to the corresponding sulfoxides and sulfones under electrochemical conditions, where sulfoxides were formed at the current of the order of magnitude of 5 mA, whereas sulfones were the main products at 10–20 mA in MeOH. In [26], it was noted that in nonaqueous media, the sulfur-containing depolarizer itself can take part in the electrode process.

It is known that the electrooxidation of dimethyl sulfone produces dimethyl disulfone in alkaline medium [27, 28] and methanesulfonic acids and dimethyl disulfone in acidic medium [29–32]. The cathodic reduction of dimethyl sulfone in acidic medium produces dimethyl polysulfides. It is shown [33] that on the Pt electrode surface, DMSO2 forms stronger adsorption bonds as compared with hydrogen- and oxygen-containing species.

In this study, we considered the kinetics of dimethyl sulfone electrooxidation on the Pt electrode in a wide potential region and also showed for the first time the results of quantum-chemical calculations aimed at elucidation of the possible routes of anodic oxidation and cathodic reduction of dimethyl sulfone of the Pt electrode.

EXPERIMENTAL

The following reagents were used without preliminary cleaning: dimethyl sulfone CH3S(O)2CH3 (Sigma Aldrich), sulfuric acid (special grade), and sodium hydroxide (reagent grade).

As the supporting electrolyte, we used 0.1 М solutions of sulfuric acid (H2SO4) and sodium hydroxide (NaOH). Solutions were prepared using twice distilled deionized water.

The electrochemical studies were carried out in a three-electrode electrochemical cell with anodic and cathodic compartments separated by a fitted-in ceramic diaphragm. The volt-ampere measurements were carried out at temperature of 20 ± 2°С using an automatic potentiostat-galvanostat IPC-Pro MF (Volta, Russia). The potential was measured with respect to the reversible hydrogen electrode (RHE) in 1.0 М H2SO4. Hydrogen of the 99.9999% purity was produced by a HGP-6 generator (Khimelektronika, Russia). The reference electrode was brought closely to the working electrode surface by means of an electrolytic bridge and a Luggin capillary filled with the working solution. The working solution in the cell was blown though by inert gas (argon). Polycrystalline platinum with the geometrical surface of 0.016 cm2 was used as the working electrode; its true surface determined by the method of [14] was 0.02 cm2. The counter electrode was a platinum plate.

When measuring stationary polarization curves, the working electrode was exposed at each potential for 15–60 min until the current stopped to change.

The kinetic relationships were found by analyzing the stationary polarization curves in coordinates \(\eta \) vs. log (j) to determine their tangent (b) and the Tafel equation parameters Eqs. (1)–(3)

$$\eta = a + b{\kern 1pt} \log \left| {{{j}_{0}}} \right|,$$
(1)
$$a = - \frac{{RT}}{{\alpha zF}}\ln {\kern 1pt} {{j}_{0}},$$
(2)
$$b = \frac{{2.3RT}}{{\alpha zF}},$$
(3)

where η, the activation barrier (overpotential) for the electrochemical reaction, was found from Eq. (4) as the difference between the electrode potential (E, V) and the equilibrium potential (Eeq, V)

$$\eta = E - {{E}_{{{\text{eq}}}}},$$
(4)

а is the constant determined by the nature of electrode material; j0 is the exchange current (A/m2 or mA/cm2); α is the charge transfer coefficient for the cathodic (β—for anodic) reaction which characterizes the contribution made by the electric field of the electric double layer (EDL); z is the number of electrons involved in the electrode reaction; F is the Faraday constant (96 500 C/mol = 26.8 A h); R is the universal gas constant (8.315 J/(mol K); T is the absolute temperature (К).

By extrapolating the linear segments of polarization curves to η = 0, the exchange current j0 was determined [35]. The exchange current density (j0) in the equation of Butler–Volmer (5) characterizes the rate constant of the electrode reaction at the equilibrium potential:

$$j = {{j}_{0}}\left[ {\exp \left( { - \frac{{\alpha zF\eta }}{{RT}}} \right) - \exp \left( {\frac{{(1 - \alpha )zF\eta }}{{RT}}} \right)} \right],$$
(5)

when the cathodic overpotential of the electrochemical reaction exceeds (Eq. (6)) the anodic overpotential, the Bulter–Volmer equation [36] coincides with the Tafel equation and takes the following form (Eq. (7)):

$${{\eta }_{{\text{k}}}} \gg \frac{{RT}}{{zF}},$$
(6)
$${{\eta }_{{\text{k}}}} = - \frac{{RT}}{{\alpha zF}}\ln {\kern 1pt} {{j}_{0}} + \frac{{RT}}{{\alpha zF}}\ln {\kern 1pt} {{j}_{{\text{k}}}}.$$
(7)

The preparative electrolysis was carried out on platinum electrodes with the geometrical surface of 0.25 cm2 in a diaphragm electrolyzer at the controlled current density for 32 h using a dc rectifier TYPE:TR‑9252. The corresponding current densities of oxidation/reduction of DMSO2 were determined based on the stationary anodic/cathodic voltammograms. A perfluorinated cation-exchange membrane MF-4SK (Plastomer, Russia) was used as the diaphragm. At the stationary electrolysis, the temperature was maintained in the interval of 25–30°С.

To measure Raman spectra, the analyzed sample was placed in a quartz ampoule, which was first evacuated using a vacuum pump and then filled with argon and sealed. The Raman spectra of samples before and after electrolysis were measured using a spectrometer DXR Smart Raman Research (Thermo Scientific, United States) with laser excitation (λ = 532 nm and power of 10 mW), the spectrum range was 50–3500 cm–1 with resolution of 3–5 cm–1, the integration time was 20 s for each scan.

The 3D models of molecular structures were obtained using program ACD/Labs Freeware 2015. After this, their preliminary geometrical optimization was carried out by the DFT method at the level B97‑3c [37] using the program Orca 4.2.0 [38, 39]. The final calculations of the electronic structure (with the complete geometrical optimization) and the energy characteristics were carried out by the simplified method of generalized gradient approximation (PBE) with polarized triply split basis set of wave functions (def2-TZVP) by taking into account the spin multiplex character of the molecular and atomic radical species; the effect of the aqueous medium was taken into account using the polarized continuum model (CPCM) [40–43].

RESULTS AND DISCUSSION

Electrooxidation of Dimethyl Sulfone

The anodic voltammograms measured on a Pt electrode under stationary conditions in 0.1 M H2SO4 (Fig. 1) and 0.1 M NaOH (Fig. 2) as the supporting electrolytes with and without addition of DMSO2 in different concentrations demonstrated a rise of the current at the potential above 1.1 V (RHE) which was associated with the formation and evolution of oxygen (Eqs. (8) and (9)) [33].

$$\begin{gathered} {\text{In acidic medium:}}\,\,2{{{\text{H}}}_{{\text{2}}}}{\text{O}} \to {{{\text{O}}}_{2}} + 4{{{\text{H}}}^{ + }} + 4{{\bar {e}}} \\ E \geqslant 1.2\,\,{\text{V }}\left( {{\text{RHE}}} \right){\text{,}} \\ \end{gathered} $$
(8)
$$\begin{gathered} {\text{in alkaline medium:}}\,\,4{\text{H}}{{{\text{O}}}^{ - }} \to {{{\text{O}}}_{2}} + 2{{{\text{H}}}_{{\text{2}}}}{\text{O}} + 4{{\bar {e}}} \\ E \geqslant 0.4\,\,{\text{V }}\left( {{\text{RHE}}} \right). \\ \end{gathered} $$
(9)
Fig. 1.
figure 1

Anodic voltammograms of Pt electrode measured in the stationary mode in (1) 0.1 М H2SO4 and in the presence of DMSO2, М: (2) 0.1; (3) 0.2; (4) 0.4; (5) 0.8.

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Fig. 2.
figure 2

Anodic voltammograms of Pt electrode measured in the stationary mode in (1) 0.1 М NaOH and in the presence of DMSO2, М: (2) 0.1; (3) 0.2; (4) 0.4; (5) 0.8.

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The anodic voltammograms (Fig. 1) measured on the Pt electrode in aqueous 0.1 M solutions of H2SO4 with addition of DMSO2 showed that the overpotential of oxygen evolution increased at the DMSO2 concentration higher than 0.1 M. The similar effect was observed in 0.1 M NaOH solution in the presence of DMSO2 (but for all concentrations studied) (Fig. 2).

The decrease in the current observed in anodic curves 3–5 (Fig. 1) in the presence of different concentrations of DMSO2 was associated with suppression of the oxygen evolution reaction (Eq. (6)), with the exception of curve 2. In the latter case, the oxidation current of the oxygen reaction in the presence of 0.1 М DMSO2 was higher as compared with curve 1 measured in the supporting electrolyte, i.e., 0.1 М H2SO4. This could be associated either with the oxidation of dimethyl sulfone molecules or with the contribution made by the heterogeneous surface of the platinum electrode to the increase in the current density in the presence of 0.1 М DMSO2.

It deserves mention that the surface of the Pt electrode in aqueous solutions at the anodic potentials was covered by various chemisorbed oxygen-containing species [44, 45].

Based on the linear segments in anodic voltammograms (Figs. 1 and 2) at 1.7–2.1 V (RHE), the Tafel equation coefficients were determined (Tables 1 and 2).

Table 1. Tafel coefficients a, b, and β for Pt electrode in the potential interval of 1.8–2.1 V (RHE) and the exchange current j0 of the oxygen reaction (Eq. (6)) in 0.1 M H2SO4 with and without addition of DMSO2
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Table 2. Tafel coefficients a, b, and β for Pt electrode in the potential interval of 1.8–2.1 V (RHE) and the exchange current j0 of the oxygen reaction (Eq. (7)) in 0.1 M NaOH with and without addition of DMSO2
Full size table

According to Table 1 and 2, in the linear interval of 1.8–2.1 V (RHE) the open-circuit potential (coefficient а) was independent of the dimethyl sulfone concentration. The different values of the tangent (b) and the exchange current (j0) of oxygen evolution observed in acidic and alkaline media pointed to the different mechanisms of dimethyl sulfone oxidation. The change of the transfer coefficient β ≤ 0.5 was associated with the fact that the electrode reaction was asymmetric and proceeded irreversibly in the anodic direction.

The calculated exchange currents (Table 1) correlated with the data of [46, 47]. At the same time, it should be noted that the charge transfer coefficient and the exchange current for the hydrogen evolution on platinum depended on several factors, namely, the pH of the medium, the electrolyte composition and concentration.

It follows from Tables 1 and 2 that dimethyl sulfone exerted a considerable effect on the oxygen evolution rate in acidic and alkaline media. Thus, in the alkaline medium, as the DMSO2 concentration increased, the anodic oxygen evolution rate noticeably decreased, whereas in the acidic medium, the rate increased. We assumed that these differences may be associated with the different mechanism of electrochemical oxidation of dimethyl sulfone in acidic and alkaline media.

The quantum-chemical calculations by the method of generalized gradient approximation with polarized triply split basis set of wave functions (PBE/def2-TZVP) showed that in the dimethyl sulfone molecule, the break of the C–S bond is more advantageous (Table 3) as compared with the C–H bond.

Table 3. Bond breaking energy in water and dimethyl sulfone molecules
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According to [48], the break of the HO–H bond in the water molecule requires about 118.8 kcal/mol (497.1 kJ/mol) in vacuum. The calculations of the dissociation energy of water molecule shown in Table 3 took into account the effect of the solvent (water) according to CPCM [43].

From the calculated changes in the Gibbs energy (∆G, kJ/mol) of bond formation for possible recombination reactions of dimethysulfone species, which were also determined by the DFT method (Table 4), it is seen that the highest ∆G corresponds to the interaction of methylsulfo groups (CH3S∙(O)2) with OH radicals to form methanesulfonic acid.

Table 4. The changes in the Gibbs energy in possible recombination reactions of dimethyl sulfone species
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Were the С–H bond broken, then derivatives 3 and 4 would be formed from the dimethyl sulfone molecule (Table 4). The low ∆G of dimethyl sulfone (CH3–S(O)2–S(O)2–CH3) indicates that this substance is unstable and can undergo further transformations.

The analysis of NMR (Fig. 5) and Raman (Fig. 6) spectra of the final products of dimethyl sulfone electrolysis formed in the anodic compartment of a diaphragm electrolyzer at the current density of 0.01–0.03 A/cm2 confirmed only the formation of derivatives 1 and 2 (Table 4).

The 1H-NMR spectrum (500 MHz, DMSO-d6) of original DMSO2 demonstrated the following chemical shifts (δ), ppm: 3.34(1H, CH doublet), 3.13 (1H, CH3 doublet), 2.99 (5H, CH3 singlet). The 1H-NMR spectrum (500 MHz, DSO-d6) of the oxidation product of DMSO2 (Fig. 3, spectrum 2) demonstrated the following chemical shifts (δ), ppm: 3.33 (1H, CH singlet), 2.98 (5H, CH3 singlet). The 1H-NMR spectrum (500 MHz, DMSO-d6) of the second oxidation product of DMSO2 (Fig. 3, spectrum 3) demonstrated the chemical shifts (δ), ppm: 2.38 (3H, CH3 singlet); 8.38 (1H, ОН singlet). The information on chemical shifts makes it possible to assess the changes in the structure of the final product.

Fig. 3.
figure 3

1Н-NMR spectra of (1) DMSO2 and its electrooxidation products: (2) dimethyldisulfone and (3) methanesulfonic acid, in the presence of solvent DMSO-d6 = 2 : 1 (vol)., Т = 22.8°С.

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It should be noted that the presence of electron-withdrawing groups such as S–S and S–O near protons could descreen the latter, i.e., pull off the electron density from protons and shift their signals to the weaker field (δ) [49]. Figure 3 shows the differences between 1Н chemical shifts in NMR spectra 1 and 2 observed both in the weak (by 0.01 ppm) and strong fields (by 8.38 ppm). Apparently, this can be associated with the presence of the disulfone group which simultaneously contains electron-withdrawing (oxygen (O)) and electron-donating (sulfur (S)) nuclei.

Thus, the comparative analysis of 1Н-NMR spectra (Fig. 3) of initial and final products after the electrolysis of 0.2 М DMSO2 in acidic medium showed that the difference between the 1Н-NMR spectra 1 and 3 (the change in the weak field by 0.1 ppm) was associated with the presence of the S(O)2S(O)2 group that simultaneously contains electron-withdrawing (oxygen О) and electron-donating (sulfur S) nuclei, which corresponds to the molecular form of dimethyl sulfone.

The qualitative difference between the functional groups in the dimethyl sulfone electrooxidation products was also observed in Raman spectra (Fig. 4) in the “fingerprint” region (1600–100 cm–1).

Fig. 4.
figure 4

Raman spectra of (1) DMSO2 and final products of its electrooxidation: (2) dimethydisiulfone and (3) methanesulfonic acid.

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Thus, based on the results of quantum-chemical calculations is was assumed that the rise of curve 2 (Fig. 1) was associated with the electrooxidation of dimethyl sulfone in acidic medium at the concentration \({{C}_{{{\text{DMS}}{{{\text{O}}}_{{\text{2}}}}}}} \leqslant 0.1\,\,{\rm M}\) which can occur as a result of the break of C–S bond followed by the formation of methanesulfonic acid (10)–(13).

(10)
(11)
(12)
(13)

The current drop in curves 3–5 (Fig. 1) can be associated with the displacement of OH species from the platinum surface and the adsorption of a large number of dimethyl sulfone molecules. The formed methylsulfone groups dimerized by reaction (14) to form dimethyl sulfone.

(14)

The methyl radical can be oxidized by the one-electron mechanism in which one electron passes to the oxygen atom or some other oxidant. For example, the reaction of the methyl radical with the oxygen molecule (O2) produces the methylperoxide radical (CH3OO·). In this reaction, the methyl radical is oxidized to the oxidation state +1, and one of oxygen atoms in the O2 molecule decreases its oxidation state from 0 to –1/2. Such one-electron processes play the important role in biochemical reactions and fuel combustion processes. The methylperoxide radical is a very active unstable species that decomposes to various products [50]. Formaldehyde can be one of intermediate products of methyl radical oxidation. At the same time, the role of oxidant can be played by the oxygen species chemisorbed on the platinum surface, which are strong oxidants in the vicinity of 1.8 V (RHE) [44, 45]. These assumptions can be confirmed or rejected by carrying out the EPR analysis.

It was noted [51, 52] that methyl radicals are reduced sufficiently easily even in the region of relatively low anodic potentials. At the same time, the C‒S bond break and oxidation in the DMSO2 molecule in the acidic medium can proceed at the higher anodic potential of 1.8 V (RHE).

In the alkaline medium, the main product of dimethyl sulfone electrooxidation at the current density of 0.01–0.02 A/cm2 was dimethyldisulfone irrespective of the dimethyl sulfone concentration. We assumed that the C–S break in the DMSO2 molecule in 0.1 М NaOH occurred at the higher anodic potential of 1.8 V (RHE), where, in the absence of OH species on the platinum electrode surface and at the suppressed oxygen evolution, the reaction of dimerization of methylsulfo groups prevailed.

Electroreduction of Dimethyl Sulfone

The rise of the cathodic current at the potential above 0.1 V (RHE) in voltamograms of Pt electrode (Fig. 5) measured under stationary conditions in 1.0 М H2SO4 solutions with addition of DMSO2 corresponded to formation and evolution of hydrogen Eq. (15).

$${\text{2}}{{{\text{H}}}^{ + }} + {{2\bar {\text{e}}}} \to {{{\text{H}}}_{{\text{2}}}}{\text{.}}$$
(15)
Fig. 5.
figure 5

Stationary voltammograms of Pt electrode in (1) 1.0 M H2SO4 and with addition of DMSO2, M: (2) 0.001; (3) 0.01; (4) 0.05; (5) 0.1.

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In the cathodic potential region of –(0.2–0.6) V, based on the linear segments in curves 1–6 (Fig. 5), the Tafel equation coefficients were determined (Table 3).

The DMSO2 concentration had no effect on the variation of the open-circuit potential on the Pt electrode, whereas the decrease in j0 characterized the decrease in the rate of hydrogen evolution (Table 5). The changes in the coefficient α can probably be associated with the cathodic reduction of DMSO2 on the Pt surface. The Tafel coefficients and the exchange current (Table 5) adequately agreed with the literature data [53–55].

Table 5. Tafel equation coefficients a, b, and α on Pt electrode in the cathodic potential region of –0.2…–0.6 V (RHE) and the exchange current j0 for the hydrogen ion electroreduction in 1.0 М H2SO4 solution with and without DMSO2
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To find the composition of the DMSO2 reduction products formed in the cathodic compartment of the diaphragm electrolyzer, the preparative electrolysis was carried out at a current density of 0.1 A/cm2. After its completion (in 16 h), a black film coating oxidizable in air was observed on the Pt cathode surface.

The electrolysis product was analyzed by the NMR and Raman techniques.

The NMR analysis (Fig.6) was carried out in the presence of solvents (DMSO-d6 and CDCl3). The 1H-NMR spectrum (500 MHz, DMSO-d6) of the original DMSO2 (Fig. 6, spectrum 1) demonstrated the chemical shifts (δ), ppm: 3.34 (1H, CH3 doublet), 3.13 (3H, CH3S doublet), 2.99 (5H, CH3 singlet), 2.50 (6H, CH3 singlet). The 1H-NMR spectrum (500 MHz, CDCl3) of the DMSO2 electroreduction product (Fig. 6, spectrum 2) demonstrated the following chemical shifts (δ), ppm: 3.33 (1H, CH singlet), 2.99 (5H, CH3 singlet), 2.53 (6H, CH3 singlet), 2.25 (3H, CH3 singlet), 1.76 (1H, CH singlet). According to the reference data [49], for this composition, the chemical shifts of proton nuclei in the strong field in NMR spectra (Fig. 6) can be associated with the electron-donating S–S groups [49].

Fig. 6.
figure 6

1Н-NMR spectra of (1) DMSO2 and (2) its electroreduction product in the presence of solvents DMSO-d6 and CDCl3 = 2 : 1 (vol), Т = 22.8°С.

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The DMSO2 electroreduction product was also studied by the Raman spectroscopy. The characteristic bands of deformational vibrations for S—S bonds appeared in Raman spectra (Fig. 7) at 693 and 119 cm–1.

Fig. 7.
figure 7

Raman spectra of the reaction product detached from the Pt cathode surface after the electrolysis of DMSO2 in 1.0 M H2SO4.

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Based on the results of analysis, it was shown that the methylsulfone electroreduction in the acidic medium produced dimetyl polysulfide. The following mechanism was proposed for the cathodic reduction:

(16)
$$2{\text{Pt}}{-} {\text{S}}{-} {\text{C}}{{{\text{H}}}_{{\text{3}}}} \to {\text{C}}{{{\text{H}}}_{{\text{3}}}}{-} {\text{S}}{-} {\text{S}}{-} {\text{C}}{{{\text{H}}}_{{\text{3}}}} + 2{\text{Pt,}}$$
(17)
$$\begin{gathered} n{\text{C}}{{{\text{H}}}_{{\text{3}}}}{-} {\text{S}}{-} {\text{S}}{-} {\text{C}}{{{\text{H}}}_{{\text{3}}}} + n{{\bar {e}}} \\ \to n\left[ {{\text{C}}{{{\text{H}}}_{{\text{3}}}}{-} {{{{\text{(S}}{-} {\text{S)}}}}_{{n\,\,{\text{ + }}\,\,{\text{2}}}}}{-} {\text{C}}{{{\text{H}}}_{{\text{3}}}}{\text{ }}} \right] + n{{{\text{C}}}_{{\text{2}}}}{{{\text{H}}}_{{\text{6}}}}{\text{.}} \\ \end{gathered} $$
(18)

Based on experimental data obtained in this study, the generalized scheme (Fig. 8) of the electrochemical behavior of dimethyl sulfone on the Pt electrode in alkaline media was proposed.

Fig. 8.
figure 8

Scheme of the electrochemical behavior of dimethyl sulfone on Pt electrode.

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CONCLUSIONS

Based on the found kinetic parameters, it is shown that as the concentration of dimethyl sulfone in acidic and alkaline media decreases, the rates of anodic oxygen evolution and cathodic hydrogen evolution on the platinum electrode decrease. The quantum chemical calculation has shown that in the dimethyl sulfone molecule, the break of the C–S bond is more advantageous as regards energy as compared with the C–Н bond. It is assumed that the electrooxidation and electroreduction of DMSO2 proceeds by the radical-ion mechanism. The changes in the transfer coefficients α and β for reactions of hydrogen and oxygen evolution, respectively, may be associated with irreversibility of the processes of dimethyl sulfone electroreduction and electrooxidation on the Pt electrode. The results of NMR and Raman spectroscopic studies confirm that the final products of the dimethyl sulfone anodic oxidation in acidic medium are methanesulfonic acid and dimethyl disulfone; in alkaline medium, only dimethyldisulfone is formed. At the same time, the cathodic reduction product of DMSO2 is dimetyl polysulfide. A scheme of the electrochemical behavior of dimethyl sulfone on the platinum electrode is proposed.