Voltammetric Studies on the Reduction Potentials of Perfluoroalkyl Halides and Their Analogous Compounds

Abstract

Photocatalytic single-electron transfer (SET) reactions involving perfluoroalkyl halides play a crucial role in synthetic organic chemistry. However, the electrochemical data for these compounds, which are essential in the discussion of the SET process, are missing. In this study, the electrochemical reduction potentials of perfluoroalkyl halides, alkyl halides, and other analogous compounds were investigated in 0.1 M Bu₄NPF₆/CH₃CN using Ag, Pt, and glassy carbon electrodes. The Ag electrode showed remarkable catalytic properties and a positive reduction peak shift during the reduction reaction; this indicates that the Ag electrode is suitable for estimating the electrochemical potential of the SET process. This study provides a comprehensive dataset for the electrochemical measurements of perfluoroalkyl and alkyl halides, which will help synthetic organic chemists select appropriate reaction systems for these compounds.

1. Introduction

Fluorine is a unique element¹ because it has the third smallest van der Waals radius,^2,3 the highest electronegativity,⁴ a low atomic refractive index,⁵ and the ability to form strong carbon–fluorine bonds.⁶ Owing to the unique nature of fluorine, fluorinated organic compounds have a wide range of applications in several fields including pharmaceuticals,^7–11 agrochemicals,^12–15 and functional materials.^16,17 The introduction of perfluoroalkyl groups into organic compounds dramatically changes their electronic properties,¹⁸ hydrophobicity,¹⁹ and metabolic stability.^20–22 Therefore, a simple and efficient synthetic method that introduces perfluoroalkyl groups in molecules of interest is urgently required.

Numerous studies have used perfluoroalkyl halides to introduce perfluoroalkyl groups into organic compounds.²³ Perfluoroalkyl iodide is an inexpensive, readily available, and structurally diverse source of perfluoroalkyl moieties and is thus widely used in synthetic organic chemistry.²⁴ Photoredox catalysis has emerged as a powerful tool for perfluoroalkylation because it can generate perfluoroalkyl radical species via single-electron transfer (SET) reactions under mild conditions.^25–29 As the perfluoroalkylation reaction is triggered by the SET process, the redox potential of the perfluoroalkyl iodide is a key factor in determining the outcome of the reaction. However, a comprehensive database that presents the reduction potentials of a series of perfluoroalkyl halides has not been reported to date.

In this study, we comprehensively investigated the reduction potentials of various perfluoroalkyl halides (iodides, bromides, and chlorides). The reduction potentials of several alkyl halides and perfluoroalkyl dihalides were also determined to better understand the relationship between the structure of the compounds and their reduction potential. The present dataset was also compared with previously reported photoredox reactions.

2. Experimental

2.1 Chemicals

Acetonitrile (Cica-reagent, ≥99.5 %) was purchased from FUJIFILM Wako Pure Chemical Corp. In addition, 0.1 M Bu₄NPF₆/MeCN (≥98 %) was purchased from Sigma-Aldrich. All other reagents with good purities (≥98 %) were purchased from commercial sources and used without further purification. The perfluoroalkyl iodides were supplied by Tosoh Finechem Co., Ltd.

2.2 Instrumentation

Electrochemical measurements were performed using an ALS electrochemical analyzer (Model 1205C potentiostat, BAS Inc., Tokyo, Japan). All experiments were performed at 25 °C. Linear sweep voltammetry (LSV) was conducted using a three-electrode setup; a disk electrode (Φ = 3 mm) was used as the working electrode. The glassy carbon (GC), Pt, and Ag electrodes were employed for LSV. A Pt wire and Ag/AgNO₃|0.1 mol L⁻¹ (M) Bu₄NPF₆ in CH₃CN were used as the counter and reference electrodes, respectively. All potentials reported in this paper were measured against a KCl-saturated calomel electrode (SCE). This was achieved by calibrating the Ag/AgNO₃ reference system against the ferricenium/ferrocene couple (E^θ_Fc+/Fc = 0.391 V versus SCE in CH₃CN) at the end of each experiment and converting the potentials into the SCE scale. The GC (BAS Inc., GCE Glassy carbon electrode), Pt (BAS Inc., PTE Platinum electrode), and Ag (BAS Inc., AGE Silver electrode) disk electrodes were built using 3-mm-diameter disks embedded in Teflon and polyether ether ketone. Before each experiment, the GC, Pt, and Ag electrodes were polished using an alumina suspension and rinsed with CH₃CN. This polishing procedure yielded good reproducibility for the voltammetric responses of all three electrodes. The passivation of the Ag electrode was observed during the experiment, which led to poor reproducibility of the data. In this study, two types of reduction potentials (E_red) were determined: the peak potential (E_red,peak) and onset potential (E_red,onset). The E_red,onset was determined by reading the peak potential from the second derivative of the reduction wave.

3. Results and Discussion

3.1 Comparison of electrode materials

First, the effect of electrode material on the reduction potential (E_red) was investigated. Ag electrodes are known for their catalytic activity in the dehalogenation reactions of various alkyl halides such as benzyl chloride.^30,31 Thus, the Ag electrode may provide an experimental E_red close to the thermodynamic value. For comparison, we performed LSV using Pt and GC electrodes, which are commonly used for electrochemical measurements. Perfluorohexyl halides (C₆F₁₃-X, X = I, Br, or Cl) were selected as model substrates. To determine the E_red, onset potential (E_red,onset) and peak potentials (E_red,peak) were determined.

The LSV data for C₆F₁₃-X are shown in Fig. 1 and Table 1. In all cases, the most positive E_red,onset and E_red,peak values were obtained for the Ag electrode, suggesting that Ag possesses electrocatalytic properties for cathodic dehalogenation. The Pt and GC electrodes exhibited a more negative potential for the reduction process, suggesting that these electrodes had a larger overpotential. In other words, the Pt and GC electrodes overestimated the E_red values compared to Ag.

Figure 1.

Linear sweep voltammograms (LSVs) of (a) C₆F₁₃-I, (b) C₆F₁₃-Br, and (c) C₆F₁₃-Cl (10 mM) recorded at a scan rate of 0.1 V s⁻¹ using Ag, Pt, and GC working electrodes in 0.1 M Bu₄NPF₆/MeCN.

Table 1. Experimental E_red values of perfluorohexyl iodide, perfluorohexyl bromide, and perfluorohexyl chloride measured in 0.1 M Bu₄NPF₆/MeCN using different working electrodes.

	Ag		Pt		GC
	Ered,onset	Ered,peak	Ered,onset	Ered,peak	Ered,onset	Ered,peak
CF3(CF2)5-I	−0.77	−1.29	−1.07	−1.89	−1.14	−1.60
CF3(CF2)5-Br	−1.25	—	−2.33	—	−1.87	−2.11
CF3(CF2)5-Cl	−1.69	−2.03	−2.57	—	−2.57	—

(V vs. Ag/AgNO₃)

Notably, a gentle onset current was observed for the Ag electrode. Similar behavior was reported for benzyl chloride, which was attributed to the electrochemical reduction of weakly adsorbed species on the electrode surface. In addition, the linear sweep voltammogram of C₆F₁₃-Cl recorded using the Ag electrode showed a shoulder at −1.8 V. This result suggests that complex chemical and electrochemical reactions occurred because of the single-electron reduction process. Thus, we conclude that the E_red recorded using the Ag electrode provides a more thermodynamically precise value for perfluoroalkyl halides. Additionally, reporting both the E_red,onset and E_red,peak values makes the dataset more comprehensive. In the following section, we summarize the E_red,onset and E_red,peak data recorded using the Ag working electrode. The complete dataset recorded for Pt and GC electrodes is provided in the Supporting Information.

3.2 Voltammetric data

3.2.1 Perfluoroalkyl halides and alkyl halides

The E_red values of various perfluoroalkyl and alkyl halides, including diiodoperfluoroalkanes, esters, and benzyl halides, were measured; these values are listed in Table 2. In Fig. 2, it shows a scatter plot of reduction potentials for each sample. Some compounds were covered by the solvent peak, and their E_red,onset could not be determined. In the following sections, we divide these E_red values into several groups and compare the reduction potentials in detail by presenting the recorded voltammetric data.

Table 2. Voltammetric data for the reduction of 10 mM of various perfluoroalkyl halides and alkyl halides in 0.1 M Bu₄NPF₆/MeCN recorded using Ag working electrodes.

entry	Perfluoroalkyl halide/alkyl halide	Ered,onset [V vs. Ag/AgNO3]	Ered,peak [V vs. Ag/AgNO3]
1	CF3(CF2)2-I	−0.76	−1.32
2	CF3CF(CF3)-I	−0.76	−1.57
3	CF3(CF2)3-I	−0.78	−1.43
4	CF3(CF2)4-I	−0.77	−1.32
5	CF3(CF2)5-I	−0.77	−1.29
6	CF3(CF2)6-I	−0.75	1.25
7	CF3(CF2)7-I	−0.76	−1.25
8	CF3(CF2)9-I	−0.76	−1.19
9	CF3(CF2)3-Br	−1.29	−1.54
10	CF3(CF2)5-Br	−1.25	—
11	CF3(CF2)5-Cl	−1.69	−2.03
12	CH3-I	−1.56	−1.57
13	CH3CH2-I	−1.67	−1.90
14	CH3CH(CH3)-I	−1.61	−1.88
15	CH3(CH2)3-I	−1.64	−1.95
16	CH3(CH2)4-I	−1.69	−1.73
17	CH3(CH2)5-I	−1.62	−1.99
18	CH3CH2-Br	−1.68	—
19	CH3(CH2)2-Br	−1.65	—
20	CH3(CH2)3-Br	−1.65	—
21	CH3(CH2)5-Br	−1.65	—
22	CH3(CH2)6-Br	−1.64	—
23	CH3(CH2)7-Br	−1.65	—
24	CH3(CH2)8-Br	−1.64	—
25	I-(CF2)4-I	−1.04	−1.43
26	I-(CF2)6-I	−0.90	−1.52
27	I-(CF2)8-I	−0.91	−1.53
28	Br-(CF2)2-Br	−1.50	−1.73
29	Br-(CF2)6-Br	−1.52	−1.79
30	Br-(CF2)8-Br	−1.52	−1.67
31	I-CH2-I	−1.23	−1.64
32	Br-CH2-Br	−1.51	−1.77
33	Br-(CH2)2-Br	−1.45	−1.70
34	Br-(CH2)4-Br	−1.55	—
35	EtOCOCF2-I	−0.56	−1.45
36	EtOCOCH2-I	−1.55	−1.78
37	EtOCOCF2-Br	−1.55	−1.78
38	EtOCOCH2-Br	−1.44	−1.71
39	(C6F5)CF2-I	−0.44	−1.73
40	C3F7-CH2-I	−1.52	−1.66
41	CF3-CH2-I	−1.62	−1.90
42	Br-(CF2)2-I	−0.98	−1.47
43	Cl-(CF2)6-I	−1.13	−1.45
44	CH2=CH-(CF2)4-I	−1.30	−1.67
45	CH2=CH-(CF2)6-I	−1.14	−1.37

Figure 2.

E_red,onset and E_red,peak of perfluoroalkyl halides and alkyl halides.

3.2.2 Perfluorohexyl halides and hexyl halides

First, perfluorohexyl and hexyl halides with different halogen atoms at their chain ends were compared (Fig. 3). We found that perfluorohexyl halides are more susceptible to single-electron reduction in the order I > Br > Cl; this observation is consistent with the polarization trend of the carbon–halogen bonds (Table 2, entries 5, 10 and 11). However, no difference was observed in the reduction potentials of the hexyl halides owing to the terminal halogen atom. This was presumably due to the small polarization of the C–X bond in alkyl halides compared to that in perfluoroalkyl halides. Fluorine has the greatest electronegativity of all atoms,⁴ resulting in a lower electron density of carbon in perfluoroalkyl halides. Therefore, perfluoroalkyl halides have high intramolecular polarization. On the other hand, the electronegativity of hydrogen is smaller than that of fluorine, which leads to a lower intramolecular polarization than perfluoroalkyl halides. Furthermore, the perfluorohexyl halides showed considerably higher E_red values than the hexyl halides. The same trend was observed for perfluoroalkyl halides and alkyl halides with other carbon numbers.

Figure 3.

LSVs of 10 mM perfluorohexyl halides and hexyl halides with C6-chain lengths recorded at a scan rate of 0.1 V s⁻¹ using the Ag electrode in 0.1 M Bu₄NPF₆/MeCN.

3.2.3 Perfluoroalkyl iodides with different carbon chain lengths

The reduction potentials of perfluoroalkyl iodides with different chain lengths were compared to assess whether the number of carbon atoms affects the E_red value (Fig. 4) (Table 2, entries 1–8). CF₃(CF₂)₇-I and CF₃(CF₂)₉-I are solid reagents that do not fully dissolve in CH₃CN, resulting in low currents. The LSV data suggest that no significant difference in E_red values was observed based on the chain length. Although a perfluoroalkyl halide with a longer chain contains more fluorine atoms, the inductive effect of fluorine on the terminal C–I bond is limited when it is located farther away. Thus, the effect of the number of fluorine atoms, that is, the chain length, on E_red is limited.

Figure 4.

LSVs of 10 mM perfluoroalkyl iodides with different carbon chain lengths recorded at a scan rate of 0.1 V s⁻¹ using the Ag electrode in 0.1 M Bu₄NPF₆/MeCN.

3.2.4 Diiodoperfluoroalkanes

Next, the E_red values of diiodoperfluoroalkanes with different chain lengths were compared. Interestingly, a slight difference in E_red owing to the difference in carbon number was observed (Fig. 5). The four-carbon chain, I-(CF₂)₄-I, exhibited a lower reduction potential than the six- and eight-carbon chains. However, I-(CF₂)₆-I and I-(CF₂)₈-I showed almost the same reduction potential (Table 2, entries 25–27). Moreover, I-(CF₂)₆-I showed a lower reduction potential than CF₃(CF₂)₅-I, i.e., it was more difficult to reduce than CF₃(CF₂)₅-I. These results suggest that I-(CF₂)₄-I is considerably affected by the iodine atoms on the opposite sides because of its short fluorine chain. In contrast, I-(CF₂)₆-I and I-(CF₂)₈-I are less affected by the iodine atoms at the opposite ends because the carbon chains are sufficiently long to isolate each terminus.

Figure 5.

LSV of diiodo perfluoroalkanes (10 mM) with different carbon chain lengths recorded at a scan rate of 0.1 V s⁻¹ using the Ag electrode in 0.1 M Bu₄NPF₆/MeCN.

3.2.5 Polyfluoroalkyl iodide with various terminal groups

Next, we fixed the carbon number of perfluoroalkyl iodides to six and compared the effect of the terminal functional groups on the E_red of these compounds (Fig. 6). For this purpose, a series of compounds with Cl, I, and vinyl terminal groups were compared (Table 2, entries 5, 26, 42, and 44). No significant difference in E_red was observed among the compounds with halogen atoms or alkyl groups at the end of -(CF₂)₆-I. The carbon chain of X-(CF₂)₆-I was too long to be affected by the moiety at the end of the chain, suggesting little effect on the terminal iodine atoms; this observation is consistent with the trend observed for diiodoperfluoroalkanes.

Figure 6.

LSVs of different perfluorohexyl iodides (10 mM) recorded at a scan rate of 0.1 V s⁻¹ using the Ag electrode in 0.1 M Bu₄NPF₆/MeCN.

3.3 Interpretation of photoredox synthesis

Several research groups, including ours, have reported that eosin Y (EY) induces the SET reduction of perfluorohexyl halides in both reductive and oxidative quenching cycles.^32–35 Interestingly, perfluorohexyl iodide undergoes single-electron reduction in both cycles of EY, while perfluorohexyl chloride is only reduced by the oxidative quenching cycle of EY (Fig. 7). The reduction potential of EY in the reductive and oxidative quenching cycles is −1.08 V and −1.58 V (vs. SCE), respectively. Thus, the outcome of the EY-catalyzed perfluoroalkylation reaction could be explained by the difference in the E_red of perfluorohexyl iodide and perfluorohexyl chloride.

Figure 7.

Catalytic cycle of eosin Y (EY).

The E_red dataset was used as a reference to confirm this hypothesis. The dataset shown in Table 1 suggests that the E_red,onset values of CF₃(CF₂)₅-I and CF₃(CF₂)₅-Cl were −0.71 V and −1.64 V vs. SCE, respectively. Thus, it is reasonable to expect that CF₃(CF₂)₅-I could be reduced by both the reductive and oxidative quenching cycles, whereas CF₃(CF₂)₅-Cl was hardly reduced via the reductive quenching cycle. As briefly demonstrated herein, our dataset can be used to predict the outcome of a perfluoroalkylation reaction or to choose a suitable substrate for photoredox catalysis.

4. Conclusion

We comprehensively investigated the reduction potentials of perfluoroalkyl halides and their analogous compounds using LSV. The Ag electrode was found to be suitable for estimating the electrochemical potential of the SET process because of its catalytic activity for cathodic dehalogenation. Both the onset and peak potentials were determined in this study to obtain a complete understanding of the reduction process. Thus, this study provides important guidelines for the interpretation and prediction of photocatalytic perfluoroalkylation reactions.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.24541084.

• 1) Photocatalytic single-electron transfer (SET) reactions involving perfluoroalkyl halides play a crucial role in synthetic organic chemistry. However, the electrochemical data for these compounds, which are essential in the discussion of the SET process, are missing. In this study, the electrochemical reduction potentials of perfluoroalkyl halides, alkyl halides, and other analogous compounds were investigated in 0.1 M Bu₄NPF₆/CH₃CN using Ag, Pt, and glassy carbon electrodes. The Ag electrode showed remarkable catalytic properties and a positive reduction peak shift during the reduction reaction; this indicates that the Ag electrode is suitable for estimating the electrochemical potential of the SET process. This study provides a comprehensive dataset for the electrochemical measurements of perfluoroalkyl and alkyl halides, which will help synthetic organic chemists select appropriate reaction systems for these compounds.

CRediT Authorship Contribution Statement

Airi Yamaguchi: Conceptualization (Equal), Data curation (Lead), Formal analysis (Lead), Investigation (Lead), Visualization (Lead), Writing – original draft (Lead), Writing – review & editing (Equal)

Naoki Shida: Conceptualization (Equal), Data curation (Supporting), Formal analysis (Supporting), Funding acquisition (Supporting), Investigation (Supporting), Project administration (Equal), Resources (Supporting), Supervision (Lead), Validation (Equal), Visualization (Equal), Writing – original draft (Equal), Writing – review & editing (Lead)

Mahito Atobe: Funding acquisition (Supporting), Project administration (Equal), Supervision (Equal), Validation (Equal), Writing – review & editing (Supporting)

Tomoko Yajima: Conceptualization (Equal), Funding acquisition (Lead), Methodology (Supporting), Project administration (Lead), Resources (Lead), Supervision (Lead), Writing – review & editing (Supporting)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: 22H02118

Japan Society for the Promotion of Science: 21H05215

Japan Society for the Promotion of Science: 21H05219

Footnotes

N. Shida and M. Atobe: ECSJ Active Members

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