Addition of a Perfluoroalkyl Acetyl Group to the C-Vertex of a Carborane Anion to Enhance Its Solubility in Fluorinated Ether Solvents

Abstract

The modification of carborane anion monocarba-closo-dodecaborate (1) with perfluoroalkyl groups enhances its solubility in fluorinated ethers. This novel approach achieves a degree of solubility that is unattainable by using traditional lipophilic modifications or boro–vertex functionalizations of 1. A spectroscopic analysis in combination with DFT calculations confirmed that these new anions retain their weakly coordinating nature and exhibit moderate chemical stability.

1. Introduction

Monocarba-closo-dodecaborate CHB₁₁H₁₁^‒ (1; referred to as the ‘carborane anion’ in this manuscript, Figure 1a) is a rigid and symmetrical cluster in which one carbon and eleven boron atoms form a cage structure [1,2,3,4,5]. Because of its three-dimensional aromaticity, the negative charge of the carborane anion is delocalized over the entire cage, which results in extremely low nucleophilicity/basicity and reasonably good stability. The unique electronic and molecular structure of 1 make it an attractive starting material for the construction of various functional molecules in fundamental and applied science [1,2,3,4,5]. Additionally, its electrochemical stability and weakly coordinating nature [6,7,8,9,10] have attracted attention for electrolyte applications in batteries [11,12,13,14,15,16,17,18,19,20]. One promising approach to enhancing electrolyte performance is the use of fluorinated-solvent-based electrolyte solutions, which exhibit high flame retardancy and a wide liquid-state temperature range, enabling low-temperature operation without freezing. [21] Fluorinated ethers, such as tris (2,2,2-trifluoroethyl)orthoformate (TFEO), bis (2,2,2-trifluoroethyl) ether (BTFE), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE), have shown potential in maintaining solvent-in-salt structures and enhancing the performance of localized high-concentration electrolytes (LHCEs) in lithium-metal batteries [22]. In this context, carborane-anion-based electrolyte systems with fluorinated solvents have shown potential; however, they have so far been generally limited to non-fluorinated ether solvents [11,12,13,14,15,16,17,18,19,20]. The main obstacle is the poor solubility of metal salts of the carborane anion in fluorinated solvents due to the low affinity of the boron cluster toward fluorinated solvents. Here, we present a new type of carborane anion that contains perfluoro alkyl groups. We have developed synthesis methods to introduce perfluoro alkyl groups at the C-vertex of the carborane anion, enhancing its solubility in fluorinated ether solvents. (Figure 1b) A spectroscopic analysis in combination with DFT calculations corroborated that these new anions retain their weakly coordinating nature and exhibit reasonably good chemical stability.

2. Materials and Methods

2.1. Instrumentation

NMR spectra were recorded on a BRUKER AVANCE NEC400 OneBay. Chemical shifts are expressed in δ (ppm). ¹H NMR spectra were referenced to the peaks of residual amounts of the (partially) protonated solvent, which was used as the internal standard. ¹¹B NMR spectra were referenced to an external sample of BF₃ Et₂O (δ = 0.0 ppm). ¹³C NMR spectra were referenced to the solvent signal, which was used as the internal standard. The following abbreviations are used: s = singlet, d = doublet, t = triplet, m = multiplet, and bs = broad singlet. IR spectra were recorded on a Shimadzu Corporation IR Prestige-21 spectrometer (Shimadzu, Kyoto, Japan). ESI mass spectra were measured on a Bruker micrOTOF-II-SF spectrometer (Bruker, Billerica, MA, USA). Melting points were determined on a Yanaco micro melting point machine (Yanaco Analytical Systems Inc., Tokyo, Japan). Normal-phase column chromatography was performed with a Biotage Isolera One 1SW (Biotage AB, Uppsala, Sweden). Reverse-phase column chromatography was performed with a COSMOSIL^® C18-OPN purchased from Nacalai Tesque, Inc (Nacalai Tesque, Inc., Kyoto, Japan). Normal-phase column chromatography was performed with silica gel 60 (230–400 mesh) from Merck (Merck KGaA, Darmstadt, Germany).

2.2. Materials

Unless otherwise noted, materials were purchased from Aldrich Inc. (St. Louis, MO, USA), Wako Pure Chemical Industries (Osaka, Japan), Ltd., Tokyo Kasei Co. (Tokyo, Japan), and other commercial suppliers and were used after appropriate purification. [Cs⁺][CHB₁₁H₁₁]⁻ (Cs·1), [Cs⁺][CHB₁₁Me₁₁]⁻ (Cs·7) and [Cs⁺][CHB₁₁F₁₁]⁻ (Cs 8) was prepared according to a procedure published in the literature [23,24,25]. Air- and moisture-sensitive manipulations were performed with standard Schlenk techniques.

2.3. Experimental Procedures

2.3.1. General Procedure for the Perfluoroalkyl Acylation of C-Vertex of Carborane Anion

[Cs]⁺[CB₁₁H₁₂]⁻ (Cs·1) (555.0 mg 2.011 mmol) was charged in a Schlenk flask and dried under reduced pressure at 120 °C for 1 h. The flask was charged with argon, and 6 mL of anhydrous THF was added at room temperature. The solution was cooled to –78 °C, and n-BuLi (1.58 M in n-hexane, 2.11 mmol, 1.33 mL) was added. The mixture was stirred at the same temperature for 10 min and then at 0 °C for 1 h. To the reaction mixture was added methyl undecafluorohexanoate (692.2 mg, 2.11 mmol) at 0 °C. After it had been stirred at room temperature for 2 h, the reaction mixture was poured into distilled water and extracted with AcOEt. The solvent was removed under vacuum, and the obtained material was purified by silica gel flash column chromatography (eluent: MeOH/CH₂Cl₂). Further purification by reverse-phase column chromatography gave·3b (the counter cation(s) were not determined).

The counter cation exchange to cesium cation (Cs⁺) was performed as follows [26]: the AcOEt solution (ca. 20 mL) of·1 was washed with 20% aqueous CsCl solution (3 × 3 mL), and then the combined water layers were extracted with AcOEt (3 × 10 mL). The combined organic phases were evaporated, and the solvent was removed under vacuum. The crude product was purified by recrystallization from a minimum amount of boiling water to yield pure Cs·3b (872.8 mg, 76% yield based on the starting material).

Compound: [Cs⁺][1-(CO(CF₂)₂CF₃)–CB₁₁H₁₁^‒] (Cs·3a)

Using the General Procedure with methyl undecafluorohexanoate (459.3 mg, 1.41 mmol), this compound was prepared as follows. Yield: 82%; viscous oil; ¹¹B{H}-NMR (128 MHz, acetone-d₆) δ −3.1 (s, 1B), −12.3 (s, 5B), −13.8 (s, 5B); ¹H{¹¹B}-NMR (400 MHz, acetone-d₆) δ 2.00 (s, 6H, BH), 1.74 (s, 5H, BH); ¹⁹F-NMR (376 MHz, acetone-d₆) δ −80.73 (s, 3F), −110.68 (s, 2F), −125.84 (s, 2F); HR-ESI-TOF-MS m/z 339.0586 (calcd m/z 339.0500 for CB₁₁H₁₁CO(CF₂)₂CF₃ [M]⁻).

Compound: [Cs⁺][1-(CO(CF₂)₄CF₃)–CB₁₁H₁₁^‒] (Cs·3b)

Yield: 76%; brown solid; mp: 217 °C; ¹¹B {¹H} NMR (128.00 MHz, acetone-d₆) δ −3.1 (s, 1B), −12.3 (s, 5B), −13.7 (s, 5B); ¹H{¹¹B} NMR (400.00 MHz, acetone-d₆) δ 2.00 (s, 6H, BH), 1.7 (s, 5H, BH); ¹⁹F-NMR (376 MHz, acetone-d₆) δ −81.61 (s, 3F), −110.16 (s, 2F), −121.46 (s, 2F), −122.00 (s, 2F), −127.01 (s, 2F); HR-ESI-TOF-MS m/z 439.0651 (calcd m/z 439.0656 for CB₁₁H₁₁CO(CF₂)₄CF₃ [M]⁻).

Compound: [Cs⁺][1-(CO(CF₂)₈CF₃)–CB₁₁H₁₁^‒] (Cs·3c)

Using the General Procedure with methyl nonadecafluorodecanoate (744.6 mg, 1.41 mmol), this compound was prepared as follows. Yield: 82%; white solid; yield: 75%; mp: 222 °C; ¹¹B{H}-NMR (128 MHz, acetone-d₆) δ −2.9 (s, 1B), −12.3 (5B), −13.7 (s, 5B); ¹H{¹¹B}-NMR (400 MHz, acetone-d₆) δ 2.02 (s, 6H, BH), 1.75 (s, 5H, BH); ¹⁹F-NMR (376 MHz, acetone-d₆) δ −81.55 (s, 3F), −110.10 (s, 2F), −120.95 (d, J = 59.6 Hz, 4F), −122.59−122.14 (m, 8F), −123.22 (s, 2F), −126.72 (s, 2F); HR-ESI-TOF-MS m/z 639.0912 (calcd m/z 639.0952 for CB₁₁H₁₁CO(CF₂)₈CF₃ [M]⁻).

Compound: [Cs⁺][1-(CO(CF₂)₁₀CF₃)–CB₁₁H₁₁^‒] (Cs·3d)

Using the General Procedure with methyl perfluorododecanoate (885.7 mg, 1.41 mmol), this compound was prepared as follows. Yield: 73%; pale brown powder; mp: 201–202 °C; ¹¹B{H}-NMR (128 MHz, acetone-d₆) δ −3.1 (s, 1B), −12.3 (s, 5B), −13.7 (s, 5B); ¹H{¹¹B}-NMR (400 MHz, acetone-d₆) δ 2.02 (s, BH), 1.75 (s, 5H, BH); ¹⁹F-NMR (376 MHz, acetone-d₆) δ −81.70 (s, 3F), −110.10 (s, 2F), −120.94 (d, J = 63.8 Hz, 4F), −121.93–122.67 (m, 10F), −123.20 (s, 2F), −126.70 (s, 2F); HR-ESI-TOF-MS m/z 739.1120 (calcd m/z 739.1104 for CB₁₁H₁₁CO(CF₂)₁₀CF₃ [M]⁻).

Compound: [Cs⁺][1-(CO(CH₂)₄CH₃)–CB₁₁H₁₁^‒] (Cs·3e)

Using the General Procedure with methyl hexanoate (692.2 mg, 1.41 mmol), this compound was prepared as follows. Yield: 49%; white powder; mp: 117 °C; ¹¹B{¹H}-NMR (128 MHz, acetone-d₆) δ −6.1 (s, 1B), −12.9 (s, 5B), −14.2 (s, 5B); ¹H{¹¹B}-NMR (400 MHz, acetone-d₆) δ 2.55 (t, J = 7.1 Hz, 2H, C=O–CH₂–CH₂CH₂CH₂CH₃), 1.93 (s, 5H, BH), 1.84 (s, 1H, BH), 1.66 (s, 5H, BH), 1.39–1.46 (m, 2H, C=OCH₂–CH₂–CH₂CH₂CH₃), 1.17–1.33 (m, 4H, C=OCH₂CH₂–CH₂CH₂–CH₃), 0.88 (t, J = 7.3 Hz, 3H, C=OCH₂CH₂CH₂CH₂–CH₃); ¹³C{¹H}-NMR (101 MHz, acetone-d₆) δ 39.3 (CH₂), 31.0 (CH₂), 23.6 (CH₂), 22.3 (CH₂), 13.3 (CH₃); HR-ESI-TOF-MS m/z 241.1779 (calcd m/z 241.1700 for CB₁₁H₁₁CO(CH₂)₄CH₃ [M]⁻).

2.3.2. Synthesis of Tri-n-octylammonium Salt of [1-(CO(CF₂)₂CF₃)–CB₁₁H₁₁^‒] (3b)

To the H₂O solution (5 mL) of Cs·3b (100.0 mg, 0.17 mmol), during stirring at 100 °C, AgNO₃ (35.6 mg, 0.21 mmol) was added. The white precipitate was filtered off, washed with water, and dried under vacuum to yield Ag·3b (91.8 mg, 96%) as a white powder. The CH₂Cl₂ solution (5 mL) of [HN(n-octyl)₃]⁺[Cl]^‒ (42.9 mg, 0.11 mmol) and Ag·3b (60.0 mg, 0.11 mmol) was stirred at room temperature for 30 min and filtered to remove AgCl. The solution was evaporated to dryness, and the product was dried under reduced pressure (1.0 × 10⁻¹ hPa/80 °C, 6h) to afford [HN(n-octyl)₃]·3b as a white powder (41.9 mg, 30% yield based on Ag·3b).

Compound: [Ag⁺][1-(CO(CF₂)₄CF₃)–CB₁₁H₁₁^‒] (Ag·3b)

Pale brown oil; yield: 96%; ¹¹B{¹H}-NMR (128 MHz, acetone-d₆) δ −3.1 (s, 1B), −12.3 (s, 5B), −13.7 (s, 5B); ¹H{¹¹B}-NMR (400 MHz, acetone-d₆) δ 2.01 (s, 6H, BH), 1.75 (s, 5H, BH); ¹⁹F-NMR (376 MHz, acetone-d₆) δ −81.64 (s, 3F), −110.15 (s, 2F), −121.32 (s, 2F), −121.88 (s, 2F), −126.97 (s, 2F); HR-ESI-TOF-MS m/z 439.0612 (calcd m/z 439.0656 for CB₁₁H₁₁CO(CF₂)₄CF₃ [M]⁻).

Compound: [HN(octyl)₃⁺][1-(CO(CF₂)₄CF₃)–CB₁₁H₁₁^‒] (HN(octyl)₃·3b)

Viscous oil; yield: 30%; ¹¹B{¹H}-NMR (128 MHz, acetone-d₆) δ −3.2 (s, 1B), −12.3 (s, 5B), −13.7 (s, 5B); ¹H{¹¹B}-NMR (400 MHz, acetone-d₆) δ 8.40 (1H, NH), 3.44–3.49 (m, 6H, N–CH₂–CH₂CH₂CH₂CH₂CH₂CH₂CH₃), 2.01 (s, 6H, BH), 1.86–1.94 (m, 6H, NCH₂–CH₂–CH₂CH₂CH₂CH₂CH₂CH₃), 1.74 (s, 5H, BH), 1.30–1.47 (m, 30H, NCH₂CH₂–CH₂CH₂CH₂CH₂CH₂–CH₃), 0.88 (t, J = 6.9 Hz, 9H, NCH₂CH₂CH₂CH₂CH₂CH₂CH₂–CH₃); ¹³C-NMR (101 MHz, acetone-d₆) δ 53.47 (CH₂), 31.53 (CH₂), 26.26 (CH₂), 23.68 (CH₂), 22.36 (CH₂), 13.41 (CH₃); ¹⁹F-NMR (376 MHz, acetone-d₆) δ −81.63 (s, 3F), −110.11 (s, 2F), −121.31 (s, 2F), −121.94 (s, 2F), −126.99 (s, 2F); HR-ESI-TOF-MS m/z 439.0663 (calcd m/z 439.0656 for CB₁₁H₁₁CO(CF₂)₄CF₃ [M]⁻).

2.3.3. Solubility of Cesium Salts in Fluorinated Solvents

A dry cesium salt was stirred in fluorinated solvents (3–5 mL) at room temperature until saturation was confirmed visually by the presence of undissolved solids. The saturated solution was then filtered to remove any remaining solids. The filtrate was transferred into pre-weighed glass vials, which were immediately weighed to determine the total amount of cesium salt and solvent. The solvent was then evaporated, and the residue was dried under vacuum (1.0 × 10⁻¹ hPa, room temperature, 6 h). The residue was weighed to calculate the amount of cesium salt and the volume of solvent used.

3. Results

3.1. Synthesis

We commenced our study by attempting to add a perfluoroalkyl group through substitution at the C1 position. For that purpose, we conducted S_N2 reactions between lithiated carborate 2 and perfluoroalkyl iodine compounds such as I(CH₂)₂(CF₂)₅CF₃ or I(CF₂)₅CF₃. Unfortunately, these reactions did not proceed; starting material 1 remained unreacted, and the formation of byproducts was not observed. Duttwyler and coworkers reported that treatment of 2 with benzoyl chloride in THF yielded 1-(PhC=O)–CB₁₁H₁₁^‒ in 72% yield [27]. Inspired by these results, we decided to explore the use of the acetyl group as a linker. Gratifyingly, treatment of 2 with methyl perfluorobutyrate gave the desired product (3a) in moderate yield (Scheme 1). This protocol is also applicable to a wide range of perfluoroalkyl chains that vary in length (Scheme 1). The reaction could also be conducted on the gram scale, and purification of the obtained products is straightforward; separation of starting material 1 was achieved by reverse-phase chromatography. The product thus obtained was characterized by means of ¹¹B NMR and ¹H NMR spectroscopy, as well as by ESI-TOF mass spectrometry. To compare the effect of perfluoroalkyl substitution to that of simple alkyl substitution on the solubility in fluorinated solvents, we also synthesized the hexyl-acetylated product (3e) using the same protocol.

It is also worth noting here that the reduction of the carbonyl group of 3b could not be achieved under conventional conditions (Scheme 2). This may be attributed to the steric hindrance of the carborane anion and/or the reduced reactivity of the carbonyl carbon in nucleophilic reactions due to the anionic charge of the carborane anion. These findings suggest that 3b is reasonably stable toward reduction, which is important for potential applications in electrolytes. Furthermore, decomposition was not observed for Cs·3b under acidic conditions or upon heating to 200 °C (Cs·3b: mp: 217 °C).

3.2. Solubility of Cesium Salts of 3 in Fluorinated Solvents

We then investigated the influence of the addition of perfluoroalkyl groups on the solubility of the corresponding cesium salts in fluorinated solvents of varying polarity, including methyl 1,1,2,2-tetrafluoroethyl ether (4), ethyl 1,1,2,2-tetrafluoroethyl ether (5), and perfluoro (methylcyclohexane) (6) (Figure 2C). These fluorinated ethers were chosen as alternatives to those mentioned in the introduction (TFEO, BTFE, and TTE) based on their lower cost and ease of use; in other words, these solvents enabled more practical and accessible experiments while still providing valuable insights into the solubility behavior of the carborane anions 3 in fluorinated solvents. As anticipated, Cs·1 is practically insoluble in all the solvents tested (Figure 2A). In contrast, Cs·3a exhibited enhanced solubility in 4 (92 mM) and 5 (60 mM), although it was scarcely soluble in 6. Thus, the introduction of the trifluoropropyl acetyl group dramatically improves the solubility of the carborane anion 3a in fluorinated ethers. Cs·3b, which features a longer perfluoro alkyl group than Cs·3a, exhibited almost the same solubility in 4 and 5 and enhanced solubility in 6. All salts Cs·3 exhibited enhanced polarity due to the introduced carbonyl group, and the increased solubility of the cesium salt of 3a and 3b in ether-type solvents 4 and 5 compared to that in 6 is likely due to the higher favorability of the interaction between the anion and the more polar ethers compared to that with the less polar cyclohexane. To our surprise, cesium salts of more strongly fluorinated 3c and 3d showed decreased solubility in all the solvents tested. These results suggest that when the perfluoroalkyl chain is too long, the solubility is negatively affected due to either (1) increased intermolecular interactions between the extended perfluoroalkyl groups of the anions or (2) stronger interactions between the counter cation (Cs⁺) and the perfluoroalkyl groups.

To investigate whether lipophilicity contributes to the increased solubility in fluorinated solvents, we tested the solubility of the cesium salts of 3e and CB₁₁Me₁₁⁻ (7) [25]. However, Cs∙3e and Cs∙7 were found to be only sparingly soluble in all the solvents, highlighting the importance of fluorine atoms for enhanced solubility. It is also worth noting here that the cesium salt of perfluorinated CHB₁₁F₁₁^‒ (8) was sparingly soluble in all the solvents tested [24]. Given that the preparation of 3 is more convenient than that of 8, which requires the manipulation of fluorine gas, C-vertex perfluoroalkyl acetylation represents a more favorable and practical approach. Finally, the effect of 12-B substitution was examined; it has been reported that 12-B halogenation of the carborane anion dramatically increases the solubility of the corresponding magnesium salts (Mg·[12-Cl-CHB₁₁H₁₀]₂ and Mg·[12-Br-CHB₁₁H₁₀]₂) compared to that of Mg·[CHB₁₁H₁₁]₂ [17]. This has been attributed to the attenuation of the interaction between the magnesium cation and the carborane anion due to the substitution of a hydrogen atom with an electronegative, lower-charge-density, sterically shielding, and lipophilic halogen atom (Br or Cl) at the 12-B vertex. However, the cesium salt of 12-Cl-CHB₁₁H₁₀ (9) was practically insoluble in all the solvents tested, indicating that simple substitution at the 12-B vertex is insufficient to enhance solubility in fluorinated solvents.

3.3. Coordinating Ability

Subsequently, we investigated the anion stability (weakly coordinating nature) of 3b. Anions with low coordinating ability are crucial for cation–anion dissociation in electrolytes, which directly impacts conductivity and transport numbers [3]. Reed et al. developed a reliable method to evaluate the coordinating ability based on an IR analysis of the corresponding tri-n-octylammonium salts [28]. In their method, the N–H stretching frequencies of the ammonium cations of the salts are compared in the solid state. The N–H stretching vibration is observed at higher frequency for 3b (3143 cm^‒1) than for the parent carborane anion 1 (3129 cm⁻¹). This result indicates that 3b exhibits higher anion stability than does 1. Then, we carried out DFT calculations in order to better understand the unique properties of these new anions (Figure 3). It is well known that the counter cation of carborane anions tends to be located near the 12-B position, which is consistent with the electrostatic potential distribution of 1 [1,5]. In this context, the 12-B position of 3b is less negatively charged compared to that of 1, primarily due to the electron-withdrawing effect of the perfluoroalkyl acetyl group. This may be one of the reasons for the weaker coordinating nature of 3b compared with that of 1. These computational results were experimentally validated, i.e., the signal corresponding to the B12 position in the ¹¹B NMR spectra of 3 was shifted upfield by 3.8 ppm compared to that of 1, reflecting the influence of the perfluoroalkyl acetyl group (

Table 1. Experimental ¹¹B{¹H} chemical shifts of 1 and 3a–3d.

Compound/Position	B12	B7–B11	B2–B6
1	–6.9	–13.3	–16.2
3a	–3.1	–12.3	–13.9
3b	–3.1	–12.3	–13.7
3c	–2.9	–12.3	–13.7
3d	–3.1	–12.3	–14.3

). Interestingly, extending the perfluoroalkyl chain does not alter the chemical shift of B12, suggesting that the electron-withdrawing effect on the boron cluster remains consistent regardless of the chain length.

4. Conclusions

In summary, we have designed, synthesized, and characterized C-(perfluoroalkyl acetylated) carborane anions whose cesium salts exhibit significantly enhanced solubility in fluorinated solvents. By varying the length of the perfluoroalkyl chain, we achieved high solubility, which has so far been unattainable by using traditional approaches such as increasing lipophilicity or surface modifications of the boron vertices of the boron cluster, while maintaining the weakly coordinating nature required for electrolyte applications, as corroborated by experimental and computational analyses. The high chemical stability of 3 under various conditions is particularly noteworthy considering the generally reactive/sensitive nature of carbonyl groups. Further investigations to examine the solubility in fluorinated solvents, ion conductivity, and redox stability of the lithium and magnesium salts of these new anions are currently in progress in our laboratory.