JP7157966B2 - silicon-containing tetraalkylborate - Google Patents

Description

The present invention relates to silicon-containing tetraalkylborates.

An ionic liquid is a salt composed only of ions and generally has a melting point of 100° C. or less. Ionic liquids are being studied for various applications due to their properties. Many of the ionic liquids known so far contain halogen atoms such as fluorine atoms in their anions, so there is still a problem in terms of environmental load, and halogen-free ionic liquids have been desired.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a novel salt capable of becoming an ionic liquid containing no halogen atoms.

Means for Solving the Problems As a result of intensive studies to achieve the above object, the present inventors have found that the above object can be achieved by a tetraalkylborate having a trialkylsilyl group, and completed the present invention.

（式中、Ｒ¹～Ｒ³は、それぞれ独立に、炭素数１～４のアルキル基である。Ｒ⁴～Ｒ⁶は、それぞれ独立に、炭素数１～８のアルキル基である。ｎは、１～６の整数である。）
２．Ｒ¹～Ｒ³が、同一の基である１の塩。
３．Ｒ¹～Ｒ³が、全てメチル基又はエチル基である２の塩。
４．Ｒ¹～Ｒ³が、全てメチル基である３の塩。
５．Ｒ⁴～Ｒ⁶が、炭素数２～６のアルキル基である１～４のいずれかの塩。
６．Ｒ⁴～Ｒ⁶が、同一の基である１～５のいずれかの塩。
７．ｎが、１、２又は３である１～６のいずれかの塩。
８．カチオンが、有機カチオンである１～７のいずれかの塩。
９．カチオンが、リン原子含有有機カチオンである８の塩。
１０．カチオンが、窒素原子含有有機カチオンである８の塩。
１１．融点が、１００℃以下のイオン液体である１～１０のいずれかの塩。
１２．融点が、２５℃以下のイオン液体である１１の塩。 That is, the present invention provides the following silicon-containing tetraalkylborate.
1. A silicon-containing tetraalkylborate containing an anion represented by the following formula (1) and a cation.

(wherein R ¹ to R ³ are each independently an alkyl group having 1 to 4 carbon atoms; R ⁴ to R ⁶ are each independently an alkyl group having 1 to 8 carbon atoms; n is , an integer from 1 to 6.)
2. A salt of 1 wherein R ¹ to R ³ are the same groups.
3. A salt of 2 wherein R ¹ to R ³ are all methyl or ethyl groups.
4. A salt of 3 wherein R ¹ to R ³ are all methyl groups.
5. A salt of any one of 1 to 4, wherein R ⁴ to R ⁶ are an alkyl group having 2 to 6 carbon atoms.
6. A salt of any one of 1 to 5, wherein R ⁴ to R ⁶ are the same group.
7. A salt of any of 1-6 wherein n is 1, 2 or 3.
8. A salt of any of 1-7, wherein the cation is an organic cation.
9. A salt of 8, wherein the cation is an organic cation containing a phosphorus atom.
10. A salt of 8 wherein the cation is a nitrogen atom-containing organic cation.
11. A salt according to any one of 1 to 10, which is an ionic liquid having a melting point of 100° C. or lower.
12. 11 salts that are ionic liquids with a melting point of 25° C. or lower.

The silicon-containing tetraalkylborate of the present invention becomes an ionic liquid depending on the type of cation, and is halogen-free, so that it has a low environmental load.

1 is a ¹ H-NMR spectrum of octyltributylphosphonium(trimethylsilylmethyl)triethylborate produced in Example 1. FIG. 1 is a ¹³ C-NMR spectrum of octyltributylphosphonium(trimethylsilylmethyl)triethylborate produced in Example 1. FIG. ¹¹ B-NMR spectrum of octyltributylphosphonium(trimethylsilylmethyl)triethylborate prepared in Example 1. FIG. 1 is a TG-DTA chart of octyltributylphosphonium (trimethylsilylmethyl)triethylborate produced in Example 1. FIG. 1 is a DSC chart of octyltributylphosphonium(trimethylsilylmethyl)triethylborate produced in Example 1. FIG. 1 is a ¹ H-NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)triethylborate produced in Example 2. FIG. 13 is a ¹³ C-NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)triethylborate prepared in Example 2. FIG. ^11B -NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)triethylborate prepared in Example 2. FIG. 4 is a TG-DTA chart of tetrabutylammonium (trimethylsilylmethyl)triethylborate produced in Example 2. FIG. 1 is a DSC chart of tetrabutylammonium(trimethylsilylmethyl)triethylborate produced in Example 2. FIG. 1 is a ¹ H-NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)tributylborate produced in Example 3. FIG. 13 is a ¹³ C-NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)tributylborate produced in Example 3. FIG. ^11B -NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)tributylborate produced in Example 3. FIG. 1 is a TG-DTA chart of tetrabutylammonium (trimethylsilylmethyl)tributylborate produced in Example 3. FIG. 1 is a DSC chart of tetrabutylammonium(trimethylsilylmethyl)tributylborate produced in Example 3. FIG. 1 is a ¹ H-NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)triisobutylborate produced in Example 4. FIG. 13 is a ¹³ C-NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)triisobutylborate prepared in Example 4. FIG. ^11B -NMR spectrum of tetrabutylammonium(trimethylsilylmethyl)triisobutylborate prepared in Example 4. FIG. 1 is a TG-DTA chart of tetrabutylammonium (trimethylsilylmethyl) triisobutylborate produced in Example 4. FIG. 1 is a DSC chart of tetrabutylammonium(trimethylsilylmethyl)triisobutylborate produced in Example 4. FIG. 1 is a ¹ H-NMR spectrum of tetrabutylphosphonium(trimethylsilylmethyl)triethylborate prepared in Example 5. FIG. 13 is a ¹³ C-NMR spectrum of tetrabutylphosphonium(trimethylsilylmethyl)triethylborate prepared in Example 5. FIG. ^11B -NMR spectrum of tetrabutylphosphonium(trimethylsilylmethyl)triethylborate produced in Example 5. FIG. 4 is a TG-DTA chart of tetrabutylphosphonium(trimethylsilylmethyl)triethylborate produced in Example 5. FIG. 4 is a DSC chart of tetrabutylphosphonium(trimethylsilylmethyl)triethylborate produced in Example 5. FIG.

[Silicon-containing tetraalkylborate]
The silicon-containing tetraalkylborate of the present invention contains an anion represented by the following formula (1) and a cation.

In formula (1), R ¹ to R ³ are each independently an alkyl group having 1 to 4 carbon atoms. The alkyl group may be linear, branched or cyclic, and specific examples thereof include methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl and sec-butyl. group, isobutyl group, tert-butyl group, cyclobutyl group and the like.

Among these, R ¹ to R ³ are preferably C 1-3 alkyl groups, more preferably C 1-3 linear alkyl groups, and still more preferably methyl or ethyl groups. R ¹ to R ³ are all preferably the same group, more preferably all methyl groups or ethyl groups, and even more preferably all methyl groups.

In formula (1), R ⁴ to R ⁶ are alkyl groups having 1 to 8 carbon atoms. The alkyl group may be linear, branched, or cyclic, and specific examples thereof include the aforementioned alkyl groups having 1 to 4 carbon atoms, n-pentyl group, cyclopentyl group, n-hexyl group, cyclohexyl group, n-heptyl group, n-octyl group, cycloheptyl group, cyclooctyl group, isopentyl group and the like.

Among these, R ⁴ to R ⁶ are preferably alkyl groups having 2 to 6 carbon atoms, more preferably alkyl groups having 2 to 4 carbon atoms, such as ethyl group, isopropyl group, n-butyl group and tert-butyl group. is more preferred. Moreover, it is preferable that all of R ⁴ to R ⁶ are the same group.

In formula (1), n is an integer of 1 to 6, preferably 1, 2 or 3.

The cation contained in the silicon-containing tetraalkylborate of the present invention is not particularly limited, but is preferably monovalent. Further, the cation may be an inorganic cation or an organic cation, but an organic cation is preferred.

Examples of the inorganic cations include alkali metal ions such as sodium ions, potassium ions and lithium ions, metal ions such as magnesium ions, silver ions, zinc ions and copper ions.

As the organic cations, phosphorus atom-containing organic cations and nitrogen atom-containing organic cations are preferable, and specific examples include quaternary phosphonium ions, quaternary ammonium ions, imidazolium ions, pyridinium ions, pyrrolidinium ions, piperidinium ions, and the like. is preferred.

As the phosphorus atom-containing organic cation, for example, a quaternary phosphonium ion represented by the following formula (2) is preferable.

In formula (2), R ¹¹ is an alkyl group having 1 to 20 carbon atoms. The alkyl group having 1 to 20 carbon atoms may be linear, branched, or cyclic, and specific examples thereof include the aforementioned alkyl groups having 1 to 8 carbon atoms, n-nonyl groups, n- Decyl group, n-undecyl group, n-dodecyl group, n-tridecyl group, n-tetradecyl group, n-pentadecyl group, n-hexadecyl group, n-heptadecyl group, n-octadecyl group, n-nonadecyl group, n- An eicosyl group and the like can be mentioned.

In formula (2), R ¹² is an alkyl group having 1 to 20 carbon atoms or an alkoxyalkyl group represented by --(CH ₂ ) _k --OR. k is 1 or 2; R is a methyl group or an ethyl group. Examples of the alkyl group having 1 to 20 carbon atoms include those described above. The alkoxyalkyl group includes a methoxymethyl group, an ethoxymethyl group, a methoxyethyl group and an ethoxyethyl group. Among the alkoxyalkyl groups, a methoxymethyl group or a methoxyethyl group is preferable.

Among the quaternary phosphonium ions represented by the formula (2), those in which R ¹² is an alkoxyalkyl group represented by --(CH ₂ ) _k --OR easily form an ionic liquid. When R ¹² is an alkyl group, those having different structures for R ¹¹ and R ¹² tend to form an ionic liquid. In this case, the difference in the number of carbon atoms is preferably 1 or more, more preferably 2 or more, still more preferably 4 or more.

As the nitrogen atom-containing organic cation, for example, one represented by the following formula (3) is preferable.

In formula (3), R ²¹ to R ²⁴ are each independently an alkyl group having 1 to 20 carbon atoms or an alkoxyalkyl group represented by --(CH ₂ ) _k --OR. k is 1 or 2; R is a methyl group or an ethyl group. Examples of the alkyl group and alkoxyalkyl group having 1 to 20 carbon atoms are the same as those described above.

Also, any two of R ²¹ to R ²⁴ may bond with each other to form a ring together with the nitrogen atom to which they bond. Furthermore, any two of R ²¹ to R ²⁴ may be bonded together to form a ring together with the nitrogen atom to which they are bonded, and the remaining two may be bonded together to form a spiro ring having the nitrogen atom as the spiro atom. good. In this case, examples of the ring include an aziridine ring, an azetidine ring, a pyrrolidine ring, a piperidine ring, an azepane ring and the like, but a pyrrolidine ring, a piperidine ring and the like are preferable, and a pyrrolidine ring and the like are more preferable. As the spiro ring, a 1,1'-spirobipyrrolidine ring is particularly preferred.

When all of R ²¹ to R ²⁴ are alkyl groups, at least one of which has a structure different from the others tends to form an ionic liquid. In this case, the difference in carbon number is preferably 1 or more, and more It is preferably 2 or more, more preferably 4 or more.

Specific examples of the nitrogen atom-containing organic cation represented by formula (3) include quaternary ammonium ions represented by the following formula (3-1) or (3-2), the following formula (3-3) or ( Examples include pyrrolidinium ions represented by 3-4).

In formulas (3-1) to (3-4), R and k are the same as above. R ²⁰¹ to R ²⁰⁴ are each independently an alkyl group having 1 to 4 carbon atoms. R ²⁰⁵ and R ²⁰⁶ are each independently an alkyl group having 1 to 4 carbon atoms. In addition, R ²⁰⁵ and R ²⁰⁶ may combine with each other to form a ring together with the nitrogen atom to which they are bonded. Examples of the alkyl group having 1 to 4 carbon atoms include those mentioned above.

As the nitrogen atom-containing organic cation, for example, an imidazolium ion represented by the following formula (4) is also preferable.

In formula (4), R ³¹ and R ³² are each independently an alkyl group having 1 to 20 carbon atoms or an alkoxyalkyl group represented by --(CH ₂ ) _k --OR. R and k are the same as above. Examples of the alkyl group and alkoxyalkyl group having 1 to 20 carbon atoms are the same as those described above. In this case, it is easier to form an ionic liquid when R ³¹ and R ³² are different groups.

As the nitrogen atom-containing organic cation, for example, a pyridinium ion represented by the following formula (5) is also preferable.

In formula (5), R ⁴¹ is an alkyl group having 1 to 8 carbon atoms or an alkoxyalkyl group represented by --(CH ₂ ) _k --OR. R and k are the same as above. Examples of the alkyl group and alkoxyalkyl group having 1 to 8 carbon atoms are the same as those described above.

The silicon-containing tetraalkylborate of the present invention becomes an ionic liquid depending on the type of cation. For example, cations represented by formula (3-4) tend to form ionic liquids, and cations represented by formula (2) with different structures in which R ¹¹ and R ¹² are different form ionic liquids. Prone. In the present invention, the ionic liquid means a salt composed only of ions and having a melting point of 100° C. or lower. The ionic liquid composed of the silicon-containing tetraalkylborate of the present invention preferably has a melting point of room temperature (25° C.) or lower (that is, liquid at room temperature). The ionic liquid composed of the silicon-containing tetraalkylborate of the present invention is halogen-free and thus has a small environmental load.

（式中、Ｒ¹～Ｒ⁶及びｎは、前記と同じ。Ｅ⁺は、１価のカチオンである。Ｘ^-は、ハロゲン化物イオンである。） [Method for producing silicon-containing tetraalkylborate]
The silicon-containing tetraalkylborate of the present invention can be produced, for example, according to Scheme A below.

(In the formula, R ¹ to R ⁶ and n are the same as above. E ⁺ is a monovalent cation. X ^- is a halide ion.)

First, compound (1A) and compound (1B) are reacted to synthesize compound (1C) (first step). Here, when n=1 in the above formula, the compound (1C) is unstable in the air, so the reaction is preferably carried out under an inert gas. Compound (1A) can be synthesized from the corresponding halide by a known method, for example, the method described in Jikken Kagaku Koza (4th edition), Vol. 24 (Maruzen Publishing), Chapter 1.

The reaction in the first step can proceed by mixing a solution of compound (1A) and a solution of compound (1B). At this time, organic solvents such as hexane, benzene, toluene, diethyl ether, and tetrahydrofuran can be used as solvents for the solution.

In the reaction of the first step, the molar ratio of the compound represented by formula (1A) to the compound represented by formula (1B) can be about 5:1 to 1:5. Considering the cost aspect, it is preferable to perform at a ratio close to 1:1.

The reaction in the first step is preferably carried out at 30-90°C, more preferably at 50-70°C. The reaction time is usually about 4 to 8 hours.

The obtained compound (1C) can be recovered by a usual post-treatment and used in the next step.

Next, as a second step, ion exchange reaction between compound (1C) and compound (1D) is carried out. Thereby, a salt containing an anion represented by formula (1) can be obtained.

The ion exchange reaction can be performed, for example, by mixing an aqueous solution of compound (1C) and an aqueous solution of compound (1D). The reaction temperature at this time is preferably 10 to 50° C., more preferably around room temperature (around 25° C.). The reaction time is usually about 1 to 2 hours. When compound (1C) and compound (1D) are mixed, the mixture is not limited to an aqueous solution, and an organic solvent may be used as long as it dissolves both.

In the reaction of the second step, the molar ratio of the compound represented by the formula (1C) to the compound represented by the formula (1D) can be about 5:1 to 1:5. Considering the cost aspect, it is preferable to perform at a ratio close to 1:1.

After completion of the reaction, the desired product can be obtained by performing a normal post-treatment.

The silicon-containing tetraalkylborate of the present invention is an electrolyte solvent for electric storage devices such as electric double layer capacitors, lithium ion capacitors, redox capacitors, lithium secondary batteries, lithium ion secondary batteries, lithium air batteries, and proton polymer batteries. It can also be used as an electrolyte or an additive for electrolytes. The silicon-containing tetraalkylborate of the present invention can also be used as a lubricant. Furthermore, the silicon-containing tetraalkylborate of the present invention can be used as an antistatic agent or plasticizer added to polymer materials such as rubbers and plastics. In addition, since the ionic liquid comprising the silicon-containing tetraalkylborate of the present invention is a halogen-free ionic liquid, it is useful as a green solvent with low environmental load.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples. The analyzers and conditions used in Examples and the like are as follows.
[1] Nuclear magnetic resonance ( ¹ H-NMR, ¹³ C-NMR, ¹¹ B-NMR) spectrometer: ECP-500 manufactured by JEOL Ltd.
Solvent: heavy water, heavy dimethylsulfoxide or heavy chloroform [2] Melting point (Example 1 only)
Equipment: DSC6200 manufactured by Seiko Instruments Inc.
Measurement conditions: Temperature rise 10°C per minute from 20 to 40°C, hold at 40°C for 1 minute, lower temperature from 40 to -100°C by 1°C per minute, hold at -100°C for 1 minute, then -100 to 100°C Measurement was performed under the condition that the temperature was raised by 1°C per minute.
(Examples 2-5)
Apparatus: Thermo plus EVO II DSC manufactured by Rigaku Corporation
Measurement conditions: The temperature was raised from 30 to 100°C by 5°C per minute, maintained at 100°C for 1 minute, and then lowered by 5°C per minute from 100 to 30°C.
[3] Decomposition point device: Thermo plus EVO II TG-DTA manufactured by Rigaku Corporation
Measurement conditions: Measurement was performed in an air atmosphere at a temperature increase of 10°C per minute from room temperature to 500°C, and the temperature at which the weight decreased by 10% was taken as the decomposition point.

[Example 1] Synthesis of octyltributylphosphonium(trimethylsilylmethyl)triethylborate [Example 1-1] Synthesis of lithium(trimethylsilylmethyl)triethylborate All of this experimental operation was performed in a vacuum line or in a glove box. I was careful not to let air get inside. In addition, all the solvents used were freeze-deaerated and dried.
10.0 mL (10.0 mmol) of a hexane solution of triethylborane was placed in an eggplant-shaped Schlenk flask equipped with a magnetic stirrer, and 10.0 mL (9.53 mmol) of a hexane solution of (trimethylsilyl)methyllithium was placed therein under an argon stream at room temperature. ) was added. At this time, the inside of the Schlenk flask instantly became cloudy. This solid was filtered by a line operation, and the obtained solid was washed with hexane and then vacuum-dried to obtain lithium (trimethylsilylmethyl)triethylborate as air-unstable colorless crystals. Yield 1.59 g (8.23 mmol), yield 86.4%.

[Example 1-2] Synthesis of octyltributylphosphonium(trimethylsilylmethyl)triethylborate All of these experimental operations were performed in a vacuum line or in a glove box, and care was taken to prevent air from entering the system. All the solvents used were freeze-degassed.
0.72 g (1.81 mmol) of octyltributylphosphonium bromide and 0.29 g (1.52 mmol) of lithium (trimethylsilylmethyl)(triethyl)borate were placed in separate eggplant-shaped Schlenk flasks, and 5 mL of water was added to each. The octyltributylphosphonium bromide aqueous solution was added to the other eggplant-shaped Schlenk flask using a syringe, and at this time, the inside of the flask instantly became cloudy. After stirring for 1 hour, the inside of the flask separated into two liquid layers. The upper layer was washed twice with water and dried in a vacuum to obtain octyltributylphosphonium(trimethylsilylmethyl)triethylborate as a colorless viscous liquid. Yield 0.42 g (0.85 mmol), yield 55.6%.
The ¹ H-NMR spectrum of the obtained octyltributylphosphonium (trimethylsilylmethyl) triethylborate is shown in FIG. 1, the ¹³ C-NMR spectrum is shown in FIG. 2, the ¹¹ B-NMR spectrum is shown in FIG. 3, and the TG-DTA chart is shown. 4, the DSC chart is shown in FIG.

[Example 2] Synthesis of tetrabutylammonium (trimethylsilylmethyl) triethylborate Tetrabutylammonium (tributylammonium) ( Trimethylsilylmethyl)triethylborate was obtained as a white solid.
The ¹ H-NMR spectrum of the obtained tetrabutylammonium (trimethylsilylmethyl) triethylborate is shown in FIG. 6, the ¹³ C-NMR spectrum is shown in FIG. 7, the ¹¹ B-NMR spectrum is shown in FIG. 8, and the TG-DTA chart is shown. 9, the DSC chart is shown in FIG.

[Example 3] Synthesis of tetrabutylammonium (trimethylsilylmethyl) tributylborate Example 1-1 was repeated except that 10.0 mL (10.0 mmol) of a hexane solution of tributylborane was used instead of the hexane solution of triethylborane. Lithium (trimethylsilylmethyl)tributyl borate was obtained in a similar manner.
Next, Example 1-2 except that tetrabutylammonium bromide was used instead of octyltributylphosphonium bromide, and lithium (trimethylsilylmethyl)tributylborate was used instead of lithium (trimethylsilylmethyl)(triethyl)borate. Tetrabutylammonium(trimethylsilylmethyl)tributylborate was obtained as a white solid in a similar manner.
The ¹ H-NMR spectrum of the obtained tetrabutylammonium (trimethylsilylmethyl) tributylborate is shown in FIG. 11, the ¹³ C-NMR spectrum in FIG. 12, the ¹¹ B-NMR spectrum in FIG. 13, and the TG-DTA chart in FIG. 14, the DSC chart is shown in FIG.

[Example 4] Synthesis of tetrabutylammonium(trimethylsilylmethyl)triisobutylborate A hexane solution of triisobutylborane was used in place of the hexane solution of triethylborane, but the same procedure as in Example 1-1 was repeated to obtain lithium ( Trimethylsilylmethyl)triisobutylborate was obtained.
Next, Example 1-2 except that tetrabutylammonium bromide was used instead of octyltributylphosphonium bromide, and lithium (trimethylsilylmethyl)tributylborate was used instead of lithium (trimethylsilylmethyl)(triethyl)borate. Tetrabutylammonium(trimethylsilylmethyl)triisobutylborate was obtained as a white solid in a similar manner.
The ¹ H-NMR spectrum of the obtained tetrabutylammonium (trimethylsilylmethyl) triisobutylborate is shown in FIG. 16, the ¹³ C-NMR spectrum is shown in FIG. 17, the ¹¹ B-NMR spectrum is shown in FIG. 18, and the TG-DTA chart is shown. FIG. 19 shows a DSC chart in FIG.

[Example 5] Synthesis of tetrabutylphosphonium (trimethylsilylmethyl) triethylborate Tetrabutylphosphonium (trimethylsilylmethyl) triethylborate was prepared in the same manner as in Example 1-2, except that tetrabutylphosphonium bromide was used instead of octyltributylphosphonium bromide. Trimethylsilylmethyl)triethylborate was obtained as a white solid.
The ¹ H-NMR spectrum of the obtained tetrabutylphosphonium (trimethylsilylmethyl) triethylborate is shown in FIG. 21, the ¹³ C-NMR spectrum is shown in FIG. 22, the ¹¹ B-NMR spectrum is shown in FIG. 24 and the DSC chart is shown in FIG.

Table 1 shows the melting point and decomposition point of each silicon-containing tetraalkylborate obtained by DSC and TG-DTA measurements.

Claims (9)

Anion represented by the following formula (1) and a phosphorus atom-containing organic cation represented by the following formula (2) or a nitrogen atom-containing organic cation represented by the following formula (3), and having a melting point of 100 ° C. or less A silicon-containing tetraalkylborate that is an ionic liquid .

(In the formula, R ¹ to R ³ are all methyl groups. R ⁴ to R ⁶ are the same alkyl group having 2 to 4 carbon atoms. n is an integer of 1 to 6.)

(In the formula, R ¹¹ is an alkyl group having 1 to 20 carbon atoms. R ¹² is an alkyl group having 1 to 20 carbon atoms or an alkoxyalkyl group represented by —(CH ₂ ) _k —OR. k is 1 or 2. R is a methyl group or an ethyl group.)

(wherein R ²¹ to R ²⁴ are each independently an alkyl group having 1 to 20 carbon atoms or an alkoxyalkyl group represented by —(CH ₂ ) _k —OR; k is 1 or 2; R is a methyl group or an ethyl group, and any two of R ²¹ to R ²⁴ may combine with each other to form a ring together with the nitrogen atom to which they are ^bonded . Any two of R ²⁴ may be bonded together to form a ring together with the nitrogen atoms to which they are bonded, and the remaining two may also be bonded together to form a spiro ring having the nitrogen atom as the spiro atom.) 2. The salt according to claim 1, wherein the cation is a phosphorus atom-containing organic cation represented by formula (2). 3. The salt according to claim ² , wherein ^R12 is an alkyl group, R11 and ^R12 have different structures, and the difference in number of carbon atoms is 4 or more. 2. The salt according to claim 1, wherein the cation is a nitrogen atom-containing organic cation represented by formula (3). The nitrogen atom-containing organic cation is a quaternary ammonium ion represented by the following formula (3-1) or (3-2), or a pyrrolidinium ion represented by the following formula (3-3) or (3-4) 5. The salt of claim 4, wherein

(Wherein, R and k are the same as above. R ²⁰¹ ～R ²⁰⁴ each independently represents an alkyl group having 1 to 4 carbon atoms. R. ²⁰⁵ and R ²⁰⁶ each independently represents an alkyl group having 1 to 4 carbon atoms. Also, R ²⁰⁵ and R ²⁰⁶ may bond with each other to form a ring with the nitrogen atom to which they are bonded. ) A salt according to any one of claims 1 to 5 , wherein n is 1, 2 or 3. The salt according to claim 1 , which is an ionic liquid having a melting point of 25°C or lower. A salt according to claim 1, wherein the cation is an octyltributylphosphonium cation, a tetrabutylphosphonium cation or a tetrabutylammonium cation. Octyltributylphosphonium (trimethylsilylmethyl) triethylborate, tetrabutylammonium (trimethylsilylmethyl) triethylborate, tetrabutylammonium (trimethylsilylmethyl) tributylborate, tetrabutylammonium (trimethylsilylmethyl) triisobutylborate, or tetrabutylphosphonium ( A salt according to claim 1 which is trimethylsilylmethyl)triethylborate.

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