Synthesis of Disubstituted Carboxonium Derivatives of Closo-Decaborate Anion [2,6-B10H8O2CC6H5]−: Theoretical and Experimental Study

A comprehensive study focused on the preparation of disubstituted carboxonium derivatives of closo-decaborate anion [2,6-B10H8O2CC6H5]− was carried out. The proposed synthesis of the target product was based on the interaction between the anion [B10H11]− and benzoic acid C6H5COOH. It was shown that the formation of this product proceeds stepwise through the formation of a mono-substituted product [B10H9OC(OH)C6H5]−. In addition, an alternative one-step approach for obtaining the target derivative is postulated. The structure of tetrabutylammonium salts of carboxonium derivative ((C4H9)4N)[2,6-B10H8O2CC6H5] was established with the help of X-ray structure analysis. The reaction pathway for the formation of [2,6-B10H8O2CC6H5]− was investigated with the help of density functional theory (DFT) calculations. This process has an electrophile induced nucleophilic substitution (EINS) mechanism, and intermediate anionic species play a key role. Such intermediates have a structure in which one boron atom coordinates two hydrogen atoms. The regioselectivity for the process of formation for the 2,6-isomer was also proved by theoretical calculations. Generally, in the experimental part, the simple and available approach for producing disubstituted carboxonium derivative was introduced, and the mechanism of this process was investigated with the help of theoretical calculations. The proposed approach can be applicable for the preparation of a wide range of disubstituted derivatives of closo-borate anions.


Introduction
Molecular modelling enables the study of the main features of chemical compounds in a clear and simple way [1][2][3][4][5]. Geometric parameters, chemical bonding, atomic charges, electrostatic potential and many other phenomena can be analyzed quickly and with great accuracy [6][7][8][9][10]. In addition, many methods have been developed for the investigation of the reactivity of molecules and reaction mechanisms [11][12][13]. Utilizing the methods of quantum chemistry, it is possible to investigate potential energy surface and study intermediates and transition states of various reactions [14][15][16]. Methods such as QTAIM (Quantum Theory of Atoms in Molecules), NBO (Natural Bond Orbitals) and ELF (Electron Localization Function) enable the definition of peculiarities in the electronic structure of key intermediates and transition states [17][18][19][20][21]. Conceptual DFT enables an evaluation of the electrophile/nucleophile nature of a chemical system through merely estimating the energy of HOMO/LUMO orbitals [22][23][24][25][26]. Thus, molecular modelling is a powerful tool for examining the chemical transformation of molecules.  [60], but, during this work, some synthetic protocols were improved. Synthesis of [B 10 H 8 O 2 CCH 3 ] − was based on the interaction of [B 10 H 11 ] − and acetic acid CH 3 COOH. Acetic acid was chosen as a solvent for synthesis performing. In this case, such a synthetic protocol is not possible because of the high melting temperature of benzoic acid C 6 H 5 COOH. Thus, several approaches for the preparation of this derivative were carried out.
The first synthetic route was based on interactions between [B 10 H 11 ] − and C 6 H 5 COOH. This process occurred in a dichloromethane CH 2 Cl 2 solution at boiling point temperature (41 • C). In the first stage of this process, mono-substituted derivatives of the general form [B 10 H 9 OC(OH)C 6 H 5 ] − were obtained. Progress of the process was monitored using a 11 B NMR spectra. The spectra of mono-substituted derivatives are identical to those of [B 10 H 9 OC(OH)CH 3 ] − . The signal of one in an apical position lay at 2.3 ppm. The signal from another apical position appeared as a broadband signal in the range of 0.3 to −4.6 ppm. This broadening of the signal can be connected with the intramolecular interaction between a hydrogen atom of the boron cluster and a proton from the hydroxy group of organic moieties. The signal from the substituted position lay at −7.7 ppm. Signals from equatorial boron atoms lay at −23.8, −24.8 and −29.7 ppm. The isolation and purification of mono-substituted derivative is a complicated task, due to its thermal instability. Thus, it is possible to use monosubstituted derivative without additional manipulation; the evaporation of dichloromethane solution and washing with ether are enough.
At the next stage, the heating of mono-substituted derivative [B 10 H 9 OC(OH)C 6 H 5 ] − led to intramolecular cyclisation when [B 10 H 8 O 2 CC 6 H 5 ] − was obtained. It is noteworthy that the cyclisation process was conducted in the absence of solvent. For this process, a rotary evaporator was used with an air bath oven. 11 3 ] − was carried out in the authors' previous work [61]. The 11 B NMR spectra of [B 10 H 8 O 2 CC 6 H 5 ] − had four signals: a signal at 0.0 ppm with integral intensities I = 2, corresponding to the boron atoms from equivalent substituted positions (B2, B6), a signal at −7.1 with integral intensities I = 2, corresponding to boron atoms from equivalent apical positions (B1, B10), signals from equatorial positions appearing at −17.6 (I = 2, B3, B9) and a signal at −30.0 (I = 4, B4, B5, B7, B8).
The process of obtaining [B 10 H 8 O 2 CC 6 H 5 ] − can be performed in one stage, without the separation and purification of [B 10 H 9 OC(OH)C 6 H 5 ] − . In this case, the reaction between [B 10 H 11 ] − and C 6 H 5 COOH was carried out in an autoclave at 70 • C. Dichloromethane CH 2 Cl 2 was also chosen as a solvent. The main advantage of a one-pot preparation of the target disubstituted product is the significant decrease in the total time required for the synthesis, compared with the two-stage approach. The overall yield of the target product is similar to the two-stage route.
The crystal structure of ((C 4 H 9 ) 4 N)[B 10 H 8 O 2 CC 6 H 5 ] salt was established using an X-ray diffraction experiment (Figure 1). The B 2 O 2 C cycle has an almost planar conformation, with torsion angles lying within the interval of 2 to 6 • . The B-O bond lengths were in the range of 1.519 to 1.522 Å.  Several approaches to obtaining the target carboxonium derivative of the general form [B10H8O2CC6H5] − have been proposed (Scheme 1, for more details also see Scheme S1). It has been shown that the formation of this product proceeds stepwise, through the formation of a mono-substituted product [B10H9OC(OR)C6H5] − , R = H, C2H5. Formation of monosubstituted derivative was confirmed with the help of 11 B NMR spectroscopy. However, it is possible to avoid the mono derivative preparation stage and obtain the target product [B10H8O2CC6H5] − in one step.

Reaction Mechanism Investigation Based on DFT Calculations
The process of interaction between [B10H11] − and C6H5COOH was studied using DFT calculations. ωB97X-D3 was chosen as DFT functional. The given level of theory is that it is appropriate for a wide range of theoretical calculation issues such as atomic charges, chemical reactivity and covalent and non-covalent interactions [62][63][64]. In addition, the authors successfully used this method for the investigation of closo-borate structures, reactivity and NMR properties [46,61,65]. This mechanism study was conducted to identify the main stages of the process. It is necessary to find out the reason of regioselectivity of formation of disubstituted product. All calculations were carried out with dichlomethane CH2Cl2 as the solvent. The process of carboxonium derivative formation is related to the EINS process (Electrophile Induced Nucleophilic Substitution). It is well known that the initial step in the EINS process is the elimination of H2 from the [B10H11] − anion, with the formation of [B10H9] − intermediate. The starting point was the calculation of the orientation complex between [B10H11] − and C6H5COOH. The structure of this orientation complex is presented below (Figure 2). Several approaches to obtaining the target carboxonium derivative of the general form [B 10 H 8 O 2 CC 6 H 5 ] − have been proposed (Scheme 1, for more details also see Scheme S1). It has been shown that the formation of this product proceeds stepwise, through the formation of a mono-substituted product [B 10 H 9 OC(OR)C 6 H 5 ] − , R = H, C 2 H 5 . Formation of monosubstituted derivative was confirmed with the help of 11 B NMR spectroscopy. However, it is possible to avoid the mono derivative preparation stage and obtain the target product [B 10 H 8 O 2 CC 6 H 5 ] − in one step. Several approaches to obtaining the target carboxonium derivative of the general form [B10H8O2CC6H5] − have been proposed (Scheme 1, for more details also see Scheme S1). It has been shown that the formation of this product proceeds stepwise, through the formation of a mono-substituted product [B10H9OC(OR)C6H5] − , R = H, C2H5. Formation of monosubstituted derivative was confirmed with the help of 11 B NMR spectroscopy. However, it is possible to avoid the mono derivative preparation stage and obtain the target product [B10H8O2CC6H5] − in one step.

Reaction Mechanism Investigation Based on DFT Calculations
The process of interaction between [B10H11] − and C6H5COOH was studied using DFT calculations. ωB97X-D3 was chosen as DFT functional. The given level of theory is that it is appropriate for a wide range of theoretical calculation issues such as atomic charges, chemical reactivity and covalent and non-covalent interactions [62][63][64]. In addition, the authors successfully used this method for the investigation of closo-borate structures, reactivity and NMR properties [46,61,65]. This mechanism study was conducted to identify the main stages of the process. It is necessary to find out the reason of regioselectivity of formation of disubstituted product. All calculations were carried out with dichlomethane CH2Cl2 as the solvent. The process of carboxonium derivative formation is related to the EINS process (Electrophile Induced Nucleophilic Substitution). It is well known that the initial step in the EINS process is the elimination of H2 from the

Reaction Mechanism Investigation Based on DFT Calculations
The process of interaction between [B 10 H 11 ] − and C 6 H 5 COOH was studied using DFT calculations. ωB97X-D3 was chosen as DFT functional. The given level of theory is that it is appropriate for a wide range of theoretical calculation issues such as atomic charges, chemical reactivity and covalent and non-covalent interactions [62][63][64]. In addition, the authors successfully used this method for the investigation of closo-borate structures, reactivity and NMR properties [46,61,65]. This mechanism study was conducted to identify the main stages of the process. It is necessary to find out the reason of regioselectivity of formation of disubstituted product. All calculations were carried out with dichlomethane CH 2 Cl 2 as the solvent. The process of carboxonium derivative formation is related to the EINS process (Electrophile Induced Nucleophilic Substitution). It is well known that the initial step in the EINS process is the elimination of H 2 from the [B 10  However, the formation of the complex between [B10H11] − and C6H5COOH is an endergonic process. It can be concluded that, in the first stages of proton migration, the C6H5COOH molecule does not take part. Thus, initially, H fac migrates to the equatorial boron atom with the formation of a [B10H9(H2)] − anion. The formation of [B10H9(H2)] − occurred through the formation of a transition state. This transition state has a structure in which one boron atom coordinates with dihydrogen, H2. The distance between the boron atom and the hydrogen atom was equal to 1.35 Å, the distance between the hydrogen atoms in the H2-fragment was equal to 0.86 Å and the energy barrier of the transition state was equal to 59 kJ/mol. The overall process of proton migration was endergonic, and Gibbs energy of the isomerization reaction was 30 kJ/mol ( Figure 3). This anion, as in the case of the transition state, had a structure in which one boron atom coordinated with the molecule of dihydrogen, H2. The distance between the boron atom and the hydrogen atom was equal to 1.30 Å, while the distance between the hydrogen atoms in the H2-fragment was equal to 0.86 Å. Thus, compared with the transition state structure, the B-H bond length increased, and the H-H contact length decreased.  However, the formation of the complex between [B 10 H 11 ] − and C 6 H 5 COOH is an endergonic process. It can be concluded that, in the first stages of proton migration, the C 6 H 5 COOH molecule does not take part. Thus, initially, H fac migrates to the equatorial boron atom with the formation of a [B 10 H 9 (H 2 )] − anion. The formation of [B 10 H 9 (H 2 )] − occurred through the formation of a transition state. This transition state has a structure in which one boron atom coordinates with dihydrogen, H 2 . The distance between the boron atom and the hydrogen atom was equal to 1.35 Å, the distance between the hydrogen atoms in the H 2 -fragment was equal to 0.86 Å and the energy barrier of the transition state was equal to 59 kJ/mol. The overall process of proton migration was endergonic, and Gibbs energy of the isomerization reaction was 30 kJ/mol ( Figure 3). This anion, as in the case of the transition state, had a structure in which one boron atom coordinated with the molecule of dihydrogen, H 2 . The distance between the boron atom and the hydrogen atom was equal to 1.30 Å, while the distance between the hydrogen atoms in the H 2 -fragment was equal to 0.86 Å. Thus, compared with the transition state structure, the B-H bond length increased, and the H-H contact length decreased. However, the formation of the complex between [B10H11] − and C6H5COOH is an endergonic process. It can be concluded that, in the first stages of proton migration, the C6H5COOH molecule does not take part. Thus, initially, H fac migrates to the equatorial boron atom with the formation of a [B10H9(H2)] − anion. The formation of [B10H9(H2)] − occurred through the formation of a transition state. This transition state has a structure in which one boron atom coordinates with dihydrogen, H2. The distance between the boron atom and the hydrogen atom was equal to 1.35 Å, the distance between the hydrogen atoms in the H2-fragment was equal to 0.86 Å and the energy barrier of the transition state was equal to 59 kJ/mol. The overall process of proton migration was endergonic, and Gibbs energy of the isomerization reaction was 30 kJ/mol ( Figure 3). This anion, as in the case of the transition state, had a structure in which one boron atom coordinated with the molecule of dihydrogen, H2. The distance between the boron atom and the hydrogen atom was equal to 1.30 Å, while the distance between the hydrogen atoms in the H2-fragment was equal to 0.86 Å. Thus, compared with the transition state structure, the B-H bond length increased, and the H-H contact length decreased.  analyzed. The value of ρ (r) was equal to 0.126 e Å −3 , the Laplacian of electron density was equal to 0.350 e Å −5 , the total energy at the bcp was equal to −0.097 h e −1 and the delocalization index was equal to 0.342. In the contour line map of the Laplacian of electron density for the B-(H 2 ) fragment, the charge delocalization between boron atom and two hydrogen atoms was observed.
Molecules 2023, 28, x FOR PEER REVIEW 6 of 15 graph of electron density distribution. The main descriptors of electron density at the bond critical point (bcp) corresponding to the interaction between the boron atom and H2 were analyzed. The value of ρ(r) was equal to 0.126 e Å −3 , the Laplacian of electron density was equal to 0.350 e Å −5 , the total energy at the bcp was equal to −0.097 h e −1 and the delocalization index was equal to 0.342. In the contour line map of the Laplacian of electron density for the B-(H2) fragment, the charge delocalization between boron atom and two hydrogen atoms was observed. The structure of [B10H9OC(OH)C6H5] − was analyzed ( Figure 5). Experimentally, the structure of this particle was established only by 11 B-NMR spectroscopy data. Using theoretical modeling, the data on the structure of this derivative have been supplemented. The organic substituent can rotate freely, relative to the cluster fragment. The rotation barrier of the organic fragment was ~8 kJ/mol. The two lowest energy isomers were localized on the potential energy surface, and the difference in Gibbs energy between them was 3 kJ/mol. The structures of these isomers were stabilized with the formation of an intramolecular dihydrogen bond between the proton of the hydroxy group of the organic substituent and hydrogen from the boron cluster. The B-O bond length was equal to 1.50 Å. The bond length of the C = O bond was equal to 1.25 Å, which was longer than in the case of initial benzoic acid (1.20 Å), whereas the bond length of the C-O bond was equal to 1.29, which was shorter than that of benzoic acid (1.33 Å). The structure of [B 10 H 9 OC(OH)C 6 H 5 ] − was analyzed ( Figure 5). Experimentally, the structure of this particle was established only by 11 B-NMR spectroscopy data. Using theoretical modeling, the data on the structure of this derivative have been supplemented. The organic substituent can rotate freely, relative to the cluster fragment. The rotation barrier of the organic fragment was~8 kJ/mol. The two lowest energy isomers were localized on the potential energy surface, and the difference in Gibbs energy between them was 3 kJ/mol. The structures of these isomers were stabilized with the formation of an intramolecular dihydrogen bond between the proton of the hydroxy group of the organic substituent and hydrogen from the boron cluster. The B-O bond length was equal to 1.50 Å. The bond length of the C = O bond was equal to 1.25 Å, which was longer than in the case of initial benzoic acid (1.20 Å), whereas the bond length of the C-O bond was equal to 1.29, which was shorter than that of benzoic acid (1.33 Å).
At the next step, the proton from the hydroxy group migrated to the boron cluster. This process occurred without the formation of a transition state. [B 10 H 9 OC(O)C 6 H 5 *H fac ] − was formed. The process of proton migration from the oxygen atom to the cluster cage was endergonic. As in the case of [B 10 H 9 OC(OH)C 6 H 5 ] − , the given anion can have different isomers. The proton atom can be localized on different shapes of the boron cluster. The difference between two isomers is less than 1 kJ/mol based on the DFT calculations at the ωB97X-D3/def2-TZVPP level of theory (see Section 3.5. Computational details). Previously, the authors investigated an analogous protonated structure, for [B 10   has several isomers. The structure of two of them is represented below. The structure with the boron atom bonded to H2, located in the same equatorial belt as exo-polyhedral substituent, had lower Gibbs energy than the structure where the analogous boron atom was located in the opposite equatorial belt to the exo-polyhedral substituent. As in the case of [B10H9(H2)] − , one boron atom coordinates the molecule of hydrogen, H2. The distance between the boron atom and the hydrogen atom was equal to 1.30 Å. The distance between two hydrogen atoms was equal to 0.87 Å.       (Figure 7). This anion had two isomers. The first isomer [2,6- Then, as in case of [B10H9(H2)] − , the H2 eliminated. For the [B10H8OC(O)C6H5(H2)] − isomer, in which the boron atom was bonded to H2 in the same equatorial belt as the exopolyhedral substituent, the [B10H8OC(O)C6H5] − molecular specie was localized on the potential energy surface. In the case of the isomer, in which the boron atom was in the opposite equatorial belt to the exo-polyhedral substituent, [B10H8OC(O)C6H5] − was not localized, because the process of geometry optimization led to disubstituted anion [2,6-B10H8O2CC6H5] − .
Finally, the process of intramolecular cyclisation occurred, and [B10H8O2CC6H5] − was formed (Figure 7). This anion had two isomers. The first isomer [2,6-B10H8O2CC6H5] − had substituted boron atoms in different equatorial belts, whereas the second isomer [2,3-B10H8O2CC6H5] − had two substituted boron atoms in one equatorial belt. The first isomer [2,6-B10H8O2CC6H5] − had significantly lower Gibbs energy than the second isomer [2,3-B10H8O2CC6H5] − (the difference between two isomers is equal to 97.4 kJ/mol). This fact is the reason of high regioselectivity for the formation process of the disubstituted carboxonium derivative. The main geometric parameters of [2,6-B10H8O2CC6H5] − were considered. The B-O bond lengths were equal to 1.535 Å. The C-O bond lengths were equal to 1.269 Å. Obtained parameters were similar to those previously obtained for the [2,6-B10H8O2CCH3] − anion. Thus, the reaction pathway for the formation of [2,6-B10H8O2CC6H5] − was investigated (Scheme 2). This process was based on the EINS mechanism and, as in the case of a previously described analogous process, the key role was played by intermediate anion species in which one boron atom coordinates two hydrogen atoms. The process of [2,6-B10H8O2CC6H5] − formation started with hydrogen migration and the formation of [B10H9(H2)] − . Then the H2 molecule was eliminated, and [B10H9] − was formed. The process of H2 elimination is endergonic, and the ∆G of this process is equal to 49 kJ/mol. Next, [B10H9] − interacted with C6H5COOH and mono-substituted derivative [B10H9OC(OH)C6H5] − was formed. This process is quite exergonic and the ∆G of this

IR Spectra
The IR spectra of prepared compounds were recorded on an Infralyum FT 02 Fourier transform spectrometer (Lumex Instruments Research and Production Company, Vancouver, BC, Canada) in the region of 4000 to 300 cm −1 and with a resolution of 1 cm −1 . Samples were prepared as dichloromethane CH2Cl2 solution.

NMR Spectra
The NMR ( 1 H, 11B, 13C) spectra of solutions of the studied compounds in CD3CN were recorded on a Bruker (Billerica, MA, USA) Avance II 300 spectrometer operating at 300.3, 96.32 and 75.49 MHz, respectively, using an internal deuterium lock. Tetramethylsilane and Boron trifluoride etherate were used as external references.

Electrospray Ionisation Mass Spectrometry (ESI-MS)
The LC system consisted of two LC-20AD pumps (Shimadzu, Kyoto, Japan), and an autosampler was coupled online with an LCMS-IT-TOF mass spectrometer equipped with an electrospray ionization source (Shimadzu, Kyoto, Japan). The HRMS spectra were acquired in direct injection mode without column. The samples were prepared as CH3CN

IR Spectra
The IR spectra of prepared compounds were recorded on an Infralyum FT 02 Fourier transform spectrometer (Lumex Instruments Research and Production Company, Vancouver, BC, Canada) in the region of 4000 to 300 cm −1 and with a resolution of 1 cm −1 . Samples were prepared as dichloromethane CH 2 Cl 2 solution.

NMR Spectra
The NMR ( 1 H, 11B, 13C) spectra of solutions of the studied compounds in CD 3 CN were recorded on a Bruker (Billerica, MA, USA) Avance II 300 spectrometer operating at 300.3, 96.32 and 75.49 MHz, respectively, using an internal deuterium lock. Tetramethylsilane and Boron trifluoride etherate were used as external references.

Electrospray Ionisation Mass Spectrometry (ESI-MS)
The LC system consisted of two LC-20AD pumps (Shimadzu, Kyoto, Japan), and an autosampler was coupled online with an LCMS-IT-TOF mass spectrometer equipped with an electrospray ionization source (Shimadzu, Kyoto, Japan). The HRMS spectra were acquired in direct injection mode without column. The samples were prepared as CH3CN solutions. Detection parameters: Detector Voltage 1.55 kV; Nebulising Gas 1.50 L/min; CDL Temperature 200.0 • C.

X-ray Crystal Structure Determination
The single-crystal X-ray diffraction data for X were collected on a three-circle Bruker D8 Venture diffractometer using ϕ and ω scan mode. The data were indexed and integrated using the SAINT program (V7.60A) [66] and then scaled and corrected for absorption using the SADABS program (version 2008/1) [67]. For details, see Table S1. The structures were determined by direct methods and refined by the full-matrix least squares technique on F2 with anisotropic displacement parameters for non-hydrogen atoms. The hydrogen atoms in all compounds were placed in calculated positions and refined within the riding model with fixed isotropic displacement parameters [Uiso(H) = 1.5Ueq(C) for the CH3-groups and 1.2Ueq(C) for the other groups]. All calculations were carried out using the SHELXTL program (version 2018/2) [68] and OLEX2 program package (version 1.5) [69].
Crystallographic data for all investigated compounds have been deposited with the Cambridge Crystallographic Data Centre, CCDC 2,227,408. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CHB2 1EZ, UK (Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www.ccdc.cam.ac.uk).

Computational Details
A complete geometry optimization of all model structures was performed at the ωB97X-D3/def2-TZVPP theory level using the ORCA 4.2.1 software package (the atompairwise dispersion correction with the zero-damping scheme was utilized) [70][71][72]. All calculations were performed using the RIJCOSX approximation with the def2/J auxiliary basis set [73]. Tight criteria of SCF convergence ("Tight SCF") were employed for the calculations. The keywords "Grid5", "FinalGrid6" and "GridX5" were used as parameters for the spatial integration grid. For the model structures with closed electron shells, spinrestricted approximation was utilized. Symmetry operations were not applied during the geometry optimization procedure for all model structures. The Hessian matrices were calculated numerically for all optimized model structures in order to prove the location of correct minima on the potential energy surfaces (no imaginary frequencies for all reactants, intermediates, and final products; only one imaginary frequency for transition states). The connectivity of each reaction step was also confirmed using the intrinsic reaction coordinate (IRC) calculations from the transition states [74][75][76]. Solvent effects were considered using the Solvation Model based on Density (SMD) [77]. The natural bond orbital (NBO) method was employed, using the NBO7 program package [78,79]. Topological analysis of the electron density distribution, based on the Quantum Theory of Atoms in Molecules (QTAIM) formalism developed by Bader [80,81], was employed with the Multiwfn program (version 3.7) [82]. The Cartesian atomic coordinates for all optimized equilibrium model structures are presented in the Supplementary Materials as the xyz-files. Visualization of the optimized structures was carried out with the help of ChemCraft program version 1.7 [83].

Materials
Solvents of reagent and special purity grades Sigma-Aldrich (Burlington, MA, USA) and Panreac (Darmstadt, Germany) (99.7%), were used without any additional purification. were dissolved in dichloromethane CH 2 Cl 2 (10 mL). The resulting mixture was heated in an atmosphere of dry argon at 40 to 43 • C for 12 h. After completion of the reaction process, the solution was cooled to room temperature and the solvent was evaporated using a rotary pump vacuum. The residue was washed with Et 2 O (2 × 10 mL) and heated in a vacuum drying oven at 105 • C for 4 h. The product was purified by column chromatography on silica gel, eluting with CHCl 3 /CH 3 CN mixture (9:1). The yield was 0.   (10 mL). The resulting mixture was heated in an autoclave at 75 • C for 4 h. After completion of the reaction process, the solution was cooled to room temperature and the solvent was evaporated using a rotary pump vacuum. The residue was washed with Et 2 O (2 × 10 mL) and purified by column chromatography on silica gel, eluting with CHCl 3 /CH 3 CN mixture (9:1). The yield was 0.255 g (77%) ((C 4 H 9 ) 4 N)[2,6-B 10 H 8 O 2 CC 6 H 5 ].

Conclusions
The process of anion [2,6-B 10 H 8 O 2 CC 6 H 5 ] − preparation was considered both theoretically and experimentally. This process featured the EINS mechanism. The synthesis of the target product was based on the interaction between [B 10 H 11 ] − and benzoic acid C 6 H 5 COOH. The formation of this product proceeded stepwise through the formation of a mono-substituted product [B 10 H 9 OC(OH)C 6 H 5 ] − . The proposed approach is characterized by a simple apparatus configuration and good yields of final products. This approach can be used for synthesis of disubstituted carboxonium derivative with various natures. Using DFT calculations, the main stages of obtaining [2,6-B 10