New Aspects of the Synthesis of closo -Dodecaborate Nitrilium Derivatives [B 12 H 11 NCR] − (R = n -C 3 H 7 , i -C 3 H 7 , 4-C 6 H 4 CH 3 , 1-C 10 H 7 ): Experimental and Theoretical Studies

: The preparation of novel nitrilium derivatives of closo -dodecaborate anion [B 12 H 11 NCR] − , R = n -C 3 H 7 , i -C 3 H 7 , 4-C 6 H 4 CH 3 , 1-C 10 H 7 is described. Target compounds were obtained in good yields (up to 73%). The synthesis of target borylated nitrilium derivatives was characterised by the simplicity of the chemical apparatus and the absence of the necessity for the puriﬁcation of desired compounds. The crystal structures of previously obtained [B 12 H 11 NCCH 3 ] − and novel [B 12 H 11 NCC 3 H 7 ] − were established with the help of X-ray structure analysis. DFT-analysis of several nitrilium derivatives [B 12 H 11 NCR] − , R = CH 3 , C 3 H 7 , 4-CH 3 C 6 H 4 was carried out. The main peculiarities of the C ≡ N bond of the exo -polyhedral substituent were revealed in terms of bond lengths, bond orders and atomic charges. The LUMO orbitals of the systems considered were examined for understanding of the electrophilic nature of the nitrilium derivatives of the closo -dodecaborate anion.

Nitrilium derivatives are another important class of cluster systems with exo-polyhedral B-N bonds.Nitrilium derivatives of boron clusters contain a polarised N≡C bond and have similar chemical properties to transition metal complex compounds, with nitrile ligands [31][32][33].Nitrilium derivatives readily enter nucleophilic addition reactions.This peculiarity enables nitrilium derivatives to be applied as a starting point for obtaining different borylated systems.Various borylated imine amides, iminols, imidates, amidines, including those based on amino acids, and oligopeptides have been obtained on the basis of nitrilium derivatives [34,35].
In several works, nitrilium derivatives of closo-dodecaborate anions were prepared [B 12 H 11 NCR] − , R = CH 3 , CH 2 CH 3 [40,52] and different reaction protocols were applied.In paper [52], the most optimal method was established.This method was based on obtaining the target substances in a glass autoclave at an oil bath temperature above 150 • C. The CF 3 COOH was used as an electrophilic inducer.In the present work, this approach was extended to obtaining nitrile complexes [B 12 H 11 The main goal of the current research was to establish the best conditions for the obtaining derivatives, with both aliphatic and aromatic substituents.In addition, theoretical investigation was devoted to the impact of boron clusters on the N≡C bond nature in target derivatives.

Synthesis of closo-Dodecaborate Nitrilium Derivatives
As previously stated, the main goal of the present research was to establish the best conditions for the preparation of derivatives with both aliphatic and aromatic substituents.
The approach used was one that had been employed by the authors in previous research [52].This process involved obtaining the target substances in a glass autoclave at an oil bath temperature above 150 • C. The CF 3 COOH was used as an electrophilic inducer (Scheme 1).As in the case of the acetonitrile derivative, the complete conversion of the initial salt of closo-dodecaborate anion with tetrabutylammonium cations ((n-C 4 H 9 ) 4 N) + occurred in about 30 min.Increasing the reaction time had a negative effect on the yield of the target nitrile derivative due to formation of additional by-products such as carboxylic derivatives of closo-dodecaborate anion.
Nitrilium derivatives are another important class of cluster systems with exo-polyhedral B-N bonds.Nitrilium derivatives of boron clusters contain a polarised N≡C bond and have similar chemical properties to transition metal complex compounds, with nitrile ligands [31][32][33].Nitrilium derivatives readily enter nucleophilic addition reactions.This peculiarity enables nitrilium derivatives to be applied as a starting point for obtaining different borylated systems.Various borylated imine amides, iminols, imidates, amidines, including those based on amino acids, and oligopeptides have been obtained on the basis of nitrilium derivatives [34,35].
In several works, nitrilium derivatives of closo-dodecaborate anions were prepared [B12H11NCR] − , R = CH3, CH2CH3 [40,52] and different reaction protocols were applied.In paper [52], the most optimal method was established.This method was based on obtaining the target substances in a glass autoclave at an oil bath temperature above 150 °C.The CF3COOH was used as an electrophilic inducer.In the present work, this approach was extended to obtaining nitrile complexes [B12H11NCR] − , R = n-C3H7, i-C3H7, 4-C6H4CH3, 1-C10H7.The main goal of the current research was to establish the best conditions for the obtaining derivatives, with both aliphatic and aromatic substituents.In addition, theoretical investigation was devoted to the impact of boron clusters on the N≡C bond nature in target derivatives.

Synthesis of closo-Dodecaborate Nitrilium Derivatives
As previously stated, the main goal of the present research was to establish the best conditions for the preparation of derivatives with both aliphatic and aromatic substituents.The approach used was one that had been employed by the authors in previous research [52].This process involved obtaining the target substances in a glass autoclave at an oil bath temperature above 150 °C.The CF3COOH was used as an electrophilic inducer (Scheme 1).As in the case of the acetonitrile derivative, the complete conversion of the initial salt of closo-dodecaborate anion with tetrabutylammonium cations ((n-C4H9)4N) + occurred in about 30 min.Increasing the reaction time had a negative effect on the yield of the target nitrile derivative due to formation of additional by-products such as carboxylic derivatives of closo-dodecaborate anion.A signal from the substituted boron atom lay in the range −12.0-− 12.3  NCR] − , R = C 10 H 7 the low yields is additionally related to the reduced nucleophilic capacity of C 7 H 10 CN compared with other nitriles.The yields of the target products were at an acceptable level and losses of target products were compensated by the ease of their isolation.
Apart from 11 B{H} NMR, the target closo-dodecaborate derivatives were characterised using 1 H and 13 C{H} NMR, IR and ESI MS methods.In 1 H NMR spectra, signals from the tetrabutylammonium cation and a signal from the side R group of nitrilium derivatives were observed.
In 13 C{H} NMR spectra, tetrabutylammonium cation signals were observed at 59.4, 24.4,20.2 and 13.9 ppm for (N(n-C 4 H 9 ) 4 ) + .Signals from carbon atoms of the nitrilium group were in the interval 115.The target derivatives can be obtained without complex purification procedures.All derivatives obtained are stable in air and can be stored without specific conditions (such as an argon atmosphere or a vacuum).Over time, these compounds experience no more than slight hydrolysis processes of the nitrile group.These circumstances are additional advantages of utilising [B 12 H 11 NCR] − as potential molecular species for various applications.

X-ray Analysis
The crystals for X-ray diffraction experiments were obtained by isothermal evaporation of acetonitrile solutions of the corresponding derivative, in the presence of ( In the crystal structure of compound 1a, cation-anion layers of [B 12 H 11 NCCH 3 ] − anions and (C 2 H 5 ) 4 N + cations were formed, parallel to the plane ab (Figure 2a).The anions bound to each other in dimeric pairs by weak CH . . .HB(B) contacts were located sequentially above each other along the a-axis.The cations and anions were also connected by a network of weak CH . . .HB(B) interactions, which are shown as red spots on the Hirschfeld surface of the anion (Figure 2b).H .In the packing of compound 1b, the picture is similar to that of compound 1a: the (C6H5)4P + cations and [B12H11NCn-C3H7] − anions formed cation-anion layers parallel to the ab plane, which were bonded together, due to weak CH...HB(B) interactions (Figure 3a).The antiparallel [B12H11NCn-C3H7] − anions were similarly bonded by CH...HB(B) contacts between the α-methylene groups of the exo-polyhedral substituent and the boron back-

DFT Calculation
One of the main tasks of the present work was to reveal how the formation of the complex with closo-dodecaborate anion affected the properties of the nitrile group.To investigate this phenomenon, several nitriles RCN, R = CH3, n-C3H7, 4-CH3C6H4 and nitrilium derivatives [B12H11NCR] − , R = CH3, n-C3H7, 4-CH3C6H4 were chosen for theoretical study (Figure 4).All DFT calculations were carried out both in the gas phase and considering solvation effects.Firstly, the bond lengths of initial nitriles and borylated complexes were examined (Table 1).It is worth noting that the CN bond lengths were practically the same in the initial nitriles and their borylated analogues.For the estimation of bond orders, the Wiberg and Mayer approaches were used (Table 1).With the Mayer approach, the values of bond orders were higher than with the Wiberg approach.Both Wiberg and Mayer bond orders slightly decreased during the formation of the complex.These changes indicated that the nature of the interaction between nitrogen and carbon atoms had modified.The role of electrostatic interaction between carbon and nitrogen atoms was increased.The carbon atom of the nitrilium group of [B12H11NCR] − became much more positively charged than the initial nitrile.This increase in positive charge also indicated that the carbon atom of the nitrilium group should be much more easily attacked by nucleophile molecules.In the case of CH3CN and CH2Cl2 solutions, mean values of carbon atomic charges were higher than in the gas phase model.The nature of the R-substituent of [B12H11NCR] − , R = CH3, n-C3H7, 4-CH3C6H4 had a slight effect on the values of the bond lengths and atomic charges.Firstly, the bond lengths of initial nitriles and borylated complexes were examined (Table 1).It is worth noting that the CN bond lengths were practically the same in the initial nitriles and their borylated analogues.For the estimation of bond orders, the Wiberg and Mayer approaches were used (Table 1).With the Mayer approach, the values of bond orders were higher than with the Wiberg approach.Both Wiberg and Mayer bond orders slightly decreased during the formation of the complex.These changes indicated that the nature of the interaction between nitrogen and carbon atoms had modified.The role of electrostatic interaction between carbon and nitrogen atoms was increased.The carbon atom of the nitrilium group of [B 12 H 11 NCR] − became much more positively charged than the initial nitrile.This increase in positive charge also indicated that the carbon atom of the nitrilium group should be much more easily attacked by nucleophile molecules.In the case of CH 3 CN and CH 2 Cl 2 solutions, mean values of carbon atomic charges were higher than in the gas phase model  5).In addition to the carbon atom of the nitrilium group, atoms from the benzene ring also made a significant contribution to LUMO.The increase in electrophilicity can also be correlated with the energy of LUMO (lowest unoccupied orbital).The carbon atom of the nitrilium group makes the largest contribution to LUMO orbitals.As can be seen from the data obtained for the gas phase, the LUMO of [B12H11NCR] − had more positive values of energies than the initial nitriles.When solvation effects were taken into account, the opposite was true.The ratio of LUMO energies explains the much greater reactivity of activated nitriles.This pattern was common to all nitriles considered and their derivatives [B12H11NCR] − .The value of LUMO for [B12H11NC(4-C6H4CH3)] − was less positive than for [B12H11NCR] − , R = CH3, n-C3H7.In addition, the shape of LUMO for [B12H11NC(4-C6H4CH3)] − was quite different to [B12H11NCR] − , R = CH3, n-C3H7 (Figure 5).In addition to the carbon atom of the nitrilium group, atoms from the benzene ring also made a significant contribution to LUMO.The increased reactivity of borylated nitriles compared with initial nitriles is a wellknown and established fact, which can be explained in terms of changes in the nature of the CN chemical bond upon formation of the target complex.In the case of nitrilium derivatives, the CN bond has a more ionic nature than that of initial nitriles.In addition, a change in the energy of the LUMO orbital during the formation of the nitrilium complex also plays a significant role.The decrease in LUMO energy during complex formation provides greater reactivity with nucleophile molecules.These effects are, however, characteristic only when solvent influence is taken into account.In the gas phase, uncoordinated nitriles have lower positive LUMO values than cluster derivatives.The increased reactivity of borylated nitriles compared with initial nitriles is a wellknown and established fact, which can be explained in terms of changes in the nature of the CN chemical bond upon formation of the target complex.In the case of nitrilium derivatives, the CN bond has a more ionic nature than that of initial nitriles.In addition, a change in the energy of the LUMO orbital during the formation of the nitrilium complex also plays a significant role.The decrease in LUMO energy during complex formation provides greater reactivity with nucleophile molecules.These effects are, however, characteristic only when solvent influence is taken into account.In the gas phase, uncoordinated nitriles have lower positive LUMO values than cluster derivatives.

IR Spectra
IR spectra of the compounds were recorded on an Infralum FT-08 IR Fourier spectrophotometer (NPF Lumex AP) in the region 400-4000 cm −1 , with a resolution of 1 cm −1 .Samples were prepared as thin film in CH 2 Cl 2 .

NMR Spectra
1 H, 13 C{H} and 11 B{H} NMR spectra of solutions of the studied substances in CD 3 CN or CD 2 Cl 2 were recorded on a Bruker MSL-300 pulsed Fourier spectrometer (Germany), at frequencies of 300.3, 75.49 and 96.32 MHz, respectively, with internal deuterium stabilisation.Tetramethylsilane or boron trifluoride ether was used as the external standard.

Electrospray Ionisation Mass Spectrometry (ESI-MS)
The LC system consisted of two LC-20AD pumps (Shimadzu, Japan) and an autosampler was coupled online with an LCMS-IT-TOF mass spectrometer equipped with an electrospray ionisation source (Shimadzu, Japan), The HRMS spectra were acquired in direct injection mode without column.The samples were prepared as CH 3 CN solutions.Detection parameters: Detector Voltage 1.55 kV; Nebulising Gas 1.50 L/min; CDL Temperature 200.0 • C.

X-ray Diffraction
The single-crystal X-ray diffraction data for 1 and 2 were collected using a threecircle Bruker D8 Venture (Centre of Joint Equipment of Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences), in ϕ and ω scan mode.The data were indexed and integrated using the SAINT program [No Title.Bruker, SAINT, Bruker AXS Inc., Madison, WI, 2018.].Absorption correction based on measurements of equivalent reflections (SADABS) was applied [53].The structures were determined by direct methods and refined using 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 with 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 [54] and OLEX2 program package [55].For details, see Table S1 (Electronic Supporting Information).
The crystallographic data were deposited with the Cambridge Crystallographic Data Center, CCDC 2206019-2206020.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).
As it stated previously the crystals for X-ray diffraction experiments were obtained by isothermal evaporation of acetonitrile solutions of the corresponding derivative, in the presence of (C 2 H 5 ) 4 NCl or (C 6 H 5 ) 4 PCl.

Hirshfeld Surface Analysis
The Crystal Explorer 17.5 program was [56] used to analyse the interactions within the crystal.The donor-acceptor groups were visualised using a standard (high) surface resolution and d norm surfaces were mapped over a fixed colour scale of −0.640 (red) to 0.986 (blue) a.u.

Computational Details
DFT calculations were made using the ORCA 4.2.1 program package [57].Geometries of all model structures were fully optimised at the ωB97X-D3/def2-TZVPP level of the-ory [56,58].All calculations were performed using the RIJCOSX approximation with the def2/J auxiliary basis set [59].Tight criteria of SCF convergence (Tight SCF) were employed for the calculations.the keywords Grid5 FinalGrid6 GridX5 were used as parameters for the spatial integration grid All of the nitrilium derivatives considered had closed electron shells and the spin restricted approximation was applied.During the geometry optimisation procedure, symmetry operations were not applied for the structures considered.The Hessian matrices were calculated numerically for all model structures, to prove the location of correct minima on potential energy surfaces; (in all cases, no imaginary frequencies were found).Solvent effects were taken into account using the Solvation Model based on Density (SMD) [60].The natural bond orbital (NBO) method (for calculation of Wiberg bond indices in Natural Atomic Orbitals) was employed using the NBO7 program package [61].The Cartesian atomic coordinates for all optimised model structures of nitrlium derivatives of closo-dodecaborate anion are presented in the Supporting Information.The visualisation of optimised structures and their LUMO was performed with the help of the ChemCraft program (version 1.7) [62].
Solvents of reagent and special purity grades Sigma-Aldrich and Panreac (99.7%), were used without any additional purification.H 12 ] were dissolved in 10 mL of acetonitrile CH 3 CN.The solution was placed in a glass pressure vessel, it was purged with argon and then 0.3 mL trifluoroacetic acid CF 3 COOH was added.The reaction solution was heated to 150 • C.After 30 min of heating, the solution was cooled to room temperature and concentrated in a rotary evaporator.To this solution, 15 mL of glacial acetic acid CH3COOH was added.The precipitate was filtered through a glass filter and washed with 30 mL glacial acetic acid and 30 mL diethyl ether.The product was dried in vacuo.Yield: 76%.

Scheme 1 .
Scheme 1. Scheme of preparation of [B12H11NCR] − .Scheme 1. Scheme of preparation of [B 12 H 11 NCR] − .The completeness of the reaction was monitored using 11 B NMR.As in the case of [B 12 H 11 NCCH 3 ] − , there were two signals observed in the spectra of the target compounds.
5-103.3 ppm in the case of nitrilium derivatives with aliphatic backbones R = n-C 3 H 7 , i-C 3 H 7 .These values were more positive than with [B 12 H 11 NCCH 3 ] − .In the case of borylated nitriles with aromatic backbones R = 1-C 10 H 7 , 4-C 6 H 4 CH 3 , signals from carbon atoms of the nitrilium group were in the interval 103.3-104.2ppm.In the IR-spectra of [B 12 H 11 NCR] − , R = n-C 3 H 7 , i-C 3 H 7 , 4-C 6 H 4 CH 3 , 1-C 10 H 7 there were the two most significant absorption bands: from BH valence vibrations in the range 2490-2500 cm −1 and from CN valence vibrations in the range 2304-2338 cm −1 .

Figure 1 .
Figure 1.Structures of [B12H11NCCH3] − and [B12H11NCC3H7] − .In the crystal structure of compound 1a, cation-anion layers of [B12H11NCCH3] − anions and (C2H5)4N + cations were formed, parallel to the plane ab (Figure 2a).The anions bound to each other in dimeric pairs by weak CH...HB(B) contacts were located sequentially above each other along the a-axis.The cations and anions were also connected by a network of weak CH...HB(B) interactions, which are shown as red spots on the Hirschfeld surface of the anion (Figure 2b).H...H contacts accounted for 92.5% of the anion surface, while H...C/C...H and H...N/N...H contacts accounted for 5.5% and 2.0% of the anion surface, respectively.

2. 3
. DFT Calculation One of the main tasks of the present work was to reveal how the formation of the complex with closo-dodecaborate anion affected the properties of the nitrile group.To investigate this phenomenon, several nitriles RCN, R = CH 3 , n-C 3 H 7 , 4-CH 3 C 6 H 4 and nitrilium derivatives [B 12 H 11 NCR] − , R = CH 3 , n-C 3 H 7 , 4-CH 3 C 6 H 4 were chosen for theoretical study (Figure 4).All DFT calculations were carried out both in the gas phase and considering solvation effects.
. The nature of the R-substituent of [B 12 H 11 NCR] − , R = CH 3 , n-C 3 H 7 , 4-CH 3 C 6 H 4 had a slight effect on the values of the bond lengths and atomic charges.The increase in electrophilicity can also be correlated with the energy of LUMO (lowest unoccupied orbital).The carbon atom of the nitrilium group makes the largest contribution to LUMO orbitals.As can be seen from the data obtained for the gas phase, the LUMO of [B 12 H 11 NCR] − had more positive values of energies than the initial nitriles.When solvation effects were taken into account, the opposite was true.The ratio of LUMO energies explains the much greater reactivity of activated nitriles.This pattern was common to all nitriles considered and their derivatives [B 12 H 11 NCR] − .The value of LUMO for [B 12 H 11 NC(4-C 6 H 4 CH 3 )] − was less positive than for [B 12 H 11 NCR] − , R = CH 3 , n-C 3 H 7 .In addition, the shape of LUMO for [B 12 H 11 NC(4-C 6 H 4 CH 3 )] − was quite different to [B 12 H 11 NCR] − , R = CH 3 , n-C 3 H 7 (Figure