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Article

Reduction of Triple Bond in [B12H11NCR]− Anions by Lithium Aluminum Hydride: A Novel Approach to the Synthesis of N-Monoalkylammonio-Substituted closo-Dodecaborates

by
Alexey V. Nelyubin
1,
Nikolay K. Neumolotov
1,
Nikita A. Selivanov
1,
Alexander Yu. Bykov
1,
Ilya N. Klyukin
1,
Alexander S. Novikov
2,3,
Alexey S. Kubasov
1,
Andrey P. Zhdanov
1,*,
Konstantin Yu. Zhizhin
1,* and
Nikolay T. Kuznetsov
1
1
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii Pr. 31, 119991 Moscow, Russia
2
Institute of Chemistry, Saint Petersburg State University, Universitetskaya Nab. 7-9, 199034 Saint Petersburg, Russia
3
Research Institute of Chemistry, Peoples’ Friendship University of Russia (RUDN University), Miklukho-Maklaya St. 6, 117198 Moscow, Russia
*
Authors to whom correspondence should be addressed.
Inorganics 2024, 12(1), 2; https://doi.org/10.3390/inorganics12010002
Submission received: 7 November 2023 / Revised: 12 December 2023 / Accepted: 16 December 2023 / Published: 20 December 2023
Browse Figures
Versions Notes

Abstract

:
By reacting nitrilium derivative of the closo-dodecaborate anion, Bu4N[B12H11N≡CR] (where R = Me, Et, nPr, iPr, p-tolyl), with lithium aluminum hydride (LiAlH4), N-alkylammonium derivatives of the closo-dodecaborate anion, and Bu4N[B12H11NH2CH2R], were obtained. The reduction reaction procedure was optimized, achieving yields close to quantitative (90–95%). The structure of the compound Bu4N[B12H11NH2CH2CH3] was determined using X-ray structural analysis. It was found that substituting lithium aluminum hydride (LiAlH4) with sodium borohydride (NaBH4) leads to the same products but only upon heating, while the reaction with LiAlH4 proceeds at room temperature.

1. Introduction

Higher polyhedral boranes [BnHn]2− (n = 10, 12) are the subjects of numerous studies. Traditionally, boron cluster anions are used as agents for boron neutron capture therapy (BNCT) [1,2,3,4,5], high-energy materials, non-coordinating anions, and ligands [6,7,8,9,10,11,12]. In recent years, this class of compounds has been utilized as optical [13,14,15,16,17] and magnetic [18,19,20,21] materials, chemical reaction catalysts [22,23,24,25], and materials for photovoltaics [26,27,28].
The closo-dodecaborate dianion [B12H12]2− is a spatially aromatic system with delocalized electrons, which accounts for the high thermal and kinetic stability of its derivatives and their tendency to undergo substitution reactions of the exo-polyhedral hydrogen atom [29]. Due to the high symmetry of the closo-dodecaborate anion [B12H12]2− (symmetry group Ih), the anion lacks a distinct reaction center. Consequently, obtaining monosubstituted derivatives of the closo-dodecaborate anion is often complicated by the formation of poly-substituted products.
Hydrogen atom substitution reactions can proceed via radical mechanisms. Such processes include reactions of exhaustive halogenation of the free anion [B12H12]2− and its substituted derivatives [30,31,32,33], as well as the formation of closomer anions [B12(OH)12]2− and [B12(OH)11NH3]− [34,35]. Electrophilic processes are used to obtain carbonyl derivatives [36], halogen-closo-dodecaborates with low degrees of substitution [37] and alkyl and aryl derivatives [38]. The reaction of [B12H12]2− anion with hydroxylamine-O-sulfonic acid proceeds through an electrophilic mechanism [39]. This process can be characterized as stepwise; it strongly depends on the reaction conditions and the ratio of reagents and often leads to the formation of a mixture of mono- and di-substituted products in the form of 1,2-, 1,7-, and 1,12-isomers [32]. The latter can be separated by chromatographic methods or fractional crystallization. The most selective process for the preparation of mono-substituted closo-dodecaborates is nucleophilic substitution proceeding via the EINS mechanism [40]. This approach allows the preparation of derivatives based on cyclic and simple esters, organic acids and amides based on them, and thioureas [29,41]. Methods based on the modification of substituents introduced into the cluster are actively used for the directed synthesis of functionalized closo-dodecaborates. Such processes include alkylation and acylation reactions of hydroxy groups and thiols [42,43,44,45,46,47], opening of oxonium substituents [41,48,49], ipso-substitution of halogen atoms [50,51], and iodonium substituents [52,53].
Previously, alkylammonium derivatives of the closo-dodecaborate anion were obtained through the reduction of the reaction products of ammonium derivatives with aldehydes using sodium borohydride (NaBH4) [54]. Using this method, derivatives with the composition [B12H11NH2CH2R] were successfully obtained but only with aryl substituents (R = 2-C6H4-OMe, 3,4-C6H3O2CH2, 4-C6H4NHCOMe, 4-C6H4CN). It is noteworthy that the work lacks data on the synthesis of derivatives with aliphatic substituents.
Another method for obtaining alkylammonium derivatives is the direct alkylation of [B12H11NH3]−, and the products depend on the nature of the alkyl group [55,56]. For instance, when R = CH3 or C2H5, alkylation leads to the formation of trialkyl-substituted derivatives [B12H11NR3]−. Increasing the length of the substituent chain results in the formation of disubstituted derivatives [B12H11NHR2]− due to steric factors. The synthesis of monoalkyl-substituted derivatives is complicated because, even with a 1:1 ratio, a mixture of mono- and poly-substituted products is formed. Therefore, the challenge of selectively obtaining monoalkyl-substituted derivatives of closo-dodecaborate anion with different substituent types remains unresolved.
Nitrilium derivatives of the closo-dodecaborate anion [B12H11N≡CR]− have great potential for modification due to the labile N≡C bond [57,58,59]. For a long time, it was believed that these derivatives could not be isolated in their non-hydrolyzed form [58]. However, a new synthesis method involving the use of absolute organic solvents allows the preparation of these compounds in pure form and for further modifications [60].
Previously in our laboratory, it was demonstrated that the reduction of nitrile derivatives of the closo-dodecaborate anion [B10H9N≡CR]− using LiAlH4 in an anhydrous, aprotic solvent leads to the formation of alkylammonium derivatives with the composition [B10H9NHCH2R]− (R = CH3, C2H5, n-Pr, t-Bu, Ph, 1-Naph) [61]. In the present study, we applied this approach to obtain monoalkylammonium derivatives of the closo-dodecaborate anion [B12H11NHCH2R]− with alkyl and aryl substituents.

2. Results

Lithium aluminum hydride (LAH) is widely used for the reduction of various functional groups, including carbonyl groups [62,63], nitriles [64,65,66,67], and others [68,69]. Nitriles containing the -CN functional group can be reduced by LAH to primary amines [70]. This process is of great importance because amines are significant building blocks in organic synthesis, including the development of pharmaceuticals, agrochemicals, and other specialized organic compounds. The reduction mechanism of LAH nitriles involves several key steps [71]. First, lithium aluminum hydride interacts with the nitrile group, leading to the formation of an imine and an aluminate complex. Next, the imine is reduced to the primary amine. An important feature of this reaction is its high stereospecificity and ability to form products in high yields.
The reduction of nitriles by LAH is a preferred method over other reducing agents such as sodium borohydride (NaBH4) because of its higher reactivity and ability to reduce a wider range of functional groups [72]. In addition, LAH efficiently reduces nitriles under mild conditions, which minimizes the risk of degradation of sensitive functional groups or formation of byproducts. In addition, the choice of solvent for reaction with LAH is crucial. Typically, ether solvents such as diethyl ether or tetrahydrofuran (THF) are used because they stabilize the alumohydride anion and prevent spontaneous decomposition of the reagent.
As previously demonstrated, the reduction of nitrile derivatives of the closo-decaborate anion (Bu4N)[B10H9N≡CR] with lithium aluminum hydride leads to the formation of monoalkylammonium derivatives with the substituent RCH2−. In the present study, a similar approach was used for the first time to obtain a series of monoalkylammonio-closo-dodecaborates with the composition (Bu4N)[B12H11NH2CH2R] (R = Me, Et, nPr, iPr, p-tolyl) from the corresponding nitrile derivatives (Bu4N)[B12H11N≡CR]. The reaction proceeds selectively under mild conditions with near quantitative in situ yields (90–95%) and high isolated yields (79–90%).
The reaction was conducted in anhydrous tetrahydrofuran (THF) at room temperature with a 7-fold excess of Li[AlH4] (Scheme 1) within 2 h. A multiple excess of LAH is due to the reagent grade of the reagent (95%). Purification of the resulting product was not complicated and featured few steps. Firstly, 10 mL of 1N HCl was added to the solution until evaluation of gas ceased. After collecting, precipitate was washed with 20 mL of acetonitrile, and filtrate solution was concentrated. The last purification step was to remove any remaining byproducts by washing with 20 mL of diethyl ether. When the suspension of Li[AlH4] was added to the nitrile derivative solution, the mixture acquired a light-yellow color, which later faded. A similar change in color was observed when sodium borohydride (Na[BH4]) was used. This phenomenon can be explained by the formation of a colored intermediate during the reaction. Also, in contrast to the closo-decaborate anion, the formation of degradation products of the boron cluster (and the formation of boronium adducts) is not observed in the reaction mixture due to the greater stability of the twelve-vertex polyhedron.
The reactions were monitored using 11B NMR spectroscopy by taking an aliquot of reacting THF solution and quenching with 1N HCl until pH < 7. Purity of resulted individual compounds was determined via the HPLC method (40% of MeCN/60% of 0.2% CF3COOH in H2O). The spectra of nitrile derivatives (Bu4N)[B12H11N≡CR] differ from those of (Bu4N)2[B12H12] by the presence of a small shoulder in the region close to the main peak (−15.1 ppm). Depending on the nature of the substituent, the signal from the substituted position (B1) can be well distinguished (e.g., a singlet at −12.0 ppm for (Bu4N)[B12H11N≡CC2H5]) or completely merge with the signal from the unsubstituted positions (as in the case of (Bu4N)[B12H11N≡CC6H4CH3]). This difference in the nitrile derivatives from other derivatives of the closo-dodecaborate anion is related to the presence of a cone of magnetic anisotropy in the -N≡C-bond.
During the reduction of the triple bond, the shielding effect significantly weakens. For example, in the 11B NMR spectrum of (Bu4N)[B12H11NH2CH2C2H5], three signals are clearly distinguishable: −5.5 ppm (B1, singlet), −16.8 ppm (B2-B11, doublet), and −19.6 ppm (B12, singlet). The two hydrogen atoms attached to the nitrogen atom (-NH2CH2C2H5) are detected in the 1H NMR spectrum as a broad signal at 4.87 ppm. The hydrogen atoms of the propyl group are observed as multiplets at 2.83 (CH2CH2CH3), 1.58 (2H, CH2CH2CH3), and a triplet at 0.88 ppm (CH2CH2CH3). Additionally, characteristic signals from the Bu4N+ cation are present in the spectrum at 3.15, 1.61, 1.45, and 1.01 ppm.
The eleven hydrogen atoms from the B-H bonds of the cluster are in a broad range from 0 to 2.5 ppm. The carbon atoms of the propyl group are detected in the 13C NMR spectrum. The signals at 50.6, 22.2, and 11.3 ppm correspond to the α, β, and γ carbon atoms, respectively.
The change in the exo-polyhedral substituent structure is also evident from the IR spectra data. Two characteristic types of absorption bands are observed in the spectra of [B12H11NH2CH2R]− products. The absorption band of the valence vibrations of the B-H bond is in the region of 2495–2490 cm−1. Two absorption bands in the ranges 3290–3230 and 3206–3195 cm−1 correspond to the valence vibrations of the N-H bonds of the ammonium group. Also, the absorption bands of the C≡N bond in the region 2330–2300 cm−1 are absent in the spectra of the reduction products [60].
The compound 2a crystallizes in a monoclinic crystal system (space group P21/n, Table 1). The crystallographically independent part of the elementary cell contains one cation Bu4N⁺ and one anion [B12H11NH2Et]−. The boron cluster is an icosahedron with slightly distorted geometry, which may be due to the presence of a charged exo-polyhedral substituent. The lengths of B-B bonds are in the range 1.760(5)–1.808(5) Å (Table 1). The B-N bond length in the anion is 1.587(4) Å (Figure 1), which is slightly more than in the unsubstituted ammonium derivatives [32,73], but corresponds well with the previously described anion [B12H11NH2iPr]− [56]. Also, this parameter is comparable to the B-N bond length in the anion [2-B10H9NH2CH2CH3]− (1.569 Å) [61] and borane adducts with primary amines (n-propylamine B-N bond 1.593 Å [74] and ethylenediamine 1.60 Å [75]). The N1-C1 bond length is 1.432(4) Å, which is slightly shorter than the analogous bond in the substituted N-ethylammonio closo-decaborate and borane adducts. The values of the B1-N1-C1 and N1-C1-C2 angles indicate a distorted tetrahedral environment of the N1 and C1 atoms. The ethylammonium substituent itself in the crystal has a geometry close to planar. Thus, the dihedral angle B1-N1-C1-C2 is 167.9(3)°. This geometry of the substituent is explained by the formation of rather strong intermolecular dihydrogen bonds. In solutions these interactions are probably leveled, which is consistent with the 1H NMR data. Thus, no enantiomeric splitting of signals of methylene protons of NH2-CH2 groups, which is characteristic of molecules with difficult rotation of the substituent around the N-C bond, is observed [76,77]. In addition, in IR spectra of solutions of the obtained compounds, there is no additional cleavage of B-H bond bands.
In the crystal, the anions [B12H11NH2Et]− connected to each other via numerous non-covalent contacts. For the investigation of non-covalent contact, combined Hirshfeld surface analysis and the QTAIM approach were used (Figure 2b and Figure S21). According to Hirshfeld analysis, the major contribution to the connection between anions is made by NH…HB contacts (Figure 2b, red spots on the Hirshfeld surface of the anion), which are shown on the fingerprint plots as two adjacent peaks with di and de alternately equal to 1.00 Å and 1.03 Å. The distances of NH…HB contacts lie in the range between 2.15 and 2.20 Å. According to QTAIM analysis, the values of electron density p(r) at bond critical points (bcp) related to NH…HB contacts are equal to 0.008 e Å−3. The values of Laplacian of electron density ∇2p(r) lie in the range 0.024–0.025 e Å−5. The values of total energy Hb are positive and equal to 0.001 h e−1. It is interesting to compare present results with those previously obtained for the [B12H11NH=C(CH3)NH2]− anion [58]. For the [B12H11NH=C(CH3)NH2]− system, intramolecular NH…HB contacts were considered, whereas in the case of [B12H11NH2Et]− analogous contacts are intermolecular. Based on data of QTAIM analysis, one can conclude that intramolecular NH…HB contacts are stronger compared to intermolecular ones. The NH…HB contacts’ cluster anions are linked to each other by CH…HB contacts and form a column parallel to the a-axis, surrounded by tetrabutylammonium cations. One hundred percent of the anion’s surface is accounted for by H…H contacts with tetrabutylammonium cations and adjacent anions. The distances of CH…HB contacts lie in the range between 2.42 and 2.56 Å. The values of electron density p(r) at bcp related to CH…HB contacts lie in the range 0.004–0.005 e Å−3. The values of Laplacian of electron density ∇2p(r) lie in the range 0.013–0.016 e Å−5. The values of total energy have three zeros after the point and have significant figures only at 4 decimal places; thus, these values can be considered as approximately zero. The third type of contact between cluster anions is the BH…HB one. The distances of BH…HB contacts are equal to 2.81 Å. The values of electron density p(r) at bcp related to BH…HB contacts are equal to 0.003 e Å−3. The values of Laplacian of electron density ∇2p(r) are equal to 0.010 e Å−5. The values of total energy are approximately zero as in the case of CH…HB contacts.
NH…HB contacts have large descriptor values compared to CH…HB and BH…HB contacts. Given contacts have the greatest values of electron density p(r) and total energy Hb at bcp, whereas BH…HB contacts have the lowest values of these descriptors. Thus, one can conclude that the NH…HB contacts are the stronger ones, and BH…HB contacts are the weakest ones based on joint QTAIM and Hirshfeld analysis.

3. Materials and Methods

3.1. IR Spectra

IR spectra of the compounds were recorded on an IR Fourier-transform spectrophotometer Infralum FT-08 (“Lumex”, St. Petersburg, Russia) in the range of 4000–400 cm−1 with a resolution of 1 cm−1. Samples were prepared in the form of solutions in chloroform.

3.2. 1H NMR, 11B NMR, and 13C NMR

1H NMR, 11B NMR, and 13C NMR spectra of solutions of the investigated substances in CD3CN were recorded on a pulse Fourier-transform spectrometer Bruker AVANCE-300 (Germany) at frequencies of 300.3, 96.32, and 75.49 MHz, respectively, with internal deuterium stabilization. Tetramethylsilane or boron trifluoride etherate was used as the external standard.

3.3. Electrospray Ionization Mass Spectrometry (ESI-MS)

An LC system consisting of two LC-20AD pumps (Shimadzu, Kyoto, Japan) and autosampler was coupled online with an LCMS-IT-TOF mass spectrometer equipped with an electrospray ionization source (Shimadzu, Japan). The HRMS spectra were acquired in direct injection mode without a column. Mass spectra were obtained in the m/z range from 120 to 700 Da (for negative ionization mode) and 100–700 for positive mode. Other MS parameters were as follows: detector voltage: 1.55 kV, nebulizing gas: 1.50 L/min, CDL temperature: 200.0 °C CDL, heat block temperature: 200.0 °C, ESI voltage: 4.50 kV, event time: 300 ms, repeats: 3, ion accumulation: 30 ms. Instrument tuning (mass calibration and sensitivity check) was carried out before analysis.

3.4. Preparative HPLC

Preparative HPLC was carried out on an isocratic HPLC system “STAYER” (“Akvilon”, Moscow, Russia), with a spectrophotometric detector (UVV 104, with variable wavelength from 200 to 360 nm) using a Knauer Eurospher 110 Si column.

3.5. Single-Crystal X-ray Diffraction

Single-crystal X-ray diffraction data for 1 were collected on a three-circle Bruker D8 Venture diffractometer (T = 100 K, graphite monochromator, ω and ϕ scanning mode). The data were indexed and integrated using the SAINT (V7.60A) program [78] and then scaled and corrected for absorption using the SADABS (version 2008/1) program [79]. For details, see Table S1.
The structures were determined using direct methods and refined with the full-matrix least squares technique on F2 with anisotropic displacement parameters for non-hydrogen atoms. The hydrogen atoms were placed in calculated positions and refined within the riding model with fixed isotropic displacement parameters (Uiso(H) = 1.5Ueq(O), 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) [80] and OLEX2 program (version 1.5) package [81].
The B-type alerts in crystal 2a are due to poor crystal quality.
Crystallographic data for all investigated compounds have been deposited with the Cambridge Crystallographic Data Center, CCDC 2305164. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336033, e-mail: [email protected], or www.ccdc.cam.ac.uk (accessed on 15 December 2023).

3.6. Computational Details

For estimation of non-covalent contacts in the crystal structure of (Bu4N)[B12H11NH2Et], the part of crystal unit including three boron clusters was chosen. The ORCA 4.2.1 software package was applied to perform calculation. A single-point computation for the model structure was performed at the ωB97X-D3/def2-TZVPP theory level using the atom-pairwise dispersion correction with the zero-damping scheme [82,83,84]. All calculations were performed using the RIJCOSX approximation with the def2/J auxiliary basis set [85]. 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, spin-restricted approximation was utilized. Topological analysis of the electron density distribution, based on the Quantum Theory of Atoms in Molecules (QTAIM) formalism developed by Bader [86,87], was employed with the Multiwfn program (version 3.7) [88].

3.7. Synthesis of Compounds

The reagent grade solvents were used without additional purification. Commercially available reagent grade lithium aluminum hydride (LAH) (95%) was purchased from Sigma Aldrich.
Synthesis of nitrile derivatives of closo-dodecaborate anion (Bu4N)[B12H11NCR]:
The nitrilium derivatives (1 a–e) were obtained using a known methodology [58,62].
Synthesis of N-alkylammonium derivatives of the closo-dodecaborate anion. (Bu4N)2[B12H11NH2CH2R]
(Bu4N)[B12H11(NH2CH2CH3)] 2a
General procedure
First, 0.38 g (10 mmol) of LiAlH4 and 2 mL of tetrahydrofuran (THF) were placed into a flask. Then, 0.424 g (1 mmol) of (Bu4N)[B12H11NCCH3] was dissolved in 5 mL of THF. The solution of the nitrile derivative was slowly added to the LiAlH4 suspension, and the mixture was stirred at room temperature for 2 h. To the reaction mixture, 10 mL of 1 N HCl was added dropwise until gas evolution ceased. The precipitated solid was filtered off and washed with 20 mL of acetonitrile. The filtrate was concentrated using a rotary evaporator, and 20 mL of diethyl ether was added to it. The mixture was then treated with ultrasound. The resulting precipitate was filtered, washed with diethyl ether, and dried under vacuum. Yield: 90%. 11B{H} NMR (CD3CN, ppm.): −4.6 (s, 1B, B-N), −15.8 (s, 10B, B-H(B2-11)), −17.9 (s, 1B, B-H (B12)). 1H NMR (CD3CN, ppm.): 2.5–0.0 (m, 11H, B-H), 3.08 (8H, Bu4N), 1.6 (8H, Bu4N), 1.35 (8H, Bu4N), 0.96 (12H, Bu4N), 4.86 (s, 2H, NH2), 2.91 (m, 2H, CH2CH3), 1.16 (t, 3H, CH2CH3 J = 7.26 Hz). 13C{H} NMR (CD3CN, ppm.): 59.4 (Bu4N), 24.4 (Bu4N), 20.,2 (Bu4N), 13.9 (Bu4N), 44.0 (CH2CH3), 14.1(CH2CH3). IR(CH2Cl2,cm−1): 3224, 3205 ν (N-H), 2491 ν (B-H). MS(ESI) m/z: 186.2650 (A refers to the molecular weight of [B12H11(NH2CH2CH3)], calculated for {[A]-} 186.2629).
(Bu4N)[B12H11(NH2nC3H7)] 2b
Yield: 89%. 11B{H} NMR (CD3CN, ppm.): −4.6 (s, 1B, B-N), −15.9 (s, 10B, B-H(B2-11)), −17.9 (s, 1B, B-H (B12)). 1H NMR (CD3CN, ppm.): 2.5–0.0 (m, 11H, B-H), 3.15 (8H, Bu4N), 1.61 (8H, Bu4N), 1.45 (8H, Bu4N), 1.01 (12H, Bu4N), 4.87 (s, 2H, NH2), 2.83 (m, 2H, CH2CH2CH3), 1.58 (m, 2H, CH2CH2CH3), 0.88 (t, 3H, CH2CH2CH3 J = 7.46 Hz). 13C{H} NMR (CD3CN, ppm.): 59.4 (Bu4N), 24.4 (Bu4N), 20.2 (Bu4N), 13.9 (Bu4N), 50.6 (CH2CH2CH3), 22.2 (CH2CH2CH3), 11.3 (CH2CH2CH3). IR(CH2Cl2, cm−1): 3229, 3206 ν (N-H), 2491 ν (B-H). MS(ESI) m/z: 200,2810 (A refers to the molecular weight of [B12H11(NH2CH2CH2CH3)], calculated for {[A]-} 200,2785).
(Bu4N)[B12H11(NH2nC4H9)] 2c
Yield: 86%. 11B{H} NMR (CD3CN, ppm.): −4.5 (c, 1B, B-N), −15.8 (s, 10B, B-H(B2-11)), −17.9 (s, 1B, B-H (B12)). 1H NMR (CD3CN, ppm.): 2.5–0.0 (m, 11H, B-H), 3.15 (8H, Bu4N), 1.61 (8H, Bu4N), 1.45 (8H, Bu4N), 1.01 (12H, Bu4N), 4.85 (s, 2H, NH2), 2.85 (m, 2H, CH2CH2CH2CH3), 1.59 (m, 2H, CH2CH2CH2CH3), 1.33 (m, 2H, CH2CH2CH2CH3), 0.89 (t, 3H, CH2CH2CH2CH3 J = 7.36 Hz). 13C{H} NMR (CD3CN, ppm.): 59.4 (Bu4N), 24.4 (Bu4N), 20.2 (Bu4N), 13.9 (Bu4N), 48.8 (CH2CH2CH2CH3), 31.0 (CH2CH2CH2CH3), 20.5 (CH2CH2CH2CH3), 14.0(CH2CH2CH2CH3). IR(CH2Cl2, cm−1): 3277, 3206 ν (N-H), 2492 ν (B-H). MS(ESI) m/z: 214,2966 (A refers to the molecular weight of [B12H11(NH2CH2CH2CH2CH3)], calculated for {[A]-} 214,2942).
(Bu4N)[B12H11(NH2iC4H9)] 2d
Yield: 79%. 11B{H} NMR (CD3CN, ppm.): −4.4 (s, 1B, B-N), −15.8 (s, 10B, B-H(B2-11)), −17.9 (s, 1B, B-H (B12)). 1H NMR (CD3CN, ppm.): 2.5–0.0 (m, 11H, B-H), 3.15 (8H, Bu4N), 1.61 (8H, Bu4N), 1.45 (8H, Bu4N), 1.01 (12H, Bu4N), 4.76 (s, 2H, NH2), 2.73 (m, 2H, CH2CH(CH3)2), 1.88 (m, 1H, CH2CH(CH3)2), 0.9 (d, 6H, CH2CH(CH3)2 J = 6.7 Hz). 13C{H} NMR (CD3CN, ppm.): 59.4 (Bu4N), 24.4 (Bu4N), 20.2 (Bu4N), 13.9 (Bu4N), 56.3 (CH2CH(CH3)2), 27.7 (CH2CH(CH3)2), 20.3, 20.1 (CH2CH(CH3)2). IR (CH2Cl2, cm−1): 3293, 3201 ν (N-H), 2493 ν (B-H). MS(ESI) m/z: 214,2968 (A refers to the molecular weight of [B12H11(NH2CH2CH(CH3)2], calculated for {[A]-} 214,2942).
(Bu4N)[B12H11(NH2C6H4CH3)] 2e
This compound was obtained using a similar methodology.
Yield: 90%. 11B{H} NMR (CD3CN, ppm.): −4.3 (s, 1B, B-N), −15.8 (s, 10B, B-H(B2-11)), 17.8 (s, 1B, B-H(B12)). 1H NMR (CD3CN, ppm.): 2.5–0.0 (m, 11H, B-H), 7.25, (d, 2H, C6H4 J = 7.91 Hz), 7.17 (d, 2H, C6H4 J = 7.83 Hz), 5.23 (s, 2H, NH2), 4.01 (M, 2H, NH2CH2C6H4CH3), 3.08 (8H, NBu4), 2.32 (s, 3H, NH2CH2C6H4CH3), 1.60 (8H, NBu4), 1.35 (8H, NBu4), 0.97 (12H, NBu4). 13C NMR (CD3CN, ppm): 139.2, 133.2, 130.1 (NH2CH2C6H4CH3), 59.4 (NBu4), 52.7 (NH2CH2C6H4CH3), 24.3 (NBu4), 21.1 (NH2CH2C6H4CH3), 20.3 (NBu4), 13.8 (NBu4). IR (CH2Cl2, cm−1): 3288, 3197, 3122 ν (N-H), 2491 ν (B-H), 1635 ν (C=C). MS (ESI) m/z: 262.2964 (A refers to the molecular weight of [B12H11(NH2CH2C6H4CH3)], calculated for {[A]-} 262.2942).

4. Conclusions

During this work, a technique for selectively obtaining N-alkylammonium derivatives of the closo-dodecaborate anion [B12H11NH2CH2R]− by reducing the corresponding nitrile derivatives [B12H11NCR]− with lithium aluminum hydride (LiAlH4) was developed. A series of N-alkylammonium derivatives with different substituents R were obtained, including Me (methyl), Et (ethyl), nPr (n-propyl), nPr (isopropyl), and p-tolyl. The analysis of the crystal structure (Bu4N)[B12H11(NH2CH2CH3)] showed that anions form dimeric pairs linked by NH…HB dihydrogen bonds.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics12010002/s1, Figure S1: 11B{1H} NMR spectrum (Bu4N)[B12H11NH2CH2CH3] 2a; Figure S2: 1H NMR spectrum (Bu4N)[B12H11NH2CH2CH3] 2a; Figure S3: 13C NMR spectrum (Bu4NN)[B12H11NH2CH2CH3] 2a; Figure S4: ESI-MS spectrum (Bu4N)[B12H11NH2CH2CH3] 2a (negative area) 1; Figure S5: 11B{1H} NMR spectrum (Bu4N)[B12H11NH2CH2CH2CH3] 2b; Figure S6: 1H NMR spectrum (Bu4N)[B12H11NH2CH2CH2CH3] 2b; Figure S7: 13C NMR spectrum (Bu4N)[B12H11NH2CH2CH2CH3] 2b; Figure S8: ESI-MS spectrum (Bu4N)[B12H11NH2CH2CH2CH3] 2b (negative area).; Figure S9: 11B{1H} NMR spectrum (Bu4N)[B12H11NH2CH2CH2CH2CH3] 2c; Figure S10: 1H NMR spectrum (Bu4N)[B12H11NH2CH2CH2CH2CH3] 2c; Figure S11: 13C NMR spectrum (Bu4N)[B12H11NH2CH2CH2CH2CH3] 2c; Figure S12: ESI-MS spectrum (Bu4N)[B12H11NH2CH2CH2CH2CH3] 2c (negative area).; Figure S13: 11B{1H} NMR spectrum (Bu4N)[B12H11NH2CH2CH(CH3)2] 2d; Figure S14: 1H NMR spectrum (Bu4N)[B12H11NH2CH2CH(CH3)2] 2d; Figure S15: 13C NMR spectrum (Bu4N)[B12H11NH2CH2CH(CH3)2] 2d; Figure S16: ESI-MS spectrum (Bu4N)[B12H11NH2CH2CH(CH₃)2] 2d (negative area).; Figure S17: 11B{1H} NMR spectrum (Bu4N)[B12H11NH2CH2C6H4CH3] 2e; Figure S18: 1H NMR spectrum (Bu4N)[B₁₂H₁₁NH2CH2C6H4CH3] 2e; Figure S19: 13C NMR spectrum (Bu4N)[B12H11NH2CH2C6H4CH3] 2e; Figure S20: ESI-MS spectrum (Bu4N)[B12H11NH2CH2C6H4CH3] 2e (negative area).; Figure S21: Molecular graph showing the results of the topological analysis of the electron density distribution in the model structure of the trimer of [B12H11NH2CH2CH3]−.

Author Contributions

Manuscript conception, A.P.Z. and K.Y.Z.; writing and original draft preparation, A.V.N., N.K.N. and A.P.Z.; synthesis of derivatives, A.V.N. and N.K.N.; NMR analysis, N.A.S. and A.Y.B.; X-ray analysis and Hirshfeld surface analysis, A.S.K.; editing, data analysis, and interpretation, I.N.K., A.S.N., A.P.Z. and K.Y.Z.; supervision, K.Y.Z. and N.T.K. All authors have read and agreed to the published version of the manuscript.

Funding

The work was supported by the Ministry of Science and Higher Education of the Russian Federation. Agreement No. 075-15-2020-782.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

A.S.N. is grateful to the RUDN University Strategic Academic Leadership Program.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 12 00002 sch001
Scheme 1. Reaction of nitrilium derivatives with lithium aluminum hydride.
Scheme 1. Reaction of nitrilium derivatives with lithium aluminum hydride.
Inorganics 12 00002 sch001
Inorganics 12 00002 g001
Figure 1. The structure of the anion [B12H11NH2Et]− according to X-ray structural analysis. Thermal ellipsoids are shown with a 50% probability. Selected bond lengths and angles in the structure of 2a: lengths, Å: B1-N1 1.587(4); N1-C1 1.432(4); C1-C2 1.487(5); B-B 1.760(5)-1.808(5) (1.783 avg.); angles, °: B1-N1-C1 119.4(3); N1-C1-C2 116.4(3).
Figure 1. The structure of the anion [B12H11NH2Et]− according to X-ray structural analysis. Thermal ellipsoids are shown with a 50% probability. Selected bond lengths and angles in the structure of 2a: lengths, Å: B1-N1 1.587(4); N1-C1 1.432(4); C1-C2 1.487(5); B-B 1.760(5)-1.808(5) (1.783 avg.); angles, °: B1-N1-C1 119.4(3); N1-C1-C2 116.4(3).
Inorganics 12 00002 g001
Inorganics 12 00002 g002
Figure 2. (a) Packing diagram, (b) dnorm surface of [B12H11NH2Et]− anion for compound 2a, (c) full two-dimensional fingerprint plot for 2a. The area of NH…HB contacts, that are less than the sum of the van der Waals radii of hydrogen atoms, is highlighted with a red oval.
Figure 2. (a) Packing diagram, (b) dnorm surface of [B12H11NH2Et]− anion for compound 2a, (c) full two-dimensional fingerprint plot for 2a. The area of NH…HB contacts, that are less than the sum of the van der Waals radii of hydrogen atoms, is highlighted with a red oval.
Inorganics 12 00002 g002
Table 1. Crystal data and structure refinement for 2a.
Table 1. Crystal data and structure refinement for 2a.
Compound2a
Empirical formulaC18H54B12N2
Formula weight428.35
Temperature/K150.00
Crystal systemmonoclinic
Space groupP21/n
a/Å10.292(12)
b/Å12.938(10)
c/Å21.49(3)
β/°90.15(6)
90
Volume/Å32862(5)
Z4
ρcalcg/cm30.994
μ/mm−10.050
F(000)944.0
RadiationMoKα (λ = 0.71073)
2Θ range for data collection/°3.674 to 51.994
Reflections collected11,524
Independent reflections5562 [Rint = 0.0379, Rsigma = 0.0722]
Data/restraints/parameters5562/7/295
Goodness of fit on F21.057
Final R indexes [I >= 2σ (I)]R1 = 0.0783, wR2 = 0.2136
Final R indexes [all data]R1 = 0.1340, wR2 = 0.2438
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Nelyubin, A.V.; Neumolotov, N.K.; Selivanov, N.A.; Bykov, A.Y.; Klyukin, I.N.; Novikov, A.S.; Kubasov, A.S.; Zhdanov, A.P.; Zhizhin, K.Y.; Kuznetsov, N.T. Reduction of Triple Bond in [B12H11NCR]− Anions by Lithium Aluminum Hydride: A Novel Approach to the Synthesis of N-Monoalkylammonio-Substituted closo-Dodecaborates. Inorganics 2024, 12, 2. https://doi.org/10.3390/inorganics12010002

AMA Style

Nelyubin AV, Neumolotov NK, Selivanov NA, Bykov AY, Klyukin IN, Novikov AS, Kubasov AS, Zhdanov AP, Zhizhin KY, Kuznetsov NT. Reduction of Triple Bond in [B12H11NCR]− Anions by Lithium Aluminum Hydride: A Novel Approach to the Synthesis of N-Monoalkylammonio-Substituted closo-Dodecaborates. Inorganics. 2024; 12(1):2. https://doi.org/10.3390/inorganics12010002

Chicago/Turabian Style

Nelyubin, Alexey V., Nikolay K. Neumolotov, Nikita A. Selivanov, Alexander Yu. Bykov, Ilya N. Klyukin, Alexander S. Novikov, Alexey S. Kubasov, Andrey P. Zhdanov, Konstantin Yu. Zhizhin, and Nikolay T. Kuznetsov. 2024. "Reduction of Triple Bond in [B12H11NCR]− Anions by Lithium Aluminum Hydride: A Novel Approach to the Synthesis of N-Monoalkylammonio-Substituted closo-Dodecaborates" Inorganics 12, no. 1: 2. https://doi.org/10.3390/inorganics12010002

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