Synthesis and Structure of Nido-Carboranyl Azide and Its “Click” Reactions

Novel zwitter-ionic nido-carboranyl azide 9-N3(CH2)3Me2N-nido-7,8-C2B9H11 was prepared by the reaction of 9-Cl(CH2)3Me2N-nido-7,8-C2B9H11 with NaN3. The solid-state molecular structure of nido-carboranyl azide was determined by single-crystal X-ray diffraction. 9-N3(CH2)3Me2N-nido-7,8-C2B9H11 was used for the copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene, alkynyl-3β-cholesterol and cobalt/iron bis(dicarbollide) terminal alkynes to form the target 1,2,3-triazoles. The nido-carborane-cholesterol conjugate 9-3β-Chol-O(CH2)C-CH-N3(CH2)3Me2N-nido-7,8-C2B9H11 with charge-compensated group in a linker can be used as a precursor for preparation of liposomes for Boron Neutron Capture Therapy (BNCT). A series of novel zwitter-ionic boron-enriched cluster compounds bearing a 1,2,3-triazol-metallacarborane-carborane conjugated system was synthesized. Prepared conjugates contain a large amount of boron atom in the biomolecule and potentially can be used for BNCT.


Introduction
Nido-Carborane (7,8-dicarba-nido-undecaborate anion) and its derivatives attract attention due to its unique electronic structure, namely, electron delocalization, which is often considered as an unusual three-dimensional aromaticity [1,2], and also due to its electron-withdrawing character of the skeletal carbon atoms. Such structural and physicochemical properties of 7,8-dicarba-nido-undecaborate anion induce the preparation of new functional derivatives of nido-carboranes with a variety of practical applications. These compounds attract the continued interest of researchers working in various fields, such as medical chemistry (boron neutron capture therapy (BNCT) of malignant tumors [3][4][5], HIV protease inhibitors [6], reagents for radioimaging of tumors [7]), and creation of new materials [1] (carborane-containing polymers, ionic liquids, liquid crystals).
It is well known that BNCT is a binary method of cancer treatment, in which it is necessary that boron compounds selectively accumulate in the tumor tissue at the required therapeutic concentration for its subsequent irradiation with thermal neutrons [8][9][10]. The advantage of using derivatives of nido-carboranes in BNCT is their stability and a high content of boron atoms in the molecule. In this regard, the new methods for the synthesis of various functional mono-substituted derivatives of nido-carborane should be developed. Currently, there are several methods for the preparation of mono-substituted nido-carboranes: C-substituted derivatives can be obtained by modification of the carborane cage at carbon atoms of the parent ortho-carborane followed by its deboronation to the corresponding nido-carborane. Last time, the most cited methods of synthesis B-substituted functional derivatives of nido-carboranes are ring opening of their cyclic oxonium derivatives under the action of nucleophilic agents [11][12][13][14], alkylation of their methylsulfide derivatives [15][16][17][18], Cu-promoted synthesis of their ammonium derivatives [19] and nucleophilic addition of alcohols and mercaptans to highly polarized triple bond B-N + ≡CR [20].
Synthesis of mono-substituted derivatives of nido-carborane with a functional group makes it possible to attach a carborane fragment to various bio-and macromolecules and thus to obtain compounds with a given set of properties. The substituent introduced can be a biologically active derivative that acts as a tumor targeting vector, or a simple functional group, which can be used for conjugation with high molecular weight biomolecules using standard methods of bioorganic chemistry. Recently the Cu(I)-catalyzed reaction of 1,3-dipolar [3 + 2]-cycloaddition of azides with alkynes ("click" reaction) has found more and more widespread use for the bioconjugation of molecules [21][22][23][24][25]. Such reactions must proceed rapidly under ambient conditions, resulting in a high yield of desired 1,2,3-triazole. The 1,2,3-triazole scaffold is known to be one of the most valuable in the chemistry of biologically active compounds [26], which exhibit anticancer [27][28][29], anti-HIV [27,29,30], antibacterial [27,29,31,32], antioxidant [27,29,33,34] activities. In addition, the 1,2,3-triazole in the molecules between boron-containing and the biologically active fragments (porphyrins, nucleosides) is a linking unit that mimics geometry and electronic properties of the peptide bond is a more stable to hydrolysis reactions [14,35]. However, the preparation of a suitable nido-carborane-containing substrate for a "click" reaction often requires multistage syntheses, for instance, either the incorporation of a distant or cage-bound azide group or a terminal alkyne. Azide is one the most popular biorthogonal functional groups due to its small size coupled with stability to water and inertness towards endogenous biological functionalities [36]. It should be noted that only a few examples of azido-containing nidocarboranes are known to date. Recently we have obtained C-substituted nido-carboranyl azides [7-N 3 CH 2 CH 2 OCH 2 CH 2 O-nido-7,8-C 2 B 9 H 11 ] − and [7-N 3 CH 2 CH 2 OCH 2 CH 2 S-nido-7,8-C 2 B 9 H 11 ] − which were synthesized by alkylation of 1-mercapto-ortho-carborane with bis(2-chloroethyl) ether followed by introduction of azide group and by the conversion of closo-derivative to nido-form [37]. B-substituted [10-N 3 CH 2 CH 2 CH 2 CH 2 O-nido-7,8-C 2 B 9 H 11 ] − and [10-N 3 CH 2 CH 2 OCH 2 CH 2 O-nido-7,8-C 2 B 9 H 11 ] − were prepared by the ring-opening reactions of the corresponding cyclic oxonium derivatives with sodium azide [11,14].

Results and Discussion
2.1. Synthesis of 9-N 3 (CH 2 ) 3 Me 2 N-nido-7,8-C 2 B 9 H 11 and Its "Click" Reaction with Phenylacetylene The reaction of 9-Cl(CH 2 ) 3 Me 2 N-nido-7,8-C 2 B 9 H 11 1 with NaN 3 in the presence of NaI as a catalyst in DMF upon prolonged heating under 50 • C for 7 days results in 9-N 3 (CH 2 ) 3 Me 2 N-nido-7,8-C 2 B 9 H 11 2 with 90% yield (Scheme 1). It was isolated as a white non-hydroscopic solid soluble in common organic solvents like CH 2 Cl 2 , CH 3 CN, alcohols and non-soluble in hydrocarbons and water. The 11 B-NMR spectrum contains eight signals indicating a nonsymmetrical monosubstituted structure. The 1 H-NMR spectrum of the signal of the methylene group bonded to the nitrogen atom exhibits a singlet at 3.37 ppm. It should be noted that the splitting reduces as the distance from the nitrogen atom increases resulting in singlet and multiplet at 2.15 and 3.50 ppm for the second and third methylene groups. In addition, the signals Scheme 1. Reaction of 9-Cl(CH 2 ) 3 Me 2 N-nido-7,8-C 2 B 9 H 11 with NaN 3 . The 11 B-NMR spectrum contains eight signals indicating a nonsymmetrical monosubstituted structure. The 1 H-NMR spectrum of the signal of the methylene group bonded to the nitrogen atom exhibits a singlet at 3.37 ppm. It should be noted that the splitting reduces as the distance from the nitrogen atom increases resulting in singlet and multiplet at 2.15 and 3.50 ppm for the second and third methylene groups. In addition, the signals of the CH carb groups and the extra-hydrogen are observed approx. at 2.59 and -3.4 ppm, correspondingly. In the 13 C-NMR spectrum the most characteristic is signal of CH 2 N 3 group. As observed earlier [19], the signal of the CH 2 Cl group in the 13 C-NMR spectrum appears at 43.0 ppm for 1. The substitution of chlorine for azide results in the low-field shift to 65.2 ppm. The azide stretching band in the IR spectrum of 2 is located at 2075 cm -1 .
Conditions of "click" reactions in the preparation of various boron-containing biomolecules vary particularly wide. Earlier "click" reactions have been successfully used for the synthesis of conjugates of bis(dicarbollide) metallacarboranes and nido-carborane with thymidine [14]. The synthesis was carried out in a mixture of tert-butanol/water (1:1) at ambient temperature using copper(II) sulfate pentahydrate with potassium ascorbate as a catalyst. The same reaction for synthesis of conjugate dodecaborate dianion with thymidine proceed in CH 3 CN at ambient temperature with copper(II) sulfate pentahydrate with sodium ascorbate [38]. Conjugates of chlorine e 6 with a cobalt bis(dicarbollide) anion or closo-dodecaborate dianion were obtained using CuI and Et 3 N in acetonitrile at ambient temperatures [35]. Series of 1,2,3-triazoles bearing closo-dodecaborate fragment was obtained using CuI as a catalyst and Et 3 N as a base under reflux in ethanol [39].
In the present work, we studied the behavior of azide 2 in copper(I)-catalyzed azidealkyne cycloaddition using simple terminal alkyne as a pilot compound. It was showed that it readily reacts with phenylacetylene in ethanol in the presence of diisopropylethylamine (DIPEA) and catalytic amount of CuI to give the corresponding 1,2,3-triazole 3 with 85 % yield (Scheme 2). Scheme 1. Reaction of 9-Cl(CH2)3Me2N-nido-7,8-C2B9H11 with NaN3.
The 11 B-NMR spectrum contains eight signals indicating a nonsymmetrical monosubstituted structure. The 1 H-NMR spectrum of the signal of the methylene group bonded to the nitrogen atom exhibits a singlet at 3.37 ppm. It should be noted that the splitting reduces as the distance from the nitrogen atom increases resulting in singlet and multiplet at 2.15 and 3.50 ppm for the second and third methylene groups. In addition, the signals of the CHcarb groups and the extra-hydrogen are observed approx. at 2.59 and -3.4 ppm, correspondingly. In the 13 C-NMR spectrum the most characteristic is signal of CH2N3 group. Аs observed earlier [19], the signal of the CH2Cl group in the 13 C-NMR spectrum appears at 43.0 ppm for 1. The substitution of chlorine for azide results in the low-field shift to 65.2 ppm. The azide stretching band in the IR spectrum of 2 is located at 2075 cm -1 .
Conditions of "click" reactions in the preparation of various boron-containing biomolecules vary particularly wide. Earlier "click" reactions have been successfully used for the synthesis of conjugates of bis(dicarbollide) metallacarboranes and nido-carborane with thymidine [14]. The synthesis was carried out in a mixture of tert-butanol/water (1:1) at ambient temperature using copper(II) sulfate pentahydrate with potassium ascorbate as a catalyst. The same reaction for synthesis of conjugate dodecaborate dianion with thymidine proceed in CH3CN at ambient temperature with copper(II) sulfate pentahydrate with sodium ascorbate [38]. Conjugates of chlorine e6 with a cobalt bis(dicarbollide) anion or closo-dodecaborate dianion were obtained using CuI and Et3N in acetonitrile at ambient temperatures [35]. Series of 1,2,3-triazoles bearing closo-dodecaborate fragment was obtained using CuI as a catalyst and Et3N as a base under reflux in ethanol [39].
In the present work, we studied the behavior of azide 2 in copper(I)-catalyzed azidealkyne cycloaddition using simple terminal alkyne as a pilot compound. It was showed that it readily reacts with phenylacetylene in ethanol in the presence of diisopropylethylamine (DIPEA) and catalytic amount of CuI to give the corresponding 1,2,3-triazole 3 with 85 % yield (Scheme 2). Scheme 2. "Сlick" reaction of nido-carboranyl azide with phenylacetylene.
The structure of the nido-carborane 3 was confirmed by the data of 1 H-, 11 B-and 13 C-NMR, IR spectroscopy and HRMS. The 1 H-and 13 C-NMR spectra of compounds 3 along with the signals of the heteroaliphatic chain and the phenyl group contain the characteristic signals of the triazole cycle in the 1 H-NMR spectrum signal of the CHtriazole hydrogen appears at 8.62 ppm. In the 13 C-NMR spectrum, the signal of the CHtriazole carbon is observed at 122.0 ppm, whereas the signal of the Ctriazole carbon appears at 146.9 ppm. In the 1 H-NMR spectrum, the signal of the methylene group next to the triazole cycle is observed Scheme 2. "Click" reaction of nido-carboranyl azide with phenylacetylene.
The structure of the nido-carborane 3 was confirmed by the data of 1 H-, 11 B-and 13 C-NMR, IR spectroscopy and HRMS. The 1 H-and 13 C-NMR spectra of compounds 3 along with the signals of the heteroaliphatic chain and the phenyl group contain the characteristic signals of the triazole cycle in the 1 H-NMR spectrum signal of the CH triazole hydrogen appears at 8.62 ppm. In the 13 C-NMR spectrum, the signal of the CH triazole carbon is observed at 122.0 ppm, whereas the signal of the C triazole carbon appears at 146.9 ppm. In the 1 H-NMR spectrum, the signal of the methylene group next to the triazole cycle is observed at 4.54 ppm and the characteristic signal of the Me 2 N hydrogens appear at 2.88 ppm. The IR spectrum of 3 demonstrates an absence of the azide band stretching and the appearance of the band of the triazole cycle at 1462 cm −1 .

Synthesis of Nido-Carboranyl Cholesterol Derivative with Charge-Compensated Group
Furthermore, compound 2 was used for synthesis of boronated cholesterol as precursor for the preparation of liposomes. The usage of liposomes is the important approach directed to selective delivery of therapeutics into tumors [40][41][42]. The development of selective, non-toxic boron delivery agents that can preferentially deliver a high concentration of boron to the tumor is probably the greatest need for the future progress of BNCT [43]. Due to the high permeability of the walls of blood vessels inside the tumor, stagnant blood flow occurs and lymphatic outflow is disrupted. These changes lead to the EPR (enhanced permeability and retention) effect, due to which macromolecules and nanoparticles such as liposomes penetrate from the bloodstream of the tumor vessel into the intercellular space and accumulate mainly in the tumor tissue [44][45][46]. Cholesterol is the major component of the cell membrane and most liposomal formulations. Therefore, the development of boronated derivatives of cholesterol is an effective approach for the selective delivery of boron clusters into the cancer cells via liposomes. Recently, using "click" reactions we obtained a series of mono-negative charged conjugates of cobalt bis(dicarbollide) with cholesterol [47], conjugates of cobalt/iron of bis(dicarbollide) and cholesterol with similar length spacer but with zwitter-ionic character of target molecule [48] and conjugates of closo-dodecaborate dianion with cholesterol [49]. It has been only recently shown that the inclusion of lipophilic boron-containing species in the liposome bilayer provides an attractive method to increase the gross boron content of the liposomes in the formulation [50,51]. In addition, it has been found that PEGylated liposome encapsulating nido-carborane by hydrating thin lipid films significantly suppresses tumors in boron neutron capture therapy [52].
In the present work, we use the "click" methodology to obtain new conjugate of nido-carborane with cholesterol suitable for the preparation of boron-containing liposomes as potential drugs for boron neutron capture therapy of cancer. Usage of nido-carborane 2 for synthesis of carborane-cholesterol conjugates leads to zwitter-ionic character of product structure. Its reaction proceeded in a slight excess of alkynyl-3β-cholesterol 4 in the presence of a CuI catalyst and diisopropylethylamine (DIPEA) as a base in ethanol upon prolonged reflux for 8 h to give novel boron conjugate 5 with 85 % yield (Scheme 3).

Synthesis of Nido-Carboranyl Cholesterol Derivative with Charge-Compensated Group
Furthermore, compound 2 was used for synthesis of boronated cholesterol as precursor for the preparation of liposomes. The usage of liposomes is the important approach directed to selective delivery of therapeutics into tumors [40][41][42]. The development of selective, non-toxic boron delivery agents that can preferentially deliver a high concentration of boron to the tumor is probably the greatest need for the future progress of BNCT [43]. Due to the high permeability of the walls of blood vessels inside the tumor, stagnant blood flow occurs and lymphatic outflow is disrupted. These changes lead to the EPR (enhanced permeability and retention) effect, due to which macromolecules and nanoparticles such as liposomes penetrate from the bloodstream of the tumor vessel into the intercellular space and accumulate mainly in the tumor tissue [44][45][46]. Cholesterol is the major component of the cell membrane and most liposomal formulations. Therefore, the development of boronated derivatives of cholesterol is an effective approach for the selective delivery of boron clusters into the cancer cells via liposomes. Recently, using "click" reactions we obtained a series of mono-negative charged conjugates of cobalt bis(dicarbollide) with cholesterol [47], conjugates of cobalt/iron of bis(dicarbollide) and cholesterol with similar length spacer but with zwitter-ionic character of target molecule [48] and conjugates of closo-dodecaborate dianion with cholesterol [49]. It has been only recently shown that the inclusion of lipophilic boron-containing species in the liposome bilayer provides an attractive method to increase the gross boron content of the liposomes in the formulation [50,51]. In addition, it has been found that PEGylated liposome encapsulating nido-carborane by hydrating thin lipid films significantly suppresses tumors in boron neutron capture therapy [52].
In the present work, we use the "click" methodology to obtain new conjugate of nidocarborane with cholesterol suitable for the preparation of boron-containing liposomes as potential drugs for boron neutron capture therapy of cancer. Usage of nido-carborane 2 for synthesis of carborane-cholesterol conjugates leads to zwitter-ionic character of product structure. Its reaction proceeded in a slight excess of alkynyl-3β-cholesterol 4 in the presence of a CuI catalyst and diisopropylethylamine (DIPEA) as a base in ethanol upon prolonged reflux for 8 h to give novel boron conjugate 5 with 85 % yield (Scheme 3). The 1 H-NMR spectrum of complex 5 contains a signal for the proton of the triazole group at 8.01 ppm. The characteristics signal of the alkyne CH st hydrogen of cholesterol in the conjugate 5 is observed at 5.37 ppm. The spectral characteristics of the CH protons of cholesterol are in good agreement with the literature data [53]. The 13 C-NMR spectrum of 5 exhibits signals for two carbon atoms of the triazole fragment at 145.8 ppm for CH triazole atom and at 123.1 ppm for C triazole . In the 11 H-NMR spectrum, the signal of the extrahydrogen, as expected, is observed approx. at −3.4 ppm. The IR spectrum of compound 5 exhibits absorption bands characteristic of the BH group 2685 cm −1 and the triazole ring 1392 cm −1 .
Based on synthesized compounds the boronated liposomes are planned to prepare in order to deliver boron clusters into a cancer cell for the BNCT experiment.

Synthesis of Zwitter-Ionic Boron-Enriched Cluster Compounds Bearing a 1,2,3-Triazol-Metallacarborane-Nido-Carborane Conjugated Systems
As was mentioned above, functionalized nido-carboranes can be used as building blocks for design and construction of boron-containing compounds for various medical applications [3,4,54]. For example, one of the important requirements of BNCT is the synthesis of structures with a higher content of boron atoms in the molecule than in the clinically used compounds [55,56]. In this contribution, we propose to combine two boron clusters into one molecule: the bis(dicarbollide) cluster serves as a boron-containing substituent providing low toxicity [57,58] and amphiphilicity [59,60] of the molecule and the nido-carborane cage serves as a boron-containing base for attachment to molecules by the "click" reaction. Penetration of various substances through biological membranes, their accumulation and retention in cells largely depend on their charge. It is known that positively charged particles have better penetration through biological membranes than negatively changed ones [61][62][63]. It motivated us that the synthesis of such compounds is based on the introduction of two ammonium centers in a spacer: the first one compensating the negative charge of the nido-carborane fragment and the second one compensating the charge of the cobalt/iron bis(dicarbollide) moiety. This allows us to double the boron content of the biomolecule as compared to the single cage approach. Moreover, by changing the type and the size of a spacer between these two boron cages, it is possible to control, to some extent, the hydrophilic/hydrophobic balance of the compounds.
At the first step, cobalt and iron bis(dicarbollide) terminal alkynes with chargecompensated group 6-9 were prepared by the cleavage reactions of oxonium derivatives of cobalt/iron bis(dicarbollide) with N,N-dimethylprop-2-yn-1-amine [48,64]. It was found that alkynes prepared from 1,4-dioxane and tetrahydropyran derivatives of cobalt bis(dicarbollide) 6 and 7 [48] readily undergo "click" reactions with a small excess of azidoderivative of nido-carborane 2 to give novel boron conjugates 10 and 11. High preparative yields (85 %) of the desired products 10 and 11 was achieved using CuI in the presence diisopropylethylamine (DIPEA) as a catalyst and running the reaction for 8 h under reflux. At the same time, usage of the alkyne synthesized from tetrahydropyran derivative of iron bis(dicarbollide) 9 in "click" reaction with 2 leads to dramatically decrease of yield of product 12 (54 %). However, the alkyne synthesized from 1,4-dioxane derivatives of iron bis(dicarbollide) 8 does not react with azido-derivative 2 at all under the same conditions as for the compounds 6, 7, 9 and leads to the recovery of the starting materials. It should be noted that a similar difference of behavior of iron and cobalt complexes was observed earlier in the reaction of cobalt/iron bis(dicarbollide) terminal alkynes 6-9 with azido-cholesterol [48]. Alkyne 8 had not reacted with azido-cholesterol and had not led to target triazole 12 in contrast to alkynes 6, 7 and 9. These results pushed us to carry out the reaction of alkyne 8 with methyl azidoacetate under the same conditions as for compounds 6, 7 and 9. However, we did not get the desired result. Employing another amine and/or solvent (e.g., Et 3 N in CH 3 CN, Et 3 N in EtOH, DIPEA in CH 3 CN) did not provide the required material as well. The use of a 3-fold excess of CuI, EtOH as a solvent, DIPEA as a base and running the reaction for 8 h under reflux also did not lead to formation of the target 1,2,3-triazole (Scheme 4). Structures of the compounds 10-12 were established by 1 H-, 11 B-and 13 C-NMR and IR spectra. In IR spectra of them the absorption bands of BH (2524-2531 cm -1 ) and 1,2,3triazole (1461-1464 cm −1 ) were observed. In the 1 H-NMR spectra of the obtained compounds, the characteristics signals of the CHtriazole hydrogens appear in the region at 6.52-8.64 ppm. In the 13 C-NMR spectra, the signals of the CHtriazole carbons for 10-12 are observed in the range of 126.8-129.1 ppm, whereas the signals of the Ctriazole carbons appear in the range 133.8-135.9 ppm. In IR spectra of 10-12 the characteristic bands 1,2,3-triazoles (1461-1464 cm −1 ) were observed.
The compounds obtained are of potential interest for the delivery of boron-enriched drugs in boron neutron capture cancer therapy. Structures of the compounds 10-12 were established by 1 H-, 11 B-and 13 C-NMR and IR spectra. In IR spectra of them the absorption bands of BH (2524-2531 cm -1 ) and 1,2,3-triazole (1461-1464 cm −1 ) were observed. In the 1 H-NMR spectra of the obtained compounds, the characteristics signals of the CH triazole hydrogens appear in the region at 6.52-8.64 ppm.
In the 13 C-NMR spectra, the signals of the CH triazole carbons for 10-12 are observed in the range of 126.8-129.1 ppm, whereas the signals of the C triazole carbons appear in the range 133.8-135.9 ppm. In IR spectra of 10-12 the characteristic bands 1,2,3-triazoles (1461-1464 cm −1 ) were observed.
The compounds obtained are of potential interest for the delivery of boron-enriched drugs in boron neutron capture cancer therapy.

Single-Crystal X-ray Diffraction Studies
The structure of 9-N 3 CH 2 Me 2 N-nido-7,8-C 2 B 9 H 11 2 was additionally confirmed by the single crystal X-ray diffraction study (Figure 1). Crystals of 2 suitable of single crystal X-ray analysis were grown from the dichloromethane solution layered with hexane. Scheme 4. Synthesis of conjugates of nido-carborane with cobalt/iron bis(dicarbollide)s.
Structures of the compounds 10-12 were established by 1 H-, 11 B-and 13 C-NMR and IR spectra. In IR spectra of them the absorption bands of BH (2524-2531 cm -1 ) and 1,2,3triazole (1461-1464 cm −1 ) were observed. In the 1 H-NMR spectra of the obtained compounds, the characteristics signals of the CHtriazole hydrogens appear in the region at 6.52-8.64 ppm. In the 13 C-NMR spectra, the signals of the CHtriazole carbons for 10-12 are observed in the range of 126.8-129.1 ppm, whereas the signals of the Ctriazole carbons appear in the range 133.8-135.9 ppm. In IR spectra of 10-12 the characteristic bands 1,2,3-triazoles (1461-1464 cm −1 ) were observed.
The compounds obtained are of potential interest for the delivery of boron-enriched drugs in boron neutron capture cancer therapy.

Single-Crystal X-ray Diffraction Studies
The structure of 9-N3CH2Me2N-nido-7,8-C2B9H11 2 was additionally confirmed by the single crystal X-ray diffraction study (Figure 1). Crystals of 2 suitable of single crystal Xray analysis were grown from the dichloromethane solution layered with hexane. Structural features of 2 are expected for this class of compounds (see, for instance [19]) with the only exception of rotation of the substituent at the B9 atom ( Figure 1). Namely, the compact conformation of substituent is observed that results in rather short intramolecular contact between the N4 atom of azide function and the H1C extra-hydrogen atom of nido-carborane cage (3.089 Å with non-normalized B-H bond lengths). A similar contact of the extra hydrogen atom with the acetylene fragment previously found in the crystal of 10-Me3SiC≡CCH2(Me)S-7,8-C2B9H11 was attributed to the attractive B-H…π Structural features of 2 are expected for this class of compounds (see, for instance [19]) with the only exception of rotation of the substituent at the B9 atom ( Figure 1). Namely, the compact conformation of substituent is observed that results in rather short intramolecular contact between the N4 atom of azide function and the H1C extra-hydrogen atom of nido-carborane cage (3.089 Å with non-normalized B-H bond lengths). A similar contact of the extra hydrogen atom with the acetylene fragment previously found in the crystal of 10-Me 3 SiC≡CCH 2 (Me)S-7,8-C 2 B 9 H 11 was attributed to the attractive B-H . . . π interaction which hindered free rotation of substituent in a solution [16]. For 2, the DFT calculation of the isolated molecule was performed to get more insight into the nature of the H1C . . . N4 contact. It was found that the conformation of 2 is rather rigid and is influenced by media effects only slightly: the best overlap for non-hydrogen atoms of crystal and isolated molecular structures produces a rather small rmsd value of 0.17 Å (Figure 2). Despite the most deviations are observed for the coordinates of azide fragment, the H1C . . . N4 contact lengthens insignificantly upon the transition into the isolated state (3.217 Å). According to the analysis of theoretical electron density function ρ(r) in terms of the R. Bader's "Atoms in Molecules" [65] theory, the compact conformation of 2 can be indeed stabilized by intramolecular non-covalent interactions. In particular, there is a bond path connecting the H1C and N4 atoms which can be considered as the privileged channel of interatomic exchange interaction (Figure 3) [66,67]. This bond path is significantly curved in the area of the azide fragment and less curved in the area of the H1C atom. Implying a non-directional character of the interaction the bond path curvature allows to consider at least the π-electrons of the N3-N4 fragment to be involved into the non-covalent bonding with the H1C atom. This is in concordance with the another real-space descriptor of non-covalent interactions based on the reduced density gradient formalism [68]. The distribution of the sign (λ 2 )·ρ(r) function (λ 2 is the intermediate eigenvalue of the ρ(r) Hessian) mapped onto the 0.4 isosurface of reduced density gradient (see Figure 3) is characterized by a rather wide region of negative values of sign (λ 2 )·ρ(r) which correspond to the concentration of electronic charge between H1C, N4 and N3 atoms. It is interesting to note that the distribution of Bader's integral atomic charges does not allow to unambiguously treat this interaction as the H-bond (the charges of H1C, N4 and N3 atoms equal to −0.56e, +0.12e and −0.16e, respectively). Finally, according to the ρ(r) surface integrals scheme [69], this non-covalent interaction of the extra-hydrogen atom is rather strong (2.1 kcal·mol −1 ) and, thus, can be indeed regarded as the important factor which stabilizes the compact conformation of 2 and, probably, relative compounds. of the R. Bader's "Atoms in Molecules" [65] theory, the compact conformation of 2 can be indeed stabilized by intramolecular non-covalent interactions. In particular, there is a bond path connecting the H1C and N4 atoms which can be considered as the privileged channel of interatomic exchange interaction ( Figure 3) [66,67]. This bond path is significantly curved in the area of the azide fragment and less curved in the area of the H1C atom. Implying a non-directional character of the interaction the bond path curvature allows to consider at least the π-electrons of the N3-N4 fragment to be involved into the non-covalent bonding with the H1C atom. This is in concordance with the another realspace descriptor of non-covalent interactions based on the reduced density gradient formalism [68]. The distribution of the sign (λ2)·ρ(r) function (λ2 is the intermediate eigenvalue of the ρ(r) Hessian) mapped onto the 0.4 isosurface of reduced density gradient (see Figure 3) is characterized by a rather wide region of negative values of sign (λ2)·ρ(r) which correspond to the concentration of electronic charge between H1C, N4 and N3 atoms. It is interesting to note that the distribution of Bader's integral atomic charges does not allow to unambiguously treat this interaction as the H-bond (the charges of H1C, N4 and N3 atoms equal to −0.56e, +0.12e and −0.16e, respectively). Finally, according to the ρ(r) surface integrals scheme [69], this non-covalent interaction of the extra-hydrogen atom is rather strong (2.1 kcal·mol −1 ) and, thus, can be indeed regarded as the important factor which stabilizes the compact conformation of 2 and, probably, relative compounds.
Single crystal X-ray diffraction experiment for 2 was performed using with a Bruker SMART APEX 2 Duo CCD diffractometer [λ(MoKα) = 0.71072Å, ω-scans, 2θ < 60 • ] (Bruker AXC, Maddison, Wisconsin, USA). The structure was solved by the direct method and refined by the full-matrix least-squares technique against F2 in the anisotropic-isotropic approximation. The hydrogen atoms were found in the difference Fourier synthesis and were further refined in the isotropic approximation without restraints. All calculations were performed using SHELX2018 [70]. The CCDC 2048027 contains the Supplementary Crystallographic Data for 2. These data can be obtained free of charge via http://www. ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge, CB21EZ, UK; or deposit@ccdc.cam.ac.uk).
The DFT calculations were done in the Gaussian09 program (rev. D01) [71] using the PBE0 functional [72,73] with the Grimme's D3 dispersion correction [74] and Becke-Jonson damping [75]. The standard def2tzvp basis set and ultrafine integration grids were used. The geometry optimization procedure for the isolated molecule of 2 was performed using standard criteria on displacements and forces. The analysis of the Hessian of potential energy surface for 2 revealed the correspondence of the isolated structure to the energy minimum. The analysis of electron density based on the "Atoms in Molecules" theory together with the reduced density gradient and sign(λ2)·ρ(r) computations were performed in the AIMAll program [76].
3.2. Synthesis of 9-N 3 (CH 2 ) 3 Me 2 N-nido-7,8-C 2 B 9 H 11 2 Compound 1 (1.00 g, 0.0034 mol) was dissolved in 50 mL of DMF and NaN 3 (0.44 g, 0.0068 mol) and anhydrous NaI (0.05 g, 0.0003 mol) were added. The reaction mixture was heated at 50 • C for 7 days. Then the reaction mixture was cooled to room temperature, and 50 mL of H 2 O was added. The formed precipitate was filtered and dried in the air, then it was purified on silica column by using CH 2 Cl 2 -CH 3 CN (5:1) as eluent to give white solid of 2 (0.80 g, yield 90%). 1

Conclusions
In this work we prepared and characterized novel nido-carboranyl azide 9-N 3 (CH 2 ) 3 Me 2 Nnido-7,8-C 2 B 9 H 11 derived from the reaction of 9-Cl(CH 2 ) 3 Me 2 N-nido-7,8-C 2 B 9 H 11 with NaN 3 in the presence of NaI as a catalyst in strong conditions. The possibility of using "click" approach in regard to the obtained compound was demonstrated on the reaction of 9-N 3 (CH 2 ) 3 Me 2 Nnido-7,8-C 2 B 9 H 11 with phenylacetylene. We also studied the behaviour of nido-carboranyl azide in the copper(I)-catalyzed azide-alkyne cycloaddition reaction with alkynyl-cholesterol and obtained new the nido-carborane-cholesterol conjugate with charge-compensated group in a linker in a good yield. Based on the synthesized compound, the boronated liposomes are planned for preparation in order to deliver boron clusters into a cancer cell for a BNCT experiment. It should be concluded that "click" reactions between 9-N 3 (CH 2 ) 3 Me 2 N-nido-7,8-C 2 B 9 H 11 and cobalt/iron bis(dicarbollide) terminal alkynes lead to novel zwitter-ionic boron-enriched cluster compounds bearing a 1,2,3-triazol-metallacarborane-carborane conjugated system. By changing the type and the size of a spacer between these two boron cages, it is possible to control the hydrophilic/hydrophobic balance of the compounds. Prepared conjugates contain a large amount of boron atom in the biomolecule potentially can be used for boron neutron capture therapy of cancer (BNCT).
The solid-state molecular structure of the novel nido-carboranyl azide 9-N 3 (CH 2 ) 3 Me 2 Nnido-7,8-C 2 B 9 H 11 was determined by single-crystal X-ray diffraction studies. Structural feature of nido-carboranyl azide is rather rigid and compact conformation of substituent is observed that results in rather short intramolecular contact between the N4 atom of azide function and the H1C extra-hydrogen atom of nido-carborane cage (3.089 Å with non-normalized B-H bond lengths).