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Article

How to Protect ortho-Carborane from Decapitation—Practical Synthesis of 3,6-Dihalogen Derivatives 3,6-X2-1,2-C2B10H10 (X = Cl, Br, I)

by
Akim V. Shmal’ko
1,
Sergey A. Anufriev
1,
Kyrill Yu. Suponitsky
1,2 and
Igor B. Sivaev
1,2,*
1
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str., 119334 Moscow, Russia
2
Basic Department of Chemistry of Innovative Materials and Technologies, G.V. Plekhanov Russian University of Economics, 36 Stremyannyi Line, 117997 Moscow, Russia
*
Author to whom correspondence should be addressed.
Inorganics 2022, 10(11), 207; https://doi.org/10.3390/inorganics10110207
Submission received: 18 October 2022 / Revised: 2 November 2022 / Accepted: 9 November 2022 / Published: 13 November 2022
(This article belongs to the Special Issue Fifth Element: The Current State of Boron Chemistry)
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Abstract

:
The 3-halogen and 3,6-dihalogen derivatives of ortho-carborane 3-X-1,2-C2B10H11 and 3,6-X2-1,2-C2B10H10 (X = Cl, Br, I) were prepared by Cu-assisted halodeboronation of the corresponding pinacolborate derivatives 3-Bpin-1,2-C2B10H11 and 3,6-(Bpin)2-1,2-C2B10H10. It was shown that decapitation of 3-Cl-1,2-C2B10H11, similarly to the corresponding bromo and iodo derivatives, proceeds regioselectively with the retention of the B-Cl bond. Crystal structures of 3,6-Cl2-1,2-C2B10H10 and Cs [3-Cl-7,8-C2B9H11] were determined by single crystal X-ray diffraction.

Graphical Abstract

1. Introduction

The discovery of the decapitation of ortho-carborane under the action of strong nucleophiles with the formation of nido-carborane in the 1960s [1,2] initiated the development of at least two main directions in the development of carborane chemistry. The first one was the use of dicarbollide ligands, which are formed upon the deprotonation of nido-carboranes with strong bases, for the synthesis of π-complexes of transition metals analogous to complexes with cyclopentadiene ligands, the so-called metallacarboranes [3,4,5,6,7,8,9,10,11]. Another direction was the transformation of closo-carborane derivatives into the corresponding nido-carboranes in order to increase their water solubility for use in boron neutron capture therapy for cancer [12,13,14,15,16,17,18,19], radio-immunodiagnostics and radio-immunotherapy [20,21,22], as well as some other medical applications [23,24,25,26,27]. Later, the use of nido-carboranes in the design of carborane-containing luminescent materials was reported [28,29,30,31,32,33,34]. However, the transformation of the closo-carborane cage into the nido-carborane one leads to the appearance of an anionic charge, which implies the presence of a counterion, and also results in a significant decrease in the thermal and chemical stability of the carborane cage. In addition, during decapitation, the strong electron-withdrawing effect of the C-carboranyl group changes to an electron-donating one [30,31,35,36,37,38]. Therefore, for many important applications of carboranes in material chemistry [39,40,41,42,43,44,45,46,47,48,49,50,51,52], their decapitation is highly undesirable. Therefore, the protection of the ortho-carborane cage from decapitation is one of the urgent problems in the design of new carborane-based materials.
It was earlier demonstrated that the substitution of hydrogen atoms in positions 3 and 6 of ortho-carborane with phenyl groups successfully protects the carborane cage from decapitation [53]. However, the rather large size of the phenyl groups precludes substitution at adjacent carborane carbons and obstructs the rational design of ortho-carborane-based materials. Therefore, the goal of this study was to develop convenient methods for the synthesis of 3,6-dihalogen-substituted derivatives of ortho-carborane 3,6-X2-1,2-C2B10H10, the substituents in which have the smallest size that do not prevent further modification.

2. Results and Discussion

The introduction of substituents into positions 3 and 6 of the ortho-carborane cage by a direct route is impossible and usually involves several steps. The simplest approach involves two consecutive decapitations of ortho-carborane to nido-carborane followed by the insertion of the “missing” boron vertex with the corresponding substituent BX. It is this approach that was first used for the synthesis of 3,6-diiodo-ortho-carborane [54,55].
An important issue here is the retention of the substituent during the decapitation of 3-substituted ortho-carboranes, which in general can go through both the free position 6 and the substituted position 3. Earlier, it was shown that the decapitation of 3-bromo- and 3-iodo-ortho-carboranes proceeds with the retention of the substituent [56]. The retention of a substituent is also characteristic of 3-alkyl-, 3-aryl-, and 3-alkynyl-ortho-carboranes [53,57,58,59,60], as well as of 3-amino-ortho-carborane and other derivatives with a B-N bond [61,62,63,64,65]. At the same time, decapitation of 3-fluoro [66] and 3-hydroxy [67] derivatives of ortho-carborane leads to mixtures of the parent nido-carborane and the corresponding substituted nido-carboranes, i.e., is not selective. Thus, both the fluorine and the hydroxy group cannot be used to protect ortho-carborane from decapitation.
Alternatively, the first decapitation-insertion sequence can be replaced by diazotization of the 3-amino derivative formed by the reduction of ortho-carborane with sodium metal in liquid ammonia, followed by oxidation of the resulting product with KMnO4 or CuCl2, followed by replacement of the diazo group with various nucleophiles [68,69,70,71]. Recently, the direct way to the synthesis of the 3-amino derivative by the Ir-catalysed reaction of the parent ortho-carborane with ammonia in tetrahydrofuran has been proposed [72]. This approach is also applicable to various derivatives of ortho-carborane, including 3-substituted derivatives, which makes it possible to avoid the use of highly aggressive boron trihalides and liquid ammonia.
Recently, the simultaneous introduction of substituents at positions 3 and 6 of the ortho-carborane cage was reported using the Ir-catalysed reaction of the parent ortho-carborane with bis(pinacolato)diboron B2pin2 followed by the replacement of the pinacolborane groups in 3,6-(Bpin)2-1,2-C2B10H10. In particular, the authors used this approach to obtain 3,6-dibromo- and 3,6-diiodo-derivatives of ortho-carborane [73]. Wishing to use this approach, we prepared 3-pinacolborane (1) and 3,6-di(pinacolborane) (2) derivatives by reaction of the parent ortho-carborane with B2pin2 in the presence of a [(cod)Ir(μ-Cl)]2 iridium catalyst according to the previously described procedure modified by us at the product separation stage (Scheme 1). However, in our hands, the further Pd-catalysed substitution reactions described by them did not give the desired result.
To solve the problem, we have developed a method for the Cu-catalysed halodeboronation of pinacolborane derivatives of ortho-carborane, similar to that used in organic chemistry for halodeboronation of aryl boronic acids [74,75,76,77,78,79].
The reactions of 3-Bpin-1,2-C2B10H11 (1) with three equivalents of N-chloro- and N-bromosuccinimides in the presence of three equivalents of CuX2·2H2O in boiling acetonitrile for 24 h gave the corresponding 3-chloro (3) and 3-bromo (4) derivatives of ortho-carborane in high yields (Scheme 2). The 3-iodo derivative 3-I-1,2-C2B10H11 (5) was prepared using I2 and Cu(OAc)2·H2O instead of the corresponding N-halogen-succinimide and copper(II) halogenide (Scheme 2). It should be noted that in the absence of an iodine source, the latter reaction leads to the formation of the corresponding acetoxy derivative 3-AcO-1,2-C2B10H11 (6).
Similarly, the reactions of 3,6-(Bpin)2-1,2-C2B10H10 (2) with six equivalents of N-chloro- and N-bromosuccinimides in the presence of six equivalents of CuX2·2H2O in boiling acetonitrile for 24 h result in the corresponding 3,6-dichloro (7) and 3,6-dibromo (8) derivatives of ortho-carborane. The 3,6-diiodo derivative 3,6-I2-1,2-C2B10H10 (9) was prepared by the treatment of the 3,6-di(pinacolborate) derivative 2 with six equivalents of sodium iodide NaI and copper acetate Cu(OAc)2·H2O or iodine I2 and copper fluoride CuF2·H2O in refluxing acetonitrile (Scheme 3). The reaction with copper acetate in the absence of NaI results in the known 3,6-di(acetoxy) derivative 3,6-(AcO)2-1,2-C2B10H10 (10). It should be noted that the reaction of the 3,6-di(pinacolborate) derivative 2 with copper acetate in the presence of NaBr also led to the 3,6-di(acetoxy) derivative 10, while the reactions with N-bromosuccinimide or (Bu4N)Br and Br2 gave mixtures of 3,6-dibromo 8 and 3,6-(diacetoxy) 10 derivatives. The reactions of the 3,6-di(pinacolborate) derivative 2 with CuF2·2H2O in the presence of CsF, KHF2, or MeI were found to result in the parent ortho-carborane 1,2-C2B10H12 in 7 h.
The structure of 3,6-dichloro derivative 3,6-Cl2-1,2-C2B10H10 (7) was determined by single crystal X-ray diffraction. The general view of 7 is presented in Figure 1. In the crystal, the molecule occupies special positions located at the two-fold symmetry axis. The B3-Cl1 bond (1.757(5) Å) is noticeable shorter than the B-Cl bonds in the 9,12-isomer 9,12-Cl2-1,2-C2B10H10 (1.798(2) Å) [80]. In contrast to the crystal structure of the 3,6-diiodo derivative, which is characterized by the presence of the I···I dihalogen bonds of type II (I···I distance 4.067 Å, B-I···I angle is 91.19°) [81], there was not any strong intermolecular interactions in the crystal structure of the titled compound. Weak Cl···Cl dihalogen bonds of type I with a distance of 3.768(3) Å (that is longer than the sum of Van der Waals radii (3.65 Å) [82]) linked molecules into chains along the (101) direction while all the other interactions were Van der Waals (Figure 2).
Since the decapitations of 3-bromo and 3-iodo derivatives of ortho-carborane occur selectively with the retention of the substituent, and in the case of the 3-fluoro derivative non-selectively, it was important to understand how the decapitation of the 3-chloro derivative 3 occurs. We found that heating 3 with CsF in ethanol led to selective decapitation of the unsubstituted vertex to form nido-carborane Cs [3-Cl-7,8-C2B9H11] (11) (Scheme 4). Thus, unlike fluorine, the chlorine atom protects the boron atom bound to two carbon atoms from nucleophilic attack.
The crystal structure of Cs [3-Cl-7,8-C2B9H11] (11) was determined by single crystal X-ray diffraction. The general view of 3-chloro-nido-carborane with Cs counterion is given in Figure 3. The B3-Cl1 bond length was somewhat longer than that in 7 (1.790 (10) Å). In the crystal there was not even any weak Cl···Cl and Cl···H contacts that was probably due to the presence of the Cs cation which governs crystal packing motifs by the formation of numerous Cs···H and Cs···Cl contacts. Through those contacts, each Cs cation simultaneously binds six carborane cages (Figure 4).
The 3,6-dihalogen derivatives 3,6-X2-1,2-C2B10H10 (X = Cl, Br, I) were found to resist decapitation under the conditions which were used for decapitation of the 3-chloro derivative of ortho-carborane (refluxing with CsF in ethanol).

3. Materials and Methods

3.1. General Methods

Acetonitrile and tetrahydrofuran were dried using standard procedures [83]. All other chemical reagents were purchased from Sigma Aldrich (Burlington, MA, USA), Acros Organics (Geel, Belgium), and ABCR (Karlsruhe, Germany) and used without purification. The reaction progress was monitored by thin-layer chromatography (Merck F254 silica gel on aluminium plates and Macherey-Nagel ALUGRAM Xtra SIL G UV254) and visualized using 0.5 % PdCl2 in 1% HCl in aq. MeOH (1:10). Acros Organics (0.060–0.200 mm) silica gel was used for column chromatography. The NMR spectra at 400 MHz (1H), 128 MHz (11B) and 100 MHz (13C) (See Supplementary Materials) were recorded with a Varian Inova 400 spectrometer (Palo Alto, CA, USA). The residual signal of the NMR solvent relative to Me4Si was taken as the internal reference for the 1H and 13C NMR spectra. The 11B NMR spectra were referenced using BF3∙Et2O as external standard. Mass spectra (MS) were measured using the Shimadzu LCMS-2020 instrument (Kyoto, Japan) with DUIS ionization (ESI—electrospray ionization and APCI—atmospheric pressure chemical ionization). The measurements were performed in a negative ion mode with a mass range from m/z 50 to m/z 2000.

3.2. Synthesis of 3-Bpin-1,2-C2B10H11 (1) and 3,6-(Bpin)2-1,2-C2B10H10 (2)

Under an argon atmosphere, ortho-carborane (1.440 g, 10.00 mmol), bis(pinacolato)diboron B2pin2 (1.016 g, 40.00 mmol), and bis(1,5-cyclooctadiene)diiridium (I) dichloride [(cod)Ir(μ-Cl)]2 (235 mg, 0.35 mmol) were placed in a 50 mL two-neck flask and dry THF (10 mL) was added. Then, 2-methylpyridine (196 mg, 207 µL, 2.10 mmol) was added and heated under reflux for 12 h, monitoring the progress of the reaction by thin-layer chromatography (dichloromethane) and 11B NMR spectroscopy. After cooling the reaction mixture to ambient temperature, silica gel was added, and all volatiles were removed on a rotary evaporator. The resulting solid residue was subjected to column chromatography using dichloromethane as the eluent. The first and second fractions were collected and concentrated on a rotary evaporator to obtain white compounds 1 (648 mg, 24%) and 2 (2574 mg, 65%), respectively.
3-Bpin-1,2-C2B10H11 (1): 1H NMR (CDCl3, ppm): 3.56 (2H, br.s, CHcarb), 1.25 (12H, s, CH3). 11B NMR (CDCl3), δ: 33.3 (1B, s, Bpin), −1.6 (2B, d, J = 145 Hz), −7.7 (1B, d, J = 155 Hz), −8.3 (1B, d, J = 146 Hz), −12.8 (5B, d + s, J = 163 Hz), −14.3 (1B, d, J = 177 Hz).
3,6-(Bpin)2-1,2-C2B10H10 (2): 1H NMR (CDCl3, ppm): 3.55 (2H, br.s, CHcarb), 1.26 (24H, s, CH3). 11B NMR (CDCl3), δ: 33.7 (2B, s, Bpin), −0.7 (2B, d, J = 145 Hz), −6.4 (2B, d, J = 155 Hz), −11.7 (6B, d + s, J = 146 Hz).

3.3. General Procedure for the Synthesis of 3-Halogen-ortho-carboranes 3-X-1,2-C2B10H11 (X = Cl (3), Br(4))

3-Bpin-1,2-C2B10H11 (1) (50.0 mg, 0.185 mmol), N-X-succinimide (0.555 mmol) and CuX2 (0.555 mmol) were placed in a 25 mL round bottom flask and acetonitrile (5 mL) was added. The reaction mixture was heated under reflux for ~ 24 h until complete conversion according to 11B NMR and allowed to cool to room temperature. The solvent was removed in vacuo and the resulting solid residue was subjected to column chromatography on silica using a mixture of chloroform and petroleum ether (2:1, v/v) as the eluent.
3-Cl-1,2-C2B10H11 (3): According to the general procedure using N-chlorosuccinimide (74.0 mg) and CuCl2·2H2O (75.0 mg), 25.8 mg (76% yield) of a white crystalline compound 3 was obtained. 1H NMR (CDCl3, ppm): 3.81 (2H, br.s, CHcarb). 11B NMR (CDCl3, ppm): −2.7 (2B, d, J = 151 Hz), −5.5 (1B, s), −8.9 (1B, d, J = 152 Hz), −12.4 (2B, d, J = 184 Hz), −13.9 (3B, d, J = 174 Hz), −14.8 (1B, d, J = 115 Hz).
3-Br-1,2-C2B10H11 (4): According to the general procedure using N-bromosuccinimide (98.8 mg) and CuBr2·2H2O (144.0 mg), 30.1 mg (73% yield) of a white crystalline compound 4 was obtained. 1H NMR (CDCl3, ppm): 3.84 (2H, br.s, CHcarb). 11B NMR (CDCl3, ppm): −2.2 (2B, d, J = 151 Hz), −8.3 (1B, d, J = 155 Hz), −11.9 (2B, d, J = 175 Hz), −12.4 (1B, s), −13.4 (4B, d, J = 174 Hz).

3.4. Synthesis of 3-Iodo-ortho-Carborane 3-I-1,2-C2B10H11 (5)

3-Bpin-1,2-C2B10H11 (1) (50.0 mg, 0.185 mmol), NaI (83.2 mg, 0.555 mmol) and Cu(OAc)2·H2O (110.8 mg, 0.555 mmol) were placed in a 25 mL round bottom flask and acetonitrile (5 mL) was added. The reaction mixture was heated under reflux for 15 h until complete conversion according to 11B NMR and allowed to cool to room temperature. The solvent was removed in vacuo and the resulting solid residue was subjected to column chromatography on silica using a mixture of chloroform and petroleum ether (2:1, v/v) as the eluent. The first boron-containing fraction was collected and concentrated on a rotary evaporator under reduced pressure to obtain a white crystalline compound 5 (43.4 mg, 87% yield). 1H NMR (CDCl3, ppm): 3.84 (2H, br.s, CHcarb). 11B NMR (CDCl3, ppm): −1.3 (2B, d, J = 151 Hz), −7.1 (1B, d, J = 156 Hz), −11.0 (3B, d, J = 174 Hz), −12.4 (2B, d, J = 174 Hz), −13.1 (1B, d, J = 188 Hz), −29.3 (1B, s).

3.5. Synthesis of 3-Acetoxy-ortho-Carborane 3-AcO-1,2-C2B10H11 (6)

3-Bpin-1,2-C2B10H11 (1) (50.0 mg, 0.185 mmol) and Cu(OAc)2·H2O (110.8 mg, 0.555 mmol) were placed in a 25 mL round bottom flask and acetonitrile (5 mL) was added. The reaction mixture was heated under reflux for 30 h until complete conversion according to 11B NMR and allowed to cool to room temperature. The solvent was removed in vacuo and the resulting solid residue was subjected to column chromatography on silica using a mixture of chloroform and petroleum ether (2:1, v/v) as the eluent. The first boron-containing fraction was collected and concentrated on a rotary evaporator under reduced pressure to obtain a white crystalline compound 6 (33.2 mg, 89% yield). 1H NMR (acetone-d6, ppm): 4.86 (2H, br.s, CHcarb), 2.12 (3H, s, CH3). 11B NMR (acetone-d6, ppm): −4.0 (1B, s), −5.5 (2B, d, J = 153 Hz), −11.2 (1B, d, J = 178 Hz), −13.9 (2B, d, J = 174 Hz), −14.8 (1B, d), −15.6 (2B, d, J = 188 Hz), −17.0 (1B, d, J = 162 Hz).

3.6. General Procedure for the Synthesis of 3,6-Dihalogen-ortho-carboranes 3,6-X2-1,2-C2B10H10 (X = Cl (7), Br(8))

3,6-(Bpin)2-1,2-C2B10H10 (2) (1 equiv.), N-X-succinimide (6 equiv.) and CuX2 (6 equiv.) were placed in a 25 mL round bottom flask and acetonitrile (5 mL) was added. The reaction mixture was heated under reflux for ~24 h until complete conversion according to 11B NMR and allowed to cool to room temperature. The solvent was removed in vacuo and the resulting solid residue was subjected to column chromatography on silica using a mixture of chloroform and petroleum ether (2:1, v/v) as the eluent.
3,6-Cl2-1,2-C2B10H10 (7): According to the general procedure using 3,6-(Bpin)2-1,2-C2B10H10 (396.0 mg, 1.000 mmol), N-chlorosuccinimide (801.0 mg, 6.000 mmol) and CuCl2·2H2O (1022.7 mg, 6.000 mmol), 168.3 mg (79% yield) of a white crystalline compound 7 was obtained. 1H NMR (CDCl3, ppm): 4.07 (2H, br.s, CHcarb). 1H NMR (acetone-d6, ppm): 5.40 (2H, br.s, CHcarb). 11B NMR (CDCl3, ppm): −3.2 (2B, d, J = 153 Hz, B(9) + B(12)), −4.4 (2B, s, B(3) + B(6)), −12.6 (4B, d, J = 169 Hz, B(4) + B(5) + B(7) + B(11)), −14.6 (2B, d, J = 159 Hz, B(8) + B(10)). 11B NMR (acetone-d6, ppm): −4.2 (4B, d + s, J = 147 Hz, B(3) + B(6)+ B(9) + B(12)), −12.6 (4B, d, J = 167 Hz, B(4) + B(5) + B(7) + B(11)), −14.7 (2B, d, J = 150 Hz, B(8) + B(10)). 13C NMR (acetone-d6, ppm): 63.5 (CHcarb). MS (DUIS), m/z: found: 212.2 (M–H)−; calculated for C2H9B10Cl2 (M–H)−: 212.1.
3,6-Br2-1,2-C2B10H10 (8): According to the general procedure using 3,6-(Bpin)2-1,2-C2B10H10 (50.0 mg, 0.126 mmol), N-bromosuccinimide (133.50 mg, 0.756 mmol) and CuBr2·2H2O (194.5 mg, 0.756 mmol), 27.0 mg (71% yield) of a white crystalline compound 8 was obtained. 1H NMR (CDCl3, ppm): 4.14 (2H, br.s, CHcarb). 1H NMR (acetone-d6, ppm): 5.47 (2H, br.s, CHcarb). 11B NMR (CDCl3, ppm): −1.9 (2B, d, J = 151 Hz), −11.6 (8B, m). 11B NMR (acetone-d6, ppm), δ: −2.8 (4B, d, J = 151 Hz), −10.9 (2B, d + s, J = 156 Hz), −11.5 (4B, d, J = 156 Hz), −12.5 (2B, d, J = 127 Hz).

3.7. Synthesis of 3,6-Diiodo-ortho-Carborane 3,6-I2-1,2-C2B10H11 (9)

(a) 3,6-(Bpin)2-1,2-C2B10H10 (2) (50.0 mg, 0.126 mmol), NaI (112.0 mg, 0.756 mmol) and Cu(OAc)2·H2O (150.0 mg, 0.756 mmol) were placed in a 25 mL round bottom flask and acetonitrile (5 mL) was added. The reaction mixture was heated under reflux for 15 h until complete conversion according to 11B NMR and allowed to cool to room temperature. The solvent was removed in vacuo and the resulting solid residue was subjected to column chromatography on silica using a mixture of chloroform and petroleum ether (2:1, v/v) as the eluent. The first boron-containing fraction was collected and concentrated on a rotary evaporator under reduced pressure to obtain a white crystalline compound 9 (43.8 mg, 88% yield).
(b) 3,6-(Bpin)2-1,2-C2B10H10 (1) (50.0 mg, 0.126 mmol), I2 (192.0 mg, 0.756 mmol), and CuF2·H2O (104.0 mg, 0.756 mmol) were placed in a 25 mL round bottom flask and acetonitrile (5 mL) was added. The reaction mixture was heated under reflux for 20 h until complete conversion according to 11B NMR and allowed to cool to room temperature. The solvent was removed in vacuo and the resulting solid residue was subjected to column chromatography on silica using a mixture of chloroform and petroleum ether (2:1, v/v) as the eluent. The first boron-containing fraction was collected and concentrated on a rotary evaporator under reduced pressure to obtain a white crystalline compound 9 (42.8 mg, 86% yield).
1H NMR (CDCl3, ppm): 4.13 (2H, br.s, CHcarb). 11B NMR (CDCl3, ppm): −0.2 (2B, d, J = 144 Hz), −8.9 (2B, d, J = 182 Hz), −9.9 (4B, d, J = 151 Hz), −27.9 (2B, s).

3.8. Synthesis of 3,6-Diacetoxy-ortho-Carborane 3,6-(AcO)2-1,2-C2B10H10 (10)

3,6-(Bpin)2-1,2-C2B10H10 (2) (50.0 mg, 0.126 mmol) and Cu(OAc)2·H2O (150.0 mg, 0.756 mmol) were placed in a 25 mL round bottom flask and acetonitrile (5 mL) was added. The reaction mixture was heated under reflux for 25 h until complete conversion according to 11B NMR and allowed to cool to room temperature. The solvent was removed in vacuo and the resulting solid residue was subjected to column chromatography on silica using a mixture of chloroform and petroleum ether (2:1, v/v) as the eluent. The first boron-containing fraction was collected and concentrated on a rotary evaporator under reduced pressure to obtain a white crystalline compound 10 (28.2 mg, 86% yield). 1H NMR (acetone-d6, ppm): 5.21 (2H, br.s, CHcarb), 2.12 (6H, s, CH3). 11B NMR (acetone-d6, ppm): −4.3 (2B, s), −8.1 (2B, d, J = 150 Hz), −15.8 (4B, d, J = 164 Hz), −18.6 (2B, d, J = 152 Hz).

3.9. Synthesis of Cesium 3-chloro-7,8-Dicarba-Nido-Undecaborate Cs [3-Cl-7,8-C2B9H11] (11)

3-Cl-1,2-C2B10H11 (3) (25.0 mg, 0.140 mmol) and CsF (64.0 mg, 0.420 mmol) were placed in a 25 mL round bottom flask and ethanol (5 mL) was added. The reaction mixture was heated at 60 °C for 15 h until complete conversion according to 11B NMR and allowed to cool to room temperature. The solvent was removed in vacuo, to the residue dichloromethane (15 mL) was added and resulting solution was washed with water (3 × 15 mL). The organic fraction was collected and dried over Na2SO4, filtered and concentrated in vacuo to obtain a white crystalline compound 11 (43.8 mg, 88% yield). 1H NMR (acetone-d6, ppm): 1.90 (2H, br.s, CHcarb), −2.68 (BHB, br.m). 11B NMR (acetone-d6, ppm), δ: −8.5 (1B, s, B(3)), −10.7 (2B, d, J = 138 Hz, B(9) + B(11)), −16.8 (2B, d, J = 138 Hz, B(5) + B(6)), −21.4 (2B, d, J = 152 Hz, B(2) + B(4)), −37.6 (2B, d, J = 139 Hz, B(1) + B(10)). 13C NMR (acetone-d6, ppm): 63.5 (CHcarb). MS (DUIS), m/z: found: 168.2 (M)−; calculated for C2H11B9Cl (M)−: 168.1.

3.10. Single Crystal X-ray Diffraction Study

The single crystals of 3,6-Cl2-1,2-C2B10H10 (3) and Cs [3-Cl-7,8-C2B9H11] (11) were grown by slow evaporation of a solution in chloroform and acetone, respectively, at room temperature. Single crystal X-ray diffraction experiments were carried out using a SMART APEX2 CCD diffractometer (λ(Mo-Kα) = 0.71073 Å, graphite monochromator, ω-scans) at 120 K. Collected data were processed by the SAINT and SADABS programs incorporated into the APEX2 program package [84]. The structures were solved by the direct methods and refined by the full-matrix least-squares procedure against F2 in anisotropic approximation. The refinement was carried out with the SHELXTL program [85]. Details of the refinement are provided in Table 1. The CCDC numbers (221,3255 for 3, and 221,3256 for 11) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

4. Conclusions

A convenient two-stage method for the preparation of 3-halogen and 3,6-dihalogen ortho-carborane derivatives 3-X-1,2-C2B10H11 and 3,6-X2-1,2-C2B10H10 (X = Cl, Br, I) through Cu-assisted halodeboronation of the corresponding pinacolborate derivatives has been proposed. This approach allows to avoid the use of highly aggressive boron trihalides and liquid ammonia. It was demonstrated that a chlorine atom effectively protects the boron atom bound to two carbon atoms from nucleophilic attack. Crystal structures of 3,6-Cl2-1,2-C2B10H10 and Cs [3-Cl-7,8-C2B9H11] were determined by single crystal X-ray diffraction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics10110207/s1, Crystallographic data for compounds 3 and 11, NMR and MS spectra of compounds 1–11.

Author Contributions

Synthesis, A.V.S.; synthesis and NMR spectroscopy studies, S.A.A.; single crystal X-ray diffraction experiments, K.Y.S.; supervision and manuscript concept, I.B.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Russian Science Foundation (Grant No. 21-13-00345).

Data Availability Statement

Crystallographic data for the structures of 3,6-Cl2-1,2-C2B10H10 (3) and Cs [3-Cl-7,8-C2B9H11] (11) were deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 2213255 (for 3) and 2213256 (for 11). The Supplementary Information contains crystallographic data for compounds 3 and 11, and NMR and MS spectra of compounds 1–11.

Acknowledgments

The single crystal X-ray diffraction and NMR spectral data were obtained by using equipment from the Center for Molecular Structure Studies at A.N. Nesmeyanov Institute of Organoelement Compounds, operating with financial support from the Ministry of Science and Higher Education of the Russian Federation.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Inorganics 10 00207 sch001 550
Scheme 1. Synthesis of 3-Bpin-1,2-C2B10H11 (1) and 3,6-(Bpin)2-1,2-C2B10H10 (2).
Scheme 1. Synthesis of 3-Bpin-1,2-C2B10H11 (1) and 3,6-(Bpin)2-1,2-C2B10H10 (2).
Inorganics 10 00207 sch001
Inorganics 10 00207 sch002 550
Scheme 2. Synthesis of 3-halogen derivatives 3-X-1,2-C2B10H11 (X = Cl (3), Br (4), I (5)).
Scheme 2. Synthesis of 3-halogen derivatives 3-X-1,2-C2B10H11 (X = Cl (3), Br (4), I (5)).
Inorganics 10 00207 sch002
Inorganics 10 00207 sch003 550
Scheme 3. Synthesis of 3,6-dihalogen derivatives 3,6-X2-1,2-C2B10H10 (X = Cl (7), Br (8), I (9)).
Scheme 3. Synthesis of 3,6-dihalogen derivatives 3,6-X2-1,2-C2B10H10 (X = Cl (7), Br (8), I (9)).
Inorganics 10 00207 sch003
Inorganics 10 00207 g001 550
Figure 1. General view of 3,6-Cl2-1,2-C2B10H10 along with atomic numbering. Thermal ellipsoids are shown at 50% probability level.
Figure 1. General view of 3,6-Cl2-1,2-C2B10H10 along with atomic numbering. Thermal ellipsoids are shown at 50% probability level.
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Inorganics 10 00207 g002 550
Figure 2. Crystal packing fragment of 3,6-Cl2-1,2-C2B10H10. The Cl⋯Cl contacts are shown by dashed lines.
Figure 2. Crystal packing fragment of 3,6-Cl2-1,2-C2B10H10. The Cl⋯Cl contacts are shown by dashed lines.
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Inorganics 10 00207 sch004 550
Scheme 4. Decapitation of 3,6-Cl2-1,2-C2B10H10 (7).
Scheme 4. Decapitation of 3,6-Cl2-1,2-C2B10H10 (7).
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Inorganics 10 00207 g003 550
Figure 3. General view of Cs [3-Cl-7,8-C2B9H11] along with atomic numbering. Thermal ellipsoids are shown at 50% probability level.
Figure 3. General view of Cs [3-Cl-7,8-C2B9H11] along with atomic numbering. Thermal ellipsoids are shown at 50% probability level.
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Figure 4. Crystal packing fragment of Cs [3-Cl-7,8-C2B9H11]. The closest environment of the Cs cation is shown by solid dashed lines. The Cs1···Cl1 distances are 3.762 (2) Å and 3.776 (2) Å, the Cs-H distances are in the range of 3.15–3.35 Å.
Figure 4. Crystal packing fragment of Cs [3-Cl-7,8-C2B9H11]. The closest environment of the Cs cation is shown by solid dashed lines. The Cs1···Cl1 distances are 3.762 (2) Å and 3.776 (2) Å, the Cs-H distances are in the range of 3.15–3.35 Å.
Inorganics 10 00207 g004
Table
Table 1. Crystallographic data for compounds 3,6-Cl2-1,2-C2B10H10 (3) and Cs [3-Cl-7,8-C2B9H11] (11).
Table 1. Crystallographic data for compounds 3,6-Cl2-1,2-C2B10H10 (3) and Cs [3-Cl-7,8-C2B9H11] (11).
3,6-Cl2-1,2-C2B10H10 (3)Cs [3-Cl-7,8-C2B9H11] (11)
FormulaC2H10B10Cl2Cs+C2B9H11Cl−
FW213.10300.76
Crystal systemMonoclinicOrthorhombic
Space groupC2/cPbca
a, Å14.746(7)10.693(2)
b, Å6.805(4)11.149(2)
c, Å11.485(6)18.174(4)
β, deg115.259(14)90
V, Å31042.4(9)2166.6(8)
Z48
ρcalc, g·cm−31.3581.844
F(000)4241120
μ, mm−10.5573.599
θ range, deg3.06–26.082.24–26.15
Independent reflections10302141
Completeness to theta θ, %99.098.8
Refined parameters85122
GOF (F2)0.9841.037
Reflections with I > 2σ(I)5871549
R1(F) (I > 2σ(I)) a0.05920.0579
wR2(F2) (all data) b0.14730.1382
Largest diff. peak/hole, e·Å−30.400/−0.4780.977/−1.299
a R1 = ∑|Fo—|Fc||/∑(Fo); b wR2 = (∑[w(Fo2—Fc2)2]/∑[w(Fo2)2 ]½.
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Shmal’ko, A.V.; Anufriev, S.A.; Suponitsky, K.Y.; Sivaev, I.B. How to Protect ortho-Carborane from Decapitation—Practical Synthesis of 3,6-Dihalogen Derivatives 3,6-X2-1,2-C2B10H10 (X = Cl, Br, I). Inorganics 2022, 10, 207. https://doi.org/10.3390/inorganics10110207

AMA Style

Shmal’ko AV, Anufriev SA, Suponitsky KY, Sivaev IB. How to Protect ortho-Carborane from Decapitation—Practical Synthesis of 3,6-Dihalogen Derivatives 3,6-X2-1,2-C2B10H10 (X = Cl, Br, I). Inorganics. 2022; 10(11):207. https://doi.org/10.3390/inorganics10110207

Chicago/Turabian Style

Shmal’ko, Akim V., Sergey A. Anufriev, Kyrill Yu. Suponitsky, and Igor B. Sivaev. 2022. "How to Protect ortho-Carborane from Decapitation—Practical Synthesis of 3,6-Dihalogen Derivatives 3,6-X2-1,2-C2B10H10 (X = Cl, Br, I)" Inorganics 10, no. 11: 207. https://doi.org/10.3390/inorganics10110207

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