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Review

Charge-Compensated Derivatives of Nido-Carborane

by
Marina Yu. Stogniy
1,2,*,
Sergey A. Anufriev
1 and
Igor B. Sivaev
1,3
1
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str., 119334 Moscow, Russia
2
M.V. Lomonosov Institute of Fine Chemical Technology, MIREA—Russian Technological University, 86 Vernadsky Av., 119571 Moscow, Russia
3
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 2023, 11(2), 72; https://doi.org/10.3390/inorganics11020072
Submission received: 9 January 2023 / Revised: 25 January 2023 / Accepted: 30 January 2023 / Published: 2 February 2023
(This article belongs to the Special Issue Fifth Element: The Current State of Boron Chemistry)
Browse Figures
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Abstract

:
This review summarizes data on the main types of charge-compensated nido-carborane derivatives. Compared with organic analogs, onium derivatives of nido-carborane have increased stability due to the stabilizing electron-donor action of the boron cage. Charge-compensated derivatives are considered according to the type of heteroatom bonded to a boron atom.

1. Introduction

The synthesis of the first polyhedral boranes, carboranes, and metallacarboranes in the early 1960s was one of the major highlights in the development of inorganic chemistry over the last century [1]. The first reports on the synthesis of icosahedral carboranes appeared almost sixty years ago, at the end of 1963 when both the United States and the Soviet Union almost simultaneously declassified documents about their boron fuel projects [2,3,4,5,6]. A few months later, the nucleophile-promoted removal of one boron atom from the icosahedral ortho-carborane cage to form the 11-vertex nido-carborane cage species (Figure 1) was reported [7,8]. It was one of the most significant findings in the early years of the development of carborane chemistry, and now, more than five decades later, it remains indispensable for the synthesis of numerous metallacarboranes [9,10,11,12,13,14,15,16,17] and hydrophilic functionalized carboranes for medical [18,19,20,21,22,23,24,25,26,27] and other [28,29,30,31,32,33,34,35,36,37,38,39,40] applications.
Metallacarboranes based on the dicarbollide ligand [7,8-C2B9H11]2−, which is formed upon the deprotonation of nido-carborane with strong bases, resemble the well-known transition metal cyclopentadienyl complexes. However, the dicarbollide ligand differs from the cyclopentadienyl ligand in a number of ways. In addition to its 3D character, the dicarbollide ligand is a significantly stronger donor than the cyclopentadienyl one and has a double charge. The donor nature of the dicarbollide ligand can be largely tuned via the introduction of substituents of various natures. At the same time, the charge of the ligand can be partially compensated by introducing into the dicarbollide ligand the so-called charge-compensating substituents of an onium nature (ammonium, phosphonium, sulfonium, etc.). This significantly brings the properties of the dicarbollide and cyclopentadienyl complexes closer together and causes a high interest in metallacarboranes based on charge-compensated dicarbollide ligands [41,42,43,44,45,46,47,48,49,50].
In this study, we review the synthesis and properties of charge-compensated nido-carborane derivatives in which the onium center is bonded to the boron atom directly or through a short single-atom spacer and, therefore, not only reduces the ligand charge but also has a significant effect on its electron-donating properties. Therefore, derivatives containing charge-compensating substituents at the carbon atoms [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67] or bound to the carborane basket through a longer spacer, for example, obtained by opening cyclic oxonium derivatives [68] and some others [69,70,71,72], are beyond the scope of this review.
As a rule, charge-compensating substituents are groups in which the positive charge is localized on the atoms of Group 5 (nitrogen, phosphorus, and arsenic) or Group 6 (oxygen, sulfur, and selenium) elements. In most cases, this atom is bonded directly to the boron atom of the nido-carborane basket. Therefore, the classification of charge-compensated derivatives of nido-carborane according to the type of boron–element bond is the most convenient. Another important factor is the position of the substituent, which can be located in the upper (open) belt (positions 9, 10, and 11) or lower (positions 1, 2, 3, 4, 5, and 6) of nido-carborane. Substituents located in the upper belt of nido-carborane in some cases can affect the coordination environment of the metal in metallacarboranes based on them both due to steric factors and in the presence of additional donor groups. In addition, for the synthesis of derivatives with substituents in the upper belt, substitution reactions in the nido-carborane itself are mainly used, while the preparation of derivatives with substituents in the lower belt is based on the decapitation of the corresponding ortho-carborane derivatives. It should also be kept in mind that positions 1, 3, and 10 are in the plane of symmetry of the nido-carborane cage, and, therefore, the substitution of hydrogen atoms in these positions leads to symmetrically substituted derivatives. At the same time, the substitution of hydrogen atoms in positions 2, 4, 5, 6, 9, and 11 leads to asymmetrically substituted derivatives, which are racemic mixtures of the corresponding isomers.

2. Charge-Compensated Derivatives of Nido-Carborane with Boron–Nitrogen Bond

Due to the great diversity of nitrogen chemistry, the charge-compensated derivatives of nido-carborane with the B-N bond are characterized by the greatest variety of forms. The first example of the synthesis of charge-compensated derivatives of nido-carborane with a B-N bond was the reaction of the parent nido-carborane with pyridine in benzene in the presence of anhydrous FeCl3, leading to the asymmetrically substituted pyridinium derivative 9-Py-7,8-C2B9H11 (Scheme 1) [73], the structure of which was later supported via a single-crystal X-ray diffraction study (Figure 2) [74]. When FeCl3·6H2O was used instead of anhydrous FeCl3, the by-product of the reaction was the disubstituted pyridinium derivative 9,11-Py2-7,8-C2B9H9 [74]. The reaction with 7,8-dimethyl-nido-carborane proceeds in a similar way, giving 9-Py-7,8-Me2-7,8-C2B9H9 [73]. In a similar way, the reaction of nido-carborane with methyl isonicotinate in the presence of FeCl3 in refluxing benzene results in 9-(4′-MeO(O)CC5H3N)-7,8-C2B9H11 [75].
The asymmetrically substituted 9-pyridinium derivative of nido-carborane was also prepared via the reaction of the parent ortho-carborane with pyridine in the presence of copper acetate and water. Similar reactions with C-monosubstituted ortho-carboranes give a mixture of the corresponding isomeric pyridinium derivatives 9-Py-7-R-7,8-C2B9H10 and 11-Py-7-R-7,8-C2B9H10 (R = Me, Ph). In the case of 1-XCH2 derivatives of ortho-carborane (X = Cl, Br, OH), in addition to a mixture of the corresponding 9- and 11-pyridinium derivatives of nido-carborane, the reaction gives the pyridinium methyl derivative 7-PyCH2-7,8-C2B9H11 [76].
The reaction of nido-carborane with pyridine in the presence of HgCl2 in refluxing benzene gives a mixture of the symmetrically and asymmetrically substituted pyridinium derivatives 10-Py-7,8-C2B9H11 and 9-Py-7,8-C2B9H11 in a ratio of 2:1 (Scheme 2) [50,77]. The reaction of 7,8-dimethyl-nido-carborane [7,8-Me2-7,8-C2B9H10]− with pyridine proceeds in a similar way [77].
The symmetrically substituted pyridinium derivative 10-Py-7,8-C2B9H11 was prepared via the reaction of the 10-diphenylsulfonium derivative 10-Ph2S-7,8-C2B9H11 with pyridine in refluxing chloroform (Scheme 3, Figure 2) [78].
The reaction of nido-carborane with 4-phenylpyridine in 1,2-dimethoxyethane in the presence of 2,3-dichloro-5,6-icyanobenzoquinone (DDQ) as an oxidizing agent leads to the corresponding asymmetrically substituted pyridinium derivative [9-(4′-PhC5H4N)-7,8-C2B9H11] (Scheme 4) [79].
This approach is also applicable to various pyridine derivatives and C,C’-disubstituted nido-carboranes. The reaction was shown to be tolerant to the halogen, methoxy, methylcarboxy, amino, and vinyl substituents (Scheme 5, Figure 3) [79]. The same products can be prepared via reagent-free electrocatalyzed direct B-N oxidative couplings of nido-carboranes with pyridines (Scheme 4, Figure 3) [80,81]. In the case of C-monosubstituted nido-carboranes, the reaction leads to mixtures of the 9- and 11-pyridinium derivatives [79,81].
The reaction of 7,8-diphenyl-nido-carborane with 4,4′-vinylenedipyridine in the presence of DDQ in 1,2-dimethoxyethane gives the corresponding pyridinium derivative containing two nido-carboranyl units (Figure 4) [79].
The electrocatalyzed B-N oxidative couplings of nido-carboranes with pyridines were used for the synthesis of a nido-carborane-based amino acid and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives (Figure 5 and Figure 6) [80].
The use of quinoline and isoquinoline and their derivatives instead of pyridine leads to the corresponding quinolinium and isoquinolinium derivatives of nido-carborane (Scheme 6, Figure 7) [79,80].
The oxidation of C-substituted nido-carboranes containing a pendant pyridine or quinoline fragment leads to intramolecular cyclization with the formation of the corresponding pyridinium and quinolinium derivatives (Scheme 7, Figure 8) [79,80,81].
Bromination and iodination of 9-Py-7,8-C2B9H11 with bromine and iodine in acetic acid leads to 11-Br-9-PyN-7,8-C2B9H10 and 11-I-9-PyN-7,8-C2B9H10, correspondently (Figure 9) [82,83].
The oxidative coupling can be also applied for the synthesis of nido-carboranyl derivatives of other azaheterocycles, including pyrazole, imidazole, oxazole, thiazole, pyrimidine, and azaindoles (Figure 10 and Figure 11) [79,80,81].
The 9-trimethylammonium derivative of nido-carborane 9-Me3N-7,8-C2B9H11 was prepared via the reaction of the potassium salt of the parent nido-carborane with copper(II) sulfate in the presence of aqueous ammonia and trimethylammonium chloride (Scheme 8) [74]. This approach was also used for the synthesis of a series of 9-alkyldimethylamino derivatives of nido-carborane, 9-RMe2N-7,8-C2B9H11 (R = CH2Ph, CH2C≡N, CH2C≡CH, (CH2)3Cl, (CH2)2OH, (CH2)3OH, (CH2)2NMe2) (Scheme 8, Figure 12) [84].
An attempt to crystallize 9-HO(CH2)3Me2N-7,8-C2B9H11 from acetone led to intramolecular BH-activation with the formation of μ-9,4-Me2N(CH2)3O-7,8-C2B9H10 (Figure 13) [84].
The derivative with the N,N,N′,N′-tetramethylethylenediamine substituent 9-Me2N(CH2)2Me2N-7,8-C2B9H11 can be also obtained via the nucleophilic substitution of iodine in the 9-iodo derivative [9-I-7,8-C2B9H11]− with tetramethylethylenediamine (TMEDA) in the presence of t-BuOK (Scheme 9) [85].
Alkylation of 9-Me2N(CH2)2Me2N-7,8-C2B9H11 with allyl chloride or propargyl bromide leads to the corresponding cationic derivatives of nido-carborane (Scheme 10) [84].
The azido derivative of 9-N3(CH2)3Me2N-7,8-C2B9H11 (Figure 12) was prepared by heating the corresponding chloride with sodium azide in DMF in the presence of NaI. The copper(I)-catalyzed azide-alkyne cycloaddition reactions of 9-N3(CH2)3Me2N-7,8-C2B9H11 with various terminal alkynes, including phenylacetylene and alkyne derivatives of cholesterol and cobalt and iron bis(dicarbollides), results in the corresponding 1,2,3-triazoles (Scheme 11) [86]. The zwitterionic nido-carborane–cholesterol conjugate was also prepared via the Cu(I)-catalyzed cycloaddition of the alkyne derivative of nido-carborane 9-HC≡CCH2Me2N-7,8-C2B9H11 with 3β-(2-azidoethoxy)-5-cholestene (Scheme 12) [87].
The same approach can be used for the preparation of trimethylammonium derivatives of C-substituted nido-carboranes. The reaction of 7,8-bis(methylthio)-nido-carborane with trimethylammonium chloride in the presence of copper(II) sulfate in an aqueous ammonia solution results in 9-Me3N-7,8-(MeS)2-7,8-C2B9H9, whereas the similar reaction of 7-methylthio-nido-carborane gives a mixture of 9-Me3N-7-MeS-7,8-C2B9H10 (major isomer) and 11-Me3N-7-MeS-7,8-C2B9H10 (minor isomer) (Scheme 13, Figure 14) [88].
The symmetrically substituted triethylammonium derivative 10-Et3N-7,8-C2B9H11 was prepared in low yield via the reaction of the 10-diphenylsulfonium derivative 10-Ph2S-7,8-C2B9H11 with triethylamine in refluxing chloroform (Scheme 14, Figure 15) [78].
The asymmetrically substituted triethylammonium derivative 9-Et3N-7,8-C2B9H11 was prepared in low yield via the reaction of the 9-iodo derivative [9-I-7,8-C2B9H11]− with triethylamine in the presence of t-BuOK under reflux conditions [85].
A series of cyclic 9-dialkylammonium derivatives of 7,8-diphenyl-nido-carborane was prepared via the photoredox coupling of [7,8-Ph2-7,8-C2B9H10]− with secondary amines under blue LED light irradiation in the presence of 9-mesityl-10-methylacridinium perchloratee as the photocatalyst (Scheme 15, Figure 16) [89]. In a similar way, unsymmetrical and acyclic dialkylammonium derivatives of nido-carborane can be prepared using tetrahydroisoquinoline and methylbenzylamine, respectively (Figure 16) [89].
This approach is also applicable to the synthesis of various primary aliphatic and heteroaromatic amines (Scheme 16, Figure 17) [89].
Halogenation of the 9-trimethylammonium derivative 9-Me3N-7,8-C2B9H11 was studied. The reaction with an equimolar amount of Cl2 in dichloromethane at -25°C results in a mixture of 11-Cl-9-Me3N-7,8-C2B9H10 and 6-Cl-9-Me3N-7,8-C2B9H10 isolated in 14% and 45% yields, respectively, whereas the reaction with an excess of Cl2 under similar conditions gives 6,11-Cl2-9-Me3N-7,8-C2B9H9 isolated in a 26% yield [90]. The reaction of 9-Me3N-7,8-C2B9H11 with an equimolar amount of Br2 in dichloromethane at -25°C results in a mixture of 11-Br-9-Me3N-7,8-C2B9H10 and 6-Br-9-Me3N-7,8-C2B9H10 isolated in 81% and 11% yields, respectively [90]. The reaction with an excess of Br2 in dichloromethane under reflux gave a mixture of 6,11-Br2-9-Me3N-7,8-C2B9H9] and 1,6,11-Br3-9-Me3N-7,8-C2B9H8, both isolated with a yield of 12% (Figure 18) [90]. The reaction of 9-Me3N-7,8-C2B9H11 with I2 in acetic acid under reflux leads to 11-I-9-Me3N-7,8-C2B9H10 (Figure 18) as a single product [90].
A convenient method for the functionalization of nido-carborane, leading to the formation of symmetrically substituted derivatives with a B-N bond, is the synthesis and subsequent modification of its nitrilium derivatives. The first nitrilium derivatives of nido-carboranes were synthesized via reactions of the potassium salts of the parent nido-carborane and its 7,8-dimethyl derivative with acetonitrile in the presence of FeCl3. In both cases, the nitrilium derivatives were obtained as mixtures of asymmetrically and symmetrically substituted isomers 9-MeC≡N-7,8-R2-7,8-C2B9H9 (R = H, Me) [73]. Later, it was found that the reaction of (Me4N)[7,8-C2B9H12] with AlCl3 in acetonitrile in the presence of acetone led solely to the symmetric product [10-MeC≡N-7,8-C2B9H11]; however, the product yield was rather low [78]. Recently it was found that the reaction of the potassium salt of nido-caborane K [7,8-C2B9H12] with HgCl2 in a mixture of refluxing acetonitrile or propionitrile and benzene also leads selectively to the corresponding symmetrically substituted nitrilium derivatives 10-RC≡N-7,8-C2B9H11 (R = Me, Et) in close to quantitative yields. Hydrolysis of 10-EtC≡N-7,8-C2B9H11 leads to the protonated iminol 10-EtC(OH)=HN-7,8-C2B9H11, which upon treatment with triethylamine, gives the corresponding amide (Et3NH)[10-EtC(O)HN-7,8-C2B9H11] (Scheme 17) [91].
A similar approach can also be applied to C-substituted derivatives of nido-carborane. The reaction of K[7-BnOOCCH2-7,8-C2B9H11] with acetonitrile in refluxing benzene in the presence of HgCl2, followed by hydrolysis of the resulting nitrilium derivative and acid hydrolysis of the amide, produces the corresponding ammonium derivative of nido-carborane [7-HOOCCH2-10-NH3-7,8-C2B9H10] [92].
The reactions of 10-EtC≡N-7,8-C2B9H11 with alcohols (MeOH, EtOH, i-PrOH, and n-BuOH) and thiols (EtSH, n-BuSH, and n-HxSH) result in the corresponding imidates 10-EtC(OR)=HN-7,8-C2B9H11 and thioimidates 10-EtC(SR)=HN-7,8-C2B9H11 as mixtures of E- and Z-isomers, which can be separated using column chromatography on silica (Scheme 18, Figure 19) [91].
In a similar way, the reactions of 10-EtC≡N-7,8-C2B9H11 with primary amines (MeNH2, EtNH2, n-PrNH2, i-BuNH2, BnNH2, PhNH2, HOCH2CH2NH2, HOCH2CH2CH2NH2, MeOCH2CH2NH2, and Me2NCH2CH2NH2) lead to the corresponding amidines 10-EtC(NHR)=HN-7,8-C2B9H11 as mixtures of E- and Z-isomers (Scheme 19) [93,94]. The obtained carboranyl amidines were shown to be promising ligands for the synthesis of various metallacarboranes [94,95,96]. The reaction of 10-EtC≡N-7,8-C2B9H11 with ethylenediamine proceeds with the elimination of imidazoline, resulting in the 10-ammonium derivative 10-H3N-7,8-C2B9H11 (Scheme 19) [94]. The reactions of 10-EtC≡N-7,8-C2B9H11 with secondary amines (Me2NH, Et2NH, piperidine, and morpholine) produce the corresponding amidines 10-EtC(NR2)=HN-7,8-C2B9H11 as single E-isomers (Scheme 19, Figure 20) [93].
The reaction of the potassium salt of nido-caborane K[7,8-C2B9H12] with bis(2-cyanoethyl) ether in the presence of HgCl2 results in the corresponding nitrilium derivative 10-NCCH2CH2OCH2CH2C≡N-7,8-C2B9H11; however, treatment of the iminol formed after its hydrolysis with triethylamine unexpectedly leads to side-chain shortening with the formation of the acrylamide derivative (Et3NH)[10-CH2=CHC(O)HN-7,8-C2B9H11] (Scheme 20) [97].
Surprisingly, the reactions of 10-NCCH2CH2OCH2CH2C≡N-7,8-C2B9H11 with alcohols and thiols proceed in a different manner: the reactions with thiols lead to the expected thioimidates, 10-NCCH2CH2OCH2CH2C(SR)=HN-7,8-C2B9H11 (R = Et, Bu), as mixtures of E- and Z-isomers, which can be separated using column chromatography on silica, whereas the reactions with alcohols result in side-chain shortening with the formation of a mixture of two imidates, 10-CH2=CHC(OR)=HN-7,8-C2B9H11 and 10-HOCH2CH2C(OR)=HN-7,8-C2B9H11 (R = Me, Et, i-Pr, n-Bu) (Scheme 21, Figure 21). The reaction with diethylamine gives amidine 10-HOCH2CH2C(NEt2)=HN-7,8-C2B9H11 (Scheme 21) [97].
Amidine 10-CH3C(NCPh2)=HN-7,8-C2B9H11 (Figure 22) was obtained from the reaction of the tetramethylammonium salt of nido-carborane with benzophenone imine in acetonitrile in the presence of acetyl chloride. It is assumed that the reaction proceeds through the formation of the acetonitrilium derivative of nido-carborane, followed by the addition of the imine to the activated C≡N triple bond [98].
The 3-Ammonium derivative of nido-carborane 3-H3N-7,8-C2B9H11 was prepared via the deboronation of 3-amino-ortho-carborane 3-H2N-1,2-C2B10H11 with an alkali in refluxing ethanol (Scheme 22, Figure 23) [99,100]. The same approach can be used for the synthesis of the 3-ammonium derivatives of C-substituted nido-carboranes 3-H3N-7-R-7,8-C2B9H12 (R = i-Pr, CH2COOH, CH2COOBn) (Figure 23) [99,101]. The 3-dimethylammonium and 3-benzylammonium derivatives of nido-carborane were prepared via the deboronation of the corresponding derivatives of ortho-carborane (Scheme 22) [99]. The 3-trimethylammonium derivative of nido-carborane 3-Me3N-7,8-C2B9H11 was obtained via the treatment of the 3-ammonium derivative with methyl iodide in the presence of K2CO3 (Scheme 22) [99].
Heating 9-amido-ortho-carboranes 9-RCONH-1,2-C2B10H11 (R = H, Alk, or Ar) with 10 mol.% of Pd(OAc)2, 2 equiv. of AgOAc, and 2 equiv. of K2CO3 in 1,4-dioxane at 100°C results in successive deboronation and cyclization reactions with the formation of the corresponding N-protonated nido-carborane fused oxazoles 5,10-μ-RCNHO-7,8-C2B9H10. The reaction is applicable to various N-carboranylamides including formamide and alkyl- and arylamides, as well as to C,C’-substituted N-carboranylamides (Scheme 23, Figure 24) [102]. Interestingly, in the case of 9-PhCONH-1,2-μ-C6H4(CH2)2-1,2-C2B10H9, the reaction results in a mixture of the 5,10-μ-PhCNHO-7,8-μ-C6H4(CH2)2-7,8-C2B9H8 and 5,9-μ-PhCNHO-7,8-μ-C6H4(CH2)2-7,8-C2B9H8 isomers (Figure 24) in a 1:1 ratio [102]. It was found that the Pd catalyst plays an important role in promoting deboronation, while the presence of AgOAc is critical for the cyclization reaction [102].

3. Charge-Compensated Derivatives of Nido-Carborane with Boron–Phosphorus Bond

Unlike derivatives with a boron–nitrogen bond, derivatives of nido-carboranes with a boron–phosphorus bond can be prepared using electrophilic substitution reactions. Heating the potassium salt of nido-carborane K[7,8-C2B9H12] with Ph2PCl in tetrahydrofuran at reflux leads to the P-protonated diphenylphosphonium derivative 9-Ph2PH-7,8-C2B9H11, which can be alkylated with MeI under reflux in ethanol in the presence of EtONa as a base to give the methyldiphenylphosphonium derivative 9-MePh2P-7,8-C2B9H11 (Scheme 24, Figure 25) [103,104].
The symmetrically substituted phosphonium derivatives 10-Ph2PH-7,8-C2B9H11 and 10-MePh2P-7,8-C2B9H11 (Figure 25) were prepared in a similar way using the disodium dicarbollide salt Na2[7,8-C2B9H11] as a starting material (Scheme 25) [104].
Phosphonium derivatives of nido-carborane also can be prepared via Lewis-acid-mediated nucleophilic substitution reactions. The reaction of the potassium salt of nido-carborane K[7,8-C2B9H12] with PPh3 in the presence FeCl3 in benzene at 80 °C leads to a mixture of triphenylphosphonium 9-Ph3P-7,8-C2B9H11 and 10-Ph3P-7,8-C2B9H11, which were separated using column chromatography on silica (Scheme 26) [103,104].
The asymmetrical triphenylphosphonium derivative 9-Ph3P-7,8-C2B9H11 was also obtained via the reaction of triphenylphosphine with the dithallium dicarbollide salt Tl2[7,8-C2B9H11] in dichloromethane and in the presence of AgBr at ambient temperature (Scheme 27) [105].
Similar to the pyridinium derivatives, a series of asymmetrically substituted phosphonium derivatives 9-R’R2P-7,8-Ph2-7,8-C2B9H9 was prepared via electrocatalyzed B-P oxidative couplings of 7,8-diphenyl-nido-carborane with various phosphines and phosphites (Scheme 28, Figure 26) [106].
However, the largest number of phosphonium derivatives of nido-carborane was obtained through transition-metal-catalyzed cross-coupling reactions [107]. A series of triphenylphosphonium derivatives of nido-carborane, X-Ph3P-7,8-C2B9H11 (X = 1, 3, 5, 9), were synthesized via the reactions of the corresponding iodo derivatives with PPh3 in the presence of 10 mol.% of [(Ph3P)4Pd] in 1,4-dioxane at 90 °C. This approach can be used to synthesize derivatives containing substituents in different positions of the nido-carborane cage, including the upper and lower belts, as well as the bottom of the basket (Scheme 29) [108].
The 5-triphenyl- and 5-bis(t-butyl)phosphonium derivatives of nido-carborane 5-Ph3P-7,8-C2B9H11 and 5-tBu2PH-7,8-C2B9H11 were obtained in low yields directly by heating 9-iodo-ortho-carborane with an excess of AgF and catalytic amounts of [(Ph3P)4Pd] or [(tBu3P)2Pd] in DMF at 140 °C (Scheme 30, Figure 27) [109].
The symmetrically substituted diiodo derivatives [5,6-I2-7,8-C2B9H10]− and [9,11-I2-7,8-C2B9H10]− under the same conditions give the corresponding mono-coupling products 5-I-6-Ph3P-7,8-C2B9H10 and 9-I-11-Ph3P-7,8-C2B9H10 in good yields, whereas the reaction of the asymmetrical diiodo derivative with substituents at boron atoms in both pentagonal belts [6,9-I2-7,8-C2B9H10]− results in selective coupling at the open pentagonal belt to form 6-I-9-Ph3P-7,8-C2B9H10 in a moderate yield (Scheme 31, Figure 28) [108].
The bis(triphenylphosphonium) derivatives 5,6-(Ph3P)2-7,8-C2B9H10 and 9,11-(Ph3P)2-7,8-C2B9H10 can be prepared in a similar way with the addition of Cs2CO3 as a base to remove the bridging hydrogen (Scheme 32, Figure 29) [108]. 9,11-(Ph3P)2-7,8-C2B9H10 can also be obtained in a two-step process through 11-Ph3P-9-I-7,8-C2B9H10 (Scheme 32) [108].
In the case of the 5,6,9-triiodo derivative of nido-carborane [5,6,9-I3-7,8-C2B9H9]−, the substitution proceeds selectively in the open pentagonal belt and in the most distant position in the lower pentagonal belt to form 6,9-(Ph3P)2-5-I-7,8-C2B9H8 (Scheme 33, Figure 30). Alternatively, this compound can be obtained via the reaction of the diiodo derivative of nido-carborane [5,6-I2-7,8-C2B9H10]− with PPh3 in the presence of a sub-equimolar amount of [(Ph3P)4Pd] in dioxane at 90°C (Scheme 33) [108]. In this case, both BI and BH activation takes place.
The reaction of nido-carborane with 0.5 equiv. of [(Me2PhP)2PdCl2] in dichloromethane at room temperature followed by heating with NaBH4 in benzene leads to the BH-activation of nido-carborane with the formation of a mixture of isomeric phosphonium derivatives 9-Me2PhP-7,8-C2B9H11 and 10-Me2PhP-7,8-C2B9H11 (Scheme 34, Figure 31) [110].
The reaction of ortho-carborane with triphenylphosphine in the presence of catalytic amounts of PdCl2 in benzene at 80 °C directly results in the 5-triphenylphosphonium derivative of nido-carborane 5-Ph3P-7,8-C2B9H11 in a moderate yield (Scheme 35, Figure 32) [111].
The reactions of sulfide [7,8-µ-S(CH2)3-7,8-C2B9H10]− with 1 equiv. of palladium(II) complexes [L2PdCl2] (L = PPh3, PPh2Me) in boiling ethanol led to selective BH-activation at the nearest-t-sulfur boron atom of the upper pentagonal belt with the formation of 11-R’R2P-7,8-µ-(S(CH2)3)-7,8-C2B9H9 (Scheme 36, Figure 33) [112]. It is assumed that the alkyl sulfide substituent plays the role of a directing group.
The 2-pyridyl substituent can also act as a directing group. The reactions 2-pyridyl-substituted nido-carborane [7-(2′-Py)-7,8-C2B9H11]−, formed by heating the corresponding ortho-carborane 1-(2′-Py)-1,2-C2B9H12 in aqueous acetonitrile, with various phosphines in the presence of catalytic amounts of PdCl2 in a mixture of toluene, water, and acetonitrile at 120°C lead to the corresponding phosphonium derivatives 11-R’R2P-7-(2′-Py)-7,8-C2B9H10 (Scheme 37, Figure 34). The reaction is tolerant to the presence of alkyl and aryl substituents at the second carbon atom of the nido-carborane cage (Scheme 37) [113].
Various substituted pyridines (5-, 6-methyl-, and 4-trifluoromethylpyridine), as well as 2-benzoxazolyl and diphenylphosphine groups, can be used as guide groups as well (Scheme 38) [113].
The reaction of [7-(NC5H3-3′-Me-2′-)-7,8-C2B9H11]− with triphenylphosphine in the presence of 10 mol. % of PdCl2 unexpectedly led to a mixture of 11- and 2-triphenylphosphonium derivatives (Scheme 39, Figure 35) isolated in 31% and 25% yields, respectively. It was found that the addition of 20 mol. % of 2-amino-5-methyl-pyridine as a ligand leads to a change in the product ratio, with an increase in the 2-isomer yield of up to 64% (the yield of the 11-isomer is 14% in this case) [113,114].
This reaction is applicable to a wide variety of phosphines; however, the expected 2-phosphonium derivatives are formed in rather low yields varying from 12 to 45% (Scheme 40) [114].
Another group that directs the substitution to position two of the nido-carborane cage under similar conditions is the isoquinolin-1-yl group (Scheme 41). The reaction is tolerant to the presence of a substituent at the second carbon atom of the nido-carborane cage (Scheme 42) [114].
It is also worth mentioning the formation of a symmetrically substituted triphenylphosphonium derivative of nido-carborane during the migration of the phosphine ligand from the metal atom to the dicarbollide ligand in nickelacarborane 3,3-(Ph3P)2-3,1,2-NiC2B9H11 (Scheme 43) [115,116].

4. Charge-Compensated Derivatives of Nido-Carborane with Boron–Arsenic and Boron–Antimony Bonds

The charge-compensated derivatives of nido-carborane with boron–arsenic and boron–antimony bonds are rare and are limited to a few examples. Similar to the triphenylphosphonium derivative, the asymmetrically substituted triphenylarsonium and tetraphenylstilbonium derivatives 9-Ph3X-7,8-Ph2-7,8-C2B9H9 (X = As, Sb) were prepared via electrocatalyzed oxidative couplings of 7,8-diphenyl-nido-carborane with Ph3As and Ph3Sb, respectively (Scheme 44, Figure 36) [106].
The reaction of 2-pyridyl-substituted nido-carborane [7-(2′-Py)-7,8-C2B9H11]− with triphenylarsine in the presence of catalytic amounts of PdCl2 in a mixture of toluene, water, and acetonitrile at 120°C results in the corresponding triphenylarsonium derivative 11-Ph3As-7-(2′-Py)-7,8-C2B9H10 (Scheme 45) [113].

5. Charge-Compensated Derivatives of Nido-Carborane with Boron–Oxygen Bond

Alkyloxonium salts are much less stable than ammonium and phosphonium salts, and some of them are used in organic chemistry as strong alkylating agents. Nevertheless, strong electron-withdrawing of polyhedral boron hydride clusters substituted at boron atoms and, in particular, nido-carborane [117], is capable of stabilizing their oxonium derivatives [68,118]. The first example of such a derivative was obtained very soon after the discovery of nido-carborane via the reaction of the potassium salt of nido-carborane with tetrahydrofuran in the presence of FeCl3 in benzene. As a result, a mixture of two isomeric tetrahydrofuran derivatives of nido-carborane was obtained. The reaction with the C,C′-dimethyl derivative of nido-carborane proceeds in a similar way (Scheme 46) [73].
It was later found that the replacement of FeCl3 with HgCl2 in this reaction leads to the selective formation of the symmetrically substituted tetrahydrofuran derivative 10-(CH2)4O-7,8-C2B9H11 [77,119]. The symmetrically substituted derivative was also obtained via the reaction of the tetramethylammonium salt of nido-carborane with AlCl3 in a mixture of tetrahydrofuran and acetone [78] and by the treatment of the potassium salt of nido-carborane with tetrahydrofuran in the presence of acetaldehyde or formaldehyde and hydrochloric acid in a mixture of water and toluene [120] (Scheme 47).
Oxonium derivatives of nido-carborane with other cyclic ethers were synthesized as well. The symmetrically substituted 1,4-dioxane derivative 10-O(CH2CH2)2O-7,8-C2B9H11 can be prepared via the reaction of the potassium salt of nido-carborane with 1,4-dioxane in the presence of HgCl2 in benzene [119] or in the presence of acetaldehyde and hydrochloric acid in a water–toluene mixture [120]. The 1,4-dioxane derivative can also be synthesized via the heating of the protonated form of nido-carborane 7,8-C2B9H13 with 1,4-dioxane [121] (Scheme 48). The molecular structure of the 1,4-dioxane derivative of nido-carborane was determined using single-crystal X-ray diffraction (Figure 37) [122].
The reaction of the potassium salt of nido-carborane with tetrahydropyran in the presence of mercury(II) chloride in benzene results in the tetrahydropyran derivative 10-(CH2)5O-7,8-C2B9H11 (Scheme 49) [123,124].
The most important property of the cyclic oxonium derivatives of nido-carborane is their tendency for ring-opening reactions under the action of nucleophiles. This makes it possible to modify the nido-carborane cluster and introduce various terminal groups including functional groups as side substituents. At the same time, depending on the oxonium derivative used, it is possible to obtain compounds with different spacer lengths between the terminal group and the cluster (Scheme 50). In this way, nido-carborane-based carboxylic acids [119,125], azides [119,126,127], calixarenes [122], and coumarins [128], as well as hydroxy [122,129,130], halogen [122], ammonium [121,122,124], and mercapto [122,131] derivatives, and others, were prepared. The use of two equivalents of oxonium derivatives in the reaction with dinucleophiles allowed us to obtain podands, which were used for the synthesis of crown ethers [129,131] (Scheme 50).
It should be noted that the use of neutral nucleophiles, such as ammonia, in ring-opening reactions leads to charge-compensated derivatives in which the charges are separated by a spacer formed during the opening of the oxonium ring (Figure 38) [122].
There are several examples of acyclic oxonium derivatives of nido-carborane. The reaction of the potassium salt of nido-carborane with dimethoxymethane in the presence of mercury(II) chloride in a benzene solution leads to a mixture of the asymmetrically and symmetrically substituted dimethyloxonium derivatives 9-Me2O-7,8-C2B9H11 and 10-Me2O-7,8-C2B9H11, along with the corresponding methoxy derivatives [9-MeO-7,8-C2B9H11]− and [10-MeO-7,8-C2B9H11]− [132] (Scheme 51).
The symmetrically substituted diethyloxonium derivative of nido-carborane can be obtained via the reaction of the potassium salt of nido-carborane with diethyl ether in the presence of formaldehyde or acetaldehyde and hydrochloric acid in a mixture of water and toluene [120], or via the reaction of the potassium salt of nido-carborane with diethyl ether in the presence of HgCl2 in benzene [50] (Scheme 52).
The symmetrically and asymmetrically substituted diethyloxonium derivatives of nido-carborane 9-Et2O-7,8-C2B9H11 and 10-Et2O-7,8-C2B9H11 along with the 9-diethyloxonium-11-chloro derivative [9-Et2O-11-Cl-7,8-C2B9H10] were found to form as by-products in the reaction of the dicarbollide dianion with PhBCl2 in diethyl ether [133]. The structure of the 10-diethyloxonium derivative 10-Et2O-7,8-C2B9H11 was determined using single-crystal X-ray diffraction (Figure 39) [133].
The dialkyloxonium derivatives of nido-carboranes easily lose the alkyl group when they react with nucleophiles and therefore can be considered alkylating agents [132,133].
An interesting example of a charge-compensated derivative of nido-carborane is 10-Me2SO-7,8-µ-(CH2)3-7,8-C2B9H9, which can be obtained via the reaction of the potassium salt of [7,8-µ-(CH2)3-7,8-C2B9H10]− with a dimethyl sulfoxide/water solution in the presence of concentrated H2SO4, or via the reaction of the trimethylammonium salt with DMSO in dry 1,2-dichloroethane in the presence of triflic acid [134] (Scheme 53). The structure of 10-Me2SO-7,8-µ-(CH2)3-7,8-C2B9H9 was determined using single-crystal X-ray diffraction (Figure 40) [134].

6. Charge-Compensated Derivatives of Nido-Carborane with Boron–Sulfur Bond

Compared with the oxonium derivatives, the sulfonium derivatives of nido-carborane are represented by a wider variety of derivatives and synthetic methods for their preparation. However, the most studied of them are the dimethylsulfonium derivatives of nido-carborane, which are widely used for the synthesis of metallacarboranes [41,50,135].
It should be noted that symmetrically and asymmetrically substituted dimethylsulfonium derivatives of nido-carborane are usually obtained in different ways, which excludes the formation of mixtures of their isomers. The asymmetrically substituted 9-dimethylsulfonium derivative of nido-carborane 9-Me2S-7,8-C2B9H11 was prepared via the reaction of the parent nido-carborane with dimethylsulfoxide in the presence of sulfuric acid at 80°C [74,136,137]. The reactions of the C,C’-substituted derivatives of nido-carborane proceed in a similar way, leading to the corresponding dimethylsulfonium derivatives 9-Me2S-7,8-R2-7,8-C2B9H9 (R = Me, Ph) [74,138] (Scheme 54). These conditions are similar to those used for the synthesis of the dimethylsulfonium derivatives of the closo-decaborate [139] and closo-dodecaborate [140] anions. The C,C′-substituted derivatives 9-Me2S-7,8-Me2-7,8-C2B9H9 and 9-Me2S-7,8-μ-(1′,2′-C6H4(CH2)2)-7,8-C2B9H9 were prepared via the reactions of the corresponding nido-carboranes with dimethylsulfoxide in the presence of triflic acid in 1,2-dichloroethane [134] (Scheme 54). The structures of 9-Me2S-7,8-R2-7,8-C2B9H9 (R = H, Ph) were determined using single-crystal X-ray diffraction (Figure 41) [141,142].
The asymmetrically substituted dimethylsulfonium derivatives 9-Me2S-7,8-Me2-7,8-C2B9H9 and 9-Me2S-7,8-µ-(CH2OCH2)-7,8-C2B9H9 were prepared via the reactions of the corresponding nido-carboranes with dimethylsulfide in the presence of Fe(NO3)3 in aqueous ethanol [49].
In the case of C-substituted nido-carboranes, such as K[7-Ph-7,8-C2B9H11], the introduction of a Me2S group results in a mixture of 9-Me2S-7-Ph-7,8-C2B9H10 and 11-Me2S-7-Ph-7,8-C2B9H10 isomers, which can be separated using column chromatography [137]. The molecular structure of 11-Me2S-7-Ph-7,8-C2B9H10 was determined using single-crystal X-ray diffraction (Figure 41) [138].
In a similar way, the reactions of the cesium salts of the 5-methyl or 5-bromo derivatives of nido-carborane K[5-R-7,8-C2B9H11] (R = Me or Br) with dimethylsulfide in the presence of iron(III) chloride FeCl3 in aqueous ethanol result in mixtures of the 9-Me2S-5-R-7,8-C2B9H10 and 11-Me2S-5-R-7,8-C2B9H10 isomers, which were separated using column chromatography on silica (Scheme 55) [143].
The reaction of the tetramethylammonium salt of 9-methyl-nido-carborane (Me4N)[9-Me-7,8-C2B9H11] with dimethyl sulfide under the same conditions results in the introduction of a Me2S group into position nine of the nido-carborane cage. However, the reaction is accompanied by a rearrangement of the carborane cage, leading to a mixture of the 9-Me2S-3-Me-7,8-C2B9H10 (main isomer), 9-Me2S-4-Me-7,8-C2B9H10, 9-Me2S-2-Me-7,8-C2B9H10, 9-Me2S-1-Me-7,8-C2B9H10, and 9-Me2S-10-Me-7,8-C2B9H10 isomers, which were separated using column chromatography (Scheme 56) [143]. It was assumed that the reaction proceeds through the oxidative closure of the nido-carborane cage followed by a series of rearrangements of the resulting 11-vertex B-substituted closo-carborane with a subsequent reopening of its isomers under the action of dimethylsulfide. The structures of 9-Me2S-3-Me-7,8-C2B9H10 and 9-Me2S-4-Me-7,8-C2B9H10 were determined using single-crystal X-ray diffraction (Figure 42) [143].
The 9-dimethylsulfonium derivative of nido-carborane can be demethylated with strong bases (sodium naphthalenide in tetrahydrofuran at room temperature [144,145]; 1,1′-bis(diphenylphosphino)ferrocene (dppf) in toluene at 80 °C [146]; sodium in liquid ammonia at −40 °C [145]; and boiling TMEDA, morpholine or triethylamine [147], and sodium amide in boiling toluene [147]) to the 9-methylthio derivative [9-MeS-7,8-C2B9H11]−, the subsequent alkylation of which gives a whole series of new dialkylsulfonium derivatives of nido-carborane 9-R(Me)S-7,8-C2B9H11, including derivatives with various functional groups. The resulting nido-carboranyl esters, nitriles, and phthalimides can be converted into corresponding carboxylic acids and amines using acid hydrolysis and deprotection with hydrazine, respectively (Scheme 57, Figure 43) [147,148,149].
The reaction of the 2′-bromoethyl(methyl)sulfonium derivative 9-BrCH2CH2(Me)S-7,8-C2B9H11 with K2CO3 in ethanol leads to the ethoxy derivative 9-EtOCH2CH2(Me)S-7,8-C2B9H11, while the reaction in chloroform results in the vinylsulfonium derivative 9-CH2=CH(Me)S-7,8-C2B9H11 (Scheme 58, Figure 43) [148].
The asymmetrically substituted diethylsulfonium derivative 9-Et2S-7,8-C2B9H11 was prepared via electrocatalyzed B-S oxidative couplings of the tetramethylammonium salt of nido-carborane with diethylsulfide (Scheme 59). The reaction is applicable to various C,C’-dialkyl and diaryl derivatives of nido-carborane (Scheme 59, Figure 44) [106]. This approach can be used for the synthesis of carboranyl sulfonium derivatives containing various alkyl and aryl groups (Scheme 60, Figure 44) [106].
This approach was used for the synthesis of nido-carboranyl analogs of some drugs including ibuprofen, indomethacin, ciprofibrate, and probenecid [106].
Halogenation of the 9-dimethylsulfonium derivative 9-Me2S-7,8-C2B9H11 was studied. The reaction with an equimolar amount of N-chlorosuccinimide in acetonitrile produces 11-Cl-9-Me2S-7,8-C2B9H10 [150], while bubbling gaseous Cl2 through a solution of 9-Me2S-7,8-C2B9H11 in dichloromethane results in the dichloro derivative 6,11-Cl2-9-Me2S-7,8-C2B9H9 (Figure 45) [151,152]. The reaction of 9-Me2S-7,8-C2B9H11 with an equimolar amount of Br2 in dichloromethane results in a mixture of 11-Br-9-Me2S-7,8-C2B9H10 (Figure 45) and 6-Br-9-Me2S-7,8-C2B9H10 isolated in 61% and 18% yields, respectively, while the reaction with an excess of Br2 gives 6,11-Br2-9-Me2S-7,8-C2B9H9 (Figure 45) in an 88% yield [150]. The reaction of 9-Me2S-7,8-C2B9H11 with I2 in acetic acid under reflux leads to 11-I-9-Me2S-7,8-C2B9H10 (Figure 45) as a single product isolated in a 37% yield [150]. The same product was prepared in a 78% yield via the reaction of the iodo derivative of nido-carborane [9-I-7,8-C2B9H11]− with dimethylsulfoxide in the presence of concentrated sulfuric acid [150].
The symmetrically substituted sulfonium derivatives of nido-carboranes 10-RR’S-7,8-C2B9H11 were prepared via the reaction of the potassium salt of nido-carborane with various alkyl sulfides, HCl, and acetaldehyde in a mixture of water and toluene (Scheme 61, Figure 46) [120,153]. The symmetrically substituted sulfonium derivatives of C- and C,C′-substituted nido-carboranes were prepared in the same way (Scheme 61, Figure 46) [44,153,154].
When acetaldehyde is replaced with formaldehyde under similar conditions, a mixture of 9-R2SCH2-7,8-C2B9H11 (as the main product) and 10-R2S-7,8-C2B9H11 is formed (Scheme 62) [120].
A convenient method for the synthesis of the 10-dimethylsulfonium derivative of nido-carborane is a two-step reaction of nido-carborane with dimethysulfide in toluene in the presence of a strong acid [155] (Scheme 63). However, the reaction of (Me3NH)[3-Ph-7,8-C2B9H12] with Me2S under similar conditions leads to the sulfonium product 10-SMe2-3-Ph-7,8-C2B9H12 in only a 15% yield [50].
In a similar way, the reaction of (Me4N)[7,8-µ-(CH2)3-7,8-C2B9H10] with triflic acid in a mixture of DMSO and 1,2-dichloroethane mixture results in the symmetrically substituted dimethylsulfonium derivative 10-Me2S-7,8-µ-(CH2)3-7,8-C2B9H10 (Scheme 64) [132].
Like the symmetrically substituted oxonium derivatives, the symmetrically substituted dialkylsulfonium derivatives of nido-carborane can be prepared by reacting the potassium or cesium salt of nido-carborane with alkyl sulfides in the presence of HgCl2 (Scheme 65) [50].
The symmetrically substituted sulfonium derivatives 10-Me2S-7,8-R2-7,8-C2B9H9 were prepared via the reaction of the tetramethylammonium salts of the corresponding nido-carboranes with Me2S in the presence of FeCl3 in benzene [49] (Scheme 66).
The reaction of the tetramethylammonium salt of nido-carborane with tetrahydrothiophene in refluxing acetone in the presence of AlCl3 results in 10-(CH2)4S-7,8-C2B9H11 (Scheme 67) [78].
The symmetrically substituted diphenylsulfonium derivative of nido-carborane 10-Ph2S-7,8-C2B9H11 was prepared via the reaction of the tetramethylammonium salt of nido-carborane with Ph3CBF4 in dichloromethane at −78 °C (Scheme 68, Figure 47) [78].
Similar to 9-Me2S-7,8-C2B9H11, the symmetrically substituted dimethylsulfonium derivative of nido-carborane 10-Me2S-7,8-C2B9H11 can be demethylated with sodium amide in boiling toluene [155] and re-alkylated with various alkylating agents in boiling chloroform or ethanol to give the corresponding sulfonium derivatives 10-R(Me)S-7,8-C2B9H11 (Scheme 69, Figure 48). The resulting nido-carboranyl esters, nitriles, and phthalimides can be converted into corresponding carboxylic acids and amines using acid hydrolysis and deprotection with hydrazine, respectively (Scheme 68) [155,156].
A mixture of asymmetrically and symmetrically substituted dimethylsulfonium derivatives 9-Me2S-7,8-C2B9H11 and 10-Me2S-7,8-C2B9H11 was obtained via the reaction of the potassium salt of nido-carborane with H2SO4 and K2Cr2O7 in a mixture of water and chloroform, followed by the addition of dimethylsulfide [157]. The reaction proceeds through the formation of di-nido-carborane C4B18H22 followed by its splitting using dimethyl sulfide as a Lewis base.
The only example of the introduction of a dialkylsulfonium substituent into the lower belt of nido-carborane described in the literature is the reaction of the 9-mercapto derivative of ortho-carborane 9-HS-1,2-C2B10H11 with KOH and MeI in methanol (Scheme 70) [136].
In the chemistry of the closo-dodecaborate anion, an approach was previously developed for the preparation of its practically important mercapto derivative [B12H11SH]2− through the reaction of the parent closo-dodecaborate with thioureas or thioamides in an acidic medium followed by alkaline hydrolysis of the resulting charge-compensated S-thiouronium and S-thioimidolium derivatives [140]. The reaction of the formed in situ protonated form of nido-carborane C2B9H13 in oluene under reflux conditions gave a mixture of the asymmetrically and symmetrically substituted thiouronium derivatives 9-(H2N)2CS-7,8-C2B9H11 and 10-(H2N)2CS-7,8-C2B9H11. These derivatives were hydrolyzed with NaOH in water, and the formed mercapto derivatives were alkylated with benzyl bromide in chloroform to give the corresponding dibenzylsulfonium derivatives 9-Bn2S-7,8-C2B9H11 and 10-Bn2S-7,8-C2B9H11, which were separated using column chromatography on silica (Scheme 71, Figure 49) [158,159].
The reactions of the tetramethylammonium salt of nido-carborane with thioacetamide and N,N-dimethylthioacetamide in refluxing acetone in the presence of AlCl3 leads to the formation of the asymmetrically and symmetrically substituted S-thioimidolium derivatives 9-i-PrHNC(Me)S-7,8-C2B9H11 and 10-Me2NC(Me)S-7,8-C2B9H11, respectively (Scheme 72, Figure 50) [98].
The 5-dimethylsulfonium derivative of nido-carborane was synthesized via the reaction of orto-carboran-9-yl(phenyl) iodonium tetrafluoroborate [9-PhI-1,2-C2B10H11][BF4] with dimethylsulfoxide (Scheme 73, Figure 51) [160].

7. Charge-Compensated Derivatives of Nido-Carborane with Boron–Selenium and Boron–Tellurium Bonds

The charge-compensated derivatives of nido-carborane with boron–selenium and boron–tellurium bonds are rather rare. Similar to the dialkyl- and diarylsulfonium derivatives, a series of asymmetrically substituted triakyl(aryl)selenium and triaryltellurium derivatives 9-RR’X-7,8-Ph2-7,8-C2B9H9 (X = Se, Te) were prepared via electrocatalyzed oxidative couplings of 7,8-diphenyl-nido-carborane with RR’Se and R2Te, respectively (Scheme 74, Figure 52) [106].

8. Some Other Charge-Compensated Derivatives of Nido-Carborane

The asymmetrically substituted 9-carbonyl derivative of nido-carborane 9-O≡C-7,8-C2B9H11 and the 3,3,8-(CO)3-3,1,2-CoC2B9H10 cobaltacarborane based on its symmetrically substituted analog as a ligand were isolated as minor products of the reaction of the parent nido-carborane with [Co2(CO)8] [161].
The symmetrically substituted cobaltacenium derivative of nido-carborane 10-{CpCo(C5H4)}-7,8-Me2-7,8-C2B9H9 (Figure 53) was prepared along with the 3-Cp-1,2-Me2-3,1,2-CoC2B9H9 cobaltacarborane in the reaction of the dithallium dicarbollide salt Tl2[7,8-Me2-7,8-C2B9H9] with CpCo(CO)I2 in acetonitrile [162].

9. Some Comments on Substitution Mechanisms in Nido-Carborane

In conclusion, we would like to touch upon the issue of substitution mechanisms in nido-carborane. As mentioned above, the introduction of substituents into the lower belt of the nido-carborane cage passes through the stage of substitution in the closo-carborane followed by deboronation and, in our opinion, does not require special comments. The synthesis of derivatives with substituents at the boron atoms in the upper belt of the nido-carborane cage can proceed according to various reaction mechanisms and, depending on this, lead to both symmetrically and asymmetrically substituted derivatives.
The secondary substitution reaction mechanisms, such as Pd-catalyzed/promoted cross-coupling reactions of the iodo derivatives of nido-carborane or functional group-directed B-H activation via transition metal complexes, are quite obvious and do not require discussion.
As a rule, substitution in polyhedral boron hydrides can occur via two mechanisms [163]. The first one is analogous to well-known aromatic electrophilic substitution, while the second one involves the attack of electrophile E+ followed by its elimination together with hydride (-EH). Then, the resulting electrophilic center is attacked by a nucleophile. This mechanism is called electrophilically induced nucleophilic substitution (EINS). In the simplest case, a proton can act as an electrophile; in this case, the reaction proceeds according to the mechanism of acid-assisted nucleophilic substitution (AANS). Since the first stage of the reaction in any case involves the attack of the electrophile, regardless of the mechanism, the substitution should proceed at the boron atom with the largest negative charge. The electrophilic center on a boron atom can also arise when the most-hydride hydrogen atom is removed by a Lewis acid. However, since the most-hydride hydrogen atom is bonded to the boron atom with the largest negative charge, the substitution position does not change. It is known that halogenation reactions, which are the simplest example of electrophilic substitution, occur at positions B(9) and B(11) of the nido-carborane cage [164,165,166]. Thus, if the substitution proceeded only via the aforementioned mechanisms, it would lead exclusively to asymmetrically substituted derivatives.
However, in the case of nido-carborane, there are a number of possibilities for substitution to occur in a different way. A characteristic feature of nido-carborane is the presence of the “extra”-hydrogen, which is able to migrate between the boron atoms of the open pentagonal face. Therefore, the intramolecular migration of the “extra” hydrogen to the electrophilic center formed in position B(9) can occur faster than the attack of the nucleophile. This should lead to the transfer of the electrophilic center to position B(10), the attack of which by the nucleophile will lead to symmetrically substituted derivatives.
On the other hand, strong bases can remove the “extra” hydrogen, leading to the formation of the dicarbollide anion [7,8-C2B9H11]2− with a different electron density distribution than nido-carborane. In this case, the substitution proceeds at position B(10) with the formation of symmetrically substituted derivatives [104]. Interestingly, in the case of the protonated form of nido-carborane 7,8-C2B9H13, substitution also leads to the formation of symmetrically substituted derivatives [121,155]. This can be caused by the elimination of a hydrogen molecule with the closure of an unstable 11-vertex closo-polyhedron, which, before rearranging into a stable 2,3-isomer [167], is attacked by a nucleophile to form a substituted nido-carborane. It should be noted that in this case, as in the case of oxidative addition [79,80,81,84,106], we should consider these reaction pathways more like potential opportunities than established mechanisms, since there are no detailed studies on the mechanisms of these reactions. We can talk somewhat more definitely about the mercury-promoted substitution reactions, since in these cases, the formation of η1-B(10)-mercuracarboranes, which can be considered as the initial products of the reaction, were reliably established with single-crystal X-ray diffraction studies [168,169,170].
Nevertheless, despite the fact that the mechanisms of formation of various charge-compensated nido-carborane derivatives remain largely unknown, analysis of the available literature data allows the targeted synthesis of these derivatives with high selectivity and good yields.

Author Contributions

Conceptualization, M.Y.S. and I.B.S.; writing—original draft preparation, M.Y.S., S.A.A. and I.B.S.; writing—review and editing, I.B.S.; supervision, I.B.S.; project administration, M.Y.S. and 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 (21-73-10199).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The idealized structure and atom numbering of nido-carborane [7,8-C2B9H12]−.
Figure 1. The idealized structure and atom numbering of nido-carborane [7,8-C2B9H12]−.
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Scheme 1. The synthetic process to obtain 9-Py-7,8-C2B9H11.
Scheme 1. The synthetic process to obtain 9-Py-7,8-C2B9H11.
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Figure 2. Crystal molecular structures of 9-Py-7,8-C2B9H11 (left) and 10-Py-7,8-C2B9H11 (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 2. Crystal molecular structures of 9-Py-7,8-C2B9H11 (left) and 10-Py-7,8-C2B9H11 (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 2. Synthesis of 10-Py-7,8-C2B9H11 and 9-Py-7,8-C2B9H11 via the reaction of nido-carborane with pyridine in the presence of HgCl2.
Scheme 2. Synthesis of 10-Py-7,8-C2B9H11 and 9-Py-7,8-C2B9H11 via the reaction of nido-carborane with pyridine in the presence of HgCl2.
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Scheme 3. The synthetic process to obtain 10-Py-7,8-C2B9H11.
Scheme 3. The synthetic process to obtain 10-Py-7,8-C2B9H11.
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Scheme 4. The synthesis of [9-(4′-PhC5H4N)-7,8-C2B9H11].
Scheme 4. The synthesis of [9-(4′-PhC5H4N)-7,8-C2B9H11].
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Scheme 5. The metal-free or electrocatalyzed direct B-N oxidative couplings of nido-carboranes with pyridines.
Scheme 5. The metal-free or electrocatalyzed direct B-N oxidative couplings of nido-carboranes with pyridines.
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Figure 3. Crystal molecular structures of 9-C5H5N-7,8-Ph2-7,8-C2B9H9 (top left), 9-(4′-MeOC5H4N)-7,8-Ph2-7,8-C2B9H9 (top right), 9-(3′-H2NC5H4N)-7,8-Ph2-7,8-C2B9H9 (middle left), 9-(4′-CH2 = CHC5H4N)-7,8-Ph2-7,8-C2B9H9 (middle right), 9-(4′-MeO(O)CC5H4N)-7,8-Ph2-7,8-C2B9H9 (bottom left), and 9-(4′-PhC5H4N)-7,8-Ph2-7,8-C2B9H9 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 3. Crystal molecular structures of 9-C5H5N-7,8-Ph2-7,8-C2B9H9 (top left), 9-(4′-MeOC5H4N)-7,8-Ph2-7,8-C2B9H9 (top right), 9-(3′-H2NC5H4N)-7,8-Ph2-7,8-C2B9H9 (middle left), 9-(4′-CH2 = CHC5H4N)-7,8-Ph2-7,8-C2B9H9 (middle right), 9-(4′-MeO(O)CC5H4N)-7,8-Ph2-7,8-C2B9H9 (bottom left), and 9-(4′-PhC5H4N)-7,8-Ph2-7,8-C2B9H9 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
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Figure 4. Crystal molecular structure of 9,9′-μ-(NC5H4-p-CH=CH-p-C5H4N)(7,8-Ph2-7,8-C2B9H9)2. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 4. Crystal molecular structure of 9,9′-μ-(NC5H4-p-CH=CH-p-C5H4N)(7,8-Ph2-7,8-C2B9H9)2. Hydrogen atoms of organic substituents are omitted for clarity.
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Figure 5. The nido-carborane-based amino acid and BODIPY derivatives.
Figure 5. The nido-carborane-based amino acid and BODIPY derivatives.
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Figure 6. Crystal molecular structure of the BODIPY-labeled nido-carborane. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 6. Crystal molecular structure of the BODIPY-labeled nido-carborane. Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 6. The synthetic process to obtain quinolinium and isoquinolinium derivatives of nido-carborane.
Scheme 6. The synthetic process to obtain quinolinium and isoquinolinium derivatives of nido-carborane.
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Figure 7. Crystal molecular structures of 9-quinoline-1-yl-7,8-C2B9H11 (left) and 9-isoquinoline-2-yl-7,8-Ph2-7,8-C2B9H9 (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 7. Crystal molecular structures of 9-quinoline-1-yl-7,8-C2B9H11 (left) and 9-isoquinoline-2-yl-7,8-Ph2-7,8-C2B9H9 (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 7. The synthesis of pyridinium and quinolinium derivatives of nido-carborane.
Scheme 7. The synthesis of pyridinium and quinolinium derivatives of nido-carborane.
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Figure 8. Crystal molecular structures of μ-7,11-NC7H6(2′-Bu)CH2-7,8-C2B9H10 (top left), μ-7,11-NC7H6(1″,4″-dioxan-2″-yl)CH2-7,8-C2B9H10 (top right), μ-7,11-NC7H6(2″-tetrahydrofuryl)CH2-7,8-C2B9H10 (middle left), μ-7,11-NC7H7CH2-8-Me-7,8-C2B9H9 (middle right), μ-7,11-NC7H7CH2-8-Ph-7,8-C2B9H9 (bottom left), and μ-7,11-NC7H7CH2-8-Bu-7,8-C2B9H9 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 8. Crystal molecular structures of μ-7,11-NC7H6(2′-Bu)CH2-7,8-C2B9H10 (top left), μ-7,11-NC7H6(1″,4″-dioxan-2″-yl)CH2-7,8-C2B9H10 (top right), μ-7,11-NC7H6(2″-tetrahydrofuryl)CH2-7,8-C2B9H10 (middle left), μ-7,11-NC7H7CH2-8-Me-7,8-C2B9H9 (middle right), μ-7,11-NC7H7CH2-8-Ph-7,8-C2B9H9 (bottom left), and μ-7,11-NC7H7CH2-8-Bu-7,8-C2B9H9 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
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Figure 9. Crystal molecular structure of 11-I-9-Py-7,8-C2B9H10. Hydrogen atoms of pyridyl substituent are omitted for clarity.
Figure 9. Crystal molecular structure of 11-I-9-Py-7,8-C2B9H10. Hydrogen atoms of pyridyl substituent are omitted for clarity.
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Figure 10. The N-heterocyclic charge-compensated nido-carborane derivatives.
Figure 10. The N-heterocyclic charge-compensated nido-carborane derivatives.
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Figure 11. Crystal molecular structures of 9-N-methylimidazolium-7,8-Ph2-7,8-C2B9H9 (left) and 9-N-isopropylimidazolium-7,8-Ph2-7,8-C2B9H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 11. Crystal molecular structures of 9-N-methylimidazolium-7,8-Ph2-7,8-C2B9H9 (left) and 9-N-isopropylimidazolium-7,8-Ph2-7,8-C2B9H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 8. The synthetic route to obtain 9-trimethylammonium and 9-alkyldimethylamino derivatives of nido-carborane.
Scheme 8. The synthetic route to obtain 9-trimethylammonium and 9-alkyldimethylamino derivatives of nido-carborane.
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Figure 12. Crystal molecular structures of 9-C6H5CH2Me2N-7,8-C2B9H11 (top left), 9-Me2NCH2CH2Me2N-7,8-C2B9H11 (top right), 9-N3CH2CH2CH2Me2N-7,8-C2B9H11 (middle left), 9-N≡CCH2Me2N-7,8-C2B9H11 (middle right), and 9-HC≡CCH2Me2N-7,8-C2B9H11 (bottom). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 12. Crystal molecular structures of 9-C6H5CH2Me2N-7,8-C2B9H11 (top left), 9-Me2NCH2CH2Me2N-7,8-C2B9H11 (top right), 9-N3CH2CH2CH2Me2N-7,8-C2B9H11 (middle left), 9-N≡CCH2Me2N-7,8-C2B9H11 (middle right), and 9-HC≡CCH2Me2N-7,8-C2B9H11 (bottom). Hydrogen atoms of organic substituents are omitted for clarity.
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Figure 13. Crystal molecular structure of μ-9,4-Me2N(CH2)3O-7,8-C2B9H10. Hydrogen atoms of organic substituent are omitted for clarity.
Figure 13. Crystal molecular structure of μ-9,4-Me2N(CH2)3O-7,8-C2B9H10. Hydrogen atoms of organic substituent are omitted for clarity.
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Scheme 9. The synthesis of 9-Me2N(CH2)2Me2N-7,8-C2B9H11.
Scheme 9. The synthesis of 9-Me2N(CH2)2Me2N-7,8-C2B9H11.
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Scheme 10. The alkylation of 9-Me2N(CH2)2Me2N-7,8-C2B9H11.
Scheme 10. The alkylation of 9-Me2N(CH2)2Me2N-7,8-C2B9H11.
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Scheme 11. The synthesis of azido derivative of nido-carborane 9-N3(CH2)3Me2N-7,8-C2B9H11 and its copper(I)-catalyzed azide–alkyne cycloaddition reactions with various terminal alkynes.
Scheme 11. The synthesis of azido derivative of nido-carborane 9-N3(CH2)3Me2N-7,8-C2B9H11 and its copper(I)-catalyzed azide–alkyne cycloaddition reactions with various terminal alkynes.
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Scheme 12. The reaction of 9-HC≡CCH2Me2N-7,8-C2B9H11 with 3β-(2-azidoethoxy)-5-cholestene.
Scheme 12. The reaction of 9-HC≡CCH2Me2N-7,8-C2B9H11 with 3β-(2-azidoethoxy)-5-cholestene.
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Scheme 13. Synthesis of trimethylammonium derivatives of C-substituted nido-carboranes.
Scheme 13. Synthesis of trimethylammonium derivatives of C-substituted nido-carboranes.
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Figure 14. Crystal molecular structure of 11-Me3N-7-MeS-7,8-C2B9H10. Hydrogen atoms of methyl groups are omitted for clarity.
Figure 14. Crystal molecular structure of 11-Me3N-7-MeS-7,8-C2B9H10. Hydrogen atoms of methyl groups are omitted for clarity.
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Scheme 14. The synthesis of 10-Et3N-7,8-C2B9H11 from 10-Ph2S-7,8-C2B9H11.
Scheme 14. The synthesis of 10-Et3N-7,8-C2B9H11 from 10-Ph2S-7,8-C2B9H11.
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Figure 15. Crystal molecular structure of 10-Et3N-7,8-C2B9H11. Hydrogen atoms of ethyl groups are omitted for clarity.
Figure 15. Crystal molecular structure of 10-Et3N-7,8-C2B9H11. Hydrogen atoms of ethyl groups are omitted for clarity.
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Scheme 15. Synthesis of the cyclic 9-dialkylammonium derivatives of 7,8-diphenyl-nido-carborane.
Scheme 15. Synthesis of the cyclic 9-dialkylammonium derivatives of 7,8-diphenyl-nido-carborane.
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Figure 16. Crystal molecular structures of 9-(CH2)5NH-7,8-Ph2-7,8-C2B9H9 (left) and 9-O(CH2CH2)2NH-7,8-Ph2-7,8-C2B9H9 (right). Hydrogen atoms of alkyl and aryl substituents are omitted for clarity.
Figure 16. Crystal molecular structures of 9-(CH2)5NH-7,8-Ph2-7,8-C2B9H9 (left) and 9-O(CH2CH2)2NH-7,8-Ph2-7,8-C2B9H9 (right). Hydrogen atoms of alkyl and aryl substituents are omitted for clarity.
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Scheme 16. The synthesis of nido-carborane-based primary aliphatic and heteroaromatic amines.
Scheme 16. The synthesis of nido-carborane-based primary aliphatic and heteroaromatic amines.
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Figure 17. Crystal molecular structure of 9-Bn(Me)NH-7,8-Ph2-7,8-C2B9H9. Hydrogen atoms of alkyl and aryl substituents are omitted for clarity.
Figure 17. Crystal molecular structure of 9-Bn(Me)NH-7,8-Ph2-7,8-C2B9H9. Hydrogen atoms of alkyl and aryl substituents are omitted for clarity.
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Figure 18. Crystal molecular structures of [6,11-Br2-9-Me3N-7,8-C2B9H9] (top left), [1,6,11-Br3-9-Me3N-7,8-C2B9H8] (top right), and [11-I-9-Me3N-7,8-C2B9H10] (bottom). Hydrogen atoms of methyl groups are omitted for clarity.
Figure 18. Crystal molecular structures of [6,11-Br2-9-Me3N-7,8-C2B9H9] (top left), [1,6,11-Br3-9-Me3N-7,8-C2B9H8] (top right), and [11-I-9-Me3N-7,8-C2B9H10] (bottom). Hydrogen atoms of methyl groups are omitted for clarity.
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Scheme 17. The synthesis of 10-RC≡N-7,8-C2B9H11 (R = Me, Et) and some of their reactions.
Scheme 17. The synthesis of 10-RC≡N-7,8-C2B9H11 (R = Me, Et) and some of their reactions.
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Scheme 18. The reactions of 10-EtC≡N-7,8-C2B9H11 with alcohols and thiols.
Scheme 18. The reactions of 10-EtC≡N-7,8-C2B9H11 with alcohols and thiols.
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Figure 19. Crystal molecular structures of 10-(E)-EtC(OEt)=HN-7,8-C2B9H11 (top left), 10-(E)-EtC(OiPr)=HN-7,8-C2B9H11 (top right), 10-(E)-EtC(OBu)=HN-7,8-C2B9H11 (bottom left), and 10-(Z)-EtC(SEt)=HN-7,8-C2B9H11 (bottom right). Hydrogen atoms of alkyl substituents are omitted for clarity.
Figure 19. Crystal molecular structures of 10-(E)-EtC(OEt)=HN-7,8-C2B9H11 (top left), 10-(E)-EtC(OiPr)=HN-7,8-C2B9H11 (top right), 10-(E)-EtC(OBu)=HN-7,8-C2B9H11 (bottom left), and 10-(Z)-EtC(SEt)=HN-7,8-C2B9H11 (bottom right). Hydrogen atoms of alkyl substituents are omitted for clarity.
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Scheme 19. The reactions of 10-EtC≡N-7,8-C2B9H11 with primary and secondary amines and ethylenediamine.
Scheme 19. The reactions of 10-EtC≡N-7,8-C2B9H11 with primary and secondary amines and ethylenediamine.
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Figure 20. Crystal molecular structures of 10-(E)-EtC(NEt2)=HN-7,8-C2B9H11 (left) and 10-(E)-EtC(N(CH2CH2)2O)=HN-7,8-C2B9H11 (right). Hydrogen atoms of alkyl substituents are omitted for clarity.
Figure 20. Crystal molecular structures of 10-(E)-EtC(NEt2)=HN-7,8-C2B9H11 (left) and 10-(E)-EtC(N(CH2CH2)2O)=HN-7,8-C2B9H11 (right). Hydrogen atoms of alkyl substituents are omitted for clarity.
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Scheme 20. The synthesis of 10-NCCH2CH2OCH2CH2C≡N-7,8-C2B9H11 and some of its reactions.
Scheme 20. The synthesis of 10-NCCH2CH2OCH2CH2C≡N-7,8-C2B9H11 and some of its reactions.
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Scheme 21. Reactions of 10-NCCH2CH2OCH2CH2C≡N-7,8-C2B9H11 with alcohols, thiols, and diethylamine.
Scheme 21. Reactions of 10-NCCH2CH2OCH2CH2C≡N-7,8-C2B9H11 with alcohols, thiols, and diethylamine.
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Figure 21. Crystal molecular structure of 10-(E)-H2C=CHC(OEt)=HN-7,8-C2B9H11. Hydrogen atoms of alkyl substituents are omitted for clarity.
Figure 21. Crystal molecular structure of 10-(E)-H2C=CHC(OEt)=HN-7,8-C2B9H11. Hydrogen atoms of alkyl substituents are omitted for clarity.
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Figure 22. Crystal molecular structure of 10-CH3C(NCPh2)=HN-7,8-C2B9H11. Hydrogen atoms of methyl and phenyl groups are omitted for clarity.
Figure 22. Crystal molecular structure of 10-CH3C(NCPh2)=HN-7,8-C2B9H11. Hydrogen atoms of methyl and phenyl groups are omitted for clarity.
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Scheme 22. The synthetic routes to ammonium derivatives of nido-carborane.
Scheme 22. The synthetic routes to ammonium derivatives of nido-carborane.
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Figure 23. Crystal molecular structures of 3-H3N-7,8-C2B9H11 (left) and 3-H3N-7-BnOOCCH2-7,8-C2B9H10 (right). Hydrogen atoms of alkyl substituents are omitted for clarity.
Figure 23. Crystal molecular structures of 3-H3N-7,8-C2B9H11 (left) and 3-H3N-7-BnOOCCH2-7,8-C2B9H10 (right). Hydrogen atoms of alkyl substituents are omitted for clarity.
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Scheme 23. The synthetic route to obtain N-protonated nido-carborane oxazoles.
Scheme 23. The synthetic route to obtain N-protonated nido-carborane oxazoles.
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Figure 24. Crystal molecular structures of 5,10-μ-PhCNHO-7,8-C2B9H10 (top left), 5,10-μ-PhCNHO-7,8-Me2-7,8-C2B9H8 (top right), 5,10-μ-PhCNHO-7,8-μ-C6H4(CH2)2-7,8-C2B9H8 (bottom left), and 5,9-μ-PhCNHO-7,8-μ-C6H4(CH2)2-7,8-C2B9H8 (bottom right). Hydrogen atoms of alkyl and aryl substituents are omitted for clarity.
Figure 24. Crystal molecular structures of 5,10-μ-PhCNHO-7,8-C2B9H10 (top left), 5,10-μ-PhCNHO-7,8-Me2-7,8-C2B9H8 (top right), 5,10-μ-PhCNHO-7,8-μ-C6H4(CH2)2-7,8-C2B9H8 (bottom left), and 5,9-μ-PhCNHO-7,8-μ-C6H4(CH2)2-7,8-C2B9H8 (bottom right). Hydrogen atoms of alkyl and aryl substituents are omitted for clarity.
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Scheme 24. The synthesis of 9-Ph2PH-7,8-C2B9H11 and its alkylation.
Scheme 24. The synthesis of 9-Ph2PH-7,8-C2B9H11 and its alkylation.
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Figure 25. Crystal molecular structures of 9-MePh2P-7,8-C2B9H11 (left) and 10-MePh2P-7,8-C2B9H11 (right). Hydrogen atoms of phenyl and methyl groups are omitted for clarity.
Figure 25. Crystal molecular structures of 9-MePh2P-7,8-C2B9H11 (left) and 10-MePh2P-7,8-C2B9H11 (right). Hydrogen atoms of phenyl and methyl groups are omitted for clarity.
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Scheme 25. The synthesis of 10-Ph2PH-7,8-C2B9H11 and its alkylation.
Scheme 25. The synthesis of 10-Ph2PH-7,8-C2B9H11 and its alkylation.
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Scheme 26. Reaction of nido-carborane with triphenylphosphine in the presence of FeCl3.
Scheme 26. Reaction of nido-carborane with triphenylphosphine in the presence of FeCl3.
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Scheme 27. Synthesis of 9-Ph3P-7,8-C2B9H11.
Scheme 27. Synthesis of 9-Ph3P-7,8-C2B9H11.
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Scheme 28. The electrocatalyzed B-P oxidative coupling of triarylphosphines, diarylalkylphosphines, and trialkylphosphines with 7,8-diphenyl-nido-carborane.
Scheme 28. The electrocatalyzed B-P oxidative coupling of triarylphosphines, diarylalkylphosphines, and trialkylphosphines with 7,8-diphenyl-nido-carborane.
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Figure 26. Crystal molecular structure of 9-Ph3P-7,8-Ph2-7,8-C2B9H9. Hydrogen atoms of phenyl rings are omitted for clarity.
Figure 26. Crystal molecular structure of 9-Ph3P-7,8-Ph2-7,8-C2B9H9. Hydrogen atoms of phenyl rings are omitted for clarity.
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Scheme 29. Synthesis of triphenylphosphonium derivatives of nido-carborane X-Ph3P-7,8-C2B9H11 (X = 1, 3, 5, 9) via transition-metal-catalyzed cross-coupling reactions.
Scheme 29. Synthesis of triphenylphosphonium derivatives of nido-carborane X-Ph3P-7,8-C2B9H11 (X = 1, 3, 5, 9) via transition-metal-catalyzed cross-coupling reactions.
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Scheme 30. Synthesis of 5-Ph3P-7,8-C2B9H11 and 5-tBu2PH-7,8-C2B9H11.
Scheme 30. Synthesis of 5-Ph3P-7,8-C2B9H11 and 5-tBu2PH-7,8-C2B9H11.
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Figure 27. Crystal molecular structure of 5-t-Bu2HP-7,8-C2B9H11. Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 27. Crystal molecular structure of 5-t-Bu2HP-7,8-C2B9H11. Hydrogen atoms of alkyl groups are omitted for clarity.
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Scheme 31. Synthesis of 9-I-11-Ph3P-7,8-C2B9H10, 5-I-6-Ph3P-7,8-C2B9H10, and 6-I-9-Ph3P-7,8-C2B9H10.
Scheme 31. Synthesis of 9-I-11-Ph3P-7,8-C2B9H10, 5-I-6-Ph3P-7,8-C2B9H10, and 6-I-9-Ph3P-7,8-C2B9H10.
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Figure 28. Crystal molecular structures of 9-Ph3P-11-I-7,8-C2B9H10 (left) and 9-Ph3P-6-I-7,8-C2B9H10 (right). Hydrogen atoms of phenyl groups are omitted for clarity.
Figure 28. Crystal molecular structures of 9-Ph3P-11-I-7,8-C2B9H10 (left) and 9-Ph3P-6-I-7,8-C2B9H10 (right). Hydrogen atoms of phenyl groups are omitted for clarity.
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Scheme 32. Synthesis of 9,11-(Ph3P)2-7,8-C2B9H10 and 5,6-(Ph3P)2-7,8-C2B9H10.
Scheme 32. Synthesis of 9,11-(Ph3P)2-7,8-C2B9H10 and 5,6-(Ph3P)2-7,8-C2B9H10.
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Figure 29. Crystal molecular structure of 5,6-(Ph3P)2-7,8-C2B9H9. Hydrogen atoms of phenyl groups are omitted for clarity.
Figure 29. Crystal molecular structure of 5,6-(Ph3P)2-7,8-C2B9H9. Hydrogen atoms of phenyl groups are omitted for clarity.
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Scheme 33. Two synthetic processes to obtain 6,9-(Ph3P)2-5-I-7,8-C2B9H8.
Scheme 33. Two synthetic processes to obtain 6,9-(Ph3P)2-5-I-7,8-C2B9H8.
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Figure 30. Crystal molecular structure of 6,9-(Ph3P)2-5-I-7,8-C2B9H8. Hydrogen atoms of phenyl groups are omitted for clarity.
Figure 30. Crystal molecular structure of 6,9-(Ph3P)2-5-I-7,8-C2B9H8. Hydrogen atoms of phenyl groups are omitted for clarity.
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Scheme 34. Synthesis of 9-Me2PhP-7,8-C2B9H11 and 10-Me2PhP-7,8-C2B9H11.
Scheme 34. Synthesis of 9-Me2PhP-7,8-C2B9H11 and 10-Me2PhP-7,8-C2B9H11.
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Figure 31. Crystal molecular structure of 9-PhMe2P-7,8-C2B9H11. Hydrogen atoms of substituents are omitted for clarity.
Figure 31. Crystal molecular structure of 9-PhMe2P-7,8-C2B9H11. Hydrogen atoms of substituents are omitted for clarity.
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Scheme 35. Synthesis of 5-Ph3P-7,8-C2B9H11.
Scheme 35. Synthesis of 5-Ph3P-7,8-C2B9H11.
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Figure 32. Crystal molecular structure of 5-Ph3P-7,8-C2B9H11. Hydrogen atoms of phenyl groups are omitted for clarity.
Figure 32. Crystal molecular structure of 5-Ph3P-7,8-C2B9H11. Hydrogen atoms of phenyl groups are omitted for clarity.
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Scheme 36. The synthesis of 11-R’R2P-7,8-µ-(S(CH2)3)-7,8-C2B9H9.
Scheme 36. The synthesis of 11-R’R2P-7,8-µ-(S(CH2)3)-7,8-C2B9H9.
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Figure 33. Crystal molecular structure of 9-Ph3P-7,8-(μ-(CH2)3S)-7,8-C2B9H9. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 33. Crystal molecular structure of 9-Ph3P-7,8-(μ-(CH2)3S)-7,8-C2B9H9. Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 37. Synthesis of triarylphosphonium, diarylalkylphosphonium, and trialkylphosphonium derivatives 11-R′R2P-7-(2′-Py)-7,8-C2B9H10 starting from 2-pyridyl-substituted nido-carborane.
Scheme 37. Synthesis of triarylphosphonium, diarylalkylphosphonium, and trialkylphosphonium derivatives 11-R′R2P-7-(2′-Py)-7,8-C2B9H10 starting from 2-pyridyl-substituted nido-carborane.
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Figure 34. Crystal molecular structure of 11-Ph3P-7-(NC5H4-2′-)-7,8-C2B9H10. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 34. Crystal molecular structure of 11-Ph3P-7-(NC5H4-2′-)-7,8-C2B9H10. Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 38. Synthesis of triphenylphosphonium derivatives starting from different C-substituted nido-carborane pyridines as well as from the C-2-benzoxazolyl and C-diphenylphosphine derivatives.
Scheme 38. Synthesis of triphenylphosphonium derivatives starting from different C-substituted nido-carborane pyridines as well as from the C-2-benzoxazolyl and C-diphenylphosphine derivatives.
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Scheme 39. Reaction of [7-(NC5H3-3′-Me-2′-)-7,8-C2B9H11]− with triphenylphosphine in the presence of PdCl2.
Scheme 39. Reaction of [7-(NC5H3-3′-Me-2′-)-7,8-C2B9H11]− with triphenylphosphine in the presence of PdCl2.
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Figure 35. Crystal molecular structures of 11-Ph3P-7-(NC5H3-3′-Me-2′-)-7,8-C2B9H10 (left) and 2-Ph3P-7-(NC5H3-3′-Me-2′-)-7,8-C2B9H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 35. Crystal molecular structures of 11-Ph3P-7-(NC5H3-3′-Me-2′-)-7,8-C2B9H10 (left) and 2-Ph3P-7-(NC5H3-3′-Me-2′-)-7,8-C2B9H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 40. Reaction of [7-(NC5H3-3′-Me-2′-)-7,8-C2B9H11]− with triarylphosphines and diarylalkylphosphines in the presence of PdCl2.
Scheme 40. Reaction of [7-(NC5H3-3′-Me-2′-)-7,8-C2B9H11]− with triarylphosphines and diarylalkylphosphines in the presence of PdCl2.
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Scheme 41. Reaction of C-substituted isoquinolin-1-yl derivative of nido-carborane with triarylphosphines and diarylalkylphosphines in the presence of PdCl2.
Scheme 41. Reaction of C-substituted isoquinolin-1-yl derivative of nido-carborane with triarylphosphines and diarylalkylphosphines in the presence of PdCl2.
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Scheme 42. Reactions of C-isoquinolin-1-yl derivatives of nido-carborane with triphenylphosphine in the presence of PdCl2.
Scheme 42. Reactions of C-isoquinolin-1-yl derivatives of nido-carborane with triphenylphosphine in the presence of PdCl2.
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Scheme 43. Formation of 10-Ph3P-7,8-C2B9H11 from nickelacarborane 3,3-(Ph3P)2-3,1,2-NiC2B9H11.
Scheme 43. Formation of 10-Ph3P-7,8-C2B9H11 from nickelacarborane 3,3-(Ph3P)2-3,1,2-NiC2B9H11.
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Scheme 44. Synthesis of 9-Ph3As-7,8-Ph2-7,8-C2B9H9 and 9-Ph3As-7,8-Ph2-7,8-C2B9H9.
Scheme 44. Synthesis of 9-Ph3As-7,8-Ph2-7,8-C2B9H9 and 9-Ph3As-7,8-Ph2-7,8-C2B9H9.
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Figure 36. Crystal molecular structure of 9-Ph3As-7,8-Ph2-7,8-C2B9H9. Hydrogen atoms of phenyl groups are omitted for clarity.
Figure 36. Crystal molecular structure of 9-Ph3As-7,8-Ph2-7,8-C2B9H9. Hydrogen atoms of phenyl groups are omitted for clarity.
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Scheme 45. Synthesis of 11-Ph3As-7-(2′-Py)-7,8-C2B9H10.
Scheme 45. Synthesis of 11-Ph3As-7-(2′-Py)-7,8-C2B9H10.
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Scheme 46. Synthesis of 9-(CH2)4O-7,8-C2B9H11 and 10-(CH2)4O-7,8-C2B9H11 via the reaction of nido-carborane with FeCl3 in THF–benzene mixture.
Scheme 46. Synthesis of 9-(CH2)4O-7,8-C2B9H11 and 10-(CH2)4O-7,8-C2B9H11 via the reaction of nido-carborane with FeCl3 in THF–benzene mixture.
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Scheme 47. Different pathways for synthesis of 10-(CH2)4O-7,8-C2B9H11.
Scheme 47. Different pathways for synthesis of 10-(CH2)4O-7,8-C2B9H11.
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Scheme 48. Different pathways for synthesis of 10-O(CH2CH2)2O-7,8-C2B9H11.
Scheme 48. Different pathways for synthesis of 10-O(CH2CH2)2O-7,8-C2B9H11.
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Figure 37. Crystal molecular structure of 10-O(CH2CH2)2O-7,8-C2B9H11. Hydrogen atoms of organic substituent are omitted for clarity.
Figure 37. Crystal molecular structure of 10-O(CH2CH2)2O-7,8-C2B9H11. Hydrogen atoms of organic substituent are omitted for clarity.
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Scheme 49. Different pathways for synthesis of 10-(CH2)5O-7,8-C2B9H11.
Scheme 49. Different pathways for synthesis of 10-(CH2)5O-7,8-C2B9H11.
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Scheme 50. The ring-opening reactions of the cyclic oxonium derivatives of nido-carborane under action of nucleophiles.
Scheme 50. The ring-opening reactions of the cyclic oxonium derivatives of nido-carborane under action of nucleophiles.
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Figure 38. Crystal molecular structure of 10-H3N(CH2CH2O)2-7,8-C2B9H11. Hydrogen atoms of methylene groups are omitted for clarity.
Figure 38. Crystal molecular structure of 10-H3N(CH2CH2O)2-7,8-C2B9H11. Hydrogen atoms of methylene groups are omitted for clarity.
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Scheme 51. Synthesis of the dimethyloxonium derivatives of nido-carborane.
Scheme 51. Synthesis of the dimethyloxonium derivatives of nido-carborane.
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Scheme 52. Different pathways for synthesis of 10-Et2O-7,8-C2B9H11.
Scheme 52. Different pathways for synthesis of 10-Et2O-7,8-C2B9H11.
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Figure 39. Crystal molecular structure of 10-Et2O-7,8-C2B9H11. Hydrogen atoms of organic substituent are omitted for clarity.
Figure 39. Crystal molecular structure of 10-Et2O-7,8-C2B9H11. Hydrogen atoms of organic substituent are omitted for clarity.
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Scheme 53. Synthesis of 10-Me2SO-7,8-µ-(CH2)3-7,8-C2B9H9.
Scheme 53. Synthesis of 10-Me2SO-7,8-µ-(CH2)3-7,8-C2B9H9.
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Figure 40. Crystal molecular structure of 10-Me2SO-7,8-µ-(CH2)3-7,8-C2B9H9. Hydrogen atoms of organic substituent are omitted for clarity.
Figure 40. Crystal molecular structure of 10-Me2SO-7,8-µ-(CH2)3-7,8-C2B9H9. Hydrogen atoms of organic substituent are omitted for clarity.
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Scheme 54. Synthesis of the asymmetrically substituted dimethylsulfonium derivatives of nido-carborane 9-Me2S-7,8-R2-7,8-C2B9H9 (R = H, Me, Ph; RR = μ-1′,2′-C6H4(CH2)2).
Scheme 54. Synthesis of the asymmetrically substituted dimethylsulfonium derivatives of nido-carborane 9-Me2S-7,8-R2-7,8-C2B9H9 (R = H, Me, Ph; RR = μ-1′,2′-C6H4(CH2)2).
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Figure 41. Crystal molecular structures of 9-Me2S-7,8-C2B9H11 (top), 11-Me2S-7-Ph-7,8-R2-7,8-C2B9H10 (bottom left), and 9-Me2S-7,8-Ph2-7,8-C2B9H9 (bottom right). Hydrogen atoms of organic substituent are omitted for clarity.
Figure 41. Crystal molecular structures of 9-Me2S-7,8-C2B9H11 (top), 11-Me2S-7-Ph-7,8-R2-7,8-C2B9H10 (bottom left), and 9-Me2S-7,8-Ph2-7,8-C2B9H9 (bottom right). Hydrogen atoms of organic substituent are omitted for clarity.
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Scheme 55. Reaction of the 5-substituted derivatives of nido-carborane 5-X-7,8-C2B9H11 (X = Br, Me) with dimethylsulfide in the presence of aq. FeCl3.
Scheme 55. Reaction of the 5-substituted derivatives of nido-carborane 5-X-7,8-C2B9H11 (X = Br, Me) with dimethylsulfide in the presence of aq. FeCl3.
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Scheme 56. Reaction of the 9-methyl derivative of nido-carborane 9-Me-7,8-C2B9H11 with dimethylsulfide in the presence of aqueous iron(III) chloride.
Scheme 56. Reaction of the 9-methyl derivative of nido-carborane 9-Me-7,8-C2B9H11 with dimethylsulfide in the presence of aqueous iron(III) chloride.
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Figure 42. Crystal molecular structures of 9-Me2S-3-Me-7,8-C2B9H10 (left) and 9-Me2S-4-Me-7,8-C2B9H10 (right). Hydrogen atoms of methyl groups are omitted for clarity.
Figure 42. Crystal molecular structures of 9-Me2S-3-Me-7,8-C2B9H10 (left) and 9-Me2S-4-Me-7,8-C2B9H10 (right). Hydrogen atoms of methyl groups are omitted for clarity.
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Scheme 57. The synthetic pathway to obtain 9-R(Me)S-7,8-C2B9H11 derivatives.
Scheme 57. The synthetic pathway to obtain 9-R(Me)S-7,8-C2B9H11 derivatives.
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Figure 43. Crystal molecular structures of 9-ClCH2(Me)S-7,8-C2B9H11 (top), 9-Bn(Me)S-7,8-C2B9H11 (bottom left), and 9-EtOCH2CH2(Me)S-7,8-C2B9H11 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 43. Crystal molecular structures of 9-ClCH2(Me)S-7,8-C2B9H11 (top), 9-Bn(Me)S-7,8-C2B9H11 (bottom left), and 9-EtOCH2CH2(Me)S-7,8-C2B9H11 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 58. Reaction of 9-BrCH2CH2(Me)S-7,8-C2B9H11 with K2CO3.
Scheme 58. Reaction of 9-BrCH2CH2(Me)S-7,8-C2B9H11 with K2CO3.
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Scheme 59. The electrocatalyzed B-S oxidative coupling of nido-carboranes with diethylsulfide.
Scheme 59. The electrocatalyzed B-S oxidative coupling of nido-carboranes with diethylsulfide.
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Figure 44. Crystal molecular structures of 9-Et2S-7,8-(4′-ClC6H4)2-7,8-C2B9H9 (left) and 9-(4′-BrC6H4)EtS-7,8-Ph2-7,8-C2B9H9 (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 44. Crystal molecular structures of 9-Et2S-7,8-(4′-ClC6H4)2-7,8-C2B9H9 (left) and 9-(4′-BrC6H4)EtS-7,8-Ph2-7,8-C2B9H9 (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 60. The electrocatalyzed direct B-S oxidative coupling of nido-carboranes with sulfides.
Scheme 60. The electrocatalyzed direct B-S oxidative coupling of nido-carboranes with sulfides.
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Figure 45. Crystal molecular structures of 6,11-Cl2-9-Me2S-7,8-C2B9H9 (top left), 11-Br-9-Me2S-7,8-C2B9H10 (top right), 6,11-Br2-9-Me2S-7,8-C2B9H9 (bottom left), and 11-I-9-Me2S-7,8-C2B9H10 (bottom right). Hydrogen atoms of methyl groups are omitted for clarity.
Figure 45. Crystal molecular structures of 6,11-Cl2-9-Me2S-7,8-C2B9H9 (top left), 11-Br-9-Me2S-7,8-C2B9H10 (top right), 6,11-Br2-9-Me2S-7,8-C2B9H9 (bottom left), and 11-I-9-Me2S-7,8-C2B9H10 (bottom right). Hydrogen atoms of methyl groups are omitted for clarity.
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Scheme 61. Synthesis of symmetrically substituted sulfonium derivatives of nido-carborane.
Scheme 61. Synthesis of symmetrically substituted sulfonium derivatives of nido-carborane.
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Figure 46. Crystal molecular structures of 10-Me2S-7,8-C2B9H11 (top left), 10-(CH2)4S-7,8-C2B9H11 (top right), and 10-Me2S-7,8-Ph2-7,8-C2B9H9 (bottom). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 46. Crystal molecular structures of 10-Me2S-7,8-C2B9H11 (top left), 10-(CH2)4S-7,8-C2B9H11 (top right), and 10-Me2S-7,8-Ph2-7,8-C2B9H9 (bottom). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 62. Synthesis of 9-R2SCH2-7,8-C2B9H11.
Scheme 62. Synthesis of 9-R2SCH2-7,8-C2B9H11.
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Scheme 63. Synthesis of 10-Me2S-7,8-C2B9H11.
Scheme 63. Synthesis of 10-Me2S-7,8-C2B9H11.
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Scheme 64. Synthesis of 10-Me2S-7,8-μ-(CH2)3-7,8-C2B9H9.
Scheme 64. Synthesis of 10-Me2S-7,8-μ-(CH2)3-7,8-C2B9H9.
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Scheme 65. Synthesis of symmetrically substituted dialkylsulfonium derivatives of nido-carborane in the presence of HgCl2.
Scheme 65. Synthesis of symmetrically substituted dialkylsulfonium derivatives of nido-carborane in the presence of HgCl2.
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Scheme 66. Synthesis of symmetrically substituted dialkylsulfonium derivatives of nido-carborane in the presence of FeCl3.
Scheme 66. Synthesis of symmetrically substituted dialkylsulfonium derivatives of nido-carborane in the presence of FeCl3.
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Scheme 67. Synthesis of 10-(CH2)4S-7,8-C2B9H11.
Scheme 67. Synthesis of 10-(CH2)4S-7,8-C2B9H11.
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Scheme 68. Synthesis of 10-Ph2S-7,8-C2B9H11.
Scheme 68. Synthesis of 10-Ph2S-7,8-C2B9H11.
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Figure 47. Crystal molecular structure of 10-Ph2S-7,8-C2B9H11. Hydrogen atoms of phenyl groups are omitted for clarity.
Figure 47. Crystal molecular structure of 10-Ph2S-7,8-C2B9H11. Hydrogen atoms of phenyl groups are omitted for clarity.
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Scheme 69. The synthetic pathway to obtain 10-R(Me)S-7,8-C2B9H11 derivatives.
Scheme 69. The synthetic pathway to obtain 10-R(Me)S-7,8-C2B9H11 derivatives.
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Figure 48. Crystal molecular structures of 10-Bn(Me)S-7,8-C2B9H11 (top left), 10-C6H4(CO)2NCH2CH2(Me)S-7,8-C2B9H11 (top right), 10-HC≡CCH2(Me)S-7,8-C2B9H11 (bottom left), and 10-Me3SiC≡CCH2(Me)S-7,8-C2B9H11 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 48. Crystal molecular structures of 10-Bn(Me)S-7,8-C2B9H11 (top left), 10-C6H4(CO)2NCH2CH2(Me)S-7,8-C2B9H11 (top right), 10-HC≡CCH2(Me)S-7,8-C2B9H11 (bottom left), and 10-Me3SiC≡CCH2(Me)S-7,8-C2B9H11 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 70. Synthesis of 5-Me2S-7,8-C2B9H11.
Scheme 70. Synthesis of 5-Me2S-7,8-C2B9H11.
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Scheme 71. Reaction of nido-carborane with thiourea under acidic conditions.
Scheme 71. Reaction of nido-carborane with thiourea under acidic conditions.
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Figure 49. Crystal molecular structure of 9-Bn2S-7,8-C2B9H11. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 49. Crystal molecular structure of 9-Bn2S-7,8-C2B9H11. Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 72. Synthesis of S-thioimidolium derivatives of nido-carborane.
Scheme 72. Synthesis of S-thioimidolium derivatives of nido-carborane.
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Figure 50. Crystal molecular structures of 9-i-PrHNC(Me)S-7,8-C2B9H11 (left) and 10-Me2NC(Me)S-7,8-C2B9H11 (right). Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 50. Crystal molecular structures of 9-i-PrHNC(Me)S-7,8-C2B9H11 (left) and 10-Me2NC(Me)S-7,8-C2B9H11 (right). Hydrogen atoms of alkyl groups are omitted for clarity.
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Scheme 73. Synthesis of the 5-dimethylsulfoxonium derivative of nido-carborane 5-Me2(O)S-7,8-C2B9H11.
Scheme 73. Synthesis of the 5-dimethylsulfoxonium derivative of nido-carborane 5-Me2(O)S-7,8-C2B9H11.
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Figure 51. Crystal molecular structure of 5-Me2(O)S-7,8-C2B9H11. Hydrogen atoms of methyl groups are omitted for clarity.
Figure 51. Crystal molecular structure of 5-Me2(O)S-7,8-C2B9H11. Hydrogen atoms of methyl groups are omitted for clarity.
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Scheme 74. Synthesis of 9-dialkyl(aryl)selenium and 9-diaryltellurium derivatives of nido-carborane 9-RR’X-7,8-Ph2-7,8-C2B9H9.
Scheme 74. Synthesis of 9-dialkyl(aryl)selenium and 9-diaryltellurium derivatives of nido-carborane 9-RR’X-7,8-Ph2-7,8-C2B9H9.
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Figure 52. Crystal molecular structure of 9-Ph2Te-7,8-Ph2-7,8-C2B9H9. Hydrogen atoms of phenyl groups are omitted for clarity.
Figure 52. Crystal molecular structure of 9-Ph2Te-7,8-Ph2-7,8-C2B9H9. Hydrogen atoms of phenyl groups are omitted for clarity.
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Figure 53. Crystal molecular structure of 10-{CpCo(C5H4)}-7,8-Me2-7,8-C2B9H9. Hydrogen atoms of methyl groups are omitted for clarity.
Figure 53. Crystal molecular structure of 10-{CpCo(C5H4)}-7,8-Me2-7,8-C2B9H9. Hydrogen atoms of methyl groups are omitted for clarity.
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Stogniy, M.Y.; Anufriev, S.A.; Sivaev, I.B. Charge-Compensated Derivatives of Nido-Carborane. Inorganics 2023, 11, 72. https://doi.org/10.3390/inorganics11020072

AMA Style

Stogniy MY, Anufriev SA, Sivaev IB. Charge-Compensated Derivatives of Nido-Carborane. Inorganics. 2023; 11(2):72. https://doi.org/10.3390/inorganics11020072

Chicago/Turabian Style

Stogniy, Marina Yu., Sergey A. Anufriev, and Igor B. Sivaev. 2023. "Charge-Compensated Derivatives of Nido-Carborane" Inorganics 11, no. 2: 72. https://doi.org/10.3390/inorganics11020072

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