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Review

Combining Two Types of Boron in One Molecule (To the 60th Anniversary of the First Synthesis of Carborane)

by
Igor B. Sivaev
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str., 119334 Moscow, Russia
Chemistry 2023, 5(2), 834-885; https://doi.org/10.3390/chemistry5020059
Submission received: 4 March 2023 / Revised: 7 April 2023 / Accepted: 9 April 2023 / Published: 11 April 2023
(This article belongs to the Special Issue Recent Advances in Modern Inorganic Chemistry: Featured Reviews)
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Abstract

:
This review is an attempt to bring together the data from the literature on the synthesis and properties of icosahedral carborane derivatives, in which exo-polyhedral three- and four-coordinated boron substituents are attached directly to the carborane cage through boron–carbon or boron–boron bonds. Various classes of compounds are considered, including carboranyl aryl boranes, boronic acids and their derivatives, boroles, diazaboroles, etc. Particular attention is paid to carborane-fused heterocycles containing boron atoms.

1. Introduction

The synthesis of carboranes in the early 1960s [1,2,3,4,5] revolutionized the chemistry of boron, giving researchers boron compounds with extraordinary stability and, at the same time, practically unlimited possibilities for their modification [6,7]. At the same time, this development led to the split of organic boron chemistry into two practically non-overlapping branches: the chemistry of classical organoboron compounds, which operates with compounds of the BR3 type and their derivatives; and the chemistry of polyhedral carboranes and boranes. Despite the first attempt to bind organic boron to the carborane cage being undertaken shortly after the synthesis of the carboranes themselves [8], as well as a number of other studies in this area [9,10,11,12,13], this direction did not undergo significant development until relatively recently due to the rather low stability of the C-B bond in such compounds. Moreover, the use of carboranyl boronic acid esters has been proposed as a convenient method for the synthesis of C-hydroxy carboranes and derivatives thereof [14,15,16,17,18,19,20,21,22,23,24,25,26]. The use of generated in situ carboranyl boronic acid esters in Pd-catalyzed cross-coupling reactions has also been described [27]. The situation changed only about 10 years ago due to good prospects for the use of some carborane derivatives as luminescent materials [28,29,30], which in turn stimulated a notable increase in interest in further research in the field of carborane derivatives with the C-B bond. Subsequent research, especially over the last five years, has led to significant progress in this area. At the same time, no serious attempt has yet been made to generalize the results obtained in this area.
Therefore, the purpose of this work was to collect and summarize the fragmented information available in the literature on the preparation and properties of carborane derivatives with exo-polyhedral C-B and B-B bonds.

2. Early Work on Carborane Derivatives with exo-Polyhedral Carbon-Boron Bonds

Most of the early work on carborane derivatives with exo-polyhedral carbon-boron bonds was more of an exploratory foray into this territory than targeted research aimed at its development. Perhaps the exception here is a series of little-known works by Shagova and Mikhailov in the 1970s, who described the preparation of a fairly large number of such compounds despite the difficulties with their identification due to imperfections of the instruments of that time.
The reactions of the lithium derivatives of C-alkyl-ortho-carboranes with BCl3 or BBr3 led to the corresponding ortho-carboranylboron dihalides 1-X2B-2-R-1,2-C2B10H10 (X = Cl, R = Et, i-Pr; X = Br, R = Me) (Scheme 1, Figure 1) [9,31,32,33]. The resulting carboranylboron dichlorides react with two equivalents of alcohols, mercaptans, and amines to form the corresponding esters, thioethers, and amides of ortho-carboranylboronic acids, respectively (Scheme 1) [9]. The reaction of the dimethyl ester 1-(MeO)2B-2-i-Pr-1,2-C2B10H10 with an excess of methanol leads to the cleavage of the C-B bond to give C-isopropyl-ortho-carborane [9]. The dimethyl ester 1-(MeO)2B-2-i-Pr-1,2-C2B10H10 can also be prepared by the direct reaction of the lithium derivative of C-isopropyl-ortho-carborane with (MeO)2BCl (Scheme 1, Figure 1) [33]. Similarly, the pinacol ester 1-PinB-2-i-Pr-1,2-C2B10H10 was prepared by the reaction of the lithium derivative of C-isopropyl-ortho-carborane with PinBCl (Scheme 1) [33]. The reaction of 1-(MeO)2B-2-i-Pr-1,2-C2B10H10 with an excess of triallylborane results in the corresponding allylmethoxyboryl derivative 1-All(MeO)B-2-i-Pr-1,2-C2B10H10 (Scheme 1) [33].
The reaction of 1-Cl2B-2-i-Pr-1,2-C2B10H10 with 2 equiv. of pentafluorophenol in dichloromethane leads to μ-1,1′-O(1-(C6F5O)B-2-i-Pr-C2B10H10)2 (Figure 2) as a product of dehydration of the carboranyl boronic acid pentafluorophenyl monoester [33].
The dimethoxyboryl derivative of unsubstituted ortho-carborane 1-(MeO)2B-1,2-C2B10H11 was prepared by the reaction of 6,9-bis(diethylsulfide)decaborane with ethynyldimethoxyborane HC≡CB(OMe)2 in refluxing benzene [8]. The 1,2-bis(dimethoxyboryl) derivative 1,2-((MeO)2B)2-1,2-C2B10H10 was prepared by the reaction of the dilithium derivative of ortho-carborane with bis(dimethoxy)boron chloride (MeO)2BCl [34].
The reaction of the lithium derivative of C-isopropyl-ortho-carborane with Bu2BCl in hexane produces the corresponding ortho-carboranyl dibutylborane 1-Bu2B-2-i-Pr-1,2-C2B10H10 [35]. Its reactions with aliphatic aldehydes at 50–100 °C lead to the corresponding ortho-carboranyl butylborinate esters 1-RO(Bu)B-2-i-Pr-1,2-C2B10H10 (R = Et, Pr, Bu), whereas the reaction with benzaldehyde requires heating at 100–200 °C (Scheme 2) [36]. It should be noted that the reactions with aliphatic aldehydes at 140–150 °C proceed with the cleavage of the C-B bond, with the release of C-isopropyl-ortho-carborane [36]. The reaction of 1-Bu2B-2-i-Pr-1,2-C2B10H10 with benzaldehyde in polar donor solvents, such as pyridine or hexamethylphosphoramide (HMPA), at 100–120 °C proceeds through addition to the C=O double bond with cleavage of the Ccarb-B bond [36]. Similarly, the reactions of 1-Bu2B-2-i-Pr-1,2-C2B10H10 with aromatic nitriles in hexamethylphosphoramide at 80–150 °C proceed through addition of the C≡N triple bond with cleavage of the Ccarb-B bond [37]. At the same time, the reaction with benzonitrile at 150 °C in the absence of HMPA leads to ortho-carboranyl phenylaldiminobutylborane 1-PhCH=N(Bu)B-2-i-Pr-1,2-C2B10H10 (Scheme 2) [38]. The reaction of 1-Bu2B-2-i-Pr-1,2-C2B10H10 with 1 equivalent of glacial acetic acid at 80 °C results in removal of one butyl group with the formation of the ortho-carboranyl acetoxybutylborane 1-AcO(Bu)B-2-i-Pr-1,2-C2B10H10, while the reaction with an excess of acetic acid at 100 °C leads to the ortho-carboranyl pyroboroacetic anhydride μ-1,1′-O(1-(AcO)B-2-i-Pr-1,2-C2B10H10)2 (Scheme 2) [35,38].
The reaction of 1-Bu2B-2-i-Pr-1,2-C2B10H10 with 1 equivalent of butylthiol at 160 °C results in the thioester 1-BuS(Bu)B-2-i-Pr-1,2-C2B10H10, while the reactions with methanol at 120 °C and isobutylamine at 160–190 °C give ester 1-MeO(Bu)B-2-i-Pr-1,2-C2B10H10 and aminoborane 1-i-BuNH(Bu)B-2-i-Pr-1,2-C2B10H10, respectively, together with C-isopropyl-ortho-carborane as the product of the C-B bond cleavage (Scheme 2) [35,38].
It should be noted that all the carborane derivatives given above, with the exception of the acetoxy derivatives, are vacuum-distillable oils or low-melting solids. However, the dibutylborane derivative 1-Bu2B-2-i-Pr-1,2-C2B10H10, which is a viscous, colorless liquid, when heated to 200–230 °C, splits off one butyl group with release 1- and 2-butene, and it undergoes cyclization to form 1,2-μ-(1′-butyl-3′-methyl-1′-borapropylene)-ortho-carborane with release of dihydrogen (Scheme 3) [39,40]. Similarly, heating 1-Bu2B-2-H2C=CH-1,2-C2B10H10, 1-Bu2B-2-H2C=C(Me)-1,2-C2B10H10, and 1-(i-Bu)2B-2-H2C=C(Me)-1,2-C2B10H10 at 200–230 °C results in the corresponding five-membered cyclization products 1,2-μ-(BuBCH2CH2)-1,2-C2B10H10, 1,2-μ-(BuBCH=CMe)-1,2-C2B10H10, and 1,2-μ-(i-BuBCH=CMe)-1,2-C2B10H10, respectively (Scheme 3) [40]. As in the case of the acyclic dibutylborane derivative 1-Bu2B-2-i-Pr-1,2-C2B10H10, the resulting cyclic derivatives react with acetaldehyde to form the corresponding ethoxy derivatives 1,2-μ-(EtOBCH2CHMe)-1,2-C2B10H10, 1,2-μ-(EtOBCH2CH2)-1,2-C2B10H10, and 1,2-μ-(EtOBCH=CH)-1,2-C2B10H10 (Scheme 3) [40].
Reduction of 1,2-μ-(EtOBCH2CHMe)-1,2-C2B10H10 with LiAlH4 in diethyl ether yields the corresponding borane isolated as the complex with triethylamine 1,2-μ-(Et3N·HBCH2CHMe)-1,2-C2B10H10 [41].
The para-carboranyl boronic acid 1-(HO)2B-12-H(O)C-1,12-C2B10H10 was synthesized by the reaction of the diacetal protected para-carboranyl aldehyde with n-butyllithium, followed by treatment with trimethylborate B(OMe)3 and deprotecting the aldehyde function with trifluoroacetic acid (Scheme 4) [13].

3. Anionic Carboranes with BX3 Substituents and Derivatives Thereof

The anionic trihydridoborate derivative of ortho-carborane Cs[1-H3B-1,2-C2B10H11] was obtained by heating the cesium salt of nido-carborane in triethylamine-borane Et3N·BH3 at 200 °C. It was suggested that the formation of the trihydridoborate derivative occurs upon the interaction of the resulting deprotonated ortho-carborane with an excess of triethylamine-borane. Indeed, the reaction of ortho-carborane with triethylamine-borane in refluxing diglyme in the presence of NaH, followed by the addition of tetramethylammonium chloride in water, leads to (Me4N)[1-H3B-1,2-C2B10H11], while the reaction of ortho-carborane with triethylamine-borane at 200 °C gives the triethylammonium-dihydridoborate derivative 1-Et3NBH2-1,2-C2B10H11 (Scheme 5) [11]. Short-term (30 min) heating of Cs[1-H3B-1,2-C2B10H11] in an acidic aqueous solution leads to its complete hydrolysis to the parent ortho-carborane [11].
The anionic trihydridoborate derivatives of ortho-carborane Li[1-H3B-2-R-1,2-C2B10H10] (R = Me, Ph) and Li2[1,2-(H3B)2-1,2-C2B10H10] were prepared by the reactions of the corresponding lithium derivatives of ortho-carborane with trimethylamine-borane Me3N·BH3 in 1,2-dimethoxyethane. Using the same approach, a series of functionalized dihydridoborate derivatives of ortho-carborane were synthesized (Scheme 6) [41].
The difluorohydridoborate derivative of (Me4N)[1-HF2B-1,2-C2B10H11] (Figure 3) was unexpectedly isolated as a side product of the ortho-carborane deboronation reaction with anhydrous (Me4N)F in tetrahydrofuran. The formation of the difluorohydridoborate derivative was rationalized by partial deprotonation of ortho-carborane with F– to result in [closo-1,2-C2B10H11]−, which participates in the deboronation of the second equivalent of closo-1,2-C2B10H10. (Me4N)[1-HF2B-9,12-I2-1,2-C2B10H9] (Figure 3) was obtained similarly, starting from the 9,12-diiodo derivative of ortho-carborane [42].
The treatment of Cs[1-H3B-1,2-C2B10H11] in dimethylsulfide with iodine in benzene results in the dimethylsulfonium-dihydridoborate derivative 1-Me2SBH2-1,2-C2B10H11, which in turn reacts with amines in benzene to form of the corresponding amine-dihydridoborate derivative 1-LBH2-1,2-C2B10H11 (L = Py, Me3N, Et3N) (Scheme 7) [11].
The dimethylsulfonium- and pentamethyleneoxonium-dihydridoborate derivatives 1-Me2SBH2-2-i-Pr-1,2-C2B10H10 and 1-(CH2)5OBH2-2-i-Pr-1,2-C2B10H10 were synthesized by reactions of 1-(MeO)2B-2-i-Pr-1,2-C2B10H10 with Me2S·BH3 in diethyl ether and BH3 in tetrahydrofuran, correspondingly (Scheme 8) [33]. The tetrahydrofuran molecule in 1-(CH2)5OBH2-2-i-Pr-1,2-C2B10H10 can be replaced by 4-dimethylaminopyridine to give 1-p-Me2NC5H4NBH2-2-i-Pr-1,2-C2B10H10 (Figure 4) [33].
The trimethylammonium-dihydridoborate derivative 1-Me3NBH2-1,2-C2B10H11 can be also prepared by the reaction of the lithium derivative of ortho-carborane with Me3N·BH2I in benzene (Scheme 9) [10,43]. In contrast to the trihydridoborate derivative [1-H3B-1,2-C2B10H11]−, the trimethylammonium-dihydridoborate derivative is resistant to acid and alkaline hydrolysis. At the same time, the CH group in 1-Me3NBH2-1,2-C2B10H11 exhibits properties typical of carboranes. In particular, its treatment with n-butyllithium followed by methyl iodide gives 1-Me3NBH2-2-Me-1,2-C2B10H10 [10]. Heating 1-Me3NBH2-1,2-C2B10H11 under reflux in piperidine results in the replacement of trimethylamine with piperidine to form the piperidinium-dihydridoborate derivative 1-C5H10NHBH2-1,2-C2B10H11 (Scheme 9) [10].
The trimethylammonium-dihydridoborate derivative 1-Me3NBH2-1,2-C2B10H11 has also been obtained by thermal decomposition of the tetramethylammonium salt of reduced ortho-carborane (Me4N)[C2B10H13] at 200 °C [10].
Hydroboration of hex-1-ene and phenylacetylene with 1-Me2SBH2-2-i-Pr-1,2-C2B10H10 in diethyl ether results in the corresponding di(n-hexyl)boryl- and di(phenylethylenyl) carboranes 1-(n-Hx)2B-2-i-Pr-1,2-C2B10H10 and 1-(PhCH=CH)2B-2-i-Pr-1,2-C2B10H10, respectively (Scheme 10) [33].
The triethylammonium-dihydridoborate derivative of meta-carborane 1-Et3NBH2-1,7-C2B10H11 in low yield was obtained by heating meta-carborane in triethylamine-borane at 200 °C (Scheme 11) [11]. The trimethylammonium-dihydridoborate derivative of meta-carborane 1-Me3NBH2-1,7-C2B10H11 was prepared by the reaction of the lithium derivative of meta-carborane with Me3N·BH2I in benzene. The treatment of 1-Me3NBH2-1,2-C2B10H11 with n-butyllithium in benzene, followed by heating with Me3N·BH2I, leads to 1,7-(Me3NBH2)2-1,7-C2B10H10. Heating 1-Me3NBH2-1,7-C2B10H11 under reflux in piperidine results in the replacement of trimethylamine with piperidine to form the piperidinium-dihydridoborate derivative 1-C5H10NHBH2-1,7-C2B10H11 (Scheme 11) [10].
An interesting example of compounds in which the carborane cage is bonded to an exo-polyhedral four-coordinated boron atom are ortho-carboranyl-capped iron(II) clatrochelates (Figure 5) [32,34].

4. Arylboryl Derivatives of Carboranes

A series of C-ortho-carboranyl diarylboranes 1-Ar2B-2-R-1,2-C2B10H10 (Ar = Ph, R = Me; Ar = Mes, R = H, Me, Ph) was prepared by the reactions of lithium derivatives of carboranes with diphenylboron chloride or dimesitylboron fluoride (Scheme 12, Figure 6) [12,44,45]. 1-Ph2B-2-Me-1,2-C2B10H10 was found to undergo complete hydrolysis in boiling water to the corresponding carborane and diphenylborinic acid within 8 h [12]. 1-Mes2B-2-R-1,2-C2B10H10 is considered to be rather water stable; however, it slowly hydrolyzes in the solid state on prolonged exposure to air (within three weeks for R = H and within 18 months for R = Ph) to give mesitylene and the corresponding carboranyl boronic acid 1-(HO)2B-2-R-1,2-C2B10H10. However, in the presence of fluoride ions, they easily hydrolyze in acetonitrile solution to the corresponding carboranes and dimesitylborinic acid [44]. The bis(pentafluorophenyl)boryl derivative 1-(C6F5)2B-2-Me-1,2-C2B10H10 was prepared by the reaction of the corresponding dichlorocarboranylborane with C6F5MgBr in diethyl ether (Scheme 12, Figure 6) [33].
Initial attempts to prepare tris(ortho-carboranyl)borane from the lithium derivative of ortho-carborane and boron trichloride in diethyl ether were unsuccessful [8]. Later, bis(C-isopropyl-ortho-carboranyl)boron chloride and bis(C-isopropyl-ortho-carboranyl)boron fluoride were prepared by the reactions of C-isopropyl-ortho-carboranylboron dichloride with the lithium derivative of C-isopropyl-ortho-carborane or the lithium derivative of C-isopropyl-ortho-carborane with boron trifluoride etherate, respectively. Both are easily hydrolyzed to form (C-isopropyl-ortho-carboranyl)borinic acid (Scheme 13) [46].
Bis(ortho-carboranyl)arylboranes μ-1,1′-ArB(2-R-1,2-C2B10H10)2 (Ar = Ph, R = Me; Ar = R = p-Tol) were prepared by the reactions of the lithium derivatives of the corresponding carboranes with diarylboron dichloride or dibromide (Scheme 14, Figure 7) [12,47]. μ-1,1′-PhB(2-Me-1,2-C2B10H10)2 undergoes hydrolysis by warm (40-50 °C) water over 2 h, giving 1-methyl-ortho-carborane and phenylboronic acid [12].
More recently, tris(ortho-carboranyl)borane 1,1′,1″-μ3-B(1,2-C2B10H11)3 was prepared by the reaction of the lithium derivative of ortho-carborane with BCl3 or BBr3 in toluene (Scheme 15, Figure 8) [48].
Tris(ortho-carboranyl)borane forms stable complexes with benzaldehyde, acetonitrile, and triethylphosphine oxide (Figure 8). A single crystal X-ray diffraction study validated stronger binding of the donor molecules to 1,1′,1″-μ3-B(1,2-C2B10H11)3 than B(C6F5)3. In solution, the Gutmann–Beckett studies indicated that the Lewis acidity of tris(ortho-carboranyl)borane exceeds the values in the literature for known fluorinated aryl boranes [48,49].
The reaction of the dilithium derivative of meta-carborane with diphenylboron chloride in toluene leads to the disubstituted carborane derivative 1,7-(Ph2B)2-1,7-C2B10H10, while the reaction of the lithium derivative with phenylboron dichloride produces μ-1,1′-PhB(1,7-C2B10H11)2 (Scheme 16). μ-1,1′-PhB(1,7-C2B10H11)2 undergoes complete hydrolysis in boiling water to the corresponding carborane and diphenylborinic acid within 8 h, whereas the disubstituted derivative 1,7-(Ph2B)2-1,7-C2B10H10 was found to resist boiling water for 10 h, and a similar period in contact with 4 M NaOH at 80–90 °C was needed for its complete hydrolysis [12].
Benzanullated boroles or 9-borafluorenes can be described as biphenyl units linked by a tricoordinate boron center or as an antiaromatic BC4 ring with two fused arene rings. Lewis bases can coordinate with the boron center across a wide range of Lewis acidities for 9-borafluorenes, dependent on the substitution at boron. As a result of the vacant p orbital extending π-conjugation through the tricyclic system, this class of compounds has interesting photophysical properties, much like polycyclic aromatic hydrocarbons. Adduct formation results in a disruption of conjugation, as well as altered fluorescence, which can be exploited in molecular sensing [50,51].
The ortho-carboranyl derivative of 9-borafluorene 1-C12H8B-2-Me-1,2-C2B10H10 was prepared by the reaction of the lithium derivative of 1-methyl-ortho-carborane with 9-bromo-9-borafluorene (Scheme 17, Figure 9) [52].
The synthesized ortho-carboranyl derivative of 9-borafluorene readily form adducts with various Lewis bases, including 4-dimethylaminopyridine, pyridine, tetrahydrofuran, and acetone (Figure 10). Its Lewis acidity, determined by the Gutmann–Beckett method, is comparable to that of 9-bromo-9-borafluorene [52].
The reactions of ortho-carboranyl-9-borafluorene with alkenes lead to the alkene insertion to form the corresponding 6,7-dihydroborepines (Scheme 18, Figure 11) [52].
An attempt to synthesize the disubstituted 9-borafluoren-9-yl derivative of ortho-carborane 1,2-(C12H8B)2-1,2-C2B10H10 by the reaction of the dilithium derivative of ortho-carborane with 9-bromo-9-boradibenzofluorene in toluene unexpectedly gave a rearrangement product as a result of C-H activation of one 9-boradibenzofluorenyl substituent. The other 9-borafluorene unit undergoes a ring opening process, resulting in an ortho-biphenyl substituent (Scheme 19, Figure 12). The crystallization of the product from toluene via hexane diffusion in the presence of a small amount of tetrahydrofuran leads to a complex with one THF molecule, indicating higher Lewis acidity of the five-membered BC4 ring (Figure 12) [53].
The similar reaction of the dilithium derivative of ortho-carborane with 9-bromo-9-boradibenzofluorene in tetrahydrofuran gave an adduct of expected bis(9-borafluorenyl) derivative with two molecules of 5-bromopentanol as a product of the THF ring opening, while the reaction in dimethylsulfide produced the complex of ortho-carboranyl-9-borafluorene with dimethylsulfide Me2S·1-C12H8B-2-MeS-1,2-C2B10H10 (Figure 13) [53].

5. Aminoboryl Derivatives of Carboranes and Their Analogs

The first C-aminoboryl-ortho-carborane 1-(Me2N)2B-1,2-C2B10H11 was prepared by the reaction of the lithium derivative of ortho-carborane with bis(dimethylamino)boron chloride (Me2N)2BCl in diethyl ether (Scheme 20) [8]. The reaction of the dilithium derivative of ortho-carborane with diethylaminoboron dichloride Et2NBCl2 in toluene leads to 1,2-(Et2N(Cl)B)2-1,2-C2B10H10, while the reaction with diisopropylaminoboron dichloride i-Pr2NBCl2 results in a mixture of the corresponding mono- and disubstituted aminoboryl-ortho-carboranes 1-i-Pr2N(Cl)B-1,2-C2B10H11 and 1,2-(i-Pr2N(Cl)B)2-1,2-C2B10H10 (Scheme 20, Figure 14) [54]. The reaction of the dilithium derivative of ortho-carborane with 1,2-dichloro-1,2-bis(dimethylamino)-diborane(4) (Me2NBCl)2 in toluene results in 1-(Me2N)2B(Me2N)B-1,2-C2B10H11 (Scheme 20, Figure 14) [55].
The dilithium derivative of ortho-carborane with diisopropylamino(inden-3-yl)boryl chloride gave the indenyl derivative 1-(i-Pr2N)(C9H7)B-1,2-C2B10H11 (Scheme 21, Figure 15) [56], which was used to synthesize a series of indenyl-carborane complexes with lanthanides and the group 4 metals [56,57].
The 1,3-diethyl-1,3,2-benzodiazaborolyl group is known as a good π-donor comparable with the methoxy and dimethylamino groups [58], which has led to the synthesis of numerous “push-pull” type organoboron compounds as promising emitting materials for potential application in electro-optical devices [59]. On the other hand, the C-ortho-carboranyl fragment has strong electron-withdrawing properties [6]. Therefore, ortho-carborane derivatives have been tested as electron-withdrawing substituents in systems in which benzodiazaboroles act as donors. It was expected that the obtained 1,3,2-benzodiazaborolyl-ortho-carboranes could form a new family of donor-acceptor systems. The simplest compound of this type, 1-(1′,3′-H2-1′,3′,2′-N2BC6H4-2′-)-2-Me-1,2-C2B10H10 (Figure 15), was synthesized by the cyclocondensation of C-carboranylboron dibromide 1-Br2B-2-Me-1,2-C2B10H10 with 1,2-phenylene diamine (Scheme 22) [32]. A series of N,N′-dialkyl and N,N′-diaryl substituted 1,3,2-benzodiazaborolyl-ortho-carboranes 1-(1′,3′-R’2-1′,3′,2′-N2BC6H4-2′-)-2-R-1,2-C2B10H10 (R = H, R′ = Et, i-Pr, Ph; R = Me, R′ = Et, Ph; R = t-Bu; R′ = Et, Ph; R′ = SiMe3, R′ = Et, i-Pr, Ph; R = Ph, R′ = Et) and 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H2Me2-2′-)-2-R-1,2-C2B10H10 (R = H, t-Bu, SiMe3) were prepared by the reactions of lithium derivatives of ortho-carboranes with the corresponding bromo-1,3,2-benzodiazaboroles (Scheme 22) [32,60,61]. The solid-state structures of the synthesized 1,3,2-benzodiazaborolyl-ortho-carboranes were determined by single crystal X-ray diffraction (Figure 16, Figure 17 and Figure 18), and their photophysical and electrochemical properties were studied [32,60,61].
In most synthesized compounds, the plane of the 1,3,2-benzodiazaborolyl group was found to be nearly perpendicular to the carborane C-C bonds (ψ ≈ 90°), and these compounds demonstrate remarkable low-energy fluorescence emissions with large Stokes shifts of 15,100–20,260 cm−1 and quantum yields of up to 70% on excitation in the solid state. In contrast, the compounds with a different orientation of the 1,3,2-benzodiazaborolyl group (ψ ≈ 0°), namely 1-(1′,3′-R2-1′,3′,2′-N2BC6H4-2′-)-1,2-C2B10H11 (R = i-Pr, Ph) and 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H2Me2-2′-)-1,2-C2B10H11, do not emit low-energy radiation upon excitation in the solid state [32,60,61].
The ortho-carborane derivative containing two 1,3,2-benzodiazaborolyl groups 1,2-(1′,3′-R2-1′,3′,2′-N2BC6H4-2′-)2-1,2-C2B10H10 (R = Et, Ph) was prepared by the reaction of the dilithium derivative of ortho-carborane with the corresponding 2-bromo-1,3,2-benzodiazaboroles (Scheme 23, Figure 19) [61].
The disubstituted derivatives demonstrate the low-energy fluorescence typical for the 1,3,2-benzodiazaborolyl derivatives of ortho-carborane Stokes shifts between 17,330 and 21,290 cm−1; however, the quantum yields are very low both in solution and in the solid state [61].
The meta- and para-carborane derivatives with the same substituents 1,7-(1′,3′-R2-1′,3′,2′-N2BC6H4-2′-)2-1,7-C2B10H10 and 1,12-(1′,3′-R2-1′,3′,2′-N2BC6H4-2′-)2-1,12-C2B10H10 (R = Et, Ph) were prepared similarly (Scheme 24, Figure 20) [61].
The bis(1,3,2-benzodiazaborolyl) derivatives of meta- and para-carboranes were found to demonstrate low-energy emissions with Stokes shifts between 8320 and 15,170 cm−1. The fluorescence quantum yields for bis(1,3,2-benzodiazaborolyl)-meta- and -para-carboranes depend on the substituents at the nitrogen atoms of the heterocycle. Thus, the para-carborane derivative with N-ethyl substituents has a quantum yield value of 41% in cyclohexane solution and only 9% in the solid state, whereas the analogous derivative with N-phenyl substituents demonstrates quantum yields of 3% in cyclohexane solution and 72% in the solid state [61].
A series of 1,3,2-benzodioxaborolyl-ortho-carboranes 1-(1′,3′,2′-O2BC6H4-2′-)-2-R-1,2-C2B10H10 (R = H, Me, Ph) were prepared by the reactions of lithium derivatives of the corresponding ortho-carboranes with 2-bromo-1,3,2-benzodioxaborole (Scheme 25, Figure 21) [45,62].
Similarly, 1-(1′,3′,2′-benzodioxaborolyl)-7-diphenylphosphino-meta-carborane 1-(1′,3′,2′-O2BC6H4-2′-)-2-Ph2P-1,7-C2B10H10 (Figure 22) was prepared by the reaction of the lithium derivatives of the 1-diphenylphosphino-meta-carborane with 2-bromo-1,3,2-benzodioxaborole [45].

6. Carboranyl-Fused Boron-Containing Heterocycles

The carborane derivative with the fused exo-polyhedral C2B2C2 heterocycle 1,2-μ-(1′,2′-C6H4(BNiPr2)2)-1,2-C2B10H10 was synthesized by the reaction of the dilithium derivative of ortho-carborane with the 1,2-diborylbenzene derivative 1,2-C6H4(B(NiPr2)Cl)2 in toluene (Scheme 26) [55]. The NBC2 fragments in the structure of 1,2-μ-(1′,2′-C6H4(BNiPr2)2)-1,2-C2B10H10 are planar, indicating the double character of the B=N bonds; however, the C2B2C2 ring is bent along the B-B axis by ~55° with CBC angles of ~109° (Figure 23). The analogous reactions with 1,1-bis(dimethylaminochloroboryl)ethane MeCH(B(NMe2)Cl)2 and with 1,3-dichloro-1,2,3-tris(dimethylamino)triborane(5) afforded the corresponding carborane derivative with the fused exo-polyhedral five-membered C2B2C and C2B3 rings (Scheme 26) [55]. The C2B3 ring in the structure of 1,2-μ-(Me2NB)3-1,2-C2B10H10 is puckered by ~35°with CBB and BBB angles of ~104° (Figure 23).
The reaction of iminocarborane 1-DippN=(tBu)C-1,2-C2B10H11 (Dipp = 2′,6′-C6H3iPr2) with n-BuLi in toluene, followed by the addition of boron tribromide, leads to the closure of the five-membered ring with the formation of iminocarboranyldibromoborane 1,2-μ-Br2BN(Dipp)=CtBu-1,2-C2B10H10. Its reduction of dibromoborane with KC8 in the presence of 1,3-diisopropyl-4,5-dimethylimidazole-2-ylidene (MeIiPr) in toluene results in the N-heterocyclic carbene-stabilized carborane-fused azaborole 1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10, which in turn can be reoxidized with silver acetate in tetrahydrofuran to give iminocarboranyldiacetoxyborane 1,2-μ-(AcO)2BN(Dipp)=CtBu-1,2-C2B10H10 (Scheme 27, Figure 24) [63].
The one-electron oxidation of the carborane-fused azaborole 1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10 with CuCl or CuI in tetrahydrofuran results in a carbene-stabilized carborane-fused azaborolyl radical cation isolated as the corresponding salts [1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10][CuCl2] and [1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10]2[Cu2I4] (Scheme 28). The geometry of the resulting radical cation is close to that of the starting compound, except for some redistribution of bond lengths in the carborane-fused five-membered heterocycle. The dihedral angles of the azaborole and N-heterocyclic carbene planes are 67.9° and 67.4° for the [CuCl2]− and [Cu2I4]2− salts, respectively, which are very close to that of 65.3° observed in the starting compound (Figure 24). These data strongly suggest that the carbene acts solely as a σ donor, not as a π acceptor [64].
The reaction of 1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10 with elemental sulfur in tetrahydrofuran at room temperature unexpectedly leads to the deboronation of the ortho-carborane cage with the formation of the dicarbollide-fused azaborole 7,8-μ-(MeIiPr)BN(Dipp)CtBu-7,8-C2B9H9. The open pentagonal C2B3 face is fused with another C3BN five membered ring with a dihedral angle of 122.2° (Figure 25). The quantum chemical calculations demonstrate the π conjugation between two fused five-membered rings formally bearing charges of +2 (C3BN) and −2 (C2B3) [64].
Irradiation of solution of the carborane-fused azaborole 1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10 in tetrahydrofuran with a Xe light lamp (400-1500 nm, 300 W) at ambient temperature under N2 atmosphere results in its photoisomerization to the carbene-stabilized carborane-fused borirane 1,2-μ-DippN=(tBu)C(MeIiPr)B-1,2-C2B10H10. This transformation is completely reversible under an inert atmosphere and is accelerated by heating (Scheme 29, Figure 26) [65].
The treatment of the carborane-fused borirane 1,2-μ-DippN=(tBu)C(MeIiPr)B-1,2-C2B10H10 with CuCl in tetrahydrofuran leads to the disclosure of carborane-fused three-membered ring with the formation of a new BNC three-membered ring, giving 1-cyclo-(Dipp)N=(tBu)C(MeIiPr)B-2-ClCu-1,2-C2B10H10. The reaction of the carborane-fused borirane with HCl in 1,4-dioxane also proceeds with the ring disclosure, resulting in 1-DippN=(tBu)C(MeIiPr)ClB-1,2-C2B10H11. Unexpectedly, the reaction of the carborane-fused borirane with elemental sulfur in tetrahydrofuran at room temperature leads to the ring expansion with the formation of 1,2-μ-DippN=(tBu)C(MeIiPr)BS-1,2-C2B10H10 (Scheme 30, Figure 26) [65].
Another carborane-fused borirane was prepared in a more straightforward manner. The reaction of the lithium derivative of ortho-carborane with boron tribromide BBr3 in the presence of 1,3-bis(2′,6′-diisopropyl)imidazol-2-ylidene (Idipp) gives the N-heterocyclic carbene-coordinated carboranyl boron dibromide 1-(Idipp)Br2B-1,2-C2B10H11, which on the treatment with lithium bis(trimethylsilyl)amide (LiN(TMS)2) in toluene at 80 °C produces the carborane-fused borirane 1,2-μ-(Idipp)BrB-1,2-C2B10H10. Subsequent treatment of 1,2-μ-(Idipp)BrB-1,2-C2B10H10 with AgOTf in toluene at ambient temperatures results in the replacement of bromide with triflate, giving 1,2-μ-(Idipp)(TfO)B-1,2-C2B10H10. The last one on heating to 50 °C isomerizes with the three-membered cycle disclosure to give oxoborane 1-(Idipp)(O)B-2-CF3O2S-1,2-C2B10H10 (Scheme 31, Figure 27) [66].
The carboranyl oxoborane 1-O=(Idipp)B-2-CF3O2S-1,2-C2B10H10 reacts with GeCl2·1,4-dioxane to give the corresponding Lewis acid-base adduct 1-Cl2Ge·O=(Idipp)B-2-CF3O2S-1,2-C2B10H10. On the other hand, the reactions with PBr3 and SiCl4 result in 1-(Idipp)BrPOBr2B-2-CF3O2S-1,2-C2B10H10 and 1-(Idipp)Cl2SiOCl2B-2-CF3O2S-1,2-C2B10H10 (Scheme 32) [66].
The reaction of the dilithium derivative of ortho-carborane with bis(trimethylsilyl)amino dichloroborane (Me3Si)2NBCl2 in toluene results directly in the corresponding symmetrical carborane-fused borirane 1,2-μ-(Me3Si)2NB-1,2-C2B10H10 (Scheme 33, Figure 28) [67]. A similar reaction with trimethylsilyl-t-butylamino dichloroborane, along with 1,2-μ-(Me3Si)tBuNB-1,2-C2B10H10, gives 1-(cyclo-H2CMe2SitBuN=B)-1,2-C2B10H11, 1,2-μ-Me2SitBuNClB-1,2-C2B10H10, and 1,2-μ-Me2SitBuNMeB-1,2-C2B10H10 (Scheme 33, Figure 28) [68].
The treatment of the carborane-fused borirane 1,2-μ-RR’NB-1,2-C2B10H10 (R = R′ = SiMe3; R = SiMe3, R′ = t-Bu) in hexane with THF as a Lewis base leads to ring disclosure with the formation of the corresponding iminoboryl carborane 1-RN≡B-2-Me3Si-C2B10H10 (R = SiMe3, t-Bu) (Scheme 34, Figure 29) [69].
In the presence of catalytic amounts of tert-butylisonitrile t-BuNC in benzene, 1-Me3SiN≡B-2-Me3Si-C2B10H10 undergoes dimerization with the formation of the central B2N2 ring of a butterfly geometry (interplanar angle of 156.0°). In the presence of an excess of tert-butylisonitrile, one molecule of t-BuNC is coordinated to the central B2N2 ring, changing its geometry to planar (Figure 30) [69].
The reaction of the carborane-fused borirane 1,2-μ-(Me3Si)2NB-1,2-C2B10H10 with 1,3-diisopropyl-4,5-dimethylimidazole-2-ylidene (MeIiPr) in benzene results in the disclosure of the borinane ring with the formation of the N-heterocyclic carbene iminoborane adduct 1-Me3SiN=(MeIiPr)B-2-Me3Si-1,2-C2B10H10 (Scheme 35, Figure 31) [68].
Another symmetrical carborane-fused borirane 1,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10 was prepared by the reaction of the dilithium salt of ortho-carborane with 2,6-dimesitylphenyl boron dichloride in toluene at 80 °C. An attempt to determine its Lewis acidity by the Gutmann–Beckett method resulted in the formation of the carborane derivative with fused C2BOP five-membered heterocycle 1,2-μ-(Et3PO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 (Scheme 36, Figure 32) [70].
The reactions of 1,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10 with benzaldehyde and benzophenone in toluene produce the five- and seven-membered carborane-fused heterocycles 1,2-μ-(PhHCO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 and 1,2-μ-(o-C6H5=(Ph)CO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10, respectively (Scheme 37, Figure 33) [70].
The reactions with nitriles in toluene also lead to the five-membered carborane-fused heterocycle 1,2-μ-(RC=N(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 (R = Me, Ph) (Scheme 37, Figure 33) [70].

7. Bis(Carboranyl)-Fused Boron-Containing Heterocycles

The three-dimensional inorganic analogs of 9-borafluorene 2,2′-μ-XB-1,1′-bis(ortho-carborane) (X = Cl, Br) (Figure 34) were synthesized by the reaction of the dilithium derivative of 1,1′-bis(ortho-carborane) with BX3 in hexane (Scheme 38) [71], while the reaction of the dilithium derivative of 1,1′-bis(ortho-carborane) with BBr3 in toluene unexpectedly resulted in the anionic dibromoborate derivative Li+[2,2′-μ-Br2B-1,1′-bis(ortho-carborane)]− (Figure 34) [71].
Due to their high Lewis acidity, the synthesized bis(carboranyl) 9-borafluorene analogs form stable complexes with diethyl ether 2,2′-μ-Et2O·(X)B-1,1′-bis(ortho-carborane) (X = Cl, Br), the structure of which in the solid state was determined by a single crystal X-ray diffraction study (Figure 35), and the existence in solution was established by 1H and 11B NMR spectroscopy [71].
The reaction of 2,2′-μ-BrB-1,1′-bis(ortho-carborane) with Et3SiH leads to the anionic dihydride derivative (Et3Si)+[2,2′-μ-H2B-1,1′-bis(ortho-carborane)]−. An attempt to obtain the neutral hydride derivative by removing the silane in a high vacuum was unsuccessful; however, its complex with diethyl ether 2,2′-μ-Et2O(H)B-1,1′-bis(ortho-carborane) was synthesized by the reaction of the complex of the bromo derivative with diethyl ether 2,2′-μ-Et2O(H)B-1,1′-bis(ortho-carborane) 2,2′-μ-Et2O(Br)B-1,1′-bis(ortho-carborane) with Et3SiH. The 1H NMR spectrum of the hydride complex displays an upfield shift of the signal of the α-proton of the coordinated diethyl ether (3.31 ppm) with respect to that of the chloro and bromo complexes (3.55 and 3.58 ppm), indicating its relatively weaker Lewis acidity [71].
The aryl derivatives of the bis(carboranyl) analog of 9-borafluorene 2,2′-μ-RB-1,1′-bis(ortho-carborane) (R = Ph, Mes) were prepared by the reaction of the dilithium derivative of 1,1′-bis(ortho-carborane) with the corresponding aryldichloroborane RBCl2 in hexane or fluorobenzene (Scheme 38) [71,72]. A single crystal X-ray diffraction study of the aryl derivatives revealed that they have notable structural differences. In the phenyl derivative, the aryl group and the C4B ring are nearly coplanar, as indicated by the small dihedral angle (10.7°), while in the mesityl derivative, the aryl group displays a significantly enlarged dihedral angle of 65.47° (Figure 36), which can be explained by steric repulsion between the ortho-methyl groups of the mesityl substituent and the BH groups of the carborane units, which presumed a hindered rotation of the mesityl group around the B-C bond [71]. As a result, the phenyl derivative was found to form a complex with diethyl ether 2,2′-μ-Et2O·(Ph)B-1,1′-bis(ortho-carborane) (Figure 37), while the mesityl derivative is unable to do so [71].
Most notably, the significant difference in the rotation of the aryl group on boron in the phenyl and mesityl derivatives led to their different photophysical properties. In particular, the absorption maximum in the UV-visible spectrum of the mesityl derivative (334 nm) is red-shifted by ~55 nm compared to that of the phenyl derivative (279 nm). Moreover, while the mesityl derivative demonstrates bluish-violet fluorescence, the phenyl derivative displayed no distinct fluorescent properties. This outcome is presumably caused by the nonradiative relaxation of the excited states, associated with rotation of the exocyclic phenyl group, which is largely hindered for the mesityl group. The emission maximum of for the mesityl derivative is strongly bathochromically shifted in polar solvents, thus indicating that the emission should arise from an intramolecular charge-transfer (ICT) transition [71].
The reaction of the phenyl derivative with Ph3CBr in pentane proceeds with bromide ion transfer from carbon to boron, producing the anionic phenyl bromide derivative (Ph3C)+[2,2′-μ-Ph(Br)B-1,1′-bis(ortho-carborane)]− (Figure 38), which can be regarded as a weakly coordinating anion [71].
Due to the unique spatial environment of the trigonal planar boron in the phenyl derivative of the bis(carboranyl) analog of 9-borafluorene, it has good potential for use in frustrated Lewis pair chemistry. Indeed, in contrast to purely organic 9-borafluorenes and B(C6F5)3, which form stable complexes with triphenylphosphine, the phenyl derivative of the bis(carboranyl) analog of 9-borafluorene is not capable of doing so. However, the exposure of a 1:1 mixture of phenyl derivative and triphenylphosphine to 1 atm. H2 at room temperature leads to the activation of the latter with the formation of the triphenylphosphonium salt of the anionic phenyl hydride derivative (Ph3PH)+[2,2′-μ-Ph(H)B-1,1′-bis(ortho-carborane)]−. Likewise, the treatment of a 1:1 mixture of the phenyl derivative and triphenylphosphine with excess Et3SiH results in the similar triethysilyltriphenylphosphonium salt (Ph3PSiEt3)+[2,2′-μ-Ph(H)B-1,1′-bis(ortho-carborane)]−, the structure of which was determined by single crystal X-ray diffraction (Figure 39) [71].
Other carboranyl analogs of 9-borafluorene 1,1′-bis(ortho-carborane) 2,2′-μ-(i-Pr)2NB-1,10-bis(ortho-carborane) and 2,2′-μ-(i-Pr)2NB-1,1′-bis(ortho-carborane-8,9,10,12-Me4) were synthesized by the reaction of the dipotassium derivatives of 1,1′-bis(ortho-carborane) and its methylated analog with (i-Pr)2NBCl2 in tetrahydrofuran (Scheme 39, Figure 40) [72].
The Lewis acidity of the synthesized bis(carboranyl) analogs of 9-borafluorene 2,2′-μ-XB-1,1′-bis(ortho-carborane) was determined by the Gutmann–Beckett method, and it was found that it decreases in the series Br > Cl > Ph > NiPr2 [45,71,72].
The three-dimensional inorganic analogs of 9,10-diboraanthracene 1,1′,2,2′-μ-(XB)2-(ortho-carborane)2 (X = Cl, Br) (Figure 41) were synthesized by the reaction of the dilithium derivative of ortho-carborane with BX3 in dichloromethane (Scheme 40) [73].
Further reactions of the synthesized halogen derivatives of the carboranyl analog of 9,10-diboraanthracene with trimethylsilylazide in toluene result in the azido derivative 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2 (Scheme 40, Figure 42) [74], while the reactions with the Grignard reagent RMgBr lead to the corresponding methyl and phenyl derivatives 1,1′,2,2′-μ-(RB)2-(ortho-carborane)2 (R = Me, Ph) (Scheme 40, Figure 43) [73]. The Lewis acidity of the synthesized carboranyl analogs of 9,10-diboraanthracene 1,1′,2,2′-μ-(XB)2-(ortho-carborane)2 decreases in the series Br > Cl > Ph > N3 > Me [73,74]. Their Lewis acidities are somewhat less than that of the corresponding derivatives of the bis(carboranyl) analog of 9-borafluorene but much greater than that of B(C6F5)3 [49]. The azido derivative was found to form a stable complex with acetonitrile 1,1′,2,2′-μ-(MeCN(N3)B)2-(ortho-carborane)2 (Figure 42) [74].
The reactions of 1,1′,2,2′-μ-(XB)2-(ortho-carborane)2 with Et3SiH in benzene result in the borane-silane adduct (Et3Si)2[1,1′,2,2′-μ-(H2B)2-(ortho-carborane)2] (Figure 44) [73]. The bridging B-H bonds (1.33 Å) are notably longer than the terminal B-H bonds (1.09 Å) but are comparable to the bridging three-center two-electron B-H-B bonds in diborane derivatives, including species such as [HBMes2]2 (1.288 Å) [75].
A number of addition reactions to the azido derivative 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2 have been studied. The reaction with triphenylphosphine in toluene results in the corresponding iminophosphorane 1,1′,2,2′-μ-(Ph3P=N=B)2-(ortho-carborane)2 as the Staudinger reaction product (Scheme 41, Figure 45) [74]. The Staudinger reaction is known to proceed via the nucleophilic addition of triarylphosphine to the terminal nitrogen atom of the organic azide, followed by the elimination of dinitrogen. The reaction with 9-borabicyclo [3.3.1]nonane (9-BNN) in toluene at 60 °C produces the corresponding aminoborane 1,1′,2,2′-μ-((9″’-BBN)-NHB)2-(ortho-carborane)2 (Scheme 41) [74].
Similarly, the reactions of the azido derivative with tertiary silanes R3SiH in toluene at 60 °C result in the corresponding silyl aminoborane 1,1′,2,2′-μ-(R3SiNHB)2-(ortho-carborane)2 (SiR3 = SiEt3, SiMe2Ph); however, if the same reactions are conducted at -30 °C, it is possible to isolate the initial products of 1,2-addition of the silane 1,1′,2,2′-μ-(R3Si(N2)NHB)2-(ortho-carborane)2 as the reaction intermediate (Figure 46, Scheme 42). The complete solid-state conversion of 1,1′,2,2′-μ-(Et3Si(N2)NHB)2-(ortho-carborane)2 to 1,1′,2,2′-μ-(Et3SiNHB)2-(ortho-carborane)2 with N2 elaboration was observed within three days at 38 °C (Figure 47, Scheme 42) [74].
In contrast with the tertiary silane R3SiH, the reaction of the azido derivative with Et3GeH in toluene at 60 °C stops at the first stage with the formation of 1,1′,2,2′-μ-(Et3Ge(N2)NHB)2-(ortho-carborane)2 (Scheme 43, Figure 48). Moreover, the reaction is reversible, and an increase in temperature promotes the dissociation of the addition product [74].

8. Carborane Derivatives with exo-Polyhedral Boron-Boron Bond

Carborane derivatives in which a tri- or four-coordinated boron atom is bonded to the carborane cage by an exo-polyhedral boron–boron bond have received much less attention than derivatives with a carbon–boron bond. Interest in such derivatives arose relatively recently and was caused by the development of new methods for the functionalization of carboranes through substitution at boron atoms.
The 3-pinacolborate derivative of ortho-carborane 3-pinB-1,2-C2B10H11 was prepared by reacting the parent carborane with dipinacolborane B2pin2 in tetrahydrofuran in the presence of 3.5 mol.% [(cod)IrCl]2 and 20 mol.% 2,6-lutidine. When using an excess of B2pin2 and 20 mol.% 2-picoline, the main reaction product is the 3,6-di(pinacolborate) derivative 3,6-(pinB)2-1,2-C2B10H10 (Scheme 44, Figure 49). The same approach can be used for the synthesis of pinacolborate derivatives of ortho-carborane containing various substituents at the boron and carbon atoms of the carborane cage [75,76].
The 4-pinacolborate derivative of ortho-carborane 4-pinB-1,2-C2B10H11 was prepared by the iridium-catalyzed borylation of 1-t-BuMe2Si-ortho-carborane with B2pin2 in tetrahydrofuran in the presence of 2,2′-bipyridine, followed by removal of the silyl group with CsF (Scheme 45, Figure 50) [75]. The 9-pinacolborate derivative of ortho-carborane 9-pinB-1,2-C2B10H11 was prepared by the reaction of 9-iodo-ortho-carborane with B2pin2 in N,N-dimethylformamide in the presence of 5 mol.% dppf-Pd-G3 pre-catalyst (Scheme 45, Figure 50) [77].
The picoline protection in 3-pinB-1,2-C2B10H11, 9-pinB-1,2-C2B10H11, and 3,6-(pinB)2-1,2-C2B10H10 can be easily removed by treatment with HCl in ethanol or diethanolamine to give the corresponding boronic acids 3-(HO)2B-1,2-C2B10H11, 9-(HO)2B-1,2-C2B10H11, and 3,6-((OH)2B)2-1,2-C2B10H10 [75,77].
The picolinate groups in 3-pinB-1,2-C2B10H11 and 3,6-(pinB)2-1,2-C2B10H10 can be replaced by a wide variety of functional groups, including amine [75], acetate [75,76], azide [75], halogens [76], allyl [75], and aryl [75] groups.
The 2-pinacolborate derivative of meta-carborane 2-pinB-1,7-C2B10H11 was prepared by the reaction of meta-carborane with dipinacolborane B2pin2 in tetrahydrofuran in the presence of 3.5 mol.% [(cod)IrCl]2 and 20 mol.% 2-picoline (Scheme 46, Figure 51) [75]. The 9-pinacolborate derivative of meta-carborane 9-pinB-1,7-C2B10H11 was prepared by the reaction of 9-iodo-ortho-carborane with B2pin2 in N,N-dimethylformamide in the presence of 5 mol.% dppf-Pd-G3 pre-catalyst (Scheme 46, Figure 51) [77].
The picoline protection in 9-pinB-1,7-C2B10H11 can be removed by treatment with HCl in ethanol of the corresponding boronic acid 9-(HO)2B-1,7-C2B10H11. The reaction of 9-pinB-1,7-C2B10H11 with potassium fluoride (KF) in a mixture of acetonitrile and methanol results in K[9-F3B-1,7-C2B10H11]. Some reactions of the synthesized carboranyl boronic acid and tetrafluoroborate were described [77].

Funding

This research was supported by the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2021-1027 from 4 October 2021).

Data Availability Statement

No new data were created.

Conflicts of Interest

The author declares no conflict of interest.

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Chemistry 05 00059 sch001 550
Scheme 1. Syntheses of the X2B-ortho-carborane derivatives (X = Cl, Br, OR, SR, NR2).
Scheme 1. Syntheses of the X2B-ortho-carborane derivatives (X = Cl, Br, OR, SR, NR2).
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Figure 1. Single crystal structures of 1-Cl2B-2-i-Pr-1,2-C2B10H10 (left) and 1-(MeO)2B-2-i-Pr-1,2-C2B10H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 1. Single crystal structures of 1-Cl2B-2-i-Pr-1,2-C2B10H10 (left) and 1-(MeO)2B-2-i-Pr-1,2-C2B10H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Figure 2. Single crystal structure of μ-1,1′-O(1-(C6F5O)B-2-i-Pr-C2B10H10)2. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 2. Single crystal structure of μ-1,1′-O(1-(C6F5O)B-2-i-Pr-C2B10H10)2. Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 2. Synthesis of 1-Bu2B-2-i-Pr-ortho-carborane and its transformations.
Scheme 2. Synthesis of 1-Bu2B-2-i-Pr-ortho-carborane and its transformations.
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Scheme 3. Thermal cyclization of 1-R2B-2-R’-ortho-carboranes.
Scheme 3. Thermal cyclization of 1-R2B-2-R’-ortho-carboranes.
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Scheme 4. Synthesis of the para-carboranyl boronic acid 1-(HO)2B-12-H(O)C-1,12-C2B10H10.
Scheme 4. Synthesis of the para-carboranyl boronic acid 1-(HO)2B-12-H(O)C-1,12-C2B10H10.
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Scheme 5. Synthesis of the trihydridoborate derivative of ortho-carborane and its complex with triethylamine.
Scheme 5. Synthesis of the trihydridoborate derivative of ortho-carborane and its complex with triethylamine.
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Scheme 6. Synthesis of the trihydridoborate derivatives of ortho-carborane and their analogs containing various functional groups.
Scheme 6. Synthesis of the trihydridoborate derivatives of ortho-carborane and their analogs containing various functional groups.
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Figure 3. Single crystal structures of [1-HF2B-1,2-C2B10H11]− (left) and [1-HF2B-9,12-I2-1,2-C2B10H9]− (right) anions.
Figure 3. Single crystal structures of [1-HF2B-1,2-C2B10H11]− (left) and [1-HF2B-9,12-I2-1,2-C2B10H9]− (right) anions.
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Scheme 7. Synthesis of amine complexes with the dihydridoborate derivative of ortho-carborane.
Scheme 7. Synthesis of amine complexes with the dihydridoborate derivative of ortho-carborane.
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Scheme 8. Synthesis of Lewis base complexes with the dihydridoborate derivative of 1-isopropyl-ortho-carborane.
Scheme 8. Synthesis of Lewis base complexes with the dihydridoborate derivative of 1-isopropyl-ortho-carborane.
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Figure 4. Single crystal structure of 1-p-Me2NC5H4NBH2-2-i-Pr-1,2-C2B10H10. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 4. Single crystal structure of 1-p-Me2NC5H4NBH2-2-i-Pr-1,2-C2B10H10. Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 9. Synthesis of amine complexes with the dihydridoborate derivative of ortho-carborane.
Scheme 9. Synthesis of amine complexes with the dihydridoborate derivative of ortho-carborane.
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Scheme 10. Hydroboration of double and triple bonds with 1-Me2SBH2-2-i-Pr-1,2-C2B10H10.
Scheme 10. Hydroboration of double and triple bonds with 1-Me2SBH2-2-i-Pr-1,2-C2B10H10.
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Scheme 11. Synthesis of amine complexes with the trihydridoborate derivative of meta-carborane.
Scheme 11. Synthesis of amine complexes with the trihydridoborate derivative of meta-carborane.
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Figure 5. Single crystal structures of ortho-carboranyl-capped iron(II) clatrochelates. Hydrogen atoms are omitted for clarity.
Figure 5. Single crystal structures of ortho-carboranyl-capped iron(II) clatrochelates. Hydrogen atoms are omitted for clarity.
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Scheme 12. Synthesis of diarylboryl derivatives of ortho-carborane.
Scheme 12. Synthesis of diarylboryl derivatives of ortho-carborane.
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Figure 6. Single crystal structures of 1-Mes2B-1,2-C2B10H11 (top left), 1-Mes2B-2-Me-1,2-C2B10H10 (top right), 1-Mes2B-2-Ph-1,2-C2B10H10 (bottom left), and 1-(C6F5)2B-2-i-Pr-1,2-C2B10H10 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 6. Single crystal structures of 1-Mes2B-1,2-C2B10H11 (top left), 1-Mes2B-2-Me-1,2-C2B10H10 (top right), 1-Mes2B-2-Ph-1,2-C2B10H10 (bottom left), and 1-(C6F5)2B-2-i-Pr-1,2-C2B10H10 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 13. Synthesis of bis(carboranyl)boron derivatives 1,1′-(2-i-Pr-1,2-C2B10H10)2BX (X = F, Cl, OH).
Scheme 13. Synthesis of bis(carboranyl)boron derivatives 1,1′-(2-i-Pr-1,2-C2B10H10)2BX (X = F, Cl, OH).
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Scheme 14. Synthesis of bis(ortho-carboranyl) aryl boranes.
Scheme 14. Synthesis of bis(ortho-carboranyl) aryl boranes.
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Figure 7. Single crystal structure of μ-1,1′-p-TolB(2-p-Tol-1,2-C2B10H10)2. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 7. Single crystal structure of μ-1,1′-p-TolB(2-p-Tol-1,2-C2B10H10)2. Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 15. Synthesis of tris(ortho-carboranyl) borane 1,1′,1″-μ3-B(1,2-C2B10H11)3.
Scheme 15. Synthesis of tris(ortho-carboranyl) borane 1,1′,1″-μ3-B(1,2-C2B10H11)3.
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Figure 8. Single crystal structures of tris(ortho-carboranyl)borane 1,1′,1″-μ3-B(1,2-C2B10H11)3 (top left) and their complexes PhC(O)H·1,1′,1″-μ3-B(1,2-C2B10H11)3 (top right), MeCN·1,1′,1″-μ3-B(1,2-C2B10H11)3 (bottom left), and Et3PO·1,1′,1″-μ3-B(1,2-C2B10H11)3 (bottom right). Hydrogen atoms of the ethyl groups in the Et3PO molecule are omitted for clarity.
Figure 8. Single crystal structures of tris(ortho-carboranyl)borane 1,1′,1″-μ3-B(1,2-C2B10H11)3 (top left) and their complexes PhC(O)H·1,1′,1″-μ3-B(1,2-C2B10H11)3 (top right), MeCN·1,1′,1″-μ3-B(1,2-C2B10H11)3 (bottom left), and Et3PO·1,1′,1″-μ3-B(1,2-C2B10H11)3 (bottom right). Hydrogen atoms of the ethyl groups in the Et3PO molecule are omitted for clarity.
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Scheme 16. Synthesis of carboranyl aryl derivatives of meta-carborane.
Scheme 16. Synthesis of carboranyl aryl derivatives of meta-carborane.
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Scheme 17. Synthesis of the ortho-carboranyl derivative of 9-borafluorene 1-C12H8B-2-Me-1,2-C2B10H10.
Scheme 17. Synthesis of the ortho-carboranyl derivative of 9-borafluorene 1-C12H8B-2-Me-1,2-C2B10H10.
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Figure 9. Single crystal structure of the ortho-carboranyl derivative of 9-borafluorene 1-C12H8B-2-Me-1,2-C2B10H10. Hydrogen atoms of the methyl group are omitted for clarity.
Figure 9. Single crystal structure of the ortho-carboranyl derivative of 9-borafluorene 1-C12H8B-2-Me-1,2-C2B10H10. Hydrogen atoms of the methyl group are omitted for clarity.
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Figure 10. Single crystal structures of ortho-carboranyl-9-borafluorene complexes with axetone (top left), tetrahydrofuran (top right), pyridine (bottom left), and p-dimethylaminopyridine (bottom right). Hydrogen atoms of the methyl group and Lewis bases are omitted for clarity.
Figure 10. Single crystal structures of ortho-carboranyl-9-borafluorene complexes with axetone (top left), tetrahydrofuran (top right), pyridine (bottom left), and p-dimethylaminopyridine (bottom right). Hydrogen atoms of the methyl group and Lewis bases are omitted for clarity.
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Scheme 18. Alkene insertion into the ortho-carboranyl derivative of 9-borafluorene.
Scheme 18. Alkene insertion into the ortho-carboranyl derivative of 9-borafluorene.
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Figure 11. Single crystal structures of 5-ortho-carboranyl-7-R-6,7-dihydrodibenzo[b,d]borepines (R = t-Bu (left), R = Ph (center), and R = p-C6H4Br (right)). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 11. Single crystal structures of 5-ortho-carboranyl-7-R-6,7-dihydrodibenzo[b,d]borepines (R = t-Bu (left), R = Ph (center), and R = p-C6H4Br (right)). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 19. Reaction of the dilithium derivative of ortho-carborane with 9-bromo-9-boradibenzofluorene.
Scheme 19. Reaction of the dilithium derivative of ortho-carborane with 9-bromo-9-boradibenzofluorene.
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Figure 12. Single crystal structures of the product of the reaction of the dilithium derivative of ortho-carborane with 9-bromo-9-borafluorene (left) and its complex with THF (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 12. Single crystal structures of the product of the reaction of the dilithium derivative of ortho-carborane with 9-bromo-9-borafluorene (left) and its complex with THF (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Figure 13. Single crystal structures of the products of the reaction of the dilithium derivative of ortho-carborane with 9-bromo-9-borafluorene in tetrahydrofuran (left) and in dimethylsulfide (right). Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 13. Single crystal structures of the products of the reaction of the dilithium derivative of ortho-carborane with 9-bromo-9-borafluorene in tetrahydrofuran (left) and in dimethylsulfide (right). Hydrogen atoms of alkyl groups are omitted for clarity.
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Scheme 20. Preparation of aminoboryl derivatives of ortho-carborane.
Scheme 20. Preparation of aminoboryl derivatives of ortho-carborane.
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Figure 14. Single crystal structures of 1,2-(Et2N(Cl)B)2-1,2-C2B10H10 (top left), 1-i-Pr2N(Cl)B-1,2-C2B10H11 (top right), 1,2-(i-Pr2N(Cl)B)2-1,2-C2B10H10 (bottom left), and 1-(Me2N)2B(Me2N)B-1,2-C2B10H11 (bottom right). Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 14. Single crystal structures of 1,2-(Et2N(Cl)B)2-1,2-C2B10H10 (top left), 1-i-Pr2N(Cl)B-1,2-C2B10H11 (top right), 1,2-(i-Pr2N(Cl)B)2-1,2-C2B10H10 (bottom left), and 1-(Me2N)2B(Me2N)B-1,2-C2B10H11 (bottom right). Hydrogen atoms of alkyl groups are omitted for clarity.
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Scheme 21. Synthesis of the indenyl derivative of ortho-carborane 1-(i-Pr2N)(C9H7)B-1,2-C2B10H11.
Scheme 21. Synthesis of the indenyl derivative of ortho-carborane 1-(i-Pr2N)(C9H7)B-1,2-C2B10H11.
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Figure 15. Single crystal structures of 1-(i-Pr2N)(inden-3-yl)B-1,2-C2B10H11 (left) and 1-(1′,3′-H2-1′,3′,2′-N2BC6H4-2′-)-2-Me-1,2-C2B10H10 (right). Hydrogen atoms of isopropyl and indenyl groups are omitted for clarity.
Figure 15. Single crystal structures of 1-(i-Pr2N)(inden-3-yl)B-1,2-C2B10H11 (left) and 1-(1′,3′-H2-1′,3′,2′-N2BC6H4-2′-)-2-Me-1,2-C2B10H10 (right). Hydrogen atoms of isopropyl and indenyl groups are omitted for clarity.
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Scheme 22. Synthesis of N,N′-dialkyl and N,N′-diaryl substituted 1,3,2-benzodiazaborolyl-ortho-carboranes.
Scheme 22. Synthesis of N,N′-dialkyl and N,N′-diaryl substituted 1,3,2-benzodiazaborolyl-ortho-carboranes.
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Figure 16. Single crystal structures of 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-1,2-C2B10H11 (top left), 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-2-Me-1,2-C2B10H10 (top middle), 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-2-t-Bu-1,2-C2B10H10 (top right), 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-2-Me3Si-1,2-C2B10H10 (bottom left), 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-2-Ph-1,2-C2B10H10 (bottom middle), and 1-(1′,3′-(i-Pr)2-1′,3′,2′-N2BC6H4-2′-)-1,2-C2B10H11 (bottom right). Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 16. Single crystal structures of 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-1,2-C2B10H11 (top left), 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-2-Me-1,2-C2B10H10 (top middle), 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-2-t-Bu-1,2-C2B10H10 (top right), 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-2-Me3Si-1,2-C2B10H10 (bottom left), 1-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)-2-Ph-1,2-C2B10H10 (bottom middle), and 1-(1′,3′-(i-Pr)2-1′,3′,2′-N2BC6H4-2′-)-1,2-C2B10H11 (bottom right). Hydrogen atoms of alkyl groups are omitted for clarity.
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Figure 17. Single crystal structures of 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)-1,2-C2B10H11 (top left), 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)-2-Me-1,2-C2B10H10 (top right), 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)-2-t-Bu-1,2-C2B10H10 (bottom left), and 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)-2-Me3Si-1,2-C2B10H10 (bottom right). Hydrogen atoms of alkyl and phenyl groups are omitted for clarity.
Figure 17. Single crystal structures of 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)-1,2-C2B10H11 (top left), 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)-2-Me-1,2-C2B10H10 (top right), 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)-2-t-Bu-1,2-C2B10H10 (bottom left), and 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)-2-Me3Si-1,2-C2B10H10 (bottom right). Hydrogen atoms of alkyl and phenyl groups are omitted for clarity.
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Figure 18. Single crystal structures of 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H2Me2-2′-)-1,2-C2B10H11 (left), 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H2Me2-2′-)-2-t-Bu-1,2-C2B10H10 (middle), and 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H2Me2-2′-)-2-Me3Si-1,2-C2B10H10 (right). Hydrogen atoms of alkyl and phenyl groups are omitted for clarity.
Figure 18. Single crystal structures of 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H2Me2-2′-)-1,2-C2B10H11 (left), 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H2Me2-2′-)-2-t-Bu-1,2-C2B10H10 (middle), and 1-(1′,3′-Ph2-1′,3′,2′-N2BC6H2Me2-2′-)-2-Me3Si-1,2-C2B10H10 (right). Hydrogen atoms of alkyl and phenyl groups are omitted for clarity.
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Scheme 23. Synthesis of 1,2-bis(N,N′-diethyl- and N,N′-phenyl-1,3,2-benzodiazaborolyl) derivatives of ortho-carborane.
Scheme 23. Synthesis of 1,2-bis(N,N′-diethyl- and N,N′-phenyl-1,3,2-benzodiazaborolyl) derivatives of ortho-carborane.
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Figure 19. Single crystal structures of 1,2-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)2-1,2-C2B10H10 (left) and 1,2-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)2-1,2-C2B10H10 (right). Hydrogen atoms of alkyl and phenyl groups are omitted for clarity.
Figure 19. Single crystal structures of 1,2-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)2-1,2-C2B10H10 (left) and 1,2-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)2-1,2-C2B10H10 (right). Hydrogen atoms of alkyl and phenyl groups are omitted for clarity.
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Scheme 24. Synthesis of 1,2-bis(1,3,2-benzodiazaborolyl) derivatives of meta- and para-carboranes.
Scheme 24. Synthesis of 1,2-bis(1,3,2-benzodiazaborolyl) derivatives of meta- and para-carboranes.
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Figure 20. Single crystal structures of 1,7-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)2-1,7-C2B10H10 (top), 1,7-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)2-1,7-C2B10H10 (middle), and 1,12-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)2-1,12-C2B10H10 (bottom). Hydrogen atoms of ethyl and phenyl groups are omitted for clarity.
Figure 20. Single crystal structures of 1,7-(1′,3′-Et2-1′,3′,2′-N2BC6H4-2′-)2-1,7-C2B10H10 (top), 1,7-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)2-1,7-C2B10H10 (middle), and 1,12-(1′,3′-Ph2-1′,3′,2′-N2BC6H4-2′-)2-1,12-C2B10H10 (bottom). Hydrogen atoms of ethyl and phenyl groups are omitted for clarity.
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Scheme 25. Synthesis of 1,3,2-benzodioxaborolyl derivatives of ortho-carborane.
Scheme 25. Synthesis of 1,3,2-benzodioxaborolyl derivatives of ortho-carborane.
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Figure 21. Single crystal structures of 1-(1′,3′,2′-O2BC6H4-2′-)-1,2-C2B10H11 (left), 1-(1′,3′,2′-O2BC6H4-2′-)-2-Me-1,2-C2B10H10 (middle), and 1-(1′,3′,2′-O2BC6H4-2′-)-2-Ph-1,2-C2B10H10 (bottom). Hydrogen atoms of methyl and phenyl groups are omitted for clarity.
Figure 21. Single crystal structures of 1-(1′,3′,2′-O2BC6H4-2′-)-1,2-C2B10H11 (left), 1-(1′,3′,2′-O2BC6H4-2′-)-2-Me-1,2-C2B10H10 (middle), and 1-(1′,3′,2′-O2BC6H4-2′-)-2-Ph-1,2-C2B10H10 (bottom). Hydrogen atoms of methyl and phenyl groups are omitted for clarity.
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Figure 22. Single crystal structure of 1-(1′,3′,2′-O2BC6H4-2′-)-7-Ph2P-1,7-C2B10H10. Hydrogen atoms of phenyl groups are omitted for clarity.
Figure 22. Single crystal structure of 1-(1′,3′,2′-O2BC6H4-2′-)-7-Ph2P-1,7-C2B10H10. Hydrogen atoms of phenyl groups are omitted for clarity.
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Scheme 26. Synthesis of carborane-fused six- and five-membered heterocycles.
Scheme 26. Synthesis of carborane-fused six- and five-membered heterocycles.
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Figure 23. Single crystal structures of 1,2-μ-(1′,2′-C6H4(BNiPr2)2)-1,2-C2B10H10 (left) and 1,2-μ-(Me2NB)3-1,2-C2B10H10 (right). Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 23. Single crystal structures of 1,2-μ-(1′,2′-C6H4(BNiPr2)2)-1,2-C2B10H10 (left) and 1,2-μ-(Me2NB)3-1,2-C2B10H10 (right). Hydrogen atoms of alkyl groups are omitted for clarity.
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Scheme 27. Synthesis of the N-heterocyclic carbene-stabilized carborane-fused azaborole 1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10.
Scheme 27. Synthesis of the N-heterocyclic carbene-stabilized carborane-fused azaborole 1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10.
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Figure 24. Single crystal structures of 1,2-μ-Br2BN(Dipp)=CtBu-1,2-C2B10H10 (top left), 1,2-μ-(AcO)2BN(Dipp)=CtBu-1,2-C2B10H10 (top right), 1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10 (bottom left), and the [1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10]•+ radical (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 24. Single crystal structures of 1,2-μ-Br2BN(Dipp)=CtBu-1,2-C2B10H10 (top left), 1,2-μ-(AcO)2BN(Dipp)=CtBu-1,2-C2B10H10 (top right), 1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10 (bottom left), and the [1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10]•+ radical (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 28. Synthesis of carborane-fused azaborolyl radical cation [1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10]•+.
Scheme 28. Synthesis of carborane-fused azaborolyl radical cation [1,2-μ-(MeIiPr)BN(Dipp)CtBu-1,2-C2B10H10]•+.
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Figure 25. Single crystal structure 7,8-μ-(MeIiPr)BN(Dipp)CtBu-7,8-C2B9H9. Hydrogen atoms of organic substituents are omitted for clarity.
Figure 25. Single crystal structure 7,8-μ-(MeIiPr)BN(Dipp)CtBu-7,8-C2B9H9. Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 29. Synthesis of the carbene-stabilized carborane-fused borirane 1,2-μ-DippN=(tBu)C(MeIiPr)B-1,2-C2B10H10.
Scheme 29. Synthesis of the carbene-stabilized carborane-fused borirane 1,2-μ-DippN=(tBu)C(MeIiPr)B-1,2-C2B10H10.
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Figure 26. Single crystal structures of 1,2-μ-DippN=(tBu)C(MeIiPr)B-1,2-C2B10H10 (top left), 1-cyclo-(Dipp)N=(tBu)C(MeIiPr)B-2-ClCu-1,2-C2B10H10 (top right), 1,2-μ-DippN=(tBu)C(MeIiPr)BS-1,2-C2B10H10 (bottom left), and 1-DippN=(tBu)C(MeIiPr)ClB-1,2-C2B10H11 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 26. Single crystal structures of 1,2-μ-DippN=(tBu)C(MeIiPr)B-1,2-C2B10H10 (top left), 1-cyclo-(Dipp)N=(tBu)C(MeIiPr)B-2-ClCu-1,2-C2B10H10 (top right), 1,2-μ-DippN=(tBu)C(MeIiPr)BS-1,2-C2B10H10 (bottom left), and 1-DippN=(tBu)C(MeIiPr)ClB-1,2-C2B10H11 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 30. Reactions of the carborane-fused borirane 1,2-μ-DippN=(tBu)C(MeIiPr)B-1,2-C2B10H10.
Scheme 30. Reactions of the carborane-fused borirane 1,2-μ-DippN=(tBu)C(MeIiPr)B-1,2-C2B10H10.
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Scheme 31. Synthesis and reactions of the carborane-fused borirane 1,2-μ-(Idipp)BrB-1,2-C2B10H10.
Scheme 31. Synthesis and reactions of the carborane-fused borirane 1,2-μ-(Idipp)BrB-1,2-C2B10H10.
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Figure 27. Single crystal structures of 1,2-μ-(Idipp)(TfO)B-1,2-C2B10H10 (left) and 1-(Idipp)(O)B-2-CF3O2S-1,2-C2B10H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 27. Single crystal structures of 1,2-μ-(Idipp)(TfO)B-1,2-C2B10H10 (left) and 1-(Idipp)(O)B-2-CF3O2S-1,2-C2B10H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 32. Reactions of carboranyl oxoborane 1-O=(Idipp)B-2-CF3O2S-1,2-C2B10H10.
Scheme 32. Reactions of carboranyl oxoborane 1-O=(Idipp)B-2-CF3O2S-1,2-C2B10H10.
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Scheme 33. Synthesis of carborane-fused boriranes 1,2-μ-RR’NB-1,2-C2B10H10.
Scheme 33. Synthesis of carborane-fused boriranes 1,2-μ-RR’NB-1,2-C2B10H10.
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Figure 28. Single crystal structures of 1-Me3SiN≡B-2-Me3Si-C2B10H10 (top left), 1,2-μ-(Me3Si)tBuNB-1,2-C2B10H10 (top right), 1-(cyclo-H2CMe2SitBuN=B)-1,2-C2B10H11 (bottom left), and 1,2-μ-Me2SitBuN(Me/Cl)B-1,2-C2B10H10 (bottom right).
Figure 28. Single crystal structures of 1-Me3SiN≡B-2-Me3Si-C2B10H10 (top left), 1,2-μ-(Me3Si)tBuNB-1,2-C2B10H10 (top right), 1-(cyclo-H2CMe2SitBuN=B)-1,2-C2B10H11 (bottom left), and 1,2-μ-Me2SitBuN(Me/Cl)B-1,2-C2B10H10 (bottom right).
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Scheme 34. THF-promoted rearrangement of the carborane-fused borirane 1,2-μ-RR’NB-1,2-C2B10H10.
Scheme 34. THF-promoted rearrangement of the carborane-fused borirane 1,2-μ-RR’NB-1,2-C2B10H10.
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Figure 29. Single crystal structures of 1-Me3SiN≡B-2-Me3Si-C2B10H10 (left) and 1-t-BuN≡B-2-Me3Si-C2B10H10 (right). Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 29. Single crystal structures of 1-Me3SiN≡B-2-Me3Si-C2B10H10 (left) and 1-t-BuN≡B-2-Me3Si-C2B10H10 (right). Hydrogen atoms of alkyl groups are omitted for clarity.
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Figure 30. Single crystal structures of the products of dimerization of 1-Me3SiN≡B-2-Me3Si-C2B10H10 in the presence of t-BuCN. Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 30. Single crystal structures of the products of dimerization of 1-Me3SiN≡B-2-Me3Si-C2B10H10 in the presence of t-BuCN. Hydrogen atoms of alkyl groups are omitted for clarity.
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Scheme 35. Reaction of 1,2-μ-(Me3Si)2NB-1,2-C2B10H10 with N-heterocyclic carbene MeIiPr.
Scheme 35. Reaction of 1,2-μ-(Me3Si)2NB-1,2-C2B10H10 with N-heterocyclic carbene MeIiPr.
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Figure 31. Single crystal structure of the products of dimerization of 1-Me3SiN=(MeIiPr)B-2-Me3Si-1,2-C2B10H10. Hydrogen atoms of alkyl groups are omitted for clarity.
Figure 31. Single crystal structure of the products of dimerization of 1-Me3SiN=(MeIiPr)B-2-Me3Si-1,2-C2B10H10. Hydrogen atoms of alkyl groups are omitted for clarity.
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Scheme 36. Synthesis of the carborane-fused borirane 1,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10.
Scheme 36. Synthesis of the carborane-fused borirane 1,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10.
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Figure 32. Single crystal structures of 1,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10 (left) and 1,2-μ-(Et3PO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 32. Single crystal structures of 1,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10 (left) and 1,2-μ-(Et3PO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 (right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 37. Ring-expansion reactions of 1,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10.
Scheme 37. Ring-expansion reactions of 1,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10.
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Figure 33. Single crystal structures of 1,2-μ-(PhHCO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 (top left), 1,2-μ-(o-C6H5=(Ph)CO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10, (top right), 1,2-μ-(PhC=N(2′,6′-Mes2C6H3)B)-1,2-C2B10H101,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10 (bottom left), and 1,2-μ-(MeC=N(2′,6′-Mes2C6H3)B)-1,2-C2B10H101,2-μ-(Et3PO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
Figure 33. Single crystal structures of 1,2-μ-(PhHCO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 (top left), 1,2-μ-(o-C6H5=(Ph)CO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10, (top right), 1,2-μ-(PhC=N(2′,6′-Mes2C6H3)B)-1,2-C2B10H101,2-μ-(2′,6′-Mes2C6H3B)-1,2-C2B10H10 (bottom left), and 1,2-μ-(MeC=N(2′,6′-Mes2C6H3)B)-1,2-C2B10H101,2-μ-(Et3PO(2′,6′-Mes2C6H3)B)-1,2-C2B10H10 (bottom right). Hydrogen atoms of organic substituents are omitted for clarity.
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Scheme 38. Synthesis of bis(carboranyl) analogs of 9-borafluorene 2,2′-μ-XB-1,1′-bis(ortho-carborane) (X = Cl, Br, Ph, Mes).
Scheme 38. Synthesis of bis(carboranyl) analogs of 9-borafluorene 2,2′-μ-XB-1,1′-bis(ortho-carborane) (X = Cl, Br, Ph, Mes).
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Figure 34. Single crystal structures of μ-2,2′-ClB-1,1′-bis(ortho-carborane) (top left), μ-2,2′-BrB-1,1′-bis(ortho-carborane) (top right), and the [μ-2,2′-Br2B-1,1′-bis(ortho-carborane)]− anion (bottom).
Figure 34. Single crystal structures of μ-2,2′-ClB-1,1′-bis(ortho-carborane) (top left), μ-2,2′-BrB-1,1′-bis(ortho-carborane) (top right), and the [μ-2,2′-Br2B-1,1′-bis(ortho-carborane)]− anion (bottom).
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Figure 35. Single crystal structures of μ-2,2′-Et2O(Cl)B-1,1′-bis(ortho-carborane) (left) and μ-2,2′-Et2O(Br)B-1,1′-bis(ortho-carborane) (right). The hydrogen atoms of Et2O are omitted for clarity.
Figure 35. Single crystal structures of μ-2,2′-Et2O(Cl)B-1,1′-bis(ortho-carborane) (left) and μ-2,2′-Et2O(Br)B-1,1′-bis(ortho-carborane) (right). The hydrogen atoms of Et2O are omitted for clarity.
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Figure 36. Single crystal structures of μ-2,2′-PhB-1,1′-bis(ortho-carborane) (left) and μ-2,2′-MesB-1,1′-bis(ortho-carborane) (right).
Figure 36. Single crystal structures of μ-2,2′-PhB-1,1′-bis(ortho-carborane) (left) and μ-2,2′-MesB-1,1′-bis(ortho-carborane) (right).
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Figure 37. Single crystal structure of μ-2,2′-Et2O·(Ph)B-1,1′-bis(ortho-carborane). The hydrogen atoms of Et2O are omitted for clarity.
Figure 37. Single crystal structure of μ-2,2′-Et2O·(Ph)B-1,1′-bis(ortho-carborane). The hydrogen atoms of Et2O are omitted for clarity.
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Figure 38. Single crystal structure of the [μ-2,2′-Ph(Br)B-1,1′-bis(ortho-carborane)]− anion.
Figure 38. Single crystal structure of the [μ-2,2′-Ph(Br)B-1,1′-bis(ortho-carborane)]− anion.
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Figure 39. Single crystal structure of (Ph3PSiEt3)+[2,2′-μ-Ph(H)B-1,1′-bis(ortho-carborane)]−. The hydrogen atoms of the ethyl groups are omitted for clarity.
Figure 39. Single crystal structure of (Ph3PSiEt3)+[2,2′-μ-Ph(H)B-1,1′-bis(ortho-carborane)]−. The hydrogen atoms of the ethyl groups are omitted for clarity.
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Scheme 39. Synthesis of 2,2′-μ-(i-Pr)2NB-1,1′-bis(ortho-carborane) and 2,2′-μ-(i-Pr)2NB-1,1′-bis(ortho-carborane-8,9,10,12-Me4).
Scheme 39. Synthesis of 2,2′-μ-(i-Pr)2NB-1,1′-bis(ortho-carborane) and 2,2′-μ-(i-Pr)2NB-1,1′-bis(ortho-carborane-8,9,10,12-Me4).
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Figure 40. Single crystal structure of 2,2′-μ-(i-Pr)2NB-1,1′-bis(ortho-carborane). The hydrogen atoms of the isopropyl groups are omitted for clarity.
Figure 40. Single crystal structure of 2,2′-μ-(i-Pr)2NB-1,1′-bis(ortho-carborane). The hydrogen atoms of the isopropyl groups are omitted for clarity.
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Scheme 40. Synthesis of the bis(carboranyl) analog 9,10-diboraanthracene 1,1′,2,2′-μ-(XB)2-(ortho-carborane)2 (X = Cl, Br, N3, Me, Ph).
Scheme 40. Synthesis of the bis(carboranyl) analog 9,10-diboraanthracene 1,1′,2,2′-μ-(XB)2-(ortho-carborane)2 (X = Cl, Br, N3, Me, Ph).
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Figure 41. Single crystal structures of 1,1′,2,2′-μ-(ClB)2-(ortho-carborane)2 (left) and 1,1′,2,2′-μ-(BrB)2-(ortho-carborane)2 (right).
Figure 41. Single crystal structures of 1,1′,2,2′-μ-(ClB)2-(ortho-carborane)2 (left) and 1,1′,2,2′-μ-(BrB)2-(ortho-carborane)2 (right).
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Figure 42. Single crystal structures of 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2 (left) and 1,1′,2,2′-μ-(MeCN(N3)B)2-(ortho-carborane)2 (right).
Figure 42. Single crystal structures of 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2 (left) and 1,1′,2,2′-μ-(MeCN(N3)B)2-(ortho-carborane)2 (right).
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Figure 43. Single crystal structures of 1,1′,2,2′-μ-(MeB)2-(ortho-carborane)2 (left) and 1,1′,2,2′-μ-(PhB)2-(ortho-carborane)2 (right).
Figure 43. Single crystal structures of 1,1′,2,2′-μ-(MeB)2-(ortho-carborane)2 (left) and 1,1′,2,2′-μ-(PhB)2-(ortho-carborane)2 (right).
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Figure 44. Single crystal structure of (Et3Si)2[1,1′,2,2′-μ-(H2B)2-(ortho-carborane)2]. The hydrogen atoms of the ethyl groups are omitted for clarity.
Figure 44. Single crystal structure of (Et3Si)2[1,1′,2,2′-μ-(H2B)2-(ortho-carborane)2]. The hydrogen atoms of the ethyl groups are omitted for clarity.
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Scheme 41. Reactions of 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2.
Scheme 41. Reactions of 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2.
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Figure 45. Single crystal structure of 1,1′,2,2′-μ-(Ph3P=N=B)2-(ortho-carborane)2. The hydrogen atoms of the phenyl groups are omitted for clarity.
Figure 45. Single crystal structure of 1,1′,2,2′-μ-(Ph3P=N=B)2-(ortho-carborane)2. The hydrogen atoms of the phenyl groups are omitted for clarity.
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Figure 46. Single crystal structures of 1,1′,2,2′-μ-(Et3Si(N2)NHB)2-(ortho-carborane)2 (left) and 1,1′,2,2′-μ-(PhMe2Si(N2)NHB)2-(ortho-carborane)2 (right). Hydrogen atoms of alkyl and aryl groups are omitted for clarity.
Figure 46. Single crystal structures of 1,1′,2,2′-μ-(Et3Si(N2)NHB)2-(ortho-carborane)2 (left) and 1,1′,2,2′-μ-(PhMe2Si(N2)NHB)2-(ortho-carborane)2 (right). Hydrogen atoms of alkyl and aryl groups are omitted for clarity.
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Figure 47. Single crystal structure of 1,1′,2,2′-μ-(PhMe2SiNHB)2-(ortho-carborane)2. The hydrogen atoms of the phenyl groups are omitted for clarity.
Figure 47. Single crystal structure of 1,1′,2,2′-μ-(PhMe2SiNHB)2-(ortho-carborane)2. The hydrogen atoms of the phenyl groups are omitted for clarity.
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Scheme 42. Reactions of 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2.
Scheme 42. Reactions of 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2.
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Scheme 43. Reaction of 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2 with Et3GeH.
Scheme 43. Reaction of 1,1′,2,2′-μ-(N3B)2-(ortho-carborane)2 with Et3GeH.
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Figure 48. Single crystal structure of 1,1′,2,2′-μ-(Et3Ge(N2)NHB)2-(ortho-carborane)2. Hydrogen atoms of the ethyl groups are omitted for clarity.
Figure 48. Single crystal structure of 1,1′,2,2′-μ-(Et3Ge(N2)NHB)2-(ortho-carborane)2. Hydrogen atoms of the ethyl groups are omitted for clarity.
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Scheme 44. Synthesis of 3-pinB-1,2-C2B10H11 and 3,6-(pinB)2-1,2-C2B10H10.
Scheme 44. Synthesis of 3-pinB-1,2-C2B10H11 and 3,6-(pinB)2-1,2-C2B10H10.
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Figure 49. Single crystal structures of 3-pinB-1,2-C2B10H11 (left) and 3,6-(pinB)2-1,2-C2B10H10 (right). Hydrogen atoms of the methyl groups are omitted for clarity.
Figure 49. Single crystal structures of 3-pinB-1,2-C2B10H11 (left) and 3,6-(pinB)2-1,2-C2B10H10 (right). Hydrogen atoms of the methyl groups are omitted for clarity.
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Scheme 45. Synthesis of 4-pinB-1,2-C2B10H11 and 9-pinB-1,2-C2B10H11.
Scheme 45. Synthesis of 4-pinB-1,2-C2B10H11 and 9-pinB-1,2-C2B10H11.
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Figure 50. Single crystal structure of 4-pinB-1,2-C2B10H11. Hydrogen atoms of the methyl groups are omitted for clarity.
Figure 50. Single crystal structure of 4-pinB-1,2-C2B10H11. Hydrogen atoms of the methyl groups are omitted for clarity.
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Scheme 46. Synthesis of 2-pinB-1,7-C2B10H11 and 9-pinB-1,7-C2B10H11.
Scheme 46. Synthesis of 2-pinB-1,7-C2B10H11 and 9-pinB-1,7-C2B10H11.
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Figure 51. Single crystal structures of 2-pinB-1,7-C2B10H10 (left) and 9-pinB-1,7-C2B10H10 (right). Hydrogen atoms of the methyl groups are omitted for clarity.
Figure 51. Single crystal structures of 2-pinB-1,7-C2B10H10 (left) and 9-pinB-1,7-C2B10H10 (right). Hydrogen atoms of the methyl groups are omitted for clarity.
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Sivaev, I.B. Combining Two Types of Boron in One Molecule (To the 60th Anniversary of the First Synthesis of Carborane). Chemistry 2023, 5, 834-885. https://doi.org/10.3390/chemistry5020059

AMA Style

Sivaev IB. Combining Two Types of Boron in One Molecule (To the 60th Anniversary of the First Synthesis of Carborane). Chemistry. 2023; 5(2):834-885. https://doi.org/10.3390/chemistry5020059

Chicago/Turabian Style

Sivaev, Igor B. 2023. "Combining Two Types of Boron in One Molecule (To the 60th Anniversary of the First Synthesis of Carborane)" Chemistry 5, no. 2: 834-885. https://doi.org/10.3390/chemistry5020059

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