Icosahedral Carboranes

Chapter 8

Icosahedral Carboranes: Closo-CB11 Clusters 8.1

OVERVIEW

The prototype boron clusters, the extremely stable B12H12 2
dianion and its neutral dicarbon counterparts 1,2-, 1,7-, and 1,12-C2B10H12 (o-, m-, and p-carborane respectively), are 26-electron icosahedral systems that are closely related to elemental boron, all of whose allotropic forms incorporate B12 icosahedra. Their isoelectronic analogue CB11 H12
is similarly resistant to cage degradation and offers important synthetic advantages: unlike B12 H12 2
, it is polar and can be selectively functionalized at its carbon vertex or at three distinct boron sites, and in contrast to the electrically neutral C2B10H12 isomers, it is highly soluble in water. In recent years CB11 H12
, and especially its B-polyhalogenated derivatives, have assumed an increasingly important role as weakly coordinating anions that can form stable salts with extremely electrophilic cations such as R3Si+, protonated benzene, C60  + , H3O+, and other elusive species, as discussed later in this chapter. In addition, the water-solubility and stability of CB11 H12
derivatives in aqueous media make them potentially useful in boron neutron capture therapy (BNCT). These and other evolving applications of monocarbon carborane chemistry are elaborated in Chapters 16 and 17. Comprehensive reviews of the chemistry of CB11 H12
[1,318] and related neutral CB11 cluster systems [2] have been published.

8.2 8.2.1

SYNTHESIS AND STRUCTURE Parent CB11 H12


The original synthesis by Knoth [3,4] involved either disproportionation of Cs+nido-7-CB10 H13
(prepared from B10H14 as described in Section 7.2) or boron insertion using triethylamine-borane: 300
320°C



CB10 H
13 ƒƒƒƒƒƒ! CB10 H11 + CB11 H12 + 2H2 Et3 NBH3


CB10 H
13 ƒƒƒƒƒƒ! CB11 H12 180°C

The latter approach was improved by Czech workers [5], who found that heating 7-(Me3N)CB10H12 with Et3NBH3 at 180-200 °C gives (Me2NH)CB11H11, which is methylated to produce (Me3N)CB11H11; reduction of the latter compound with sodium in liquid ammonia affords the CB11 H12
anion. A more recent, and important, synthesis is based on the conversion of B11 H14
(prepared [6] from the inexpensive bulk chemical NaBH4) to CB11 species via carbon insertion [7,8]: CHCl3


B11 H
14 ƒƒƒƒƒ! CB11 H12

In a different type of carbon insertion, the reaction of HC^CdC5H4N with closo-B11 H11 2
yields a 1-(NC5H4-CH2)CB11H10 product in which one carbon is incorporated into the cage and the pyridyl nitrogen is bound to a boron vertex [9]. Closo-CB11 derivatives are also formed via Lewis base-promoted extraction of a cage carbon atom from 13-vertex C2B11 clusters as described in Section 11.5.1.2. For example, treatment of 13-vertex 1,2-(CH2)4-1,2-C2B11H11 with (4-MeC6H4)SNa affords [m-1,2-(CH2)4CHS(4-MeC6H4)-1-CB11H10]
. The extracted carbon, located at the end of the polymethylene chain, binds to the B(2) adjacent to the remaining cage carbon, forming an exocyclic C5 unit [283].

Carboranes. http://dx.doi.org/10.1016/B978-0-12-801894-1.00008-1 Copyright © 2016 Elsevier Inc. All rights reserved.

249

250

Carboranes

The icosahedral structure of the CB11 H12
ion (slightly distorted from ideal Ih symmetry), shown in Figure 1-2, is well established from X-ray diffraction, NMR, and other spectroscopic studies on the parent species and on a number of derivatives (Table 8-1) [1,10]. In some of its salts, BdHX hydrogen-bonding interactions are found between the carborane and the associated cation (even forming 3-D networks) [11], while in others such bonding is effectively absent [12]. The CB11 H12
cluster can function as an electron donor, for example, forming a charge-transfer complex with a methyl viologen [13]. The nature and extent of cation-CB11 H12
interactions is significant because of the interest in this carborane and its derivatives as weakly coordinating anions, a topic that is further developed below and in Chapter 17.

8.2.2

Substitution at Carbon

The acidity of the CH proton in CB11 H12
is comparable to that of acetylene, and C-lithiation via treatment with n-butyllithium followed by reaction with electrophiles such as EtBr, Ph3SiCl, CF3I, and Ph2PCl, generates RCB11 H11
products, where R is an alkyl, silyl, phosphino, carboxylic acid, hydroxy, alcohol, alkenyl, aryl, or other group (Table 8-1) [14–18,294]. The method also works well with most B-substituted derivatives of CB11 H12
[4,17,19–21], and can be employed to prepare B-perhalogenated 1-(NC)CB11 X11
anions (X ¼ F, Cl, Br) [22]. High-yield syntheses of a wide range of RCB11 H5 Br6
derivatives from the C-lithio anion have been described [185]. Manipulation of the C-amino groups in zwitterionic (Me2NR+)CB11 H11
compounds, where R is H or Me, produces other C-substituted species. For example, (Me2NH)CB11H11 is easily demethylated with I2 in alkaline solution to give (MeNH2)CB11H11, which, in turn, can be treated with formaldehyde and I2 to produce (NMe]CHOH)CB11 H11
[23]. Deamination of (H3N)CB11H11 with nitrous acid leads instead to (HO)CB11 H11
, which can be methylated to produce (MeO)CB11 H11
. If excess SMe2 is present, reaction of (H3N)CB11H11 with nitrous acid forms (Me2S)CB11H11 in good yield [15]. The versatile (Me2NH)CB11H11, a stable solid that is soluble in polar solvents, can also be methylated, ethylated, and benzoylated to generate (Me3N)CB11H11, (EtMeNH)CB11H11, and [Ph(OH)C]NMe]CB11 H11
, respectively [23]. The compounds (MenNH3-n)CB11H11 (n ¼ 0-2) are weak acids (pKa  6 in 50% ethanol) and readily lose an amino proton to form the corresponding anion. C-aryl derivatives can be prepared via several routes [24]. As described in Section 6.2, the reaction of B10H14 and PhCHO gives nido-6-PhCB9 H11
. Heating this 10-vertex species with Me3NBH3 at 210 °C generates a 2:1 mixture of the Et4N+ salts of PhCB11 H11
and 4
PhCB9 H9
that can be separated on an ion exchange column by conversion to their Cs+ salts, followed by removal of the much less soluble Cs+ PhCB9 H9
via fractional crystallization from cold water [25]. Alternatively, if Me2SBH3 is employed as the boronating agent in a Cl2C2H4 solution, Et4N+ PhCB11 H11
is obtained in 92% yield [26]. The stoichiometry of these cage-expansion reactions can be represented as
PhCB9 H
11 + 2LBH3 ! PhCB11 H11 + 3H2 + 2L

Complexation of N-heterocyclic carbenes (NHC’s) with CB11 H12
ions leads to symmetric NHC-bridged biscarboranyl species (e.g., 8-1a) [313]. In an extension of this work, the unsymmetrical zwitterionic imidazolium NHC 8-1b was prepared and deprotonated to afford the monoanion 8-1c, while double deprotonation gave the dianion 8-1d, with the latter two species characterized by X-ray crystallography [339]. Taken together, these studies demonstrate the reaction-directing role of the monocarbon carborane substituents. 2 C

N

N

N

C

8-1a

N

C

8-1c 2

+ N

N

8-1b

C

N

N

8-1d

C

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

251

In a very different approach, treatment of B11 H14
with arylhalocarbenes (p-XC6H4)CCl: (X ¼ H, F, Cl, Br, I, Ph) achieves carbon atom insertion to form (p-XC6H4)CB11 H11
ions [7,27]. The PhCB11 H11
ion has also been obtained via palladium-catalyzed cross-coupling of phenyl triflate with CIZnCB11 H11
[8]. Copper-mediated C-C cross-coupling has been developed as a versatile route to aryl, carboxyl, halo, alkenyl, alkynyl, thio, and other C-substituted RCB11 H12
derivatives (Table 8-1) [296,317]. A general route to RCB11H10-12-R
and HCB11H9-2-R-12-R0
difunctional species, in which one substituent is bound either to the cage carbon or to B(2) and the other is at the antipodal B(12) vertex, entails iodination at B(12) followed by microwave-assisted Kumada crosscoupling [305]. Microwave-assisted syntheses of HCB11H5I6
and HCB11I11
have also been reported [352]. C-halo derivatives, XCB11 H11
, can be prepared by treating the parent CB11 H12
or CuCB11 H11
with N-halosuccinimides (for X ¼ Cl, Br, and I) and N-fluorobis(benzenesulfonyl) amine (for X ¼ F) [28]; under carefully controlled conditions, the XCB11 H11
products are obtained in high yields. An improved method for the synthesis of the chloro, bromo, and iodo compounds, using methyl triflate in sulfolane in the presence of CaH to extract the triflic acid formed, has been reported [29,30]. This technique also allows the synthesis of XCB11 Me11 - D6
anions (X ¼ Cl, Br, I) from HCB11 Me11
salts, allowing easy access to C-alkenyl derivatives that undergo O2-promoted polymerization, as described below [20,31]. C-alkylation of HCB11Cl11
to give RCB11Cl11
products (R ¼ Me, Et, n-C4H9, n-C6H13) can be achieved in high yield by reaction with KO(CMe3) and RI reagents [277].

8.2.3

Substitution at Boron by Electrophilic Reagents

The introduction of exo-polyhedral atoms or groups at specific boron locations in CB11 H12
is controlled by the charge distribution in the cage skeleton, which gives rise to three different vertex types: the “upper belt” borons adjacent to carbon [B(2)-B(6)], the “lower belt” [B(7)-B(11)], and the unique B(12) that occupies a location antipodal to the carbon atom. As the negative charge increases with increasing distance from the CH vertex, B(12)-H is most negative and hence most susceptible to electrophilic attack, followed by the adjacent (lower-belt) BH groups; the upper-belt borons nearest the carbon are much less reactive, but can nevertheless be functionalized under suitable conditions. This pattern has been clearly revealed by MO calculations, 11B NMR chemical shifts, deuteration experiments, and reactivity studies involving halogens and other electrophiles. Thus, electrophilic deuteration of the parent carborane with DCl/D2O, monitored by 11B NMR, proceeds in the order B(12) > B(7-11), to give CB11H6-7,8,9,10,11,12
D6
as the final product [32]. The treatment of CB11 H12
with Br2 or N-bromosuccinimide (NBS) affords CB11H11-12-Br
; with an excess of NBS, CB11H107,12-Br2
is also obtained [14]. Chlorination with N-chlorosuccinimide (NCS) in a 1:1 mole ratio yields CB11H11-12Cl
, with the 7,12-dichloro product also forming if excess NCS is present. HCB11H10-n-X
anions X ¼ F, Cl; (n ¼ 7, 12) can be converted to the ammoniated derivatives HCB11H10-n-NH3 via microwave-assisted reaction with LiN(SiMe3)2; methylation of the 12-NH3 species yields the zwitterion HCB11H10-12NMe3 [287]. Similarly, microwave-promoted cross-coupling of the parent anion with CuCN generates the B(12)-CN derivative as well as the HCB11H9-7,12-ðCNÞ2
ion, all in high yield [297]. Reaction of the HCB11H10-12-CN
anion with Me3O+BF4
or MeOTf generates the ylide 1-HCB11 H10
-12-CNMe2 + , which on treatment with NH3 gives 1-HCB11 H10
-12-C(NHMe)¼NH2 + [331]. Hydrolysis of HCB11H9-7,12-ðCNÞ2
with HCl/acetic acid affords the B(7,12)-dicarboxylic acid, which is the only known carborane with carboxyl groups occupying adjacent vertexes [333]. Its weak acidity (pKa > 6.35) is as expected, given the electron-donating character of boron toward attached substituents. The parent anion combines with I2 to generate CB11H11-12-I
[15,33], which in turn serves as a precursor to 12-alkyl derivatives via palladium-cross-coupling (Figure 8-1A) [33]. The tropylium ylide CB11 H11
-12-C7 H6 + , a highly polar molecule (11.2 D) of interest in nonlinear optical (NLO) applications [34] (Chapter 17), is obtained directly from CB11 H12
via reaction with C7 H7 + ion, as shown in Figure 8-1B [33]. The Pd-catalyzed amination of 1-RCB11H11-12I
anions affords the amino derivatives 1-RCB11H11-12-NH2 -- , which can be diazotized to give the corresponding B(12)-pyridinium zwitterions 1-RCB11H10-12-(p-MeOC5H4N) [324], listed in Table 8-1. Palladium-cross-coupling reactions have also been employed to prepare 7-alkynyl and 7,12-dialkynyl derivatives (Table 8-1) from their corresponding iodo- and diiodo-carboranyl precursors [35,36]. More generally, Pd-catalyzed microwave-assisted cross-coupling with RCB11H10-12-I
anions and Grignard reagents affords a wide variety of B(12)-substituted alkyl, alkenyl, alkynyl, alkoxy, silyl, and other derivatives (Table 8-1) [272]. A new method developed by Kaszynski and Ringstrand employs 10-B-aryliodinium zwitterions, 1-RCB11H10
-12-IPh+ [R ¼ H, C5H11] which react with nucleophiles to give B(10)-substituted carbonitrile, pyridinium, sulfonium, thiol, actoxy, and amino derivatives [343]. The same approach also works with closo-RCB9H10
and 3-CoðC2 B9 H11 Þ2
anions.

252

Carboranes

H C

H 1

C

6

2

5 11

4 10

8

9 12

I

A

RMgX/Pd(PPh3)2

3

THF

7

R = Me, Et, n-C4H9, n-C6H13, Ph

CB10H11-12-R− R

CB10H11-12-I−

H

H C

C

+ H2O/toluene

CB10H−12

CB10H−11-12-C7H6+

+ B FIGURE 8-1 Synthesis of CB11H11-12-R derivatives (in Chapters 8 through 17, carborane structures are depicted in the style of Figure 8-1, with unlabeled vertices representing B or BH units unless otherwise indicated.).

Derivatives of CB11 H12
that are functionalized at the antipodal 1 and 12 positions have received special attention because their linear “carborod” [37] geometry (8-2) invites possible application in NLO materials, liquid crystals, conducting polymers, and other areas. A number of these have been characterized (Table 8-1) and include homosubstituted (L1 ¼ L2) species and heterosubstituted (L1 6¼ L2) systems; the ligands can be aryl, alkyl, alkenyl, alkynyl, silyl, halo, amino, metallo, or other groups [33,38–44]. Measurements of pKa values for [HO(O)C]CB11H10-12-X
anions, where X is an electron donor or acceptor (Table 8-1), have been used to estimate the efficiency of transmission of electronic effects through the cage, which corresponds to 65% of the corresponding value for a benzene ring [276]. Addition insight is gained from detailed experimental and theoretical studies of 1-alkynyl-12-R bifunctional derivatives where R is a carboxyl, cyano, isocyano, amido, or other group [322]. In general, 1, 12-disubsituted derivatives are prepared by the introduction of an L2 moiety at B(12) on an L1CB11 substrate, as in the preparation of the 1,12-diphenyl derivative from Cs+ PhCB11 H11
[40]:

8-2 L1

MeCðOÞOH

C

L2

PhMgBr



PhCB11 H
11 + I2 ƒƒƒƒƒƒ! PhCB11 H10 -12-I ƒƒƒƒƒƒ! PhCB11 H10 -12-Ph PdCl2 ðPPh3 Þ2

Fluorinated derivatives of CB11 H12
are obtained by treating it with anhydrous HF [45] or N-fluoro reagents [46]. At 23 °C with anhydrous liquid HF, CB11H11-12-F is obtained almost quantitatively, while higher temperatures yield CB11H10-n, 12-F2 (n ¼ 2,7) and CB11H9-7,9,12-F3, the latter product being formed at 180 °C. Electrophilic B-substitution of CB11 H12
with H2NOSO3H proceeds according to an apparently different mechanism, affording CB11H11-7-NMe3 as the main product, together with a minor amount of the expected 12-NMe3 isomer [32]; this somewhat anomalous result has apparently not been further explored.

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

8.2.4

253

Synthesis of B-Substituted Derivatives via Boron Insertion

As mentioned at the start of this section, parent CB11 H12
can be obtained from smaller boron clusters by incorporation of boron. This approach can be extended to generate CB11 derivatives bearing substituents at different locations, either by employing starting carboranes that are already B-functionalized, or by using RBX2 as the attacking reagent. Examples of the former method include the conversion of 11-vertex closo-2-(Me3N)CB10H8IX (X ¼ I, CH2]CHCH2) to (Me3N) CB11H9-2-Ph-8-X products [43] and the cage-expansion of nido-(BrC6H4)CB9H11 by reaction with Me2SBH3 and MeC6H4MgBr to give (MeC6H4-C6H4)CB11H10-12-C6H4Me [38]. Insertion using RBX2 reagents, where R is Ph, C6H4Me, F, O(CH2)4Cl, NH2, or NMe3, has been employed in the synthesis of CB11H11-2-R
products from CB10 H11 3
salts [47,48,250]. An interesting illustration of this method is the reaction of nido-7-(Me3N)CB10H10 with (Me2CH)2NBCl2 to form 8-3 in 19% isolated yield, in the course of which a methyl group is eliminated from the trimethylamine group and CH2 is inserted into the exo-polyhedral BdN bond [49]: NMe3

NMe2

C

C

HN(CHMe2)2

(Me2CH)2NBCl2 −2 Cl−

8-3

The reaction is proposed to proceed via a sterically induced activation of CdH in which H is transferred from a methyl group to the (Me2CH)2N unit, in the process forming an unstable intermediate that disproportionates to an amine and a carbene, generating 8-3 [49]. In this case the carborane cluster is clearly responsible for the unusual reactivity in which a carbene is inserted into a BdN bond.

8.2.5

Polyhalogenation at Boron

Electrophilic halogenation can place up to six Cl, Br, or I atoms on boron atoms in CB11 H12
or its C-substituted derivatives, with substitution usually restricted to the more negatively charged B(12) and “lower belt” B(7)-B(11) vertices (Table 8-1). A straightforward approach involves elemental halogens [15]: MeCðOÞOH


CB11 H
12 + excess Cl2 ƒƒƒƒƒƒ! CB11 H7 -7, 8, 10, 12-Cl5 20°C

MeCðOÞOH


CB11 H
12 + excess Cl2 ƒƒƒƒƒƒ! CB11 H6 -7, 8,9, 10, 11,12-Cl6 100°C

MeCðOÞOH


CB11 H
12 + excess Br2 ƒƒƒƒƒƒ! CB11 H6 -7, 8,9, 10, 11,12-Br6 80°C

MeCðOÞOH


CB11 H
12 + excess I2 ƒƒƒƒƒƒ! CB11 H10 -7, 12-I2 80°C

Further iodination can be achieved with other reagents; for example, the reaction of PhCB11 H11
with iodine monochloride generates the hexaiodo species PhCB11H5-7,8,9,10,11,12-I6
[50]. Perhalogenated species, of considerable interest as weakly coordinating anions and in other applications (Chapter 17), have been synthesized for all four halogens (Table 8-1). The B-perfluorinated anion HCB11 F11
can be prepared by direct fluorination in liquid HF at room temperature [51]: HFliq

+
Cs + CB11 H
12 + 11F2 ƒƒƒƒƒƒ! Cs HCB11 F11 + 11HF 25°C, 48h

254

Carboranes

Similar treatment of the boron-ammoniated derivatives 1-HCB11H10-n-NH3 (n ¼ 2, 7, 12) yields the corresponding B-perfluorinated 1-HCB11F10-n-NH3 compounds, which on hydrolysis are slowly deprotonated to form the 1-HCB11H10-n-NH2
anions [354]. The HCB11 F11
ion is stable in 5M aqueous acid and moderately stable in 3 M KOH, slowly converting to HCB11 ðOHÞF10
and HCB11 ðOHÞ2 F9
over many hours. NMR evidence indicates that HCB11 F11
in strong aqueous base is partially deprotonated to CB11 F11 2
[51]. This dianion, with its “bare” carbon, forms a CudCldCB11F11 coordination complex that features a linear CldCudC array, illustrating the ability of monocarbon carboranes to coordinate to transition metals [51]. The hydrolysis of (NC)CB11 F11
proceeds similarly and is faster in basic than in acidic media, with (NC) CB11F10-6-OH
being identified as an intermediate [22]. The 1-(H2N)CB11 F11
anion, on treatment with lithium diisopropylamide, undergoes a concerted extrusion of carbon from the icosahedral framework to form the 11-vertex closo-3-(NC)B11 F10 2
borane dianion in 90% yield [52]. Carbon extraction/polyhedral contraction is also observed in supra-icosahedral carboranes (Chapter 11) but is otherwise unknown in carborane chemistry as a single-step process; however, 1,2-Me2C2B10H10 can be degraded to CB9 or CB10 clusters via multistep reactions as described in Chapters 6 and 7, respectively [53,54]. The B-perchloro, B-perbromo, and B-periodo HCB11 X11
anions, as well as a number of their C-substituted analogues (RCB11 X11
, R ¼ alkyl, aryl, phosphino, or other groups), have been well characterized (Table 8-1). Their synthesis generally requires fairly stringent conditions, and several approaches have been explored [21,55,56]; however, a recently reported preparation of HCB11 Cl11
and HCB11 Br11
using SOCl2 or SbCl5 is simple, efficient, and relatively safe [57]. Another approach employs the reaction of RCB11 H11
(R ¼ H or Me) with halogenating agents in sealed tubes at elevated temperature [21]: ICl=CF3 SO3 H

ð1ÞBuLi

200°C

ð2ÞMeI



CB11 H
12 ƒƒƒƒƒƒ! HCB11 Cl11 ƒƒƒƒƒƒ! MeCB11 Cl11 Br2 =CF3 SO3 H

Br2 =CF3 SO3 H

200°C

250°C



CB11 H
12 ƒƒƒƒƒƒ! HCB11 Br11 ƒƒƒƒƒƒ! BrCB11 Br11 ð1ÞBuLi


HCB11 Br
11 ƒƒƒƒƒƒ! MeCB11 Br11 ð2ÞMeI

ICl

ICl=CF3 SO3 H

200°C

200°C



CB11 H
12 ƒƒƒƒƒƒ! HCB11 I11 ƒƒƒƒƒƒ! MeCB11 Cl11 ICl


MeCB11 H
11 ƒƒƒƒƒƒ! MeCB11 I11 220°C

The sealed-tube reactions are reported to afford the halogenated products in high yield [21], although this has been questioned [29]. It will be noted that iodine monochloride in the presence of triflic acid generates the B-perchlorinated carborane, but in the absence of triflic acid only the B-iodinated species is obtained. This finding is interpreted in terms of electrophilic substitution to give HCB11 I11
, followed by triflic acid-promoted nucleophilic replacement of the I substituents by Cl to form HCB11 Cl11
[21]. MeCB11 H11
is similarly per-B-iodinated by ICl, but when triflic acid is present the main product is the hexachloro-pentaiodo derivative, MeCB11 Cl6 I5
, suggesting that nucleophilic chlorination is partially inhibited by electron donation from the methyl group. The BrCB11 Br11
ion is the only fully halogenated CB11 carborane to be characterized thus far; it is quantitatively converted to HCB11 Br11
on treatment with aqueous AgNO3 [21]. The sealed-tube method affords an efficient route to mixed-halogen species of the type HCB11 X6 Y5
(X, Y ¼ Cl, Br, I), as illustrated by the synthesis of the hexabromo-pentaiodo derivative [58]: I2 =CF3 SO3 H


HCB11 H5 -7, 8, 9,10, 11, 12-Br
6 ƒƒƒƒƒƒ! HCB11 -2, 3,4, 5, 6-I5 -7, 8,9, 10, 11,12-Br6

In general, the boron-halogen bonds in the B-perhalo anions are very unreactive, indeed nearly inert. However, basepromoted cycloaddition of organic azides with the HCB11Cl11
anion following lithiation at carbon (8-4) forms products of the type 1,2-(cyclo-N3R)CB11Cl10
(8-5) in a “click”-type reaction [289]; a competing, and dominant, process affords a Csubstituted carboranyl azide, 1-(N3)CB11H11
, which reacts with PPh3 to give a stable phosphazide [299]. Similar treatment of the polybromo species CB11Br11
and CB11H5Br6
gives only the C-azido derivatives, and no triazole product is formed [325].

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

Li

+ Li

Li

+

N N

Cl C Cl Cl

Cl Cl

Cl

Cl

N3R

Cl

Cl Cl

Cl Cl

Cl

Li

C

Cl

Cl

Li +

+

Cl Cl Cl

Cl

Cl

R

N

N Cl Cl

Cl

Cl

Cl

R

+

LiCl

Cl Cl

Cl Cl

8-4

N

N Cl C

Cl Cl

255

8-5

R = Ph, 4-FC6H4, 4-MeOC6H4, 2-MeC6H4, C6H2Me3, n-C4H9, adamantyl

Computational studies indicate a stepwise cyclization pathway. Interestingly, analysis of the NMR chemical shifts in 8-5 suggests that both the carborane cage and the exo-polyhedral ring have aromatic character, in contrast to “benzocarborane” (9-34, Section 9.4.2.11) where no aromaticity is found in the exo-ring. Methylation of 8-5 (R ¼ Ph) forms a triazole zwitterion, which on chemical or electrochemical reduction generates a radical anion, 1,2-(cyclo-N3PhMe)CB11 H10
[309]. A general method for preparing water-soluble triazolium and tetrazolium salts has been described [357].

8.2.6

Polyalkylation at Boron

In the search for weakly coordinating anions, an alternative to replacing the BH hydrogens with halogens is to replace them with alkyl or hydroxy substituents. B-peralkylated derivatives lack the electron lone pairs that are associated with halogen atoms (possibly leading to even lower basicity), and also provide an outer sheath of hydrocarbon groups that promotes their solubility in nonpolar solvents. However, the cumulative effects of the electron donation from the alkyl groups can be expected to alter the properties of the CB11 cluster to some degree. Methylation of all of the boron vertices in CB11 H12
is achieved upon reaction with methyl triflate; similar treatment of ICB11 H11
affords the dodecamethyl anion [8,19]: CF3 SO3 Me


HCB11 H
11 ƒƒƒƒƒƒ! HCB11 Me11 CF3 SO3 Me


ICB11 H
11 ƒƒƒƒƒƒ! CB11 Me12

In general, this reaction is allowed to proceed to completion, as it is relatively nonselective and is not useful in the preparation of specific partially alkylated derivatives. However, the placement of a bulky substituent such as Si(CHMe2)3 on the carbon vertex can be used to block the entry of methyl groups at nearby boron vertices; treatment with methyl triflate, followed by the removal of the silyl group with water, produces HCB11H5-7,8,9,10,11,12-Me6
in a good yield [29]. Iodine can also be used as a blocking agent, as its presence on a given boron atom tends to prevent methylation at adjacent borons [29]. Methylation employing trideuteriomethyl triflate generates both B-CD3 and B-CHD2 products, indicating formation of an intermediate in which hydrogen scrambling occurs between the BH unit and the methyl group. DFT calculations suggest that the intermediate species is a 3-center bonded BH-CD3 sigma adduct [298]. Alternatively, B-alkylation can be accomplished by heating CB11 H12
salts with alkyl halides in sealed tubes [59]: MeBr


RCB11 H
11 ƒƒƒƒƒƒ! RCB11 Me11 R ¼ H, Me EtBr


HCB11 H
11 ƒƒƒƒƒƒ! HCB11 Et11

Microwave-assisted alkylation affords a very convenient route to peralkylated derivatives, particularly those bearing F or other strongly electron-withdrawing groups at C(1) or B(12) [354]. The methyl group at B(12) in Li(dioxaborole)+CB11 Me11
(8-6) can be replaced by a p-bromophenyl substituent upon treatment with p-Me3SiC6H4Br at 190 °C to give 8-7 [41].

256

Carboranes

Me

Me

Li+ O

− B

8-6

Me Me

O

Me

C

Me3Si-C6H4-Br Δ

Me Me Me Me Me

Me

Li+ B

8-7

Me

−

O

O

Me

C

Br

Me Me

Me Me

Fluorination of CB11 Me12
, in a two-step process that involves a reaction with F2 followed by treatment with K2NiF6 in HF, affords the dodecakis(trfluoromethyl) cluster CB11 ðCF3 Þ12
(8-8) whose molecular structure is a nearly spherical ball ˚ that is defined by the 36 fluorines on its perimeter [60]. While 8-8 is stable in 20% KOH/EtOH, with a radius of 8 A concentrated H2SO4, and anhydrous CF3COOH, and is unreactive toward O2 + AsF6
in anhydrous HF, the Cs+ salt is shock-sensitive and detonates on scraping with a metal implement. The properties of CB11 Me12
are unusual, and in some respects unique, and are further addressed below. Fluorination of partly methylated HCB11H11
anions yields HCB11(CF3)nF11-n
salts (n ¼ 5,6,10,11), of which the species having n ¼ 11 is highly explosive (as is Cs+ CB11ðCF3 Þ12
), but salts of the others are reportedly stable. These anions exhibit high gas-phase electron-detachment energies and form neutral-colored EPR-active HCB11(CF3)nF11-n radicals [356].

8.2.7

Mixed Alkyl-Halo Derivatives

Clusters combining alkyl and halogen substituents—of interest for “fine-tuning” the electronic properties and basicity of the CB11 system (see below)—are obtained from hexahalo clusters via reaction with methyl triflate at elevated temperature [61]: CF3 SO3 Me


HCB11 H5 X
6 ƒƒƒƒƒƒ! HCB11 Me5 X6 X ¼ Cl, Br, I

8.2.8

Acid-Base Properties

The CB11 H12
ion, and especially its polyhalo and polyalkyl derivatives, are among the least nucleophilic species known [62–64], and their conjugate acids are extremely strong Brønsted acids—by several measures, the strongest found to date [65,66]. The CB11 cage is highly aromatic (see Chapter 2), but in contrast to benzene, which is stabilized by p-delocalization in the planar ring, the carborane system features 3-dimensional s aromaticity in which the negative charge is delocalized over the entire icosahedral framework. A measure of this stabilization is the enormous HOMO-LUMO energy gap, which exceeds by far that of arenes and other p-aromatic systems [67]. As a consequence, the CB11 skeleton is chemically very robust and remains intact under a wide range of conditions (for example, surviving direct fluorination with F2 as mentioned earlier). The CB11 H12
parent ion does interact with aromatic molecules in the solid state: a DFT study has shown that it binds to benzene via C-H p bonds and B-H  H-C dihydrogen bonds [252]. Moreover, a kinetic study of 1-aryl-CB11H11
anions in solution has revealed conjugation between the p-aromatic aryl ring and the sigmaaromatic carborane cage, in which the rate of iodination of 1-C6H4R-CB11H11
at the antipodal B(12) vertex was shown to be dependent on the nature of the R substituent [351]. The already weak coordinating ability [1] of the parent CB11 H12
ion is further lowered when the hydrogen atoms are replaced with halogens and/or alkyl groups; halogen substituents withdraw their electron density from the cage and render it even less nucleophilic [66,68], while polyalkyl substitution provides a protective hydrocarbon sheath, affording increased solubility in organic solvents [8]. These two approaches offer competing strategies that are aimed at achieving the synthesis of “least coordinating” anions. For some purposes, mixed-ligand HCB11 Me5 X6
species combine the advantages of both types; thus, R + HCB11 Me5 X6
ions (R ¼ Me, Et, CHMe2; X ¼ Cl, Br) are powerful alkylating agents that are stronger than alkyl triflates and can convert benzene to protonated toluene [61,69]:

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

257

+
Me + HCB11 Me5 Br
6 + C6 H6 ! CH3 C6 H6 HCB11 Me5 Br6

The same carborane reagents extract H
from alkanes at room temperature or below, forming carbocations that in many cases can be isolated as stable salts [70]: +
Me + HCB11 Me5 Br
6 + Cn Hn + 2 ! Cn Hn + 1 HCB11 Me5 Br6 + CH4

Among the elusive species characterized in this way are the tert-butyl and tert-pentylcations. Indeed, CMe3+ HCB11Cl11
is quite stable and affords strong evidence of intramolecular C-H hydrogen bonding with resulting partial double-bond character in the C-C bonds, as seen via comparison of its solid-state IR spectrum with that of the CMe2(CH2D)+ cation [284]. As proton donors, partially or fully halogenated derivatives of CB11 H12
are significantly more powerful than conventional superacids such as CF3SO3H or FSO3H, as shown by their ability to protonate mesityl oxide in liquid SO2 via the reaction

Moreover, the fact that the carborane acids are nonoxidizing, and therefore benign toward proton acceptors, offers an important additional advantage. As measured by the 13C NMR chemical shift between Cb and Ca, the acids H+ HCB11 Cl11
and H + HCB11 H5 X6
(X ¼ Cl, Br, I), for which Dd > 83, both move the above equilibrium entirely to the right [68,71]. (In comparison, values of Dd for FSO3H, CF3SO3H, HN(SO2CF3)2, and H2SO4 are respectively 74, 73, 72, and 64) [71]. DFT calculations on a variety of H+ CB11 Xn H12
n
species (see Table 8-1) show a strong dependence of gas-phase acidity on the nature and location of the substituents [74]. The extreme acidity of H+HCB11Cl11
notwithstanding, it has been supplanted by H+HCB11F11
as the strongest known Brønsted acid. This remarkable superacid protonates hydrocarbons at room temperature, combining with benzene to form the benzenium salt C6 H7 + HCB11F11
, with n-butane to generate (surprisingly) crystalline CMe3 + HCB11F11
, and with n-hexane to afford an isolable hexyl carbocation salt. The same acid protonates water to yield H3O+HCB11F11
[308]. The extreme proton-donating capability of these monocarborane acids has been exploited, especially by Reed and coworkers [68], to stabilize many species that had previously escaped definitive structural characterization, in many cases allowing their isolation and structural study as crystalline salts under ambient conditions. The list is striking, and includes oxonium ions [271]); HðH2 OÞn + (hydrated protons) [75–79,308,319(review)]; (n-C4H9)3Sn+ [80]; protonated arenes such as C6 H7 + (benzenium), C6 MeH6 + and C6Me6H+ [81,82,308]; protonated organic solvents [83]; HC60 + (protonated fullerene) [84] and C59N+ [84]; (azafullerene) [85,86]; C76 + [87]; fluorinated carbocations [88]; protonated porphyrins; [89] imidazolium salts; [90,91,321] R3Si+ salts [92,93]; (mes)3Si + CB11 Br6 Me5
(mes ¼ 2,4,6-trimethylphenyl);[94] silyl-stabilized allyl cations [95]; bridged heterocyclium dications [249]; R3E+ salts (E ¼ Ge, Sn, Pb) [251]; chloronium ions (Et2Cl+) [262]; digermyl and germylsilyl cations [95,312]; and CIR2 + (chloronium ions, R ¼ Me or Et) [96], among others (Table 8-1). The silylated species represents the closest approach yet to a true R3Si+ (silylium) ion. In an important application having potential environmental consequences, silylium salts of HCB11 H5 X6
(X ¼ H, Cl, or Br) have been shown to efficiently catalyze the replacement of fluorine atoms by hydrogen in pefluoroalkanes under mild conditions [97,98] and R3Si+ cations stabilized by HCB11H5Cl6+ are found to selectively activate C(sp3)-F bonds in saturated fluorocarbons [320]. In another example, the isonitrylium ion (Me3Si)2NNCSiMe3+ in the presence of HCB11H5Br6
catalyzes the trimerization of disilylated diazomethane [280]. Other examples of catalysis exploiting nearly noncoordinating CB11 clusters as counterions are given in Chapter 15. The novel silylium zwitterion 8-9, a strong Lewis acid, is a valuable precursor to elusive cationic species, for example converting to Ph4P+ Me2SiCl-CH2-CB11Cl11
and CMe3+ Me2SiClCH2CB11Cl11
on treatment with Ph4PCl and Me3CCl in liquid SO2, respectively [307]. Me

Me

H

Cl

C

Cl

Cl

Cl Cl

Cl

Cl

Cl

Cl Cl

NaH, ClCH2SiMe2H

Me

CH 2

H

Cl

Cl

C

Cl

Cl Cl

+

Me

Si

Cl

Cl Cl

C

8-9

Cl Cl Cl

Cl

Cl

Cl

Cl Cl

CH2

Cl

Cl Cl

Cl

Cl

[Ph3C][B(C6H5)4]

Si

Cl Cl

258

Carboranes

Silylium salts of HCB11Cl11
abstract F from C6H5F at room temperature, forming 1-HCB11Cl11-n-Ph products (n ¼ 7,12) containing a carborane-Cl-phenyl linkage [255]. Moreover, R3Si+ in the presence of a CB11H6Cl6
counterion enables Friedel-Crafts proton-catalyzed coupling of fluoroarenes in high yield [263]:

F

R3Si+ CB11H6Cl6 H

+ R3SiF

+

Terphenylsilylium cations in HCB11H6Cl6
salts are stabilized by two competing modes, i.e., p-arene and lone pair halogen coordination [266,358]. Some comparative observations may help to place this chemistry in perspective. For example, although C60 and other fullerenes had escaped protonation despite years of effort, HC60 + CB11 H5 Cl6
has been isolated as a stable salt and its structure has been characterized in detail by 13C CPMAS techniques [99,100]. The sharp singlet exhibited by the 13C NMR spectrum of HC60 + indicates a rapid movement of the proton, rendering all 60 carbon atoms equivalent on the NMR time scale [84]. Hydrated protons have been isolated in benzene solvent as H3O+, H5 O2 + , H7 O3 + , and H9 O4 + [76,101,102], the first of which has been shown from crystallographic studies to be stabilized via hydrogen-bonded interactions between the cation and the p-system of benzene [101]. Infrared studies of tertiary carbocation salts of HCB11Me5X6
and HCB11X11
(X ¼ Cl, Br, I) suggest that both hydrogen bonding and hyperconjugation are important in stabilizing these cations [293,346]. HCB11X6Y5
ions (X ¼ H, Cl, Br; Y ¼ H, Me) are well suited as supporting electrolyte anions for electrochemical studies [359]. The tris(n-butyl)tin cation, previously unknown as a structurally characterized entity, is generated [80] by oxidation of Sn2(n-C4H9)6 by the stable free radical CB11 Me12  (further discussed below), which itself is formed on electrochemical oxidation of CB11 Me12
[103]. 1=2ðn-C4 H9 Þ3 Sn-Snðn-C4 H9 Þ3 + CB11 Me12  ! ðn-C4 H9 Þ3 Sn + CB11 Me
12 In contrast to covalently bound R3SnX compounds, which feature short Sn-X bonds, the (n-C4H9)3Sn+ ion is only weakly coordinated to methyl groups of the carborane cation.

8.2.9

Metal Complexation

The low nucleophilicity of CB11 H12
and its derivatives, especially the halogen- and alkyl-substituted species, can also be used to advantage in constructing novel metal coordination polymers. The architecture and properties of such systems (Table 8-1) are strongly influenced by the nature of the counterions, especially their size, shape, and degree of coordination to the metal cations. The stable and versatile CB11 clusters, and other monocarbon carboranes, have attracted attention because their binding to metals can be controlled and engineered via the choice of exo-polyhedral substituents; hence they are of major interest in supramolecular crystal engineering. For example, phosphino derivatives of the CB11 H12
anion have found a role as chelating agents in the construction of “weak-link approach” (WLA) complexes of platinum(II), palladium(II), and rhodium(I), in which both the electron-withdrawing (at carbon) and electron-donating (at boron) properties of the carborane cage are exploited. One compound, the unique salt Rh[Ph2P(CH2)2S-9-1,7-C2B10H11]+2 Rh[Ph2P(CH2)3S9-1,7-C2B10H11]
2 , incorporates metallacarborane cages in both the anion and cation [302]. There is considerable variation in the role played by carborane anions in metal systems: some interact with metal centers through BdH⋯M binding [17,104–110,301]; certain halogenated CB11 H12
derivatives bind via their halogen atoms [17,21,111–115,268]; silver salts of HCB11 Me11
show strong CH3⋯Ag interactions [116] (see below); the same carborane anion undergoes B-CH3 activation and BdC bond cleavage in the presence of Cp2ZrMe+, H2 IrðPPh3 Þ2 + and Pt½PðCHMe2 Þ3 2 2 + [117]. In other cases, there is only weak interaction and the carborane functions mainly as a spectator ion [118,119]. Some of these findings are counterintuitive, such as the fact that the polybrominated species CB11 H6 Br6
is

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

259

actually less nucleophilic than CB11 H12
, despite the lone pairs on the bromines and the much higher polarity of the brominated versus the parent carborane, a discovery that has been attributed to steric effects [114]. The bis(perfluorocarboranyl)mercury dianion HgðCB11 F11 Þ2 2
is a remarkably strong Lewis acid, notwithstanding its dinegative charge; its mercury(II) center coordinates strongly to acetonitrile and water. In contrast, the singly charged  PhHg(CB11F11)
ion does not coordinate to these molecules [257], nor do the dianions Hg CB11 X11 Þ2
2 (X ¼ H, Cl, Br) [270]. In the case of X ¼ H, theoretical analysis indicates that this non-coordination is due to low Lewis acidity of the mercury(II) center, but for the species in which X is Cl or Br it is a consequence of shielding of Hg by the bulky ligands [270]. Linear mercury(II) carboranyl dianions 8-10 have been prepared by two different routes and characterized as building blocks for supramolecular assemblies held together by weak HgI and CsI interactions [328].

2

2

I

n-BuLi

CH

THF -C4H10

I

HgCl2

C

C

I

THF

Hg

C

I

R C C MgBr PdCl2 (PPh3)2 THF

2 R

C

C

C

R = H, Si(CHMe2)3, C5H4FeCp

Hg

C

C

C

8-10

R

HgCl 2 THF 2

R3Si

C

C

CH

n-BuLi THF −C4H10

R3Si

C

C

C

An example of crystal engineering in this area is the coordination polymer shown in Figure 8-2, which incorporates (4,4bipyridine)Ag+ and PhCB11 H5 I6
units and consists of zig-zag chains that are held together by interactions between the silver ion and adjacent iodine atoms on the carborane and by silver-bipyridine binding [50]. In contrast, the related complex Ag2(C14H10N4)(BrC6H4)CB11 H11
[C14H10N4 ¼ 2,3-bis(2-pyridyl)pyridine] has no direct interactions between the metal centers and the carborane anions [50]. A novel type of metal-organic framework (MOF) has been prepared in which PhCB11H5I6
anions are linked by DABCO (diazabicylcooctane) featuring Ag  H-B interactions [291]. Essentially nonbinding RCB11 F11
clusters serve as counterions that can stabilize species such as the AlMe2+ cation [286] and the formerly elusive copper carbonyls, as in CuðCOÞ2+ PhCH2CB11 F11
and CuðCOÞ4+ EtCB11 F11
, the latter of which is the first structurally characterized copper(I) tetracarbonyl [120]. The salt Ag[CH(C6H5)3]+MeCB11 F11
is the first example of a trigonal-planar AgðareneÞ3 + complex [121]. The parent CB11 H12
anion forms host-guest crystal lattices of the type M[2.2.2]cryptate+CB11 H12
Ni(TMTAA)]3, where M is Na or K and Ni(TMTAA) is tetraazacyclotetradecinenickel(II). In these perovskite-like structures, each cryptate-encapsulated Na+ or K+ cation is surrounded by six Ni(TMTAA) units in a cubic arrangement, with the non-interacting carborane ions located at the corners of the cube [12]. Other examples of novel crystal engineering include the construction of M+CB11 H6 X6
CTVH2OL coordination networks (X ¼ Cl, Br; L ¼ CF3CH2OH, MeCN; M ¼ Na, K, Rb, Cs), which feature bowl-shaped cyclotriveratrylene (CTV) host molecules and Group 1 cations [122–124] (analogous CTV-CB9 cluster arrays such as 6-31 are described in Chapter 6, Section 6.3.1.3). The versatility of CB11 monocarbon carboranes has been exploited in still other ways. The complex (Ph3P)Ag+ CB11 H6 Br6
, when present in only 0.1 mol%, catalyzes the hetero-Diels-Alder reaction between N-benzylidene and Danishefsky’s diene and affords quantitative yields of 8-11 at room temperature [108,125]. The same reaction is catalyzed by (Ph3P)Ag+ CB11 H12
but more slowly.

260 Carboranes

C I

I

I I

I Ag

N

N

N

N

Ag

Ag

I

I

I

I

I

I

I

I

I

C

FIGURE 8-2

I

I

I

C

Structure of [(4,4-C10H8N2)Ag][PhCB11H5I6] (MeCN solvent not shown). OMe

Ph

+ SO

Ph

(Ph3P)Ag+CB11H6Br−6

N

N

CH2Cl2 H

Ph

O

8-11 Ph

Also, silylium derivatives such as R3Si+HCB11 Cl11
and R3Si+HCB11 R5 Br6
(R ¼ H, Me) have been shown to catalyze the ring-opening polymerization of the cyclo-N3P3Cl6 chlorophosphazene trimer [126]. The zwitterionic gold (I) complex C4H4S-Au-P(Me2CH)2CB11H11 catalyzes the hydroamination of alkynes with amines at a very high turnover rate [288], and RCB11Cl11
(R ¼ H, n-C4H9) anions are useful counterions in cycloaddition and C-H activation reactions of Ta alkylidynes [336]. The zwitterionic palladium allyl complex (Z3-C3H7)Pd+-P(CHMe2)2-CB11Cl11
catalyzes the vinyl addition polymerization of norbornene [342]. Numerous chelated metal cations have been shown to form stable, isolable salts with HCB11H11
or its polyhalogenated derivatives, including those of aluminum [258], chromium [259], tin [259], and others listed in Table 8-1. Introduction of vinyl groups at selective boron vertices via a rhodium phosphine complex is illustrated in Figure 8-3. Hydrogenation of these vinyl substituents forms the 2,4,8,10,12-pentaethyl derivative 8-12, as shown in Figure 8-4 [106].

8.2.10

Special Properties of CB11 Me12
, HCB11Me11
, and CB11 Me12 

Peralkylated derivatives of CB11 H12
, like their B12 Me12 2
counterpart [127], are effectively spherical hydrocarbons whose CB11 core is sterically protected, so that interactions with solvents and attacking species must necessarily take place at the peripheral alkyl groups. As remarked earlier, its salts dissolve even in low-polarity organic solvents, though this property varies with the cation, with Li + CB11 Me12
having the highest solubility; saturated solutions of the lithium salt

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

−

H

H

H

−

−

H2C=CH2

+

CH2Cl2

H

C

C

C

H H

H

261

Rh+

Rh

Rh+

PPh3

PPh3

PPh3

Ph3P

Ph3P

Ph3P

norbornadiene Rh(PPh3)2 −

(nbd)+

Rh(PPh3)2(nbd)+

H −

C

H C

+

FIGURE 8-3 Synthesis of (Ph3P)2Rh(nbd)+CB11H11-n-CH]CH2 2 (n 5 12, 7) from (Ph3P)2Rh+CB11 H12 2 via (Ph3P)2Rh+CB11H11n-CH]CH2 2 isomers.

R −

Rh(PPh3)2(nbd)+

C 1) C2H4 H H

repeat x 6

norbornadiene

2) H2

R

−

C

8-12

Rh+ PPh3

Ph3P

FIGURE 8-4 Synthesis of (Ph3P)2Rh(nbd)+CB11H7-2,4,8,10,12
Et5 2 (8-12).

in benzene are electrically conductive. The CB11 Me12
ion is stable toward air, strong bases, and in dilute aqueous strong acids, but decomposes on exposure to concentrated H2SO4 or triflic acid [8]. Electrochemical studies of the CB11 Me12
, HCB11Me11
, and HCB11Me10 -12-H
anions show reversible 1-electron oxidations in a variety of solvents [290]. The methyl groups in CB11 Me12
and HCB11 Me11
have substantial CH3
(methide) character according to DFT calculations [128], and readily bind to transition metal [116,129,130] and main group metal [128,131,132] cations, forming the salts listed in Table 8-1. (Some transition metal salts, such as [MoCp(CO)3]2(m-I)+ HCB11 Me11
[133], do not exhibit significant interaction between the metal cation and the carborane methyls). In certain silver-phosphine complexes of HCB11 Me11
, the Ag⋯H3C binding is sufficiently strong to persist even in CD2Cl2 solution [116]. DFT calculations on the second-order NLO properties of RCB11Me11
complexes as a function of the R group demonstrate that molecular oxidation occurs in the cage framework, and that the molecular geometry is influenced by both electron-donating and electron-withdrawing substituents [306]. Depending on the metal-containing cation, the metal-methyl group interaction can be quite strong and, as noted earlier, can lead to BdCH3 bond cleavage [117]. In the main-group metal compounds such as Me3 M + CB11 Me12
, where M is Ge,

262

Carboranes

Sn or Pb, actual Me3MdCH3 covalent bonding takes place as CH3
is extracted from the carborane, leaving a neutral CB11Me11 cluster (8-13) that has a cage boron atom with a vacant bonding orbital in place of a substituent. This “naked boron” species, described as a boronium ylide, is air-sensitive but stable as a solid below
60 °C and can be generated via reactions of the CB11 Me12  radical, as discussed below [134]. Me Me

C

C

Me

Me

Me Me

Me

8-13

Me Me

8-14

Me

Me Me

Me Me

Me Me

Me

Me

Me Me

A different CB11Me11 isomer (8-14), which has a vacant orbital on the cage carbon atom and is described as a carbenoid-carbonium ylide, has been postulated as an intermediate during the extraction of the L substituent from LdCB11Me11 carboranes (L ¼ BrCH2CH2 or (CF3)2CHO) by electrophiles [2,31]. This species reacts with arenes in the presence of (CF3)2CHOH to generate 1-aryl-CB11Me11 products [135]. The low nucleophilicity of CB11 Me12
can be exploited in organic synthesis, as illustrated by the use of its lithium salt to catalyze pericyclic rearrangements such as the conversion of quadricyclane to norbornadiene (8-15), cubane to cuneane, diademane to triquinacene, and others [136]. The nearly-nonbinding permethylated carborane anion renders the Li+ cation highly active and effective in catalysis. Related lithium salts of 1,12-dialkylated anions, e.g., Li+ RCB11Me11
(R ¼ Me, Et, i-C4H9, n-C8H17), are effective Lewis acid catalysts [253].

Li+CB11Me−11 C6D6

8-15

A different Li+-catalyzed process is the radical polymerization of the alkenylcarborane salts of the type Li+[CH2]CH (CH2)n-2]CB11 Me11
, which occurs in the solid state or in solution, and is initiated by O2, azoisobutylnitrile, or di-tertbutylperoxide [20,31,137]. These salts are generated from the LiCB11 Me11
anion, which is obtained via reaction of BrCB11 Me11
or ICB11 Me11
with n-butyllithium at
78 °C:   n-BuLi
TosOðCH2 Þn
2 CH¼CH2 +
XCB11 Me
11 ƒƒƒƒƒƒ! LiCB11 Me11 ƒƒƒƒƒƒƒƒƒƒƒƒƒ! Li CH2¼CHðCH2 Þn
2 CB11 Me11 X¼Br, I n¼3
7 In general, although the monomer conversions are relatively high, comparatively low molecular weight oligomers are obtained under normal conditions [260]. However, when conducted in air at 25 °C or with di-tert-butyl peroxide at 80 °C, polymerization of isobutylene in the presence of CB11 Me12
ion affords both linear and highly branched polyisobutylene having molecular weights up to 50,000 and 26,000, respectively [261,282]. The branched product is identical with that obtained under nonoxidizing conditions, but the novel linear polymer contains a bound CB11 Me12
ion at the end of the chain. The evidence suggests that the process involves initial formation of a neutral CB11Me12 radical which transfers a CH3 radical to isobutylene, initiating polymerization via a radical mechanism; the remaining CB11Me11 borenium ylide, a strong Lewis acid, induces formation of the linear polymer by a cationic mechanism [261]. Organic salts having CB11 Me11
as a counterion display unusually high solubility, making them attractive candidates for the development of “molecular wires” that employ polyarene oligomers (Chapter 17) [138]. Studies of intra- and intermolecular electron exchange in the phenothiazine system have been facilitated by the isolation and structural characterization of the cation radical salt o-(phenothiazinyl)2C6 H4 + CB11 Me11
[139]. This radical has also been employed as a p-type dopant in electron-delocalized hexaarylbenzene cation-radical salts [140]. Stable, isolable biradicals of the type Me11B11CdRdCB11 Me11  , where R is CH]CH or C^C, can be prepared by two-electron electrochemical oxidation of the corresponding Me11B11CdRdCB11 Me11 2
dianions [141].

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

263

The high stability of these B-permethylated species is illustrated by cyclic voltammetry on 1-R-CB11Me10-12-R0
anions where R and/or R0 are alkyl, carboxyl, halo, alkoxy, or other groups (Table 8-1). These studies, supported by DFT calculations, for most species show fully reversible one-electron oxidation even at low scan rates; exceptions to this behavior are found for R0 ¼ I and for R ¼ C(O)OH and R0 ¼ H [275]. Doubly trimethylene-bridged paracyclophane cations are also stabilized by HCB11Me11
anions [300]. The permethylated neutral free radical CB11 Me12  , which is obtained via oxidation of CB11 Me12
, as mentioned earlier, is a remarkably stable solid that dissolves easily in nonpolar solvents, sublimes at 150 °C, and is even stable in air for short periods [103]. It is, however, highly reactive and can extract electrons from aromatic hydrocarbons, amines, tetrathiofulvalene [142], and organometallics having MdM or MdC bonds [103,128]. CB11 Me12  can also function as a methyl transfer agent; for example, it reacts in a 2:1 ratio with Si2(CMe3)6 to give two equivalents of MeSi(CMe3)3, and in the process is converted to the boronium ylide CB11Me11 that has been described earlier [2]. The possible application of CB11 Me12  as a building-block for conducting polymers has been suggested [2]. In a spin-density investigation of CB11MenðCD3 Þ12-n - radicals (n ¼ 0-12), all 16 isomers having 5-fold substitution symmetry were synthesized and the average hyperfine coupling constants aH for the CH3 groups located at C(1) and the B(2-6), B(7-11) and B(12) locations, were determined from the variation in width of their respective EPR signals [279].

TABLE 8-1 CB11H122 Derivativesa Informationb

References

CB11 H12
derivatives as weakly coordinating anions

Review (2002)

[8]

Compound Synthesis and Characterization

CB11 anions (weakly coordinating anions)

Review of structural and NQR data

[113]

CB11 H12
C-aryl derivatives

Review

[24]

C7 H6 + CB11 H12


Review of NLO properties of carboranes

[34]

S, H

[5]

X(variable temperature), thermal studies

[143]

B

[3,5,32,46,144,145]

C

[144,146]

E

[145]

IR

[3,147–149]

IR(actual spectrum)

[144]

MAG, COND

[147]

Raman (actual spectrum)

[144]

Non-transition Metal Derivatives No Substituents on Boron CB11 H12


UV

[147,149]


Charge-transfer complexes with MV PF6 MV ¼ methyl viologen; triplet excited state generated by 308 nm laser excitation

[13]

Counterion for bridged heterocyclium dications Me3N
N3C2H2
CH2
X
CH2
N3C2H2-NMe3 2 + X ¼ CH2¼CH2, C C

[249]

Counterion for (PNP)Pd+ fragment in heterolytic cleavage of B-H bonds in catecholborane and B-B bond in catecholdiboron PNP ¼ bis(Pr2Mephenyl) amino

[267]

+

Continued

264

Carboranes

TABLE 8-1 CB11H122 Derivatives—cont’d Compound

Information

References

Cs CB11 H12 in MeCN or ethylene carbonatedimethylcarbonate solvents; adsorption electrolyte salt for electrochemical capacitors

E, ionic conductivity, impedance spectroscopy, capacitance

[314]

1-[HO(O)C]CB11H11


pKa, B, H

[276]

S, X(CHMe2), H, B, C, P, F, MS

[294]

1-(RC6H4)CB11 H12 R ¼ C(O)Me, C(O)OMe, CN, NO2, H, n-C12H25, OMe, OTBS

S, X(Cu-mediated C-C cross-coupling), X[C6H4C(O) OMe], H, B, C, MS

[296]

[(F3C)2C6H3]CB11 H12


S, X(Cu-mediated C-C cross-coupling), H, B, C, MS

[296]

1-XCB11 H12 X ¼ C10H8, C4H2SMe, C4H3S-C4H3S, C5H4N, CH¼CHPh, C CPh

S, X(Cu-mediated C-C cross-coupling), H, B, C, MS

[296]

p-C6H4ðCB11 H12 Þ2 2


S, X(Cu-mediated C-C cross-coupling), H, B, C, MS

[296]

S, X(Cu-mediated C-C cross-coupling), H, B, C, MS

[296]

S, COND, permeation of bilayer lipid membranes

[281]

S, H, C, Li

[313]




+




1-(R2P)CB11H11 R ¼ H, Cl, CHMe2





0

0

0

1 ,3 ,5 -C6H3ðCB11 H12 Þ3

3


Porphyrin[C6F4-p-CB11H11]2Ph2

2


2Cs

H11B11C
N¼CH
CH¼N
CB11 H11 S2ðCB11 H11 Þ2

+

2


2


S, H, B, C, MS

[302]

HS-CB11H11


S, X, H, B, C, MS

[302]

H2N-CB11H11


S, H, B(2d), C, MS

[305]

p-RC6H4-CB11H11
R ¼ C(O)OMe, CN, NO2, H, n-C12H25, OMe, OCH2SiCMe3

S(Pd-catalyzed C-C cross-coupling), UV, mesogenic activity, androgen-receptor binding

[317]

1-(H2N)CB11H11


S, X, B, C, MS, IR (actual spectrum), Raman (actual spectrum)

[144]

(n-C8H17)3NH+ CB11 H12


IR (nN-H as a measure of acid strength)

[73]

4,4-bipyridineH+ CB11 H12
salts H-bonded networks with N
HN and C
HN interactions

S, X, IR

[11]

CB11 H12
(2-10B)

S, B

[32]

S, X, H, B, C, MS

[17]

S, X, H, B, C

[25,26]

S, H, B, P

[14]

Li RCB11H11 R ¼ Et, n-C3H7, n-C4H9, n-C6H13, 2-EtC6H12

S, H, C, B, IR, MS

[253]

M+(NC)CB11H11
X ¼ H, F. Cl, Br, I M+ ¼ Cs+, Et4N+

S, X(Cs,H; Et4N, Cl), H, B, C, IR, MS, Raman

[22]

H

[3]

(H3N)CB11H11

S, H, B

[155]

(Me3N)CB11H11

S, X, H, B, IR, thermal analysis

[156]

LCB11H11 L ¼ H3N, MeH2N, Me2HN, Me3N

S, H, B, MS

[15]

(Me2S)CB11H11

S, H, B, MS, IR

[15]

LCB11H11 L ¼ H2N, MeHN, Me2N, MeO, HO, HO(O)C

S, H, B

[15]

RCB11H11
R ¼ NHC(O)Me, NMe3 + , NHMe2 + , NH3 + , succinylamino

S, H, B, MS

[159]

Me3NH+ CB11 H12


S (improved: 2 steps from NaBH4 + BF3OEt2 via nidoB11 H14
)

[7]

MeCB11H11 PhCB11H11










RCB11H11 R ¼ Li, H, Et, Ph3Si, CF3, Ph2P, CH2Ph


+

(Me3Si)CB11H11







Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

265

TABLE 8-1 CB11H122 Derivatives—cont’d Compound


XCB11H11 X ¼ F, Cl, Br, I


XCB11H11 X ¼ Cl, Br, I

Information

References

S, B, C, UV, MS

[28]

S(improved; methyl triflate), X




[30]


(p-XC6H4)CB11H11 X ¼ H, F, Cl, Br, I, Ph

S (insertion of arylhalocarbenes into nido-B11 H14 ), X(F), H, B, C, IR, UV, MS

[27]

(15NC)CB11H11


S, H, B, C, N, IR, Raman

[322]

S, X, H, B, C, P, IR, MS

[353]

S, B(2d)

[32]

CB11H11-2-R R ¼ Ph, C6H4Me, F, O(CH2)4Cl, NMe3

S, B(2d), H, F(F), X(O[CH2]4Cl)

[48]

RCB11Me11
R ¼ H, Me

S, H, B, C, MS, E

[29]

CB11H11-12-R R ¼ Me, Et, n-C4H9, hexyl, Ph

S, H, B, C, IR, MS

[33]

S, X, H, B, C, IR, Raman, DSC

[36]

1-[P(CHMe2)2IrCl(C8H12)]CB11H11




D or Hydrocarbon Substituents on Boron CB11H6-7,8,9,10,11,12-D6






HCB11H10-12-C CH

HCB11H9-7,12-ðC CHÞ2




S, X, H, B, C, IR, Raman, DSC

[36]

MeCB11H10-12-C CH


S(microwave-assisted Pd-promoted cross-coupling), H, B, C, P

[272]

H2N-CB11H10-12-R
R ¼ C CH, C C-Ph, C C-SiEt3

S, X(C C-Ph), H, B(2d), C, MS, Raman

[305]

Me3N-CB11H10-12-C CH

S, X, H, B(2d), C, MS, Raman

[305]

[HO(O)C]CB11H10-12-C CR
R ¼ H, SiEt3

S, X(H), H, B, C, IR, Raman, MS

[322]

R+ CB11Me5X6
R ¼ Me, CHMe2; X ¼ Cl, Br) very strong alkylating agents

S, X(CHMe2, Br), H, C, IR

[69]

1,2-[cyclo-(CH2)4]CB11 H10


S, X, H, B, C, IR

[283]

PhCB11H10-12-Ph

S, X, H, B, C

[40]

CB11H11-12-C7H6

S, H, B, C, IR, MS, NLO [1st hyperpolarizability (b), later corrected (J. Am. Chem. Soc. 2000, 122, 11274)]

[33]

HCB11Me11


S, H, B, C, IR, MS

[31]

Li CB11Me11


S (improved)

[20]

S, H, C, B, IR, MS

[253]

1-XCB11Me10-12-Y X ¼ H, Me, I, Br, F, C(O)OH, OH, B(OH)2, Et, n-C3H7, n-C4H9, n-C6H13, 2-EtC6H12; Y ¼ F, I

S, H, C, B, IR, MS

[254]

RCB11H10-2-R0
R ¼ H, Ph, NH2, NH(CMe2C6H4-pMe); R0 ¼ NH2, Ph, NMe3 +

S,X(Ph,NH2; NH2,Ph; Ph, NMe3 + ),H, B, IR, MS, DSC, pKa

[47]

[CH2¼CH(CH2)n
2]CB11Me11
Cs+ n ¼ 2–7

S (improved)

[20]

S, X, H, B, MS

[133]

S, H, B, C, IR, MS

[59]

S(improved; methyl triflate), X

[30]

S, X, H, B, Sn, MS, IR, UV

[80]

X

[142]

S, X, E, ESR, UV

[103]




+

Li RCB11Me11
R ¼ Me, Et, i-C4H9, n-C8H17 +




+

Ph3C HCB11Me11 0







0

RCB11R 11 R ¼ H, Me; R ¼ Me, Et





XCB11Me11 , XCB11H5Me6 X ¼ Cl, Br, I +

Snðn
C4 H9 Þ3 CB11 Me12





TTF CB11 Me12 TTF ¼ tetrathiofulvalene +



CB11 Me12 stable radical

Continued

266

Carboranes

TABLE 8-1 CB11H122 Derivatives—cont’d Compound 1-(HMe2N)CB11H11 Me11B11C
CH¼CH
CB11 Me11 Me11B11C
C C
CB11 Me11

2


Me11B11C
C C
CB11 Me11 

CB11 Me12

2


stable biradical




Information

References

S, H, B, C, IR, MS

[141]

S, H, B, C, IR, MS

[141]

S, H, B, C, IR, MS

[141]

S, IR, MS

[141]

S, X, H, B, E, IR, MS (electrospray)

[19]

0

[135]

CB11 Me11 reactive intermediate in aromatic substitution ArH + CB11Me11 ! Ar
CB11Me11
+ H+ Li(MeC6H5)+/MðC6 H6 Þ2 + CB11 Me12
M ¼ Tl, Cs, Rb, K, Na

X

[131]

Me3M+ CB11 Me12
(M ¼ Ge, Sn, Pb) Me3M+Me interactions

S, X, EXAFS, B, C, Sn, Pb

[128]

H(arene)+ HCB11R5X6
R ¼ H, Me; X ¼ Cl, Br; arene ¼ benzene, toluene, m-xylene, mesitylene, hexamethylbenzene

S, X, C, H, IR

[81]

R+ MeCB11Me5X6
R+ ¼ CMe3 + , CMe2Et+, cycloMeC5 H8 + ; X ¼ Cl, Br isolation of CMe3 + cation, etc., at room temperature

S, X, H, C

[70]

CB11Me11 boronium ylide; naked B(12) vertex

S, H, B, C

[134]

CB11H11-12-R R ¼ C CPh, C CSiMe3, F, I, Cl, Br

S, X(C CPh, Cl), H, B, C, IR (actual spectrum), Raman (actual spectrum), MS

[35]

PhCB11H10-12-C CR
R ¼ Ph, SiMe3

S, H, B, C, IR, Raman, MS

[35]

S, H, B, C, IR, Raman, MS

[35]

XCB11Me10-12-Y X ¼ Me, Et, Pr, Bu, OMe, F, Br, I, H, C(O)OH, C(O)OMe, CH2CHEtBu, C6H13 Y ¼ H, Me, F, Cl, Br, I

S, E(oxidation potential)

[275]

RO-C5H5N+CB11H10-12-Cn2n+1 n ¼ 5, 6, 10 R ¼ C7H15, CHMeC6H13 liquid crystals

S, H, B, UV, powder X-ray diffraction, dielectric anisotropy, polarizing optical microscopy, DSC

[311]

1-RC6H4-CB11H11
R ¼ H, Me, m/p-OMe, m/p -C (O)OMe, m/p -CF3, m/p -CN, p-F, p-Cl, p-I conjugation between CB11 cage and C6 ring

Kinetic study of rate of iodination at B(2)

[351]

S(microwave-assisted Pd-promoted cross-coupling), X(Ph, C6H4OMe, C C
CMe¼CH2), H, B, C, P

[272]

CB11H11-7-NR3 R ¼ H, Me

S, H, B(2d) MS

[32]

[HO(O)C]CB11H10-12-NMe3

S, pKa, B, H

[276]

HCB11H10-12-PPh3

S(microwave-assisted Pd-promoted cross-coupling), H, B, C, P

[272]

CB11H11-12-NH3

S, X, H, B, C, MS

[287]

CB11H10-7-NH3-12-X X ¼ F, Cl

S, H, B, C, MS

[287]

CB11H11-12-NMe3

S, H, B, C, MS

[287]




CB11H10-7,12-ðC CPhÞ2

2





Si-Containing Substituents on Boron HCB11H10-12-R
R ¼ SiMe3, C CSiEt3, C CSi (CHMe2)3, C CC4H9, C CPh, C C
CMe¼CH2, Ph, C6H4-p-SiMe3, C6H4-p-OMe, C6H4-iC CSiMe3, Et, CH2SiMe3, CH2CH¼CH2, C CSiMe3 N- or P-Containing Substituents on Boron

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

267

TABLE 8-1 CB11H122 Derivatives—cont’d Compound

Information

References

S, X, H, B, C, MS, IR

[297]

S, H, B, C, MS, IR

[297]

S, B, MS

[169]

S, H, B, IR, MS

[170]

S, X, IR

[75]

S, H, B

[7]

S, X, H, B, IR, MS

[170]

1,2-Me2N-CB11H10-2-R R ¼ SH , SMe2

S, X, B(2d), H, MS

[172]

CB11H11-12-L L ¼ Me2S, MeSCH2SMe

S, H, B, MS, IR

[15]

S, B, F, MS

[46]

S, X(Ag salt), B, C, F

[45]

S, B, F, MS

[46]

S, H, B, IR, Raman

[250]

S, X, H, B, IR, Raman

[250]

S, H, B, IR, Raman

[250]




CB11H10-12-CN

CB11H9-7,12-ðCNÞ2




O- or S-Containing Substituents on Boron HCB11 ðOHÞ11



CB11H11-12-OH CB11H(OH)5Br6




CB11H11-2-OEt +

CB11H11-12-O(CH2)2O dioxane zwitterion


F-, Cl-, Br-, or I-Containing Substituents on Boron CB11H11-7-F



CB11H11-12-F




+

(H2N)CB11H10-2-F K (H3N)CB11H10-2-F


(H2N)CB11I10-2-F Et4N CB11H10-7,12-F2

+




S, B, F, MS

[46]

CB11H9-7,9,12-F3


S, B, C, F

[45]

CB11ðCF3 Þ12


S, B(2d), F, MS, IR, Raman

[60]

S, B, F, MS

[46]

HCB11F11


S, H, B, C, MS, F

[51]

1-(H2N)CB11F11


S, X, B(2d, actual spectrum), C, F(2d, actual spectrum), MS, IR (actual spectrum), Raman (actual spectrum)

[144]

1-(H2N)CB11F10-6-OH
, 1-(H2N)CB11F9-4, 6-ðOHÞ2


S, X, B(2d, actual spectrum), F(2d, actual spectrum), MS

[144]

MeCB11F11


S, H, B, MS, F

[51]

SiMe3 + RCB11F11
R ¼ H, Et

S, H, B, C, Si, COND

[173]

Cs+ HCB11F11


S, H, B, F, MS

[308]

CPh3 + HCB11F11


S, H, IR

[308]

(Et2Si)2H+ HCB11F11


S, IR

[308]

H+ HCB11F11


S, IR

[308]

Me2Si+ CH2-CB11Cl11


S, X, H, C, Si

[307]

CMe3 + Me2SiCl
CH2
CB11Cl11


S, H, C, Si

[307]

1-XCB11Me10-12-Y
X ¼ H, Me, I, Br, F, C(O)OH, OH, B(OH)2, Et, n-C3H7, n-C4H9, n-C6H13, 2-EtC6H12; Y ¼ F, I

S, H, B, C, IR, MS

[254]

HCB11(CF3)nF11
n
n ¼ 5, 6, 10, 11

S, H, B, C, IR, S

[256] Continued

268

Carboranes

TABLE 8-1 CB11H122 Derivatives—cont’d Compound

Information

References

S, X(n¼7,12), H, B, C, IR, MS

[255]

[2,6-(2,6-R2C6H3)2C6H3]2AlEt+ HCB11X6H5
X ¼ Cl, I R ¼ CHMe2, Cl

S, X, H, B, C

[258]

B(N3C15Ar3)+ HCB11H5Br6
Ar ¼ Ph, p-C6H4Br, p-C6H4OMe; N3C15Ar3 ¼ subporphyrin; borenium cations

S, X(Ph), H, B, UV(absorption, fluorescence)

[265]

C9H14N+ HCB11Cl11
N-butylpyridinium

S, X

[269]

S, H, B, C, MS

[277]

XCB11Me10-12-Y X ¼ Me, Et, Pr, Bu, OMe, F, Br, I, H, C(O)OH, C(O)OMe, CH2CHEtBu, C6H13 Y ¼ H, Me, F, Cl, Br, I

S, E(oxidation potential)

[275]

[HO(O)C]CB11H10-12-I


pKa, B, H

[276]

S, X, IR

[284]

PhCB11H5I6 incorporation into DABCO (diazabicylcooctane) MOFs Ag  H
B interactions

S, X

[291]

CB11 Me12
, HCB11Me11
, HCB11Me10-12-H
, HCB11Me10-12-I
isoflurane (ClF3C2H
O
CF2H) as a nonpolar electrochemical solvent

E(isoflurane, SO2, MeCN, CH2Cl2)

[290]

CB11H11-n-I
n ¼ 7,12

S, X, B

[292]

R HCB11X11 R ¼ CMe3 , i-C5H11 , 2,3Me2C4H+, MeC5H4 + X ¼ Cl, Br hyperconjugation and H-bonding

IR(solid state, C
H bonds) hyperconjugation and H-bonding

[293]

CMe3 + HCB11X11
X ¼ F, Cl, Br stabilization in condensed phases via hyperconjugation and H-bonding

S, H, IR

[346]

1-(N3)CB11H10-2-Cl
azide

S, B

[299]

S, X, H, B(2d), C

[305]

S, X, H, B(2d), C

[305]

H4(tetratolylporphyrin) [CB11H5X6 ]2 X ¼ Cl, Br, H N-H—p bonding; evidence against “sitting-atop” metalloporphyrin complexes

S, X(Cl), H

[89]

MeCB11H5X6
X ¼ Cl, Br, I

S, X, H, B, C, IR, MS

[17]

S, X (molecular and extended crystal)

[174]

R HCB11Me5X6 R ¼ CMe3 , i-C5H11 , 2,3Me2C4H+, MeC5H4 + X ¼ Cl, Br, I hyperconjugation and H-bonding

IR(solid state, C
H bonds) hyperconjugation and H-bonding

[293]

CB11H11-2-Cl


S, H, B

[7]

S (electrochemical), MS

[42]

S, UV

[82]

S, H, C, IR

[82]

S

[175]

X

[176]

S, H, B

[15]

1-HCB11Cl11-n-Ph n ¼ 7,12 silylium HCB11Cl11 abstracts F from C6H5F to Si at r.t. +







RCB11Cl11 R ¼ Me, Et, n-C4H9, n-C6H13





+

CMe3 HCB11Cl11 C—H hydrogen bonding





+

+

+

+




H2N-CB11H9-2-F-12-I H2N–CB11H10-7-I







2+




Cs 1-HCB11H5X6 X ¼ Cl, Br +




+

+

+

+

(Me2HN)CB11H10-12-Cl +

C6Me6 HCB11H6Cl6 C6 H7 + CB11H6Cl6







+

Ph3C CB11H6-7,8,9,10,11,12-Cl6 CB11H6Cl6




CB11H7-7,8,9,10,12-Cl5







Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

269

TABLE 8-1 CB11H122 Derivatives—cont’d Compound

Information

References

M CB11H6X6 CTVH2OL X ¼ Cl, Br; L ¼ CF3CH2OH, MeCN; M ¼ Na, K, Rb, Cs; CTV ¼ cyclotriveratrylene coordination chains

S, X, IR

[122]

Na+ (CTV)(H2O)CB11H6Cl6 + cyclotriveratrylene host-guest

S, X

[123]

H+ CB11H6Cl6


S, H, C, UV, IR

[84]

S, H, C, UV, IR

[84]

S, H, C, UV, IR, ESR

[84]

S, H, B

[15]

Acid strength

[71]




+

+

HC60 CB11H6Cl6 HC60

+




CB11H6Cl6





CB11H6-7,8,9,10,11,12-X6 X ¼ Cl, Br +

H HCB11Cl11







+

Cs CB11Cl11 improved synthesis via SbCl5

S, H, B, C, MS

[57] +

IR study of H(H2O)n structure

[78]

IR (gas and solid), X

[72]

Acid strength

[71]

S, X

[92]

H3O 3C6H6 HCB11Cl11 C6H6 hydronium ion-benzene p-complex

S, X, IR

[76]

H3O+ HCB11Cl11


S, X, IR

[101]

S, IR

[77]

S, X, IR

[96]




+

H HCB11Cl11 Cl–H–Cl bridged linear polymer


H HCB11H5X6 X ¼ Cl, Br, I +




+

Et3Si HCB11Cl11 silylium ion


+

+

H2 O5 HCB11Cl11







R2Cl HCB11Cl11 R ¼ Me, Et chloronium ions +




+

(n-C8H17)3NH HCB11Cl11 / HCB11H5Cl6




IR (nN-H as a measure of acid strength)

[73]

H3O+ HCB11H5X6
X ¼ Cl, Br, I

S, X, IR

[101]

H3O+ HCB11Me5X6
X ¼ Cl, Br, I

S, X(Br), IR

[101]

H2 O5 + HCB11Me5Br6


S, IR

[77]

Ph3C+ CB11H11Br


S, H, B

[133]

CB11H11-12-X
X ¼ Cl, Br

S, X, H, B(Br)

[14]

CB11H11-2-Br


S, X, H, B, C, MS

[7]

C½C13 H6 ðOMeÞ2 SMe22 + [HCB11H5Br6
]2 hexacoordinate carbon cation

S, X, H, C, IR

[181]

HðC3 H7 Þ3 + HCB11H5X6
R ¼ CHMe2, C6H11, cyclo-C5H9; X ¼ H, Br

S, H, B, P

[182]

H(arene)+ HCB11R5X6
R ¼ H, Me; X ¼ Cl, Br; arene ¼ benzene, toluene, m-xylene, mesitylene, hexamethylbenzene

S, X, C, H, IR

[81]

(Me2CH)3Si+ CB11H6-7,8,9,10,11-Br6


S, X, B, Si

[93]

S, X

[58]

S, X, H, B, C, MS

[57]

X

[58]

S, UV, ESR, IR, COND

[87]

S, H, B, IR

[114]

HCB11Br5I6





+

Cs CB11Br11 improved synthesis via SbCl5 HCB11Br11 +




C76 CB11H6Br6 +




Ph3C CB11H6-7,8,9,10,11,12-Br6




Continued

270

Carboranes

TABLE 8-1 CB11H122 Derivatives—cont’d Compound +

Na CB11H6-7,8,9,10,11,12-Br6







HCB11X11 X ¼ Cl, Br, I


HCB11X11 X ¼ Cl, I CB11Br12

Information

References

Luminescence/phosphorescence

[184]

S, X, H, B, C, IR, MS(Br)

[21]

IR study of




H(H2O)n+

structure

[78]

S, X, H, B, C, IR, MS

[21]

X

[58]

HCB11Cl5Br6 , HCB11Br5I6 , HCB11Br6I5 , HCB11Br11


S, H, B, C, IR, MS (negative ion)

[58]

HCB11Me5X6
X ¼ Cl, Br, I

S, X(Br), Si (in Me2CHSi+ salts)

[61]

(2,4,6-C6Me3H2)3Si HCB11Me5Br6 trimesitylsilyl cation; silylium ion isolated

S, X, Si

[94]

MeCB11X11
X ¼ Cl, Br, I

S, X, H, B, C, IR, MS(Br)

[21]

S, H, B, C

[40]

S, H, B, C

[50]

HCB11Cl5Br6














+




PhCB11H10-12-I

PhCB11H5-7,8,9,10,11-I6 +

H9O4 CB11H6Br6







S, X

[102]

MeCB11H10-12-I


S, H, B, C, IR, MS

[33]

Li+ RCB11H10-12-I
R ¼ Et, n-C3H7, n-C4H9, n-C6H13, 2-EtC6H12

S, H, C, B, IR, MS

[253]

HCB11H10-12-I


S (improved)

[33]

S, B(2d[Cl]), H

[14]

S, H, B

[15,89]

S, H, B, B(2d), C, MS

[189]

S, H, B, C, IR, MS

[33]

1,2-(cyclo-N3R)CB11Cl10 R ¼ Ph, p-C6H4F, o/p-C6H4OMe, C6H2Me3, n-C4H9, adamantyl

S, X(Ph), H, B, C, MS

[289]

HCB11Cl6Br5
, HCB11Cl5Br6
, HCB11Cl6I5
, RCB11Cl11
R ¼ H, Me

S, H, B, C, IR, MS (negative ion)

[58]

(n-C8H17)3NH+ HCB11H5I6


IR (nN-H as a measure of acid strength)

[73]

S, H, B, IR, MS

[55]

S, X

[191]




HCB11H9-7,12-X2 X ¼ Cl, Br, I HCB11H9-7,12-I2





HCB11H9-7-I-12-X X ¼ F, Cl, Br, OH MeCB11H5-7,8,9,10,11,12-I6







HCB11I11





HCB11I11 enclosing nanotubes

H(H2O)n+

cations in

(H3N)CB11I11

S, H, B, IR, MS

[55]




S, H, B, C, IR, MS (negative .ion)

[58]




(Me2CH)3Si CB11H6-7,8,9,10,11,12-X6 X ¼ Cl, Br, I

S, X, B, Si

[175]

H solventÞ2 + HCB11R5X6
R ¼ H, Me; X ¼ Cl, Br; solvent ¼ organic O-donor: OEt2, THF, benzophenone, nitrobenzene) O–H–O bonding oxonium ions

S

[83]

Et2Al+ HCB11H5X6
X ¼ Cl, Br

S, X, H, B

[111]

S, X, H, B, C

[268]

S, X(Hg), IR, DSC

[257]







HCB11Br5I6 , HCB11Br6I5 , HCB11Cl6I5 +

Transition Metal s- and m-Complexes HCB11H11Zn(Z3-C6H6)Et Hgð1-CB11 F11 Þ2

2


Lewis acidity

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

271

TABLE 8-1 CB11H122 Derivatives—cont’d Compound

Information

References

S, X, H, B, C, Hg, MS

[270]

S, X(H, Br), H, B, C, Hg, MS

[270]

MS

[270]

S, X, M€ ossbauer, MAG, H, IR

[195]

S, X, M€ ossbauer, MAG

[198]

S, IR, MAG, UV, COND, thermal analysis

[199]

S, X, B, F

[51]

S, X, H(2d), B(2d), C, IR

[203]

S, X, H, B, C

[204]

X, H, B, IR

[133]

(Z -C8H12)Rh(Z -CB11H12) (2 B
H
Rh)

S, X, H, B

[208]

(Ph3P)2Rh(m-H)2-CB11H10

S, X, H, B, P

[210]

S, X, H, B

[107]

S, X

[212]

exo-closo-(R3P)2Rh-CB11H12 R3P ¼ P(OMe)3, P(MeC6H4CHMe2), 0.5Ph2PCH2CH2PPh2

S, X [P(MeC6H4CHMe2)], H, B, P

[209]

1,2,3-cyclo-(C8H12)Ir[PðCHMe2 Þ2 + -1-CB11H11
2 Ir
H
B agostic bonds

S, X, H(var T), B(var T), IR

[301]

AgðMeCNÞ2 + /AgðNCCH2 CNÞ2 + / CB11 H12


S, X, IR

[110]

{[2,6-(MeO)2C6H3]SnPh2 }2 AgðCB11 H12 Þ3 stabilization by pincer-type triaryltin ligand

S, X, H, B, Sn, IR, MS

[150]

Ag+ CB11H6X6
X ¼ Cl, Br, I

X

[115]

S, H, B, IR

[114]

S, X, B, P, IR

[108]

S, X, H, B, C, IR, MS, Raman

[85]

Fe(TPP)(C8H10) AgðCB11 H6 Br6 Þ2 TPP ¼ tetraphenylporphyrinate

S, X

[193,194]

Fe(TPP)+ CB11 H12


ESR, Mossbauer, MAG (Fe spin-state mixing and ligand field strength)

[218]

CB11H6Br3-(m-Br)3PtMe3

S, X, H, B

[221]

S, B, IR, Raman

[44]

S, X, IR, H(Sm), C(Sm), B

[119]

Electrophilic substitution, deuteration

[32]

Fluorination

[46]

Thermolysis

[149]

Pd-catalyzed cross-coupling

[33]

HgðCB11 X11 Þ2

2


PhHg(CB11I11)

X ¼ H, Cl, Br




HgðCB11 I11 Þ2 2
Fe(TPP)+ CB11H6Br6
+

Fe(Ph4 porphyrinate) CB11 H12 MðphenanthrolineÞ3 ClCu-CB11F11





2+

[CB11 H12 ]2 M ¼ Co, Ni




+

Cp2ZrMe (12-m-Me)CB11HMe10




2

Cp2Zr(Z -CH2Ph)-CB11H12 B
H
Zr


+

MoCp COÞ3 CB11H11Br 4

2

+

(Ph3P)2Rh(norbornadiene) HCB11H6Et5 +

[Cp*RhCl ]2CB11H 12




2





+

+

Ag CB11H6-7,8,9,10,11,12-Br6 +

(Ph3P)Ag CB11 H12


2


C59N AgðCB11 H6 Cl6 Þ +




azafullerenes


+

(Me3N)CB11(HgOCOCF3)11 +

[1,3-(Me3Si)2C5H3]2LnðTHFÞ2 CB11H6Br6 Ln ¼ Sm, Er




Other Experimental Studies Reactivity and Kinetics CB11 H12


Continued

272

Carboranes

TABLE 8-1 CB11H122 Derivatives—cont’d Compound

Information

References

Catalysis of radical polymerization of 1-alkenes

[260]

Catalysis of radical polymerization of isobutylene

[222]

(Me3N)CB11H11

Controlled chemical and electrochemical substitution Cl, D, OH, CO, COOH, B(2d), H, MS

[42]

MðenÞ3 2 + [CB11 H12
]2 M ¼ Co, Ni

Thermal decomposition

[200]

X(CH2)nCB11Me11
X ¼ Cl, Br; n ¼ 3,4,7

Side-chain C
C cleavage

[31]




+

Li salts of CB11 H12 derivatives


CB11 Me12 Li

HCB11Me11

+




1-(H2N)CB11F11

Cage decomposition; B-C bond activation


Extrusion of C to form closo- 3-(NC)B11F10

[117] 2


[52]

Catalysis Li+ CB11 Me12


Promotes polymerization of isobutylene with air or di-tert-butylperoxide initiators to form highly branched and linear polyisobutylene

[261,282]

Ag+CB11 H12


Activation of Co/Ni/Pd catalysts for vinyl addition/ polymerization of norbornene

[214]

R3Si+ HCB11HnCl11
n
R ¼ Et, n-C6H13; n ¼ 0, 5

Hydrodehalogenation catalysts for C
F, C
Cl and C
Br bonds

[97]

RhCl(cod)+ CB11 H12


Catalyst for addition of arylboronic acids to aldehydes (weakly coordinating anion)

[223]

(L)Rh PPh3 Þ2+ CB11H6Br6
L ¼ Z4-C6H8, Z4-C7H10

Intermediate in catalytic dehydrogenation of cyclohexane to benzene

[211]

(norbornadiene)Rh(PPh3)+ CB11 H12
/CB11H6Br6


Catalytic hydrogenation of alkenes

[210]

(Ph3P)Ag CB11 H12
½ Ph3 PÞAg + CB11 H12
2 2
, (Ph3P)Ag+ CB11H6Br6
, (Ph3P)2Ag+ CB11H6Br6


Catalysis of a Diels-Alder reaction

[108]

Et2Al+ HCB11H5X6
X ¼ Cl, Br

Lewis acid activation of ethylene; catalytic oligomerization of ethylene; catalytic polymerization of cyclohexane oxide

[111]

HCB11H5X6
X ¼ H, Cl, Br

Counterion for hydrodefluorination of perfluoroalkyl groups by SiR3 R ¼ Et, n-C6H13 Si-H / C-F metathesis

[98]

HCB11X6Y5
X ¼ Br, Cl; Y ¼ H, Cl, Me

Gas phase acidity; electron binding energy

[224]

[-(CH2)n-NH(CB11H11)
]n polymers

Copolymerization of CB11H11
with dibromoalkanes, TLC

[155]

HCB11Br11


Ion exchanger in cation-selective polymeric membrane electrode sensors

[226]

RCB11H10-12-R0
R ¼ H, C7H6; R0 ¼ C7H6, H

Hyperpolarizability; NLO

[227]

¼ H, Me; X ¼ Cl, Br; H(arene) arene ¼ benzene, toluene, m-xylene, mesitylene, hexamethylbenzene

Stabilization of benzenium ion salts; carborane superacids

[81]

Me3NH+ HCB11X11
X ¼ Cl, Br, I; Me3NH+ HCB11H5Br6


Ion-pairing ability in membranes; selectivity; ion-selective electrodes cation exchangers; ionophorebased sensing platforms; natural population analysis charges

[228]

HCB11Me5X6
X ¼ Cl, Br, I

Cation interactions (least nucleophilic anion)

[61]

+

Other Applications

+

HCB11R5X6
R

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

273

TABLE 8-1 CB11H122 Derivatives—cont’d Compound

Information

References

X

[121]

S, H, B, MS, IR, UV

[220]

Ab initio

[230,231]

EI [energy indexes]; stabilities

[233]

Antipodal effect

[234]

Optimized geometry, IR-active vibrations, charge distribution

[235]

Vibrational modes

[148]

Influence of charge, spin, substituents, and atom encapsulation on volume of cage

[236]

DFT: solid state interaction with benzene via C
H p bonds and B
H- - -H
C dihydrogen bonds

[252]

[H11B11C
C C
CB11H11] biradical with singlettriplet near- degeneracy; candidate for molecular magnet

DFT, ab initio

[239]

X@CB11 H12
X ¼ Li+, Be2
carboraneencapsulated ions

DFT, stability

[240]

CB11 R12
n ¼ 0,1 R ¼ H, Me

Energies, ionization potentials

[241]

TTF+ CB11 Me12
TTF ¼ tetrathiofulvalene

Electronic structure

[142]

C7 H6 + CB11H11
tropylium

(Ab initio) charge-transfer

[242]

(C7H6)CB11H10-12-C5H4
R ¼ H, C5H4


b (first hyperpolarizability); NLO

[151]

exo-closo-(R3P)2Rh-CB11H12 R3P ¼ P(OMe)3, P(MeC6H4CHMe2), 0.5 Ph2PCH2CH2PPh2

DFT isomers

[209]

Et2Al+ HCB11H5X6
X ¼ Cl, Br

DFT geometry and electrostatic Al-CB11 bonding

[244]

UV-vis and transient absorption spectra

[86]

DFT: bridged linear polymer structure

[72]

DFT: isomers

[134]

DFT, electron transport in a single molecule junction

[264]

DFT

[266]

R1R2R3C9H12 + CB11H11
R1 ¼ R2 ¼ H, R3 ¼ n-C3H7 oxatriquinanes R3O+…H+ bond

DFT

[271]

XCB11Me10-12-Y
X ¼ Me, Et, Pr, Bu, OMe, F, Br, I, H, C(O)OH, C(O)OMe, CH2CHEtBu, C6H13 Y ¼ H, Me, F, Cl, Br, I

Oxidation potential

[275]

CB11MenðCD3 Þ12
n
 n ¼ 0–12 radicals 16 isomers

DFT: spin density distribution

[279]

CB11H11-12-NH3

DFT: structure

[287]

1,2-(cyclo-N3R)CB11Cl10
R

DFT: electronic structure

[289]




+

Ag[CH(C6H5)3] MeCB11F11 super-weak anion


(chlorin)M-CB11H11 Cs M ¼ Pd, Sn(OH)2, Zn light-independent cytotoxicity / photodynamic tumor therapy +

Theoretical Studies CB11 H12


+

+

+

C59N Ag [CB11H6Cl6 ]2 +

H HCB11Cl11




CB11Me11 bornium ylide; naked B(12) vertex


X @CB11 H12 X ¼ Li, Na +

+

Me2SiC6H3-2,6-ðC6 H2 Me2 XÞ2 HCB11H5Cl6 X ¼ F, Cl stable terphenylsilylium ions

+

¼ Ph, p-C6H4F, o/p-C6H4OMe, C6H2Me3, n-C4H9, adamantyl

Continued

274

Carboranes

TABLE 8-1 CB11H122 Derivatives—cont’d Compound

Information

References

1,2-(cyclo-N3PhMe)CB11H10
triazole radical anion

S, X, ESR, E, UV

[309]

CB11H11-n-I
n ¼ 7,12

Electronic structure

[292]

H2N-CB11H10-12-R R ¼ H, C C-H, C C-Ph, C C-SiEt3

DFT, bond lengths, NMR

[305]

Me2Si+ CH2-CB11Cl11


DFT: structure, 29Si NMR

[307]

RO-C5H5N CB11H10-12-Cn2n+1 n ¼ 5, 6, 10 R ¼ C7H15, CHMeC6H13 liquid crystals

DFT: energy levels

[311]

p-RC6H4-CB11H11
R ¼ C(O)OMe, CN, NO2, H, n-C12H25, OMe, OCH2SiCMe3

DFT: energy levels

[317]

B

[144,245]

GIAO

[35]

B, F

[144]

GIAO Ag–B interactions in solution

[216]

(Me2CH)3Si+ CB11H6-7,8,9,10,11,12-X6
X ¼ Cl, Br, I

IGLO

[175]

CB11H11-n-I
n ¼ 7,12

B

[292]

HCB11H9-7,12-ðC CHÞ2


DFT: structure

[36]

CB11FnH12
n
n ¼ 1,6,12

Brønsted acidity

[62]

H+ CB11H11X
X ¼ CF3SO2, NO2, NMe2, CMe3

DFT: structure, gas phase Brønsted acidity

[74]

HCB11H11
X ¼ H, Me, Cl, F

Molecular electrostatic potential; acidity

[65]

Catalytic activity in olefin hydrogenation

[118]




+

NMR Calculations CB11 H12



CB11H11-12-R R ¼ C CPh, Cl 1-(H2N)CB11F11




+

[AgðIMesÞ2 ]2 Ag2ðcloso
CB11 H12 Þ4 dimesitylimidazol-2-ylidene

2


IMes ¼

Reactivity Calculations

+

(Ph3P)2H2Ir CB11H6Cl6

+

Me3M CB11 Me12
M ¼ Ge, Sn, Pb +

Me3M Me interactions +

[128]

1-CB11 H12
-12-

Electron-donating strength determined from H NMR of b-H

[303]

CB11H11-12-NH3

DFT: pKa

[287]

DFT: Lewis acidity

[270]

DFT: Lewis acidity

[270]

Molecular dynamics in gas flow- or electric fielddriven rotors

[274]

DFT: molecular orbitals

[309]

HgðCB11 X11 Þ2

2


PhHg(CB11X11)

1

X ¼ H, Cl, B




X ¼ H, Cl, Br, I

Molecular Dynamics Calculations 1,10-[tricyclo-C(CH2)3C]2CB11L5 L ¼ B,B0 -attached planar hydrocarbon "blade”; molecular rotor Other Calculations 1,2-(cyclo-N3PhMe)CB11H10
triazole radical anion a

Supplemental data for this table is available in Appendix C (electronic version) and on the book’s companion website http://booksite.elsevier.com/ 9780128018941/. S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; N, 15N NMR; F, 19F NMR; P, 31P NMR; Si, 29Si NMR; Li, 7Li NMR; Pt, 195Pt NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; E, electrochemical data; ESR, electron spin resonance data; MAG, magnetic susceptibility; COND, electrical conductivity; OR, optical rotation; NLO, nonlinear optical properties; NQR, nuclear quadrupole resonance; DSC, differential scanning calorimetry. b

Icosahedral Carboranes: Closo-CB11 Clusters Chapter 8

275

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