Decaborane: From Alfred Stock and Rocket Fuel Projects to Nowadays ^†

Abstract

The review covers more than a century of decaborane chemistry from the first synthesis by Alfred Stock to the present day. The main attention is paid to the reactions of the substitution of hydrogen atoms by various atoms and groups with the formation of exo-polyhedral boron–halogen, boron–oxygen, boron–sulfur, boron–nitrogen, boron–phosphorus, and boron–carbon bonds. Particular attention is paid to the chemistry of conjucto-borane anti-[B₁₈H₂₂], whose structure is formed by two decaborane moieties with a common edge, the chemistry of which has been intensively developed in the last decade.

1. Introduction

Decaborane [B₁₀H₁₄] plays a central role in the chemistry of polyhedral boron hydrides. Decaborane is an essential boron reagent for the preparation of medium and higher carboranes C₂B_nH_n+2 (n = 8–10) [1] and the carba-closo-decaborate anions [CB₉H₁₀]⁻ [2]. Until recently, the synthesis of the closo-dodecaborate [B₁₂H₁₂]²⁻ [3,4] and the carba-closo-dodecaborate [CB₁₁H₁₂]⁻ [5,6] anions was also based on the use of decaborane, and it is still used for the synthesis of the closo-decaborate anion [B₁₀H₁₀]²⁻ [7,8]. In addition, decaborane can be used to prepare boron coatings [9,10,11,12,13], nanoparticles [14], microcrystals [15,16], boron nitride nanosheets [17], and nanotubes [18], as well as various metal boride thin films [19,20,21,22,23,24]. Recently, a decaborane-based fuel cell power source with a high energy density was developed [25]. The intensive development of the chemistry of decaborane is associated with the 1950s to the early 1960s, when the main types of its transformations were discovered and described. These early studies were reviewed in the 1960s by Hawthorne [26] and Zakharkin et al. [27]. This area was also partly elucidated in Boron Hydride Chemistry [28] and Comprehensive Inorganic Chemistry I [29]. Recent studies in the field of decaborane chemistry, deepening and expanding the previously described conclusions using modern instrumental methods, were briefly covered in Comprehensive Inorganic Chemistry III [30]. Therefore, the purpose of this review is to give the most complete picture of the current state of the chemistry of decaborane and its derivatives.

2. Synthesis, Structure, and General Properties

The formation of this ten-vertex cluster during the pyrolysis of diborane B₂H₆ was first described by Alfred Stock and co-workers more than 100 years ago [31,32]. The best yields of decaborane(14) were obtained by heating diborane to 120 °C for 47 h. The low volatility of decaborane allows it to be easily separated from other volatile boron hydrides while being volatile enough to be easily separated from non-volatile products. Decaborane is a colorless, air-stable, easily subliming, malodorous, crystalline solid that melts at 99.7 °C and boils with decomposition at 213 °C [33]. For a long time, interest in the chemistry of boron hydrides was mainly academic but was supported by the fact that boranes and some related compounds did not comply with the usual rules relating the chemical composition to the classical theory of valence. At the same time, various assumptions were made about the structure of B₁₀H₁₄, including linear [34] or naphthalene-like [35] structures.

Practical interest in boron hydrides, and decaborane in particular, arose shortly after World War II, when the United States government launched programs (Projects Hermes, Zip, and HEF (High-Energy Fuels)) [36,37] whose purpose was to develop borane-based aviation and rocket fuels capable of generating much higher energy than conventional kerosene-based fuels [38,39,40]. As a part of this program, two chemical companies, Callery Chemical Company and Olin-Mathieson Corporation, developed eight pilot and production plants and produced an array of borane-derived energetic products, including methyldecaborane (HEF-4), ethyldecaborane (HEF-3), and ethylacetylenedecaborane (HEF-5), to be tested as additives to propellants and explosives [41]. Amost at the same time, due to the development of various physical research methods, such as single-crystal X-ray diffraction, neutron diffraction, and gas phase electron diffraction, the molecular structure of [B₁₀H₁₄] was determined [42,43,44,45,46,47,48,49,50]. The decaborane molecule was found to be shaped like a boat built from ten BH-units, with four additional BHB bridges decorating its bow and stern (Figure 1).

Shortly thereafter, the future Nobel Winner Lipscomb and his collaborators developed bond counting rules and topological principles that made it possible to describe bonding in boron hydrides. According to the topological formalism, the binding in the decaborane molecule can be described by a combination of four 3c-2e B-H-B bonds, six closed or fractional closed 3c-2e B-B-B bonds, and two 2c-2e B-B bonds [51,52,53]. Some time later, molecular orbital theory in the form of the extended Hückel theory was originated and applied to decaborane to provide an alternative to this topological approach [54,55,56]. Subsequently, both the logical basis and the parameters for these molecular orbitals were greatly improved using the more rigorous molecular self-consistent field (SCF) method [57,58]. More recently, the electronic structure of decaborane has been described in terms of the BadeR′s theory “Atoms in Molecules” (AIM) [59].

A powerful tool for determining the structure of polyhedral boron hydrides is NMR spectroscopy, the practical birth of which coincided with a wave of interest in the chemistry of boron hydrides. Therefore, it is not surprising that decaborane was one of the first molecules to be investigated using NMR spectroscopy [60,61,62]. The subsequent development of the instrumental base and methods of NMR spectroscopy caused repeated studies [63,64,65,66,67,68,69,70]. The decaborane molecule has also been characterized by IR [59,71,72], Raman [59], electron [73,74,75], NQR [76,77], photoelectron [78], and electron energy loss [79] spectroscopy. The ionization potentials of decaborane and ¹¹B-enriched decaborane were determined to be 11.0 eV [80] and 10.26 eV [81], respectively. The dipole moment of decaborane was determined by measuring the dielectric constants of benzene, cyclohexane, and carbon disulfide solutions and varies from 3.17 D in carbon disulfide to 3.62 D in benzene [82]. The magnetic susceptibility of decaborane is −116 ± 1.5 × 10⁻⁶ emu mol⁻¹ [82]. The heat of formation of decaborane was determined to be −66.1 kJ/mol [83]. The heat capacity of decaborane has been measured, and the derived thermodynamic functions have been calculated [84,85]. The heats of melting and vaporization [85], as well as the vapor pressure of decaborane [85,86], were also determined. Pressure-induced room temperature transformations of decaborane up to 131 GPa were studied using in situ optical spectroscopy techniques [87].

The industrial production of boron hydrides, which involved more than 2000 people, was accompanied by various accidents, which led to the discovery of the high toxicity of decaborane and its derivatives [88,89,90,91,92]. With decaborane, intoxication, headaches, tremors, impaired coordination, confusion, anxiety, photophobia, and other symptoms are observed. Moreover, intoxication can occur from relatively small amounts of decaborane. Decaborane can be detected by its odor at or near its maximum acceptable concentration, but there is considerable olfactory fatigue. Repeated exposure to decaborane can cause severe damage to the nervous system [93]. The effects of decaborane on various animals have also been studied [93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111]. A number of studies were directed to study the mechanism of decaborane action on living organisms [112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130].

The production of decaborane, established in the 1950s, was based on the pyrolytic conversion of diborane proposed by Alfred Stock [33]. At the same time, attempts were made to find an alternative to this dangerous process, among which the use of the CW CO₂ laser is worth mentioning [131]. Almost at the same time, a convenient and effective method was proposed, which is based on the oxidation of sodium tetrahydroborate NaBH₄ to the octahydrotriborate anion [B₃H₈]⁻, followed by its pyrolysis in diglyme at 105 °C to the tetradecahydro-nido-undecaborate anion [B₁₁H₁₄]⁻ [132]. The subsequent mild oxidation of [B₁₁H₁₄]⁻ gives decaborane [B₁₀H₁₄] (Scheme 1) [133,134,135,136]. Decaborane can also be obtained by the cage-opening of the closo-decaborate anion [B₁₀H₁₀]²⁻ on protonation with strong acids such as sulfuric acid [137].

Decaborane(14) has an acidic character [138] and can be deprotonated with strong bases, such as sodium hydride [139], tetraalkylammonium hydroxides [140], diethylamine [140], triethylamine [140,141], methylenetriphenylphosphorane [141,142], or a Proton Sponge (PS) [143,144] to give the corresponding salts of the tridecahydro-nido-decaborate [B₁₀H₁₃]⁻ anion (Scheme 2). The pK_a value of decaborane(14) in aqueous ethanol was found to vary from 2.41 to 3.21 depending on the water content [145].

The (Et₃NH)[B₁₀H₁₃] and (Et₄N)[B₁₀H₁₃] salts obtained by the deprotonation of decaborane with Et₃N and (Et₄N)OH, respectively, have been found to trigger the hypergolic reactivity of some polar aprotic organic solvents, such as tetrahydrofuran and ethyl acetate [146].

The solid state structures of (Et₃NH)[B₁₀H₁₃] [147], (BnNMe₃)[B₁₀H₁₃] [148], and (HPS)[B₁₀H₁₃] [144] were determined by single-crystal X-ray diffraction. The solid state structure of the [B₁₀H₁₃]⁻ anion (Figure 2) can be derived from the structure of B₁₀H₁₄ by μ-H(9,10) deprotonation.

In the solution, the [B₁₀H₁₃]⁻ anion exists as a mixture of symmetrical and unsymmetrical H-tautomers with different arrangements of bridging hydrogens (Figure 2) [143,149], with an interconversion ΔG^‡ value of less than 7 kcal/mol [144].

Strong bases such as sodium hydride in ether solvents are able to remove two protons from decaborane(14) to form the [B₁₀H₁₂]²⁻ dianion [140,150,151]. The latter is unstable in solution and transforms into other decaborates and their derivatives [151]. According to quantum chemical calculations, the [B₁₀H₁₂]²⁻ anion has the C₂-symmetric structure with μ-B(5)HB(6) and μ-B(8)HB(9) bridging hydrogens [152].

The reduction of decaborane(14) with KBH₄ in water results in the formation of the [arachno-B₁₀H₁₄]²⁻ anion with a boron cage geometry near the same as that of the starting [nido-B₁₀H₁₄] (Scheme 3), which was isolated by precipitation from an aqueous solution in the form of rubidium, cesium, or tetramethylammonium salts [153,154]. The structure of the [B₁₀H₁₄]²⁻ anion was proposed using ¹¹B NMR spectroscopy [155]. The solid state structure of (Me₄N)₂[B₁₀H₁₄] was determined by single-crystal X-ray diffraction [154]. It was supposed that the reaction proceeds by hydride transfer with the formation of the [B₁₀H₁₅]⁻ anion. The latter is unstable in the solution and loses hydrogen to form [nido-B₁₀H₁₃]⁻ [156,157].

It should be borne in mind that decaborane itself has pronounced reducing properties. Due to this, the possibility of its use as a reducing agent in organic synthesis was studied. In particular, decaborane can be used for the reduction of acetals to ethers [158], the reductive esterification of aromatic aldehydes [159,160], and the reductive amination of acetals with aromatic amines [161]. Decaborane can also be used for chemoselective reduction aldehydes and ketones [162,163,164], the dehalogenation of α-halocarbonyl compounds [165], and the hydrogenation of alkenes or alkynes [166].

The bridging hydrogens in decaborane(14) were found to exchange rapidly for deuterium atoms with D₂O in 1,4-dioxane or acetonitrile to give [μ₄-B₁₀H₁₀D₄] [72,167,168]. The use of DCl in 1,4-dioxane makes it possible to obtain the decadeuterated decaborane [μ₄-5,6,7,8,9,10-B₁₀H₄D₁₀] [169]. The treatment of decaborane(14) with DCl in a carbon disulfide solution and in the presence of AlCl₃ results in the tetradeuterated decaborane [1,2,3,4-B₁₀H₁₀D₄] [169,170]. If the reaction is carried out under heating in sealed ampoule, the product is the octadeuterated decaborane [1,2,3,4,5,7,8,10-B₁₀H₆D₈] [72]. The tetradeuterated decaborane [5,7,8,10-B₁₀H₁₀D₄] was obtained by the reaction of [1,2,3,4,5,7,8,10-B₁₀H₆D₈] with HCl in a carbon disulfide solution and in the presence of AlCl₃ [72]. The octadeuterated decaborane [μ₄-1,2,3,4-B₁₀H₆D₈] was prepared by the reaction of [1,2,3,4-B₁₀H₁₀D₄] with D₂O in acetonitrile [72], whereas the dodecadeuterated decaborane [μ₄-1,2,3,4,5,7,8,10-B₁₀H₂D₁₂] was obtained by heating [μ₄-B₁₀H₁₀D₄] with DCl in carbon disulfide in sealed ampoule in the presence of AlCl₃ [72]. Deuterated aromatic solvents can also act as a source of deuterium. For example, heating decaborane(14) in benzene-d₆ in the presence of AlCl₃ under reflux leads to the the tetradeuterated decaborane [1,2,3,4-B₁₀H₁₀D₄], while the reaction of decaborane(14) with AlCl₃ in toluene-d₈ at 5 °C results in the dideuterated decaborane [2,4-B₁₀H₁₂D₂] [171]. The reaction of [1,2,3,4-B₁₀H₁₀D₄] with AlCl₃ in benzene leads to [1,3-B₁₀H₁₂D₂] [172].

3. Halogen Derivatives

Stock first reported the preparation of halogen derivatives of decaborane(14) by the direct reaction of decaborane with halogens in a sealed tube [33]. These reactions were re-investigated in the 1960s. It was found that the reaction of decaborane(14) with 1 equiv. iodine at 110–120 °C leads to the formation of a mixture of 1- and 2-iodo derivatives of decaborane in a ratio of ~1:2 [173,174]. The resulting mixture of isomers can be separated by fractional crystallization from low-boiling alkanes (pentane, hexane, heptane) [173] or chromatographically [175]. The assignment of the substitution position was made based on the ¹¹B NMR spectra [174,176,177]; however, the 1-isomer was initially erroneously assigned as the 5-isomer [173,178]. The subsequent reaction of 2-iododecaborane with iodine at 110 °C results in a mixture of the 1,2- and 2,4-diiodo derivatives [1,2-I₂-B₁₀H₁₂] and [2,4-I₂-B₁₀H₁₂] in a nearly equal ratio, while the similar reaction of 1-iododecaborane produces mainly [1,2-I₂-B₁₀H₁₂]. The reaction of decaborane(14) with an excess of iodine was found to give a mixture of [1,2-I₂-B₁₀H₁₂] and [2,4-I₂-B₁₀H₁₂] in a ratio of ~1:2 [173]. The reaction of decaborane(14) with iodine or iodine chloride in carbon disulfide in the presence of AlCl₃ was found to give a mixture of the 1- and 2-iodo derivatives of decaborane in the same ratio of ~1:2 [179]. Later, the 1- and 2-iodo derivatives of decaborane were synthesized by the reaction of decaborane(14) with iodine chloride in refluxing dichloromethane in the presence of AlCl₃ and reliably characterized by NMR spectroscopy [180]. The solid state structures of [1-I-B₁₀H₁₃] [181], [2-I-B₁₀H₁₃] (Figure 3) [180], and [2,4-I₂-B₁₀H₁₂] [182] were determined by single-crystal X-ray diffraction.

The 5- and 6-iodo derivatives of decaborane were prepared in an indirect way. The treatment of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] with anhydrous HI in benzene under reflux conditions results in a mixture of the 5- and 6-iodo derivatives of decaborane [5-I-B₁₀H₁₃] and [6-I-B₁₀H₁₃] [174], which was separated by fraction crystallization from hexane [174] or chromatographically [175]. The reaction of [arachno-6,9-(Et₂S)₂-B₁₀H₁₂] with anhydrous HI in benzene at room temperature was found to give the 5-iodo derivative [5-I-B₁₀H₁₃] [183,184,185]. The reaction of (NH₄)₂[closo-B₁₀H₁₀] with anhydrous HCl in a mixture of AlI₃ and 1-butyl-3-methylimidazolium iodide (bmimI) at 70 °C proceeds with the boron cage opening and results in the 6-iodo derivative of decaborane [6-I-B₁₀H₁₃] [186]. The 6-iodo derivative isomerizes to the 5-iodo derivative [5-I-B₁₀H₁₃] in the presence of a catalytic amount of triethylamine in toluene at 60 °C. It is assumed that the isomerization occurs through the transformation of [6-I-B₁₀H₁₃] into the [6-I-B₁₀H₁₃]⁻ anion, followed by its isomerization [187]. The 6-iodo derivative [6-I-B₁₀H₁₃] was also found to undergo photochemical isomerization to [5-I-B₁₀H₁₃] under UV-irradiation in a solution [187]. The solid state structures of [5-I-B₁₀H₁₃] [187] and [6-I-B₁₀H₁₃] [186] were determined by single-crystal X-ray diffraction (Figure 4). The 6-iodo derivative [6-I-B₁₀H₁₃] was also obtained by the reaction of (NH₄)₂[closo-B₁₀H₁₀] with AlI₃, followed by the hydrolysis of the resulting intermediate [188].

The reactions of decaborane(14) with bromine in dichloromethane in the presence of AlCl₃ [189] or AlBr₃ [190], or in carbon disulfide in the presence of AlCl₃ [174], lead to the formation of a mixture of 1- and 2-bromo derivatives of decaborane [1-Br-B₁₀H₁₃] and [2-Br-B₁₀H₁₃], which can be separated by fractional crystallization from hexane [173,189,190] or chromatographically [175].

The 5- and 6-bromo derivatives of decaborane were also prepared in an indirect way. The treatment of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] with anhydrous HBr in benzene under reflux conditions results in a mixture of the 5- and 6-bromo derivatives of decaborane [5-Br-B₁₀H₁₃] and [6-Br-B₁₀H₁₃] in a ratio of ~1:4, which was separated by fraction crystallization from hexane [174] or chromatographically [175,190]. The reactions of [arachno-6,9-(R₂S)₂-B₁₀H₁₂] (R = Me, Et) with anhydrous HBr in benzene at room temperature were found to give the 5-bromo derivative [5-Br-B₁₀H₁₃] [183,184,185]. The reaction of (NH₄)₂[closo-B₁₀H₁₀] with anhydrous HBr in a mixture of AlBr₃ and 1-butyl-3-methylimidazolium bromide (bmimBr) at 70 °C proceeds with the boron cage opening and results in the 6-bromo derivative of decaborane [6-Br-B₁₀H₁₃] [186]. The 6-bromo derivative isomerizes to the 5-bromo derivative [5-Br-B₁₀H₁₃] in the presence of a catalytic amount of triethylamine in toluene at 60 °C [187]. The solid state structures of [5-Br-B₁₀H₁₃] [187] and [6-Br-B₁₀H₁₃] [186] were determined by single-crystal X-ray diffraction (Figure 5). The 6-bromo derivative [6-Br-B₁₀H₁₃] was also obtained by the reaction of (NH₄)₂[closo-B₁₀H₁₀] with AlBr₃, followed by the hydrolysis of the resulting intermediate [188].

The reaction of dimethylstannaundecaborane [nido-Me₂SnB₁₀H₁₂] with bromine in carbon disulfide leads to the oxidative removal of tin with the formation of the 5,10-dibromo derivative of decaborane [5,10-Br₂-B₁₀H₁₂] (Figure 6) [191].

The reaction of decaborane(14) with chlorine in dichloromethane in the presence of AlCl₃ results in a mixture of the 1- and 2-chloro derivatives of decaborane [1-Cl-B₁₀H₁₃] and [2-Cl-B₁₀H₁₃] [174,189]. Unexpectedly, a mixture of the 1- and 2-chloro derivatives of decaborane was obtained in the reaction of decaborane(14) with 1,1-difluoroethane in carbon disulfide in the presence of AlCl₃ [192]. The isomers were separated by fractional crystallization from pentane or hexane [174,189,192] or chromatographically [175], and the substitution position was assigned using ¹¹B NMR spectroscopy [174,193].

Similar to the corresponding iodo and bromo derivatives, the 5- and 6-chloro derivatives of decaborane were prepared in an indirect way. The treatment of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] with anhydrous HCl in benzene under reflux conditions results mainly in the 6-chloro derivative of decaborane [6-Cl-B₁₀H₁₃] with some amount of the 5-chloro isomer [174]. The reactions of [arachno-6,9-(Et₂S)₂-B₁₀H₁₂] with anhydrous HCl or HgCl₂ in benzene at room temperature were also found to give the 6-chloro derivative [6-Cl-B₁₀H₁₃] [183,184,185]. The reaction of (NH₄)₂[closo-B₁₀H₁₀] with anhydrous HCl in a mixture of AlCl₃ and 1-butyl-3-methylimidazolium chloride (bmimCl) at 70 °C proceeds with the boron cage opening and results in the 6-chloro derivative of decaborane [6-Cl-B₁₀H₁₃] [186]. The 6-chloro derivative of decaborane can also be prepared by the reactions of (NH₄)₂[closo-B₁₀H₁₀] with triflic acid in dichloromethane, respectively [186]. The 6-chloro derivative isomerizes to the 5-chloro derivative [5-Cl-B₁₀H₁₃] in the presence of a catalytic amount of triethylamine in toluene at 60 °C [187]. It was assumed that the isomerization occurs through the transformation of [6-Cl-B₁₀H₁₃] into the [6-Cl-B₁₀H₁₃]⁻ anion, followed by its isomerization. The solid state structures of [6-Cl-B₁₀H₁₃], (HPS)[6-Cl-B₁₀H₁₂], and (HPS)[5-Cl-B₁₀H₁₂] were determined by single-crystal X-ray diffraction (Figure 7) [186,187]. The B-H-B bridge deprotonation at the site adjacent to the halogenated boron atoms was revealed [187].

The 6-chloro derivative [6-Cl-B₁₀H₁₃] was also obtained by the reaction of (NH₄)₂[closo-B₁₀H₁₀] or (Et₄N)₂[closo-B₁₀H₁₀] with AlCl₃, followed by the hydrolysis of the resulting intermediate [188,194,195].

The 6-fluoro derivative of decaborane [6-F-B₁₀H₁₃] was first obtained by the reaction of [arachno-6,9-(Et₂S)₂-B₁₀H₁₂] with anhydrous HF in benzene at room temperature [183,184,185]. Later, the 6-fluoro derivative was prepared by the reaction of (NH₄)₂[closo-B₁₀H₁₀] with triflic acid in 1-fluoropentane [186]. The solid state structure of [6-F-B₁₀H₁₃] was determined by single-crystal X-ray diffraction (Figure 8) [186].

4. Derivatives with a B-O Bond

Due to the lack of electrophilic reagents for the introduction of oxygen substituents, the corresponding decaborane derivatives with substituents localized in the “bottom” of the decaborane basket have not yet been obtained. Alkoxy derivatives of decaborane [5-RO-B₁₀H₁₃] (R = Me, Et, Pr, Bu) were first obtained in low (13–20%) yields by trying to iodinate Na[B₁₀H₁₃] in the corresponding esters [196]. The phenoxy derivative [5-PhO-B₁₀H₁₃] was obtained in the same way, using anisole as a solvent [196]. The substitution position was determined using ¹¹B NMR spectroscopy [197]. It was assumed that their formation proceeds through the formation of oxonium derivatives of arachno-decaborane [R₂O-B₁₀H₁₃]⁻, followed by the elimination of one alkyl group [26]. The reaction of Na[B₁₀H₁₃] with SnCl₄ in diethyl ether leads to a mixture of 6- and 5-alkoxy derivatives of decaborane [6-EtO-B₁₀H₁₃] and [5-EtO-B₁₀H₁₃] in a ratio varying from 85:15 to 70:30 depending on the reaction temperature. The isomers were separated using column chromatography on silica [198]. It should be noted that the direct reactions of decaborane(14) with alcohols and phenols ROH leads to its complete degradation to the corresponding trialkyl- or triarylborates (RO)₃B [199]. The trimethylsiloxy derivative [6-Me₃SiO-B₁₀H₁₃] was obtained in a low yield (5–20%) from the reactions of Na[B₁₀H₁₃] and Na₂[B₁₀H₁₂] with Me₃SiCl in diethyl ether [198]. The report on the preparation of a trimethylsilyl derivative [Me₃Si-B₁₀H₁₃] under similar conditions [200] should apparently be considered erroneous.

The reactions of 5-bromo derivative [5-Br-B₁₀H₁₃] with alcohols ROH in the presence of NaHCO₃ in dichloromethane lead to 6-alkoxy derivatives [6-RO-B₁₀H₁₃] (R = Me, Et, t-Bu, c-Hx, CH₂CH₂SH, CH₂CH₂I, CH₂CH₂OCH₂CH₂Cl, (CH₂)₃C≡CH, CH(CH₂CH=CH₂)₂), while the reactions of 6-bromo derivative [6-Br-B₁₀H₁₃] with alcohols ROH in the presence of NaHCO₃ in dichloromethane lead to 5-alkoxy derivatives [5-RO-B₁₀H₁₃] (R = Me, t-Bu, cHx, CH₂CH₂SH, CH₂CH₂I, CH₂CH₂N(CO)₂C₂H₄, CH₂CH₂OCH₂CH₂Cl, (CH₂)₃C≡CH, CH₂C≡CCH₃, CH(CH₂CH=CH₂)₂). The reactions of [5-Br-B₁₀H₁₃] and [6-Br-B₁₀H₁₃] with 1,4-cyclohexyldiol lead to the compounds [μ-6,6′-(OC₆H₁₀O)-(B₁₀H₁₃)₂] and [μ-5,5′-(OC₆H₁₀O)-(B₁₀H₁₃)₂], respectively. The reactions of alcohols with [6-Br-B₁₀H₁₃] proceed quickly at room temperature, while those with [5-Br-B₁₀H₁₃] require heating (70 °C) to achieve completion. The reaction of [6-Br-B₁₀H₁₃] with ethanol was largely complete after 12 h at room temperature, but the reactions with 2-iodoethanol (~20 h), 2-bromoethanol (~40 h), 2-chloroethanol (~100 h), and 2-fluoroethanol (~125 h) all took increasingly longer times. The reactions with chloro- and iodo-derivatives of decaborane were found to proceed in a similar way; however, the reaction rate decreases in the halogen series I~Br > Cl [198]. The solid state structures of [5-MeO-B₁₀H₁₃] (Figure 9), [6-t-BuO-B₁₀H₁₃] (Figure 9), [5-ClCH₂CH₂OCH₂CH₂O-B₁₀H₁₃], [6-ClCH₂CH₂OCH₂CH₂O-B₁₀H₁₃], [5-MeC≡CCH₂O-B₁₀H₁₃], and [μ-6,6′-(OC₆H₁₀O)-(B₁₀H₁₃)₂] (Figure 9) were determined by single-crystal X-ray diffraction [201].

The 6-triflate derivative of decaborane [6-TfO-B₁₀H₁₃] was prepared by the reaction of Cs₂[closo-B₁₀H₁₀] with neat triflic acid at an ambient temperature [202,203]. In contrast, the reaction of (NH₄)₂[closo-B₁₀H₁₀] with triflic acid in 1-butyl-3-methylimidazolium triflate at 60 °C results in the 5-triflate derivative of decaborane [5-TfO-B₁₀H₁₃] [204]. It was found that the reaction proceeds through the formation of the 6-triflate derivative, which, upon heating, isomerizes into the 5-triflate derivative. In the presence of a catalytic amount of triethylamine, the isomerization of [6-TfO-B₁₀H₁₃] to [5-TfO-B₁₀H₁₃] proceeds even at room temperature [204]. The reactions of [5-TfO-B₁₀H₁₃] with methanol and 4-methoxyphenol in 1,2-dichloroethane at 70 °C result in the corresponding ethers [6-RO-B₁₀H₁₃] (R = Me, C₆H₄-4-OMe) [204]. The solid state structures of [6-TfO-B₁₀H₁₃] [202] and [5-TfO-B₁₀H₁₃] [204] were determined by single-crystal X-ray diffraction (Figure 10).

The reactions of Na₂[closo-B₁₀H₁₀] with alcohols ROH (R = Me, Et, i-Pr, Bu, Ph) in hexane in the presence of trimethylsilyl triflate lead to the corresponding 6-alkoxy derivatives of decaborane [6-RO-B₁₀H₁₃] [205]. The reaction with water under the same conditions results in the 6-trimethylsiloxy derivative [6-Me₃SiO-B₁₀H₁₃] [205].

In a similar way, the reaction of (NH₄)₂[closo-B₁₀H₁₀] with sulfuric acid produces the 6-hydroxy derivative of decaborane [6-HO-B₁₀H₁₃] [206]. The 6-hydroxy derivative was also obtained as a by-product of the reaction of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] with sulfuric acid in benzene [207].

The 6-acetoxy derivative of nido-decaborane [6-AcO-B₁₀H₁₃] was obtained by the reaction of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] with mercury acetate [208]. The 6-acetoxy derivative of the arachno-decaborate anion [arachno-6-AcO-B₁₀H₁₃]²⁻ was obtained by the reaction of decaborane with 1-ethyl-3-methylimidazolium acetate (C₂mim)(OAc). The solid state structure of (C₂mim)₂[6-AcO-B₁₀H₁₃] was determined by single-crystal X-ray diffraction (Figure 11) [209].

The bis(decaboranyl) ether [μ-6,6′-O-(B₁₀H₁₃)₂] was prepared by the reaction of [arachno-6,9-(R₂S)₂-B₁₀H₁₂] (R = Me, Et) with sulfuric acid in benzene [210,211]. Its structure was determined by ¹¹B NMR spectroscopy [183,211] and supported by single-crystal X-ray diffraction [212]. The bis(decaboranyl) ether [μ-6,6′-O-(B₁₀H₁₃)₂] was also obtained by the dehydration of (H₃O)₂[closo-B₁₀H₁₀] [213].

The preparation of decaborane derivatives with amides [arachno-6,9-(MeRN(R′)CO)₂-B₁₀H₁₂] (R = H, R′ = H, Me; R = Me, R′ = H, Me), triphenylphosphine oxide [arachno-6,9-(Ph₃PO)₂-B₁₀H₁₂], and dimethylsulfoxide [arachno-6,9-(Me₂SO)₂-B₁₀H₁₂] has also been reported [214,215,216,217].

5. Derivatives with a B-S Bond

The reaction of decaborane(14) with sulfur in the presence of AlCl₃ at 120 °C results in a mixture of the mercapto derivatives [1-HS-B₁₀H₁₃], [2-HS-B₁₀H₁₃], and [1,2-(HS)₂-B₁₀H₁₂] [218,219]. The solid state structures of these mercapto derivatives were determined by single-crystal X-ray diffraction (Figure 12) [219].

The reactions of Na₂[closo-B₁₀H₁₀] with thiols RSH (R = i-Pr, i-Bu, c-Hx, C₆H₄-p-Me, C₆H₄-p-F) in hexane in the presence of trimethylsilyl triflate led to the corresponding 6-alkyl- and 6-arylsulfides [6-RS-B₁₀H₁₃] [205]. It should be noted that the direct reactions of decaborane(14) with alkyl thiols RS lead to its complete degradation to the corresponding trialkylthioborates (RS)₃B [220].

Due to its use in the synthesis of carboranes, the 6,9-bis(dimethylsulfonium) derivative of the arachno-decaborate anion [arachno-6,9-(Me₂S)₂-B₁₀H₁₂], which is formed by refluxing decaborane with dimethyl sulfide in ether or benzene, is the most known decaborane derivative with a B-S bond [214,221,222]. The solid state structure of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] was determined by single-crystal X-ray diffraction [223]. The 6,9-bis(dimethylsulfonium) derivative was studied by X-ray and X-ray photoelectron spectroscopy [224,225,226], and its diamagnetic susceptibility was determined [227]. The Me₂S substituents in [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] can be easily replaced by stronger Lewis bases [214,228]. A series of other bis(dialkylsulfonium) derivatives ([arachno-6,9-(RR′S)₂-B₁₀H₁₂] (R = R′ = Et, Pr; RR′ = (CH₂)₄, (CH₂CH₂)₂S, (CH₂CH₂)₂O)) have been prepared in a similar manner [215,220,228,229].

The bis(diethylsulfonium) derivative [arachno-6,9-(Et₂S)₂-B₁₀H₁₂] can also be obtained by the reaction of (NH₄)₂[closo-B₁₀H₁₀] with anhydrous hydrogen chloride in diethylsulfide [230]. This approach has been extended to other salts of the closo-decaborate anion and other strong acids, including (H₃O)₂[closo-B₁₀H₁₀] [231,232].

The reactions of 2-halogen derivatives of decaborane [2-X-B₁₀H₁₃] (X = Cl, Br, I) with dimethylsulfide Me₂S give the corresponding 2-halogen-6,9-bis(dimethylsulfonium) derivatives [arachno-2-X-6,9-(Me₂S)₂-B₁₀H₁₁] [233,234]. In a similar way, the reactions of 5-halogen derivatives of decaborane [5-X-B₁₀H₁₃] (X = F, Br, I) with dialkylsulfides R₂S (R = Me, Et) result in the corresponding 5-halogen-6,9-bis(dialkylsulfonium) derivatives [arachno-5-X-6,9-(R₂S)₂-B₁₀H₁₁], while the reactions of the 6-chloro derivative proceed with the halogen displacement, giving [arachno-6,9-(R₂S)₂-B₁₀H₁₂] [185]. The solid state structure of [5-Br-6,9-(R₂S)₂-B₁₀H₁₁] was determined by single-crystal X-ray diffraction [235]. The reactions of 2,4-dichloro- and 1,2,4-trichloro derivatives of decaborane [nido-2,4-Cl₂-B₁₀H₁₂] and [nido-1,2,4-Cl₃-B₁₀H₁₁] with dimethylsulfide were found to proceed with the boron cage rearrangement, resulting in [arachno-1,7-Cl₂-6,9-(Me₂S)₂-B₁₀H₁₀] and [arachno-1,3,7-Cl₃-6,9-(Me₂S)₂-B₁₀H₉], respectively [236]. The solid state structures of [1,7-Cl₂-6,9-(Me₂S)₂-B₁₀H₁₀] and [1,3,7-Cl₃-6,9-(Me₂S)₂-B₁₀H₉] were determined by single-crystal X-ray diffraction [236].

The reaction of the 5-triflato derivative of decaborane [5-TfO-B₁₀H₁₃] with dimethylsulfide in toluene results in the 5-triflato-6,9-bis(dialkylsulfonium) derivative [arachno-5-TfO-6,9-B₁₀H₁₁(SMe₂)₂], while the similar reaction of the 5-triflato derivative [6-TfO-B₁₀H₁₃] proceeds with the substitution of the triflate group, giving [arachno-6,9-B₁₀H₁₂(SMe₂)₂] [204]. The solid state structure of [5-TfO-6,9-(Me₂S)₂-B₁₀H₁₁] was determined by single-crystal X-ray diffraction [204].

The 5-dimethylsulfonium derivative of decaborane [nido-5-Me₂S-B₁₀H₁₂] was obtained by heating [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] in toluene or mesitylene, and its solid state structure was determined by single-crystal X-ray diffraction (Figure 13) [215,222,237,238]. It was found that the B-H-B bridge deprotonation occurs at the site adjacent to the substituted boron atom, and thus, the structure of [5-Me₂S-B₁₀H₁₂] is similar to the structure of the [6-Cl-B₁₀H₁₂]⁻ anion [187].

The preparation of the bis(dimethylthioformamide) [arachno-6,9-(Me₂N(H)CS)₂-B₁₀H₁₂] [215] and the bis(di(alkyl/aryl)thiourea) [arachno-6,9-((RHN)₂CS)₂-B₁₀H₁₂] (R = Et, Ph) [217,239] derivatives has also been reported.

6. Derivatives with a B-N Bond

Heating decaborane(14) in acetonitrile under reflux proceeds with hydrogen elimination, resulting in the 6,9-bis(acetonitrilium) derivative of the arachno-decaborate anion [arachno-6,9-(MeC≡N)₂-B₁₀H₁₂] [240], which was the first structurally characterized decaborane derivative [217,241,242,243] (Figure 14). The 6,9-bis(acetonitrilium) derivative was studied by X-ray photoelectron spectroscopy [226], and its diamagnetic susceptibility was determined [227]. The 6,9-bis(propionitrilium) and 6,9-bis(benzonitrilium) derivatives [arachno-6,9-(RC≡N)₂-B₁₀H₁₂] (R = Et, Ph) were prepared in a similar way from decaborane(14) and propionitrile [244] or benzonitrile [217], respectively. The reaction of the 6,9-bis(acetonitrilium) derivative with diethylcyanamide in diethyl ether results in [arachno-6,9-(Et₂NC≡N)₂-B₁₀H₁₂] [214,245]. The reaction of the 2-bromo derivative of decaborane [2-Br-B₁₀H₁₃] gives [arachno-6,9-(MeC≡N)₂-2-Br-B₁₀H₁₁] [233].

The 6,9-bis(acetonitrilium) derivative [arachno-6,9-(MeC≡N)₂-B₁₀H₁₂] reacts with N-nucleophiles (primary and secondary amines and hydrazine), giving the corresponding amidines [arachno-6,9-(RR′N(Me)C=HN)₂-B₁₀H₁₂] (R = H, R′ = Et, Pr, Bu, Ph, NH₂, NHMe; R = R′ = Et, Pr, Bu] [246,247,248]. The solid state structures of the amidines [6,9-(Bu₂N(Me)C=HN)₂-B₁₀H₁₂] and [6,9-(PhHN(Me)C=HN)₂-B₁₀H₁₂]·Et₂O were determined by single-crystal X-ray diffraction (Figure 15) [248]. It should be noted that the reactions with primary amines produce mainly the ZE isomers, whereas the reactions with secondary amines result only in the EE isomers.

The reaction of the 6,9-bis(acetonitrilium) derivative with methanol results in the formation of the corresponding imidate [arachno-6,9-(MeO(Me)C=HN)₂-B₁₀H₁₂] [248]. Nowadays, the addition of nucleophiles to the activated triple -C≡N- bond of nitrilium derivatives of various polyhedral boron hydrides has become a widely used method for their modification [249,250,251,252,253,254,255,256,257,258].

The reactions of the 6,9-bis(acetonitrilium) derivative with tertiary amines in refluxing benzene or toluene lead to the corresponding 6,9-bis(trialkylammonium) derivatives [arachno-6,9-(R₃N)₂-B₁₀H₁₂] (R = Me, Et) [214,245]. The 6,9-bis(ammonium) derivative [arachno-6,9-(H₃N)₂-B₁₀H₁₂] was prepared by the reaction of decaborane(14) with ammonia in benzene or toluene [259,260]. The solid state structures of [6,9-(H₃N)₂-B₁₀H₁₂] (Figure 16) [243,261], [6,9-(Me₃N)₂-B₁₀H₁₂] [262], and [6,9-(Et₃N)₂-B₁₀H₁₂] [263] were determined by single-crystal X-ray diffraction. [6,9-(H₃N)₂-B₁₀H₁₂] and [6,9-(Et₃N)₂-B₁₀H₁₂] were studied by X-ray photoelectron and X-ray fluorescence spectroscopy [225,226,264]. The diamagnetic susceptibilities of [6,9-(Me₃N)₂-B₁₀H₁₂] and [6,9-(Et₃N)₂-B₁₀H₁₂] were determined [227]. The thermal decomposition of the 6,9-bis(ammonium) derivative [arachno-6,9-(H₃N)₂-B₁₀H₁₂] was studied [260,265,266].

The reaction of decaborane(14) with diethylamine in cyclohexane results in the diethylammonium derivative of the arachno-decaborate anion (Et₂NH₂)[arachno-6-Et₂HN-B₁₀H₁₃] [267]. Similar alkylammonium derivatives (Me₄N)[arachno-6-RR′R″N-B₁₀H₁₃] (R = Et, R′ = R″ = H; R = R′ = Et, R″ = H; R = R′ = R″ = Et; RR′ = (CH₂)₅, R″ = H) were prepared by the reactions of Na[B₁₀H₁₃] with the corresponding amines, followed by precipitation with (Me₄N)Cl [267]. Heating (Et₂NH₂)[arachno-6-Et₂HN-B₁₀H₁₃] in THF under reflux gives the 6,9-bis(diethylammonium) derivative [arachno-6,9-(Et₂HN)₂-B₁₀H₁₂], whereas the similar reaction in acetonitrile leads to [arachno-6-Et₂HN-9-Et₂N(Me)C=HN-B₁₀H₁₂] [267]. The reactions of Na[arachno-6-Et₂HN-B₁₀H₁₃] with acetonitrile and dimethylsulfide in the presence of dry HCl result in [arachno-6-Et₂NH-9-MeC≡N-B₁₀H₁₂] and [arachno-6-Et₂NH-9-Me₂S-B₁₀H₁₂], respectively [267]. In a similar way, the reaction of (Me₄N)[arachno-6-Et₃N-B₁₀H₁₃] with acetonitrile leads to [arachno-6-Et₃N-9-MeC≡N-B₁₀H₁₂] [221]. The reactions of Na[arachno-6-Et₂HN-B₁₀H₁₃] with amines in THF produce the corresponding 6,9-bis(alkylammonium) derivatives [arachno-6-Et₂NH-9-RR′R′’N-B₁₀H₁₂] (R = Et, R′ = R″ = H; R = R′ = Et, R″ = H; R = R′ = R″ = Me) [267].

The reactions of decaborane(14) with pyridines, quinolines, and isoquinoline lead to the corresponding [arachno-6,9-L₂-B₁₀H₁₂] (L = pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, 2-ethynylpyridine, 2-cyanopyridine, quinoline, 2-methylquinoline, 8-methylquinoline, isoquinoline) derivatives (Scheme 4) [214,236,268,269]. A more convenient way to prepare 6,9-bis(pyridinium) derivatives is the nucleophilic substitution of the dialkylsulfide groups in [arachno-6,9-(R₂S)₂-B₁₀H₁₂] (R = Me, Et). In this way, the [arachno-6,9-L₂-B₁₀H₁₂] (L = pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, 2,3-dimethylpyridine, 2,4-dimethylpyridine, 2,5-dimethylpyridine, 2,6-dimethylpyridine, 3,4-dimethylpyridine, 2-methyl-5-ethylpyridine, 2-phenylpyridine, 4-benzylpyridine, 4-styrylpyridine, 2-methoxypyridine, 4-methoxypyridine, 3-chloropyridine, 4-chloropyridine, 2-bromopyridine, 4-bromopyridine, 3,5-dibromopyridine, 3-cyanopyridine, 4-cyanopyridine, 4-acetylpyridine, quinoline) derivatives were synthesized (Scheme 4) [229,270,271,272].

All compounds of this series are brightly colored from yellow to red, which is the reason for the interest in their study by UV and luminescent spectroscopy [229,271,272,273]. The 6,9-bis(pyridinium) derivative [6,9-Py₂-B₁₀H₁₂] was studied by X-ray photoelectron and X-ray fluorescence spectroscopy [225,226] and, its diamagnetic susceptibility was determined [227]. The thermal decomposition of the 6,9-bis(pyridinium) and 6,9-bis(quinolinium) derivatives was studied [274]. The solid state structures of [6,9-Py₂-B₁₀H₁₂] [275], [6,9-(HC≡C-o-C₅H₄N)₂-B₁₀H₁₂] [269], and [6,9-(N≡C-o-C₅H₄N)₂-B₁₀H₁₂]·CH₂Cl₂ [269] were determined by single-crystal X-ray diffraction (Figure 17).

It should be noted that the reaction of decaborane(14) with pyridine at low temperatures was found to form the 6,6-bis(pyridinium) derivative [arachno-6,6-Py₂-B₁₀H₁₂], which, upon refluxing in dry degassed pyridine, converts into the more stable 6,9-isomer (Scheme 5) [268].

The reaction of [6,9-(Me₂S)₂-B₁₀H₁₂] with pyrazine in dichloromethane gives the pyrazine-bridged derivative [μ-6,6′-pyrazine-(9-Me₂S-B₁₀H₁₂)₂], whereas the reaction with 4,4′-bipyridine leads to the product of the substitution of the Me₂S groups with azaheterocycle [6,9-(NC₅H₄C₅H₄N)₂-B₁₀H₁₂] (Figure 18) [276].

The similar reactions of [6,9-(Me₂S)₂-B₁₀H₁₂] with 1,4-bis[β-(4-pyridyl)vinyl]benzene and 1,4-bis[β-(4-quinolyl)vinyl]benzene were found to produce mixtures of the corresponding bridged and terminal substituted derivatives (Scheme 6) [277].

The reactions of Na[arachno-6-Et₂HN-B₁₀H₁₃] and (Me₄N)[arachno-6-Et₃N-B₁₀H₁₃] with pyridine in THF produce the corresponding 6-alkylammonium-9-pyridinium derivatives [arachno-6-Et₂RN-9-Py-B₁₀H₁₂] (R = H, Et) [267,278].

The reactions of decaborane(14) with imidazoles in refluxing benzene result in the corresponding 6,9-bis(imidazolium) derivatives [arachno-6,9-(RIm)₂-Me₂S-B₁₀H₁₂] (R = H, Me, Et, Bu) (Figure 19). The hypergolic properties of the 6,9-bis(imidazolium) derivatives prepared were studied [279].

The reactions of decaborane(14) with 2-isopropyl- and 2-methyl-5-(2-chloroethyl) tetrazoles in benzene result in the corresponding 6,9-bis(tetrazolium) derivatives [arachno-6,9-L₂-Me₂S-B₁₀H₁₂] [280].

The 6-isothiocyanato derivative [6-SCN-B₁₀H₁₃] was prepared by the reaction of [6,9-(R₂S)₂-B₁₀H₁₂] (R = Me, Et) with mercury isothiocyanate [208]. Alternatively, the 6-isothiocyanato derivative can be prepared by the reaction of decaborane(14) with NaSCN in 1,2-dimethoxyethane in the presence of dry HCl [281]. The solid state structure of [6-SCN-B₁₀H₁₃] was determined by single-crystal X-ray diffraction (Figure 20) [281].

The reaction of [6,9-(Me₂S)₂-B₁₀H₁₂] with excess HN₃ in toluene results in the 6-azido-μ-5,6-amino derivative [6-N₃-μ-5,6-NH₂-B₁₀H₁₁], the structure of which was determined by single-crystal X-ray diffraction (Figure 20) [282].

7. Derivatives with a B-P Bond

The reaction of decaborane(14) with triphenylphosphine in diethyl ether under reflux results in the 6,9-bis(triphenylphosphonium) derivative of the arachno-decaborate anion [arachno-6,9-(Ph₃P)₂-B₁₀H₁₂]; the same product can be prepared by the reaction of [6,9-(MeC≡N)₂-B₁₀H₁₂] with triphenylphosphine in hot acetonitrile [214,245,283]. The solid state structure of [arachno-6,9-(Ph₃P)₂-B₁₀H₁₂]·2DMF·H₂O was determined by single-crystal X-ray diffraction [284]. The 6,9-bis(triphenylphosphonium) derivative was also studied by X-ray emission and X-ray photoelectron spectroscopy [279,280], and its diamagnetic susceptibility was determined [227]. The reaction of [6,9-(Me₂S)₂-2-Br-B₁₀H₁₁] with triphenylphosphine in benzene results in the corresponding 6,9-bis(triphenylphosphonium) derivative [6,9-(Ph₃P)₂-2-Br-B₁₀H₁₁] [233].

The reaction of decaborane(14) with PhMe₂P at low temperatures (~200 K) gives a mixture of exo,exo- and exo,endo-isomers of [6,9-(PhMe₂P)₂-arachno-B₁₀H₁₂], which were separated chromatographically [285,286]. The structure of both isomers was confirmed by single-crystal X-ray diffraction (Figure 21) [286].

Under similar conditions, the reaction of the 2,4-dichloro derivatives of decaborane with PhMe₂P solely produces the exo,endo-isomer of [6,9-(PhMe₂P)₂-2,4-Cl₂-arachno-B₁₀H₁₀], while the reaction of 2-bromo leads to mixtures of exo,exo- and exo,endo-isomers and [6,9-(PhMe₂P)₂-2-Br-arachno-B₁₀H₁₁]. It is interesting to note that in the reaction of the 2-bromo derivative, it is precisely the 6,9-exo,endo-isomer that is formed, without any traces of the 9,6-exo,endo-isomer. The solid state structure of the exo,endo-isomer [6,9-(PhMe₂P)₂-2-Br-arachno-B₁₀H₁₂] was determined by single-crystal X-ray diffraction (Figure 22) [286].

The reaction of decaborane(14) with triethylphosphine in benzene results in the 6,9-bis(triethylphosphonium) derivative [arachno-6,9-(Et₃P)₂-B₁₀H₁₂] [287], whereas the 6,9-bis(diphenylphosphonium) and 6,9-bis(phenylphosphonium) derivatives [arachno-6,9-(Ph₂HP)₂-B₁₀H₁₂] and [arachno-6,9-(PhH₂P)₂-B₁₀H₁₂] were prepared by the reactions of the corresponding phosphines with [arachno-6,9-(Et₂S)₂-B₁₀H₁₂] [217].

The phosphite, phosphinite, and thiophosphite derivatives of the arachno-decaborate anion [arachno-6,9-(R₂R′P)₂-B₁₀H₁₂] (R = R′ = OMe, OEt, OPh; R = Ph, R′ = OEt; R = OBu, R′ = Ph; R = R′ = SEt) were prepared by the direct reactions of decaborane(14) with the corresponding phosphorus compounds or via substitution of the Me₂S and MeCN groups in [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] and [arachno-6,9-(MeC≡N)₂-B₁₀H₁₂], respectively [217,288,289,290].

The reaction of decaborane(14) with diphenylchlorophosphine in diethyl ether gives the 6,9-bis(chlorodiphenylphosphonium) derivative [arachno-6,9-(ClPh₂P)₂-B₁₀H₁₂], which, upon the treatment with dimethylamine in alcohols, lead to the corresponding phosphonites [6,9-(ROPh₂P)₂-B₁₀H₁₂] (R = Me, Et, CH₂CH₂OH) [290]. The reaction of [6,9-(ClPh₂P)₂-B₁₀H₁₂] with dimethylamine in aqueous solution water results in the bis(dimethylammonium) salt of the corresponding acid (Me₂NH₂)₂[6,9-(OPh₂P)₂-B₁₀H₁₂] [290]. The 6,9-bis-(hydroxydiphenylphosphonium) derivative [6,9-(HOPh₂P)₂-B₁₀H₁₂] was prepared by the reaction of [6,9-(ClPh₂P)₂-B₁₀H₁₂] with water in acetone [290].

The 6,9-bis(chlorodiphenylphosphonium) derivative reacts with ammonia, hydrazine, primary aliphatic amines, and ethylenimine in alcohols to form the corresponding 6,9-bis(aminodiphenylphosphonium) derivatives [6,9-(R′RNPh₂P)₂-B₁₀H₁₂] (R = H, R′ =NH₂, Me, Bu; R = R′ = CH₂CH₂) [290]. The reactions of decaborane(14) or [arachno-6,9-(R₂S)₂-B₁₀H₁₂] (R = Me, Et) with dimethylaminophosphines in refluxing benzene lead to the corresponding 6,9-bis(dimethylaminophosphonium) derivatives [6,9-(RR′(Me₂N)P)₂-B₁₀H₁₂] (R = R′ = NMe₂; R = NMe₂, R′ = Ph, Cl; R = R′ = Ph; R = Ph, R′ = Cl; RR′ = OCH₂CH₂O) [217].

The reaction of [6,9-(ClPh₂P)₂-B₁₀H₁₂] with NaN₃ in ethanol results in the 6,9-bis-(azidodiphenylphosphonium) derivative [6,9-(N₃Ph₂P)₂-B₁₀H₁₂] [290], which, upon the treatment with triphenylphosphine in refluxing benzene, gives the 6,9-bis(triphenylphosphineiminodiphenylphosphonium) derivative [6,9-(Ph₃P=NPh₂P)₂-B₁₀H₁₂] [291].

The bifunctional derivatives [6,9-(XPh₂P)₂-B₁₀H₁₂] (X = Cl, OH, N₃) were used for the synthesis of decaborane-based polymers [291,292]. The chemistry of decaborane-based polymers is considered in detail in the review [293]. The formation of decaborane-based polymers along with a small amount of [6,9-(dppf)₂-B₁₀H₁₂] has also been reported in the reaction of decaborane(14) with 1,1′-bis(diphenylphosphino)ferrocene (dppf) [294].

The reaction of Na[B₁₀H₁₃] with tributylphosphine in acetonitrile in the presence of dry HCl leads to [arachno-6-Bu₃P-9-MeC≡N-B₁₀H₁₂] [221]. The reaction of Na[B₁₀H₁₃] with diphenylchlorophosphine in diethyl ether leads to the diphenylphosphine derivative [nido-μ-5,6-Ph₂P-B₁₀H₁₃] [295], whose structure was determined by single-crystal X-ray diffraction [296]. The same compound was reported to be formed in the reaction of the so-called “Grignard derivative” [B₁₀H₁₃MgI] formed by the treatment of decaborane(14) with MeMgI, with diphenylchlorophosphine in diethyl ether [297]. The diphenylphosphine derivative [nido-μ-5,6-Ph₂P-B₁₀H₁₃] is easily deprotonated with triethylamine or sodium hydroxide to form the corresponding salts [295,297]. The solid state structure of the triphenylmethylphosponium salt (Ph₃PMe)[arachno-μ-6,9-Ph₂P-B₁₀H₁₂] was determined by single-crystal X-ray diffraction (Figure 23) [298].

It should be noted that the reactions of [6,9-(MeC≡N)₂-B₁₀H₁₂] with low-coordinated phosphorus compounds, such as phosphaalkynes RC≡P (R = t-Bu, Ad), do not lead to substitution o hydrogens but to the incorporation of phosphorus into the decaborane basket with the formation of 11-vertex phosphoboranes [nido-RC(H)=PB₁₀H₁₃] [299,300].

8. Derivatives with a B-As Bond

The 6,9-bis(trialkyl/arylarsonium) derivatives [6,9-(R₃As)₂-B₁₀H₁₂] (R = Et, Ph) were prepared by the reactions of decaborane(14) or [6,9-(MeC≡N)₂-B₁₀H₁₂] with the corresponding arsines in benzene or toluene [287]. The reaction of decaborane(14) with triethoxyarsine in benzene leads to [6,9-((EtO)₃As)₂-B₁₀H₁₂] [289].

9. Derivatives with a B-C Bond

Decaborane derivatives with a B-C bond are probably the most studied area of decaborane chemistry. Like the halogenation of decaborane, the direct alkylation reactions result in the substitution of hydrogen atoms at “the bottom” of the decaborane basket. The reaction of decaborane(14) with methyl bromide in carbon disulfide in the presence of AlCl₃ at 80 °C gives a mixture of the 2-methyl [2-Me-B₁₀H₁₃], 1,2- and 3,4-dimethyl [1,2-Me₂-B₁₀H₁₂] and [2,4-Me₂-B₁₀H₁₂], 1,2,3- and 1,2,4-trimethyl [1,2,3-Me₃-B₁₀H₁₁] and [1,2,4-Me₃-B₁₀H₁₁], 1,2,3,4- and 1,2,3,5(or 8)-tetramethyl [1,2,3,4-Me₃-B₁₀H₁₀], and [1,2,3,5(or 8)-Me₄-B₁₀H₁₀] derivatives, which were chromatographically separated [301]. The methylation of decaborane(14) was also studied using methyl chloride [302,303].

The reaction of decaborane(14) with neat methyl iodide in the presence of AlCl₃ at room temperature gives the 1,2,3,4-tetramethyl derivative [1,2,3,4-Me₄-B₁₀H₁₀], whereas the reaction at 120 °C leads to the octasubstituted product [1-I-2,3,4,5,6,7,8-Me₇-B₁₀H₆]. The similar octasubstituted derivative [1-TfO-2,3,4,5,6,7,8-Me₇-B₁₀H₆] was obtained by the reaction of decaborane(14) with TfOMe in the presence of a catalytic amount of triflic acid at 120 °C. The solid state structures of [1-I-2,3,4,5,6,7,8-Me₇-B₁₀H₆] and [1-TfO-2,3,4,5,6,7,8-Me₇-B₁₀H₆] were determined by single-crystal X-ray diffraction (Figure 24) [304]. The methyl derivatives prepared can be easily deprotonated with a Proton Sponge to give the corresponding salts [304].

The reaction of decaborane(14) with ethyl bromide in carbon disulfide in the presence of AlCl₃ under reflux gives a mixture of mono-, di-, and triethyl derivatives [305]. The monoethyl derivative of decaborane was prepared by the reaction of decaborane(14) with neat ethyl bromide in the presence of AlCl₃ [306]. The solid state structure of the 1-ethyl derivative of decaborane [1-Et-B₁₀H₁₃] was determined by single-crystal X-ray diffraction [307].

The reaction of decaborane(14) with MeLi in benzene followed by treatment with HCl has been reported to give a mixture of the 6-methyl-, 6,5(or 8)- and 6,9-dimethyl derivatives of decaborane [308]. The reaction with EtLi in benzene gives the 6-ethyl derivative [6-Et-B₁₀H₁₃] [308]. The reaction of decaborane(14) with MeMgI has been shown to proceed by two routes. The major reaction yields the so-called “Grignard derivative” [B₁₀H₁₃MgI] and methane, and the minor reaction produces the 6-methyl derivative of decaborane. The reaction of the “Grignard derivative” with dimethyl sulfate produces a mixture of the 5- and 6-methyl derivatives of decaborane [309]. In a similar way, the reaction of decaborane(14) with EtMgI produces the “Grignard derivative” as the main product and the 6-ethyl derivative of decaborane as a by-product. The reaction of the “Grignard derivative” with [Et₃O]BF₄ or diethyl sulfate gives the 5-ethyl derivative of decaborane [5-Et-B₁₀H₁₃] [309]. A series of alkyl derivatives [R-B₁₀H₁₃] (R = butyl, amyl, hexyl, cyclohexyl, heptyl, octyl) was prepared by the reactions of the “Grignard derivative” with the corresponding alkyl fluorides [310]. The 6-benzyl derivative of decaborane [6-Bn-B₁₀H₁₃] can be prepared by the reaction of the “Grignard derivative” with benzyl chloride or Na[B₁₀H₁₃] with benzyl bromide [309,311,312].

Later, these reactions were re-examined, and it was shown that the first stage of the reaction of decaborane(14) with the alkyllithium reagents RLi is deprotonation of decaborane with the formation of Li[B₁₀H₁₃]. The reaction with the second equivalent of RLi produces Li₂[arachno-6-R-B₁₀H₁₃], which, when treated with HCl, gives Li[arachno-6-R-B₁₀H₁₄] and then [nido-6-R-B₁₀H₁₃] (R = Me, n-Bu, t-Bu). The use of pre-prepared salts of the [B₁₀H₁₃]⁻ anion makes it possible to reduce the formation of by-products [237,313].

Another approach to the 6-alkyl derivatives of decaborane includes the reactions of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] with alkenes in dichloromethane, resulting in [nido-6-R-8-Me₂S-B₁₀H₁₁] (R = cyclohexyl, cyclohexenyl, hexyl, octyl, 2,3-dimethyl-1-butyl, 2,3-dimethyl-2-butyl, 2-methyl-2-butyl, (1R)-(+)-α-pinene, and (1S)-(−)-β-pinene), which can be reduced using Superhydride Li[Et₃BH] in THF to [6-R-B₁₀H₁₂]⁻ and then protonated with HCl/Et₂O to [6-R-B₁₀H₁₃] [222,237,313,314,315]. From the point of view of organic chemistry, these reactions can be considered as the hydroboration reactions. The solid state structures of [6-Chx-8-Me₂S-B₁₀H₁₁] [316], [6-Thx-8-Me₂S-B₁₀H₁₁] [237] (Figure 25), and [6-Thx-B₁₀H₁₃] [237] (Figure 26) were determined by single-crystal X-ray diffraction.

Another convenient method for the synthesis of 6-alkyl derivatives of decaborane is based on the use of ionic liquids as a solvent. The reactions of decaborane(14) with terminal alkenes in biphasic ionic-liquid/toluene mixtures lead to the corresponding 6-alkyl derivatives [6-R-B₁₀H₁₃] (R = C₆H₁₃, C₈H₁₇, C₁₆H₃₃, CH(i-Pr)CH₂CHMe₂, (CH₂)₂C₆H₅, (CH₂)₃C₆H₅, (CH₂)₆Br, (CH₂)₄CH=CH₂, (CH₂)₆CH=CH₂, (CH₂)₃OC₃H₇, (CH₂)₃SiMe₃, (CH₂)₄COMe, (CH₂)₆OAc, (CH₂)₃OBn, (CH₂)₃OH, and (CH₂)₃Bpin, norbornenyl) [317,318,319,320]. The best results were observed for reactions with [bmim]X (1-butyl-3-methylimidazolium, X = Cl⁻ or BF₄⁻) and bmpyX (1-butyl-4-methylpyridinium, X = Cl⁻ or BF₄⁻). The reaction mechanism includes the ionic-liquid-promoted formation of the [B₁₀H₁₃]⁻ anion, its addition to the alkene to form the [6-R-B₁₀H₁₂]⁻ anion, and, finally, the protonation of the last one to form the final product [6-R-B₁₀H₁₃] [314]. The solid state structures of [6-Me₃Si(CH₂)₃-B₁₀H₁₃] and [6-MeC(O)(CH₂)₄-B₁₀H₁₃] were determined by single-crystal X-ray diffraction (Figure 26) [318].

The 6-cyclohexyl derivative of decaborane [nido-6-C₆H₁₁-B₁₀H₁₃] was obtained in a low yield from the reaction of Cs₂[closo-B₁₀H₁₀] with triflic acid in cyclohexane (Figure 27) [202,203]. In a similar way, the 6-hexyl derivative [nido-6-C₆H₁₃-B₁₀H₁₃] was isolated from the reaction of (NH₄)₂[closo-B₁₀H₁₀] with concentrated nitric acid in hexane [321].

The Cp₂Ti(CO)₂-catalyzed reactions of decaborane(14) with terminal alkenes have been found to result in the high-yield formation of 6-alkyl derivatives of decaborane [6-R-B₁₀H₁₃] (R = C₆H₁₃, C₈H₁₇, (CH₂)₃SiMe₃) [322,323]. The reactions of decaborane(14) with equimolar amounts of bifunctional alkenes such as diallyldimethylsilane, 1,5-hexadiene, 1,4-cyclohexadiene, 1,5-cyclooctadiene, and 2,5-norbornadiene produce the corresponding decaborane derivatives with a double bond in the substituent [6-R-B₁₀H₁₃] (R = (CH₂)₃SiMe₂CH₂CH=CH₂, (CH₂)₄CH=CH₂, 4-cyclohexenyl, 5-cyclooctenyl, 5-norbornenyl) [322,323,324,325]. The solid state structures of [6-H₂C=CHCH₂SiMe₂(CH₂)₃-B₁₀H₁₃] (Figure 26) [322], [6-(4′-cyclohexenyl)-B₁₀H₁₃] (Figure 27) [325], and [6-(5′-norbornenyl)-B₁₀H₁₃] (Figure 27) [324] were determined by single-crystal X-ray diffraction.

The reactions of multifunctional alkenes with an excess amount of decaborane(14) produce the saturated linked-cage compounds with two ([μ-6,6′-Me₂Si-(6-(CH₂)₃-B₁₀H₁₃)₂], [μ-6,6′-(CH₂)₆-(B₁₀H₁₃)₂], [μ-6,6′-(1″,5″-cyclooctyl)-(B₁₀H₁₃)₂], and [μ-6,6′-(2″,5″-norbornyl)-(B₁₀H₁₃)₂]) or four ([μ⁴-6,6′,6″,6‴-Si-(6-(CH₂)₃-B₁₀H₁₃)₄]) decaborane units [322,323,325]. The solid state structures of [μ-6,6′-(CH₂)₆-(B₁₀H₁₃)₂], [μ-6,6′-(2″,5″-norbornyl)-(B₁₀H₁₃)₂], and [μ⁴-6,6′,6″,6‴-Si-(6-(CH₂)₃-B₁₀H₁₃)₄] were determined by single-crystal X-ray diffraction (Figure 28) [322,323,325].

The derivatives with the two decaborane units [μ-6,6′-(1″,5″-cyclooctyl)-(B₁₀H₁₃)₂] and [μ-6,6′-(2″,5″-norbornyl)-(B₁₀H₁₃)₂] can also be prepared by the titanium-catalyzed reactions of decaborane(14) with [6-(5′-cyclooctenyl)-B₁₀H₁₃] and [6-(5′-norbornenyl)-B₁₀H₁₃], respectively [325].

The 6-alkyl derivatives of decaborane with substituents containing double bonds in the side chain (hexenyl, norbornenyl, etc.) are used for the synthesis of decaborane-based polymers and boron-containing ceramics [319,324,325,326,327,328,329,330,331,332].

The reactions of decaborane(14) with terminal alkenes in the presence of catalytic amounts of PtBr₂ or H₂PtCl₆ lead to the 6,9-dialkyl derivatives nido-[6,9-R₂-B₁₀H₁₂] (R = C₂H₅, C₃H₇, C₄H₉, C₅H₁₁) [333]. The reactions of [5-TfO-B₁₀H₁₃] and [5-I-B₁₀H₁₃] with 1-pentene in the presence of a catalytic amount of PtBr₂ at 55 °C lead to the corresponding 6,9-dialkyl derivatives [6,9-(C₅H₁₁)₂-5-TfO-B₁₀H₁₁] and [6,9-(C₅H₁₁)₂-5-I-B₁₀H₁₁] [204]. The solid state structure of [6,9-(C₅H₁₁)₂-5-I-B₁₀H₁₁] was determined by single-crystal X-ray diffraction (Figure 29) [204].

The reactions of decaborane(14) with terminal alkynes in toluene in the presence of [Cp*IrCl₂]₂ or [(p-cymene)RuCl₂]₂ as catalysts lead to the corresponding 6,9-di(β-alkenyl) derivatives of decaborane [6,9-((E)-RCH=CH)₂-B₁₀H₁₂] (R = H, C₆H₁₃, C₆H₅, (CH₂)₂Br, (CH₂)₃Cl, SiMe₃) [334,335]. The solid state structures of [6,9-((E)-Br(CH₂)₂CH=CH)₂-B₁₀H₁₂] and [6,9-((E)-Me₃SiCH=CH)₂-B₁₀H₁₂] were determined by single-crystal X-ray diffraction (Figure 30) [334,335].

In contrast to [(p-cymene)RuCl₂]₂, the reactions of decaborane(14) with terminal alkynes in the presence of [(p-cymene)RuI₂]₂ result in the 6,9-di(α-alkenyl) derivatives [6,9-(R(H₂C=)C)₂-B₁₀H₁₂] (R = C₆H₁₃, CH₂-c-C₆H₁₁, (CH₂)₂Br, (CH₂)₃Cl) [334,335]. The solid state structure of [6,9-(c-C₆H₁₁CH₂(H₂C=)C)₂-B₁₀H₁₂] was determined by single-crystal X-ray diffraction (Figure 31) [334,335].

In a similar way, the reactions of 6-alkyldecaboranes [6-R-B₁₀H₁₃] with terminal alkynes in the presence of [Cp*IrCl₂]₂ give asymmetrically substituted 6-alkyl-9-alkenyl-derivatives [6-R-9-((E)-R′CH=CH)₂-B₁₀H₁₂] (R = (CH₂)₃SiMe₃, R′ = H, C₆H₅, C₆H₄-m-CH≡CH; CH₂CH=CH₂; R = C₅H₁₁, R′ = H). The solid state structure of [6-Me₃Si(CH₂)₃-9-(E)-m-HC≡CC₆H₄CH=CH-B₁₀H₁₂] was determined by single-crystal X-ray diffraction (Figure 32) [334,335].

While [Cp*IrCl₂]₂ proved to be inactive for inducing the hydroboration of simple olefins, such as 1-pentene, by either decaborane or the 6-alkyl-decaboranes, it was found to catalyze the hydroboration of 6-alkyl-9-vinyldecaboranes [6-R-9-CH₂=CH-B₁₀H₁₂] (R = C₅H₁₁, (CH₂)₃SiMe₃) by 6-alkyl-decaboranes [6-R-B₁₀H₁₃] (R = C₅H₁₁, (CH₂)₃SiMe₃) to yield linked-cage products [9,9′-μ-CH₂CH₂-(6-R-B₁₀H₁₂)₂] (R = C₅H₁₁, (CH₂)₃SiMe₃) (Figure 33) [335].

The vinyl derivative [6-Me₃Si(CH₂)₃-9-CH₂=CH)₂-B₁₀H₁₂] was found to readily undergo both homo- and cross-metathesis reactions in the presence of Grubbs’ II catalyst, giving the corresponding products [9,9′-μ-CH=CH-(6-Me₃Si(CH₂)₃-B₁₀H₁₂)₂] (Figure 34) and [6-Me₃Si(CH₂)₃-9-RCH=CH-B₁₀H₁₂] (R = C₃H₇, (CH₂)₄Br, CH₂SiMe₃) [335].

Heating [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] with silylated acetylenes Me₃SiC≡CR (R = Me, Bu, SiMe₃) leads to the corresponding trimethylsilyl alkenyl derivatives [nido-6-Me₃Si(R)C=CH-5-Me₂S-B₁₀H₁₁] [336,337]. The solid state structures of [6-Me₃Si(Me)C=CH-5-Me₂S-B₁₀H₁₁] [336], [6-Me₃Si(Bu)C=CH-5-Me₂S-B₁₀H₁₁] [337], and [6-(Me₃Si)₂C=CH-5-Me₂S-B₁₀H₁₁] [337] were determined by single-crystal X-ray diffraction (Figure 35).

The reaction of [6,9-(Me₂S)₂-B₁₀H₁₂] with the phosphaalkyne t-BuC≡P in refluxing benzene leads to [μ-6(C),6′(C),5′(P)-C(t-Bu)PH-(nido-8-Me₂S-B₁₀H₁₁)(nido-B₁₀H₁₂)], in which two decaborane units are linked by the C(t-Bu)PH-bridge (Figure 36) [338].

The derivatives [μ-(exo-6(C),endo-6(N)-CH=CH-o-C₅H₄N)-9(N)-HC≡C-o-C₅H₄N-arachno-B₁₀H₁₁] (Figure 34) [265] and [μ-(exo-6(C),endo-6(N)-(closo-1′,2′-C₂B₁₀H₁₀-2′-)-o-C₅H₄N)-μ-(exo-8(C),exo-9(N)-CH₂CH₂-o-C₅H₄N)-arachno-B₁₀H₁₀] (Figure 37) [339] were isolated in minor amounts as products of the intramolecular hydroboronation of [arachno-6,9-(HC≡C-o-C₅H₄N)₂-B₁₀H₁₂] during its thermolysis in 1,2-dichloroethane. In the latter compound, the formation of the ortho-carborane fragment occurs as a result of the reaction of the acetylene group of the substituent with the second molecule of the decaborane derivative.

The reactions of Cs₂[closo-B₁₀H₁₀] with triflic acid or (NH₄)₂[closo-B₁₀H₁₀] with sulfuric acid in the presence of aromatic hydrocarbons produce the corresponding 6-aryl derivatives of decaborane [nido-6-Ar-B₁₀H₁₃] (Ar = Ph, C₆H₄-4-Me, C₆H₃-3,5-Me₂, C₆H₂-2,4,6-Me₃, C₆H₂-2,4,6-ⁱPr₃, C₆H₄Cl, C₆H₄CF₃) [202,203,321,340]. The solid state structures of [6-Ph-B₁₀H₁₃], [6-p-Tol-B₁₀H₁₃], and [6-(2′,4′,6′-ⁱPr₃-C₆H₂-B₁₀H₁₃] were determined by single-crystal X-ray diffraction (Figure 38) [202,203,340].

The 6-phenyl derivative [6-Ph-B₁₀H₁₃] was also obtained by the reaction of decaborane(14) with PhLi, followed by the treatment with HCl in Et₂O [237] as well as by the solid state pyrolysis of [nido-Ph₂SnB₁₀H₁₂] at 95 °C [198].

The pyrolysis of decaborane(14) in benzene at 200 °C gives the 5-phenyl derivative [5-Ph-B₁₀H₁₃] as the main product together with some amounts of the 6-isomer and 5,8-diphenyl derivative [5,8-Ph₂-B₁₀H₁₂] [341]. The solid state structures of [5-Ph-B₁₀H₁₃] and [5,8-Ph₂-B₁₀H₁₂] were determined by single-crystal X-ray diffraction (Figure 39) [341].

The pyrolysis of decaborane(14) in toluene at 250 °C affords the novel microporous polymer named “activated borane”, in which the decaborane clusters are interconnected by toluene moieties. Activated borane displays a high surface area of 774 m² g⁻¹, a thermal stability up to 1000 °C (under Ar), and a sorption capacity to emerging pollutants exceeding the capacity of commercial activated carbon [342].

Heating (HPS)[B₁₀H₁₃] in acetonitrile under reflux results in the formation of the bridged imino derivative (HPS)[arachno-μ-6(C),9(N)-MeC=NH-B₁₀H₁₂] (Figure 40) [343].

The reaction of decaborane(14) with sodium cyanide in water followed by the addition of CsCl gives Cs₂[arachno-endo-6-N≡C-B₁₀H₁₃] [215]. The solid state structure of the trimethylphenylammonium salt (Me₃NPh)₂[endo-6-N≡C-B₁₀H₁₃] (Figure 40) [344] and lead complex {(Bipy)₂Pb[endo-6-N≡C-B₁₀H₁₃]} [345] were determined by single-crystal X-ray diffraction.

The reactions of decaborane(14) with sodium cyanide and dimethylsulfide or tetrahydrothiophene lead to the corresponding Na[arachno-6-N≡C-9-RR′S-B₁₀H₁₂] (R = R′ = Me, RR′ = (CH₂)₄). Na[6-N≡C-9-Me₂S-B₁₀H₁₂] can also be prepared by the reaction of [6,9-(Me₂S)₂-B₁₀H₁₂] with sodium cyanide in dimethylsulfide [215].

10. Derivatives with an exo-Polyhedral B-B Bond

Decaborane derivatives with an exo-polyhedral B-B bond are rare, since the reaction of decaborane(14) with boron hydrides usually leads to the completion of the polyhedral backbone with the formation of tetradecahydro-nido-undecaborate [B₁₁H₁₄]⁻ [346] and dodecahydro-closo-dodecaborate [B₁₂H₁₂]²⁻ [3] anions and their derivatives.

The reactions of decaborane(14) with sterically hindered (alkyl/arylimino)(2,2,6,6-tetramethylpiperidino)boranes lead to the corresponding 6-substituted aminoborane derivatives [nido-6-(RNH)(C₅H₆Me₄NH)B-B₁₀H₁₃] (R = t-Bu, C₆H₃-2,6-ⁱPr₂) (Figure 41) [347].

The reaction of Na[B₁₀H₁₃] with 9-bora[3.3.1]bicyclononane (9-Br-BBN) in dichloromethane results in the formation of [μ-5,6-(9-BBN)-B₁₀H₁₃], where the 9-BBN group appears in the role of an asymmetric bridge between the B(5) and B(6) positions of the decaborane basket (Figure 41). It is noteworthy that upon the deprotonation of [μ-5,6-(9-BBN)-B₁₀H₁₃] with a Proton Sponge in dichloromethane, the 9-BBN bridging group migrates from the B(6) atom to the B(10) atom, which leads to the formation of an 11-vertex nido-structure (HPS)[μ-7,7-CH(CH₂CH₂CH₂)₂CH-B₁₁H₁₂], being a formal derivative of the [B₁₁H₁₄]⁻ anion [348].

Decaborane derivatives with an exo-polyhedral B-B bond also include isomeric conjuncto-decaboranes [B₁₀H₁₃]₂, which consist of two nido-B₁₀ units linked by a direct B-B bond. These compounds were first identified as trace impurities in technical decaborane (l4) [349]. In principle, there can be 11 different geometric isomers of conjuncto-decaborane [B₁₀H₁₃]₂, 4 of which are in the form of enantiomeric pairs. Therefore, various routes (photolysis [350,351], pyrolysis [350,351], γ-irradiation [352], high-energy electron bombardment [351], silent electrical discharge [353]) for synthesizing these compounds have been developed. All of them, as a rule, lead to the formation of mixtures with different isomeric compositions. The solid state structures of the 1,1′- [353], 1,2′- [354], 1,5′- [352], 1,6′- [238], 2,2′- [355,356], and 2,6′- [356] isomers have been determined by single-crystal X-ray diffraction, whereas the 2,5′- [351], 5,5′- [351], and 6,6′- [357] isomers have been characterized by NMR spectroscopy.

The asymmetric derivatives [arachno-1-(6′-nido-B₁₀H₁₃)-6,9-(Me₂S)₂-B₁₀H₁₁] and [nido-4-(2′-nido-B₁₀H₁₃)-5-Me₂S-B₁₀H₁₁] (Figure 42) were isolated from the reaction of [1,5′-(nido-B₁₀H₁₃)₂] with dimethylsulfide under reflux [238]. The first compound was also obtained by the thermolysis of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] in refluxing toluene [238].

The isomeric tridecaboranyl species [arachno-1,5-(6′-nido-B₁₀H₁₃)₂-6,9-(Me₂S)₂-B₁₀H₁₀] (Figure 43) and [arachno-1,3-(6′-nido-B₁₀H₁₃)₂-6,9-(Me₂S)₂-B₁₀H₁₀] were isolated from the products of the thermolysis of [arachno-6,9-(Me₂S)₂-B₁₀H₁₂] in refluxing benzene [238,358].

Here, it is also worth mentioning an unusual structure, in which two hydrogen atoms at positions 5 and 6 of the decaborane basket are replaced by pentaborane moieties—5-(nido-pentaboran-2-yl)-6-(nido-pentaboran-1-yl)-nido-decaborane, formed as a result of the long-term storage (23 years) of a sealed, under-vacuum pentaborane(9) sample under ambient lighting and temperature conditions (Figure 44) [339].

11. Decaborane-Related conjucto-Boranes [B₁₈H₂₂]

Another type of compound worth considering here are the conjuncto-boranes [B₁₈H₂₂], in which two decaborane baskets are connected by a common edge. Octadecaborane(22) [B₁₈H₂₂] in the form of a mixture of syn- and anti-isomers (Figure 45) is formed on the hydrolysis of (H₃O)₂[trans-B₂₀H₁₈]·nH₂O and can be separated by fractional crystallization [359]. More recently, a convenient method has been proposed for the synthesis of anti-[B₁₈H₂₂] by mild oxidation of (Me₄N)[nido-B₉H₁₂] with iodine in toluene, which gives excellent yields (~80%) and thus provides a large-scale and safe route to this important polyborane cluster [360].

The photophysics of both isomers have been studied by UV–vis spectroscopic techniques and quantum chemical calculations. In an air-saturated hexane solution, anti-[B₁₈H₂₂] shows fluorescence with a high quantum yield, Φ_F = 0.97, and singlet oxygen O₂(¹Δ_g) production (Φ_Δ ~ 0.008). Conversely, the isomer syn-[B₁₈H₂₂] shows no measurable fluorescence, instead displaying a much faster, picosecond nonradiative decay of excited singlet states [361]. Due to this, anti-[B₁₈H₂₂] can be considered as a potential blue laser material. The photophysical properties of anti-[B₁₈H₂₂] can be tuned by the partial substitution of hydrogen atoms with various functional groups. Because of this, combined with its high stability [362,363,364,365,366], anti-[B₁₈H₂₂] is attracting increasing research interest, while the syn-[B₁₈H₂₂] isomer has received much less attention.

The reaction of anti-[B₁₈H₂₂] with chlorine generated in situ from N-chlorosuccinimide (NCS) with HCl/dioxane in dichloromethane leads to the 7-chloro derivative anti-[7-Cl-B₁₈H₂₁] (Figure 46) [367]. The reaction of anti-[B₁₈H₂₂] with AlCl₃ in tetrachloromethane results in a mixture of the 3,3′- and 3,4′-dichloro derivatives anti-[3,3′-Cl₂-B₁₈H₂₀] (Figure 46) and anti-[3,4′-Cl₂-B₁₈H₂₀] (Figure 45) together with minor amounts of the other isomeric dichloro derivatives anti-[4,4′-Cl₂-B₁₈H₂₀] (Figure 46), anti-[3,1′-Cl₂-B₁₈H₂₀] (Figure 46), and anti-[7,3′-Cl₂-B₁₈H₂₀], as well as the 3- and 4-chloro derivatives anti-[3-Cl-B₁₈H₂₁] and anti-[4-Cl-B₁₈H₂₁] and the 3,4,3′- and 3,4,4′-trichloro derivatives anti-[3,4,3′-Cl₃-B₁₈H₁₉] and anti-[3,4,4′-Cl₃-B₁₈H₁₉], which were separated chromatographically [368].

The bromination of anti-[B₁₈H₂₂] with bromine in dichloromethane in the presence of AlCl₃ leads to the 4-bromo or 4,4′-dibromo derivatives anti-[4-Br-B₁₈H₂₁] or anti-[4,4′-Br₂-B₁₈H₂₀] depending on the reagent ratio (Figure 47) [369,370].

The reaction of anti-[B₁₈H₂₂] with iodine in ethanol leads to the 4-iodo derivative anti-[7-I-B₁₈H₂₁] (Figure 48) [359,371], while the reaction with I₂ or ICl in the presence of AlCl₃ in dichloromethane results in the 4-iodo- and 4,4′-diiodo derivatives anti-[4-I₂-B₁₈H₂₁] and anti-[4,4′-I₂-B₁₈H₂₀] (Figure 48), depending on the reagent ratio [367,371].

The iodine atom in anti-[7-I-B₁₈H₂₁] can be substituted by various nucleophiles: the reaction with trifluoroacetamide in toluene in the presence of K₃PO₄ gives the corresponding N-boronated amide anti-[7-CF₃CONH-B₁₈H₂₁]; the reactions with t-BuOK, 4-FC₆H₄OK, and 1-AdSK in toluene or tetrahydrofuran lead to the corresponding (thio)ethers anti-[7-RX-B₁₈H₂₁]. The reaction with potassium 2,6-dimethylthiophenolate in toluene results in the corresponding thioether anti-[7-(2′,6′-Me₂C₆H₃S)-B₁₈H₂₁], while the reaction in tetrahydrofuran produces anti-[7-(2′,6′-Me₂C₆H₃S(CH₂)₄O)-B₁₈H₂₁] [372]. The Pd-catalyzed reactions of anti-[7-I-B₁₈H₂₁] with CF₃CONH₂, t-BuOK, and 2,6-Me₂C₆H₃OK in the presence of catalytic amounts of RuPhos Pd G4 and RuPhos in 1,4-dioxane lead to the corresponding derivatives with B-N and B-O bonds anti-[7-X-B₁₈H₂₁] [372].

The reaction of anti-[B₁₈H₂₂] with neat methyl iodide in the presence of AlCl₃ at room temperature results in the 3,3′,4,4′-tetramethyl derivative anti-[3,3′,4,4′-Me₄-B₁₈H₁₈] (Figure 49) as the main product, together with minor amounts of the 4,4′-dimethyl derivative anti-[4,4′-Me₂-B₁₈H₂₀] (Figure 49), the 3,4,4′- and 3,3′,4-trimethyl derivatives anti-[3,4,4′-Me₃-B₁₈H₁₉] (Figure 47) and anti-[3,3′,4-Me₃-B₁₈H₁₉], as well as the 1,3,3′,4,4′- and 3,3′,4,4′,8-pentamethyl derivatives anti-[1,3,3′,4,4′-Me₅-B₁₈H₁₇] and anti-[3,3′,4,4′,8-Me₅-B₁₈H₁₇] and the 1,1′,3,3′,4,4′-hexamethyl derivative anti-[1,1′,3,3′,4,4′-Me₆-B₁₈H₁₆] [373]. The similar reaction with ethyl iodide gives the 3,3′,4,4′-tetraethyl derivative anti-[3,3′,4,4′-Et₄-B₁₈H₁₈] (Figure 49) [373].

The dichloroundecamethyl anti-[2,2′-Cl₂-1,1′,3,3′,4,4′,7,7′,8,8′,10′-Me₁₁-B₁₈H₉], dichlorododecamethyl anti-[2,2′-Cl₂-1,1′,3,3′,4,4′,7,7′,8,8′,10,10′-Me₁₂-B₁₈H₈] (Figure 50), and dichlorotridecamethyl anti-[2,2′-Cl₂-1,1′,3,3′,4,4′,7,7′,8,8′,9,10,10′-Me₁₃-B₁₈H₇] derivatives were obtained by the reaction of anti-[B₁₈H₂₂] with methyl iodide in the presence of AlCl₃ in dichloromethane at 55 °C [373,374].

The reaction of anti-[B₁₈H₂₂] with elemental sulfur in the presence of AlCl₃ at 125 °C leads to the 4,4′-dimercapto derivative anti-[4,4′-(HS)₂-B₁₈H₂₀] (Figure 51) [375].

The reaction of anti-[B₁₈H₂₂] with pyridine in refluxing chloroform or benzene unexpectedly results in a two-fold substitution in one of the B₁₀-baskets to form nido-arachno-[6′,9′-Py₂-B₁₈H₂₀] (Figure 52) together with some amount of anti-[8′-Py-B₁₈H₂₁] (Figure 52) and [3′,8′-Py₂-B₁₆H₁₈] as the main degradation product. In contrast to the thermochromic fluorescence of nido-arachno-[6′,9′-Py₂-B₁₈H₂₀] (from 620 nm brick red at room temperature to 585 nm yellow at 8 K), anti-[8′-Py-B₁₈H₂₁] exhibits no luminescence [376,377]. The 6′,9′-disubstituted derivatives with 4-picoline [377], isoquinoline [378], and 5-hydroxyisoquinoline [379] were prepared in a similar way.

The reaction of anti-[B₁₈H₂₂] with methyl isonitrile MeNC in benzene leads to anti-[7-{(MeNH)C₃N₂HMe₂}-B₁₈H₂₀], in which a reductive trimerization of MeNC gives an unusual imidazole-based carbene, {(MeNH)C₃N₂HMe₂}, that is stabilized by coordination to the macropolyhedral boron cluster (Figure 53) [380].

The reaction with tert-butyl isonitrile in 1,2-dichloroethane results in anti-[7-{(t-BuNHCH){t-BuNHC(CN)}CH₂}-B₁₈H₂₀], in which a reductive oligomerization of t-BuNC has given the complex polynitrogen base {(t-BuNHCH){t-BuNHC(CN)}CH₂:} formally as a zwitterionic carbene attached to the macropolyhedral boron cluster (Figure 54) [381].

The synthesis of few substituted derivatives of syn-[B₁₈H₂₂] was reported. Heating syn-[B₁₈H₂₂] with sulfur in the presence of anhydrous AlCl₃ at 125 °C results in a mixture of the isomeric mercapto derivatives syn-[1-HS-B₁₈H₂₁], syn-[3-HS-B₁₈H₂₁], and syn-[4-HS-B₁₈H₂₁], which were all separated by chromatography on silica (Figure 55) [382].

The 3- and 4-mercapto derivatives of syn-[B₁₈H₂₂], syn-[3-HS-B₁₈H₂₁], and syn-[4-HS-B₁₈H₂₁] were obtained as byproducts of thermolysis of nonathiaborane arachno-[SB₈H₁₂] in boiling cyclohexane [383]. The mercapto derivatives obtained are brightly luminescent under UV irradiation, making these compounds rare examples of a luminescent derivative of syn-[B₁₈H₂₂] [382,383].

The deprotonation of syn-[B₁₈H₂₂] with NaH in 1,2-dimethoxyethane, followed by the reaction with iodine and dimethylsulfide under reflux, results in the 7-dimethylsulfonium derivative syn-[7-Me₂S-B₁₈H₂₀] (Figure 56) [384].

12. Conclusions

The purpose of this review was to give the most complete picture of the current state of the chemistry of decaborane and its derivatives. After its rapid development on the verge of the 1950s and 1960s, associated with the study of the chemistry of decaborane as a potential component of rocket fuels, the chemistry of decaborane was studied by many research groups. However, no comprehensive review elucidating it in full has appeared for more than 50 years, which certainly hindered the development of this important area of boron cluster chemistry. We would like to hope that this review will be useful both for young researchers just starting their way in boron chemistry and for researchers actively working in this field.