Synthesis and characterization of synthetically useful salts of the weakly-coordinating dianion [B₁₂Cl₁₂]²⁻

Abstract

The closo-dodecahydrododecaborate [NEt₃H]₂[B₁₂H₁₂] has been prepared on a lab scale by an improved synthesis from cheap and readily available starting materials Na[BH₄] and I₂ in diglyme (diethylene glycol dimethyl ether). Subsequent chlorination with elemental chlorine in aqueous solution at normal pressure yielded the per-chlorinated weakly coordinating [B₁₂Cl₁₂]²⁻ anion. By simple metathesis reaction a variety of useful salts [cation]₂[B₁₂Cl₁₂] (cation = [NEt₃H]⁺, [NBu₄]⁺, Li⁺, Na⁺, K⁺, Cs⁺) is available. These salts are useful starting materials, which have the potential to open up the chemistry of [B₁₂Cl₁₂]²⁻ as a weakly coordinating dianion. Exemplarily, they were used in further reactions to prepare [NO]₂[B₁₂Cl₁₂], [PPN]₂[B₁₂Cl₁₂], and [CPh₃]₂[B₁₂Cl₁₂]. The crystal structures of Cs₂[B₁₂Cl₁₂]·SO₂, [CPh₃]₂[B₁₂Cl₁₂]·2C₂H₄Cl₂, and [CPh₃]₂[B₁₂Cl₁₂]·2SO₂ and preliminary crystal structures of [NO]₂[B₁₂Cl₁₂]·SO₂ and [PPN]₂[B₁₂Cl₁₂]·CH₂Cl₂ were determined. The crystal structure of the SO₂ solvate Cs₂[B₁₂Cl₁₂]·SO₂ is related to the crystal structure of solvent free Cs₂[B₁₂Cl₁₂]. [CPh₃]₂[B₁₂Cl₁₂]·2C₂H₄Cl₂ and [CPh₃]₂[B₁₂Cl₁₂]·2SO₂ have very similar structures in the solid state. In both cases the [CPh₃]⁺ cations form only very weak contacts to the [B₁₂Cl₁₂]²⁻ anion and SO₂ or C₂H₄Cl₂solvent molecules respectively. The averaged experimental B–B (178.7 pm) and B–Cl (178.9 pm) bond lengths within [B₁₂Cl₁₂]²⁻ are essentially unchanged in all determined structures and are reproduced well by PBE0/TZVPP quantum chemical calculations (B–B 178.6 pm, B–Cl 179.3 pm). All results indicate that [B₁₂Cl₁₂]²⁻ is a readily accessible weakly-coordinating dianion.

---

1. Introduction

Weakly-coordinating anions (WCA) have become a major player in fundamental and applied chemical research.¹ Among the large variety of WCAs halogenated carborane anions² have been shown to be the most stable weakly-coordinating anions and allowed the synthesis and characterization of some very reactive cations, which include [Et₂Al]⁺,^3a [C₆₀H]⁺,^3b[t-butyl]⁺,^3c [R₃Si]⁺.^3d Unfortunately, the synthesis of the carborane anions is time consuming and expensive,⁴ which prevented their widespread application so far. Halogenated closo-dodecaborates [B₁₂X₁₂]²⁻ (X = F, Cl, Br, I)⁵ posses a similar stability and they are easier and cheaper to prepare. The higher −2 charge on [B₁₂X₁₂]²⁻ dianions is unusual for a weakly-coordinating anion (a typical WCA is a large (fluorinated) anion with a charge of −1)¹ and prevents the often desired “pseudo-gas phase conditions”.^1b,c Therefore the closo-dodecaborates [B₁₂X₁₂]²⁻ have been widely neglected so far. The perchlorinated derivative [B₁₂Cl₁₂]²⁻ is the most promising candidate for future applications among the series [B₁₂X₁₂]²⁻ (X = F, Cl, Br, I), because it is more resistant against oxidation than [B₁₂Br₁₂]²⁻ and [B₁₂I₁₂]²⁻, but more readily accessible than [B₁₂F₁₂]²⁻, which is prepared by the reaction of [B₁₂H₁₂]²⁻ with elemental fluorine.⁶ Salts containing the [B₁₂Cl₁₂]²⁻ dianion have already found several applications in the past, some of which are stabilization of cationic Pd and Ni complexes,⁷ as ionic liquids,⁸ evaluation as non-aqueous electrolytes in solid cathode lithium batteries⁹ and in fuel cells.¹⁰ Recently the fluorinated [B₁₂F₁₂]²⁻ dianion⁶ was tested as electrolyte for electrochemical devices.¹¹ Therefore a simple and cheap lab scale synthesis of [B₁₂Cl₁₂]²⁻ and its predecessor [B₁₂H₁₂]²⁻ is key for their widespread investigation. Although [B₁₂H₁₂]²⁻¹² and [B₁₂Cl₁₂]²⁻⁵ are well known compounds, full experimental details for a cheap, simple and reproducible lab scale synthesis are not available.

In this paper we present an optimized synthesis of closo-dodecaborate [B₁₂H₁₂]²⁻ from cheap starting materials Na[BH₄] and I₂ avoiding toxic diborane and expensive decaborane, the complete chlorination of [B₁₂H₁₂]²⁻ to [B₁₂Cl₁₂]²⁻ in aqueous solution with elemental Cl₂, and the preparation and characterization of some synthetically useful salts of the [B₁₂Cl₁₂]²⁻ anion, i.e.Li⁺, Cs⁺, [NEt₃H]⁺, [NBu₄]⁺, [CPh₃]⁺, and [NO]⁺. The crystal structures of Cs₂[B₁₂Cl₁₂]·SO₂ (1), [CPh₃]₂[B₁₂Cl₁₂]·2C₂H₄Cl₂ (2), and [CPh₃]₂[B₁₂Cl₁₂]·2SO₂ (3) are also reported.

2. Experimental

General remarks

Air and moisture sensitive solid reagents were manipulated using standard vacuum and Schlenk techniques or in a glove box with an atmosphere of dry argon (H₂O and O₂ <1 ppm). The reactions using liquid sulfur dioxide as a solvent were carried out in H-shaped glass vessels with J. Young Teflon in glass valves and an incorporated fine frit. The solvents liquid sulfur dioxide (Schick), 1,2-dichloroethane (Riedel-de-Haën), and deuterated solvents (Deutero) were dried over CaH₂ (Merck) and distilled prior to use. CH₃CN, CH₂Cl₂, and n-pentane were purified using a Grubbssolvent purification system. Na[BH₄] (Acros, 98%, powder), I₂ (Grüssing, p.a.), diglyme (diethylene glycol dimethyl ether) (Acros), NEt₃ (Fluka), LiOH (Merck), NaOH (Grüssing, p.a.), KOH (Roth, p.a.), CsOH (Acros), Cl₂ (SBF), [NBu₄]Cl (Fluka), [NO][BF₄] (Acros, 97%), and CPh₃Cl (Alfa Aesar) are commercially available and were used as received. Commercial [CPh₃][BF₄] (Acros) was purified by washing with n-pentane. [PPN]Cl (bis(triphenylphosphine)iminium chloride) was prepared by a modified literature procedure.¹³

IR spectra were recorded on a Nicolet Magna-IR 760 equipped with a diamond ATR attachment and FT-Raman spectra were recorded in flame-sealed capillaries on a Bruker Vertex 70 IR spectrometer equipped with a Bruker RAM II Raman module using a highly sensitive Ge detector and a Nd:YAG-Laser (1064 nm). ¹H, ¹¹B, ¹³C and ³¹P NMR spectra were measured on a Bruker Avance II 400 WB spectrometer in 5 mm NMR tubes at room temperature. Chemical shifts are given with respect to Me₄Si (¹H, ¹³C), BF₃·OEt₂ (¹¹B), H₃CNO₂ (¹⁴N), and 85% H₃PO₄ (³¹P). Melting and decomposition points were obtained on a Setaram DSC 131. All actual spectra are included with the ESI.†

[NHEt₃]₂[B₁₂H₁₂]. A solution of iodine (205.58 g, 0.810 mol) in 350 mL of diglyme was added dropwise over a period of 6 h to a suspension of Na[BH₄] (97.91 g, 2.588 mol) in diglyme (400 ml) at 100 °C. During the addition of iodine the amount of insoluble Na[BH₄] decreased and at the end a yellow color of the reaction mixture was observed. The reaction mixture was stirred over night at 100 °C to complete the formation of [B₃H₈]⁻ followed by additional 24 h reflux to completely disproportionate [B₃H₈]⁻ to [B₁₂H₁₂]²⁻ and [BH₄]⁻ (at this stage the yellow color disappeared and a white precipitate started to form). The solvent was distilled off under reduced pressure giving a large amount of white solid. The white solid was dissolved in 600 mL of water and 280 mL of concentrated hydrochloric acid were added carefully to the reaction mixture (caution: hydrogen evolution). The acidified clear solution was stored at +6 °C over night. Colourless crystals of boric acid (ca. 15 g) precipitated and were removed by filtration. The filtrate was treated with 400 mL Et₃N (pH = 9–10) and readily a white precipitate formed (ca. 80 g, the white precipitate contained the product [NEt₃H]₂[B₁₂H₁₂] and remaining boric acid (¹¹B NMR in D₂O), which was collected by filtration. The solid was re-suspended in water (ca. 250 ml), stirred for two hours, and then filtrated to remove the more soluble boric acid impurity. The product [NEt₃H]₂[B₁₂H₁₂] (28.50 g, 0.082 mol, 51%) remained as a white solid. ¹H NMR (CD₃CN): δ = 0.50–1.90 (mult., br., 12 H) [B₁₂H₁₂]²⁻, 1.19 (t, ³J_HH = 7.3 Hz, 18 H, CH₃), 3.11 (q, ³J_HH = 7.3 Hz, 12 H, CH₂). ¹¹B NMR (CD₃CN): δ = −15.3 (d, ¹J_BH = 125 Hz). A full account of the preparation including all details has been deposited as ESI.†

M₂[B₁₂H₁₂] (M = Na, K, Cs). Solid [NHEt₃]₂[B₁₂H₁₂] was added to a solution of 2.1 equivalents of MOH (M = Na, K) dissolved in water in a polypropylene beaker. Typically about 100 ml water was used for 10 g [NHEt₃]₂[B₁₂H₁₂]. The suspension was heated on a water bath until it became a clear solution. Then the solution was evaporated to dryness. Solvent free K₂[B₁₂H₁₂] can be prepared by drying the remaining solid in vacuum at 80 °C. The Cs⁺ salt is much less soluble than the other alkali metal salts and precipitates immediately after CsOH addition.¹⁴ It can be re-crystallized from boiling water. ¹H NMR (D₂O): δ = 0.50–1.90 (mult., br.) [B₁₂H₁₂]²⁻. ¹¹B NMR (D₂O): δ = −15.3 (d, ¹J_BH = 125 Hz) [B₁₂H₁₂]²⁻.

Chlorination of Na₂[B₁₂H₁₂]. In a typical reaction 10 g Na₂[B₁₂H₁₂] were dissolved in 100 ml of water. Chlorine was bubbled through the reaction mixture for five hours at rt and additional 24 hours at 100 °C. Completeness of chlorination was checked by ¹¹B NMR spectra of aliquots every few hours. When the chlorination was complete the excess of chlorine was removed by purging the reaction mixture with argon. The resulting very acidic clear colourless solution of Na₂[B₁₂Cl₁₂] was directly used to prepare [NEt₃H]₂[B₁₂Cl₁₂] and Cs₂[B₁₂Cl₁₂] (see below).

[NHEt₃]₂[B₁₂Cl₁₂]. An excess of NEt₃ was added to the acidic aqueous solution from the chlorination reaction (see above) and immediately white [NEt₃H]₂[B₁₂Cl₁₂] precipitated. The [NEt₃H]₂[B₁₂Cl₁₂] precipitate was separated by filtration and washed with water (the filtrate was checked for chloride with Ag[NO₃]) to give the product as a white solid. ¹H NMR (CD₃CN): δ = 1.19 (t, ³J_HH = 7.3 Hz, 18 H, CH₃), 3.11 (q, ³J_HH = 7.3 Hz, 12 H, CH₂). ¹¹B NMR (CD₃CN): δ = −12.7 (s) [B₁₂Cl₁₂]²⁻. IR (cm⁻¹): ν = 3181 (m), 2988 (w), 1471 (m), 1458 (m), 1397 (m), 1304 (w), 1289 (w), 1172 (w), 1155 (w), 1027 (vs) [B₁₂Cl₁₂]²⁻, 835 (w), 793 (w), 677 (w), 533 (vs) [B₁₂Cl₁₂]²⁻, 506 (w), 458 (m), 407 (m). Raman (cm⁻¹): ν = 3187 (5), 2988 (50), 2948 (50), 1453 (17), 300 (90) [B₁₂Cl₁₂]²⁻, 128 (100) [B₁₂Cl₁₂]²⁻.

M₂[B₁₂Cl₁₂] (M = Li, Na, K). Solid [NHEt₃]₂[B₁₂Cl₁₂] was added to a solution of 2.1 equivalents of MOH (M = Li, Na, K) dissolved in water in a polypropylene beaker. Typically about 100 ml water was used for 10 g [NHEt₃]₂[B₁₂H₁₂]. The suspension was heated on a water bath until it became a clear solution. The solution was evaporated to dryness yielding M₂[B₁₂Cl₁₂] as a white solid in quantitative yield. Solvent free M₂[B₁₂Cl₁₂] can be prepared by drying in vacuum at 150–200 °C. The product is highly soluble in H₂O, and very soluble in liquid SO₂ and CH₃CN and has low solubility in CH₂Cl₂. M₂[B₁₂Cl₁₂] (M = Li, Na, K, Cs) gives essentially identical spectroscopic data. ¹¹B NMR (D₂O): δ = −12.7 (s) [B₁₂Cl₁₂]²⁻. IR (cm⁻¹): ν = 1029 (vs), 532 (vs), 160 (s), 136 (w), 129 (m). Raman (cm⁻¹): ν = 305 (100), 132 (70). mp > 450 °C.

Cs₂[B₁₂Cl₁₂]. 2.1 equivalents of CsOH were added to the acidic aqueous solution from the chlorination reaction (see above). The Cs₂[B₁₂Cl₁₂] precipitate was separated by filtration and re-crystallized from hot water. Crystals of Cs₂[B₁₂Cl₁₂]·SO₂ suitable for X-ray diffraction were prepared by dissolving Cs₂[B₁₂Cl₁₂] in liquid SO₂ and slow removal of the solvent. Cs₂[B₁₂Cl₁₂] is soluble in THF and CH₃CN, has low solubility in SO₂, and is insoluble in toluene, CH₂Cl₂ and 1,2-C₆H₄F₂.

[NBu₄]₂[B₁₂Cl₁₂]. An excess of [NBu₄]Br was added to the acidic aqueous solution from the chlorination reaction (see above). The [NBu₄]₂[B₁₂Cl₁₂] precipitate was separated by filtration and washed with water (the filtrate was checked for chloride with Ag[NO₃]) to give the product as a white solid. [NBu₄]₂[B₁₂Cl₁₂] is soluble in CH₃CN, CH₂Cl₂ and SO₂. ¹H NMR (CD₃CN): δ = 0.96 (t, ³J_HH = 7.5 Hz, 24H, CH₃), 1.34 (sextet, ³J_HH = 7.5 Hz, 16H, CH₂-CH₃), 1.62 (mult., 16H, CH₂), 3.09 (mult., 16H, NCH₂). ¹¹B NMR (D₂O): δ = −12.7 (s) [B₁₂Cl₁₂]²⁻.¹³C NMR (CD₃CN): δ = 12.7 (CH₃), 19.3 (CH₂-CH₃), 23.2 (CH₂), 58.1 (NCH₂). IR (cm⁻¹): ν = 2966 (m), 2936 (w), 2878 (w), 1469 (s), 1384 (w), 1169 (w), 1031 (vs) [B₁₂Cl₁₂]²⁻, 995 (sh), 925 (w), 880 (m), 794 (w), 740 (m), 577 (w), 531 (vs) [B₁₂Cl₁₂]²⁻. Raman (cm⁻¹): ν = 3145 (2), 2972 (30), 2937 (80), 2875 (30), 2756 (18), 1453 (2), 1355 (1), 1318 (2), 1271 (1), 1111 (6), 1068 (10), 906 (2), 882 (2) 300 (100) [B₁₂Cl₁₂]²⁻, 127 (80) [B₁₂Cl₁₂]²⁻. mp 420 °C (dec.).

[PPN]₂[B₁₂Cl₁₂]. Li₂B₁₂Cl₁₂ (0.172 g, 0.3 mmol) and [PPN]Cl (0.352 g, 0.6 mmol) were dissolved in 50 ml CH₂Cl₂. The reaction mixture was stirred at rt for 3 h. A white precipitate (LiCl) formed, which was removed by filtration through a fine frit. Removing of the solvent in vacuum yielded [PPN]₂[B₁₂Cl₁₂] as a white solid in quantitative yield. Crystals of [PPN]₂[B₁₂Cl₁₂] suitable for X-ray diffraction were prepared by dissolving [PPN]₂[B₁₂Cl₁₂] in CH₂Cl₂ followed by slow evaporation of the solvent (see ESI). ¹H NMR (CH₂Cl₂/CDCl₃): δ = 7.45–7.55 (mult., 48H, o-/m-C₆H₅), 7.60–7.70 (mult., 12H, p-C₆H₅). ¹¹B NMR (CH₂Cl₂/CDCl₃): δ = −12.6 (s, [B₁₂Cl₁₂]²⁻). ³¹P NMR (CH₂Cl₂/CDCl₃): δ = 21.1 (s, Ph₃N=P=NPh₃). IR (cm⁻¹): ν = 3059 (w), 2930 (w), 1729 (w), 1587 (w), 1483 (w), 1394 (s), 1303 (m), 1262 (m), 1184 (m), 1115 (m), 1024 (s) [B₁₂Cl₁₂]²⁻, 997 (s), 854 (w), 799 (w), 750 (m), 721 (s), 691 (2), 617 (w), 527 (s) [B₁₂Cl₁₂]²⁻, 497 (s), 448 (m). Raman (cm⁻¹): ν = 3062 (61), 1588 (54), 1111 (17), 1027 (31), 1000 (100), 659 (24), 616 (20), 296 (63) [B₁₂Cl₁₂]²⁻, 235 (29), 129 (89) [B₁₂Cl₁₂]²⁻. mp 303 °C.

[CPh₃]₂[B₁₂Cl₁₂], method A. [CPh₃][BF₄] (0.41 g, 1.23 mmol) and Cs₂[B₁₂Cl₁₂] (0.50 g, 0.61 mmol) were dissolved in dry acetonitrile and stirred over night. All volatiles were removed in vacuum and the product was extracted with 1,2-C₂H₄Cl₂ giving [CPh₃]₂[B₁₂Cl₁₂]·2(1,2-C₂H₄Cl₂) quantitatively as a orange crystalline solid (isolated yield: 0.64 g, 0.52 mmol, 85%). C₂H₄Cl₂ solvate molecules can be removed by dissolving the product in liquid sulfur dioxide followed by drying at 50 °C over night. Crystals suitable for X-ray diffraction of 3 were obtained by dissolving [CPh₃]₂[B₁₂Cl₁₂] in liquid SO₂ followed by slow removal of the solvent and crystals of 2 by slow removal of the solvent from a saturated solution in 1,2-dichloroethane. ¹H NMR (CD₃CN): δ = 7.73 (d, ³J_HH = 7.2 Hz, 12 ortho-H), 7.90 (t, ³J_HH = 8.0 Hz, 12 meta-H), 8.30 (t, ³J_HH = 7.4 Hz, 6 para-H). ¹¹B NMR (CD₃CN): δ = −12.7 (s, [B₁₂Cl₁₂]²⁻). ¹³C NMR (CD₃CN): δ = 130.6 (meta-C), 139.8 (ipso-C), 142.7 (ortho-C), 143.4 (para-C), 210.9 ([CPh₃]⁺). IR (cm⁻¹): ν = 3064 (w), 1577 (vs), 1480 (s), 1448 (vs), 1352 (vs), 1292 (vs), 1230 (m), 1182 (s), 1166 (sh), 1025 (vs) [B₁₂Cl₁₂]²⁻, 994 (vs), 983 (sh), 945 (m), 915 (m), 847 (sh), 835 (m), 806 (m), 770 (s), 702 (vs), 687 (sh), 657 (sh), 622 (s), 608 (s), 531 (vs) [B₁₂Cl₁₂]²⁻, 464 (m), 426 (w), 402 (s). Raman ([CPh₃]₂[B₁₂Cl₁₂]·2(1,2-C₂H₄Cl₂), cm⁻¹): ν = 3069 (20), 1596 (40), 1580 (100), 1483 (15), 1358 (45), 1296 (5), 1186 (30), 1167 (5), 1027 (10), 998 (40), 916 (10), 849 (5), 772 (5), 708 (5), 623 (10), 469 (15), 405 (40), 298 (5) [B₁₂Cl₁₂]²⁻, 285 (40), 131 (50) [B₁₂Cl₁₂]²⁻, 94 (70). We did not succeed in obtaining a Raman spectrum of solvent free [CPh₃]₂[B₁₂Cl₁₂] presumably due to loss of crystallinity.

[CPh₃]₂[B₁₂Cl₁₂], method B. 10 ml SO₂ were condensed onto a mixture of CPh₃Cl (0.80 g, 2.88 mmol) and Li₂[B₁₂Cl₁₂] (0.82 g, 1.45 mmol). The reaction mixture turned yellow on warming and was stirred at rt for one hour. The volatiles were removed in vacuum and the product was extracted with 1,2-C₂H₄Cl₂ as described for method A giving [CPh₃]₂[B₁₂Cl₁₂] as yellow crystalline solid in essentially quantitative yield (isolated yield 1.05 g, 1.01 mmol, 70).

[NO]₂[B₁₂Cl₁₂]. 10 ml SO₂ were condensed onto a mixture of [NO][BF₄] (0.21 g, 1.79 mmol) and Li₂[B₁₂Cl₁₂] (0.46 g, 0.81 mmol). The reaction mixture was stirred over night giving a red solution and some white precipitate (Li[BF₄], IR), which was separated by filtration. Removal of all volatiles in vacuum yielded [NO]₂[B₁₂Cl₁₂] as a red solid in quantitative yield (0.40 g, 0.65 mmol, 80%). Red crystals suitable for X-ray diffraction were obtained by slow removal of the solvent (see ESI†). ¹¹B NMR (SO₂, unlocked): δ = −12.6 (s, [B₁₂Cl₁₂]²⁻). ¹⁴N NMR (SO₂, unlocked): δ = 15 (s, Δω_1/2 220 Hz, [NO]⁺). IR (cm⁻¹): ν = 2214 (m, broad) [NO]⁺, 1026 (s) [B₁₂Cl₁₂]²⁻, 530 (s) [B₁₂Cl₁₂]²⁻. Raman (cm⁻¹): ν = 2220 (100) [NO]⁺, 1067 (4), 990 (1), 305 (5) [B₁₂Cl₁₂]²⁻, 144 (6), 126 (4) [B₁₂Cl₁₂]²⁻.

X-Ray crystallography

Single-crystal X-ray structure determinations were carried out on a Rigaku R-AXIS Spider image plate (1 and 3) or a Bruker APEX II CCD diffractometer (2) using Mo Kα (0.71073 Å) radiation. The crystals were mounted onto a kryo loop using fluorinated oil and frozen in the cold nitrogen stream of the goniometer. Details of the crystallographic data collection and refinement parameters are given in Table 1. The structures were solved by direct methods (SHELXS).¹⁵ Subsequent least-squares refinement on F² (SHELXL 97-2) located the positions of the remaining atoms in the electron density maps.¹⁵ All atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions using a riding model and were refined isotropically in blocks. The data were corrected for absorption. Graphical representations of the structures were prepared with the program DIAMOND.¹⁶

Table 1 Crystallographic parameters

	Cs2[B12Cl12]·SO2 (1)	[CPh3]2[B12Cl12]·2C2H4Cl2 (2)	[CPh3]2[B12Cl12]·2SO2 (3)
a R 1 = ∑‖Fo| − |Fc‖/∑|Fo|, wR2 = (∑[w(Fo^2−Fc^2)^2]/∑[wFo^4])^1^/2.
Formula	B12Cl12Cs2O2S	C42H38B12Cl16	C38H30B12Cl12O4S2
M	885.00	1239.64	1169.86
Crystal system	Trigonal	Monoclinic	Monoclinic
Space group (no.)	R3̄ (148)	P21/n (14)	C2/c (15)
a/pm	1014.32(14)	993.18(2)	1620.5(3)
b/pm	1014.32(14)	1498.79(4)	1661.7(3)
c/pm	2066.8(4)	1792.43(4)	1901.0(4)
α/°	90	90	90
β/°	90	91.622(1)	102.60(3)
γ/°	120	90	90
U /nm^3	1.8415(5)	2667.08(11)	4.9958(17)
T/K	110(2)	114(2)	173(2)
Z	3	2	4
μ(Mo Kα)/mm^−1	4.361	0.857	0.790
No. of data collected	3466	62722	43480
No. of unique data	936	6807	5725
R int	0.0418	0.0414	0.0434
R 1, wR2 (I >2σ(I))^a	0.0346, 0.0703	0.0306, 0.0749	0.0355, 0.0889
R 1, wR2 (all data)	0.0409, 0.0721	0.0393, 0.0802	0.0456, 0.0937

---

Quantum chemical calculations

DFT (BP86 and PBE0) calculations were performed with SV(P), TZVPP, and QZVPP basis sets as implemented in the program TURBOMOLE.¹⁷ Frequency calculations were performed at the BP86/SV(P) level for all species. All calculated species are true minima on the energy hyper surface as shown by the absence of imaginary frequencies. Calculated coordinates, frequencies and total energies are included with the ESI.†

3. Results and discussion

3.1. Synthesis of [B₁₂H₁₂]²⁻ from [BH₄]⁻ and I₂

The closo-dodecahydrododecaborate [B₁₂H₁₂]²⁻ is an important starting material in boron chemistry,¹² and various syntheses are known.¹² Unfortunately, most synthetic routes use either expensive (e.g. [B₁₀H₁₄]) or potentially dangerous (e.g. B₂H₆) starting materials. A cheap and experimentally simple route is the reaction of M[BH₄] (M = Na, K) with I₂ in diglyme (diethylene glycol dimethyl ether) giving the anion [B₃H₈]⁻ as an intermediate in a first step (eqn (1)), which subsequently in a second step can be thermally decomposed to [B₁₂H₁₂]²⁻ (eqn (2)).

3 NaBH₄ + I₂→ NaB₃H₈ + 2 NaI + 2 H₂↑ (1)

5 Na[B₃H₈] → Na₂[B₁₂H₁₂] + 3 Na[BH₄] + 8 H₂↑ (2)

Both steps are well known independently of each other,^18,19 but only limited experimental details are available for the complete two-step synthesis.^20–22 We modified and scaled up the published procedures and improved the synthesis of [B₁₂H₁₂]²⁻ from Na[BH₄] and I₂ to give reproducible yields of 50–60% based on I₂ used. The obtained product did not contain smaller boron clusters (e.g. [B₆H₆]²⁻, [B₁₀H₁₀]²⁻) as evident from ¹¹B NMR spectra, which showed a doublet at −15.3 ppm for [B₁₂H₁₂]²⁻ only.²³ Our improvements include a modified stoichiometry, longer reaction times, and a modified work up procedure. A discussion of the synthesis including full experimental details is included with the ESI.†

3.2. Chlorination of [B₁₂H₁₂]²⁻ to [B₁₂Cl₁₂]²⁻

The chlorination of [B₁₂H₁₂]²⁻ is known since the pioneering work of Muetterties et al.,⁵ who reacted [B₁₂H₁₂]²⁻ with elemental chlorine in aqueous solution giving partly chlorinated compounds, which were subsequently reacted with an excess of chlorine in an autoclave to give [B₁₂Cl₁₂]²⁻ as the final product (eqn (3)).

[B₁₂H₁₂]²⁻ + 12 Cl₂→ [B₁₂Cl₁₂]²⁻ + 12 HCl (3)

However, incomplete chlorination (i.e. the product consists of a mixture of [B₁₂Cl₁₂]²⁻, [B₁₂HCl₁₁]²⁻, and [B₁₂H₂Cl₁₀]²⁻) has been reported.^8,10,24 The absence of the B–H vibration in the IR spectrum was suggested as diagnostic tool for checking the completeness of the chlorination process.¹⁰ We prepared Na₂[B₁₂Cl₁₂] under avoiding a high pressure autoclave^5,25 by passing elemental chlorine through an aqueous solution of Na₂[B₁₂H₁₂] at room temperature. The sodium salt is preferred over the caesium salt because of its much higher solubility in water.¹⁴ The reaction speed decreases with progressing chlorination and therefore after a few hours the reaction mixture was heated to 100 °C. The chlorination progress was checked repeatedly by ¹¹B NMR. In contrast to previous reports¹⁰ we found the absence of a B–H vibration in the IR spectrum to be non suitable for checking the chlorination completeness, because the B–H vibration becomes increasingly weak with progressing chlorination and may not be observable even if the chlorination is still incomplete. Fig. 1 shows ¹¹B NMR spectra of [B₁₂H₁₂]²⁻ and [B₁₂Cl₁₂]²⁻ together with several ¹¹B NMR spectra of samples taken from the reaction mixture during chlorination over a time period of 40 h. The spectra of the intermediate steps show two separate regions for H-bonded and Cl-bonded boron atoms, which shift to higher field with progressing chlorination. A preliminary investigation of the composition of the intermediate reaction mixture may be found in the ESI. The existence of a single peak only at about −12.8 ppm indicates complete chlorination.

Fig. 1 ¹¹B NMR spectra in D₂O at rt of aliquots taken from the reaction of Na₂[B₁₂H₁₂] with Cl₂ in H₂O over a time period of 40 h.

The mechanism of the chlorination remains unknown, but electrophilic substitution and electrophile-induced nucleophilic substitution (EINS) have been suggested.⁴ The stereochemistry of the substitution and the corresponding substitution pattern for the chlorination is controversial. Preetz et al. investigated the first three steps of the chlorination and suggested an ortho substitution pattern to give [1,2-B₁₂H₁₀Cl₂]²⁻ and [1,2,3-B₁₂H₉Cl₃]²⁻ based on ¹¹B NMR and vibrational spectroscopy.²⁶ In contrast a detailed NMR spectroscopic study (based on ¹¹B–¹¹B cosy spectra) together with structure determination of some intermediates for the corresponding fluorination by Kuznetsov et al. showed unambiguously a preferred meta substitution pattern. Correspondingly [1,7-B₁₂F₂H₁₀]²⁻ and [1,7,9-B₁₂H₉F₃]²⁻ were identified.²⁷ Only two partly chlorinated clusters have been structurally characterized, i.e. [B₁₂H₁₁Cl]²⁻²⁸ and [B₁₂H₂Cl₁₀]²⁻,²⁴ which give no evidence for either of the substitution patterns. Isolation and characterization of the intermediate products of the chlorination will be a part of a future project to experimentally proof the theoretically predicted sequential substitution pattern.²⁹

3.3. Synthesis and characterization of synthetically useful salts of the dianion [B₁₂Cl₁₂]²⁻

Li₂[B₁₂Cl₁₂], Cs₂[B₁₂Cl₁₂], and [NBu₄][B₁₂Cl₁₂]. Alkali metal and ammonium salts are fundamental starting materials for exploring the chemistry of [B₁₂Cl₁₂]²⁻. Cs[B₁₂Cl₁₂] is the least soluble alkali metal salt and can be prepared directly from the chlorination reaction mixture by precipitating with CsOH (eqn (4). Similarly, neutralization of the acidic chlorination reaction mixture with NEt₃ affords only sparingly soluble [NEt₃H]₂[B₁₂Cl₁₂], which can be reacted with MOH (M = Li, K) in hot water in a second step to obtain the more soluble alkali metal salts (eqn (5)).

M = Li, Na, K, Cs. The lithium, caesium and tetrabutylammonium salts of [B₁₂Cl₁₂]²⁻ are especially useful, because they show complementary properties (e.g. solubility in different solvents)³⁰ and have been turned out to be ideal starting materials. For instant Li₂[B₁₂Cl₁₂] has some solubility in CH₂Cl₂ and undergoes easily metathesis reactions with chlorides to form the desired [B₁₂Cl₁₂]²⁻ salts and LiCl. This methodology may be useful to prepare low melting salts or ionic liquids containing the [B₁₂Cl₁₂]²⁻ anion. Exemplarily, we prepared [PPN]₂[B₁₂Cl₁₂] from Li₂[B₁₂Cl₁₂] and [PPN]Cl in CH₂Cl₂ (eqn (6)).

A preliminary crystal structure of [PPN]₂[B₁₂Cl₁₂] has been determined and shows well defined [B₁₂Cl₁₂]²⁻ anions and heavily disordered [PPN]⁺ cations, which prevent a detailed discussion of the structure (see ESI†).

[CPh₃][B₁₂Cl₁₂]. Trityl (= [CPh₃]⁺) salts have been recognized as important reagents for hydride and alkyl abstraction.^1b Therefore, [CPh₃]₂[B₁₂Cl₁₂] is a worthwhile synthetic target to extend the chemistry of [B₁₂Cl₁₂]²⁻. We developed two independent synthetic routes to [CPh₃]₂[B₁₂Cl₁₂]. Cs₂[B₁₂Cl₁₂] reacts with [CPh₃][BF₄] in CH₃CN in a metathesis reaction to [CPh₃]₂[B₁₂Cl₁₂] and Cs[BF₄] (eqn (7)).

However, this reaction uses expensive [CPh₃][BF₄] as a starting material. Thus we investigated routes using the cheaper trityl chloride as a starting point. Li₂[B₁₂Cl₁₂] reacts with CPh₃Cl in liquid SO₂ to [CPh₃]₂[B₁₂Cl₁₂] and LiCl (eqn (8)).

The cleavage of the C–Cl bond is overcompensated by the high lattice enthalpy of LiCl. The solid state reaction is favourable by about 100 kJ mol⁻¹ as evident from the Born–Fajans–Haber cycle in Fig. 2. However, in acetonitrile, which dissolves the starting materials and the products very well, the reaction is not complete. Instead, in acetonitrile solution there is a reversible balance between the starting materials and the products. In contrast, in SO₂LiCl is insoluble, which forces the reaction to the product side.

Fig. 2 Born–Fajans–Haber cycle for the formation of [CPh₃]₂[B₁₂Cl₁₂] from CPh₃Cl and Li₂[B₁₂Cl₁₂]. Details on the calculation of the gas phase and the lattice enthalpies are included with the ESI.†

Independent of the synthetic route used [CPh₃]₂[B₁₂Cl₁₂] has to be separated from the insoluble side-products Cs[BF₄] or LiCl respectively. It turned out that 1,2-dichloroethane seems to be the only solvent, in which [CPh₃]₂[B₁₂Cl₁₂] has significant solubility and the side-products are insoluble. Unfortunately, the solvent gets incorporated in the crystal lattice and thus solid [CPh₃]₂[B₁₂Cl₁₂] contains two equivalents of 1,2-dichloroethane (¹H NMR and crystal structure determination), which cannot be removed in vacuum. The solvated 1,2-dichloroethane can be replaced by other solvent molecules (e.g. SO₂, CH₃CN, CH₂Cl₂) by dissolving the compound in an excess of the respective solvent and subsequent removal of all volatiles. The solid state structures of the 1,2-dichloroethane and the sulfur dioxide solvates have been determined by single crystal X-ray diffraction (see below). Solvate free [CPh₃]₂[B₁₂Cl₁₂] is available by dissolving the compound in liquid sulfur dioxide followed by drying in vacuum at 50 °C over night.

[NO]₂[B₁₂Cl₁₂]. [NO]⁺ salts are strong oxidizers and important reagents in organic chemistry.³¹ Recently [NO]⁺ salts of some weakly-coordinating anions have been reported.^32,33a We decided to prepare [NO]₂[B₁₂Cl₁₂], because it has the potential to oxidize a substrate and at the same time gives direct access to desired salts [substrate][B₁₂Cl₁₂]. It has been shown before that the metathesis of lithium salts of weakly coordinating anions with [cation][MF₆] (M = As, Sb) in liquid sulfur dioxide represents a simple straight forward route to the respective salts of the weakly-coordinating anion.³³ Following this methodology the metathesis reaction of Li₂[B₁₂Cl₁₂] with [NO][BF₄] in liquid sulfur dioxide yielded [NO]₂[B₁₂Cl₁₂] as a red solid in quantitative yield (eqn (9)).

The ¹⁴N NMR spectrum in liquid SO₂ shows a signal at 15 ppm for [NO]⁺, which is shifted downfield by about 20 ppm compared to previously published data.^33a,34 The NO stretching frequency at 2214 cm⁻¹ (IR) and 2220 cm⁻¹ (Raman) fits well into the wide range of nitrosonium ion stretching frequencies ranging from 2150 to 2400 cm⁻¹ and is similar to [NO]⁺ stretching frequencies in other salts containing chlorinated counter anions, cf. [NO][SbCl₆] 2189 cm⁻¹, [NO]₂[SnCl₆] 2191 cm⁻¹, [NO][AlCl₄] 2242 cm⁻¹.³⁵ In general, the [NO]⁺ stretching frequency in a salt with a chlorinated counter anion is smaller than in a salt with a similar fluorinated counter anion (e.g.[NO][SbCl₆] 2189 cm⁻¹vs. [NO][SbF₆] 2385 cm⁻¹), which has been explained in terms of polarization.³⁵

A preliminary crystal structure of [NO]₂[B₁₂Cl₁₂]·SO₂ has been determined. The [NO]⁺ cations are disordered, which prohibits a detailed discussion of the structure (see ESI† for X-ray data).

3.4. X-Ray single crystal structure determinations

Several single crystal structures containing the [B₁₂Cl₁₂]²⁻ anion have been reported in recent years, i.e. Cs₂[B₁₂Cl₁₂] (three polymorphs are known),^24,25,36 Cs₂[B₁₂Cl₁₂]·nH₂O (n = 1, 2),²⁴ Cs₂[B₁₂Cl₁₂]·2CH₃CN,²⁴ Ag₂[B₁₂Cl₁₂],³⁷ and [C₂mim]₂[B₁₂Cl₁₂].⁸

Cs₂[B₁₂Cl₁₂]·SO₂ (1). Cs₂[B₁₂Cl₁₂]·SO₂ (1) crystallizes in the trigonal space groupR3̄ in a structure type similar to solvate-free Cs₂[B₁₂Cl₁₂] and the hydrates Cs₂[B₁₂Cl₁₂]·nH₂O (n = 1, 2).²⁵Fig. 3 shows the unit cell of 1. The SO₂ molecule inserts between two Cs⁺ cations along the c-axis and increases the Cs⁺⋯Cs⁺ distance between these two cations from 763 pm in solvate-free Cs₂[B₁₂Cl₁₂]²⁵ to 924 pm in 1. The Cs–O distance of 338 pm is in good agreement with the sum of the ionic radius of Cs⁺ and the van der Waals radius of oxygen (345 pm),³⁸ which indicates that the interaction is weak and the compound may be described as a solvate rather than a Cs–OSO complex. This is in agreement with almost unchanged O–S bond lengths (137(9)–140(8)) compared to free SO₂ (142.97(4)).³⁹ The Cs⁺ cation is surrounded by four [B₁₂Cl₁₂]²⁻cluster and one molecule SO₂ giving a total coordination number of ten (Fig. 4).

Fig. 3 Unit cell of Cs₂[B₁₂Cl₁₂]·SO₂. The SO₂ molecules are rotationally disordered over six positions. Selected averaged bond length [pm]: B–B 178.5, B–Cl 178.8, O1–S1 138.5.

Fig. 4 Coordination sphere around Cs⁺ in Cs₂[B₁₂Cl₁₂]·SO₂. Selected bond length [pm]: O1–Cs1 337.9(9), Cl1–Cs1 365.4(2) (3×), Cl2–Cs1 349.5(2) (3×), Cl2′–Cs1 373.7(2) (3×).

[CPh₃]₂[B₁₂Cl₁₂]·2C₂H₄Cl₂ (2) and [CPh₃]₂[B₁₂Cl₁₂]·2SO₂ (3). Fig. 5 shows the crystal structures of 2 and 3. Unit cell representations are deposited as ESI.† Both structures are very similar. The dianion [B₁₂Cl₁₂]²⁻ forms two weak contacts via the chlorine atoms in 1 and 12 position to the carbenium centers of the planar trityl cations (sum of the angles around C1 is 360°). These contacts (Cl5⋯C1 346.1(2) (2), 350.4(2) (3) are longer than the sum of the van der Waals radii (328 pm)³⁸ indicating a very weak interaction. In addition, on the opposite side of the trityl cation solvate molecules 1,2-C₂H₄Cl₂ (2) or SO₂ (3), respectively, coordinate very weakly to the carbenium center (C1⋯Cl8 347.9(2) (2), C1⋯O1 309.5(3) (3)). The C1–O1 distance is a little bit shorter than the sum of the van der Waals radii (325 pm),³⁸ however, the SO₂ molecule shows no distortion and bond length (142.0(2) and 142.3(2) pm) in agreement with free SO₂ (142.97(4)).³⁹ A similar structure without incorporated solvate molecules has been found for [CPh₃]₂[B₁₂F₁₂],⁶ with C⋯F distance of 308.7(2) pm in agreement with the sum of the van der Waals radii (300 pm).³⁸ These results indicate that [B₁₂Cl₁₂]²⁻, despite being a dianion, is only weakly coordinating and comparable to its fluorinated relative [B₁₂F₁₂]²⁻.

Fig. 5 Crystal structures of [CPh₃]₂[B₁₂Cl₁₂]·2SO₂ (top) and [CPh₃]₂[B₁₂Cl₁₂]·2C₂H₄Cl₂ (bottom). Thermal ellipsoids are shown at the 50% probability level. Selected bond length [pm] and angles [°]: 2: av. B–B 178.2, av. B–Cl 178.8, Cl8–C21 176.5(3), C21–C20 147.3(5), C20–Cl7 182.3(4), Cl5–C1–Cl7 167.82(5), Cl5⋯C1 346.1(2), C1⋯Cl8 347.9(2); 3: av B–B 178.5, av. B–Cl 179.6, O1–S1 142.0(2), S1–O2 142.3(2), Cl5–C1–O1 166.67(7), Cl5⋯C1 350.4(2), C1⋯O1 309.5(3).

[NO]₂[B₁₂Cl₁₂]·SO₂ and [PPN]₂[B₁₂Cl₁₂]·CH₂Cl₂. Preliminary crystal structures of [NO]₂[B₁₂Cl₁₂]·SO₂ and [PPN]₂[B₁₂Cl₁₂]·CH₂Cl₂ have been determined. The [B₁₂Cl₁₂]²⁻ anions are well defined in both structures. Unfortunately, in both cases the cations are highly disordered, which prevents a detailed discussion. Crystal structure details and figures are included with the ESI.†

3.5. Structural and spectroscopic properties of [B₁₂Cl₁₂]²⁻

In all determined solid state structures the [B₁₂Cl₁₂]²⁻ anion has almost I_h symmetry. A comparison of all known [B₁₂Cl₁₂]²⁻ data (Table 2) gives an averaged B–B bond length of 178.7 and an averaged B–Cl bond length of 178.9 pm. These values are reproduced well by PBE0/TZVPP quantum chemical calculations which give 178.6 and 179.3 pm respectively. In accordance with its high I_h symmetry the vibrational spectra of [B₁₂Cl₁₂]²⁻ show only few signals. The IR spectra has two intensive signals at 533 (ν_B–Cl) and 1028 cm⁻¹ (ν_B–Cl) (plus three signals in the FIR region at 160 (δ_BBCl), 136, and 129 cm⁻¹) and the Raman spectra two intense signals at 306 (ν_B–B) and 133 cm⁻¹. Actual vibrational spectra are included with the ESI.† A detailed assignment of the vibrational spectra can be found in the literature.⁴⁰ A slight distortion from ideal I_h symmetry of the [B₁₂Cl₁₂]²⁻ anion in the solid state does not result in observable splitting of the vibrational frequencies. The limited number of distinct signals due to [B₁₂Cl₁₂]²⁻ makes vibrational spectroscopy a valuable tool for the investigation of the nature of the counter cations. In the ¹¹B NMR spectrum pure [B₁₂Cl₁₂]²⁻ shows only one sharp resonance at −12.8 ppm.

Table 2 Comparison of averaged experimental (X-ray diffraction) and calculated (PBE0/TZVPP) B–B and B–Cl bond lengths

	B–B distance/pm	B–Cl distance/pm	Reference
a C2mim = 1-etyl-3-methylimidazolium b Preliminary crystal structure data has been deposited as ESI.^1
α-Cs2[B12Cl12]	178.9	179.3	25
β-Cs2[B12Cl12]	178.9	178.8	24
γ-Cs2[B12Cl12]	179.4	177.6	24
Cs2[B12Cl12]·2CH3CN	178.7	179.4	24
Ag2[B12Cl12]	178.4	178.6	37
[C2mim]2[B12Cl12]^a	178.6	179.2	8
Cs2[B12Cl12]·SO2 (1)	178.5	178.8	This work
[CPh3]2[B12Cl12]·2C2H4Cl2 (2)	178.2	178.8	This work
[CPh3]2[B12Cl12]·2SO2 (3)	178.5	179.6	This work
[NO]2[B12Cl12]·SO2	178.8	179.1	This work^b
[PPN]2[B12Cl12]·CH2Cl2	178.5	178.5	This work^b
calc. (PBE0/TZVPP)	178.6	179.3	This work

---

Acknowledgements

The authors are grateful to Professor Dr Ingo Krossing for support and helpful discussions, Boumahdi Benkmil for help with the X-ray diffraction measurements, and all the students who got involved in this work while doing their advanced inorganic chemistry labs. This work was supported by the Albert-Ludwigs Universität Freiburg and the Deutsche Forschungsgemeinschaft (DFG).

References

1. (a) See for reviews: S. H. Strauss, Chem. Rev., 1993, 93, 927–942; (b) I. Krossing and I. Raabe, Angew. Chem., 2004, 116, 2116–2142; I. Krossing and I. Raabe, Angew. Chem. Int. Ed., 2004, 43, 2066–2090; (c) I. Krossing and A. Reisinger, Coord. Chem. Rev., 2006, 250, 2721–2744.
2. (a) Selected references: C. A. Reed, Acc. Chem. Res., 1998, 31, 133–139; (b) C. A. Reed, Chem. Commun., 2005, 1669–1677; (c) S. V. Ivanov, J. J. Rockwell, O. G. Polyakov, C. M. Gaudinski, O. P. Anderson, K. A. Solntsev and S. H. Strauss, J. Am. Chem. Soc., 1998, 120, 4224–4225; (d) T. Küppers, E. Bernhardt, R. Eujen, H. Willner and C. W. Lehmann, Angew. Chem., 2007, 119, 6462–6465; T. Küppers, E. Bernhardt, R. Eujen, H. Willner and C. W. Lehmann, Angew. Chem. Int. Ed., 2007, 46, 6346–6349; (e) A. Herzog, R. P. Callahan, C. L. B. Macdonald, V. M. Lynch, M. F. Hawthorne and R. J. Lagow, Angew. Chem., 2001, 113, 2179–2181; A. Herzog, R. P. Callahan, C. L. B. Macdonald, V. M. Lynch, M. F. Hawthorne and R. J. Lagow, Angew. Chem. Int. Ed., 2001, 40, 2121–2123.
3. (a) K. C. Kim, C. A. Reed, G. S. Long and A. Sen, J. Am. Chem. Soc., 2002, 124, 7662–7663; (b) C. A. Reed, K. C. Kim, R. D. Bolskar and L. J. Mueller, Science, 2000, 289, 101–104; (c) T. Kato and C. A. Reed, Angew. Chem., 2004, 115, 2968–2971; T. Kato and C. A. Reed, Angew. Chem. Int. Ed., 2004, 43, 2907–2911; (d) C. A. Reed, Acc. Chem. Res., 1998, 31, 325–332.
4. S. Körbe, P. J. Schreiber and J. Michl, Chem. Rev., 2006, 106, 5208–5249 ; and references therein.
5. W. H. Knoth, H. C. Miller, J. C. Sauer, J. H. Balthis, Y. T. Chia and E. L. Muetterties, Inorg. Chem., 1964, 3, 159–167.
6. S. V. Ivanov, S. M. Miller, O. P. Anderson, K. A. Solntsev and S. H. Strauss, J. Am. Chem. Soc., 2003, 125, 4694–4695.
7. (a) I. A. Zakharova, N. T. Kuznetsov and Y. L. Gaft, Inorg. Chim. Acta, 1978, 28, 271–274; (b) Y. L. Gaft, I. A. Zakharova and N. T. Kuznetsov, Russ. J. Inorg. Chem., 1980, 25, 726–729; Y. L. Gaft, I. A. Zakharova and N. T. Kuznetsov, Zh. Neorg. Khim., 1980, 25, 1308–1313; (c) M. G. Voronkov, V. B. Pukhnarevich, I. I. Tsykhanskaya, N. I. Uhakova, Y. L. Gaft and I. A. Zakharova, Inorg. Chim. Acta, 1983, 68, 103–105; (d) Y. L. Gaft, N. T. Kuznetsov and L. M. Sukova, Russ. J. Inorg. Chem, 1983, 28, 89–92; Y. L. Gaft, N. T. Kuznetsov and L. M. Sukova, Zh. Neorg. Khim, 1983, 28, 162–167.
8. M. Nieuwenhuyzen, K. R. Seddon, F. Teixidor, A. V. Puga and C. Viñas, Inorg. Chem., 2009, 48, 889–901.
9. (a) A. N. Dey and J. Miller, J. Electrochem. Soc., 1979, 126, 1445–1451; (b) J. W. Johnson and M. S. Whittingham, J. Electrochem. Soc., 1980, 127, 1653–1654; (c) J. W. Johnson and A. H. Thompson, J. Electrochem. Soc., 1981, 128, 932–933; (d) J. W. Johnson and J. F. Brody, J. Electrochem. Soc., 1982, 129, 2213–2219.
10. M. W. Rupich, J. S. Foos and S. B. Brummer, J. Electrochem. Soc., 1985, 132, 119–122.
11. (a) S. V. Ivanov, W. J. Casteel Jr, G. P. Pez and M. Ulman, EP 1513215 A2, 2005; (b) G. P. Pez, S. V. Ivanov, G. Dantsin, W. J. Casteel and J. F. Lehmann, EP 1763099 A2, 2007.
12. See for review: I. B. Sivaev, V. I. Bregadze and S. Sjöberg, Collect. Czech. Chem. Commun., 2002, 67, 679–727.
13. J. K. Ruff and W. J. Schlientz, Inorg. Synth., 1974, 15, 84–90.
14. N. A. Zhukova, E. A. Malinina and N. T. Kuznetsov, Russ. J. Inorg. Chem., 1985, 30, 735–736; N. A. Zhukova, E. A. Malinina and N. T. Kuznetsov, Zh. Neorg. Khim., 1985, 30, 1292–1294.
15. G. M. Sheldrick, SHELX-97 Programs for Crystal Structure Analysis (Release 97-2), Institut für Anorganische Chemie der Universität Göttingen, 1997.
16. Diamond-Visual Crystal Structure Information System, Crystal Impact, Bonn, Germany.
17. (a) TURBOMOLE Version 5.10; COSMOlogic; (b) R. Ahlrichs, M. Bär, M. Häser, H. Horn and C. Kölmel, Chem. Phys. Let., 1989, 162, 165–169.
18. (a) K. C. Nainan and G. E. Ryschkewitsch, Inorg. Nucl. Chem. Let., 1970, 6, 765–766; (b) G. E. Ryschkewitsch and K. C. Nainan, Inorg. Synth., 1974, 15, 113–118; (c) K. G. Myakishev, I. I. Gorbacheva and V. V. Volkov, Russ. J. Inorg. Chem., 1984, 29, 526–529; K. G. Myakishev, I. I. Gorbacheva and V. V. Volkov, Zh. Neorg. Khim., 1984, 29, 912–916.
19. (a) L. V. Titov, E. R. Eremin and V. Y. Rosolovskii, Russ. J. Inorg. Chem., 1982, 27, 500–520; L. V. Titov, E. R. Eremin and V. Y. Rosolovskii, Zh. Neorg. Khim., 1982, 27, 891–895; (b) W. Preetz and G. Peters, Eur. J. Inorg. Chem., 1999, 1831–1846.
20. (a) G. Brauer, Handbuch der Präparativen Anorganischen Chemie, 3rd edn, vol. 2, F. Enke Verlag, Stuttgart, 1979, pp. 794–795; (b) Synthetic Methods of Organometallic and Inorganic Chemistry, ed. W. A. Herrmann, N. Auner and U. Klingebiel, Thieme Verlag, Stuttgart, 1996, p. 67.
21. M. Komura, K. Aono, K. Nagasawa and S. Sumimoto, Chem. Express, 1987, 2, 173–176.
22. F. Wang, Y. Wang and X. Wang, Yingyong Huaxue (Chin. J. Appl. Chem.), 1998, 15, 111–112 , (only available in Chinese).
23. W. L. Smith, J. Chem. Ed., 1977, 54, 469–473.
24. I. Tiritiris, Doctoral Thesis, Universität Stuttgart, 2004.
25. I. Tiritiris and T. Schleid, Z. Anorg. Allg. Chem., 2004, 630, 1555–1563.
26. H. G. Srebny, W. Preetz and H. C. Marsmann, Z. Naturforsch., 1984, 39B, 189–196.
27. K. A. Solntsev, S. V. Ivanov, S. G. Sakharova, S. B. Katser, A. S. Chernyavskii, N. A. Votinova, E. A. Klyuchishche and N. T. Kuznetsov, Russ. J. Coord. Chem., 1997, 23, 369–376; K. A. Solntsev, S. V. Ivanov, S. G. Sakharova, S. B. Katser, A. S. Chernyavskii, N. A. Votinova, E. A. Klyuchishche and N. T. Kuznetsov, Koord. Khim., 1997, 23, 403–409.
28. O. Haeckel and W. Preetz, Z. Anorg. Allg. Chem., 1995, 621, 1454–1458.
29. R. Hoffmann and W. N. Lipscomb, J. Chem. Phys., 1962, 37, 520–523.
30. N. A. Zhukova, E. A. Malinina and N. T. Kuznetsov, Russ. J. Inorg. Chem., 1985, 30, 735–736; N. A. Zhukova, E. A. Malinina and N. T. Kuznetsov, Zhur. Neorg. Khim., 1985, 30, 1292–1294.
31. G. I. Borodkin and V. G. Shubin, Russ. Chem. Rev, 2001, 70, 211–230; G. I. Borodkin and V. G. Shubin, Usp. Khim., 2001, 70, 241–261 ; and references therein.
32. (a) E. Bernhardt, M. Finze and H. Willner, Z. Allg. Anorg. Chem., 2006, 632, 248–250; (b) A. M. Khenkin and R. Neumann, J. Am. Chem. Soc., 2008, 130, 11876–11877.
33. (a) A. Decken, H. D. B. Jenkins, G. B. Nikiforov and J. Passmore, Dalton Trans., 2004, 2496–2504; (b) C. Knapp, A. Mailman, G. B. Nikiforov and J. Passmore, J. Fluorine Chem., 2006, 127, 916–919; (c) A. Decken, C. Knapp, G. B. Nikiforov, J. Passmore, M. Rautiainen, X. Wang and X. Zeng, Chem. Eur. J., 2009 , submitted.
34. J. Mason and K. O. Christe, Inorg. Chem., 1983, 22, 1849–1853.
35. D. W. A. Sharp and J. Thorley, J. Chem. Soc., 1963, 3557–3560.
36. I. Tiritiris and T. Schleid, Z. Anorg. Allg. Chem., 2002, 628, 2212.
37. I. Tiritiris and T. Schleid, Z. Anorg. Allg. Chem., 2003, 629, 581–583.
38. (a) J. Huheey, E. Keiter and R. Keiter, Anorganische Chemie, 2nd edn, de Gruyter, Berlin, 1995; (b) S. S. Batsanov, Inorg. Mater., 2001, 37, 871–855; S. S. Batsanov, Neorg. Mater., 2001, 37, 1031–1046.
39. R. Mews, E. Lork, P. G. Watson and B. Görtler, Coord. Chem. Rev., 2000, 197, 277–320.
40. (a) L. A. Leites, S. S. Bukalov, A. P. Kurbakova, M. M. Kaganski, Y. L. Gaft, N. T. Kuznetsov and I. A. Zakharova, Spectrochim. Acta, 1982, 38A, 1047–1056; (b) S. J. Cyvin, B. N. Cyvin and T. Mogstad, Spectrochim. Acta, 1986, 42A, 985–989.

---

Footnote

† Electronic supplementary information (ESI) available: Full experimental details and discussion of the synthesis of [NEt₃H]₂[B₁₂H₁₂] and additional data for the chlorination of [B₁₂H₁₂]²⁻, all actual NMR, IR, and Raman spectra, details on the thermodynamics and quantum chemical calculations, preliminary crystal structure determinations of [NO]₂[B₁₂Cl₁₂]·SO₂ and [PPN]₂[B₁₂Cl₁₂]·CH₂Cl₂, figures of the unit cells of [CPh₃]₂[B₁₂Cl₁₂]·2C₂H₄Cl₂ (2) and [CPh₃]₂[B₁₂Cl₁₂]·2SO₂ (3). CCDC reference numbers 710773–715826. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b821030f

---