Chlorination of 1-Carba-closo-dodecaborate and 1-Ammonio-closo

Oct 5, 2016 - Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, United States...
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Chlorination of 1‑Carba-closo-dodecaborate and 1‑Ammonio-closododecaborate Anions Mahmoud Saleh,† Douglas R. Powell,‡ and Rudolf J. Wehmschulte*,† †

Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, United States Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019-5251, United States

‡

S Supporting Information *

ABSTRACT: Fully chlorinated carborate and dodecaborate cages such as [CHB11Cl11]− and [Me3NB12Cl11]− are prominent examples of valuable and chemically rather inert weakly coordinating anions. While both anions can be obtained by chlorination of the precursors [CH12B11]− and [H3NB12H11]− with SO2Cl2 followed by methylation for the synthesis of [Me3NB12Cl11]−, best results were found using photochemical chlorination with SO2Cl2 for [CH12B11]− and thermal chlorination with SO2Cl2 for [H3NB12H11]−. The hexachlorinated anion [n-Pr3NB12H5Cl6]− was formed readily within 30 min by chlorination of [n-Pr3NB12H11]−, but attempts to synthesize isopropyl-substituted ammonio-dodecaborates with a higher chlorination number resulted in the formation of mixtures and partial decomposition. The silver and trityl salts of the anions [CHB11Cl11]−, [Me3NB12Cl11]−, and [n-Pr3NB12H5Cl6]− as well as the contact ion-pair [Et2Al][Me3NB12Cl11] were also prepared, and the compounds [Ag(NCMe)][Me3NB12Cl11], [Et2Al][Me3NB12Cl11], and [Et4N][i-Pr3NB12H5Cl6] were also characterized by X-ray crystallography.

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and I2.10 Formal substitution of one of the hydride substituents with the NH 3 molecule leads to the monoanion [H3NB12H11]−,11 which after chlorination and N-methylation would be converted into [Me3NB12Cl11]−, thus a more readily available potential alternative for the carborane anion [CHB11Cl11]−. While we were working on the synthesis and reactivity of [Me3NB12Cl11]− and related species, procedures for the synthesis of our target compound using SbCl5 and, more recently, SO2Cl2 as the chlorination agents were published.12 Similarly, a reliable but rather protracted high-temperature synthesis of [CHB11Cl11]− from [CH12B11]− and SO2Cl2 was also reported.13 Apparently, the chlorination of [H3NB12H11]− with SO2Cl2 faces similar reproducibility issues as that of [CH12B11]− with SO2Cl2.8b,12a Our primary goal for this work was to find reliable and “fast” synthetic procedures for the monoanions [H3NB12Cl11]− and [CHB11Cl11]− using SO2Cl2 instead of SbCl5 as the chlorination reagent mainly due to the more difficult and unpleasant workup involving SbCl5. We report here a fast chlorination procedure of [CH12B11]− using a photochemical route and an elevated temperature route to [H3NB12Cl11]− including a procedure to restart a stalled chlorination. Our findings are complementary to the recent reports, and we also disclose the syntheses of the silver, trityl, and [Et2Al]+ salts of the [Me3NB12Cl11]− anion and the synthesis of the [n-Pr3NB12H5Cl6]− anion and its [Et4N]+ and silver salts (Figure 1).

INTRODUCTION The synthesis of very strong Brønsted and cationic Lewis acids requires the use of weakly coordinating (or very weakly basic) counterions. Classic examples of such weakly coordinating anions (WCAs) include [BF4]−, [PF6]−, [Sb2F11]−, or [CF3SO3]−,1 which were subsequently replaced in organometallic chemistry by borates and alanates such as [B{C6H3(CF3)2-3,5}4]−, [B(C6F5)4]−, and [Al{OC(CF3)3}4]− due to their lower basicity, higher stability, and higher solubility.2 Although these anions have found widespread application and are either commercially available or can be readily prepared, they are still not sufficiently stable to allow the isolation of very strong Lewis acids such as [iBu2Al]+ or unsolvated Brønsted super acids such as [H]+[B(C6F5)4]−.3 Polyhalogenated carborane anions [CHnB11X12−n]− have been introduced and developed during the past two decades as a new and chemically inert class of WCAs,4 and [H][CHB11Cl11]5 and [H][CHB11F11]6 followed each other as the strongest Brønsted acids. Despite their excellent properties, the application of these anions is limited due to their rather involved syntheses. For example, the best syntheses of the parent anion [CH12B11]− require several steps starting from the expensive and toxic precursor decaborane(14), B10H14.4b,7 Furthermore, the most reliable perchlorination procedure uses SbCl5 at elevated temperatures over a period of several days,8 and the synthesis of [CHB11F11]− necessitates the use of fluorine gas.9 While the [CH12B11]− anion is not easy to obtain, the related dianion [B12H12]2− is significantly less expensive, and it can also be readily synthesized in batches of 20 g or higher from NaBH4 © XXXX American Chemical Society

Received: August 1, 2016

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DOI: 10.1021/acs.inorgchem.6b01867 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Drawings of the anions used and synthesized in this study.

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The following procedure was optimized to isolate the tetraethylammonium salt in high purity from Cs2[B12H12]. Cs2[B12H12] (10.02 g, 24.58 mmol) was heat-dissolved in water (100 mL; oil bath temperature 90 °C) in a round-bottom flask. A solution of H3NOSO3 (6.95 g, 2.5 equiv, 61.5 mmol) in 30 mL of water was added dropwise to the warm Cs2[B12H12] solution over a period of 5 min. The reaction mixture was stirred at 90 °C for 1.5 h and allowed to cool to room temperature. The resulting clear solution was concentrated to ca. 20 mL under dynamic vacuum, upon which a white precipitate started to form. (This precipitate contains Cs2[B12H12], Cs[H3NB12H11], and (H3N)2B12H10.) Acetonitrile (200 mL) was added to the reaction mixture, which was refluxed overnight to extract the product. (This step was repeated if the remaining precipitate contained a considerable amount of the product.) The acetonitrile soluble fraction was filtered using a fine frit. The acetonitrile was removed to yield a white precipitate. (This precipitate contains Cs[H3NB12H11] and (H3N)2B12H10.) The precipitate was redissolved in water (ca. 125 mL), and Et4NBr (6.0 g, 28.5 mmol) was added. Allowing the mixture to stand overnight at 4 °C afforded a white precipitate. The precipitate was filtered off and recrystallized from hot water to give [Et4N][H3NB12H11] as a colorless crystalline solid 3.54 g, 12.29 mmol, 50.0%. 1H NMR (400.13 MHz, CD3CN, 297 K): δ = 4.69 (br, w1/2 = 71.74 Hz, NH3, 3H), 3.17 (q, J = 7.3 Hz, N− CH2-CH3, 8H), 1.21 (tt, J = 7.3 Hz, JNH = 1.8 Hz, N−CH2−CH3, 12H), 1.73−0.39 (m, B-H, 11H). 13C{1H} NMR (100.61 MHz, CD3Cl, 297 K): δ = 53.17 (N-CH2−CH3), 7.78 (N−CH2−CH3). 11B NMR (128.38 MHz, CD3CN, 297 K): δ = −7.23 (s, 1B), −15.78 (d, 10B), −18.85 (d, 1B). Photochemical (Radical) Chlorination. Synthesis of Cs[CHB11Cl11]·H2O, Cs[1], from Cs[CH12B11]. A 100 mL quartz roundbottom flask equipped with a small magnetic stirrer, a Claisen adapter, and a reflux condenser (Figure S1) was charged with dried Cs[CH12B11] (from Katchem, 1.00 g, 3.62 mmol; Figure S1 demonstrates the reaction setup). The reaction flask was heat-dried under vacuum and placed under a nitrogen atmosphere. SO2Cl2 (ca. 100 mL altogether) was added dropwise at 0 °C through a rubber septum using a PTFE tube (Caution! The initial reaction is violent and produces harmf ul gases and heat). After ca. 20 mL was added, the ice bath was removed, and the reaction mixture was allowed to warm to room temperature. After the addition of SO2Cl2 was completed, the reaction flask was heated in an oil bath (oil bath temperature set at 100 °C) and allowed to reflux for 1 h then an additional 2 h (oil bath temperature of 50 °C). The reaction flask was then removed from the oil bath and placed in a Rayonet UV photochemical reactor for 1 h. The progress of the reaction was monitored by 11B NMR spectroscopy every half an hour (see Figure S2). After the reaction was complete, the remaining SO2Cl2 was distilled under reduced pressure and

EXPERIMENTAL SECTION

General Procedures. Cs[CH12B11] was purchased from Katchem spol. s.r.o. or prepared from [Me3NH][CH12B11] according to literature procedures.7,14 SO2Cl2 was purchased from Acros Organics and was fractionally distilled prior to use, disposing of the first pale yellow fraction, giving a colorless liquid. MeI (Aldrich) and n-PrI (Acros Organics) were distilled prior to use. Hydroxylamine-Osulfonic acid (H2NOSO3H, Acros Organics), trityl chloride (Ph3CCl, Acros Organics), and Cs2[B12H12] (Strem Chemicals) were used without further purification. All chlorination reactions were performed under a nitrogen atmosphere using Schlenk techniques or under a stream of nitrogen if possible. A polytetrafluoroethylene (PTFE) tube was used to transfer SO2Cl2 into the reaction flask instead of a regular stainless steel cannula to prevent any interaction. No silicon grease was used in these experiments; instead, Glindemann sealing rings (PTFE) for glass conical joints were used. The synthesis of air- and moisturesensitive compounds was performed under dry nitrogen using standard Schlenk techniques or a Vacuum Atmospheres drybox. All glassware was dried in an oven and heated under vacuum to remove any trace amount of moisture. All reactions were performed in a wellventilated fume hood, in N2 atmosphere, and a blast shield was used for the high-pressure reaction. Photochemical reactions were performed in a Rayonet photoreactor (The Southern New England Ultraviolett Co, model RPR-100) with a wavelength of 350 nm. NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. 1H NMR chemical shift values were determined relative to the residual C6D5H in benzene-d6, CHCl3 in deuterochloroform, CD2HCN in acetonitrile-d3, and (CD3)(CD2H)CO in acetone-d6 as internal reference (δ = 7.16, 7.26, 1.94, and 2.05 ppm). 13C NMR spectra were referenced to the solvent signal (δ = 128.39, 77.23, 1.32, and 29.84, respectively). 11B NMR spectra were referenced to an external solution of F3B·OEt2 in C6D6, and the chemical shifts are reproducible with an error of less than 1 ppm. Negative-ion mass spectra were recorded using an AccuTOF (JEOL) analyzer equipped with a direct analysis in real time (DART) ion source (from IonSense) or an AXIMA-LNR MALDI-TOF mass spectrometer using 2,5-dihydroxybenzoic acid (DHB) as the matrix. Safety Notes. Caution! The chlorination reactions using SO2Cl2 must be performed with extreme care in a f ully f unctional f ume hood. The reaction is exothermic and produces HCl and SO2 gases. High-pressure reactions were rigorously monitored, and although no incidents occurred, venting the system regularly to prevent pressure buildup especially for largescale reactions is recommended. The employment of pyrophoric Et3Al must also be performed with care under an inert atmosphere. Synthesis of [Et4N][H3NB12H11], [Et4N][2]. [Et4N][H3NB12H11] was prepared in a manner similar to that reported by Hertler and Raasch.11 B

DOI: 10.1021/acs.inorgchem.6b01867 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry collected into a liquid nitrogen trap leaving behind a white precipitate. Ice cold water (ca. 50 mL) was added and then heated to dissolve the precipitate. The solution was made slightly basic by slow addition of an NaOH (1 M) solution. The solution was then topped to 100 mL with water, and CsCl (3.00 g) was added forming a white precipitate. Heat dissolving and allowing the solution to stand at 3 °C overnight yielded Cs[CHB11Cl11]·H2O as a crystalline solid (1.58 g, 2.35 mmol, 64.7%). The crystals were collected by filtration and washed with ice-cold water (2 × 10 mL). A second fraction was obtained by concentrating the mother liquor to ca. 50 mL and cooling at 3 °C (0.33 g, 0.49 mmol, 13.5%). Yield: 1.91 g, 2.84 mmol, 78.5%. 1H NMR (400.13 MHz, CD3CN, 297 K): δ = 4.09 (s, 1H), 2.14 (s, H2O, 1.8H). 13 C{1H} NMR (100.61 MHz, CD3CN, 297 K): δ = 46.44 (s, cage carbon). 11B NMR (128.38 MHz, CD3CN, 297 K): δ = −2.56 (s, 1B), −9.98 (s, 5B), −13.02 (s, 5B). Synthesis of Cs[CHB11Cl11]·H2O, Cs[1]·H2O, from [Me3NH][C H12B11]. Dried [Me3NH][CH12B11] (0.75 g, 3.69 mmol) was charged into a 100 mL quartz round-bottom flask equipped with a small magnetic stirrer, a Claisen adapter, and a reflux condenser. The reaction flask was heat-dried under vacuum and placed under a nitrogen atmosphere. SO2Cl2 (ca. 30 mL) was added dropwise at 0 °C through a rubber septum using a PTFE tube (Caution! The initial reaction is violent and produces harmf ul gases and heat). After ca. 10 mL was added, the ice bath was removed, and the reaction mixture was allowed to stir at room temperate for 1 h. The reaction flask was placed in a Rayonet photochemical UV reactor for 2 h. An additional 50 mL of SO2Cl2 was then added. The reaction mixture was irradiated for an additional 5 h. The progress of the reaction was monitored by 11 B NMR spectroscopy. (The reaction was practically over after 3 h of irradiation; see Figure S7.) The remaining SO2Cl2 was distilled under reduced pressure and collected into a liquid nitrogen trap, leaving behind a white precipitate. Ice-cold water (ca. 50 mL) was added and heated to dissolve the precipitate. The solution was made slightly basic by slow addition of NaOH (1 M) solution. CsCl (5.0 g) was added, forming a white precipitate. Heat dissolving and allowing the solution to stand at 3 °C overnight yielded Cs[CHB11Cl11]·H2O as a crystalline solid. Yield: 1.63 g, 2.42 mmol, 65.6%. Synthesis of [Et4N][H3NB12Cl11], [Et4N][3]. Na[H3NB12H11] was prepared by passing a solution of [Et4N][2] (1.20 g, 4.17 mmol) in 70% v/v acetonitrile in water (ca. 50 mL) through an ion-exchange column using Amberlyst 15 in the H+ form. The solution was then neutralized by the slow addition of a solution of NaOH (0.17 g, 4.25 mmol), followed by removing the solvent and drying the solid under vacuum for 5 h at 140 °C. Isolated yield: 0.65 g, 3.59 mmol, 86.1%. A 100 mL quartz round-bottom flask equipped with a small magnetic stirrer, a Claisen adapter, and a reflux condenser was charged with carefully dried Na[H3NB12H11] (0.30 g, 1.66 mmol). The reaction flask was heat-dried under vacuum and then placed under nitrogen atmosphere. SO2Cl2 (ca. 100 mL altogether) was added dropwise at 0 °C through a rubber septum using a PTFE tube (Caution! The initial reaction is violent and produces harmf ul gases and heat). After ca. 50 mL was added, the ice bath was removed, and the reaction mixture was allowed to warm to room temperature. After the addition of SO2Cl2 was completed, the reaction flask was heated in an oil bath (oil bath temperature set at 56 °C) and allowed to stand for 2 h. The reaction flask was then removed from the oil bath and placed in a Rayonet UV photochemical reactor for 20 h (No change was observed after 15 h). The remaining SO2Cl2 was distilled off under reduced pressure and collected into a liquid nitrogen trap, giving a white precipitate. The product was dissolved in 50 mL of ice-cold water, precipitated by the addition of Et4NBr (3.0 g, 14.25 mmol), filtered, and then dried in air. Yield (0.63 g, 0.94 mmol, 56.6%). Thermal (Electrophilic) Chlorination. Attempted Synthesis of Cs[CHB11Cl11], Cs[1], from the Reaction in a Glass Pressure Vessel. A 100 mL thick-walled glass pressure vessel (Chemglass CG 1880) equipped with a small magnetic stirrer and a PTFE screw cap and Oring was charged with dried Cs[CH12B11] (0.50 g, 1.81 mmol) and placed in an ice bath. Freshly distilled SO2Cl2 (50 mL) was then added dropwise under a stream of nitrogen using a PTFE tube. (Caution! The initial reaction is violent and produces harmf ul gases and heat). After

the addition was complete, the reaction vessel was allowed to warm and was temporarily fitted with a reflux condenser and allowed to reflux overnight under a nitrogen atmosphere. The pressure vessel was then tightly sealed and stirred at 120 °C for 4 d followed by another 4 d at 140 °C. (Note: The reaction vessel was vented every day to prevent pressure buildup by allowing the flask to cool to room temperature, then carefully loosening the valve.) The progress of the reaction was monitored by 11B NMR spectroscopy every day (see Figure S15). Distilling the SO2Cl2 and dissolving the remaining solid in water yielded a cloudy solution. Filtration through diatomaceous earth and the addition of CsCl (3.0 g) afforded the Cs+ salt as a white precipitate. Recrystallization from hot water afforded a crystalline solid. Yield: 0.72 g, 1.07 mmol, 59.1%. Matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) analysis of the sample showed that the product is predominantly Cs[CHB11Cl11]−, but it also contains ∼22% of Cs[CH2B11Cl10]− and 2% of Cs[CH3B11Cl9]−. Attempted Synthesis of [Me3NH][CHB11Cl11], [Me3NH][1], Using a High-Pressure Vessel. A high-pressure vessel (Parr Non-Stirred Vessels module 4748) equipped with a PTFE lined cup was charged with Cs[CH12B11] (0.10 g, 0.36 mmol) and SO2Cl2 (10 mL) under a stream of nitrogen. The reaction vessel was sealed and placed on a sand bath. The temperature was gradually increased and maintained at 235 °C for 4 d. The progress of the reaction was monitored by 11B NMR spectroscopy every day by allowing the reaction vessel to cool followed by carefully opening it under a stream of nitrogen. The excess SO2Cl2 was removed by distillation to obtain a white precipitate that was dissolved in deionized water (ca. 50 mL). The solution was treated with Me3NHCl (0.50 g, 5.23 mmol). The white precipitate was removed by filtration, washed with water, and then dried to give [Me3NH][1] as a white solid. Yield: 0.17 g, 0.29 mmol, 80.6%. DARTMS of the isolated white precipitate showed that it is composed of predominantly the [CHB11Cl11]− anion but also contained 1% of the [CH2B11Cl10]− species (see Figure S18). The product also shows new sets of peaks at 430, 546, and 614 amu that were not removed even after recrystallization and are yet to be identifed. Synthesis of [Et4N][H3NB12Cl11], [Et4N][3], with SO2Cl2 Using a Glass Pressure Vessel. A 50 mL thick-walled Pyrex Schlenk tube equipped with a Teflon screw cap (Chemglass AF-0092), a small magnetic stirrer bar, and an external bubbler was charged with dry [Et4N][H3NB12H11] (1.00 g, 3.47 mmol). The reaction flask was placed in an ice bath, and SO2Cl2 (25 mL) was added dropwise using a PTFE tube. (Caution! The initial reaction is violent and produces harmf ul gases and heat.) The reaction mixture was allowed to warm to room temperature, and then the flask was sealed and heated for 12 h (oil bath temperature at 100 °C). The reaction vessel was allowed to cool and was carefully opened to release the pressure. (The pressure was vented with extreme care; a large amount of HCl and SO2 are produced during the chlorination and must be removed.) The reaction vessel was resealed and heated for an additional 24 h at 120 °C. (Upon further chlorination, the ammonium salt melts and forms a separate phase but crystallizes upon standing at room temperature; see Figure S19). Chlorine gas was also observed during this reaction and is believed to be due to the thermal decomposition of SO2Cl2 at elevated temperatures. After the reaction, SO2Cl2 was distilled under reduced pressure and collected into a liquid nitrogen trap, affording a white precipitate. This residue was treated with 5.0 mL of ice-cold water, and acetone (ca. 50 mL) was added to dissolve the crude product. The clear solution was transferred into a 250 mL round-bottom flask, and excess [Et4N]Br (5.0 g) and water (100 mL) were added. (The addition of the excess ammonium helps in isolating the product as a white precipitate; not adding [Et4N]Br will result in separation of the product as an oil once the acetone is removed, because the reaction also causes some chlorination on the ethyl group.) Removal of the acetone by rotary evaporation gave the product as an off-white solid. The product was removed by filtration, washed three times with water, and was then recrystallized from 70% v/v methanol in water (100 mL). Allowing the methanol to slowly evaporate gave [Et4N][H3NB12Cl11] as a colorless crystalline solid (2.12 g, 3.18 mmol, 91.6%). 1H NMR (400.13 MHz, (CD3)2CO, 297 K): δ = 6.42 (t, 3H, JNH = 47.1 Hz, NH3), 3.49 (q, 8H, J = 7.22 Hz, N−CH2-CH3), 1.40 C

DOI: 10.1021/acs.inorgchem.6b01867 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

was resuspended in water (100 mL), filtered, washed with water, and dried in air. Recrystallization from hot 90% methanol in water yielded [Et4N][n-Pr3NB12H11], [Et4N][5], as colorless crystals (2.63 g, 6.35 mmol, 91.5%). 1H NMR (400.13 MHz, CD3CN, 297 K): δ = 3.16 (q, J = 7.27 Hz, 8H, N−CH2-CH3), 3.04 (m, 6H, N−CH2-CH2−CH3), 1.81 (m, 6H, N−CH2−CH2-CH3), 1.21 (tt, J = 7.24 Hz, JNH = 1.80 Hz, 12H, N−CH2−CH3), 0.84 (t, 9H, N−CH2−CH2−CH3), 1.82− 0.59 (m, 11H, B-H); 13C{1H} NMR (100.61 MHz, CD3CN, 297 K): δ = 63.42, 53.15, 18.40, 11.20, 7.75; 11B NMR (128.38 MHz, CD3CN, 297 K): δ = 1.4 (s, 1B, B1-N(n-Pr)3), −16.3 (d, 1JBH = 128 Hz, 11B, B(2−12)-H). Synthesis of [Et4N][n-Pr3NB12H5Cl6], [Et4N][6]. A Schlenk flask equipped with a small magnetic stirrer was charged with dried [Et4N][n-Pr3NB12H11], [Et4N][5] (1.24 g, 3.0 mmol), and dry acetonitrile (20 mL). SO2Cl2 (20 mL) was added at room temperature using a PTFE tube over a period of 5 min. The progress of the reaction was monitored by 11B NMR spectroscopy every 10 min. After 30 min, SO2Cl2 was quickly distilled under reduced pressure and collected into a liquid nitrogen trap giving a white precipitate. The product was suspended in water, filtered, and left to dry in air. [Et4N][nPr3NB12H5Cl6], [Et4N][6], was obtained as colorless crystals suitable for X-ray crystallography by suspending it in methanol (40 mL), followed by the addition of acetone until dissolved (ca. 20−30 mL), then allowing the solvent to slowly evaporate. Isolated yield: 1.72 g, 2.8 mmol, 93.3%. 1H NMR (400.13 MHz, CD3CN, 297 K): δ = 3.16 (q, 1J = 7.27 Hz, 8H, N−CH2-CH3), 2.98 (m, 6H, N−CH2-CH2−CH3), 1.75 (m, 6H, N−CH2−CH2-CH3), 1.21 (tt, J = 7.28 Hz, JNH = 1.84 Hz, 12H, N−CH2−CH3), 0.86 (t, J = 7.28 Hz, 9H, N−CH2−CH2− CH3), 2.48−1.19 (m, 5H, B-H); 13C{1H} NMR (100.61 MHz, CD3CN, 297 K): δ = 64.12, 53.13, 18.25, 10.99, 7.76; 11B NMR (128.38 MHz, CD3CN, 297 K): δ = −8.0 (s, 1B, B1-N(n-Pr)3, obscured by B-Cl signal), −8.0 (s, 6B, B(7−12)-Cl), −21.2 (d, 1JBH = 140 Hz, 5B, B(2−6)-H). Attempted Synthesis of [Et4N][n-Pr3NB12Cl11] with SO2Cl2 from Acetonitrile. (A) A small Schlenk flask equipped with a small magnetic stirrer was charged with dried [Et4N][n-Pr3NB12H11] (0.10 g, 0.24 mmol) and dry acetonitrile (10 mL). SO2Cl2 (10 mL) was added at room temperature using a PTFE tube. The reaction mixture was allowed to stir for 5 d at room temperature resulting in a dark brown solution. Removing the solvent under vacuum produced a dark sticky substancemainly due to the reaction of SO2Cl2 with the solventfrom which we were unable to isolate the product. On the basis of 11B NMR spectroscopy and MALDI-TOF mass spectrometry the product contained a mixture of the [n-Pr3NB12H5Cl6]− and [nPr3NB12H4Cl7]− anions (Figures S36 and S37). (B) A Schlenk flask equipped with a small magnetic stirrer was charged with dried [Et4N][n-Pr3NB12H11] (0.10 g, 0.24 mmol) and dry acetonitrile (10 mL). SO2Cl2 (10 mL) was added at room temperature using a PTFE tube over a period of 10 min. The reaction mixture was then placed in an oil bath at 85 °C. The progress of the reaction was monitored by 11B NMR spectroscopy and MALDI-TOF mass spectrometry. After 26 h, an additional 5 mL of SO2Cl2 was added, and the reaction mixture stirred for a further 18 h at 85 °C, upon which the mixture turned to a light brown color from which the product was not isolated. MALDI-TOF mass spectrometry of the reaction mixture showed negative ions with masses higher than that expected from the [n-Pr3NB12Cl11]− anion (m/z = 663) indicating partial chlorination of the alkyl chains (Figures S38 and S39). Synthesis of Ag[Me3NB12Cl11], Ag[4]. [Et4N][Me3NB12Cl11] (0.50 g, 0.7 mmol) in 70% v/v acetonitrile in water (40 mL) was passed through the strong acid ion-exchange resin Amberlyst 15 in the H+ form. The acetonitrile was removed by rotary evaporation, and the slightly cloudy aqueous solution was topped to 50 mL with DI water, then treated with activated carbon and filtered through a fine frit lined with diatomaceous earth. The solution was acidified with concentrated

(tt, 12H, J = 7.28 Hz, JNH = 1.76 Hz, N−CH2−CH3). 13C{1H} NMR (100.61 MHz, (CD3)2CO, 297 K): δ = 53.15 (N-CH2−CH3), 7.80 (N−CH2−CH3). 11B NMR (128.38 MHz, (CD3)2CO, 297 K): δ −11.2 (s, 1B, B12-Cl), −12.3 (s, 5B, B(7−11)-Cl), −13.4 (s, 5B, B(2− 6)-Cl), −17.8 (s, 1B, B1-NH3). Restarting a Stalled Synthesis of [Et4N][3] Using Chlorine Gas. A 50 mL thick-walled Pyrex tube equipped with a Teflon screw cap, a small magnetic stirrer, and an external bubbler was charged with dry [Et4N][H3NB12H11] (2.02 g, 7.01 mmol). The reaction flask was placed in an ice bath, and SO2Cl2 (30 mL) was added dropwise using a PTFE tube. (Caution! The initial reaction is violent and produces harmf ul gases and heat.) The reaction mixture was allowed to warm to room temperature, and the tube was then sealed and heated for 12 h (oil bath temperature at 100 °C). The reaction vessel was allowed to cool and was carefully opened to release the pressure as described above. The reaction vessel was resealed and heated for an additional 48 h at 120 °C. No chlorine gas was observed, and some substrate remained in a solid form. After it cooled, Cl2 gas was bubbled through the reaction mixture for 10 min at −30 °C. The reaction vessel was sealed once more and heated at 120 °C for 24 h. (The ammonium salt melted after the addition of chlorine gas, indicating that the completion of the reaction requires the presence of Cl2.) SO2Cl2 was distilled under reduced pressure and collected into a liquid nitrogen trap, affording a white precipitate. The residue was treated with 5.0 mL of ice-cold water, and acetone (2 × 50 mL) was added to dissolve and extract the product. The clear solution was transferred into a round-bottom flask, and excess [Et4N]Br (6.0 g) was added. Removing the acetone by rotary evaporation gave the product as an off-white solid. The product was removed by filtration and washed three times with water followed by recrystallization from 70% v/v methanol in water (200 mL). Allowing the methanol to slowly evaporate afforded [Et4N][H3NB12Cl11] as a colorless crystalline solid (4.43 g, 6.64 mmol, 94.7%). Synthesis of [Me 3NH][Me3 NB12Cl 11 ], Me3 NH[4]. [Me 3NH][Me3NB12Cl11] was prepared in a manner similar to the recently published procedure.12a A Schlenk flask was charged with dry [Et4N][H3NB12Cl11] (1.58 g, 2.37 mmol), finely powdered KOH (2.01 g, 35.82 mmol, ca. 15 equiv), and dry acetonitrile (50 mL). MeI (1.14 g, 0.50 mL, 8.03 mmol, ca. 3.4 equiv) was added dropwise via a syringe. The suspension was stirred at room temperature for 24 h. The reaction mixture was then filtered through a medium frit to remove insoluble KOH and KI. Excess [Et4N]Br (2.0 g) was added to the filtrate, and the solvent was removed to give an off-white precipitate. The precipitate was resuspended in water, filtered, washed with water, and dried in air. The precipitate was then dissolved in 70% v/v acetonitrile in water and passed though the strong acid ion-exchange resin Amberlyst 15 in the H+ form. The acetonitrile was removed by rotary evaporation, and the slightly cloudy aqueous solution was topped to 50 mL with deionized (DI) water and filtered through a fine frit lined with diatomaceous earth. [Me3NH]Cl (2.0 g, 20.9 mmol) was added, forming a voluminous white precipitate. The precipitate was filtered and recrystallized from hot 90% v/v methanol in water (200 mL) to give [Me3NH][Me3NB12Cl11], [Me3NH][4], as colorless crystals (1.37 g, 2.14 mmol, 90.3%). 1H NMR (400.13 MHz, (CD3)2CO, 297 K): δ = 4.28 (s, br, w1/2 = 207 Hz, 1H, NH), 3.47 (s, 9H, B1−N(CH3)3), 3.19 (s, 9H, N(CH3)3); 13C{1H} NMR (100.61 MHz, (CD3)2CO, 297 K): δ = 57.50 (B1−N(CH3)3), 46.36 (N(CH3)3); 11B NMR (128.38 MHz, (CD3)2CO, 297 K) δ = −9.2 (s, 1B, B12-Cl), −10.6 (s, 5B, B(7−11)-Cl), −14.0 (s, 5B, B(2−6)-Cl), −15.8 (s, 1B, B1-N(CH3)3). Synthesis of [Et 4 N][n-Pr 3 NB 12 H 11 ], [Et 4 N][5]. [Et 4 N][nPr3NB12H11] was prepared in a manner similar to the published procedure by Gabel et al.15 A Schlenk flask was charged with dry [Et4N][H3NB12H11] (2.0 g, 6.94 mmol), finely powdered KOH (5.90 g, 105.15 mmol, ca. 15 equiv), and dry acetonitrile (70 mL). The n-PrI (3.83 g, 2.20 mL, 22.53 mmol, ca. 3.2 equiv) was added dropwise via a syringe. The suspension was stirred at room temperature for 24 h. The reaction mixture was filtered through a medium frit to remove insoluble KOH and KI. Excess [Et4N]Br (4.0 g) was added, and the solvent was removed to give an off-white precipitate. The precipitate D

DOI: 10.1021/acs.inorgchem.6b01867 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystal Data and Summary of Data Collection and Refinement Details for Ag[4], [Et2Al][4], and [Et4N][6]

empirical formula formula weight crystal system space group a, Å b, Å c, Å β (deg) V, Å3 Z, Z′ Dcalcd, Mg/m3 wavelength, Å T, K F(000) 2θ range (deg) reflections collected wR2a R1 goodness-of-fit, F2 observed data [I > 2s(I)] data/restraints/parameters a

Ag[Me3NB12Cl11]·MeCN

[Et2Al] [Me3NB12Cl11]

Ag[4]·MeCN

[Et2Al][4]

[Et4N] [n-Pr3NB12H5Cl6] [Et4N][6]

C20H48Ag4B48Cl44N8 2910.82 monoclinic Cc 19.978(2) 29.280(4) 18.961(2) 118.716(2) 9727.2(19) 4, 1 1.988 0.71073 100(2) 5600 2.708 to 62.598 125 394 0.0634 0.0258 0.984 27 455 29 575/8/1118

C7H19AlB12Cl11N 663.88 monoclinic P21/n 19.552(2) 9.8336(13) 27.237(4) 91.846(2) 5234.0(12) 8, 2 1.685 0.71073 100(2) 2624 2.526 to 52.000 39 156 0.1796 0.0634 1.017 27 588 39 156/0/590

(C8H20N)+ (C9H25.85B12Cl6.15N)− 626.06 monoclinic P21/c 10.2771(16) 17.981(3) 19.097(3) 114.6508(19) 3207.4(9) 4, 1 1.297 0.71073 100(2) 1305 3.262 to 51.994 36 013 0.1633 0.0622 0.997 5114 6313/81/382

wR2 = {Σ[w(F02 − Fc2)2]/Σ[w(F02)2]}1/2; R1 = Σ∥F0| − |Fc∥/Σ|F0|.

nitric acid (0.5 mL). The addition of silver nitrate (490 mg, 2.88 mmol, ca. 4 equiv) formed an immediate white solid that dissolved upon heating. Allowing the reaction mixture to slowly cool afforded Ag[Me3NB12Cl11] as pale yellow well-shaped crystals. (In other attempts, the silver salt did not dissolve in water but remained a solid even when heated; it is believed that this is due to presence of small amounts of acetonitrile. In this case, the white precipitate was filtered through a fine frit and dried under vacuum.) Isolated yield: 358 mg, 0.52 mmol, 74%. The crystals were dried under vacuum at 150 °C. On the basis of 1H NMR spectroscopy, the silver salt still contains ∼0.16 equiv of H2O. 1H NMR (400.13 MHz, CD3CN, 297 K): δ = 3.35 (s, 9H, B1−N(CH3)3); 11B NMR (128.38 MHz, CD3CN, 297 K): δ = −15.7 (s, 1B, B1-N(CH3)3), −13.9 (s, 5B, B(2−6)-Cl), −10.8 (s, 5B, B(7−11)-Cl), −9.5 (s, 1B, B12-Cl); 13C{1H} NMR (100.61 MHz, CD3CN, 297 K): δ = 57.8 (B1−N(CH3)3). Synthesis of Ag[n-Pr3NB12H5Cl6], Ag[6]. A solution of [Et4N][nPr3NB12H5Cl6] (0.81 g, 1.30 mmol) in 70% v/v acetonitrile in water (40 mL) was passed through the strong acid ion-exchange resin Amberlyst 15 in the H+ form. The acetonitrile was removed by rotary evaporation, and the solution was topped to 50 mL with DI water and acidified with concentrated nitric acid (0.5 mL). The addition of silver nitrate (1.20 g, 7.06 mmol, ca. 4 equiv) formed an immediate voluminous white precipitate, which is poorly soluble in water and methanol. The precipitate was collected on a fine frit and dried under vacuum. Isolated yield: 0.36 g, 0.60 mmol, 46%. 1H NMR (400.13 MHz, CD3CN, 297 K): δ = 3.14 (m, 2H, N−CH2-CH2−CH3, from the Cl7 interaction), 2.98 (m, 6H, N−CH2-CH2−CH3), 1.75 (m, 6H, N−CH2−CH2-CH3), 0.86 (t, J = 7.28 Hz, 9H, N−CH2−CH2−CH3), 2.39−1.18 (m, 5H, B-H); 13C{1H} NMR (100.61 MHz, CD3CN, 297 K): δ = 64.12, 63.51, 63.25, 18.25, 10.98, 10.17; 11B NMR (128.38 MHz, CD3CN, 297 K): δ = −7.9 (s, 1B, B1-N(n-Pr)3, obscured by BCl signal), −8.0 (s, 6B, B(7−12)-Cl), −21.2 (d, 1JBH = 140 Hz, 5B, B(2−6)-H). Synthesis of [Ph3C][CHB11Cl11], [Ph3C][1]. A Schlenk flask was charged with dry Cs[CHB11Cl11] (1.50 g, 2.29 mmol), trityl chloride (1.28 g, 4.58 mmol, 2 equiv), and CH2Cl2 (50 mL). Upon addition of dichloromethane the reaction mixture turned yellow. The reaction mixture was then heated for 5 h (oil bath temperature at 40 °C) and left to stir for an additional 12 h at room temperature. The orange solution was filtered through a frit lined with diatomaceous earth to

remove CsCl. The residue was washed with CH2Cl2 (30 mL) and filtered. The combined clear orange solution was concentrated to ca. 40 mL under reduced pressure. Hexane (ca. 50 mL) was slowly layered on top of the CH2Cl2 resulting in the formation of a fine crystalline dark orange solid. (In other attempts, the rapid addition of hexane results in the formation of a fine yellow precipitate, but in some cases a separation of an oily phase was observed once the trityl salt reaches solubility limits. Upon standing for several hours, this oily phase tends to solidify.) The mother liquor was decanted, and the crystals were washed with hexane (2 × 10 mL) and dried under vacuum at 70 °C for 2 h to give [Ph3C][1] as a dark orange solid. Yield: 1.65 g, 2.16 mmol, 94.3% (based on Cs salt). 1H NMR (400.13 MHz, CDCl3, 297 K): δ = 8.29 (t, J = 7.5, p-C6H5, 3H), 7.93 (t, J = 7.6, m-C6H5, 6H), 7.71 (d, J = 7.2, o-C6H5, 6H), 3.09 (s, 1H, cage C−H). 13C{1H} NMR (100.61 MHz, CDCl3, 297 K): δ = 210.84 [C(C6H5)3], 143.94 (p-C), 143.04 (o-C), 140.14 (i-C), 131.15 (m-C), 47.09 (CHB11Cl11). 11B NMR (128.38 MHz, CD3Cl, 297 K): δ = −2.5 (1B), −10.1 (5B), −13.1 (5B). Synthesis of [Ph3C][Me3NB12Cl11], [Ph3C][4]. A Schlenk flask was charged with dry Ag[Me3NB12Cl11] (0.80 g, 1.17 mmol), trityl chloride (0.39 g, 1.40 mmol, 1.2 equiv), and CH2Cl2 (50 mL). Upon addition of methylene chloride the reaction mixture turned yelloworange. The reaction mixture was then heated for 5 h (oil bath temperature at 40 °C) and left to stir for 24 h at room temperature. The orange solution was filtered through a frit lined with diatomaceous earth to remove AgCl, the residue was washed with CH2Cl2 (2 × 10 mL), and the combined clear solution was concentrated to ca. 40 mL under reduced pressure. Hexane (ca. 40 mL) was then layered carefully on top of the solution to obtain the product as an orange solid. The mother liquor was decanted, and the product was washed with hexane (2 × 10 mL) and dried under vacuum at 70 °C for 3 h to give [Ph3C][4] as a dark orange solid. Yield: 0.73 g, 0.89 mmol, 76.1% (based on Ag salt). 1H NMR (400.13 MHz, CD3CN, 297 K): δ = 8.29 (t, J = 7.5 Hz, p-C6H5, 3H), 7.88 (t, J = 7.8 Hz, m-C6H5, 6H), 7.72 (d, J = 7.4 Hz, o-C6H5, 6H), 3.35 (s, N(CH3)3, 9H). 13C{1H} NMR (100.61 MHz, CD3CN, 297 K): δ = 212.98 (C(C6H5)3), 144.19 (p-C), 144.12 (o-C), 141.24 (i-C), 131.21 (m-C), 57.78 (N(CH3)3). 11B NMR (128.38 MHz, CD3CN, 297 K): δ = −15.8 (s, B1-N(CH3)3, 1B), −14.0 (s, B(2−6)-Cl, 5B), −10.9 (s, B(7−11)-Cl, 5B), −9.6 (s, B12-Cl, 1B). E

DOI: 10.1021/acs.inorgchem.6b01867 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1

■

Synthesis of [Et2Al][Me3NB12Cl11], [Et2Al][4]. A small grease-free Schlenk tube equipped with a Teflon screw valve and a small magnetic stirrer bar was charged with [Ph3C][Me3NB12Cl11] (200 mg, 243 μmol), benzene (1.0 mL), and Et3Al (52 mg, 455 μmol). The reaction mixture was heated at 80 °C for 3 h, during which time the color of the yellow-orange trityl salt faded to afford a colorless solution. Hexane (ca. 1.5 mL) was layered carefully on top of the solution to yield [Et2Al][Me3NB12Cl11] as colorless crystals after 2 d at room temperature. The mother liquor was decanted off, and the crystals were washed with hexane (2 × 0.5 mL) and dried under vacuum. Isolated yield: 69 mg, 104 μmol, 42.8% (based on trityl salt). 1H NMR (400.13 MHz, C6D6, 297 K): δ = 2.44 (s, N(CH3)3, 9H), 0.86 (t, J = 8.2 Hz, Al-CH2−CH3, 6H), 0.33 (q, J = 8.2 Hz, Al−CH2-CH3, 4H). 13 C{1H} NMR (100.61 MHz, C6D6, 297 K): δ = 56.72 (N(CH3)3), 7.79 (Al-CH2−CH3), 5.53 (Al-CH2−CH3). 11B NMR (128.38 MHz, C6D6, 297 K): δ = −14.2 (s, B1-N(CH3)3, 1B), −13.1 (s, B(2−6)-Cl, 5B), −10.5 (s, B(7−11)-Cl, 5B), −9.2 (s, B12-Cl, 1B). X-ray Crystallography. Crystals of compounds [Et4N][6], Ag[4]· MeCN, and [Et2Al][4] were grown as described above. The data were collected using a diffractometer with a Bruker APEX ccd area detector and graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) at 100(2) K. The data were corrected for absorption by the empirical method, and the structures were solved and refined using the SHELXS and SHELXL package (Table 1).16,17 The structure of Ag[4] forms a one-dimensional polymer along the b axis. The asymmetric unit contains four molecular units of Ag[4]·MeCN, the geometric data of which are very similar. Restraints on the displacement parameters of one atom, C4 of residue 4, were required. The intensity data exhibit pseudo B centering due to the relative positions of the silver atoms; the light atoms break this symmetry. The structure also exhibits racemic twinning with a refined twin ratio of 0.483(14). The selected crystal of [Et2Al][4] was integrated as a four-component twin with refined twin ratios of 0.2064(8), 0.0506(5), and 0.0377(4). There were two molecules per asymmetric unit of the cell. The displacement ellipsoids were drawn at the 50% probability level. One of the propyl side arms of the anion in the structure of [Et4N][6] was disordered. The occupancies of atoms C7−C9 refined to 0.513(6) and 0.487(6) for the unprimed and primed atoms. Either hydrogens or chlorines were attached to borons B5 and B6. The occupancies refined to 0.940(5) for H5, to 0.913(5) for H6, to 0.087(5) for Cl6, and to 0.060(5) for Cl5. Restraints on the positional parameters of the disordered propyl group and the displacement parameters of all disordered atoms were required.

RESULTS AND DISCUSSION Although SbCl5 is a proven chlorination agent for the [CH12B11]− and [H3NB12H11]− anions,8a,12a the high reaction temperatures (160−180 °C), long reaction times, and the unpleasant workup and separation of the SbCl3 side product prompted us to give SO2Cl2 another look. The side products are the gases SO2 and HCl, and excess SO2Cl2 can easily be removed under vacuum. SO2Cl2 is able to fully chlorinate the [B 12 H 12 ] 2− dianion, 18 and even the carborane anion [CH12B11]− can be fully chlorinated by SO2Cl2, although the latter reaction appears to be dependent on yet unknown catalytic(?) impurities (or the absence of those).8 A similar dependence on impurities may also exist for the chlorination of the [H3NB12H11]− anion. On the one hand, attempted chlorination using SO2Cl2 in refluxing acetonitrile did not work in our hands or in those of the Jenne group.12a On the other hand, the Duttwyler group reported a successful chlorination after 36 h.12b Very recently, a prolonged hightemperature process using SO2Cl2 (two weeks at 115 °C in a pressure tube) was shown to reliably afford [CHB11Cl11]− in good yields.13 While mechanistic details are not known, typical chlorination reactions using SbCl5, Cl2, ICl, or SO2Cl2 most likely follow electrophilic pathways. An alternative route, a radical pathway initiated by UV light irradiation, has been used successfully for the reaction of o-carborane, 1,2-C2H12B10, with elemental chlorine to afford the decachlorinated species 1,2C2H2B10Cl10,19 but has not yet been reported for the chlorination of [CH12B11]− or [H3NB12H11]− (Scheme 1). Furthermore, SO2Cl2 has been employed as chlorinating agent for C−H bonds in a radical mechanism.20 Our approach here focuses on using SO2Cl2 as solvent and reagent and the investigation of its activity in thermal and photochemical chlorination. Photochemical (Radical) Chlorination. Synthesis of Cs[CHB11Cl11]·H2O, Cs[1]·H2O. After some experimentation, the best procedure was found to be a combination of electrophilic and photochemical chlorination using SO2Cl2 as both reagent and solvent (1−2 g Cs[CH12B11] and 50−100 mL SOCl2). The addition of SO2Cl2 is exothermic and should be performed with ice cooling. A brief reflux afterward led to a F

DOI: 10.1021/acs.inorgchem.6b01867 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 2

Scheme 3

formation of a yet unknown higher mass B−Cl species (m/z = 614) were also observed. During the writeup of this submission, Tanaka et al. reported the synthesis of Cs[1] through the chlorination of Cs[CH12B11] with SO2Cl2 under pressure at 115 °C for two weeks.13 Synthesis of [Et4N][3] Using a Glass Pressure Vessel. Several attempts to chlorinate the [H3NB12H11]− anion were performed in the presence of various counter cations. The poor solubility of the [Me4N]+, [Et4N]+, and Cs+ salts in SO2Cl2 under reflux conditions led us to attempt the chlorination with SO2Cl2 at higher temperatures. Surprisingly, only the [Et4N]+ salt was fully chlorinated when the reaction was performed in a pressure vessel at 120 °C. Again, the color change of the reaction mixture to yellow indicates the presence of chlorine gas. When heated for 12 h, the suspended solids melted and formed a separate dense phase that crystallized when cooled to room temperature (see Figure S19). 1H NMR spectra indicated that the ethyl groups of the cation had been partially chlorinated, which could explain the lower melting point of the salt, since that correlates to the nature of the counterion. On one occasion, no chlorine formation took place, and the reaction stopped at a partial chlorination stage. This could be mitigated by the addition of chlorine gas to the reaction mixture followed by heating at 120 °C as before. The reason for this is not yet understood, possibly an impurity in the starting material, but it is important to know that even such a stalled reaction can be restarted. Use of an excess of [Et4N]Br allows the precipitation of fairly pure [Et4N][3] in good yields. Alkylated Chlorinated Ammonio-Dodecaborates. Synthesis of [Me3NH][Me3NB12Cl11], [Me3NH][4]. The ammonio dodecaborates [H3NB12H11]− and [H3NB12Cl11]− are slightly acidic, and the latter anion can be deprotonated in aqueous solution by a strong base such as KOH.12a Thus, for many applications in organometallic chemistry the ammonium nitrogen center needs to be peralkylated. Although N-alkylation with various alkyl groups can be achieved for the nonchlorinated precursor [H3NB12H11]− 15 and the perfluorinated anion [H3NB12F11]−,22 only the trimethylated ammonium salt was accessible from the undecachlorinated anion.12a The large size of the Cl atom compared to H and F atoms likely hinders the trisubstitution with bigger groups like Et and n-Pr.

mixture of partially chlorinated species (see Figure S2). Irradiation with UV light in a photoreactor essentially completed the chlorination within 1 h. Although mass spectra showed the presence of a small amount of the decachlorinated anion (4%), additional irradiation did not complete the reaction but resulted in slow decomposition of the CB11 cage, and boric acid was detected by 11B NMR spectroscopy. The pure product Cs[CHB11Cl11]·H2O, 1, was isolated after recrystallization from hot water in 78% yield. The ammonium salt [Me3NH][CH12B11] can also be chlorinated by this procedure, but its lower solubility in SO2Cl2 requires a longer reaction time. Synthesis of [Et4N][H3NB12Cl11], [Et4N][3]. Because of the low solubility of the precursors Cs[H3NB12H11] and [Et4N][H3NB12H11] in SO2Cl2, only the sodium salt Na[H3NB12H11] could be completely chlorinated using the same approach as for the synthesis of Cs[1]. Even so, the radiation time was 15 h for just 0.3 g of Na[H3NB12H11], and the yield was only 57%. Thermal (Electrophilic) Chlorination. The addition of SO2Cl2 to the carborate and dodecaborate anions [CH12B11]− and [H3NB12H11]− results in an exothermic reaction, during which the anions are partially chlorinated. The reaction is more intense for the carborate anion. The completion of this reaction under reflux conditions (bp = 69 °C21) is usually not possible, although it has been achieved for some batches of Cs[CH12B11].8a We were interested, if higher reaction temperatures (and pressures) would result in complete chlorination, and the thermal chlorination of the anions [CH12B11]− and [H3NB12H11]− was investigated. Caution! While the reactions discussed below can be performed using regular glassware, care must be taken to regularly vent the gaseous side products (mostly HCl and SO2), and the use of blast shields is strongly recommended. Attempted Synthesis of Cs[1] Using Pressure Vessels. Increase of the reaction temperature to 120 °C for 4 d using a thick-walled glass pressure vessel followed by an additional 4 d at 140 °C did indeed result in a higher degree of chlorination, but the isolated product still contained significant amounts of the anions [CH2B11Cl10]− and [CH3B11Cl9]−. The yellow color of the reaction mixture suggests the presence of chlorine gas. With even more forcing conditions, namely, 4 d at 235 °C in a Teflon lined steel autoclave, the amount of the target species did increase, but some decomposition to boric acid and the G

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also to a partial decomposition of the solvent. For example, after 5 d at room temperature, the mixture contained [nPr3NB11H5Cl6]− and [n-Pr3NB11H4Cl7]− in an ∼1:3 ratio (Figures S36 and S37). A 6 h reflux resulted in the formation of mostly the octachlorinated species [n-Pr3NB12H3Cl8]− based on mass spectrometry. The 11B NMR spectrum shows broad unresolved signals suggesting a mixture of isomers. After 44 h at 85 °C the mass spectrum shows peaks corresponding to anions with more than 11 chlorine atoms indicating partial chlorination of the side chains. Furthermore, the 11B NMR spectrum is still more complex than that of [Me3NB11Cl11]− including a broad peak at −20 ppm (likely B−H, Figures S38 and S39). Salts of the Anions [CHB11Cl11]−, [1]−, [Me3NB11Cl11]−, [4]−, and [n-Pr3NB12H5Cl6]−, [6]−. Several salts of the anions [1]−, [4]−, and [6]− were prepared including the trityl salts of [1]− and [4]−, the silver salts of [4]− and [6]−, and the tight ion pair [Et2Al][4]. Silver Salts of [4]− and [6]−. The silver salts were prepared by simple metathesis of AgNO3 with the acids [H3O][4] and [H3O][6], which were generated in situ by passing aqueous acetonitrile solutions of the ammonium salts [Et4N][4] and [Et4N][6] through an ion-exchange column. Removing the acetonitrile followed by acidifying the solution with HNO3 and the addition of AgNO3 gave the silver salts as voluminous white precipitates (eq 1).

Attempted alkylation using CsOH or the successive addition of n-BuLi followed by R-Br or R-I addition were unsuccessful and only resulted in partial alkylation (Scheme 2). Synthesis of [Et4N][n-Pr3NB12H5Cl6], [Et4N][6]. As the alkylation of the anion [H3NB12Cl11]− was unsuccessful for alkyl groups other than methyl, the chlorination of the alkylated anion [n-Pr3NB12H11]−, [5], was attempted as an alternative approach. Addition of SO2Cl2 at room temperature to a solution of [Et 4 N][5] in acetonitrile resulted in fast chlorination (Scheme 3), and the hexachlorinated species [Et4N][n-Pr3NB12H5Cl6], [Et4N][6], was formed already after 30 min at room temperature. A quick workup led to the isolation of a sample that, based on MALDI-TOF mass spectrometry, 1H NMR spectroscopy, and X-ray diffraction data, was an ∼4:1 mixture of [Et4N][6] and the higher chlorinated species [Et4N][n-Pr3NB12H4Cl7]. The crystal structure analysis (Figure 2) shows that the hemisphere opposite to the ammonium group is chlorinated. A

H 2O

[H3O][A] + AgNO3 ⎯⎯⎯→ Ag[A] + HNO3 + H 2O A = Me3NB12Cl11, n‐Pr3NB12H5Cl 6

(1)

The formation of the silver salts requires the starting materials to be of high purity to prevent any undesired reduction of the silver. This can be achieved by careful crystallization of the ammonium salts followed by treatment with activated carbon. The silver salts precipitate as white solids, but change to pale yellow upon standing for 5 min. The products are slightly lightsensitive, and the crystalline forms seems to be more stable under ambient light. Yet, they should be protected from light and stored in a dark container. The low yield for the salt Ag[6] is likely due to losses during the cation exchange on the Amberlyst column. In addition, the solubility of Ag[6] is lower than that of Ag[4]. Crystals obtained from the recrystallization of crude Ag[4], which still contained some acetonitrile, from hot methanol were used for an X-ray diffraction study (Figure 3). Similar to silver salts of other icosahedral carborane anions,25 the silver salt of [4]− forms a one-dimensional polymeric chain (Figure S55). The silver center resides in a distorted trigonal bipyramidal environment coordinated to four chloride substituents from two neighboring anions and one acetonitrile molecule. The silver center coordinates to one anion on the lower belt with Ag···Cl(11) and Ag···Cl(21) distances of 2.6522(9) and 2.8796(9) Å, respectively. The Ag··· Cl contacts to the other anion involve one contact to the lower belt and one to the upper belt with values of 2.7631(9) (Ag··· Cl(104)) and 2.8919(9) Å (Ag···Cl(114)). It has recently been shown that the coordination behavior of the [Me3NB12Cl11]− anion is similar to that of the carborate anion [CHB11Cl11]−, where cations tend to coordinate to the lower belt or the 12position of the anion because of the higher electron density of these positions.12a It is worth noting that the Ag−Cl distances with the upper belt chlorides in Ag[4] are the longest, since substituents on the upper belt are known to bear less negative

Figure 2. Thermal ellipsoid plot (50%) of the anion in [Et4N][6]. Hydrogen atoms were omitted for clarity. Selected bond distances (Å): N(1)−B(1) = 1.598(5), N(1)−C(1) = 1.602(7), N(1)−C(4) = 1.524(5), N(1)−C(7) = 1.501(6), Cl(7)−B(7) = 1.810(4), Cl(8)− B(8) = 1.811(4), Cl(9)−B(9) = 1.813(4), Cl(10)−B(10) = 1.814(4), Cl(11)−B(11) = 1.815(4), Cl(12)−B(12) = 1.786(4).

similar reactivity pattern was reported for the halogenation of the related carborate anion [CH12B11]−, which readily affords the hexahalogenated anions [CH6BX6]−, in which the halogen substitutents also occupy the hemisphere opposite to the CH group.23 The B−Cl distances in the meta belt (1.813 Å average (avg)) are slightly longer than those reported for [Me3NH][4] (1.793 Å avg) 12a but closely resemble those of the [CH6B11Cl6]− anion (1.807 Å avg),24 whereas the B−Cl distance opposite of the ammonium group is shorter by 0.03 Å with a value of 1.786(4) Å. The corresponding B−Cl distance in [Me3NH][4] is only slightly shorter than the average with 1.791(2) Å. The B−B distances average 1.777 Å, a bit less than the 1.792 Å average observed for [Me3NH][4]. Longer reaction times at room temperature or prolonged refluxing led to the addition of more chloride substituents but H

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the basis of this goal, the ionlike compound [Et2Al][4], which contains the very strong Lewis acid [Et2Al]+, was prepared. The reaction of the trityl salt [Ph3C][4] anion with Et3Al in benzene proceeded cleanly by abstracting an ethyl group via β-hydrogen elimination and the formation of ethylene and afforded [Et2Al][4] as a colorless crystalline solid in moderate yields (eq 4). Et3Al + [Ph3C][Me3NB12Cl11] benzene

⎯⎯⎯⎯⎯⎯⎯→ [Et 2Al][Me3NB12Cl11] + Ph3CH + CH 2CH 2

(4)

This compound is readily soluble in benzene suggesting strong ion pairing in solution, which is also found in the solid state. The structure of [Et2Al][4] (Figure 4) consists of a bent

Figure 3. Thermal ellipsoidal plot (50%) of part of the coordination polymer in Ag[4]·CH3CN showing one Ag+ cation, the two bridging [Me3NB12Cl11]− anions, and the coordinated CH3CN molecule. Selected bond distances (Å) and angles (deg): Ag(1)−N(11) = 2.209(3), Ag(1)···Cl(11) = 2.6522(9), Ag(1)···Cl(21) = 2.8796(9), Ag(1)···Cl(104) = 2.7631(9), Ag(1)···Cl(114) = 2.8919(9), Cl(11)− Ag(1)−Cl(21) = 82.87(2), Cl(104)−Ag(1)−Cl(114) = 78.00(3), Cl(11)−Ag−Cl(114) = 159.90(3).

charge than the lower ones, but the overall Ag−Cl distances are in agreement with previously reported compounds.25e,g The Ag−N distance with a value of 2.209(3) Å is also similar to those observed in other [Ag(CH3CN)n]+ systems.25d,g Trityl Salts of [1]− and [4]−. Trityl ([Ph3C]+) salts of weakly coordinating anions are among the most important precursors for the synthesis of highly electrophilic cationic or ionlike species through β-hydride elimination or alkide abstraction from organometallic substrates. The trityl salt of [1]− is usually prepared from the silver salt Ag[1],5 and the trityl salt of [4]− has been synthesized using Ph3CCl as the trityl source and liquid SO2 as the solvent.12a It is found that [Ph3C][1] can easily be prepared following a silver-free route from the reaction of Cs[1] with Ph3CCl in dichloromethane in essentially quantitative yield (eq 2). CH2Cl2

Cs[CHB11Cl11] + Ph3CCl ⎯⎯⎯⎯⎯⎯⎯→ [Ph3C][CHB11Cl11] + CsCl↓

Figure 4. Thermal ellipsoidal plot (50%) of one of the independent molecules in the structure of [Et2Al][4]). Hydrogen atoms were omitted for clarity. Selected bond distances (Å) and angles (deg): Al(1)−C(1) = 1.923(8), Al(1)−C(3) = 1.923(9), Al(1)···Cl(1) = 2.476(3), Al(1)···Cl(2) = 2.383(3), B(1)−Cl(1) = 1.825(9), B(5)− Cl(5) = 1.775(10), B(12)−N(1) = 1.591(11), C(1)−Al(1)−C(3) 127.7(4), Cl(1)−Al(1)−Cl(2) 92.33(11).

(2)

[Et2Al]+ moiety, which is rather strongly coordinated by two chloride substituents from the anion with Al···Cl contacts of 2.476(3) and 2.383(3) Å. These values are comparable to the Al···Cl distances in [Et2Al][CH6B11Cl6] (2.4295(10) and 2.4400(9) Å)24 and [Et2Al]2[B12Cl12] (2.479(3) and 2.490(3) Å).26 The aluminum center is in a distorted tetrahedral environment. The C−Al−C angle of 127.7(4)° is far from the ideal tetrahedral 109.5° and the linear 180° and falls between those for [Et 2 Al] 2 [B 12Cl 12 ] (124.3(12)°) and [Et 2 Al][CH6B11Cl6] (136.6°). The coordination of the Cl substituents to the aluminum Lewis acid slightly increases these B−Cl distances with respect to free B−Cl groups (1.826 vs 1.785 Å). For comparison, the C−Al−C angle in the related [Me2Al][MeCB11F11] is rather wide with 147.6(2)°, although density functional theory calculations have shown that that is likely due to packing effects and not the lower basicity of the [MeCB11F11]− anion.27

CH2Cl2

Ag[Me3NB12Cl11] + Ph3CCl ⎯⎯⎯⎯⎯⎯⎯→ [Ph3C][Me3NB12Cl11] + AgCl↓

(3)

The product can be isolated as a powder or microcrystalline yellow-orange solid by carefully layering hexane onto the dichloromethane solution. Using 2 equiv of Ph3CCl gives better yields, and the identity of the product was confirmed by 1H and 13 C NMR spectroscopy by comparison with the reported data. Although it seems likely that a similar approach should afford the trityl salt of [4]−, we chose the silver route in this case because of the ready availability of Ag[4]. Compound [Ph3C][4] was obtained in good yields as a dark orange solid similar to the synthesis of Ag[1]4b but employing dichloromethane as the solvent (eq 3), thus avoiding the use of large amounts of toluene4b or liquid SO2.12a [Et2Al][4]. The synthesis of the chlorinated ammoniododecaborates [4]− and [6]− is part of a study to find more readily available but chemically inert weakly coordinating anions. On I

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Borates and Dodecaborates. In Comprehensive Inorganic Chemistry II, 2nd ed.; Poeppelmeier, K., Reedijk, J., Eds.; Elsevier: Amsterdam, 2013; p 651. (5) Juhasz, M.; Hoffmann, S.; Stoyanov, E.; Kim, K.-C.; Reed, C. A. The strongest isolable acid. Angew. Chem., Int. Ed. 2004, 43, 5352. (6) Nava, M.; Stoyanova, I. V.; Cummings, S.; Stoyanov, E. S.; Reed, C. A. The Strongest Brønsted Acid: Protonation of Alkanes by H(CHB11F11) at Room Temperature. Angew. Chem., Int. Ed. 2014, 53, 1131. (7) Franken, A.; Bullen, N. J.; Jelinek, T.; Thornton-Pett, M.; Teat, S. J.; Clegg, W.; Kennedy, J. D.; Hardie, M. J. Structural chemistry of halogenated monocarbaboranes: the extended structures of Cs[1HCB9H4Br5], Cs[1-HCB11H5Cl6] and Cs[1-HCB11H5Br6]. New J. Chem. 2004, 28, 1499. (8) (a) Gu, W.; McCulloch, B. J.; Reibenspies, J. H.; Ozerov, O. V. Improved methods for the halogenation of the [HCB11H11]− anion. Chem. Commun. 2010, 46, 2820. (b) Gu, W.; McCulloch, B. J.; Reibenspies, J. H.; Ozerov, O. V. Improved methods for the halogenation of the [HCB11H11]− anion. [Erratum to document cited in CA153:037209]. Chem. Commun. 2010, 46, 9263. (9) Ivanov, S. V.; Rockwell, J. J.; Polyakov, O. G.; Gaudinski, C. M.; Anderson, O. P.; Solntsev, K. A.; Strauss, S. H. Highly Fluorinated Weakly Coordinating Monocarborane Anions. 1-H-CB11F11−, 1-CH3CB11F11−, and the Structure of [N(n-Bu)4]2[CuCl(CB11F11)]. J. Am. Chem. Soc. 1998, 120, 4224. (10) Geis, V.; Guttsche, K.; Knapp, C.; Scherer, H.; Uzun, R. Synthesis and characterization of synthetically useful salts of the weakly-coordinating dianion [B12Cl12]2‑. Dalton Trans. 2009, 2687. (11) Hertler, W. R.; Raasch, M. S. Chemistry of Boranes. XIV. Amination of B10H10−2 and B12H12−2 with Hydroxylamine-O-sulfonic Acid. J. Am. Chem. Soc. 1964, 86, 3661. (12) (a) Bolli, C.; Derendorf, J.; Jenne, C.; Scherer, H.; Sindlinger, C. P.; Wegener, B. Synthesis and Properties of the Weakly Coordinating Anion [Me3NB12Cl11]−. Chem. - Eur. J. 2014, 20, 13783. (b) Zhang, Y.; Liu, J.; Duttwyler, S. Synthesis and Structural Characterization of Ammonio/Hydroxo Undecachloro-closo-Dodecaborates [B12Cl11NH3]−/[B12Cl11OH]2‑ and Their Derivatives. Eur. J. Inorg. Chem. 2015, 2015, 5158. (13) Tanaka, N.; Shoji, Y.; Fukushima, T. Convenient Route to Monocarba-closo-dodecaborate Anions. Organometallics 2016, 35, 2022. (14) Franken, A.; King, B. T.; Rudolph, J.; Rao, P.; Noll, B. C.; Michl, J. Preparation of [closo-CB11H12]− by dichlorocarbene insertion into [nido-B11H14]−. Collect. Czech. Chem. Commun. 2001, 66, 1238. (15) (a) Justus, E.; Rischka, K.; Wishart, J. F.; Werner, K.; Gabel, D. Trialkylammoniododecaborates: anions for ionic liquids with potassium, lithium and protons as cations. Chem. - Eur. J. 2008, 14, 1918. (b) Justus, E.; Vö g e, A.; Gabel, D. N-alkylation of ammonioundecahydro-closo-dodecaborate(1-) for the preparation of anions for ionic liquids. Eur. J. Inorg. Chem. 2008, 2008, 5245. (16) Sheldrick, G. M. SADABS. Program for Empirical Absorption Correction of Area Detector Data. University of Göttingen: Germany, 2001. (17) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112. (18) Gu, W.-X.; Ozerov, O. V. Exhaustive Chlorination of [B12H12]2‑ without Chlorine Gas and the use of [B12Cl12]2‑ as a Supporting Anion in Catalytic Hydrodefluorination of Aliphatic C-F Bonds. Inorg. Chem. 2011, 50, 2726. (19) Zakharkin, L. I.; Ogorodnikova, N. A. Preparation and study of decachloro-o-carborane (o-B10Cl10C2H2). J. Organomet. Chem. 1968, 12, 13. (20) Moussa, V. N. Sulfuryl Chloride: A Versatile Alternative to Chlorine. Aust. J. Chem. 2012, 65, 95. (21) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, U.K, 1997. (22) Ivanov, S. V.; Davis, J. A.; Miller, S. M.; Anderson, O. P.; Strauss, S. H. Synthesis and Characterization of Ammonioundecafluoro-closo-

SUMMARY The undecachloro weakly coordinating anions [CHB11Cl11]−, [1]−, and [H3NB12Cl11]−, [3]−, can be obtained by chlorination with SO2Cl2 under either photochemical or thermal conditions. Whereas the N−H-free anion [Me3NB12Cl11]− is easily prepared by methylation of [3]− with methyl iodide under basic conditions, the alkylation with other alkyl groups such as ethyl or n-propyl does not go to completion. An alternative approach, the chlorination of [n-Pr3B12H11]− with SO2Cl2, affords the hexachloro species [n-Pr3B12H5Cl6]− with relative ease, but attempts to introduce additional chlorine substituents led to mixtures and eventual decomposition. There are now several methods available for the synthesis of the weakly coordinating anion [CHB11Cl11]−, [1]−. For groups with no easy access to [CH12B11]−, weakly coordinating anions based on the ammoniododecaborate framework are a versatile alternative. This is demonstrated here with the syntheses of the silver and trityl salts of the anions [4]− and [6]− and the contact ion-pair [Et2Al][Me3NB12Cl11] and their crystal structures. This and the more facile synthesis of the trityl salt of [1]− is expected to contribute to the wider use of these very weakly basic, chemically inert, and also aesthetically pleasing anions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01867. NMR and mass spectra, an additional figure of the crystal structure of Ag[4] (PDF) X-ray data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: rwehmsch@fit.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Petroleum Research Fund administered by the American Chemical Society (PRF No. 52856-ND3) and the 2013 NASA/KSC Chief Technologist Center Innovation Fund is gratefully acknowledged.

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REFERENCES

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K

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