Syntheses of Tricyanofluoroborates M[BF(CN)3] (M = Na, K): (CH3)

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Syntheses of Tricyanofluoroborates M[BF(CN)3] (M = Na, K): (CH3)3SiCl Catalysis, Countercation Effect, and Reaction Intermediates Jan A. P. Sprenger,† Johannes Landmann,† Michael Drisch,† Nikolai Ignat’ev,§ and Maik Finze*,† †

Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany Merck KGaA, PM-ATI, Frankfurter Straße 250, 64293, Darmstadt, Germany

§

S Supporting Information *

ABSTRACT: Potassium tricyanofluoroborate, K[BF(CN)3], which is the starting material for tricyanofluoroborate roomtemperature ionic liquids [N. Ignat’ev et al. J. Fluorine Chem., submitted] was obtained on a molar scale (140 g) from Na[BF4] and (CH3)3SiCN with a purity of up to 99.9%. The initial product of the reaction that was catalyzed by (CH3)3SiCl was Na[BF(CN)3]·(CH3)3SiCN that was characterized by multinuclear NMR and vibrational spectroscopy, elemental analysis, differential scanning calorimetry, and single-crystal X-ray diffraction. Na[BF(CN)3]·(CH3)3SiCN was converted to K[BF(CN)3] via a simple extraction protocol. The catalytic effect of (CH3)3SiCl was evaluated and some intermediates of the reaction, including the isocyanoborate anion [BF(NC)(CN)2]−, were identified using multinuclear NMR and vibrational spectroscopy. K[BF2(CN)2] also reacted with (CH3)3SiCN in the presence of (CH3)3SiCl, to result in K[BF(CN)3]. The interpretation of the experimental observations was supported by data derived from density functional theory (DFT) calculations. In addition, the influence of selected countercations of the tetrafluoroborate anion on the progress of the (CH3)3SiCl-catalyzed reaction was studied. The fastest reaction was observed for Na[BF4], while the conversion of [BF4]− to [BF(CN)3]− was slower with the countercation K+. Li[BF4] and [Et4N][BF4] were converted under the reaction conditions applied to Li[BF2(CN)2] and [Et4N][BF2(CN)2] only.

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INTRODUCTION Cyanoborate anionsfor example, the tetracyanoborate anion [B(CN)4]− (see ref 1) and the tricyanofluoroborate anion [BF(CN)3]− (see ref 2)are highly valuable building blocks for low-viscosity ionic liquids (ILs) that exhibit high thermal, chemical, and electrochemical stabilities.3,4 Because of these and other appealing properties, ILs that contain cyanoborate anions can be used in many applications. Especially in electrochemical devices such as dye-sensitized solar cells (Grätzel cells5), they are of interest as components of the electrolyte, e.g., EMIM[B(CN)4] (EMIM = 1-ethyl-3-methylimidazolium), which was shown to result in an improved performance of the cell, compared to other ILs.6 The [B(CN)4]− anion, which can be regarded as the archetype of a highly thermally stable and chemical as well as electrochemical robust cyanoborate anion,3,1,4 is currently widely applied for the preparation of various organic and inorganic salts, and many properties of the respective ILs have been investigated.6c,7 Salts of mixed cyanofluoroborate anions (e.g., the [BF(CN)3]− anion) often exhibit properties that are similar to those of the related tetracyanoborates, for example, high thermal and chemical stability.1c,2b,3 Especially because of the lower symmetry of the [BF(CN)3]− anion (C3v) in comparison to the [B(CN)4]− anion (Td) and due to the lower mass of the [BF(CN)3]− anion,8 ILs with the tricyanofluoroborate anion © 2015 American Chemical Society

are less viscous (e.g., compare EMIM[BF(CN)3] to EMIM[B(CN)4]).3,9 Since a lower viscosity is often advantageous for electrochemical applications, in which the IL is a component of the electrolyte, salts of the [BF(CN)3]− anion are of interest. In addition, several transition-metal compounds with cyanoborate anions have been studied. Discrete complexes, in which the respective borate anion is either coordinated to the metal center or serves as a weakly coordination counteranion, as well as coordination polymers with cyanoborate anions as linkers, have been described.1a,2a,10 The copper(I) and silver(I) salts of the tetracyanoborate and the tricyanofluoroborate anion M[B(CN)4]1a,10b and M[BF(CN)3]·CH3CN2a (M = Cu, Ag) are examples for such three-dimensional coordination polymers. Many applications exist for coordination compounds with simple cyanoborate anions of this type. For example, complex salts that contain cationic octahedral cobalt(II) and cobalt(III) complexes with aromatic N-donor ligands, e.g., [Co(bipy)3][B(CN)4]n11 and [Co(bipy)3][BF(CN)3]n (n = 2, 3; bipy = 2,2′-bipyridine),12 can serve as redox shuttles in dye-sensitized solar cells. The tricyanofluoroborate anion was identified by NMR spectroscopy as an intermediate of the synthesis of salts of the Received: December 23, 2014 Published: March 18, 2015 3403

DOI: 10.1021/ic503077c Inorg. Chem. 2015, 54, 3403−3412

Article

Inorganic Chemistry [B(CN)4]− anion for the first time.13 Later, the crystal structures of M[BF(CN)3]·CH3CN (M = Cu, Ag) were published, but no synthetic details were given.2a In 2003, the first syntheses of alkali metal tricyanofluoroborates starting from Li[BF4] or K[BF4] and (CH3)3SiCN were described. However, the reaction times were 1 week in the case of Li[BF4] and one month if K[BF4] was used as the starting compound.2b Furthermore, the conversions were not complete and 5% of the respective alkali metal salt of the [BF2(CN)2]− anion was obtained (Scheme 1). The spectroscopic, structural, and

strong demand for an easy and high yield synthesis for M[BF(CN)3] (M = alkali metal). In this contribution, we report on (CH3)3SiCl-catalyzed syntheses that enable the preparation of sodium and potassium salts of the [BF(CN)3]− anion on a scale of up to 140 g (K[BF(CN)3]) within a few hours and in yields of up to 92%.18 The influence of the amount of (CH3)3SiCl and of the alkali-metal cation (Li+, Na+, and K+) on the reaction will be discussed. The thus-far unknown isocyanoborate anion [BF(NC)(CN)2]− was identified as an intermediate in the formation of the [BF(CN)3]− anion.

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Scheme 1. Syntheses of Tricyanofluoroborates, Using Li[BF4] and K[BF4] as the Starting Materials2b and Using Tetraalkylammonium or Tetraalkylphosphonium Tetrafluoroborates as the Starting Materials15

RESULTS AND DISCUSSION Optimized Syntheses of M[BF(CN)3] (M = Na, K). Na[BF(CN)3]·(CH3)3SiCN was synthesized from sodium tetrafluoroborate and trimethylsilyl cyanide in the presence of trimethylsilyl chloride in a closed glass vessel (see Scheme 2).18 In the first step of the reaction, a suspension of Na[BF4] in (CH3)3SiCN/(CH3)3SiCl (∼1:0.1 v/v) was heated to 50 °C for 1 h to yield a mixture of cyanofluoroborates and Na[BF4]. The overpressure in the reaction flask, which was mainly due to the (CH3)3SiF formed, was released by short opening of the valve. The reaction mixture was subsequently stirred at 100 °C for six additional hours and the maximum pressure was 99%, according to 11B NMR spectroscopy). An alternative workup procedure for the mixture obtained from the reaction of Na[BF4] with (CH3)3SiCN in the presence of (CH3)3SiCl started with the removal of all volatiles. The pale-red solid residue (see Figure S6 in the Supporting Information) was transferred into the potassium salt via an extraction procedure including treatment with aqueous H2O2 and K2CO3 as described above. K[BF(CN)3] was obtained in a yield of 91% and with a purity of >98.5%. The isotopically enriched salts K[10BF(CN)3], K[11BF(CN)3], K[10BF(13CN)3], and K[11BF(13CN)3] also have been obtained using this workup procedure. They were characterized by multinuclear NMR (see Figure 2, as well as Table S6 in the Supporting Information) and vibrational spectroscopy (see Figures S7−S10 and Table S7 in the Supporting Information). The reaction of Na[BF4] with (CH3)3SiCN in the presence of (CH3)3SiCl was also performed at ambient pressure in an

Figure 1. Coordination around the Na+ cation in the crystal of Na[BF(CN)3]·(CH3)3SiCN [ellipsoids are drawn at the 50% probability level; H atoms are omitted for clarity]. Selected bond lengths [Å] of the [BF(CN)3]− anion: B−F, 1.399(2); B−C1, 1.614(2); B−C2, 1.6105(13); C1−N1, 1.142(2); C2−N2, 1.1445(13). Selected bond angles [°] of the [BF(CN)3]− anion: F−B−C1, 109.97(11); F−B−C2, 111.44(3); C1−B−C2, 107.40(3); C2−B− C2′, 109.03(5); B−C1−N1, 179.47(14); B−C2−N2, 177.12(9). Selected bond lengths [Å] of (CH3)3SiCN: Si−C3, 1.8937(15); Si− C4, 1.8437(12); Si−C5, 1.854(2); C3−N3, 1.145(2). Selected bond lengths [Å] in the Na environment: Na···N1, 2.4379(13); Na···N2, 2.4143(9); Na···N3, 2.2475(14); Na···F, 2.2959(9). Selected bond angles [°] in the Na environment: F···Na···N1, 167.17(4); N2···Na··· N3, 115.85(2); N2···Na···N2′, 128.00(3); N1···Na···N2, 89.22(2); N1···Na···N3, 85.80(5); F···Na···N2, 96.36(2); F···Na···N3, 81.36(4).

(Tonset). The DSC curve is depicted in Figure S5 in the Supporting Information. The trimethylsilyl cyanide is slowly released already at room temperature and ambient pressure. Solid Na[BF(CN)3] melts at 256 °C (DSC, Tonset) as confirmed visually and dismutates at 273 °C (DSC, Tonset) to yield a 3:1 mixture of Na[B(CN)4] and Na[BF4], as proven by 11 B NMR spectroscopy. The conversion of potassium

Figure 2. 19F (left), 11B and 10B (middle), as well as 13C (right) NMR spectra of isotopically enriched [11BF(13CN)3]− and [10BF(13CN)3]−, respectively [degree of enrichment: 97%, with respect to 13C; 99%, with respect to 10B and 11B]. 3405


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this two-step reaction is the reduction of the amount of the expensive (CH3)3SiCN. The excess (CH3 ) 3 SiCN that was employed in all aforementioned reactions with Na[BF4] or K[BF2(CN)2] was recovered as a mixture with the catalyst (CH3)3SiCl and with varying amounts of the byproduct (CH3)3SiF. Distillation using a short column resulted in a mixture of (CH3)3SiCN and the catalyst (CH3)3SiCl. This mixture was used for subsequent syntheses of Na[BF(CN)3]·(CH3)3SiCN after addition of the (CH3)3SiCN consumed in the previous reaction. Pure (CH3)3SiCN also was obtained by fractional distillation. Catalytic Effect of (CH3)3SiCl. Sodium tetrafluoroborate reacted with neat (CH3)3SiCN in 2 h at 80 °C to yield a mixture of Na[BF2(CN)2] and the starting material Na[BF4] (Table 1, entry 5). After 24 h, all Na[BF4] was converted to Na[BF2(CN)2] but no Na[BF(CN)3] had formed, as proven by 11 B and 19F NMR spectroscopy. The addition of trimethylsilyl chloride at the start of the reactions resulted in (i) a much faster consumption of the Na[BF4], which was hardly soluble in neat (CH3)3SiCN and in the mixtures of (CH3)3SiCN and (CH3)3SiCl, and (ii) the transformation of Na[BF2(CN)2] to Na[BF(CN)3] (Table 1, entries 1−4). Increasing the amount of trimethylsilyl chloride led to a faster reaction and, in the case of the addition of a very large amount of (CH3)3SiCl, almost all sodium tetrafluoroborate was reacted to Na[BF(CN)3] (96% conversion, Table 1, entry 1). Sodium tetrafluoroborate reacted with (CH3)3SiCN in the presence of a large amount of (CH3)3SiCl, even at room temperature, to give Na[BF(CN)3] slowly as the main product. In Figure 3, the 11B and 19F NMR spectra of a representative

inert atmosphere. The (CH3)3SiF that was formed was distilled off through a reflux condenser that prevented any loss of (CH3)3SiCN and (CH3)3SiCl. Only little Na[BF(CN)3]· (CH3)3SiCN crystallized. Most probably, this was a result of the removal of the (CH3)3SiF, which resulted in a higher solubility of the sodium salt in the reaction mixture. However, the purity of the K[BF(CN)3] that was obtained after aqueous workup with H2O2 and K2CO3 (>96%) was lower, compared to the reactions performed in closed vessels. K[BF(CN)3] was obtained in an alternative approach using K[BF2(CN)2] as starting material that was reacted with (CH3)3SiCN in the presence of (CH3)3SiCl with a purity of >96%, according to 11B NMR spectroscopy (see Scheme 3). Scheme 3. Two-Step Synthesis of K[BF(CN)3] from BF3· CH3CN via K[BF2(CN)2]

K[BF2(CN)2] is easily accessible from BF3·CH3CN as demonstrated herein or from BF3·OEt2.2b,4 The advantage of

Table 1. Reactions of Na[BF4] (2.3 mmol) and (CH3)3SiCN (8 mL) with Different Amounts of (CH3)3SiCl at 80 °C for 2 ha

Composition of the Borate Anions [mol %] Determined by 11B and 19F NMR Spectroscopy

(CH3)3SiCl entry

V [mL]

n [mmol]

mol % to 3n([BF4]−)b

[BF4]−

[BF2(CN)2]−

[BF(NC)(CN)2]−

[BF(CN)3]−

1 2 3 4 5

4.00 0.80 0.40 0.03 0

31.4 6.3 3.1 0.2 0

455 91 46 3.4 0

1 7 25 64 83

1 14 29 21 17

2 5 7 12

96 74 39 3

Details on the reactions are given in the Supporting Information. b3n([BF4]−) is the molar amount of fluorine substituents that have to be replaced upon conversion of [BF4]− into [BF(CN)3]−; n([BF4]−) = 2.3 mmol. a

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Figure 3. 11B and 19F NMR spectra of the mixture obtained from the reaction of Na[BF4] (1.01 mmol) with (CH3)3SiCN (6 mL) in the presence of (CH3)3SiCl (1 mL, 260 mol % to 3n([BF4]−)) after 10 d at room temperature.

Scheme 4. Proposed Mechanism for the Catalytic Effect of (CH3)3SiCl, Which Acts as a Lewis Acid, Exemplified for the Transformation of [BF2(CN)2]− into Either [BF(CN)3]− (Left) or [BF(NC)(CN)2]− (Right)

strong interaction between (CH3)3SiCN and (CH3)3SiCl, because ν̃ (CN) of the mixtures is identical to the value found and reported19 for neat (CH3)3SiCN (2190 cm−1). The [BF3(CN)]− anion was not observed in either of the sodium borate mixtures that have been studied by NMR spectroscopy. This result is in agreement to earlier observations on the reactions of M[BF4] (M = Li, K) with neat (CH3)3SiCN and it supports the earlier comment that the reactivity of the exchange of F− against CN− decreases in the order [BF(CN)3]− > [BF2(CN)2]− > [BF(CN)3]−.2b This decrease in reactivity is paralleled by an increase in calculated B−F and B− CN bond strengths (see Table S4 in the Supporting Information). However, the preparation of K[BF3(CN)] was achieved from BF3·OEt2 and potassium cyanide1c and its crystal structure was described.14 In addition to the NMR signals of [BF4]−, [BF2(CN)2]−, and [BF(CN)3]−, a further signal was observed in most of the 11B and 19F NMR spectra (δ(11B) = −15.7 ppm; δ(19F) = −183.5 ppm). The maximum content of the new borate anion in a reaction mixture was 29%, together with [BF4]− and mixed cyanofluoroborate anions. In the 13C and 13C{11B} NMR spectra two signals were assigned to this borate anion. The one at δ(13C) = 127.4 ppm corresponds to two cyano groups and the second signal at δ(13C) = 170.8 ppm (see Figure S12 in the Supporting Information) is in the region typical for isocyano groups bonded to boron, e.g., [BHx(NC)4−x]− (x = 1, 2),22 [BH(NC)(CN)2]−,23 [(CF3)3B(NC)]−,24 and [RB(NC)(CN)2]− (R = CH3, nBu).25 Furthermore, in the IR and Raman spectra of this sample, a band at 2171 cm−1 was observed that is (i) in the typical region for ν̃ (CN) of isocyanoborate anions22−25 and (ii) not present in the vibrational spectra of K[BFx(CN)4−x] (x = 4−0). Therefore, the new borate anion was unambiguously identified as [BF(NC)(CN)2]−. In addition, calculated chemical shifts and wavenumbers for [BF(NC)(CN) 2 ] − at the B3LYP/6311+G(d) level of theory are in excellent agreement with the experimental data (see Tables S5 and S6 in the Supporting Information).

reaction mixture, which was stirred at room temperature for 10 days, are depicted. The [BF(CN)3]− anion was formed with a purity of more than 96%. There was no evidence for the consumption of (CH3)3SiCl in any of the reactions performed. No NaCl precipitated in the course of reactions. Furthermore, no chlorine-containing boron species were identified as side products or as intermediates. These observations provide some indication that (CH3)3SiCl acts as a catalyst. To prove the catalytic effect of (CH3)3SiCl, Na[BF4] was treated with (CH3)3SiCN in the presence of 50 mol % of (CH3)3SiCl (calculated to n(Na[BF4])) for 1 day at 80 °C. Na[BF(CN)3] was the only boron containing product. In contrast, Na[BF(CN)3] was not observed if no (CH3)3SiCl was added, even after 3 days at 80 °C, and Na[BF2(CN)2] was the major product (∼96%; details on the reactions are given in the Supporting Information (Figure S1)). Probably, trimethylsilyl chloride acts as a Lewis-acid catalyst in the synthesis of salts of the [BF(CN)3]− anion. The Si center of (CH3)3SiCl interacts with a fluorine substituent of the respective fluoroborate anion, which results in a weakening of the B−F bond. The Si···F interaction activates the borate anion and leads to an easier nucleophilic attack of trimethylsilyl cyanide. In Scheme 4, this mechanism is shown for the last exchange step, which results in the formation of the [BF(CN)3]− anion if trimethylsilyl isocyanide (CH3)3SiNC acts as the nucleophile. Since (CH3)3SiNC is the minor isomer and the concentration of the major isomer (CH3)3SiCN is much higher,19 nucleophilic attack of trimethylsilyl cyanide also can occur. This explains the formation of the [BF(NC)(CN)2]− anion, as discussed in a subsequent section of this contribution. Pentacoordinated silicon intermediates, e.g., (CH3)3SiCl··· Si(CH3)3(CN) and (CH3)3SiCl···Si(CH3)3(NC), provide another possible explanation for the catalytic effect of trimethylsilyl chloride. Related cyanosilicate derivatives are known20 and have been discussed as intermediates in cyanide transfer reactions.19,21 Therefore, mixtures of (CH3)3SiCN with 10%, 50%, and 90% of (CH3)3SiCl (v/v) were studied by Raman spectroscopy. However, there is no evidence for a 3407


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Inorganic Chemistry Presumably, the [BF(NC)(CN)2]− anion was formed by the reaction of [BF2(CN)2]− with (CH3)3SiCN, as outlined in Scheme 4. The [BF(NC)(CN)2]− anion was transformed to [BF(CN)3]− at 80 °C in (CH3)3SiCN in the presence of (CH3)3SiCl. In contrast, a sample of cyanoborates that contained the [BF(NC)(CN)2]− anion did not show any isomerization in neat (CH3)3SiCN at 80 °C. Hence, the transformation of [BF(NC)(CN)2]− to [BF(CN)3]− at 80 °C was promoted by (CH3)3SiCl (see Scheme 5) and a simple intramolecular isomerization is unlikely at this relatively low temperature.

Influence of the Countercation M+ of M[BF4] (M = Li, Na, K, [Et4N]). The countercation of the tetrafluoroborate anion has a strong influence on the reaction of M[BF4] with (CH3)3SiCN and (CH3)3SiCl as a catalyst (see Table 2). Li[BF4] reacted with (CH3)3SiCN in the presence of (CH3)3SiCl at 80 °C to yield only Li[BF2(CN)2]. However, at higher temperatures, the Li[BF4] reacted with (CH3)3SiCN to yield Li[BF(CN)3]. In contrast, the reactions of M[BF4] (M = Na, K) gave a mixture of the alkali-metal salts of the anions [BF4]−, [BF2(CN)2]−, [BF(NC)(CN)2]− and [BF(CN)3]−. The [BF3(CN)]− anion was the only boron-containing species that was observed after reaction of tetraethylammonium tetrafluoroborate with neat (CH3)3SiCN at 25 °C in the presence of different amounts of (CH3)3SiCl (see Table S1 and Figure S2 in the Supporting Information). In contrast, without (CH3)3SiCl no reaction was observed and [Et4N][BF4] was recovered. Stirring for further 2 h at 50 °C resulted in complete transformation to [Et4N][BF3(CN)] (neat (CH3)3SiCN and with little (CH3)3SiCl [1 mL (CH3)3SiCN + 0.1 mL (CH3)3SiCl]) or in a mixture of [Et4N][BF3(CN)] and [Et4N][BF2(CN)2] (1 mL (CH3)3SiCN + 0.2 mL or more (CH3)3SiCl). After further 20 h at 80 °C all reaction mixtures contained only [Et4N][BF2(CN)2]. After an additional 12 h at 100 °C, the [BF2(CN)2]− was consumed in part and converted to unknown boron species and traces of [BF(CN)3]− in some cases. So, (CH3)3SiCl does not seem to be beneficial for the preparation of [Et4N][BF(CN)3] from [Et4N][BF4]. In a separate experiment, [Et4N][BF4] was reacted with neat (CH3)3SiCN to yield [Et4N][BF(CN)3] at 180 °C. Similar reactions of other tetraalkylammonium tetrafluoroborates with (CH3)3SiCN to give tetraalkylammonium tricyanofluoroborates at 140−200 °C were reported earlier.15 The results of the comparative study on the reaction of M[BF4] (M = Li, Na, K, [Et4N]) shows the influence of two different factors on the progress of the exchange reaction of the fluorine substituents against cyano groups. The solubility of the respective tetrafluoroborate influences the start of the exchange process. This is evident by the presence of Na[BF4] and

Scheme 5. Reaction of [BF(NC)(CN)2]− To Yield [BF(CN)3]−

This assumption is in agreement with the intramolecular isomerization of K[(CF3)3B(NC)] to give K[(CF3)3B(CN)] at temperatures above 130 °C.24 The activation energy calculated for the intramolecular isomerization of [BF(NC)(CN)2]− to give [BF(CN)3]− of 145.3 kJ mol−1 (see Table S3 and Figure S13 in the Supporting Information) indicates that the reaction should occur at elevated temperatures similar to the formation of K[(CF3)3B(CN)] from K[(CF3)3B(NC)] (Ea(calcd) = 160.2 kJ mol−1; Ea(expt) = 180 ± 20 kJ mol−1).24 In case of the earlier synthesis of M[BF(CN)3] (M = Li, K) without (CH3)3SiCl,2b which had to be performed at higher temperatures and at much longer reaction times (Scheme 1), the intermediate [BF(NC)(CN)2]− was not observed, which is most likely due to its conversion to [BF(CN)3]− via an intramolecular rearrangement.

Table 2. Reactions of M[BF4] (1.1 mmol) and (CH3)3SiCN (4 mL) with (CH3)3SiCl at 80 °C for 2 ha,b

(CH3)3SiCl salt d

K[BF4] Na[BF4] Li[BF4]

Composition of the Borate Anions [mol %] − c

V [mL]

mol % to 3n([BF4] )

[BF4]

0.2 0.2 0.2

49 49 49

57 38

−

[BF2(CN)2]−

[BF(NC)(CN)2]−

[BF(CN)3]−

17 28 100

20 12

6 22

a

Details on the reactions are given in the Supporting Information. b11B and 19F NMR spectra were recorded in aqueous solution after removal of all volatiles of the respective reaction mixture in a vacuum. c3n([BF4]−) is the number of fluorine substituents that have to be replaced upon conversion of [BF4]− to [BF(CN)3]−. dK[BF4] was incompletely dissolved in H2O (checked by 11B and 19F NMR spectroscopy). 3408


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Inorganic Chemistry

extractions of the respective cyanoborate with ether was secured with Quantofix Peroxide 100 test sticks (Machery-Nagel). Chemicals. All standard chemicals were obtained from commercial sources. (CH3)3SiCN and (CH3)3SiCl were obtained from Chemische Fabrik Karl Bucher GmbH (Germany) and Wacker Chemie AG (Germany), respectively, and used without further purification. BF3· CH 3 CN in acetonitrile was purchased from Sigma-Aldrich. (CH3)3Si13CN was synthesized from (CH3)3SiCl and K13CN (99% enrichment).30 Na[10BF4] and Na[11BF4] were prepared from the respective isotopically labeled boronic acids (99% enrichment) and NaHF2 in anhydrous HF (aHF). The isotopically labeled boronic acids were obtained from Katchem spol. s.r.o. (Prague, Czech Republic). Single-Crystal X-ray Diffraction. Colorless crystals of Na[BF(CN)3]·(CH3)3SiCN suitable for a X-ray diffraction study were obtained from the reaction of Na[BF4] with (CH3)3SiCN and (CH3)3SiCl as a catalyst upon cooling the reaction mixture to room temperature. A crystal was investigated with a Bruker X8-Apex II diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 100 K. Na[BF(CN)3]·(CH3)3SiCN crystallized in the orthorhombic space group Pnma (No. 62) with Z = 4, and unit-cell parameters of a = 14.6004(9), b = 8.8498(7), c = 10.0689(10), and V = 1301.01(18) Å3; ρcalcd = 1.175 Mg m−3, μ(Mo Kα) = 0.199 mm−1, F(000) = 472. A total of 14 585 reflections were collected (2.46° < θmax < 25.99°). The structure was solved by intrinsic phasing methods (SHELXT),31 and refinement was based on full-matrix least-squares calculations on F2 (SHELXL) with 1364 independent reflections [1272 independent reflections with I > 2σ(I)].31b,32 All non-hydrogen atoms were refined anisotropically. For CH, idealized bond lengths and angles were used. One of the methyl groups is located on a mirror plane and the hydrogen atoms were therefore disordered over two positions. The final refinement resulted in R1[F02 > 2σ(F02)] = 0.0228, wR2 = 0.0598 (all data), Δρmax/Δρmin = 0.302 and −0.209 e Å−3. Calculations were carried out using the SHELXLE graphical interface.33 Molecular structure diagrams were drawn with the program Diamond 3.2i.34 Supplementary crystallographic data for this publication are deposited in the Supporting Information or can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (CCDC No. 1027935). Quantum Chemical Calculations. Density functional calculations (DFT)35 were carried out using Becke’s three-parameter hybrid functional and the Lee−Yang−Parr correlation functional (B3LYP)36 using the Gaussian09 program suite.37 Geometries were optimized, and energies were calculated with the 6-311++G(d,p) basis sets. Diffuse functions were incorporated because improved energies are obtained for anions.38 Structures represent true minima with no imaginary frequency on the respective hypersurface. DFT-GIAO39 NMR shielding constants σ(11B), σ(13C), and σ(19F) were calculated at the B3LYP/6-311++G(2d,p) level of theory using the geometries computed as described. The 11B, 13C, and 19F NMR shielding constants were calibrated to the respective chemical shift scale δ(11B), δ(13C), and δ(19F) using predictions on diborane(6), Me4Si, and CFCl3 with chemical shifts of 16.6 ppm for B2H640 and 0 ppm for Me4Si as well as for CFCl3.41 Spin−spin coupling constants were calculated at the same level as the NMR shielding constants. Calculations of all NMR parameters were performed with the Gaussian09 program suite.37 Transition states exhibit one imaginary frequency, and IRC calculations were performed to verify that the transition states connect products and reactants, respectively.42 Preparation of K[BF(CN)3] in a Closed Reaction Flask from Na[BF4]: Method A. A 2-L round-bottom flask equipped with a valve with a PTFE stem and fitted with a gas-tight magnetic stirrer head (Bola, Germany) and a PTFE-jacketed stirrer shaft was charged with Na[BF4] (120.0 g, 1.09 mol), (CH3)3SiCN (1.3 L, 9.75 mol), and (CH3)3SiCl (0.1 L, 0.79 mol). The reaction mixture was stirred for 1 h at 50 °C (temperature of the heat-on attachment of the magnetic stirrer (Heidolph, Germany)). Subsequently, the overpressure was released, the vessel was closed, and the reaction mixture was heated to 100 °C (temperature of the heat-on attachment of the magnetic stirrer (Heidolph, Germany)) for 6 h. Upon cooling to room temperature,

K[BF4] in the reaction mixtures of the experiments listed in Table 2. After the first two exchange steps to yield M[BF2(CN)2], a strong countercation effect becomes obvious for the further replacement of fluorine against a cyano group. In the case of [Et4N][BF2(CN)2] and Li[BF2(CN)2], no further exchange at 80 °C was observed, whereas in the presence of Na+ and K+, the reaction continued. The interaction of Na+ and K+ with the fluorine substituents of the anions seems to facilitate the exchange via electrophilic assistance26 (B−F···M+). Surprisingly, Li[BF2(CN)2] is less reactive toward a further exchange than Na[BF2(CN)2] and K[BF2(CN)2], although the Li+ cation should interact more strongly with the fluorine substituents. Probably, the Lewis acidity of Li+ is reduced due to the coordination with N-donor atoms of the cyanoborate anions as well as the (CH3)3SiCN molecules. In the case of Na[BF4], the degree of conversion (Table 2) was much higher, compared to K[BF4], which may be related, at least in part, to the very low solubility of K[BF4] in (CH3)3SiCN. Hence, the Na + cation and therefore Na[BF4 ] provides the best compromise in solubility and electrophilic assistance.

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SUMMARY AND CONCLUSIONS The alkali-metal tricyanofluoroborates M[BF(CN)3] (M = Na, K) have become easily accessible on a large scale (125 g) and in high purity (99.9%). The high yield syntheses employ Na[BF4] and (CH3)3SiCN as starting materials and (CH3)3SiCl as a catalyst. The catalytic effect of trimethylsilyl chloride was unambiguously proven. Furthermore, it was shown that the countercation (Li+, Na+, K+, and [Et4N]+) has a strong influence on the reaction and best results were obtained with Na[BF4]. The new isocyanoborate anion [BF(NC)(CN)2]− was identified as an intermediate in the formation of the [BF(CN)3]− anion. K[BF(CN) 3] is a valuable starting material for the preparation of low-viscosity room-temperature ILs3,4 and for the potassium salt of the boron-centered nucleophile, the tricyanoborate dianion, B(CN)32−.27

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EXPERIMENTAL SECTION

General Methods. 1H, 10B, 11B, 13C, 19F, and 29Si NMR spectra were recorded at 25 °C in (CD3)2CO on a Bruker Avance 200 NMR spectrometer, a Bruker Avance 500 NMR spectrometer, or a Bruker Avance III HD 300 NMR spectrometer. The NMR signals were referenced against TMS (1H and 13C), BF3·OEt2 in CDCl3 with Ξ(11B) = 32.083974 MHz and Ξ(10B) = 10.743658 MHz, as well as CFCl3 with Ξ(19F) = 94.094011 MHz as external standards.28 1H and 13 C chemical shifts were calibrated against the residual solvent signal and the solvent signal, respectively (δ(1H): (CD3)(CD2H)CO 2.05 ppm; δ(13C): (CD3)2CO 206.26 and 29.84 ppm.29 IR spectra were recorded at room temperature with a Bruker Alpha spectrometer with an apodized resolution of 1 cm−1 in the attenuated total reflection (ATR) mode in the region of 4000−500 cm−1, using either a setup with a diamond or a Ge crystal. Raman spectra were recorded at room temperature on a Bruker IFS-120 spectrometer with an apodized resolution of 2 cm−1 using the 1064 nm excitation line of a Nd/YAG laser on crystalline samples contained in melting point capillaries in the region of 3500−100 cm−1. Elemental analysis (C, H, N) were performed with a Euro EA3000 instrument (HEKA-Tech, Germany). All reactions were performed in standard glassware. DSC measurements were performed with a TA DSC Q1000 V 8.1 instrument (TA Instruments) and a heating rate of 10 K min−1. Excess (CH3)3SiCN as well as the (CH3)3SiCl were recovered from the reaction mixtures and reused in subsequent reactions after removal of most of the byproduct (CH3)3SiF. Complete removal of H2O2 during workup before 3409


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Inorganic Chemistry crystalline Na[BF(CN)3]·(CH3)3SiCN formed, and this was filtered off. The solvate molecules were removed in a vacuum at 80 °C overnight. The crude product was dissolved in H2O (50 mL) and the solution was gradually treated with an aqueous solution of H2O2 (35% v/v, 50 mL) and K2CO3 (150 g) until the red solution decolorized. Excess H2O2 was destroyed by the addition of K2S2O3. The aqueous layer was extracted with THF (10 × 80 mL). The combined organic layers were dried with K2CO3, filtered, and the solution was concentrated to a volume of ∼50 mL. Dichloromethane (200 mL) was added, which resulted in the formation of a colorless precipitate that was collected by filtration and dried in a vacuum. Yield: 140.6 g (0.96 mol, 88%, purity according to 11B NMR spectra 99.9%). 13C NMR ((CD3)2CO, δ ppm): 126.8 (s, 3C, 1J(13C,11B) = 74.7 Hz, 1 13 10 J( C, B) = 25.0 Hz, 2J(19F,13C) = 37.3 Hz). 11B NMR ((CD3)2CO, δ ppm): −17.8 (d, 1B, 1J(19F,11B) = 44.1 Hz, 1J(13C,11B) = 74.7 Hz). 10 B NMR ((CD3)2CO, δ ppm): −17.8 (d, 1B, 1J(19F,10B) = 14.6 Hz, 1 13 10 J( C, B) = 25.0 Hz). 19F NMR ((CD3)2CO, δ ppm): −212.2 (q, 1F, 1 19 11 J( F, B) = 44.1 Hz, 1J(13C,11B) = 74.7 Hz, 1J(19F,10B) = 14.6 Hz, 2 19 13 J( F, C) = 37.3 Hz, 1Δ19F(10/11B) = 0.0951 ppm). Anal. Calcd for C3BFKN3: C, 24.52; N, 28.59. Found: C, 24.96; N, 28.18. A second fraction of K[BF(CN)3] was isolated from the filtrate, which was obtained after removal of the crystalline Na[BF(CN)3]· (CH3)3SiCN. The trimethylsilyl cyanide was removed under reduced pressure and the brownish semisolid remainder was purified and transferred to the potassium salt, as described for the workup of the crystalline Na[BF(CN)3]·(CH3)3SiCN. The purity of the K[BF(CN)3] was lower compared to the first fraction. Yield of the second fraction: 6.2 g (0.04 mol, 4%, purity according to 11B NMR spectra >99%). Na[BF(CN)3]·(CH3)3SiCN. The colorless crystalline (CH3)3SiCN solvate of Na[BF(CN)3] was isolated from a reaction mixture performed as described for the preparation of K[BF(CN)3], according to method A. Yield: 80%−90%. [The exact yield was not determined, because even careful drying resulted in partial removal of the solvate molecules.] Anal. Calcd for a large crystal of C7H9BFN4NaSi: C, 36.55; H, 3.94; N, 24.35. Found: C, 36.62; H, 3.69; N, 24.57. 1H NMR ((CD3)2CO, δ ppm): 0.02 (s, 9H, Si(CH3)3). 13C NMR ((CD3)2CO, δ ppm): 126.8 (s, 3C, 1J(13C,11B) = 74.7 Hz, 1J(13C,10B) = 25.0 Hz, 2 19 13 J( F, C) = 37.3 Hz), 129.7 (s, 1C, SiCN), 1.1 (s, 3C, Si(CH3)3). 11B NMR ((CD3)2CO, δ ppm): −17.8 (d, 1B, 1J(19F,11B) = 44.1 Hz). 19F NMR ((CD3)2CO, δ ppm): −213.1 (q, 1F, 1J(19F,11B) = 44.1 Hz). IR/ Raman (cm−1): 2234 (ν(BCN)), 2208 (ν(SiCN)), 1085/1046 (ν(B−F)). Na[BF(CN)3]. Na[BF(CN)3]·(CH3)3SiCN was heated to 80 °C in a vacuum overnight giving Na[BF(CN)3] in quantitative yield. Anal. Calcd for C3BFN3Na: C, 27.54; N, 32.11. Found: C, 27.91; N, 31.86. IR/Raman (cm−1): 2239 (ν(BCN)), 1084/1045 (ν(B−F)). Preparation of K[BF(CN)3] in a Closed Reaction Flask from Na[BF4]: Method B. A glass tube (50 mL) equipped with a valve with a PTFE stem and fitted with a magnetic stirring bar was charged with Na[BF4] (2.50 g, 22.77 mmol), (CH3)3SiCN (30.0 mL, 224.97 mmol), and (CH3)3SiCl (2.50 mL, 19.79 mmol). The reaction mixture was stirred at 50 °C (oil bath temperature) for 1 h and then the reaction vessel was vented. The vessel was closed again and the mixture was heated to 100 °C (oil bath temperature) for further 2 h. The mixture was kept at room temperature, all volatiles were removed under reduced pressure, and the light-red, solid residue was dissolved in a minimum amount of H2O (∼5 mL). The aqueous solution was treated with an aqueous solution of H2O2 (35% v/v, 1−2 mL) and K2CO3 (2−3 g) until the red solution became colorless. All volatiles were removed using a rotary evaporator. The remaining white solid was extracted with acetone (2 × 100 mL), the combined organic layers were dried with K2CO3, filtered, and concentrated to a volume of ∼15 mL. CH2Cl2 (20 mL) was added and a colorless precipitate formed that was collected by filtration and dried in a vacuum. Yield: 3.05 g (20.75 mmol, 91%, purity, according to 11B NMR spectra >98.5%). K[11BF(CN)3], K[10BF(CN)3], K[11BF(13CN)3], and K[10BF(13CN)3]. The isotopically enriched potassium tricyanofluoroborates have been obtained as described for K[BF(CN)3], according to method B. The

NMR spectroscopic data including isotopic shifts are listed, together with the NMR data of K[BF(CN)3] in the previous example. Preparation of K[BF(CN)3] from Na[BF 4] at Ambient Pressure. A 2-L three-necked round-bottom flask equipped with a dropping funnel with a valve with a PTFE stem, an inside thermometer, and a reflux condenser that was cooled to 25−35 °C was fitted with a magnetic stirring bar and charged with Na[BF4] (100.0 g, 0.91 mol), (CH3)3SiCN (1.2 L, 8.99 mol), and (CH3)3SiCl (100.0 mL, 0.79 mol). The heat-on attachment of the magnetic stirrer (Heidolph, Germany) was heated to 100 °C. The mixture was stirred for 5 h, and the temperature inside the vessel reached a maximum of 87 °C. Most of the (CH3)3SiF (Tbp = 16−18 °C) that was formed in the course of the reaction passed through the reflux condenser (25−35 °C). No significant loss of (CH3)3SiCl (Tbp = 56−57 °C) and (CH3)3SiCN (Tbp = 114−117 °C) was observed. The (CH3)3SiF was collected at 0 °C. After cooling of the reaction mixture to room temperature, all volatiles were distilled off. The reddish brown residue was dried at 50 °C under vacuum overnight. The brownish black solid remainder was dissolved in aqueous H2O2 (35% v/v, 100.0 mL) and K2CO3 (150 g) was added in portions. Excess H2O2 was quenched with K2S2O3 and the aqueous layer was subsequently extracted with THF (5 × 80 and 5 × 50 mL). The combined organic layers were dried with K2CO3, filtered, and the solution was concentrated to ∼40− 50 mL. The addition of CH2Cl2 (150 mL) resulted in the precipitation of a colorless solid that was collected by filtration and dried in a vacuum. Yield: 107.6 g (732 mmol, 80%, purity according to 11B NMR spectra >96%). Preparation of K[BF(CN)3] in a Closed Reaction Flask from K[BF2(CN)2]. A glass finger (50 mL) equipped with a valve with a PTFE stem and fitted with a magnetic stirring bar was charged with K[BF2(CN)2] (5.0 g, 35.73 mmol), (CH3)3SiCN (24.0 mL, 179.97 mmol), and (CH3)3SiCl (1.60 mL, 12.66 mmol). The reaction mixture was heated to 100 °C (oil bath temperature) for 3 h. All volatiles were removed under reduced pressure. The solid residue was dissolved in aqueous H2O2 (35% v/v, 20.0 mL) and solid K2CO3 (10 g) was added in small portions while stirring. After 1 h, all volatiles were removed using a rotary evaporator. The white solid remainder was extracted with THF (3 × 15 mL). The combined organic layers were dried with K2CO3, filtered, and concentrated to 3−4 mL. CHCl3 (30 mL) was added and immediately a colorless precipitate formed that was collected by filtration and dried in a vacuum. Yield: 4.4 g (29.94 mmol, 84%, purity according to 11B NMR spectra >96%). Preparation of K[BF2(CN)2]. The synthesis is a modification of a procedure described in the literature.2b,4 A round-bottom flask (250 mL) fitted with a magnetic stirring bar and a dropping funnel (50 mL) was charged with KCN (13.4 g, 205.8 mmol) and acetonitrile (25 mL) in an argon atmosphere. The suspension was cooled to 0 °C and a solution of BF3·CH3CN in acetonitrile (15.2−16.8% BF3, ∼2.05 mmol mL−1, 100 mL, ∼205 mmol) was added dropwise within 4 h. The yellow suspension was filtered at 0 °C. All volatiles were removed under reduced pressure, the yellow solid was taken up into aqueous H2O2 (35% v/v, 20.0 mL) at room temperature, and the solution was stirred for additional 4 h. All volatiles were removed with a rotary evaporator and the solid residue was extracted with CH3CN (2 × 100 mL). The combined organic phases were dried with K2CO3, filtered, and the solution was concentrated to a volume of 5 mL. Upon addition of CH2Cl2 (50 mL) light-yellow K[BF2(CN)2] precipitated that was filtered off and dried in a vacuum. Yield: 9.27 g (66.2 mmol, ∼65%, purity according to 11B and 19F NMR spectroscopy 95%). 13C NMR ((CD3)2CO, δ ppm): 129.7 (qt, 2C, 1J(13C,11B) = 80.8 Hz, 2 19 13 J( F, C) = 50.6 Hz, BCN). 11B NMR ((CD3)2CO, δ ppm): −7.3 (t, 1B, 1J(19F,11B) = 40.5 Hz). 19F NMR ((CD3)2CO, δ ppm): −154.7 (q, 2F, 1J(19F,11B) = 40.5 Hz). Preparation of K[BF(CN)3] from Na[BF4] and (CH3)3SiCN in the Presence of (CH3)3SiCl at Room Temperature. A glass tube (15 mL) equipped with a valve with a PTFE stem and fitted with a magnetic stirring bar was charged with Na[BF4] (110 mg, 1.01 mmol), (CH3)3SiCN (6.0 mL, 44.99 mmol), and (CH3)3SiCl (1.0 mL, 7.92 mmol). The reaction mixture was stirred at room temperature for 10 d and all volatiles were removed under reduced pressure. The remainder 3410


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Inorganic Chemistry

(2) (a) Hamilton, B. H.; Ziegler, C. J. Chem. Commun. 2002, 842− 843. (b) Bernhardt, E.; Berkei, M.; Willner, H.; Schürmann, M. Z. Anorg. Allg. Chem. 2003, 629, 677−685. (3) Ignat’ev, N.; Finze, M.; Sprenger, J. A. P.; Kerpen, C.; Bernhardt, E.; Willner, H. Submitted to J. Fluorine Chem. (4) Welz-Biermann, U.; Ignatiev, N. V.; Bernhardt, E.; Finze, M.; Willner, H. (Merck GmbH). Patent WO2004072089, 2004. (5) O’Regan, B.; Grätzel, M. Nature (London) 1991, 353, 737−740. (6) (a) Kuang, D.; Wang, P.; Ito, S.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Chem. Soc. 2006, 128, 7732−7733. (b) Kuang, D.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. (Ecole Polytechnique Federale de Lausanne (EPFL)). WO2007093961, 2007. (c) Marszalek, M.; Fei, Z.; Zhu, D.-R.; Scopelliti, R.; Dyson, P. J.; Zakeeruddin, S. M.; Grätzel, M. Inorg. Chem. 2011, 50, 11561−11567. (7) (a) Neves, C. M. S. S.; Kurnia, K. A.; Coutinho, J. A. P.; Marrucho, I. M.; Canongia Lopes, J. N.; Freire, M. G.; Rebelo, L. P. N. J. Phys. Chem. B 2013, 117, 10271−10283. (b) Koller, T.; Rausch, M. H.; Schulz, P. S.; Berger, M.; Wasserscheid, P.; Economou, I. G.; Leipertz, A.; Fröba, A. P. J. Chem. Eng. Data 2012, 57, 828−835. (c) Monta-Martinez, M. T.; Althuluth, M.; Kroon, M. C.; Peters, C. J. Fluid Phase Equilib. 2012, 332, 35−39. (d) Heitmann, S.; Krings, J.; Kreis, P.; Lennert, A.; Pitner, W. R.; Górak, A.; Schulte, M. M. Sep. Purif. Technol. 2012, 97, 108−114. (e) Twu, P.; Zhao, Q.; Pitner, W. R.; Acree, W. E., Jr.; Baker, G. A.; Anderson, J. L. J. Chromatogr. A 2011, 1218, 5311−5318. (f) Meindersma, G. W.; Hansmeier, A. R.; de Haan, A. B. Ind. Eng. Chem. Res. 2010, 49, 7530−7540. (g) Tong, J.; Liu, Q.-S.; Fang, Y.-X.; Fang, D.-W.; Welz-Biermann, U.; Yang, J.-Z. J. Chem. Eng. Data 2010, 55, 3693−3696. (h) Mahurin, S. M.; Lee, J. S.; Baker, G. A.; Luo, H.; Dai, S. J. Membr. Sci. 2010, 353, 177−183. (8) Barthen, P.; Frank, W.; Ignatiev, N. Ionics 2015, 21, 149−159. (9) Kawata, K.; Ignatyev, N.; Schulte, M.; Yoshizaki, H. (Merck GmbH). Patent WO2012041437A2, 2012. (10) (a) Berkei, M.; Bernhardt, E.; Schürmann, M.; Mehring, M.; Willner, H. Z. Anorg. Allg. Chem. 2002, 628, 1734−1740. (b) Küppers, T.; Bernhardt, E.; Willner, H.; Rohm, H. W.; Köckerling, M. Inorg. Chem. 2005, 44, 1015−1022. (c) Nitschke, C.; Köckerling, M. Inorg. Chem. 2011, 50, 4313−4321. (d) Yao, H.; Kuhlman, M. L.; Rauchfuß, T. B.; Wilson, S. R. Inorg. Chem. 2005, 44, 6256−6264. (e) Finze, M.; Bernhardt, E.; Willner, H.; Lehmann, C. W. Organometallics 2006, 25, 3070−3075. (11) (a) Tsao, H. N.; Comte, P.; Yi, C.; Grätzel, M. ChemPhysChem 2012, 13, 2976−2981. (b) Nazeeruddin, M.; Grätzel, M.; Baranoff, E.; Kessler, F.; Yum, J.-H.; Yella, A.; Tsao, H. N. (Ecole Polytechnique Federale de Lausanne (EPFL)). WO2012114315A1, 2012. (12) Ignatyev, N.; Schulte, M.; Kawata, K. (Merck GmbH). Patent WO2014082706A1, 2014. (13) Bernhardt, E.; Henkel, G.; Willner, H.; Pawelke, G.; Bürger, H. Chem.Eur. J. 2001, 7, 4696−4705. (14) Bernhardt, E.; Willner, H. Z. Anorg. Allg. Chem. 2009, 635, 2511−2514. (15) Rijksen, C.; Ott, L.; Sievert, K.; Harloff, J.; Schulz, A.; Ellinger, S. (Lonza, Ltd.). Patent WO2014029833A1, 2014. (16) Ellinger, S.; Sievert, K.; Harloff, J.; Schulz, A.; Rijksen, C.; Ott, L. (Lonza, Ltd.). Eur. Patent EP2772495A1, 2014. (17) Kawata, K.; Yoshizaki, H.; Shinohara, H.; Kirsch, P.; Ignatyev, N.; Schulte, M.; Sprenger, J.; Finze, M.; Frank, W. (Merck GmbH). Patent WO2011085965, 2011. (18) Ignatyev, N.; Sprenger, J. A. P.; Landmann, J.; Finze, M. (Merck GmbH). Patent WO2014198401, 2014. (19) Rasmussen, J. K.; Heilmann, S. M.; Krepski, L. R. Adv. Silicon Chem. 1991, 1, 65−187. (20) (a) Couzijn, E. P. A.; Slootweg, J. C.; Ehlers, A. W.; Lammertsma, K. Z. Anorg. Allg. Chem. 2009, 635, 1273−1278. (b) Dixon, D. A.; Hertler, W. R.; Chase, D. B.; Farnham, W. B.; Davidson, F. Inorg. Chem. 1988, 27, 4012−4018. (21) Soli, E. D.; Manoso, A. S.; Patterson, M. C.; DeShong, P.; Favor, A. D.; Hirschmann, R.; Smith, A. B., III. J. Org. Chem. 1999, 64, 3171− 3177.

was dissolved in H2O (5 mL) and the clear solution was investigated by 11B and 19F NMR spectroscopy. The mixture contained [BF(NC)(CN)2]− (96%). The 11B and 19 F NMR spectra are depicted in Figure 3. The NMR spectroscopic data of the [BF(NC)(CN)2]− anion are reported with the following example. NMR Study on the Formation of [BF(NC)(CN)2]−. A glass tube (40 mL) equipped with a valve with a PTFE stem and fitted with a magnetic stirring bar was charged with Na[BF4] (1.0 g, 9.10 mmol) and (CH3)3SiCN (20.0 mL, 149.98 mmol). The suspension was stirred at 80 °C (oil bath temperature) for 48 h. A sample of the suspension was dissolved in acetone to give a clear solution. According to the 11B and 19F NMR spectra, the [BF2(CN)2]− anion is the sole boron-containing species present. (CH3)3SiCl (0.5 mL, 3.96 mmol) was added and the reaction mixture was stirred for additional 18 h at 45 °C (oil bath temperature). A sample of the solution on top of the solid residue was separated and all volatiles were removed in a vacuum. The solid residue was investigated by 11B and 19F NMR spectroscopy in (CD3)2CO and it contains the anions [BF(NC)(CN)2]− (29%), [BF2(CN)2]− (27%), and [BF(CN)3]− (44%). The respective 11B, 19F, and 13C NMR spectra are depicted in Figure S12 in the Supporting Information. NMR spectroscopic data of [BF(NC)(CN)2]−: 13C NMR ((CD3)2CO, δ ppm): 170.8 (s, br, 1C, BNC), 127.4 (qd, 2C, 1 13 11 J( C, B) = 80.1 Hz, 2J(19F,13C) = 41.9 Hz, BCN). 11B NMR ((CD3)2CO, δ ppm): −15.7 (d, 1B, 1J(19F,11B) = 40.3 Hz). 19F NMR ((CD3)2CO, δ ppm): −183.5 (q, 1F, 1J(19F,11B) = 40.3 Hz).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and tables on comparative reactions of Cat[BF4] (Cat = Li, Na, K, [Et4N]) with (CH3)3SiCN, either in the absence or presence of (CH3)3SiCl and figures with the corresponding 11B and 19F NMR spectra; tables containing calculated energies and free energies of selected boron species, calculated energies on the isomerization of isocyanoborate anions to cyanoborate anions, F−, CN−, as well as NC− affinities, calculated and experimental bond parameters, vibrational spectroscopic data; figures of crude reaction products, the unit cell of Na[BF(CN)3]·(CH3)3SiCN, the DSC curve of Na[BF(CN)3]·(CH3)3SiCN, IR and Raman spectra of isotopically enriched K[BF(CN)3], NMR spectra of reaction mixtures containing [BF(NC)(CN)2]−; and crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: maik.fi[email protected]. Funding

Financial support by Merck KGaA (Darmstadt, Germany) is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. R. Bertermann (Julius-MaximiliansUniversität Würzburg) for technical support and helpful discussions.

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REFERENCES

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Syntheses of Tricyanofluoroborates M[BF(CN)3] (M = Na, K): (CH3)

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