Syntheses of tricyanofluoroborates M[BF(CN)3] (M = Na, K): (CH3)3SiCl catalysis, countercation effect, and reaction intermediates

Article abstract of DOI:10.1021/ic503077c

Potassium tricyanofluoroborate, K[BF(CN)₃], which is the starting material for tricyanofluoroborate room-temperature ionic liquids [N. Ignatev 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.

Full text of DOI:10.1021/ic503077c

Article
pubs.acs.org/IC
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 fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, 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 room-
temperature 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 charac-
terized 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.
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 poly-
mers. 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]-
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
(Gratzel cells5), they are of interest as components of the
̈
electrolyte, e.g., EMIM[B(CN)4] (EMIM = 1-ethyl-3-methyl-
imidazolium), 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 electro-
chemical 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
11
[B(CN)4]n 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
© 2015 American Chemical Society
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[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 identi-
fied as an intermediate in the formation of the [BF(CN)3]−
anion.
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 <3 bar.
Upon cooling to room temperature, almost colorless crystals of
Na[BF(CN)3]·(CH3)3SiCN formed in a yield of 80%−90%
that were separated from the reaction mixture by filtration (see
Figure S3 in the Supporting Information).
Na[BF(CN)3]·(CH3)3SiCN was characterized by multi-
nuclear NMR and vibrational spectroscopy as well as by
elemental analysis. A single crystal of Na[BF(CN)3]·
(CH3)3SiCN was studied by X-ray diffraction. In Figure 1,
the coordination environment of the Na+ cation in the
coordination polymer is depicted, and selected interatomic
distances and angles are listed in the figure caption. An
extended section of the solid-state structure and of the unit cell
is shown in Figure S4 in the Supporting Information. The bond
parameters of the [BF(CN)3]− anion in the crystal of
Na[BF(CN)3]·(CH3)3SiCN are similar to those reported for
the potassium salt2b,14 and M[BF(CN)3]·CH3CN (M = Cu,
Ag) earlier.2a Furthermore, they are close to values derived
from DFT calculations at the B3LYP/6-311+G(d) level of
theory (see Table S5 in the Supporting Information).
thermal properties of K[BF(CN)3] were studied in detail.1c,2b,14
The potassium salt melts at 215 5 °C and it dismutates at
245
5 °C to give a mixture of K[B(CN)4] and K[BF4],
according to differential scanning calorimetry (DSC) measure-
ments and 11B NMR spectroscopy.1c Tetraalkylammonium and
tetraalkylphosphonium tetrafluoroborates have been used as
starting materials for the respective tricyanofluoroborates as
depicted in Scheme 115 and [nBu4N][BF(CN)3] was converted
to Ag[BF(CN)3], which was used as starting material for the
preparation of K[BF(CN)3].16
Alkali metal tricyanofluoroborates are valuable starting
materials for the preparation of ILs4,17 and coordination
compounds with the [BF(CN)3]− anion. Since all known
syntheses to yield M[BF(CN)3] (M = alkali metal) have
drawbacks in that (i) they either require long reaction times or
(ii) laborious metathesis reactions are necessary, there is a
Solvate-free Na[BF(CN)3] was obtained via removal of the
(CH3)3SiCN at 80 °C overnight in a vacuum. According to
DSC measurements, the trimethylsilyl cyanide is lost at 80 °C
Scheme 2. Optimized Synthesis of Na[BF(CN)3]·(CH3)3SiCN in a Closed Vessel and Its Conversion into K[BF(CN)3]
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cyanofluoroborate to K[B(CN)4] and K[BF4] and related
reactions of other cyanofluoroborates was reported earlier, but
no yields were given.1c,2b
Na[BF(CN)3]·(CH3)3SiCN was converted to K[BF(CN)3]
by removal of the (CH3)3SiCN at 80 °C overnight in a vacuum
followed by an extraction procedure. Treatment with aqueous
hydrogen peroxide and K2CO3 in the course of the extraction
step ensured that trace amounts of impurities that were present
in the crystalline Na[BF(CN)3]·(CH3)3SiCN were removed
and up to 140 g of pure, white K[BF(CN)3] (99.9% purity
according to 11B NMR spectroscopy) was obtained in a yield of
88% (see Scheme 2).
From the dark-red mother liquor, which was obtained after
separation of the crystalline Na[BF(CN)3]·(CH3)3SiCN (see
Figure S3 in the Supporting Information), a second fraction of
K[BF(CN)3] was obtained in a yield of 4% after aqueous
workup with hydrogen peroxide and K2CO3. However, the
purity of the second fraction was slightly lower, compared to
the one of the first fraction (>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).
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
11B NMR spectroscopy. The conversion of potassium
The reaction of Na[BF4] with (CH3)3SiCN in the presence
of (CH3)3SiCl was also performed at ambient pressure in an
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].
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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).
this two-step reaction is the reduction of the amount of the
expensive (CH3)3SiCN.
The excess (CH3)3SiCN 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
11B 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).
Scheme 3. Two-Step Synthesis of K[BF(CN)3] from BF3·
CH3CN via K[BF2(CN)2]
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
K[BF2(CN)2] is easily accessible from BF3·CH3CN as
demonstrated herein or from BF3·OEt2.2b,4 The advantage of
a
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 h
Composition of the Borate Anions [mol %] Determined by 11B and 19F NMR
(CH3)3SiCl
n [mmol]
Spectroscopy
b
entry
V [mL]
mol % to 3n([BF4]−)
[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
1
14
29
21
17
2
5
96
74
39
3
25
64
83
7
12
a
b
Details on the reactions are given in the Supporting Information. 3n([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.
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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)
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%.
strong interaction between (CH3)3SiCN and (CH3)3SiCl,
because ν (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
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.
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 ν
̃
(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/6-
311+G(d) level of theory are in excellent agreement with the
experimental data (see Tables S5 and S6 in the Supporting
Information).
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
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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
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 temper-
atures 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.
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
ab
,
Table 2. Reactions of M[BF4] (1.1 mmol) and (CH3)3SiCN (4 mL) with (CH3)3SiCl at 80 °C for 2 h
(CH3)3SiCl
mol % to 3n([BF4]−)
Composition of the Borate Anions [mol %]
c
salt
V [mL]
[BF4]−
[BF2(CN)2]−
[BF(NC)(CN)2]−
[BF(CN)3]−
d
K[BF4]
0.2
0.2
0.2
49
49
49
57
38
17
28
20
12
6
Na[BF4]
Li[BF4]
22
100
a
b
Details on the reactions are given in the Supporting Information. 11B and 19F NMR spectra were recorded in aqueous solution after removal of all
c
volatiles of the respective reaction mixture in a vacuum. 3n([BF4]−) is the number of fluorine substituents that have to be replaced upon conversion
d
of [BF4]− to [BF(CN)3]−. K[BF4] was incompletely dissolved in H2O (checked by 11B and 19F NMR spectroscopy).
3408
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Article
extractions of the respective cyanoborate with ether was secured
with Quantofix Peroxide 100 test sticks (Machery-Nagel).
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.
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·
CH3CN 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
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.
2
final refinement resulted in R1[F02 > 2σ(F0 )] = 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).
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
Quantum Chemical Calculations. Density functional calcula-
tions (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
27
2−
tricyanoborate dianion, B(CN)3
.
EXPERIMENTAL SECTION
■
1
General Methods. H, 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
13C 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 measure-
ments 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
40
CFCl3 with chemical shifts of 16.6 ppm for B2H6 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,
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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,
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[BF4] 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%).
2
1J(13C,10B) = 25.0 Hz, J(19F,13C) = 37.3 Hz). 11B NMR ((CD3)2CO,
1
1
δ ppm): −17.8 (d, 1B, J(19F,11B) = 44.1 Hz, J(13C,11B) = 74.7 Hz).
10B NMR ((CD3)2CO, δ ppm): −17.8 (d, 1B, J(19F,10B) = 14.6 Hz,
1
1J(13C,10B) = 25.0 Hz). 19F NMR ((CD3)2CO, δ ppm): −212.2 (q, 1F,
1
1
1J(19F,11B) = 44.1 Hz, J(13C,11B) = 74.7 Hz, J(19F,10B) = 14.6 Hz,
1
2J(19F,13C) = 37.3 Hz, Δ19F(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,
1
1
δ ppm): 126.8 (s, 3C, J(13C,11B) = 74.7 Hz, J(13C,10B) = 25.0 Hz,
2J(19F,13C) = 37.3 Hz), 129.7 (s, 1C, SiCN), 1.1 (s, 3C, Si(CH3)3). 11
B
NMR ((CD3)2CO, δ ppm): −17.8 (d, 1B, J(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)).
1
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
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
1
NMR ((CD3)2CO, δ ppm): 129.7 (qt, 2C, J(13C,11B) = 80.8 Hz,
2J(19F,13C) = 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,
1
2F, J(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
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Article
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]− (<4%) and [BF(CN)3]− (>96%). The 11B and
19F NMR spectra are depicted in Figure 3. The NMR spectroscopic
data of the [BF(NC)(CN)2]− anion are reported with the following
example.
(2) (a) Hamilton, B. H.; Ziegler, C. J. Chem. Commun. 2002, 842−
843. (b) Bernhardt, E.; Berkei, M.; Willner, H.; Schurmann, 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.;
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,
1J(13C,11B) = 80.1 Hz, 2J(19F,13C) = 41.9 Hz, BCN). 11B NMR
Willner, H. (Merck GmbH). Patent WO2004072089, 2004.
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̈
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1
((CD3)2CO, δ ppm): −15.7 (d, 1B, J(19F,11B) = 40.3 Hz). 19F NMR
1
((CD3)2CO, δ ppm): −183.5 (q, 1F, J(19F,11B) = 40.3 Hz).
ASSOCIATED CONTENT
* Supporting Information
■
S
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, vibra-
tional 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 isotopi-
cally 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.finze@uni-wuerzburg.de.
(13) Bernhardt, E.; Henkel, G.; Willner, H.; Pawelke, G.; Burger, H.
̈
■
Chem.Eur. J. 2001, 7, 4696−4705.
(14) Bernhardt, E.; Willner, H. Z. Anorg. Allg. Chem. 2009, 635,
2511−2514.
Funding
(15) Rijksen, C.; Ott, L.; Sievert, K.; Harloff, J.; Schulz, A.; Ellinger, S.
(Lonza, Ltd.). Patent WO2014029833A1, 2014.
Financial support by Merck KGaA (Darmstadt, Germany) is
gratefully acknowledged.
(16) Ellinger, S.; Sievert, K.; Harloff, J.; Schulz, A.; Rijksen, C.; Ott, L.
(Lonza, Ltd.). Eur. Patent EP2772495A1, 2014.
Notes
(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.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank Dr. R. Bertermann (Julius-Maximilians-
■
Universitat Wurzburg) for technical support and helpful
̈
̈
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