Tetracyanidoborates with sterically demanding phosphonium cations - Thermally resistant ionic liquids

Article abstract of DOI:10.1002/zaac.200900469

The reaction of K[B(CN)₄] with [Ph₄P]Cl, [nBu ₄P]Br, [nBuPh₃P]Br, or [EtPh₃P]Cl (Ph = phenyl, nBu = n-butyl, Et = ethyl) affords [Ph₄P][B(CN)₄] (1), [nBu₄P] [B(CN)₄] (2), [EtPh₃P][B(CN) ₄] (3), and [nBuPh₃P] [B(CN)₄] (4) in high yields. The structures of the compounds 1-3, which contain the tetrahedral [B(CN)₄]^- ion, were determined by single-crystal X-ray diffraction. 1 crystallizes in the tetragonal space group I42d (no. 122), a = 12.948(2), c = 29.382(6) A, Z = 8, R1(F)/wR2(F²) = 0.0335/0.0953; 2 and 3 with orthorhombic symmetry; 2: space group Pnna (no. 52), a = 18.2187(6), b = 12.0707(3), c = 11.6704(4) A, Z= 4, R1(F)/wR2(F ²) = 0.0489/0.1532; 3: space group Pnma (no. 62), a = 14.1777(3), b = 13.4214(3), c = 12.3199(2) A, Z = 4, R1(F)/wR2(F²) = 0.0477/0.1505. The IR, Raman, and NMR spectra as well as thermal data derived from DSC and TGA measurements are reported. Depending on the thermal stability of the phosphonium cation some of the tetracyanidoborates are stable up to ～370 °C with the liquid state having a range of up to 300 K. Such salts could be very useful as weakly coordinating reaction media.

Full text of DOI:10.1002/zaac.200900469

ARTICLE
DOI: 10.1002/zaac.200900469
Tetracyanidoborates with Sterically Demanding Phosphonium Cations –
Thermally Resistant lonic Liquids
Anke Flemming,[a] Martina Hoffmann,[a] and Martin Köckerling*[a]
Dedicated to Professor Martin Jansen on the Occasion of His 65th Birthday
Keywords: Boron; Tetracyanidoborates; Phosphonium salts; X-ray diffraction; Vibrational spectroscopy; Thermal data
Abstract. The reaction of K[B(CN)4] with [Ph4P]Cl, [nBu4P]Br, 12.0707(3), c = 11.6704(4) Å, Z = 4, R1(F)/wR2(F2) = 0.0489/0.1532;
[nBuPh3P]Br, or [EtPh3P]Cl (Ph = phenyl, nBu = n-butyl, Et = ethyl) 3: space group Pnma (no. 62), a = 14.1777(3), b = 13.4214(3), c =
affords [Ph4P][B(CN)4] (1), [nBu4P][B(CN)4] (2), [EtPh3P][B(CN)4] 12.3199(2) Å, Z = 4, R1(F)/wR2(F2) = 0.0477/0.1505. The IR, Raman,
(3), and [nBuPh3P][B(CN)4] (4) in high yields. The structures of the and NMR spectra as well as thermal data derived from DSC and TGA
compounds 1–3, which contain the tetrahedral [B(CN)4]– ion, were measurements are reported. Depending on the thermal stability of the
determined by single-crystal X-ray diffraction. 1 crystallizes in the te- phosphonium cation some of the tetracyanidoborates are stable up to
¯
tragonal space group I42d (no. 122), a = 12.948(2), c = 29.382(6) Å,
~370 °C with the liquid state having a range of up to 300 K. Such
Z = 8, R1(F)/wR2(F2) = 0.0335/0.0953; 2 and 3 with orthorhombic salts could be very useful as weakly coordinating reaction media.
symmetry; 2: space group Pnna (no. 52), a = 18.2187(6), b =
actions between the corresponding phosphonium halide and
potassium tetracyanidoborate, according to Scheme 1.
Introduction
In recent years the substance class of tetracyanidoborates,
salts with the [B(CN)4]– ion, has received intense interest in
science, applied research and technological development. Be-
cause of the presence of a weakly coordinating and chemically
as well as thermally stable anion, tetracyanidoborates are used
in Ionic Liquids [1–3], electrolytes [4, 5], especially in solar
cells [6–9], as precursors for the synthesis of new and unusual
materials [10–17], and for the modification of membranes [18–
21]. Since the improvement of the synthesis of K[B(CN)4] in
2003 [22] many interesting compounds with tetracyanidoborate
anions were synthesized. So far, examples include salts with
monovalent cations, e.g. alkali metal cations [23–26], Tl+,
[NH4]+, [nBu4N]+ and also some with transition metal cations
[27–30]. Many of them crystallize with fascinating structures.
In this contribution the syntheses, X-ray structures, vibra-
tional and NMR spectra, and thermophysical properties of tet-
racyanidoborates with phosphonium-based organic cations are
described and compared with other tetracyanidoborates.
Scheme 1. Reaction sequence for the synthesis of phosphonium tetra-
cyanidoborate salts
Using acetonitrile as solvent for the metathesis reaction, KCl
or KBr precipitates immediately. The alkyl-/arylphosphonium
tetracyanidoborate salts 1–4 are obtained from the remaining
solution in high purity and high yield by evaporation of the
solvent. They are easily soluble in polar organic solvents and
are stable when exposed to air.
Crystal Structures
The single-crystal X-ray structures of 1, 2, and 3 were estab-
lished by X-ray diffraction analysis. Crystal data and parame-
ters of the structure determinations are given in Table 1, and
selected bond lengths in Table 2. Suitable crystals were grown
by slow evaporation of the solvent from saturated solutions of
the salts in acetonitrile at room temperature. All three com-
pounds consist of isolated [B(CN)4]– ions and the correspond-
ing phosphonium cations.
Results and Discussion
Syntheses
The synthesis of the tetracyanidoborate salts with organic
phosphonium cations 1–4 is easily achieved by metathesis re-
* Prof. Dr. M. Köckerling
Fax: +49-381-498-6382
The tetraphenylphosphonium salt 1 crystallizes in the acen-
E-Mail: Martin.Koeckerling@uni-rostock.de
[a] Institut für Chemie
¯
tric tetragonal space group I42d with 8 formula units in the
Abteilung Anorganische Chemie/Festkörperchemie
Albert-Einstein-Str. 3a
unit cell. Both the phosphorus atom of the tetraphenylphospho-
18059 Rostock, Germany
nium cation and the boron atom of the [B(CN)4]– anion are
562
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Z. Anorg. Allg. Chem. 2010, 636, 562–568

Tetracyanidoborates with Phosphonium Cations – Thermally Resistant lonic Liquids
Table 1. Crystal, X-ray diffraction data, data collection and refinement parameters for 1, 2, and 3.
Compound
1
2
3
Formula
C28H20BN4P
454.26
C20H36BN4P
374.31
C24H20BN4P
406.22
Fw /g·mol–1
T /K
293(2)
298(2)
293(2)
Crystal system
Space group
a /Å
tetragonal
orthorhombic
Pnna, no.52
18.2187(6)
12.0707(3)
11.6704(4)
2566.5(2), 4
0.969
orthorhombic
Pnma, no. 62
14.1777(3)
13.4214(3)
12.3199(2)
2344.29(8), 4
1.151
¯
I42d, no. 122
12.948(2)
12.948(2)
29.382(6)
4925(2), 8
1.225
b /Å
c /Å
v /Å3, Z
Dcalc. /g·cm–3
µ /mm–1
0.135
0.116
0.134
2θ range /°
Collected reflections
Unique reflections, Rint
Variables
5.22 to 68.74
78826
4.14 to 53.04
31042
4.48 to 60.16
57553
5373, 0.0401
156
2645, 0.0444
192
3574, 0.0286
155
Goof on F2
R1, wR2 [I>2θ(I)]a), b)
R1, wR2 (all data)a), b)
1.052
1.087
1.090
0.0334, 0.0846
0.0477, 0.0952
0.0566, 0.44
0.297, –0.182
0.0489, 0.1402
0.0677, 0.1532
0.0904, 0.19
0.305, –0.142
0.0477, 0.1395
0.0607, 0.1505
0.0780, 0.34
0.256, –0.240
0.015(2)
b)
A, R
Res. dens. / e·A–3
Extinction coefficient
a) R1 = Σ||Fo|–|Fc|| / Σ|Fo|; b) wR2 = √ Σ{w(Fo –Fc ) } / Σ{w(Fo ) }; w = 1/[(σ2(Fo )+(A·P)2+B·P]; P = (Fo + 2Fc )/3
2
2 2
2 2
2
2
2
Table 2. Selected bond lengths /Å for 1, 2, and 3. For 2 only the bond lengths of the major orientation of the disordered cation, labelled with
A are given.
1
2
3
atoms
bond length
1.7906(9)
1.7958(9)
1.793
atoms
bond length
1.796(2)
1.790(2)
1.793
atoms
bond length
1.796(2)
1.793(2)
1.797(2)
1.795
1.131(3)
1.125(2)
1.130(4)
1.129
1.591(3)
1.580(2)
1.585(4)
1.585
1.387(2)
1.392(2)
1.389(2)
1.367(3)
1.352(3)
1.392(3)
1.394(2)
1.384(2)
1.378(2)
1.382
P1–C3
P1A–C3A
P1A–C7A
average P–C
C1–N1
P1–C4
P1–C9
P1–C10
P1–C14
average P–C
C1–N1
average P–C
C1–N1
1.141(2)
1.146(2)
1.1435
1.135(2)
1.136(2)
1.136
C2–N2
C2–N2
average C–N
B1–C1
average C–N
B1–C1
C2–N2
C3–N3
1.596(2)
1.580(2)
1.588
1.579(3)
1.581(2)
1.580
1.532(4)
1.533(6)
1.483(5)
1.510(4)
1.508(4)
1.469(5)
1.506
B1–C2
B1–C2
average C–N
B1–C1
average B–C
C3–C8
average B–C
C3A–C4A
C4A–C5A
C5A–C6A
C7A–C8A
C8A–C9A
C9A–C10
average C–C
1.392(2)
1.397(2)
1.385(2)
1.388(2)
1.377(2)
1.387(2)
1.395(2)
1.401(2)
1.392(2)
1.381(2)
1.391(2)
1.384(2)
1.389
B1–C2
B1–C3
C3–C4
C4–C5
average B–C
C4–C5
C5–C6
C6–C7
C4–C9
C7–C8
C5–C6
C9–C10
C9–C14
C10–C11
C11–C12
C12–C13
C13–C14
average C–C
C6–C7
C7–C8
C8–C9
C10–C11
C11–C12
C12–C13
average C–C
(within the phenyl rings)
(within the phenyl rings)
tetrahedrally coordinated. Instead of being located on the 4a Further rows of ions are shifted by ½ in the a and b direction
¯
or 4b Wyckoff sites of 4 symmetry, they are arranged on the of the unit cell (along 0,½,z; ½,0,z; ½,½,z and so on). Along
8c site of twofold rotational symmetry. The structure of both the c direction each row is shifted with respect to the four
ions is shown in Figure 1 as thermal ellipsoid plot with the neighboring rows by ¼. This arrangement of ions is shown
atom numbering scheme included. Within the unit cell the bo- in Figure 2. Along the crystallographic c direction the se-
ron and phosphorus atoms (and thereby the [B(CN)4]– anions quence of the aligned discrete ions is ···cation–cation–anion–
and the [Ph4P]+ cations) are aligned in rows along the cell anion···. An interesting fact about the environment of each cat-
¯
edges parallel to the crystallographic c axis along the 4 axis. ion (and of each anion vice versa) is that it consists not only
Z. Anorg. Allg. Chem. 2010, 562–568
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A. Flemming, M. Hoffmann, M. Köckerling
ARTICLE
of anions (cations, respectively) as could be expected for a [B(CN)4]– ion is given in Figure 4. The carbon atoms of the
“normal” ionic salt, but of three anions and three other cations n-butyl chains of the cation are disordered and refined with
(and as necessary for a cation-anion ratio of 1:1, the same type two orientations A and B, which are shown in Figure 5. The
of environment is found for the tetracyanidoborate anions). occupation of the orientation A refines to 83 % (and B to the
The anions are separated from each other with B···B distances remaining 17 %). Both ions of 2 are located on sites within
of 6.490 Å (2x) and 7.808 Å (1x), whereas the anion···cation the unit cell, which have twofold rotational symmetry (Wyck-
distances (B···P) are 6.494 Å (2x) and 6.837 Å. This structural off site 4d of Pnna).
situation needs of course to be taken into account considering
the melting point of this compound (see below).
Figure 3. Packing of the [nBu4P]+ and the [B(CN)4]– ions in crystals
of 2. Only the orientation with the larger occupation of the disordered
n-butyl chains is shown. The anions are shown in a polyhedral repre-
sentation.
Figure 1. Structure of the [Ph4P]+ and the [B(CN)4]– ion in crystals of
1 with atom numbering scheme. Atomic displacement ellipsoids are
shown at the 50 % probability level.
Figure 4. Structure of the [nBu4P]+ and the [B(CN)4]– ion in crystals
of 2 with atom numbering scheme. Atomic displacement ellipsoids are
Figure 2. Packing of the tetrahedral [Ph4P]+ and [B(CN)4]– ions in the
shown at the 50 % probability level, hydrogen atoms are omitted for
unit cell of 1. Hydrogen atoms are omitted for clarity. The environment
clarity. Only one out of two orientations of the disordered n-butyl
of the boron and the phosphorus atoms is shown in a polyhedral repre-
chains is shown.
sentation. The discrete ions are arranged along the horizontal c direc-
tion in rows of the sequence ···cation–cation–anion–anion··· .
With respect to the ionic environment the structure of
1 the environment of the ions in [EtPh3P][B(CN)4] (3) is similar to the structure of 2. Each an-
In contrast to
[nBu4P][B(CN)4] (2) is closer to the expectations for a “nor- ion is surrounded by five cations with P···B distances of 6.313
mal” salt. Each [nBu4P]+ ion is surrounded by five tetracyani- (1x), 6.364 (1x), 6.729 (2x), and 7.116 Å (1x). The cations are
doborate anions and vice versa with P···B distances of 6.042 Å separated from each other by a P···P length of 7.187 Å. A
(2x), 6.204 Å (1x), and 6.518 Å (2x). The B···B and P···P dis- packing plot of the ions is shown in Figure 6. Figure 7 gives
tances are significantly longer and measure 8.795 Å (4x), and a thermal ellipsoid plot of both the cation and the anion. In the
8.860 Å (2x), respectively. The packing of the ions in crystals unit cell both ions are located on sites with local mirror sym-
of 2 is shown in Figure 3. Compound 2 crystallizes in the or- metry (Wyckoff site 4c). The ethyl group of the cation is disor-
thorhombic space group Pnna with 4 formula units in the unit dered on both sides of the mirror plane with half occupation,
cell. A thermal ellipsoid plot of the [nBu4P]+ and the consistent with the mirror symmetry.
564
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Tetracyanidoborates with Phosphonium Cations – Thermally Resistant lonic Liquids
Figure 7. Structure of the [EtPh3P]+ and the [B(CN)4]– ion in crystals
of 3 with atom numbering scheme. Atomic displacement ellipsoids are
shown at the 50 % probability level. Only one of the two orientations
of the disordered ethyl group is shown.
Figure 5. View of the disordered n-butyl chains in the [nBu4P]+ cation
of 2. The minor orientation B (17 %) of carbon atoms is marked with
striped circles.
Vibrational Spectroscopy
The data of the IR and Raman spectra of the four title com-
pounds, which show typical bands of both the cations and the
anions, are compiled in the experimental section. The charac-
teristic CN stretching mode of the [B(CN)4]– anion typically
appears between 2220 and 2295 cm–1 [26]. The position of the
CN stretching mode depends strongly on the chemical environ-
ment of the CN groups. Both the IR and the Raman spectra of
the four title compounds show one peak around 2222 cm–1,
which indicates that the environment of all four CN groups of
the tetracyanidoborate anions is similar. These values agree
with the one found for [nBu4N][B(CN)4] [26]. Compared to
tetracyanidoborates with metal cations, e.g. Co[B(CN)4]·2H2O
{(νCN, Raman) = 2270 cm–1 and (νCN, IR) = 2275 cm–1 [30]},
the bands of the CN stretching mode are not shifted to higher
wavelengths, which indicates that only weak interionic con-
tacts exist between the cations and the anions.
NMR Spectroscopy
All four analyzed substances contain anions and cations with
NMR active cores. The signal of the carbon atoms of the tetra-
cyanidoborate anion of the investigated compounds appears in
the 13C NMR spectrum at 123.2 ppm. It is split into a quartet
(J ≈ 70.9 Hz) because of the coupling with 11B. Also a septet
should be visible as a result of the coupling between 13C and
10B [26], but the intensity apparently is too weak and the septet
is therefore not observed.
Figure 6. Packing of the [EtPh3P]+ and the [B(CN)4]– ions in crystals
of 3. The anions are shown in a polyhedral representation.
The bond lengths within the ionic units of the three com-
pounds are all within the estimated range. The angles around
the boron atoms of the tetracyaniodoborate anions and around
the phosphorus atoms of the phosphonium cations are all close
Thermal Properties
Thermal data of the title compounds have been measured
to tetrahedral, as expected. The average B···C distances using DSC (differential scanning calorimetry) and TG (thermal
(1.588 Å in 1, 1.580 Å in 2, and 1.585 Å in 3) compare well gravimetric) analyses in the temperature range from room tem-
with those in other tetracyanidoborates, where values between perature to 800 °C. The curves of the four title compounds
1.581 Å {Tl[B(CN)4]} and 1.595 Å {K[B(CN)4]} are found are similar. As a typical example the DSC and TG curve of
[24, 26]. The same holds for the other bond lengths.
[Ph4P][B(CN)4] (1) is shown in Figure 8. The TG curves in
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A. Flemming, M. Hoffmann, M. Köckerling
ARTICLE
general show no mass loss up to a temperature around 400 °C hardly visible, very faint yellow color. This indicates good
and the DSC curves an endothermic peak below 200 °C for 1 thermal stability.
and below 100 °C for 2 to 4, that corresponds to the melting
points. The mass loss above ~400 °C is accompanied by exo-
Experimental Section
thermic effects (DSC curves) indicating that the compounds
decompose at this temperature. The temperatures of the onset
of the exothermic effects are interpreted as decomposition tem-
peratures, whereas the temperatures of the onset of the endo-
thermic effects are annotated as melting points. Table 3 sum-
marizes the results of the thermal analyses. We assume that the
thermal stability of the title compounds is limited by that of
the organic phosphonium cations and not by the stability of
the [B(CN)4]– ion due to the fact that the decomposition of
tetracyanidoborates with alkaline metal cations occurs above
500 °C [24, 26]. Whereas the tetraphenylphosphonium salt 1
melts at a comparatively high temperature 2, 3, and 4 melt
below 100 °C and have temperature ranges of the liquid state
of almost 300 K. This means that these three compounds might
be used as Ionic Liquids containing the weakly coordinating
[B(CN)4] anion within a temperature range from ~100 °C to
~350 °C. In this temperature range, only a very limited number
of polar solvents is available.
Materials
[Ph4P]Cl (Chempur, 98+%), [nBu4P]Br (ABCR, 98 %), [EtPh3P]Cl
(Acros Organics, 98 %), and [nBuPh3P]Br (Alfa Products, 98 %) were
used as received. CH3CN and Et2O were freshly distilled before use.
K[B(CN)4] was synthesized by means of a high temperature process
according to reference [22].
Syntheses
K[B(CN)4] (250 mg 1.624 mmol) and the equivalent amount of the
respective phosphonium halide {[Ph4P]Cl (608.6 mg), [nBu4P]Br
(551.0 mg), [EtPh3P]Cl (530.6 mg), or [nBuPh3P]Br (648.4 mg)} were
each dissolved in CH3CN (15 mL). After fast combination of both
solutions colorless KCl or KBr precipitated immediately. The mixture
was stirred for 15 minutes and subsequently centrifuged (15 min,
4000 rpm). The solution was decanted and the solvent was evaporated
by means of a rotating evaporator. The products 1–4, which were ob-
tained as colorless solids, were washed with Et2O and dried under
reduced pressure at room temperature. In order to obtain single crystals
suitable for X-ray structure determination and also pure material for
the investigation of their properties, the compounds were recrystallized
from CH3CN.
Tetraphenylphosphonium tetracyanidoborate (1): Yield: 657 mg,
89 %, m. p. 193 °C. C28H20BN4P: calcd. (%) C 74.03, H 4.44, N
12.33; found C 73.85, H 4.23, N 12.80. 1H NMR (500 MHz, CD3CN):
δ = 7.95–7.90 (m, 1 H, H–Cpara), 7.78–7.72 (m, 2 H, H–Cmeta), 7.72–
7.66 (m, 2 H, H–Cortho). 13C NMR (126 MHz, CD3CN): δ = 136.5 (d,
Ph–Cpara, JP,C ≈ 3.0 Hz), 135.7 (d, Ph–Cortho, JP,C ≈ 10.1 Hz), 131.2 (d,
Ph–Cmeta, JP,C ≈ 12.9 Hz), 118.6 (d, Ph–CP, JP,C ≈ 88.9 Hz), 123.0 (q,
C–N, J ≈ 70.6 Hz). 31P NMR (202 MHz, CD3CN): δ = 23.6. 11B NMR
(160 MHz, CD3CN): δ = –39.0. IR (ATR, 25 °C): ν = 618 (w), 687
(s), 718 (s), 762 (s), 816 (w), 837 (w), 857 (w), 932 (s), 967 (m), 996
(m), 1026 (w), 1106 (s), 1165 (w), 1187 (w), 1320 (w), 1341 (w),
1406(w), 1435 (s), 1487 (w), 1588 (w), 1622 (w), 2226 (w), 2324 (w),
2355 (w), 2997 (w), 3011 (w), 3057 (w), 3082 (w), 3117 (w), 3173
(w) cm–1. Raman (25 °C): ν = 112 (s), 113 (s), 114 (s), 206 (w), 253
(w), 287 (vw), 491 (w), 520 (vw), 618 (w), 681 (w), 937 (vw), 985
(w), 1004 (s), 1031 (w), 1103 (w), 1166 (w), 1195 (w), 1575 (w), 1589
(m), 2226 (s), 2963 (vw), 3000 (vw), 3014 (vw), 3072 (vs), 3120 (vw),
3149 (w), 3174 (vw) cm–1.
Figure 8. DSC and TG curve of [Ph4P][B(CN)4] (1) in the temperature
range from room temperature to 800 °C (solid line DSC, dotted line
TG).
Table 3. Thermal data for 1, 2, 3, and 4.
m.p. /°C decomp.
temp. /°C
temp. range of the
liquid state /K
Tetra-n-butylphosphonium tetracyanidoborate (2): Yield: 553 mg,
91 %, m. p. 73 °C. C20H36BN4P: calcd. (%) C 64.18, H 9.69, N 14.97;
1
[Ph4P][B(CN)4] (1)
[nBu4P][B(CN)4] (2)
193
73
390
372
370
374
197
299
272
301
found C 64.14, H 9.70, N 14.93. H NMR (300 MHz, CD3CN): δ =
2.18–1.98 (m, 2 H, H–Ca (C3)), 1.58–1.38 (m, 4 H, H–Cb (C4),H–Cc
(C5)), 0.99–0.91 (t, 3 H, H–Cd (C6)). 13C NMR (63 MHz, CD3CN):
δ = 123.2 (q, C–N, J ≈ 70.9 Hz), 13.5 (s, Cd (C6)), 18.8 (d, Ca (C3),
J ≈ 48.1 Hz), 23.8 (d, Cc (C5), J ≈ 4.6 Hz), 24.4 (d, Cb (C4), J ≈
15.6 Hz). 31P NMR (121 MHz, CD3CN): δ = 33.7. 11B NMR
(160 MHz, CD3CN): δ = –39.0. IR (ATR 25 °C): ν = 701 (m), 716
(m), 751 (w), 776 (w), 801 (m), 818 (m), 839 (w),909 (s),934 (vs),
967 (m), 998 (m), 1052 (w), 1084 (m), 1102 (m), 1190 (w), 1210 (w),
1227 (w), 1241 (w), 1283 (w), 1316 (w), 1347 (w), 1381 (w), 1404
(w), 1457 (m), 1466 (m), 2222 (w), 2874 (m), 2936 (m), 2963 (m)
[EtPh3P][B(CN)4] (3) 98
[nBuPh3P][B(CN)4] (4) 73
The long term thermal stability of Ionic Liquids is difficult
to be estimated from fast DSC and TG scans, as shown for
example in ref. [33]. Therefore further DSC/TG scans were
conducted with a sample of 1, which was heated above the
melting point at 309 °C for 4 hours. No mass loss was ob-
served. After cooling to room temperature, the sample had a cm–1. Raman (25 °C): ν = 121 (w), 146 (w), 244 (vw), 487 (vw), 522
566
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Z. Anorg. Allg. Chem. 2010, 562–568

Tetracyanidoborates with Phosphonium Cations – Thermally Resistant lonic Liquids
(vw), 672 (vw), 869 (vw), 890 (vw), 908 (vw), 938 (vw), 969 (vw), analyzer, which allows simultaneous thermogravimetric and DSC
1006 (vw), 1052 (w), 1100 (vw), 1203 (vw), 1230 (vw), 1280 (vw), measurements. Amounts of about 5–15 mg of the carefully dried sam-
1316 (vw), 1351 (vw), 1415 (vw), 1451 (w), 2223 (s), 2877 (s), 2915
ples were sealed in aluminum crucibles and studied in the temperature
range of 20 to 800 °C with heating rates of 5 °C·min–1. After determi-
nation of the decomposition temperatures further heating and cooling
runs were performed in the temperature range from 20 to 230 °C. Dur-
ing all measurements the furnace was flushed with dry argon. In order
to test for long term stability, a sample of the tetraphenylphosphonium
salt (1) was heated at 309 °C in the TG apparatus for 4 hours and
visually checked after cooling down to room temperature.
(vs), 2940 (s), 2965 (m) cm–1.
Ethyl-triphenylphosphonium tetracyanidoborate (3): Yield:
614 mg, 93 %, m. p. 98 °C. C24H20BN4P: calcd. (%) C 70.96, H 4.96,
N 13.79; found C 70.93, H 4.92, N 13.76. 1H NMR (250 MHz,
CD3CN): δ = 7.95–7.69 (m, Ph–H), 3.25 (m, 2 H, H–Ca (C14)), 1.34
(m, 3 H, H–Cb (C15)). 13C NMR (75 MHz, CD3CN): δ = 136.0 (d,
Ph–Cpara, JP,C ≈ 3.0 Hz), 134.6 (d, Ph–Cortho, JP,C ≈ 9.9 Hz), 131.2 (d,
Ph–Cmeta, JP,C ≈ 12.6 Hz), 123.2 (q, C–N, J ≈ 70.9 Hz), 119.0 (d, Ph–
CP), 16.8 (d, Ca (C14), J ≈ 53.2 Hz), 6.8 (d, Cb (C15), J ≈ 5.2 Hz).
31P NMR (202 MHz, CD3CN): δ = 25.5. 11B NMR (160 MHz,
CD3CN): δ = –39.0. IR (ATR 25 °C): ν = 614 (w), 668 (m), 689 (vs),
722 (s), 735 (vs), 753 (m), 776 (m), 818 (w), 841 (w), 855 (w), 932
(vs), 965 (m), 996 (m), 1015 (w), 1038 (w), 1075 (w), 1113 (s), 1163
(vw), 1192 (w), 1231 (vw), 1266 (vw), 1304 (vw), 1325 (w), 1345
(w), 1395 (w), 1439 (m), 1457 (vw), 1489 (w), 1590 (w), 2222 (vw),
2693 (vw), 2822 (vw), 2849 (vw), 2891 (vw), 2916 (w), 2957 (w),
2994 (vw), 3032 (vw), 3044 (vw), 3061 (w), 3098 (vw) cm–1. Raman
(25 °C): ν = 140 (w), 194 (vw), 248 (vw), 258 (vw), 281 (vw), 312
(vw), 385 (vw), 489 (vw), 520 (vw), 616 (vw), 670 (w), 935 (vw),
1000 (s), 1033 (w), 1106 (w), 1164 (vw), 1197 (vw), 1442 (vw), 1579
(w), 1592 (m), 2227 (s), 2769 (vw), 2825 (vw), 2894 (vw), 2919 (w),
2960 (w), 3002 (vw), 3069 (vs), 3152 (vw), 380 (vw) cm–1.
Elemental Analysis
The elemental analyses (CHN) were performed using a Thermoquest
Flash EA 1112.
IR/Raman Spectroscopy
The IR spectra in the range of 4000–500 cm–1 were obtained with a
Nicolet 380 FT-IR spectrometer with a Smart Endurance ATR device.
Raman spectra were recorded at room temperature with a Bruker Ver-
tex 70 Raman spectrometer (Bruker, Karlsruhe, Germany) using the
1064 nm exciting line of a Nd/YAG laser. The crystalline samples were
flame sealed in melting point capillaries.
n-Butyl-triphenylphosphonium tetracyanidoborate (4): Yield:
670 mg, 95 %, m. p. 73 °C. C26H24BN4P: calcd. (%) C 71.91, H 5.57,
N 12.90; found C 71.66, H 5.48, N 12.82. 1H NMR (300 MHz,
CD3CN): δ = 7.91–7.81 (m, 3 H, Ph–Cpara), 7.76–7.65 (m, 12 H, Ph–
X-ray Structure Analysis
Transparent, colorless crystals of 1, 2, and 3 were mounted on the tips
of thin glass fibers for the single-crystal X-ray diffraction measure-
ments. Data were collected on a Bruker-Nonius Apex X8 diffractome-
ter equipped with a CCD detector. Measurements were done using
monochromatic Mo-Kα radiation (λ = 0.71073 Å). Preliminary data of
the unit cell were obtained from the reflex positions of 12 frames, each
measured in three different directions of the reciprocal space. After
completion of the data measurements the intensities were corrected for
Lorentz, polarization, and absorption effects using the Bruker-Nonius
software [31]. The structure solution and refinement was done with
the aid of the SHELX-97 program package [32]. All non-hydrogen
atoms were refined anisotropically. The hydrogen atoms were added
on idealized positions and refined using riding models. The n-alkyl
chains of the phosphonium cation in [nBu4P][B(CN)4] (2) are disor-
dered. The disorder was treated by a split-model describing two orien-
tations with a ratio of 83 % for the first and 17 % for the second
orientation. Crystal data, data collection, and refinement parameters
are collected in Table 1. Crystallographic data for the structural analy-
ses have been deposited with the Cambridge Crystallographic Data
Centre, CCDC-718591 for the tetraphenylphosphonium salt 1, CCDC-
718592 for the tetra-n-butylphosphonium salt 2, and CCDC-718593
for the ethyl-triphenylphosphonium salt 3. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ,
UK; Fax: +44-1223-336-033; or E-Mail: deposit@ccdc.cam.ac.uk.
C
meta,ortho), 3.24–3.10 (m, 2 H, H–Ca), 1.69–1.46 (m, 4 H, H–Cb+c),
0.97–0.87 (t, 3 H, Cd). 13C NMR (63 MHz, CD3CN): δ = 136.0 (d,
Ph–Cpara, JP,C ≈ 3.0 Hz), 134.5 (d, Ph–Cortho, JP,C ≈ 10.0 Hz), 131.2 (d,
Ph–Cmeta, JP,C ≈ 12.6 Hz), 123.2 (q, C–N, J ≈ 70.9 Hz), 118.0 (d, Cp,
J ≈ 86.4 Hz), 24.9 (d, Cc, J ≈ 4.4 Hz), 24.3 (d, Cb, J ≈ 16.8 Hz), 22.3
(d, Ca, J ≈ 51.7 Hz), 13.5 (s, Cd). 31P NMR (121 MHz, CD3CN): δ =
23.6. 11B NMR (160 MHz, CD3CN): δ = –39.0. IR (ATR 25 °C): ν =
616 (w), 685 (vs), 722 (s), 731 (s), 743 (m), 783 (w), 809 (w), 826
(w), 849 (w), 868 (w), 907 (m), 924 (s), 934 (s), 965 (m), 998 (m),
1030 (w), 1057 (w), 1071 (w), 1113 (s), 1163 (w), 1190 (w), 1225 (w),
1275 (vw), 1296 (vw), 1322 (w), 1343 (w), 1356 (w), 1385 (w), 1437
(s), 1462 (w), 1487 (w), 1590 (w), 1615 (vw), 1634 (vw), 1674 (vw),
1682 (vw), 1819 (vw), 2222 (vw), 2697 (vw), 2737 (vw), 2753 (vw),
2874 (w), 2934 (w), 2967 (w), 3013 (vw), 3032 (vw), 3063 (w), 3098
(vw), 3154 (vw) cm–1. Raman (25 °C): ν = 148 (w), 184 (vw), 209
(vw), 256 (w), 283 (vw), 296 (vw), 385 (vw), 398 (vw), 485 (vw), 522
(vw), 616 (w), 678 (w), 869 (vw), 929 (vw), 1002 (s), 1033 (m), 1058
(vw), 1071 (vw), 1108 (w), 1166 (w), 1197 (vw), 1297 (vw), 1347
(vw), 1446 (vw), 1581 (w), 1592 (m), 2223 (s), 2757 (vw), 2875 (w),
2915 (w), 2938 (w), 2971 (w), 2998 (vw), 3015 (vw), 3068 (vs), 3154
(vw), 3179 (vw) cm–1.
NMR Spectroscopy
1H, 13C, 31P, 11B NMR spectra were recorded with the Bruker instru-
ments AC250 and ARX 300. The used internal standard was TMS for
1H NMR and 13C NMR spectroscopy. B(OCH3)3 was used for 11B
NMR and a 83 % solution of H3PO4 as external standard for 31P NMR
spectra.
Acknowledgement
We thank Prof. Dr. Christoph Schick and Renate Nareyka (University
of Rostock/Germany, Institute of Physics) for DSC and TG measure-
ments, Prof. Dr. Ralf Ludwig and Alexander Wulf (University of Ros-
DSC/TG Measurements
The differential scanning calorimetry (DSC) and thermogravimetric tock, Institute of Chemistry) for recording Raman data, Prof. Dr. Hel-
measurements (TG) were carried out by using a Setram Labsys thermal mut Reinke (University of Rostock, Institute of Chemistry) for
Z. Anorg. Allg. Chem. 2010, 562–568
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
567

A. Flemming, M. Hoffmann, M. Köckerling
ARTICLE
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 562–568