The composition and structure of lithium tetrakis(pentafluorophenyl)borate diethyletherate

Article abstract of DOI:10.1016/j.ica.2009.09.013

Treatment of LiC₆F₅ with B(C₆F₅)₃ in equal volumes of light petroleum and diethyl ether at low temperature followed by slow warming to room temperature precipitates a microcrystalline solid which dries under vacuum to a material with the composition [Li(OEt₂)₃][B(C₆F₅)₄]. Crystallization from diethyl ether yields solvent dependent [Li(OEt₂)₄][B(C₆F₅)₄], which has been crystallographically characterised.

Full text of DOI:10.1016/j.ica.2009.09.013

Inorganica Chimica Acta 363 (2010) 275–278
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Note
The composition and structure of lithium tetrakis(pentafluorophenyl)
borate diethyletherate
*
Eddy Martin, David L. Hughes, Simon J. Lancaster
Wolfson Materials and Catalysis Centre, School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of LiC6F5 with B(C6F5)3 in equal volumes of light petroleum and diethyl ether at low
temperature followed by slow warming to room temperature precipitates a microcrystalline solid
which dries under vacuum to a material with the composition [Li(OEt2)3][B(C6F5)4]. Crystallization from
diethyl ether yields solvent dependent [Li(OEt2)4][B(C6F5)4], which has been crystallographically
characterised.
Received 6 August 2009
Accepted 1 September 2009
Available online 8 September 2009
Keywords:
Ó 2009 Elsevier B.V. All rights reserved.
X-ray crystal structures
Weakly coordinating anions
Borate
1. Introduction
activated 4 Å molecular sieves and degassed by several freeze–
thaw cycles. NMR spectra were recorded using a Bruker DPX300
The tetrakis(pentafluorophenyl)borate anion was first reported
in 1963 [1]. The remarkable stability and low basicity of this anion
has led to a huge number of applications as a weakly coordinating
anion [2–4]. Its importance is reflected by the commercial availabil-
ity of its salts. In particular a number of suppliers provide the
lithium salt [Li(Et2O)n][B(C6F5)4]. However, there is considerable
variation in the value of n, the number of associated ether mole-
cules. The most frequently quoted composition is n = 2.5. It is
obvious that the success of subsequent salt metathesis reactions,
especially where the product is difficult to purify will be dependent
upon anion purity and precise stoichiometry. We report here a
procedure to isolate lithium tetrakis(pentafluorophenyl)borate
diethyletherate in good yield with excellent anion purity. The
composition of the isolated materials is compared to that of the
crystalline salt, the solid state structure of which is reported herein.
spectrometer. Chemical shifts are reported in ppm and referenced
to residual solvent resonances (1H); 19F is relative to CFCl3;
11B is relative to F3BÁOEt2. (C6F5)3BÁOEt2 was prepared according
to the published procedure [5]. Elemental analyses were per-
formed at the University of East Anglia. Additional reagents were
purchased from Aldrich or Fluorochem and used without further
purification.
2.2. Synthesis of [Li(OEt2)3][B(C6F5)4] (1)
A 1.65 M solution of n-butyl lithium in hexanes (14.5 mL) was
added slowly to a solution of bromopentafluorobenzene (3 mL,
24.4 mmol) in
a mixture of diethyl ether/light petroleum
(200 mL/200 mL) at À78 °C and stirred for 1 h. Caution! LiC6F5 is
thermally unstable and may explode on warming; it must be pre-
pared and maintained at À78 °C. This was followed by the drop-
wise addition of (C6F5)3BÁOEt2 (14.3 g, 24.4 mmol) dissolved in
diethyl ether (300 mL). After 1 h at À78 °C the resulting suspension
was warmed slowly to room temperature (ca. 20 °C/h). This gave a
colourless microcrystalline solid, which was separated by filtra-
tion. The solid was dried under vacuum to give the desired salt
as a white powder (16.35 g, 75%). The composition [Li(OEt2)3]
[B(C6F5)4] was determined using the internal reference standard
F3CC6H4CH3. Anal. Calc. for C36H30BF20LiO3: C, 47.60; H, 3.33.
Found: C, 46.79; H, 3.23%. 1H NMR (300 MHz, CD3CN): d 1.14
(t, 18H, JH–H = 7 Hz, CH3 Et2O), 3.43 (q, 12H, JH–H = 7 Hz, CH2
Et2O). 11B NMR (96 MHz, CD3CN): d À16.7. 19F (282 MHz, CD3CN):
dÀ133.8 (m, 8F, o-F), À163.9 (td, 4F, JF–F = 20 and 7 Hz, p-F), À168.4
(m, 8F, m-F).
2. Experimental
2.1. Material and methods
Syntheses were performed under anhydrous oxygen-free nitro-
gen using standard Schlenk techniques. Solvents were distilled
over sodium-benzophenone (diethyl ether) or sodium (light petro-
leum, b.p. 40–60 °C). The NMR solvent (CD3CN) was dried over
* Corresponding author. Tel.: +44 1603 592009; fax: +44 1603 592003.
E-mail address: S.Lancaster@uea.ac.uk (S.J. Lancaster).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.09.013

276
E. Martin et al. / Inorganica Chimica Acta 363 (2010) 275–278
2.3. Crystallographic analysis
Colourless blocks of [Li(OEt2)4][B(C6F5)4] (1a) suitable for X-ray
diffraction were obtained by cooling the solution obtained in 2.2.
to À28 °C overnight.
2.3.1. Crystal data
ꢀ
C16H40O4Li, C24F20B, M = 982.5. Triclinic, space group P1 (no. 2),
Scheme 1. Preparation of 1.
a = 11.1432(2), b = 11.1625(2), c = 17.8069(4) Å,
86.058(2),
= 89.857(2)°, V = 2166.97(7) Å3. Z = 2, Dc = 1.506 g
cmÀ3, F(000) = 1000, T = 140(1) K, ) = 1.53 cmÀ1, k(Mo
(Mo K
) = 0.71073 Å.
Crystals of [Li(OEt2)4][B(C6F5)4] (1a) were examined under per-
a = 78.734(2), b =
c
l
a
diethyl ether and light petroleum [12]. When a B(C6F5)3 solution
in diethyl ether was added at À78 °C to a solution of C6F5Li in a
1:1 mixture of diethyl ether and light petroleum and the reaction
mixture allowed to warm up slowly to room temperature, a micro-
crystalline solid precipitated, which after drying corresponds to a
75% yield of [Li(OEt2)3][B(C6F5)4]. The 19F NMR of the precipitate
indicated very high (>98%) anion purity. Apparently any impurities
and by-products remain in solution under these conditions
(Scheme 1).
Ka
fluoropolyether oil, one, ca 0.56 Â 0.44 Â 0.21 mm, was mounted
on a glass fibre and fixed in the cold nitrogen stream on an Oxford
Diffraction Xcalibur-3 CCD diffractometer equipped with Mo K
radiation and graphite monochromator. Intensity data were mea-
sured by thin-slice - and u-scans. Total no. of reflections re-
corded, to hmax = 25.0°, was 39729 of which 7618 were unique
(Rint = 0.038); 5670 were ‘observed’ with I > 2 (I).
a
x
r
The number of associated diethyl ether molecules was deter-
Data were processed using the CRYSALIS-CCD and -RED [6] programs.
The structure was determined by the direct methods routines in
the SHELXS program [7a] and refined by full-matrix least-squares
methods, on F2’s, in SHELXL [7b]. The non-hydrogen atoms were re-
fined with anisotropic thermal parameters. Hydrogen atoms were
included in idealised positions and their Uiso values were set to ride
on the Ueq values of the parent carbon atoms. At the conclusion of
the refinement, wR2 = 0.102 and R1 = 0.070 [7b] for all 7618 reflec-
mined by comparing results for integrating the 1H and 19F NMR
spectra of 1 using ca 0.02 mL
a,a,a-trifluoro-p-xylene (F3CC6-
H4CH3) as a reference (the spectra are given in the Supplementary
Information). The number of diethyl ether molecules coordinated
to the lithium cation (n) was found to be three, which is in good
agreement with the results of the elemental analysis.
tions weighted w = [r +
2(F2o) + (0.049P)2 + 0.667P]À1 with P = (Fo2
3.2. Molecular structure
2F2)/3; for the ‘observed’ data only, R1 = 0.041.
c
In the final difference map, the highest peak (ca 0.59 e ÅÀ3) was
close to C71.
In order to better understand the composition of 1, its solid
state structure was determined. To the best of our knowledge,
the solid state structure of a material with the formulation
[Li(Et2O)n][B(C6F5)4] has not been reported. There are six crystal
structures of lithium tetrakis(pentafluorophenyl)borate salts in
the Cambridge Structural Database, but none with diethyl ether
donors [13,14].
Crystallisation of 1, from a saturated diethyl ether solution at
À28 °C, yielded crystals which were very sensitive to solvent loss
and required storage under the mother liquor.
Scattering factors for neutral atoms were taken from reference
[8]. Computer programs used in this analysis have been noted
above, and were run through WINGX [9] on a Dell Precision 370 PC
at the University of East Anglia.
Selected bond lengths and angles are given in Table 1.
3. Results and discussion
The structure was determined to be [Li(OEt2)4][B(C6F5)4] (1a) in
which the asymmetric unit consists of an ion pair (Fig. 1). Both ions
have a tetrahedral distribution of ligands around the central
atoms; bond lengths and angles are reported in Table 1. There is
nothing exceptional about the molecular structures of the cation
and anion, which closely resemble those previously reported
[14,15] and there is no close contact between the lithium centre
and the anion.
Evidently, in diethyl ether solution, the lithium cation of lithium
tetrakis(pentafluorophenyl)borate is fully solvated with four coor-
dinated molecules of diethyl ether. This salt can be crystallized
from diethyl ether solution. The facile loss of a solvent as volatile
as diethyl ether leading to crystal degeneration is a common phe-
nomenon but in this case it is not lattice solvent molecules but
those donors associated with the cation that are lost. In our hands
simple drying of the solid under vacuum at room temperature re-
sults in the loss of one molecule of diethyl ether per lithium centre
giving [Li(OEt2)3][B(C6F5)4]. Presumably continued vacuum-drying
perhaps at elevated temperatures might result in the slow loss of
further molecules of diethyl ether.
3.1. Synthesis
Since its discovery, and because of its importance, there have
been numerous revised syntheses of lithium tetrakis(pentafluor-
ophenyl)borate [10]. We required a convenient laboratory route
to a material with high anion purity and well defined stoichiome-
try. Song et al. reported that an ether-free salt could be obtained by
treating B(C6F5)3 with C6F5Li in light petroleum [11]. However, in
our hands this method yielded an off-white solid with several sets
of 19F resonances indicating poor purity. Marks has shown that re-
lated anions can be prepared cleanly using a solvent mixture of
Table 1
Selected bond lengths (Å) and angles (°) for [Li(OEt2)4][B(C6F5)4] (1a).
B–C11
B–C41
B–C31
B–C21
1.651(3)
1.653(3)
1.654(3)
1.658(3)
C11–B–C41
C11–B–C31
C41–B–C31
C11–B–C21
C41–B–C21
C31–B–C21
114.46(17)
101.85(16)
113.49(17)
113.17(17)
101.60(16)
112.77(17)
Close inspection of the supramolecular architecture of 1a
(Fig. 2) reveals a lattice structure consisting of layers of the anion
alternating with layers of the cation along the b-axis parallel to
the (1 0 1) plane. These oppositely charged layers are attracted
by coulombic interactions, there are no short charge-assisted C–
HÁÁÁF–C hydrogen bonds.
Li–O8
Li–O5
Li–O7
Li–O6
1.936(4)
1.942(4)
1.951(4)
1.957(4)
O8–Li–O5
O8–Li–O7
O5–Li–O7
O8–Li–O6
O5–Li–O6
O7–Li–O6
104.20(18)
109.49(18)
109.0(2)
110.3(2)
116.90(19)
106.80(19)

E. Martin et al. / Inorganica Chimica Acta 363 (2010) 275–278
277
Fig. 1. Molecular structure of 1a. Thermal ellipsoids are shown at the 50% probability level. H-atoms on carbon have been omitted for clarity. One of the ethyl chains bound to
O7 is disordered and both orientations were resolved and are represented.
loses the first equivalent of ether. The solid state structure consists
of layers of cations and anions.
Acknowledgements
The authors are grateful to the University of East Anglia and Ox-
ford Diffraction for financial support and Prof. Manfred Bochmann
for helpful discussion.
Appendix A. Supplementary material
CCDC 743194 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2009.09.013.
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