Ionic liquids: Synthesis and characterisation of triphenylphosphonium tosylates

Article abstract of DOI:10.1002/ejic.200500769

Ionic liquids are currently attracting considerable interest from chemists, partly due to their potential as "green" alternatives to volatile molecular organic solvents. Phosphonium ionic liquids have received much less attention than ammonium salts in th

Full text of DOI:10.1002/ejic.200500769

FULL PAPER
DOI: 10.1002/ejic.200500769
Ionic Liquids: Synthesis and Characterisation of Triphenylphosphonium
Tosylates
Ludovic G. Bonnet[a] and Benson M. Kariuki*[a]
Keywords: Structure elucidation / Ionic liquids / Solid-state structures / Melting points
Ionic liquids are currently attracting considerable interest
from chemists, partly due to their potential as “green” alter-
natives to volatile molecular organic solvents. Phosphonium
ionic liquids have received much less attention than ammo-
nium salts in the past, but the situation is changing. The salts
are applied in the liquid state but in this work, salts with
higher melting points than is generally the case are investi-
gated. They are solid at room temperature, thus enabling fac-
ile separation of liquid reaction products. Nine triphenyl-
phosphonium tosylates have been prepared and character-
ised; their crystal structures have been determined and their
melting points have been scrutinized, showing some sensitiv-
ity to the duration of drying. The structural information is
expected to be invaluable in the understanding of the prop-
erties of the molten state.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
date, their potential as ionic solvents has been recognized
for a long time. For example, the first palladium-catalysed
reaction in an ionic liquid was carbon–carbon coupling in
tributylhexadecylphosphonium bromide.[6] Tetrabutylphos-
phonium halides (TBPHs) featured prominently in the ear-
lier application of phosphonium salts as ionic solvents. One
of the first reported uses of a phosphonium salt as a solvent
in an organic reaction was of TBPHs in regioselective o-
alkylation leading to an efficient and recyclable catalytic
system.[7] Several years earlier, in 1981, TBPHs had been
used in the ruthenium-catalysed hydrogenation of carbon
monoxide to ethyleneglycol.[8] Hydroformylation of olefins
in TBPHs and other quaternary phosphonium salts has
also been demonstrated.[9,10] Interest in application as alter-
native solvents and co-catalysts continues to grow.[11]
In this investigation, we consider triphenylphosphonium
salts with higher melting points than conventional ionic li-
quids, focusing exclusively on the tosylates shown in
Scheme 1. The efficacy of some of the tosylates has already
been demonstrated. The first reported use of a phospho-
nium molten salt with a tosylate counterion was as the sol-
vent for catalytic hydroformylation of hex-1-ene to heptanal
and 2-methylhexanal using 1, 3, 5 and 9.[12] This established
that alteration of the substituents in the cation could affect
product distribution. Diels–Alder reactions of isoprene with
methyl acrylate, but-3-en-2-one and acrylonitrile have also
been performed in phosphonium tosylates, including 1, 3, 5
and 9.[13] The results indicate that product selectivity varies
with cation substituent and also that the solvents are more
suitable for the reactions of oxygen-containing dienophiles
than for nitrogenous ones, showing high regioselectivity.
Phosphonium salts including 1 and 9 have also been re-
ported to be good co-catalysts for the Baylis–Hillman reac-
Introduction
Although ionic liquids were first described as long ago
as 1914,[1] it is only relatively recently that they have re-
ceived much attention from chemists.[2–5] The last few dec-
ades have witnessed increased application, initially as media
for organic synthesis and later on with multiple uses. A
number of properties make ionic liquids particularly suited
to application as solvents; they have high thermal stability,
low vapour pressure and the ability to dissolve a large range
of inorganic, organic and polymeric materials. In addition,
they have a wide liquid range and are relatively cheap and
easy to prepare.
The terms “ionic liquid” and “room temperature ionic
liquid” often refer to ionic salts with melting points below
ca. 100 °C. Many ionic liquids studied to date fall within
this category and, in particular, salts containing quaternary
nitrogen cations have been investigated extensively. A
number of phosphonium salts also fall within this low melt-
ing point class. A factor in the growing interest in these
materials is their potential as “green” alternatives to mol-
ecular organic solvents; replacement of volatile organic sol-
vents by ionic liquids, which are involatile, would signifi-
cantly reduce their detrimental impact on the environment.
This aspect will undoubtedly become increasingly impor-
tant as the level of awareness and concern for the habitat
intensifies.
Although phosphonium salts have attracted much less
interest in the literature than quaternary nitrogen salts to
[a] School of Chemistry, University of Birmingham,
Edgbaston, Birmingham B15 2TT, UK
Fax: +44-121-414-4403
E-mail: B.M.Kariuki@bham.ac.uk
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L. G. Bonnet, B. M. Kariuki
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tion with the results showing that neither modification of as the number of available cations and anions is immense.
the cation substituents nor a change in the anion has a Any investigation, therefore, needs to be performed system-
major effect on the yield.[14] The tosylates have also been atically.
used in rhodium-catalysed transfer hydrogenation of aceto-
Melting points, which are an indication of the lower tem-
perature limit of application of ionic solvents, can be related
to the crystal structures and are looked at in some detail in
this study. However, ionic salts are applied in the liquid
state, where the challenge is to understand properties such
as viscosity and ion diffusion. The more ordered crystalline
state can provide important information (such as molecular
geometry, intra- and intermolecular contacts) to assist in
the process of beginning to understand and possibly model
the properties of the liquid state. This is a realistic approach
as shown by studies of imidazolium ionic liquids in which
a correlation has been found between the structural proper-
ties of the solid state and those of the liquid state.[16–20] To
the best of our knowledge, no structural information has
been published for any triphenylphosphonium tosylates. An
aim of this study has been to obtain this information, and
to this end, the salts shown in Scheme 1 have been synthe-
sised and characterised.
phenone.[15]
Results and Discussion
Melting Points
Melting points are invaluable in the characterisation of
materials. In the case of ionic liquids, accurate melting
points (along with boiling points and decomposition tem-
peratures) are essential as they give an indication of the
range of application. Melting points are also sensitive to
the purity of the material – an important consideration as
impurities may change the nature of ionic solvents. The
melting points determined during this study are shown in
Table 1. The “initial” values in the Table were obtained af-
ter sample preparation and some showed significant devia-
tions from previously reported values. There were also some
Scheme 1. The phosphonium tosylates under investigation.
In addition to the general properties of ionic liquids men-
tioned above, phosphonium salts have a number of other
advantages. They can be prepared without halide contami-
nation and therefore be used with no adverse effect on me-
tal catalysts. They are also more thermally stable than qua-
ternary nitrogen salts and thus can be applied under more
demanding reaction conditions. The tosylates are relatively
inexpensive, easily available, noncorrosive and are stable in
air as well as in moisture. A high melting point has been
found to be beneficial for the following reason. At the reac-
tion temperature (above the melting point of the salt), the
phosphonium salts act as solvents (for catalysts and rea-
gents), allowing homogeneous conditions. On cooling, the
salts crystallise out and trap the catalyst thereby enabling
easy separation of liquid products. For example, in the hy-
droformylation,[12] Diels–Alder[13] and hydrogenation[15] re-
actions mentioned above, product separation involved fil-
tration or decantation of the liquid organic product. The
catalyst system or ionic solvent was subsequently reusable.
The growing interest in these materials as well as their
potential as alternatives to other ionic and volatile organic
solvents makes more fundamental research into their prop-
erties imperative. The properties of triphenylphosphonium
salts can be affected by a change in the nature of both cat-
ion and anion; as a consequence, solvents can be tailor-
made for a specific reaction by tuning their properties. For
this to be achievable, the correlation between the physico-
chemical properties of the solvents and the molecular struc-
Table 1. Melting points (Tmp) for the triphenylphosphonium tosyl-
ates.
Tmp [°C]
Salt
n
initial
–
after additional drying literature
–
0
1
–
137–139[21]
147–149[22]
94–95[12]
82–83[21]
141–143[21]
116–117[12]
139–140[21]
–
1
118–120
139–141
2
3
2
3
142–143
130–131
143–145
132–134
4
5
4
7
120–122
–
123–125
129–131[a]
70–71[12]
70–78[21]
94–96[23]
–
–
6
7
8
9
9
–
–
154–156
173–175
–
164–166
175–176
–
–
–
74–76
74–75
81–83[12]
ture needs to be established.[4] This is a sizeable challenge [a] Solvate.
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Ionic Liquids: Synthesis and Characterisation of Triphenylphosphonium Tosylates
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discrepancies between the various reports,[12,21–23] and
hence further examination was undertaken.
The initial samples were rinsed using a volatile solvent
and did not appear to be wet, but additional drying was
found to raise the melting points significantly for some. The
drying process was repeated until no further change was
observed. Thus, for example, the melting point of 1 rose
from 118–120 °C to 139–141 °C on storage under vacuum
for several days and that of 6 increased from 154–156 °C to
164–166 °C under the same treatment. With the exception
of 5, which is a solvate, none of the analytical techniques
indicated the presence of significant amounts of solvent in
the initial samples and powder X-ray diffraction confirmed
that no observable change in phase or crystallinity occurred
during the drying process.
Figure 1. Melting points for the alkyltriphenylphosphonium tosyl-
ates.
The final values of melting points obtained are also
shown in Table 1. One literature value for 3 and that for 9
are significantly higher than the final values from this study,
suggesting that even higher melting points might be ob-
Incorporation of solvent of crystallisation is clearly a
tained by further treatment. The final values from this consideration when selecting a salt for use as a solvent. Un-
study, along with the literature values for the n-alkyltri- like the other samples investigated, the structure of 5 (n =
phenylphosphonium tosylates, are plotted in Figure 1. The 7) included the solvent (dichloromethane). Two values have
need for reliable values is clearly illustrated by 1. Going been reported previously for the melting point of 5, and it
by the previously reported melting point, 1 falls within the is noteworthy that although they are close to each other,
conventional definition of ionic liquids (i.e. m.p. Ͻ 100 °C), the range is quite large for one (8 °C). If an assumption is
whereas a considerably higher value has been obtained dur- made that melting points decrease gradually with increasing
ing this study. It is noted in passing, however, that 1, 3 and n for nonsolvated triphenylphosphonium tosylates (the
5 have been used at temperature as low as 80 °C, 40 °C and dashed line in Figure 1), then a possible discrepancy is ap-
60 °C[13] respectively although the melting points obtained parent for 5, with the reported values falling well below that
for the pure phases are ca. 140 °C, 133 °C and 130 °C, predicted.
respectively. This is attributable to the “impurity” effect of
the reagents on the ionic solvents.
The DSC results obtained indicate that endothermic pro-
cesses occur at temperatures consistent with the melting
Table 2. Crystallographic data for the alkyltriphenylphosphonium tosylates.
Salt
1
2
3
4
5
Formula
C20H20P+ C7H7O3S– C21H22P+ C7H7O3S– C22H24P+ C7H7O3S– C23H26P+ C7H7O3S– C26H32P+ C7H7O3S–
CH2Cl2
FW
T [K]
462.52
296
476.54
296
490.57
296
504.59
296
631.60
296
λ [Å]
0.71069
monoclinic
P21/n
13.663(2)
12.695(2)
13.811(2)
90.967(5)
2395.2(7)
4
0.71069
monoclinic
P21/c
8.7830(8)
16.244(2)
17.643(2)
93.975(4)
2511.0(4)
4
0.71069
monoclinic
P21/a
12.672(1)
17.227(2)
12.451(2)
97.426(9)
2695.1(5)
4
1.54178
monoclinic
P21/a
12.6695(5)
17.2562(7)
12.6892(5)
98.347(2)
2744.82(19)
4
1.54178
monoclinic
P21/a
12.782(3)
17.906(4)
15.554(4)
104.553(7)
3446(1)
4
Crystal system
Space group
a [Å]
b [Å]
c [Å]
β [°]
V [Å3]
Z
Dcalcd. [Mg·m–3]
μ [mm–1]
F(000)
1.283
0.228
976
1.261
0.220
1008
1.209
0.207
1040
1.221
1.819
1072
1.218
2.940
1336
Crystal size [mm]
Total reflections
Unique reflections
Rint
0.50×0.15×0.10
13305
4380
0.042
1.048
0.50×0.15×0.10
12967
4319
0.034
1.047
0.10×0.08×0.01
12939
4377
0.065
1.047
0.34×0.10×0.04
27287
4917
0.096
1.007
0.24×0.12×0.06
8675
2313
0.094
1.037
S
R1 [I Ͼ 2σ(I)]
wRp
0.052
0.123
0.054
0.136
0.063
0.089
0.045
0.097
0.096
0.26
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Table 3. Crystallographic data for the tosylates of triphenylphosphonium esters.
Salt
6
7
8
9
Formula
FW
T [K]
C21H20O2P+ C7H7O3S–
506.53
296
C22H22O2P+ C7H7O3S–
520.55
293
C28H26O2P+ C7H7O3S–
596.64
296
C14H32P+ C7H7O3S–
402.55
296
λ [Å]
0.71069
monoclinic
Cc
9.9121(9)
16.843(1)
14.8464(9)
90.844(5)
2478.3(3)
4
0.71069
orthorhombic
P212121
13.048(1)
18.379(2)
10.9741(8)
–
2631.8(3)
4
1.314
0.71069
monoclinic
Cc
17.752(2)
9.6829(7)
18.935(2)
109.629(2)
3065.6(4)
4
0.71069
monoclinic
P21/n
8.137(3)
13.249(6)
22.652(9)
96.594(7)
2426(2)
4
Crystal system
Space group
a [Å]
b [Å]
c [Å]
β / °
V [Å3]
Z
Dcalcd. [Mg·m–3]
μ [mm–1]
F(000)
1.358
0.233
1064
1.293
0.199
1256
1.102
0.215
880
0.221
1096
Crystal size [mm]
Total reflections
Unique reflections
Rint
0.30×0.20×0.02
7224
4006
0.072
1.078
0.40×0.10×0.05
15070
4636
0.047
1.192
0.50×0.40×0.40
9088
4630
0.030
1.041
0.40×0.30×0.10
6127
2702
0.103
1.045
S
R1 [I Ͼ 2σ(I)]
wRp
x (Flack)
0.056
0.132
0.01(13)
0.064
0.126
–0.04(12)
0.039
0.101
0.08(7)
0.118
0.305
–
points. The peaks are sharp for the alkyltriphenyl com-
pounds (1–4 and 9), and there is no indication of thermal
instability within the timescale and temperature range of
the experiment. These salts have high thermal stability, are
tolerant toward air and are reusable as solvents.[12,13] On
the other hand, DSC shows broader features in the cases of
5–7; loss of the solvent is clearly a factor in 5, whereas the
results may be an indication of thermal transformation
close to the melting point for the esters 6 and 7.
Structural Characterisation
Crystallographic data for 1–5 are shown in Table 2 and
data for 6–9 are shown in Table 3. The asymmetric units
are shown in Figure 2(a) to Figure 10(a).
Alkyltriphenylphosphonium Tosylates
In the structure of 1, tosylate ions are linked by C(3)–
H(3)···O(2) interactions (with a H···O distance of 2.4 Å, a
C–H···O angle of 170°) to form zigzag chains along the b-
axis. These chains are arranged side-by-side to form sheets
parallel to the (1,0,–1) plane, leading to a layered structure
(Figure 2, b). In the structure, a number of edge-to-face in-
teractions with “nonideal” geometry are observed. For ex-
ample, an aromatic ring of the cation is the donor to two
anion rings with planes at ca. 60° and 70° to it and both
with CH···π(ring-plane) distances of ca. 3 Å. In the anion,
the oxygen closest to the plane of the aromatic ring forms
a torsion angle [O(3)–S(1)–C(1)–C(6)] of 16.9(3)°.
Figure 2. (a) The asymmetric unit and (b) the crystal structure of
1 viewed down the b-axis. Hydrogen atoms have been omitted for
clarity.
For 2, the propyl chains of the cations are in trans con- tacts involve aryl hydrogen atoms [with angles C(26)–
formation relative to the phosphorus atoms to which they H(26B)···O(3) 160.9(4)° and C(13)–H(13)···O(1)
=
=
are attached. In the crystal, the cations form channels, 164.0(3)° and distances O(3)···H(26B) = 2.39(1) Å and O(1)···
which run parallel to the a-axis and are occupied by pairs H(13) = 2.33(1) Å]. In the crystal structure, pairs of cations
of anion chains (Figure 3, c). The two closest C–H···O con- are linked by edge-to-face interactions involving all six
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Ionic Liquids: Synthesis and Characterisation of Triphenylphosphonium Tosylates
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phenyl rings as shown in Figure 3, (b) and, in addition, 173.4(4)° and contact distances O(2)···H(16) = 2.43(1) Å,
close CH···π contact occurs between a proton from a ring O(1)···H(26B) = 2.29(1) Å, O(1)···H(24) = 2.37(1) Å]. The
of the anion and one cation ring. For these interactions, the oxygen closest to the plane of the aromatic ring of the anion
C–H···π(ring-plane) distances are in the 2.9–3.3 Å range. forms a torsion angle O(2)–S(1)–C(1)–C(2) of 12.8(4)°.
The oxygen closest to the plane of the aromatic ring of the
anion forms a torsion angle O(2)–S(1)–C(1)–C(2) of
–16.2(4)°.
Figure 3. (a) The asymmetric unit, (b) a pair of cations and (c) the
crystal structure of 2 viewed down the a-axis. Hydrogen atoms have
Figure 4. (a) The asymmetric unit of 3, (b) a pair of cations with
the dashed lines showing CH···π contacts and (c) the crystal struc-
ture viewed down the c-axis. Hydrogen atoms have been omitted
from (a) and (c) for clarity.
been omitted from (a) and (c) for clarity.
The butyl chain in the crystal structure of 3 is also all-
trans in conformation. In the structure (Figure 4, c), the
tosylate anions are isolated from each other, with no direct
In the structure of 4, (Figure 5, c) the pentyl group of
contact between them. Pairs of symmetry related cations the cation is disordered with two components of 63% and
interact through edge-to-face contacts as shown in Figure 4, 37% occupancy. For the major component, the chain
b. Each cation acts as both a donor and acceptor, with C– adopts an all-trans conformation, except for the terminal
H···π(ring-centre) distance of ca. 2.64 Å. The two ions are methyl group which is gauche. As suggested by the similar-
also involved in π···π contact with a distance of ca. 3.5 Å ity in the cell parameters and symmetry, the crystal struc-
between the planes of rings showing ca. 50% overlap. In the tures of 3 and 4 are almost identical. Thus pairs of mole-
structure, a second type of π···π interaction occurs between cules interact in a manner similar to those discussed for 3,
phenyl rings of neighbouring cations with a separation dis- with C–H···π(ring-centre) distance of ca. 2.67 Å and π···π
tance of ca. 4.1 and an overlap of about 1/3 of a ring. The contact interplanar distance of ca. 3.5 Å for rings showing
closest C–H···O contacts involve aryl–H and HC–H from 50% overlap (Figure 5, b). The separation between the pla-
the cation [with angles C(16)–H(16)···O(2) = 168.6(5)°, nes of the pair of rings involved in the second type of π···π
C(24)–H(24)···O(1) = 175.2(5)° and C(26)–H(26B)···O(1) = interaction (1/3 ring overlap) is also 4.1 Å. Aryl and CH2
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L. G. Bonnet, B. M. Kariuki
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hydrogen atoms from the cation involved in the closest C– terplanar distance of ca. 3.5 Å for rings showing an overlap
H···O contacts have CH···O distances: O(1)···H(19B) = of ca. 1/3 of a ring. The rings involved are also donors in
2.4(1) Å, O(1)···H(11) = 2.4(1) Å, O(2)···H(19A) = 2.4(1) Å C–H···π interactions with –H···π distances of ca. 2.7 Å.
and C–H···O angles: C(11)–H(11)···O(1)
= 174.9(5)°,
C(19)–H(19B)···O(1) = 170.6(5)°, C(19)–H(19A)···O(2) =
156.9(5)°. The oxygen closest to the plane of the aromatic
ring of the anion forms a torsion angle O(3)–S(1)–C(24)–
C(29) of 11.5(4)°.
Figure 6. (a) The asymmetric unit, (b) a pair of cations with the
dashed lines showing CH···π contacts and (c) the crystal structure
of 5 viewed down the a-axis. Hydrogen atoms have been omitted
from (a) and (c) for clarity.
Figure 5. (a) The asymmetric unit of 4, (b) a pair of cations with
the dashed lines showing CH···π contacts and (c) the crystal struc-
ture viewed down the c-axis. Hydrogen atoms have been omitted
from (a) and (c) for clarity.
Tosylates of Triphenylphosphonium Esters
In the crystal structure of 6, the cations form channels,
parallel to the a-axis, which are occupied by straight chains
of tosylate anions (Figure 7, b). A number of edge-to-face
interactions are observed with the tosylate anion as the do-
Unlike the other salts, the crystals of 5 included the nor and the cation as the acceptor. However, the planes of
dichloromethane solvent (Figure 6, b). The structure con- the rings involved are not perpendicular. For example, a C–
sists of channels, parallel to the a-axis, which are occupied H···π(ring-plane) distance of 2.9 Å is observed between
by the anions. The closest C–H···O contacts have distances: rings with planes oriented ca. 60° relative to each other. The
O(1)···H(26B) = 2.2(2) Å, O(1)···H(10) = 2.4(2) Å, O(2)··· closest CH···O contacts involve cation methylene groups
H(34A) = 2.4(2) Å and C–H···O angles: C(26)–H(26B)··· and tosylate oxygen atoms, with distances O(1)···H(26A) =
O(1) = 178.7(9)°, C(10)–H(10)···O(1) = 176.0(9)°, C(34)– 2.24(1) Å, O(2)···H(26B) = 2.26(1) Å, and angles C(26)–
H(34A)···O(2) = 135.3(9)°. The oxygen closest to the plane H(26A)···O(1) = 153.3(6)°, and C(26)–H(26B)···O(2) =
of the aromatic ring of the anion forms a torsion angle 164.6(6)°. The oxygen closest to the plane of the aromatic
O(2)–S(1)–C(1)–C(6) = 16.6(9)°. Interactions of π···π type ring of the anion forms a torsion angle O(1)–S(1)–C(1)–
also occur for a pair of cations (Figure 6, b), with an in- C(6) is –19.4(6)°.
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Figure 7. (a) The asymmetric unit and (b) the crystal structure of
6 viewed down the a-axis. Hydrogen atoms have been omitted for
clarity.
Figure 8. (a) The asymmetric unit and (b) the crystal structure of
7 viewed down the a-axis. Hydrogen atoms have been omitted for
clarity.
In the crystal structure of 7, the cations also form chan-
nels, running parallel to the a-axis (Figure 8, b), which are
occupied by zigzag chains of tosylate anions. In the crystal
structure, the closest C–H···π distance for an edge-to-face
type of interaction involves a cation donor and anion ac-
ceptor with a distance of ca. 3.1 Å for rings oriented ca.
100° apart. The shortest C–H···O contact involves the terti-
to the plane of the aromatic ring of the anion forms a tor-
sion angle O(2)–S(1)–C(1)–C(2) of –29.2(8)°.
Tributyl(ethyl)phosphonium Tosylate
In the crystal structure of 9, the methyl group of one
ary proton of the cation and one tosylate oxygen with a butyl chain of the cation is disordered with two components
distance O(2)···H(10) of 2.17(4) Å and angle C(10)–H(10)··· of 60% and 40% occupancy. Considering just the major
O(2) of 175(3)°. The oxygen closest to the plane of the aro- component, all the alkyl chains adopt trans conformation,
matic ring of the anion forms a torsion angle O(3)–S(1)– apart from one, in which the methyl group is gauche. In the
C(1)–C(2) of –11.7(4)°.
structure, the tosylate anions are isolated from each other,
The crystal structure of 8 (Figure 9, b) does not contain each being surrounded by cations. The structure can be
channels. The cations pack in layers, parallel to the ab visualised as being composed of sheets parallel to the ab
plane, between which isolated anions are located. Edge-to- (Figure 10, b), with intermolecular C–H···O contacts occur-
face interactions are found and involve one cation ring act- ring within the sheets. The shortest CH···O contacts involve
ing as donor to two rings of two neighbouring cations. The one tosylate oxygen with distances: O(1)···H(18A) =
two acceptor rings are almost perpendicular to the donor 2.4(3) Å, O(1)···H(12A) = 2.4(2) Å and angles: C(18)–
ring, with a C–H···π(ring-plane) distance of ca. 3 Å. The H(18A)···O(1) = 173(1)°, C(12)–H(12A)···O(1) = 153(1)°.
shortest CH···O contact involves an aryl proton and a tosyl- The oxygen closest to the plane of the aromatic ring of the
ate oxygen with a distance O(3)···H(17) of 2.40(1) Å and anion forms a torsion angle O(1)–S(1)–C(1)–C(2) of
angle C(17)–H(17)···O(3) of 141.5(4)°. The oxygen closest –15.1(2)°.
Eur. J. Inorg. Chem. 2006, 437–446
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
443

L. G. Bonnet, B. M. Kariuki
FULL PAPER
Figure 10. (a) The asymmetric unit and (b) the crystal structure of
9 viewed down the a-axis. Hydrogen atoms have been omitted for
clarity.
Figure 9. (a) The asymmetric unit and (b) the crystal structure of
8 viewed down the b-axis. Hydrogen atoms have been omitted for
clarity.
halide or π) are a recurring theme in the field of ionic li-
quids and have been described extensively for, for example,
imidazolium salts.[16–20]
Conclusions
The observations made during this study show that some
The cations in the n-alkyltriphenylphosphonium salts 1–
care needs to be exercised when determining and using the 5 tend to pack in pairs (with the exception of 1, in which
melting points of these materials. The values obtained sug- the short alkyl chain may allow more favourable alternative
gest that minute amounts of solvent can have a significant packing in the crystal). This motif involving pairs of
effect on the melting points, a possible explanation for the (phenyl)3P groups is not unusual and has been observed in,
inconsistencies in the previously reported values.
for example, neutral triphenylphosphane oxides.[24] In the
Being ionic, coulombic interactions obviously exist in all structures of 3–5, the pairs of rings involved in C–H···π in-
these materials. Another feature common to all nine salts is teractions are almost perpendicular, with interplanar angles
the absence of strong hydrogen-bond donors. There is thus of 86°, 85° and 81° respectively. The C–H···π distances (ca,
no single strong directional interaction to influence crystal 2.7 Å) in these three structures are also the shortest ob-
packing. Despite this, a number of structural patterns, served during this study and are similar to the average value
which may be valuable when modelling their liquid proper- obtained in
a
survey of organometallic materials
ties, are evident. In the tosylate anion the angles between (2.79 Å).[25] The donor rings in the ion pairs in 3–5 are also
the ring and the plane through the C–S–O group containing involved in π···π contacts, clearly leading to a stable motif.
the oxygen atom closest to the ring fall within the range Although only a few structures have been determined, some
11–20° (except for 8 in which the angle is ca. 30°). Apart clear differences are observed between the n-alkyl salts on
from 5, none of the salts contains solvent of crystallisation, the one hand and ester salts on the other. In the case of the
a desirable characteristic in ionic solvents. All structures (1– esters, cation pairs are not observed and, in addition, the
9) show some degree of C–H···O contact with the tosylate three structures are noncentrosymetric.
ion acting as the acceptor. In addition, ring edge-to-face C–
We believe that as in the case of imidazolium salts re-
H···π interactions of variable geometry are also observed ported before, the structural information from this study
(the exception in the C–H···π case is 9, in which the cation will be important in future attempts to gain insight into the
has no phenyl groups). C–H···X interactions (e.g. with X = structural characteristics of ionic liquids of this type and
444
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 437–446

Ionic Liquids: Synthesis and Characterisation of Triphenylphosphonium Tosylates
FULL PAPER
Ethyltriphenylphosphonium Tosylate (1): Ethyl p-toluenesulfonate
(3.8 g, 19.0 mmol) and triphenylphosphane (5.0 g, 19.1 mmol) af-
forded a crystalline white solid (8.1 g, 92%). M.p. 139–141 °C
(ref.[21] 82–83 °C, ref.[12] 94–95 °C). FAB+: m/z (%) = 291 (100)
[M – C7H7SO3]+, 753 (8) [2M – C7H7SO3]+. 1H NMR (CDCl3,
will aid in the understanding of the nature of interactions
with solutes.
Experimental Section
3
3
300 MHz): δ = 1.34 (dt, JH,H = 7.4, JP–H = 20.1 Hz, 3 H,
CH2CH3), 2.31 (s, 3 H, CCH3), 3.60 (m, 2 H, CH2), 7.08 (d, 3JH,H
=
Techniques: NMR spectra were recorded with a Bruker AC-300
(1H, 300 MHz; 13C, 75.5 MHz) and
a
Bruker AV-300 (1H,
8.0 Hz, 2 H, tosylateAr-H), 7.63–7.79 (m, 17 H, Ar-H) ppm. 13C{1H}
300 MHz; 13C, 75.5 MHz). Chemical shifts are described in parts
per million (ppm) downfield shift from SiMe4 and are reported
consecutively as position (δH or δC), multiplicity (s = singlet, d =
doublet, t = triplet, q = quadruplet, m = multiplet, dd = doublet
of doublet), coupling constant (J/Hz), relative integral and assign-
ment. Proton NMR spectra were referenced to the chemical shift
of residual proton signals (CHCl3 δ = 7.27 ppm). Carbon spectra
were referenced to a 13C resonance of the solvent (CDCl3 δ =
77.16 ppm). 31P NMR spectra were referenced internally on H2PO4
5%/D2O (δ, 0.00). Mass spectra were recorded with a VG Zabspec
spectrometer. LSIMS (ceasium ionisation) was performed using 3-
nitrobenzyl alcohol as the matrix. DSC results for 1–7, 9 were ob-
tained using a Perkin–Elmer Pyris 1 instrument between 25 and
200 °C at a heating rate of 20°/min with nitrogen purge gas and
aluminium sample pans.
2
NMR (CDCl3, 75.5 MHz): δ = 5.6 (d, JP,C = 5.2 Hz, CH2CH3),
1
1
14.8 (d, JP,C = 52.3 Hz, CH2), 20.3 (CCH3), 116.8 (d, JP,C
86.2 Hz, phosphoniumAr-Cipso), 125.1, 127.3 (tosylateAr-CH), 129.4,
129.5, 132.4, 132.5, 134.0
phosphoniumAr-CH), 137.6, 143.5
tosylateAr-Cipso) ppm. 31P NMR (D2O, 121 MHz): δ = 26.3
phosphoniumP) ppm.
=
(
(
(
Triphenyl(propyl)phosphonium Tosylate (2): Propyl p-toluenesul-
fonate (4.0 g, 18.7 mmol) and triphenylphosphane (4.9 g, 18.7 mmol)
afford a crystalline white solid (8.4 g, 94%). M.p. 143–145 °C
(ref.[21] 141–143 °C). FAB+: m/z (%) = 305 (100) [M – C7H7SO3]+,
1
781 (5) [2M – C7H7SO3]+. H NMR (CDCl3, 300 MHz): δ = 1.18
(m, 3 H, CH2CH3), 1.61 (m, 2 H, CH2CH3), 2.33 (s, 3 H, CCH3),
3
3.52 (m, 2 H, PCH2), 7.04 (d, JH,H = 8.0 Hz, 2 H, tosylateAr-H),
7.62–7.84 (m, 17 H, Ar-H) ppm. 13C{1H} NMR (CDCl3,
3
2
75.5 MHz): δ = 14.7 (d, JP,C = 17.2 Hz, CH2CH3), 16.0 (d, JP,C
= 4.0 Hz, CH2CH3), 20.9 (CCH3), 23.1 (d, 1JP,C = 50.6 Hz, PCH2),
Single crystals were obtained by slow diffusion of diethyl ether into
a dichloromethane solution of the salt. Single-crystal diffraction
data for 1–3, 6–9 were recorded at ca. 296 K with a Rigaku R-
axis II diffractometer equipped with a molybdenum rotating anode
source and an image plate detector system. Data for 4 and 5 were
recorded with a Bruker Smart 6000 diffractometer equipped with
a CCD detector and a copper-tube source. Structures were solved
and refined using SHELXL.[26] During final refinement, aromatic
C–H atoms were riding on the atoms they are bonded to with a
bond length of 0.93 Å. Powder diffraction data confirm that the
crystal structures obtained are representative of the bulk samples,
except for 8, where the amount of crystalline material obtained was
too small for powder diffraction.
1
117.9 (d, JP,C
=
85.6 Hz, phosphoniumAr-Cipso), 125.8, 127.8
(
tosylateAr-CH), 130.0, 130.2, 133.0, 133.2, 134.5 (phosphoniumAr-
CH), 137.8, 144.5
tosylateAr-Cipso ppm. 31P NMR (D2O,
121 MHz): δ = 23.9 (phosphoniumP) ppm.
(
)
n-Butyltriphenylphosphonium Tosylate (3): n-Butyl p-toluenesul-
fonate (3.5 g, 15.3 mmol) and triphenylphosphane (4.0 g, 15.3 mmol)
afforded a crystalline white solid (7.0 g, 93%). M.p. 132–134 °C
(ref.[21] 139–140 °C, ref.[12] 116–117 °C). FAB+: m/z (%) = 319 (100)
1
[M – C7H7SO3]+, 809 (11= [2M – C7H7SO3]+. H NMR (CDCl3,
3
300 MHz): δ = 0.86 (t, JH,H = 7.1 Hz, 3 H, CH2CH3), 1.52 (m,
4 H, CH2CH2CH3), 2.28 (s, 3 H, CCH3), 3.50 (m, 2 H, PCH2), 7.01
3
(d, JH,H = 7.9 Hz, 2 H, tosylateAr-H), 7.57–7.79 (m, 17 H, Ar-H)
ppm. 13C{1H} NMR (CDCl3, 75.5 MHz): δ = 13.3 (CH2CH3), 20.9
CCDC-277854 to -277862 contain the supplementary crystallo-
graphic 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.
1
2
(CCH3), 21.3 (d, JP,C = 50.6 Hz, PCH2), 23.2 (d, JP,C = 16.7 Hz,
3
1
PCH2CH2), 24.5 (d, JP,C = 4.0 Hz, CH2CH3), 118.0 (d, JP,C
85.6 Hz, phosphoniumAr-Cipso), 125.9, 127.8 (tosylateAr-CH), 130.1,
130.2, 133.1, 133.2, 134.6
phosphoniumAr-CH), 137.9, 144.5
tosylateAr-Cipso). 31P NMR (D2O, 121 MHz): δ = 24.2 (phosphoniumP)
ppm.
=
(
Syntheses: All manipulations were performed under argon using
standard Schlenk techniques. All solvents were dried with the ap-
propriate drying agent and distilled under dinitrogen: sodium
benzophenone ketyl (diethyl ether, toluene) or calcium hydride
(dichloromethane). Unless otherwise stated, all reagents were pur-
chased from Aldrich, Acros or Lancaster and used as supplied. All
the p-toluenesulfonates were obtained in good yield and high purity
using known literature methods.[27]
(
n-Pentyltriphenylphosphonium Tosylate (4): n-Pentyl p-toluenesul-
fonate (3.8 g, 15.6 mmol) and triphenylphosphane (4.1 g,
15.6 mmol) afforded a crystalline white solid (6.9 g, 87%). M.p.
123–125 °C. FAB+: m/z (%) = 333 (100) [M – C7H7SO3]+, 837 (9)
3
[2M – C7H7SO3]+. 1H NMR (CDCl3, 300 MHz): δ = 0.76 (t, JH,H
= 7.1 Hz, 3 H, CH2CH3), 1.24 (m, 2 H, CH2CH3), 1.53 (m, 4 H,
PCH2CH2CH2CH2), 2.31 (s, 3 H, CCH3), 3.61 (m, 2 H, PCH2),
7.08 (d, 3JH,H = 8.0 Hz, 2 H, tosylateAr-H), 7.62–7.84 (m, 17 H, Ar-
H) ppm. 13C{1H} NMR (CDCl3, 75.5 MHz): δ = 13.2 (CH2CH3),
20.8 (CCH3), 21.3 (d, 1JP,C = 48.9 Hz, PCH2), 21.6 (CH2CH3), 21.7
The same method was used for all salts. Salts 1–5 were obtained
by refluxing a 1:1 molar ratio solution of the alkyl p-toluenesul-
fonate (ca. 4 g) and triphenylphosphane (ca. 5 g) in toluene (ca.
50 mL) for 18 hours under argon. The solvent was then removed
under reduced pressure and the residual was washed with diethyl
ether. The resulting solid was recrystallised from dichloromethane/
diethyl ether to a crystalline white solid.
3
2
(d, JP,C = 4.6 Hz, CH2CH2CH3), 31.8 (d, JP,C = 15.5 Hz,
1
PCH2CH2), 117.9 (d, JP,C = 85.6 Hz, phosphoniumAr-Cipso), 125.7,
127.7
(
(
tosylateAr-CH), 130.0, 130.1, 133.0, 133.1, 134.5
The appropriate methoxycarbonylalkyl p-toluenesulfonate was
used for 6 and 7. The recrystallised product obtained for 6 had a
pale brown colouration. For 8, methyl 3-phenyl-2-(tolyl-4-sul-
fonyloxy)propionate was used and the solution was refluxed for
42 hours. The final recrystallised product was pale yellow in colour.
A solution of ethyl p-toluenesulfonate and tributylphosphane was
refluxed as described above to yield 9.
phosphoniumAr-CH), 137.8, 144.4 (tosylateAr-Cipso) ppm. 31P NMR
(D2O, 121 MHz): δ = 24.1 (phosphoniumP) ppm.
n-Octyltriphenylphosphonium Tosylate (5): n-Octyl p-toluenesul-
fonate (2.5 g, 8.8 mmol) and triphenylphosphane (2.3 g, 8.8 mmol)
afforded a crystalline white solid (3.1 g, 65%). M.p. 129–131 °C,
(ref.[12] 70–71 °C, ref.[21] 70–78 °C). FAB+: m/z (%) = 375 (100)
Eur. J. Inorg. Chem. 2006, 437–446
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
445

L. G. Bonnet, B. M. Kariuki
FULL PAPER
[M – C7H7SO3]+, 922 (6) [2M – C7H7SO3]+. 1H NMR (CDCl3, CH2CH2CH3), 23.0 (d, JP,C = 15.5 Hz, PCH2CH2), 125.2, 127.5
2
3
300 MHz): δ = 0.79 (t, JH,H = 6.6 Hz, 3 H, CH2CH3), 1.11 (m, 8
(
tosylateAr-CH), 137.8, 144.1 (tosylateAr-Cipso) ppm. 31P NMR (D2O,
H, CH2CH3), 1.48 (m, 4 H, PCH2CH2CH2CH2), 2.26 (s, 3 H, 121 MHz): δ = 34.6 (phosphoniumP) ppm.
3
CCH3), 3.46 (m, 2 H, PCH2), 7.00 (d, JH,H = 8.1 Hz, 2 H,
tosylateAr-H), 7.62–7.76 (m, 17 H, Ar-H) ppm. 13C{1H} NMR
Acknowledgments
(CDCl3, 75.5 MHz): δ = 13.6 (CH2CH3), 20.8 (CCH3), 21.4 (d,
1JP,C = 50.6 Hz, PCH2), 22.0, 28.4, 28.6 (PCH2CH2CH2CH2CH2),
The University of Birmingham is thanked for continued support.
2
22.1 (CH2CH3), 29.8 (d, JP,C
= 15.5 Hz, PCH2CH2), 31.2
(CH2CH2CH3), 117.8 (d, JP,C = 85.6 Hz, phosphoniumAr-Cipso),
1
[1] P. Walden, Bull. Acad. Imp. Sci. St. Petersbourg 1914, 1800.
[2] R. D. Rogers, K. R. Seddon, in: Ionic Liquids: Industrial Appli-
cations to Green Chemistry, ACS Symposium Series 818,
American Chemical Society, Washington, DC, 2002.
[3] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis,
Wiley-VCH: Weinheim, 2003.
125.8, 127.7 (tosylateAr-CH), 129.9, 130.1, 133.0, 133.1, 134.5
(
phosphoniumAr-CH), 137.9, 144.1 (tosylateAr-Cipso) ppm. 31P NMR
(D2O, 121 MHz): δ = 24.3 (phosphoniumP) ppm.
[(Methoxycarbonyl)methyl]triphenylphosphonium
Tosylate
(6):
(Methoxycarbonyl)methyl p-toluenesulfonate (2.9 g, 11.9 mmol)
and triphenylphosphane (3.1 g, 11.9 mmol) afforded a crystalline
pale brown solid (4.7 g, 78%). M.p. 164–166 °C. FAB+: m/z (%) =
335 (100) [M – C7H7SO3]+, 841 (9) [2M – C7H7SO3]+. 1H NMR
(CDCl3, 300 MHz): δ = 2.32 (s, 3 H, CCH3), 3.59 (s, 3 H, OCH3),
[4] C. Chiappe, D. Pieraccini, J. Phys. Org. Chem. 2005, 18, 275–
297.
[5] a) C. M. Gordon, Appl. Catal., A 2001, 222, 101–117; b) J. G.
Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer,
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248, 2459–2477; f) J. Dupont, R. F. de Souza, P. A. Z. Suarez,
Chem. Rev. 2002, 102, 3667–3692.
2
3
5.30 (d, JP–H = 13.5 Hz, 2 H, PCH2), 7.08 (d, JH,H = 8.0 Hz, 2
H, tosylateAr-H), 7.62–7.87 (m, 17 H, Ar-H) ppm. 13C{1H} NMR
1
(CDCl3, 75.5 MHz): δ = 21.2 (CCH3), 31.3 (d, JP,C = 58.0 Hz,
PCH2), 53.2 (OCH3), 117.9 (d, 1JP,C = 89.1 Hz, phosphoniumAr-Cipso),
126.1, 127.1 (tosylateAr-CH), 130.1, 130.3, 133.7, 133.8, 135.0
[6] D. E. Kaufmann, M. Nouroozin, H. Henze, Synlett 1996,
2
(
phosphoniumAr-CH), 138.3, 144.3 (tosylateAr-Cipso), 165.3 (d, JP,C
=
1091–1092.
3.4 Hz, CO) ppm. 31P NMR (D2O, 121 MHz):
phosphoniumP) ppm.
δ = 20.8
[7] M. Badri, J. J. Brunet, R. Perron, Tetrahedron Lett. 1992, 33,
4435–4438.
(
[8] J. F. Knifton, J. Am. Chem. Soc. 1981, 103, 3959–3961.
[9] J. F. Knifton, J. Mol. Catal. 1987, 43, 65–77.
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Zhou, Green Chem. 2003, 5, 143–152.
[12] N. Korodia, S. Guise, C. Newlands, J. Andersen, Chem. Com-
mun. 1998, 2341–2342.
[1-(Methoxycarbonyl)ethyl]triphenylphosphonium Tosylate (7): 1-
(Methoxycarbonyl)ethyl p-toluenesulfonate (1.4 g, 5.4 mmol) and
triphenylphosphane (1.42 g, 5.4 mmol) in toluene (50 mL) afforded
crystalline white needles (1.8 g, 64%). M.p. 175–176 °C. FAB+: m/
1
z (%) = 349 (100) [M – C7H7SO3]+, 869 (4) [2M – C7H7SO3]+. H
3
2
NMR (CDCl3, 300 MHz): δ = 1.67 (dd, JH,H = 7.0, JP–H
=
18.6 Hz, 3 H, CHCH3), 2.33 (s, 3 H, CCH3), 3.54 (s, 3 H, OCH3), [13] P. Ludley, N. Korodia, Tetrahedron Lett. 2001, 42, 2011–2014.
3
6.36 (m, 1 H, PCH), 7.11 (d, JH,H = 7.7 Hz, 2 H, tosylateAr-H),
[14] C. L. Johnson, R. E. Donkor, W. Nawaz, N. Karodia, Tetrahe-
dron Lett. 2004, 45, 7359–7361.
[15] C. Comyns, N. Karodia, S. Zeler, J. Andersen, Catal. Lett.
2000, 67, 113–115.
[16] J. Dupont, J. Braz. Chem. Soc. 2004, 15, 341–350.
[17] A. Elaiwi, P. B. Hitchcock, K. R. Seddon, N. Srinivasan, Y.-M.
Tan, T. Welton, J. A. Zoraa, J. Chem. Soc., Dalton Trans. 1995,
3467–3472.
[18] J. van den Broeke, M. Stam, M. Lutz, H. Kooijman, A. L.
Spek, B.-J. Deelman, G. van Koten, Eur. J. Inorg. Chem. 2003,
2798–2811.
[19] C. S. Consorti, P. A. Z. Suarez, R. F. de Souza, R. A. Burrow,
D. H. Farrar, A. J. Lough, W. Loh, L. H. M. da Silva, J. Du-
pont, J. Phys. Chem. B 2005, 109, 4341–4349.
[20] , J. Dupont, P. A. Z. Suarez, R. F. de Souza, R. A. Burrow, J.-
P. Kinkzinger, Chem. Eur. J. 2000, 6, 2377–2377.
[21] D. Klamann, P. Weyerstahl, Ber. 1964, 97, 2534–2538.
[22] D. E. Bugner, J. Org. Chem 1989, 54, 2580–2586.
[23] W. G. Dauben, J. M. Gerdes, R. A. Bunce, J. Org. Chem. 1984,
49, 4293–4295.
[24] P. Calcagno, B. M. Kariuki, S. Kitchin, J. M. A. Robinson, D.
Philp, K. D. M. Harris, Chem. Eur. J. 2000, 6, 2338–2349.
[25] D. Braga, F. Grepioni, E. Tedesco, Organometallics 1998, 17,
2669–2672.
7.64–7.96 (m, 17 H, Ar-H) ppm. 13C{1H} NMR (CDCl3,
2
75.5 MHz): δ = 13.0 (d, JP,C = 13.1 Hz, CHCH3), 21.2 (CCH3),
1
1
36.2 (d, JP,C = 48.2 Hz, PCH), 53.3 (OCH3), 118.0 (d, JP,C
79.7 Hz, phosphoniumAr-Cipso), 126.1, 128.2 (tosylateAr-CH), 130.0,
130.2, 134.0, 134.1, 134.7
phosphoniumAr-CH), 138.7, 144.5
=
(
2
(
tosylateAr-Cipso), 166.1 (d, JP,C = 3.2 Hz, CO). 31P NMR (D2O,
121 MHz): δ = 28.0 (phosphoniumP) ppm.
[1-(Methoxycarbonyl)-2-phenylethyl]triphenylphosphonium Tosylate
(8): Methyl 3-phenyl-2-(tolyl-4-sulfonyloxy)propionate (0.5 g,
1.5 mmol) and triphenylphosphane (0.4 g, 1.5 mmol) in toluene
(20 mL) afforded a crystalline pale yellow solid (0.5 g, 56%). The
material was the most difficult to obtain and it was not possible to
carry out full characterisation.
Tributyl(ethyl)phosphonium Tosylate (9): Ethyl p-toluenesulfonate
(2.0 g, 10.0 mmol) and tributylphosphane (2.0 g, 10.0 mmol) in tol-
uene (50 mL) afforded a crystalline white solid (3.7 g, 93%). M.p.
74–76 °C (ref.[12] 81–83 °C). FAB+: m/z (%) = 231 (100) [M –
C7H7SO3]+. 1H NMR (CDCl3, 300 MHz): δ = 0.97 (m, 9 H,
CH2CH2CH3), 1.22 (m, 3 H, PCH2CH3), 1.46 (m, 12 H,
PCH2CH2CH2), 2.28 (m, 6 H, PCH2CH2), 2.31 (s, 3 H, CCH3),
[26] G. M. Sheldrick, SHELXL. Program package for crystal struc-
ture determination. University of Göttingen, Germany.
[27] L. F Fieser, M. Fieser, in: Reagents for Organic Synthesis,
Wiley, New York, 1967.
3
2.42 (m, 2 H, PCH2), 7.10 (d, JH,H = 8.0 Hz, 2 H, tosylateAr-H),
3
7.78 (d, JH,H = 8.0 Hz, 2 H, tosylateAr-H) ppm. 13C{1H} NMR
2
(CDCl3, 75.5 MHz): δ = 5.1 (d, JP,C = 5.7 Hz, CH2CH3), 11.5 (d,
1
1JP,C = 48.9 Hz, PCH2CH3), 12.6 (CH2CH2CH3), 17.2 (d, JP,C
=
Received: August 31, 2005
Published Online: November 9, 2005
3
47.7 Hz, PCH2CH2), 20.4 (CCH3), 22.7 (d, JP,C
=
4.6 Hz,
446
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 437–446