Introduction of hydrophilic groups onto the ortho-position of benzoate anions induced phase separation of the corresponding ionic liquids with water

Article abstract of DOI:10.1039/c3cc45671d

Ionic liquids (ILs) composed of tetrabutylphosphonium cations and benzoate anion derivatives with hydrophilic hydroxyl or carboxyl groups introduced onto the ortho-position were found to be less hydrophilic and showed phase separation with water, whereas unmodified benzoate-type IL was freely miscible with water.

Full text of DOI:10.1039/c3cc45671d

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Introduction of hydrophilic groups onto the ortho-
position of benzoate anions induced phase separation
of the corresponding ionic liquids with water†
Cite this: Chem. Commun., 2013,
4
9, 10248
Received 25th July 2013,
Accepted 10th September 2013
ab
ab
ab
ab
Takashi Ando, Yuki Kohno, Nobuhumi Nakamura and Hiroyuki Ohno*
DOI: 10.1039/c3cc45671d
www.rsc.org/chemcomm
Ionic liquids (ILs) composed of tetrabutylphosphonium cations and
benzoate anion derivatives with hydrophilic hydroxyl or carboxyl
groups introduced onto the ortho-position were found to be less
hydrophilic and showed phase separation with water, whereas
unmodified benzoate-type IL was freely miscible with water.
Ionic liquids (ILs), organic salts with very low melting tempera-
tures, provide a variety of interesting properties as liquids, and
have been considered as novel solvents which are alternatives to
molecular solvents. Suitable design of component ions of the
ILs enables us to control liquid properties of the ILs including
polarity, viscosity, and miscibility with other molecular solvents.
Fig. 1 Structure of component ions used in this study. Positions of the hydrophilic
groups are denoted as -o-, -m-, and -p- for ortho-, meta-, and para-, respectively.
1
–3
Much interest has been focused on the ILs showing functions, both species and position of the introduced hydrophilic groups
4
now classified as task-specific ILs or functional ILs. We have and phase behaviour of the corresponding ILs with water.
+
prepared a series of functional ILs that can be used to dissolve
Various ILs composed of tetrabutylphosphonium cations ([P4444] )
and benzoate anions having hydroxyl or carboxyl groups on the
benzene ring (Fig. 1) have been prepared. The obtained salts were
5
6
7
celluloses, stabilise proteins, realise liquid crystalline properties,
8,9
or to show dynamic phase transition with water.
To prepare functional ILs, introduction of suitable functional mixed with an equal weight of pure water. Table 1 shows both
9
–11
groups onto the component ions is an easy method.
In this thermal properties of thus prepared ILs and their phase behaviour
context, benzoate anion derivatives would be one of the attractive after mixing with water. It was found that phase behaviour of the
candidates for introducing the functional groups on them. Some prepared IL–water mixtures depended strongly on both species
ILs composed of ions with a benzene moiety have been reported and position of the hydrophilic groups on the benzoate anions.
to show various functions such as mediators for indirect electro- [P4444][Bz] was miscible with water regardless of the mixing ratio. On
12
13
lysis and buffers in non-aqueous media. Through the basic the other hand, [P4444][Bz-o-OH], in which the hydroxyl group was
study of functional ILs, we have prepared several ILs composed of attached onto the ortho-position of the anion, was found to show
benzoate derivatives having hydroxyl or carboxyl groups, both of liquid–liquid type phase separation at 60 1C after mixing with water.
which are typical hydrophilic groups. As a preliminary study, This result shows that [P4444][Bz-o-OH] became less hydrophilic than
a few ILs having these hydrophilic groups have been found to be [P4444][Bz] in spite of the introduction of the hydrophilic hydroxyl
less hydrophilic, and underwent clear phase separation with group. The other two salts with the hydroxyl group attached onto
water. This finding pushed us to analyse the relationship between the meta- and para-position were solid at 60 1C with poor water
solubility. The elevated melting point was explained to be due to
a
hydrogen bonding. A hydroxyl group acts as both a proton donor
and acceptor. However, the effect of the hydroxyl group was different
from other cases when it was introduced onto the ortho-position.
Department of Biotechnology, Tokyo University of Agriculture and Technology,
-24-16, Naka-cho, Koganei, Tokyo 184-8588, Japan. E-mail: ohnoh@cc.tuat.ac.jp;
2
Fax: +81 42 388 7024; Tel: +81 42 388 7024
b
Functional Ionic Liquid Laboratories, Graduate School of Engineering,
Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei,
Tokyo 184-8588, Japan
m
[P4444][Bz-o-OH] gave lower T than other isomers.
We have already proposed a hydrophilicity index (HI) value, the
number of water molecules per ion pair in the phase-separated IL
†
Electronic supplementary information (ESI) available: Synthetic procedure and
phase at 60 1C, as an index for evaluating the hydrophilicity of the
hydrophobic ILs. In this study, we also applied the HI value to
characterisation of ILs. CCDC 950650. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3cc45671d
14
1
0248 Chem. Commun., 2013, 49, 10248--10250
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Table
1
Physico-chemical properties of ILs and their phase behaviour showed liquid–liquid type phase separation with water, whereas the
with water
hydrophilicity of the salts was different. The HI value of [P4444]-
[
Bz-o-COOH] and [P4444][Bz-m-COOH] was 3.5 Æ 0.1 and 4.4 Æ 0.3,
Pure ILs
Abbr.
IL–water mixture (50 wt%)
respectively. It means that the introduction of a carboxyl group onto
the ortho-position of [Bz] gave less hydrophilic ILs than that onto
the meta-position. The results of [P4444][Bz-o-COOH] suggest
that the introduction of the hydrophilic carboxyl group onto the
ortho-position is also effective in obtaining less hydrophilic ILs.
c
d
T
m
(1C)
T
g
(1C)
Phase state
HI
À
b
[P
[P
[P
[P
[P
[P
[P
4444][Bz]
44
59
118
93
47
140
82
–54
—
—
—
M
L
S
—
a
4444][Bz-o-OH]
4444][Bz-m-OH]
4444][Bz-p-OH]
4444][Bz-o-COOH]
4444][Bz-m-COOH]
9.1 Æ 0.9
a
b
—
a
b
S
—
a
—
L
L
M
3.5 Æ 0.1
À
Anomalously, a [Bz-p-COOH] -based salt was yielded as a dianionic
IL ([P4444] [Bz-p-COO]) (see ESI†), and became homogeneously
2
0
4.4 Æ 0.3
a
b
4444
]
2
[Bz-p-COO]
—
—
miscible with water.
M: miscible, L: liquid–liquid phase separation, S: solid–liquid phase
separation. Not detected. Not measured. At 60 1C. Number of
water molecules per ion pair in the separated IL phase at 60 1C.
a
b
c
d
The HI value of thus prepared ILs in this study varied widely with
both the structure and position of the introduced hydrophilic groups.
The HI value of [P4444][Bz-o-COOH] was lower than that of [P4444]-
compare the hydrophilicity of thus prepared ILs. [P4444][Bz-o-OH] [Bz-o-OH]. Since [P4444][Bz-o-COOH] has a HI value below 7, it showed
underwent phase separation with water, and the HI value was stable phase separation with water regardless of temperature. As
determined to be 9.1 Æ 0.9. This value was sufficient to show the [P4444][Bz-m-COOH] had a HI value below 7, we expected that this IL
temperature-responsive lower critical solution temperature (LCST)- also showed the same phase behaviour as [P4444][Bz-o-COOH], but it
type phase transition, in which the separated phases turn into a was not the case. After mixing with water, the hydrated [P4444]-
8
homogeneous phase upon cooling. We have reported that hydro- [Bz-m-COOH] phase became a gel-like solid upon cooling at room
phobic ILs having a HI value of less than 7 make stable phase temperature (see Fig. S1, ESI†) and [P4444][Bz-m-COOH] showed stable
separation with water regardless of temperature, but those having phase separation with water upon heating at only around 60 1C.
a HI value of more than 7 showed LCST-type phase transition with
Some scientists have previously reported that the increase in
9
water. As expected, [P4444][Bz-o-OH] underwent the LCST beha- formula weight of ILs by introducing alkyl chains onto ions decreased
16,17
viour with water. Fig. 2 shows the phase separation temperature the miscibility with water.
To compare the relation between the
(
Tc) of the [P4444][Bz-o-OH]–water mixtures as a function of water formula weight and hydrophilicity of the ILs, the HI value of ILs
+
content. The T value at a water content of 50% was 30 1C, which prepared in this study was compared with that of other [P4444] -type
was comparable to that of [P4444] -based ILs containing trifluoro- ILs reported previously as a function of formula weight. We adopted
acetate or 2,4,6-trimethylbenezenesulfonate ([TMBS] ) anions. It several [P4444] -type ILs with benzenesulfonate derivatives here since
c
+
18
À
15
+
is clear that the introduction of the hydroxyl group onto the ortho- other prepared ILs with benzoate derivatives except for [P4444]-
À
position of [Bz] resulted in lowering of the hydrophilicity of ILs. [Bz-o-OH], [P4444][Bz-o-COOH] and [P4444][Bz-m-COOH] were miscible
In the case of ILs having a suitable HI value as a result of with water as described below. The HI value of ILs composed of
intramolecular interaction of hydrophilic groups, they show benzenesulfonate anion derivatives having different numbers of alkyl
À
À
LCST-type phase separation with water at ambient temperature. chains (tosylate ([TsO] ), 2,4-dimethylbenzenesulfonate ([DMBS] ),
À
This should be categorised as a unique method to lower the and [TMBS] ) gradually decreased by increasing their formula weight.
hydrophilicity of ILs without introducing hydrophobic groups. Against these, the HI value of [P4444][Bz-o-OH] was much lower than
Since the carboxyl group is also hydrophilic and forms hydrogen that of [P4444][TsO] in spite of smaller formula weight (see Fig. S2(&),
bonding, the effect of introduction of the carboxyl group onto the ESI†). Furthermore, the HI value of [P4444][Bz-o-COOH] was the lowest
benzoate anions on the phase behaviour of the corresponding ILs among ILs in this study (Fig. S2(’), ESI†). The HI value of [P4444]-
has been studied. Both [P4444][Bz-o-COOH] and [P4444][Bz-m-COOH] [Bz-m-COOH] was higher than that of [P4444][Bz-o-COOH]. Introduction
of the hydrophilic groups onto the ortho-position was an important
factor to lower the hydrophilicity of ILs through intramolecular
interaction without drastic increase in their formula weight.
To analyse the role of attached hydrophilic groups in the phase
behaviour with water, we first focused on the hydroxyl group and
prepared tetrabutylphosphonium o-methoxybenzoate ([P4444]-
[
Bz-o-OCH
group was introduced onto the ortho-position of the [Bz] . The
resulting IL ([P4444][Bz-o-OCH ]) was mixed with water to compare
the phase behaviour to that of the [P4444][Bz-o-OH]–water mixture.
3
]), where the methoxy group instead of the hydroxyl
À
3
[
P4444][Bz-o-OCH ] was found to be miscible with water in spite of
3
the empirical fact that the methoxy group is much less hydrophilic
than the hydroxyl group. The obtained results cannot be explained
by simple hydrophilicity of the introduced groups. Based on the
aforementioned results, we explained the less hydrophilicity of
[P4444][Bz-o-OH] and [P4444][Bz-o-COOH] with ‘‘proton sharing’’
Fig. 2 Phase transition temperature of the [P4444][Bz-o-OH]–water mixture at
different amounts of water.
between the carboxylate anion and the attached group as shown
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anion was studied using infra-red spectroscopy (see Fig. S6, ESI†).
In the case of [P4444][Bz], [P4444][Bz-o-OH], and [P4444][Bz-o-COOH],
the symmetric stretching bands derived from carboxylate anions
À1
were observed at around 1350 cm . On the other hand, the CQO
stretching band of the uncharged carboxyl group was observed at
À1
1
709 cm . This band clearly shows that the negative charge of
À
À
[Bz-o-SO
3
]
exists on the SO
3
group. The IR spectra also imply
Fig. 3 Possible anion structure of both [P4444][Bz-o-OH] (left) and [P4444][Bz-o-COOH]
(
right).
that the uncharged carboxyl group should form intermolecular
hydrogen bonding between the anions. This indicates that not all
in Fig. 3. For offering further insight into the existence of the proton of the introduced hydrophilic groups always form the proton
sharing, we analysed the crystal structure of [P4444][Bz-o-OH] by single- sharing, and a suitable choice of the hydrophilic group is impor-
crystal X-ray diffraction measurement. The obtained single crystal was tant to design less hydrophilic ILs. All the aforementioned results
19
analysed with R-AXIS RAPID. As illustrated in Fig. S3 (ESI†), both indicate that introduction of the suitable hydrophilic groups such
À
the hydrogen atom of the hydroxyl group and the oxygen atom onto as hydroxyl and carboxyl groups onto the ortho-position of [Bz]
the carboxylate anion were found to lie in the same plane.
gives less hydrophilic ILs, derived from the proton sharing. This
There should be a few interpretations on intramolecular hydrogen would provide a novel method to obtain less hydrophilic ILs
bonding especially in the case where the geometry is deviated from without introduction of fluorine atoms or long alkyl chains onto
the collinear position. It is true that proton sharing seen in both anions, usually applied to lower the hydrophilicity of ILs. It also
salicylic acid and phthalic acid is accepted as ‘‘intramolecular suggests that ILs composed of benzoate anion derivatives would
2
0,21
hydrogen bonding’’.
the hydrogen atom of the hydroxyl group and the oxygen atom of with their tuneable hydrophilicity.
the carboxylic acid group for salicylic acid were 1.785 Å and 142.851, In conclusion, the introduction of adequate hydrophilic groups
The estimated distance and angle between provide the structural platform to design functional ILs together
22
À
respectively. In the case of [P4444][Bz-o-OH], the distance and the onto the ortho-position of the [Bz] was effective to lower the
angle were found to be 1.507 Å and 163.091 (see Fig. S4 and S5, hydrophilicity of the corresponding ILs due to the proton sharing,
ESI†). According to these data, the hydrogen atom of the OH group and thus prepared ILs underwent phase separation after mixing
was located closer to the oxygen atoms as compared to salicylic acid, with water.
making the angle more collinear. These results suggest that proton
The authors would like to thank Prof. Dr Keiichi Noguchi for
sharing seen in [P4444][Bz-o-OH] is regarded to be distorted hydrogen valuable comments and advice on single-crystal X-ray diffrac-
bonding. However, the angle for O–H–O of [P4444][Bz-o-COOH] tion measurement. This study was supported by a Grant-in-Aid
should be too sharp to form a hydrogen bond, suggesting the for Scientific Research from the Japan Society for the Promo-
proton sharing. Since a single-crystal of [P4444][Bz-o-COOH] has not tion of Science (Grant No. 21225007).
been obtained yet, we cannot discuss the detailed position of shared
protons of [P4444][Bz-o-COOH]. Here, we discuss the proton sharing Notes and references
including the distorted hydrogen bonding. In any case, the proton
1
2
T. Welton, Chem. Rev., 1999, 99, 2071.
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We have also investigated the effect of other hydrophilic
groups on the phase behaviour of the resulting ILs with water.
3 R. D. Rogers and K. R. Seddon, Science, 2003, 792, 202.
4
E. D. Bates, R. D. Mayton, I. Ntai and J. H. Davis Jr, J. Am. Chem. Soc.,
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2
+
We prepared [P4444] -type IL with 2-aminobenzoate as an anion
5
(
[P4444][Bz-o-NH
2
]), where hydrophilic amino groups were intro-
6 K. Fujita, D. R. MacFarlane and M. Forsyth, Chem. Commun., 2005, 4804.
7 T. Ichikawa, M. Yoshio, S. Taguchi, J. Kagimoto, H. Ohno and
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À
duced onto the ortho-position of [Bz] , and analysed the phase
behaviour with water. [P4444][Bz-o-NH ] was found to be miscible
2
with water. We next prepared the ILs composed of [P4444] and
benzoate anion derivatives attached with the sulfonic acid group
8
K. Fukumoto and H. Ohno, Angew. Chem., Int. Ed., 2007, 46, 1852.
+
9 Y. Kohno and H. Ohno, Chem. Commun., 2012, 48, 7119.
0 B. E. Gurkan, J. C. Fuente, E. M. Mindrup, L. E. Ficke, B. F. Goodrich,
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1
À
À
(2-sulfobenzoate ([Bz-o-SO ] ), 3-sulfobenzoate ([Bz-m-SO ] ), and
3
3
À
4
-sulfobenzoate ([Bz-p-SO
3
] )), and analysed the phase behaviour 11 R. Giernoth, Angew. Chem., Int. Ed., 2010, 49, 2834.
1
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of these ILs after mixing with water. Both [P4444][Bz-o-SO
4444][Bz-m-SO ] were found to show liquid–liquid type phase
separation after mixing with water, whereas [P4444][Bz-p-SO
3
] and
2
[P
3
1
3
] was 14 Y. Kohno and H. Ohno, Phys. Chem. Chem. Phys., 2012, 14, 5063.
1
1
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hardly soluble in water and showed solid–liquid type phase
separation with water. Table S2 (ESI†) shows that the HI value of
[
P4444][Bz-o-SO ] was 1.1; this was the lowest value for ILs analysed 17 J. Kagimoto, S. Taguchi, K. Fukumoto and H. Ohno, J. Mol. Liq.,
3
2010, 153, 133.
in this study. Introduction of the sulfonic acid group onto the
benzoate anion suppressed the dissociation of the carboxyl group
1
1
8 Y. Kohno and H. Ohno, Aust. J. Chem., 2012, 65, 91.
9 CCDC 950650.
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3, 16.
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between the introduced hydrophilic group and the carboxylate 22 P. Munshi and T. N. G. Row, Acta Crystallogr., Sect. B, 2006, 62, 612.
9
the less hydrophilicity of [P4444][Bz-o-SO
from proton sharing but in a totally different manner. Interaction
3
] should not be derived
2
9
1
0250 Chem. Commun., 2013, 49, 10248--10250
This journal is c The Royal Society of Chemistry 2013