| summary: | Introduction
Molecular design of ionic liquids, which are entirely composed
of ions and melt below room temperature (RT) or 100
°C, appears to be one of the most grown areas in chemistry.1 A
special fascination of the ionic liquids is that the selection of
component ions not only can tailor the liquid properties such
as melting point, miscibility, solubility, polarity, electrochemical
window, viscosity, and ionic conductivity1,2 but also can
introduce new functionalities such as paramagnetic,3 luminescence,
4 and redox properties.5 For these reasons, the ionic liquids
increasingly came to be called the “designer solvents”.6
However, although it is known that this sort of anions exerts a
drastic effect on various liquid properties,2,7 the main developments
of the ionic liquids have tended to focus on the
modification of the cationic structures while keeping the anion
invariant. This is mainly caused by the inorganic nature of most
anions such as BF4
-, PF6
-, AlCl4
-, N(CN)2
-, and N(SO2CF3)2
-.
They are originally difficult chemically modify, with some
exceptions such as BF3X- (X ) CnH2n+1,8 CnF2n+1,9 or MeCHCN10),
PF3(CnF2n+1)3
-,11 N(SO2CnF2n+1)X- (X ) SO2CmF2m+1
or COCF3),7b,12 and C(CN)2X- (X ) H, NO, NO2,13 or CN13,14)
anions. It is thus obvious that combination with the chemically
tunable organic anions would open up not only an ongoing
exploration of new and more versatile ionic liquids but also an
in-depth understanding of the relationship between the molecular
structures of component ions and liquid properties. However,
despite the presence of some ionic liquids formed with organic
anions,1e,3d,e,7h,j,15 there are no systematic studies on the influence
of the substituents of anions, especially in terms of the electronwithdrawing
or -donating ability, on liquid properties.
In this work, we prepared a series of 1-ethyl-3-methylimidazolium
(EMI) and 1-butyl-3-methylimidazolium (BMI) salts
made by combining the 1,1,3,3-tetracyanoallyl (TCA) anion and
its derivatives, replacing the 2-hydrogen with methyl (2-methyl-
1,1,3,3-tetracyanoallyl; Me-TCA), methoxy (2-methoxy-1,1,3,3-
tetracyanoallyl; MeO-TCA), cyano (1,1,2,3,3-pentacyanoallyl;
PCA), and cyanomethyl (2-cyanomethyl-1,1,3,3-tetracyanoallyl;
CM-TCA) groups (Scheme 1). Their liquid properties such as
melting and decomposition temperatures, viscosity, ionic conductivity,
ion association, and ion solvation (Gutmann’s acceptor
(AN) and donor numbers (DN)16) were characterized. Comparable
anion sizes, namely, van der Waals (vdW) radii (ra) of
3.08 Å for TCA, 3.21 Å for Me-TCA, 3.28 Å for MeO-TCA,
3.22 Å for PCA, and 3.37 Å for CM-TCA anions (Table 1),17
with the same basic skeleton give us a good opportunity for
studying the influence of the electron-withdrawing or -donating
ability of the 2-substituents on liquid properties. Many anions
including natural amino anions1e,7j used so far are composed of
a neutral moiety with attached acidic groups such as carboxylic
* To whom correspondence should be addressed. E-mail: yyoshida@
ccmfs.meijo-u.ac.jp (Y.Y.); gsaito@ccmfs.meijo-u.ac.jp (G.S.).
† Meijo University.
‡ Kyoto University.
SCHEME 1: Molecular Structures of TCA-Type Anions
8960 J. Phys. Chem. B 2009, 113, 8960–8966
10.1021/jp903242w CCC: $40.75 2009 American Chemical Society
Published on Web 06/11/2009
Downloaded by AUSTRIA CONSORTIA on July 6, 2009
Published on June 11, 2009 on http://pubs.acs.org | doi: 10.1021/jp903242w
and sulfonic groups,3d,e,15d which would result in the decrease
of both fluidity and ionic conductivity due to the confined
negative charge to the acidic group. On the other hand, the
negative charge is well delocalized over the TCA-type anions
(vide infra) as a consequence of the multiple cyano groups with
high electron-withdrawing ability, and thus, the high fluidity
and ionic conductivity are anticipated.
Ionic liquids have been considered as green solvents mainly
associated with their negligible vapor pressure, and environmental
and safety problems arising from the volatility of
molecular solvents could be avoided by the use of such involatile
liquids.18 Typical ionic liquids are composed of halogencontaining
anions (e.g., BF4
-, PF6
-, AlCl4
-, and N(SO2CF3)2
-),
and the presence of halogen may cause serious conce
s when
the hydrolysis stability of the anions is poor and/or when a
thermal treatment of spent ionic liquids is required. Therefore,
it appears that these situations empower the chemically tunable
ionic liquids composed only of light elements (C, H, N, and O)
as the present TCA-type salts to serve as solvents for many
aspects of green chemistry.
Results and Discussion
Preparation. Silver salts of TCA-type anions were synthesized
according to the literature procedures.19 The EMI or BMI
salts were prepared by the metathesis of [EMI]I or [BMI]Br
with a slight excess of the appropriate silver salts in distilled
water at RT, filtered and washed by distilled water, and dried
in vacuo at RT for 2 days. Some salts were decolorized by
activated carbon treatment and/or by column chromatography
on silica. Resulting salts were yellowish, reddish, or brown in
color. It must be because of an absorption band centered at
330-410 nm, which could be assigned to the intramolecular
transition between nonbonding HOMO (nπ) and LUMO (π*)
orbitals.20 We note that needle-shaped single crystals of
[EMI][PCA] obtained by recrystallization are yellowish in color.
For all of the salts, no impurity signal was observed in their 1H
NMR spectra, and no trace of halogen and silver was detected
for energy-dispersive X-ray spectroscopy (EDS). The present
salts are much less hygroscopic than EMI and BMI salts with
other cyano-containing anions such as SCN-, N(CN)2
-, and
C(CN)3
-.
Electron Density Distribution. The electron density distribution
in TCA-type anions was calculated based on ab initio
restricted Hartree-Fock (RHF) method with the 6-31G* basis
set.17 The average electron density on N4-N7 is on the order
Me-TCA (-0.523e) > MeO-TCA (-0.519e) > TCA (-0.516e)
> CM-TCA (-0.508e) > PCA (-0.496e), which coincides with
the reciprocal order of the electron-withdrawing ability, as
quantified by the Hammett σ value,21,22 of the 2-substituents of
the anions. It is possible that the limited electron density on
N4-N7 for the PCA and CM-TCA anions could have a
favorable effect on the decrease of interionic interactions,
especially between 2-hydrogen of imidazolium cations and
cyano groups of anions.
Thermal Behavior. Table 2 summarizes glass transition (Tg),
crystallization (Tc), solid-solid transition (Ts-s), and melting
(Tm) temperatures determined by the analysis of differential
scanning calorimetry (DSC) traces, as well as decomposition
temperatures (Td) determined by thermogravimetric analysis
(TGA). Among the EMI salts, only [EMI][MeO-TCA] and
[EMI][CM-TCA] are in liquid form at RT, which derives from
a significant orientational freedom in methoxy and cyanomethyl
groups. The highest Tm of [EMI][PCA] (115 °C), although lower
than those of PCA salts formed with several heterocyclic cations
(>124.8 °C) reported by Shreeve et al.,23 would arise from the
increased number of cyano groups and high local symmetry of
the PCA anions. The former effect reminds us of the higher
melting point of the EMI salt formed by the PF6 anion with six
fluorides (58-62 °C7b,c,24) than that formed by BF4 anion with
four fluorides (11-15 °C7b,c,9a,24b,25). |
