First observation for dynamic solvent effect in ionic liquids

Article abstract of DOI:10.1246/cl.151169

We observed pressure effects on the rate of thermal fading of colored chromene 1 photochemically generated from 2 in ionic liquids. The reaction rates were retarded with increasing pressure in [C_4-mim][CS], [bzl-mim][Tf₂N], and [mnp-mim][Tf₂N], whereas the reaction rate increased with pressures in [C_4- mim][Tf₂N]. These pressure-induced retardations, so-called dynamic solvent effects, result from the slow thermal fluctuations of solvents.

Full text of DOI:10.1246/cl.151169

Received: December 19, 2015 | Accepted: January 10, 2016 | Web Released: January 16, 2016
CL-151169
First Observation for Dynamic Solvent Effect in Ionic Liquids
Satoshi Kitaoka,*1 Kaoru Nobuoka,2 Junji Miura,2 Yasushi Ohga,2 and Yuichi Ishikawa2
1Department of Biotechnology and Chemistry, Faculty of Engineering, Kindai University,
1 Umenobe, Takaya, Higashihiroshima, Hiroshima 739-2116
2Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192
(E-mail: kitaoka@hiro.kindai.ac.jp)
We observed pressure effects on the rate of thermal fading of
h
䃜
colored chromene 1 photochemically generated from 2 in ionic
liquids. The reaction rates were retarded with increasing pressure
in [C4-mim][CS], [bzl-mim][Tf2N], and [mnp-mim][Tf2N],
whereas the reaction rate increased with pressures in [C4-
mim][Tf2N]. These pressure-induced retardations, so-called
dynamic solvent effects, result from the slow thermal fluctua-
tions of solvents.
O
䂴
O
1-(3,3-diphenylprop-2-enylidene)
naphthalen-2-one
3,3-diphenyl-3H-naphtho[2,1-b]pyran
1
2
Figure 1. Photo and thermal isomerization between 1 and 2.
Keywords: Ionic liquid
| Solvation behavior |
Dynamic solvent effect
Tf2N-
N
Room-temperature ionic liquids could be suitable and
environmentally safer replacements for volatile, toxic, and
flammable organic solvents, and ionic liquids have been used
in the synthetic and catalytic reactions.1 There are various
reports about using ionic liquids as media for Friedel-Crafts,2
Diels-Alder,3 Heck,4 hydrogenation,5 and porphyrin prepara-
tion.6 Properties of medium, such as polarity, viscosity, and
hydrogen-bonding ability, affect the reaction rate in condensed
phase.7 Unlike ordinal molecular solvents, which build up with
a single component, ionic liquids are composed of a cation and
an anion. Furthermore, the size of a cation and an anion are
relatively larger than that of molecular solvents. These features
also play a key role in the solvation behavior of ionic liquids.
The solvation dynamics of the dipole probe in ionic liquids or
ionic deep eutectic solvents were investigated previously.8
However, the solvation behavior of ionic liquids is still poorly
understood in the electronic ground state because it is difficult to
observe the solvent molecular movement around the substrates.
By combining high pressure and viscous molecular sol-
vents, Asano et al. demonstrated that the thermal isomerization
of molecules in their electronic ground state are retarded with an
increase in pressure (dynamic solvent effect).9 In general, ionic
liquids have higher viscosity than molecular solvents. Herein,
we firstly examined the observation of the dynamic solvent
effect on thermal isomerization in the electronic ground state in
ionic liquids.
N R
[R-mim][Tf2N]
R= -(CH2)3CH3,
[C4-mim][Tf2N]
-(CH2)7CH3, [C8-mim][Tf2N]
-(CH2)2OH,
-CH2Ph,
[C2OH-mim][Tf2N]
[bzl-mim][Tf2N]
-CH2-C(CH3)3, [mnp-mim][Tf2N]
X-
N
N
(CH2)3CH3
[C4-mim][X]
X= (CF3SO2)2N, [C4-mim][Tf2N]
(CN)2N,
[C4-mim][(N(CN)2]
O
O
N
-
SO3
N
O
S
S O
O
O
O
O
[C4-mim][Ace]
[C4-mim][Sac]
[C4-mim][CS]
Figure 2. The structure of ionic liquids.
We selected the thermal cyclization of a colored hexa-
dienone from 3,3-diphenyl-3H-naphtho[2,1-b]pyran as a probe
reaction in ionic liquids (Figure 1). The thermal cyclization
reaction is an appropriate probe for discussing the solvation
behavior in ionic liquids because the reaction has been studied
in detail in molecular solvents.10 Hexadienone 1 was photo-
chemically generated in situ from 2 and its decay was
followed spectrophotometrically. The experiments were carried
out according to the literature.11
as shown in Figure 2. [R-mim][Tf2N] are composed of the
alkylimidazolium cations with various substituents and the Tf2N
anion. [bmim][X] are composed of the 1-butyl-3-methyllimida-
zolium cation and different type of anions. These ionic liquids
were prepared according to the published procedures,12 and
preparation details are shown in Supporting Information.
Figure 3 illustrates the pressure dependence of the reaction
rate in [R-mim][Tf2N], which have different imidazolium
cations. Open plots represent the results in the molecular
solvents. The reaction was moderately accelerated by a pressure
In order to exhibit the effect of anion and cation structures
of the ionic liquids on a dynamic solvent effect, the ionic liquids
employed in this study can be categorized into two groups,
Chem. Lett. 2016, 45, 385–387 | doi:10.1246/cl.151169
© 2016 The Chemical Society of Japan | 385

Figure 3. Pressure dependence of the cyclization rate (kobs
¹1) of 1 in [R-mim][Tf2N] and molecular solvents (EtOH and
MPD) at 25 °C.
/
Figure 4. Pressure dependence of the cyclization rate (kobs
s
/
¹1) of 1 in [C4-min][X] at 25 °C.
s
Table 1. Activation volume of thermal fading of colored
chromene 1 to 2 in ionic liquids at 25 °C
increase in ethanol. On the other hand, the reaction rate was
retarded with an increase in pressure in 2-methyl-2,4-pentane-
diol (MPD). The difference between ethanol and MPD shows
slow solvent fluctuations of the MPD under high pressure and
it is a typical example of dynamic solvent effects. The reaction
rate increased with increasing pressure because of a negative
activation volume.9 However, a further increase in pressure
resulted in a retardation. The viscosity of viscous solvents such
as MPD is increased by high pressure and the increase of
viscosity makes the solvent reorganization slow. In this case,
the transition state theory (TST) is invalid, and the reaction was
retarded by pressure. The closed plots represent the reaction
in [R-mim][Tf2N]. In [bzl-mim][Tf2N] and [mnp-mim][Tf2N]
containing benzyl and methylneopentyl groups, respectively,
the dynamic solvent effects were observed at higher pressures.
[mnp-mim][Tf2N] has a branching structure as well as MPD. It
is reasonable to surmise that the branching structure of [mnp-
mim][Tf2N] give a high viscosity at high pressure as well as
MPD, and the retardation of the reaction was observed. In case
of [bzl-mim][Tf2N], it is surmised that the increasing pressure
exerts an effect on the formation of the π-π stacking interaction
among the benzyl group of the cations, and such a rigid π-π
interaction increased the viscosity of [bzl-mim][Tf2N].
‡
¹1
Ionic liquids
¦V0 /cm3 mol
[C4-mim][CS]
[C4-mim][N(CN)2]
[C4-mim][Sac]
¹6.4
¹8.4
¹11
[C4-mim][Tf2N]
[mnp-mim][Tf2N]
[bzl-mim][Tf2N]
¹10
¹10
¹11
mim][CS] is very high (1.9 © 104 cP at 25 °C, 0.1 MPa).
Although we cannot evaluate the viscosity of ionic liquids at
high pressure, it is surmised that [C4-mim][CS] become more
viscous at high pressure. Then, it seems to surmise that the
reaction rate in [C4-mim][CS] was decelerated considerably. The
pressure effect based on TST may be extrapolated to TST-invalid
viscosities if the reaction mechanism is the same at all pressures.
This extrapolation provides an estimate for the rate constant
expected from TST, kTST. The kTST values in [C4-mim][CS]
at higher pressures were estimated by extrapolations of the
lower-pressure data. Isobaric activation energies from kTST are
¹1
almost the same in all pressures (73.0 kJ mol at 0.1 MPa and
¹1
Figure 4 illustrates the pressure dependence of the reaction
rate in [C4-mim][X], which have different anions. In the ionic
72.3 kJ mol at 600 MPa). Therefore, it seems reasonable to
conclude that the pressure effects in [C4-mim][CS] at higher
pressures are not a static effect.
¹
¹
¹
liquids containing the bulky anion such as Asc , Sac , and CS ,
the dynamic solvent effects were observed. The bulky anion
structures of these ionic liquids play a critical role on the
dynamic solvent effect. In particular, the viscosity of [C4-
‡
¹1
Finally, we discuss the activation volume (¦V0 /cm3 mol
)
in each ionic liquid (Table 1). The activation volume of thermal
fading of colored chromene 1 to 2 is defined as a difference
386 | Chem. Lett. 2016, 45, 385–387 | doi:10.1246/cl.151169
© 2016 The Chemical Society of Japan

between the partial molar volumes of the activated complex and
the reactant. The estimated activation volumes were almost the
same levels in all ionic liquids except for [C4-mim][CS]. Only
[C4-mim][CS] gave a small negative activation volume. There
are two factors that can change the volume. First, a loss of
internal freedom of motion with cyclization decreases the
volume. Second, the polarity of the reactant is decreased in the
cyclization pathway and the volume of the reactant is increased
because of desolvation. The second factor is closely related to
the solvation. Therefore, an extremely sterically bulky CS anion
desolvates around 1 and such a large desolvation gave the small
negative activation volume. The CS anion is an O-base anion,
whereas the Sac anion is an N-base anion. The interaction of
the O-base anion with chromene is weaker than that of the
isomerization N-base anion. The difference between the O-base
and N-base anions may affect the desolvation of the chromene.
In conclusion, the reaction rates in [bzl-mim][Tf2N], [mnp-
mim][Tf2N], [C4-mim][Sac], [C4-mim][Ace], and [C4-mim][CS]
were retarded with increasing pressure. Such pressure-induced
retardations, the so-called dynamic solvent effects, are the results
of slow solvent thermal fluctuations caused by high viscosities.
The bulky structures and aromatic ring of the ionic liquids caused
high viscosity under the high pressure. The activation volumes of
thermal cyclization of 1 show almost the same levels in all ionic
liquids except for [C4-mim][CS], which gave a small negative
activation volume. It is reasonable to assume that the desolvation
around 1 was effectively increased by the ionic liquids contain-
ing a bulky ion like a CS anion. In order to elucidate the
difference of solvation behavior between [C4-mim][CS] and
other ionic liquids in detail, we conducted further experiments
by using chromene derivatives with different polar substituents.
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This work was supported by JSPS KAKENHI Grant
Numbers 25810109, 26600028, 15K05598 and JST A-STEP
Number AS242Z02498M.
Supporting Information is available on http://dx.doi.org/
10.1246/cl.151169.
References and Notes
1
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therein.
Chem. Lett. 2016, 45, 385–387 | doi:10.1246/cl.151169
© 2016 The Chemical Society of Japan | 387