On the characterization of some [bmim][X]/co-solvent binary mixtures: a multidisciplinary approach by using kinetic, spectrophotometric and conductometric investigations

Article abstract of DOI:10.1016/j.tet.2007.11.033

In order to study the intrinsic characteristics and to evaluate the structural variations determined by the addition of a co-solvent to 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) we examined the behaviour of some probes in [bmi

Full text of DOI:10.1016/j.tet.2007.11.033

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 672e680
www.elsevier.com/locate/tet
On the characterization of some [bmim][X]/co-solvent binary
mixtures: a multidisciplinary approach by using kinetic,
spectrophotometric and conductometric investigations
a
a
a,
Francesca D’Anna a, , Vincenzo Frenna , Sandra La Marca , Renato Noto
*
*
,
Vitalba Pace a, Domenico Spinelli b,
*
a Dipartimento di Chimica Organica ‘E. Paterno`’, Universita` degli Studi di Palermo, Viale delle Scienze-Parco d’Orleans II, 90128 Palermo, Italy
b Dipartimento di Chimica Organica ‘A. Mangini’, Universita` degli Studi di Bologna, Via S. Giacomo 11, 40126 Bologna, Italy
Received 2 October 2007; received in revised form 29 October 2007; accepted 8 November 2007
Available online 12 November 2007
Abstract
In order to study the intrinsic characteristics and to evaluate the structural variations determined by the addition of a co-solvent to 1-butyl-3-
methylimidazolium tetrafluoroborate ([bmim][BF4]) we examined the behaviour of some probes in [bmim][BF4]/co-solvent binary mixtures.
The rate constants of the piperidino-catalyzed rearrangement of the Z-phenylhydrazone of the 3-benzoyl-5-phenyl-1,2,4-oxadiazole into the rel-
evant 4-benzoylamino-2,5-diphenyl-1,2,3-triazole as well as the spectrometric properties of pyrene and Nile Red were evaluated. With the same
purpose also 1H NMR and conductivity measurements were carried out. By comparison the behaviour of 1-butyl-3-methylimidazolium bis-(tri-
fluoromethylsulfonylimide)/1,4-dioxane mixtures has been analyzed. The whole of data confirms the presence of weak interactions that deter-
mine a partially preorganized structure for [bmim][X]. This is perturbed at some degrees by quantity and nature of guest molecules. For example
conductivity measurements support the idea that different charged aggregates are present in the [bmim][X]/co-solvent binary mixtures.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Ionic liquids; Binary mixtures; MHR reaction
1. Introduction
For practical reasons in several cases investigations are not
carried out in pure RTILs. Thus, the reaction medium is a
binary mixture more or less rich in RTIL and the co-solvent
employed might be able to affect solvent parameters of RTILs
as well as their organized structure. Thus, understanding of the
structural and physico-chemical properties of the mixed
systems appears desirable and recently it has been the main
subject of some papers.6
Within this line of research we have examined the proper-
ties of some binary mixtures constituted by the RTIL
[bmim][BF4] (bmim¼1-butyl-3-methylimidazolium) with vari-
able amounts of solvents largely used in RTIL studies, showing
different properties such as polarity, hydrogen bond donor or
acceptor ability, density and viscosity.7
Room temperature ionic liquids (RTILs) are defined as
‘green’ solvents because of their non-volatility and non-flam-
mability.1 To completely develop their laboratory and indus-
trial use, a knowledge of their physico-chemical properties
as well as of their effect on chemical reactivity appears neces-
sary.2 The latter has been explained on the grounds of a par-
tially preorganized structure of RTILs.3 Alternatively the
effect of medium has been attributed to a combination of their
solvent properties such as hydrogen bond acidity (a) and ba-
sicity (b) as well as polarizability effects (p*).4 Recently,
a comparison between two models has been discussed.5
The RTIL [bmim][NTf2] [NTf2¼bis-(trifluoromethylsulfo-
nylimide)] has been also tested to evaluate if, and how, the
change of the anion counter-part could influence the behaviour
of imidazolium RTILs.
* Corresponding authors. Tel.: þ39 091596919; fax: þ39 091596825 (F.D.,
R.N.); tel.: þ39 0512095689; fax: þ39 0512095688 (D.S.)
E-mail addresses: fdanna@unipa.it (F. D’Anna), rnoto@unipa.it (R. Noto),
domenico.spinelli@unibo.it (D. Spinelli).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.033

F. D’Anna et al. / Tetrahedron 64 (2008) 672e680
673
For this, we have now studied the effect of some binary
mixtures on the kinetics of piperidino-catalyzed MRH (mono-
nuclear rearrangement of heterocycles, see Scheme 1) of the
Z-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole
(1) into the relevant 4-benzoylamino-2,5-diphenyl-1,2,3-tria-
zole (2)8 and the spectroscopic (UVevis and fluorescence,
respectively) properties of Nile Red9 and pyrene10 (Scheme
1) in the presence of the same mixtures; with the same aim
occurring via a bicyclic quasi-aromatic (10 p electrons) tran-
sition state.11 It has been largely investigated in conventional
organic solvents,8e as well as in organized (micelles)12 or in
preorganized (cyclodextrins)13 systems and, recently, in
RTILs.14
Kinetic study in different binary mixtures containing
[bmim][BF4] has been carried out at different piperidine
(Pip) concentrations: according to previous reports,2p this
amine did not induce significant variations in organized
RTIL structure in the range of concentrations used, unlike
some other cyclic secondary amines tested.
1
also some H NMR and conductivity measurements in binary
mixtures were carried out.
In our previous investigations of the same reaction in
RTILs,14 we have added the reagents dissolved in 75 mL of
Diox to 500 mL of RTIL. In order to verify the influence of
the nature of the co-solvent used, we have carried out new
kinetic measurements both in pure RTIL ([bmim][BF4]) or
adding the same volume (75 mL) of different co-solvents.
Moreover we have also carried out kinetic measurements in
the presence of an equal number of moles (w8.80ꢀ10ꢁ3 mol)
of different co-solvents.
Ph
Ph
H
N
N
N
+δ
B
−δ
Ph
Ph
N
N
H
B
N
N
N
Ph
Ph
O
O
N
N
N
H
Ph
N
Ph
O
1
TS
2
MRH reaction
In all the cases considered, a linear dependence of the ob-
served rate constant (kA,R) on [Pip] was evidenced and a nega-
tive intercept was always calculated thus confirming some our
previous observations.14a
N
O
N
O
In Table 1 the second-order rate constants (kII calculated
at 298 K) are reported as a function of the co-solvent used.
Moreover the relative permittivity (3)15 of co-solvents used,
Pyrene
Nile Red
Scheme 1.
9
10
the ENR and RI/III values measured by us (see below) are
reported. Data in Table 1 indicate that the reaction rates are
affected by the nature of the co-solvent used. For example,
at constant volume of co-solvent, a reactivity ratio w2.3 is
calculated between the kII in [bmim][BF4]/H2O and in [bmim]-
[BF4]/CH2ClCH2Cl.
2. Results and discussion
2.1. Kinetic measurements
The presence of the co-solvent seems capable of increasing
the reaction rates: indeed the reaction always proceeds faster
in the binary mixtures than in neat RTIL, the chlorinated sol-
vents appearing the less efficient and the polar co-solvents the
most efficient in accelerating the reaction.
2.1.1. Kinetic study of the rearrangement of 1 into 2 in
[bmim][BF4]/co-solvent at constant volume of co-solvent
and at constant molarity
The mononuclear rearrangement of heterocycles (MRH) is
a peculiar intramolecular nucleophilic substitution (SNi)
Table 1
Second-order rate constants (kII) for the MRH of 1 into 2 at 298 K in [bmim][BF4]/co-solvent mixtures, polarity parameter (3) of co-solvents and polarity para-
meters (RI/III and ENR) of the binary mixtures
Vd (mL)
kII
(Mꢁ1 sꢁ1
)
(kII/kII,IL
)
RI/III
a, e
b, e
ENR
(kJ/mol)
c, e
a, f
b, f
ENR
(kJ/mol)
c, f
Entry
Co-solvent
3
kII
(Mꢁ1 sꢁ1
(kII/kII,IL
)
RI/III
)
1
2
3
4
5
6
7
8
9
a
0.481 (0.041)
1.23 (0.11)
1.20 (0.03)
1
217.6
211.6
217.6
216.1
217.9
217.6
218.2
217.6
217.5
1.74
0.78
1.67
1.70
1.48
1.42
1.39
1.53
1.65
0.481 (0.041)
1.27 (0.04)
1
217.6
216.1
218.5
216.5
217.6
217.4
217.9
217.1
217.5
1.74
0.99
1.57
1.63
1.54
1.61
1.41
1.58
1.65
H2O
80.10
36.64
33
2.6
2.5
2.5
1.1
1.3
1.7
1.3
1.8
15
45
35
70
55
85
70
75
2.6
1.8
3.4
1.1
1.1
1.6
1.2
1.8
MeCN
MeOH
CH2ClCH2Cl
CH2Cl2
AcOEt
CHCl3
Diox
0.889 (0.044)
1.63 (0.03)
1.20 (0.03)
10.42
8.93
6.08
4.81
2.22
0.525 (0.017)
0.618 (0.027)
0.816 (0.030)
0.604 (0.023)
0.844 (0.028)
0.538 (0.017)
0.545 (0.022)
0.793 (0.042)
0.585 (0.025)
0.844 (0.028)
Standard deviations are given in parenthesis. The rate constants were reproducible within ꢂ3%.
The ENR values were reproducible within 0.2 unities.
b
c
d
e
f
The RI/III values were reproducible within ꢂ3%.
Amount of co-solvent.
Data collected in [bmim][BF4]/co-solvent (3:23, v/v) mixtures.
Data collected in [bmim][BF4]/co-solvent (Xco-solventw0.25) mixtures.

674
F. D’Anna et al. / Tetrahedron 64 (2008) 672e680
Similar variations in the reaction rates, as a function of the
co-solvent used, have been recently observed by Harper
et al.,2k in the base-catalyzed solvolysis of 3-chloro-3,7-dime-
thyloctane in [bmim][NTf2], in the presence of methanol or
benzyl alcohol.
1.2
1.1
1
(a)
In order to rationalize the observed trends, we tried to relate
rate constants to polarity of the co-solvents used, but data in
Table 1 show that no relationship can be evidenced between
kinetic and polarity data. As a matter of fact, comparable
rate constants were calculated in mixtures of co-solvents hav-
ing significantly different polarity (see entries 2 and 4, column
4). In contrast, in mixtures with co-solvents having rather sim-
ilar 3 values, quite different reactivity data have been mea-
sured (see entries 7 and 8), thus indicating the occurrence of
significantly different kinetic effects.
0.9
0.8
0.7
0.6
0.5
0.4
2.1.2. Kinetic study of the rearrangement of 1 into 2 in
[bmim][BF4]/co-solvent at variable mole fraction of RTIL
A comparison between data (columns 4 and 9) reported in
Table 1 shows that the reactivity is affected also by the amount
of co-solvent used. On the basis of the latter finding we have
undertaken a kinetic study of the title reaction, as a function of
RTIL mole fraction (different mole fraction ranges have been
examined because of different relative solubilities). To carry
out this investigation we have chosen only three solvents
with different polarities and hydrogen bond abilities, namely
Diox, AcOEt and MeOH.
In Figure 1, kII values are reported as a function of RTIL
mole fraction [data at 298 K for different binary mixtures
are collected in Supplementary data (Table 3)]. In all the cases
examined the plot of kII versus the mole fraction of RTIL does
not show a monotonic trend. Moreover, the MeOH co-solvent
appears capable of inducing the largest increase in reactivity
with respect to neat [bmim][BF4]. Furthermore, the mole frac-
tion corresponding to the highest reactivity depends on the
very nature of the co-solvent used, going from XMAX¼0.40
(Diox) to ca. 0.80 (AcOEt and MeOH).
0.2
0.3
0.4
0.5
0.6
ΧRTIL
0.7
0.8
0.9
1
1
1
(b) 0.85
0.8
0.75
0.7
0.65
0.6
0.55
0.5
0.45
0.4
0.5
0.6
0.7
ΧRTIL
0.8
0.9
1.8
1.6
1.4
1.2
1
(c)
2.1.3. Kinetic measurements of the rearrangement of 1 into
2 in [bmim][NTf2]/Diox mixtures
In order to ascertain the role played by the anion of RTIL in
affecting the reaction rates in binary mixtures, we carried out
further kinetic measurements for the [bmim][NTf2]/Diox mix-
tures. In Figure 2, kII values are reported as a function of RTIL
mole fraction [data at 298 K for the studied binary mixtures
are in Supplementary data (Table 4)].
They show a trend similar to that related to the
[bmim][BF4]/Diox mixtures with a different mole fraction cor-
responding to the highest reactivity (XMAXw0.3).
0.8
0.6
0.4
A
comparison between the two binary systems
0.2
0.3
0.4
0.5
0.6
ΧRTIL
0.7
0.8
0.9
[bmim][BF4]/Diox and [bmim][NTf2]/Diox shows that in the
former case the presence of the co-solvent induces a higher re-
activity variation with respect to neat RTIL (DkIIw0.682 for
[bmim][BF4] and DkIIw0.260 for [bmim][NTf2]). On the
other hand, the reactivity ratios ([bmim][BF4]/[bmim][NTf2]),
going from neat RTILs to mixtures (XRTIL¼0.3), are w4.6 and
w2.4, respectively. Probably, the co-solvent has a greater
Figure 1. Plots of second-order rate constants (kII) at 298 K versus RTIL
mole fraction concerning the binary mixtures: (a) [bmim][BF4]/Diox, (b)
[bmim][BF4]/AcOEt and (c) [bmim][BF4]/MeOH.

F. D’Anna et al. / Tetrahedron 64 (2008) 672e680
675
Table 2
Activation parametera values for the MRH of 1 in [bmim][X]/co-solvent
binary mixtures
0.4
0.35
0.3
RTIL
Co-solvent
DHs,b
(kJ/mol) (J/K mol) (kJ/mol) (J/K mol)
DSs,b
DHs,c
DSs,c
[bmim][BF4]
[bmim][NTf2]
[bmim][BF4]
[bmim][BF4]
[bmim][BF4]
[bmim][BF4]
[bmim][BF4]
[bmim][BF4]
[bmim][BF4]
[bmim][BF4]
50
45
52
38
48
ꢁ122
ꢁ144
ꢁ105
ꢁ152
ꢁ118
ꢁ121
ꢁ172
ꢁ130
ꢁ144
ꢁ158
ꢁ135
H2O
MeCN
MeOH
40
36
49
50
42
50
42
38
ꢁ145
ꢁ161
ꢁ114
ꢁ121
ꢁ145
ꢁ116
ꢁ145
ꢁ158
0.25
0.2
CH2ClCH2Cl 50
CH2Cl2
AcOEt
CHCl3
Diox
34
46
43
38
48
0.15
0.1
[bmim][NTf2] Diox
a
0.2
0.3
0.4
0.5
0.6
ΧRTIL
0.7
0.8
0.9
1
At 298 K the maximum errors are 3 kJ/mol and 8 J/K mol, respectively.
Data calculated in [bmim][BF4]/co-solvent (3:23, v/v) mixtures.
Data calculated in [bmim][BF4]/co-solvent (Xco-solventw0.25) mixtures.
b
c
Figure 2. Plot of second-order rate constants (kII) at 298 K versus RTIL mole
fraction for [bmim][NTf2]/Diox binary mixtures.
Dioxewater mixtures,8c in the base-catalyzed pathways,
shows higher values of enthalpic parameter (DHs¼92ꢂ2 kJ/
mol) and more positive values for entropic contribute (DSs
ranges from ꢁ44.3 J/K mol to 2.1 J/K mol) according to the
occurrence of strong substrateesolvent interactions.
In order to get deeper information about the organization
degree and properties of the studied binary mixtures, we have
used other probes, carrying out some other determinations.
disorganizing effect on [bmim][BF4], having a higher cross-
linking degree than [bmim][NTf2].
2.1.4. Activation parameters
Bearing in mind that RTILs containing aromatic moieties
are capable of displaying self-organization, the presence of
co-solvent could induce significant changes in their organized
structure. Because even modest temperature variations can
cause significant differences in the packing of RTILs, we
thought it could be interesting to study the probe reaction,
for different solvent mixtures, at different temperatures.
For kinetics carried out in RTILs, it has been reported that
a sharp curvature in Arrhenius or Eyring plots could be ob-
served, as a consequence of some significant structural
changes in the RTIL.16 So, for a careful analysis of the temper-
ature effect, the reaction was carried out at five temperatures
going from 293 K up to 313 K.
In all the cases considered, an excellent linear correlation of
log(kA,R/T ) versus 1/T was obtained, indicating that, in the an-
alyzed range, the above upsetting effect is not operating and
that the calculated activation parameters are only dependent
on the nature of the MRH process. The activation parameters
are reported in Table 2, kinetic data collected at different tem-
peratures are available in the Supplementary data (Tables 5
and 6).
1
2.2. Spectroscopic, H NMR and conductivity
measurements
2.2.1. Polarity measurements
Since the literature 3 values (data in Table 1) refer to the
pure co-solvent, it appeared more correct to have polarity pa-
rameters related to the binary mixtures. We choose pyrene and
Nile Red, as solvatochromic probes: their planar and aromatic
structures seemed suitable to intercalate between imidazolium
layers, without excessively perturbing the RTIL structure.
In Table 1 the determined ENR and RI/III values, collected
for [bmim][BF4]/co-solvent binary mixtures are reported.
It is worthwhile that the two solvatochromic probes seem to
be scarcely affected by different micro-environments. In fact,
narrow ranges of ENR values (211.6e218.2 kJ/mol and 216.1e
218.5 kJ/mol, see Table 1) were determined. Significantly
larger variations (0.78e1.74 and 0.99e1.74, see Table 1)
were calculated for RI/III values. Furthermore, it must be no-
ticed that for both parameters, by excluding data concerning
the [bmim][BF4]/H2O mixture, the above ranges become
even narrower. The effect of H2O on the RTIL structure has
been extensively studied by Mele et al.,17 by means of NMR
measurements, and it has been shown that the presence of wa-
ter makes the imidazoliumeimidazolium association looser,
shifting the RTIL structure towards a different organization
with a lower degree of ring stacking. Also in this case polarity
parameters of binary mixtures were used for attempting to ex-
plain kinetic data. Once more they seem to be inadequate. For
example, the ENR parameter foresees no variation of polarity,
A narrow range of activation parameters was calculated. In
particular, at constant volume of co-solvent, mean values of
DHs¼44ꢂ5 kJ/mol and DSs¼ꢁ138ꢂ19 J/K mol were calcu-
lated. The same values, DHs¼43ꢂ5 kJ/mol and DSs¼ꢁ138ꢂ
16 J/K mol, were calculated at constant number of moles of
co-solvent. These values are not significantly different from
those calculated for neat [bmim][BF4]. Moreover similar re-
sults were also obtained for neat [bmim][NTf2] and [bmim]-
[NTf2]/Diox binary mixtures.
Above data confirm14b that weak interactions, scarcely sen-
sitive to composition variation of binary mixtures, are operat-
ing in imidazolium-based ILs. Indeed, the studied reaction in

676
F. D’Anna et al. / Tetrahedron 64 (2008) 672e680
even if significant differences in reactivity were measured (see
entries 3 and 6 of Table 1). The same conclusions can be
pointed out considering the RI/III polarity parameter.
(see also Table 10 of Supplementary data). For [bmim][BF4]
mixtures (Fig. 3a), Dd values are comparable and smaller in
the presence of Diox (3¼2.22) and MeOH (3¼33), but higher
in the presence of AcOEt (3¼6.08): once more the variation in
physico-chemical properties cannot be related only to the co-
solvent polarity.
By comparing [bmim][BF4]/Diox and [bmim][NTf2]/Diox
mixtures (Fig. 3b) a higher variation in H-2 chemical shift
for the first binary mixture has been evidenced, in line with
the higher [bmim][BF4] organization.
In Figures 5e7 and in Tables 7e9 of Supplementary data
the ENR and RI/III values, as a function of RTIL mole fraction,
related to [bmim][BF4]/co-solvent (Diox, AcOEt and MeOH)
and [bmim][NTf2]/Diox mixtures, are reported. In general
RI/III parameter, with the exception of [bmim][NTf2]/Diox
mixtures, has a non-monotonic trend, with maximum polarity
values quite similar to those calculated from reactivity data. In
contrast, ENR’s give monotonic polarity trend decreasing in the
presence of Diox and AcOEt and increasing in the presence of
MeOH.
A comparison among polarity parameters concerning
[bmim][BF4] and [bmim][NTf2] binary mixtures indicates
that the different nature of RTIL anion is able to affect the an-
swer of solvatochromic probe to solvent characteristics, giving
rise to completely different polarity trend. On the other hand,
it can be also supposed that the same probes affect in different
ways, the structure of binary mixtures of the two RTILs, as
a consequence of the different possible cationeanion interac-
tions. Under this light, the different trends of ENR and RI/III
values could be considered a result of different binary mix-
tures’ organization around the solute molecules.
2.2.3. Conductivity measurements
In Figure 4 conductivity values (L), collected at 298 K, for
the same three above examined binary mixtures, as a function
of RTIL mole fraction, are reported (see also Table 11 in Sup-
plementary data).
In the presence of variable amounts of MeOH, the conduc-
tivity values give a smooth curve. A similar result was previ-
ously obtained by Han et al.,20 on studying the [bmim][PF6]/
(a) 0.5
1.4-Diox
MeOH
AcOEt
0.4
0.3
0.2
0.1
0
1
2.2.2. H NMR measurements
The RTIL/co-solvent interactions were studied also by H
1
NMR spectrometry recording spectra of binary mixtures of
[bmim][BF4] with Diox, AcOEt and MeOH.
Firstly, we analyzed the effect of Diox addition to RTIL ob-
serving significant variations of NMR spectrum. For instance,
the signal due to H-2 proton of imidazolium was split in a cou-
ple of signals of different intensities, the signal most shielded
had the lowest intensity and was broad (see Fig. 9a of Supple-
mentary data).
In general, for all of the binary mixtures examined, the co-
solvent is capable of affecting both the chemical shift and the
1
multiplicities of H NMR signals. This fact has been previ-
0.2
0.3
0.4
0.5
0.6
ΧRTIL
0.7
0.8
0.9
1
0.4
0.35
0.3
(b)
ously ascribed to the existence of different ion pairs in solu-
1
bmimNTf2
bmimBF4
tion.18 Furthermore, H NMR measurements have underlined,
in some cases, in the presence of co-solvents, the formation of
nanostructures with polar and non-polar regions.19b Among
the co-solvents tested, AcOEt induces the highest signal
broadening, whereas MeOH the lowest (see Fig. 8 of Supple-
mentary data). This result could be related to different size,
shape and structural flexibility of the three solvents used. We
have extended the comparison to [bmim][NTf2]/Diox mixtures,
observing a smaller broadening of signals with respect to
[bmim][BF4]/Diox mixtures (see Fig. 9 of Supplementary data).
In general the co-solvent shifts imidazolium signals to
downfield and, in all of the cases considered, variations of
chemical shift are higher for H-2. Interestingly enough, varia-
tions in chemical shifts (Dd¼dbinary mixtureꢁdRTIL), as a func-
tion of the RTIL mole fraction, shows a monotonic trend.
For the binary mixtures examined, the comparisons among
Dd values concerning H-2 proton of imidazolium cation, as
a function of RTIL mole fraction, are reported in Figure 3
0.25
0.2
0.15
0.1
0.05
0
0.2
0.3
0.4
0.5
0.6
ΧRTIL
0.7
0.8
0.9
1
Figure 3. Variations of chemical shift (Dd) relative to H-2 proton of imidazo-
lium ion, versus RTIL mole fraction, for: (a) [bmim][BF4]/Diox, [bmim][BF4]/
AcOEt and [bmim][BF4]/MeOH mixtures and (b) [bmim][BF4]/Diox and
[bmim][NTf2]/Diox mixtures.

F. D’Anna et al. / Tetrahedron 64 (2008) 672e680
677
(a)
(b)
8
7
6
5
4
3
7
6.5
6
5.5
5
4.5
4
3.5
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.4
0.5
0.6
0.7
0.8
0.9
ΧRTIL
ΧRTIL
25
20
15
10
5
7
6.5
6
(c)
(d)
5.5
5
4.5
4
3.5
3
0.3
0.4
0.5
0.6
ΧRTIL
0.7
0.8
0.9
1
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
ΧRTIL
1
Figure 4. Plots of conductivity values (L) at 298 K, versus RTIL mole fraction, relative to: (a) [bmim][BF4]/Diox, (b) [bmim][BF4]/AcOEt, (c) [bmim][BF4]/
MeOH and (d) [bmim][NTf2]/Diox binary mixtures.
MeCN binary mixtures. This was attributed to the miscibility
on the molecular level of the two components, giving binary
mixtures where the RTIL is present mainly as separate free
ions. In our case this hypothesis seems further supported by
the variation in conductivity values induced by the presence
of co-solvent.
mole fraction quite similar to that observed in kinetic measure-
ments and, accordingly, the value increases going from
[bmim][NTf2] mixtures (XMAXw0.3) to [bmim][BF4] mix-
tures (XMAXw0.4). In the presence of AcOEt, the RTIL
mole fraction corresponding to the maximum value of conduc-
tivity is similar to that calculated in the presence of Diox
(XMAXw0.4), but quite different from that calculated by
kinetic data.
Non-monotonic experimental traces, such as the ones ob-
tained by us, have been previously reported.20 It is noteworthy
that similar results were obtained also by FTIR21 and calorimet-
ric measurements.19 In all the cases considered, these experi-
mental trends were ascribed to the coexistence of different
species in solution originated from RTIL, such as free ions, con-
tact ion pairs and charged aggregates. The presence of a maxi-
mum has been considered as result of a balance between the
contribution to conductivity of the different species present
and solution viscosity.21 ESI-MS investigations have shown
that each species has a different prevalence range.22
Among the solvents tested, MeOH induces the largest vari-
ation. As a matter of fact in the range of mole fraction consid-
ered (from 0.2 up to 1.0) conductivity values decrease from
25 mS up to 3.3 mS. In contrast, in the range examined for
AcOEt and Diox (from 0.30 and 0.32 up to 1.0, respectively)
the conductivity variations observed are, as expected on the
grounds of the different physical characteristics of the three sol-
vents examined, much smaller: the former inducing a slightly
higher variation (DL¼3.85 mS) than the latter (DL¼3.26 mS).
On the other hand, a comparison between the two RTILs
considered by us evidences that the variation in conductivity
is slightly higher for [bmim][NTf2] mixtures than for
[bmim][BF4] mixtures (DL¼3.26 mS and 3.82 mS for
[bmim][BF4]/Diox and [bmim][NTf2]/Diox, respectively),
once more confirming a higher organization of [bmim][BF4].
With the exception of [bmim][BF4]/MeOH mixtures, in all
of the other cases, conductivity values show a non-monotonic
trend, quite similar to that concerning the kII data. Both in
[bmim][BF4]/Diox mixtures and in [bmim][NTf2]/Diox mix-
tures the maximum of conductivity is achieved at RTIL
The presence of co-solvent also affects the viscosity of the
RTIL, indeed it can induce dramatic decreases in this param-
eter.23 However, the viscosity and conductivity trends are not
always in agreement between themselves. For dilute solutions
in RTIL, low viscosity corresponds to low conductivity, as
neutral ion pairs should be the predominant species. For con-
centrated solutions, a high conductivity, due to the presence of

678
F. D’Anna et al. / Tetrahedron 64 (2008) 672e680
charged aggregates, will be counterbalanced by a high and un-
favourable viscosity.
hypothesis seems to be supported by both kinetic and 1H
NMR data, which give higher variations for [bmim][BF4] binary
mixtures.
In our case, starting from high RTIL mole fraction, the re-
action rate increases up to a maximum. Probably, in this range,
the decrease in viscosity favours the progress of the reaction,
as a consequence of easier intramolecular and intermolecular
motions. Furthermore, in the presence of Diox, at the
same mole fraction, the variation in kII values, respect to
neat RTIL, is higher for the [bmim][BF4] mixtures than for
3. Conclusions
Data collected here once more show that the imidazolium
RTILs are very intriguing solvent media, acting as ‘organized
solvents’. As a consequence, any reaction carried out in these
solvents could be strongly and differently affected by the
RTILs’ nature because of their peculiar structural characteris-
tics. In some cases, they may act as ‘entropic drivers’.3b In
imidazolium-based RTILs, different components are kept to-
gether by means of feeble but directional and cooperative
forces. So, the resulting solvent media are differently sensitive,
with respect to conventional organic solvents, to upsetting ef-
fects due to the presence of guest molecules, such as reactants
or co-solvents. This agrees with the data previously reported
by Armstrong et al. about the dependence of polarity measure-
ments on the probe used.26
As indicated by kinetic, conductivity and NMR data, the
disorder degree, induced by co-solvents, depends on the nature
of both RTIL components and co-solvent, resulting greater for
the most cross-linked [bmim][BF4] than for [bmim][NTf2].
However, the effect that binary mixtures [bmim][X]/co-sol-
vents can exert on a given reaction cannot be ascribable to
simple polarity, viscosity or conductivity effects, but it is evi-
dent that a whole of parameters has to be considered in order
to rationalize it.
the [bmim][NTf2] mixtures (XDiox¼0.54, DkII[bmim][BF4]
¼
0.654 Mꢁ1 sꢁ1; DkII[bmim][NTf2]¼0.158 Mꢁ1 sꢁ1). This result
can be ascribed to a higher viscosity of [bmim][BF4] (n¼
233 cP) with respect to [bmim][NTf2] (n¼52 cP).24
As a consequence, the former RTIL should be more af-
fected by addition of a second component having a lower vis-
cosity. After the reaching of the maximum value, kII values
begin to decrease. As previously reported,14 for the probe re-
action the reactivity increases in imidazolium RTILs. This ac-
celerating effect with respect to conventional organic solvents,
was not simply ascribed to a higher polarity of RTILs, but to
their self-organized structure and to their organizing ability,
which may allow the presence of feeble but cooperative inter-
actions, such as pep interactions. These, occurring between
the imidazolium rings and the bicyclic quasi-aromatic transi-
tion state, stabilize the latter and favour the progress of the re-
action. It is expected that the charged aggregates are more
efficient with respect to neutral ion pairs or free ions in carry-
ing out this stabilizing effect.
On decreasing the RTIL mole fraction there will be a de-
crease in aggregate concentration. Thus, the favourable effect
due to a lower viscosity will be largely counterbalanced by the
minor stabilization of the transition state, so globally inducing
a decrease in reaction rates. This hypothesis seems to be sup-
ported by conductivity trends. Therefore, the co-solvent seems
to have an upsetting effect on the structure of the imidazolium
RTILs.
Furthermore, by using kinetic data in the attempt of charac-
terizing binary mixtures, one must remember that the extent of
the co-solvent disorganizing effect depends on the nature of
reaction probe used. Probably, the disorganizing effect in-
creases when the reactions are characterized by highly ordered
transition states.
Furthermore, disorganization process (with increase of the
mole fraction of co-solvent added) seems to be affected by
co-solvent nature. Considering the RTIL mole fraction, corre-
sponding to the maximum kII value, such as the transition
point among two or more different species, this process is
early in the presence of polar solvents, such as AcOEt and
MeOH (XMAXw0.8), but rather late in the presence of apolar
solvents, such as Diox (XMAXw0.4).
The effect of co-solvent changes with symmetry and coor-
dination ability of the present RTIL anion. These determine
a different degree of cross-linking for the two [bmim][X]25
and consequently a different concentration of charged aggre-
gates, capable of catalyzing the probe reaction, giving rise to
slower reactions.
This hypothesis seems to be also supported by DL variation
(see above). This parameter is slightly higher for [bmim][NTf2]
mixtures, which could indicate a higher concentration of free
ions in its solutions. Furthermore, as it has a less ordered struc-
ture, it should be less affected by the presence of co-solvents.
This could explain why the disorganization process of RTIL
structure in the presence of [bmim][NTf2] is delayed. This
4. Experimental section
4.1. Materials
Commercial H2O, MeCN, MeOH, CH2ClCH2Cl, CH2Cl2,
AcOEt, CHCl3, Diox, pyrene and Nile Red were used without
any further purification. Commercial [bmim][BF4] was dried
on a vacuum line at 60 ꢃC for at least 2 h and stored in a dryer
under argon and over calcium chloride. [bmim][NTf2] was
prepared according to a procedure previously reported.25b
Pip was freshly distilled before use. Compound 1 was pre-
pared according to the method reported.27
Binary mixtures were prepared by mixing suitable volumes,
by using micro-syringes, of both co-solvent and RTIL. Differ-
ent mole fraction ranges have been examined because of dif-
ferent relative solubilities.
4.2. Kinetic measurements and calculations
UVevis spectra and kinetic measurements were carried out
by using a spectrophotometer equipped with a Peltier

F. D’Anna et al. / Tetrahedron 64 (2008) 672e680
679
temperature controller, able to keep the temperature constant
Supplementary data
within 0.1 K. Kinetic runs were carried out over the tempera-
ture range 293e313 K. The sample for a typical kinetic run
was prepared by mixing the proper RTIL, co-solvent and sub-
strate solution into a quartz cuvette (optical path 0.2 cm). The
solution obtained was thermostated, the proper volume of Pip
solution (at six concentrations) added and the reaction rates
measured by the disappearance of 1 at its lMAX. The concen-
tration of substrate was constant and equal to 1.9ꢀ10ꢁ4 M; the
amine concentration ranged from 2.2 M to 17ꢀ10ꢁ3 M. The
course of the reactions was followed over at least three half-
lives. Kinetic data were analyzed by means of the Kaleida-
Graph 3.0.1 software. The apparent first-order rate constants
obtained were reproducible within ꢂ3%.
Supplementary data associated with this article can be
found in the online version, at doi:10.1016/j.tet.2007.11.033.
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