Solvent effect on a model SNAr reaction in ionic liquid/water mixtures at different compositions

Article abstract of DOI:10.1039/c7nj04820c

The reaction of phenyl 2,4,6-trinitrophenyl ether and piperidine was kinetically evaluated in BMIMBF₄/water mixtures as the reaction media. This study shows the dramatic effect of the mixture composition on the reacting pair and its reaction rate, highlighting two strongly demarcated zones. The first one, rich in water, is characterized by strong variations in the rate coefficient values, suggesting the presence of preferential solvent effects in the aqueous phase. The second zone shows high rate coefficient values independent of the composition of the solvent mixture, suggesting predominant "anion" solvent effects. These results were validated using fluorescence spectroscopy and the Kamlet-Taft parameter.

Full text of DOI:10.1039/c7nj04820c

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Solvent effect on a model SNAr reaction in ionic liquid/water
mixtures at different compositions
Bruno Sanchez,a Cristian Calderón,b Constanza Garrido,a Renato Contrerasa and Paola R.
Campodonicob†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The reaction of phenyl 2,4,6-trinitrophenyl ether and piperidine was kinetically evaluated in mixtures BMIMBF4/water as
reaction media. This study shows the dramatical effect of the mixture compositions over the reacting pair and its reaction
rate, highlightning two zones strongly demarcated. The first one, rich in water is characterized by strong variations in the
rate coefficient values, suggesting the presence of a preferential solvent effects in the aqueous phase. The second zone
shows high rate coefficient values independent of the composition of the solvent mixture, pointing out to a predominant
“anion” solvent effects. These results were validated using fluorescence spectroscopy and Kamlet-Taft parameter.
www.rsc.org/
surface (PES). These specific interactions could produce
catalytic effects and/or changes in the rate determining step
(RDS) on the reaction mechanisms.[8] Note that, in this context,
Introduction
In the last decades the Hydrogen Bond (HB) effect has been
subject of active discussion in nucleophilic aromatic
the solute is considered as the reacting pair. Solute-solvent
interactions are much more complex in solvent mixtures than
in pure solvents. Therefore, the study in solvent mixtures
opens up the possibility to evaluate: i) preferential solvation
a
substitutions (SNAr) reactions.[1-3] Bernasconi et al. explained
the reactivity trends based on the existence of an
intramolecular HB between
a hydrogen atom of the
and/or iso-solvation phenomena[8-10]
; ii) properties and
nucleophile and the o-NO2 group of the substrate
(electrophile).[4] Recently, this HB effect was followed along
the intrinsic reaction coordinate (IRC) profile showing that it
promotes the activation of both the electrophile and the
nucleophile.[5] The HB complexes at the transition state (TS)
structures of the reaction enhance the reactivity of the
reacting pair, by diminishing the activation barriers.[5,1,6] This
effect would be present in electrophiles without NO2- group in
its structures and involved in SNAr reactions.[7] On the other
hand, the HB effect could be involved in solute-solvent
interactions playing a key role in the chemical reactivity
beyond the stabilization of species along the potential energy
structural changes and iii) catalysis. Preferential solvation has
been defined as the difference between the local and bulk
composition of the solutes with respect to the composition of
[8,10,11-14]
the solvents in a binary mixture.
Iso-solvation has
been introduced as a concept describing the composition of a
mixture in which the solute under consideration is solvated,
approximately, by an equal number of co-solvent molecules in
[15-17]
the solvent mixture.
Up to date, studies based on
mixtures of solvents have been broadly published. These
works involve mixtures of: i) conventional solvents (CS),[8-10,18]
ii) CS/ionic liquids[19,20] and iii) ionic liquids.[21-23]
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Ionic liquids are salts with a low melting point, usually weakening the polar network given by cation-anion
liquids at room temperature (RTILs).[24,25] RTILs are interactions of the RTIL and building some clusters embedded
technological materials that provides an alternative to CSs for in the polar network. Note that, this effect occurs prior to the
both scientific and industrial purposes, taking advantage of saturation of the sites of the anion available to establish HB
their potential as novel reaction media.[24-26] The presence of with the water molecules; ii) high quantities of water showed
water or CS in RTILs can modify their physical and chemical local aggregation of the water molecules inside of the polar
properties. For instance: density, vapour pressure, viscosity, network because of the gradual loss of cation-anion
electrical conductivity, solvation and solubility properties. interactions in the RTIL, producing a saturation of the sites on
RTILs are hygroscopic and they can absorb water from the the anionic moiety available to establish a HB and displacing
atmosphere.[27,28] Seddon et al.[29], was pioneer in the study of the cationic moiety. This fact produces repulsive interactions
the effect of impurities and additives in RTILs based on between cation moieties weakening the structural network of
imidazolium cation. He evidenced the change in these RTILs the RTIL, allowing water-anion moiety and water-water
properties with the molar fraction of added water. The interactions by HB.
relevant role of the HB in mixtures RTILs/water was also
The aforementioned studies led us to analyse the solvation
reported by Seddon et al.[28] and its incidence in the structural effects based on the reactivity of the reacting pair expressed in
changes produced when the RTILs is mixed with a co-solvent. terms of the nucleophilic rate coefficients for an SNAr reaction
Linden-Bell et al.[30] showed by molecular dynamic simulations, in
a range of aqueous mixtures of 1-butyl-3-methyl
that in mixtures of water/dialkylimidazolium RTILs, low imidazolium tetrafluoroborate (BMIMBF4). A SNAr reaction
quantities of water behaves as small and independent clusters, involves a nucleophilic addition followed by elimination of a
while that at high quantities of water a continuous water leaving group (LG) and it requires the presence of at least one
network appears thereby changing the properties of the strong electron-withdrawing substituent[1] in the ring of the
mixture. Fazio et al.[31] performed a complete experimental electrophile to stabilize the intermediate, called Meisenheimer
analysis in RTILs/water mixtures, showing that: i) at low Complex (MC).[4,7,8,32] The first step of the reaction mechanism
quantities of water in the mixture, these water molecules corresponds to the formation of an MC. In a second step, the
established interactions with the anionic moiety of the RTIL, LG detaches after an intramolecular proton transfer (RLPT)
2 | J. Name., 2012, 00, 1-3
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from the nucleophile (see more details ahead).[10,32] The highlighting the key role of the HB interaction between the amine
studied SNAr reaction in this work takes place between phenyl and the solvent.[10]
2,4,6-trinitrophenyl ether (PTNPE) and piperidine, depicted in
Table 1. Values of K1k2 and K1k3 for the studied reaction for each BMIMBF4/water
mixtures at 25°C.
Scheme 1. This reaction was revisited from a previous work,
where piperidine was the only amine of a series of secondary
alicyclic amines sensitive to solvation effects in ethanol/water
mixtures, establishing a specific electrophilic solvation by the
water molecules of the mixtures.[10] The main goals of this
work are: i) to determine the relevance of the HB on the
studied reaction at several RTIL/water compositions and its
relationship with catalytic processes or solvation phenomena;
ii) to determine the behaviour of each composition using
probes in order to sense the micro-domains in the studied
solvent mixtures and its incidence over the reaction
mechanisms, solvent effects or catalytic processes for the
studied reaction.
χBmimBF4
K1k2 (M-1s-1)
K1k3 (M-1s-2)
0,000
0,030
0,086
0,150
0,220
0,540
0,790
1,000
1,75 ± 0,19
240,53 ± 10,93
661,29 ± 67,22
752,58 ± 48,71
1006,1± 84,60
1162,55 ± 97,91
1186,3 ± 92,40
1209,9 ±22,46
1293,81 ± 99,90
0
0
0
1,73 ± 0,86
0
0
0,20 ± 0,33
The values accompanying K1k2 and K1k3, correspond to the error associated to
the intercept and slope, respectively to obtain these values. More details in SI.
Results and Discussion.
Figure 1 shows the relationships between the kinetic data (K1k3)
and the composition of the solvent mixtures expressed in molar
fraction (χ) of BMIMBF4 in the mixtures. This Figure shows two
zones strongly demarcated. The first one is a zone rich in water at
the range of compositions between 0 to 0,2 of molar fraction,
In ethanol/water mixtures, the studied reaction followed a
stepwise mechanism, where the loss of a proton from the MC
is the rate determining step (i.e., RLPT mechanism) instead of
the expulsion of the LG, followed by a dual secondary step:
catalyzed and non-catalyzed routes (see Scheme 2).[10] Under
the experimental conditions for the studied reaction, a single
product named 1-(2,4,6-trinitrophenyl) piperidine was
observed and monitored by UV-Vis spectrophotometry. Plots
of kobs vs. free piperidine concentration for the studied
reaction in mixtures of BMIMBF4 with water were in
accordance with a RLPT mechanism. More details are given in
Tables S1–S7 in Electronic Supplementary Information (ESI).
These plots were in accordance with
a second order
polynomial equation (Eq. 2) derived from Eq. 1, assuming that
ꢄ
ꢇ
⁄
ꢀꢁ ꢂ ꢀꢃ ꢅꢆ ≪ ꢀꢈꢉ, where ꢊꢉ ꢋ ꢀꢉ ꢀꢈꢉ
.
ꢄ
ꢇ
ꢄ
ꢄ
ꢇꢁ
ꢇ
ꢀꢉꢀꢁ ꢅꢆ ꢂ ꢀꢉꢀꢃ ꢅꢆ
ꢀꢌꢍꢎ
ꢋ
ꢏ1ꢐ
ꢏ2ꢐ
ꢀꢈꢉ ꢂ ꢀꢁ ꢂ ꢀꢃ ꢅꢆ
ꢄ ꢇ
ꢄ ꢇꢁ
ꢅ
ꢆ
ꢀꢌꢍꢎ ꢋ ꢊꢉꢀꢁ
ꢅ
ꢂ ꢊꢉꢀꢃ
ꢆ
Figure 1. Plot of K1k3 vs. molar fraction to respect BMIMBF4 for
BMIMBF4/water mixtures.
The kinetic data were fitted with Eq. 2. The resulting curves are
shown in Figures S1-S7 in ESI. According to Scheme 2, the reaction
proceeds through two intermediates (MC1 and MC2) corresponding
to a zwitterionic adduct and its deprotonated form, respectively.
Table 1 summarizes all the kinetic data determined at different
compositions of BMIMBF4/water in each mixture. Note that, the
contribution of the non-catalyzed route is negligible (to promote
the LG departure) and all the kinetic weight is in K1k3 constants (to
promote the stabilization of MC2). This result may be traced to the
presence of a second molecule of amine that could establish a more
favoring interaction to the MC1 compared to that of the solvent
molecules. Then, plots of ꢀꢌꢍꢎ/ꢄꢅꢇꢆ VS. ꢄꢅꢇꢆ are linear if the
reaction proceeds as in Scheme 2.[33] Therefore, all the kinetic data
in BMIMBF4/water mixtures follow the catalyzed route, thereby
Figure 2. Plot of K1k3 vs. the parameter β of Kamlet-Taft at
different composition of BMIMBF4/water mixtures.
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characterized by strong variations in the rate coefficient values in the water molecules to bond the amine and the saturation of
this narrow range of compositions, suggesting the beginning of a anionic sites by the RTIL cations.
preferential solvation process.[8,10] The second zone associated to
Based on the previous analysis, a second goal was
this Figure corresponds to a plateau, suggesting that the rate contemplated in this work in order to analyze the features of
coefficient values would be independent of the composition of the each micro-environment given by the composition of the
solvent mixture. The response obtained at χ ≤ 0,2 suggests that the solvent mixtures, specifically close to borderline compositions
anionic moiety is establishing a HB interaction with the amine, (0,2 ≤
χ ≥ 0,2). Figure 3 shows the relationships between
⁄
promoting the strength of the nucleophile and improving the pyrene ꢑꢉ ꢑꢃ emission intensity ratio and the composition
reactivity of the studied substrate. This fact also suggests that in BMIMBF4 on the solvent mixtures. See details in Table S9 in
this composition range, preferential solvation is taking place by the ESI. Pyrene (Py) is one of the most used fluorescent probes for
addition of BMIMBF4. The second zone, χ ≥ 0,2 shows a catalytic the characterization of polarity[36] in both homogeneous and
effect exerted by the RTIL. However, the rate coefficient values are heterogeneous environments (Scheme 3 shows the chemical
structure of pyrene). The polarity scale based on Py
Scheme 3. Structures of pyrene and coumarin 343
⁄
fluorescence is defined by the ꢑꢉ ꢑꢃ emission intensity ratio,
ꢏ
ꢐ
where ꢑꢉ corresponds to the ꢒꢉ ꢓ ꢋ 0 ꢒꢔꢏꢓ ꢋ 0ꢐ transition,
which reaches an increase on its intensity in polar
ꢏ ꢐ
environments, and ꢑꢃ, associated to the ꢒꢉ ꢓ ꢋ 0 ꢒꢔꢏꢓ ꢋ 1ꢐ
transition, insensitive to polarity, derived from the hyperfine
structure of Py fluorescence spectra.[36]
⁄
The change on the ꢑꢉ ꢑꢃ ratio observed in Figure 3 ranges
⁄
Figure 3. Relationships between ꢑꢉ ꢑꢃ emission intensity ratio and the
from 1,55 (pure water) followed by a steep decrease up to a
value of 1,28 at a molar fraction of BMIMBF4 close to 0,2. Note
that, at χ ≤ 0,2 the polarity of the mixtures diminishes
dramatically (0,30 u.e.) with the addition of RTIL, in
accordance to the decreasing of the water content in the
composition of BMIMBF4 on the BMIMBF4/water solvent mixtures.
in a narrow range of variations.
In order to find an explanation for the observed kinetic
behavior and its relationship with the preferential solvation
⁄
mixture. Beyond of χ ≥ 0,2 the variation on the ꢑꢉ ꢑꢃ
emission intensity ratio is negligible (0,05 u.e.), reaching a
proposed in the zone close to
basicity based on Kamlet-Taft (KT) model was performed.[34,35]
More details are in Table S8 in ESI. Figure 2 shows the
χ ≤ 0,2 a complete study of
value of 1,15 at pure BMIMBF4, equivalent to the polarity
⁄
evaluated in ethanol ( ꢑꢉ ꢑꢃ=1,10) in accordance with previous
reports that shows the polarity of the RTILs becoming close to
that shown by alcohols.[37] The kinetic response is in
relationships between β vs. χBMIMBF4. The β parameter of KT
model, is related to the ability of the solvent to accept HB.
accordance with the polarity study suggesting that close to χ=
Note that in the first part of the Figure 2, there is an
0,2 there is a borderline regime, where the mixture properties
are different or changes. For instance, χ ≤ 0,2 is attributable a
reaction media with high degrees of freedom and more
susceptible to establish HB. On the other hand, χ ≥ 0,2
suggests minor possibilities to establish HB with compositions
rich in BMIMBF4. Note that, in each extremes zone the
improvement in the
β values. After of χ ≥ 0,2 the parameter β
shows high values practically constants and close to those
shown in pure BMIMBF4 . Note that, Figure 2 shows similar
trend that Figure 1. However, the second experiment is related
only to the environment, it does not contemplate the reacting
pair. This fact suggests a competition between the water
molecules and the RTIL anion moiety to establish an HB. For
-
-
probabilities of interactions are: H2O(H)-BF4 and BMIM+-BF4 ,
respectively highlighting the key role of the anion moiety of
the RTIL in each mixtures. In order to analyze the anion effect
both Figures (1 and 2) and considering
χ ≤ 0,2, this bond
promotes the ability of the oxygen atom of the water molecule
to link the hydrogen atom of the amine enhancing the strength
of the nucleophile, which is observed through the increase of
-
(BF4 ) over the polarity of the mixtures, experiments based on
the absorbance change of coumarin 343 (C343) in BMIMBF4
and pure water and solutions at different concentrations of
NaBF4 in water were carried out. Scheme 3 shows the chemical
structure of C343 and Table S10 shows data in ESI. In a first
the rate coefficients and the increased
β values, respectively.
However, if the amount of RTIL increases in the mixtures (χ
0,2), the effect is constant due to a lower degree of freedom of
≥
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step of this experiment were considered all the range of these ratios, show an important shift towards longer
mixtures BMIMBF4/water studied. However, each showed absorption wavelengths. At a ratio C343/NaBF4 equal to 1,1
χ
the same absorption wavelength, except at very high diluted the wavelength of the absorption maxima (458 nm) matches
mixtures. See Table S11 in ESI. This fact, might suggest that: i) the one measured in pure BMIMBF4 (457 nm). This result
the composition of the solvation shell close to the probe will enhances the influence of the “anion effect” into the solvation
not correlate linearly with the composition of the solvent bulk shell. Inversely, decreasing concentrations of NaBF4 readily
or/and ii) the anionic moiety of the RTIL will be associated with approximates to the value of C343 in water. This fact could be
attributable to the ionization state of the carboxyl moiety of
C343. Note that, in aqueous media the absorption maximum
of C343 at pH values of 7,0 and 2,0 are reported at 431nm and
452 nm (see Figure S8 in ESI), respectively. This bathochromic
shift shows a relationship with the values measured in water
and pure BMIMBF4, respectively. This fact could indicate that
-
protonated C343 would prioritize interactions BMIM+-BF4
-
while interactions H2O(H)-BF4 would be relevant for non-
protonated C343. The analysis is in accordance with the kinetic
results, where we found that the larger increase on the rate
coefficient values is achieved at low compositions of BMIMBF4
(
χ ≤ 0,2). This result is in agreement with the proposed
preferential solvation exerted in the first part of the mixture
compositions, where the anionic moiety of BMIMBF4 might be
located on the first solvation shell surrounding the reacting
pair. At χ ≥ 0,2 suggests a catalytic effect mediated by the
anion of the RTIL in the mixture. This fact, enhances the
“anion
effect over the reaction promoted by the anion moiety which
”
improves the reactivity of the reacting pair. [24]
Experimental
Kinetic Measurements
an anion-rich solvation shell surrounding the probe. In order to
-
evaluate specific interactions between C343 and BF4 moiety
Kinetic measurement was carried out spectrophotometrically
(300-550 nm range) by means of diode array
a
were carried out UV/Vis absorption of C343 in aqueous
solutions of NaBF4. On this way, Figure 4 shows the absorption
maxima of C343 as a function of the ratio between C343 and
spectrophotometer in mixtures of ionic liquid/water at 25 ±
0,1 ºC. The reactions, studied under excess of the amine over
the substrate, were started by injection of a substrate stock
solution in acetonitrile into the amine solution in RTIL-water
mixture (1000 µL in the spectrophotometric cell). Under these
conditions, pseudo first order rate coefficients (kobs) were
found. The experimental data are listed in Tables S1-S10 and
Figures S1-S8 in the Electronic Supplementary Information.
NaBF4 concentrations. C343 belongs to
aminocoumarin derivatives[38]
which has been used
extensively as laser dyes and in studies regarding solvation
dynamics in different medias.[39]
The most important
a family of 7-
properties associated to C343 are: rigidity, strong radiative
rates, solubility in aqueous and organic media and high
solvatochromism where its absorption and emission spectra
display strong dependence with the environment.[37,38] The
solvatochromic behaviour of C343 is influenced by the Kamlet-
Taft parameters denoted by a: i) partial polarizability (π*) and
ii) high sensitive to β. This sensitivity of the C343 molecule
towards β parameter is increased upon its excitation due to a
weakened intramolecular HB established between the
carboxylic and esters groups of C343.[40] Figure 4 shows the
maximum absorbance wavelength (λabs) for BMIMBF4 pure at
457 nm while that in pure water is observed at 441 nm. Based
on the relationships between C343 and β parameter, it is
possible to analyse the micro-environments given by the
compositions of the mixtures.
Product Study
The presence of 1-(2,4,6-trinitrophenyl) piperidine as product
of the reactions was determined spectrophotometrically by
comparison of the UV-visible spectra at the end of the
reactions with those of authentic samples under the same
conditions.
Determination of Kamlet-Taft parameters
The solvent parameters was measured by injecting the binary
mixtures of IL (950 μL) into a quartz cuvette of optical path 1,0
cm with the probes (4-nitroaniline, and N,N-diethyl-4-
nitroaniline) previously prepared in acetone (50 μL of stock
solution of the probes was evaporated to dryness for 30
Note that in Figure 4, at high concentrations of NaBF4 in
water (lower C343/NaBF4 ratios) the probe senses a similar
polarity to that of pure RTIL. The absorption spectra of C343 at
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minutes). The concentration of the stock solution was 1,6x10-3 anion of the RTIL over the studied reaction, where the
M. All the solutions were thermostated at 25 ± 0,1 ºC.
predominant interactions are between anion-cation.
Coumarin 343 UV-Vis spectrophotometry and Pyrene steady
state fluorescence measurements
Conflicts of interest
Aliquots from freshly prepared stock solutions of C343
dissolved in ethanol were added to pure water, vacuum dried
BMIMBF4 as well as NaBF4 aqueous solutions, incorporated
into ambered glass vials. The resulting solutions were
incorporated into a rotary evaporator and further treated with
a stream of nitrogen in order to remove the ethanol used to
dissolve the probe. C343 absorbance was measured in an
Agilent 8453 UV-VIS spectrometer using 1 cm path quartz cells.
Similarly, fluorescence measurements were carried out by
using pyrene as fluorescent probe, adding the adequate
volume of a stock solution of pyrene (dissolved in methanol) to
water, vacuum dried BMIMBF4 and mixtures of these solvents,
in order to achieve a final pyrene concentration of 1 uM.
Methanol removal from the solutions was performed using the
same procedure used for C343 solutions. Pyrene fluorescence
spectra were recorded in a Perkin Elmer LS-55 fluorescence
spectrometer, using an excitation wavelength of 340 nm, and
the final spectra considered for each measurement
corresponds to the average between 20 collected spectra.
There are no conflicts to declare.
Acknowledgements
This work was supported by project RC-130006 CILIS, granted
by Fondo de Innovación para la Competitividad del Ministerio
de Economía, Fomento y Turismo, Chile; Fondecyt Grants
1150759 and Proyecto de Mejoramiento Institucional
postdoctoral fellowship (CC) PMI-UDD.
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Conclusions
A complete experimental study on the SNAr reaction in a
series of mixtures of BMIMBF4/water has been presented. This
reaction was selected as model system because its kinetic in
EtOH/water is well documented in literature. The main
outcomes of the present study are solvation phenomena of
this reaction in mixtures of BMIMBF4/water at different
compositions. The results shown that, in the studied range of
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