Spectrometric and kinetics studies involving anionic chromogenic chemodosimeters based on silylated imines in acetonitrile or acetonitrile-water mixtures

Article abstract of DOI:10.1039/c6ra23156j

Three chromogenic anionic chemodosimeters (1-3) based on silylated imines were synthesized and characterized. Solutions of compound 1 in acetonitrile with 4.0% (v/v) of water are colorless, but with the addition of several anions only CN^-, and

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Spectrometric and kinetics studies involving anionic chromogenic
chemodosimeters based on silylated imines in acetonitrile or
acetonitrile-water mixtures
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
C. R. Nicoleti,a L. G. Nandi,a G. Ciancaleonia and V. G. Machado*a
www.rsc.org/
Three chromogenic anionic chemodosimeters (1-3) based on silylated imines were synthesized and characterized.
Solutions of compound 1 in acetonitrile with 4.0% (v/v) of water are colorless, but with the addition of several anions only
CN-, and to a lesser extent F-, changed the color of the solutions to orange. However, compounds 2 and 3 were selective
toward F- in acetonitrile. The nucleophilic attack of F- or CN- on the silicon center of the chemodosimeters, through an
SN2@Si mechanism, released colored phenolates as leaving groups. PGSE NMR data corroborated the mechanism
postulated for the reaction. Kinetics studies were carried out, revealing that a higher second-order rate constant was
obtained for the reaction of 1 with F-. The addition of water to the system reduces the nucleophilicity of F-, showing a
slower second-order rate constant in relation to CN-, the latter anion being less hydrated and the more reactive species for
the nucleophilic attack on the silicon center of 1.
Keywords: chromogenic chemodosimeters, imines, naked eye detection, fluoride, cyanide
associated with high costs.21-24 In the search for easy, efficient,
and low cost alternatives, many research groups have reported
chromogenic and fluorogenic chemodosimeters for the
Introduction
Anion species are fundamental in many biological,
environmental and industrial systems due to their structural
features and reactivity.1-13 Two examples of anions of great
importance in many fields are cyanide (CN-) and fluoride (F-).
CN- is a very toxic species due to its strong binding affinity with
the active site of the enzyme cytochrome-oxidase, causing a
decrease in the oxidative metabolism and cell death.14 In
selective detection of anionic species.25-27 This strategy is
based on irreversible chemical reactions between
a
nucleophile and
centers.28-30
a
chemodosimeter with electrophilic
Various chemodosimeters have been designed in recent
years for the detection and quantification of CN-31-37 and F-38-46
in different media. Many of these systems have been designed
for the detection of F- based on the cleavage of the Si-O bond
in organic,47-50 organic-water,40, 51-55 and aqueous media,56-61
considering the strong affinity between F- and silicon. Recently,
we reported a chromogenic chemodosimeter based on 3,5-
dibromo-N-(4-nitrobenzylidene)-4-[(triisopropylsilyl)oxy]anili-
addition, CN- is found in some fruit seeds,15 roots,16 and
18
insects,17,
the anion being delivered by means of the
hydrolysis of cyanogenic glycosides. The other anion, F-,
exhibits a beneficial effect regarding dental care.19 However,
acute F- toxicity can result in gastric and kidney disorders,
skeletal fluorosis, and even death. Moreover, some studies
have suggested that F- exposure is toxic to the brain.20
ne (1
; Figure 1) for the highly selective detection of CN- in an
aqueous micellar system.62 This study represented the first
report found in the literature involving the chemodosimeter
approach based on the cleavage of the Si-O bond for the
detection of CN- in purely aqueous solutions. This system is
able to detect CN- in water and in human blood plasma, with
high sensitivity and specific selectivity.62
Various methods have been used to determine F- and CN-,
but often they require sophisticated instrumentation and are
The features exhibited by compound
1 as chemodosimeter
have motivated us to study the reaction of this and related
compounds with anionic species using different techniques, in
order to provide evidences on the mechanism of the reaction.
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Thus, herein we report studies involving compound 1 and two to the fact that the dye generated (4b) is a strongly
) as chemodosimeters for the solvatochromic species.64, 65 The fact that the addition of a
novel silylated imines (2, 3
detection specifically of CN- and F- anions in acetonitrile and small amount of water makes the system highly selective
1
acetonitrile-water mixtures, using UV-vis, H NMR, and HRMS toward CN- in comparison with other anions has been
techniques. The phenols 4a
to compare their corresponding phenolates with the products chromogenic chemosensors based on acid-base strategies. The
-
6a were also synthesized in order exploited in the literature,63, 66-70 especially in studies involving
of the reactions of
1-
3
with the anions.
explanation relates to the fact that the free energies of
hydration for H2PO4- (-465 kJ mol-1), F- (-465 kJ mol-1), and AcO-
(-365 kJ mol-1) are high in comparison to that observed for CN-
(-295 kJ mol-1).71, 72 CN- is less hydrated with the addition of
water, being a more strongly nucleophilic species than the
other species and therefore it is more able to carry out a
nucleophilic attack on the silicon center in the compound 1,
displacing the colored species 4b as a leaving group.
Figure 1. Molecular structures of chemodosimeters 1-3 and of phenols 4a-
6a.
Results and Discussion
Scheme 1 shows the synthesis of the compounds. Silylated
imines (
synthesized following
literature62, 63 by condensation of the corresponding amines
12 with 4-nitrobenzaldehyde in the presence of a small
1-
3
) and their corresponding phenols (4a
-6a) were
a
methodology described in the
7-
amount of acetic acid. The novel compounds 2, 3, and 6a were
characterized by means of IR, 1H NMR, 13C NMR, and HRMS
spectrometric techniques.
Figure 2. Solutions of 1-3 in (A) acetonitrile and acetonitrile with (B) 1.0%,
(C) 2.0%, and (D) 4.0% (v/v) of water, without anion added and after the
-
-
-
addition of HSO4 , H2PO4 , NO3 , CN-, AcO-, F-, Cl-, Br, and I- as tetra-n-
butylammonium salts. The concentration of 1 and 3 was 4.0×10-5 mol L-1 and
of 2 was 5.0×10-5 mol L-1, while the concentration of the anions was 1.0×10-3
mol L-1. The pictures were taken 5 min after the addition of the anions.
Figure
3 shows the UV-vis spectra and the relative
absorbance values for solutions of
presence of the anions. For compound
1
in the absence and
in acetonitrile (Figure
1
3A) a band with a maximum at 360 nm appears, which
-
decreases in intensity with the addition of H2PO4 , CN-, AcO-
and F- due to Si-O bond cleavage, with the simultaneous
appearance of another band with a maximum at 516 nm,
related to phenolate 4b. With the addition of 4.0% (v/v) of
water (Figure 3B), CN-, and to a lesser extent F-, causes the
appearance of the band in the visible region, with a maximum
at 484 nm.
Scheme 1. Synthesis of compounds 1-3 and 4a-6a.
Figure 2A shows the influence of the individual addition of
-
-
-
different anions (HSO4 , H2PO4 , NO3 , CN-, AcO-, F-, Cl-, Br-, and
I-) as tetra-n-butylammonium salts to solutions of compounds
1-3 in acetonitrile. The solutions of these compounds are
-
Figure 4A,B shows the UV-vis spectra for solutions of
in the absence and presence of the anions and Figure 4C,D
exhibits their corresponding relative absorbance values.
Compounds and in acetonitrile (Figure 4A,B) have bands
2 and
colorless, but in the presence of the anions only H2PO4 , CN-,
3
AcO- and F- caused the appearance of a pink color for
compound
able to cause changes to violet for compound
compound . The colors of the resulting solutions match the
colors of their corresponding phenolate dyes 4b 6b obtained
1
. Only F- (and to a much lesser extent CN-) was
and blue for
2
3
2
with maximum absorbance at 372 nm and 380 nm,
respectively. The addition of CN- and F- decreases the intensity
of the bands, due to Si-O bond cleavage, with the
simultaneous appearance of another band, with a maximum at
3
-
through the deprotonation of compounds 4a-6a (see
Supplementary Information).
550 nm for
5b, and at 614 nm for compound
verified for compound 6b
2
, coinciding with that verified for the phenolate
Figure 2B-D shows that on the addition of a small amount
(4.0%) of water to the solutions of
, only CN-, and to a lesser
extent F-, caused a change in the solution color to orange. The
change in the color of the resulting solution from pink in
acetonitrile to orange with the addition of 4.0% of water is due
3, which coincides with that
1
.
2 | J. Name., 2012, 00, 1-3
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Simultaneously, the spectrum of the product exhibited the
signals of a singlet at δ 8.51 ppm (Hd), a doublet at δ 7.96 ppm
(He), and a distorted doublet at δ 1.02 ppm (Hf). More detailed
1H NMR analysis showed that the chemical shifts of the final
spectrum coincide with the 1H NMR spectrum of 4a in the
presence of CN- in excess (Figure S2), indicating that the
species generated (4b) is the same in the two cases. The
1.1
CNꢀ, AcOꢀ, Fꢀ
H2PO4ꢀ
0.66
0.55
0.44
0.33
0.22
0.11
(A)
(B)
other anions
1.0
0.9
0.8
CNꢀ
0.7
other anions
0.6
0.5
0.4
0.3
0.2
0.1
Fꢀ
results of the titration experiments obtained employing
1
1 with
F- using the H NMR technique were similar to those found for
350
400
450
500
550
600
350
400
450
500
550 600
650
1
CN-. Similar H NMR spectrometry results were obtained for
2,
Wavelength / nm
Wavelength / nm
3
, and their corresponding phenols (5a and 6a) in CD3CN with
1.0
0.8
0.6
0.4
0.2
0.0
1.0
(D)
the addition of several equivalents of F- (Figures S4-S7).
(C)
0.8
0.6
0.4
0.2
0.0
-
-
-
-
HSO4- H2PO4 NO3 CN- AcO- F-
Cl-
Br-
I-
HSO4- H2PO4 NO3 CN- AcO-
F-
Cl-
Br-
I-
-
-
Figure 3. UV-vis spectra for solutions of 1 in the presence of HSO4 , H2PO4 ,
-
NO3 , CN-, AcO-, F-, Cl-, Br-, and I-, as tetra-n-butylammonium salts (A) in
acetonitrile and (B) in acetonitrile with 4.0% (v/v) of water and respective
relative absorbance values (C) in acetonitrile and (D) in acetonitrile with
4.0% of water. For concentrations of 1 and the anions, see Figure 1. All
spectra were made 5 min after the addition of the anions.
Fꢀ
(A)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
(B)
other anions
Fꢀ
other anions
H2PO4ꢀ
CNꢀ
CNꢀ
AcOꢀ
350 400 450 500 550 600 650 700 750 350 400 450 500 550 600 650 700 750 800
Figure 5. 1H NMR spectra (400 MHz, CD3CN) of 1 (A) in the absence and in
the presence of (B) 0.3, (C) 0.8, and (D) 1.3 equivalents of tetra-n-
Wavelength / nm
Wavelength / nm
butylammonium cyanide
.
1.0
1.0
0.8
0.6
0.4
0.2
0.0
(D)
(C)
0.8
0.6
0.4
0.2
0.0
A similar titration experiment was carried out to study the
interaction between and tetra-n-butylammonium fluoride in
CD3CN (Figure 6). Analysis of the H NMR spectra shows that
after the addition of 1.2 equivalents of F- to a solution of
(2.7
mmol L-1) in CD3CN the peaks relative to
are completely
1
1
1
1
HSO4- H2PO4- NO3 CN- AcO-
F- Cl-
Br-
I-
-
HSO4-H2PO4 NO3 CN- AcO-
F-
Cl-
Br-
I-
-
-
absent. In particular, the doublet related to the isopropyl
groups shifts from δ 1.17 to 1.07 ppm and the septet of the
same groups shifts from δ 1.60 to 1.13 ppm, assuming a more
complex pattern. On the other hand, a sharp signal at δ -187
ppm appears in the 19F NMR spectrum (Fg, see Figure 6).
Figure 4. UV-vis spectra for solutions of (
A
)
2
and (
B) 3 in the presence
-
-
-
of HSO4 , H2PO4 , NO3 , CN-, AcO-, F-, Cl-, Br-, and I-, as tetra-n-
butylammonium salts in pure acetonitrile and (
absorbance values. For concentrations of
Figure 1
C
3
,
D
) respective relative
, and the anions, see
2
,
In order to verify the nature of the product(s), a series of
Pulse Gradient Spin-Echo (PGSE) NMR experiments were
carried out. This technique allows the hydrodynamic volume of
the species in solution to be measured,73 aiding their
identification (see details in the Experimental Section). From
the plot in Figure 7, it is evident that the peaks corresponding
to Hd and Hf relate to species with different self-diffusion
coefficients, indicating that they do not belong to the same
molecular system. In particular, Hd relates to a species with a
hydrodynamic volume (VH) of 481 Å3 (entry 2, Table 1) and Hf
.
All spectra were made 5 min after the addition of the anions.
1H NMR studies were performed for compound
with the addition of consecutive aliquots of CN- (Figure 5).
Compound shows the singlet at
8.67 ppm (Ha),
1 in CD3CN
1
δ
corresponding to the proton of the imine group, the doublet at
δ 8.10 ppm (Hb) of the aromatic protons and another doublet
assigned to the triisopropyl group at δ 1.17 ppm (Hc). After the
addition of 1.3 equivalents of CN- these signals disappeared.
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Table 1. Diffusion coefficients (Dt, 10–10 m2 s–1), hydrodynamic radii (rH, Å),
and hydrodynamic volumes (VH, Å3) of the species present in the solution (1
+ TBAX).
to a different species with a VH of 159 Å3 (entry 3, Table 1).
Therefore, we can reasonably conclude that, since the peaks Hf
and Fg show comparable VH values, they belong to the same
molecular system. A hydrodynamic volume around 150 Å3 is
perfectly compatible with the FSi(Pri)3 species and also the
chemical shift of fluorine is compatible with a FSiR3 moiety.74
On the other hand, a volume of 481 Å3 is compatible with the
Entry
1
Solution
Dt
rH
VH
1
12.4
5.11
559
2
3
4
5
6
1 + 1.2 eq Fꢀ (Hd)
1 + 1.2 eq Fꢀ (Hf)
1 + 1.2 eq Fꢀ (Fg)
1 + 1.3 eq CNꢀ (Hd)
1 + 1.3 eq CNꢀ (Hf)
13.3
22.5
23.5
13.6
21.0
4.86
3.36
3.27
4.80
3.52
481
159
146
463
183
phenolate 4b
.
Scheme 2 describes the chemodosimeter approach based
on compounds
. The nucleophilic attack of F- or CN- on the
silicon center, through an SN2@Si mechanism,75-77 releases the
colored species 4b 6b as leaving groups.
1-3
-
Figure 6. 1H and 19F NMR spectra (400 MHz, CD3CN) of 1 (A) in the absence
and in the presence of (B) 0.4, (C) 0.8, and (D) 1.2 equivalents of tetra-n-
butylammonium fluoride
.
Scheme 2. Reaction of F- or CN- with 1-3 to generate colored species 4b-6b
in solution.
Kinetic measurements
The reaction of compounds 1-3 with the anionic species occurs
quantitatively in acetonitrile at 25.0 oC. Rate constants for the
reactions were determined by following the appearance of the
corresponding phenolates 4b, 5b, and 6b at 516, 550, and 614 nm,
respectively, and the reactions studied are strictly second order.
Kinetic runs were performed under first-order conditions with at
least a 10-fold excess of the anionic nucleophile. The observed
pseudo-first-rate constant (kobs) values obtained were plotted as a
function of c (anion), being linear with zero intercepts, providing
the second-order rate constants (k2). The k2 values were
reproducible to within ± 2%.
Figure 8 shows plots of the kobs values for the reaction of
compounds 1-3 with F- in acetonitrile at 25.0 oC, which were used to
provide the corresponding k2 values. Table 2 shows the k2 values
obtained for the reactions involving compounds 1-3 and F-. Data
show that the k2 values decrease in the order: 1 > 2 > 3. This order
suggests that the substituents at the 2,6 position in the compounds
change the reactivity of the compounds due to a change in the
nature of the phenolate leaving group. Thus, the negative inductive
Figure 7. Plot of ln(I/I0) versus G2 for a solution of
tetra-n-butylammonium fluoride in CD3CN
1 + 1.2 equivalent of
.
Similar results can be obtained after the addition of TBACN
to a solution of
(see Figure S30). In this case Hd shows a VH of
1
463 Å3 and Hf of 183 Å3 (entries 5 and 6, Table 1). These values
are similar to those obtained with TBAF, within the
experimental error, confirming the same fragmentation in the
two cases.
In addition, ESI mass spectra were obtained for
1 in the
presence of F- and CN- (see Figure S31), showing m/z peaks of
[M]- at 398.8764, after the addition of F-, and [M]- at 398.8780,
after the addition of CN-, corresponding to that of 4b. Thus,
the characterization by both NMR and MS spectrometric
techniques demonstrated the Si-O bond cleavage in the
effect of bromo substituents in compound
1 makes the
corresponding phenolate a better leaving group than that verified
for compound 2. On the other hand, the positive inductive effect of
the methyl groups in compound 3 makes its phenolate a poorer
leaving group than those of compounds 1 and 2. These data are
corroborated by comparison with the pKa values of the phenols 4a-
6a in water and their corresponding estimated pKa values in
chemodosimeter by F- and CN- with the formation of 4b
.
4 | J. Name., 2012, 00, 1-3
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acetonitrile (Table 3).63 Data show that the pKa values increase in becomes strongly solvated by water molecules, which makes the
the order: 4a < 5a < 6a, which is intrinsically related to the ability of hydrated species less able to act as a nucleophile. However, CN-
the conjugated bases to act as leaving groups.
interacts more weakly with water molecules and therefore acts
more efficiently as a nucleophile in these experimental conditions,
reacting faster with the silicon center of 1.
0.20
0.15
0.10
0.05
0.00
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.0
0.5
1.0
1.5
2.0
c(Fꢀ) / 10ꢀ3 mol Lꢀ1
0.00
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
c(anion) / 10ꢀ3 mol Lꢀ1
Figure 8. Influence of c(F-) on the kobs value for the reaction of 1 (▲), 2 (●),
and 3 (■) with F- in acetonitrile at 25.0 oC.
-
Figure 9. Influence of the concentration of CN- (●), AcO- (■), and H2PO4 (▲)
on the kobs values of 1 with the anions in acetonitrile at 25.0 oC.
Table 2. Values of k2 for the reaction of compounds 1-3 with F- in
acetonitrile at 25.0 oC.
Table 4. The k2 values obtained for the reaction of 1 with the anions in
acetonitrile at 25.0 oC.
Compound
k2 (L molꢀ1 sꢀ1)
r2
1
2
3
ꢀ
Anion
k2 (L molꢀ1 sꢀ1)
r2
Fꢀ
CNꢀ
109
AcOꢀ
16.3
H2PO4
5.23
182
39.3
11.0
0.9999
0.9970
0.9996
182
0.9999
0.9997
0.9989
0.9992
Table 3. pKa values of compounds 4a-6a obtained in water at 25.0 oC and
estimated in acetonitrile.
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0.06
(A)
(B)
0.05
0.04
0.03
0.02
0.01
0.00
Compound
pKa in H2O
4a
5a
6a
7.37a
22.2
10.6b
27.6b
11.8
29.6
pKa in CH3CN
a,b Data from the literature.62, 63
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
c(Fꢀ) / 10ꢀ3 mol Lꢀ1
c(CNꢀ) / 10ꢀ3 mol Lꢀ1
Figure 10. Influence of the concentration of (A) CN- and (B) F- on the kobs
values of 1 in acetonitrile at 25.0 oC in the presence of 1.0% (■), 2.0% (◄),
Since compound
1
was the most reactive of the
chemodosimeters investigated, its reaction with CN-, H2PO4 and
AcO- was studied in acetonitrile at 25.0 oC to provide the
corresponding k2 values (Figure 9). Table 4 shows that the k2 values
for the reaction of 1 with the anions follow the decreasing order of
-
and 4.0% (●) of water
.
Table 5. Values of k2 obtained for the reaction of 1 with F- and CN- in
acetonitrile with 1.0%, 2.0%, and 4.0% of water at 25.0 oC.
nucleophilicity in acetonitrile: F- > CN- > AcO- > H2PO4 , which
-
reflects the nucleophilic potential of these species in a non-
hydrogen-bond donor solvent, as is the case of acetonitrile.
1.0% water
2.0% water
4.0% water
The addition of small amounts of water leads to a decrease in
the kobs values of 1 with the anions. Figure 10 shows plots of the kobs
values as a function of the CN- and F- concentrations for compound
1 in acetonitrile with different amounts of water, while Table 5
shows the corresponding k2 values obtained. The rate constants
decreased with the addition of water, which is consistent with other
data published in the literature.51, 54 This is due to the fact that F-
Anion
k2 (L molꢀ1 sꢀ1)
r2
Fꢀ
CNꢀ
Fꢀ
CNꢀ
10.8
Fꢀ
CNꢀ
4.41
14.6
19.4
4.21
1.13
0.9990
0.9993
0.9995
0.9999
0.9995
0.9992
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25.0 °C, using a 1 cm quartz square cuvette. The maximum
absorption on the UV-vis spectra (λmax) was calculated from
the first derivative of the absorption spectrum. Melting points
were obtained on a Kofler hot stage and were uncorrected.
The nuclear magnetic resonance (NMR) spectra were obtained
using a Bruker Avance 200, Avance 400 or Varian AS-400
spectrometers. The 1H NMR data are reported as chemical
shifts (δ in ppm), with the solvent resonance as the internal
standard. Data are reported as follows: chemical shift,
multiplicity (s = singlet, d = doublet, and m = multiplet),
coupling constants (Hz) and integration. The 13C NMR data are
reported as chemical shifts (δ in ppm). Infrared (IR) spectra
were recorded on a Shimadzu spectrophotometer (model
Prestige-21), with KBr pellets. High-resolution mass spectra
(HRMS) analysis was carried out using an electro-spray
ionization (ESI) quadrupole time-of-flight (QTOF) mass
spectrometer. The instrument was calibrated using an internal
calibration standard (low concentration tuning mix) supplied
by Agilent Technologies.
Conclusions
Silylated imines (1-3) and their corresponding phenols (4a-
6a
)
were successfully synthesized and characterized.
Compounds 1-3 were applied in a strategy based on a
chemodosimeter approach for the selective detection of F- or
CN- in pure acetonitrile or acetonitrile-water mixtures.
Experimental studies verified the nucleophilic attack of F- or
CN- on the silicon center of the compounds as well the species
generated by oxygen-silicon bond cleavage, these studies
being monitored using UV-vis, HRMS, and 1H NMR techniques.
PGSE NMR experiments were utilized in the studies, by
analyzing the hydrodynamic volumes of the products
generated in solution. The data obtained corroborated the
mechanism postulated for the reaction, through an SN2@Si
mechanism, with the nucleophilic attack of F- or CN- on the
silicon center.
Kinetics studies showed that the silylated imines react
faster with F- in pure acetonitrile in relation to other anions,
due to their greater nucleophilic potential. In addition,
compound
compounds. This is due to the fact that the negative inductive
effect of bromo substituents present in improves the nature
of the leaving group. The addition of water to the medium
caused a decrease in the k2 values of
with F- and CN- due to
1 exhibited higher k2 values in relation to the other
Synthesis of the compounds and characterization
The 4-aminophenols
corresponding silylated derivatives
prepared as described previously.35, 62 Compounds
are the same as those described in a recent publication.62
(
8,
10,
and 12
)
and their
were
and 4a
1
(
7,
9,
and 11
)
1
1
the fact that water solvates the anions, hindering their action
as nucleophiles. However, the interaction of CN- with the
water molecules is weaker than that of F-, leading to the
reaction with CN- being faster than that with F- in aqueous
medium.
Compound 5a was obtained according to
described in the literature.63
a procedure
General procedure for the synthesis of the imines (2, 3, and
6a)
The results reported herein provide a useful approach to
the design of new molecules for the development of anionic
chemodosimeters that can act in different ways and with
selectivity.
4-Nitrobenzaldehyde (0.50 g; 3.31 mmol) and the respective
amine (3.31 mmol) were dissolved in dry ethanol (5 mL) in a
round-bottom flask containing acetic acid (one drop). The
reaction mixture was then stirred for 3 h at room temperature.
The reaction was monitored by thin layer chromatography
(TLC) eluting with n-hexane:ethyl acetate (3:1; v/v). The
precipitate in the reaction medium was filtered off under
vacuum, washed with iced ethanol and recrystallized from
ethanol.
Experimental Section
Reagents
Acetonitrile (HPLC grade, Sigma-Aldrich) was purified
according to a previously described procedure and then
stored.78 The deionized water used in the measurements was
boiled and bubbled with nitrogen and kept in a nitrogen
atmosphere to avoid the presence of carbon dioxide. Acetone,
acetonitrile, and absolute ethanol were purchased from Vetec
and dried and stored over 4 Å molecular sieves in sealed
(E)-N-(4-nitrobenzylidene)-4-[(triisopropylsilyl)oxy]aniline (2)
o
.
Yield: 50%; yellow needles, m.p. 62-63 C; IR (KBr, max/cm–1):
υ
2941, 2864 (C–H), 1600 (C=N), 1636, 1501 (C=C), 1522, 1341
1
(N=O), 1260, 909 (Si(CH3)n); H NMR (400 MHz, CDCl3) δ/ppm:
8.56 (1H, s), 8.30 (2H, d, J = 8.6 Hz), 8.04 (2H, d, J = 8.6 Hz),
7.23 (2H, d, J = 9.0 Hz), 6.92 (2H, d, J = 9.0 Hz), 1.26 (3H, m),
1.11 (18H, d, J = 7.4 Hz); 13C NMR (100 MHz, CDCl3) δ/ppm:
154.76, 148.92, 143.77, 141.94, 129.03, 123.96, 122.49,
120.54, 104.97, 17.88, 12.63; HRMS (ESI, TOF): m/z calcd for
C22H31N2O3Si [M + H]+ 399.2098, found 399.2092.
bottles. All anions (HSO4 , H2PO4 , NO3 , CN-, AcO-, F-, Cl-, Br-,
and I-) were used as tetra-n-butylammonium salts with purity
greater than 94-99 %. The anions were purchased from Fluka
-
-
-
-
-
(Cl-, > 98 %; NO3 , > 97 %; H2PO4 , > 97 %), Vetec (Br-, > 99 %; I-,
> 99 %; HSO4 , > 99 %) and Sigma-Aldrich (F-, > 97 %; AcO-, > 97
%; CN-, > 94%) and were dried over P4010 under vacuum before
use.
-
(E)-3,5-dimethyl-N-(4-nitrobenzylidene)-4-
[(triisopropylsilyl)oxy] aniline (3). Yield: 60%; yellow needles,
m.p. 135-136 C; IR (KBr, max/cm–1): 2923, 2866 (C–H), 1599
o
υ
Instrumentation
(C=N), 1634, 1479 (C=C), 1522, 1342 (N=O), 1246, 883
1
(Si(CH3)n), 1234 (C-O); H NMR (400 MHz, CDCl3) δ/ppm: 8.56
UV-vis measurements were performed with an HP 8452A
spectrophotometer equipped with a thermostated bath at
(1H, s), 8.30 (2H, d, J = 8.6 Hz), 8.03 (2H, d, J = 8.6 Hz), 6.98
6 | J. Name., 2012, 00, 1-3
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(2H, s), 2.29 (6H, s), 1.31 (3H, m), 1.12 (18H, d, J = 7.4 Hz); 13C magnetogyric ratio. The shape of the gradients was
NMR (100 MHz, CDCl3) δ/ppm: 154.47, 153.40, 148.84, 143.31, rectangular, their length d was 4–5 ms, and their strength G
142.09, 128.99, 128.93, 123.95, 121.71, 104.97, 17.98, 14.22; was varied during the experiments. The semi-logarithmic plots
HRMS (ESI, TOF): m/z calcd for C24H35N2O3Si [M + H]+ 427.2411, of ln(I/I0) versus G2 were fitted using a standard linear
found 427.2412.
regression algorithm, and a correlation factor better than 0.99
(E)-2,6-dimethyl-4-[(4-nitrobenzylidene)amino]phenol (6a)
υ
.
was always obtained. Different G values and numbers of
o
Yield: 70%; red solid, m.p. 140-141 C; IR (KBr, max/cm–1): transients were used for different samples.
3192 (OH), 2951, 2921 (C–H), 1597 (C=N), 1656, 1481 (C=C),
1
The self-diffusion coefficient Dt, which is directly
1515, 1338 (N=O), 1228 (C-O); H NMR (400 MHz, DMSO-d6) proportional to the slope m of the regression line obtained by
δ/ppm: 8.78 (1H, s), 8.51 (1H, s), 8.33 (2H, d, J = 8.6 Hz), 8.12 plotting ln(I/I0) versus G2 (Figure 7), was estimated by
(2H, d, J = 8.6 Hz), 7.06 (2H, s), 2.19 (6H, s). 13C NMR (100 MHz, determining the proportionality constant for a sample of HDO
DMSO-d6) δ/ppm: 154.49, 153.16, 148.27, 142.23, 141.53, (5%) in D2O (known diffusion coefficients in the range 274–318
128.99, 124.93, 123.97, 121.75, 16.72; HRMS (ESI, TOF): m/z K80 under exactly the same conditions as the sample of
calcd for C15H15N2O3 [M + H]+ 271.1077, found 271.1073.
interest. TMS was used as the internal standard. The Dt data
were treated as described in the literature in order to derive
the hydrodynamic dimensions.73 Error propagation analysis
UV-vis study of compounds 1-3 with the anions
A stock solution of 1-3 was prepared in anhydrous acetone yielded a standard deviation of approximately 3–4 % in the
in a concentration of 1.2×10-2 mol L-1. An aliquot of the hydrodynamic radius.
solution, sufficient to give a concentration of 4.0×10-5 mol L-1
for
microsyringe and placed in
evaporation of the solvent the solid was dissolved in dry
acetonitrile. These solutions were then used to prepare the described. All reactions were initiated by mixing an aliquot of
solutions of each anion in concentrations of 6.0×10-4 mol L-1 these solutions in
hermetically sealed quartz cuvette
for , in 5 mL containing 1.5 mL of dry acetonitrile. The final concentrations
, 1.0×10-3 mol L-1 for , and 7.0×10-4 mol L-1 for
volumetric flasks. Subsequently, these solutions were of in all kinetic experiments were the same as those shown
1
and
3
, and 5.0×10-5 mol L-1 for
2
, was collected with a Kinetics of the reaction of 1-3 with anions in acetonitrile and
volumetric flask. After acetonitrile-water mixtures
A stock solution of
a
1-3 was prepared as previously
a
1
2
3
1-3
transferred to quartz cuvettes hermetically sealed with a previously to UV-vis study. After the thermal equilibrium at
rubber septum and UV-vis spectra were recorded. Similar 25.0 oC was reached, aliquots of a stock solution of 3.0×10-2
experiments were carried out with acetonitrile-water mixtures mol L-1 of the anions in dry acetonitrile and acetonitrile-water
to evaluate the influence of the medium on the performance mixtures were added to the cuvette and the kinetics data were
of 1. The maximum of the UV-vis spectra (λmax) were calculated collected. The reaction was monitored through the change in
from the first derivative of the absorption spectrum.
absorbance (A) at 516, 550, and 614 nm for
respectively, as a function of the reaction time t. Experiments
were also performed for in acetonitrile containing water in
1, 2, and 3,
NMR titrations
1
1H NMR titrations were carried out in CD3CN solutions at proportions (v/v) of 1.0%, 2.0%, and 4.0% and the absorbance
concentrations of 2.70×10-3, 2.51×10-2, 2.34×10-2, 2.50×10-2, values were collected at 498, 490, and 484 nm, respectively.
4.13×10-2, and 3.70×10-2 mol L-1 for
1,
2,
3,
4a,
5a, and 6a
,
The kobs values were obtained through nonlinear least-squares
respectively. Aliquots of 0.5 mol L-1 F- and CN- (as tetra-n- regression curve-fitting using the program ORIGIN 6.1.
butylammonium salts) stock solutions were added to the NMR
tubes containing the compounds and the 1H NMR spectra were
obtained after 3 min at 22.0 oC.
Acknowledgements
The authors are grateful to the Brazilian governmental
agencies Conselho Nacional de Desenvolvimento Científico e
PGSE NMR experiments
1H and 19F PGSE NMR measurements were performed
using the double stimulated echo sequence with a longitudinal
eddy current79 at 293 K without spinning. The dependence of
the resonance intensity (I) on a constant waiting time and on a
varied gradient strength G is described by the following
equation:
Tecnológico (CNPq), Fundação de Amparo
à Pesquisa e
Inovação do Estado de Santa Catarina (FAPESC), and
Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior (CAPES) for their financial support, as well as to the
Laboratório Central de Biologia Molecular (CEBIME/UFSC) and
UFSC.
ꢁ
ꢁꢂ
ꢆ
3
ꢄ
ꢇꢈ
ln ꢀ ꢃ = − ꢅꢆ ꢉꢊ ꢀΔ − ꢃ ꢋꢈ
Notes and references
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Graphical Abstract
The nucleophilic attack of F- or CN- on the silicon center of the
chemodosimeters, through an SN2@Si mechanism, released
colored phenolates as leaving groups. NMR, PGSE NMR, and UV-
vis studies provided evidences on the mechanism of the reaction
of the compounds with the anionic species.
10 | J. Name., 2012, 00, 1-3
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