A new mechanism for selective recognition of cyanide in organic and aqueous solution

Article abstract of DOI:10.1002/ejoc.202000342

A simple colorimetric and fluorimetric chemosensor 3,5-dinitro-(N-phenyl)benzamide (DNBA), was synthesized for selective determination of cyanide anion in organic and aqueous solutions via novel chemodosimeter approach. The chemosensor DNBA showed a chrom

Full text of DOI:10.1002/ejoc.202000342

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: A mechanism for selective recognition of cyanide in organic and
aqueous solution: nucleophilic aromatic substitution of hydrogen
and occurring biologically important benzisoxazole
Authors: Ergin Keleş, Burcu Aydıner, Yahya Nural, Nurgül Seferoğlu,
Ertan Şahin, and Zeynel Seferoğlu
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of the final Version of Record (VoR). This work is currently citable by
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content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000342
Link to VoR: https://doi.org/10.1002/ejoc.202000342
01/2020

10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
A new mechanism for selective recognition of cyanide in organic
and aqueous solution: cyanide-induced nucleophilic aromatic
substitution of hydrogen (NASH) and occurring biologically
important benzisoxazole
Ergin Keleş[a], Burcu Aydıner[a], Yahya Nural[b,c], Nurgül Seferoğlu[d], Ertan Şahin[e] and Zeynel
Seferoğlu*[a]
[a]
E. Keleş, Assoc. Prof. Dr. B. Aydıner, Prof. Dr. Z. Seferoğlu
Gazi University, Department of Chemistry, 06560, Ankara, Turkey. Tel.: +90 312 2021525. E-mail: znseferoglu@gazi.edu.tr (Z.SEFEROĞLU).
http://w3.gazi.edu.tr/~znseferoglu/index.html
[b]
Assoc. Prof. Dr. Y. Nural
Department of Analytical Chemistry, Faculty of Pharmacy, Mersin University, 33169, Mersin, Turkey
Advanced Technology, Research and Application Center, Mersin University, 33343 Mersin, Turkey
Prof. Dr. N. Seferoğlu
[c]
[d]
Department of Advanced Technology, Gazi University, 06560 Ankara, Turkey
Prof. Dr. E. Şahin
[e]
Department of Chemistry, Faculty of Science, Atatürk University, 25240, Erzurum, Turkey
Supporting information for this article is given via a link at the end of the document.
Abstract: A simple colorimetric and fluorimetric chemosensor 3,5-
dinitro-(N-phenyl)benzamide (DNBA), was synthesized for selective
determination of cyanide anion in organic and aqueous solutions via
novel chemodosimeter approach. The chemosensor DNBA showed
a chromogenic and fluorogenic selective response to CN− against
competing anions such as F−, AcO− and H2PO4− in organic (DMSO
and ACN) and in aqueous solutions (in DMSO/H2O: 8/2, v/v). A
mechanism between DNBA and cyanide which was named as
aromatic substitution of cyanide to the dinitrophenyl ring via
abstraction of hydrogen was presented. The intensive colorimetric
and fluorimetric color changes were observed in ambient light and UV-
light (λex. 365 nm) after cyanide interacted with DNBA. To explain
colorimetric and fluorimetric changes that comes from dominantly the
cyanide addition to dinitrophenyl ring in DNBA, N-methyl-3,5-dinitro-
N-phenylbenzamide (DNBAM) was synthesized. A method that can
be used in the synthesis of new biologically active benzisoxazole
compound was described by the reaction of DNBA with TBACN and
KCN in DMSO or DMSO/H2O, respectively. All interaction
mechanisms between DNBA and cyanide and fluoride anions were
demonstrated by experimental studies using various spectroscopic
methods such as UV-vis, fluorescence, 1H/13C NMR and mass
spectrometry as well as X-ray diffraction method. In addition, the
experimental results were also explained with theoretical data. The
spectroscopic results showed that cyanide interacts with three
different mechanisms named as deprotonation, nucleophilic aromatic
substitution, and formation of benzisoxazole ring. In addition, DNBA
has lower LOD (Limit of detection) than WHO (World Health
Organization) value for the detection of cyanide.
cations have become of great significance due to simultaneous
analysis of these analytes, which may surmount the challenges
encountered with loading multiple indicators.[2] In the past
decades, various anions have been found to play an important
role in the advancement of dental health,[3] metabolic processes,[4]
energy storage, and signal transduction systems.[5] Therefore,
lots of efforts have been committed towards the design and
development of new protocols and anion probes for the detection
of fluoride, cyanide, acetate, etc., in the last decades. Fluoride is
the most electronegative anion and it has strong hydrogen
bonding ability with different probes having acidic -NH and -OH
groups. Hence, many probes which have acidic proton have been
developed for sensing fluoride.[6] In the meanwhile, to the
detection of cyanide, many different strategies such as hydrogen
bonding,[7] chemodosimeters,[8] displacement approaches[9] and
addition mechanism[10] have been developed. In addition, various
approaches in sensing cyanide anion have been formulated to
enhance the sensitivity, selectivity, and dynamical working range
recently.[11] However, till now cyanide detection via nucleophilic
aromatic substitution of cyanide to electron-deficient dinitroarenes
has not been published.
DNBA has an amide functional group as the receptor part and
can easily be synthesized by the well-known procedure.[12]
Because of having acidic hydrogen on the amide functional group,
it may be used as a simple colorimetric chemosensor for some
−
important anions such as CN−, F−, AcO−, H2PO4 etc. The anion
sensitivity of DNBA towards some selected anions such as F− and
CN− via deprotonated process or H-bonding interaction in
acetonitrile (ACN) was published in a previous article.[12] In that
article, the color changes were seen easily with the naked-eye
after interaction of DNBA with CN− and F− from colorless to pink
and yellow, respectively. However, color changes were not
observed in methanol:H2O (9:1) binary solvents. Interestingly, in
that article both mechanisms were proposed using color change
only in ambient-light. Herein, previously proposed mechanisms
between DNBA and CN− were improved and rebutted by using
well-known spectroscopic methods such as UV-vis, fluorescence,
Introduction
The colorimetric and fluorimetric chemosensor for sensing
anions have attracted much interest as a result of being applied
in detecting hazardous anions, observation of enzyme reactions
associated with anions, and bioimaging of anions.[1] In addition,
single molecular optical probes that can sense multiple anions or
1
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
and 1H NMR titration methods and mass spectrometry analysis as
well as X-ray structural analysis technique. Besides, the
deprotonation of amide proton, according to our proposed
mechanism cyanide reacts with dinitrophenyl fragment via
nucleophilic aromatic substitution of hydrogen (NASH). In addition,
while isolating products, benzisoxazole ring formation was swiftly
observed via ring closure reaction by cyano and nitro group in
dicyano derivative of DNBA.[13]
Scheme 2. Synthesis of CN-2a.
The reactions for the synthesizing of the benzisoxazole
derivative were performed in DMSO or DMSO-H2O mixture using
TBACN or KCN as a nucleophile source. When the synthesis of
the benzisoxazole compound was performed by reaction of
DNBA and 3.0 equiv KCN in DMSO or DMSO/H2O for 96 h, the
obtained yield is 3% and 8%, respectively. However, when 3.0
equiv TBACN was used as a cyanide source and the reaction was
performed in DMSO, it was observed by TLC that the reaction was
completed in two hours, and after flash chromatography, CN-2a
was obtained in 35% yield. The structure of the CN-2a was fully
characterized by single-crystal X-ray diffraction analysis (Figure
1-2, and Table S1-5) and other analytical techniques such as UV-
vis and Fluorescence spectroscopy (Supporting Information,
Figures S10-19). Even though 1.0, 2.0 or 3.0 equiv of TBACN or
KCN were used in the reactions, nucleophilic addition products of
mono or di-cyano substituted could not be observed in any
isolated products. Moreover, when the reaction was performed by
DNBA and 3.0 equiv TBACN, expected product as a result of
triple nucleophilic aromatic substitution of the cyano group could
not be isolated in a significant yield (less than 1%). When 3.0
equiv KCN used instead of TBACN, this product could not be
observed. These results showed cyano group firstly bind to the
ortho position (according to carbonyl) of the 3,5-dinitrophenyl and
the second nucleophilic aromatic addition of cyanide is to the para
position of the 3,5-dinitrophenyl ring. And then, the ring closing
reaction was occurred by o-cyano and one of the nitro groups in
the dinitrophenyl ring.
For anion selectivity study, DNBA has interacted with ten
selective anions (I−, Br−, Cl−, F−, CN−, AcO−, H2PO4 , NO3 , ClO4−,
and HSO4−) in organic solvents (DMSO and ACN) and aqueous
solution (DMSO/H2O: 8/2). While F− interacted with DNBA via
partial the deprotonated process or H-bonding interaction in ACN
and DMSO, CN− interacted with DNBA via the deprotonation
process and nucleophilic aromatic substitution to the electron
deficient dinitrophenyl fragment in ACN and DMSO, and also in
−
−
aqueous
solution.
In
addition,
N-methyl-3,5-dinitro-N-
phenylbenzamide (DNBAM) was synthesized for explain
colorimetric and fluorimetric change that comes from dominantly
the cyanide addition to dinitrophenyl ring in DNBA. DNBA for the
detection of CN− displayed a lower limit of detection (LODs) than
the WHO value. Finally, proposed mechanisms were confirmed
with theoretical calculations.
Results and Discussion
Preparation of DNBA, DNBAM, tricyano derivatives of DNBA
and 3-amino-7-cyano-6-nitro-N-phenylbenzo[c]isoxazole-4-
carboxamide (CN-2a). DNBA and DNBAM were obtained in
good yields by reacting aniline and N-methylaniline with 3,5-
dinitrobenzoyl chloride, respectively (Scheme 1). The structure of
the both synthesized compounds was confirmed by FT-IR, 1H/13C-
NMR and LCMS/HRMS spectroscopic techniques as well as
elemental analysis (Supporting Information, Figures S1-9).
Scheme 1. Synthesis of DNBA and DNBAM.
To isolate mono, di- and tricyano substituted DNBA
derivatives, many reactions were done in organic solvents and a
binary solvent such as DMSO/H2O. In these reactions, while
monocyano and dicyano derivatives of DNBA could be not
isolated, tricyano derivative was obtained in trace amounts (less
than 1%) (Supporting Information, Figure S9) and it could be
characterized only by 1H NMR spectrum. However, the novel
benzisoxazole compound CN-2a was obtained by ring closing
reaction via dicyano derivatives of DNBA while isolating of the
compounds from the crude product (Scheme 2).
Figure 1. The crystal structure of CN-2a prepared via slow evaporation from a
solution of ethyl acetate/n-hexane and analysed at 273 K (40% probability
contours).
2
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
Anion interaction study in organic solvents
bands can be attributed to a different mechanism between
cyanide and DNBA. While the observed short wavelength at 399
nm may be attributed mono cyanide addition to dinitrophenyl
fragment, the longer wavelengths values at 550 and 627 nm may
attributed di and tricyano addition, respectively. To attribute
observed three absorption maxima in visible region and
colorimetric and fluorimetric changes, same titration condition
was applied between cyanide and DNBAM and almost same
results were obtained. The results show that these observed
absorption maxima and color changes can be attributed
dominantly cyanide addition to dinitrophenyl ring (Supporting
Information, Figures S20-25).
The selectivity of the various anions (I−, Br−, Cl−, F−, CN−, AcO−,
−
−
−
−
H2PO4 , NO3 , ClO4 and HSO4 with tetrabutylammonium as a
counter cation) towards the chemosensor DNBA (c = 30 µM in
DMSO) were examined. The chemosensor DNBA showed a
selective chromogenic response to CN− and F−. The UV-vis and
the fluorescence spectra were recorded in order to study the
sensing abilities of the prepared DNBA in various concentrations
and different mixture of solvents (organic and aqueous). DNBA is
a colorless compound and it has no absorption band in the studied
titration method in the range of 250-700 nm in DMSO or ACN and
in DMSO/H2O: 8/2 (v/v).
Scheme 3. Proposed deprotonation and hydrogen bonding mechanisms.
To determine the solvent effect on interaction study, DMSO
was changed with ACN. While ACN was used instead of DMSO
as a solvent, the almost same results were observed for cyanide
addition (Figure 3b). However, there are some differences for
fluoride addition in ACN. Fluoride has hygroscopic and strong
hydrogen bonding properties in protic solvents. It has a strong
base behavior especially in aprotic organic solvents that have no
acidic proton such as DMSO, THF, ACN, etc. but it exhibits a
weak base property in protic solvents such as methanol, water,
etc.[14] In our proposed mechanism, fluoride removed a proton
from ACN and occurred a carbanion from deprotonated ACN
(−:CH2CN). The fluoride or −:CH2CN firstly removed a proton from
−
amide and it was obtained deprotonated DNBA. Then, :CH2CN
attack to electron-deficient dinitrophenyl ring. Upon addition of 20
equiv of fluoride to DNBA solution (c= 30 µM) in ACN, the
obtained lowest absorption band at 310 nm can be attributed to
deprotonation of DNBA with -:CH2CN or fluoride. The wavelength
observed in longer wavelength (at 395 and 617 nm in ACN,
respectively) can be attributed to the mono or di addition of
−:CH2CN to the dinitrophenyl fragment (Scheme 4). According to
Figure 2. H-bonding geometry (up) and Stacking motif of CN-2a with the unit
cell viewed down along the a-axis (down).
With respect to the UV-vis spectrum of DNBA (c = 30 µM), as
depicted in Figure 3a, the addition of 20 equiv of F− anion led to
the appearance of a new dominant band at 313 nm and a broad
weak band at 425 nm. The titration of DNBA with CN− was studied
by adding increasing concentrations of the anion. In addition, as
can be seen in Figure 3a, upon addition cyanide to DMSO
solution of DNBA the new three absorption bands with maxima
were observed at 399, 550 and 627 nm. The absorption titration
results show that DNBA interact with fluoride via partial
deprotonation and hydrogen-bonding interaction processes
(Scheme 3, A and B). However, the observed three absorption
−
proposed mechanism, :CH2CN bind to the both ortho positions
(according to carbonyl) of the 3,5-dinitrophenyl because of steric
hindrance of para position. These values are consistent with
obtained absorption maxima values upon the addition of cyanide
to DNBA in ACN.
The colorimetric and fluorimetric naked-eye detection of
anions are very important for the applicability of the determination
of anions in a real sample. While DNBA showed the change in
coloration from colorless to purple for CN−, light yellow color was
3
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
observed upon addition of 20 equiv of the fluoride anion.
−
−
Meanwhile, the other competing anions (I−, Br−, Cl−, NO3 , ClO4 ,
−
and HSO4 ) did not show any color changes as can be seen in
Figure 3 (inset). These results indicated that DNBA can be used
as a selective colorimetric sensor for colorimetric separation of
fluoride and cyanide without any interference from other
competitive anions. In addition, detailed mechanisms between
1
cyanide/fluoride and DNBA were proved by using H NMR and
mass spectroscopic methods (see sensing mechanism part).
Scheme 4. Proposed mechanisms on acetonitrile addition to dinitrophenyl
fragment in basic fluoride solution.
Figure 3. The absorption spectrum of DNBA (c= 30 µM), (a) in DMSO (b) in
ACN after the addition of 20 equiv of all the anions. Insets: color changes after
the addition of 20 equiv of anions.
To explain the observed deprotonation, −:CH2CN and CN−
addition mechanism for both anions, a reversibility test was done.
TFA was added to a solution of DNBA-F− in ACN, in order to study
the reversibility of the chemosensor. As depicted in Figure 4a, the
addition of 100 equiv of TFA to a solution of DNBA-F− with 20
equiv of TBAF led to a removal of the absorption band at 310 nm
in the UV-vis spectra, and change in coloration of the solution
from yellow to colorless. However, the intensities of absorbance
at 395 nm and 617 nm wavelengths are still observed. These
absorption maxima are attributed to addition of −:CH2CN to
dinitrophenyl fragment and the plausible mechanism that can be
seen in Scheme 4. However, after the addition of 100 equiv of
TFA to a solution of DNBA-CN−, it was only observed that the
absorption bands of intensities at 400, 553 and 627 nm, were
decreased because of partial protonation of the anionic
intermediates such as Meisenheimer complex. Besides three
absorption maxima, new absorption maximum was obtained in a
shorter wavelength at 250 nm. It can be attributed to the
protonation of the amide group (Figure 4b). These results
supported that CN− added to the dinitrophenyl fragment via
nucleophilic aromatic substitution of hydrogen and they also
showed that the interaction of DNBA with fluoride anion can be
reversed by adding TFA in DMSO (Scheme 3).
Figure 4. Absorption spectrum of DNBA (c= 30 µM, (a) upon addition 20 equiv
of F− and 100 equiv of TFA in ACN, respectively (b) upon the addition 20 equiv
of CN− and 100 equiv of TFA in ACN, respectively. Insets: color changes after
the addition of 20 equiv of fluoride/cyanide and TFA, respectively.
4
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
DNBA is almost non-fluorescence in DMSO. However, the
emission spectrum of the interaction of DNBA with CN− anion
showed the existence of a new dominant band at 671 nm with a
shoulder at around 605 nm and also a weak band was observed
at 468 nm. As can be seen in Figure 5, only CN− responded
towards DNBA in DMSO, while the remaining anions did not show
any appreciable change in the spectral properties, even fluoride
that interacted with DNBA in DMSO via deprotonation of the
amide functional group. The color of the solution became red
under UV irradiation (λex. 365 nm) after the addition of 10 equiv of
CN− anion as shown in Figure 5 (inset). This result shows that
deprotonation does not affect observed emission. The same
titration was applied between cyanide and DNBAM and almost
same results were obtained (Supporting Information, Figures
S22-25).
Figure 6. The emission spectrum of DNBA (c= 30 µM in DMSO) after the
addition of 85 equiv of cyanide anion. Insets: fluorescence color changes after
the addition of 85 equiv of cyanide under UV lamp (λex. 365 nm).
Figure 5. The emission spectrum of DNBA (c= 30 µM in DMSO) after the
addition of 10 equiv of all the anions. Insets: fluorescence color changes after
the addition of 10 equiv of cyanide under UV lamp (λex. 365 nm).
Figure 7. The emission spectrum of DNBA (c= 30 µM in ACN) upon the addition
of 45 equiv of F− in ACN under UV lamp (λex. 365 nm).
As seen from Figure 6, upon the addition of 20 equiv of CN−
to a solution of DNBA, the new emission band at 670 nm was
increased. After excess addition (from 25 to 85 equiv) of cyanide
anion, the emission band decreased and new band in a shorter
wavelength at 468 nm appeared (red-colored lines). Interestingly,
while the same conditions applied for fluoride anion, fluorescence
changing was not observed in DMSO (Figure 5). As shown in Fig.
6-inset under UV irradiation (λex. 365 nm), while upon addition of
20 equiv of CN− the color of the solution became red, upon
addition of 85 equiv of CN− the color of the solution changed as
green. It shows that cyanide adds to the dinitrophenyl fragment
through a few steps such as mono, di and tricyano addition.
However, when ACN was used instead of DMSO, both anions
showed fluorescence enhancement. While, after cyanide
interacts with DNBA, the same emission curves were observed in
ACN, different colors were obtained (Figures 7 and 8). The
obtained data from fluorescence titration studies showed that the
increasing fluorescence can be attributed to the addition of
cyanide (for cyanide addition) and −:CH2CN (for fluoride addition)
to the dinitrophenyl fragment, respectively (Schemes 4 and 6).
Figure 8. The emission spectrum of DNBA (c= 30 µM in ACN) upon the addition
of 45 equiv of CN− in ACN under UV lamp (λex. 365 nm).
5
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
Sensing Mechanism Study of DNBA
The mechanism of nucleophilic aromatic substitution of
cyanide to the dinitrophenyl fragment and the deprotonation or
hydrogen bonding interaction with fluoride were determined by 1H
NMR titration method in DMSO-d6. At first, DNBA (c= 10 mM) was
titrated with F− (up to 2 equiv) in DMSO-d6. As seen from Figure
9 and Scheme 5, after the addition of fluoride, the phenyl protons
(H4, H5, and H6) and H1 were shifted slightly to the upper field.
These small shifts (Δδ≈0.24) and the broadband at 16.04 ppm
indicated that an amide -NH is completely deprotonated but has
a strong interaction with fluoride via hydrogen bonding interaction.
Figure 10. Partial 1H-NMR spectral change of DNBA (c= 10 mM) with titration
of TBACN (c= 1 M) in DMSO-d6.
When titration was continued to 1 equiv of cyanide, the
deprotonation process was completed as black colored signals
that belong to DNBA and -NH signal disappeared. All these
changes at signals suggest that two different mechanisms such
as deprotonation and addition began at the same time but at the
end of the titration, the major product was the addition of three
cyanide to dinitrophenyl ring that was shown as purple signals.
The mechanism of this system was decided from the
disappearance of all aromatic hydrogens belonging to that ring.
To further understand the mechanism, HRMS spectra were taken
with freshly added potassium cyanide to DNBA in ACN (c= 50
µM). After the addition of 1 equiv of CN−, two different signals
were obtained. One of them belonged to DNBA, while the other
one had a mass of 313.0578 ([M+H]+) that was consistent with the
addition of cyanide to the dinitrophenyl fragment (σH-complex 1)
(Supporting Information, Figure S26). When cyanide was
increased to 5 equiv, beside σH-complex 1 a new mass was
encountered as 338.0531 ([M+H]+) that consist of second cyanide
addition to the ring (σH-complex 2) (Supporting Information,
Figure S27). Even though different media, cyanide source (KCN
and TBACN), and concentrations were used for the titration
methods, the HR-MS (c = 50 µM, ACN) and the 1H NMR (c= 10
mM, DMSO) results are compatible with the proposed mechanism
(Schemes 5 and 6).
Figure 9. Partial 1H-NMR spectral change of DNBA (c= 10 mM) with titration of
TBAF (c= 1 M) in DMSO-d6.
After the interaction mechanism of the fluoride was determined,
the DMSO-d6 solution of DNBA (c= 10 mM) was titrated with CN−
up to 2 equiv. As seen from Figure 10, upon the addition of 0.25
equiv of CN−, the new signals began to appear. While the blue
colored ones were easily determined as deprotonation products
in which cyanide acts as a base, new signals at the lower field
(10.07 and 9.76 ppm) and upper field (5.67 and 5.41 ppm)
indicated that there was another process at the same time. The
orange and the cyan colored signals at the upper field were
defined as σH-adduct as intermediate. This result show that
cyanide may be attacked to simultaneously the ortho and para
position of dinitrophenyl ring (Schemes 5 and 6).
Scheme 5. Proposed intermediates during cyanide titration of DNBA.
6
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
Scheme 6. Proposed mechanisms on cyanide additions to dinitrophenyl
fragment.
Scheme 7. Proposed mechanism on ring closing reaction via dicyano derivative
of DNBA.
The plausible mechanism was shown in Scheme 6. Firstly,
cyanide exhibited nucleophilic property and attacked at the ortho
position of dinitrophenyl that is an electron-deficient system,
leading to the intermediate σH-complex 1, followed by the
departure of a proton through an eliminative pathway that
produced the CN-1 product.[15] The addition of cyanide causes the
reaction to continue and a second a nucleophilic attack was at the
para position of the dinitrophenyl ring (according to carbonyl),
followed by an intermediate σH-complex 2 and then the
deprotonation of hydrogen to get CN-2. The last step was the third
cyanide addition to the other ortho position of the dinitrophenyl
ring (σH- complex 3 then CN-3). The proposed mechanism by
Anion interaction study in Aqueous Medium
The applicability of the chemosensor DNBA was investigated
for the determination of cyanide in the real sample, by the titration
between the anions and the probe DNBA in aqueous solution.
The interactions of the chemosensor with CN− anion was first
studied in DMSO/H2O binary solvent mixtures varying from 9/1 to
1/9. The best response in terms of UV-vis and fluorescence
spectra was obtained in a system containing DMSO/H2O (8/2, v/v)
(Figure 11). Upon addition of 20 equiv of all studied anions to an
aqueous solution of DNBA, the chemosensor DNBA showed
selectivity only towards CN− anion besides competing anions
1
using H NMR titration methods was also supported by the UV-
vis titration method. While at the 250 nm absorption band belong
to the deprotonation of the amide proton, at 400, 553 and 627 nm
in ACN, the absorption bands can be attributed to mono, di and
tri addition of cyanide to dinitrophenyl fragment, respectively.
After the addition of cyanide, a new intermediate species (σH-
complex 1, 2 and 3) which have high conjugation in a ring can be
obtained and absorption maxima shifted bathochromically
because of the conjugation between the allylic anion and the
dien/nitro or carbonyl and highly conjugated. In addition, the
observed three absorption band may also be attributed to mono-,
di- or tri cyanodinitro phenyl (see theoretical results, Scheme 6).
−
such as F−, AcO− and H2PO4 , also DNBAM showed same
behavior towards CN- in binary solvent mixtures (Figures 11-13,
Supporting Information, Figures S28-31).
To determine the limit of detection (LOD), absorption titration
of DNBA with CN− was carried out by adding aliquots of
micromolar concentration of TBACN in DMSO and DMSO/H2O
(8:2, v/v) mixture. The LOD was calculated using the equation:
detection limit = 3σ/k, where σ is the standard deviation of blank,
and k is the slope of the plot of the absorbances versus [CN−].
The absorbance of DNBA at λmax = 404 nm was measured 5 times
in replicate, and the standard deviation of blank was determined.
The absorbance (at 404 nm) was plotted vs. [CN-] to obtain the
slope (Supporting Information, Figures S32 and 33). The value of
LOD obtained as 0.8 µM in DMSO, 1.5 µM in DMSO/ H2O (8/2,
v/v) binary solution. While the calculated LOD is lower than 2.7
µM which refers to WHO value for cyanide in aqueous solution.[16]
Moreover, detection limits obtained in the present study are
comparable with the reported values (Supporting Information,
Table S6).
To support the reaction mechanism, the proposed structures
need to isolate as pure products. To isolate of all the addition
products, a few experiments were done and attempted to get pure
products by using different purification methods (see
experimental, syntheses and characterization parts). It is
interesting that, while obtaining dicyano substituted DNBA
derivatives, benzisoxazole derivative was obtained ring closing
reaction via dicyano derivative of DNBA (Scheme 7). The product
corresponding to benzisoxazole was characterized by X-ray
crystallography (Figure 1-2, see Supporting Information).
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Application in real CN− samples in tap water
The sufficient results obtained in DMSO/H2O: 8/2 (v/v), for
DNBA, encouraged us to put into practice the use of tap water,
instead of distilled water. In this experiment, TBACN solved in tap
water (50 µM) was used instead of TBACN in DMSO and
fluorescence titration experiment was done. The results showed
a turn-on in the fluorescence. Figure 14 represents the response
of DNBA upon the addition of 20 equiv of TBACN, and an
intensive red fluorescence increment can be seen in the
photograph (Fig. 14-inset). This result proved that the detection
of CN− ions can be realized using a simple UV lamp without any
analytical devices.
Figure 11. The absorption spectra of DNBA (c= 100 µM in DMSO/H2O: 8/2, v/v)
upon addition of 20 equiv of all used anions.
Figure 12. The photograph of DNBA (c= 100 µM) in DMSO/H2O (the water
contents were increased from left to right) (v/v) with 20 equiv of TBACN under
ambient light (top) and UV lamp (λex 365 nm, bottom).
Figure 14. The emission spectra of tap water experiment (c= 50 µM in
DMSO/Tap water: 8/2, v/v) upon the addition of 20 equiv of TBACN dissolved
in tap water. Inset: photograph of DNBA (1 µM) in DMSO/Tap water (8/2, v/v)
with 20 equiv of TBACN under UV lamp (λex. 365 nm).
The usable colorimetric test strips for fast determination of CN–
could be important and useful for environmental samples. For this,
especially inexpensive test strips can be produced by filter
papers/TLC plates. In this study, the DNBA solution in ACN (1
mM) were dropped in TLC plate and dried in air. Cyanide solution
in ACN (1 mM) were added one drop on this plate and the color
change from colorless to blue was observed (Figure 15). This
experiment reflects that the DNBA-based test strips can be proper
detect CN– in solutions without any other tools.
Figure 15. The photograph of the TLC plates loaded with DNBA were detected
to sense CN– in ACN solution with naked eye.
Figure 13. (a) Absorption spectra of DNBA (c= 100 µM in DMSO/H2O: 8/2); (b)
Emission spectra of DNBA (c= 50 µM in DMSO/H2O: 8/2) after addition of 20
and 80 equiv of cyanide, respectively.
8
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
Theoretical results
To support the sensing mechanisms of DNBA with F– and CN–,
the structural and electronic properties of DNBA and its adducts
were obtained within the DFT calculations at B3LYP/6311+g(d,p)
level. The optimized geometries of DNBA and its complexes
formed via deprotonation with F– and CN– were shown in Figure
16. In the deprotonation process, F– and CN– interact with amide
-NH proton and the distance N12-H13 changes significantly as
seen in Figure 16. For DNBA and its fluoride or cyanide adducts
formed via the deprotonation process, phenyl and amide were
almost planar with the dihedral angle at about -177o. The dihedral
angle between amide and dinitrophenyl ring was at about -156o.
Upon interaction with F– or CN– via deprotonation process, there
were no any geometrical changes, but the conjugation of the
phenyl ring increases with the increase of the negative charge on
nitrogen atom. The calculated absorption spectra of the
deprotonated DNBA with F– in DMSO have a peak at 318 nm with
oscillator strength f=0.3986 and a broad peak at 494 nm with very
small f value (f=0.0178) (Table S7), which are consistent with
experimental results.
Figure 16. The optimized structures of a) DNBA and its complex formed via
deprotonation with b) F–, c) CN–.
The complexes for the addition of F– or CN– given in Scheme
4 and 6 were optimized in gas phase and the obtained geometries
were illustrated in Figure 17 and Figure S34 (in Supporting
Information). In case of addition CN–, amide and dinitrophenyl ring
became more planar than DNBA for σH-complex 1 (the dihedral
angle~173o) and for CN-1, CN-2 and CN-3 (the dihedral
angle~180o) while it was the same with DNBA for σH-complex 2
(the dihedral angle~149o), σH-complex
3
(the dihedral
angle~156o). Similar geometrical changes were also observed for
the addition −:CH2CN adducts (Supporting Information, Figure
S34). However, conjugations between the allylic anion and
diene/nitro or carbonyl in σH-complexes and also conjugations in
the aromatic cyano-substituted dinitrophenyl ring in adducts (CN-
1, CN-2, CN-3 or ACN-1, ACN-2) are expected to cause
bathochromic shifts in the absorption spectra.
9
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
application of DNBA as a portable and fast strip for real-time
naked-eye detection of CN−. As a result, DNBA can be used to
detect CN− for environmental real water sample analysis with
satisfactory results.
Experimental Section
Materials and reagents. The TBA salts of anions and all other
reagents were purchased from Sigma-Aldrich. The solvents of
analytical grade were purchased from Avra Chemicals and used
without further purification.
General Information. All commercially available chemicals were
reagent grade and used without further purification. Thin-layer
chromatography (TLC) was used for monitoring the reactions
using pre-coated silica gel 60 F254 plates. FT-IR spectra were
recorded on a Mattson 1000 FT-IR spectrophotometer (Gazi
University Department of Chemistry, Turkey) in KBr (ν, in cm-1).
NMR spectra were measured on a Bruker Avance III 400 MHz
(1H: 400 MHz, 13C: 100 MHz) NaNoBay FT-NMR spectrometer at
the Mersin University Advanced Technology Education, Research
and Application Center (MEITAM), VARIAN (Agilent) MERCURY
400 MHz (Varian, Palo Alto, CA, USA) at a proton resonance
frequency of 400 and 100 MHz for carbon at Ankara University
Faculty of Pharmacy, and Bruker Avance 300 (1H: 300 MHz,
13C:75 MHz) spectrometers at 20 °C (293 K). Chemical shifts (δ)
are given in parts per million (ppm) using the residue solvent
peaks as reference relative to TMS. Coupling constants (J) are
given in hertz (Hz). Signals are abbreviated as follows: broad. br;
singlet. s; doublet. d; doublet-doublet. dd; doublet-triplet dt; triplet.
t; multiplet. Elemental analysis was performed using a Thermo
Scientific Flash 2000 at the Gazi University Department of
Chemistry. The X-ray crystal structure data were collected at the
Mersin University, MEITAM by a Bruker APEX-II charge-coupled
device (CCD) diffractometer. High-resolution mass spectra
(HRMS) were recorded at Gazi University Faculty of Pharmacy
using electron ionization (EI) mass spectrometry (Waters-LCT-
Premier-XE-LTOF (TOF-MS) instruments; in m/z (rel. %). The
melting points were measured using the Electrothermal IA9200
apparatus. Absorption spectra were recorded on a Shimadzu
1800 spectrophotometer; fluorescence spectra were recorded on
a Hitachi F-7000 fluorescence spectrophotometer.
Figure 17. The optimized structures of the adducts formed via the addition of
CN– to DNBA.
In case of the addition of CN– to the dinitrophenyl fragment, the
peaks are observed at 401 nm and 478 nm for mono addition (σH-
complex 1), at 425 nm and 525 nm for di addition σH-complex
2), at 494 nm for tri addition σH-complex 3). After the
deprotonation of hydrogen in each σH -complexes, the absorption
spectra obtained for CN-1, CN-2 and CN-3 have a main peak at
396 nm, 492 nm and 597 nm, respectively (Table S8). In addition,
the obtained absorption maxima of the complexes of the –:CH2CN
addition are consistent with the experimental results (Table S7).
The comparison of the calculated and experimental data resulted
that the peaks observed in the experimental spectrum can be
attributed to the coexistence of different species.
Conclusion
In summary, DNBA was synthesized for the selective
colorimetric and fluorimetric determination of cyanide anion in
organic and aqueous solution. The proposed mechanism in the
previously published study was improved and explained by
spectroscopic titration analyses. The chemosensor DNBA
showed a chromogenic and a fluorogenic selective response to
Theoretical Methods
The geometries of the structures were optimized using Gaussian
09 package program[17] with Density Functional Theory (DFT)
calculations at B3LYP/6311+g(d,p) level.[18] Frequency
calculations performed on the optimized geometries confirm that
the geometries correspond to local minima in a potential surface
by the absence of imaginary frequency. The absorption spectrum
was calculated using TD-DFT calculations at the same level. The
solvent effect was considered using the integral equation
formalism of the Polarizable Continuum Model (PCM).[19]
−
CN− against competing anions such as F−, AcO−, H2PO4 in
DMSO and ACN. Three proposed sensing mechanisms named
as deprotonation, nucleophilic aromatic substitution, and
formation of benzisoxazole ring were confirmed by the UV-vis,
fluorescence and 1H NMR and mass spectroscopy techniques as
well as X-ray crystallography. In addition, the experimental results
were explained by the theoretical studies. DNBA showed that the
detection of cyanide with LODs of 0.8 µM in DMSO, 1.5 µM in
DMSO/H2O (8:2, v/v) mixture, which was lower than those in
WHO value for cyanide in aqueous solution. In addition, prepared
colorimetric test strips with DNBA indicated
a promising
10
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10.1002/ejoc.202000342
European Journal of Organic Chemistry
FULL PAPER
X-ray diffraction analysis
dropped. The temperature of the reaction mixture was raised to
room temperature and continued to stir overnight. After that, water
(20 mL) was added. The residue was filtered and crystallized with
petroleum ether. Yield 70%, m.p. 160 °C; 1H NMR (DMSO-d6, 400
MHz): δ 8.69 (t, J = 2.2 Hz, 1H), 8.43 (s, 2H), 7.33 (d, J = 7.5 Hz,
2H), 7.29 (t, J = 7.7 Hz, 2H), 7.19 (t, J = 7.1 Hz, 1H), 3.42 (s, 3H).
13C NMR (DMSO-d6, 100 MHz): δ 165.5, 147.8, 143.7, 139.7,
129.9, 128.9, 127.8, 127.7, 119.6. HRMS (TOF ES+) (m/z),
[M+H]+ calcd for C14H11N3O5, 302.0777; found, 302.0769.
For the crystal structure determination, a single-crystal of the
compound X was used for the data collection on a four-circle
Rigaku R-AXIS RAPID-S diffractometer (equipped with a two-
dimensional area IP detector). Graphite-monochromated Mo-Kα
radiation (λ= 0.71073 Å) and oscillation scans technique with ∆w
= 5º for one image were used for data collection. The lattice
parameters were determined by the least-squares methods
based on all reflections with F2 > 2σ (F2). Integration of the
intensities, correction for Lorentz and polarization effects and cell
refinement were performed using CrystalClear (Rigaku/MSC Inc.,
2005) software.[20] The structures were solved by direct methods
using SHELXS-97[21] which allowed for the location of most of the
heaviest atoms, with the remaining non-hydrogen atoms being
located from different Fourier maps calculated from successive
full-matrix least-squares refinement cycles on F2 using SHELXL-
97.[21] All non-hydrogen atoms were refined using anisotropic
displacement parameters. Hydrogens attached to carbons were
located at their geometric positions using appropriate HFIX
instructions in SHELXL. The final difference Fourier maps showed
no peaks of chemical significance. Crystal data for CN-2a:
C15H9N5O4, crystal system, space group: monoclinic, P21/c;
(no:14); unit cell dimensions: a = 9.959(4), b = 7.148(3), c =
19.758(5) Å, α= 90, β = 92.28(2), γ = 90o; volume; 1405.4(9) Å3,
Z=4; calculated density: 1.528 g/cm3; absorption coefficient:
0.116 mm-1; F(000): 664; θ-range for data collection 2.9-28.3o;
refinement method: full-matrix least-squares on F2; da-
ta/parameters: 3068/218; goodness-of-fit on F2: 0.948; final R-
indices [I > 2 σ (I)]: R1 = 0.076, wR2 = 0.183; largest diff. peak
Synthesis
of
3-amino-7-cyano-6-nitro-N-
phenylbenzo[c]isoxazole-4-carboxamide (CN-2a)
Path 1: To a stirred solution of the DNBA (1 mmol) in DMSO (15
mL) was added a solution of tetrabutylammonium cyanide (3.0
mmol) in (5 mL) DMSO at room temperature and the reaction
mixture was stirred for 2 h. After completion of the reaction, 50 mL
of deionized water was added and the mixture was extracted
three times with ethyl acetate. The crude mixture was purified by
column chromatography (ethyl acetate: hexane / 1:4) and the pure
product was obtained as dark crystals in 35% yield. Furthermore,
2,4,6-tricyano-3,5-dinitro-N-phenylbenzamide
(CN-3)
was
obtained with a trace amount (yield less than 1%) after
chromatographic purification.
Path 2: To a stirred solution of the DNBA (1.0 mmol) in DMSO
(16 mL) was added a solution of potassium cyanide (3.0 mmol) in
H2O (4 mL) at room temperature and the reaction mixture was
stirred for 96 h, but the reaction was not complete. Then, 50 mL
of deionized water was added and the mixture was extracted
three times with ethyl acetate. The crude mixture was purified by
column chromatography (ethyl acetate:hexane/1:4) and the pure
product was obtained as dark crystals in 8% yield.
m.p. 219 °C; FT-IR (cm-1): 3386, 3273, 3225, 2233, 1981, 1659,
1616, 1595. 1H NMR (DMSO-d6, 400 MHz): δ 10.99 (s, 1H), 9.02
(s, 2H), 7.90 (s, 1H), 7.77-7.75 (m, 2H), 7.44-7.40 (m, 2H), 7.23-
7.19 (m, 1H); 13C (DMSO-d6, 100 MHz): δ 169.1, 162.7, 156.3,
153.0, 137.63, 137.58, 128.7 (2 x C), 125.1, 121.6 (2 x C), 112.2,
111.2, 99.5, 92.1. HRMS (TOF ES+) (m/z), [M+H]+ calcd for
C15H10N5O4, 324.0733; found, 324.0727.
and hole: 0.199 and -0.210 e Å-3.
Deposition CCDC Number 1980003 contains the supplementary
crystallographic data for this paper. These data are provided free
of charge via the joint CCDC/FIZ Karlsruhe deposition service
www.ccdc.cam.ac.uk/structures.
Characterization data and synthetic procedures. Synthesis of
3,5-dinitro-(N-phenyl)benzamide (DNBA); A solution containing
aniline (1 mmol) and K2CO3 (1 mmol) in THF (5 mL) was cooled
to 0 °C in an iced bath. To this solution, at 0 °C, 3,5-dinitrobenzoyl
chloride (1 mmol) was slowly dropped. The temperature of the
reaction mixture was raised to room temperature and continued
to stir overnight. After that, water (20 mL) was added. The residue
was filtered and crystallized with petroleum ether. Yield 75%, m.p.
233 °C, lit. m.p. 238.8[12]; FT-IR (cm-1): 3280, 3103, 1652, 1630,
1599. 1H NMR (DMSO-d6, 300 MHz): δ 10.86 (s, 1H), 9.18 (d, J =
2.1 Hz, 2H), 9.01 (t, J = 2.1 Hz, 1H), 7.85 – 7.74 (m, 2H), 7.42 (dd,
J = 8.5, 7.4 Hz, 2H), 7.20 (d, J = 7.4 Hz, 1H); 13C NMR (DMSO-
d6, 75 MHz): δ 160.8, 147.7, 137.8, 137.0, 128.4, 127.6, 124.1,
120.7, 120.2. HRMS (TOF ES+) (m/z), [M+H]+ calcd for C13H9N3O5,
288.0576; found, 288.0593 and [M+H+CH3CN]+ calcd for
C13H9N3O5, 329.0808; found, 329.0845. Elemental analysis: anal.
calc. for C13H9N3O5; C, 54.36; H, 3.16; N, 14.63; found: C, 54.48;
H, 3.26; N, 14.24.
1H NMR data for (CN-3): 1H NMR (DMSO-d6, 400 MHz): δ 7.76 -
7.63 (m, 5H).
Acknowledgements
This research was supported by Gazi University. The numerical
calculations reported in this paper were fully performed at
TUBITAK ULAKBIM, High Performance and Grid Computing
Center (TRUBA resources).
Keywords:
Chemosensor
•
Colorimetric/Fluorimetric
Nucleophilic
chemosensor
•
Selective cyanide detection •
aromatic substitution of hydrogen (NASH) • SNH • Benzisoxazole
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