Synthesis and studies of selective chemosensors for anions and cations by azo-containing salicylaldimine-based receptors

Article abstract of DOI:10.1080/00958972.2012.752818

Three new chromogenic receptors containing electron-withdrawing groups appended to the azophenol moiety were synthesized, characterized, and their chromogenic behavior toward various anions and cations investigated. These chemosensors show visual changes towards biologically relevant anions like F^- and OH^-, and also towards various cations such as Cu2⁺, Co2⁺, Ni2⁺, Mn2⁺, and Zn2 ⁺. Yet, other anions such as Cl^-, Br^-, and NO3^- could not cause any color change. The sensing action was further confirmed by UV-Vis titration. 1H-NMR experiments were carried out to explore the nature of interaction between receptors with F^- and OH ^-. The binding constants (Ka) of the host-guest complexes formed were determined.

Full text of DOI:10.1080/00958972.2012.752818

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Synthesis and studies of selective
chemosensors for anions and cations by
azo-containing salicylaldimine-based
receptors
Raziyeh Arabahmadi a & Saeid Amani a
a Faculty of Sciences, Department of Chemistry, Arak University,
Arak, Iran
Accepted author version posted online: 27 Nov 2012.Version of
record first published: 14 Jan 2013.
To cite this article: Raziyeh Arabahmadi & Saeid Amani (2013): Synthesis and studies of selective
chemosensors for anions and cations by azo-containing salicylaldimine-based receptors, Journal of
Coordination Chemistry, 66:2, 218-226
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Journal of Coordination Chemistry
Vol. 66, No. 2, 20 January 2013, 218–226
Synthesis and studies of selective chemosensors for anions
and cations by azo-containing salicylaldimine-based receptors
RAZIYEH ARABAHMADI and SAEID AMANI*
Faculty of Sciences, Department of Chemistry, Arak University, Arak, Iran
(Received 8 July 2012; in final form 11 October 2012)
Three new chromogenic receptors containing electron-withdrawing groups appended to the azophe-
nol moiety were synthesized, characterized, and their chromogenic behavior toward various anions
and cations investigated. These chemosensors show visual changes towards biologically relevant
anions like Fꢀ and OHꢀ, and also towards various cations such as Cu2+, Co2+, Ni2+, Mn2+, and
Zn2+. Yet, other anions such as Clꢀ, Brꢀ, and NO3 could not cause any color change. The sens-
ꢀ
ing action was further confirmed by UV–Vis titration. 1H-NMR experiments were carried out to
explore the nature of interaction between receptors with Fꢀ and OHꢀ. The binding constants (Ka)
of the host–guest complexes formed were determined.
Keywords: Chemosensor; Transition metal azo complexes; Receptors
1. Introduction
Considerable attention has been focused on recognition and sensing of anions and cations
since these play important roles in biology, industry, and catalysis [1]. Generally, pyrrole,
urea/thiourea, amine, or phenol act as binding sites for anions [2–4] and cations [5–8]. In
particular, sensing of fluoride has attracted growing attention due to unique properties com-
pared to its congeners as a result of high electronegativity, smaller size, and high basicity.
Among anions, fluoride is of particular interest owing to its established role in treatment
of osteoporosis and analysis of drinking water [9, 10]. Currently available methods for
detection of fluoride include spectrophotometry, electrochemistry, and ion chromatography
[11–13]. From the pharmacological and environmental importance of fluoride, there is need
to develop selective and sensitive methods for fluoride detection in the environment that is
not easily served by conventional ion selective electrodes. In this context, colorimetric
chemosensors are of particular interest due to their simplicity. Colorimetric chemosensors
allow “naked-eye” detection of anions without need to resort to spectroscopic instrumenta-
tion [14]. Most colorimetric chemosensors only respond to anions or cations separately
[15–18]. Development of chemosensors for detection of transition metal cations is active
due to potential applications as diagnostic tools in medical, physiological, and environmen-
tal applications [19]; transition metal ions are present as essential elements in biological
systems [20].
*Corresponding author. Email: s-amani@araku.ac.ir
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online Ó 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.752818

Synthesis and studies of selective chemosensors
219
We report for the first time the colorimetric studies of cation and anion sensing by chro-
mogenic receptors 2a–2c possessing a phenolic OH, which binds Fꢀ and OHꢀ via H–bond
interaction or deprotonation, with an electron-withdrawing nitro which acts as a chromo-
genic signaling unit. The nature of these Schiff bases was altered by the position of nitro
in the receptors, which was able to tune anion recognition selectivity. These receptors are
selective sensors for Cu2+, Co2+, Ni2+, Mn2+, and Zn2+ in DMSO (Scheme 1).
CHO
2- (2-amino ethoxy) ethanol
NaNO2, HCl
NH2
N=N
R3
OH
R3
Reflux, Ethanol
OH
CHO
R2
R1
R2
R1
R1 , R2=H R3=NO2
R1 , R2=H R3=NO2
1a
1b
R1=Cl , R2=H R3=NO2
R1=H , R2=NO2 , R3=Cl
R1=Cl , R2=H R3=NO2
R1=H , R2=NO2 , R3=Cl 1c
H
C
N
O
OH
R3
N=N
OH
R2
R1
R1 , R2=H R3=NO2
2a
2b
R1=Cl , R2=H R3=NO2
R1=H , R2=NO2 , R3=Cl 2c
Scheme 1. Syntheses of receptors (2a–2c).
2. Experimental
2.1. Physical measurements
C, H, and N analyses were undertaken using an elemental analysis system Gmb H Vario
EL III. Electronic spectra of receptors in DMSO were recorded on a Perkin-Elmer Lambda
15 spectrophotometer. FT-IR spectra of the compounds as KBr disks were obtained from
4000 to 300 cmꢀ1 with a Galaxy series FT-IR 5000 spectrophotometer. Melting points of
1
all newly prepared compounds were determined on an Electrothermal 9200 apparatus. H-
NMR spectroscopy was performed using a Bruker 300 MHz spectrometer.
2.2. Materials
All anions as tetrabutylammonium salts were purchased from Fluka and used without
purification. All solvents were of reagent grade quality, purchased from Merck Chemical
Company and used as received. The Cu2+, Co2+, Ni2+, Mn2+, and Zn2+ were added as
chloride solutions in DMSO.

220
R. Arabahmadi and S. Amani
2.3. Synthesis
Azo dyes (1a–1c) were synthesized according to the well-known literature procedure [21].
2.3.1. Synthesis of the Schiff bases. A mixture of 0.02 M of 2-(2-aminoethoxy) ethanol
and 0.02 M of the related azo dye (1a–1c) was dissolved in 50 mL absolute ethanol with
a few drops of glacial acetic acid as a catalyst. The resulting mixture was allowed to stir
magnetically under reflux for 1–2 h. The product was filtered under vacuum and washed
with a small amount of heated ethanol.
Synthesis of 1-{3-[(1-ethoxy-2-hydroxyethyl-imino)methyl]-4-hydroxyphenylazo}-4-nitro
benzene (2a). Brownish red, yield: 63%, m.p.: 174 °C. Anal. Calcd for C17H18N4O5·0.5-
CH3OH: C, 56.17; H, 5.34; N, 14.96. Found: C, 56.70; H, 6.17; N, 15.22%. FT-IR (KBr,
cmꢀ1): 3110–3650 (–OH group), 3105 (C–H, aromatic), 2874 (C–H, aliphatic), 1651
(C = N), 1614 (C = C, aromatic), 1421 (N = N), 1271 (C–O, phenolic) cmꢀ1. UV–Vis
(DMSO, nm): λmax = 260 (14,150), 407 (16,125), 449 (16,775) nm. 1H-NMR (CDCl3):
δ = 10–11 (1H, s), 8.46 (1H, s), 8.39 (2H, d, J = 9.0 Hz), 8.03 (2H, dd, J = 9 Hz), 7.99 (2H,
d, J = 7 Hz), 7.97 (1H, d, J = 2.4 Hz) 7.0 (1H, d, J = 9.03 Hz), 3.87 (4H, m, J = 3.6 Hz),
3.77 (2H, t, J = 3.3 Hz), 3.64 (2H, t, J = 4.02 Hz) ppm.
Synthesis of 1-{3-[(1-ethoxy-2-hydroxethyl-imino)methyl]-4-hydroxyphenylazo}-2-chloro-
4-nitrobenzene (2b). Dark brown, yield: 46%, m.p.:185 °C. Anal. Calcd for
C17H17N4O5Cl·H2O: C, 49.71; H, 4.62; N, 13.63. Found: C, 50.40; H, 4.60; N, 13.87%.
FT-IR (KBr, cmꢀ1): 3120–3700 (–OH group), 3115 (C–H, aromatic), 2889 (C–H,
aliphatic), 1651 (C = N), 1614 (C = C, aromatic), 1408 (N = N), 1269 (C–O, phenolic)
cmꢀ1. UV–Vis (DMSO, nm): λmax = 260 (13,525), 407 (14,575), 469 (17,375) nm. 1H-
NMR (CDCl3): δ = 10.2 (1H, s), 8.44 (1H, s), 8.42 (1H, d, J = 2.2 Hz), 8.1 (1H, dd, J = 8.7,
2.2 Hz), 8.0 (1H, dd, J = 9.0 Hz), 7.9 (1H, d, J = 2.0 Hz), 7.7 (1H, d, J = 8.8 Hz), 7.0 (1H,
d, J = 9.0 Hz), 3.86 (4H, m, J = 4.8 Hz), 3.78 (2H, t, J = 4.5 Hz), 3.65 (2H, t, J = 4.2 Hz)
ppm.
Synthesis of 1-{3-[(1-ethoxy-2-hydroxyethyl-imino)methyl]-4-hydroxyphenylazo}-4-chloro-3-
nitrobenzene (2c). Light brown, yield: 32%, m.p.: 223 °C. Anal. Calcd for C17H17N4O5Cl·-
CH3OH: C, 50.90; H, 4.94; N, 13.18. Found: C, 50.40; H, 3.69; N, 13.89%. FT-IR (KBr,
cmꢀ1): 3300–3700 (–OH group), 3200 (C–H, aromatic), 2918 (C–H, aliphatic), 1647
(C = N), 1616 (C = C, aromatic), 1406(N = N), 1288 (C–O, phenolic) cmꢀ1. UV–Vis
(DMSO, nm): λmax = 262 (17,600), 387 (19,575), 430 (18,875) nm. 1H-NMR (CDCl3):
δ = 10 (1H, s),8.47 (1H, s), 8.36 (1H, d, J = 2.2 Hz), 8.06 (1H, dd, J = 17.4, 2.2 Hz), 8.02
(1H, d, J = 2.3 Hz), 7.9 (1H, d, J = 2.3 Hz), 7.7 (1H, d, J = 3.6 Hz), 7.0 (1H, d, J = 9 Hz),
3.8 (4H, m, J = 3.3 Hz), 3.7 (2H, t, J = 4.5 Hz), 3.6 (2H, t, 3.9 Hz) ppm.
3. Results and discussion
3.1. Anion sensing
Recognition properties of 2a–2c towards different anions were studied by methods such as
1
naked-eye examination, the H-NMR titration, and UV–visible titration.

Synthesis and studies of selective chemosensors
221
3.1.1. Colorimetric investigations. In the naked-eye experiments, receptors 2a–2c
showed dramatic color changes in the presence of two equivalents of tetrabutylammonium
fluoride (5 ꢁ 10ꢀ4 M in DMSO). Receptor 2a displays noticeable color change from light
pale orange to violet and 2b and 2c gave dramatic color changes from yellow to blue and
orange, respectively. In addition, 2a–2c could sense OHꢀ (see Supplementary Material,
figure S The receptors 2a–2c did not show color change for Clꢀ, Brꢀ, and NO3ꢀ. The
receptors bound anions through hydrogen bonds to form stable complexes and the electron
density in the supramolecular system was increased to enhance the charge-transfer interac-
tions between the electron-rich and electron-deficient moieties, resulting in a visible color
change [22]. Restoration of the original spectrum of the sensor system from this receptor
Fꢀ adduct upon addition of a trace amount of water/methanol suggests that complexations
between Fꢀ and OHꢀ with 2a–2c are reversible in nature [23].
Electronic spectra of the receptors showed three transitions in DMSO. The first band at
260 nm was assigned to excitation of the π-electrons of the aromatic system. The second
band observed at 387 and 407 nm could be due to the transition between the π-orbital
localized on the central bond of the azomethine group (CH = N). The third band located at
449, 469, and 430 nm is due to an intramolecular charge transfer (CT) transition within the
whole molecule. This important band in salicylaldimine compounds indicates the presence
of a strong intramolecular hydrogen bond between hydroxyl and azomethine nitrogen that
causes planarity and facilitates CT bands which are more sensitive to solvent changes than
bands resulting from local transitions [24, 25].
3.1.2. 1H-NMR spectral studies. We investigated the interaction of receptors with
1
1
anions through H-NMR studies. In H-NMR spectra of the receptors, singlet resonances
in the 10–10.3 ppm region can be attributed to protons of phenol. All three receptors exhi-
1
bit singlets at 8.36–8.46 ppm, which were attributed to imine protons. H-NMR spectra
were recorded by treating the receptor with Fꢀ and OHꢀ. The peak at 10.3 ppm corre-
sponding to proton of phenolic OH disappeared on addition of one equivalent of Fꢀ or
OHꢀ. This demonstrates the interaction of anion through the phenolic OH is more effec-
tive than interaction through the alcoholic OH groups present in the receptor. Deprotona-
tionꢀof the receptor can occur and we observed such deprotonation with formation of
1
HF2 in H-NMR spectra of 2b as a new signal at 16 ppm [figure 1(a)]. A similar case
can be observed for addition of OHꢀ as shown in figure 1(b) [26]. The interesting part of
the NMR studies is in figure 1(B) trace C. There is a large displacement in the signals.
1
Further H-NMR studies are in progress to understand this behavior.
3.1.3. UV–Vis spectroscopic studies. The sensing behavior of 2a–2c towards Fꢀ,
Clꢀ, Brꢀ, NO3ꢀ, and OHꢀ was also investigated by UV–Vis spectroscopy. UV–Vis
titration was carried out by incremental addition of tetrabutylammonium salts to 2a–2c
and the spectra are shown in figure 2. When increasing the concentration of Fꢀ in 2a–
2c, a new red shifted absorption at 565, 583, and 503 nm was gradually enhanced,
while the intensity of absorptions at 449, 469, and 387 nm decreased correspondingly.
Clear isosbestic points were observed at 492, 524, and 403 nm for 2a–2c, respectively.
The appearance of a single isosbestic point indicates the presence of only two species,
neutral host and its anion in solution. Figure 2(d) shows the gradual decrease and

222
R. Arabahmadi and S. Amani
Figure 1. (a) Partial 1H-NMR (300 MHz) spectra of 2b in CDCl3, (A) in the absence, (B) presence of 1.0
equiv., and (C) presence of 3.0 equiv. of TBAF. (b) Partial 1H-NMR (300 MHz) spectra of 2b in CDCl3, (A) in
the absence, (B) presence of 1.0 equiv., and (C) presence of 3.0 equiv. of TBAOH.
increase in the concentration of these two species with increase in equivalents of tetra-
butylammonium fluoride, respectively. Similar changes could be observed during the
addition of OHꢀ (see Supplementary Material, figure S2). The negative charge brought
about by anion-induced deprotonation increases the dipole moment and stabilizes the
excited state, causing a red shift of 2a–2c. These results demonstrate that complex-for-
mation of 2a–2e with Fꢀ and OHꢀ takes place via hydrogen bondingꢀelectrostatic
interactions (table 1). No change was observed with Clꢀ, Brꢀ, and NO3 in DMSO
[figure 2(e)] (see Supplementary Material, figures S3–S5).
The chromogenic diazo unit is introduced at the 5-position of salicylaldehyde to achieve
direct resonance between them [27–33]. As shown in figure 3, the electron-withdrawing
nitro group at the end increases not only the degree of π-conjugation but also the hydrogen
bond donor ability of the phenolic OH. The strong hydrogen bonding to or deprotonation/
protonation of the phenolic moiety might modulate the electronic properties of the chromo-
phore and give rise to large color changes [25].

Synthesis and studies of selective chemosensors
223
Figure 2. (a–c) UV–Vis spectra of 2a–2c in the course of titration of DMSO solutions of 2a–2c (4 ꢁ 10ꢀ5 M)
with a standard solution of TBAF (c ≈ 5 ꢁ 10ꢀ4 M) at 25 °C; inset shows the visual color change on addition of
Fꢀ. (d) Plot of 2a absorption vs. concentration of Fꢀ showing decrease and increase in absorption of the 449 nm
and 565 nm bands, respectively. (e) UV–Vis absorption spectra of 2a upon addition of a particular anion salt.
Table 1. Data obtained from the UV–Vis spectra upon titration of 2a–2c with n-Bu4N+Fꢀ in DMSO.
Receptor, λmax
Complex, λmax
Bathochromic shift,
Isosbestic point
(nm)
Detection
limit (M)
(nm)
(nm)
Δλmax (nm)
Ka (Mꢀ1
)
2a
2b
2c
449
469
387
565
583
503
114
117
98
492
524
403
8.616 ꢁ 103
1.262 ꢁ 103
1.64 ꢁ 10ꢀ6
2.19 ꢁ 10ꢀ6
2.55 ꢁ 10ꢀ6
3.7 ꢁ 103

224
R. Arabahmadi and S. Amani
3.2. Calculation of binding constant
3.2.1. UV–Vis titration. The binding constants of 2a–2c were calculated from the fluo-
ride-induced absorption changes using equation (1) [34],
b
1
1
1
¼
ꢁ
þ
ð1Þ
DA StKaDe ½Lꢂ StDe
where ΔA is the change in absorbance, b is path length, St is total concentration of sub-
strate, Ka is association constant, and Δɛ is the change in molar absorption coefficient. A
plot can be made of 1/ΔA as a function of 1/[L]. The binding constant can be obtained by
the ratio of intercept and slope (see Supplementary Material, figures S6 and S7). The
detection limits of the sensors toward Fꢀ and OHꢀ were obtained from UV–Vis titration.
The result can also be calculated from the fluoride quantities added, which made the initial
changes in absorbance of the sensors during UV–Vis titration [35]. The binding constants
and detection limits are summarized in table 1.
3.3. Cation sensing
We investigated the recognition ability of 2a–2c (2 ꢁ 10ꢀ5 M in DMSO) by naked-eye col-
orimetric experiments for Cu2+ (2.5 ꢁ 10ꢀ3 M), Co2+ (8 ꢁ 10ꢀ3 M), Ni2+ (3 ꢁ 10ꢀ2 M),
Mn2+ (6 ꢁ 10ꢀ2 M), and Zn2+ (6 ꢁ 10ꢀ2 M) (for color change of 2a–2c, see Supplemen-
tary Material, figures S8–S10).
The sensing behavior of 2a–2c towards Cu2+, Co2+, Ni2+, Mn2+, and Zn2+ was moni-
tored by the spectrophotometric method in DMSO. A solution of 2a–2c was titrated with
increasing volume of concentrated solution of a given cation. On incremental addition of
Cu2+ to 2a, the peak at 260 nm gradually increased with a red shift and a new band arose
at 885 nm. On further addition of Co2+ to 2a a new band was formed at 623, 685 nm. On
gradual addition of Ni2+ to 2a, a new absorption band at 805 nm appeared. On successive
addition of Mn2+ and Zn2+, the spectrum of 2a shifted from 460 to 470 and 447 nm. Simi-
lar changes were observed for 2b and 2c. The insert plot in the UV–Vis spectrum shows
the variation of absorbance at a particular wavelength vs. equivalents of the corresponding
ion added (for titration spectrum, see Supplementary Material, figures S11–S13).
N
N
OH
O
O
O
HC
H
HC
O
O
HA
H
A-
N
N
N
N
O
N +
-
O
+
N
-
O
-
O
Figure 3. Resonance structures of phenolic OH and chromogenic diazo scaffold.

Synthesis and studies of selective chemosensors
3.4. Sensing in the presence of competing ions
225
Since 2a–2c sense both transition metal ions and Fꢀ and OHꢀ, we examined the sensing
ability in the presence of competing ions. These experiments were carried out in two dif-
ferent manners. First, the receptors were treated with two equivalents of anions and to
this, successive addition of cations (20–200 μl) was done; in another set of experiments,
the receptors were first treated with two equivalents of cations and to that 20–200 μl of
anion was added incrementally. For this dual-sensing experiment, Cu2+ and Fꢀ were cho-
sen. The spectral changes are shown in figures S14–S16 (see Supplementary Material).
When 2a was treated with two equivalents of Cu2+, a new band was observed at
805 nm which might be due to the d–d band of 2a·Cu2+ complex. Addition of Fꢀ (0.5–
2 equiv.) ions to the above (2a·Cu2+) solution leads to reduction in intensity of the band
at 805 nm, possibly due to formation of a Cu:F ion pair ion in solution. In the second
experiment, the titration was carried out in the presence of two equivalents of Fꢀ to
which Cu2+ was added incrementally. When 2a was added to two equivalents of Fꢀ, the
intensity of the band at 449 nm decreased and a new band at 565 nm indicated formation
of 2a·Fꢀ complex. Upon successive addition of Cu2+ the band at 565 nm gradually
decreased, suggesting sequestering of the Fꢀ to form an ion pair with Cu2+ in solution.
Intensity of the d–d band increased slightly, possibly due to interaction between 2a and
Cu2+ ions.
In another experiment, 2a was titrated with a mixture of Fꢀ and Cu2+ (using 15, 30, 45
and 60 equiv.). When the mixture of Fꢀ with Cu2+ was introduced to the DMSO solution
of 2a, the intensity of the band at 260 nm increased and enhancement of the d–d band
occurred, due to formation of a Cu2+ complex with 2a. The formation of the band at
565 nm was not observed in this case because Fꢀ in solution engaged the metal ions in
ion-pair formation rather than in complexation with these receptors. Similar changes were
observed for 2b and 2c (see Supplementary Material, figures S17–S22). We repeated simi-
lar experiments with Fꢀ and Co2+ showing the receptors 2a–2c can be utilized for the
detection of Co2+ in the presence of competing Fꢀ (mixture of Fꢀ and Co2+) in DMSO by
change in absorption spectra.
4. Conclusion
Receptors 2a–2c are prepared via diazo coupling and Schiff base condensation, and pos-
sess the ability to sense both anions and cations colorimetrically. These Schiff base recep-
tors form metal complexes and sense anions through hydrogen bonding interaction through
the phenolic OH. In 2a, the presence of the electron-withdrawing NO2 enhances hydrogen
bonding and exhibits significant difference in anion binding by providing colorimetric
spectral responses. These receptors sense Cu2+ selectively in the presence of Fꢀ, confirmed
by the absorbance spectra of the competing studies.
Acknowledgement
The authors would like to thank the Research Council of Arak University for financial
support of this research.

226
R. Arabahmadi and S. Amani
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