A new class of efficient 4-[(nitro substituted-phenyl)-hydrazonomethyl]-1-phenyl-1H-pyrazole-3-carboxylate derived colorimetric chemosensor for selective sensing of fluoride and other biologically important anions

Article abstract of DOI:10.1007/s12039-016-1142-9

A new class of efficient colorimetric chemosensors derived from 4-[(nitro substituted-phenyl)-hydrazonomethyl]-1-phenyl-1H-pyrazole-3-carboxylate have been synthesized and characterized. The synthesized receptors exhibit instant color change from yellow t

Full text of DOI:10.1007/s12039-016-1142-9

c
J. Chem. Sci. Vol. 128, No. 9, September 2016, pp. 1451–1457. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1142-9
A new class of efficient 4-[(nitro substituted-phenyl)-hydrazonomethyl]-
1-phenyl-1H-pyrazole-3-carboxylate derived colorimetric chemosensor
for selective sensing of fluoride and other biologically important anions
SUMAN SWAMI, ARUNAVA AGARWALA, BABITA MALIK and RAHUL SHRIVASTAVA∗
Department of Chemistry, Manipal University Jaipur, VPO-Dehmi Kalan, Teshil-Sanganer, Off Jaipur Ajmer
Expressway, Jaipur, Rajasthan, 303 007, India
e-mail: chem.rahul@gmail.com
MS received 20 April 2016; revised 11 July 2016; accepted 11 July 2016
Abstract. A new class of efficient colorimetric chemosensors derived from 4-[(nitro substituted-phenyl)-
hydrazonomethyl]-1-phenyl-1H-pyrazole-3-carboxylate have been synthesized and characterized. The synthe-
sized receptors exhibit instant color change from yellow to dark purple along with significant bathochromic
1
shifts when interacted with fluoride ions. The UV-Visible and H NMR titration experiments revealed that
4-[(4-nitro-phenyl)-hydrazonomethyl]-1-phenyl-1H-pyrazole-3-carboxylate derivatives showed selective sens-
ing of fluoride ions in preference to Cl−, Br−, I−, PF6−, HSO4−, ClO4−, CH3COO− and H2PO4 ions while
−
4-[2,4-dinitro-phenyl)-hydrazonomethyl]-1-phenyl-1H-pyrazole-3-carboxylate derivatives showed sensing of
acetate, dihydrogen phosphate ion and fluoride ion in organic media.
Keywords. Pyrazole; receptor; sensor; anions; fluoride ion.
bonds.9 The interaction of anion with receptor triggers a
change in photophysical response of the chromophoric
unit to allow sensing through visual color changes.10
The selectivity and colorimetric response of receptors
depend on the structure of the hydrogen bond com-
plexes and basicity of anions. Among the anions, flu-
oride ion, usually forms strong hydrogen bond with a
hydrogen bond donor fragment of the artificial recep-
tor at lower concentration, and at higher concentration
the interaction would likely stimulate the proton trans-
fer reaction, which depends on the acidity of hydrogen
bond donor group of the receptor. The colorimetric sen-
sors based on the deprotonation of the binding moiety
by anions are highly sensitive and selective in nature.
Heterocyclic systems like imidazole, pyrrole, naph-
thyridine, pyrazole etc., belong to an important class of
nitrogen containing systems and they are of consider-
able pharmacological interest in view of their uses as
anti-microbial,11a anti-cancer,11b anti-inflammatory,11c
anti-tubercular,11d anti-parasitic,11e anti-pyretic11f and
anti-fungal11g agents. In continuation of our work
on anion sensing,12 it was envisaged that function-
alized pyrazole derivative like 4-formyl-3-carboxylate
pyrazole might display interesting anion binding and
sensing characteristic because of its structural fea-
tures which involve methine proton of pyrazole and
basic hydrogen bond acceptor fragment in condensed
heterocyclic system integrated with formyl and ester
1. Introduction
The selective sensing of anionic species through artificial
receptors have gained immense interest in the recent
past1 due to involvement of various anions in biological,
chemical, environmental and pathological events of
varying complexity.2 Among the biologically important
anions, sensing of fluoride ions is a significant research
area3 because of its functional diversity which is both
beneficial and detrimental. For example, it is well-
established that fluoride ion plays important role in pre-
venting dental caries and treatment of osteoporosis4
while excess of fluoride ion causes several adverse
effects like fluorosis, kidney disorder, bone and skeletal
cancer.5 Amongst several approaches adopted6 for the
development of fluoride ion receptors, sensing of fluo-
ride through colorimetric neutral receptors seems to be
of utmost significance as they require minimum instru-
mentation and quick response through anion induced
color changes of solution.7 Previous reports on colori-
metric sensing of fluoride ions8 include molecular hosts
having precise alignment of optical signaling chromo-
phoric units with suitable anion binding sites such as
urea, thiourea, amide and sulfonamides, which can pro-
vide one or more H-bond donor sites for selective re-
cognition of anionic species through N–H–X hydrogen
∗For correspondence
1451

1452
Suman Swami et al.
functionality which can be modulated for the construc- 110.2, 118.47, 120.24, 121.89, 125.20, 126.38, 128.31,
tion of effective recognition sites for noncovalent inter- 130.44, 138.18, 138.38, 140.92, 152.45, 161.68. Com-
actions. In this paper, we report the synthesis of pyra- pound 5e: Yield (88%); M.p.: 234◦C; IR (KBr), νmax
zole based molecular receptors for sensing of fluoride (cm−1): 1601 (C=N), 1698 (C=O). TOF-MS ES+ m/z
ion and other biologically and environmentally impor- calcd: 458.27. found: 459.34 (M+1); Anal. Calcd. for
tant anions. To design the receptors, we introduced C19H16BrN5O4: C 49.80, H 3.52, Br 17.44, N 15.28, O
1
hydrazone functionality substituted with electron with- 13.97%. Found: C 49.88, H 3.43, N 15.37%; HNMR
drawing nitro unit which are probably responsible for (400 MHz, CDCl3): δ 1.12 (t, 3H, COOCH2CH3), δ
the changes in color as well as to increase hydro- 4.10 (q, 2H, COOCH2CH3), δ 6.75 (d, 2H, ArH), δ 7.29
gen bond donor tendency.12,13 It has been observed (d, 2H, ArH), δ 7.44 (d, 2H, ArHnitrophenyl), δ 7.75 (d,
that these synthesized receptor molecules (5a and 5e) 2H, ArHnitrophenyl), δ 8.21 (s, 1H, CHazomethine), δ 8.24 (s,
exhibit significant binding ability for fluoride ions in 1H, CHpyrazole), δ 10.55 (s, 1H, NH). 13C NMR (CDCl3):
prefere−nce to other anions which include Cl−, Br−, I−, 14.7, 63.4, 110.1, 113.7, 118.3, 124.0, 125.1, 126.8,
−
−
H2PO4 , HSO4 , CH3COO−, ClO4− and PF6 .
127.0, 128.1, 132.6, 140.5, 149.8, 150.6, 152.o, 158.7.
2.3 General procedure for the synthesis of 5b
2. Experimental
To 4a (1 g, 4.1 mmol) in 20 mL ethanol, 2,4-dinitro
phenylhydrazine (0.972 g, 4.9 mmol) was added. The
reaction mixture was refluxed for 45 min and then
cooled to room temperature. Orange solid was fil-
tered off, washed with hot ethanol (3 × 10 mL) and
then dried under vacuum to give the desired com-
pound 5b as orange solid. Compound 5b: Yield (93%);
M.p.: 232◦C; IR (KBr), νmax (cm−1): 1598 (C=N),
1712 (C=O). TOF-MS ES+ m/z calcd: 424.11 found:
425.22 (M+1); Anal. Calcd.for C19H16N6O6: C 53.77,
H 3.80, N 19.80, O 22.62%. Found: C 53.63, H 3.74,
N 19.69%; 1H NMR (300 MHz, CDCl3): δ 1.43 (t, 3H,
COOCH2CH3), δ 4.45 (q, 2H, COOCH2CH3), δ 7.31
(d, 2H, ArH), δ 7.73 (d, 2H, ArH), δ 7.82 (m, 1H,
ArH), δ 8.10 (m, 1H, ArHDNP), δ 8.29 (m, 1H, ArHDNP),
δ 8.67 (s, 1H, CHPyrazole), δ 8.93 (s, 1H, CHazomethine),
δ 9.04 (m, 1H, ArHDNP), δ 11.58 (s, 1H, NH). 13C
NMR (CDCl3): 14.4, 61.5, 103.3, 113.7, 114.6, 119.8,
124.2, 127.0, 128.7, 130.3, 138.9, 142.5, 147.31, 161.8.
Compound 5c: Yield (95%); M.p.; 235◦C; IR (KBr),
νmax(cm−1): 1609 (C=N), 1720 (C=O). TOF-MS ES+
2.1 General procedure for the synthesis of 4a
To a cooled and anhydrous dimethylformamide (80
mL), POCl3 (25.5 mL) was added drop wise at 0◦C
and stirred the reaction mixture for 30 min at same
temperature. Phenyl hydrazone (13.5 g, 1 mmol) (3a)
was added at 0◦C. The reaction mixture was stirred at
room temperature for 2 h. After completion of reac-
tion as monitored by TLC, the reaction mixture was
poured over crushed ice under stirring. Thereafter the
mixture was neutralized with 10% NaHCO3 solution
which resulted in the formation of white solid. The solid
product was filtered under vacuum and dried in air to
give a compound 4a.
2.2 General procedure for the synthesis of 5a
To 4a (1 g, 4.1 mmol) in 20 mL ethanol, 4-nitro phenyl-
hydrazine (0.736 g, 4.8 mmol) was added. The reac-
tion mixture was refluxed for 45 min and then cooled
to room temperature. A yellow solid product was fil-
tered off, washed with hot ethanol (3 × 10 mL) and
then dried under vacuum to give desired compound
5a as yellow solid. Selected analytical data for com-
pounds 5a and 5e are given below: Compound 5a:
Yield (90%); M.p.: 238◦C; IR (KBr), νmax (cm−1): 1605
(C=N), 1704 (C=O). TOF-MS ES+ m/z calcd: 379.37.
found: 380.23 (M+1); Anal. Calcd.for C19H17N5O4: C
60.15, H 4.52, N 18.46, O 16.87%. Found: C 60.08, H
1
m/z calcd: 438.39 found:.461.22 (M+Na); H NMR
(300 MHz, CDCl3): δ 1.29 (t, 3H, COOCH2CH3), δ
2.19 (s, 3H, CH3), δ 4.31 (q, 2H, COOCH2CH3), δ 7.37-
7.46 (m, 4H, ArH), δ 8.04 (m, 2H, ArHDNP), δ 8.76-8.79
(m, 2H, CHPyrazole+CHazomethine), δ 8.99 (s, 1H, ArHDNP),
δ 11.78 (bs, 1H, NH). 13C NMR (CDCl3): 14.4, 17.9,
61.7, 116.5, 119.9, 123.6, 126.3, 126.9, 129.9, 130.8,
131.5, 134.0, 138.2, 138.9, 141.2, 142.0, 144.9, 161.3.
1
4.36, N 18.35%; H NMR (400 MHz, CDCl3): δ 1.41
(t, 3H, COOCH2CH3), δ 4.42 (q, 2H, COOCH2CH3), δ
7.14 (d, 1H, ArH), δ 7.36 (m, 2H, ArH), δ 7.47–7.51 (m,
3. Results and Discussion
2H, ArH), δ 7.77 (d, 2H, ArHnitrophenyl), δ 8.11 (d, 2H, In our effort to prepare receptor molecules (5a–5e),
ArHnitrophenyl), δ 8.41(bs, 2H, CHpyrazole+CHazomethine), first we prepared 4-formyl-1-substituted phenyl-1H-
10.45 (s, 1H, NH). 13C NMR (CDCl3): 14.2, 60.8, pyrazole-3-carboxylate derivatives (4a–4d) in 90-95%

Fluoride, dihydrogenphosphate and acetate sensing
1453
yield via two step synthetic protocol. In the first step,
The anion binding characteristics of receptors 5a–5e
alkyl-2-oxopropanoate (1a–1b) reacted with substituted with various anions were examined spectrophotomet-
phenyl hydrazine (2a–2b) in presence of trifluroacetic rically and visual colorimetric observation in DMSO-
acid in water at room temperature for 5–15 minutes to CH3CN (1.0 : 9.0 v/v). It was observed that receptor 5a
give 3a–3d in more than 90% yield. Compound 3a-3d showed an instant change in color from yellow to dark
was then reacted with DMF-POCl3 at 0◦C to provide purple with fluoride ion while no change in color was
4a–4d in more than 92% yield.14 Reaction of 4a– obser−ved wi−th other anions such as Cl−−, Br−, I−, HSO4 ,
−
4d with various phenyl hydrazine reagents afforded ClO4 , PF6 , CH3COO− and H2PO4 in form of their
the receptor 5a–5e in 88–95% yield as depicted in tetrabutylammonium salts as depicted in Figure 1a. On
Scheme 1.
the other hand, it was observed that receptor 5b gave
Receptor compounds (5a–5e) were characterized by a noticeable color change not only with fluoride ion
1
elemental and spectroscopic analysis. For instance, H but also with dihydrogen phosphate and acetate anions
NMR spectrum of 5a showed a triplet and quartet at δ (Figure 1b) under similar experimental conditions.
1.44 and δ 4.45 for CH3 and CH2 protons of ester group,
The anion binding ability of receptors 5a-5e was
respectively. The azo-methine proton and aromatic pro- also examined by UV-Visible spectroscopy in DMSO-
ton of pyrazole unit appeared as a non-exchangeable CH3CN (1.0 : 9.0 v/v) at pH 7.7. The receptor 5a
singlet at δ 8.43 and δ 8.44, respectively. The indi- showed an absorption peak at 402 nm that could be
vidual assignments of the aromatic proton of pyrazole ascribed to the absorption maximum for the hydrazone
and azo-methine protons were confirmed by compari- unit present in the receptor molecule. The binding char-
son of their spectra with those of precursor compounds. acteristics receptor 5a with various anions was stud-
A singlet of NH protons appeared at δ 10.45 which ied by using their tetrabutylammonium salt of F−, Cl−−,
−
−
−
disappeared on deuteration with D2O. Similarly, other Br−, I−, HSO4 , ClO4 , PF6 , CH3COO− and H2PO4
synthesized compounds were characterized by various in DMSO-CH3CN (1.0 : 9.0 v/v). It was observed that
spectroscopic techniques.
receptor 5a selectively recognizes fluoride ion from a
mixture of various anions. For example, when 2.2 ×
10−5 M solution of receptor 5a was treated with tetra-
butyl ammonium fluoride, it showed a bathochromic
shift (ꢀλmax) of 155 nm in its UV-Visible spectrum with
the appearance of a new absorption peak at 557 nm
R1
O
NHNH2
(i)
COOR
R1
NH
N
2a-2b
1a-1b
ROOC
3a-3d
(ii)
ROOC
R1
N
N
R1
(iii)
HC
N
N
N
HN
R3
R2
ROOC
CHO
5a-5e
4a-4d
Comp. No. R
R1
H
H
R2
H
R3
NO2
5a
5b
5c
5d
5e
Et
Et
Et
Me
Et
NO2 NO2
CH3 NO2 NO2
H
Br
NO2 NO2
NO2
H
Figure 1. Selectivity of 5a for fluoride ion over other
anions. Color changes of 5a (a) and 5b (b) in (22 μM) in
DMSO: CH3CN (1.0 : 9.0 v/v) with the addition of TBA
Scheme 1. Synthesis of receptors 5a–5e. Reagents and
conditions: (i) TFA, H2O, room temperature, 15 min;
(ii) DMF-POCl3, 0◦C, 2.5–3 h; (iii) substituted phenyl
hydrazine derivative, ethanol, reflux, 1–2 h.
anions (5 × 10−3 M): A = Receptor, B = F−, C = Cl−, D =
Br−, E = I−, F = CH3COO−, G = HSO4−, H = H2PO4
.
−

1454
Suman Swami et al.
while no significant shift in absorption maximum at
The binding affinity of synthesized receptor 5a
402 nm was o−bserved−with other anionic species (Cl−−, towards fluoride ion was again examined by H NMR
1
−
1
Br−, I−, HSO4 , ClO4 , PF6 , CH3COO− and H2PO4 ) titration experiments in CDCl3. The H NMR spec-
in similar experimental conditions (Figure 2a). These trum of 5a on gradual addition of tetrabutylammonium
behavior of 5a could be explained by the formation of fluoride (Figure 3) indicated that addition of fluoride
more stabilized excited state of 5a by interaction with ions a broadening of hydrazone NH signal and a down-
fluoride as compared to other anions.12,13
field shift in methine proton of pyrazole and aromatic
Further, it was observed that on gradual addition proton of phenyl unit (attached to pyrazole moiety)
of a standard solution of tetrabutylammonium fluoride was observed whereas an unexpected upfield shift in
(5 × 10−3 M), the intensity of absorption peak at 402 nm aromatic proton of nitrophenyl moiety was observed.
progressively decreased with simultaneous increase in Analysis of the 1H NMR titration spectra indicated that
the intensity of absorption peak at 557 nm in UV- addition of increasing equivalent of F− ions to 5a results
Visible spectrum. Critical analysis of UV-Visible titration in a broadening of hydrazone NH signals at δ 10.45
data indicated that the limiting value of the absorption which completely disappeared after the addition of two
maximum at 557 nm was reached at three equivalent of equivalents of fluoride ions with appearance of a broad
1
fluoride ion instead of one equivalent (Figure 2b) and signal at 16 ppm in H NMR spectrum that could be
the receptor can detect fluoride ion in μM range. These ascribed to the formation of stable bifluoride [HF−2 ].16
observations can be accounted by the hypothesis that However, addition of fluoride ions more than two equiv-
initial addition of one equivalent of fluoride ion estab- alents to the solution of 5a resulted in upfield chemical
lishes a hydrogen bond interaction with NH proton shift in aromatic proton at δ 8.11 of the nitro phenyl
of the receptor, and further addition of fluoride ion units while downfield chemical shift in methine proton
triggers deprotonation of NH proton,15 which results of pyrazole at δ 8.41 and aromatic proton of phenyl unit
in bathochromic shift in absorption maximum. The at δ 7.14 which reached its limiting value on addition
change in color occurs possibly through efficient charge of three equivalents of fluoride ion. These experimental
transfer from donor to acceptor substituents.
observations indicate that initial one equivalent of fluo-
ride ion interact through hydrogen bonds with the NH
protons of hydrazone subunits of receptor 5a whereas
addition beyond one equivalent of fluoride results in the
deprotonation of the NH protons to induce an upfield
shift of the aromatic proton of nitrophenyl unit due
Figure 2. (a) Absorption spectra of 5a (2.2 × 10−5 M, path
length of the cuvette 1 cm) upon addition of F−,−Cl−, Br−, I−,
1
H2PO4−, HSO4−, CH3COO−, ClO4 and PF6 ions (tetra-
−
Figure 3. Partial H NMR (400 MHz, CDCl3) spectra of
butyammonium salt) (5 × 10−3 M) in DMSO-CH3CN (1.0
: 9.0 v/v) (b) UV-Visible titration of 5a with 0 to 3 equiv.
of F−.
5a (15 mM) upon addition of various equiv. of TBAF (45
mM). Numbers at left side indicate the equivalent amount of
F− added ( = ArHpyrazole
,
= ArHnitrophenyl
,
= ArH).

Fluoride, dihydrogenphosphate and acetate sensing
1455
R1OOC
R1OOC
R1OOC
N
N
N
N
N
H
N
HC
H
HC
N
H
F-
3 equiv F-
HC
1equiv F-
H
F-
H
H
N
N
O
O
NO2
NO2
N
N
N
N
Scheme 2. A proposed binding mode and deprotonation of 5a with fluoride ion.
Table 1. Shift in absorption peak (λmax) for receptors 5a–e
with the fluoride iona.
Receptor (L) λmax/nm (L)a λmax/nm (L+F−) ꢀλmax/nm
5a
5b
5c
5d
5e
402
386
385
385
401
557
492
493
492
556
155
106
108
107
155
a Absorption spectra were taken at a concentration of 22 μM
in DMSO-CH3CN (1.0: 9.0 v/v) solution.
Figure 4. Absorption spectra of 5b (2.2 × 10−5 M, path
length of the cuvette 1 cm) upon addition of F−,−Cl−, Br−, I−,
H2PO4−, HSO4−, CH3COO−, ClO4 and PF6 ions (tetra-
−
butyammonium salt) (5 × 10−3 M) in DMSO-CH3CN (1.0 :
to through-bond propagation of electron density. The
downfield shift of methine proton and aromatic proton
of the phenyl unit on addition of three equivalents of
fluoride ion in CDCl3 indicated that both the methine
and aromatic protons of phenyl unit participate in the
formation of cooperative hydrogen bonds with anionic
guest (Scheme 2).17
Similar experiments were performed with 5b, 5c
and 5d. It was observed that these receptors exhibited
bathochromic shifts of lower magnitude as compared
to 5a and 5e in the UV-Visible spectrum with fluoride
ions (Table 1), possibly due to the extended conjuga-
tion present in 5a and 5e. Additionally, it was inter-
esting to observe that receptors 5b, 5c and 5d showed
bathochromic shifts of similar magnitude in their λmax
9.0 v/v).
EtOOC
EtOOC
N
N
N
H
N
H
HC
N
O
O
HC
N
CH3COO-
C
CH3
H
H
NO2
N
NO2
N
O2N
O2N
Scheme 3. A proposed binding mode of 5b with acetate
ion.
Similarly,−the titration profiles of 5b with CH3COO−
values on addition of tetrabutylammoniumdihydrogen and H2PO4 ion was further studied from UV-
phosphate and acetate ions as shown in Figure 4. The visible spectrum in DMSO-CH3CN (1.0 : 9.0 v/v). A
additional binding ability of receptors 5b and 5d for bathochromic shift (ꢀλmax) of 105 nm in its UV-Visible
dihydrogen phosphate and acetate ions can possibly spectrum was observed with the appearance of a new
be ascribed to the comparatively more acidic nature absorption peak at 491 nm which reached the limiting
of the NH protons due to the presence of two elec- values on addition of two equivalents of the respective
tron withdrawing nitro substituents. The UV-Visible anions (Figures S2 and S3). These observations sug-
titration experiments of receptor 5b with F− ion indi- gest that the F− ion interact with 5b in a similar man-
−
cated that intensity of absorption at 386 nm progres- ner as that of 5a, while the CH3COO− and H2PO4
sively decreased with gradual appearance of a new peak ions interact with 5b exclusively through hydrogen
at 492 nm along with a color change from yellow to bonding (Scheme 3). The receptors 5e and 5d showed
purple which reached a saturation point on addition similar behavior in the colorimetric and UV-Visible
of three equivalents of fluoride ions (Figure S1, see experiments as that of 5a and 5b, respectively, thereby
Supplementary Information).
suggesting that the aromatic proton of the phenyl unit

1456
Suman Swami et al.
3. (a) Gai L, Mack J, Lu H, Nyokong T, Li Z, Kobayashi
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(attached to pyrazole) (in 5a and 5b) when replaced by
methyl and bromine (in 5c and 5e), respectively, do not
affect the binding mode of anions.
4. Conclusions
In conclusion, a new class of efficient colorimetric chemo-
sensors derived from 4-[(nitro substituted-phenyl)-
hydrazonomethyl]-1-phenyl-1H-pyrazole-3-carboxylate
(5a–5e) have been synthesized and characterized. The
synthesized receptors exhibit significant bathochromic
shifts when interacted with fluoride ions along with
prominent color change. The UV-Visible and 1H NMR
titration experiments revealed that receptor 5a and 5e
showed selective sensing of fluoride ions via hydrogen
−
bond−interaction in preference to Cl−, Br−, I−, HSO4 ,
−
−
ClO4 , PF6 , CH3COO− and H2PO4 ions whereas
receptors 5b, 5c and 5d showed sensing of acetate and
dihydrogenphosphate ions along with fluoride ion in
organic media.
Acknowledgements
RS and AA acknowledge Manipal University Jaipur for
seed money grant. SS acknowledges MUJ for a teaching
assistantship. Material Research Center, MNIT, Jaipur
is gratefully acknowledged for recording NMR, FT-IR
and Mass spectra reported in this paper.
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