Spectroscopic, colorimetric and theoretical investigation of Salicylidene hydrazine based reduced Schiff base and its application towards biologically important anions

Article abstract of DOI:10.1016/j.saa.2012.02.028

A reduced Schiff base anionic receptor 1 [N,N′-bis-(2-hydroxy-5- nitro-benzyl)hydrazine] has been synthesized, characterized and reported as a selective chromogenic receptor for fluoride, acetate and phosphate anions over the other tested anions such as c

Full text of DOI:10.1016/j.saa.2012.02.028

Spectrochimica Acta Part A 92 (2012) 131–136
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic, colorimetric and theoretical investigation of Salicylidene
hydrazine based reduced Schiff base and its application towards biologically
important anions
Sankar Janaa,1, Sasanka Dalapatia,1, Md. Akhtarul Alamb,∗,2, Nikhil Guchhaita,∗,1
a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata-700 009, India
b Department of Chemistry, Aliah University, DN-41 & 47, Sector-V, Saltlake, Kolkata-700 091, India
a r t i c l e i n f o
a b s t r a c t
A reduced Schiff base anionic receptor 1 [N,Nꢀ-bis-(2-hydroxy-5-nitro-benzyl)hydrazine] has been synthe-
sized, characterized and reported as a selective chromogenic receptor for fluoride, acetate and phosphate
anions over the other tested anions such as chloride, bromide, iodide and hydrogensulphite. Colorimetric
naked-eye detection and UV–vis absorption spectroscopic techniques were used to distinguish the recog-
nition behaviours towards various anions. The receptor–anion complexation mainly occurs via hydrogen
bonding interactions which facile to generate the charge transfer band in the UV–vis spectra and cause
large bathochromic shift as well as naked-eye colour change. Complexation stoichiometry, binding con-
stant and free energy change due to complex formation were determined from Benesi–Hildebrand plot.
The binding constant and the free energy change values are well interactive for spontaneous complexa-
tion. The experimental results have been correlated with the theoretical calculations using B3LYP hybrid
functional and 6-311++G(d,p) basis set for both the receptor and complex by Density Functional Theory
(DFT) method.
Article history:
Received 25 October 2011
Received in revised form 6 February 2012
Accepted 10 February 2012
Keywords:
Anion recognition
Schiff base receptor
N,Nꢀ-bis-(2-hydroxy-5-nitro-
benzyl)hydrazine
Naked eye colorimetric anion sensor
DFT
Hydrogen bonding interaction
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
these significant applications, detection of anions with the help of
easily synthesized receptor and minimal instrumental assistance
Design and synthesis of an artificial anion receptor demand sig-
nificant interest in supramolecular chemistry. The recognition and
sensing of more than one anion by a single receptor is a great
achievement in anion recognition chemistry [1–3] due to their
biological, industrial and environmental relevance [4–6]. Although
anions are essential for living organism, excessive amount of anion
accumulated in human body causes permanent damaging effect
of several organs like kidney, bone, liver and central nervous sys-
tem. For example, it is well known that a small quantity of fluoride
ion is present in biological fluids, tissues and especially in bone
and tooth. However, excess fluoride anion causes several serious
diseases such as fluorosis, depression of thyroid activity, bone dis-
order and disruption of immune systems [7,8]. On the other hand,
carboxylate and phosphate anions are important for specific bio-
chemical behaviour of enzymes and antibodies and are also critical
components of numerous metabolic processes [7,8]. Because of
are desirable towards practical applications. In the reported artifi-
cial anion sensing receptors, the preparation requires a complicated
synthetic routes or troublesome purification procedures [9–11]
and the recognition processes are costly based on complicated
steady state and time resolved fluorescence techniques [12,13]
or by other ways [14,15], followed by theoretical optimization
[1,13,16]. Therefore, colorimetric and spectrophotometric sensing
of anions by an easily synthesized receptor has become an advan-
tageous field of interest due to its simplicity and cost effectiveness.
Again reduced Schiff bases are used as important chemicals in co-
ordination chemistry due to their flexible binding affinity towards
various metal ions [17,18]. On the other hand, Schiff bases and
reduced bases have some synthetic and purification facilities such
as (a) Schiff bases are easily obtained by condensation of com-
mercially available aldehydes and amines, (b) in most cases, the
products are precipitated as a pure solid and with higher percentage
of yield and finally, (c) the reduced Schiff bases are easily obtained
by reduction of Schiff bases with sodium borohydride (NaBH4) in
a common organic solvents, such as methanol and ethanol. In this
regard, the anion recognition possibility of reduced Schiff bases has
gained faster attention in current chemical research. Here, we have
investigated the anion recognition possibility of a reduced Schiff
base receptor.
∗
Corresponding authors.
E-mail addresses: alam iitg@yahoo.com (Md.A. Alam), nguchhait@yahoo.com
(N. Guchhait).
1
Tel.: +91 33 23508386; fax: +91 33 23519755.
Tel.: +91 33 27062125; fax: +91 33 27062124.
2
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.02.028

132
S. Jana et al. / Spectrochimica Acta Part A 92 (2012) 131–136
Before going to our complete experimental evaluation, it is
anions. The binding or association constant (K) for the complex
formed between receptor 1 and anion has been determined by
using Benesi–Hildebrand (B–H) relation [20,21]. The association
constant of the complex has been determined from the following
complexation equilibrium:
important to mention that most of the reported receptors (having
NH or OH group) show an anion recognition capability through
hydrogen bonding interaction with the anion. Whereas the strength
of H-bonding interaction depends upon the basicity and structure
of the anion [3]. Therefore, hydrogen bonding solvents such as
water, ethanol, and methanol can not be used for the anion recog-
nition process by reduced Schiff base receptor [19]. Thus, aprotic
solvent like acetonitrile is used in anion recognition studies based
on hydrogen bonds.
L + mXn− ↔ (XmL)mn−
[XmLmn−
]
m
K =
[L][Xn−
]
In this work, we report the synthesis, characterisation and
use of a reduced Schiff base receptor [N,Nꢀ-bis-(2-hydroxy-5-nitro-
benzyl)hydrazine] (1), which can recognize biologically important
F−, AcO− and H2PO4− anions by both the naked-eye colour changes
as well as UV–vis spectral changes. The UV–vis analysis shows that
receptor 1 can form 1:2 stable complexes with fluoride, acetate
and phosphate anions. Furthermore, we have tried to compare our
experimental results with the theoretical calculations performed
by Density Functional Theory (DFT) method with the help of B3LYP
hybrid functional and 6-311++G(d,p) basis set for both the receptor
and its complexes.
Benesi–Hildebrand relation with m = 1 for 1:1 complex can be
expressed in terms of optical density (A) as follow
A0 + A1K[Xn−
]
A =
1 + K[Xn−
]
Or,
1
1
1
=
=
(A1 − A0)K[Xn−
]
(A − A0)
(A1 − A0)
where [Xn−], [L] and [(XmL)mn−] are the concentrations of anions
added, receptor 1 and the complex formed between anion and
receptor 1, respectively. A0, A and A1 indicate the absorbance at
a particular wavelength of receptor 1 without anion, after adding
anion at each step and excess amount of anion added, respectively.
The terms K, m and n are the binding constant of the complex, moles
of anion and charge of the anion, respectively. The binding constant,
K (M−1 or M−2) was determined from the ratio of intercept and slope
2. Experimental
2.1. Reagents
All reagents and solvents were received from Spectrochem
Pvt. Ltd., India and used without further purification. The
anions, tetrabutylammonium fluoride (Bu4N+F−) hydrate (98%),
tetrabutylammonium chloride (Bu4N+Cl−) hydrate (98%), tetra-
butylammonium bromide (Bu4N+Br−) (98%), tetrabutylammonium
iodide (Bu4N+I−) (98%), tetrabutylammonium dihydrogenphos-
phate (Bu4N+H2PO4−) (97%), and tetrabutylammonium acetate
(Bu4N+AcO−) (97%) were purchased from Sigma–Aldrich Chem-
ical Company Pvt. Ltd. Tetrabutylammonium hydrogensulphite
(Bu4N+HSO3−) (97%) was purchased from Spectrochem Pvt. Ltd.,
India. All solvents of spectroscopic grade were used for spectral
measurement.
of Benesi–Hildebrand plot. The free energy change ꢀG (kJ mol−1
)
for receptor–anion complexation was determined at experimen-
tal temperature (298 K) from the value of K by using the relation
ꢀG = −RT ln K.
2.5. Synthesis of compound 1
[N,Nꢀ-bis-(2-hydroxy-5-nitro-benzyl)hydrazine]
To the methanolic solution (15 ml) of 5-nitro salicylaldehyde
(2.00 g, 11.9 mmol), methanolic solution of hydrazine monohy-
drate (0.19 ml, 5.83 mmol) was added and stirred for 1 h at room
temperature [22,23]. A light yellow colour solid of compound 2
(Scheme S1, in supplementary data) was precipitated. The reaction
mixture was filtered and washed with cold methanol. The resultant
solid was dried under vacuum, yield 82% (1.62 g, 4.9 mmol).
2.2. Apparatus
Electronic absorption spectra were recorded on a Hitachi UV–vis
(Model U-3501) spectrophotometer equipped with two quartz cells
of path length 10 mm. In all the cases spectra were recorded from
250 nm to 550 nm wavelength range after subtraction with the
proper solvent background at 298 K temperature. IR spectra (KBr
pellet, 4000–400 cm−1) were recorded on a Perkin Elmer model
883 infrared spectrophotometer. 1H NMR spectra were recorded
on a Bruker, Avance 300 spectrometer, where chemical shifts (ı in
ppm) were determined with respect to tetramethylsilane (TMS) as
internal standards.
To the methanolic (50 ml) suspension of compound 2 (1.00 g,
3.03 mmol), sodiumborohydride (NaBH4, 0.34 g, 3 equiv.) was
added slowly at 273 K temperature. During the progress of reduc-
tion a clear red solution was appeared. The solvent was evaporated
to dryness and then cold water (15 ml) was added to dissolve
the reaction mixture. The resulting mixture was neutralized with
dilute HCl. A light yellow colour solid of 1 (Scheme 1, in supple-
mentary data) was precipitated. The precipitate was filtered and
washed with distilled water and dried under vacuum, yield: 84%
(0.86 g, 2.57 mmol), 1H NMR in d6-DMSO, 300 MHz, ı (ppm): 8.22
(d, J = 3 Hz, 2H), 6.89 (d, J = 9 Hz, 2H), 6.74 (d, J = 9 Hz, 2H), 4.55 (s,
CH2−, 4H). IR (KBr): 3344, 3282, 3085, 1596, 1483, 1336, 1284,
2.3. Preparation of sample solutions for titration
Stock solutions of anions were prepared in acetonitrile solvent
and concentrations were maintained to 10−3 M. These stock solu-
tions were used in spectroscopic titration and all other experiments
after proper dilution. Stock solution of the receptor 1 (10−3 M) was
prepared in acetonitrile and diluted to 10−6 M during the titration.
1088, 928, 832, 744, 640 cm−1
.
2.6. Computational details
To investigate the complexation between receptor 1 with F−,
AcO−and H2PO4 ions, theoretical calculations have been p−er-
−
2.4. Benesi–Hildebrand plot, association constant determination
(K), stoichiometry and Gibbs’ free energy change (ꢀG) calculation
for receptor–anion complexation
formed for the free receptor 1 and 1(F−)2, 1(AcO−)2, 1(H2PO4
)
2
complexes in vacuo. All quantum chemical calculations were
carried out using Gaussian 03 software [24]. The global min-
ima optimized structure of receptor 1 and 1(F−)2, 1(AcO−)2, and
1(H2PO4−)2 complexes in vacuo were obtained by optimizing them
UV–vis titration spectra were used for the determination of sto-
ichiometry and the association constant between receptor 1 and

S. Jana et al. / Spectrochimica Acta Part A 92 (2012) 131–136
133
with Density Functional Theory (DFT) method using 6-311++G(d,p)
basis set. For simplification of theoretical calculations, only F−,
AcO−and H2PO4 ions have been considered by neglecting their
−
tetrabutylammonium counter cations [1,13,16]. This type of theo-
retical methods have been used for producing reliable results for
such types of receptor–anion complexes and reported in many
recent scientific publications [13,16,25–28].
3. Results and discussion
3.1. Visual colour change
The colorimetric recognition of anions by the reduced Schiff
base receptor has been investigated by addition of each anion to a
fixed concentration of the receptor in acetonitrile solvent keeping
total volume constant. As seen in Fig. S1 (in supplementary data),
a naked-eye detectable bright yellow colour was appeared on
addition of 30 ␮M F− ion to the colourless solution of 1 (16 ␮M).
Whereas under similar experimental condition, a very light yellow
colour was observed by addition of AcO− ion. In contrast, in the
Fig. 2. B–H plot for 1:2 complexation between 1 and F− ion. Inset shows B–H plot
for 1:1 complexation.
−
presence of H2PO4 ion (30 ␮M) the receptor exhibits a very faint
yellow colour (Fig. S1). Other anions such as Cl−, Br−, I− and HSO3
.
−
isosbestic point at 342 nm was observed during the titration pro-
cess between the receptor 1 and F− ion, which clearly indicates the
formation of complex between 1 and F− ion and the presence of
equilibrium between the reactants and the product complex. The
stoichiometry, binding constant (K) and free energy change (ꢀG)
due to complexation between 1 and F− ion were determined from
the UV–vis titration spectra using Benesi–Hildebrand (B–H) rela-
tion [20,21]. As shown in Fig. 2, the B–H plot of 1/[A − A0] vs 1/[F−]2
from the titration of 1 with F− ion provides a straight line through-
out the entire range of F− ion concentration indicating 1:2 complex
formation with binding constant (K) 1.035 × 1010 M−2 and binding
free energy change (ꢀG) −57.134 kJ mol−1 (Table 1). To confirm
our assumption, we also constructed the B–H plot for 1:1 complex-
ation as shown in inset in Fig. 2. The nonlinear plot in this case
again supports that the 1(F−)2 complexation occurs only through
1:2 stoichiometry.
Under the similar experimental conditions, addition of acetate
(AcO−) ion to the acetonitrile solution of 1 (Fig. 3), a new absorp-
tion band at 425 nm was appeared gradually with decreasing the
intensity of the initial band of 1 at 298 nm. During the titration an
isosbestic point at 348 nm was observed which indicates an equi-
librium between bare molecules and 1(AcO−)2 complex (Fig. 3). The
B–H plot of 1/[A − A0] vs 1/[AcO−]2 also exhibits a 1:2 complexation
did not exhibit any detectable naked-eye colour change. F−, AcO−
and H2PO4 ions can form hydrogen bonded complexes with the
−
receptor 1. Such types of hydrogen bonding interactions increase
the electron density on phenolic ‘O’ atom of OH group (donor) and
a charge transfer process from electron rich ‘O’ atom to the elec-
tron deficient NO2 group (acceptor) is facilitated, resulting in the
appearance of the yellow colour [19,29]. It is noted that the yel-
low colour of the complex disappears and the original colourless
solution reappears on addition of protic solvents such as H2O and
methanol, because, protic solvents compete for H-bonding interac-
tion with the receptor 1 compared to the anions.
3.2. UV–vis spectroscopic titration
The anion recognition properties of receptor 1 have been inves-
tigated by monitoring UV–vis spectral change upon addition of
different anions to the acetonitrile solvent. Receptor 1 exhibited a
broad absorption band at 298 nm (Fig. 1). Upon addition of increas-
ing amount of F− ion to the acetonitrile solution of receptor 1
(10−5 M), the absorption peak at 298 nm gradually decreases its
intensity and a new red shifted strong absorption band at 429 nm
gradually appears with a shoulder at 359 nm (Fig. 1). A distinct
0.4
IX
0.3
0.2
0.1
I
0.0
300
375
450
525
Wavelength(nm)
Fig. 1. UV–vis titration spectra of receptor 1 (16 ␮M) upon addition of F− ion in
acetonitrile solvent. I → XVIII; 0, 1.49, 2.98, 4.47, 5.96, 7.45, 8.93, 11.9, 14.12, 16.34,
18.56, 20.77, 22.98, 25.18, 28.12, 32.51, 39.8, 54.28 ␮M F− ion.
Fig. 3. UV–vis spectral changes of receptor 1 (16 ␮M) upon addition of AcO− ion in
acetonitrile solvent. I → IX; 0, 1.49, 2.98, 4.47, 5.96, 7.45, 8.94, 15.61, 30.32 ␮M AcO−
ion.

134
S. Jana et al. / Spectrochimica Acta Part A 92 (2012) 131–136
Table 1
Some useful experimental parameters of receptor 1 and its complexes with anions.
b
c
Complex
ꢁabs (nm)a
K × 10−10 (M−2
)
−ꢀG (kJ mol−1
)
Rd
1
298
429
425
425
–
–
–
1(F−
)
2
1.035
3.99
0.062
57.134
60.478
50.321
0.9979
0.9953
0.9989
1(AcO−
1(H2PO4
)
2
−
)
2
a
Absorption maxima.
Association constant.
Free energy change due to complexation between receptor and anions.
Linear regression for linear fitting.
b
c
d
−
Fig. 6. B–H plot for 1:2 complexation between 1 and H2PO4 ion. Inset shows B–H
plot for 1:1 complexation.
Fig. 4. B–H plot for 1:2 complex formed between 1 and AcO− ion. Inset: B–H plot
for 1:1 complexation.
The sensitivity of the receptor 1 towards all the anions is
shown in bar plot (Fig. 8), which is the plot of relative absorbance
at 429 nm vs different anions under similar experimental con-
ditions.Interestingly, the selectivity and sensitivity of−receptor 1
towards F−, AcO−, H2PO4−, Cl−, Br−, I−, and HSO3 ions can
be rationalized on the basis of their basicity as well as struc-
tural orientation. The Y-shaped structure of acetate anion [3] with
doubly H-bonding capability can strongly form complex through H-
bonding interaction with OH and NH protons (Fig. 9, structure
of receptor 1). On the other hand, due to low basicity and tetrahe-
dral structure phosphate anion is unable to bind in a similar fashion
and for these reasons the binding constant obtained from B–H plot
between 1 and acetate ion (AcO−) (Fig. 4) with K = 3.99 × 1010 M−2
and ꢀG = −60.478 kJ mol−1. Similar type of absorption spectral
changes were observed during the titration of receptor 1 with
H2PO4− ion (Fig. 5) and this also follows 1:2 complexation equilib-
rium (Fig. 6) with K = 0.0622 × 1010
M
−2 and ꢀG = −50.321 kJ mol−1
(Table 1). On the −other hand, addition of other anions such as Cl−,
Br−, I− and HSO3 did not show any noticeable spectral or colour
change in the similar experimental conditions, which indicate no
interaction or complexation of these anions with the receptor 1
(Fig. 7).
−
Fig. 5. UV–vis spectral changes of receptor 1 (16 ␮M) upon addition of H2PO4 ion
in acetonitrile solvent. I → XV; 0, 1.49, 2.98, 7.45, 14.86, 19.29, 23.72, 28.12, 32.51,
Fig. 7. UV–vis spectral changes of receptor 1 (16 ␮M) upon addition of different
anions (30 ␮M) in acetonitrile solvent.
−
36.51, 41.26, 45.61, 49.96, 54.28, 59.12 ␮M H2PO4 ion.

S. Jana et al. / Spectrochimica Acta Part A 92 (2012) 131–136
135
Table 2
Some useful optimized parameters of receptor 1 and its complexes.
Complex
H1 O1 (Å)
C
O1 (Å)
O1–A− (Å)a
N1–A− (Å)
1
0.963
1.442
1.550
1.035
1.363
1.279
1.273
1.324
–
–
1(F−
)
2
2.433
2.561
2.505
3.198
3.280
3.061
1(AcO−
1(H2PO4
)
2
−
)
2
a
−
F−, AcO− and H2PO4 anions.
on the nature of bonding interaction with anions. The ground
state optimization was carried out in vacuum for all compounds
using Gaussian 03 [24] software with B3LYP hybrid functional and
6-311++G(d,p) basis set by applying Density Functional Theory
method. The optimized global minimum structure of receptor 1
shows that the two nitro-salicylidene moieties are placed in the
opposite direction (Fig. 9). This kind of structure of the recep-
tor facilitates the incoming anion to interact with the receptor 1
through opposite direction with respect to the molecular base-
line. As a result the receptor 1 can form the possible 1:2 stable
complexes with F−, AcO−and H2PO4−ions (Figs. S2–S4, in supple-
mentary data). The optimized structure of each complex shows
that two hydrogen bonding interactions exist within the acceptable
Fig. 8. Bar plot for absorbance change of the receptor 1 (16 ␮M) at 429 nm in the
presence of various anions (30 ␮M), where AL is the absorbance of receptor 1 at
429 nm.
follow the trend AcO− > F− ꢁ H2PO4−. The trend of binding con-
stant also reflects in their free energy change (Table 1). Complexes
with higher binding constants also have greater free energy change.
Considering the basic−ity of the anion, weakly basic anion such as
Cl−, Br−, I− and HSO3 anion did not interact with OH and NH
protons, and hence these anions are unable to change the colour
of the solution as well as in the UV–vis spectra. In contrast, as the
basicity order follow the trend F− > AcO− > H2PO4−, fluoride ion is
capable to enhance greater absorbance at 429 nm compared to oth-
ers (Fig. 7), which may be due to deprotonation of the receptor 1 in
the presence of excess fluoride anion. The detailed UV–vis spectral
analysis of 1 in the presence of various anions (Fig. 7) clearly dis-
criminates their H-bond interacting behaviors in the complexation
process and the bathochromic shift of the absorption spectra can
be explained in terms of a facile charge transfer from OH to NO2
groups.
donor–acceptor distances (Table 2) [30,31]. The optimized struc-
ture shows that for 1(F−)2, 1(AcO−)2, and 1(H2PO4
) complexes,
2
−
the O1 H1 bond distance increases as the hydrogen bonding abil-
ity of the anion increases in the order F−> AcO−> H2PO4, except,
for the AcO− anion where we observed the largest O1 H1 distance
(Table 2). This is mainly due to the Y-shaped structure of the acetate
anion which facilitates the formation of hydrogen bonded complex
with the receptor 1 [3]. On the other hand, for the same reason,
we observed the decrease in C O1 bond distance in same manner
(Table 2), which clearly indicates that, hydrogen bonding interac-
tion between 1 and anions promotes the receptor 1 to undergo a
facile charge transfer state which is mainly from OH to nitro group.
Theoretical optimization clearly explains that the nature of bind-
ing of structurally symmetrical receptor 1 with fluoride, acetate and
phosphate anions are almost similar fashion. Finally, proper orien-
tation of the receptor and anions, predicted stoichiometry and the
nature of hydrogen bonding interactions obtained from theoretical
structural optimization corroborates well the outcome of UV–vis
titration experiment and B–H plot.
3.3. Theoretical modeling and optimization
To build up the base of our successful achievements in the
experimental findings, we have tried to find out the global min-
ima optimized structure of receptor 1 and its complexes focusing
Fig. 9. Global minimum optimized geometry of 1 using B3LYP hybrid functional and 6-311++G(d,p) basis set.

136
S. Jana et al. / Spectrochimica Acta Part A 92 (2012) 131–136
4. Conclusions
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In summary, we have demonstrated an easily prepared
reduced Schiff base receptor
1
(N,Nꢀ-bis-(2-hydroxy-5-nitro-
benzyl)hydrazine), which can recognize F−, AcO−and H2PO4
anions by naked-eye colour change as well as UV–vis spectral
change in acetonitrile solvent. Color changes and bathochromic
shift of the absorption spectra can be explained on the basis of
formation of 1:2 hydrogen bonded complex, which facilitates the
charge transfer between the phenolic oxygen and the electron
withdrawing nitro group of the receptor after complexation. Fur-
thermore, predicted stoichiometries of the complexes and nature
of complexation between 1 and anions obtained from Density
Functional Theory (DFT) calculation corroborate well with the
experimental results.
−
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NG gratefully acknowledges the financial support received
from Department of Science and Technology, India (Project no.
SR/S1/PC-26/2008). SJ and SD would like to acknowledge UGC for
Fellowship. MAA thank the VC of AU for giving him permission to
work at CU.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2012.02.028.
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