Solvent-assisted naked eye sensing of Hg2+ by a chemoreceptor derived from diazocoupling of sulfathiazole with diethyl malonate

Article abstract of DOI:10.1080/10426507.2010.536187

2-{[4-(Thiazol-2-ylsulfamoyl)-phenyl]-hydrazono}-malonic acid diethyl ester (R1) and 2-{[4-(5-methyl-isoxazol-3-ylsulfamoyl)-phenyl]-hydrazono}-malonic acid diethyl ester (R2) were synthesized through diazocoupling of sulfathiazole and sulfamethoxazole, r

Full text of DOI:10.1080/10426507.2010.536187

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Solvent-Assisted Naked Eye Sensing of
Hg2+ by a Chemoreceptor Derived from
Diazocoupling of Sulfathiazole with
Diethyl Malonate
K. K. Upadhyay a , Shalini Upadhyay a , Ajit Kumar a , Kamlesh
Thapliyal b & Santosh K. Srivastava a
a Department of Chemistry , Faculty of Science, Banaras Hindu
University , Varanasi, India
b Department of Chemistry , Indian Institute of Technology Kanpur ,
Kanpur, India
Published online: 11 Aug 2011.
To cite this article: K. K. Upadhyay , Shalini Upadhyay , Ajit Kumar , Kamlesh Thapliyal & Santosh
K. Srivastava (2011) Solvent-Assisted Naked Eye Sensing of Hg2+ by a Chemoreceptor Derived from
Diazocoupling of Sulfathiazole with Diethyl Malonate, Phosphorus, Sulfur, and Silicon and the Related
Elements, 186:8, 1820-1834, DOI: 10.1080/10426507.2010.536187
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Phosphorus, Sulfur, and Silicon, 186:1820–1834, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.536187
SOLVENT-ASSISTED NAKED EYE SENSING OF Hg2+ BY A
CHEMORECEPTOR DERIVED FROM DIAZOCOUPLING OF
SULFATHIAZOLE WITH DIETHYL MALONATE
K. K. Upadhyay,1 Shalini Upadhyay,1 Ajit Kumar,1
Kamlesh Thapliyal,2 and Santosh K. Srivastava1
1Department of Chemistry, Faculty of Science, Banaras Hindu University,
Varanasi, India
2Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, India
GRAPHICAL ABSTRACT
Abstract 2-{[4-(Thiazol-2-ylsulfamoyl)-phenyl]-hydrazono}-malonic acid diethyl ester (R1)
and 2-{[4-(5-methyl-isoxazol-3-ylsulfamoyl)-phenyl]-hydrazono}-malonic acid diethyl ester
(R2) were synthesized through diazocoupling of sulfathiazole and sulfamethoxazole, respec-
tively, with diethyl malonate. They were characterized through various spectroscopic and mass
spectral studies. R2 was also characterized through a single crystal X-ray diffraction (XRD)
study. Only R1 selectively recognized Hg2+ from a wide range of metal ions through naked-eye
change. A color change from orange to olive green was observed upon addition of 1.0 equiva-
lent of Hg2+ as its chloride salt to the 1 × 10−3 M DMSO solution of R1. The role of DMSO
in the sensing process appears to be the crucial one, because the solvent-assisted band of R1
at 482 nm observed in its UV-Vis spectrum in DMSO did not appear in its spectra recorded
in nujol or in a polar aprotic solvent. The UV-Vis and 1H NMR titrations revealed that the
formation of six-membered 1:1 chelate between R1 and Hg2+ triggering the desolvation of R1
Received 12 August 2010; accepted 22 October 2010.
The authors are thankful to the coordinator SAP for enabling us to perform DFT calculations using Gaussian-
03. S. U. is thankful to CSIR, New Delhi, for providing financial assistance in the form of a fellowship.
Address correspondence to K. K. Upadhyay, Department of Chemistry, Faculty of Science, Banaras Hindu
University, Varanasi-221005, India. E-mail: drkaushalbhu@yahoo.co.in
1820

SENSING OF Hg2+ BY A CHEMORECEPTOR
1821
as the key step towards its sensing activity. The determination of binding stoichiometry between
R1 and Hg2+ along with binding constant has also been discussed.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.
Keywords Binding constant; diazo derivatives; Hg2+ sensing; naked eye sensing; titrations
INTRODUCTION
In recent years, the design and development of chemosensors has expanded signif-
icantly.1–4 The synthesis of new chemosensors for metal ions of biological/environmental
relevance has been of interest to chemists for the last few decades. A variety of metal ions
play important role in living systems by occupying the active sites of several enzymes
such as Zn2+ in carboxypeptidase5 as well as carbonic anhydrase6 and Cu2+, Zn2+, Mn2+
in superoxide dismutase.7 Besides the enzymatic systems, the role played by Fe2+ in the
transport of O2 is also well known.8 There are a number of cations that play harmful
roles also. In this context, Cr6+/Cr3+, Ni2+, Cd2+, Hg2+ 9–12 are worth mentioning. Among
these, the most notorious and toxic metal ion is Hg2+, which affects living beings even at
very low concentrations. It affects vital organs such as the kidney, digestive system, and
especially the nervous system.13 The famous Minamata disease of Japan,14 in the 1950s
was also the result of mercury poisoning. In view of its high potential in causing irre-
versible environmental damages, its recognition at the qualitative and quantitative level
has received much attention. Among many Hg2+ selective chemosensors developed so far,
chromogenic/naked-eye receptors are especially convenient and user friendly, because the
analyte determination is carried out with the “naked-eye” without the use of any costly
instrumentation; no pretreatment of the sample15 is needed.
Out of the receptors R1 and R2, only one showed solvatochromism and solvent-
assisted naked-eye sensing of Hg2+ as its chloride salt at millimolar level in DMSO solution.
The selection of sulfa drugs as the starting material for the present study was based on their
easy availability, low price, and presence of some appropriate ligating groups. The present
work is a part of our continuous effort towards design, synthesis, and evaluation of naked-
eye chemoreceptors for biologically relevant ions through simple reaction protocols.16–24
RESULTS AND DISCUSSION
Single-Crystal X-Ray Diffraction Studies
Small crystals of R2 were grown and the structure determined by single-crystal X-
ray diffraction studies (CCDC No-739701). Crystals of a suitable size for R1 could not be
readily grown. The crystal structure was solved by direct methods using SHELXL-97.25
The ORTEP diagram for R2 has been given in Figure 1, which suggested the existence of
the same in its hydrazo form. The crystal data of same revealed that the N(3) N(4) bond
length is 1.31 Å, which matches well with literature value for N N single bond.26 The long
range intermolecular interactions in R2 involved five types of interactions, viz. N1 and N2
of one R2 with N2 and N1 of another R2 with interatomic distance of 2.986 Å (b view),
N3 and O4 of same R2 (2.618 Å), N3 of one R2 with O3 of another R2, and O3 of one R2
with N3 of another R2 (3.051 Å) (c view) (see Supplemental Materials; Figure S1).
The corresponding crystal data and details of the structure determinations are given
in Table 1.

1822
K. K. UPADHYAY ET AL.
Figure 1 Molecular structure of R2. The thermal ellipsoids are drawn at the 50% probability level.
Density Functional Theory (DFT) Studies
The DFT calculations also confirmed the existence of R2 in its hydrazo form on the
similar line of XRD pattern for the same. Full optimization for hydrazo and azo forms of
R1 and R2 revealed that the hydrazo forms are more stable than the azo ones by 18.86
and 26.65 kcal/mol, respectively (Table 2). The corresponding optimized structures of the
hydrazo and azo forms of R1 are shown in Figure 2.
Table 1 Crystal structure and data refinement parameters of R2
Compound
R2
Empirical formula
Formula weight
C17H20N4O7S
424.43
Crystal system/space group
a/Å
b/Å
Triclinic/P-1
5.382(4)
8.141(5)
22.544(3)
81.185(5)
84.308(4)
74.302(5)
938.0(9)
2
c/Å
α/◦
β/◦
γ /◦
V/Å3
Z
D calc (g/cm3)
1.503
0.223
µ (mm−1
)
Crystal size (mm)
Color/shape
Temp (K)
0.16 × 0.15 × 0.13
Light yellow/block
100(2)
Theta range for collection
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
2.62–28.38
6134
3565
4434/0/270
1.181
0.0748
0.1014
Largest difference peak/hole
0.770/−1.118

SENSING OF Hg2+ BY A CHEMORECEPTOR
1823
Table 2 The Potential Energy for the Hydrazo and Azo Forms of R1 and R2 Calculated Through Density
Functional Theory Using B3LYP/6-31G∗∗
Total electronic energy (a.u.)
Tautomers
Compound name
Hydrazo
Azo
R1
R2
−2087.19003431
−1803.52713173
−2087.15998541
−1803.48467000
UV-Visible Studies
The UV-Vis spectra of R1 and R2 were recorded in nujol (see Supplemental materials;
Figure S2) as well as in different solvents of the polar aprotic type such as DMSO, DMF,
acetonitrile, acetone, and nonpolar aprotic like chloroform at 1 × 10−3 M concentration.
The corresponding absorption wavelengths with their respective absorbances have been
given in Table 3.
The UV-Vis spectra of the R1 and R2 showed broad absorption bands in nujol at 379
(0.7202) and 363 (2.464), 310 (2.144) nm respectively. R1 absorbed at 482 (1.139), 380
(3.866), 313 (3.967) in its 1.0 × 10−3 M DMSO solution, while the R2, although it showed
variation in its absorption pattern in DMSO as compared to nujol, it did not absorb in the
range of ∼480–490 nm. Since R1 showed a major change in its UV-Vis spectrum upon
going from nujol to DMSO in the form of a solvent assisted band in 480–490 nm, it was
further studied in a series of solvents of varying polarity. The absorption bands at 488, 482,
and 479 nm in DMF, DMSO, and acetone, respectively, were not seen in nujol, acetonitrile,
and chloroform spectra of R1 (Table 3, Figure 3). Hence, it is quite clear that the polarity
of these solvents played a crucial role in the genesis of this band, hence justifying the term
solvent-assisted band. The polar solvents have been reported in the literature, taking part
in the formation of intermolecular hydrogen bonding with receptors having NH, SH, and
OH groups.27 At the same time, a considerable hyperchromic shift was also observed in the
same solvent-assisted band on moving from acetone–DMSO–DMF (Figure 3).
Nevertheless, it is also worth mentioning that the solvent polarity alone cannot be
taken as the sole factor, because in the case of acetonitrile, where the polarity index is
higher than for acetone, this solvent-assisted band was not observed. Yet another reason
Figure 2 Energy-minimized structure of the hydrazo and azo form of R1.

1824
K. K. UPADHYAY ET AL.
Table 3 UV-Vis absorption bands of R1 and R2 in solid and solution state
UV-Vis bands (nm)
S. No.
Medium
Nujol
DMSO
DMF
Acetonitrile
Acetone
Chloroform
Receptor R1
Receptor R2
1.
2.
3.
4.
5.
6.
−, 379 (0.7202)
363 (2.464), 310 (2.144)
336 (3.786), 311 (3.890)
339 (3.688)
334 (3.830), 301 (3.939)
351 (3.809)
482 (1.139), 380 (3.866), 313 (3.967)
488 (1.354), 375 (3.836), 336 (3.88)
−, 380 (3.410), 315 (3.652)
479 (0.981), 371 (3.592), 351 (3.707)
−, 381 (3.386), 316 (3.609)
373 (3.365), 308 (3.857)
that is expected to be operating here is supramolecular interactions of DMF, DMSO, and
acetone through hydrogen bonding between NH at the N-2 position of R1 and X O of
the above solvents where X C for DMF and acetone, while X S for DMSO as shown in
Scheme 1.
Scheme 1 Interaction of solvent molecule with R1 through hydrogen bonding.
Metal Binding Studies
In order to study the naked-eye sensing behavior of R1, its 1 × 10−3 M DMSO
solution was treated separately with 1 equivalent of a variety of metal ions. Only Hg2+
showed naked-eye change with R1, i.e., from orange to olive green (Figure 4a).
It is quite clear that among the chosen range of metal ions, only Hg2+ produced a
visible change from orange to olive green, leading to the disappearance of 482 nm band
in the UV-Vis spectrum of R1. The summary of the corresponding spectral changes in the
UV-Vis absorption characteristics of R1 on its interaction with Hg2+ along with Zn2+and
Cd2+ have been given in Table 4.
The vanishing of the 482 nm band with Hg2+ may be understood in terms of its soft
nature,28,29 and hence its preferential coordination with sulfur atom of thiazole and one of
the oxygen atom of vicinal SO2 leading to formation of six-membered chelate (Scheme 2).
Although oxygen is a hard donor and not preferred generally by Hg2+, the formation of a
stable six-membered chelate is the driving force here,30 and in the literature, there are many
instances where mercury did bind with oxygen,31,32 though not as strongly as with sulfur.

SENSING OF Hg2+ BY A CHEMORECEPTOR
1825
CH3
H3C
S
H3C
CH3
O
O
S
H
N
O
H
N
N
H
N
H
N
O
S
N
OCH2CH3
OCH2CH3
O
O
OCH2CH3
N
C
S
N C
S
O
S
O
OCH2CH3
O
O
HgCl2
O
H
N
H
N
N
O
O
S
OCH2CH3
N
C
+
S
S
O
OCH2CH3
H3C
CH3
O
Hg
Cl
Cl
Scheme 2 Mechanism of Hg2+ binding with R1.
This chelation was further supported by the bathochromic shifts of 24 and 4 nm in 313 nm
and 380 nm absorption bands respectively upon interaction of R1 with Hg2+
.
The chelate formation between R1 and Hg2+ (Scheme 2) leads to transfer of some
electron density back from Hg2+ to the donors of the receptor R1 (M L π bonding)
and hence enriching the electron density on NH leading to breaking of its intermolecular
hydrogen bonding with DMSO, which is finally observed in the form of vanishing of
482 nm band. This view was further supported by our 1H NMR titrimetric studies (described
1
below under the section H NMR spectral studies), where NH at N-2 underwent upfield
shifting (towards TMS) on the concomitant additions of different equivalents of Hg2+ to
the 1 × 10−3 M DMSO-d6 solution of R1.
The UV-Vis spectral changes in the absorption characteristics of R1 on separate
additions of Zn2+ and Cd2+ were almost identical except the change in 313 nm absorption
band of R1 (Table 4). For Zn2+, there was a bathochromic shift of 22 nm in the 313 nm
absorption band, while no changes in 380 nm and 482 nm absorption band of R1. This
Figure 3 Absorption spectra of 1 × 10−3 M DMSO solution of R1 in different solvents (410–600 nm) and their
corresponding color changes.

1826
K. K. UPADHYAY ET AL.
Figure 4 (a) Color of 1 × 10−3 M DMSO solution of R1 up on addition of 1.0 equivalent of different metal ions
(where Mn+ = Na+, K+, Mg2+, Ca2+, Ba2+, Pb2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+). (b)
Absorption spectra of 1 × 10−3 M DMSO solution of R1 on addition of 1.0 equivalent each of Zn2+, Cd2+, and
Hg2+ in the range of 420–600 nm (the spectral patterns for other metal ions have not been shown to avoid the
overcrowding).
bathochromic shift as well as non-vanishing of 482 nm band in the case of Zn2+ may be
understood in terms of the hard nature of Zn2+ in comparison to Hg2+, hence there was
no participation of sulfur of thiazole. The above-mentioned red shift of 22 nm in 313 nm
band of R1 may be a consequence of the formation of six-membered chelate of Zn2+
with nitrogen (N-1) and one of the oxygen of SO2. It seems that Cd2+ was probably not
able to develop any binding with R1, because there was virtually no change in its UV-Vis
Table 4 UV-Vis spectral bands and color changes of 1 × 10−3 M DMSO solution of R1 upon separate addition
of 1 equivalent each of Zn2+, Cd2+, and Hg2+ as their chloride salts
Species
Color
Observed wavelength (nm)
R1
Orange
Orange
Orange
Olive green
482 (1.139), 380 (3.866), 313 (3.967)
482 (1.117), 380 (3.845), 335 (3.843)
482 (1.116), 380 (3.837), 316 (3.976)
482 (0.692), 384 (3.732), 337 (3.823)
R1:Zn2+
R1:Cd2+
R1:Hg2+

SENSING OF Hg2+ BY A CHEMORECEPTOR
1827
Figure 5 Absorption spectra of 1 × 10−3 M DMSO solution of R1 upon concomitant addition of 0.0–2.0
equivalent of Hg2+ (420–600 nm).
absorption characteristics upon the addition of its one equivalent excepting a marginal
bathochromic shift of 3 nm in the 313 nm absorption band of R1 (Table 4).
In order to have a quantitative idea about the binding of Hg2+ with R1, a UV-
Vis titration was performed between the 1 × 10−3 M DMSO solution of R1 and HgCl2
(0.0–2.0 equivalent as DMSO solution). The interaction between R1 and Hg2+ was studied
by monitoring the regular changes in the 482 nm absorption band of the receptor in terms
of its intensity on the concomitant additions of different equivalent of Hg2+. The full family
of titration curves has been shown below (Figure 5).
The titration data were best fitted for 1:1 host–guest binding, and the binding constant
was evaluated to be Kass = (1.34 0.28) × 103 M−1. The corresponding binding curve for
1:1 fitting is shown in Figure 6.
The host–guest binding stoichiometry was further confirmed through Job’s plot
(Figure 7) as well as from mass spectral pattern of R1 + HgCl2 (Figure 8).
Here it is worth mentioning that the very high boiling point of DMSO (189◦C)
restricted us from developing single crystals of the above mercury complex of suitable
sizes from an XRD point of view.
Figure 6 Binding curve for 1:1 binding between Hg2+ as its chloride salt and R1 in DMSO.

1828
K. K. UPADHYAY ET AL.
Figure 7 Job’s plot showing 1:1 binding stoichiometry between Hg2+ and R1.
The similar separate additions of the entire range of metal ions (as mentioned above)
as their chloride salts to the 1 × 10−3 M DMSO solution of R2 did not produce any visual
changes through the naked eye, although spectral changes in the UV range (lower than
400 nm) were observed (Figure 9 and Table 5). Since R2 did not show any solventassisted
band in the visible region as by R1 in DMSO/other polar aprotic solvents mentioned above,
no metal ion could have a chance to modulate it, finally resulting in no visible color changes.
Hence R2 did not serve as a naked-eye receptor.
1H NMR Spectral Studies
1
The H NMR spectral data for R1 were recorded in DMSO-d6. No signal for the
azoic proton was observed, even upon scanning up to 20.00 δ ppm, which is in accordance
Figure 8 Mass spectrum of R1 +HgCl2 in DMSO.

SENSING OF Hg2+ BY A CHEMORECEPTOR
1829
Figure 9 Color of 1 × 10−3 M DMSO solution of R2 upon addition of 1.0 equivalent of different metal ion
(where Mn+ = Na+, K+, Mg2+, Ca2+, Ba2+, Pb2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+
,
Al3+).
with our conclusion regarding the structure of R1 on the basis of X-ray data and DFT
calculations. Out of two NHs of the R1, the proton at N-2 absorbed at downfield (12.68
δ ppm) in comparison to N-3 (11.88 δ ppm). This was confirmed through dilution study
with CDCl3. (For the full range NMR spectra, see the Supplemental Materials, Figure S3).
1
The H NMR spectral titration data of R1 against different equivalents of Hg2+
1
(0.0–1.0) are presented in Table 6, and the truncated H NMR spectra in the range of
9.0–13.0 δ ppm have been given in Figure 10 (for the full range NMR spectra, see the
Supplemental Materials, Figure S4). The most important change is the gradual vanishing of
NH at N-2 position at 12.68 δ ppm and the origin of a new signal at ∼9.90 δ ppm. Upon the
full addition of 1.0 equivalent of Hg2+, the NH at N-2 position was totally vanished, and a
new peak at 9.88 δ ppm was fully developed. In fact, this is nothing but an upfield shifting
of NH at the N-2 position on the coordination of Hg2+ with appropriate donors of R1, as
has been proposed above on the basis of UV-Vis studies for the same. The co-ordination
of R1 with Hg2+ leads to its desolvation, i.e., removal of DMSO-d6 from NH at the N-2
position in R1 as concluded above on the basis of UV-Vis spectral titrations.
Table 5 UV-Vis spectral bands and color changes of 1 × 10−3 M DMSO solution of R2 upon separate addition
of 1.0 equivalent each of Zn2+, Cd2+, and Hg2+ as their chloride salts
Species
Color
Observed wavelength (nm)
R2
Olive green
Olive green
Olive green
Olive green
363 (3.486), 336 (3.786), 311 (3.890)
365 (3.551), 337 (3.773), 303 (3.835)
369 (3.386), 319 (3.812), 307 (3.826)
365 (3.427), 337 (3.603), 313 (3.728)
R2:Zn2+
R2:Cd2+
R2:Hg2+

1830
K. K. UPADHYAY ET AL.
Figure 10 Truncated 1H NMR spectra (9.0–13.0 δ ppm) of R1 on the concomitant addition of 0.0–1.0 equivalent
of Hg2+ as its chloride salt to the 1 × 10−3 M DMSO-d6 solution of R1.
EXPERIMENTAL
Apparatus
Melting points were recorded on a capillary melting point apparatus and is uncor-
rected. The IR spectra were recorded in KBr on a Varian 3100 FT-IR spectrophotometer.
The NMR spectral studies along with titrations were performed on a JEOL AL 300 FT
NMR spectrometer. The chemical shifts are reported in δ values in the units of ppm relative
to the tetramethylsilane (TMS) as the internal standard. The UV-Vis spectral studies along
Table 6 1H NMR spectral titration data of 1 × 10−3 M DMSO-d6 solution of R1 upon the concomitant addition
of Hg2+ (0.0–1.0 equivalent) as its chloride salt (9.0–13.0 δ ppm)
Chemical shifts in (δ ppm)
Different equiv. of Hg2+
N
H(N-2)
N
H(N-3)
New peak
0.00
0.25
0.50
0.75
1.00
12.68
12.69
12.69
12.69
–
11.88
11.88
11.88
11.88
11.88
–
9.94
9.93
9.91
9.88

SENSING OF Hg2+ BY A CHEMORECEPTOR
1831
with titrations were performed on UV-1700 pharmaspec, UV-Vis spectrophotometer. Mass
spectrometric analysis was carried out on a MDS Sciex API 2000 LCMS spectrometer.
Reagents and Chemicals
The sulfathiazole and sulfamethoxazole were purchased from Sigma (USA) and used
without further purification. The sodium nitrite and sodium acetate both were purchased
from Qualigens Fine Chemicals, Mumbai, India. Diethyl malonate was purchased from
Spectrochem Pvt. Ltd., Mumbai, India. Hydrochloric acid was purchased from Central
Drug House (P) Ltd., Delhi, India. The chloroform, DMF, acetone, acetonitrile, and DMSO
of spectroscopic grade were purchased from Central Drug House (P) Ltd., Delhi, India.
Reaction Scheme
R1 and R2 were synthesized according to the procedure in the literature.33 The
various spectra of the synthesized compounds are given as Figures S5–S8 (Supplemental
Materials).
H
N
O
S
H
N
O
S
NaNO2 +HCl
(0-50C)
+
N
R
NCl
R
NH2
O
O
O
OC2H5
OC2H5
CH3COONa
(0-50C)
H2C
O
O
C
O
H
N
O
S
H
N
OC2H5
OC2H5
R
N
O
O
N
N
and
R=
H3C
S
R1
Scheme 3 Synthesis of R1 and R2.
R2

1832
K. K. UPADHYAY ET AL.
2-{[4-(Thiazol-2-ylsulfamoyl)-phenyl]-hydrazono}-malonic acid diethyl
ester (R1). Yield = 82%; mp = 174–176◦C; CHN calculated for C16H18N4O6S2; C =
45.06%, H = 4.25%, N = 13.14% and found; C = 45.58%, H = 4.24%, N = 12.66%;
IR (KBr/cm−1): 3453 (νN of HN N C<), 3152 (νN of NHSO2), 2923 (νC H),
H
H
1725 (ν>C O of COOC2H5), 1535 (ν>C
νC H aromatic ring), 1412 (NH bending vib),
N +
1
1300, 1231, 1145 (νSO2), 645 (νC S); H NMR (DMSO-d6/δ ppm): 12.68 (1H, NH
at N-2), 11.88 (1H, NH at N-3), 6.81–7.79 (6H, Ar H), 4.21–4.34 (4H, 2 >CH2),
1.03–1.31 (6H, 2 CH3);13C NMR (DMSO-d6/δ ppm): 162.01, 161.26 [C(OEt) O],
145.09, 136.54, 127.45, 124.29, 124.07, 114.73, 108.01 (Aromatic C), 61.41, 60.93
(>CH2), 13.96, 13.73 ( CH3); Mass (M + H): 427.3.
2-{[4-(5-Methyl-isoxazol-3-ylsulfamoyl)-phenyl]-hydrazono}-malonic acid
diethyl ester (R2). Yield = 79%; mp = 128–130◦C; CHN calculated for C17H20N4O7S;
C = 48.11%, H = 4.75%, N = 13.20% and found; C = 49.64%, H = 4.93%, N = 12.63%; IR
(KBr/cm−1): 3445 (νN H of HN N C<), 3171 (νN H of NHSO2), 2985 (νC H), 1679,
1724 (ν>C O of COOC2H5), 1531 (ν>C N +νC H aromatic ring), 1469 (NH bending vib),
1
1314, 1161 (νSO2), 690 (νC S); H NMR (DMSO d6/δ ppm): 11.88 (1H, NH at N-3),
11.35 (1H, NH at N-2), 6.12–7.83 (5H, Ar H), 4.22–4.35 (4H, 2 > CH2), 1.25–2.50
(9H, 3 CH3); 13C NMR (DMSO-d6/δ ppm): 170.08 (>C N , exo), 161.94, 161.14
[C(OEt) O], 157.72 (>C N , endo) 146.11, 133.35, 128.56, 124.85, 114.96 (Aromatic
C),61.53, 61.05 (>CH2), 13.96, 13.73, 11.98 ( CH3); Mass (M+H): 425.0.
X-Ray Diffraction Studies
Suitable crystals of R1 could not be developed, despite our best possible efforts.
Crystals of R2 of suitable sizes from XRD point of view were grown by the slow evaporation
of their supersaturated solutions in acetone and leaving for few days. A crystal of suitable
size of R2 was selected from the mother liquor and immersed in paraffin oil, and then it was
mounted on the tip of a glass fiber and cemented using epoxy resin. Single crystal X-ray data
were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite-
monochromated MoKα radiation (λ = 0.71069 Å). The linear absorption coefficients,
scattering factors for the atoms, and the anomalous dispersion corrections were taken from
International Tables for X-Ray Crystallography. The data integration and reduction were
processed with SAINT34 software. An empirical absorption correction was applied to the
collected reflections with SADABS35 using XPREP.36 The structure was solved by direct
methods using SHELXTL37 and was refined on F2 by full-matrix least-squares technique
using the SHELXL-9725 program package. For all cases, the non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were geometrically fixed and treated as riding
atoms using SHELXL default parameters. The crystal data for R2 were deposited to the
Cambridge Crystallographic Data Centre as CCDC No-739701.38
Density Functional Theory (DFT) Calculations
The hydrazo and azo tautomeric structures for R1 and were optimized using density
functional theory (DFT) at the B3LYP/6-31G∗∗ level39–41 employing the Windows versions
of the Gaussian 03 (G03W)42 program.

SENSING OF Hg2+ BY A CHEMORECEPTOR
1833
CONLUSIONS
1
Hence, on the basis of UV-Vis, H NMR titrations, and mass spectral pattern of
R1 with HgCl2 in DMSO, it was revealed that the supramolecular association of DMSO
with R1 through intermolecular hydrogen bonding leads to the occurrence of a solvent-
assisted band at 482 nm. The addition of Hg2+ to this solution leads to the formation of
its six-membered chelate and rupture of hydrogen bonding between DMSO and R1, which
ultimately leads to vanishing of 482 nm solvent-assisted bands and visual color change of
R1.
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