Novel colorimetric anion sensors based on N-acetylglyoxylic amides containing nitrophenyl signalling units

Article abstract of DOI:10.1016/j.saa.2013.11.108

N-acetylglyoxylic amides 4 and 5 bearing pendant 4-nitrophenyl and 2,4-dinitrophenyl groups respectively were synthesized and evaluated as anion sensors. A crystal structure of 4 was obtained by X-ray crystallography. Compounds 4 and 5 behaved as colorimetric sensors for CN^- and F ^-, and exhibited naked eye-detectable color changes upon the addition of these anions. The chromogenic properties of 4 and 5 were assessed by UV-Vis and ¹H NMR spectroscopy.

Full text of DOI:10.1016/j.saa.2013.11.108

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 662–669
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Novel colorimetric anion sensors based on N-acetylglyoxylic amides
containing nitrophenyl signalling units
Venty Suryanti a,b, Mohan Bhadbhade c, Har Mohindra Chawla d, Ethan Howe a, Pall Thordarson a,
David StC Black a, Naresh Kumar a,
⇑
a School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia
b Department of Chemistry, The University of Sebelas Maret, Surakarta, Jawa Tengah 57126, Indonesia
c Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, NSW 2052, Australia
d Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
N-acetylglyoxylic amides based anion colorimetric sensors 4 and 5 demonstrated recognition of CNꢁ and
Fꢁ via hydrogen bonds and subsequent NH deprotonation which led to the intramolecular charge-
transfer resulting in visible color changes.
ꢀ
We designed and synthesized
N-acetylglyoxylic amide-based
colorimetric anion sensors.
ꢀ
ꢀ
Sensors 4 and 5 showed obvious color
changes upon addition of CNꢁ and Fꢁ.
CNꢁ and Fꢁ formed hydrogen bonds
with the receptors and induced
subsequent deprotonation.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2013
Received in revised form 21 November 2013
Accepted 25 November 2013
Available online 4 December 2013
N-acetylglyoxylic amides 4 and 5 bearing pendant 4-nitrophenyl and 2,4-dinitrophenyl groups respec-
tively were synthesized and evaluated as anion sensors. A crystal structure of 4 was obtained by X-ray
crystallography. Compounds 4 and 5 behaved as colorimetric sensors for CNꢁ and Fꢁ, and exhibited naked
eye-detectable color changes upon the addition of these anions. The chromogenic properties of 4 and 5
were assessed by UV–Vis and 1H NMR spectroscopy.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Glyoxylamides
Anion receptors
Colorimetric sensors
Hydrogen bonding
Deprotonation
Introduction
increasing attention has been focused upon the development of
sensors capable of selectively recognizing and sensing anions. A
Important environmental and human health consequences can
result from the presence of excess anions [1–9]. Therefore,
number of molecular receptors that contain different types of
anion-binding groups have been reported [1,10,11]. In particular,
NH fragments, such as amide, urea/thiourea [12], pyrrole [13]
and imidazolium [14] groups that can act as H-bond donors toward
anions have been widely used as anion binding sites for
⇑
Corresponding author. Tel.: +61 2 9385 4698; fax: +61 2 9385 6141.
E-mail address: n.kumar@unsw.edu.au (N. Kumar).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.11.108

V. Suryanti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 662–669
663
recognition and sensing applications. Typically, anions are recog-
Synthesis of 2-(2-acetamidophenyl)-N-(2,4-dinitrophenyl)-2-
oxoacetamide 5
nized via hydrogen bonding or deprotonation of protons on the
receptor-NH group. Isatin has been used as a scaffold for anion rec-
ognition. However, only limited number of isatin derivatives have
been reported as colorimetric anion sensors and anion receptors
[15–18].
This compound was prepared by the same method as compound
4
from N-acetylisatin (5.29 mmol, 1.01 g), 2,4-dinitroaniline
(5.29 mmol, 0.97 g) and triethylamine (2.5 mL) as an off-white solid
(1.11 g, 56%). M.p. 174 °C; UV(MeOH): kmax 228 (e -
50,086 cmꢁ1
Glyoxylamides represent ideal candidates for the potential
development of anion sensors due to their NH hydrogen bonding
donor group. Nevertheless, the glyoxylamide moiety has not been
used previously for the anion recognition. It was therefore of
interest to introduce a chromophoric group into N-acetylglyoxylic
amides derived from N-acetylisatin in order to obtain novel color-
imetric sensors for anions. In particular, nitrophenyl groups were
selected as signalling units because they could be covalently
linked to the glyoxylamide NH moiety to enhance both hydro-
gen-bond donor tendency and acidity of the NH group. Further-
more, the optical properties of the chromogenic nitrophenyl
fragment would likely be altered following anion binding, thus
providing colorimetric and spectral sensing of the recognition
event [19]. It was envisaged that the target anion sensors could
be obtained by ring-opening of N-acetylisatins with nitrophenyl
amines.
Mꢁ1), 261 (29,350), 330 (26,650); IR (KBr): mmax 3448, 3336, 3226,
3110, 2770, 2497, 1957, 1739, 1632, 1581, 1520, 1453, 1388,
1324, 1262, 1168, 1127, 1061, 982, 925, 834, 743 cmꢁ1 1H NMR
,
(300 MHz, acetone-d6): d 2.20 (s, 3H, COCH3), 7.23–7.28 (m, 2H,
Ar-H), 7.67–7.74 (m, 1H, ArH), 7.82 (dd, J = 8.0, 1.6 Hz, 1H, ArH),
7.94 (s, 1H, COCONH), 8.22 (dd, J = 9.3, 2.7 Hz, 1H, ArH), 8.61(dd,
J = 8.5, 0.9 Hz, 1H, ArH), 8.92 (d, J = 2.7 Hz, 1H, ArH), 10.87 (s, 1H,
NHCO); 13C NMR (75 MHz, acetone-d6): d 24.10 (COCH3), 117.84,
119.37, 120.19, 141.79, 149.5 (5 ꢂ ArC), 119.45, 120.27, 122.60,
123.14, 129.24, 133.20, 136.25 (7 ꢂ ArCH), 164.16, 168.78,
190.93 (3 ꢂ C@O); MS (TOF–ESI) m/z calculated for C16H13N4O7
(M + H)+ 373.07. Found 373.08; Anal. Calcd. for C16H12N4O7.
H2O: C = 49.24; H = 3.62; N = 14.35. Found: C, 49.57; H, 3.49; N,
14.26%.
Anion binding studies
Titration experiments of compounds 4 or 5 with anions were per-
formed in acetone with a constant concentration of compounds 4 or 5
through the stepwise addition of a standard solution containing both
the respective compound and anions (as tetrabutylammonium salts).
Experimental
General
Melting points were obtained using a Mel-Temp melting point
apparatus. Infrared spectra were recorded with a Thermo Nicolet
370 FTIR spectrometer. UV–Vis spectra were recorded using a
Varian Cary 100 Scan spectrometer. 1H and 13C NMR spectra were
recorded on a Bruker Avance DPX300 (300 MHz) or a Bruker
DMX600 (600 MHz) spectrometer and were internally referenced
relative to the solvent nuclei. Mass spectrometric analysis was car-
ried out at the Biomedical Mass Spectrometry Facility, UNSW.
Microanalyses were performed on a Carlo Erba Elemental Analyzer
EA 1108 at the Campbell Microanalytical Laboratory, University of
Otago, New Zealand.
Structure determination
A suitable single crystal of 4, selected under the polarizing
microscope (Leica M165Z), was picked up on a MicroMount (MiTe-
Gen, USA) consisting of a thin polymer tip with a wicking aperture.
The X-ray diffraction measurement was carried out on a Bruker
KAPPA APEX II CCD diffractometer at 155 K using graphite-mono-
chromated Mo-Ka radiation (k = 0.710723 Å). The single crystal,
mounted on the goniometer using cryo loops for intensity mea-
surements, was coated with paraffin oil and then quickly trans-
ferred to the cold stream using an Oxford Cryo stream
attachment. Symmetry related absorption corrections using the
program SADABS [20] were applied and the data were corrected
for Lorentz and polarization effects using Bruker APEX2 software
[21]. Structure was solved by direct methods and the full-matrix
least-squares refinement was carried out using SHELXL [22]. The
non-hydrogen atoms were refined anisotropically. The molecular
graphic was generated using Mercury [23].
Synthesis
Synthesis of 2-(2-acetamidophenyl)-N-(4-nitrophenyl)-2-
oxoacetamide 4
A mixture of N-acetylisatin (5.29 mmol, 1.02 g), 4-nitroaniline
(5.29 mmol, 0.73 g) and triethylamine (2.5 mL) in dry dichlorometh-
ane (50 mL) was stirred at room temperature for 24 h under
nitrogen. The resulting precipitate was collected and washed with
hydrochloric acid (0.5 M, 75 mL) and subsequently with
dichloromethane (75 mL). The title compound was obtained as an
off-white solid (1.09 g, 63%). M.p. 232 °C; UV(MeOH): kmax 230
Results and discussion
Synthesis of N-nitrophenylglyoxylic amides
(e
28,050 cm–1 Mꢁ1), 321 (22,050); IR (KBr): mmax 3244, 3213,
3051, 3018, 1702, 1671, 1654, 1591, 1542, 1508, 1453, 1410, 1374,
1335, 1296, 1243, 1165, 1041, 997, 853, 763 cmꢁ1 1H NMR
The starting material N-acetylisatin 1 was prepared by treat-
ment of isatin with acetic anhydride at reflux for 4 h as previously
described by Silva et al. [24]. N-acetylisatin 1 was then reacted
with 4-nitroaniline 2 or 2,4-dinitroaniline 3 in the presence of tri-
ethylamine at room temperature for 24 h in dry dichloromethane
under nitrogen (Scheme 1). The desired products 4 and 5 were ob-
tained in 63% and 56% yields, respectively.
;
(300 MHz, acetone-d6): d 2.13 (s, 3H, COCH3), 7.24–7.28 (m, 1H,
ArH), 7.64–7.69 (m, 1H, ArH), 7.97 (dd, J = 8.0, 1.6 Hz, 1H, ArH),
8.13 (d, J = 9.3 Hz, 2H, ArH), 8.18 (dd, J = 8.4, 1.0 Hz, 1H, ArH), 8.32
(d, J = 9.3 Hz, 2H, ArH), 10.42 (s, 1H, COCONH), 10.48 (s, 1H, NHCO),
13C NMR (75 MHz, acetone-d6): d 23.6 (COCH3), 119.4, 120.71, 120.4,
123.0, 124.8, 132.7, 135.0 (7 ꢂ ArCH), 119.9, 124.3, 125.3, 143.9
(4 ꢂ ArC), 162.0, 168.8, 189.9 (3 ꢂ C@O); HRMS (ESI) m/z calculated
for C16H14N3O5 (M + H)+ 328.0855. Found 328.0922; Anal. Calcd. for
Analysis of the N-nitrophenylglyoxylic amides 4 and 5 by 1H
NMR spectroscopy revealed the characteristic singlet resonance
for glyoxylamide NH protons at 10.42 and 7.94 ppm respectively,
which is indicative of successful ring opening of N-acetylisatin 1.
The 1H NMR spectra also revealed resonances for aromatic protons
with an integration of eight and seven aromatic protons for
C16H13N3O5: C, 58.72; H, 4.00; N, 12.84. Found: C, 58.98; H, 3.95; N,
13.00%.

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V. Suryanti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 662–669
Scheme 1. Reagents and conditions: triethylamine, dry CH2Cl2, N2, r.t., 24 h.
intercarbonyl distances in the range of 2.526–3.101 Å, and C@OꢃꢃꢃC
and OꢃꢃꢃC@O angles from 77° to 109°. The molecule also exhibits
strong and weak hydrogen bonding interactions (NAHꢃꢃꢃO and
CAHꢃꢃꢃO). The D (Donor)ꢃꢃꢃA (Acceptor) and HꢃꢃꢃA bond lengths
range from 2.696(3) to 2.884(3) and 2.31 to 2.55 Å, respectively.
The DAHꢃꢃꢃO angles are in the range of 97–116°.
Anion recognition capacity of N-nitrophenylglyoxylic amides
Visual observation of N-nitrophenylglyoxylic amides
A solution of 4 (2.3 ꢂ 10ꢁ5 M) changed from colorless to light
yellow upon the addition of 0.5 eq. of Fꢁ or CNꢁ as their
tetrabutylammonium salts. The coloration of the solution was
intensified with increasing anion concentration. On the other hand,
treatment of a solution of 5 (2.3 ꢂ 10ꢁ5 M) with 2 eq. of CNꢁ re-
sulted in a color change from colorless to light purple. However,
Fꢁ required a higher concentration (4 eq.) to reach a similar color
intensity. Similar to 4, the solution exhibited a gradual darkening
with increasing anion concentration. The color changes of 4 and
5 upon the addition of Fꢁ and CNꢁ ions was detectable by the
naked-eye. Representative photographs showing the color change
of solutions of 4 and 5 induced by Fꢁ ions are shown in Figs. 2
and 3, respectively.
Fig. 1. X-ray crystal diagram of 4 showing intramolecular interactions.
Table 1
Crystal data collection and structure refinement parameters of crystal forms 4.
Chemical formula
Mr
C16H13N3O5
327.29
Crystal system
Space group
a, b, c (Å)
Monoclinic
Pc
Under similar experimental conditions, the addition of H2POꢁ to
7.3289 (9), 10.7810 (13),
10.0984 (11)
110.886 (3)
745.47 (15)
2
4
solutions of compounds 4 or 5 did not result in any color change
apart from a faint yellow coloration observed at higher concentra-
tion (9.5 ꢂ 10ꢁ5 M, ca. 4 eq.). Less basic anions, such as HSOꢁ4 , Clꢁ,
b (°)
V (Å3)
Z
l
(mmꢁ1
)
0.11
Brꢁ, NO2ꢁ NO3ꢁ, Iꢁ and ClOꢁ4 , also induced no detectable color
,
changes. Further studies on Fꢁ and CNꢁ recognition by compounds
4 and 5 were then carried out using UV–Vis and 1H NMR
spectroscopy.
Crystal size (mm)
Tmin, Tmax
No. of measured, independent and observed
0.35 ꢂ 0.27 ꢂ 0.12
0.962, 0.986
5008, 2104, 1979
[I > 2
Rint
R[F2 > 2
r
(I)] reflections
0.018
0.026, 0.106, 1.22
r
(F2)], wR(F2), S
Anion recognition studies by UV–Vis spectroscopy
No. of reflections
No. of parameters
No. of restraints
2104
218
2
0.22, ꢁ0.28
The ability of compounds 4 and 5 to recognize Fꢁ and CNꢁ were
investigated by monitoring UV–Vis spectroscopy. Fig. 4 shows the
change in UV–Vis spectra of 4 (2.3 ꢂ 10ꢁ5 M) during titration with
Fꢁ. Upon addition of Fꢁ, the peak at 329 nm decreased in intensity,
while a new peak at 380 nm and a shoulder at 425 nm appeared.
These spectral features were accompanied by a visual color change
D
imax
,
D
imin (e Åꢁ3
)
compounds 4 and 5, respectively. The aromatic protons in the
nitrophenyl ring of compound 4 were identified by the presence
of doublets at 8.32 and 8.13 ppm, with a coupling constant of
9.3 Hz. The aromatic protons in the nitrophenyl ring of compound
5 were identified by the presence of a doublet at 8.92 ppm
(J = 2.7 Hz), a doublet of doublet at 8.22 ppm (J = 9.3, 2.7 Hz) and
a multiplet at 7.67–7.74 ppm.
The structure of N-mononitrophenylglyoxylic amide 4 was fur-
ther confirmed by single crystal X-ray structure determination
(Fig. 1). The crystal of 4 was obtained by the slow evaporation of
an acetone solution at room temperature. Experimental details
for data collection and structure refinement are summarized in
Table 1. The molecular structure of 4 along with intramolecular
hydrogen bonding interactions are shown in Fig. 1. C@OꢃꢃꢃC@O
dipolar contacts involving three C@O groups were observed, with
0
0.5
1
2
3
4
5
6
equivalents of F¯
Fig. 2. Photograph image of compound 4 (2.3 ꢂ 10ꢁ5 M) in acetone with different
concentrations of Fꢁ.

V. Suryanti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 662–669
665
0
1
2
3
4
5
6
7
8
equivalents of Fˉ
Fig. 3. Photograph image of compound 5 (2.3 ꢂ 10ꢁ5 M) in acetone with different
concentrations of Fꢁ.
Fig. 5. Changes in UV–Vis spectra for compound 4 in acetone (2.3 ꢂ 10ꢁ5 M) upon
addition of CNꢁ.
Fig. 4. Changes in UV–Vis spectra for compound 4 in acetone (2.3 ꢂ 10ꢁ5 M) upon
addition of Fꢁ.
Fig. 6. Changes in UV–Vis spectra for 5 in acetone (2.3 ꢂ 10ꢁ5 M) upon addition of
Fꢁ.
of the solution from colorless to yellow. The development of the
new absorption peaks upon increasing concentration of Fꢁ is
attributed to the deprotonation of compound 4 and subsequent
intramolecular charge-transfer transitions (Scheme 2).
UV–Vis spectral changes were also observed upon addition of
CNꢁ to a solution of 4 (Fig. 5). In the absence of anion, compound
4 exhibited a strong absorbance at 332 nm. As the concentration
of CNꢁ was increased, the absorbance intensity at 332 nm de-
creased and a new absorbance peak appeared at 407 nm. This
was accompanied by a visual color change from colorless to yellow.
The development of new bands was similarly attributed to the
deprotonation of compound 4 and subsequent intramolecular
charge-transfer transitions.
The deprotonation of compound 4 took place at low concentra-
tions of both Fꢁ and CNꢁ. The color change could be detectable by
the naked eye from 0.5 eq. of anions (Fig. 2). In the UV–Vis spectra,
the shoulder at 425 nm and the peak at 407 nm appeared at less
than 1 eq. of Fꢁ and CNꢁ ions, respectively. A clear isosbestic point
was not observed. Instead, the spectra showed an approximate
isosbestic point at 350 nm and 353 nm for Fꢁ and CNꢁ respectively,
which indicated that deprotonation was not complete and that the
hydrogen-bonded complex and the deprotonated compound 4
were both present. These results suggest that Fꢁ and CNꢁ initially
formed hydrogen bonding interactions with compound 4, which
was followed by deprotonation of compound 4 upon further addi-
tion of anions.
Fig. 6 shows the changes in the UV–Vis spectra of compound 5
(2.3 ꢂ 10ꢁ5 M) upon titration with Fꢁ in acetone. The UV–Vis spec-
trum of free 5 exhibited a band at 340 nm and a shoulder at
382 nm. With the stepwise addition of Fꢁ to 5, the absorbance at
340 nm decreased in intensity, the shoulder at 382 nm increased
in intensity and a new band at 530 nm developed. These features
could be attributed to fluoride induced deprotonation of 5 and
resulting intramolecular charge-transfer transitions (Scheme 3).
Similar to compound 4, no isosbestic point was observed. Higher
concentrations of Fꢁ were required for compound 5 to induce
deprotonation, with an obvious color change from colorless to light
purple occurring upon addition of 4 eq. Fꢁ (Fig. 3).
The above observations can be explained by initial hydrogen
bond formation between NH and Fꢁ followed by deprotonation
Scheme 2. Resonance representation of the deprotonated form of compound 4.

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V. Suryanti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 662–669
Scheme 3. Resonance representation of the deprotonated form of compound 5.
mononitrophenyl substituent. Also the mobility of the amide NH
proton would be affected by the presence of nitro groups as repre-
sented in Schemes 2 and 3. The observations can be explained by
increased acidity of the amidic NH in 5 compared with 4, and
would imply that deprotonation would be faster in 5 than in 4. This
would also be consistent with the non-observance of an isosbestic
point was observed on addition of fluoride to 5. Accordingly, while
change in color on addition of fluoride to 4 could be a two-step
process (hydrogen bond formation and ionization), in the case of
5, it could be a direct ionization with very little hydrogen bond
formation.
UV–Vis spectral changes were also observed for the titration of
5 with CNꢁ (Fig. 7). The intensity of the band at 334 nm decreased
upon increasing concentration of CNꢁ, indicating the formation of
hydrogen bonding interactions between compound 5 and CNꢁ.
An isosbestic point at 358 nm was observed during the titration
process. Upon the addition of 1 eq. of CNꢁ, a new shoulder at
385 nm and a new peak at 532 nm appeared, which was accompa-
nied by a color change of the solution from colorless to purple.
These events could be attributed to the deprotonation of the gly-
oxylamide NH in compound 5. We hypothesize that the anion
sensing process first involves hydrogen bonding interactions be-
tween compound 5 and CNꢁ. This is followed by deprotonation
of the glyoxylamide NH upon further addition of CNꢁ.
Fig. 7. Changes in UV–Vis spectra for compound 5 in acetone (2.3 ꢂ 10ꢁ5 M) upon
addition of CNꢁ.
of amidic NH which leads to resonance stabilized structures repre-
sented in Scheme 2. Accordingly one would also expect similar
structural changes in 5, though the color changes are as expected.
The non observance of isosbestic point in 5 and higher concentra-
tion of fluoride required for 5 to observe an intensity ratios similar
to those observed for 4, indicate that the phenomenon is more
complex. This conclusion is reasonable as 5, containing a dinitro-
phenyl substituent is expected to be more acidic than 4, with a
Compounds 4 and 5 were selective for CNꢁ and Fꢁ over the
other anions tested. This can be rationalized on the basis of the
Fig. 8. Partial 1H NMR spectra of compound 4 in acetone-d6 (1.0 ꢂ 10ꢁ2 M) at different concentrations of Fꢁ. The spectra were recorded after the addition of (i) 0, (ii) 1 and (iii)
2 eq. of Fꢁ.

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667
Fig. 9. Partial 1H NMR spectra of compound 5 in acetone-d6 (1.0 ꢂ 10ꢁ2 M) at different concentrations of Fꢁ. The spectra were recorded after the addition of (i) 0, (ii) 1, (iii) 2,
(iv) 3, (v) 4 and (vi) 5 eq. of Fꢁ.
hydrogen bonding ability and/or basicity of the anions. As the
hydrogen bonding ability and basicity of Fꢁ and CNꢁ ions are high-
er than that of the other tested anions, they have a greater ability
to undergo hydrogen bonding interactions and deprotonation
reactions with 4 and 5. In comparison, Fꢁ is a stronger base and
a weaker nucleophile, while CNꢁ is weaker base and a stronger
nucleophile. Therefore, these results suggest that compounds 4
and 5 can be used as selective colorimetric sensors for Fꢁ and
CNꢁ ions.
had taken place. Additionally, the aromatic protons of the nitro-
phenyl ring, initially present as doublets at 8.32 (Ha) and 8.13
(Hb) ppm were shifted upfield to 7.96 and 6.74 ppm, respectively.
This could be attributed to the increased shielding of those protons
upon formation of a quinonoid structure 7 through resonance
(Scheme 2). Shifts in the aromatic protons of the isatin moiety
were also observed, where peaks at 7.24–7.28 (Hc), 7.64–7.69
(Hd) and 7.97 (He) ppm were shifted upfield to 7.06–7.12, 7.46–
7.51 and 7.82 ppm, respectively. Moreover, the peak at 8.18 (Hf)
ppm was shifted downfield to 8.62 ppm.
Anion recognition studies by 1H NMR spectroscopy
As previously mentioned, the deprotonation of 4 occurred quite
early in the Fꢁ titration, as revealed by the appearance of the band
at 380 in the UV–Vis spectrum at less than 1 eq. of Fꢁ. This obser-
vation corroborated with the 1H NMR titration results, where the
addition of 1 eq. of Fꢁ resulted in ca. 50% deprotonation of com-
pound 4. It was initially presumed that deprotonation of 4 upon
the addition of 2 eq. of Fꢁ would be accompanied by the formation
of the hydrogen-bonded dimer HFꢁ2 . However, the characteristic
signal for HFꢁ2 at 16.1 ppm was absent even after the addition of
2 eq. of Fꢁ, indicating no formation of HFꢁ2 [25,26]. Thus, we
hypothesize that the deprotonation of 4 upon the additional of
the second equivalent of fluoride ion instead resulted in the release
of hydrogen fluoride (HF).
A solution of 4 in acetone was titrated with Fꢁ, and the 1H NMR
spectrum of the mixture was recorded after each addition (Fig. 8).
In the absence of anions, the amide and glyoxylamide NH peaks of
4 occurred at 10.47 and 10.42 ppm, respectively. At 1 eq. of F–,
these peaks were shifted downfield to 10.96 and 12.09 ppm,
respectively, indicating the formation of hydrogen bonding
interactions between Fꢁ and 4. Furthermore, it was deduced that
compound 4 was in approximately equal concentration with a sec-
ond species. Upon addition of a second equivalent of Fꢁ, complete
conversion to the second species was apparent, which was consid-
ered to be the deprotonated form of 4. The disappearance of the
glyoxylamide NH peak indicated that deprotonation of that group

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V. Suryanti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 662–669
Similar changes in the 1H NMR spectra were observed upon the
the ability of 5 to form hydrogen bonding with the anions that is
necessary for deprotonation.
addition of CNꢁ to compound 4. The addition of 1 eq. CNꢁ was suf-
ficient to complete the deprotonation of the glyoxylamide NH in
this instance.
Conclusions
When 1 eq. of Fꢁ was added to 5, the amide and glyoxylamide
NH proton signals at 10.96 and 7.96 ppm, respectively, were
shifted downfield to 11.76 and 8.12 ppm, respectively (Fig. 9). Fur-
thermore, the aromatic protons of the nitrophenyl ring underwent
variable changes in chemical shifts. The peak at 8.22 (Hb) was
shifted upfield, while the multiplet at 7.23–7.28 ppm (correspond-
ing to one proton from the nitrophenyl ring and one proton from
the isatin ring) was split into a doublet (Hc) which shifted down-
field to 7.31 ppm and a multiplet (Hd) which shifted upfield to
7.07–7.13 ppm. The peaks of the aromatic protons of the isatin
moiety were also affected. Peaks at 7.82 ppm (Hf) were shifted
downfield, while peaks at 7.67–7.74 ppm (He) were shifted upfield.
Upon addition of up to 3 eq. of Fꢁ, the amide and the glyoxylamide
NH proton signals shifted further downfield and broadened. The
proton corresponding to the nitrophenyl ring Hc shifted further
downfield, while Hb shifted further upfield. Moreover, the proton
corresponding to the aromatic ring of the isatin moiety Hf shifted
further downfield, while He shifted upfield. This evidence suggests
that hydrogen bonding interactions were formed between com-
pound 5 and the Fꢁ.
We have developed two novel colorimetric sensors 4 and 5
which utilize glyoxylamide based isatin as a new molecular scaf-
fold for the anion recognition. These sensors, containing two NH
groups as anion binding sites and a nitrophenyl moiety as a signal-
ling unit, were easily synthesized by one step ring-opening reac-
tions of N-acetylisatin with 4-nitrophenyl and 2,4-dinitrophenyl
amines, respectively. Fluoride and cyanide ions exhibited hydrogen
bonding interactions with the compounds 4 and 5 and induced
deprotonation of those compounds. The subsequent intramolecu-
lar charge-transfer transitions of the compounds 4 and 5 then ar-
ose resulting in the UV–Vis spectra changes and visible color
changes. These findings established that glyoxylamide based isatin
derivatives could provide very important contribution to the field
of anion recognition and sensing. This is the first reported example
of glyoxylamides capable of sensing anions. We believe that these
sensors, with naked-eye responses, can be used for many practical
applications in chemical, environmental and biological systems.
Further studies in this area would extend the development of gly-
oxylamide based sensors for biological anions or cations.
Upon the addition of 4 eq. of Fꢁ, the glyoxylamide NH signal dis-
appeared, indicating that deprotonation had occurred. Further-
more, the aromatic protons of the nitrophenyl ring were affected,
with Ha shifting further downfield and Hb and Hc shifting upfield.
This suggests that the deprotonated receptor could form structure
9 through resonance. Similar to the previous case, no new signal
appeared at 16.1 ppm after 4 eq. of Fꢁ were added, suggesting that
HFꢁ2 was not formed [25,26]. The addition of CNꢁ to 5 was found to
produce similar changes in the 1H NMR spectra as those observed
with Fꢁ. Two equivalents of CNꢁ were required to completely
deprotonate the glyoxylamide NH moiety of compound 5.
An electron-withdrawing nitro (NO2) group is often introduced
into anion receptors in order to enhance the acidity of the anion
recognition moiety. This facilitates the deprotonation of the hydro-
gen bond donor and subsequent transfer of H+ to the basic anion
[27–33]. The deprotonation processes can be utilized to colorimet-
rically sense anions [1,25], and is likely the mechanism of anion
sensing by receptors 4 and 5. Initially, hydrogen bond formation
occurred between the receptors 4 and 5 and the basic anions. After
further addition of Fꢁ or CNꢁ, the receptors underwent deprotona-
tion. The lack of clear isosbestic point in the UV–Vis titration of 4
with Fꢁ or CNꢁ suggested that the two stages could occur simulta-
neously, with the hydrogen-bonded complex and the deprotonated
receptor both present during the course of the titration. The depro-
tonation of the receptors were confirmed by 1H NMR spectroscopy,
which showed the loss of the glyoxylamide NH resonance upon
addition of Fꢁ and CNꢁ.
Supplementary material
X-ray crystallographic information files (CIF) of crystal 4 has
been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication number CCDC 952279. A copy of
the data can be obtained free of charge from CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
We thank the Directorate General of Higher Education, Ministry
of National Education, Indonesia through the University of Sebelas
Maret, Indonesia for a PhD scholarship to Venty Suryanti.
References
[1] R. Martinez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476.
[2] C.R. Bondy, S.J. Loeb, Coord. Chem. Rev. 240 (2003) 77–99.
[3] P.A. Gale, Coord. Chem. Rev. 213 (2001) 79–128.
[4] M. Melaimi, F.P. Gabbai, J. Am. Chem. Soc. 127 (2005) 9680–9681.
[5] V. Thiagarajan, P. Ramamurthy, D. Thirumalai, V.T. Ramakrishnan, Org. Lett. 7
(2005) 657–660.
[6] D.H. Burns, K. Calderon-Kawasaki, S. Kularatne, J. Org. Chem. 70 (2005) 2803–
2807.
[7] J. Yoon, S.K. Kim, N.J. Singh, K.S. Kim, Chem. Soc. Rev. 35 (2006) 347–355.
[8] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486–516.
[9] P.A. Gale, R. Quesada, Coord. Chem. Rev. 250 (2006) 3219–3244.
[10] C. Caltagirone, P.A. Gale, Chem. Soc. Rev. 38 (2009) 520–563.
[11] V. Amendola, L. Fabbrizzi, L. Mosc, Chem. Soc. Rev. 39 (2010) 3889–3915.
[12] C. Perez-Casas, A.K. Yatsimirsky, J. Org. Chem. 73 (2008) 2275–2284.
[13] M.P. Wintergerst, T.G. Levitskaia, B.A. Moyer, J.L. Sessler, L.H. Delmau, J. Am.
Chem. Soc. 130 (2008) 4129–4139.
The high basicity of Fꢁ and the high nucleophilicity of CNꢁ facil-
itate strong hydrogen bonding interactions with compounds 4 and
5, and promote the subsequent deprotonation of the recꢁeptors. On
the other hand, weakly basic anions such as H2SOꢁ4 , HSO4 , Clꢁ, Brꢁ,
NOꢁ2 NOꢁ, Iꢁ and ClOꢁ4 were unable to induce observable changes
[14] S.K. Kim, B.G. Kang, H.S. Koh, Y.J. Yoon, S.J. Jung, B. Jeong, K.D. Lee, J. Yoon, Org.
Lett. 6 (2004) 4655–4658.
[15] H. Su, J. Li, H. Lin, H. Lin, J. Braz. Chem. Soc. 21 (2010) 541–545.
[16] J. Shaoa, Y. Wanga, H. Lin, J.J. Li, H. Lina, Sens. Actuators B 134 (2008) 849–853.
[17] S. Hu, Y. Guo, J. Xu, S. Shao, Spectrochim. Acta Part A 72 (2009) 1043–1046.
[18] S. Hu, Y. Guo, J. Xu, S. Shao, Org. Biomol. Chem. 6 (2008) 2071–2075.
[19] H. Miyaji, J.L. Sessler, Angew. Chem. Int. Ed. 40 (2001) 154–157.
[20] Bruker, SADABS, Bruker AXS Inc., Madison, Wisconsin, USA, 2001.
[21] Bruker, APEX2 and SAINT, Bruker AXS Inc., Madison, Wisconsin, USA, 2001.
[22] G.M. Sheldrick, Acta Cryst. A64 (2008) 112–122.
[23] C.F. Macrae, I.J. Bruno, J.A. Chisholm, P.R. Edgington, P. McCabe, E. Pidcock, L.
Rodriguez-Monge, R. Taylor, J. van de Streek, P.A. Wood, J. Appl. Cryst. 41
(2008) 466–470.
[24] J.F.M.D. Silva, S.J. Garden, A.C. Pinto, J. Braz. Chem. Soc. 12 (2001) 273–324.
[25] T. Gunnlaugsson, M. Glynn, G.M. Tocci, P.E. Kruger, F. Pfeffer, Coord. Chem. Rev.
250 (2006) 3094–3117.
,
3
in the color of the receptors. Higher concentrations of Fꢁ were re-
quired to deprotonate 4 and 5 compared to that required by CNꢁ.
This result can be rationalized by the higher solvation energy of
Fꢁ than that of CNꢁ. Additionally, higher concentrations of anions
were required to deprotonate receptor 5 compared to receptor 4.
This was an unexpected result as the two nitro groups in 5 should
have conferred greater hydrogen bond donor capacity and acidity
to the glyoxylamide NH moiety compared to the single nitro group
in 4. We envisage that the additional nitro group at the C2 position
might have presented additional steric hindrance, which reduced

V. Suryanti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 662–669
669
[26] K. Adil, M.A. Saada, A.B. Ali, M. Body, M.T. Dang, A.H. Ribaud, M. Leblanc, V.
Maisonneuve, J. Fluorine Chem. 128 (2007) 404–412.
[27] V. Amendola, D.E. Gomez, L. Fabbrizzi, M. Licchelli, Acc. Chem. Res. 39 (2006)
343–353.
[30] D. Esteban-Gòmez, L. Fabbrizzi, M. Licchelli, J. Org. Chem. 70 (2005) 5717–
5720.
[31] S. Camiolo, P.A. Gale, M.B. Hursthouse, M.E. Light, A.J. Shi, Chem. Commun.
(2002) 758–759.
[28] M. Boiocchi, L. Del Boca, D.E. Go´mez, L. Fabbrizzi, M. Licchelli, E. Monzani, J.
[32] P.A. Gale, K. Navakhun, S. Camiolo, M.E. Light, M.B. Hursthouse, J. Am. Chem.
Soc. 124 (2002) 11228–11229.
Am. Chem. Soc. 126 (2004) 16507–16514.
[29] L.S. Evans, P.A. Gale, M.E. Light, R. Quesada, Chem. Commun. (2006)
965–967.
[33] T. Gunnlaugsson, P.E. Kruger, P. Jensen, F.M. Pfeffer, G.M. Hussey, Tetrahedron
Lett. 44 (2003) 8909–8913.