A benzimidazole-based Co3+ complex for electrochemical and spectroscopic recognition of I- and HSO4- in semi-aqueous media

Article abstract of DOI:10.1016/j.tetlet.2013.08.051

A benzimidazole-based Co³⁺ complex was synthesized and evaluated as a sensor for I^- and HSO4- in aqueous media. The complex showed electrochemical changes with I^- over other anions, whereas it had a ratiometric response to HSO4- with UV-visible spectroscopy even in the presence of other anions.

Full text of DOI:10.1016/j.tetlet.2013.08.051

Tetrahedron Letters xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A benzimidazole-based Co3+ complex for electrochemical and
spectroscopic recognition of IÀ and HSO4À in semi-aqueous media
c,
Hemant Sharma a, Vimal K. Bhardwaj a, Navneet Kaur b, Narinder Singh a, , Doo Ok Jang
⇑
⇑
a Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
b Centre for Nanoscience & Nanotechnology, Panjab University, Chandigarh 160014, India
c Department of Chemistry, Yonsei University, Wonju 220-710, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
A benzimidazole-based Co3+ complex was synthesized and evaluated as a sensor for IÀ and HSOÀ4 in aque-
ous media. The complex showed electrochemical changes with IÀ over other anions, whereas it had a
ratiometric response to HSOÀ4 with UV–visible spectroscopy even in the presence of other anions.
Ó 2013 Published by Elsevier Ltd.
Article history:
Received 11 June 2013
Revised 7 August 2013
Accepted 16 August 2013
Available online xxxx
Keywords:
Iodide
Hydrogen sulfate
Recognition
Benzimidazole
Electrochemical method
Co(III) complex
The design and synthesis of receptors for selective anion sens-
ing are of great interest in many areas such as environmental
chemistry and biology.1 Iodide plays important roles in neurologi-
cal activities and thyroid function. Iodide deficiency causes physi-
ological disorders, which lead to growth and maturation of organ
systems.2 Thus, it is strongly desirable to develop selective and
sensitive receptors to detect iodide. However, it is difficult to de-
sign receptors for iodide detection because iodide anion has a large
ionic radius, low charge density, and low hydrogen-bonding abil-
ity, which is reflected in the few reports of iodide sensors.3 Hydro-
gen sulfate release from industrial processes has a deleterious
effect on the environment.4 Thus, quantifying these anions in bio-
logical and environmental samples is of great importance.5
Developing a receptor to detect anions in aqueous media is
challenging because of competition between anions and water
molecules for the receptor binding sites.6,7 Thus, most hydrogen
bonding-based receptors for anions are not compatible with an
aqueous system. Metal complexes have been used to overcome this
problem. Metal displacement from a complex8 or electrostatic
interactions between the metal center of a complex and an anion
can cause changes in its photophysical properties.9 UV–vis and
fluorescence spectroscopic analysis techniques are frequently em-
ployed to detect these changes.
In the last few years, we have reported benzimidazole-based
imine receptors that act as sensors for anions and cations.10 It
had been thought that insertion of a cobalt metal ion into the coor-
dination sphere of a benzimidazole-based receptor could provide
sensor activity through electrochemical behavior, and the photo-
physical profile of the metal complex could offer a second channel
for sensor activity. In this Letter, we wish to report the develop-
ment of a sensor system that can simultaneously detect two anions
using the photophysical properties of the sensor as well as its elec-
trochemical profile. The combination of photophysical properties
with electrochemistry may increase the scope of such a sensor,
since electrochemistry has many advantages including a lower
detection limit and a measurement of redox potential.11,12 Anion
binding to the benzimidazole-based Co3+ complex leads to obser-
vable changes in its ring current and redox potential. Moreover,
this approach has the added advantage that electrostatic interac-
tions between the metal center and an anion can be interpreted
even in the presence of competing solvent molecules including
water. To the best of our knowledge, this is the first attempt to
use a benzimidazole-based sensor to recognize two different an-
ions via these two different analysis techniques.
Ligand 2 was synthesized by a condensation reaction of 2-ami-
nobenzimidazole and 2-hydroxybenzaldehyde.13 Reaction of li-
gand
2 with Co(NO3)2 gave a complex, which was further
subjected to air oxidation to obtained the desired complex 1
(Scheme 1). Complex 1 was characterized by 1H NMR, IR, elemental
analysis, and mass spectroscopy.
⇑
Corresponding authors.
E-mail addresses: nsingh@iitrpr.ac.in (N. Singh), dojang@yonsei.ac.kr (D.O. Jang).
0040-4039/$ - see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2013.08.051
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2
H. Sharma et al. / Tetrahedron Letters xxx (2013) xxx–xxx
Site for anion recognition
NH
N
HN
N
N
N
1) Co(NO3)2, MeOH
2) TBANO3, O2
HN
Co3+
O O
N
HO
N
2
1
Scheme 1.
Figure 1. Changes in the DPV profile of complex 1 (1
lM) upon addition of various
tetrabutylammonium anion salts (40 M) in MeOH/H2O (8:2, v/v).
l
The stoichiometry of complex 1 was established by a Job’s
plot,14 which had a maximum at 0.7 that corresponded to a 2:1
(host:guest) stoichiometry (Fig. S1). In spite of our numerous ef-
forts, we were unfortunate to grow the single crystals to determine
the exact structure of new material. The obtained crystals were in a
range of sub-micrometer size. We analyzed the sample with pow-
der diffraction technique. The data interpretation is much simpler
in powder diffraction when the purpose is to compare the sample
with pure metal ion and pure organic receptor. Figure S2 repre-
sents the comparison of PXRD pattern of ligand 2, complex 1,
and cobalt nitrate. The diffraction pattern of ligand 2 shows scat-
tering angles (2h) 10.04, 11.50, 14.13, 17.13, 19.52, 20.27, 21.07,
23.34, and 28.35 and cobalt nitrate has sharp peaks at (2h) 15.25,
16.40, 19.31, 27.20, 28.31, 29.18, 30.71, and 41.43. However, the
diffraction pattern of complex 1 ((2h) 8.98, 10.37, 11.65, 17.49,
17.91, 21.60, 22.09, 23.61, 24.36, 24.83, and 30.82) neither matches
with ligand 2 nor with cobalt nitrate. It implies that metal complex
1 has unique composition and represents the new material. The
formula of the metal complex was supported by massÀspectra
Figure 2. CV profile of complex 1 (1
v). Reference electrode: AgNO3/Ag; supporting electrolyte: (nBu)4NClO4.
l
M) and complex 1 + IÀ in MeOH/H2O (8:2, v/
showing
a
m/z peak at 684.2 ([1(H2O)2(CH3OH)(NO3 )]+Na+)
(Fig. S3). The coordination sphere of Co3+ consisted of a weakly
bound anion (NOÀ) and solvent molecules (H2O and CH3OH). These
3
ligands can easily be replaced with other anions. Thus, the complex
had the potential of being an anion sensor. In order to investigate
the effect of pH on the sensing behavior of complex 1, pH titrations
were performed. In acidic medium, the absorption profile of com-
plex 1 remained intact up to pH 4.0, and further lowering of the pH
resulted in the development of a new band at 400 nm and a de-
crease in the absorbance at 350 nm as shown in Figure S4A. An in-
crease in pH beyond 9.5 resulted in the development of a new band
at 372 nm (Fig. S4B). Therefore, the pH range between 4.0 and 9.5
is favorable for the optimum operation of complex 1. The effect of
ionic strength on the absorption profile of complex 1 was studied
with different concentrations of tetrabutylammonium perchlorate.
It was found that the absorption profile of complex 1 was not sig-
nificantly modulated even with up to 100 mM of tetrabutylammo-
nium perchlorate (Fig. S5). The absorption spectra of complex 1
were recorded in different solvents such as MeOH, EtOH, DMSO,
THF, and DMF to examine a possible solvatochromic effect. It
was noticed that a new band at 365 nm was observed in polar sol-
vents such as DMSO and DMF (Fig. S6). The effect of water content
was investigated with different ratios of MeOH/H2O (Fig. S7). It
was observed that water content greater than 20% led to the for-
mation of aggregates. Particle size analysis via dynamic light scat-
tering (DLS) revealed that these aggregates were approximately
50 nm in size (Fig. S8). Therefore, all recognition studies were car-
ried out in MeOH/H2O (8:2, v/v).
Figure 3. Changes in the DPV profile of complex 1 (1
lM) upon continuous addition
of tetrabutylammonium iodide (0–40 M) in MeOH/H2O (8:2, v/v).
l
and cyclic voltammetry (CV) in MeOH/H2O (8:2, v/v). Tetrabutyl-
ammonium perchlorate (0.1 M) was used as the supporting elec-
trolyte. The DPV profile of complex 1 showed an oxidation peak
at À0.235 V (Fig. 1). Upon the addition of various anions (FÀ, ClÀ,
BrÀ, IÀ, NOÀ, CNÀ, CH3COOÀ, HSOÀ, and H2POÀ) to the solution of
3
4
4
complex 1, only IÀ caused a change in the electrochemical signal.
The CV profile of complex 1 showed one reversible oxidation wave
with E1/2 = À0.752 V and an irreversible oxidation peak at 0.121 V
as shown in Figure 2. The irreversible CV peak at 0.121 V disap-
peared upon the addition of IÀ, and simultaneously, there was a
shift in E1/2 from À0.752 to À0.897 V.
The interaction of complex 1 withÀtetrabutylammonium salts of
various anions (FÀ, ClÀ, BrÀ, IÀ, NO3 , CNÀ, CH3COOÀ, HSOÀ4 , and
Changes in the DPV profile of complex 1 upon the addition of
various amounts of IÀ are shown in Figure 3. Upon the addition
H2POÀ) was investigated by differential pulse voltammetry (DPV)
4
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H. Sharma et al. / Tetrahedron Letters xxx (2013) xxx–xxx
3
0.6
B
0.5
0.4
0.3
0.2
0.1
Figure 4. DPV profile of complex 1 + IÀ and complex 1 + IÀ + other anions in MeOH/
H2O (8:2, v/v). Reference electrode: AgNO3/Ag; supporting electrolyte: TBAClO4.
0.0
250
300
350
400
450
500
550
Wavelength(nm)
Figure 6. Changes in the absorption profile of complex 1 (1
lM) with continuous
addition of tetrabutylammonium HSOÀ4 (0–200
lM) in MeOH/H2O (8:2, v/v).
Figure 5. Absorption spectra of complex 1 (1 lM) upon addition of different anions
(FÀ, ClÀ, BrÀ, IÀ, NOÀ3 , CNÀ, CH3COOÀ, HSO4À, CO23À, SO42À, PO43À and H2POÀ4 ) in MeOH/
H2O (8:2, v/v).
of IÀ, the oxidation potential of complex 1 shifted from À0.235 to
À0.512 V. After the addition of 40 equiv of IÀ, the potential became
constant. In order to evaluate the practical utility of complex 1, the
selectivity of complex 1 toward IÀ was examined in competition
experiments. Testing was performed in the presence of 4 equiv of
IÀ and an equimolar amoÀunt of another anion (FÀ, ClÀ, BrÀ, IÀ,
NO3À, CNÀ, CH3COOÀ, HSO4 , and H2PO4À) in MeOH/H2O (8:2, v/v)
(Fig. 4). The competition experiments illustrate that complex 1
had nearly the same potential changes with IÀ even in the presence
of other anions.
Figure 7. Detection of HSOÀ4 with complex 1 (1
lM) in the presence of an equimolar
amount of other anions in MeOH/H2O (8:2; v/v) at 400 nm.
It was observed that the addition of HSOÀ4 to a solution of com-
plex 1 gave a visible color change, even though HSOÀ4 had not in-
duced any significant electrochemical changes. This result
encouraged us to evaluate the anion binding profile of complex 1
through UV–visible spectroscopy. The anion binding ability of
complex 1 toward various tetrabutylammonium anion salts (FÀ,
ClÀ, BrÀ, IÀ, NOÀ3 , CNÀ, CH3COOÀ, HSO4À, and H2POÀ4 ) was investi-
gated by UV–visible spectrophotometry. Interestingly, complex 1
showed selectivity for HSOÀ4 , which was totally different from the
mechanism of HSOÀ4 to complex 1, a titration was performed be-
tween complex 1 and HSOÀ4 in MeOH/H2O (8:2, v/v) (Fig. 6). The
ratiometric changes at 400 and 375 nm are displayed. The emer-
gence of new band at 400 nm attributes to hydrogen bond interac-
tion between HSOÀ4 and N–H of benzimidazole. There are various
reports in the literature, which advocate that C–H/N–H—anion
interactions are main leading force behind the recognition.16 The
ratiometric changes of complex 1 at 400 and 375 nm were propor-
tional to the amount of added HSOÀ4 at concentrations between
electrochemical results (Fig. 5). The addition of HSOÀ4 (200
into a stock solution of complex 1 (1 M) produced a new band
lM)
l
at 400 nm with a simultaneous decrease of the band at 347 nm.
These results afforded a ratiometric determination for HSOÀ4 with
an isosbestic point at 375 nm. A ratiometric estimation is more
preferable to monitoring at a single wavelength because such an
analysis is free from errors due to receptor concentration and
instrumental error.15 To collect further information on the binding
0.01 and 140 lM (Inset of Fig. 6). The stoichiometry of complex
1. HSOÀ4 was investigated using a Job’s plot.14 The graph was plot-
ted between [HG] and [H]/([H]+[G]), which showed a 1:1 stoichi-
ometry (Fig. S9). The association constant of complex 1 and
HSOÀ4 , calculated using the Benesi–Hildebrand plot for a 1:1
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H. Sharma et al. / Tetrahedron Letters xxx (2013) xxx–xxx
stoichiometry, was 1.2 ( 0.12) Â 103 MÀ1 17
.
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tive titrations were performed. The absorption spectra of complex
1 with HSOÀ4 were measured in the absence and presence of com-
peting anions (Fig. 7). It was found that complex 1 showed high
selectivity even in the presence of competÀing anions such as FÀ,
ClÀ, BrÀ, IÀ, NOÀ, CNÀ, CH3COOÀ, and H2PO .
3
4
In summary, a benzimidazole-based Co3+ complex was pre-
pared and evaluated for anion binding in aqueous media using cyc-
lic voltammetry and UV–visible spectrophotometry. Complex 1
showed changes in its electrochemical behavior with iodide and
had high selectivity even in the presence of other anions. On the
other hand, complex 1 had a different UV–visible spectrophotom-
etry recognition profile, which exhibited a ratiometric response to
HSOÀ4 compared to other competing anions.
Acknowledgments
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This work was supported by the Korean and Indian Government
for the India-Korea Joint Program of Cooperation in Science & Tech-
nology (NRF-2011-0027710) and the CSIR project (project no:
01(2417)/10/EMR-II).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.08.
051.
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