An efficient ferrocene derivative as a chromogenic, optical, and electrochemical receptor for selective recognition of Mercury(II) in an aqueous environment

Article abstract of DOI:10.1021/om201073g

The synthesis, electrochemical, optical, and cation-sensing properties of two triazole-tethered ferrocenyl benzylacetate derivatives, C₃₆H ₃₆O₆N₆Fe (2) and C₂₃H ₂₃O₃N₃

Full text of DOI:10.1021/om201073g

Article
pubs.acs.org/Organometallics
An Efficient Ferrocene Derivative as a Chromogenic, Optical, and
Electrochemical Receptor for Selective Recognition of Mercury(II)
in an Aqueous Environment
Arunabha Thakur and Sundargopal Ghosh*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
S
* Supporting Information
ABSTRACT: The synthesis, electrochemical, optical, and cation-sensing
properties of two triazole-tethered ferrocenyl benzylacetate derivatives,
C36H36O6N6Fe (2) and C23H23O3N3Fe (3), are presented. The binding
event of both the receptors can be inferred either from a redox shift
(2, ΔE1/2 = 106 mV for Hg2+ and ΔE1/2 = 187 mV for Ni2+; 3, ΔE1/2
=
167 mV for Hg2+ and ΔE1/2 = 136 mV for Ni2+) or a highly visual output
response (colorimetric) for Hg2+, Ni2+, and Cu2+ cations. Remarkably, the
redox and colorimetric responses toward Hg2+ are preserved in the
presence of water (CH3CN/H2O, 2/8), which can be used for the
selective colorimetric detection of Hg2+ in an aqueous environment over
Ni2+ and Cu2+ cations. The changes in the absorption spectra are
accompanied by the appearance of a new low-energy (LE) peak at
625 nm for both compounds 2 and 3 (2, ε = 2500 M−1 cm−1; 3, ε =
1370 M−1 cm−1), due to a change in color from yellow to purple for Hg2+ cations in CH3CN/H2O (2/8).
molecular recognition.15−24 Thus, from a synthetic standpoint,
INTRODUCTION
■
ferrocene is a very attractive building block for redox-active ligands
because of the robustness of ferrocene under aerobic conditions
and the ease of functionalization. These facts, coupled with its
electrochemical and UV−vis spectroscopic properties, which can
be perturbed by the proximity of bound guests, reveal that ferro-
cene is an important functional tentacle in this research area. In this
article, the host−guest complexation properties of two triazole-
tethered ferrocenyl benzyl acetate derivatives toward Hg2+ and Ni2+
metal cations have been investigated.
The development of selective and sensitive imaging tools capable
of monitoring heavy- and transition-metal ions has attracted
interest because of their wide use in various fields of science and
the subsequent effects of these metals in the environment and
nature.1 The Hg2+ ion is considered a highly toxic element, and its
detection is currently a task of key importance for environmental
and biological concerns.2−5 However, the design and the
advancement of new and practical chemosensors, which offer a
promising advance for mercuric ion detection, is still a great
challenge in supramolecular chemistry.6−11 On the other hand, the
Ni2+ ion is also considered as a highly toxic trace element which
causes chronic bronchitis and lung and kidney function problems.
The progress of multichannel (chromogenic/optical/electro-
chemical) Ni2+ selective chemosensors is, as far as we know, an
unexplored subject and only a very few receptors have been
described,12 where the binding event of the Ni2+ ion can be
inferred only on the basis of the redox shift of ferrocene unit.
As a result, the development of good sensors for the detection
of these toxic metal cations through multiple channels is of key
importance for the environment and human health.13 To develop
sensitive sensors, various receptors consisting of a mercuric ion re-
cognition unit and a probe exhibiting physical responses upon co-
ordination of Hg2+ have been reported.14 One of the most elegant
ways of achieving sensor design is to functionalize a receptor
capable of both selective substrate binding with a metal cation and
reporting on the recognition event through a variety of physical
responses. In this context, the design of redox-active receptors in
which a change in electrochemical behavior can be used to monitor
complexations of guest species is an increasingly important area of
EXPERIMENTAL SECTION
■
Materials and Methods. Perchlorate salts of Li+, Na+, K+, Ag+,
Ca2+, Mg2+, Cr2+, Co2+, Cu2+, Fe2+, Zn2+, Cd2+, Ni2+, Pb2+, and Hg2+,
propargyl bromide, butyllithium, and tetramethylethylenediamine
(TMEDA) purchased from Aldrich were used directly without further
purification. Ferrocene, sodium ascorbate, benzyl-2-bromo acetate,
sodium azide, and acetonitrile were of analytical grade and were used
without further purification. DMF was purchased from Aldrich and
freshly distilled prior to use. Chromatography was carried out on 3 cm
of silica gel in a 2.5 cm diameter column. Column chromatography was
carried out using 60−120 mesh silica gels. All the solvents were dried by
conventional methods and distilled under a N2 atmosphere before use.
Benzyl 2-azidoacetate25 and compounds 1a,b [Fc(CH2OCH2CCH)n]26
(1a, n = 2; 1b, n = 1; Fc = ferrocene) were synthesized as per literature
procedures. The cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) were performed with a conventional three-electrode
configuration consisting of glassy carbon as the working electrode, platinum
Received: January 1, 2011
Published: January 26, 2012
© 2012 American Chemical Society
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1
as an auxiliary electrode, and Ag/Ag+ as a reference electrode. The
experiments were carried out with a 10−3 M solution of sample in CH3CN
containing 0.1 M (n-C4H9)4NClO4 (TBAP) as supporting electrolyte.
Deoxygenation of the solutions was achieved by bubbling nitrogen for at
least 10 min, and the working electrode was cleaned after each run. The
cyclic voltammograms were recorded at a scan rate 0.1 V s−1. The UV−vis
spectra were carried out in CH3CN solutions at c = 1 × 10−4 M.
Instrumentation. The 1H and 13C NMR spectra were recorded on
Bruker 400 and 500 MHz FT-NMR spectrometers, using tetra-
methylsilane as the internal reference. Electrospray ionization mass
spectrometry (ESI-MS) measurements were carried out on a QTOF
Micro YA263 HRMS instrument. The absorption spectra were
recorded with a JASCO V-650 UV−vis spectrophotometer at 298 K.
The CV and DPV measurements were performed on a CH Model
660B potentiostat.
3: H NMR (CDCl3, 400 MHz) δ 7.62 (s, 1H, Htriazole), 7.32−7.39
(m, 5H, Ar H), 5.21 (s, 2H, OCH2Ph), 5.16 (s, 2H, NCH2CO), 4.64
(s, 2H, OCH2), 4.35 (S, 2H, OCH2), 4.17 (s, 4H, HCp), 4.08−4.14 (t,
5H, HCp); 13C NMR (CDCl3, 100 MHz) δ 166.17 (OCO), 146.06
(Ctriazole), 134.52 (Ar C), 128.90 (Ar C), 128.80 (Ar C), 128.64 (Ar C),
123.95 (Ctriazole), 82.87 (OCH2Ph), 69.65 (NCH2CO), 68.74 (Ccp),
68.70 (Ccp), 68.53 (Ccp), 68.06 (Ccp), 63.14 (OCH2), 50.84 (OCH2);
electrospray MS m/z (relative intensity) 446 (M+ + 1), 469 (M+ + 23).
Anal. Calcd for C23H23FeO3N3: C, 62.04; H, 5.21; N, 9.44. Found: C,
62.08; H, 4.95; N, 9.30.
X-ray Crystallographic Analysis. Suitable X-ray-quality crystals
of 3 were grown by slow diffusion of a hexane/EtOAc (6/4 v/v)
solution, and a single-crystal X-ray diffraction study was undertaken. X-
ray single-crystal data were collected using Mo Kα (λ = 0.710 73 Å)
radiation on a Bruker APEX II diffractometer equipped with a CCD
area detector. Data collection, data reduction, and structure solution/
refinement were carried out using the software package of SMART
APEX. All structures were solved by direct methods and refined in a
routine manner. In most of the cases, non-hydrogen atoms were
treated anisotropically. Whenever possible, the hydrogen atoms were
located on a difference Fourier map and refined. In other cases, the
hydrogen atoms were geometrically fixed.
Caution! Metal perchlorate salts are potentially explosive under
certain conditions. All due precautions should be taken while handling
perchlorate salts.
Synthesis of 2 and 3. To a well-stirred solution of 1a (0.5 g,
1.55 mmol) and benzyl 2-azidoacetate (0.592 g, 3.1 mmol) in 15 mL of
acetone/H2O (2/1) was added an aqueous solution of CuSO4·5H2O
(0.077 g, 0.31 mmol). To this resultant mixture was added freshly
prepared sodium ascorbate solution (0.122 g, 0.62 mmol), and the
mixture was stirred at room temperature for 12 h. A 30 mL portion of
ethyl acetate was added to the reaction mixture, and the organic layer
was washed several times with water and finally with brine (15 mL)
and dried over anhydrous sodium sulfate. The solvent was removed
under reduced pressure, and the crude product was purified by silica
gel column chromatography. Elution with EtOAc/hexane (8/2 v/v)
yielded yellow 2 (0.97 g, 89%).
Crystal data for 3: formula C23H23FeO3N3; crystal system, space
group monoclinic, C2/c; unit cell dimensions a = 49.128(3) Å, b =
5.7090(3) Å, c = 36.0273(17) Å, β = 125.257(3); Z = 16. calcd density
1.434 Mg/m3; final R indices (I > 2σ(I)) R1 = 0.0355, wR2 = 0.0474
(all data); index ranges −13 ≤ h ≤ 13, −10 ≤ k ≤ 10, −13 ≤ l ≤ 12,
29 688 reflections collected, 1826 independent reflections, Rint
=
0.0705; wR2 = 0.0496 (I > 2σ(I)), R1 = 0.0772, goodness of fit on F2
0.954.
The compound 3 was prepared by following the procedure adopted
for 2, where the amounts used were alkyne 1b used 0.5 g (1.96 mmol),
benzyl 2-azidoacetate 0.374 g (1.96 mmol), aqueous CuSO4·5H2O
0.097 g (0. 392 mmol), and sodium ascorbate 0.15 g (0.784 mmol).
The crude product was purified by silica gel column chromatography
and elution with EtOAc/ hexane (7/2 v/v) to yield pure yellow 3
(0.74 g, 84%).
RESULT AND DISCUSSION
■
The precursors 1a,b for ferrocenyl benzyl acetate derivatives 2
and 3 were obtained from following literature procedures.26 As
shown in Scheme 1, the precursors 1a,b both undergo “click
reactions” with benzyl 2-azidoacetate to generate compounds 2
and 3 in 89% and 84% yields, respectively. Compounds 2 and 3
1
2: H NMR (CDCl3, 400 MHz) δ 7.59 (s, 2H, Htriazole), 7.26−7.29
1
(m, 10H, Ar H), 5.13 (s, 4H, OCH2Ph), 5.11 (s, 4H, NCH2CO), 4.55
(s, 4H, OCH2), 4.25 (s, 4H, OCH2), 4.05−4.12 (d, 8H, HCp); 13C
NMR (CDCl3, 100 MHz) δ 165.19 (OCO), 144.77 (Ctriazole),
133.48, (Ar C), 127.81 (Ar C), 127.72 (Ar C), 127.56 (Ar C), 123.06
(Ctriazole), 82.54 (OCH2Ph), 68.96 (NCH2CO), 68.13 (Ccp), 67.33
(Ccp), 66.96 (Ccp), 62.10 (OCH2), 49.77 (OCH2); electrospray MS
m/z (relative intensity) 727 (M++23, 100). Anal. Calcd for
C36H36FeO6N6: C, 61.37; H, 5.15; N, 11.93. Found: C, 60.96; H,
4.89; N, 11.12.
have been characterized by ESI-MS spectrometry, IR and H
and 13C NMR spectroscopy, and analytical techniques. The
solid-state structure of 3 was unambiguously established by X-
ray analysis and is in full agreement with spectroscopic data.
Compounds 2 and 3 are both moderately stable and could be
stored in the freezer for months. The complexation properties
of the receptors 2 and 3 have been investigated by
electrochemistry and UV−vis spectroscopic measurements.
Scheme 1. Synthesis of Mono- and Diferrocene Benzyl Acetate Derivatives 2 and 3
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Figure 1. Molecular structure of 3 with thermal ellipsoids drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): C(Cp)−
Fe = 2.016(3)−2.046(3), C11−O1 = 1.434(3)−1.440(3); triazole ring NN = 1.308(3)−1.311(2), N−N = 1.332(3)−1.336(2), N−C =
1.333(4)−1.337(2), N−C = 1.334(3)−1.353(2), C−C = 1.348(3)−1.351(4); ester C−O = 1.310(3)−1.326(3), CO = 1.167(3)−1.184(3), C−O =
1.453(3)−1.467(3), C−C = 1.493(3)−1.510(4); Ar C−C = 1.345(5)−1.392(5), C−C−Fe = 69.47(14)−70.64(17); triazole ring C−C−N =
105.8(2)−108.8(3), NN−N = 106.92(18)−106.98(12); Ar C−C−C = 119.5(4)−120.7(4); ester O−C−O = 125.6(3)−126.3(3).
X-ray Structure Analysis. Single-crystal X-ray analysis
revealed that the organometallic compound 3 crystallized in the
monoclinic centrosymmetric space group C2/c (Figure 1). The
asymmetric unit consists of two molecules of 3, which are
crystallographically independent. From the crystal data, it is
clear that the C−N bond distances of the five-membered rings
for both crystallographically independent molecules are in the
ideal range (C−N = 1.307(3)−1.370(4) Å). Moreover, in both
molecules one of the N−N bond distances is slightly shorter
(NN = 1.311(3)−1.313(3) Å) than the other (N−N =
1.336(2)−1.354(4) Å), which is in fact due to the double-bond
character of the triazole ring. The carbonyl CO oxygen of the
ester group is involved in hydrogen bonding with CH2 via a C−
H···O interaction (C···O = 3.405(5) Å; ∠C−H···O = 150(2)°),
that leads to the formation of a one-dimensional (1D)
hydrogen-bonded network (Figure 2b). Such 1D networks
sheet (Figure 2c). Interestingly, 2D sheets are packed in a
slightly offset manner sustained by a C−H···N interaction
(C···N = 3.483(3) Å; ∠C−H···N = 135(4)°) (Figure 2d).
Electrochemical Studies. Chemical sensors bearing
ferrocene nuclei as part of the sensing unit have been broadly
studied.29 Earlier, the complexation of ferrocene with a variety
of binding ligands have been studied by cyclic voltammetry and
it has shown a positive shift of the Fe(II)/Fe(III) redox couple
as a result of metal−ligand complexation.30 The metal-
recognition properties of receptors 2 and 3 were evaluated by
cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) analysis. The reversibility and relative oxidation
potential of the redox process were determined by CV and
DPV in CH3CN solutions containing 0.1 M [(n-Bu)4N]ClO4 as
the supporting electrolyte. Compounds 2 and 3 both display a
reversible one-electron-oxidation process at E1/2 = 0.494 and
0.482 V, respectively, due to the ferrocene/ferrocenium redox
couple. No perturbation of the CV and DPV voltammograms
of 2 and 3 were observed in the presence of several metal
cations such as Li+, Na+, K+, Ag+, Ca2+, Mg2+, Cr2+, Zn2+, Co2+,
Fe2+, Cd2+, and Pb2+ as their appropriate salts, even in large
excess. However, as shown in Figures 3 and 4, the original peak
gradually decreased upon stepwise addition of Hg2+ and Ni2+
ions, while a new peak, associated with the formation of a
complexed species, appeared at 0.60 and 0.681 V, respectively,
for 2. In the case of 3 the peak appears at 0.649 and 0.618 V for
Hg2+ and Ni2+ ions, respectively (Supporting Information,
Figure S3). DPV experiments show that the peak correspond-
ing to uncomplexed species completely disappeared upon
addition of 1 equiv of these metal cations. Interestingly, the
redox responses toward Hg2+ cations for compounds 2 and 3
are preserved in the presence of CH3CN/H2O (2/8).
Interestingly, as shown in Figure 5, no additional peak appeared
upon addition of Cu2+ ion for the compounds 2 and 3
(Supporting Information, Figure S4); only the voltammetric
wave shifted toward more cathodic current. In addition, linear
sweep voltammetry (LSV) studies were carried out upon addition
of Hg2+, Cu2+, and Ni2+ to CH3CN solutions of receptors 2 and 3,
which are shown in Figure 6 and in the Supporting Information
(Figure S5), respectively. The shift of the voltammetric wave
toward cathodic current in the case of the Cu2+ cation indicates
that this metal cation promotes the oxidation of the free receptors
2 and 3 with concomitant reduction to Cu+. This is in agreement
with the CV as shown in Figure 5 (for 2) and Figure S4 (for 3,
Figure 2. Crystal structure description of 3: (a) molecular structure of
3 with thermal ellipsoids drawn at the 50% probability level (two
crystallographically independent molecules were present in the
asymmetric unit); (b) 1D hydrogen-bonded network formed by the
C−H···O hydrogen-bonding interaction; (c) 2D hydrogen-bonded
corrugated sheets via C−H···π; (d) offset packing of 2D sheets via C−
H···N interactions.
are further self-assembled via C−H···π (C···π = 3.822(2) Å)
interactions27,28 and form a 2D hydrogen-bonded corrugated
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Figure 3. Evolution of CV (a) and DPV (b) of 2 (10−3 M) in CH3CN/H2O (2/8) using [(n-Bu)4N]ClO4 as supporting electrolyte when Hg(ClO4)2
is added up to 0−1 equiv.
Figure 4. Evolution of CV (a) and DPV (b) of 2 (10−3 M) in CH3CN using [(n-Bu)4N]ClO4 as supporting electrolyte when Ni(ClO4)2 is added up
to 0−1 equiv.
shown in the Supporting Information). In contrast, the same
experiment was carried out upon addition of Hg2+ and Ni2+
cations, which revealed a shift of the LSV toward positive
potential for the receptor 2 (Figure 6b,c), which is in agreement
with the complexation process previously observed by CV
(Figures 3a and 4a).
UV−Visible Absorption Studies. The UV−vis binding
interaction studies of receptors 2 and 3 in CH3CN (1 ×
10−4 M) against cations of environmental relevance, such as Li+,
Na+, K+, Ag+, Ca2+, Mg2+, Cr2+, Zn2+, Co2+, Fe2+, Cd2+, and Pb2+
as perchlorate salts, show selective response to Hg2+, Ni2+, and
Cu2+. The change in the UV−vis absorbance spectra of
receptors 2 and 3 in CH3CN due to the stepwise addition of
Hg2+ ion are shown in Figure 7 and in the Supporting
Information (Figure S7), respectively. Upon addition of 1 equiv
of Hg2+ to 2 the high-energy (HE) absorption band at λ 432
nm disappeared and a new peak at ca. 294 nm appeared.
Further, as shown in Figure 7, a new and weak lower-energy (LE)
absorption band appeared at λ 625 nm (ε = 2500 M−1 cm−1)
for 2, due to the change in color. Binding assays using the
method of continuous variations (Job plot) strongly suggest
1:1 (cation/receptor) complex formation with Hg2+ ion for
compound 2 (Figure 7, inset). Further, the stoichiometry of the
complex has also been confirmed by ESI-MS, where peaks at
m/z 905 for [2·Hg2+] and m/z 646 for [3·Hg2+] are observed
(Supporting Information, Figure S8). The binding constant
determined from the increasing absorption intensity at 630 nm
for 2 and 3 is K ( 15%) = 2.35 × 103 and 1.9 × 103 M−1,
respectively.
Likewise, the addition of increasing amount of Cu2+ ions to a
solution of 2 and 3 showed progressive appearance of one new
LE band at 637 nm for 2 and 625 nm for 3. Well-defined
isosbestic points at 350 nm for 2 and 442 nm for 3 were found,
indicating that only one spectrally distinct complex was present.
The new LE bands at 637 and 625 nm are accountable for the
change of color from yellow to bluish green. The UV−vis
spectral change, shown in Figure 8, suggests that the ferrocene
moiety is oxidized upon interaction with Cu2+ ion and the
change of color to green is characteristic of the formation of
ferrocenium ion.31 Note that there was no significant spectral
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change observed upon addition of Cu+ and Ag+ cations,
which indicates that these cations do not promote the
oxidation of the ferrocene moiety (Supporting Information,
Figure S9).
The change in the UV−vis absorbance spectra of receptors
2 and 3 in CH3CN due to the stepwise addition of Ni2+ ion
are shown in Figure 9. Upon addition of 1 equiv of Ni2+, the
high-energy (HE) absorption band at λ 361 nm was reduced
with the concomitant increase of the peak at λ 435 nm (ε =
1380 M−1 cm−1) for the receptors 2 and 3. Well-defined isos-
bestic points at ca. 395 and 289 nm indicated the presence of a
unique complex with a neutral ligand. Binding assays using the
method of continuous variations (Job plot) clearly indicate 1:1
(cation/receptor) complex formation with Ni2+ ion for both
compounds 2 and 3 (Supporting Information, Figure S10). The
binding constants determined32 from the increasing absorption
intensity at 630 nm for 2 and 3 are K ( 15%) = 2.6 × 103 and
2.36 × 103 M−1, respectively.
Visual Detection of Hg2+, Ni2+, and Cu2+. When an
excess of different metal cations (Li+, Na+, K+, Ag+, Ca2+, Mg2+,
Cr2+, Zn2+, Ni2+, Fe2+, Co2+, Cd2+, Hg2+, and Pb2+) as their
perchlorate salts are separately added to solutions of 2 and 3 in
CH3CN (10−3 M) no significant color change was observed,
except for Hg2+, Ni2+, and Cu2+. As shown in Figure 10, Hg2+
shows a drastic color change from yellow to purple and Ni2+
from yellow to pale blue, whereas Cu2+ shows a color change
Figure 5. Evolution of the CV of 2 (10−3 M) in CH3CN using [(n-
Bu)4]ClO4 as supporting electrolyte upon addition of increasing
amounts of Cu2+ metal cation up to 1 equiv. Arrows indicate the
movement of the wave during the experiments.
Figure 6. Evolution of LSV of 2 (10−3 M) in CH3CN during addition of (a) Cu2+, (b) Ni2+, and (c) Hg2+ using [(n-Bu)4N]ClO4 as supporting
electrolyte and a scan rate of 0.1 V s−1.
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from yellow to bluish green (due to the oxidation of ferrocene
to ferrocenium ion). The sensing potential of 3 toward Hg2+,
Ni2+, and Cu2+ in solution is very similar to that of 2.
absorption of the 2−Cu2+ system is influenced by the Ni2+ and
Hg2+ ions, but Cu2+ did not interfere with the absorption of 2−
Ni2+ and 2−Hg2+. Except for Hg2+ ion, the absorption intensity
of 2 in the presence of 10 equiv of the Ni2+ ion was almost
unaffected by the addition of 10 equiv of competing metal ions
(for example, Li+, Na+, K+, Ca2+, Mg2+, Cr2+, Co2+, Cu2+, Fe2+,
Zn2+, Cd2+, and Pb2+). On addition of an equal amount of Hg2+
ion into a solution of 2·Ni2+ complex, the absorption intensity
was shown to be dominated by the Hg2+ ion. Therefore, 2
could be used for the detection of Hg2+ in the presence of other
competing metal ions, including Cu2+ and Ni2+ ion. In addition,
2 can be utilized for the detection of Ni2+ ion in the presence of
other competing ions, except Hg2+ ion.
Most remarkably, as shown in the Figure 11, the colorimetric
response toward Hg2+ is preserved in the presence of water.
Thus, addition of Hg2+ cations to a solution of receptor 2 or 3 in
CH3CN/H2O (2/8) induced a red shift of the absorption band
from 312 to 326 nm (Δλ = 14 nm). These changes are
responsible for the change of color from yellow to purple that can
be used for the selective colorimetric detection of Hg2+ in an
aqueous environment, because no changes were observed in the
absorption spectrum after addition of either Ni2+ or Cu2+ cations.
The competition experiments were carried out for the
interaction of 2 between Hg2+, Ni2+, and Cu2+ in pure CH3CN
solution in order to ascertain the selectivity. It shows that the
Reversibility Interaction of 2 and Hg2+. The reversible
interaction between 2 and 3 with Hg2+ was confirmed by the
introduction of I− into the system containing 2 (10−4 M) and
Hg2+ (2 equiv). The experiment, shown in Figure 12, showed
that the introduction of 5 equiv of I− into Hg2+ immediately
quenched the absorption of 2. When Hg2+ was further added to
the system, the absorption of 2 was enhanced again. This
process could be repeated several times without loss of
sensitivity of the absorbance, which clearly demonstrates the
high degree of reversibility of the complexation/decomplex-
ation process between 2 and Hg2+ ions. On the other hand, the
reversibility of the complexation process of Ni2+ ions was
performed by an extraction experiment,33 which specifies the
high level of reversibility of the complexation/decomplexation
process (Supporting Information, Figure S11).
To support the results obtained by electrochemical and spectro-
scopic experiments, and to get an insight about the coordination
mode of these metal cations by receptor 2, we performed the 1H
NMR spectroscopic analysis in CD3CN solution. The binding of
metal ion by a host molecule is accompanied by conformational
and electronic changes, which in turn are reflected by variations in
1
the H chemical shifts of those centers close to the site of
complexation.34 The most significant spectral changes (1H NMR;
Supporting Information, Figure S12) observed upon addition of
increasing amounts of Hg2+ ions to a solution of the free receptor
2 are the following: (i) Hg2+ metal ions caused broadening of
Figure 7. Changes in the absorption spectra of 2 (10−4 M) in
CH3CN/H2O (2/8) upon addition of increasing amounts of Hg2+ up
to 1 equiv. Inset: Job plot of 2 with Hg2+ cations, indicating the
formation of a 1:1 complex.
1
the −CH2COO and −OCH2Ph protons in the H NMR
spectrum; (ii) the hydrogen atom within the triazole ring showed
Figure 8. Changes in the absorption spectra of (a) 2 and (b) 3 (10−4 M) in CH3CN upon addition of increasing amounts of Cu2+ up to 1 equiv.
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Figure 9. Changes in the absorption spectra of (a) 2 and (b) 3 (10−4 M) in CH3CN upon addition of increasing amounts of Ni2+ up to 1 equiv.
Figure 10. Visual features observed in CH3CN solution of 2 (10−3 M) after addition of 10 equiv of different metal cations tested.
a significant downfield shift by ca. 0.10 ppm; (iii) the observed
downfield shifts for the cyclopentadienyl ring hydrogen atoms in
ferrocene were not prominent. Similar types of chemical shifts
were also observed for the Ni2+ metal ion. The above metal-ion-
induced chemical shift changes support that metal cations (Hg2+
and Ni2+) are bound to the triazole (N1) ring and to the −OCH2
units of the ester group.
CONCLUSION
Figure 11. Visual color changes observed in a CH3CN/H2O (2/8)
solution of 2 (10−3 M) after addition of Hg2+, Ni2+, and Cu2+ cations.
■
In conclusion, we have described two simple triazole-based,
easy-to-synthesize, multisignaling chemosensors 2 and 3, which
selectively bind with Hg2+ and Ni2+ through multiple channels.
Interestingly, the redox and colorimetric response toward Hg2+
are preserved in the presence of water, which can be used for
the selective colorimetric (naked-eye) detection of Hg2+ in an
aqueous environment over other cations, including the strong
competitors Ni2+, Pb2+, and Cd2+. To the best of our
knowledge, the ferrocene-based ligand 2 is the rare example
of a dual electrochemical−optical Ni2+ ion sensor in which the
Fc/Fc+ redox couple is significantly shifted (ΔE1/2 = 187 mV)
on complexation, with a concomitant change in color from
yellow to blue, that allows its potential use for “naked eye”
detection.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Figures giving H and 13C NMR and ESI-MS data of 2 and 3,
electrochemical data for 2 and 3 upon titration with Cu2+ and
different metal ions, UV−vis spectra upon titration with
different metal ions, ESI-MS spectrum of [2·Hg2+] and
[3·Hg2+], evolution of 1H NMR spectra of 2 in CD3CN
upon addition of increasing amounts of Hg2+, reversibility
Figure 12. Reversibility of the interaction between 2 and Hg2+ by the
introduction of I− to the system. Inset: Stepwise complexation/
decomplexation cycles carried out in CH3CN with 2 and Hg2+.
825
dx.doi.org/10.1021/om201073g | Organometallics 2012, 31, 819−826

Organometallics
Article
experiments of 2 with Ni2+ ions, and quantitative binding data for
2 and 3 with Hg2+ and Ni2+ and a CIF file giving crystallographic
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: (+91) 44 2257 4230. Fax: (+91) 44 2257 4202. E-mail:
sghosh@iitm.ac.in.
■
(29) (a) Zhu, X. J.; Fu, S. T.; Wong, W. K.; Guo, J. P.; Wong, W. Y.
Angew. Chem. 2006, 45, 3222. (b) Molina, P; Tar
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(30) (a) Lopez, J. L.; Tarraga, A.; Espinosa, A.; Velasco, M. D.;
́
raga, A.; Caballero, A.
ACKNOWLEDGMENTS
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́
́
Generous support of the Indo-French Centre for the
Promotion of Advanced Research (IFCPAR-CEFIPRA), No.
4405-1, New Delhi, India, is gratefully acknowledged. A.T. is
grateful to the Council of Scientific and Industrial Research
(CSIR) of India for research fellowships.
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