A colorimetric chemosensor for both fluoride and transition metal ions based on dipyrrolyl derivative

Article abstract of DOI:10.1039/b510469f

The synthesis, characterization and ion binding studies of 2,3-di(1H-2-pyrrolyl)pyrido[2,3-b]pyrazine (1) have been described. 1, which has been targeted with a view to sensing both F^- and transition metal ions, exhibits binding-induced color changes from yellowish green to red/brown observable by the naked eye. The binding site for the metal ion in the system has been unambiguously established by single-crystal X-ray diffraction study of a Ni(ii) complex of 1. While the estimated value of the binding constant of 1 with F^- is 4.9 × 10³ M^-1, the binding constants for the cations are found to be two orders higher in magnitude in acetonitrile. Even though 1 possesses two separate binding sites for F ^- and metal ions, it is shown that the presence of the cation influences the binding of the anion and vice versa. The binding constant values of an ion in the presence of oppositely charged species are measured to be significantly lower. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b510469f

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PAPER
www.rsc.org/dalton | Dalton Transactions
A colorimetric chemosensor for both fluoride and transition metal ions based
on dipyrrolyl derivative
Tamal Ghosh, Bhaskar G. Maiya† and Anunay Samanta*
Received 22nd July 2005, Accepted 23rd December 2005
First published as an Advance Article on the web 13th January 2006
DOI: 10.1039/b510469f
The synthesis, characterization and ion binding studies of 2,3-di(1H-2-pyrrolyl)pyrido[2,3-b]pyrazine
(1) have been described. 1, which has been targeted with a view to sensing both F− and transition metal
ions, exhibits binding-induced color changes from yellowish green to red/brown observable by the
naked eye. The binding site for the metal ion in the system has been unambiguously established by
single-crystal X-ray diffraction study of a Ni(II) complex of 1. While the estimated value of the binding
constant of 1 with F− is 4.9 × 103 M−1, the binding constants for the cations are found to be two orders
higher in magnitude in acetonitrile. Even though 1 possesses two separate binding sites for F− and
metal ions, it is shown that the presence of the cation influences the binding of the anion and vice versa.
The binding constant values of an ion in the presence of oppositely charged species are measured to be
significantly lower.
types of sensors have the dual advantage of possessing a built-
in chromophore and being readily accessible in two steps from
Introduction
Development of chemosensors capable of recognizing and sensing
commercially available materials.
cations and anions is one of the most challenging fields, not only
We have recently embarked upon designing systems that can
from the viewpoint of organic and supramolecular chemistry but
sense both anions and metal ions. To begin with, we concentrated
also from its potential clinical applications.1–3 Chemosensors con-
on systems that can sense both transition metal ions and fluoride
sist of signaling (chromophore/fluorophore) and guest binding
ion. In order to accomplish this goal, we have exploited the fluoride
(receptor) moieties, either separated by a spacer or integrated
ion signaling potential of the DPP derivatives and transition metal
into one unit. Generally, N or O donor centers act as binding
sites for cations1 and pyrrole, –OH, –NH2, –CONH, –NH3 etc.
(capable of forming hydrogen bonds) or boron, silicon (capable of
ion binding ability of the pyridine moiety. Amalgamating these
approaches we have developed 1, wherein a pyridine ring has been
+
fused with the pyrazine frame. We demonstrate here unequivocally
forming Lewis adducts) centers act as binding sites for anions.2
that it is indeed the nitrogen atom of the pyridine moiety that
Simultaneous sensing of both types of charged analytes has been
binds the transition metal ions. It is shown that this new DPP
achieved by integrating cationic and anionic guest binding sites
derivative binds the metal ions much more strongly compared to
in a single molecule, where the cation binding site binds with
the previously reported DPP derivative which lacks the additional
a transition (Ni2+, Cu2+, Ag+, Cd2+) or alkali (Li+, Na+, K+) or
alkaline earth metal ion (Mg2+, Ca2+).4,5
fused pyridine ring.16–18 The sensing of F− and metal ions (Co2+,
Ni2+, Cu2+, Zn2+ and Cd2+) by 1 in acetonitrile is observable by
change in color from yellowish green to red in the case of the
former and to brown or light brown in the later cases. The binding
constants of the cations are found to be two orders of magnitude
higher than that of F−.
During recent years, there is an upsurge in the field of
colorimetric sensing of alkali, alkaline-earth and transition metal
ions by organic molecules.6 Among the cations, special attention is
devoted to develop chemosensors for transition metal ions: usually
they symbolize an environmental concern when present in uncon-
trolled amount but at the same time some of them such as iron,
cobalt, copper, zinc are present as essential elements in biological
systems. On the other hand, the halides, especially fluoride ion
is of major interest among the anions due to its detrimental role
(fluoresis and other diseases).7 The recently reported pyrrole-based
sensors namely, calix[4]pyrrole,8 dipyrrolylquinoxaline (DPQ)9–15
and dipyrrolylpyrazine (DPP)16–18 families of compounds have
been used for anion sensing by exploiting the hydrogen bond
forming ability of pyrrole NH with anions, with subsequent
change in color or fluorescence or redox potential. The last two
Experimental
Syntheses
The starting material 1,2-di(1H-2-pyrrolyl)-1,2-ethanedione (2)9,19
used for synthesis of 1 and the model compound pyrido[2,3-
b]pyrazine (3)20 were prepared by procedures published elsewhere.
The new sensor investigated in this study and its Ni(II) complex
were synthesized as detailed below.
2,3-Di(1H-2-pyrrolyl)pyrido[2,3-b]pyrazine (1). Diketone
2
(190 mg, 1.0 mmol) was dissolved in toluene (40 mL), and a
catalytic amount of BF3·Et2O was added to it. A hot solution of
2,3-diaminopyridine (120 mg, 1.1 mmol) in toluene (30 mL) was
School of Chemistry, University of Hyderabad, Hyderabad, 500 046, India.
E-mail: assc@uohyd.ernet.in; Fax: (+91) 40-2301-2460
† Deceased.
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added with stirring. The resulting mixture was heated at reflux
under nitrogen atmosphere for 8 h. Then, toluene was distilled
off under vacuum and the residue was taken up in a mixture
of CH2Cl2 (50 mL) and water (50 mL). The organic layer was
separated off and the aqueous layer was extracted with CH2Cl2
(3 × 30 mL). The organic layers were combined and dried over
anhydrous sodium sulfate, filtered and evaporated to dryness.
Column chromatography over neutral alumina (eluent: CH2Cl2–
MeOH, 99 : 1, v/v) afforded 1 (160 mg, 61%) as a yellow solid.
Anal. Calc. for C15H11N5: C, 68.96; H, 4.21; N, 26.83. Found: C,
was refluxed for 3 h and the solvent was evaporated to dryness.
The red solid was recrystallised from acetonitrile. Anal. Calc. for
C30H22N10NiCl2: C, 55.19; H, 3.37; N, 21.46. Found: C, 55.34; H,
3.42; N, 21.28%.
Instrumentation and methods
Hydrated perchlorate salts of the metals were used for cation titra-
tions. All anions, in the form of tetrabutylammonium salts, were
stored in a dessicator under vacuum containing self-indicating
silica and used for titration without any further purification.
Pyrrole (Ranbaxy Acros, Belgium) was dried over CaH2 (Ranbaxy,
Mumbai, India) and distilled under reduced pressure. All solvents
used in the spectroscopic studies were purchased from Ranbaxy
(Mumbai, India) and were distilled as per requirement by the
published procedures.21 CDCl3, (CD3)2SO and CD3OD were
purchased from Merck (Germany).
Care was taken to avoid the entry of direct, ambient light into
the samples in all the spectroscopic experiments described below.
Unless otherwise specified, all the experiments were carried out at
298 1 K.
The single crystals of 1 and 4 for X-ray diffraction studies
were grown by slow evaporation of acetonitrile solutions of the
compounds. Single-crystal X-ray data was collected on a Bruker-
Nonius SMART APEX CCD single-crystal diffractometer at
298 K, equipped with a graphite monochromator and a Mo-Ka
1
69.19; H, 4.28; N, 26.54%. H NMR (400 MHz, CDCl3, TMS)
d/ppm: 6.25–6.28 (1H, m), 6.30–6.33 (1H, m), 7.04–7.06 (2H, m),
7.10–7.12 (1H, m), 7.17–7.19 (1H, m), 7.50 (1H, dd, J = 4.0 Hz, 8.0
Hz), 8.21 (1H, dd, J = 2.0 Hz, 8.0 Hz), 8.91 (1H, dd, J = 2.0 Hz, 4.0
Hz), 9.60 (1H, br s), 10.04 (1H, br s). 13C NMR (50 MHz, CDCl3–
CD3OD, 3 : 1, v/v, TMS) d/ppm: 159.5, 152.8, 150.0, 148.7, 147.2,
138.7, 135.8,◦129.7, 125.1, 124.8, 123.2, 116.5, 114.9, 111.2, 110.9;
mp 155 1 C. FAB-MS: m/z 262 (M + H)+; UV-Vis (CH3CN)
kmax/nm (e/M−1 cm−1): 204 (25190), 263 (18220), 312 (7080), 350
(7810), 421 (12360).
1,2-Di(1H-2-pyrrolyl)-1,2-ethanedione (2). This was synthe-
sized as per the reported procedure published elsewhere.9,19 Anal.
Calc. for C10H8N2O2: C, 63.83; H, 4.25; N, 14.89. Found: C, 64.05;
H, 4.20; N, 14.63%. 1H NMR (200 MHz, (CD3)2SO, TMS) d/ppm:
6.25 (2H, m), 6.91 (2H, m), 7.34 (2H, m), 12.22 (2H, br s). UV-Vis
(CH2Cl2) kmax/nm (e/M−1 cm−1): 340 (15730).
˚
fine-focus sealed tube (k = 0.71073 A) operated at 2.0 kW. The
SMART software was used for data acquisition. The detector
frames were integrated by use of the program SAINT-Plus and
the intensities corrected for absorption by Gaussian integration
using the program SADABS. The structure solution was carried
out using direct methods. Full-matrix least-squares refinement
on F2 (including all data) was performed using the program
SHELXTL.22 The PLATON package was used for molecular
graphics.22 All non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms attached to pyrrole ni-
trogens were located from difference Fourier maps. All other
hydrogen atoms were fixed in their ideal geometric positions.
CCDC reference numbers 279117 and 279118.
Pyrido[2,3-b]pyrazine (3). This was synthesized as per the
reported procedure shown in Scheme 1.20 Anal. Calc. for C7H5N3:
C, 64.12; H, 3.82; N, 32.06. Found: C, 64.25; H, 3.96; N, 31.90%.
1H NMR (400 MHz, CDCl3, TMS) d/ppm: 7.77 (1H, dd, J =
3.6 Hz, 8.8 Hz), 8.51 (1H, dd, J = 2.0 Hz, 8.8 Hz), 8.70 (1H, d,
J = 1.6 Hz), 9.10 (1H, d, J = 1.6 Hz), 9.22 (1H, dd, J = 2.0 Hz,
4.0 Hz). 13C NMR (50 MHz, CDCl3, TMS) d/ppm: 154.3, 151.4,
147.8, 146.1, 138.6, 138.2, 125.4; mp 146 1 ◦C (lit.,20 146–147 ◦C).
LC-MS: m/z 131 (M+); UV-Vis (CH3CN) kmax/nm (e/M−1 cm−1):
255 (1890), 301 (6760), 308 (7010), 314 (8970), 354 (220).
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b510469f
The 1H and 13C NMR spectra were recorded on a Bruker
NR-200 AF-FT or Bruker AVANCE 400 NMR spectrometer at
ambient temperature using tetramethylsilane (TMS) as an internal
standard. UV-Visible spectra were recorded on a Shimadzu UV-
3101 PC spectrophotometer. A matched pair of quartz cuvettes
(path length = 1 cm) were employed for this purpose. Steady-
state fluorescence spectra were recorded in a four-walled, all-
transparent, quartz cell (path length = 1 cm) using a FluoroMax-
3 (Jobin Yvon-Spex) spectrofluorimeter for solutions having low
optical densities at the wavelengths of excitation. The fluorescence
quantum yield (φf) was estimated by integrating the fluorescence
band and by using 1,6-diphenyl-1,3,5-hexatriene (φf = 0.80 in
cyclohexane)23 as the standard.
Scheme 1 Synthesis of 1, 3 and 4.
Absorption titration studies
Dichlorobis(2,3-di(1H -2-pyrrolyl)pyrido[2,3-b]pyrazine)nickel-
(II) (4). 38 mg (0.14 mmol) of 1 was dissolved in methanol and
17 mg (0.07 mmol) of NiCl2 (in methanol) was added. The solution
The binding studies of sensor 1 against different cations and anions
were carried out in CH3CN. Typically, a solution of 6.5 lM of 1
796 | Dalton Trans., 2006, 795–801
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was taken in the cuvette and titrated with increasing volume of
concentrated solution of a given cation or anion. The binding
constant (K) between 1 and cation/anion was evaluated from the
change in absorbance at 490 nm by the following method.24,25
The binding constant (K) for 1 : 1 complexation can be
represented as
1 compared to a previously reported DPP derivative16–18 supports
our above justification (vide Cation sensing section).
X-Ray structures of 1 and 4
In order to determine whether one of the pyrazine nitrogen atoms
or the pyridine nitrogen atom or both are involved in interaction
with the metal ions, the Ni(II) complex of 1, i.e. 4 was synthesized
and its structure obtained by X-ray diffraction analysis.
[C]
K =
(1)
([F]0 − [C]) ([M]0 − [C])
where, [C] represents the equilibrium concentration of the complex
and [F]0 and [M]0 represent the initial concentration of the sensor
and metal ion/anion, respectively.
By substituting the term DA/De for [C] (for a path length of 1
cm) the following equation can be derived,
The solid-state structures of 1 and 4 are shown as an ORTEP
diagram in Fig. 1 and the crystallographic data are shown in
Table 1. There are major differences in the spatial orientation
of the two pyrrole rings with respect to the pyridopyrazine plane.
In the case of 1, both the pyrrole rings are directed such that the
pyrrole NH are on the same side of the pyrazine nitrogen atoms, the
dihedral angles between pyrrole rings and pyridopyrazine plane
being 23.2 and 36.1◦. The picture is quite different in the case of 4.
Ni(II) is coordinated with the pyridine nitrogens of two molecules
of 1 and two Cl−, having a distorted tetrahedral geometry with
the pyridine N–Ni–N bond angle 96.88◦ and Cl–Ni–Cl bond
angle 99.75◦. The two pyrrole NH bonds point to each other
and both are in different planes from the fused pyridopyrazine
ring plane. Here, all the dihedral angles between the pyrrole rings
and pyridopyrazine ring of two ligands are different: 18.4 and
ꢀ
ꢁ
[F]0[M]0
DA
1
1
=
[F]0 + [M]0 −
+
(2)
DA
De De KDe
where, DA is the change in absorbance due to the addition of
cation/anion, [F]0 is the initial concentration of 1, [M]0 is the
concentration of cation/anion, De is the difference between the
molar extinction coefficient of the complex and 1 and K is the
binding constant.
A plot of [F]0[M]0/DA vs. ([F]0 + [M]0 − (DA/De)) would yield
a straight line with slope 1/De and intercept 1/KDe. However,
knowledge of the unknown quantity De is needed to make this
plot.
Consequently, a tentative value of De is determined by using
data from two solutions and solving eqn (2) simultaneously for
De and K. Using this value of De a plot is made employing data
from a series of solutions, and a new value of De is determined
along with a new value of K. This procedure is repeated until a
consistent set of values for both De and K have been obtained from
two successive plots.
For anions, the binding constant was also evaluated by the
change in absorbance at 490 nm with anion concentration using
the following non-linear equation.25
DA
b
QtKDe[L]
1 + K[L]
=
(3)
where, DA refers to the change in absorbance from initial value
at the required wavelength, b is cuvette path length (in cm), Qt is
total concentration of sensors, K is the binding constant between
1 and anion, De is the difference between the molar extinction
coefficient of free and bound sensor and [L] is the concentration
of titrated anion. The K values obtained by the two methods
agreed reasonably well (within 10%).
Results and discussion
The scheme leading to the synthesis of 1, 3 and 4 is illus-
trated in Scheme 1. Reaction between 1,2-di(1H-2-pyrrolyl)-1,2-
ethanedione (2) and 2,3-diaminopyridine produced sensor 1 in
∼60% yield. Sensor 1 can be considered structurally as a DPP
derivative, well known for its potential as an F− sensor, having
an additional pyridine ring fused with the pyrazine ring as part
of its extended p framework. The justification behind the fusion
of the pyridine ring is to make 1 work as a cation sensor as well.
This is because pyridine can act as a cation binding centre in a
sensor molecule.26 In fact, a stronger metal binding property of
Fig. 1 X-Ray structures of (A) 1 and (B) 4, with all heavy atoms labeled.
The thermal ellipsoids were scaled to the 30% probability level.
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Table 1 Crystallographic data for 1 and 4
The influence of solvent on the absorption, fluorescence and
fluorescence excitation spectra of 1 has been investigated in
detail with a view to understanding the origin of the lowest
energy transition in the absorption and fluorescence spectra.
The fluorescence excitation spectrum of the system in any given
solvent is found identical with the absorption spectrum in the
same solvent. Fig. 3 shows the fluorescence and fluorescence
excitation spectra of the systems. The most noticeable feature of
these spectra is the loss of structure in polar media. That the band
positions are not so sensitive to the polarity of the medium in
the aprotic medium suggest that the charge transfer contribution
in these transitions is minimal. Reasonably high molar extinction
coefficient of the absorption band and fairly high fluorescence
efficiency of 1 (see later) suggest that the lowest energy transition
in absorption and fluorescence is p–p* in nature. However, in protic
media a blue shift of the absorption (or excitation) band suggests
that an n–p* state is in close proximity with the p–p* state.28
Since the blue shift is not observable in emission, it is evident that
irrespective of the nature of the solvent, the emission always occurs
from the p–p* state, which is the lowest excited state in the system.
Ligand/Complex
1
4
Chemical formula
Formula weight
Crystal system
Space group
C15H11N5·2H2O
297.32
NiC30H22Cl2N10
652.19
Triclinic
Triclinic
¯
¯
P1
P1
˚
a/A
6.7615(6)
9.3751(9)
12.2200(11)
83.376(2)
89.496(2)
75.250(2)
743.92(12)
2
6.9445(4)
12.8462(8)
17.3296(11)
110.3790(10)
90.8100(10)
94.5210(10)
1443.28(15)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
l/mm−1
0.093
0.898
298
13866
5058
3143
388
0.0724, 0.1991
0.1103, 0.2241
1.040
0.566, −0.532
T/K
298
8661
3453
2678
Reflections collected
Unique reflections
Reflections [I ≥ 2r(I)]
Parameters
219
R1, wR2 [I ≥ 2r(I)]
R1, wR2 (all data)
Goodness-of-fit on F2
0.0651, 0.1874
0.0792, 0.2003
1.059
1.034, −0.304
Largest peak and
−3
˚
hole/e A
20.6◦ in one ligand and 10.7 and 25.0◦ in the other. There are two
molecules of 4 in unit cell but no intermolecular hydrogen bonding
is observed like 1. It is also important to note that only pyridine
nitrogen is bonded with Ni(II), not the pyrazine nitrogen atoms.
This observation is consistent with the recent finding of Manson,
who has shown that the pyridine nitrogen atom of 3 is involved in
coordination with Cu2+.27
Spectral characterization
The UV-visible spectra of 1 and 3 in CH3CN are depicted in
Fig. 2. It can be seen that the spectrum of 1 appears as somewhat
structured but it lacks the fine structure of the spectrum of 3.
Further, the lowest energy absorption band of 1 appears at a
much longer wavelength relative to that of 3. These observations
are consistent with an extended p-conjugation and N–H · · · N
interaction between the pyrrolic and pyrazinic rings in 1.
Fig. 3 Fluorescence excitation and fluorescence spectra of 1 in different
solvents: cyclohexane (black, —), toluene (red, – –), 1,4-dioxane (green,
· · ·), tetrahydrofuran (blue, -·-), acetonitrile (light blue, —··), 2-propanol
(purple, ---) and methanol (orange, —). Conditions for fluorescence
excitation spectra of 1 in different solvents: kem = 505 nm, excitation and
emission slit = 2 nm. Conditions for fluorescence spectra of 1 in different
solvents: kexc = 395 nm, OD at 395 nm = 0.017, excitation and emission
slit = 2 nm.
1 is found to be strongly fluorescent; the fluorescence quantum
yield, φf value is estimated to be 0.29 in CH3CN. We have
also studied the fluorescence decay behavior of the system in
tetrahydrofuran and acetonitrile. A single exponential fit to the
decay profiles was found satisfactory from the v2 values and the
residuals. The measured lifetimes are 2.5 ns in acetonitrile and
3.4 ns in tetrahydrofuran.
Fig. 4 illustrates the 1H NMR spectrum of sensor 1. The
spectrum was analyzed (see Experimental section) based not
only on the chemical shift and integrated intensity data of the
various peaks appearing in the 1D spectra but also on the proton
connectivity patterns observed in the corresponding 1H–1H COSY
spectra. The four b-pyrrole protons resonate at four distinct signals
as multiplets. While two of them (Hb and Hbꢀ) appear at upfield,
other two (Hc and Hcꢀ) shift at downfield compared to Ha and Haꢀ
in 1H NMR spectrum. Also, one of the two pyrrolic NH protons
Fig. 2 Comparison of the absorption spectra of 1 (—) and 3 (---) in
CH3CN.
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Table 2 Cationa and anionb binding constants for 1 in CH3CN deter-
mined by absorption titration method at 25 ◦Cc
Ion
10−5K/M−1
Mn2+
Co2+
Ni2+
Cu2+
Zn2+
Cd2+
F−
NDd
3.2
11.0
15.0
4.8
0.68
0.049
a Counter anion was perchlorate salts for all cases. b Counter cation was
tetrabutylammonium salts. c All errors are 10%. d Change in UV-Visible
spectra was not enough to calculate binding constant.
disappears and a new band appears at a much longer wavelength
(kmax = 496 nm). The presence of an isosbestic point implies that 1
and 1·Ni2+ are in equilibrium in the course of titration. The spectral
changes observed are similar for Co2+, Cu2+, Zn2+ and Cd2+ ions.
The change in the absorbance at 490 nm has been used to calculate
the binding constant, K, using eqn (2). That the sensor 1 binds
the transition metal ions with varying degree of strength is evident
from the binding constant values shown in Table 2. Ni2+ and Cu2+
bind more strongly (K ∼ 106 M−1), compared to Co2+ and Zn2+ for
which the K values are one order of magnitude less. The metal ion
binding is associated with a color change from yellowish green to
brown and is observable by naked eye. In the case of Cd2+, the color
changes from yellowish green to light brown, whereas no change
in color of 1 is observed on addition of Mn2+. The color changes
are illustrated in Fig. 5. That an additional pyridine ring does
help metal ion binding is evident from the fact that our previously
reported DPP derivative16–18 bearing a cyano functionality does not
show a noticeable change in the absorption spectrum on addition
of Mn2+, Co2+, Ni2+, Zn2+ and Cd2+ ions. With Cu2+, a decrease in
absorbance of the 421 nm band is observed. This corresponds to a
K value of 2.27 × 104 M−1, which is two orders of magnitude less
than that observed for the present system.
Fig. 4 1H NMR spectrum of 1 (CDCl3, TMS); * indicates the solvent
signal for CDCl3.
appears 0.4 ppm downfield of the other. The pyrazine ring creates
a non-equivalent magnetic environment for the pyrrole rings by
virtue of intramolecular hydrogen bonding between pyrrole NH
and pyrazine nitrogen, and the electron withdrawing conjugation
effect of pyridine. However, the two a-pyrrole protons (Ha and
Haꢀ), being unaffected by such an effect, appear together at 7.04–
7.06 ppm.
Cation sensing
Fig. 5 shows typical changes in the absorption spectrum of 1 on
addition of the metal salts in acetonitrile. The peak around 420 nm
Anion sensing
Fig. 6 shows the change in absorption spectra of 1 on addition of
F− in CH3CN solution. Upon addition of tetrabutylammonium
fluoride (TBAF), the peak around 420 nm disappears, while a
new band at 490 nm appears with an isosbestic point at 458 nm
indicating equilibrium between 1 and 1·F− during the course of
titration. The binding is associated with a color change from
yellowish green to red and is observable by the naked eye. With
other halogen anions and ClO4−, no color change was observed.
This observation can be attributed to the small size of the F−
compared to the other anions. The binding constant for F−
complexation is estimated as 4.9 × 103 M−1. Restoration of the
original spectrum of the sensor system from this sensor-F− adduct
upon addition of a trace amount of water/methanol not only
suggests that the complexation between F− and 1 is reversible
in nature but also lends further support to the proposition that
hydrogen-bonding is involved in the binding between the sensor
and F−.
Fig. 5 Top: Absorption spectral changes observed for 1, upon addition
of Ni2+ ion in CH3CN at 298 K. [1] = 6.5 × 10−6 M; [Ni2+] = (0, 0.33,
0.66, 1.16, 1.66, 2.32, 2.98, 3.80, 4.62, 5.60, 6.58, 7.71, 9.00, 10.44, 12.03,
13.93) × 10−6 M. Bottom: Color changes observed for 1 in CH3CN upon
Comparison of the binding constant values reveals that 1 binds
the cations much more strongly (K values are two orders of
magnitude higher) compared to the F−. The X-ray structure of
−
the addition of cations (as ClO4 salts). From left to right: none, Mn2+
,
Co2+, Ni2+, Cu2+, Zn2+ and Cd2+
.
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different manners. First, we have titrated 1 with F− (using TBAF)
in the presence of 2 equivalents of metal ions, Mn+. Second, we
have titrated 1 with Mn+ in the presence of 100 equivalents of
TBAF. We chose Zn2+ and F− for the dual sensing experiment as
the Zn2+ salts are colourless and it is possible to observe distinct
changes due to metal complexation/decomplexation without any
interference from the colour of the salts. The spectral changes are
shown in Fig. 7. As can be seen, initial titration of 1 and two
equivalents of Zn2+ with TBAF results in a drastic reduction of
the intensity of the 500 nm band suggesting sequestering of the
metal ion to form an ion pair with F− ion in solution, as has been
recently observed in some cases.29,30 However, as expected, when
sequestering of the metal ion is complete, further addition of F−
ion leads to expected enhancement of the intensity of the 500 nm
band due to complexation of F− ion. The binding constant of 1
with F− in the presence of 2 equivalents of Zn2+ is estimated as
550 M−1, which is almost one order of magnitude less than that
observed in the absence of competing metal ions. In the second
experiment, which is carried out in the presence of 100 equivalents
of F−, addition of Zn2+ ion leads to a gradual reduction of the
intensity of the 500 nm band due to the sequestering of F− ion.
The fact that the concentration of F− ion in solution is far in excess
of the Zn2+ ion, the condition in which all the F− is completely
uncomplexed is never met in this case and hence, enhancement of
the 500 nm band intensity at a later stage, as has been noticed in the
previous experiment, is not observed in this case. Moreover, the
presence of large concentration of F− in the solution engages the
metal ions in ion pair formation in solution rather than engaging
them in complexation. We repeated similar experiments with other
metal ions such as Ni2+ and Cu2+ that do not have such high affinity
towards F−. With these metal ions, the sharp drop of absorption
Fig. 6 Top: Absorption spectral changes observed for 1, upon addition
of F− ion in CH3CN at 298 K. [1] = 6.5 × 10−6 M; [F−] = (0, 2.33, 3.98,
6.29, 8.92, 12.18, 16.39, 21.21, 32.88, 42.14, 54.22, 68.9) × 10−5 M. Bottom:
Color changes observed for 1 in CH3CN upon the addition of anions (as
−
TBA+ salts). From left to right: none, F−, Cl−, Br−, I−and ClO4
.
4 clearly reveals that the pyridine nitrogen atom is involved in
binding of the metal ions. It is already well known that hydrogen
bonding interaction between F− and pyrrole NH moieties is
responsible for binding of F−.9–18 Binding between the DPP moiety
and F− was explained with the help of a model,9 supported
by theoretical calculations in our previously reported work.18
According to this model, the binding of F− is facilitated by rotation
of the pyrrole rings of DPP moieties in such a way that the NH
protons are directed towards the lone pairs of the anion.9,18
A
rotation to a different plane of such type is expected to allow the
pyrrole rings to assume ‘bite angle’ suitable for the size of F− and
to position them away from the pyrazine chromophore, leading
to perturbation in the orbital overlap between the pyrrole and
pyrazine subunits.
Although, generation of a new peak around 500 nm is common
to both cation and F− titrations, careful examination of Fig. 5
and 6 reveals that the spectra for the finally formed 1·M2+
(M = Co, Ni, Cu, Zn) and 1·F− complexes are different. The
appearance of a long wavelength band on absorption titration
can perhaps be attributed to an enhanced conjugation between
the pyrrole and pyridopyrazine rings on complexation. The X-ray
crystallographic studies suggest that the dihedral angles between
the pyrrole and pyridopyrazine rings are reduced to some extent
on metal complexation allowing a better conjugation. The color
change for F− binding can be explained with the help of a model
which has been used to explain similar observation for other DPP
compounds previously.9,18
Sensing in the presence of competing ions
Fig. 7 Change in absorption spectra of (A) 1 (6.5 lM) on addition of
Zn2+ ([Zn2+] = 13 lM, spectrum 1) and then, titration with TBAF, [F−] =
(2.03, 10.12, 20.18, 40.11, 79.21, 154.51) × 10−5 M (spectra 2–7); (B) 1 (6.5
lM) on addition of TBAF ([F−] = 6.5 × 10−4 M, spectrum 1) and then,
titration with Zn2+, [Zn2+] = (0.45, 0.85, 1.16, 1.78, 2.96) × 10−5 M (spectra
2–6).
Since the present system consists of two distinct binding sites for
the metal ions and F−, we thought it appropriate to examine the
effect of competing F− ion on the binding ability of 1 for metal
ions and vice versa. These experiments have been carried out in two
800 | Dalton Trans., 2006, 795–801
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shown in Fig. 7(A) could not be observed because of high binding
constant of 1 with the metal ions and low affinity of these metal
ions with F−. Nevertheless, it is evident that although 1 contains
distinct binding sites for both anion and cation, simultaneous
binding of both the species, as observed exploiting the allosteric
effect,31 through-bond electrostatic effect32 or to the host as an
associated ion-pair,29 is not possible in this case unless the total
concentration of the two species is much less than what a particular
binding site can accommodate.
M. R. Taylor and K. P. Wainwright, Inorg. Chem., 2002, 41, 1093;
(c) L.-L. Zhou, H. Sun, H.-P. Li, H. Wang, X.-H. Zhang, S.-K. Wu and
S.-T. Lee, Org. Lett., 2004, 6, 1071.
6 (a) G. G. Talanova, H.-S. Hwang, V. S. Talanov and R. A. Bartsch,
Anal. Chem., 2001, 73, 5260; (b) J. V. Ros-Lis, R. Martinez-Manez, K.
Rurack, F. Sancenon, J. Soto and M. Spieles, Inorg. Chem., 2004, 43,
5183; (c) E. Arunkumar, A. Ajayaghosh and J. Daub, J. Am. Chem.
Soc., 2005, 127, 3156; (d) T. Gunnlaugsson, J. P. Leonard and N. S.
Murray, Org. Lett., 2004, 6, 1557; (e) L. Jia, Y. Zhang, X. Guo and X.
Qian, Tetrahedron Lett., 2004, 45, 3969.
7 (a) K. L. Kirk, Biochemistry of the Halogens and Inorganic Halides,
Plenum Press, New York, 1991, p. 58; (b) A. Wiseman, Handbook
of Experimental Pharmacology XX/2, Part 2, Springer-Verlag, Berlin,
1970, pp. 48–97.
8 (a) K. A. Nielsen, J. O. Jeppesen, E. Levillain and J. Becher, Angew.
Chem., Int. Ed., 2003, 42, 187; (b) H. Miyaji, W. Sato and J. L. Sessler,
Angew. Chem., Int. Ed., 2000, 39, 1777.
9 C. B. Black, B. Andrioletti, A. C. Try, C. Ruiperez and J. L. Sessler,
J. Am. Chem. Soc., 1999, 121, 10438.
10 P. Anzenbacher, Jr., A. C. Try, H. Miyaji, K. Jursikova, V. M. Lynch,
M. Marquez and J. L. Sessler, J. Am. Chem. Soc., 2000, 122, 10268.
11 T. Mizuno, W.-H. Wei, L. R. Eller and J. L. Sessler, J. Am. Chem. Soc.,
2002, 124, 1134.
12 P. Anzenbacher, Jr., D. S. Tyson, K. Jursikova and F. N. Castellano,
J. Am. Chem. Soc., 2002, 124, 6232.
13 J. L. Sessler, H. Maeda, T. Mizuno, V. M. Lynch and H. Furuta, Chem.
Commun., 2002, 862.
Summary
In summary, the DPP derivative 1 has been synthesized and
characterized by different methods. The crystal structures of 1
and 4, which are essential for an understanding of the change
in orientation of pyrrole rings on metal complexation and deter-
mining the coordinating site of the metal, have been determined.
The binding constants of 1 with several transition metal ions have
been found to be two orders of magnitude higher than that for
the F− in acetonitrile. The sensing of the metal ions and F− is
visible by naked eye observation of the colour change of the
system. We are currently exploring the signaling behaviour of more
such dipyrrolyl derivatives endowed with suitable cation binding
substituents.
14 J. L. Sessler, H. Maeda, T. Mizuno, V. M. Lynch and H. Furuta, J. Am.
Chem. Soc., 2002, 124, 13474.
15 D. Aldakov and P. Anzenbacher, Jr., Chem. Commun., 2003, 1394.
16 J. L. Sessler, G. D. Pantos, E. Katayev and V. M. Lynch, Org. Lett.,
2003, 5, 4141.
17 T. Ghosh and B. G. Maiya, Proc. Indian Acad. Sci. (Chem. Sci.), 2004,
116, 17.
Acknowledgements
18 T. Ghosh, B. G. Maiya and M. W. Wong, J. Phys. Chem. A, 2004, 108,
T. G. and A. S. thank the Council of Scientific and Industrial
Research, New Delhi and Department of Science and Technology,
New Delhi for financial support of this work. The UPE program
of the University Grants Commission, New Delhi is gratefully
acknowledged for some of the instrumentation facilities.
11249.
19 D. Behr, S. Brandange and B. Lindstrom, Acta Chem. Scand., 1973, 27,
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20 M. C. Venuti, Synthesis, 1982, 61.
21 D. D. Perrin, W. L. F. Armarego and D. R. Perrin, Purification of
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(b) G. M. Sheldrick, SADABS Empirical Absorption Correction Pro-
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24 S. Banthia and A. Samanta, J. Phys. Chem. B, 2002, 106, 5572.
25 A. K. Connors, Binding Constants: The Measurement of Molecular
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