Ferrocene-based multichannel molecular chemosensors with high selectivity and sensitivity for Pb(ii) and Hg(ii) metal cations

Article abstract of DOI:10.1039/c0dt00450b

The synthesis, electrochemical, optical and cation-sensing properties of ferrocene-imidazoquinoxaline dyads 6, are presented. Dyad 6a behaves as a highly selective redox, chromogenic and fluorescent chemosensor molecule for Pb ²⁺ cations in CH₃CN solutions; the oxidation redox peak is anodically shifted (ΔE_1/2 = 110 mV); in the absorption spectrum a new low-energy band appeared at λ = 463 nm, and the emission band is red-shifted (Δλ = 31 nm) along with an important chelation-enhanced fluorescence factor (CHEF = 276), upon complexation with this metal cation. The dyad 6b, bearing two additional pyridine rings as substituents, has shown its ability for sensing Hg²⁺ cations through three different channels: the oxidation peak is anodically higher shifted (ΔE_1/2 = 300 mV), a new low-energy band appears in the absorption spectrum at λ = 483 nm, and the emission band was also red-shifted (Δλ = 28 nm) and underwent an important chelation-enhanced fluorescent factor (CHEF = 227). The changes in their absorption spectra are accompanied by color changes from yellow to orange which allow their potential use for the naked eye detection of these metal cations. Linear sweep voltammetry revealed that Cu²⁺ cations induced oxidation of the ferrocene unit in both dyads, which is accompanied by an important increase of the emission band.

Full text of DOI:10.1039/c0dt00450b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Ferrocene-based multichannel molecular chemosensors with high selectivity
and sensitivity for Pb(II) and Hg(II) metal cations†
Mar´ıa Alfonso, Alberto Ta´rraga* and Pedro Molina*
Received 10th May 2010, Accepted 30th June 2010
DOI: 10.1039/c0dt00450b
The synthesis, electrochemical, optical and cation-sensing properties of ferrocene-imidazoquinoxaline
dyads 6, are presented. Dyad 6a behaves as a highly selective redox, chromogenic and fluorescent
chemosensor molecule for Pb2+ cations in CH3CN solutions; the oxidation redox peak is anodically
shifted (DE1/2 = 110 mV); in the absorption spectrum a new low-energy band appeared at l = 463 nm,
and the emission band is red-shifted (Dl = 31 nm) along with an important chelation-enhanced
fluorescence factor (CHEF = 276), upon complexation with this metal cation. The dyad 6b, bearing two
additional pyridine rings as substituents, has shown its ability for sensing Hg2+ cations through three
different channels: the oxidation peak is anodically higher shifted (DE1/2 = 300 mV), a new low-energy
band appears in the absorption spectrum at l = 483 nm, and the emission band was also red-shifted
(Dl = 28 nm) and underwent an important chelation-enhanced fluorescent factor (CHEF = 227). The
changes in their absorption spectra are accompanied by color changes from yellow to orange which
allow their potential use for the “naked eye” detection of these metal cations. Linear sweep
voltammetry revealed that Cu2+ cations induced oxidation of the ferrocene unit in both dyads, which is
accompanied by an important increase of the emission band.
Among heavy metal, lead is the most abundant and ranks
Introduction
second in the list of toxic substances and is often encountered
The development of selective and sensitive imaging tools capable
due to its wide distribution in the environment as well as its
of monitoring heavy- and transition-metal ions has attracted con-
current and previous use in batteries, gasoline and pigments.
siderable attention because of the wide use of these metal ions and
Lead pollution is an ongoing danger to the human health,
their subsequent impact on the environment and nature.1 The Hg2+
particularly in children (memory loss, irritability, anaemia, muscle
paralysis, and mental retardation),12 and the environment, as
most of the 300 million tons of this heavy metal mined to date
are still circulating in soil and groundwater.13 Despite efforts to
reduce global emissions, lead poisoning remains the worlds most
common environmentally caused disease.14 Thus, the level of this
detrimental ion, which is present in tap water as a result of
dissolution from household plumbing systems, is the object of
several official norms. The World Health Organisation established
in 1996 guidelines for drinking-water quality,15 which included
a lead maximal value of 10 mg L-1. The US Center for Disease
Control (CDC) set standards stating that a 10–19 mgdL-1 level
of lead in blood poses a potential threat and that diagnostic
testing is recommended.16 Thus, keeping in view the role of Pb2+,
the detection and monitoring of this metal cation by methods
which allow the development of selective and sensitive assays
becomes very important. As many heavy metals are known
as fluorescence quenchers via enhanced spin-orbital coupling,17
energy or electron transfer,18 development of fluorescent sensors
for Pb2+ presents a challenge. In this context, considerable efforts
have been undertaken to develop fluorescent chemosensors for
Pb2+ ions based on peptide,19 protein,20 DNAzyme,21 polymer,22
and small-molecule23 scaffolds. There is, however, a paucity of use
of multichannel receptors as potential guest reporters via multiple
signalling patterns. Specifically, as we report here, the development
of multichannel (chromogenic/fluorogenic/electrochemical) Pb2+
selective chemosensors is, as far as we know, an unexplored
subject,24 and only a few dual chromogenic and redox receptors
have been recently described.25
ion is considered a highly toxic element, and its contamination is a
global problem and a major source of human exposure stems from
a variety of natural and anthropogenic sources2 including oceanic
and volcanic emission,3 gold mining,4 solid waste incineration, and
combustion of fossil fuel.5 The exposure of mercury even at low
concentration leads to digestive, kidney, and specially neurological
diseases.6 Although the pollutant character of mercury has mainly
been focused on the occurrence of methylmercury in the marine
food chain, the mercury problem is not limited to the aquatic
food chain since rice has recently been proposed as the major
source for methylmercury intake from food in parts of the Chinese
population, and the results indicate that this derivative is more
prominent in the rice grains than would be expected from the
occurrence of methylmercury versus inorganic mercury in the soil.7
Keeping in view the roles placed by mercury in day to day life, the
development of techniques for mercury hazard assessment and
mercury pollution management has drawn worldwide attention.8
Therefore, many laboratories have focused on “colorimetric”,9
redox active10 and/or fluorescent11 highly selective mercury-
responsive small-molecule chemosensors.
Departamento de Qu´ımica Orga´nica, Universidad de Murcia, Campus de Es-
pinardo, 30100, Murcia, Spain. E-mail: pmolina@um.es, atarraga@um.es;
Fax: +34 968 364 149; Tel: +34 968 367 496
† Electronic supplementary information (ESI) available: NMR spectra
of the new compounds reported. LSV, CV and OSWV, UV-vis and
fluorescence titration data. Reversibility experiments, semi-logarithmic
plot for determining the detecting limits; 1H NMR titration data; ESI
mass spectra of the complexes formed. See DOI: 10.1039/c0dt00450b
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On the other hand, quinoxalines26 are a well known class
of fluorescent compounds with high quantum yields and have
attracted much attention due to their potential for speciality and
high-technology applications. Recently, a new class of fluorescent
anion sensor bearing extended conjugated quinoxalines has been
described, which incorporate one or several pyrrolyl units as
the anion recognition element.27 Recently, a new chemosen-
sor molecule based on a ferrocene-azaquinoxaline dyad has
been reported, which effectively recognizes Hg2+ in aqueous
environment.28
In the context of this work, it has been found that annulation
of an additional azaheterocycle, such as an imidazole ring, could
impart an interesting behaviour. In this new structural motif, one
of the nitrogen atoms of the added heterocyclic system cooperates
with the basic nitrogen atom of the quinoxaline ring and conse-
quently enhances the binding affinity towards metal cations. It is
expected that the binding ability of the resulting fused imidazo-
quinoxaline ring system could be improved if additional binding
sites in the guise of azaheterocycles are placed at appropriate
positions in the quinoxaline core. On the other hand, in ferrocene
derivatives, cation (anion) binding at an adjacent receptor site
induces a positive (negative) shift in the redox potential of the
ferrocene/ferrocenium redox couple, and the complexation ability
of the ligand can be switched on and off by varying the applied
electrochemical potential. The magnitude of the electrochemical
shift (DE1/2) upon complexation represents a quantitative measure
of the perturbation of the redox center induced by complexation
to the receptor unit.8b,29
Scheme 1 Synthesis of ligands 6a and 6b. Reagents and conditions: (a)
acetonitrile, 50 ◦C; (b) 4-amino-1,2,4-triazole, KtBuO, DMSO, 30 ◦C; (c)
N2H4·ClH, Pd/C, reflux; (d) ferrocenecarboxaldehyde, PhNO2, 60–70◦ C.
Pb2+ (DE1/2 = 110 mV), promotes remarkable responses (Fig. 1
and ESI†), addition of Li+, Na+, K+, Ca2+, Mg2+, and Ni2+
metal ions have no effect on either the LSV or on the CV or
OSWV of this receptor, even when present in a large excess.
The positive potential shift observed for the Fe(II)/Fe(III) redox
couple upon complexation by these metal cations can be due to
electrostatic repulsion effect between the bound metal cation and
the electrogenerated positive charge on the oxidized ferrocenyl
subunit. This leads to a decrease of the association constant with
the oxidized ligand and to a destabilization of the complex. Thus,
the DE1/2 reflects the balance of the interaction of the metal cation
between the neutral and the oxidized charged ligand.
Results and discussion
The target receptors 6 were prepared by the following four-
step sequence: (a) condensation of the commercially available 4-
nitro-o-phenylendiamine with the corresponding 1,2-dicarbonyl
compound 2 to give quinoxalines 3 (70–89%); (b) amination at
position 6 of the quinoxaline ring with 4-amino-1,2,4-triazole
in the presence of KtBuO in DMSO at room temperature to
provide 4 (71–90%); (c) reduction of the nitro group with hydrazine
in the presence of Pd on charcoal to afford 5 (74–93%), and
(d) imidazole-ring formation by reaction of diamines 5 with
ferrocenecarboxaldehyde in nitrobenzene at 60–70◦ to yield 6 (43–
48%) (Scheme 1).
Metal recognition properties of these receptors were evaluated
by electrochemical, (CV, LSV and OSWV) as well as through
1
Fig. 1 Evolution of the OSWV (left) and CV (right) of 6a (10-3 M) in
CH3CN using [(n-Bu)4N]ClO4 as supporting electrolyte when Pb(ClO4)2 is
added: from 0 (black) to 1 equivalent (deep green).
spectrophotometric and H NMR techniques. The titration ex-
periments were further analyzed using the computer programme
Specfit.30
At first, their electrochemical behaviour was investigated in the
presence of several metal cations such as Li+, Na+, K+, Ca2+,
Mg2+,Cu2++, Zn2+, Cd2+, Hg2+, Ni2+ and Pb2+ as their appropriate
salts.31 Each free receptor exhibited a reversible one-electron
redox wave32 typical of a ferrocene derivative, at the halfwave
potential value of E1/2 = 450 mV for 6a and E1/2 = 530 mV for
6b, calculated versus decamethylferrocene (DMFc) redox couple.
Titration studies by addition of the above-mentioned set of metal
cations to an electrochemical solution of receptor 6a (X = CH)
(c = 10-3 M) in CH3CN containing TBAP (0.1 M) as supporting
Remarkably, LSV studies carried out upon addition of Cu2+ to
the CH3CN solution of this receptor showed a significant shift
of the sigmoidal voltammetric wave towards cathodic current,
indicating that this metal cation promote the oxidation of the
free receptor (see ESI†).
Similar studies developed using receptor 6b (X = N), under the
same electrochemical conditions, demonstrate that while addition
of Zn2+ (DE1/2 = 90 mV), Cd2+ (DE1/2 = 100 mV), Ni2+ (DE1/2
=
80 mV) and Pb2+ (DE1/2 = 70 mV), promotes a moderate shift of
the oxidation wave of the Fe(II)/Fe(III) redox couple (see ESI†),
addition of Hg2+ (DE1/2 = 300 mV), induced a remarkable shift
electrolyte, demonstrate that while addition of Zn2+ (DE1/2
=
60 mV), Cd2+ (DE1/2 = 90 mV), Hg2+ (DE1/2 = 160 mV), and
8638 | Dalton Trans., 2010, 39, 8637–8645
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Table 1 Characteristic electrochemical, UV-vis and fluorescent data of the free ligands 6a and 6b and their metal complexes
a
f
Compound E1/2 (DE1/2
)
UV-vis lmax (10-3e)b
IPc
Fluorescence lemis Ud (CHEF)e Kas
Dlim
6a
450
560 (110)
270 (3.158), 313 (3.655)
253 (2.921), 313 (3.667), 350
(1.696), 463 (0.221)
270 (2.588), 313 (3.645), 350
(1.517), 463 (0.187)
270 (2.396), 313 (3.236), 350
(1.518), 463 (0.178)
258 (2.461), 313 (3.079), 350
(1.526), 463 (0.207)
277 (2.156), 306 (2.309)
284 (1.575), 311 (1.518), 365
(0.881), 483 (0.109)
284 (1.695), 310 (1.509), 365
(0.776), 483 (0.106)
284 (2.045), 311 (2.045), 365
(0.888), 483 (0.109)
284 (1.835), 310 (1.674), 365
(0.701), 483 (0.121)
484
264, 324, 391, 436 515
3.6 ¥ 10-4
[6a·Pb2+
]
]
]
0.022 (276)
8.29 ¥ 105 ( 1.39)5 5.25 ¥ 10-6
g
[6a·Zn2+
[6a·Cd2+
[6a·Hg2+
510 (60)
540 (90)
610 (160)
230, 264, 337,
391, 441
234, 346, 391, 256
—
—
—
—
—
—
2.34 ¥ 1011
( 11.41)
1.31 ¥ 10-5
1.72 ¥ 10-5
1.15 ¥ 10-5
h
h
1.56 ¥ 1011
( 11.27)
]
234, 262, 343,
389, 439
2.19 ¥ 1011
( 10.05)
6b
530
640 (70)
507
—
6.1 ¥ 10-4
—
[6b·Pb2+
[6b·Zn2]
[6b·Cd2+
[6b·Hg2+
]
262, 340
1.16 ¥ 1012
( 11.06)
7.58 ¥ 10-6
1.16 ¥ 10-5
1.50 ¥ 10-5
h
h
h
590 (90)
620 (100)
830 (300)
620 (80)
260, 346
—
—
9.57 ¥ 1011
( 12.54)
]
261, 337, 420
262, 357, 411
263, 339
—
—
5.57 ¥ 1012
( 18.25)
]
535
0.019 (227)
1.07 ¥ 106 g ( 1.93) 1.81 ¥ 10-6
[6b·Ni2+
]
284 (1.870), 311 (1.500), 365
(0.907), 483 (0.105)
6.61 ¥ 1011
( 14.26)
9.74 ¥ 10-6
h
a mV vs. DMFc. b max in nm, e in dm3 mol-1 cm-1. c Isosbestic points in nm. d Defined as in ref. 36. e Defined as in ref. 37. f Detection limits in M. g In M-1.
l
h In M-2.
-
-
of the oxidation potential (Table 1). The results obtained on the
stepwise addition of substoichiometric amounts of Hg2+ metal
cationto receptor 6b revealed atypicaltwo wave behaviour (Fig. 2),
which is characteristic of a large equilibrium constant for the
binding of this cation by the neutral receptor.33
Br-, AcO-, NO3 , HSO4 , H2PO4- anionic species had no effect on
their CV and OSWV, even when present in a large excess. Thus, the
stepwise addition of F- to receptors 6 promotes a clear cathodic
shift of the corresponding oxidation waves (DE1/2 = -300 mV for
6a and DE1/2 = -270 mV for 6b) (see ESI†). A way to reveal
the formation of hydrogen-bonded complexes under conditions of
electrochemical titration is to suppress deprotonation by adding
a small amount of acetic acid.35 Thus, when titration with
F- was performed in the presence of AcOH (20 equivalents),
no electrochemical responses were noticed. On the one hand,
upon titration with a strong base, such as Bu4NOH, the same
cathodic shift of the receptor’s oxidation wave was observe. These
results revealed that, in both cases the addition of F- leads to
deprotonation of the neutral receptors.
3-
Interestingly, the stepwise addition of HP2O7 to receptor 6a
induced the progressive appearance of two new oxidation waves,
cathodically shifted by DE1/2 = -80 mV and DE1/2 = -300 mV,
which are associated to a recognition and a deprotonation process,
respectively. The intensity of these waves is directly related to
the number of equivalents of anion added. Thus, after addition
of 0.4 equivalents the intensity of the wave associated to the
recognition event, appearing at 370 mV, is higher than that of
the wave at 160 mV, due to the deprotonation of the free ligand.
However the situation is completely reversed when 2 equivalents
Fig. 2 Evolution of the OSWV (left) and CV (right) of 6b (10-3 M) in
CH3CN using [(n-Bu)4N]ClO4 as supporting electrolyte when Hg(OTf)2 is
added: from 0 (black) to 1 equivalent (deep blue).
Remarkably, LSV studies carried out upon addition of Cu2+
to the CH3CN solution of this receptor showed not only a
significant shift of the sigmoidal voltammetric wave towards
cathodic currents, indicating that the metal cations promotes the
oxidation of the free receptor, but also a gradual shift towards more
positive potentials, which is in agreement with a complexation
process of the oxidized ferrocenium receptor with Cu2+ metal
cation (see ESI†). A similar behaviour has been observed in
ferrocene derivatives bearing pyridine rings as binding site in the
presence of Cu2+ metal cations.34
3-
of HP2O7 were added (Fig. 3(a)). Under the same conditions,
addition of HP2O73- induced a cathodic shift of DE1/2 = -90 mV in
the oxidation wave of receptor 6b (Fig. 3(b)) However, when the
addition of HP2O73- anion to receptors 6a and 6b were performed
in the presence of 20 equivalents of AcOH only an oxidation
wave, cathodically shifted by DE1/2 = -80 mV for 6a and DE1/2
=
-
Titration studies by addition of F-, Cl-, Br-, AcO-, NO3 ,
-90 mV for 6b, was observed. These results clearly indicate that
in the absence of an acidic medium both a deprotonation and a
recognition process are taking place simultaneously, while in the
presence of a small amount of acetic acid only the corresponding
hydrogen-bonded adducts are formed.
-
-
3-
HSO4 , H2PO4 , and HP2O7 anions, as their tetrabutylammo-
nium salts, have also been carried out. These studies demonstrate
that while addition of F- and HP2O7 anions induced a clear
electrochemical response of these receptors, the addition of Cl-,
3-
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227 nm (e = 2156 M-1 cm-1) and l = 306 nm (e = 2310 M-1 cm-1)
respectively (Table 1). These changes occur at approximately
three well-defined isosbestic points, at any receptor/cation ratio,
suggesting that only one spectral distinct complex was present
(Fig. 5 and ESI†). In these cases, 1 : 1 binding models were
observed for Hg2+ while 1 : 2 were found for Cd2+, Ni2+, Zn2+ and
Pb2+ metal cations.
Fig. 3 OSWV of 6a (left) and 6b (right) (1 mM) in CH3CN/[(n-Bu)4]ClO4
scanned at 0.1 V s-1 and in the present of 0.4 equivalents of HP2O73- (deep
3-
yellow) and 2 equivalents of HP2O7 (deep blue) (left) and in the present
3-
3-
of 0.8 equivalents of HP2O7 (deep green) and 2 equivalents of HP2O7
(deep pink) (right).
Recognition properties of receptors 6 towards metal cations
were also evaluated by UV-vis spectroscopy. Titration experiments
for CH3CN solutions of these ligands (c = 1 ¥ 10-4 M) and the cor-
responding cations were performed and analyzed quantitatively. It
is worth mentioning that no changes were observed in the UV-vis
spectrum of 6a upon addition of Li+, Na+, K+, Ca2+, Mg2+, Cu2+
and Ni2+ metal ions, even in a large excess; however, significant
modifications were observed upon addition of Zn2+, Cd2+, Hg2+,
and Pb2+ (Table 1). In all cases, the addition of increasing amounts
of these cations to a solution of 6a caused a progressive appearance
of two weak low-energy bands at l = 350 and 463 nm respectively,
as well as a decrease of the initial high-energy bands intensities.
Well-defined isosbestic points indicate that a neat interconversion
between the uncomplexed and complexed species occurs (Table 1).
Binding assays using the method of continuous variations (Job’s
plot) suggest a 1 : 1 (cation/receptor) binding model for Pb2+
(Fig. 4(b)) and 1 : 2 for Zn2+, Cd2+, and Hg2+ cations (see ESI†).
Fig. 5 (a) Changes in the absorption spectra of 6b (black) (5 ¥ 10-5 M)
in CH3CN upon addition of increasing amounts of Hg2+ metal cation,
until 1 equivalent was added (deep green); (b) Job’s plot for 6b and Hg2+
,
indicating the formation of 1 : 1 complexes. The total [6b] + [Pb2+] = 10-4 M.
The stoichiometry of the complex has also been confirmed by
ESI-MS, where peaks at m/z 714 for [6a·Pb2+] complex and at m/z
710 for [6b·Hg2+] complex are observed. Their relative abundance
of the isotopic clusters was in good agreement with the simulated
spectra of the complex (Fig. 6).
Fig. 4 (a) Changes in the absorption spectra of 6a (black) (10-4 M) in
CH3CN upon addition of increasing amounts of Pb2+ metal cation, until 1
equivalent was added (deep green); (b) Job’s plot for 6a and Pb2+, indicating
the formation of 1 : 1 complexes. The total [6a] + [Pb2+] = 10-4 M.
Likewise, addition of these metal cations to a solution of 6b
produces the same perturbations on its absorption spectra: a new
low-energy band at l = 365 nm and l = 483 nm appears, with
concomitant decreasing of the initial high-energy bands at l =
Fig. 6 Relative abundance of the isotopic cluster for [6a·Pb2+]: (a)
simulated; (b) experimental. Relative abundance of the isotopic cluster
for [6b·Hg2+]: (c) simulated; (d) experimental.
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On the other hand, absorption spectra of receptors 6 in the
presence of Bu4NOH displayed the same changes than those
observed with HP2O73- and F- anions. When the spectral evolution
of receptors 6 with these anions were performed in the presence of
20 equivalents of acetic acid, the spectral course of the addition
matched with that expected for a deprotonation process.
Receptor 6b also exhibits a very weak fluorescence in CH3CN,
the excitation spectrum revealing lexc = 310 nm as an ideal
excitation wavelength. The emission spectrum shows a weak and
structureless band at 507 nm, with a rather low quantum yield
(U = 6.1 ¥ 10-4). Fluorescence titration experiments by using the
same set of metal cations, demonstrate that only Hg2+ progressively
yielded an important enhancement of the emission band (CHEF =
227) along with a red-shift of the band (Dl = 28 nm) and the
quantum yield (U = 1.9 ¥ 10-2) resulted in a 31-fold increase
compared to that of the receptor 6b (Fig. 8). From the titration
data the stoichiometry of the complex was found to be 1 : 1, with
the association constant Ka = 1.07 ¥ 106 ( 1.93) M-1 and detection
limit 1.81 ¥ 10-6 M. In this case, addition of Cu2+ metal cations
induced extension a blue shift of the emission band by 49 nm and
a CHEF = 176.
Assessment of the cation affinity also came from observing
the extent to which the fluorescence intensity of receptors 6 were
affected in the presence of the selected cations. Receptor 6a exhibits
a very weak fluorescence in CH3CN (c = 1 ¥ 10-5) when excited at
lexc = 330 nm. The emission spectrum shows a structureless band
at 484 nm, with rather low quantum yield (U = 3.6 ¥ 10-4).36 This
receptor did not undergo any considerable change in its emission
spectrum, upon addition of Zn2+, Cd2+, and Hg2+. However, in the
presence of Pb2+ metal cations the emission band underwent a red-
shift at 515 nm (Dl = 31 nm) along with an important chelation-
enhanced fluorescence factor (CHEF = 276)37 and the quantum
yield (U = 2.2 ¥ 10-2) resulted in a 61-fold increase compared to
that of the receptor 6a (Fig. 7) The stoichiometry of the complex
was also confirmed by the changes in the fluorogenic response of
6a in the presence of varying concentrations of Pb2+, the results
indicating the formation of a 1 : 1 complex with a association
constant Ka = 8.29 ¥ 105 ( 1.39) M-1.
Fig. 8 (a) Changes in the fluorescence spectra of 6b (1 ¥ 10-5 M) in
CH3CN upon addition of Hg2+ (dotted line) and Cu2+ (dashed line) metal
cations (lexc = 310 nm). (b) Fluorescence emission intensity of 6b upon
addition of 0.5 equivalents of Hg2+ in the presence of 0.5 equivalents of
interference metal ions in CH3CN.
Competitive experiments were also carried out by adding Pb2+
or Hg2+ (0.5 equivalents) to the solution of 6a or 6b, respectively,
in the presence of other metal cations (Fig. 7(b) and 8(b)).
Fig. 7 (a) Changes in the fluorescence spectra of 6a (1 ¥ 10-5 M) in
CH3CN upon addition of Pb2+ (dotted line) and Cu2+ (dashed line) metal
cations (lexc = 330 nm). (b) Fluorescence emission intensity of 6a upon
addition of 0.5 equivalents of Pb2+ in the presence of 0.5 equivalents of
interference metal ions in CH3CN.
For the reported constant to be taken with confidence, we
have proved the reversibility of the complexation process by
carrying out the following experimental test: 1 equivalent of
Pb(ClO4)2 was added to a solution of the receptor 6a in CH2Cl2
1
The detection limit, calculated as three times the standard
deviation of the background noise,38 was found to be 5.25 ¥
10-6 M. Remarkably, addition of Cu2+ metal cations also induced,
albeit in a lower extension, a red shift of the emission band by
37 nm and a CHEF = 129 (Fig. 7). Such interesting result may
be explained by quenching of the fluorescence of the heterocyclic
ring system subunit in the neutral receptor 6a by the ferrocene unit.
Thus, only a weak emission band is observed. Quenching by the
ferrocene subunit may occur via either electron transfer or energy
transfer from the ferrocenyl group that act as an electron-donor,
to the excited state of the heterocyclic ring system, acting as an
electron-acceptor unit. After oxidation of the neutral receptor 6a
by the Cu2+ metal cation, as evidenced the above mentioned LSV
studies, the electron-donating ability of the ferrocene subunit is
reduced and, as a result, the electron transfer is arrested leading
to a fluorescence enhancement.39
to obtain the complex [6a·Pb2+], whose UV-vis spectra and H
NMR were recorded. The CH2Cl2 solution of the complex was
washed several times with water. The organic layer was dried, and
the optical spectrum, and 1H NMR spectrum were recorded and
they were found to be the same than that of the free receptor 6a.
Afterwards, 1 equivalent of Pb(ClO4)2 was added to this solution,
and the initial UV-vis and 1H NMR spectra of the complex
[6a·Pb2+] were fully recovered. This experiment was carried out
over several cycles, and the optical spectrum was recorded after
each step, thus demonstrating the high degree of the reversibility
of the complexation/decomplexation process. Similar studies were
carried out using the receptor 6b and Hg2+ metal cation, which
confirm the reversibility of this process between this receptor and
such cationic metal species (see ESI†).
To support the results obtained by electrochemical and spec-
troscopic (absorption and emission) experiments, and to obtain
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additional information about the coordination mode of these
metal cations by receptors 6, we also performed a 1H NMR
spectroscopic analysis in CD3CN solution. The most significant
1
features observed in the H NMR of both free ligands, 6a and
6b, are the following: (i) a singlet (d = 4.17 ppm) and two
broad singlets (d = 4.53 and d = 5.14 ppm), corresponding to
the three different set of protons existing in the unsubstituted
and monosubstituted cyclopentadienyl moieties present in the
ferrocene unit; (ii) two doublets associated to the H4 and H5
protons present in both derivatives, which appear at d = 7.89
and d = 8.06 ppm, in the case of 6a, and at d = 8.11 and
d = 8.21 ppm, in the case of 6b; (iii) the signals corresponding
to the aromatic and heteroaromatic substituents linked to the
7 and 8 positions of both heterocyclic systems. Interestingly, a
common feature observed in the complexation processes is the
appearance of a significant downfield shift of the 1H NMR signals
corresponding to the H4 (Dd = +0.22 ppm for 6a and Dd =
+024 ppm for 6b) and H5 (Dd = +0.15 ppm for 6a and Dd =
+0.38 for 6b ppm) protons (see ESI†). However, apart from this
analogous behaviour, the formation of the complexed species
is also accompanied by remarkable differences when their 1H
NMR spectra are compared. Thus, addition of Pb2+ to ligand
6a also promotes a downfield shifting of the ferrocene signals,
which peak shapes were progressively broadened as the number of
equivalents of the metal cation is increased; however, the phenyl
protons remained essentially unaffected during the complexation
process (Fig. 9). Although addition of Hg2+ to ligand 6b is also
accompanied by a downfield shift of all the 1H NMR signals
corresponding to the ferrocene unit, the two broad singlets due to
the two sets of the ferrocene equivalent protons (Ha and Hb) within
the free ligand are now clearly split into four different chemical
shift non-equivalent signals attributable to the Ha, Ha¢, Hb and
Hb¢ present in such monosubstituted cyclopentadienyl ring. On
the other hand, as the progressive addition of Hg2+ to ligand
6b is taking place two very well defined sets of sharp signals
(two doublets and two triple doublets in the d range from 6.7
to 7.8 ppm and from 7.8 to 9.2 ppm, respectively), associated
to the protons present within two different pyridine rings are
also observed (Fig. 10). This fact clearly indicates a chemical
shift non-equivalence of the protons associated to both pyridyl
substituents linked to the 7 and 8 position of the heteroaromatic
ring.
Fig. 10 Evolution of the 1H NMR spectra of 6b (top), in acetonitrile-d3,
upon addition of aliquots of Hg2+ until 1 equivalent was reached (bottom).
The above metal ion-induced chemical shift changes support
that both metal cations are bound to the imidazole (N1) and
quinoxaline (N9) units of the imidazo-quinazoline framework.
Additionally, in the case of ligand 6b the N atom present in
the 8-pyridyl substituent is acting in a synergistic way during the
complexation process of the Hg2+ metal cation.
Conclusions
The synthesis, electrochemical, optical and cation sensing prop-
erties of ferrocene-imidazo[4,5-f]quinoxalines 6, are presented.
The synthetic methodology for the preparation of the target
molecules involved initial condensation of the appropriate 1,2-
dicarbonyl compound with 4-nitro-o-phenylenediamine, followed
by amination with 4-amino-1,2,4-triazole, reduction of the nitro
group of the resulting quinoxaline and finally imidazole-ring
formation by condensation with ferrocenecarboxaldehyde under
oxidative conditions. Dyad 6a behaves as a highly selective redox,
chromogenic and fluorescent chemosensor molecule for Pb2+
cations in CH3CN solutions. Upon complexation with this metal
cation the oxidation redox peak is anodically shifted (DE1/2
=
110 mV), a new low-energy band appeared at 463 nm, in its UV-
vis spectrum, and the emission band is red-shift (Dl = 31 nm)
along with an important chelation-enhanced fluorescence factor
(CHEF = 276). On the other hand, dyad 6b, bearing two additional
pyridine rings as substituents, has shown its ability for sensing Hg2+
cations: the oxidation peak is anodically higher shifted (DE1/2
=
300 mV), a new low-energy band in the absorption spectrum
appeared at 483 nm and the emission band was also red-shifted
(Dl = 28 nm) and underwent an important chelation-enhanced
fluorescent factor (CHEF = 227). These changes in the absorption
and emission spectra of receptors 6 are accompanied by colour
changes, allowing the potential “naked eye” detection of these
metal cations (Fig. 11). Linear sweep voltammetry revealed that
Cu2+ cations induced oxidation of the ferrocene unit in both dyads,
which is accompanied by an important increase of the emission
band. Electrochemical studies on the behaviour of receptors 6
towards anions showed that F- anion induced a strong cathodic
shift of the corresponding oxidation waves (DE1/2 = -300 mV for
6a and DE1/2 = -270 mV for 6b), clearly due to a deprotonation
process. Whereas, addition of HP2O73- in the presence of acetic acid
induced the appearance of a new oxidation wave (DE1/2 = -80 mV
for 6a and DE1/2 = -90 mV for 6b), assigned to a recognition
process.
Fig. 9 Evolution of the 1H NMR spectra of 6a (top), in acetonitrile-d3,
upon addition of aliquots of Pb2+ until 1.2 equivalents were reached
(bottom).
8642 | Dalton Trans., 2010, 39, 8637–8645
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EtOH–n-hexane (1 : 1) to give 4b in 71% yield (0.37 g). Mp: 218–
220 ◦C. Found: C, 62.98; H, 3.29; N, 24.12. C18H12N6O2 requires
C, 62.79; H, 3.51; N, 24.41%. dH (400 MHz; CDCl3; Me4Si) 7.24
(1H, dd, J 4.8 and 0.8 Hz), 7.26 (1H, dd, J 4.8 and 0.8 Hz), 7.31
(1H, d, J 9.6 Hz), 7.80 (1H, td, J 7.6 and 1.6 Hz), 7.81 (1H, td, J
7.6 and 1.6 Hz), 7.91 (1H, d, 7.6 Hz), 7.97 (1H, d, 7.6 Hz), 8.33
(1H, d, J 4.8 Hz), 8.38 (1H, d, J 9.6 Hz); dC (100 MHz; CDCl3;
Me4Si) 115.2 (CH), 123.2 (CH), 123.5 (CH), 124.1 (CH), 124.3
(CH), 126.8 (CH), 127.6 (q), 132.1 (q), 136.7 (CH), 136.8 (CH),
144.2 (q), 144.3 (q), 148.5 (CH), 148.6 (CH), 150.1 (q), 155.4 (q),
156.4 (q), 156.5 (q); m/z (EI) 344 (M+, 100), 297 (93), 270 (23),
148 (23), 135 (16), 78 (38).
Fig. 11 Visual features observed in the CH3CN solutions of CH3CN
solutions of 6a after addition of Pb2+ (left), and 6b after addition of Hg2+
.
Experimental
Preparation of 5,6-diamino-2,3-di(2-pyridyl)quinoxaline, 5b.
To a solution of 5-amino-6-nitro-2,3-di(pyridyl)quinoxaline, 4b,
(0.5 g, 1.45 mmol) in ethanol (40 mL), heated at 50 ◦C, C/Pd (0.001
g) and hydrazine hydrate (0.5 mL, 10.15 mmol) were added. After
the addition of hydrazine was completed, another portion of C/Pd
catalyst (0.001 g) was added and the resulted mixture was refluxed
for 3 h. On cooling, the suspension obtained was filtered over a
Celite pad and the solvent evaporated to dryness under reduced
pressure. The resulting red solid obtained was scratched with
n-pentane, filtered and crystallized from EtOH–n-hexane (1 : 1) to
give 5b in 93% yield (0.42 g). Mp: 112–113 ◦C. Found: C, 68.50; H,
4.21; N, 26.96. C18H14N6 requires C, 68.78; H, 4.49; N, 26.74%. dH
(400 MHz; CDCl3; Me4Si) 7.20 (1H, ddd, J 7.6, 4.8 and 1.28 Hz),
7.23 (1H, ddd, J 7.6, 4.8 and 1.28 Hz), 7.28 (1H, d, J 8.8 Hz),
7.59 (1H, d, J 8.8 Hz), 7.78 (1H, td, J 7.6 and 2.0 Hz), 7.81 (1H,
td, J 7.6 and 1.6 Hz), 7.89 (1H, dt, 7.6 and 0.8 Hz), 7.98 (1H, dt,
7.6 and 0.8 Hz), 8.36 (1H, m); dC (100 MHz; CDCl3; Me4Si) 119.2
(CH), 122.1 (CH), 122.4 (CH), 122.5 (CH), 123.7 (CH), 123.9
(CH), 127.1 (q), 132.0 (q), 133.2 (q), 136.0 (CH), 136.1 (CH),
136.2 (CH), 147.9 (CH),, 148.1 (CH), 148.6 (q), 149.5 (q), 157.5
(q), 157.6 (q); m/z (EI) 314 (M+, 100), 296 (26), 286 (41), 236 (11),
156 (26), 142 (18), 105 (24), 78 (40).
General
Melting points were determined on a Kofler hot-plate melting
point apparatus and are uncorrected. H and 13C spectra were
1
recorded on a Bruker AC300, and 400. The following abbreviations
for stating the multiplicity of the signals have been used: s
(singlet), bs (broad singlet), d (doublet), dd (double of doublets),
ddd (double of doublet of doublets), dt (double of triplets), st
(pseudotriplet), td (triple of doublets) and q (quaternary carbon).
Chemical shifts refer to signals of tetramethylsilane in the case
of 1H and 13C spectra. The electron impact (EI) and electrospray
(ESI) mass spectra were recorded on a Fisons AUTOSPEC 500 VG
spectrometer. Microanalyses were performed on aCarloErba 1108
instrument. CV and OSWV techniques were performed with a
conventional three-electrode configuration consisting of platinum
working and auxiliary electrodes and a Ag/AgCl reference
electrode. The experiments were carried out with a 10-3 M solution
of sample in CH3CN containing 0.1 M (n-C4H9)4ClO4 (TBAP)
(WARNING: potential formation of highly explosive perchlorate
salts or organic derivatives) as supporting electrolyte. All the
potential values reported are relative to the decamethylferrocene
(DMFc) couple at room temperature. 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 with a scan rate increasing from
0.05 to 1.00 Vs-1, while the OSWV were recorded at a scan
rate of 100 mVs-1 with a pulse hight of 10 mV and a step
time of 50 ms. Typically, receptor (1 ¥ 10-3 M) was dissolved in
CH3CN (5 mL) and TBAP (base electrolyte) (0.170 g) added. The
guest under investigation was then added as a 0.1 M solution
in appropriate solvent using a microsyringe whilst the cyclic
voltammetric properties of the solution were monitored. DMFc
was used as an external reference both for potential calibration and
for reversibility criteria. Under similar conditions the DMFc has
E = -0.07 V vs. SCE and the anodic peak-cathodic peak separation
is 67 mV.
General procedure for the preparation of 2-ferrocenyl-7,8-
disubstituted-3H-imidazo[4,5-f ]quinoxalines, 6. To a solution of
the appropriate 5,6-diaminoquinoxaline 5 (4.37 mmol) in ni-
trobenzene (10 mL) ferrocenecarboxaldehyde (0.94 g, 4.37 mmol)
was added. Then, 0.5 mL of acetic acid was added and the
reaction mixture was stirred for 15 h at 70 ◦C. Afterwards,
an aqueous solution of NaHCO3 was added until pH = 7 was
achieved. Then, the resulting mixture was poured into water
(50 mL) and extracted with CHCl3 (2 ¥ 50 mL). The organic
phase was dried over anhydrous MgSO4, filtered and concentrated
under vacuum to give a residue which was purified by column
chromatography by using dichloromethane–n-hexane–methanol
(9 : 1 : 0.25) or dichloromethane–methanol (9 : 1) as eluent to give
compounds 6a and 6b, respectively.
Compounds 3a,40 3b,41 4a42 and 5a42 were prepared according
to the already described procedures.
2-Ferrocenyl-7,8-diphenyl-3H-imidazo[4,5-f ]quinoxalines, 6a.
48% yield (1.06 g). Mp: 201–203 ◦C. Found: C, 73.71; H, 4.60;
N, 11.29. C31H22FeN4 requires C, 73.53; H, 4.38; N, 11.06%. dH
(400 MHz; MeOD; Me4Si) 4.17 (5H, s), 4.49 (2H, st), 5.16 (2H,
st), 7.35 (1H, m), 7.45 (1H, m), 7.47 (1H, dd, J 6.4 and 2.0 Hz),
7.55 (1H, dd, J 6.4 and 1.2 Hz), 7.89 (1H, d, J 9.2 Hz), 7.98 (1H,
d, J 9.2 Hz); dH (400 MHz; CD3CN); Me4Si) 4.17 (5H, s), 4.52
(2H, bs), 5.14 (2H, bs), 7.40 (6H, m), 7.55 (3H, dd, J 6.4 and
1.6 Hz), 7.64 (1H, d, J 6.4), 7.89 (1H, d, J 8.8 Hz), 8.06 (1H, d,
Preparation of 5-amino-6-nitro-2,3-di(2-pyridyl)quinoxaline, 4b.
A solution of potassium t-butoxide (0.56 g, 5 mml) in DMSO
(10 mL) was added dropwise to a solution of 6-nitro-2,3-di-(2-
pyridyl)quinoxaline, 3b, (0.5 g, 1.5 mmol) and 4-amino-1,2,4-
triazole (0.84 g, 10 mmol) in DMSO (10 mL). The reaction mixture
was stirred at room temperature for 20 min and then poured into
a saturated aqueous solution of NH4Cl (70 mL). The precipitate
formed was filtered, washed with n-hexane and crystallized from
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 8637–8645 | 8643
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J 8.8 Hz);); dC (100 MHz; MeOD; Me4Si) 68.8 (CH), 70.8 (CH),
71.6 (CH), 73.9 (q), 124.0 (CH), 129.1 (CH), 129.3 (CH), 129.7
(CH), 129.8 (CH), 131.1 (CH), 131.2 (CH), 131.3 (q), 131.5 (q),
139.6 (q), 140.5 (q), 140.6 (q), 152.4 (q), 153.6 (q), 156.4 (q); m/z
(ESI) 507 (M++1, 100).
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6b. 43% yield (0.95 g). Mp: 197–199 ◦C. Found: C, 68.23; H,
3.81; N, 16.77. C29H20FeN6 requires C, 68.52; H, 3.97; N, 16.53%.
d
H (400 MHz; CDCl3; Me4Si) 4.10 (5H, s), 4.36 (2H, st), 4.99 (2H,
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st), 7.21 (1H, m), 7.51 (1H, t, J 7.2 Hz), 7.60 (1H, t, J 7.2 Hz),
7.75 (1H, td, J 8.0 and 1.2 Hz), 7.92 (1H, d, J 8.0 Hz), 7.95 (1H,
d, J 8.8 Hz), 8.14 (1H, d, J 8.8 Hz), 8.41 (1H, d, J 4.0 Hz), 8.51
(1H, d, J 4.4 Hz); dH (400 MHz; CD3CN); Me4Si) 4.18 (5H, s),
4.53 (2H, bs), 5.13 (2H, bs), 7.32 (2H, dd, J 7.2 and 1.8 Hz), 7.94
(3H, m), 7.95 (1H, d, J 9.0), 8.07 (1H, m); 8.08 (1H, d, J 9.0 Hz),
8.32 (2H, m); dC (100 MHz; CDCl3; Me4Si) 67.0 (CH), 69.4 (CH),
69.8 (CH), 72.9 (q), 122.4 (CH), 123.0 (CH), 123.9 (CH), 124.1
(CH), 135.8 (CH), 136.2 (CH), 138.4 (q), 148.1 (CH), 149.2 (q),
149.8 (q), 154.3 (q), 157.0 (q); m/z (EI) 508 (M+, 100), 442 (13),
430 (12), 254 (11).
Acknowledgements
We gratefully acknowledge the financial support from MICINN-
Spain, Project CTQ2008-01402 and Fundacio´n Se´neca (Agen-
cia de Ciencia y Tecnolog´ıa de la Regio´n de Murcia) project
04509/GERM/06 (Programa de Ayudas a Grupos de Excelencia
de la Regio´n de Murcia, Plan Regional de Ciencia y Tecnolog´ıa
2007/2010).
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