Selective recognition and electrochemical sensing of dicarboxylates with a ferrocene-based bis(o-trifluoroacetylcarboxanilide) receptor

Article abstract of DOI:10.1039/b606081a

A ferrocene-based bis(o-trifluoroacetylcarboxanilide) receptor selectively recognizes m-phenylene diacetate through cooperative binding; the receptor also displays a significant negative shift in the oxidation potential of ferrocene upon the guest binding. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b606081a

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Selective recognition and electrochemical sensing of dicarboxylates with
a ferrocene-based bis(o-trifluoroacetylcarboxanilide) receptor{{
Dae-Sik Kim, Hidekazu Miyaji,* Byoung-Yong Chang, Su-Moon Park* and Kyo Han Ahn*
Received (in Cambridge, UK) 28th April 2006, Accepted 13th June 2006
First published as an Advance Article on the web 28th June 2006
DOI: 10.1039/b606081a
A ferrocene-based bis(o-trifluoroacetylcarboxanilide) receptor
selectively recognizes m-phenylene diacetate through coopera-
tive binding; the receptor also displays a significant negative
shift in the oxidation potential of ferrocene upon the guest
binding.
Anions play important roles in biological and chemical processes,1
and have implications in medicine, catalysis and the environment.2
Thus, the selective recognition of anions has been a challenging
topic in recent supramolecular chemistry. Receptors that bind
guests in a cooperative manner are especially sought for the
selective recognition of a specific guest.3
Trifluoroacetophenone derivatives have been utilized as unique
ionophores for anions that reversibly form anion–ionophore
adducts by attacking the trifluoroacetyl carbonyl carbon.4 Re-
cently, we have demonstrated that introduction of an H-bonding
ditopic receptor 1 in CD3CN, both the unshifted Cp peaks
donor such as a carboxamido group to the trifluoroacetophenone
corresponding to the receptor and fully shifted adduct Cp peaks
moietystabilizestheanionicadductsandthussignificantlyenhances
appeared, which indicates that the equilibration of host–guest
the receptor’s binding affinity toward anions such as carboxylates
adduct formation is too slow on the NMR timescale. Upon
toapracticallyusefullevel.5aThisapproachofH-bondstabilization
addition of an equimolar amount of dicarboxylate 6, unshifted
also enabled us to introduce a novel fluorescence sensor toward
receptor peaks completely disappeared and only fully shifted new
cyanide.5b To develop this promising recognition motif into a
adduct peaks appeared. The changes in chemical shifts (the NH
guest-specific receptor, we have been studying bis-, tris-, and
proton: dH 5 11.03 A 11.08 ppm; the Cp ring protons: 5.00 A
hybrid derivatives of the o-trifluoroacetylcarboxanilide (TFACA)
4.80, 4.64 A 4.36 ppm; the CF3 fluorine: dF 5 5.52 A 28.47 ppm)
indicated the formation of a (1 : 1) host : guest adduct. Also
system. Here, we wish to report a ferrocene-based bis(TFACA)
system 1, which recognizes and electrochemically senses a
trifluoroacetyl carbonyl carbon signals of receptor 1 which
specific dicarboxylate in a cooperative manner.
appeared at dC 5 183.8 ppm (quartet, JC–F 5 0.4 Hz) disappeared
Ferrocene was chosen as a spacer as well as an electro-active
after addition of an equimolar amount of dicarboxylate 6 in
CDCl3. The Job plot7 for the interaction between receptor 1 and
label. In addition, the two binding motifs are expected to adjust to
form a stable ditopic complex toward a specific guest because the
each of the dicarboxylates (4, 5 and 6)§ was obtained by plotting
cyclopentadienyl (Cp) units in the ferrocene can rotate like a ball
bearing along the vertical axis.6 Compound 1 was synthesized
the molar concentration of the (1 : 1) complex vs. mole fraction of
the host, which was determined by integrating their Cp ring
from 1,19-ferrocenedicarboxylic acid by amidation with o-trifluoro-
protons (Fig. 1). The Job plot in the case of dicarboxylate 6
acetylaniline. Compound 2 was also synthesized as a control
displays a maximum at a mole fraction of 0.5 with a sharp and
receptor from ferrocenecarboxylic acid (see ESI{). All final
products were isolated, purified, and fully characterized.
symmetrical peak, suggesting a strong (1 : 1) host : guest binding
mode. In the case of dicarboxylate 4, however, the Job plot
We first investigated the molecular interactions between ditopic
displays a maximum peak at the mole fraction of 0.33, suggesting a
receptor 1 and several dicarboxylates 4–6 by NMR spectroscopy.
major (1 : 2) host : guest binding stoichiometry. Interestingly, the
When a subequimolar amount of dicarboxylate 6 was added to
Job plot also shows a maximum at the mole fraction of 0.5 with an
unsymmetrical peak shape in the case of dicarboxylate 5, which
Department of Chemistry and Center for Integrated Molecular Systems,
suggests a mixed binding mode of plausibly a dominant (1 : 1) host
POSTECH, San 31, Hyoja-dong, Pohang, 790-784, Republic of Korea.
: guest complex mixed with other higher order complexes." These
E-mail: ahn@postech.ac.kr; Fax: (+82) 54 279 3399;
results can be interpreted as: dicarboxylate 6 matches best the
Tel: (+82) 54 279 2105
{ The HTML version of this article has been enhanced with colour
images.
ditopic receptor 1 among other guests in view of the distance
between the two carboxylate moieties in the guest and the two
TFACA units in the host, and thus an efficient cooperative
binding is achieved. In the case of dicarboxylate 4, this geometrical
{ Electronic supplementary information (ESI) available: Procedures for
the synthesis of receptors 1 and 2, binding data (Job plot, ITC, MS,
NMR) and cyclic voltammetry data. See DOI: 10.1039/b606081a
3314 | Chem. Commun., 2006, 3314–3316
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Table 1 Thermodynamic data for host–guest complexation deter-
mined by isothermal titration calorimetrya
Host Guestb DHuc
2TDSuc DGuc
Kd
ne
f
1
3
220.9
14.5
5.9 211.8
26.4 3.0 6 104
25.9 1.9 6 104
1
1
2
a
5
6
3
224.7
223.2
215.4
17.1
13.2
9.4
27.6 2.8 6 105 0.70
210.0 1.6 6 107 1.06
26.0 2.1 6 104 1.00
c
Determined in CH3CN at 303 K. Bu4N+ salts. Unit: kcal/mol.
b
d
e
f
. Host–guest stoichiometry. Sequential binding.
Unit: M21
corresponds to the (1 : 1) host–guest stoichiometry suggested by
the Job plot.I Ditopic receptor 1 binds dicarboxylate 6 strongly,
with a larger DGu value (210.0 kcal mol21) compared to other
cases. It binds with dicarboxylate 6 about 760 times more strongly
than receptor 2.
Fig. 1 Job plots for ditopic receptor 1 with the tetrabutylammonium
salts of m-phenylene diacetate 6 (m), isophthalate 5 ($), and phthalate 4
(&) in CD3CN at 298 K ([H] + [G] 5 1.0 mM).
The electrochemical behavior of receptors 1 and 2 with the
carboxylates present was investigated by cyclic voltammetry in
CH3CN at a receptor concentration of 1.0 mM (ESI). As shown in
Fig. 3a, ditopic receptor 1 shows a reversible redox process
centered at Eu9 5 +0.984 V (vs. Ag/AgCl), which was obtained
from Eu9 5 (Epa + Epc)/2. Significantly, the addition of one
equivalent of the dicarboxylate 6 to the solution resulted in a large
negative shift of 2153 mV (Fig. 3c). No further change was
observed after the addition of more than one equivalent of the
guest, consistent with the formation of a strong ditopic complex
with a (1 : 1) stoichiometry. Indeed, the 1H NMR spectrum at this
equivalence point showed almost 100% complexation. In the
presence of 0.5 equivalent of dicarboxylate 6, two anodic peaks
and broad cathodic peaks appeared (Fig. 3b). Obviously, 50% of
the free host and 50% of the complex exist at this stage. The
cathodic peak for the (1 : 1) adduct becomes featureless. In the
presence of 0.7 equivalent of dicarboxylate 6, the cathodic peak of
the host was reduced and another cathodic peak of the complex
appeared more clearly, and the shift in formal potential
DEu9 5 Eu9complex 2 Eu9host was also estimated to be 2153 mV
(see ESI).
match seems to be lost and thus such a cooperative binding is no
longer operative. As a control, acetate 3 was also allowed to
interact with receptor 1, which gave a Job plot consistent with a
(1 : 2) host : guest binding mode, as was observed in the case of
dicarboxylate 4.
To obtain thermodynamic data for the binding process between
the receptors and guests, we carried out isothermal titration
calorimetry (ITC). The integrated binding isotherms for several
guests are summarized in Fig. 2, and the thermodynamic data
obtained by a non-linear least-squares curve fit for the binding
isotherms are summarized in Table 1. The DHu and 2TDSu data
in Table 1 indicate that the complex formation is driven by a
favorable enthalpy change and unfavorable entropy change in all
cases, which also supports the host–guest adduct formation. The
isothermal titration curve for ditopic receptor 1 with acetate 3 can
be best fit to a sequential binding model with a (1 : 2) host : guest
binding mode.8 The sequential association constants thus obtained
(K1, K2) are similar to that observed between receptor 2 and
acetate 3. In the case of dicarboxylate 4, we could not determine an
apparent binding mode by ITC (see ESI{). The inflection point in
the binding isotherm between ditopic receptor 1 and dicarboxylate
6 occurred near the molar ratio of 1.0 (n 5 1.06), which
As control experiments, cyclic voltammetric experiments on the
receptors 1 and 2 with acetate 3 were also carried out. Receptor 2
Fig. 3 Cyclic voltammograms of receptor 1 (1.0 mM in CH3CN): (a) in
the absence of dicarboxylate 6, (b) in the presence of 0.5 equivalent of
dicarboxylate 6 and (c) in the presence of 1.0 equivalent of dicarboxylate 6.
Note that the first redox peaks at y0.5 V vs. Ag/AgCl correspond to the
authentic Fc+/Fc pair used as an internal reference.
Fig. 2 ITC plots of (a) receptor 2 with acetate 3, (b) ditopic receptor 1
with dicarboxylate 5 and (c) ditopic receptor 1 with dicarboxylate 6 in
CH3CN at 303 K.
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We thank the Center for Integrated Molecular Systems (R11-
2000-070-070010) and the Korea Health Industry Development
Institute (A05-0426-B20616-05N1-00010A) for financial support
of this work.
Table 2 Electrochemical data of compounds 1 and 2 detailing the
shift in the formal electrode potential, DEu9 (mV), upon addition of
guests 3 and 6a
Receptor
3 (2.0 equiv)
3 (4.0 equiv)
6 (1.0 equiv)
1
2
a
2122
240
2149
240
2153
—
Notes and references
In CH3CN (1.0 mM) at 298 K.
§ Tetrabutylammonium salts were prepared according to the literature (see
T. R. Kelly and M. H. Kim, J. Am. Chem. Soc., 1994, 116, 7072). Attempts
to prepare the tetrabutylammonium salts of terephthalic acid were not
successful. Tetrabutylammonium acetate 3 was purchased from Aldrich.
" A minor complex, presumably a (2 : 1) host : guest complex, appeared
with different chemical shifts, which was not counted in the Job plot of
Fig. 1.
I A (1 : 1) binding mode that is not a chelated adduct but an open
structure may exist in a negligible amount if at all, because the association
constant observed greatly exeeds that of the (1 : 1) acetate adduct
formation, which involves a similar open structure. In fact, we could not
observe any other minor peaks in the 1H NMR spectrum of the (1 : 1)
host–guest complex.
gave a formal potential at Eu9 5 +0.710 V (vs. Ag/AgCl), showing
a lower formal potential than that of ditopic receptor 1. In the
presence of one equivalent of acetate 3, the cathodic peaks of
receptor 1 (and also 2) became broad because both free host and
complex are present. After addition of two equivalents of acetate 3,
the complex peaks became clearer and the shifts in the formal
potential (DEu9) were estimated; they are listed in Table 2. The
receptor 2 gave a negative shift of 240 mV toward acetate 3. The
ditopic receptor, 1, gave a large negative shift of 2122 mV in
the presence of two equivalents of acetate 3. At this stage, a
mixture of a (1 : 2) host–guest complex and a (1 : 1) host–guest
complex seems to be present. However, when four equivalents of
acetate 3 were added to ditopic receptor 1, almost all complexes
became a (1 : 2) host–guest species and gave a large negative shift
of 2149 mV. These results indicate that anionic complexes cause
negative shifts,9 especially di-anionic species cause large negative
shifts for the ferrocene-centered redox potentials. Only one
equivalent of dicarboxylate 6 was enough to cause the largest
negative shift of ditopic receptor 1, which again indicates that the
strong cooperative binding process effectively introduces di-anionic
charges to the ferrocene redox centre.
** Semiempirical computations were performed using Spartan ’04
Windows from Wavefunction, Inc. The computation used the AM1
parameters, and an isolated and equilibrium ground state geometry was
obtained.
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In conclusion, we have demonstrated that the redox-active
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Fig. 4 A modeled structure of the cooperative adduct between ditopic
receptor
1 and dicarboxylate 6, in which the hydrogen bonding
stabilization has not been incorporated.
3316 | Chem. Commun., 2006, 3314–3316
This journal is ß The Royal Society of Chemistry 2006