Study on anion electrochemical recognition based on a novel ferrocenyl compound with multiple binding sites

Article abstract of DOI:10.1021/jp805002a

A novel ferrocenyl anion receptor N,N,N,N-(dimethyl, ethyl, ferrocenecarboxylic amidodimethylene) ammonium fluoborate 2 with multiple binding sites was synthesized. Its anion recognition behaviors were investigated by CV, ¹H NMR and UV-vis spectrum. It was found that the combination of two interactions enforced the anion binding ability and the binding selectivity of 2 to phosphate anion. The effects of scan rate on the CV curves of 2 with phosphate were also investigated. In different scan rate, the CV curves kept stable which indicated the strong binding between 2 and phosphate. According to relationships of peak potential, peak currents and scan rate of 2 binding with phosphate, the kinetic parameters of electrode process such as diffusion coefficient D_app, surface transfer coefficient αn_α, and standard rate constant k⁰ were calculated.

Full text of DOI:10.1021/jp805002a

J. Phys. Chem. B 2008, 112, 11171–11176
11171
Study on Anion Electrochemical Recognition Based on a Novel Ferrocenyl Compound with
Multiple Binding Sites
Qiaohua Tan, Li Wang,* Liang Ma, Haojie Yu, Jianhua Ding, Qingquan Liu, Anguo Xiao, and
Guoqing Ren
State Key Laboratory of Chemical Engineering, College of Materials Science and Chemical Engineering,
Zhejiang UniVersity, Hangzhou, P. R. China, 310027
ReceiVed: June 6, 2008; ReVised Manuscript ReceiVed: June 20, 2008
A novel ferrocenyl anion receptor N,N,N,N-(dimethyl, ethyl, ferrocenecarboxylic amidodimethylene) ammonium
fluoborate 2 with multiple binding sites was synthesized. Its anion recognition behaviors were investigated
1
by CV, H NMR and UV-vis spectrum. It was found that the combination of two interactions enforced the
anion binding ability and the binding selectivity of 2 to phosphate anion. The effects of scan rate on the CV
curves of 2 with phosphate were also investigated. In different scan rate, the CV curves kept stable which
indicated the strong binding between 2 and phosphate. According to relationships of peak potential, peak
currents and scan rate of 2 binding with phosphate, the kinetic parameters of electrode process such as diffusion
coefficient Dapp, surface transfer coefficient RnR, and standard rate constant k0 were calculated.
1. Introduction
some factors such as potential scan rate and electrolyte
concentration have influences on CV behaviors.10 However, in
anion recognition study of ferrocenyl compounds by CV
reported, no one has focused on these factors, and the kinetics
of the electrode process in electrochemical anion recognition
has not been reported yet.
In this work, we designed and synthesized a ferrocenyl anion
receptor 2 with multiple binding site composed of amide group
and the quarterized nitrogen linked by dimethylene group. Its
anion recognition behaviors, its sensitivity, and its selectivity
to phosphate were studied. The influence of scan rate and
electrolyte concentration on CV responses to anions are also
discussed, and the kinetic parameters of the electrode process
anion recognition were calculated.
Anion recognition has become a hot research area because
anions play important roles in biology, medicine, and environ-
ment.1-6 People have tried to design and synthesize various
artificial anion receptors to effectively and selectively recognize
anions since the first anion receptor was synthesized in
1960s.7-17 To design a good anion receptor, the crucial point
is to find proper signal unit and design effective binding site.
In the electrochemical anion recognition area, ferrocene group
has proved to be a good signal unit because of its stable
electrochemical properties and easiness to be detected by
electrochemical methods such as cyclic voltammogram (CV).8
Besides that, the ferrocene group can participate in and modulate
the binding event through alternating between its two redox
states.8 As for binding site, amide,7 urea,9 and the hydroxyl
group10 that can form hydrogen bonds with anions are mostly
used. Quarterized nitrogen11 and positively charged pyridine12
are also chosen as binding sites on the basis of the electrostatic
interaction. Others that can provide shape complementation16
are also considered. However, for the anion receptors reported,
sensitivity and selectivity, the two basic characters of anion
receptors, are not so good and are still challenges for scientists.
Now, people realize that one single interaction in the receptor
molecule such as hydrogen bond or electrostatic interaction or
shape complementation may not be enough to improve the
selectivity and sensitivity, especially in aqueous environment.18
Thus, combination of several interactions is being considered.
It is believed that multiple binding sites which involve several
different binding groups in a certain spacial position, such as
linking of hydrogen bond, forming group and electrostatic
interaction, providing group according to a special position, will
improve the sensitivity and selectivity of anion receptors.
However, so far, anion receptors, especially ferrocenyl anion
receptors, with multiple binding sites are still seldom reported.19
Ferrocenyl compounds in anion recognition are always
investigated by CV method. Our previous work has shown that
2. Experimental Sections
2.1. Instruments and Reagents. The CV measurements of
solution were carried out with a CHI-600A electrochemical
analyzer (CH Instruments, Inc., Austin, TX) in an undivided
three-electrode cell. The working electrode was a Teflon-
shrouded Pt disk electrode (2 mm, geometric area: 0.0314 cm2),
which was polished to a mirror finish with 0.05 µm Al2Cl3 paste
on felt, cleaned by ultrasonication successively in 0.1 M NaOH,
1:1 HNO3, anhydrous ethyl alcohol, and double-distilled water,
then dried, and used for electrochemical measurements. Pt-wire
counter electrode and Ag-wire reference electrode were used.
The ohmic potential (iR) drop of the investigated solution
was measured and autocompensated with CHI-600A electro-
chemical analyzer.
All CV experiments have been done for three times to obtain
the reproducible result. In every CV experiment, the electrolyte
is Bu4NBF4, the scan rate is 0.1 V/s, and the ferrocenyl
compound concentration is 0.5 mM, unless specifically indicated.
1H NMR spectra were recorded with an AVANCE DMX-
500 NMR spectrometer by using tetamethylsilane (TMS) as
internal standard at room temperature. Mass spectrum was
recorded with an esquireplus mass spectrometer.
All the reagents in use are from commercial markets. Before
use, ferrocene carbonate acid (AR) is dried under vacuum, and
* Corresponding author. Tel: +86-571-8795-3200. Fax: +86-571-8795-
1612. E-mail: opl_wl@zju.edu.cn.
10.1021/jp805002a CCC: $40.75
2008 American Chemical Society
Published on Web 08/07/2008

11172 J. Phys. Chem. B, Vol. 112, No. 35, 2008
Tan et al.
SCHEME 1: Synthesis of Ferrocenyl Compounds 1 and 2
solvents such as dichloromethane CH2Cl2 (AR), tetrahydrofuran
(THF, AR), pyridine (AR), petroleum ether (60-90 °C, AR),
triethylamine (AR), and methanol CH3OH (AR) are distilled
twice in presence of molecular sieve. Other solid reagents such
as sodium bicarbonate (AR), anhydrous magnesium sulfate (AR)
and tetrabutylammonium tetrafluoroborate Bu4NBF4 (AR), tet-
rabutylammonium phosphate, Bu4NH2PO4 (AR), and tetrabu-
tylammonium hydrogen sulfate Bu4NHSO4 (AR) are directly
used.
2.2. Synthesis and Characterization of Ferrocenyl Com-
pounds 1 and 2. The synthesis routes of ferrocenyl compounds
1 and 2 are shown in Scheme 1.
Synthesis of ferrocenyl carbonate chloride was performed as
follows. A total of 2.00 g (8.7 mmol) of ferrocenyl carbonate
acid was suspended in a 100 mL three-necked flask containing
30 mL of CH2Cl2, and 20 drops of pyridine were added. Then,
2.50 mL (28.6 mmol) of oxalyl chloride was added dropwise
to the suspension mixture. The solution was reacted 16 h in 50
°C; then, the solvent was removed, and the excess oxalyl
chloride was obtained in vacuum. A total of 2 × 60 mL of
petroleum ether (60-90 °C) was then added, and the ferroceneyl
carbonate chloride was extracted after reflux for 20 min. We
remove the petroleum ether in vacuum, obtaining the red solid
product ferroceneyl carbonate chloride 1.68 g (Yield: 77.8%).
Synthesis of ferrocenyl compound N,N-dimethylaminoethyl-
ferrocenyl-carboxylicamide 1 was performed as follows. A total
of 1.68 g (6.7 mmol) of ferrocenyl carbonate chloride was
resolved in 5 mL CH2Cl2 and then added dropwise to a mixture
of 15 mL of CH2Cl2, 0.90 mL of triethylamine (6.7 mmol),
and 0.75 mL (13.4 mmol) of N,N-dimethylaminoethylamine.
After reaction for 16 h at room temperature, the solution was
washed with 50 mL of NaHCO3 aqueous solution (0.30 wt%)
three times and then dried by anhydrous MgSO4. A total of
1.56 g of brown solid product 1 was obtained by removing the
solvent in vacuum (Yield: 76.9%). 1H NMR (DMSO-d6): δ 2.16
(s, -N-CH3, 6H), 2.34 (t, -N-CH2- 2H), 3.25 (q,
-CONH-CH2-, 2H), 4.14 (s, C5H4, 5H), 4.30, 4.74 (t, C5H4,
4H), 7.67 (t, -CONH-, 1H).
Synthesis of ferrocenyl compound N,N,N,N-(dimethyl,ethyl,
ferrocenecarboxylic amidodimethylene) ammonium fluoborate
2 was performed as follows. A total of 0.60 g (2.0 mmol) of
ferrocenyl compound 1 was resolved in 20 mL of CH3OH, and
then, 0.25 mL (3.0 mmol) of ethyl iodide was added. After being
refluxed for 8 h, the mixture was concentrated to 5 mL and
then precipitated in 100 mL of THF for three times to obtain
the iodide ammonium. The iodide ammonium was then resolved
in 30 mL of CH3OH/water (v:v ) 50:50) mixture solvent, and
the excess NH4BF4 was added to the solution. The solution was
extracted by 5 × 30 mL of CH2Cl2. After drying by anhydrous
MgSO4, 0.33 g of orange solid product ferrocenyl compound 2
was obtained by removing the CH2Cl2 in vacuum (Yield: 40%).
1H NMR (DMSO-d6): δ 1.27 (t, -CH2CH3-, 3H), 3.07 (s,
-N-CH3, 6H), 3.38-3.42 (-CH2-N-CH2-, 4H), 3.54 (q,
CONH-CH2-, 2H), 4.17 (s, C5H4, 5H), 4.37, 4.75 (t, C5H4,
4H), 8.07 (t, -CONH-, 1H). Mass spectrum (ESI): 329
(positive fragment), 87 (negative fragment).
Figure 1. CV curves of ferrocenyl compounds 1 and 2 in CH2Cl2.
3. Result and Discussion
3.1. Anion Recognition Behaviors Investigation of 2 by
CV. We used CV method to study the electrochemical behaviors
and anion recognition properties of 2 in CH2Cl2. The CV curve
of 2 in CH2Cl2 is shown in Figure 1, with a pair of reversible
redox peak which is typical of ferrocene group. The average
peak potential Ep1/2 (Ep1/2 ) (Epc + Epa)/2) is 0.891 V, which is
larger than the 0.835 V of 1, which means that 2 is easier to
-
Figure 2. CV curves of 2 before and after addition of H2PO4 in
CH2Cl2. (a) H2PO4-/[Fe] is 0; (b) H2PO4-/[Fe] is 1.8; (c) H2PO4-/[Fe]
is 2.4.

Anion Electrochemical Recognition
J. Phys. Chem. B, Vol. 112, No. 35, 2008 11173
-
TABLE 1: Electrochemical Data when 3 equiv anions
existed in CH2Cl2
in Figure 4. The addition of H2PO4 made the amide proton
shift to low field from 8.07 to 8.33, and protons in ferrocenyl
group and methylene group linked to the quarterized nitrogen
were also influenced, indicating their involvement in the binding
events.
With the addition of H2PO4-, the UV-vis changes of 2 with
the addition of H2PO4 showed interesting responses. The
character peak of the amide group at about 237 nm changed
apparently after H2PO4- was added; however, its intensity first
decreased when less than 1 equiv H2PO4 was added (Figure
5a) and then gradually recovered when more H2PO4- was added
(Figure 5b). The relationship of the intensity and the amount
1/2 a
1/2 b
anion
Epc (V)
Epa (V)
Ep
(V)
∆Ep
(V)
receptor 1
0.865
0.635
0.783
0.956
no anion
0.801
0.391
0.652
0.911
0.833
0.513
0.717
0.937
-
H2PO4
-0.320
-
-
HSO4
-0.116
Br-
0.104
receptor 2
0.922
0.662
0.718
0.946
no anion
0.859
0.373
0.711
0.874
0.891
0.518
0.715
0.910
-
-
H2PO4
-0.373
-0.176
0.019
-
HSO4
Br-
-
of H2PO4 is shown in Figure 5c. We can find that points for
a Ep ) (Epc + Epa)/2. b ∆Ep ) Ep
- Ep
.
1/2
1/2
1/2
1/2
(bound)
(free)
1 equiv and 2 equiv are key points. Our previous work has
shown that the involvement of carbonyl group in hydrogen-
bond forming leads to the decrease of the intensity of its
character peak.17 Thus, we conclude that when H2PO4 was
added, the carbonyl group of 2 was first involved in hydrogen
bond, reflected by the decrease of the intensity of its character
peak. When more H2PO4 was added, the change of binding
model of 2 to H2PO4 from 1:1 mode to 1:2 mode somewhat
weakened the hydrogen bond of the carbonyl group because of
the steric effect, reflected by the recovery of intensity of its
character peak. The possible binding mode of 2 to H2PO4 is
illustrated by the molecular model and Kekule model in Figure
6. As for HSO4-, it was bound by 1:1 mode from the UV-vis
spectrum (Figure S2 in the Supporting Information), indicating
that the synergy effect of the positively charged quarterized
nitrogen seems not apparent.
3.3. Kinetics of Electrode Process in Electrochemical
Anion Recognition of 2. From our previous work, CV behaviors
were influenced by several factors such as scan rate and
electrolyte concentration.17 The influence of scan rate on anion
electrochemical recognition of 2 was investigated. From Figure
reduce and harder to oxidize. This may be the result of the
repulsion between the positively charged ferrocenium and the
quarterized nitrogen.
-
-
The CV responses of 2 to H2PO4 are shown in Figure 2.
-
With the addition of H2PO4-, the initial peaks of the CV curve
weakened and gradually disappeared, whereas a new pair of
peaks in the lower potential gradually appeared (the electro-
chemical data are listed in Table 1). The average potential shift
-
1/2
-
value ∆Ep is 0.373 V, which indicates the strong binding of
2 to H2PO4-20. The response of 1 to H2PO4- appears to be the
same two-pair-peak phenomena but with a smaller potential
difference (∆E ) 0.320 V, Figure S1 in the Supporting
Information), which indicates that the ability of 2 to bind
H2PO4- is better than that of 1, and the involvement of positively
charged quarterized nitrogen did enforce the interaction between
-
H2PO4 and 2.
The recognitions of HSO4 and Br- by 2 were also studied
-
-
(Figure 3). From CV curves, we can find that addition of HSO4
leads to the two-pair-peak phenomena as well but with a much
-
smaller potential change compared to that for H2PO4 (∆E )
-
2, we know that 3 equiv H2PO4 guaranteed the total binding
0.176 V), indicating that the interaction between HSO4- and 2
is much weaker than H2PO4- because of the relatively weaker
basicity.7,20 As for Br-, there’s no apparent change of CV
curves, indicating no recognition of 2 to Br- because of the
-
of H2PO4 with compound 2. Thus, we studied the CV
behaviors of 2 with 3 equiv H2PO4- in CH2Cl2 at different scan
rates (Figure 7). We found that at different scan rates, the CV
curves were stable, indicating that the binding of H2PO4 and
-
weaker basicity and the unmatching of the spatial complemen-
tation. By comparing CV responses of 2 to H2PO4-, HSO4
,
-
2 is strong and stable in the scan rate range, and we can treat
them as a whole, named 2-H2PO4-. It was shown that with
the increase of scan rate, the peak currents increased. Over a
wide scan rate range, the peak current values ip of 2 are
proportional to the square root of the scan rate, V1/2. (The results
of the linear correlation between ip and V1/2 are listed in Table
is similar to that of 2-H2PO4-. These relationships indicate
and Br-, it can be concluded that 2 showed selectivity to
-
H2PO4
.
3.2. Anion Recognition Behaviors Investigation of 2 by
1
1H NMR and UV-vis. H NMR and UV-vis methods were
used to also investigate the anion recognition of 2. The 1H NMR
spectrum of 2 before and after the addition of H2PO4- is shown
Figure 3. CV curves of 2 before and after addition of HSO4 (A) and Br- (B) in CH2Cl2. (a) [Anion]/[Fe] is 0; (b) [Anion]/[Fe] is 1.4; (c)
-
[Anion]/[Fe] is 2.0.

11174 J. Phys. Chem. B, Vol. 112, No. 35, 2008
Tan et al.
Figure 4. 1H NMR spectra of 2 before (a) and after (b) the addition of one equivalent of H2PO4
.
-
Figure 5. UV-vis spectrum of 2 with the addition of H2PO4-. (a) From top to bottom, H2PO4-/2 ranges from 0 to 1.2. (b) From top to bottom,
-
H2PO4-/2 ranger from 1.2 to 4.0. (c) Changes of UV-vis intensity at 237nm with the addition of H2PO4
.
Figure 6. Possible step-by-step binding model of 2 to H2PO4-. (A) Molecular structure of 2 binding H2PO4-, and (B) Kekule model of 2 binding
-
H2PO4
.
that the charge transport obeys Fick′s Law,21 and the electrode
processes in solution were diffusion-controlled.
According to the CV theories,21 for irreversible process, there
are relationships between peak current, peak potential, and
potential scan rate as follows,
coefficient of electroactive species (cm2/s), concentration of
electroactive species in solution (mol/cm3), surface transfer
coefficient, and standard rate constant (cm/s), respectively.
RnR, D0, and k0 of 2 and 2-H2PO4- were calculated and are
listed in Table 2. Generally, RnR represents rhw electrode
surface electron exchange efficiency, reflects the reversibility
of electrode process, and influences the peak potentials and
the symmetry of the curves. From the CV curves of 2 and
2-H2PO4-, the reversibility of 2 is better than that of
2-H2PO4-, but the trend is the opposite for RnR, which can
be explained by that in the sample of 2, where the potential
scan rate did not reach below zero and adsorption happened
ip ) 0.4985nFAC∗0
1⁄2(RnR)1⁄2D10⁄2V1⁄2
(1)
(2)
F
RT
(
)
RnRF
ip ) 0.227nFAC∗0k0 exp -
E - E0′
(
)
[
p
]
RT
where n, A, D0, RnR, and k0 are the electrochemical
stoichiometry, surface area of the electrode (cm2), diffusion

Anion Electrochemical Recognition
J. Phys. Chem. B, Vol. 112, No. 35, 2008 11175
-
Figure 7. CV curves of 2 only (A) and 2 with 3 equiv H2PO4 (B) in CH2Cl2 at different scan rates. From inner to outer, the scan rate are 0.14,
0.18, 0.22, 0.26, 0.30 V/s. Supporting electrolyte: 0.1 M Bu4NBF4, [Fe] ) 0.5 mM.
TABLE 2: Kinetic Parameters of Electrode Processes for 2
and 2-H2PO4 in CH2Cl2
us much information of the electrode process in the electro-
chemical anion recognition.
-
peak type
RnR
k0 (cm/s)
Dapp (105 cm2/s)
-
2–H2PO4
Acknowledgment. This work is supported by the National
Nature Science Foundation of China (no. 20572097 and o.
20672097) and Science and Technology Program of Ningbo.
We also thank Prof. R. James Cross at Yale University for the
kind discussions.
reduction peak
oxidation peak
0.718
0.869
1.513
0.619
0.344
0.128
2
reduction peak
oxidation peak
0.432
0.271
8.290
10.176
2.432
3.770
Supporting Information Available: CV curves of 1 after
addition of different anions, UV-vis spectra of 2 to sulfate,
1H NMR spectra, mass spectra, and linear relationship of peak
potential, peak current,and scan rate atr shown. This informa-
tion is available free of charge via the Internet at http://
pubs.acs.org.
in the electrode surface which impeded the electron exchange.
In the sample of 2, RnR of the reduction peak is larger than
that of the oxidation peak, which is in accordance with the
usual cases22-25 because of the relative instability of the
-
oxidation state. However, from the sample of 2-H2PO4
,
-
we discovered that the addition of H2PO4 can stabilize the
oxidation state because of their combination. k0 represents
the time that the redox system needs to reach the balance
and can also reflect the reversibility of the electrode process.
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-
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JP805002A