Selective anion receptor for fluoride detection using ferrocenyl-boronate derivative

Article abstract of DOI:10.1016/j.jorganchem.2015.04.003

The reaction of ferrocenecarboxaldehyde with (2-aminoethoxy)diphenylborane led to the formation of ferrocenyl-substituted boronate receptor 1, which was characterized by IR, MS, HRMS, ¹¹B, ¹H and ¹³C NMR spectra. The signaling process was confirmed by UV-vis, electrochemistry measurements as well as ¹H and ¹⁹F NMR spectroscopy. The receptor 1 exhibited high selectivity for F^- in CH₃CN solution over all the other anions in the DPVs (differential pulse voltammetry). ¹H NMR titrations indicated that F^- ion induced tautomerism of E- and Z-isomer transformations of the receptor 1 and there were three kinds of protons showing obvious highfield shifts for receptor 1 in DMSO-d₆. UV-vis titrations demonstrated that the receptor 1 could bind F^- ion forming 1:1 stoichiometric complexes and show high selectivity and sensitivity towards F^- ion.

Full text of DOI:10.1016/j.jorganchem.2015.04.003

Journal of Organometallic Chemistry 788 (2015) 17e26
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Selective anion receptor for fluoride detection using
ferrocenyleboronate derivative
, b, *
Zhi-Ming Su a, Cai-Xia Lin a, Yun-Tao Zhou a, Li-Li Xie a, Yao-Feng Yuan a
a Department of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China
b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of ferrocenecarboxaldehyde with (2-aminoethoxy)diphenylborane led to the formation of
ferrocenyl-substituted boronate receptor 1, which was characterized by IR, MS, HRMS, 11B, 1H and 13C
NMR spectra. The signaling process was confirmed by UVevis, electrochemistry measurements as well as
1H and 19F NMR spectroscopy. The receptor 1 exhibited high selectivity for Fꢀ in CH3CN solution over all
the other anions in the DPVs (differential pulse voltammetry). 1H NMR titrations indicated that Fꢀ ion
induced tautomerism of E- and Z-isomer transformations of the receptor 1 and there were three kinds of
protons showing obvious highfield shifts for receptor 1 in DMSO-d6. UVevis titrations demonstrated that
the receptor 1 could bind Fꢀ ion forming 1:1 stoichiometric complexes and show high selectivity and
sensitivity towards Fꢀ ion.
Received 28 December 2014
Received in revised form
12 March 2015
Accepted 6 April 2015
Available online 18 April 2015
Keywords:
Ferrocenyl
Anion receptor
Fluoride detection
Electrochemistry
UVevis
© 2015 Elsevier B.V. All rights reserved.
1H NMR titration
Introduction
[28e32] as well as hydrogen bonding [33,34] and other types of
interactions involving fluoride as a participant or a disrupter [35].
The design and synthesis of receptors as signaling sensing to
anions has drawn considerable attention because anions are
ubiquitous in the real world and play a fundamental role in a wide
range of organic reactions as well as in the mineral world [1e7].
Numerous efforts have been devoted to the development of che-
mosensors for anions such as dihydrogenphosphate [8e12], fluo-
ride [13e16], acetate [17] and cyanide [18,19] due to their important
role in biology, medicine, catalysis and the environment [20e23].
Among the different anions, the fluoride ion is widely used in a
number of applications such as dental care, treatment of osteopo-
rosis, fluoridation of water supplies, etc. What's more, intake of
excessive fluoride may lead to dental and skeletal fluorosis,
neurological damages, gastric and kidney problems [24e27].
In recent years, a large number of chemosensors have been
developed for detecting fluoride, and a lot of Fꢀ receptors have been
designed based on different recognition mechanism, including
strong interactions taking place between Lewis acids and fluoride
James and co-workers [36] reported a biocompatible fluoride
receptor, which had been developed where the interaction be-
tween the boronic acid and amine (NH) resulted in fluoride ion
selectivity and enhanced fluorescence quenching; Ravikanth
and co-workers [37] synthesized benzimidazole substituted boron-
dipyrromethene based reversible and reusable selective chemo-
sensor for fluoride detection; Yoon and co-workers [35] investi-
gated fluorescence and colorimetric chemosensors for fluoride-ion
detection, including sensors based on the interactions between
fluoride ions and Lewis acid, sensors utilizing hydrogen-bond in-
teractions, reaction-based sensors for the detection of fluoride ion,
etc. Despite considerable efforts having been made, Fꢀ receptors
with good preorganization and multifunctionality are nevertheless
quite rare and still highly desirable.
It is well known that the ferrocene unit has largely proved to be
a simple and remarkable redox-signaling unit, and ferrocene-based
receptors are reported frequently because of their electrochemistry
sensing abilities toward anions, these receptors are expected to
show cathodic shifts in their redox process on complexation to an
anion, as they are easier to oxidize or harder to reduce than a free
redox-active receptor [12,38e42]. The shifting magnitude of po-
tential represents the response level of anion recognition. Recently,
* Corresponding author. Department of Chemistry, Fuzhou University, Fuzhou,
Fujian 350108, China. Tel./fax: þ86 591 22866161.
E-mail address: yaofeng_yuan@fzu.edu.cn (Y.-F. Yuan).
http://dx.doi.org/10.1016/j.jorganchem.2015.04.003
0022-328X/© 2015 Elsevier B.V. All rights reserved.

18
Z.-M. Su et al. / Journal of Organometallic Chemistry 788 (2015) 17e26
a survey of literature reveals that identify units, containing ferro-
cenyl unit and various recognition units including urea, thiourea,
pyrrole, amide and imidazole, which have been designed and
synthesized [41,43e53]. However, it seems rather rare to incorpo-
rate both a redox-active group and Lewis-acidic boron group
[54e56] in one molecule to allow both electrochemical and optical
sensing output for Fꢀ. Herein, we have designed the receptor uti-
lizing a new molecule of ferrocenyl-substituted boronate receptor
1, which potential as chemosensors has been explored using
various anions, with special attention towards high selectivity for
Fꢀ ion.
Results and discussion
Synthesis of ferrocenyl-substituted boronate receptor 1
As depicted in Scheme 1, the target molecule was synthesized
from ferrocenecarboxaldehyde 2, which was prepared via for-
mylation reaction from ferrocene 4 with DMF, and it was obtained
from the literature method [57,58]. The receptor 1 was synthesized
from the reaction of compound 2 and (2-aminoethoxy)diphe-
nylborane in anhydrous toluene with catalytic amount of AcOH.
Finally, the receptor 1 was purified by recrystallization. Analysis of
receptor 1 by 1H NMR spectroscopy showed a characteristic doublet
Fig. 1. UVevis absorption spectra of receptor 1 (5 ꢁ 10ꢀ5 mol Lꢀ1 in CH3CN) in the
presence of 10 equivalents of various anions. Inset: color changes of receptor
1
(c ¼ 5 ꢁ 10ꢀ4 mol Lꢀ1) in CH3CN after addition of 1 equivalent of various anions. (For
interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
of the HC]N group at d: 8.22 ppm, as well as three characteristic
multiplets of the diphenylborane group at d: 7.38 ppm, 7.21 ppm
and 7.13 ppm, respectively. The receptor 1 displayed broad 11B NMR
signal at 12 ppm as showed in Fig. S1.
UVevisible absorption of receptor 1
The receptor 1 was soluble in common organic solvents such as
CH3Cl, dichloromethane (DCM), THF, CH3CN and DMSO, etc. To
investigate the optical properties of receptor 1, UVevis spectra
changes of receptor 1 (c ¼ 5 ꢁ 10ꢀ5 M) in CH3CN upon addition of
anions (Fꢀ, Clꢀ, Brꢀ, AcOꢀ, HSO4ꢀ, and H2PO4ꢀ) were carried out.
The receptor 1 showed high selectivity and Fꢀ ion displayed
good selectivity over other anions, as evidenced by the rapid color
changes from orange red to faint yellow, while other anions caused
no change (Clꢀ, Brꢀ, HSO4ꢀ) or a little change (AcOꢀ, H2PO4ꢀ) in
color (Fig. 1 (inset)). Fig. 1 exhibited the UVevis spectra changes of
receptor 1 towards 10 equivalents of various anions. The receptor 1
showed almost no change in absorption peak in the presence of Clꢀ
and Brꢀ, while a red-shift (Dl ¼ 15 nm) occurred from 479 nm to
494 nm in the presence of HSO4ꢀ, however there was a slight
spectral change of blue-shift (Dl ¼ 9 nm) from 285 nm to 276 nm in
the presence of AcOꢀ or H2PO4ꢀ, and a larger spectral change of
blue-shift (Dl ¼ 17 nm) in the presence of Fꢀ. As showed in Fig. 2,
when the Fꢀ was 1 equivalent, the UVevis spectra had a change of
blue-shift (Dl ¼ 8 nm) from 285 nm to 277 nm. The stoichiometry
between receptor 1 and Fꢀ was determined from the UVevis
titration data. The measured absorbance variation from 285 nm to
Fig. 2. UVevis absorption spectra of receptor 1 (5 ꢁ 10ꢀ5 mol Lꢀ1 in CH3CN) after
addition of various amounts of Fꢀ ion. Inset: Job plot of receptor 1 with Fꢀ determined
by UVevis in CH3CN.
Scheme 2, the reaction mechanism was proposed between receptor
1 and Fꢀ ion based on the Job plots.
To address the sensitivity of receptor 1 towards Fꢀ sensing, we
had carried out UVevis titration of receptor 1 (25
with Fꢀ ion (31.25 mM). Detection limit for Fꢀ ion was calculated
using UV titrations and equation, detection limit ¼ 3 /m, where
¼ standard deviation of 5 blank measurements and m ¼ slope
obtained from the graph of A (
A ¼ A0 ꢀ Ai) intensity vs
mM) in CH3CN
s
268 nm (
D
A ¼ A0 ꢀ Ai) reached maximum values when the molar
fraction of ([Fꢀ]/[1] þ [Fꢀ]) was 0.5, which indicated that receptor 1
s
and Fꢀ formed a 1:1 complexes (Fig. 2 (inset)). As showed in
D
D
Scheme 1. Synthetic procedure of receptor 1. Reaction conditions: (i) DMF, POCl3, chloroform, reflux 12 h, (ii) AcOH, toluene, reflux 5 h.

Z.-M. Su et al. / Journal of Organometallic Chemistry 788 (2015) 17e26
19
Scheme 2. The proposed mechanism of receptor 1 towards Fꢀ ion based on the Job plots in CH3CN.
concentration of Fꢀ added. The detection limit of receptor 1 to-
(Fig. 3a (inset)). Fig. S5 showed the UVevis absorption peak of re-
ceptor 1 in CH3CN:H2O ¼ 90:10 on addition of various amount of Fꢀ,
which was similar to Fig. 2, rather than the UVevis absorption
spectra of the receptor 1 alone. From the results we could draw the
conclusion that proposed reaction mechanism (Scheme 2) was
irreversible.
wards Fꢀ was evaluated to be 5.25
mM (99.7 ppb) (Fig. S4), which
was well below 4 ppm, the allowed concentration level of Fꢀ in
drinking water specified by the USEPA [59].
Experiments were explored by increasing the amount of water
gradually such as 1%, 2%, 3%, 4%, 5%, 8%, 10%, 15% and 20% aqueous
solution on the basis of receptor 1 (1 ꢁ 10ꢀ4 mol Lꢀ1 in CH3CN) with
10 equivalents' Fꢀ ion. As could be seen from Fig. 3a, there was no
remarkable change in absorption peak compared with Fig. 2.
However, the color was changed obviously from faint yellow to
pink rather than yellow (the color of receptor 1 alone in CH3CN) in
the presence of Fꢀ upon the addition of 15% aqueous solution of
Electrochemical studies of receptor 1
The electrochemical properties of receptor 1 (c ¼ 5 ꢁ 10ꢀ4 M)
were investigated by cyclic voltammetry (CV) and by differential
pulse voltammetry (DPV) in acetonitrile containing [n-Bu4N]PF6
(c ¼ 0.10 M) as a supporting electrolyte at room temperature. The
electrochemical behavior of the receptor 1 alone and in the preꢀs-
ꢀ
CH3CN:H2O ¼ 85:15, while AcOꢀ and H2PO4 caused little change
(Fig. 3b). The receptor 1 showed a larger spectral change of blue-
shift (Dl ¼ 28 nm) in the presence of 10 equivalents' Fꢀ from
479 nm to 451 nm, and by increasing the amount of water, for
example, 15% aqueous solution of CH3CN:H2O ¼ 85:15, there was
another change of red-shift (Dl ¼ 15 nm) from 451 nm to 466 nm
ence of various concentrations of anions (Fꢀ, Clꢀ, Brꢀ, AcOꢀ, HSO4
and H2PO4ꢀ) were studied.
,
Fig. 4 showed that the receptor 1 alone exhibited a reversible
redox wave, characteristic for ferrocene derivatives. Upon the
addition of Fꢀ ion to the CH3CN solutions of receptor 1, it caused a
distinct behavior in the CV. Firstly, when the increment of the Fꢀ
ion was 1 equivalent, the CV behavior compared with receptor 1
alone had two redox peaks. Then with the increase in Fꢀ ions, the
CV behavior has been changing; intriguingly when the amount of
Fꢀ ion equaled 5 equivalents, there was only one reduction peak,
while the oxidation peak still had two.
As can be seen from Fig. 5, the DPV for receptor 1 was affected by
the addition of increasing amount of Fꢀ ions. For example, when the
increment of the Fꢀ ion was 1 equivalent, the DPV behavior
compared with receptor 1 alone had two reduction peaks, one was
still in the previous reduction potential of 810 mV, the other had a
cathodic shift of 298 mV. With the increase in Fꢀ ions, the DPV
behavior has been changing; representatively when the amount of
Fꢀ ion equaled 5 equivalents, there was only one peak observed,
and its reduction peak was 512 mV completely, different from the
Fig. 3. (a) UVevis absorption spectra of receptor 1 (1 ꢁ 10ꢀ4 mol Lꢀ1 in CH3CN) with
10 equivalents' Fꢀ ion by increasing the amount of water gradually; (b) color changes
of receptor 1 (c ¼ 5 ꢁ 10ꢀ4 mol Lꢀ1) in CH3CN with 1 equivalent of Fꢀ, OAcꢀ and
ꢀ
H2PO4 upon the addition of 15% aqueous solution of CH3CN:H2O ¼ 85:15. (For
interpretation of the references to color in this figure legend, the reader is referred to
Fig. 4. CV of receptor 1 (c ¼ 5 ꢁ 10ꢀ4 M) in CH3CN on addition of various concen-
the web version of this article.)
trations of Fꢀ ion; supporting electrolyte: 0.10 M [n-Bu4N]PF6; scan rate: 100 mV sꢀ1
.

20
Z.-M. Su et al. / Journal of Organometallic Chemistry 788 (2015) 17e26
behavior showed a cathodic shift of 173 mV. The CV and DPV
behavior indicated that the complexation process between the re-
ceptor 1 and Fꢀ ion was irreversible.
In order to explain the electrochemical behaviors, we brought
forward the model of receptor 1 towards Fꢀ ion based on the
electrochemical data. Fig. 8 showed that the redox peak for receptor
1 with the increasement Fꢀ ion was cathodic shift in anhydrous
CH3CN because of strong interactions taking place between Lewis-
acid boron and Fꢀ ion as well as hydrogen bonding, including the
participation of ferrocenyl protons, which had been reported and
well illustrated that CeH of ferrocene had hydrogen bonding with
other anions [41,43e45], such as BF4ꢀ, H2PO4ꢀ, OAcꢀ. And by
increasing the amount of water, since water competed with the
hydrogen bonding of fluoride towards receptor 1, the redox peak
should be similar to the receptor 1 alone, however the strong
interaction between Lewis-acid boron and Fꢀ made its chemical
structure essentially different from receptor 1, resulted in the
different redox potential as showed in Fig. 8 and Fig. S13.
Fig. 5. DPV of receptor 1 (c ¼ 5 ꢁ 10ꢀ4 M) in CH3CN on addition of various concen-
trations of Fꢀ ion; supporting electrolyte: 0.10 M [n-Bu4N]PF6; scan rate: 100 mV sꢀ1
.
1H NMR titration studies of receptor 1
receptor 1 alone (810 mV). The electrochemical changes of DPV
curves of receptor 1 upon addition of various anions were illus-
trated in Figs. S8eS12. Fig. 6 showed the DPV behavior of receptor 1
upon addition of 5 equivalents of various anions, and the results of
the electrochemical analysis of receptor 1 were listed in Table 1. The
To gain more insight into the anion recognition properties of
receptor 1, 1H and 19F NMR titration experiments were carried out
by adding tetrabutyl-ammonium salts of the anions (TBAX, where
X ¼ Fꢀ, Clꢀ, Brꢀ, AcOꢀ, HSO4ꢀ, and H2PO4ꢀ) to the receptor 1
(c ¼ 5 ꢁ 10ꢀ3 M, DMSO-d6) solution.
shift magnitude ꢀof
DEpc values followed the order
As well known, the Schiff base has a tautomerism of E- and Z-
isomer transformations. From 1H NMR it was observed that upon
the addition of various concentrations of Fꢀ ions, Fig. 9 exhibited
that Fꢀ ion induced tautomerism and E- and Z-isomer trans-
formations of the receptor 1. And there were three kinds of protons
showing obvious highfield shifts for receptor 1, i.e., the HC]N
proton (H1); C-2 proton (H2) and C-4 proton (H4) of diphenylborate;
C-2 proton (H5) and C-3 proton (H6) of the ferrocene ring. The
highfield shifts of the three kinds of protons indicated the presence
of three main interactions between the receptor 1 and the Fꢀ ion in
DMSO-d6 solution. Representatively, the C-4 proton (H7) of the
ferrocene ring showed two kinds of 1H NMR, one was highfield shift
Fꢀ > AcOꢀ z H2PO4 > ꢀBrꢀ z Clꢀ z HSO4ꢀ, intriguingly upon the
addition of AcOꢀ/H2PO4 to the CH3CN solutions of receptor 1, the
reduction potential still had two peaks (663 mV, 512 mV or 673 mV,
501 mV), which was different from Fꢀ ion. From the results we
could draw the conclusion that the receptor 1 showed affinity and
high selectivity for Fꢀ ion in electrochemical behaviors.
Experiments on CV and DPV were also explored by increasing
the amount of water gradually such as 1%, 2%, 3%, 5%, 10%, 15%, 20%
and 25% aqueous solution on the basis of receptor 1 with 5 equiv-
alents' Fꢀ ion. As could be seen from Fig. 7, the CV behavior of re-
ceptor 1 was changed by increasing the amount of water, for
example, 15% aqueous solution of CH3CN:H2O ¼ 85:15, there was
one reversible redox wave, however the redox peak (Epa ¼ 703 mV,
(
Dd ¼ 0.2 ppm) and the other was downfield shift (Dd ¼ 0.8 ppm).
And it had a triplet around 16 ppm for the HF2ꢀ, indicating
deprotonation and fluoride ion mediated hydrogen-deuterium
exchanges [60] (Fig. S14).
Epc
¼
622 mV) was different from the receptor 1 alone
(Epa ¼ 878 mV, Epc ¼ 810 mV). Fig. S13 showed the DPV behavior of
receptor 1 with 5 equivalents' Fꢀ ion upon the addition of water,
which was different from the receptor 1 alone and the DPV
Fig. 10 exhibited 1H NMR spectra of receptor 1 upon the addition
of 3 equivalents of various anions, and the results of the 1H NMR
spectra analysis of receptor 1 were listed in Table 2. The Brꢀ, Clꢀ and
ꢀ
HSO4 had little effect on the receptor 1. Although the gradual
highfield shifts of the HC]N proton (H1), C-2 proton (H5) and C-3
proton (H6) of the ferrocene moiety resonances were followed
upon addition of H2PO4ꢀ, the receptor 1 didn't have a tautomerism
of E- and Z-isomer transformations and the C-4 proton (H7) of the
ferrocene ring had no shift change essentially. The receptor 1 had a
tautomerism upon the addition of AcOꢀ, different from fluoride ion,
it had two kinds of 1H NMR, one remained substantially the same
and the other was highfield shift. Intriguingly the C-4 proton (H7) of
the ferrocene ring was split, in which it wasn't separated
completely. From Fig. 5 and Table 2, we could draw the conclusion
that receptor 1 displayed selectivity for ꢀFꢀ ion over other anions in
ꢀ
the order Fꢀ > AcOꢀ > H2PO4 > HSO4 z Brꢀ z Clꢀ, which was
consistent with the electrochemical results.
In addition to 1H NMR, 19F NMR (Fig. 11) further confirmed that
the gradual downfield shifts of TBAF, suggesting that the Fꢀ ion has
some effect with the receptor 1, associated with CeH hydrogen
bonding. And the downfield shifts of 19F NMR of TABF is 0.5 ppm in
receptor 1 upon the addition of 6 equivalents of Fꢀ ion, which was
indirectly proved that there was an interaction between the
Fig. 6. DPV of receptor 1 (c ¼ 5 ꢁ 10ꢀ4 M) in CH3CN on addition of 5 equivalents of
various anions; supporting electrolyte: 0.10 M [n-Bu4N]PF6; scan rate: 100 mV sꢀ1
.

Z.-M. Su et al. / Journal of Organometallic Chemistry 788 (2015) 17e26
21
Table 1
The electrochemical data of DPV at the scan rate of 0.1 V sꢀ1 between the free receptor 1 and the increasement of 5 equivalents of anions.a
ꢀ
ꢀ
Ferrocene
1
1 þ Fꢀ
1 þ AcOꢀ
1 þ H2PO4
1 þ Clꢀ
1 þ Brꢀ
1 þ HSO4
Epc (mV)
431
/
810
/
512
298
663/512
147/298
673/501
137/309
760
50
744
66
792
18
D
Epc (mV)
Epc ¼ Epc (1) ꢀ Epc (X) (X ¼ Fꢀ, Clꢀ, Brꢀ, AcOꢀ, HSO4ꢀ, and H2PO4ꢀ).
a
D
receptor 1 and Fꢀ ion. Based on the 1H NMR and 19F NMR titration
spectra the proposed mechanism of receptor 1 towards Fꢀ ion on
electrochemical measurements was further confirmed, in which
there was a formation of diphenylborate and free Fꢀ ion; the fer-
rocenyl group and the HC]N group had hydrogen bonding with
the Fꢀ ion.
receptor 1 and X (X ¼ Fꢀ, AcOꢀ, H2PO4ꢀ, HSO4ꢀ, Brꢀ and Clꢀ), the
basicity of the anion, and the conformation of the receptor. UVevis
titrations demonstrated that the receptor 1 could bind Fꢀ ion
forming 1:1 stoichiometric complexes and show high selectivity
and sensitivity towards Fꢀ ion, and the detection limit of receptor 1
towards Fꢀ was evaluated to be 5.25
mM (99.7 ppb). Further in-
vestigations on ferrocenyl-substituted boronate chemosensors and
towards enhancing their selectivity and sensitivity are currently
ongoing in our laboratory.
Computational study
Further insight into this recognition process was obtained from
density functional theory (DFT) calculations, which had been
performed to understand the binding mode of Fꢀ with the re-
ceptor 1. The optimized structure of 1$2Fꢀ was represented in
Fig. 12. The remarkable result of the computed binding configu-
ration of 1$2Fꢀ complex revealed that Fc-H and N]CeH protons
had acted as donors for Fꢀ (1.762 Å, 161.3ꢂ and 1.844 Å, 169.2ꢂ).
Moreover, two additional weak AreH/FeB types of H-bonding
had also been observed, the corresponding H-bonding lengths
were listed in Table 3. This result indicated that the interaction
between Fꢀ and the receptor 1 not only had the hydrogen
bonding, but also the interaction took place between Lewis-acid
boron and fluoride ion, which was consistent with the 1H NMR
titration.
Experimental
General methods
All chemicals were commercial available and all solvents were
purified and dried in standard methods, the solution of DMF was
stirred with CaH2 overnight, and the product was got by vacuum
distillation before use. All experiments were performed under ni-
trogen atmosphere unless stated otherwise. Melting points (m.p.)
were determined using a 4ꢁ microscopic melting point instrument.
Infrared spectra (IR) were recorded on a Spectrum 2000 FTIR
spectrometer in KBr pellets and reported in cmꢀ1. 1H and 13C NMR
spectra were obtained in deuterated solvents on a Bruker Avance III
400 NMR apparatus. 11B NMR spectra were obtained in deuterated
solvents on a Bruker Avance III 500 NMR apparatus. Chemical shifts
Conclusions
d
were reported in ppm relative to internal standard TMS for both
1H and 13C NMR. Mass spectra were taken on a Thermo-Finnigan
LCQ spectrometer. High resolution mass spectrometer was ob-
tained on an Accurate-Mass Q-TOFLC/MS.
In summary, we have synthesized ferrocenyl-substituted boro-
nate receptor 1 and examined the recognition properties toward a
series of common anions. The receptor 1 showed strong electro-
chemical responses for Fꢀ ion with a remarkable phenomenon of a
cathodic shift of 290 mV, and Fꢀ ion displayed good selectivity over
other anions. 1H and 19F NMR titration experiments demonstrated
that receptor 1 displayed high selectivity for Fꢀ ion and binded with
Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) studies were performed by a CHI620C electrochemical
workstation using glassy carbon as the working electrode, platinum
electrode as counter electrode, and Ag/AgCl (3.0 M KCl) as reference
electrode at room temperature (scan rate: 100 mV sꢀ1). Acetonitrile
containing [n-Bu4N] PF6 (c ¼ 0.10 M) was used as a supporting
electrolyte and acetonitrile was refluxed with anhydrous potassium
carbonate for 5 h before use. Redox potentials were reported and
redox potential shifts were obtained after an excess of anions was
added to the solution of receptor 1 in CH3CN (c ¼ 5 ꢁ 10ꢀ3 M)
containing 0.1 M [n-Bu4N] PF6.
ꢀ
ꢀ
anions in the order Fꢀ > AcOꢀ > H2PO4 > HSO4 z Brꢀ z Clꢀ,
which was primarily determined by the interaction between
Fig. 7. CV of receptor 1 (c ¼ 5 ꢁ 10ꢀ4 M) in CH3CN with 5 equivalents' Fꢀ ion by
increasing the amount of water; supporting electrolyte: 0.10 M [n-Bu4N]PF6; scan rate:
Fig. 8. The proposed electrochemistry model of receptor 1 towards Fꢀ ion in CH3CN
upon the addition of water.
100 mV sꢀ1
.

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Z.-M. Su et al. / Journal of Organometallic Chemistry 788 (2015) 17e26
Fig. 9. 1H NMR spectra of receptor 1 (c ¼ 5 ꢁ 10ꢀ3 M) in DMSO-d6 upon the addition of various concentrations of Fꢀ ion.

Z.-M. Su et al. / Journal of Organometallic Chemistry 788 (2015) 17e26
23
Fig. 10. 1H NMR spectra of receptor 1 (c ¼ 5 ꢁ 10ꢀ3 M) in DMSO-d6 upon the addition of 3 equivalents of various anions.

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Z.-M. Su et al. / Journal of Organometallic Chemistry 788 (2015) 17e26
Table 2
Synthesis of ferrocenecarboxaldehyde (2)
1H NMR characteristic data for receptor 1 upon the addition of 3 equivalents of
various anions.a2
In a Schlenk-type apparatus, 2.1 mL (28 mmol) of DMF was
added to the solution of 5.31 g (28 mmol) of ferrocene in 30 mL of
dry chloroform, and the resulting mixture was stirred in an ice-bath
under nitrogen atmosphere for 15 min. Then, 2.6 mL (28 mmol) of
POCl3 was dropwise added to the mixture about half an hour. The
resulting reaction mixture was refluxed for 12 h. After solvent
removal, the product was poured into 100 mL ice water and filtered.
The filtrate was neutralized with Na2CO3 power, and extracted
repeatedly with ether. The crude product was obtained by evapo-
ration of the solvent and purified by silica gel (100e200 mesh)
chromatography to give a reddish-brown solid. Yield: 3.21 g, 53%.
H1 (ppm)
H5 (ppm)
H6 (ppm)
H7 (ppm)
1
8.22
5.03
4.83
4.20
4.22/4.11
4.20
4.20
4.20
M.p. 123e124 ꢂC. 1H NMR (400 MHz, CDCl3):
d 9.99 (s, 1H, CHO),
1 þ Fꢀ
1 þ AcOꢀ
1 þ H2PO4
1 þ Clꢀ
1 þ Brꢀ
8.02/7.93
8.22/8.11
8.11
8.21
8.21
4.65/4.60
5.03/4.62
4.62
5.02
5.02
4.44/4.38
4.83/4.38
4.37
4.82
4.83
4.82 (s, 2H, Cp-H), 4.63 (s, 2H, Cp-H), 4.30 (s, 5H, Cp-H). 13C NMR
ꢀ
(101 MHz, CDCl3): d 193.50 (CHO), 79.63 (Cp), 73.19 (Cp), 69.58 (Cp).
MS (ESI) m/z 214.89 [M þ H]þ Cal. 214.
4.19
4.19
ꢀ
1 þ HSO4
a
8.20
5.03
4.83
Synthesis of ferrocenyl-substituted borate receptor (1)
DMSO-d6, at 298 K.
To a solution of (2-aminoethoxy)diphenylborane 3 (1.26 g,
5.60 mmol) in dry toluene (30 mL) was added 3 drops of acetic acid.
To this was added a solution of ferrocenecarboxaldehyde 2 (1.20 g,
5.60 mmol) in dry toluene (30 mL) under a nitrogen atmosphere.
The reaction mixture was refluxed for 5 h, and then the solvent was
removed under vacuum. The resulting solid was recrystallized in an
ethyl acetate:petroleum ether ¼ 1:2 mixture to afford 1 as a dark
red solid. Yield: 1.43 g, 61%. M.p. 180e181 ꢂC. 1H NMR (400 MHz,
In the 1H NMR titration experiment, aliquots (such as 1, 2, 3 …)
of the concentrated solutions of the tetrabutylammonium salts
(0.25 M, DMSO-d6) were added to a solution of receptor 1 (0.5 mL,
5.0 mM) in DMSO-d6.
The UVevisible spectroscopy was tested by Perkin Elmer
lambda 900 UV/VIS/NIR in acetonitrile (c ¼ 5 ꢁ 10ꢀ5 M), with this
method, quartz material was used as cuvette, whose volume was
1.0 ꢁ 1.0 ꢁ 4.0 cm3.
DMSO-d6):
d
8.22 (d, 1H, J ¼ 2.4 Hz, HC]N), 7.42e7.34 (m, 4H, Ph-
Fig. 11. 19F NMR spectra of receptor 1 (c ¼ 5 ꢁ 10ꢀ3 M) in DMSO-d6 upon the addition of various concentrations of Fꢀ ion.

Z.-M. Su et al. / Journal of Organometallic Chemistry 788 (2015) 17e26
25
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