The anion recognition properties of Schiff base or its reductive system based on 2,2′-bipyridine derivatives

Article abstract of DOI:10.1016/j.saa.2009.01.013

Two novel artificial receptors, 2,2′-bipyridine derivatives containing phenol group, have been designed and synthesized. The interaction of the receptors containing Schiff base or its reductive system with biologically important anions was determined by U

Full text of DOI:10.1016/j.saa.2009.01.013

Spectrochimica Acta Part A 72 (2009) 1117–1121
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
The anion recognition properties of Schiff base or its reductive system based on
2,2ꢀ-bipyridine derivatives
Xue-Fang Shang∗
Xinxiang Medical University, Xinxiang Henan 453003, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Two novel artificial receptors, 2,2ꢀ-bipyridine derivatives containing phenol group, have been designed
and synthesized. The interaction of the receptors containing Schiff base or its reductive system with
biologically important anions was determined by UV–vis and 1H NMR titration experiments. Results
indicate that receptors 1 and 2 show the strong binding ability for dihydrogen phosphate (H2PO4−),
fluoride (F−), acetate (AcO−) and almost no binding ability for chloride (Cl−), bromide (Br−), iodide (I−).
Article history:
Received 10 August 2008
Received in revised form
28 December 2008
Accepted 14 January 2009
−
At the same time, the strongest binding ability of receptor 1 for H2PO4 among studied anions is not
Keywords:
influenced by the existence of other anions; as well as receptor 2 for F−. In addition, the binding ability
of receptor 1 (Schiff base system) with various anions is stronger than that of receptor 2 (the reductive
Schiff base system) due to the difference of electronic effect.
Anion recognition
Phenol derivative
Schiff base
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
only recently been realized and synthetic chemosensors have been
developed to date. It is important for researchers to design the
In recent years, increasing attention in the field of host–guest
chemistry has been devoted to the fast development of anion recog-
nition systems [1–9]. Considerable attention has focused on the
design of anion receptors because of the important roles of the
anion in chemical and environmental fields [10]. Anions are ubiqui-
tous throughout biological systems. They carry genetic information
(DNA is a polyanion), and the majority of enzyme substrates and
co-factors are anionic. Recently, biological anions such as pyruvate
and glutamate have attracted researches’ attention [11,12]. Among
the range of biologically important anions, the fluoride anion has
attracted growing attention due to its established role in prevent-
ing dental caries. Fluoride anion is also being explored extensively
as a treatment for osteoporosis, a type of fluoride toxicity that
generally manifests itself clinically in terms of increasing bone
density. Acetate is critical component of numerous metabolic pro-
cesses. Acetate production and oxidation rate have been frequently
used as an indicator of organic decomposition in marine sediments
[13]. Phosphate anions are very important anionic species in living
organisms. Naturally occurring phosphate-binding protein (PBP)
selectively and strongly bind hydrogen phosphate.
chemosensors which show the specific recognition ability for a
certain anion and can convert the recognition event into a signal
[20].
Nitro group (NO2) serving as chromophore and electro-
withdrawing group is often applied to the design of anion receptors.
The interaction between artificial receptors and anions is related
with hydrogen-bond. Hydroxy group (OH) is good hydrogen bond-
giving group. Based on the characteristics of NO2 and hydroxy
group, we designed and synthesized a Schiff base receptor contain-
ing hydroxy group and NO2 based on 2,2ꢀ-bipybridine (Scheme 1).
At the same time, we also synthesized reductive Schiff base system
in order to compare the binding ability of Schiff base or its reductive
system with biologically important anions. The results of UV–vis
and 1H NMR indicate that the Schiff base system (receptor 1) and
its conductive system (receptor 2) show different degree binding
ability for F−, AcO−, H2PO4 and the binding ability of H2PO4 or
F− with receptors 1 or 2 is the strongest among the above anions,
respectively; almost no binding ability for Cl−, Br− and I−. In addi-
tion, the binding ability of receptor 1 (Schiff base system) is stronger
than that of receptor 2 (reductive Schiff base system).
−
−
Artificial anion receptors have presented a lot of special applica-
tion prospects in the fields of anion sensors [14–16], phase-transfer
catalysts [17] and separation and anion-selective electrodes etc.
[18,19]. Also anions acting as the environmental pollutants have
2. Experimental
Most of the starting materials were obtained commercially
and all reagents and solvents used were of analytical grade.
All anions, in the form of tetrabutylammonium salts, were pur-
chased from Sigma–Aldrich Chemical Co., stored in a desiccator
under vacuum containing self-indicating silica, and used without
∗
Tel.: +86 373 2161489.
E-mail address: shangxuefang@mail.nankai.edu.cn.
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.01.013

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X.-F. Shang / Spectrochimica Acta Part A 72 (2009) 1117–1121
J = 3.0, 2H), 8.10 (m, J = 15.9 Hz, 4H), 7.26 (t, J = 18.3 Hz, 4H), 6.72 (m,
J = 12.0 Hz, 4H). Elemental analysis: Calc. for C24H16 N6O6 C, 59.51;
H, 3.33; N, 17.35; found: C, 59.32; H, 3.76; N, 17.43. ESI-MS (m/z):
483.2[M-H]−.
Di(2ꢀ-hydroxy-5ꢀnitrophenyl-1ꢀ-methylamino-5)-2,2ꢀ-
bipyridine (receptor 2): receptor
1 (0.1 mmol, 48.4 mg) was
dissolved in 30 mL dry methanol. NaBH4 (0.5 g) was added to
the solution slowly under ice–water condition. After the addition
was completed, the solution was stirred for 4 h at the room
temperature. Then the solvent was removed under reduced
pressure. Distilled water (10 mL) was added to the solid and the
yellow solid precipitate was formed. The mixture was filtered and
dried under vacuum. Yield: 89%. 1H NMR(400 MHz, DMSO-d6,
298 K) ı = 8.09 (d, J = 5.6, 2H), 8.02 (t, J = 12.8, 4H), 7.56 (s, 2H),
7.31 (s, 2H), 6.93 (d, J = 8.8, 2H), 6.54 (d, J = 4.0, 2H), 4.35 (d,
J = 4.4, 4H). Elemental analysis: Calc. for C24H20N6O6 C, 59.01; H,
4.13; N, 17.21; found: C, 59.18; H, 3.95; N, 17.36. ESI-MS (m/z):
487.3[M-H]−.
Scheme 1. Chemical structure of receptors.
any further purification. Dimethyl sulfoxide (DMSO) was distilled
in vacuo after dried with CaH2. Tetra-n-butylammonium salts
(such as (n-C4H9)4NF, (n-C4H9)4NCl, (n-C4H9)4NBr, (n-C4H9)4NI,
(n-C4H9)4NAcO, and (n-C4H9)4NH2PO4) need to be dried 24 h in
vacuum with P2O5 at 333 K before use. C, H, N elemental anal-
yses were made on a Vanio-EL. ESI-MS was performed with a
MARINER apparatus. 1H NMR spectra were recorded on a Varian
UNITY Plus-400 MHz Spectrometer. UV–vis Spectroscopy titra-
tion was recorded on a Shimadzu UV-2450 Spectrophotometer at
298.2 0.1 K.
3. Results and discussion
For an excellent chemosensor, high selectivity is a matter of
necessity. UV–vis spectra of receptors 1 and 2 in DMSO solution
were investigated upon addition of various anions. The spectral
changes of receptor 1 with various anions are as shown in Fig. 1.
Receptors 1 and 2 were prepared according to the route shown
in Scheme 2.
From Fig. 1 we can see that the additions of F−, AcO−, H2PO4
−
induce similar spectral changes. Receptor 1 has absorption peak
at about 300, 375 and 435 nm when the anions do not exist. With
the increase of anion concentration, the intensity of absorbance
at 300 nm decreases and the intensity of absorbance at 375 and
435 nm increases. At the same time, a clear isosbestic point forms
at 350 nm. The appeared isosbestic point shows that the sta-
ble complex forms with a certain stoichiometric ratio between
receptor 1 and anions. Although receptor 1 shows similar spec-
4,4ꢀ-diamino-2,2ꢀ-bipyridine was synthesized according to
reported literature [21]. Yield: 82%. 1H NMR (400 MHz, DMSO-d6,
298 K) ı = 8.03 (d, J = 5.6 Hz, 2H), 7.53 (d, J = 1.6 Hz, 2H), 6.45 (m,
J = 7.6 Hz, 2H), 6.01 (s, 4H).
Di(2ꢀ-hydroxy-5ꢀnitrophenyl-1ꢀ-methylimino-5)-2,2ꢀ-
bipyridine (receptor 1): a mixture of 4,4ꢀ-diamino-2,2ꢀ-bipyridine
(1 mmol, 186 mg) and 5-nitrosalicylaldehyde (2 mmol, 334 mg) in
30 mL EtOH was refluxed for 4 h. The solution was cooled to room
temperature and a yellow precipitate was formed and washed with
ethanol three times. The powder was dried under vacuum. Yield:
92%. 1H NMR(400 MHz, DMSO-d6, 298 K) ı = 10.22 (s, 2H), 8.33 (d,
tral response for F−, AcO− and H2PO4 at the same wavelength,
−
the intensity of spectral response is different upon the addition
of the same equiv. F−, AcO− and H2PO4−. This suggests the diffe−r-
ent binding ability between receptor 1 and F−, AcO− and H2PO4
.
Scheme 2. Synthesis route.

X.-F. Shang / Spectrochimica Acta Part A 72 (2009) 1117–1121
1119
−
Fig. 1. UV–vis spectral changes of receptor
1
(2.0 × 10−5 mol L−1
)
upon the addition of a) F− (0–40 × 10−5 mol L−1); b) AcO− (0–40 × 10−5 mol L−1); c) H2PO4
(0–40 × 10−5 mol L−1). Arrows indicate the direction of increasing anion concentration.
−
Fig. 2. UV–vis spectral changes of receptor
2
(2.0 × 10−5 mol L−1
)
upon the addition of (a) F− (0–40 × 10−5 mol L−1); b) AcO− (0–40 × 10−5 mol L−1); c) H2PO4
(0–40 × 10−5 mol L−1). Arrows indicate the direction of increasing anion concentration.

1120
X.-F. Shang / Spectrochimica Acta Part A 72 (2009) 1117–1121
Table 1
Affinity constants of receptors 1 and 2 with various anions.
Anion
Ks (1) mol−1
L
Ks (2) mol−1
L
F−
(2.51 0.38) × 104
(1.34 0.28) × 104
AcO−
H2PO4
Cl−
(2.98 0.46) × 104
(5.73 0.13) × 103
−
(4.49 0.53) × 104
(9.25 0.11) × 103
ND
ND
ND
ND
ND
ND
Br−
I−
ND: the spectrum change little and the affinity constant cannot be determined.
However, the additions of Cl−, Br− and I− to receptor 1 virtu-
ally lead to almost no spectral changes. This indicates that there
is almost no interaction between receptor 1 and Cl−, Br− and
I−.
Fig. 2 shows the interacted UV–vis spectrum of receptor 2 with
various anions. Receptor 2 was obtained by the reduction of Schiff
base in receptor 1. Therefore, the absorption spectrum of recep-
tor 2 is different from that of receptor 1. Receptor 2 has certain
absorption peak at about 320 and 435 nm when the anions don’t
exist. With the increase of anion concentration, the intensity of
absorbance at 320 nm decreases and the intensity of absorbance at
435 nm increases. At the same time, a clear isosbestic point forms at
375 nm. The appeared isosbestic points show that the stable com-
plex forms with a certain stoichiometric ratio between receptor 2
and anions. However, the additions of Cl−, Br− and I− to receptor
2 virtually lead to almost no spectral changes. This indicates that
there is almost no interaction between receptor 2 and Cl−, Br− and
I−.
Fig. 4. Plots of 1H NMR spectra of receptor 2 in DMSO-d6 upon the addition of various
quantities of Bu4NF.
The binding ability of receptors 1 or 2 with dihydrogen phos-
phate or fluoride anion is the strongest among the studied anions
according to affinity constant. The above results derive from the
condition that only one anion is added. Whether the binding ability
of fluoride anion occurs changes under the condition that all stud-
ied anions exist simultaneously, we conduct the UV–vis spectral
interference experiment (Fig. 3). Results indicate that the intensity
of spectral response of receptor 1 upon the addition of the mixed
anions is almost as the same as the addition of only dihydrogen
phosphate and the intensity of spectral response of receptor 2 upon
the addition of the mixed anions is almost as the same as the addi-
tion of only fluoride. It shows that the binding ability of hydrogen
phosphate with receptor 1 is not influenced by the existence of
other anions. Similarly, the binding ability of fluoride with receptor
2 is not influenced by the existence of other anions. This point has
experimental and practical importance.
The affinity constants are obtained using the method of non-
linear least square calculation according to UV–vis data [22–24].
The results are summarized in Table 1.
Obviously, the binding ability of receptor 1 (Schiff base) with
various anions is stronger than that of receptor 2 (reductive Schiff
base) with various anions. This may be related with electronic
effect. In addition, the binding ability of receptors 1 and 2 with
various anions is in the order of H2PO4− > AcO− > F− ꢁ Cl−, Br−,
I−; F− > H2PO4− > AcO− ꢁ Cl−, Br−, I−, respectively. Receptors 1
and 2 show the highest binding ability with H2PO4 and F−,
To look into the anion binding properties of receptors for studied
−
respectively. Receptor 2 was obtained by the reduction of recep-
tor 1. After the reduction of receptor 1, non-conjugated bond
appears in receptor 2. The configuration of receptor 2 (reduc-
tive Schiff base) is clearly different from receptor 1 in space. The
order that receptors bind various anions is different because the
binding ability is related with configuration match of host–guest
[25,26].
anions and the roles of CH2 and NH in reductive Schiff base, the 1
H
NMR titration experiment in DMSO-d6 between receptor 2 and F− is
shown in Fig. 4. Obviously, the proton signals in benzene ring (8.05
and 6.93 ppm) shift to the upfield direction with the increase in
F− concentration. In addition, the proton signal in NH (7.31 ppm,
reductive Schiff base) clearly shifts to the upfield direction. The
proton signal in CH2 (4.35 ppm) slightly shifts to upfield.
Fig. 3. UV–vis spectral changes of receptors 1 or 2 upon the addition of anions (1) = (2) = 2 × 10−5 mol L−1, (a) the concentration of anions is 2 equiv. receptor 1. Black line:
receptor 1; red line: 1 + H2PO4− + other studied anions, green line: 1 + H2PO4−. (b) the concentration of anions is 5 equiv. receptor 2. Black line: receptor 2; red line: 2 + F− + other
studied anions, green line: 2 + F−
.

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