The tautomerization between keto- to phenol-hydrazone induced by anions in the solution

Article abstract of DOI:10.1016/j.molstruc.2011.11.020

Two simple anion receptors, 2-[(2-hydroxy-5-nitrophenyl)methylene]hydrazone (1) and 2-[(3,5-dibromo-2-hydroxyphenyl)methylene]hydrazone (2) with -OH binding sites, were synthesized and characterized. The anion binding ability of receptors 1 and 2 with hal

Full text of DOI:10.1016/j.molstruc.2011.11.020

Journal of Molecular Structure 1010 (2012) 52–58
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
The tautomerization between keto- to phenol-hydrazone induced by anions
in the solution
a
a
b
c
Xuefang Shang a, , Jianmei Yuan , Yingling Wang , Jinlian Zhang , Xiufang Xu
⇑
a Department of Chemistry, Xinxiang Medical University, East of Jinsui Road, Xinxiang, Henan 453003, China
b School of Pharmacy, Xinxiang Medical University, East of Jinsui Road, Xinxiang, Henan 453003, China
c Department of Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two simple anion receptors, 2-[(2-hydroxy-5-nitrophenyl)methylene]hydrazone (1) and 2-[(3,5-
dibromo-2-hydroxyphenyl)methylene]hydrazone (2) with –OH binding sites, were synthesized and char-
acterized. The anion binding ability of receptors 1 and 2 with halide anions (FÀ, ClÀ, BrÀ and IÀ), AcOÀ and
Received 21 September 2011
Received in revised form 6 November 2011
Accepted 17 November 2011
Available online 29 November 2011
H2POÀ was investigated using visual (naked-eye), UV–vis titration experiments in dry DMSO together
4
with DFT theoretical calculation. The addition of FÀ, AcOÀ and H2PO4À to the host solution resulted in a
red shift of the charge-transfer absorbance band accompanied by a color change from yellow to orange
in the naked-eye experiments. Receptor 1 containing a nitro group at the para position and receptor 2
containing two bromine groups at the ortho and para positions both showed strong binding ability for
Keywords:
Anion recognition
Tautomerization
Naked-eye
Theoretical investigation
Hydrazone derivative
H2POÀ ion in the form of phenol-hydrazone. Moreover, receptor 1, induced by anion species in the solu-
4
tion, converted to the form of phenol-hydrazone from keto-hydrazone.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
without resorting to any expensive equipment [24,25]. In general,
these chemosensors are constructed according to the receptor-
Anion recognition by artificial receptors has received consider-
able attention in the field of host–guest chemistry due to the
important roles of anions in biomedicinal and chemical processes
[1–12]. The design of these receptors has been focused on having
the ability to selectively recognize and sense the biologically
important anions. Water-soluble anions (fluoride, chloride, bro-
mide, phosphate, and etc.) play crucial role in a wide range of bio-
logical phenomena and are implicated in many disease states [13].
Especially, ubiquitous phosphates are biologically relevant anions.
Phosphorylated species also play critical roles in a variety of funda-
mental processes such as energy transduction, signal processing,
genetic information storage, and membrane transport [5]. In addi-
tion, phosphates can be found in many chemotherapeutic and anti-
viral drugs [14–16]. It is for sure that phosphate originating from
the over use of agricultural fertilizers can also lead to eutrophica-
tion in inland waterways [17]. Hence, to recognize and sense phos-
phate ions and phosphorylated biomolecules is to be more and
more important than other biologically functional anions [18–21].
In many cases, a colorimetric receptor for special anionic spe-
cies is of particular interest due to its simplicity and high sensitiv-
ity [22,23]. In particular, colorimetric-based sensing is especially
attractive, as it may allow naked-eye detection of the analyte
chromophore general binomial, which involves the binding of a
special anion substrate with receptor sites and a chromophore
responsible for translating the receptor–anion association into an
optical signal [26–28]. The color variation will occur when a
charge-transfer complex is formed [29]. Although anion receptors
containing phenol group as a binding site have been researched,
recognition mechanism based on tautomeric phenol is rarely
reported in the ‘‘naked-eye’’ detection of anions [30–32]. There-
fore, it is necessary to develop a colorimetric anion receptor with
anion-induced phenol-hydrazone as a signaling mechanism. By
exploiting the optical properties of hydrazone group, many func-
tional molecules have been reported in literatures [33,34]. Phenol
group is among the most frequently used fragments to design neu-
tral receptors for the selective recognition of anions as it is able to
strongly bind anions using directional hydrogen bonding interac-
tions even in the aqueous solution [31,3]. We reasoned that a sim-
ple colorimetric anion receptor would be obtained through
coupling the phenol group as recognition site with nitro chromo-
phore as a signal group. In this paper, we designed and synthesized
a hydrazone derivative (1) containing phenol group (Scheme 1).
According to the following theoretical investigation, compound 1
existed stable in the form of keto-hydrazone for one hydroxyl moi-
ety in solid. We further anticipated that compound 1 with the NO2
group in the para position could display tautomerization between
keto- to phenol-hydrazone, stimulated by anionic species in the
⇑
Corresponding author. Tel.: +86 0373 3029128; fax: +86 0373 3029959.
E-mail address: xuefangshang@126.com (X. Shang).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.11.020

X. Shang et al. / Journal of Molecular Structure 1010 (2012) 52–58
53
OH
H
N
OH
O
OH
Br
OH
H
H
OH
R2
C
N
CH
R1
CHO
NH2NH2 H2O
CH3CH2OH
C
N
N
CH
Br
Br
R1
NNH2
NO2
R2
NO2
Br
1
2
Scheme 1. Chemical structure for compounds.
OH
R1
CHO
solution. As a comparison, we also synthesized compound 2
(Scheme 1) containing two bromine groups in the ortho- and para-
positions existing stable in the form of phenol-hydrazone in solid
in order to elucidate the anion recognition mechanism of tauto-
merization between keto- and phenol-hydrazone in the solution.
R2
OH
OH
H
C
N
N
CH
R1
R1
2. Material and methods
2.1. Materials
R2
R2
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 [(n-C4H9)4NF, (n-C4H9)4-
NCl, (n-C4H9)4NBr, (n-C4H9)4NI, (n-C4H9)4NAcO, (n-C4H9)4NH2PO4]
were purchased from Sigma–Aldrich Chemical Co., and stored in a
desiccator under vacuum containing self-indicating silica, and used
without any further purification. Tetra-n-butylammonium salts
were dried for 24 h in vacuum with P2O5 at 333 K. Dimethyl sulf-
oxide (DMSO) was distilled in vacuum after dried with CaH2. The
hydrazine monohydrate is 80%.
1: R1=H; R2=NO2
2: R1=R2=Br
Scheme 2. Synthesis route for compounds.
1H, J = 15.6). Elemental analysis: Calc. for C7H7N3O3Á4H2O: C,
33.20; H, 5.97; N, 16.59; Found: C, 33.32; H, 6.13; N, 16.27.
2.3.2. (3,5-Dibromo-2-hydroxyphenyl)methylene-hydrazone
Hydrazine monohydrate (80%, 0.5 ml) and 3,5-dibromo-salicyl-
aldehyde (278 mg, 1 mmol) were used for the preparation.
m.p. > 320 °C, 228 mg, (78%) yield. 1H NMR (400 MHz, DMSO-d6,
298 K) d = 12.65 (s, 1H), 7.86 (s, 1H), 7.59 (d, 1H, J = 2.4), 7.48 (d,
1H, J = 2.4), 7.34 (s, 2H). Elemental analysis: Calc. for
C7H6Br2N2OÁ2H2O: C, 25.48; H, 3.05; N, 8.49; Found: C, 25.81; H,
3.24; N, 8.36.
0.1 mL DMSO solution of receptor (1 Â 10À3 mol LÀ1
) was
removed to a series of 5 ml colorimetric tube, various volume of tet-
ra-n-butylammonium salts of anions in DMSO (1 Â 10À3 mol LÀ1
)
were added respectively. The mixtures were diluted with DMSO
to the scale, mixed fully and stood for a night. Then, the absorption
spectra were determined at 298 K (DMSO for reference).
2.3.3. 2-[(2-Hydroxy-5-nitrophenyl)methylene]hydrazone
(compound 1)
2.2. Apparatus
C, H, N elemental analyses were made on Vanio-EL. 1H NMR
spectra were recorded on a Varian UNITY Plus-400 MHz Spectrom-
eter. ESI-MS was performed with a MARINER apparatus. UV–vis
titration experiments were made on a Shimadzu UV2550 Spectro-
photometer at 298.2 K. The affinity constant, Ks, was obtained by
non-linear least square calculation method for data fitting.
(2-Hydroxy-5-nitrophenyl)methylene-hydrazone(180 mg,1 mmol)
and 5-nitro-salicylaldehyde (167 mg, 1 mmol) were suspended in
dry EtOH (40 mL). The mixture was refluxing for 12 h and the yellow
precipitate was separated by filtration. The solid was washed with
diethyl ether, recrystalled with EtOH, and dried under vacuum.
m.p.>320 °C, 241 mg, (73%) yield. 1H NMR (400 MHz, DMSO-d6,
298 K) d = 12.21 (s, 1H), 9.08 (s, 2H), 8.69 (d, 2H, J = 3.6), 8.26 (dd,
2H, J = 4.0, 4.0), 7.16 (d, 2H, J = 12). Elemental analysis: Calc. for
2.3. Synthesis
C
14H10N4O6: C, 50.92; H, 3.05; N, 16.96; Found: C, 50.73; H, 3.41;
N, 16.99. ESI-MS (m/z): 329.12 (M-H)À.
(2-hydroxy-5-nitrophenyl)methylene-hydrazone and (3,5-Di-
bromo-2-hydroxyl- phenyl)methylene-hydrazone were obtained
from the reaction of hydrazine monohydrate and 5-nitro-salicyal-
dehyde and 3,5-dibromo-salicyaldehyde in dry EtOH, respectively
[32,35]. Compounds 1 and 2 were synthesized by Schiff base con-
densation between the corresponding hydrazine derivatives and 5-
nitro-salicyaldehyde and 3, 5-dibromo-salicyaldehyde in dry EtOH,
respectively (Scheme 2).
2.3.4. 2-[(3,5-Dibromo-2-hydroxyphenyl)methylene]hydrazone
(compound 2)
Hydrazine monohydrate (80%, 0.5 ml) and 3,5-dibromo-salicyl-
aldehyde (278 mg, 1 mmol) were used for the preparation,
m.p.>320 °C, 337 mg, (61%) yield. 1H NMR (400 MHz, DMSO-d6,
298 K) d = 11.98 (s, 1H), 9.08 (s, 1H), 7.99 (s, 1H), 7.89 (s, 1H). Ele-
mental analysis: Calc. for C14H8Br4N2O2: C, 30.25; H, 1.45; N, 5.04;
Found: C, 30.47; H, 1.98; N, 5.29. ESI-MS (m/z): 550.48 (M-H)À,
552.63 (M-H)À.
2.3.1. (2-Hydroxyl-5-nitrophenyl)methylene-hydrazone
A solution of hydrazine monohydrate (80%, 0.5 ml) in dry EtOH
(30 mL) was added dropwise to a solution of 5-nitro-salicylalde-
hyde (167 mg, 1 mmol) in dry EtOH (15 mL) under stirring. The
mixture was heated under refluxing for 8 h and the yellow precip-
itate was separated by filtration. The solid was washed with
diethyl ether and dried under vacuum. m.p.>320 °C, 152 mg,
(84%) yield. 1H NMR (400 MHz, DMSO-d6, 298 K) d = 12.47 (s,
1H), 8.26 (d, 1H, J = 2.8), 7.78 (d, 2H, J = 8.4), 7.27 (s, 2H), 6.98 (t,
3. Results and discussion
3.1. UV–vis spectral titrations and colorimetric signaling
The anion binding ability of receptors 1 and 2 with halide an-
ions (FÀ, ClÀ, BrÀ and IÀ), AcOÀ and H2POÀ4 in dry DMSO was inves-

54
X. Shang et al. / Journal of Molecular Structure 1010 (2012) 52–58
Fig. 1. UV–vis spectral changes of 1 (2.0 Â 10À5 mol LÀ1) upon the addition of anion (a) FÀ, (b) AcOÀ, (c) H2POÀ4 , [anion] = 0–160 Â 10À5 mol LÀ1. Arrows indicate the direction
of increasing anion concentration in DMSO.
Fig. 2. Color changes inducted by the addition of anions (tetrabutylammonioum salts). From left to right: (A) compound 1, 1 + FÀ, 1 + ClÀ, 1 + BrÀ, 1 + IÀ, 1 + AcOÀ, 1 + H2PO4À;
(B) compound 2, 2 + FÀ, 2 + ClÀ, 2 + BrÀ, 2 + IÀ, 2 + AcOÀ, 2 + H2POÀ4 . The concentration of anion is 5 equiv. of receptors 1 and 2. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
tigated using UV–vis titration experiments. Solutions of
results can be rationalized on the basis of the anion-induced
tautomeric equilibrium between keto- and phenol-hydrazone in
1 Â 10À2 mol LÀ1 anions were added as tetrabutylammonium salts
to 1 Â 10À3 mol LÀ1 solutions of the receptors
1
and
2
at
DMSO [30,31]. Prior to bonding with H2POÀ, the phenol isomer
4
298.2 0.1 K.
of compound 1 dominates in the solution, to which can be attrib-
uted the strong absorption centered at 470 nm. After receptor 1
Fig. 1 showed the significant UV–vis spectral changes of com-
pound 1 upon addition of various anions. From the UV–vis absorp-
interacted with H2POÀ, the H-bonding was formed between recep-
4
tion changes of
1
(Fig. 1), compound
1
displayed obvious
tor 1 and H2POÀ ion in the form of phenol-hydrazone (Scheme 3).
4
absorption band at 470 nm with a small peak at 405 nm which
was assigned to the charge transfer of hydroxyl moiety. The addi-
tion of FÀ, AcOÀ and H2POÀ4 to DMSO solution of 1 led to the similar
spectral changes. After the anions were added gradually, the inten-
sity of absorption peaks at 405 nm and 470 nm increased and the
This will result in the red-shift phenomenon of absorption and vi-
sual color change. However, the presence of protic solvent such as
H2O, which will compete with anions for binding sites and disturb
the H-bond interactions between the host and the anionic guest,
will lead to a reversal of the visual color and the spectral changes.
In the case of weak basic ions such as ClÀ, BrÀ and IÀ, the spectral
changes were too small to calculate the corresponding affinity
constants.
absorption bands shifted to the blue (
Dk = 5 nm, from 405 nm to
400 nm) and red ( k = 12 nm, from 470 nm to 482 nm), respec-
D
tively. At the same time, the absorption peaks became sharper
which was related to the decrease of the molecular conjugacy.
The observed color changed from yellow to orange (Fig. 2). The
In order to elucidate the recognition mechanism of tautomer-
ization between phenol- and keto-hydrazone, we also synthesized

X. Shang et al. / Journal of Molecular Structure 1010 (2012) 52–58
55
a similar compound 2. Fig. 3 showed the remarkable UV–vis spec-
tral changes of compound 2 upon addition of FÀ, AcOÀ and H2PO4À.
Obviously seen from Fig. 3, compound 2 was characterized by a
broad strong absorption band centered at 375 nm and a weak
absorption band centered at 500 nm which were assigned to the
charge transfer of hydroxyl moiety. As the concentration of anion
increased stepwise, the absorption intensity at 375 nm gradually
decreased and disappeared thoroughly. At the same time, the
absorption intensity at 500 nm increased and the significant red-
NO2
NO2
O
H
O
H
HC
HC
H
O-
P
N
N
-
N
N
H2PO4
H2O
HO
O
HO
H
O
CH
CH
O
shift (Dk = 125 nm) was occurred which demonstrated the H-bond
interactions between 2 and the anionic guests affected the elec-
tronic properties of the chromophore, resulting in a new charge
transfer interaction between the electron rich –OH moiety bond
anion and the electron deficient –Br moiety along with a color
change [11]. One clear isosbestic point appeared at 410 nm, which
indicating the stable complex having a certain stoichiometric ratio
between 2 and the anion formed [36]. In contrast, the addition of
other anions (ClÀ, BrÀ, IÀ) did not trigger noticeable spectral change
of compound 2, indicating very weak or no interaction between
these anions and compound 2. The color of compound 2 deepened
NO2
NO2
phenol-hydrazone
keto-hydrazone
Br
Br
Br
O
Br
O
H
CH
CH
H
-
H2PO4
O-
after the interaction with FÀ, AcOÀ and H2POÀ, which was attrib-
N
HO
HO
N
4
P
N
uted to the clear increase of absorption intensity at 500 nm
(Fig. 2). Nevertheless, compound 2 was insensitive to the addition
of excess equiv. ClÀ, BrÀ and IÀ ions. So, a color change was not
observed.
N
H2O
O
HC
HC
H
O
H
O
Br
Br
Br
phenol-hydrazone
Br
phenol-hydrazone
3.2. Affinity constant
The job-plot analysis indicated that the spectral change could
be ascribed to the formation of 1:1 host–guest complexation. The
Scheme 3. Proposed binding mode of compound 1 and 2 with H2POÀ
.
4
Fig. 3. UV–vis spectral changes of compound 2 (2.0 Â 10À5 mol LÀ1) upon the addition of anion (a) H2POÀ4 , (b) AcOÀ, (c) FÀ, [anion] = 0–160 Â 10À5 mol LÀ1. Arrows indicate
the direction of increasing anion concentration.

56
X. Shang et al. / Journal of Molecular Structure 1010 (2012) 52–58
Table 1
Table 2
Affinity constants of receptors with various anions.
The total energy (ET) and energy gap DE(ELUMO–EHOMO) of 1 and its tautomer obtained
at B3LYP/3-21G level.
Anionsa
Ks(1)
Ks(2)
Compound
1
tautomer of 1
H2POÀ4
AcOÀ
FÀ
(7.05 0.09) Â 104
(3.25 0.08) Â 105
ET (a.u.a)
À1202.86317
À1202.89374
À80.26
(6.67 0.11) Â 104
(2.77 0.07) Â 105
Erelative (kJ molÀ1
ELUMO (a.u.)
EHOMO (a.u.)
)
0
(4.70 0.44) Â 104
(5.55 0.15) Â 104
ClÀ
NDb
ND
ND
ND
ND
ND
À0.11468
À0.24987
354.94
À0.11650
À0.23425
309.15
BrÀ
E (kJ molÀ1
)
IÀ
D
a
1 a.u. = 2625.5 kJ molÀ1
.
a
All anions were added in the form of tetra-n-butylammonium (TBA) salts.
The affinity constant could not be determined.
b
tween the host and the anionic guests. However, multiple
hydrogen-bond interactions are also necessary in high-affinity an-
ion binding sites [40]. As expected from their basicity, H2POÀ4 , AcOÀ
and FÀ will bind more strongly than the other anÀions studied; in
addition, the tetrahedral configuration of H2PO4 ion may well
match 1 and 2 in terms of shape and could form multiple hydrogen
bonding interactions with receptors (Scheme 3). Consequently,
H2POÀ ion can be selectively recognized from other anions based
4
on its affinity constant. The electron-withdrawing ability of –NO2
is stronger than that of –Br and then the anion binding ability of
the former is higher than that of the latter in theory [41]. In fact,
the binding ability of receptor 1 containing –NO2 is weaker than
that of receptor 2 containing –Br. The explanation is likely that 2
included two -Br and intramolecular hydrogen bonds, which may
improve the anion binding ability.
3.3. 1H NMR titrations
To further elucidate the nature of the intermolecular interac-
tions between anions and receptors, the binding ability of recep-
tors 1 and 2 toward selected anionic guests was investigated by
standard 1H NMR titrations, in which the host concentration was
kept constant while the guest concentration was varied (Fig. 4).
Two effects would be expected to result from the formation of
hydrogen bond between the binding sites and the anion: (1)
through-bond effects, which increase the electron density of the
benzene ring and promote upfield shifts in 1H NMR spectrum,
and (2) through-space effects, which polarize C–H bond in proxim-
ity to hydrogen bond, create the partial positive charge on the pro-
ton and cause downfield shifts [42]. Obviously, for receptor 1, the
signal at 12.21 ppm, which was assigned to -OH, disappeared upon
the addition of 0.5 equiv. of H2POÀ ion. In addition, the signals of
4
Ha (9.08 ppm), Hb (8.69 ppm), Hc (8.26 ppm) and Hd (7.16 ppm)
clearly shifted upfield with the addition of H2POÀ ion, indicative
4
of the increase in the electron density of the phenyl ring owing
to the through-bond effects. Thus, the results of 1H NMR titration
and spectrum titration implicated that the tautomeric equilibrium
occurred during the anion recognition process. As mentioned
above, the proposed anion recognition process in the solution is
shown in Scheme 3. In the case of receptor 2, the signal at
11.96 ppm, which was assigned to –OH, disappeared upon the
Fig. 4. Changes in the 1H NMR spectra with gradual addition of anion in DMSO-d6.
obtained affinity constants were listed in Table 1 using the method
of non-linear least squares calculation according to the UV–vis data
[37–39]. The selectivity trend of binding ability of receptors to an-
ions followed the order of: H2POÀ > AcOÀ > FÀ ꢀ ClÀ ꢁ BrÀ ꢁ IÀ. It
4
addition of 0.5 equiv. of H2POÀ ion. In addition, the signals of Ha
is apparent that the selectivity for specific anions can be rational-
ized on the basis of the anion’s basicity and the interactions be-
4
(9.04 ppm) and the phenyl protons Hc (7.95 ppm) and Hb
Fig. 5. Optimized geometry of compound 1, and the tautomer of 1.

X. Shang et al. / Journal of Molecular Structure 1010 (2012) 52–58
57
Fig. 6. Molecular orbital level of compound 1 and 2, (a) HOMO; (b) LUMO.
(7.84 ppm) slowly shifted to the upfield when the anion concentra-
tion was less than 1 equiv. With the further addition of anion, the
signals of Ha, Hb and Hc exhibited significant upfield shift. The
as a red shift in the UV–vis spectrum. In particular, a tautomeric
equilibrium occurred during the anion recognition process. The dif-
ferent electronic properties of the tautomer are responsible for the
observed color and spectral changes. While, compound 2 contain-
above results observed likely indicated the H2POÀ ion exhibited a
4
hydrogen-bonding interaction with 2. Thus, the results of 1H
ing two –Br groups at the ortho and para positions can bind H2POÀ
4
NMR titration also corroborated the above supposition of the inter-
ion in the form of phenol and the anion binding ability is stronger
than compound 1, exiting in the form of tautomerization, phenol-
hydrazone, in recognition process.
actions between the host and H2POÀ ion. The possible binding
4
mode in the solution was shown in Scheme 3.
Acknowledgment
4. Theory
This work was supported by the Nature Science Fund of Henan
Province (2010B150024, 112300410104), Young Teacher Fund of
Henan Province and Doctorial Starting Fund of Xinxiang Medical
University.
The geometries of compound 1 and its tautomer were optimized
(Fig. 5) using Density functional theory at B3LYP/3-21G level with
Gaussian03 program [43]. The calculation results on total energy
and the energy gap of ELUMO, EHOMO of 1 and its tautomer were listed
in Table 2. From Table 2, one can see that (1) the total energy of com-
pound 1 is higher than that of its tautomer by 80.26 kJ molÀ1; (2) the
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