Aroylhydrazone derivative as fluorescent sensor for highly selective recognition of Zn2+ ions: Syntheses, characterization, crystal structures and spectroscopic properties

Article abstract of DOI:10.1039/c0dt01590c

A new Zn²⁺ fluorescent chemosensor N′-(3,5-di-tert- butylsalicylidene)-2-hydroxybenzoylhydrazine (H₃L¹) and its complexes [Zn(HL¹)C₂H₅OH]_∞ (1) and [Cu(HL¹)(H₂

Full text of DOI:10.1039/c0dt01590c

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PAPER
Aroylhydrazone derivative as fluorescent sensor for highly selective recognition
of Zn2+ ions: syntheses, characterization, crystal structures and spectroscopic
properties†
Xiaohong Peng,a Xiaoliang Tang,a Wenwu Qin,a Wei Dou,a Yanling Guo,a Jiangrong Zheng,a Weisheng Liu*a
and Daqi Wangb
Received 16th November 2010, Accepted 10th March 2011
DOI: 10.1039/c0dt01590c
A new Zn2+ fluorescent chemosensor N¢-(3,5-di-tert-butylsalicylidene)-2-hydroxybenzoylhydrazine
(H3L1) and its complexes [Zn(HL1)C2H5OH]• (1) and [Cu(HL1)(H2O)]CH3OH (2) have been
synthesized and characterized in terms of their crystal structures, absorption and emission spectra.
H3L1 displays high selectivity for Zn2+ over Na+, K+, Mg2+, Ca2+ and other transition metal ions in
Tris–HCl buffer solution (pH = 7.13, EtOH–H2O = 8 : 2 v/v). To obtain insight into the relation
between the structure and selectivity, a similar ligand 3,5-di-tert-butylsalicylidene benzoylhydrazine
(H2L2), which lacks the hydroxyl group substituent in salicyloyl hydrazide compared with H3L1, and its
complex [Zn2(HL2)2(CH3COO)2(C2H5OH)] (3), [Co(L2)2][Co(DMF)4(C2H5OH)(H2O)] (4),
[Fe(HL2)2]Cl·2CH3OH (5), have also been investigated as a reference. H3L1 exhibits improved selectivity
for Zn2+ compared to H2L2. The findings indicate that the hydroxyl group substituent exerts an effect on
the spectroscopic properties, complex structures and selectivity of the fluorescent sensor.
the nitrogen exhibits a strong affinity for metal ions, which have
Introduction
been widely used in recognition.8–12 Schiff base fluorescent sensors
Zn2+ ion is an essential nutrient required for normal growth and
attract special attention owing to their easy synthesis, variable
development1 and for key cellular processes such as DNA repair2
structures and cheap raw materials. So the design and synthesis
and apoptosis.3 Zn2+ deficiency causes unbalanced metabolism,
which in turn can induce retarded growth in children, brain dis-
orders, and high blood cholesterol, etc.4 In this respect, of crucial
importance is the design and development of efficient complexing
agents that may be used as selective extractants of Zn2+ ion and
sensors for its detection in solution. There are two challenges in
making an effective fluorescent probe for Zn2+ detection.5 One is
to create a probe that responds selectively to Zn2+ over other metal
ions such as Ca2+, Mg2+ and Cd2+, which compete with Zn2+ for
the binding sites. The other one is the selectivity of such probes
in neutral aqueous solution. In this sense, numerous zinc sensors
based on quinoline, fluorescein, coumarin, peptide, and proteins
have been designed and reported.6,7 Moreover, Schiff bases play
a very important role in chemistry as chemosensors being that
of simple Schiff-bases as Zn2+ sensors for detection in neutral
aqueous solution have still attracted considerable attention.
To develop a simple, facile Zn2+ fluorescent chemosen-
sor, N¢-(3,5-di-tert-butylsalicylidene)-2-hydroxybenzoylhydrazine
(H3L1) (Scheme 1) was synthesized and characterized. The
absorption and fluorescence properties of H3L1 in Tris–
HCl buffer solution (pH = 7.13, EtOH–H2O = 8 : 2 v/v)
were investigated. The complexes [Zn(HL1)C2H5OH]• (1) and
[Cu(HL1)(H2O)]CH3OH (2) were characterized by elemen-
tal analysis and single-crystal X-ray structural determination.
Moreover, 3,5-di-tert-butylsalicylidene benzoylhydrazine (H2L2)
(Scheme 1) and its complex [Zn2(HL2)2(CH3COO)2(C2H5OH)]
(3), [Co(L2)2][Co(DMF)4(C2H5OH)(H2O)] (4), [Fe(HL2)2]Cl·
2CH3OH (5) were also investigated as a reference. The selectivity
aKey Laboratory of Nonferrous Metal Chemistry and Resources Utilization
of Gansu Province and State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou University,
Lanzhou, 730000, P. R. China. E-mail: liuws@lzu.edu.cn; Fax: +86-931-
8912582; Tel: +86-931-8915151
bDepartment of Chemistry, Liaocheng University, Liaocheng, 252000, China
† Electronic supplementary information (ESI) available: Crystallographic
details in CIF format for 1–5, tables of interatomic distances and angles
for 1–5, additional spectra for H3L1 and H2L2, and 1H and 13C NMR data.
CCDC reference numbers 727397, 727400, 727396, 727399, and 727401.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0dt01590c
Scheme 1 Schematic illustration of preparation of ligands H3L1 and
H2L2.
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of H3L1 and H2L2 to Zn2+ ion, the effects of the hydroxyl group
substituent in salicyloyl hydrazide on fluorescent intensity both
in solution and solid-state, and the complex structures were
investigated in detail.
Synthesis of [Zn(HL1)C2H5OH]• (1). An ethanol solution of
Zn(CH3COO)2·2H2O (0.0109 g, 0.05 mmol) was added slowly
to a magnetically stirred 5 mL ethanol solution of the ligand
(H3L1) (0.0184 g, 0.05 mmol). The mixture was stirred in air for
2 h whereby a yellow solution was formed. It was filtered and
kept in air. Yellow block single crystals of 1 suitable for X-ray
crystallography were obtained on slow evaporation of the filtrate
at ambient temperature within 5 days (yield: 0.0187 g, 63.8%).
Anal. Calcd for C52H76N4O10Zn2: C, 59.07; H, 6.076; N, 5.527.
Found: C, 59.66; H, 7.27; N, 5.35. FT-IR (KBr phase) (cm-1):
3423 w, 3199 w, 2956 s,1611 vs, 1561 vs, 1388 m, 1254 w, 1167 m,
691 m, 474 w.
Experimental
The commercially available chemicals were used without further
purification. All of the solvents used were of analytical reagent
grade. C, N and H were determined using an Elementar Vario
EL. Melting points were determined on a Kofler apparatus. IR
spectra were recorded on Nicolet FT-170SX instrument using KBr
discs in the 400–4000 cm-1 region. 1H NMR and 13C NMR spectra
were measured on a Bruker DRX 400 spectrometer in DMSO-d6
or CD3OD solution with TMS as internal standard. Chemical shift
multiplicities are reported as s = singlet, t = triplet, q = quartet and
m = multiplet. Mass spectra were recorded on a Bruker Daltonics
esquire6000 Mass spectrometer. UV absorption spectra were
recorded on a Varian Cary 100 spectrophotometer using quartz
cells of 1.0 cm path length. Fluorescence measurements were made
on a Hitachi F-4500 spectrophotometer and a shimadzu RF-
540 spectrofluorophotometer equipped with quartz cuvettes of
1 cm path length with a xenon lamp as the excitation source.
An excitation and emission slit of 2.5 nm were used for the
measurements in the solution state.
Synthesis of [Cu(HL1)(H2O)]·CH3OH (2). An methanol solu-
tion Cu(CH3COO)2·H2O (0.0100 g, 0.05 mmol) was added slowly
to a magnetically stirred 5 mL methanol solution of the ligand
(H2L2) (0.0176 g, 0.05 mmol). The mixture was stirred in air for
2 h whereby a dark green solution was formed. It was filtered
and kept in air. Dark green single crystals of 2 suitable for X-ray
crystallography were obtained on slow evaporation of the filtrate
at ambient temperature within 5 days. (yield: 0.0130 g, 47.1%).
Anal. Calcd for C23H32CuN2O5: C, 57.50; H, 6.67; N, 5.83. Found:
C, 57.61; H, 6.30; N, 5.48. FT-IR (KBr phase) (cm-1): 3370 w, 3212
w, 2955 vs, 1614 vs, 1524 vs, 1490 s, 1360 s, 1252 s, 1169 s, 752 m,
696 w, 494 w.
Synthesis of [Zn2(HL2)2(CH3COO)2(C2H5OH)] (3). An
ethanol solution of Zn(CH3COO)2·2H2O (0.0109 g, 0.05 mmol)
was added slowly to a magnetically stirred 5 mL ethanol solution
of the ligand (H2L2) (0.0176 g, 0.05 mmol). The mixture was stirred
in air for 2 h whereby a yellow solution was formed. It was filtered
and kept in air. Yellow block single crystals of 3 suitable for X-ray
crystallography were obtained on slow evaporation of the filtrate at
ambient temperature within 5 days (yield: 0.0158 g, 55.4%). Anal.
Calcd for C50H66N4O9Zn2: C, 60.11; H, 6.34; N, 5.88. Found: C,
60.24; H, 6.63; N, 5.62. FT-IR (KBr phase) (cm-1): 3424 w, 2958 s,
2869 w,1592 s, 1521 vs, 1434 s, 1386 s, 1248 m, 1165 m, 712 m,
519 w.
Preparation of ligands and their complexes
N¢-(3,5-Di-tert-butylsalicylidene)-2-hydroxybenzoyl hydrazine
(H3L1). To
a
stirred solution of 3,5-di-tert-butyl-2-
hydroxybenzaldehyde (1.17 g, 5 mmol) in ethanol was added
salicyloyl hydrazide (0.76 g, 5 mmol). The reaction mixture was
refluxed for 8 h and then cooled to room temperature. Yellow
precipitates formed were filtered and washed with ethanol and
dried under vacuum. Yield: (1.472 g) 80%. Mp: 230–232 ◦C. ESI-
MS: m/z 369.1 (M + H+). FT-IR (KBr phase) (cm-1): 3437 w, 3238
w, 2956 vs, 1641 vs, 1609 vs, 1556 m, 1489 w, 1360 s, 1306 s, 1233 s,
1
1174 m, 752 m, 669 w, 531 w. H NMR (400 MHz, CD3OD,
Synthesis of [Co2(L2)2(DMF)4(C2H5OH)(H2O)] (4). An
ethanol solution of Co(CH3COO)2·4H2O (0.0125 g, 0.05 mmol)
was added slowly to a magnetically stirred 5 mL ethanol solution
of the ligand (H2L2) (0.0176 g, 0.05 mmol). The mixture was
stirred in air for 30 min whereby a brown precipitate was formed.
Then adding DMF till the precipitate was completely dissolved.
It was filtered and kept in air. Black block single crystals suitable
for X-ray crystallography were obtained on slow evaporation of
the filtrate at ambient temperature within 7 days (yield: 0.0155 g,
51.5%). Anal. Calcd for C102H140Co3N12O14: C, 63.59; H, 7.12;
N, 8.54. Found: C, 63.26; H, 7.24; N, 8.68. FT-IR (KBr phase)
(cm-1): 3422 m, 2927 w, 2361 vs, 2338 s, 1655 m, 1544 w, 1386 m,
1114 m, 669 m, 486 w.
ppm): d 1.24 (9H, s, -CH3), 1.38 (9H, s, -CH3), 6.88–6.90 (2H, m,
Ar–H), 7.10 (1H, s, Ar–H), 7.33–7.35 (2H, m, Ar–H), 7.8 (1H, s,
Ar–H), 8.40 (1H, s, CH N). 13C NMR (400 MHz, CD3OD,
ppm): d 29.99, 31.90, 35.05, 36.04, 116.35, 118.33, 118.45, 120.51,
127.15, 127.71, 129.57, 135.37, 137.68, 142.22, 154.04, 156.61,
160.80, 166.55.
3,5-Di-tert-butylsalicylidene benzoylhydrazine (H2L2). To
a stirred solution of 3,5-di-tert-butyl-2-hydroxybenzaldehyde
(1.17 g, 5 mmol) in ethanol was added benzhydrazide (0.68 g,
5 mmol). The reaction mixture was refluxed for 8 h and then
cooled to room temperature. White precipitates formed were
filtered and washed with ethanol and dried under vacuum. Yield:
(1.30 g) 73.9%. Mp: 258.8–259.4 ◦C. ESI-MS: m/z 353.2 (M +
H+). FT-IR (KBr phase) (cm-1): 3187 w, 3030 m, 2956 vs, 1645 vs,
1645 vs, 1611 s, 1566 s, 1435 s, 1361 s, 1287 s, 1252 m, 1088 m, 958
w, 890 w, 692 s, 533 w. 1H NMR (400 MHz, DMSO-d6, ppm): d
1.28 (9H, s, -CH3),1.40 (9H, s, -CH3), 7.20–7.94 (7H, m, Ar–H),
8.57 (1H, s, CH N). 13C NMR (400 MHz, DMSO-d6, ppm): d
29.26, 31.26, 33.85, 34.62, 116.94, 125.55, 125.71, 127.61, 128.57,
132.04, 132.61, 135.63, 140.39, 151.27, 154.69, 162.77.
Synthesis of [Fe(HL2)2]Cl·2CH3OH (5). A methanol solution
of FeCl3·6H2O (0.0135 g, 0.05 mmol) was added slowly to a
magnetically stirred 5 mL methanol solution of the ligand (H2L2)
(0.0176 g, 0.05 mmol). The mixture was stirred in air for 30 min
whereby a black solution was formed. It was filtered and kept in air.
Black block single crystals of 5 suitable for X-ray crystallography
were obtained on slow evaporation of the filtrate at ambient
temperature within 7 days (yield: 0.0154 g, 49.5%). Anal. Calcd
5272 | Dalton Trans., 2011, 40, 5271–5277
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Table 1 Crystal data for 1–5
Compound reference
1
2
3
4
5
Chemical formula
Formula Mass
Crystal system
C52H76N4O10Zn2
1047.9
C23H32CuN2O5
480.05
Triclinic
C50H66N4O9Zn2
997.81
Monoclinic
16.739(2)
10.4668(11)
29.254(3)
90.00
99.362(2)
90.00
5057.1(10)
298(2)
P21/c
4
25503
8913
0.0517
0.0458
0.1024
0.0844
C102H140Co3N12O14
1935.05
Tetragonal
16.175(2)
16.175(2)
41.560(3)
90.00
90.00
90.00
10873(2)
298(2)
p43212
4
54507
9576
0.1518
0.0785
0.1913
0.1197
0.02 (3)
0.2132
C46H62ClFeN4O6
858.30
Triclinic
11.801(11)
13.610(12)
14.805(13)
88.618(13)
86.079(12)
88.823(12)
2371(4)
Triclinic
7.7586(6)
11.1224(15)
17.327(2)
96.667(2)
102.141(3)
95.722(2)
1440.0(3)
293(2)
P1
1
7433
6034
0.0438
0.0762
0.1732
0.1339
0.01 (3)
0.2072
˚
a/A
6.8520(8)
9.5110(10)
19.477(2)
99.223(2)
93.968(2)
105.547(3)
1198.6(2)
298(2)
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
Unit cell volume/A
T/K
Space group
No. of formula units per unit cell, Z
No. of reflections measured
No. of independent reflections
Rint
298(2)
¯
¯
P1
P1
2
2
6246
4134
0.0254
0.0596
0.1450
0.0827
12409
8212
0.0339
0.0483
0.0914
0.0959
Final R1 values (I > 2s(I))
Final wR(F2) values (I > 2s(I))
Final R1 values (all data)
Flack parameters
Final wR(F2) values (all data)
0.1606
0.1214
0.1035
˚
for C46H62ClFeN4O6: C, 64.37; H, 7.23; N, 6.53. Found: C, 64.26;
H, 7.24; N, 6.68. FT-IR (KBr phase) (cm-1): 3431 s, 2957 s, 1606
vs, 1546 vs, 1414 w, 1384 m, 1250 w, 1177 m, 706 m, 679 w, 543 m,
467 w (w, weak; m, medium; s, strong; vs, very strong).
Zn2 is similar to Zn1, while Zn atom is placed at 0.490 A (Zn2)
from the basal planes. The bond lengths of Zn–phenoxo (Zn1–
˚
˚
˚
O2 = 1.924(9) A; Zn1–O6 = 1.965(8) A; Zn2–O5 = 1.945(9) A;
Zn2–O3 = 1.955(9) A) are much shorter than that of Zn–N (Zn1–
N4 = 2.054(11) A; Zn2–N2 = 2.067(12) A), which shows that the
strength of Zn–O bonds are stronger than Zn–N bonds, while
Zn–O carbonyl oxygen bonds are weaker than Zn–N bonds (Zn1–
O4 = 2.197(10) A; Zn2–O1 = 2.249(11) A). The aroylhydrazone
remained in the keto form. Each of these chains is linked by
extensive O–H ◊ ◊ ◊ O hydrogen bonds involving the H atoms in L1
and O atoms in ethanol molecule into a 3D coordination network.
(Fig. 1c) Two 1D chains are further connected to a 1D double chain
coordination supramolecule by hydrogen bonds. The zigzag chains
are aligned one above the other and are held together in a three-
dimensional hydrogen-bonded network by interactions between
the ethanol.
˚
˚
˚
X-ray crystallography
The X-ray diffraction data were collected on a Bruker SMART
1000 CCD diffractometer operating at 50 KV and 30 mA
using graphite-monochromated Mo-Ka radiation source (l =
˚
˚
˚
0.71073 A). An empirical absorption correction based on the
comparison of redundant and equivalent reflections was applied
using SADABS. A summary of crystallographic data and details
of the structure refinements are listed in Table 1. The structure
was solved by direct methods and refined by full matrix least-
squares techniques on F2 with all non-hydrogen atoms treated
anisotropically. All calculations were performed with the program
package SHELXTL. The hydrogen atoms were assigned with
common isotropic displacement factors and included in the final
refinement by using geometrical restraints. The structural data
have been deposited with the Cambridge Crystallographic Data
Centre (CCDC) with reference numbers 727397, 727400, 727396,
727399, and 727401 for compounds 1–5, respectively.
Crystal structure of [Cu(HL1)(H2O)]·CH3OH (2). As shown
in Fig. 2, the phenoxo are not coordinated with copper. Copper
is four coordinate using a tridentate Schiff base ligand, and a
water molecule. The copper center adopts distorted square planar
coordination geometry.
Crystal structure of [Zn2(HL2)2(CH3COO)2(C2H5OH)] (3). As
shown in Fig. 3, a unit contains a Zn dinuclear cation, one
acetic anion, and an ethanol solvent molecule, which is obviously
different from complex 1. The geometry of each Zn2+ ion is best
described as square-pyramidal with varying amount of trigonal
bipyramidal distortion across the mononuclear units, where one
of the Zn2+ (Zn1) is coordinated by O2N of the H2L2 unit, a oxygen
atom of the acetate; while the other Zn2+ (Zn2) is coordinated by
the H2L2 unit and ethanol. These two Zn2+ centers are bridged by
the two oxygen atoms of an acetic ion, with the bond distances:
Results and discussion
X-ray diffraction studies
Crystal structure of [Zn(HL1)C2H5OH]• (1). As shown in
Fig. 1b, the crystal is a 1-D zigzag coordination polymer. The
asymmetric unit contains two crystallographically independent
Zn2+ ions (Zn1 and Zn2). Each Zn2+ center is coordinated to a
chelating H3L1 ligand, an ethanol molecule and the O donor of
a neighboring [Zn(HL1)C2H5OH] unit. The geometry around the
Zn2+ center (Zn1) can be described as a distorted square pyramid
where the oxygen atom of the ethanol molecule (O7) occupies the
apical position, the atoms O2, O6, N4, O4 define the basal plane,
˚
˚
Zn2–O4 = 1.980(3) A, Zn1–O3 = 2.020(2) A; and Zn ◊ ◊ ◊ Zn
˚
separation about 6.0406 A. There are two acetic anions in the
crystal structure. One acts as a bridge while the other one is only
coordinated with Zn2+.
˚
and the metal atom is placed at 0.512 A (Zn1) from the basal plane.
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Fig. 2 Structure of [Cu(HL1)(H2O)]·CH3OH showing the coordination
sphere of Cu2+ in complex 2 along with atom labeling schemes and 30%
thermal ellipsoids. Hydrogen atoms and crystal lattice solvent molecules
are omitted for clarity.
Fig. 3 Structure of [Zn2(HL2)2(CH3COO)2(C2H5OH)] showing the coor-
dination sphere of Zn2+ in complex 3 along with atom labeling schemes
and 30% thermal ellipsoids. Hydrogen atoms and crystal lattice solvent
molecules are omitted for clarity.
Co1 is best described as distorted octahedral. Co2+ center is six
coordinate using two tridentate Schiff base ligands.
Fig. 1 (a) Molecular structure of [Zn2(HL1)2C2H5OH]• (1) showing the
local coordination geometry along with atom labeling schemes and 30%
thermal ellipsoids. (b) Portion of the infinite zigzag chain. Hydrogen atoms
and crystal lattice solvent molecules are omitted for clarity. (c) 3D stacking
structure of complex 1. Hydrogen bonds are shown as dashed lines.
Crystal structure of [Fe(HL2)2]Cl·2CH3OH (5). The geometry
of the Fe3+ center in compound 5 is best described as a distorted
octahedral, comprising two equivalent tridentate ligands (Fig. 5).
Each of the two equivalent aroylhydrazone ligands coordinates to
the Fe3+ ion with the phenolate oxygen (O3), imine nitrogen (N3),
and keto oxygen (O4) donor atoms to form a pseudooctahedral
geometry.13
Crystal structure of [Co(L2)2][Co(DMF)4(C2H5OH)(H2O)]
(4). The crystal structure of
4
contains two crystal-
lographically independent species: one complex cation
[Co(DMF)4(C2H5OH)(H2O)]2+ and one complex anion
[Co(L2)2]2-. As shown in Fig. 4, Co2 is absolutely coordinated by
solvent molecules such as DMF, water and ethanol in a perfectly
octahedral geometry (being angles: O8–Co2–O7 = 180.00◦,
O5–Co2–O6 = 90.9◦; O5–Co2–O6 = 89.1◦). The geometry at the
Fluorescence spectra and titration
The emission spectrum of the sensor H3L1 (U = 0.00132) excited
at 336 nm exhibits the emission maximum at 505 nm in Tris–
HCl buffer solution (pH = 7.13, EtOH–H2O = 8 : 2 v/v) at
5274 | Dalton Trans., 2011, 40, 5271–5277
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is 0.03236. The experiment reveals that there is a significant
increase in fluorescence intensity for 1, the emission from
[Zn(HL1)C2H5OH]• (1) is believed to arise from an excited-state
centered on H3L1 which contains two phenolic protons, which will
deprotonate upon complexation with Zn2+.
As illustrated in Fig. 6(b), among the metal ions studied, only
Zn2+ significantly changes the fluorescence spectra of H3L1. Other
cations which exist at high concentration in living cells, such
as Na+, K+, Mg2+, and Ca2+ did not change the Zn2+-induced
emission, even when present in large excess. Paramagnetic ions,
such as Cu2+, Cr3+, Fe3+ quench the emission, owing to an electron
or energy transfer between the metal cation and fluorophore
known as the fluorescence quenching mechanism.15 Almost no
influence of the first-row transition-metal ions such as Mn2+,
Co2+, and Ni2+ (100 mM) on the fluorescence intensity of H3L1
was observed. The presence of heavy metal ions including Cd2+,
a d10 ion that often exhibits coordination properties similar to
Zn2+ (50 mM), do not interfere with the emission of the Zn2+–HL1
complex (Fig. S6†). The high selectivity of H3L1 for Zn2+ over other
metal ions should be in part attributed to the strong coordination
Fig. 4 Structure of [Co(L2)2][Co(DMF)4(C2H5OH)(H2O)] (4) showing
the coordination sphere of Co2+ in complex 4 along with atom labeling
schemes and 30% thermal ellipsoids. Hydrogen atoms are omitted for
clarity. Symmetry code: (A) y, x, -z.
Fig. 5 Structure of [Fe(HL2)2]+ showing the coordination sphere of Fe3+
in complex 5 along with atom labeling schemes and 30% thermal ellipsoids.
Hydrogen atoms and crystal lattice solvent molecules are omitted for
clarity.
25 ◦C. We measured the emission intensities of the sensor molecule
in the presence of various concentrations of Zn2+ (0–0.1 mM
concentration).
Job’s plot analysis (Fig. S5†) revealed maximum emission at 1 : 1
ratio (H3L1: Zn2+), which is in good agreement with the absorption
study results and the X-ray structure of the zinc complex. This
result has also been confirmed by ESI-MS, where a peak at m/z
477.3 corresponding to the 1 : 1 complex [HL1·Zn2+·C2H5OH]+
is observed. To observe the binding interaction with Zn2+, the
binding constant value has been determined from the emission in-
tensity data following the modified Benesi–Hildebrand equation:14
1/DF = 1/DFmax + (1/K[C])(1/DFmax). Here DF = Fx - F0 and
DFmax = F• - F0, where F0, Fx, and F• are the emission intensities
of H3L1 considered in the absence of Zn2+, at an intermediate Zn2+
concentration, and at a concentration of complete interaction,
respectively, and where K is the binding constant and [C] the Zn2+
concentration. From the plot of (F•- F0)/(Fx - F0) against [C]-1
for H3L1, the value of K extracted from the slope is 2.9792 ¥
105 M-1.
Fig. 6 (a) Increasing concentrations of Zn2+ (0, 1.0, 2.5, 5.0, 7.5, 10, 15,
20, 25, 30, 35, 40, 45, 50 mM). Inset: fluorescence intensity of H3L1 at
505 nm vs. equiv of Zn2+. (b) Fluorescent emission changes of H3L1 (1.0 ¥
10-5 mol L-1) upon addition of 1, blank; 2, Ag+; 3, Cd2+; 4, Co2+; 5, Cr3+
;
6, Cu2+; 7, Fe3+; 8, Hg2+; 9, Li+; 10, Mg2+; 11, Mn2+; 12, Ni2+; 13, Ca2+; 14,
Zn2+; 15, Zn2+ + Li+; 16, Zn2+ + Na+; 17, Zn2+ + Mg2+; 18, Zn2+ + Ca2+ (10
equiv) in Tris–HCl buffer solution (pH = 7.13, EtOH–H2O = 8 : 2 v/v) at
room temperature. Excitation wavelength was 336 nm.
Upon the addition of 10 equivalents of Zn2+, the fluorescence
intensity of H3L1 increases by 25-fold, and the quantum yield
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ability of Zn2+. To understand the fluorescence enhancement and
quench of H3L1 when binding Zn2+ and Cu2+, respectively, we
can compare the crystal structures of the two complexes. The d10
ion has distinct properties such as Zn2+, Zn2+ coordinated with
aroylhydrazone extends the rigidity of the ligand. Meanwhile, as
shown in Fig. 1 and 2, crystals 1 and 2 could act as references for
the coordination mode of Zn2+ and Cu2+ with H3L1.
Effect of pH
In addition to metal ion selectivity, we measured the fluorescence
intensity of H3L1 at various pH values in the presence and absence
of Zn2+. As can be seen from Fig. 7, the emission intensity of H3L1
decreases under acidic conditions. And essentially no change can
be observed under neutral and alkaline conditions (pH 7–12).
Interestingly, H3L1 showed good fluorescence sensing ability to
Zn2+ over a wide range of pH values. This property arises from
protonation of the phenolic hydroxyl group of the ligand. H3L1
shows no appreciable sensing ability to Zn2+ at a pH below 6.1,
which may be due to the competition of H+ below pH 6.1, but
exhibits satisfactory Zn2+ sensing abilities when the pH is increased
to the range 6.3–10, which is suitable for measurements.
Fig. 8 Fluorescent emission changes of H2L2 (1.0 ¥ 10-5 mol L-1) upon
addition of 1, blank; 2, Ag+; 3, Cd2+; 4, Co2+; 5, Cr3+; 6, Cu2+; 7, Fe3+
;
8, Hg2+; 9, Li+; 10, Mg2+; 11, Mn2+; 12, Ni2+; 13, Zn2+ (5 eq.) in ethanol.
Excitation wavelength was 316 nm.
Co2+ and Fe3+ quenched the fluorescence intensity of the H2L2. To
better understand the interaction of H2L2 and metal ions, we also
obtained the crystal structure of these two metal ions. The crystal
structure and ESI-MS spectrum (m/z 758.5 and 761.5) show that
the metals and H2L2 also form complexes of 1 : 2 stoichiometry.
Water quenched the fluorescence intensity of the Zn2+ –H2L2
system, and the fluorescence intensity of H2L2 had a very small
increase in aqueous ethanol solution upon the addition of 5
equivalents of Zn2+. The selectivity of the H2L2 for Zn2+ ion in
aqueous ethanol solution is too low to be observed.
The H3L1-Zn complex exhibits 20 times higher emission increase
than unsubstituted H2L2-Zn complex, indicating a significant
fluorescence enhancement achieved by the introduction of a
hydroxyl group.
We also studied the fluorescence intensity in solid-state Zn2+–
HL1 and Zn2+–HL2 and found that the former is much higher than
the latter under the same conditions (see supporting information
Fig. S7†). When we compare the Zn2+ complexes of the two ligands,
we can find that the hydroxyl group in salicyloyl hydrazide gets
involved in the coordination environment, which is in accord with
the crystal structure of Zn2+–HL1 and Zn2+–HL2.
Fig. 7 Fluorescence intensities of H3L1 and Zn2+ at various pH values
at room temperature, Tris–HCl buffer solution (pH = 7.13, EtOH–H2O =
8 : 2 v/v), lex = 336 nm; black line, the fluorescence intensities at 565 nm of
Zn2+-HL1 at various pH ([H3L1] = 10 mM [Zn2+] = 0.10 mM); Red line, the
fluorescence intensities at 515 nm of H3L1 at various pH ([H3L1] = 10 mM,
[Zn2+] = 100 mM).
Absorption study
The fluorescence enhancement effects of various metal ions on
H2L2 were investigated under excitation at 316 nm (Fig. 8). In the
absence of metal ions, H2L2 exhibited a very weak fluorescence
peak near 485 nm, When Zn2+ was introduced to a 10 mM
solution of H2L2 in ethanol, obvious red shift (~38 nm) and
enhancement of fluorescence spectra were observed, whereas other
ions of interest displayed much weaker response. Upon addition
of 5 equivalents of Zn2+, ligand H2L2 displayed only 5.27–fold
increase in fluorescence intensity. The fluorescence intensity had
little change when addition of more Zn2+. The quantum yield of
H2L2 is 0.00297. The value changes to 0.01728 after the addition
of Zn2+ ion in the H2L2 solution. The H2L2 and Zn2+ form
complex of 1 : 2 stoichiometry, which could be comfirmed by
ESI-MS spectrum (m/z 767.5) and crystal structure. Moreover,
The absorption spectrum of H3L1 (10 mM) exhibits a maximal
absorption at 294 nm and 349 nm before titration at room
temperature in Tris–HCl buffer solution (pH = 7.13, EtOH–
H2O = 8 : 2 v/v) (Fig. 9). Upon addition of Zn2+ (0–2 equiv), the
absorbance at 294 nm and 349 nm reduced gradually. Meanwhile,
a new absorption peak appeared at 417 nm, which could be
attributed to the interaction between the ligand moiety and zinc.
The new absorption band (417 nm) of Zn–HL1 may arise from two
effects. First, the phenolic proton of the ligand is deprotonated.
The binding of [HL1]2- to Zn2+ can form a six-membered chelate
ring with the Schiff base C N and Ar–O-, which enlarges the
conjugated system, and thus reduces the energy difference between
n and p* orbital.16 Second, in the absence of a metal ion, H3L1 does
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mental Research Funds for the Central Universities (lzujbky-
2009-k06).
Notes and references
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Conclusion
In conclusion, we have succeeded in preparing two simple
and easy-to-make Zn2+ cation chemosensors H3L1 and H2L2.
Spectroscopic properties and crystal structures with respect to
Zn2+ coordination of two aroylhydrazone derivatives have been
reported. Introduction of one hydroxyl group into H2L2, H3L1 not
only displays a strong interaction with Zn2+ but also distinguishes
this metal cation from other metal cations, including the strong
competitors Cd2+, Mg2+ and Ca2+. The increase in emission in
the presence of Zn2+ is accounted for by the formation of metal–
ligand complexes 1 and 2. An approximately 25-fold Zn2+-selective
chelation-enhanced fluorescence response of H3L1 is attributed to
the strong coordination of Zn2+ that would impose rigidity. In
our present work, H3L1 exhibits a more sensitive and selective
binding ability toward the Zn2+ ion than H2L2 in aqueous ethanol
solution, implying that the hydroxyl group substituent structure
is superior to the no substituent group for Zn2+ binding in this
case. The possible reason is the substituent hydroxyl group gives a
better chelating ability for Zn2+ ion. The different substituting
group results in different structures of the complexes. This
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[Zn2(HL2)2(CH3COO)2(C2H5OH)].
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Acknowledgements
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The authors acknowledge the financial support from the NSFC
(Grant Nos. 20771048, 20931003, 21001059) and the Funda-
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 5271–5277 | 5277
©