Synthesis, crystal structure, and properties of the cadmium complex with 2,5-bis((benzoimidazol-2-yl)methylthio)-1,3,4-thiadiazole

Article abstract of DOI:10.1080/00958972.2012.704549

A cadmium complex, [Cd(L)₂(CH₃OH)₂] ·(NO₃)₂ (L=2,5-bis((benzoimidazol-2-yl)methylthio)- 1,3,4-thiadiazole), was synthesized and characterized by elemental analysis, infrared spectra, single-crystal and powder X-ray diffraction, UV-Vis spectra, electrochemical, and fluorescence properties. Single-crystal X-ray structure analysis reveals that the complex crystallizes in the triclinic crystal system with P1 space group. The coordination geometry around cadmium is octahedral. Intermolecular hydrogen bonds result in the generation of a 1-D infinite chain structure. Both UV-Vis spectra and cyclic voltammetry studies show the Cd(II) complex could be used as a probe to distinguish ssDNA from dsDNA. Differential pulse voltammetry indicates the complex could be used to analyze quantitatively for DNA. The solid complex exhibits strong luminescence emission in the visible region at room temperature upon excitation with UV radiation.

Full text of DOI:10.1080/00958972.2012.704549

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Synthesis, crystal structure, and
properties of the cadmium complex
with 2,5-bis((benzoimidazol-2-
yl)methylthio)-1,3,4-thiadiazole
a
a
a
a
Yong-Hong Wen , Yu-Yan Chen , Hui-Ling Wen , Xing-Lei Xie
a
&
a
Lei Wang
Key Laboratory of Eco-chemical Engineering, Ministry of
Education, College of Chemistry and Molecular Engineering,
Qingdao University of Science and Technology, Qingdao 266042,
People's Republic of China
Accepted author version posted online: 19 Jun 2012.Version of
record first published: 04 Jul 2012.
To cite this article: Yong-Hong Wen , Yu-Yan Chen , Hui-Ling Wen , Xing-Lei Xie & Lei Wang (2012):
Synthesis, crystal structure, and properties of the cadmium complex with 2,5-bis((benzoimidazol-2-
yl)methylthio)-1,3,4-thiadiazole, Journal of Coordination Chemistry, 65:16, 2780-2792
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Journal of Coordination Chemistry
Vol. 65, No. 16, 20 August 2012, 2780–2792
Synthesis, crystal structure, and properties of the
cadmium complex with
,5-bis((benzoimidazol-2-yl)methylthio)-1,3,4-thiadiazole
2
YONG-HONG WEN*, YU-YAN CHEN, HUI-LING WEN, XING-LEI XIE
and LEI WANG
Key Laboratory of Eco-chemical Engineering, Ministry of Education, College of Chemistry
and Molecular Engineering, Qingdao University of Science and Technology,
Qingdao 266042, People’s Republic of China
(
Received 5 February 2012; in final form 7 May 2012)
A
cadmium
complex,
[Cd(L)
2
(CH
3
OH)
2
] ꢀ (NO
3
)
2
(L ¼ 2,5-bis((benzoimidazol-2-yl)
methylthio)-1,3,4-thiadiazole), was synthesized and characterized by elemental analysis,
infrared spectra, single-crystal and powder X-ray diffraction, UV-Vis spectra, electrochemical,
and fluorescence properties. Single-crystal X-ray structure analysis reveals that the complex
ꢀ
crystallizes in the triclinic crystal system with P1 space group. The coordination geometry
around cadmium is octahedral. Intermolecular hydrogen bonds result in the generation of a
1-D infinite chain structure. Both UV-Vis spectra and cyclic voltammetry studies show the
Cd(II) complex could be used as a probe to distinguish ssDNA from dsDNA. Differential pulse
voltammetry indicates the complex could be used to analyze quantitatively for DNA. The solid
complex exhibits strong luminescence emission in the visible region at room temperature upon
excitation with UV radiation.
Keywords: Benzimidazole;
Electrochemical property
Cadmium
complex;
Crystal
structure;
Fluorescence;
1. Introduction
Benzimidazole-containing compounds have attracted attention since the first benz-
imidazole compound, 2,5-dimethylbenzimidazole, was synthesized in 1872 [1, 2].
Benzimidazole-containing complexes with transition metals is one of the most
important subjects in coordination chemistry, crystal engineering, medicine and
materials science owing to their structural motifs [3] and characteristic properties
such as catalysis [4], luminescence [5–7], antivirus activity [8, 9], and electrochemical
0
properties [10]. Cu complexes of 1,1 -(1,5-pentanediyl)bis1H-benzimidazole) are
efficient homogeneous catalysts for oxidative coupling of 2,6-dimethylphenol [4]. Co
complex of 2-(2-pyridyl) benzimidazole has been used as an efficient fluorescent probe
for trace determination of aspartic and glutamic acids [7]. Cu and Cd complexes of
*
Corresponding author. Email: yonghwen@163.com; inorchemwl@126.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.704549

Benzoimidazole cadium complex
2781
several bis(benzimidazole) ligands have been examined as potential models of structure
and mobility of biological metal binding sites [3].
1
,3,4-Thiadiazole and its derivatives are used in pharmaceutical [11] and coordination
chemistry [12, 13]. Benzimidazole and 1,3,4-thiadiazole are both inexpensive and have
large planar ꢀ-conjugated systems. However, compounds containing both benzimid-
azole and 1,3,4-thiadiazole have not been investigated. As part of our ongoing work
on the syntheses and properties of benzimidazole-containing complexes [14], a new
thiodiazole-based bis(benzimidazole), 2,5-bis((benzoimidazol-2-yl)methylthio)-1,3,4-
thiadiazole (L), was synthesized and reacted with Cd(NO ) , giving a new coordination
3
2
compound, [Cd(L) (CH OH) ] ꢀ (NO ) , which was characterized by elemental analysis,
2
3
2
3 2
infrared (IR) spectra, single-crystal X-ray diffraction, powder X-ray diffraction,
ultraviolet and visible spectra (UV-Vis), electrochemical, and fluorescence properties.
2
2
. Experimental
.1. Materials and measurements
All commercially available reagents and chemicals were of analytical grade purity and
used without purification. Salmon sperm DNA was purchased from Shanghai Huashun
biological Engineering Company. Deionized water was obtained by passing distilled
water through a Barnstead E-pure 3-Module system. C, H, N, and S contents were
determined using an Elementar Vario EL III analyzer. IR spectra were recorded from
KBr pellets by a Nicolet 510P FT-IR spectrometer. NMR spectra were recorded on a
Bruker Avance 500 MHz spectrometer with TMS as an internal standard. Conductivity
measurement was carried out with
ꢂ3
a
DDS-11A type conductometer for
ꢂ1
ꢃ
1
.0 ꢁ 10 mol L solution in DMF at 25 C. Ultraviolet and visible spectra (UV-Vis)
spectra were measured using a Cary 500 UV-Vis-NIR spectrophotometer. Fluorescence
spectra were measured using a HITACHI F-4500 fluorescence spectrophotometer.
Powder X-ray diffraction (XRD) patterns were recorded (Bragg-Brentano) on a
˚
Siemens D5005 diffractometer using Cu-Ka radiation (ꢁ ¼ 1.5418 A) with a graphite
ꢃ
monochromator. The step size was 0.02 and the count time was 4 s.
2
.2. Synthesis
2
2
1
.2.1. Synthesis of 2,5-bis((benzoimidazol-2-yl)methylthio)-1,3,4-thiadiazole (L).
-Chloromethylbenzoimidazole was synthesized by refluxing a solution of benzene-
ꢂ1
,2-diamine, 2-chloroacetic acid and 5 mol L
hydrochloride for 4 h, followed by
adjusting the pH to 8–9 with ammonia. After stirring the 50 mL acetone solution of
,5-dimercapto-1,3,4-thiodiazole (3.00 g, 20 mmol), K CO (3.05 g, 22 mmol), and
2
2
3
KI (0.5 g) at room temperature for 30 min, a 30 mL solution of 2-chloromethylbenzoi-
midazole (6.67 g, 40 mmol) in acetone was added dropwise, and the mixture was
refluxed for 4 h. After cooling to room temperature, the mixture was washed three times
with water (3 ꢁ 6 mL) and then filtered. The resulting solid was washed three times with
acetone (3 ꢁ 6 mL) to obtain a crude product. Recrystallization of the crude product in
1
EtOH/DMF (1 : 1) afforded L as yellow crystals. Yield: 72.8%. H NMR (500 MHz,
DMSO-d , ꢂ (ppm)): 12.58 (s, 1 H, NH), 7.51–7.53 (m, 2H, ArH), 7.16–7.17
6

2
782
Y.-H. Wen et al.
1
m, 2H, ArH), 4.76 (s, 2H, CH2); C NMR (125 MHz, DMSO-d , ꢂ (ppm)): 165.17 (1C,
3
(
C of thiadiazole ring), 149.88 (1C, C in 2-position of imidazole ring), 139.06 (2C),
6
ꢂ1
1
3
1
22.34 (2C), 115.05 (2C), (C of benzene ring), 31.72 (CH ). IR (KBr pellet, cm ):
2
428s, 3248m, 2920w, 1666m, 1626w, 1531w, 1487w, 1439m, 1396m, 1381m, 1313w,
273m, 1225w, 1151w, 1050m, 854w, 749w, 737m, 621w. Anal. Calcd for C H N S :
1
8
14
6 3
C, 52.66; H, 3.44; N, 20.47; S, 23.43. Found: C, 52.34; H, 3.26; N, 20.15; S, 23.34.
.
2
.2.2. Synthesis of [Cd(L) (CH OH) ] (NO ) . A solution of Cd(NO ) ꢀ 4H O
2
3
2
3 2
3 2
2
(0.0771 g, 0.25 mmol) in 12 mL methanol was added dropwise with stirring to a
solution of L (0.2052 g, 0.5 mmol) in 15 mL methanol. The resulting mixture was stirred
at room temperature for 6 h. The precipitate was filtered, washed with methanol, and
ꢂ1
dried in vacuo over P O for 48 h. Yield: 0.2017 g, 72%. IR data (KBr pellet, cm ):
2
5
3
1
428 sb, 3057w, 2929w, 2854w, 2767w, 1622w, 1527w, 1485w, 1444s, 1431s, 1384s,
313w, 1276m, 1239w, 1152w, 1047m, 1007w, 912w, 862w, 742 s, 708w, 663w, 636w.
Anal. Calcd for C H N O S Cd: C, 40.69; H, 3.24; N, 17.48; S, 17.15. Found: C,
3
8
36 14
8 6
4
0.37; H, 3.09; N, 17.59; S, 17.26. Colorless column crystals suitable for X-ray
diffraction analysis were obtained by slow evaporation of a DMSO-methanol (1 : 2, v/v)
solution of the solid complex for 30 days.
2
.3. X-ray crystallography
Single-crystal X-ray diffraction data were collected by a Bruker SMART 1000 CCD
˚
diffactometer with graphite monochromated Mo-Ka radiation (ꢁ ¼ 0.71073 A) using !
scan mode at 113(2) K. Intensity data were corrected for Lp factors and empirical
absorption. The structure was solved by direct methods and expanded by Fourier
differential techniques with SHELXTL [15]. All non-hydrogen atoms were located with
successive difference Fourier syntheses. Hydrogen atoms were geometrically fixed and
allowed to ride on the parent to which they are attached. The structure was refined by
2
full-matrix least-squares on F with anisotropic thermal parameters for all non-
hydrogen atoms. Atomic scattering factors and anomalous dispersion corrections were
taken from International tables for X-ray crystallography [16]. A summary of the key
crystallographic data and structural refinements for the complex is presented in table 1.
Selected bond distances and angles are illustrated in table 2.
2
.4. Electrochemical studies
All electrochemical experiments were performed with a CHI 832B electrochemical
analyzer (Shanghai CHI Instrument Company, China) using a three-electrode system
composed of a glassy carbon electrode (GCE, ’ ¼ 2.96 mm) as a working electrode,
Ag/AgCl/(saturated)KCl as a reference electrode, and Pt wire as an auxiliary electrode.
ꢂ1
A 0.2 mol L Britton–Robinson buffer solution (BR buffer, the mixture of 3.39 mL
8
5% H PO , 2.95 mL HOAc and 3.0875 g H BO diluted to 250 mL, and adjusted
3 4 3 3
ꢂ1
to pH 4.0 with 0.5 mol L NaOH) was used as buffer solution. The pH was measured
with a PHS-3D pH-meter (Shanghai LeiCi Device Works, Shanghai, China) with
a combined glass-calomel electrode. Cyclic voltammetry measurements were made on
a glassy carbon electrode in BR buffer (pH 5.0) at room temperature with scan rates

Benzoimidazole cadium complex
2783
Table 1. Crystal data and structure refinement of complex.
Empirical formula
Formula weight
Crystal system
Space group
38 8 6
C H36CdN14O S
1121.64
Triclinic
P1ꢀ
Temperature (K)
Unit cell dimensions (A, )
113(2)
˚
ꢃ
a
b
c
ꢃ
ꢄ
ꢅ
9.0931(2)
11.189(2)
11.255(2)
95.48(3)
92.98(3)
91.96(3)
1137.5(4), 1
1.637
3
˚
Volume (A ), Z
Calculated density (g cm
ꢂ3
)
Absorption coefficient (mm
ꢂ1
)
0.823
3
Crystal size (mm )
0.12 ꢁ 0.10 ꢁ 0.04
1.82 – 25.02
6498
ꢃ
ꢆ range for data collection ( )
Reflections collected
Independent reflection
Data/parameters
Goodness-of-fit on F
R indices (all data)
3942
3942/311
1.001
2
a
b
R ¼ 0.0534, wR ¼ 0.1181
a
b
Final R indices [I 4 2ꢇ(I)]
R ¼ 0.0499, wR ¼ 0.1150
R ¼
PjjF
o
j ꢂ jF
c
jj/P
jF
o
j;
wR ¼ [P
w(F ꢂ F )/ w(F ) ]
P
.
a
b
2
o
2
c
2
2 1/2
o
˚
Table 2. Selected bond lengths (A) and angles ( ) for complex.
ꢃ
a
Cd(1)–N(1)
Cd(1)–O(1)
Cd(1)–N(3)
N(1)–Cd(1)–O(1)
N(1)–Cd(1)–O(1A)
N(1)–Cd(1)–N(3)
2.247(4)
2.347(3)
2.407(4)
Cd(1)–N(1A)
Cd(1)–O(1A)
2.247(4)
2.347(3)
2.407(4)
91.36(12)
88.64(12)
87.08(13)
92.92(13)
97.72(12)
82.28(12)
180.00(1)
a
a
Cd(1)–N(3A)
N(1A) –Cd(1)–O(1A)
a
a
a
91.36(12)
88.64(12)
87.08(13)
92.92(13)
97.72(12)
82.28(12)
180.00(1)
180.00(1)
a
a
a
N(1A) –Cd(1)–O(1)
a
N(1A) –Cd(1)–N(3)
a
a
N(1A) –Cd(1)–N(3)
N(1)–Cd(1)–N(3A)
a
O(1A) –Cd(1)–N(3)
O(1)–Cd(1)–N(3)
a
O(1)–Cd(1)–N(3A)
O(1A) –Cd(1)–N(3A)
O(1)–Cd(1)–O(1A)
a
a
a
N(1)–Cd(1)–N(1A)a
N(3)–Cd(1)–N(3A)
a
Symmetry code: 1 ꢂ x, 1 ꢂ y, 1 ꢂ z.
ꢂ1
of 20–200 mV s . The GCE was polished successively with 0.3 and 0.05 mm Al O3
2
slurry on emery paper. It was then rinsed with doubly-distilled water and sonicated in
ꢂ1
1
mol L HNO , acetone and doubly-distilled water for 10 min. Solutions of complex
3
was prepared by dissolution in appropriate amount of DMSO and then mixing in B–R
buffer.
3
3
. Results and discussion
.1. IR Spectral studies
When the IR spectrum of free ligand is contrasted with that of complex, the difference is
ꢂ1
apparent. In the IR spectrum of complex, there is a broad band at 3428 cm for O–H

2
784
Y.-H. Wen et al.
stretch of methanol, thus indicating the presence of methanol. The strong absorption
ꢂ1
at 1384 cm is assignable to uncoordinated nitrate [17]. These are in good agreement
with the result of the single-crystal structure analysis.
3.2. X-ray crystal structure of complex
Single-crystal X-ray diffraction analysis reveals that the crystal structure of the complex
2
þ
ꢂ
3
is composed of [Cd(L) (CH OH) ] and two NO ’s linked by hydrogen bonds and van
2
3
2
der Waals’ forces. The molecular structure of the complex with the atom numbering
scheme is shown in figure 1. The complex crystallizes in the triclinic space group P1ꢀ
with half a molecule per asymmetric unit. The other half of the molecule is generated
from the first one by the crystallographic inversion centre. The Cd(II) is six-coordinate
with four nitrogen atoms from two L’s and two oxygen atoms from two methanols. All
coordinated bond lengths (table 2) show normal values and are comparable to those in
related benzimidazole and 1,3,4-thiadiazole compounds [14, 18–21]. The coordination
geometry around Cd is best described as octahedral (figure S1 in Supplementary
Material). The equatorial positions are occupied by N(1), O(1), N(1A), and O(1A) with
the sum of the angles N(1)–Cd(1)–O(1), N(1A)–Cd(1)–O(1), N(1A)–Cd(1)–O(1A),
ꢃ
and N(1)–Cd(1)–O(1A) being 360 , and the axial positions by N(3) and N(3A) with
ꢃ
N(3)–Cd(1)–N(3A) angle being 180(1) .
L is a bidentate ligand to form a stable seven-membered ring, leaving one
benzimidazole uncoordinated. The thiadiazole and benzimidazole rings are
Figure 1. The molecular structure of the complex showing the atom-labeling scheme. Hydrogen atoms are
omitted for clarity. The unlabelled atoms are related to the labeled atoms by the symmetry operator (1 ꢂ x,
1
ꢂ y, 1 ꢂ z).

Benzoimidazole cadium complex
2785
Figure 2. The 1-D infinite chain structure of complex, formed by hydrogen bonds of nitrates, viewed down
the b axis.
˚
Table 3. Hydrogen bond data (A) and ( ) for complex.
ꢃ
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
ffDHA
O(1)–H(1)ꢀ ꢀ ꢀN(5)
N(2)–H(2)ꢀ ꢀ ꢀO(3)
N(2)–H(2)ꢀ ꢀ ꢀO(4)
N(6)–H(6)ꢀ ꢀ ꢀO(4)
0.93
0.86
0.86
0.86
0.97
0.97
0.97
2.15
2.00
2.47
1.94
2.58
2.38
2.36
2.744(4)
2.822(5)
3.012(5)
2.793(5)
2.963(6)
3.194(5)
2.842(6)
121
161
121
171
104
141
110
a
a
b
C(8)–H(8B)ꢀ ꢀ ꢀN(3)
c
C(8)–H(8B)ꢀ ꢀ ꢀO(1)
C(11)–H(11A)ꢀ ꢀ ꢀN(4)
a
b
c
Symmetry codes: 1 ꢂ x, ꢂ y, 1 ꢂ z; 1 ꢂ x, 1 ꢂ y, 2 ꢂ z; 1 ꢂ x, 1 ꢂ y, 1 ꢂ z.
essentially planar. The dihedral angles between thiadiazole and two benzimidazole
ꢃ
ꢃ
rings are 66.2(2) and 72.9(2) , respectively. The two nitrates are not coordinated
and balance the charge.
There are different types of hydrogen bonds in the complex. Nitrates form
intermolecular hydrogen bonds, N(2)–H(2)ꢀ ꢀ ꢀO(3), N(2)–H(2)ꢀ ꢀ ꢀO(4), and N(6)–
H(6)ꢀ ꢀ ꢀO(4), connecting the complex into infinite chains (figure 2 and table 3).
Methanol as hydrogen bond donors form intramolecular O(1)–H(1)ꢀ ꢀ ꢀN(5) hydrogen
bonds with uncoordinate benzimidazole (figure 2 and table 3). Finally, there exist
different types of C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀN hydrogen bonds in the complex (table 3).
The crystal structure is stabilized by these hydrogen bond interactions.
3
.3. Powder X-ray diffraction and molar conductivity
The experimental powder X-ray diffraction pattern is in good agreement with the
simulated one on the basis of the single-crystal structure (figure S2 in Supplementary
material), suggesting phase purity of the synthesized sample.

2
786
Y.-H. Wen et al.
ꢃ
2
Molar conductivity of the complex in DMF at 25 C is 151 S cm mol , indicating that
ꢂ1
the complex is a 1 : 2 electrolyte and two nitrates are out of the coordination sphere [22].
This agrees with the results of the spectral studies and single-crystal structure analysis.
3
.4. UV-Vis absorption spectra studies
Electronic absorption spectra of ligand and complex were recorded from 200 to 700 nm
with clear difference between the absorption spectra of ligand (curve a in figure 3) and
complex (curve b in figure 3), showing the formation of complex. In the absorption
spectrum of ligand (curve a in figure 3), the strong absorption at 276 nm and the broad
shoulder lower than 255 nm could be assigned to benzimidazole and thiadiazole
intraligand ꢀ!ꢀ* transitions, respectively. In the absorption spectrum of complex, a
new absorption at 270 nm was observed from one benzimidazole ring coordinated to
Cd(II) and the broad shoulder lower than 255 nm became an absorption at 242 nm from
thiadiazole ring coordinated to Cd(II) (see crystal structure section). The absorption
spectrum of complex (curve c in figure 3) had little change after adding dsDNA to the
solution, while the absorption spectrum of complex (curve d in figure 3) had obvious
decrease after adding the same concentration ssDNA. Absorptions had no red shift,
suggesting that the interaction between Cd(II) and DNA might be electrostatic [23].
The hypochromic effect by ssDNA shows the Cd(II) complex could be a probe for
recognition of ssDNA from dsDNA.
DNA binding of Cd(II) complex was studied by UV-Vis titration. For the UV-Vis
ꢂ1
ꢂ4
ꢂ5
titration experiment, 20 mL solutions of DNA (7.0 ꢁ 10 mol L
in water) were
ꢂ1
added to a 2.5 mL solution of Cd(II) complex (5.05 ꢁ 10 mol L in water) by a
micropipette. Because of the limited solubility of the complex in water, the titration
experiments were carried out in the presence of a small amount of DMSO. The intrinsic
binding constant K was evaluated from the collected absorbance data of the titration
b
curve at the specified wavelength and the following equation [24],
CDNA=ð"a ꢂ " Þ ¼ C
=ð" ꢂ " Þ þ 1=K ð" ꢂ " Þ
f
DNA
b
f
b
b
f
1
1
0
0
.5
.1
.7
.3
a
↓
d
-
0.1
2
00
250
300
350
400
Wavelength/nm
ꢂ4
ꢂ6
ꢂ1
ꢂ1
ꢂ5
ꢂ1
Figure 3. UV-Vis absorption spectra: (a) 1.01 ꢁ 10 mol L ligand; (b) 5.05 ꢁ 10 mol L complex;
ꢂ6
ꢂ1
(
c) b þ 8.4 ꢁ 10 mol L dsDNA; and (d) b þ 8.4 ꢁ 10 mol L ssDNA.

Benzoimidazole cadium complex
2787
-100
-200
-300
-400
-500
-600
-700
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
CDNA/10-5
M
Figure 4. Plot of CDNA/(" ) against CDNA
a
ꢂ "
f
.
where CDNA is the concentration of DNA, " , " , and " refer to the extinction
a
f
b
coefficients for the free complex, for each addition of DNA to the complex and for
thecomplex in the fully bound form, respectively. When CDNA/(" ꢂ " ) is plotted
a
f
against CDNA, a straight line is obtained (figure 4). The binding constant K obtained
b
4
ꢂ1
by the ratio of the slope to the intercept is 2.19 ꢁ 10 L mol . This Kb value is
comparable to those of the related bisbenzimidazole cadmium(II) complexes
4
ꢂ1
(
2.3 ꢁ 10 L mol for [Cd(L) ] ꢀ (pic) (L ¼ 1,3-bis((1-ethylbenzimidazol-2-yl)-2-thiapro-
2
2
5
ꢂ1
pane) [25] and 1.42 ꢁ 10 L mol for [Cd(L)Br ] (L ¼ 1,3-bis((1-benzylbenzimidazol-2-
2
yl)-2-thiapropane)) [26], but much higher than that of histidine cadmium(II) complex
2
ꢂ1
(
4.56 ꢁ 10 L mol for [Cd(HL) ] ꢀ H O (H L ¼ N-(2-hydroxybenzyl)-L-histidine) [27].
2
2
2
Different structures may cause different binding affinity with DNA.
3
.5. Electrochemical studies
The electrochemical behavior of the complex in the absence and presence of DNA was
ꢂ1
studied by cyclic voltammograms in 5 mL 0.04 mol L BR buffer solution (pH 5.0)
ꢂ4
ꢂ1
containing 1.003 ꢁ 10 mol L Cd complex.
As shown in figure 5 (curve a), the Cd(II) complex shows a pair of redox peaks with a
cathodic peak potential (E ) at 0.200 V and an anodic peak potential (E ) at 0.319 V.
pc
pa
The formal potential (E1/2 ¼ 1/2(E þ E )), the peak-to-peak potential separation
pa
pc
(
DE ), and the ratio of peak current (I /I ) were obtained as 0.259 V, 0.119 V, and
p
pa pc
3.75, respectively, indicating that the electrochemical behavior of Cd(II) complex is
irreversible [28].
Curves b and c in figure 5 were the voltammograms when Cd(II) complex interacted
with the same concentration dsDNA and ssDNA, respectively. In the presence of DNA,
the cyclic voltammograms exhibited significant decrease in peak current, but little shift
in formal potential. E1/2, DE , and I /I were obtained as 0.259, 0.103, and 4.90 V for
p
pa pc
dsDNA, and 0.259, 0.101 and 4.22 V for ssDNA, respectively, indicating the interaction
between the complex and the DNA. The decrease in the voltammetric currents in the

2
788
Y.-H. Wen et al.
Figure 5. Cyclic voltammograms of Cd complex in the absence (a) and presence of dsDNA (b) and ssDNA
ꢂ4
ꢂ1
ꢂ1
ꢂ1
ꢂ1
(
(
c). (a): 1.003 ꢁ 10 mol L
complex þ 0.04 mol L
B–R buffer solution (pH ¼ 5.0); (b):
ꢂ5
ꢂ5
a) þ 2.1 ꢁ 10 mol L dsDNA; (c): (a) þ 2.1 ꢁ 10 mol L ssDNA.
1
1
2
0
8
6
4
2
0
a
↓
c
0
0.04
0.08
0.12
0.16
v/V s-1
Figure 6. Plot of peak current (Ipa) against scan rate (v). (a): Complex, (b): (a) þ dsDNA, (c): (a) þ ssDNA.
presence of DNA can be attributed to slow diffusion of the complex bound to the large,
slowly diffusing DNA, and the unchanged formal potentials are probably because the
oxidation state and the reduction state of complex could interact with DNA to the same
degree [29–31]. Interaction between Cd(II) complex and ssDNA is stronger than that
between Cd(II) complex and dsDNA (figure 5) because of the exposed bases of ssDNA
[
32], consistent with the absorption spectra results.
The effect of varying the potential scan rate (v) on the oxidation peak current was
also examined. The results showed that oxidation peak currents of Cd complex
increased linearly with scan rate in the absence and presence of DNA over the range
ꢂ1
0
.02–0.16 V s (figure 6), suggesting that the oxidation of complex at the CPE is
adsorption-controlled. The decrease in slopes of straight lines in the presence of DNA
indicates the decrease in diffusion coefficient [33].
For adsorption-controlled and irreversible electrode process, according to Laviron
[
34], the oxidation peak potential (E ) can be described by the following equation:
p
0
0
0
E ¼ E þ ðRT=ꢃnFÞ lnðRTk =ꢃnFÞ þ ðRT=ꢃnFÞ ln ꢈ
p

Benzoimidazole cadium complex
2789
0
.3
0
0
0
0
.28
.26
.24
.22
-
1.8
-1.6
-1.4
-1.2
log v
-1
-0.8
p
Figure 7. Plot of oxidation peak potential (E ) against log v.
ꢂ4
ꢂ1
ꢂ1
ꢂ1
Figure 8. Differential pulse voltammograms of the complex (1.003 ꢁ 10 mol L ) in 0.04 mol L B–R
ꢂ6
buffer solution (pH ¼ 5.0) varying concentration of dsDNA (10 mol L ), (a) 0, (b) 3.4826, (c) 6.9625,
(d) 10.448, (e) 13.931, (f) 17.413, and (g) 20.895.
0
where E is the formal potential, T is the temperature, ꢃ is the transfer coefficient, n is
0
0
the number of electrons transferred in the rate-determining step, k is the electrochem-
ical rate constant, and F is the Faraday constant. The relationship between E and v was
p
investigated and E depends linearly on the logarithm of v according to the equation
p
E ¼ 0.0493 log v þ 0.3196 with a correlation coefficient of 0.9977 (figure 7). The value
p
of ꢃn, calculated from the slope of 0.0493, is 1.1658. Generally, ꢃ is assumed to be 0.5 in
a totally irreversible electrode process. Thus, two electrons are involved in the oxidation
of complex.
Differential pulse voltammograms of the complex with varying concentration of
dsDNA are shown in figure 8. The oxidation peak current decreased with increase in
concentration of dsDNA. The oxidation peak current was constant when the
ꢂ5
ꢂ1
concentration of dsDNA is 1.393 ꢁ 10 mol L , showing the complex in the fully
bound form. When DI is plotted against CDNA, a straight line is obtained (figure 9),
pa

2
790
Y.-H. Wen et al.
1
.8
.6
.4
.2
0
0
0
0
0
0
5
10
15
20
25
CDNA/10-6
M
Figure 9. Plot of DIpa against CDNA
.
9
7
5
3
1
00
00
00
00
00
Complex
L
–
100
2
50
350
450
550
650
Wavelength/nm
Figure 10. Fluorescence emission spectra of L (dot) and complex (solid) in solid state at room temperature.
indicating the complex could be used to analyze quantitatively DNA over the range
ꢂ6
ꢂ5
ꢂ1
3
.4826 ꢁ 10 ꢂ 1.393 ꢁ 10 mol L
.
3
.6. Fluorescent properties
The solid-state fluorescence of L and complex were investigated at room temperature
figure 10). L displays an intense fluorescence emission maximum at 407 nm when
(
excited at 320 nm, assigned to the intraligand (ꢀ–ꢀ*) fluorescent emission. At the same
conditions, the complex exhibits more intense fluorescence with an emission maximum
at 424 nm upon excitation at 350 nm, red-shifted 17 nm with respect to free ligand.
1
0
Enhancement of fluorescence in complex with d configuration may be attributed to
coordination of ligand to cadmium [35]. The coordination increases rigidity of ligand,
as indicated in X-ray diffraction analysis, and thus reduces the loss of energy through
a radiationless pathway. The red shift of the emission of complex is attributed to ligand-
to-metal charge transfer, which has been found in similar Cd coordination compounds
[36–38]. The strong and narrow fluorescence emission suggests the complex can be used
as a potential optical material.

Benzoimidazole cadium complex
2791
4. Conclusion
A new thiodiazole-based bis(benzimidazole), 2,5-bis((benzoimidazol-2-yl)methylthio)
,3,4-thiadiazole (L), and its cadmium complex, [Cd(L) (CH OH) ] ꢀ (NO ) , were
1
2
3
2
3 2
synthesized and characterized. The coordination geometry is octahedral. Intermolecular
hydrogen bonds in the complex result in generation of a 1-D chain. Both UV-Vis
spectra and cyclic voltammetry studies show that interaction between Cd(II) complex
and ssDNA is stronger than that with dsDNA, indicating the Cd(II) complex could be
used as a probe to distinguish ssDNA from dsDNA. The interaction of Cd(II) complex
with salmon sperm DNA might be electrostatic binding. Differential pulse voltammetry
indicates the complex could be used to analyze quantitatively DNA. The solid complex
exhibits strong luminescence emission in the visible region at room temperature upon
excitation with UV radiation, suggesting the complex could be a potential fluorescent
material.
Supplementary material
Crystallographic data for the structural analyses of complex has been deposited with
the Cambridge Crystallographic Data Center (CCDC). CCDC reference number
865269 contains the supplementary crystallographic data for this article. These data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif
Acknowledgments
This work was supported by the National Natural Science Foundation of China
(No. 20971076) and the Science and Technology Projects (Nos. BS2010NJ004,
2
009ZRB019KH) of Shandong Province, China.
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