Synthesis, crystal structure, and electrocatalytic properties of a copper(II) complex of 1,2-bis(2-benzimidazolyl)benzene

Article abstract of DOI:10.1007/s11243-012-9577-4

A new copper(II) complex based on the rigid ligand OBimB (OBimB = 1,2-bis(2-benzimidazolyl)benzene) has been synthesized and structurally characterized by X-ray single-crystal diffraction. In the structure of the complex, each Cu(II) atom is coordinated by four N atoms from two twisted V-shaped OBimB ligands in a square-planar geometry. The complex has been used as a bulk modifier to fabricate a carbon paste electrode (Cu-CPE). Electrochemical studies of the Cu-CPE in 0.1 M pH 2.0 phosphate buffer solution reveal that the Cu²⁺/Cu⁺ redox process is quasi-reversible. Electrocatalytic studies of the Cu-CPE indicate that it has good electrocatalytic activities toward reduction of bromate, hydrogen peroxide, and nitrite.

Full text of DOI:10.1007/s11243-012-9577-4

Transition Met Chem (2012) 37:309–314
DOI 10.1007/s11243-012-9577-4
Synthesis, crystal structure, and electrocatalytic properties
of a copper(II) complex of 1,2-bis(2-benzimidazolyl)benzene
•
Yue-Yi Deng Fa-Qian Liu Yan-Ling Jin
•
Received: 29 October 2011 / Accepted: 6 January 2012 / Published online: 4 April 2012
Ó Springer Science+Business Media B.V. 2012
Abstract A new copper(II) complex based on the rigid
ligand OBimB (OBimB = 1,2-bis(2-benzimidazolyl)benzene)
has been synthesized and structurally characterized by
X-ray single-crystal diffraction. In the structure of the
complex, each Cu(II) atom is coordinated by four N atoms
from two twisted V-shaped OBimB ligands in a square-
planar geometry. The complex has been used as a
bulk modifier to fabricate a carbon paste electrode
(Cu-CPE). Electrochemical studies of the Cu-CPE in 0.1 M
pH 2.0 phosphate buffer solution reveal that the Cu2?/Cu?
redox process is quasi-reversible. Electrocatalytic studies
of the Cu-CPE indicate that it has good electrocatalytic
activities toward reduction of bromate, hydrogen peroxide,
and nitrite.
formed with transition metals [12, 13], and modeling of
biological systems [14, 15].
Transition metal complexes can be used as electron
transfer mediators for modification of different kinds of
electrode materials owing to their electrochemical revers-
ibility [16]. Some metal complexes have also received
great interest as catalysts for electrochemical reactions in
recent years [16–19]. The electron transfer mediator com-
plexes can be dispersed and stabilized in different sup-
porting carbon materials to give low background currents,
wide potential windows, chemical inertness, and low cost
[20]. The electrochemical and electrocatalytic properties of
copper complexes with some polycarboxylate and N-con-
taining heterocyclic ligands have been investigated
[21–26], but studies on the complexes based on bis(2-
benzimidazolyl) ligands are scarce [26]. In this paper, we
report the synthesis and crystal structure of a new cop-
per(II) complex containing the OBimB rigid ligand,
namely [Cu(OBimB)2](NO3)2ꢀ(EtOH)2. In addition, the
electrochemical properties of the complex bulk-modified
carbon paste electrodes (Cu-CPE) have been investigated
and reveal that the modified electrode exhibits excellent
electrocatalytic activities toward the reduction of bromate,
hydrogen peroxide, and nitrite.
Introduction
Bis(2-benzimidazolyl) compounds and their metal com-
plexes have attracted much attention in recent years. They
exhibit biological properties such as antivirus activities
[1, 2] and control of liver diseases [3], as well as potential
applications as proton sponges [4], selective ion-exchange
resins [5, 6], polymerization catalysts [7–9], and electron
transfer agents [10]. The coordination chemistry of these
species has been focused on matters such as geometri-
cally constraining ligands [11], supramolecular aggregates
Experimental
All the reagents for synthesizing the ligand OBimB and the
title complex were purchased commercially and used
without further purification. K2HPO4 and KH2PO4 were
used for the preparation of 0.1 M phosphate buffer solu-
tion, and hydrochloric acid was used for pH adjustment.
Elemental analyses (C, H and N) were carried out on a
Perkin–Elmer 1400C analyzer. IR spectra were recorded in
Y.-Y. Deng (&) ꢀ F.-Q. Liu ꢀ Y.-L. Jin
Key Laboratory of Rubber-plastics, Ministry of Education,
College of Polymer Science and Engineering, Qingdao
University of Science and Technology, No. 53, Zhengzhou
Road, Qingdao 266042, People’s Republic of China
e-mail: dengyyqust@hotmail.com
123

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Transition Met Chem (2012) 37:309–314
the range of 400–4,000 cm-1 on a Nicolet 170SX spec-
trometer with pressed KBr pellets. UV–Vis spectra were
recorded on a Perkin–Elmer UV–Vis spectrometer. Vol-
tammetry was performed using a CHI 832B electrochem-
ical analysis system (China) with a three-electrode system
consisting of the modified electrode as the working elec-
trode, a saturated calomel electrode (SCE) as the reference
electrode, and a platinum wire as the auxiliary electrode.
All the electrochemical measurements were carried out in a
10 mL electrolyte cell with 0.1 M pH = 7.0 or 2.0 phos-
phate buffer solutions as electrolyte.
piece of weighing paper to achieve a mirror finish before
use.
X-ray crystallographic study
A block shaped crystal with dimensions of 0.20 9 0.15 9
0.10 mm was mounted on a Bruker SMART 1000 CCD area
detector X-ray single-crystal diffractometer with graphite-
monochromated Mo Ka radiation (k = 0.071073 nm) and a
u/x scanning mode at 293(2) K. Intensities were corrected for
Lorentz and polarization effects, and empirical absorption. The
structure was solved by direct methods using the SHELXS 97
program [28] and refined by full-matrix least-squares on F2 via
SHELXL 97 [29]. The correct positions for the copper atoms
were deduced from the E-map. Subsequent least-squares
refinement and difference Fourier calculations revealed the
positions of the remaining non-hydrogen atoms. Non-hydrogen
atoms were refined with independent anisotropic displacement
parameters. Restraints were applied to the N–H bond lengths of
the imidazole rings (DFIX restraints, N2–H2N = 0.86 nm,
N4–H4N = 0.86 nm). H atoms were positioned geometri-
cally (C–H = 0.093 and 0.096 nm) and allowed to ride on their
parent atoms with Uiso(H) = 1.2 times Ueq(C). The com-
plex crystallizes in triclinic system, P1 space group, with the
cell parameters: a = 0.8707(15) nm, b = 1.14994(19) nm,
c = 1.2181(2) nm, a = 62.765(4)°, b = 77.320(4)°, c =
73.633(4)°, V = 1.0346(3) nm3, Z = 1, and Dc = 1.445
g cm-3, the final R1 = 0.071, wR2 = 0.1831.
Preparation of the complex
OBimB was synthesized by condensing o-diaminobenzene
with o-phthalic acid according to the literature method [27]
with minor revisions. o-Diaminobenzene (4.7 g, 44 mmol)
and o-phthalic acid (3.3 g, 20 mmol) were finely ground
togetherandheatedto230 °C in polyphosphoric acid (40 mL)
with stirring for 4 h. The reaction mixture was allowed to cool
to 100 °C and then poured into ice water with vigorous stir-
ring. The precipitate was then filtered off and stirred in a
saturated sodium bicarbonate solution for 12 h. The raw solid
was recrystallized from hot methanol to give the light yellow
product (yield, 3.0 g, 38%). IR(KBr): 3,049(w), 2,878(w),
1,620(w), 1,534(w), 1,441(s), 1,317(m), 1,280(m), 1,007(m),
764(s).
Synthesis of [Cu(OBimB)2](NO3)2ꢀ(EtOH)2
Results and discussion
Cu(NO3)2ꢀ3H2O (0.072 g, 0.3 mmol) and OBimB (0.09 g,
0.3 mmol) were added to ethanol (20 mL) and acetonitrile
(10 mL), and the mixture was stirred and refluxed for 30 min.
The resulting blue solution was filtered, and the filtrate was kept
at ambient temperature for a period of about 3 days, to give blue
block crystals (31% yield based on Cu) suitable for structural
determination. Anal. Calcd(%) for C44H40CuN10O8: C 58.7, H
4.5, N 15.6; found(%): C 58.7, H 4.5, N 15.6. IR(KBr, cm-1):
3,425(br), 1,629(w), 1,578(w), 1,462(m), 1,390(s), 1,283(m),
1,229(w), 893(w), 815(w), 748(m), 682(w).
Characterization of the complex
An ORTEP view of the complex dication [Cu(OBimB)2]2?
with the atomic numbering scheme and selected bond distances
and bond angles is shown in Fig. 1, showing 30% probability
displacement ellipsoids. The central Cu atom in the complex is
coordinated by four N atoms from two OBimB ligands in a
square-planar geometry. The trans angles are all 180° for
symmetry requirement, and the cis angles are 89.63(17)° and
90.37(17)° for N1–Cu–N3 and N1–Cu–N3i (symmetry code: i,
-x ? 1, -y ? 1, -z ? 1), respectively. The Cu–N distances
Preparation of the modified CPE
˚
are 1.963(4) and 1.974(4) A, which are close to those of the
The CPE electrode modified by the complex was fabricated
by the following steps. Graphite powder (0.3 g) and com-
plex (0.03 g) were ground together with an agate mortar
and pestle for about 30 min to give an even mixture. Par-
affin oil (0.1 mL) was added to the mixture and stirred for
about 20 min. Then, a 3-mm inner diameter glass tube was
packed to a length of about 0.8 cm with the homogenized
mixture, and a copper stick was used as the electrical
contact. The surface of the modified CPE was polished on a
structurally analogous complex [Cu(bmbz)2](CF3SO3)2
(bmbz = 1,2-bis(N-methylbenzimidazol-20-yl)benzene, Cu–
˚
N = 1.961(3) and 1.981(3) A) [30]. Each OBimB ligand
affords two coordination sites and chelates a Cu(II) atom to
form a seven-membered ring, resulting in the construction of a
ML2 complex. The overall structure of the ligand can be
described as a twisted V-shaped conformation, so that the two
coordinated N atoms of the benzimidazoles are more exposed
for metal binding. The dihedral angles of the benzimidazolyl
123

Transition Met Chem (2012) 37:309–314
311
broadness of this band indicates that hydrogen bonds exist
in accordance with the crystal structure analysis. The
UV–Vis spectrum of the complex in ethanol solution shows
two intense transitions around 224 and 280 nm, which can
be assigned to intramolecular charge transfer transitions
and p ? p* transitions in the ligands. The weak absor-
bance band at *390 nm is assigned to n ? p* transitions
in the ligands.
Electrocatalytic reduction of bromate at a Cu-CPE
electrode
Because of the electrochemical reversibility of the modi-
fied electrode (Cu-CPE), using the complex as a redox
mediator in acidic media, the Cu-CPE can be used as an
electrocatalytic electrode material. Bromate is a toxic
pollutant of drinking water, which is formed by ozonation
of source waters containing bromide. The direct electro-
reduction of bromate requires a large overpotential, which
can be decreased by a suitable catalyst. Hence, the catalytic
reduction of bromate by our Cu-CPE electrode has been
investigated. The electrochemical behavior of the Cu-CPE
in pH 2 phosphate buffer solution is shown in Fig. 4. The
Cu-CPE displays a pair of well-defined redox peaks with
large currents; an anodic peak is observed at 0.341 V and a
cathodic peak at 0.186 V. The mean peak potential E1/2
Fig. 1 Structure of the complex [Cu(OBimB)2]2? at 30% probability
displacement ellipsoids with atomic numbering scheme (anions and
hydrogen atoms are omitted for clarity). Selected bonds and angles
i
˚
(A, °): Cu–N1 1.963(4), Cu–N3 1.974(4), N1–Cu–N1 180.0, N1–Cu–
N3i 90.37(17), N1–Cu–N3 89.63(17); symmetry code: i, -x ? 1, -
y ? 1, -z ? 1
ring planes (comprising imidazole ring atoms N1, C1, C6, N2,
N7 designated as plane 1 and N3, C14, C15, N4, C20, as
plane 2) with the central phenyl ring plane (comprising atoms
C8–C13) are very similar, at 39.05° and 39.07°, respectively.
The dihedral angle between the planes (planes 1 and 2) of the
benzimidazolyl groups in the ligand is 56.33°, which is smaller
than that in the complex [Cu(bmbz)2] (CF3SO3)2 (70.2°) [30];
the difference may be ascribed to the alkylation of the benz-
imidazoles in bmbz.
= (Epa ? Epc)/2 is 0.263 V, corresponding to the Cu2?
/
Cu? electrochemical process [16, 31]. The separation of
the cathodic and anodic peak potentials, DE = 0.155 V,
indicates that the electrochemical behavior of the Cu-CPE
is a quasi-reversible process. The anion peak current is
larger than the cathodic current (ipc/ipa = 0.66), indicating
a slow electron transfer process. With the addition of
bromate, there is a dramatic enhancement of the cathodic
peak current while the anodic peak current decreases
(Fig. 5), which indicates a strong catalytic effect. The
possible catalytic processes can be described by the fol-
lowing equations [32, 33]:
In the complex, the two amine NH groups of the OBimB
ligand are involved in hydrogen bonding with an ethanol
-
molecule and NO3 anion (N2 –O2, 2.792(8) A, N4 –O4,
iii
ii
˚
˚
2.738(8) A; symmetry code: ii, -x ? 1, -y ? 2, -z ? 1, iii,
x-1, y, -z), and there is also a weak hydrogen bonding
-
between the ethanol molecule and NO3 anion (O4–O2,
˚
2.798(8) A). These hydrogen bonds link the complex units
into a one-dimensional chain (Fig. 2). The 1-D chains are
further extended into a 2-D layer network by pꢀꢀꢀp stacking
interactions among the imidazole rings of the OBimB ligands,
CuðIIÞ þ eꢁ ! CuðIÞ
6CuðIÞ þ BrOꢁ3 þ 6Hþ ! 6CuðIIÞ þ Brꢁ þ 3H2O:
iv
˚
with a Cg(1)ꢀꢀꢀCg(1) distance of ca. 3.885 A (Cg(1) is the
Figure 6 shows that catalytic current is proportional to
the concentration of bromate. The plot of catalytic current
versus bromate concentration is linear in the concentration
range of 1–5 mM, which is fitted by the equation
I (lA) = 3.5513[BrO3-] ? 3.2197 lA with R2 = 0.9927.
centroid of the imidazole ring N1, C1, C6, N2, C7; symmetry
code: iv, -x, -y ? 1, -z ? 1) and intermolecular C–Hꢀꢀꢀp
iv
˚
interactions with a C4ꢀꢀꢀCg(2) distance of 3.542 A (Cg(2) is
the centroid of the imidazole ring N3, C14, C15, N4, C20;
symmetry code: iv, -x, -y ? 1, -z ? 1) (Fig. 3).
The infrared spectrum of the complex shows charac-
teristic absorptions for the OBimB ligand with slight shifts
Electrocatalytic reduction of hydrogen peroxide
and nitrite at a Cu-CPE electrode
due to coordination.
A strong absorption band at
1,390 cm-1 is assigned to the mas vibration of the nitrate
anion. A band at 3,425 cm-1 in the spectrum of the com-
plex can be assigned to mN–H of the OBimB ligand, and the
The catalytic activity of the Cu-CPE toward hydrogen
peroxide has also been investigated, as shown in Fig. 7.
123

312
Transition Met Chem (2012) 37:309–314
Fig. 2 1-D chain formed by N–HꢀꢀꢀO and O–HꢀꢀꢀO hydrogen bonds; symmetry code: ii, -x ? 1, -y ? 2, -z ? 1; iii, x - 1, y, -z
Fig. 3 2-D layer formed by pꢀꢀꢀp and C–Hꢀꢀꢀp interactions between the 1-D chains; symmetry code: iv, -x, -y ? 1, -z ? 1
10
0
10
8
a
b
-10
-20
-30
-40
-50
-60
-70
-80
-90
c
d
6
4
2
e
f
0
-2
-4
-6
-8
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Potential(V).vs.SCE
Potential(V).vs.SCE
Fig. 5 Cyclic voltammograms of Cu-CPE in 0.1 M pH 2.0 phosphate
-
Fig. 4 Cyclic voltammogram of Cu-CPE in 0.1 M pH 2.0 phosphate
buffer solution at a scan rate of 0.1 V s-1
buffer solution at a scan rate of 0.1 V s-1 with increasing BrO3
concentrations 0(a), 1(b), 2(c), 3(d), 4(e), 5 mM (f)
When H2O2 is added to the buffer solution, the cathodic
peak current increases and the anodic peak current
decreases (Fig. 7b). With increasing H2O2 concentrations,
the reduction peak currents are dramatically enhanced
(Fig. 7c, d), which indicates that the Cu-CPE has good
electrocatalytic activity toward the reduction of H2O2. The
dependence of peak current response on the concentration
of H2O2 is linear in the range 1–5 mM, as shown in the
inset of Fig. 7. The catalytic reduction of H2O2 can be
described by the following equation [25]:
2CuðIÞ þ H2O2 þ 2Hþ ! 2CuðIIÞ þ 2H2O:
Figure 8 shows the cyclic voltammograms of Cu-CPE in
buffer solutions containing different concentrations of
123

Transition Met Chem (2012) 37:309–314
313
25
20
10
y = 3.5513x + 3.2197
20
15
10
5
R2= 0.9927
0
a
-10
-20
-30
-40
-50
14
12
10
8
0
0
1
2
3
4
5
6
b
c
Concentration(mM)
6
4
Fig. 6 The plot of cathodic peak current versus bromate
concentration
2
0
d
0
2
4
6
8
Potential(V).vs.SCE
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Potential(V).vs.SCE
nitrite. With the addition of nitrite, the reduction peak
currents increase significantly, accompanied by a decrease
in the oxidation peak currents. The inset of Fig. 8 shows
that the catalytic current varies in a linear fashion with
nitrite concentration up to 6 mM. A similar mechanism for
the catalytic reduction of nitrite can be proposed as [22]:
Fig. 8 Cyclic voltammograms of Cu-CPE in 0.1 M pH 2.0 phosphate
buffer solution at a scan rate of 0.1 V s-1 with increasing NO2
concentrations 0(a), 2(b), 4(c), 6 mM (d). Inset: the variation of
-
cathodic peak currents versus nitrite concentrations
4CuðIÞ þ 2NOꢁ2 þ 6Hþ ! 4CuðIIÞ þ N2O þ 3H2O:
Conclusion
The Cu-CPE exhibits high stability. Thus, after the
electrocatalytic reaction, the Cu-CPE was investigated in
0.1 M pH 2.0 phosphate buffer solution at a scan rate of
0.1 V s-1 and showed little change in the peak potentials
and currents. When the Cu-CPE was continuously scanned
for 1 h, the peak current decreased 10%. When the Cu-CPE
was stored at room temperature for 2 months, the current
response decreased only by 3.5%. The good stability of the
Cu-CPE can be ascribed to the poor solubility of the
copper(II) complex and to the strong adsorption of the
complex on graphite powder.
A new copper(II) complex based on a rigid ligand contain-
ing bis(2-benzimidazolyl) groups, [Cu(OBimB)2](NO3)2ꢀ
(EtOH)2, had been synthesized and structurally characterized
by X-ray diffraction analysis. In the structure of the complex,
two ligands with twisted V-shaped conformations chelate the
copper(II) center, resulting in a square-planar geometry. The
bulk-modified carbon paste electrode incorporating the com-
plex exhibits good electrocatalytic activity toward reduction of
bromate, hydrogen peroxide, and nitrite, indicating that the
modified electrode has potential applications in the field of
electrocatalysis.
Supplementary data
10
CCDC 837393 contains the supplementary crystallographic
data for the complex. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Center,
12 Union Road, Cambridge CB2 1EZ, UK; fax: ?44-1223-
336033; or e-mail: deposit@ccdc.cam.ac.uk.
0
a
-10
-20
-30
-40
b
c
20
10
0
Acknowledgments The supports of Natural Science Foundation of
China (No. 20871072), Doctoral Foundation of Shandong Province
(No. 2007BS04023), and Qingdao University of Science & Tech-
nology are gratefully acknowledged.
0
1
2
3
4
5
6
d
Potential(V).vs.SCE
0.2
-0.2
0.0
0.4
0.6
0.8
Potential(V).vs.SCE
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Fig. 7 Cyclic voltammograms of Cu-CPE in 0.1 M pH 2.0 phosphate
buffer solution at a scan rate of 0.1 V s-1 with increasing hydrogen
peroxide concentrations 0(a), 1(b), 3(c), 5 mM (d). Inset: the variation
of cathodic peak currents versus hydrogen peroxide concentrations
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