Synthesis, crystal structure, electrochemistry and antioxidative activity of copper(II), manganese(II) and nickel(II) complexes containing bis(N-ethylbenzimidazol-2-ylmethyl)aniline

Article abstract of DOI:10.1002/aoc.3313

Bis(N-ethylbenzimidazol-2-ylmethyl)aniline (Etbba) and its transition metal complexes, [Cu(Etbba)(Cl)₂]·DMF (1), [Mn(Etbba)(Cl)₂] (2) and [Ni(Etbba)(Cl)₂] (3), have been synthesized and characterized on the basis of elemental analysis, molar conductivity, UV-visible, infrared and NMR spectroscopies and X-ray crystallography. The coordination environment of complex 1 can be described as distorted square-based pyramidal, while complexes 2 and 3 each have a distorted trigonal bipyramidal geometry. Cyclic voltammograms of complex 1 indicate an electrochemically quasi-reversible Cu²⁺/Cu⁺ couple. In addition, the antioxidant activities of the free ligand and its complexes were investigated using the superoxide and hydroxyl radical scavenging methods in vitro. Complexes 1, 2, 3 are found to possess potent hydroxyl radical scavenging activity and to be better than standard antioxidants like vitamin C and mannitol. Furthermore, complexes 1 and 2 exhibit significant superoxide radical activity.

Full text of DOI:10.1002/aoc.3313

Full Paper
Received: 28 December 2014
Revised: 25 February 2015
Accepted: 25 February 2015
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3313
Synthesis, crystal structure, electrochemistry
and antioxidative activity of copper(II),
manganese(II) and nickel(II) complexes
containing bis(N-ethylbenzimidazol-2-
ylmethyl)aniline
Huilu Wu*, Hongping Peng, Yanhui Zhang, Fei Wang, Han Zhang,
Cuiping Wang and Zaihui Yang
Bis(N-ethylbenzimidazol-2-ylmethyl)aniline (Etbba) and its transition metal complexes, [Cu(Etbba)(Cl)2]ꢀDMF (1), [Mn(Etbba)(Cl)2] (2)
and [Ni(Etbba)(Cl)2] (3), have been synthesized and characterized on the basis of elemental analysis, molar conductivity,
UV–visible, infrared and NMR spectroscopies and X-ray crystallography. The coordination environment of complex 1 can be
described as distorted square-based pyramidal, while complexes 2 and 3 each have a distorted trigonal bipyramidal geometry.
Cyclic voltammograms of complex 1 indicate an electrochemically quasi-reversible Cu2+/Cu+ couple. In addition, the antioxidant
activities of the free ligand and its complexes were investigated using the superoxide and hydroxyl radical scavenging methods
in vitro. Complexes 1–3 are found to possess potent hydroxyl radical scavenging activity and to be better than standard
antioxidants like vitamin C and mannitol. Furthermore, complexes 1 and 2 exhibit significant superoxide radical activity.
Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: benzimidazole; transition metal complexes; crystal structure; electrochemical property; antioxidation
typical heterocyclic ligand, the large benzimidazole rings not only
Introduction
provide potential supramolecular recognition sites for πꢀꢀꢀπ stacking
Due to the structural versatility in coordination chemistry, the study
of small-molecule chemistry of transition metal coordination
assemblies is a dynamic and thriving field which has drawn ever-
increasing research interests in recent decades.[1–3] The study of
the structural, spectroscopic and electronic properties of metal
centers present in biological systems is crucial to understand their
roles in nature.[4] These centers are often modeled by using small
molecules containing donor atoms that reproduce the coordina-
tion around the metal ion.[5] For this reason, the synthesis of
complexes oriented to mimic metal sites of various types of
metalloproteins constitutes an important branch in both inorganic
and organic chemistry.[6] In this context, copper, nickel and
manganese are present in a diverse group of metalloenzymes, in
which the metal ions are coordinated to nitrogen atoms.[7]
interactions, but also act as hydrogen bond acceptors and donors
to assemble coordination complexes.[16]
Interest in bis(2-benzimidazolyl)aniline and its derivatives is
widespread. In our previous work, we investigated the coordinating
ability of various benzimidazoles.[17,18] In the present article, the
synthesis, crystal structures and electrochemical properties of bis
(N-ethylbenzimidazol-2-ylmethyl)aniline (Etbba) and three of its
metal complexes are presented. The antioxidant activities (scaveng-
ing effects on Oꢁ2 and OHꢁ ) of the Etbba ligand and its complexes
have also been studied. Information obtained from this study will
be helpful in the development of some new antioxidants.
•
•
Experimental
Benzimidazole is a typical heterocyclic ligand with nitrogen atom
as the donor atom. Interest in exploring benzimidazole derivatives
and their metal complexes has been increasing since the
recognition that many of these materials may serve as models
which mimic both the structure and reactivity of metal sites in
biological systems and can also possess a broad spectrum of
biological activity.[8] Due to their privileged structure and
properties,[9] benzimidazoles and their derivatives exhibit a wide
variety of pharmacological activities, including antitumor,[10]
antiviral,[11] anticancer,[12] antimicrobial,[13] antihypertensive[14]
and anti-inflammatory or analgesic activities.[15] Moreover, as a
Materials and Methods
All chemicals and solvents were of reagent grade and used without
further purification. C, H and N elemental analyses were carried out
*
Correspondence to: Huilu Wu, School of Chemical and Biological Engineering,
Lanzhou Jiaotong University, Lanzhou, Gansu 730070, PR China. E-mail:
wuhuilu@163.com
School of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou, Gansu, 730070, PR China
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

H. Wu et al.
using a Carlo Erba 1106 elemental analyzer. IR spectra were
recorded in the 400–4000 cmꢁ1 region with a Nicolet FT-VERTEX
70 spectrophotometer using KBr pellets. Electronic spectra were
obtained using a Lab-Tech UV Bluestar spectrophotometer with a
spectral resolution of 0.2 nm. Absorption spectra were measured
with a Spectrumlab 722sp spectrophotometer at room tempera-
(10 ml) was added a solution of metal chloride salt (0.50 mmol:
CuCl2ꢀ4H2O, 85.24 mg; MnCl2ꢀ2H2O, 98.96 mg; NiCl2ꢀ6H2O, 118.8
mg) in EtOH (10 ml). The sediment generated rapidly. The
precipitate was filtered off, washed with ethanol and absolute
diethyl ether, and dried in vacuo. The dried precipitate was dis-
solved in DMF to form a solution into which diethyl ether was
allowed to diffuse at room temperature. Crystals suitable for X-ray
measurement were obtained after several days.
1
ture. H NMR spectra were recorded with a Varian VR 300 MHz
spectrometer and 13C NMR spectra with a Mercury plus 400 MHz
NMR spectrometer with tetramethylsilane as internal standard
and DMSO-d6 as solvent.
Complex 1. Yield 68%. Anal. Calcd for C29H34Cl2CuN6O (%): C,
56.45; H, 5.55; N, 13.62. Found (%): C, 56.40; H, 5.58; N, 13.66.
IR (KBr; ν, cmꢁ1): 750 ν(o-Ar), 1293 ν(C–N), 1452 ν(C¼N),
1620 ν(C¼C). UV–visible (DMF, λmax, nm) (εmax (l molꢁ1 cmꢁ1)):
275 (1.60 × 104), 281 (1.54 × 104), 777 (286). Molar conductance
Electrolytic conductance measurements were made with a
DDS-307 type conductivity bridge using 3 × 10ꢁ3 mol dmꢁ3 solu-
tions in DMF at room temperature. Electrochemical measurements
were performed with an LK2005A electrochemical analyzer under
nitrogen at 293 K. A glassy carbon working electrode, a platinum-
wire auxiliary electrode and an Ag/AgCl reference electrode ([Clꢁ]
= 1.0 mol lꢁ1) were used in the three-electrode measurements.
The electroactive component was at a concentration of 1.0× 10ꢁ3
(ΛM, DMF): 13 S cm2 molꢁ1
.
Complex 2. Yield 70%. Anal. Calcd for C26H27Cl2MnN5 (%): C,
58.33; H, 5.08; N, 13.08. Found (%): C, 58.35; H, 5.10; N, 13.06. IR
(KBr; ν, cmꢁ1): 761 ν(o-Ar), 1267 ν(C–N), 1453 ν(C¼N), 1601
ν(C¼C). UV–visible (DMF, λmax, nm) (εmax (lmolꢁ1 cmꢁ1)): 279
(1.59 × 104), 285 (1.49 × 104). Molar conductance: (ΛM, DMF): 21
mol dmꢁ3 with tetrabutylammonium perchlorate (0.1 mol dmꢁ3
)
used as the supporting electrolyte in DMF solution. The synthetic
route to the free Etbba ligand is shown in Scheme 1.
S cm2 molꢁ1
.
Complex 3. Yield 81%. Anal. Calcd for C26H27Cl2N5Ni (%): C, 57.92;
H, 5.05; N, 12.99. Found (%): C, 57.90, H, 5.06; N, 13.00. IR (KBr;
ν, cmꢁ1): 759 ν(o-Ar), 1267 ν(C–N), 1454 ν(C¼N), 1595 ν(C¼C).
UV–visible (DMF, λmax, nm) (εmax (lmolꢁ1 cmꢁ1)): 275 (1.55 × 104),
.282 (1.43 × 104), 773 (4.73). Molar conductance (ΛM, DMF):
Synthesis of Compounds
Synthesis of Etbba
40 S cm2 molꢁ1
.
The precursor bba was synthesized according to the procedure
reported previously.[18,19] The Etbba ligand was synthesized using
a method analogous to the literature method.[18] Yield 61.3%; m.p.
240–242°C. Anal. Calcd for C26H27N5 (%): C, 76.25; H, 6.65; N, 17.10.
Found (%): C, 76.35; H, 6.50; N, 17.15. IR (KBr; ν, cmꢁ1): 745 ν(o-Ar),
1268 ν(C–N), 1460 ν(C¼N), 1599 ν(C¼C). 1H NMR (300 MHz, CDCl3,
δ, ppm): 7.73 (d, J= 7.3 Hz, 4H, H11′), 7.22 (d, J= 7.3 Hz, 4H, H12′),
7.10–6.90 (m, 5H, Ar–H), 4.87 (s, 2H, –CH2–N), 4.05–4.16 (m, 4H, –
CH2–Me), 1.21–1.28 (m, 6H, –CH3). 13C NMR (400 MHz, DMSO-d6, δ,
ppm): 153.85 (C4), 147.22 (C6), 138.54 (C10), 134.14 (C9) 129.13
(C2), 122.84 (C12), 119.36 (C1), 115.59 (C11), 113.47 (C3), 50.10 (C5),
34.60 (C7), 18.70 (C8). UV–visible (in DMF, λmax, nm): 280, 286. Molar
Antioxidant Activities
Hydroxyl radical scavenging activity
Hydroxyl radicals were generated in aqueous medium through the
Fenton-type reaction.[20,21] Stock solutions (3 × 10ꢁ3 M) of the
compounds were prepared in absolute DMF. The reaction mixture
(3 ml) contained 1.0 ml of 0.10 mmol aqueous safranin, 1 ml of
1.0 mmol aqueous EDTA–Fe(II), 1 ml of 3% aqueous H2O2 and a
series of quantitative microadditions of solutions of the test
compound. A sample without the test compound was used as
the control. The reaction mixtures were incubated at 37°C for
conductance (ΛM, DMF solution, 297 K): 3 S cm2 molꢁ1
.
30 min in a water bath. The scavenging effect for OHꢁ was calcu-
•
Preparation of complexes
lated from the following expression:
Three complexes were prepared using a similar procedure. To a
stirred solution of ligand Etbba (0.5 mmol, 204.7 mg) in hot EtOH
Asample ꢁ Ablank
Scavanging effect ð%Þ ¼
ꢂ100
Acontrol ꢁ Ablank
where Asample is the absorbance of the sample in the presence of
the test compound, Ablank is the absorbance of the blank in the
absence of the test compound and Acontrol is the absorbance in
the absence of the test compound and EDTA–Fe(II).[22]
Superoxide radical scavenging activity
Test solutions were prepared by diluting stock solution with DMF. A
non-enzymatic system containing 0.5 ml of 3.3 × 10ꢁ5 M vitamin B2,
1 ml of 2.3 × 10ꢁ4 M nitrotetrazolium blue chloride (NBT), 1 ml of
0.05 M methionine (MET) and 2.5 ml of 0.02 M phosphate buffer
(Na2HPO4–NaH2PO4, pH = 7.8) was used to pꢁroduce the superoxide
•
•
anion (O2ꢁ ), and the scavenging rate of O2 under the influence
of 0.3–3.0 μM of the test compound was determined by monitoring
the reduction in rate of transformation of NBT to monoformazan
dye.[23] The solutions of MET, vitamin B2 and NBT were prepared
in 0.02 M phosphate buffer (pH = 7.8) avoiding light. The
reactions were monitored at 560 nm with a UV–visible spectropho-
tometer, and the rate of absorption change was determined. The
Scheme 1. Synthesis of the Etbba ligand.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)

Antioxidative activity of Cu(II), Mn(II) and Ni(II) complexes
percentage inhibition of NBT reduction was calculated using the
Table 2. Selected bond distances (Å) and angles (°) for complexes 1-3
Complex 1
following equation[24]
:
ꢀ
ꢁ
Inhibition of NBT reduction ð%Þ ¼ 1 ꢁ k′=k ꢂ100
Cu1–N3
1.9668(16)
2.2534(7)
2.5320(16)
149.07(7)
99.30(5)
Cu1–N5
Cu1–Cl2
1.9729(16)
2.2999(6)
Cu1–Cl1
where k′ and k are the slopes of the straight lines of absorbance
values as a function of time in the presence and absence of
superoxide dismutase (SOD) mimic compound, respectively. The
IC50 values for the complexes were determined by plotting the
graph of percentage inhibition of NBT reduction against the
concentration of the complex. The concentration of the complex
which causes 50% inhibition of NBT reduction is reported as IC50.
Cu1–N1
N3–Cu1–N5
N5–Cu1–Cl1
N5–Cu1–Cl2
N3–Cu1–N1
Cl1–Cu1–N1
Complex 2
Mn1–N3
N3–Cu1–Cl1
N3–Cu1–Cl2
Cl1–Cu1–Cl2
N5–Cu1–N1
Cl2–Cu1–N1
99.26(5)
94.56(5)
126.41(3)
75.33(6)
121.99(4)
94.06(5)
74.91(6)
111.61(4)
2.165(3)
2.3312(18)
2.669(3)
110.11(8)
123.52(7)
99.32(9)
69.85(9)
89.62(7)
Mn1–N5
Mn1–Cl2
2.179(2)
X-Ray Crystallography
Mn1–Cl1
2.3865(17)
Mn1–N1
Diffraction intensities for complexes 1–3 were collected using a
Bruker Smart CCD diffractometer with graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. Data reduction and cell
refinement were performed using the SMART and SAINT
programs.[25] The structure was solved by direct methods and
refined by full-matrix least-squares against F2 of data using SHELXTL
software.[26] All H atoms attached to C atoms except for
DMF groups were fixed geometrically and treated as riding with
C–H =0.93 or 0.97 Å with Uiso(H) = 1.2Ueq(C). All H atoms attached
to N atoms were fixed geometrically and treated as riding with
N–H = 0.86 Å with Uiso(H)= 1.2Ueq(N). Crystal data and details of
the refinement for complexes 1–3 are given in Table 1. Selected
bond lengths and angles are presented in Table 2.
N3–Mn1–N5
N5–Mn1–Cl1
N5–Mn1–Cl2
N3–Mn1–N1
Cl1–Mn1–N1
Complex 3
Ni1–N3
N3–Mn1–Cl1
N3–Mn1–Cl2
Cl1–Mn1–Cl2
N5–Mn1–N1
Cl2–Mn1–N1
111.64(8)
105.34(8)
104.39(6)
71.11(9)
165.93(5)
2.024(3)
2.2732(12)
2.351(3)
Ni1–N5
Ni1–Cl1
2.054(3)
Ni1–Cl2
2.3125(11)
Ni1–N1
N3–Ni1–N5
N5–Ni1–Cl2
N5–Ni1–Cl1
N3–Ni1–N1
Cl2–Ni1–N1
111.01(12)
140.90(9)
95.24(9)
N3–Ni1–Cl2
N3–Ni1–Cl1
Cl2–Ni1–Cl1
N5–Ni1–N1
Cl1–Ni1–N1
101.78(10)
102.19(9)
98.14(4)
76.47(10)
90.54(8)
77.46(11)
171.30(8)
Results and Discussion
aprotic solvents such as DMF, DMSO and MeCN, slightly soluble in
ethanol, water, ethyl acetate and chloroform, and insoluble in
Et2O and petroleum ether. The molar conductivities of complexes
1–3 in DMF solution indicate that they are non-electrolytes.
The synthetic route to the free Etbba ligand is shown in Scheme 1.
The complexes were prepared by reaction of the Etbba ligand with
CuCl2ꢀ4H2O, MnCl2ꢀ2H2O or NiCl2ꢀ6H2O in ethanol. All compounds
are stable under atmospheric conditions. They are soluble in polar
Table 1. Crystal and structure refinement data for complexes 1–3
1
2
3
Molecular formula
Molecular weight
Crystal system
Space group
a (Å)
C
29H34Cl2CuN6O
C26H27Cl2MnN5
535.37
C26H27Cl2N5Ni
539.14
617.06
Monoclinic
P21/c
Monoclinic
P21/n
Monoclinic
P21/n
9.0532(9)
14.5584(15)
22.421(2)
92.917(10)
2951.2(5)
4
12.929(13)
14.076(14)
4.464(14)
12.617(2)
b (Å)
14.524(3)
c (Å)
14.132(3)
β (°)
103.504(11)
2559(4)
104.600(2)
2506.3(8)
V (Å3)
Z
4
4
D (calculated; g cmꢁ3
F(000)
)
1.389
1.389
1.429
1284
1108
1120
Crystal size (mm)
0.30 × 028 × 0.24
1.7 to 25.5
19 897
0.29 × 0.25 × 0.21
1.9 to 25.5
17 205
0.29 × 0.26 × 0.23
1.9 to 25.5
16 978
θ range for data collection (°)
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
5441 (Rint = 0.022)
5441/0/356
1.05
4741 (Rint = 0.035)
4741/0/309
1.04
4662 (Rint = 0.079)
4662/0/309
1.02
Reflections with I > 2σ(I)
Final R1, wR2 indices (I > 2σ(I))
R1, wR2 indices (all data)
4587
3535
2825
R1 = 0.030, wR2 = 0.078
R1 = 0.039, wR2 = 0.083
R1 = 0.035, wR2 = 0.088
R1 = 0.055, wR2 = 0.099
R1 = 0.046, wR2 = 0.092
R1 = 0.098, wR2 = 0.111
Appl. Organometal. Chem. (2015)
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H. Wu et al.
IR and UV–Visible Spectra
crowding. The equatorial plane is occupied by two N atoms of the
Etbba ligand and two chloride anions. The axial position is occupied
by N1, with Cu1–N1 of 2.5320(16) Å being the longest Cu–N bond
(Table 2).
The IR spectral data for the complexes along with their assignments
are given in Table 3. In the spectrum of the free ligand, a strong band
is found around 1268 cmꢁ1 along with another less strong band
around 1460 cmꢁ1. By analogy with the spectrum of imidazole,
the former is attributed to ν(C–N) and the latter to ν(C¼N).[27,28]
The first shifts by about 20 cmꢁ1 and the second by about 7 cmꢁ1
in the spectra of the complexes, which implies direct coordination
of the imine nitrogen atoms to the metal. These are the preferred ni-
trogen atoms for coordination, as found in other metal complexes
with benzimidazoles.[29] This fact agrees with the X-ray diffraction
results (see below).
Crystal structure of complex 2
The metal atom in 2 (Fig. 2) is coordinated within a Cl2N3 donor set as
for 1 but the coordination geometry more resembles distorted trigo-
nal bipyramidal (τ = 0.70).[32] The axial sites are occupied by N1 and
Cl2, with Cl2–Mn1–N1 = 165.93(5)°. The trigonal plane is occupied
by the two ligating N atoms of the benzimidazolyl groups and a chlo-
ride atom, namely atoms N3/N5/Cl1.
DMF solutions of the Etbba ligand and its complexes show, as
expected, almost identical UV spectra. The UV bands of Etbba (279,
286 nm) are only marginally blue-shifted (2–5 nm) in the spectra of
the complexes, which is evidence of C¼N coordination to the metal
center. These bands are assigned to n →π* and π →π* (imidazole)
transitions.[30,31] Complexes 1 and 3 exhibit one absorption band,
which is assigned to d →d transition.
Crystal structure of complex 3
In complex 3 (Fig. 3), a similar mode of coordination of Etbba is
found as described for 1 and 2. The coordination geometry is
intermediate between the extremes with τ = 0.51.[33]
Electrochemical Studies
The electrochemical properties of the three complexes were
studied using cyclic voltammetry in DMF. Only complex 1
exhibits a pair of cathodic and anodic waves. The data are collected
in Table 4 and an example voltammogram is shown in Fig. 4.
The separation between the cathodic and anodic peak potentials,
ΔEp, and the current ipa/ipc indicate a quasi-reversible redox process
assignable to the Cu(II)/Cu(I) couple.[34] The neutral free Etbba
ligand proves not to be electroactive over the range ꢁ1.2 to +1.2 V.
Description of Structures
Crystal structure of complex 1
The ORTEP representation of the structure of complex 1, including
atom numbering scheme, is shown in Fig. 1 and selected bond
lengths and bond angles are listed in Table 2. The asymmetric unit
comprises neutral [Cu(Etbba)Cl2] and a DMF molecule. The Cu(II)
center is five coordinate within a Cl2N3 chromophore. The Etbba
ligand acts as a tridentate N-donor, and two chloride atoms
complete the coordination. The coordination of the Cu(II)
atom may be best described as distorted square-based pyramidal
(τ = 0.38). The parameter τ is defined as (β ꢁ α)/60 (where β = N3–
Cu1–N5, α = Cl1–Cu1–Cl2) and its value varies from 0 (in regular
square-pyramidal geometry) to 1 (trigonal bipyramidal).[32] This ge-
ometry is assumed by the Cu(II) center so as to relieve steric
Table 3. IR bands (cmꢁ1) of compounds
Compound
ν(Ar)
ν(C–N)
ν(C¼N)
ν(C¼C)
Etbba
745
750
761
759
1268
1293
1267
1267
1460
1452
1453
1454
1599
1620
1601
1613
Figure 2. Molecular structure of complex 2 with hydrogen atoms omitted
for clarity.
1
2
3
Figure 3. Molecular structure of complex 3 showing displacement
ellipsoids at the 30% probability level. Hydrogen atoms have been
omitted for clarity.
Figure 1. Molecular structure and atom numberings of complex 1 with
hydrogen atoms omitted for clarity.
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Appl. Organometal. Chem. (2015)

Antioxidative activity of Cu(II), Mn(II) and Ni(II) complexes
Table 4. Electrochemical data for complex 1a
singlet oxygen cannot be formed. The redox potential of 0.1067 V
shows that complex 1 has SOD activity.
Complex
Epc
Epa
ΔEp
E1/2
ipa
ipc
I
1
0.2748 0.3815 0.1067 0.32815 3.3 4.9 0.675
Antioxidant Activities
aΔE = Epa ꢁ Epc; E1/2 = (Epa + Epc)/2; scan rate = 0.05 V sꢁ1; I = ipa/ipc
.
Generation of reactive oxygen species is a normal process in the life
•
ꢁ
•
of aerobic organisms. OHꢁ and O2 are two clinically important
reactive oxygen species in the human body.[36] They are produced
in most organ systems and participate in various physiological and
pathophysiological processes such as carcinogenesis, aging, viral
infection, inflammation and others.[37] Consequently, in this paper,
the ligand Etbba and its complexes were studied for their
antioxidant activity by comparing their scavenging effects on
•
ꢁ
•
OHꢁ and O2
.
Hydroxyl radical scavenging activity
We first compared the abilities of the present compounds to
scavenge hydroxyl radicals with those of the well-known natural
antioxidants mannitol and vitamin C, using the same method as
reported in a previous paper.[17,38] The 50% inhibitory concentra-
tion (IC50) values of mannitol and vitamin C are about 9.6 × 10ꢁ3
and 8.7× 10ꢁ3 M, respectively. Figure 5 shows plots of hydroxyl
radical scavenging effect (%) for Etbba and its complexes.
Figure 4. Cyclic voltammogram of complex 1 recorded with a platinum
electrode in DMF solution containing tetrabutylammonium perchlorate (0.1 M).
The IC50 values of complexes 1, 2 and 3 are (5.61 0.045) × 10ꢁ6
,
(8.21 0.069) × 10ꢁ5 and (9.28 0.022) × 10ꢁ5 M, respectively;
Etbba does not have activity. The results indicate that the hy-
droxyl radical scavenging activity of three complexes follows
the order 1 > 2 > 3, which implies that these complexes exhibit
According to previous reports,[35] a transition metal complex
must have a redox potential below 0.65 V (E°(O2–O2ꢁ)) and above
ꢁ0.33 V (E°(O2–O2ꢁ)) in order to be an effective mimic of SOD, so toxic
•
Figure 5. Inhibitory effect of (a) Etbba, and complexes (b) 1, (c) 2 and (d) 3 on OHꢁ radicals; the suppression ratio increases with increasing
concentration of the test compounds.
Appl. Organometal. Chem. (2015)
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H. Wu et al.
structure of 2 may be considered distorted trigonal bipyramidal, with
3 having an intermediate coordination geometry. Electrochemical
studies show quasi-reversible redox behavior for 1. Furthermore,
1–3 exhibit good hydroxyl radical scavenging activity, and 1 and 2
display excellent antioxidant activity for superoxide radical. These
findings clearly indicate that transition metal complexes with benz-
imidazole have many potential applications, which warrants further
in vivo experiments and perhaps pharmacological assays.
Acknowledgments
This research was supported by the National Natural Science
Foundation of China (grant no. 21367017), the Fundamental
Research Funds for the Gansu Province Universities (212086),
Natural Science Foundation of Gansu Province (grant no.
1212RJZA037) and ‘Qing Lan’ Talent Engineering Funds for
Lanzhou Jiaotong University.
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