Synthesis, crystal structures, and superoxide dismutase activity of two isostructural copper(II)-zinc(II) complexes derived from N,N′-bis(4-methoxysalicylidene) cyclohexane-1,2-diamine

Article abstract of DOI:10.1080/00958970903515134

Two new isostructural copper(II)-zinc(II) complexes, [CuZnLBr₂] (1) and [CuZnLCl₂] (2) (H₂L= N,N′-bis(4-methoxysalicylidene)cyclohexane-1,2-diamine), have been synthesized and characterized by elemental analyses, infrared

Full text of DOI:10.1080/00958970903515134

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Synthesis, crystal structures, and
superoxide dismutase activity of
two isostructural copper(II)–zinc(II)
complexes derived from N,N′-bis(4-
methoxysalicylidene)cyclohexane-1,2-
diamine
Zhong-Lu You a , Li-Li Ni a , Peng Hou a , Ji-Cai Zhang a & Che
Wang a
a Department of Chemistry and Chemical Engineering , Liaoning
Normal University , Huanghe Road 850#, Dalian 116029, P.R. China
Published online: 18 Jan 2010.
To cite this article: Zhong-Lu You , Li-Li Ni , Peng Hou , Ji-Cai Zhang & Che Wang (2010) Synthesis,
crystal structures, and superoxide dismutase activity of two isostructural copper(II)–zinc(II)
complexes derived from N,N′-bis(4-methoxysalicylidene)cyclohexane-1,2-diamine, Journal of
Coordination Chemistry, 63:3, 515-523, DOI: 10.1080/00958970903515134
To link to this article: http://dx.doi.org/10.1080/00958970903515134
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Journal of Coordination Chemistry
Vol. 63, No. 3, 10 February 2010, 515–523
Synthesis, crystal structures, and superoxide dismutase
activity of two isostructural copper(II)–zinc(II)
complexes derived from N,N0-bis(4-methoxysalicylidene)
cyclohexane-1,2-diamine
ZHONG-LU YOU*, LI-LI NI, PENG HOU,
JI-CAI ZHANG and CHE WANG
Department of Chemistry and Chemical Engineering, Liaoning Normal University,
Huanghe Road 850#, Dalian 116029, P.R. China
(Received 14 February 2009; in final form 21 September 2009)
Two new isostructural copper(II)–zinc(II) complexes, [CuZnLBr2] (1) and [CuZnLCl2] (2)
(H2L¼ N,N0-bis(4-methoxysalicylidene)cyclohexane-1,2-diamine), have been synthesized and
characterized by elemental analyses, infrared spectroscopy, and single-crystal X-ray diffraction.
Both complexes crystallize in the P-1 space group. The Cu in each complex is four-coordinate
square planar with two imines and two phenolates of L. The Zn in each complex is
four-coordinate tetrahedral with two phenolates of L and two halides (Br for 1 and Cl for 2).
The superoxide dismutase (SOD) activity of the complexes indicates that both complexes are
rudimentary models for SOD.
Keywords: Schiff base; Copper; Zinc; Crystal structure; Superoxide dismutase
1. Introduction
Dinuclear complexes particularly heterobimetallic systems are of special interest
because of their relevance to physics, chemistry, and biology [1]. Superoxide dismutase
(SOD) is an important enzyme in biological systems, which catalyzes the dismutation
of the superoxide radical to hydrogen peroxide and molecular oxygen [2, 3].
Crystallographic studies show that the Cu–Zn SOD contains an imidazolato-bridged
copper(II) and zinc(II) in the active site [4]. Coordination is distorted square pyramidal
for copper(II) and tetrahedral for zinc(II). Considering that heterodinuclear Cu–Zn
complexes are very rare to date [5–7], in this article, two isostructural dinuclear Cu–Zn
complexes, [CuZnLBr2] (1) and [CuZnLCl2] (2) (H2L¼ N,N0-bis(4-methoxysalicylidene)
cyclohexane-1,2-diamine), have been synthesized and structurally characterized.
*Corresponding author. Email: youzhonglu@yahoo.com.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958970903515134

516
Z.-L. You et al.
The SOD activity of the complexes was tested. To our knowledge, the complexes
derived from the Schiff-base H2L have not been reported.
1
2
3
4
5
6
7
8
N
N
17
18
14
13
9
16
OH
H O
10
11
21
19
O
20
12
O
15
22
H L
2
2. Experimental
2.1. Materials and methods
All chemicals (reagent grade) were commercially available and used without purifica-
tion. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240C
elemental analyzer. Infrared (IR) spectra were recorded on a Nicolet AVATAR 360
spectrometer as KBr pellets from 4000 to 400 cmꢀ1. 1H NMR spectra were recorded on
a Mercury Vx300 spectrometer (300 MHz) at room temperature using CDCl3 as
solvent and tetramethylsilane as internal standard. Molar conductance was measured
with a Shanghai DDS-11A conductometer. Electronic absorption spectra were
recorded on acetonitrile solutions of the complexes on a Shimadzu UV-Vis 160
spectrophotometer.
2.2. Synthesis of H2L
To methanol solution (50 cm3) of 4-methoxysalicylaldehyde (152.1 mg, 1.0 mmol) was
added a methanol solution (30 cm3) of cyclohexane-1,2-diamine (57.1 mg, 0.5 mmol)
with stirring. The mixture was stirred for 10 min at room temperature to give a yellow
precipitate. The product was filtered, washed three times with cold methanol, and dried
in air. Yield: 96%. Anal. Calcd for C22H26N2O4 (%): C, 69.1; H, 6.8; N, 7.3. Found:
1
C, 68.7; H, 6.9; N, 7.2. H NMR (CDCl3, 300 MHz) (ppm): ꢀ ¼ 12.03 (b, OH); 8.23
(s, 2H, CH¼N); 7.23 (d, 2H, Ar–H); 6.45 (d, 2H, Ar–H); 6.33 (s, 2H, Ar–H); 3.72
(s, 6H, OCH3); 3.37 (m, 2H, chiral H); 1.83–1.40 (m, 8H, (CH2)4). 13C NMR (CDCl3,
300 MHz) (ppm): ꢀ ¼ 22.3 (C1, C2), 29.7 (C3, C4), 55.3 (C15, C22), 62.3 (C5, C6), 98.7
(C11, C20), 104.5 (C13, C18), 118.2 (C9, C16), 127.7 (C14, C17), 160.3 (C10, C21),
164.1 (C7, C8), 167.2 (C12, C19).

Copper(II)–zinc(II) complexes
517
2.3. Synthesis of 1
To methanol solution (10 cm3) of H2L (38.2 mg, 0.1 mmol) was added a methanol
solution (10 cm3) of Cu(ClO4)2 ꢁ 6H2O (37.0 mg, 0.1 mmol) and ZnBr2 (22.5 mg,
0.1 mmol) with stirring. The mixture was stirred for 30 min at room temperature
to give a clear blue solution. After keeping the solution in air for a week, blue
block-shaped crystals of 1, suitable for X-ray crystal structural determination, formed
at the bottom of the vessel on slow evaporation of the solvent. The crystals were
isolated, washed three times with methanol, and dried in air. Yield: 72%. Anal. Calcd
for C22H24Br2CuN2O4Zn (%): C, 39.5; H, 3.6; N, 4.2. Found: C, 40.1; H, 3.8; N, 4.0.
2.4. Synthesis of 2
Complex 2 was synthesized by similar method as described for 1, with ZnBr2 replaced
by ZnCl2 (13.7 mg, 0.1 mmol). Blue block-shaped crystals of 2, suitable for X-ray crystal
structural determination, were formed at the bottom of the vessel on slow evaporation
of the solvent. The crystals were isolated, washed three times with methanol, and dried
in air. Yield: 63%. Anal. Calcd for C22H24Cl2CuN2O4Zn (%): C, 45.5; H, 4.2; N, 4.8.
Found: C, 45.0; H, 4.2; N, 4.7.
2.5. X-ray structural determination
Diffraction intensities for 1 and 2 were collected at 298(2) K using a Bruker SMART
˚
CCD area detector with Mo-Ka radiation (ꢁ ¼ 0.71073 A). The collected data were
reduced using SAINT [8] and empirical absorption corrections were performed using
SADABS [9]. The structures were solved by direct methods and refined against F2 by
full-matrix least-squares using the SHELXTL package [10]. All non-hydrogen atoms
were refined anisotropically. H atoms were placed in calculated positions and
constrained to ride on their parent atoms. The crystallographic data for the two
complexes are summarized in table 1. Selected bond lengths and angles are given in
table 2.
2.6. SOD measurement
The SOD activity (the concentration of the complexes corresponding to 50% inhibition)
of the complexes was measured by the Beauchamp–Fridovich assay [11]. Superoxide
anions were produced from the riboflavin/methionine system. The indicator utilized in
this case is nitroblue tetrazolium (NBT), which reacts with Oꢀ2 to form blue formazane.
Cells containing 3.3 ꢂ 10ꢀ6 M riboflavin, 0.01 M methionine, 4.6 ꢂ 10ꢀ5 M NBT, and
0.05 M phosphate buffer pH 7.4, and 10ꢀ7–10ꢀ6 M complex or 2 ꢂ 10ꢀ9–2 ꢂ 10ꢀ8 M
native Cu–Zn SOD, were illuminated under fluorescent lamps. The absorbance at
560 nm increased linearly with time of illumination. The reduction of NBT was
measured in terms of increased absorbance at 560 nm on a Shimadzu UV-240
spectrophotometer. All photo-induced reactions were performed at 25ꢃC. Each test was
performed in triplicate.

518
Z.-L. You et al.
Table 1. Crystallographic and experimental data for 1 and 2.
Complex
1
2
Emperical formula
Formula weight
Temperature (K)
Crystal shape/color
Crystal system
Space group
C
669.16
298(2)
Block/Blue
Triclinic
P-1
22H24Br2CuN2O4Zn
C22H24Cl2CuN2O4Zn
580.24
298(2)
Block/Blue
Triclinic
P-1
ꢃ
˚
Unit cell dimensions (A,
)
a
b
c
ꢂ
ꢃ
ꢄ
7.888(1)
11.980(2)
12.877(2)
81.656(2)
83.758(3)
78.728(3)
1176.7(3), 2
1.889
7.881(1)
11.676(1)
12.794(1)
81.262(2)
84.263(2)
79.094(2)
1139.5(2), 2
1.691
3
˚
Volume (A ), Z
Calculated density (g cmꢀ3
)
Absorption coefficient m (Mo-Ka) (mmꢀ1
F(000)
)
5.350
662
2.252
590
Crystal size (mm3)
0.30 ꢂ 0.30 ꢂ 0.28
0.21 ꢂ 0.20 ꢂ 0.18
Measured reflections
Independent reflections
Observed reflections [I ꢄ 2ꢅ(I)]
Restraints
6713
4893 [R(int) ¼ 0.0243]
6838
4775 [R(int) ¼ 0.0110]
3313
0
291
4052
0
291
Parameters
Min. and max. transmission
Goodness-of-fit on F2
Final R indices, [I ꢄ 2ꢅ(I)]a
R indices (all data)a
0.297 and 0.316
1.037
R1 ¼ 0.0435, wR2 ¼ 0.0941
R1 ¼ 0.0760, wR2 ¼ 0.1059
0.729 and ꢀ0.566
0.649 and 0.687
1.053
R1 ¼ 0.0287, wR2 ¼ 0.0716
R1 ¼ 0.0359, wR2 ¼ 0.0753
0.493 and ꢀ0.494
ꢀ3
˚
Largest difference peak and hole (e A
)
P
P
aR1 ¼ Fo ꢀ Fc/Fo, wR2 ¼ [ wðFo2 ꢀ Fc2Þ/ w(Fo2)2]1/2
,
w1 ¼ [ꢅ2ðFo2Þ þ (0.0452ðFo2 þ 2Fc2Þ/3)2]ꢀ1
,
w2 ¼ [ꢅ2ðFc2Þ þ (0.0373ðFo2þ
2Fc2Þ/3)2 þ 0.4426ðFo2 þ 2Fc2Þ/3]ꢀ1
.
ꢃ
˚
Table 2. Selected bond lengths (A) and angles ( ) for 1 and 2.
1
Cu1–O1
Cu1–N1
Zn1–O1
Zn1–Br1
Cu1–Zn1
1.914(3)
1.896(3)
2.030(3)
2.322(1)
2.998(1)
Cu1–O2
Cu1–N2
Zn1–O2
Zn1–Br2
1.893(3)
1.907(4)
2.027(3)
2.323(1)
O2–Cu1–N1
N1–Cu1–N2
N1–Cu1–O1
O2–Zn1–O1
O1–Zn1–Br1
O1–Zn1–Br2
171.7(2)
O2–Cu1–N2
O2–Cu1–O1
N2–Cu1–O1
O2–Zn1–Br1
O2–Zn1–Br2
Br1–Zn1–Br2
95.5(2)
83.4(2)
171.9(2)
112.2(1)
112.7(1)
117.9(1)
86.3(2)
95.9(2)
77.3(2)
112.2(1)
117.7(1)
2
Cu1–O1
Cu1–N1
Zn1–O1
Zn1–Cl1
Cu1–Zn1
O1–Cu1–N2
N2–Cu1–O2
N2–Cu1–N1
O1–Zn1–O2
O2–Zn1–Cl2
O2–Zn1–Cl1
1.898(2)
Cu1–O2
Cu1–N2
Zn1–O2
Zn1–Cl2
1.912(2)
1.901(2)
2.040(2)
2.193(1)
1.913(2)
2.035(2)
2.193(1)
3.003(1)
170.5(1)
O1–Cu1–O2
O1–Cu1–N1
O2–Cu1–N1
O1–Zn1–Cl2
O1–Zn1–Cl1
Cl2–Zn1–Cl1
83.7(1)
95.5(1)
170.4(1)
113.4(1)
111.9(1)
118.3(1)
96.0(1)
86.4(1)
77.2(1)
111.0(1)
118.2(1)

Copper(II)–zinc(II) complexes
519
3. Results and discussion
Both complexes were synthesized by reaction of H2L with copper(II) perchlorate and
zinc(II) halide. The perchlorate anion does not coordinate, leading to formation of
square planar Cu. The phenolate O atoms are good bridging groups [12, 13] and further
coordinate to Zn forming dinuclear complexes. The halides complete the tetrahedral
coordination of Zn. Both isostructural complexes are stable at room temperature,
soluble in methanol, ethanol, and acetonitrile.
3.1. Spectroscopic study
IR spectra of H2L and the two complexes provide information about metal–ligand
bonding. Assignments are based on typical group frequencies. Weak and broad
absorptions at 3433 cmꢀ1 substantiate the presence of phenol in H2L, which disappear
in the complexes. The strong absorption at 1637 cmꢀ1 in the spectrum of H2L assigned
to the azomethine ꢆ(C¼N) [14] shifts to lower wave number in the complexes,
1617 cmꢀ1 for 1, and 1616 cmꢀ1 for 2, attributed to coordination of nitrogen to Cu. The
strong absorption of Ar–O in H2L at 1232 cmꢀ1 [15] is located at lower frequencies
for the complexes, namely 1203 cmꢀ1 for 1 and 1202 cmꢀ1 for 2. Weak absorptions
indicative of Zn–Br in 1 and Zn–Cl in 2 are at 246 and 293 cmꢀ1, respectively.
Both complexes show two typical absorption bands at about 640 and 490 nm in the
visible electronic spectra. The first corresponds to typical d–d band in a square planar
coordination around copper(II), which is in agreement with the crystal structures of
the complexes. The second could be attributed to a ligand-to-metal charge transfer
transition (LMCT).
3.2. Structure description of 1 and 2
The single-crystal X-ray diffraction shows that 1 and 2 are isostructural dinuclear
copper(II)–zinc(II) complexes (figures 1 and 2). The dihedral angles between the
corresponding benzene rings are 25.3(3)ꢃ in 1 and 27.0(3)ꢃ in 2. Both cyclohexyl rings in
the complexes adopt chair conformations. The Cu ꢁ ꢁ ꢁ Zn distances are 2.998(1) and
˚
3.003(1) A in 1 and 2, respectively.
The Cu in each complex is coordinated by two imine nitrogens and two phenolate
oxygens of L, forming a slightly distorted square plane. The two trans angles are 171.7(2)ꢃ
and 171.9(2)ꢃ in 1, and 170.4(1)ꢃ and 170.5(1)ꢃ in 2. The Cu lies in the least-squares
plane. The coordinate bond lengths related to Cu are comparable to each other and also
comparable to corresponding values observed in similar complexes [16–18].
The Zn in each complex is coordinated by two phenolate oxygens of L and two
halides (Br for 1 and Cl for 2), forming a slightly distorted tetrahedral coordination.
The Br1–Zn1–Br2 plane in 1 and the Cl1–Zn1–Cl2 plane in 2 are nearly perpendicular
to the CuL units, which can decrease the steric effects between them. The Zn–N bond
lengths in both complexes are comparable to each other, but the Zn–Br bonds in 1 are
much longer than the Zn–Cl bonds in 2. All the coordinate bond lengths are
comparable to corresponding values observed in similar complexes [16–18].

520
Z.-L. You et al.
Figure 1. The structure of 1, showing the atom-numbering scheme. Displacement ellipsoids are drawn at
30% probability and H atoms are shown as small spheres of arbitrary radii.
Figure 2. The structure of 2, showing the atom-numbering scheme. Displacement ellipsoids are drawn at
30% probability and H atoms are shown as small spheres of arbitrary radii.
3.3. SOD activity
Schiff-base metal complexes have been widely investigated [19, 20]. Most Schiff bases
and their complexes were investigated for biological activities, such as antibacterial
[21, 22], cytotoxic [22], antifungal [23], and urease inhibition [24]. Further, a number of
copper(II) complexes with Schiff bases have been investigated for their SOD activities
[23, 25, 26]. However, Cu–Zn complexes with Schiff bases have seldom been reported
for SOD activities.
The SOD activities of the two complexes in the present work are similar. The
percentage of inhibition against complex concentration was plotted in figure 3.

Copper(II)–zinc(II) complexes
521
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
Concentration (µM)
Figure 3. Percentage inhibition of NBT reduction plotted against the concentration of the complexes
(g for 1, m for 2). Each point represents the mean ꢅ SD of triplicate determination.
Table 3. SOD activities as IC50 for test materials.
Complexa
IC50 (mM)
Reference
1
2
3.506
3.127
6.15
0.17
3.16
2.25
3.9
This work
This work
[26]
[Cu(C9H7NO3)(C12H8N2)]
[Cu(N-meim)6](sal)2
[Cu(en)2](sal)2
[Cu(Pu-6-MePy)(H2O)](ClO4)2
SALCuCl2
Cu–Zn SOD
[28]
[29]
[30]
[31]
0.005
This work
aC9H7NO3 ¼ salicylideneglycine; C12H8N2 ¼ 1,10-phenanthroline; N-meim ¼ N-methylimidazole;
sal ¼ salicylate; en ¼ ethylenediamine; SALH2 ¼ 40,50-bis(salicylideneimino)benzo-15-crown-5.
The values corresponding to IC50 were 3.506 mM for 1 and 3.127 mM for 2. Native
Cu–Zn SOD was used as a reference, with IC50 of 0.005 mM. Although the two
complexes showed SOD activity, they are much weaker than native Cu–Zn SOD. The
difference of the SOD activities among the complexes and the native SOD results from
structural features. It is well-known that the active center of the native Cu–Zn SOD
consists of copper(II) and zinc(II) ions bridged by a histidyl imidazolate anion. One
water and three L-histidine molecules further coordinate copper(II) ion. Beside the
imidazolato bridge the zinc(II) ion is bonded to an L-aspartic acid and two L-histidine
molecules. The two complexes reported in this article are also Cu–Zn heterometallic
complexes; however, the copper(II) and zinc(II) ions are bridged by two phenolate O
atoms, and there are many other differences when compared with the native Cu–Zn
SOD. Even though the native Cu–Zn SOD shows excellent activities, it is quite
sensitive. Changes in temperature, solvating properties, etc. may easily lead to
denaturation (frequently irreversibly). The two complexes reported in this article are
very stable in solutions, the molar conductance values of the complexes measured in
distilled water (containing small quantity of DMSO) at the concentration of
1.0 ꢂ 10ꢀ3mol Lꢀ1 are 15.7 and 17.2 mS cmꢀ1 for 1 and 2, respectively, indicating the
non-electrolytic nature of the complexes [27].
For comparison, the IC50 values of some copper(II) complexes and the native Cu–Zn
SOD were also summarized in table 3. It can be seen that the IC50 values of the two

522
Z.-L. You et al.
complexes in this article are comparable or superior to those reported in the literature;
however, they are significantly less active than the native enzyme. Nevertheless, the two
complexes are potent SOD mimics considering their very low molecular weight
compared with that of the native enzyme.
4. Conclusions
Two isostructural heterodinuclear copper(II)–zinc(II) complexes were synthesized and
structurally characterized. Both complexes showed weak SOD activities, but are much
more stable in solutions than native Cu–Zn SOD. Further work needs to be done to
improve the mimic models of the native Cu–Zn SOD.
Supplementary material
Crystallographic data for the complexes have been deposited with the Cambridge
Crystallographic Data Center (CCDC 720314 for 1 and 720315 for 2).
Acknowledgements
This work was financially supported by the Education Office of Liaoning Province
(Project No. 2008T107).
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