Synthesis, structure and catalase-like activity of new dicopper(II) complexes with phenylglyoxylate and benzoate ligands

Article abstract of DOI:10.1016/j.molcata.2005.04.006

The preparation and characterization of dimeric Cu₂(bpy) ₂(phga)₄ (bpy: 2,2′-bipyridine; phgaH: phenylglyoxylic acid) and Cu₂(ba)₄(bpy)₂ (baH: benzoic acid) complexes are described. Crystallographic characterization of the Cu₂(bpy)₂(phga)₄ complex has shown that the coordination geometry around copper(II) ions is distorted square pyramidal (monoclinic, P21/c, a = 11.7440(4) ?, b = 20.9020(10) ?, c = 9.3980(10) ?, α = 90.00°, β = 93.2240(10)°, γ = 90.00°, V = 2303.3(3) ?³, Z = 4). These complexes were found to exhibit catalase-like activity.

Full text of DOI:10.1016/j.molcata.2005.04.006

Journal of Molecular Catalysis A: Chemical 236 (2005) 12–17
Synthesis, structure and catalase-like activity of new dicopper(II)
complexes with phenylglyoxylate and benzoate ligands
a
b
a,b,∗
c
c
´
´
´
´
, Michel Giorgi , Marius Reglier
Jozsef Kaizer , Robert Csonka , Gabor Speier
a
Research Group for Petrochemistry, Hungarian Academy of Sciences, 8200 Veszpre´m, Hungary
b
Department of Organic Chemistry, University of Veszpre´m, 8200 Veszpre´m, Hungary
c
Laboratoire de Cristallochimie et Laboratoire de Bioinorganique Structurale, Universite´ Paul Ce´zanne Aix-Marseille III,
F.S.T. Saint-Je´roˆme, Service 432, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France
Received 16 February 2005; received in revised form 6 April 2005; accepted 6 April 2005
Available online 12 May 2005
Abstract
The preparation and characterization of dimeric Cu2(bpy)2(phga)4 (bpy: 2,2ꢀ-bipyridine; phgaH: phenylglyoxylic acid) and Cu2(ba)4(bpy)2
(baH: benzoic acid) complexes are described. Crystallographic characterization of the Cu2(bpy)2(phga)4 complex has shown that the coordina-
˚
˚
˚
tion geometry around coppe◦r(II) ions is distorted square pyramidal (monoclinic, P21/c, a = 11.7440(4) A, b = 20.9020(10) A, c = 9.3980(10) A,
◦
◦
3
˚
α = 90.00 , β = 93.2240(10) , γ = 90.00 , V = 2303.3(3) A , Z = 4). These complexes were found to exhibit catalase-like activity.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Copper; Catalase; Kinetics; Carboxylates; X-ray structure
1. Introduction
in reducing oxidative stress. Catalase and glutathione perox-
idase serve this purpose. A number of manganase containing
catalases have been isolated and characterized [6–8].
The direct utilization of this natural enzyme as a phar-
maceutical agent is limited because of low membrane per-
meability as a consequence of its high molecular weight. So
considerable efforts were made in order to obtain non-toxic,
low molecular weight biomimetic molecules, which are able
to catalyze the dismutation of superoxide anion and/or de-
stroy the forming hydrogen peroxide.
A variety of low molecular weight complexes of transi-
tion metals, especially those of manganase, were prepared
and studied as Mn-catalase mimetic complex systems [9–18].
Examples of such complexes include derivatives of anti-
inflammatory drugs salicylates, aminoacids, peptides, and
amines. Compared with Mn2(II) model complex systems,
Cu2(II) systems reported are relatively rare [19–21]. Sev-
eral binuclear copper(II) carboxylates of anti-inflammatory
drugs such as salicylates [22], indomethacin [23], and lona-
zolac [24] were studied as SOD mimics, but none of simple
carboxylates such as benzoate and phenylglyoxylate. In ad-
dition, it is known that the presence of coordination sites
Oxidative stress has been defined as ‘a disturbance in the
pro-oxidant–antioxidant balance in favor of the former, lead-
ing to potential damage’ [1]. The major oxidative stress pro-
duced by superoxide, however, is derived from its partici-
pation in peroxynitrite formation [2] and its involvement in
the iron-catalyzed Haber–Weiss reaction (superoxide-driven
Fenton chemistry [3]) causing hydrogen peroxide to be con-
verted to hydroxyl radical. Hydroxyl radical, peroxynitrite
and peroxynitrite-derived products (hydroxyl radical, car-
bonate radical and nitrogen dioxide) all have the potential to
react with and damage most cellular targets including lipids,
proteins, and DNA [4]. Enzymatic antioxidants regulate su-
peroxide concentration by dismutation of superoxide to hy-
drogen peroxide (superoxide dismutase [5]), which is then
converted to water (peroxidases such as glutathione perox-
idase) or dismutated to water and dioxygen. Elimination of
hydrogen peroxide is therefore critical to the efficacy of SOD
∗
Corresponding author. Tel.: +36 88 422 0224657; fax: +36 88 427 492.
E-mail address: speier@almos.vein.hu (G. Speier).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.04.006

J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 12–17
13
73% (753 mg). Anal. Calcd. for Cu2O12C52H36N4: C, 60.3;
H, 3.5; N, 5.4. Found: C, 59.6; H, 3.7; N, 5.6.
2.4. Synthesis of Cu2(ba)4(bpy)2
A solution of 489 mg (4 mmol) of benzoic acid and 312 mg
(2 mmol) 2,2ꢀ-bipyridine in 5 cm3 acetonitrile was added
to a suspension of 251 mg (2 mmol) Cu(OMe)2 in 5 cm3
acetonitrile. The resulting blue suspension was stirred at
293 K for 2 h, filtered and kept for slow evaporation to ob-
tain blue crystals. Yield: 71% (652 mg). Anal. Calcd. for
Cu2O12C52H36N4: C, 62.4; H, 3.9; N, 6.1. Found: C, 61.8;
H, 3.7; N, 5.9.
Scheme 1.
belonging to nitrogen heteroatomic rings such as imidazoles
or pyridines is important for high SOD activity [25], and
complexes with bipyridines and phenanthroline are DNA in-
tercalators, showing ability to inhibit nucleic acid synthesis
in vivo [26].
In this paper, we report the synthesis and structure of
the binuclear copper(II) complexes Cu2(bpy)2(phga)4 and
Cu2(ba)4(bpy)2 (Scheme 1). The catalase activity of these
binuclear compounds have been measured in order to get in-
sight into the mechanism of the H2O2 dismutation process in
biological systems.
2.5. Study of catalase-like activity
All reactions were carried out at 20 ◦C in a 50 cm3 reac-
tor containing a stirring bar under air. Acetonitrile (30 cm3)
was added to the complex (0.05 mmol) and the flask was
closed with a rubber septum. Hydrogen peroxide (5 mmol)
was injected through the septum with a syringe. The reactor
was connected to a graduated burette filled with water and
dioxygen evolution was measured volumetrically at time in-
tervals of 0.5 min. Observed initial rates were expressed as
mol dm−3 s−1 by taking the volume of the solution (30 cm3)
into account and calculated from the maximum slope of curve
describing evolution of O2 versus time.
2. Experimental
2.1. Materials
2.6. X-ray structure determination of Cu2(bpy)2(phga)4
All manipulations were performed under a pure dinitro-
gen or argon atmosphere unless otherwise stated, using stan-
dard Schlenck-type inert gas techniques [27]. Solvents used
for the reactions were purified by literature methods [28]
and stored under argon. Cu(OMe)2 was prepared accord-
ing to the literature [29]. All other chemicals were com-
mercial products and were used as received without further
purification.
Single crystals of Cu2(bpy)2(phga)4 suitable for an X-ray
diffraction study were grown from acetonitrile upon standing
at room temperature for a few days. The intensity data were
collected with a Nonius Kappa CCD single-crystal diffrac-
˚
tometer using Mo K␣ radiation (λ = 0.71073 A) at 293 K.
Crystallographic data and details of the structure determi-
nation are given in Table 1. SHELX-97 [30,31] was used
for structure solution and full matrix least squares refine-
ment on F2. Crystal structure has been deposited at the Cam-
bridge Crystallographic Data Centre (deposition no. CCDC
267818).
2.2. Analytical measurements
Infrared spectra were recorded on a Specord 75 IR (Carl
Zeiss) spectrophotometer using samples mulled in Nujol be-
tween KBr plates or in KBr pellets. UV–vis spectra were
recorded on a Shimadzu UV-160 spectrophotometer using
quartz cells. Microanalyses were done by the Microanalyti-
cal Service of the University.
3. Results and discussion
3.1. Spectroscopic and single crystal X-ray structural
characterization
2.3. Synthesis of Cu2(bpy)2(phga)4
3.1.1. FT-IR and UV–vis
In the visible range, a weak and broad band was discov-
ered for these complexes at 679 and 697 nm for complexes
Cu2(bpy)2(phga)4 and Cu2(ba)4(bpy)2, respectively. These
bands can be assigned to copper(II) d–d transitions. Another
CT band was also observed at ca. 310 nm which can be at-
tributed to N → Cu(II) or O → Cu(II) charge transfer. The
positions of these bands are comparable with those found for
A solution of 601 mg (4 mmol) of phenylglyoxylic acid
and 312 mg (2 mmol) 2,2ꢀ-bipyridine in 3 cm3 acetonitrile
was added to a suspension of 251 mg (2 mmol) Cu(OMe)2 in
3 cm3 acetonitrile. The resulting blue suspension was stirred
at 293 K for 2 h, filtered and kept for slow evaporation to ob-
tain crystals suitable for X-ray structure determination. Yield:

14
J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 12–17
Table 1
Summary of the crystallographic data and structure parameters for
Cu2(bpy)2(phga)4
Formula weight
Crystal system
Crystal description
Space group
517.96
Monoclinic
Blue prism
P21/c
Unit cell dimensions
˚
a (A)
11.7440(4)
20.9020(10)
9.3980(10)
90.00
93.2240(10)
90.00
2303.3(3)
4
1.494
˚
b (A)
˚
c (A)
αχ (◦)
β (◦)
γ (◦)
3
˚
Volume (A )
Z
Calculated density (g cm−1
Crystal size (mm3)
)
0.3 × 0.2 × 0.15
Index ranges
−14 ≤ h ≤ 14
−25 ≤ k ≤ 0
0 ≤ l ≤ 10
Fig. 1. Ellipsoid drawing of Cu2(bpy)2(phga)4 with the atom numbering
scheme.
Temperature (K)
Radiation
293(2)
Mo K␣ (λ = 0.71073)
0.994
1060
4353
3621
forming a CuN2O3 chromophore in both cases. Each cop-
per(II) ion is in 4 + 1 environment with O(2), O(5), N(1),
N(2) in the basal plane and O(2) from the another bridging
carboxylate in the axial position. The in-plan Cu O bond dis-
Absorption coefficient (mm-1)
F(0 0 0)
Reflections collected
Observed reflections
˚
˚
tances average 1.945 A and Cu N bond distances 2.000 A,
which are in the normal range [33,37,41–43]. The axial Cu O
[I > 2σ(I)]
Goodness-of-fit
Final R indices
1.143
˚
bond distance (2.383(2) A) is significantly longer. The val-
R1 = 0.0521a, wR2 = 0.1204b
R1 = 0.0664, wR1 = 0.1288
ues of O(2)–Cu(1)–N(2) and O(2)–Cu(1)–O(2) bond angles
R indices (all data)
˚
(175.40(10) and 76.37(9) A, respectively) reflect the distorted
Largest difference
Peak/hole
square pyramidal geometry surrounding the copper ion. The
0.423/−0.289
˚
copper–copper separation is 3.432 A in the dinuclear com-
ꢀ
ꢀ
a
R = ||F0| − |Fc||/ |F0||.
plex, which is longer than the van der Wals distances for two
copper atoms (2.86◦). Selected bond lengths and bond angles
of the complex are listed in Table 2.
ꢀ
ꢀ
b
2
wR = [ w(|F0| − |Fc|)2/ w(|F0| ]1/2
.
structurally known binuclear copper(II) valproate with pyri-
dine [32].
3.2. Kinetic studies
The IR spectra for complex Cu2(bpy)2(phga)4 and
Cu2(ba)4(bpy)2 exhibit two νasym(CO2) and two νsym(CO2)
stretching bands, indicating the presence of two carboxylate
ligands involved in different coordination modes. The 1605
and 1380 cm−1 (ꢀν = 225 cm−1) pair are assigned to the car-
boxyl group that acts as monodentate ligand [33,34], and the
1555 and 1397 cm−1 (ꢀν = 158 cm−1) pair are assigned to
the other carboxylate that acts as asymmetric monodentate,
bridging ligand [35–37]. These results are consistent with
the proposed binuclear structures containing CuN2O3 chro-
mophores. The stretching vibrations corresponding to those
typical of coordinated bpy occur at 730, 830, 1030, 1232
and 1600 cm−1 [38,39]. The band at 1652 cm−1 in case of
Cu2(bpy)2(phga)4 can be assigned to the ν(CO) group.
The catalytic decomposition of hydrogen peroxide has
been studied as a model reaction for the catalase enzymes.
The kinetic studies on the disproportionation of H2O2 into
H2O and O2 were carried out by the method of initial
rates monitoring the increase of the evolved dioxygen and it
was found that the copper complexes Cu2(bpy)2(phga)4 and
Table 2
◦
˚
Selected bond lengths (A) and bond angles ( ) for Cu2(bpy)2(phga)4
Cu(1)–O(5)
Cu(1)–N(2)
Cu(1)–O(2)
O(1)–C(11)
O(3)–O(12)
O(5)–C(19)
N(1)–C(5)
N(2)–C(10)
O(5)–Cu(1)–O(2)
O(2)–Cu(1)–N(2)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–O(2)
1.918(2)
1.998(3)
2.383(2)
1.215(4)
1.207(4)
1.238(5)
1.343(4)
1.339(4)
90.10(10)
175.40(10)
94.57(10)
76.37(9)
Cu(1)–O(2)
Cu(1)–N(1)
N(2)–C(6)
O(2)–C(11)
O(4)–C(19)
O(6)–C(20)
N(1)–C(1)
1.971(2)
2.003(3)
1.352(4)
1.273(4)
1.220(5)
1.218(5)
1.343(4)
3.1.2. X-ray structure
Cu2(bpy)2(phga)4: an ORTEP plot showing the atomic-
numbering scheme employed is given in Fig. 1.
This complex is a dimer. Three oxygen and two nitro-
gen atoms are coordinated around each copper ion in the
distorted square-pyramidal arrangement (τ = 0.01 [40]), thus
O(5)–Cu(1)–N(2)
O(5)–Cu(1)–N(1)
N(2)–Cu(1)–N(1)
O(5)–Cu(1)–O(2)
94.36(11)
174.69(11)
80.92(11)
88.61(10)

J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 12–17
15
Table 3
Kinetic data for the Cu2(bpy)2(phga)4-catalyzed decomposition of hydrogen peroxide
Experiment number
104[Cu2] (M)
[H2O2] (M)
104V0 (M s−1
)
k1 (M−0.5s−1
)
1
2
3
4
5
6
7
8
9
4.28
8.56
12.9
17.1
25.7
17.1
17.1
17.1
17.1
17.1
0.17
0.17
0.17
0.17
0.17
0.13
0.20
0.23
0.27
0.33
1.27
2.52
2.96
3.48
4.22
3.02
5.50
6.15
5.67
7.39
0.037
0.052
0.050
0.051
0.050
0.055
0.066
0.063
0.052
0.053
10
Table 4
Kinetic data for the Cu2(ba)4(bpy)2-catalyzed decomposition of hydrogen peroxide
Experiment number
104[Cu2] (M)
[H2O2] (M)
104V0 (M s−1
)
k2 (M−1 s−1
)
1
2
3
4
5
6
7
3.32
8.33
12.5
16.7
25.0
16.7
16.7
0.17
0.17
0.17
0.17
0.17
0.10
0.23
0.32
0.90
1.18
1.76
2.62
1.00
2.65
0.59
0.65
0.57
0.63
0.63
0.60
0.68
Cu2(ba)4(bpy)2 have such activity. The catalase-like activity
of the complexes Cu2(bpy)2(phga)4 and Cu2(ba)4(bpy)2 was
examined in acetonitrile at 20 ◦C. The amount of residual
hydrogen peroxide (measured by iodometric titration) was
compared with the amounts of dioxygen formed (as calcu-
lated by volumetry) and found to satisfy the stoichiometry
implied by (Eq. (1)):
2H2O2 → 2H2O + O2
(1)
The observed initial rates for these complexes are
compiled in Table 3. When the catalase activity was
studied, complex Cu2(bpy)2(phga)4 was the more re-
active. The estimated initial rates (under same condi-
tions) were V0 = 2.96 × 10−4 mol dm−3 s−1, and V0 = 1.18 ×
10−4 mol dm−3 s−1, respectively, to the Cu2(bpy)2(phga)4
and Cu2(ba)4(bpy)2 compounds. Therefore, the Cu2-
(bpy)2(phga)4 complex is almost three-fold more reactive
than compound Cu2(ba)4(bpy)2, toward hydrogen peroxide.
In order to determine the rate dependence on the various re-
actants disproportionation runs were performed at different
substrate and catalyst concentrations (Tables 3 and 4). While
the influence of catalyst concentration on the kinetics ex-
hibited a different profile (Fig. 2), both complexes showed a
pseudo-first order dependence on the concentration of hydro-
gen peroxide (Fig. 3), resulted in the following experimental
rate equations (Eqs. (2) and (3)):
Fig. 2. Influence of the complex concentration on the initial rate of the
decompositionofhydrogenperoxide, catalyzedby:[Cu2(bpy)2(phga)4](᭹);
and [Cu2(ba)4(bpy)2] (ꢀ); [H2O2] = 0.17 M; in MeCN at 20 ◦C.
d[O2]
= kphga[Cu2(bpy)2(phga)4]0.5[H2O2]
= kba[Cu2(ba)4(bpy)2][H2O2]
(2)
(3)
dt
Fig. 3. Curves of the initial rate of the catalyzed decomposition of
hydrogen peroxide as
a function of hydrogen peroxide concentra-
d[O2]
tion. [Cu2(bpy)2(phga)4] (᭹) as the catalyst (17.10 × 10−4 M); and
[Cu2(ba)4(bpy)2] (ꢀ) as the catalyst (16.70 × 10−4 M); in MeCN at 20 ◦C.
dt

16
J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 12–17
From Eqs. (2) and (3) a mean value of the kinetic constants
kphga of 0.05 M−1/2 s−1 and kba of 0.62 M−1 s−1, at 20 ◦C
were obtained, respectively. These values matches with the
most active Mn- and Cu-catalase model systems [20,44,45].
On the basis of the kinetic data the following mechanism
for the catalytic reaction steps can be proposed (Eqs. (4)–(6)):
carboxylate complexes as catalyst. The simplicity of the sys-
tem, easy preparation of the catalysts and applicability make
copper carboxylate catalyzed decomposition of H2O2 an at-
tractive, environmentally acceptable tool. On the basis of
kinetic data a plausible mechanism is proposed assuming
(hydroxo)copper(III) species as intermediate. However, ev-
idence for its presence is still missing. It is also notewor-
thy that (ketocarboxylato)copper complexes have a much
higher activity than the corresponding (carboxylato)copper
complexes. The rate dependence on the catalyst suggests that
the benzoato copper complex is mainly in the monomeric
form, while that of the ketocarboxylato complex exist in the
dimeric form with a small K1 value.
K1
CuI2I ꢁ 2CuII
(4)
(5)
k2
2CuII + H2O2 −→ 2CuIII OH
slow
k2
2CuIII OH + H2O2 −→ 2CuII + O2 + 2H2O
(6)
The dimeric copper carboxylate complexes dissociate to
monomeric copper species (Eq. (4)) in a fast pre-equilibrium.
The monomeric complexes react then in the rate-determining
step with H2O2 undergoing oxidative addition of the per-
oxide bond to CuII giving (hydroxo)copper(III) complexes.
These react then in a fast consecutive step with further H2O2
ending-up in mononuclear Cu(II), dioxygen, and H2O. The
oxidative addition of H2O2 on Cu(II) may be supported by a
similar reaction of CuCl with dibenzoyl peroxide giving a sta-
ble Cu(III) complex of the structure of CuCl(PhCO2)2(py)2
[46]. In order to explain the difference in the concentration
dependence of the overall rate equation on the two cop-
per complexes we deduced the real rate expression for both
catalytic processes. By applying steady-state treatment for
d[CuII]/dt = 0 [47] in the case of Cu2(bpy)2(phga)4, rate equa-
tion (Eq. (7)) could be obtained.
Acknowledgment
We thank the Hungarian National Research Fund (OTKA
T 043414) for financial support of this work.
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(7)
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(8)
1
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4. Conclusion
We report here a new catalytic method for the dispro-
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