Synthesis, structure and catecholase-like activity of a new dicopper(II) complex with benzoylacetonate ligand

Article abstract of DOI:10.1016/j.molcata.2005.03.020

The preparation and characterization of tetranuclear Cu₄(bnac) ₄(μ-OEt)₄ complex is described. Crystallographic characterization of this complex has shown that the co-ordination geometry around copper(II) ions is distorted square pyramidal (monoclinic, C2/c, a = 13.3980(2) A?, b = 14.6080(2) A?, c = 25.7460(4) A?, α = 90.00°, β = 103.4090(10)°, γ = 90.00°, V = 4901.59(13) A?³, Z = 8). The in situ-generated Cu₂(bnac) ₂(μ-OEt)₄(4R-py)₄ complexes were suitable catalysts for the catalytic oxidation of 3,5-DTBCH₂ to 3,5-DTBQ with dioxygen at ambient condition in good yields. The catalytic activity was found to obey Michaelis-Menten type kinetics and increase with electron-releasing substituent on co-ordinated pyridines.

Full text of DOI:10.1016/j.molcata.2005.03.020

Journal of Molecular Catalysis A: Chemical 235 (2005) 81–87
Synthesis, structure and catecholase-like activity of a new dicopper(II)
complex with benzoylacetonate ligand
a
b
Jozsef Kaizer , Robert Csonka , Gabor Speiera,b,∗, Michel Giorgic,
´
´
´
´
c
Marius Reglier
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 8 December 2004; received in revised form 3 March 2005; accepted 3 March 2005
Abstract
The preparation and characterization of tetranuclear Cu4(bnac)4(␮-OEt)4 complex is described. Crystallographic characterization of this
˚
complex has shown that the co-ordination geometry around copp◦er(II) ions is distorted square pyramidal (monoclinic, C2/c, a = 13.3980(2) A,
◦
◦
3
˚
˚
˚
b = 14.6080(2) A, c = 25.7460(4) A, α = 90.00 , β = 103.4090(10) , γ = 90.00 , V = 4901.59(13) A , Z = 8). The in situ-generated Cu2(bnac)2(␮-
OEt)4(4R-py)4 complexes were suitable catalysts for the catalytic oxidation of 3,5-DTBCH2 to 3,5-DTBQ with dioxygen at ambient condition
in good yields. The catalytic activity was found to obey Michaelis–Menten type kinetics and increase with electron-releasing substituent on
co-ordinated pyridines.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Copper; Catecholase activity; Oxygenation; X-ray structure
1. Introduction
oxidases, in contrast to tyrosinases, catalyze exclusively
the oxidation of catechols to the corresponding o-quinones
without acting on monophenols [5]. The structures of
oxidized and reduced forms of catechol oxidase from sweet
potato were determined by X-ray crystallography [6–8].
Both consist of a binuclear copper center each co-ordinated
by three histidine nitrogen atoms. This reaction is of great
importance in medical diagnosis for the determination of the
hormonally active catecholamines adrenaline, noradrenaline
and dopa [9]. In the oxidized catechol oxidase structure,
The synthesis and reactivity studies of bimetallic cop-
per(II) complexes, as model compounds for metalloenzymes
with oxidase activity, is of particular interest for the de-
velopment of bioinspired catalysts for oxidation reactions.
The dicopper proteins hemocyanin, tyrosinase and catechol
oxidase have been shown to contain a coupled binuclear
metal center. Despite the similar geometric and electronic
structures of their dicopper centers, these proteins carry out
different biological functions: hemocyanin is the oxygen
transport and storage protein in arthropods and molluscs and
tyrosinase catalyzes the ortho-hydroxylation of phenols with
further oxidation of the catechol product to the o-quinone.
Catechol oxidase (EC 1.10.3.1), forms the third member
of this group of type 3 copper proteins [1–4]. Catechol
˚
the two cupric ions are 2.9 A apart. In addition to the six
histidine ligands, a bridging solvent molecule, most likely
a hydroxide ion completing the four-co-ordinate trigonal
pyramidal co-ordination sphere for both cupric metal ions.
EPR data reflect an antiferromagnetically coupled EPR
silent Cu(II)–Cu(II) state of the enzyme in agreement with a
solvent molecule bridging the copper atoms observed in the
crystal structure [10]. The catecholase activity of synthet-
ically prepared copper compounds with different structural
∗
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.2005.03.020

82
J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 235 (2005) 81–87
parameters has been investigated to some extent [11–18].
It has been assumed that geometry around the copper ions,
intermetallic distance, lability of exogenous ligands and
electrochemical properties are the main key factors that
determine the catecholase-like activity of the complexes.
In addition to former kinetic studies on catechol ox-
idase model systems [19–21] in this paper we report
the synthesis of a series of binuclear copper(II) com-
plexes Cu2(bnac)2(␮-OEt)4(4R-py)4 (generated from the
tetranuclear Cu4(bnac)4(␮-OEt)4 complex by the use of
4-substituted pyridines) as potential structural and func-
tional models for the active sites of catechol oxidase. The
catecholase activity of these binuclear compounds have been
measured in order to get insight into the possible role of the
basicity of donating, nitrogen-containing ligands.
with a syringe. The solution was then heated to the appropri-
ate temperature (40 ◦C), the argon was replaced by air and the
oxidation of 3,5-DTBCH2 was followed spectrophotometri-
cally by monitoring the formation of 3,5-DTBQ at 400.5 nm
(log ε = 3.21) as function of time (λmax of a typical band of
3,5-DTBQ).
2.5. X-ray structure determination of
Cu4(bnac)4(µ-OEt)4
Single crystals of Cu4(bnac)4(␮-OEt)4 suitable for an
X-ray diffraction study were grown from ethanol upon
standing at room temperature for a few days. The intensity
data were collected with a Nonius Kappa CCD single-crystal
˚
diffractometer using Mo K␣ radiation (λ = 0.71073 A) at
298 K. Crystallographic data and details of the structure
determination are given in Table 1. SHELX-97 [25,26] was
used for structure solution and full matrix least squares
refinement on F2. Crystal structure has been deposited at the
Cambridge Crystallographic Data Centre (Deposition no.
CCDC 265083).
2. Experimental
2.1. Materials
All manipulations were performed under a pure dinitro-
gen or argon atmosphere unless otherwise stated, using stan-
dard Schlenck-type inert-gas techniques [22]. Solvents used
for the reactions were purified by literature methods [23]
and stored under argon. Cu(OMe)2 was prepared according
to the literature [24]. All other chemicals were commercial
products and were used as received without further purifica-
tion.
Table 1
Summary of the crystallographic data and structure parameters for
Cu4(bnac)4(␮-OEt)4
Formula weight
Crystal system
Crystal description
Space group
454.0
Monoclinic
Dark green prism
C2/c
2.2. Analytical measurements
Unit cell dimensions
˚
a (A)
13.3980(2)
14.6080(2)
25.7460(4)
90.00
103.4090(10)
90.00
4901.59(13)
8
1.230
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.
˚
b (A)
˚
c (A)
α (◦)
β (◦)
γ (◦)
3
˚
Volume (A )
Z
Calculated density (g cm−1
Crystal size (mm3)
)
2.3. Synthesis of Cu4(bnac)4(µ-OEt)4
0.3 × 0.15 × 0.15
Index ranges
0 ≤ h ≤ 19
0 ≤ k ≤ 20
−25 ≤ l ≤ 25
293(2)
Mo K␣ (λ = 0.71073)
0.922
1912
6149
4419
A solution of 162 mg (1 mmol) of benzoylacetone in
5 cm3 ethanol was added to a solution of 125 mg (1 mmol)
Cu(OMe)2 in 5 cm3 ethanol. The resulting blue solution was
heated at 343 K for 2 h, filtered and kept for slow evaporation
to obtain crystals suitable for X-ray structure determination.
Yield: 70% (199 mg). Anal. calculated for Cu4O12C48H56:
C, 53.4; H, 5.2. Found: C, 53.6; H, 5.0.
Temperature (K)
Radiation
Absorption coefficient (mm−1
F(0 0 0)
Reflections collected
Observed reflections
)
2.4. Kinetics of the oxidation of 3,5-di-tert-butylcatechol
[I > 2σ(I)]
Goodness-of-fit
Final R indices
1.132
R1 = 0.0688a, wR2 = 0.2083b
R1 = 0.0969, wR2 = 0.2264
In a typical experiment, Cu4(bnac)4(␮-OEt)4 (0.85 ×
10−4 mol dm−3), 3,5-DTBCH2 (8.50 × 10−3 mol dm−3) and
the appropriate pyridine (1.70 × 10−3 mol dm−3) were dis-
solved in 30 cm3 of ethanol, under argon atmosphere in a
thermostated reaction vessel with an inlet for taking samples
R indices (all data)
Largest difference (peak/hole)
0.744/−0.591
ꢀ
ꢀ
a
R = ||F | − |Fc||/ ||F0||.
ꢀ0
ꢀ
b
2
Rw = [ w(|F0| − |Fc|)2/ w(|F0| ]1/2
.

J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 235 (2005) 81–87
83
3. Results and discussion
3.1. Spectroscopic and single crystal X-ray structural
characterization
3.1.1. FT-IR and UV–vis
Complex Cu4(bnac)4(␮-OEt)4 shows absorption bands in
the region of 250–330 nm in the electronic spectrum. For
the tetranuclear complex the absorption at 322 nm can be
assigned to a (␮-alkoxo) → Cu(II), ligand to metal charge-
transfer transition (LMCT) that typically appears in the
300–400 nm region [27]. Another CT band was also observed
at 256 nm and can be attributed to O → Cu(II) transfer. In ad-
dition, one weaker and partially resolved band can be found in
the visible region near 665 nm. The presence of the d–d band
in this region is consistent with the co-ordination geometry
close to square pyramidal for the Cu(II) center.
Infrared spectra shows two strong bands, 1589 and
1517 cm−1 for complex Cu4(bnac)4(␮-OEt)4, that can be as-
signed to ν(C O) and ν(C C), respectively. The correspond-
ing peaks in Cu(acac)2 were observed at 1578 and 1528 cm−1
[28–31], thus indicating that the ␲-system of the asymmetric
bnac ligands is somewhat disturbed compared to Cu(acac)2.
Fig. 2. The oxidation of 3,5-DTBCH2 catalyzed by Cu4(bnac)4(␮-OEt)4
in the presence of pyridine. Conditions: [Cu4(bnac)4(␮-OEt)4] = 5.00 ×
10−4 mol dm−3 and [3,5-DTBCH2] = 4.90 × 10−2 mol dm−3 under air at
40 ◦C in EtOH.
and smaller Cu(1)–O(1)–C(7) = 126.0(4)◦ and Cu(1)–O(2)–
C(9) = 125.2(4)◦ angles were observed. Selected bond
lengths and bond angles of the complex are listed in Table 2.
3.2. Kinetic studies
3.1.2. X-ray structure
Cu4(bnac)4(µ-OEt)4: An ORTEP plot showing the
atomic-numbering scheme employed is given in Fig. 1.
This complex is a dimer of dimer and connecting ethoxo-
bridges are located between the four copper centers. Each
copper(II) ion is in a distorted square pyramidal environ-
ment (τ = 0.04 [32]) with two ethoxo-bridged oxygen atoms
and two oxygen atoms of bnac occupying the basal plane
and another ethoxo group in the apical position with the
The catalytic oxidation of 3,5-di-tert-butylcatechol has
been studied as a model reaction for the catecholase activity
of tyrosinase and catechol oxidase. Complex Cu4(bnac)4(␮-
OEt)4 in the absence and also in the presence of 4-substituted
pyridine derivatives was suitable catalyst for the catalytic
oxidation of 3,5-DTBCH2 to 3,5-DTBQ with dioxygen at
ambient conditions in relatively good yields. According to
parallel electron spectroscopic and iodometric measurements
(Fig. 2), the stoichiometry of oxidation reactions corresponds
to Eq. (1). No other oxidation products could be detected.
˚
significantly longer bond length of 2.402(4) A. The aver-
˚
age Cu · · · Cu distance is 2.99 A. Bond distances and angles
within the bnac ligand are comparable to those previously re-
ported [33], although larger O(2)–Cu(1)–O(1) = 93.63(17)◦
3, 5-DTBCH2 + O2 = 3, 5-DTBQ + H2O2
(1)
Fig. 1. Ellipsoid drawing of Cu4(bnac)4(␮-OEt)4 with the atom numbering scheme.

84
J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 235 (2005) 81–87
Table 2
◦
˚
Selected bond lengths (A) and bond angles ( ) for Cu4(bnac)4(␮-OEt)4
Cu(1)–O(1)
Cu(1)–O(3)
Cu(1)–O(6)
Cu(2)–O(4)
Cu(2)–O(6)
Cu(2)–Cu(2)
O(2)–C(9)
O(3)–Cu(1)
O(5)–C(21)
O(6)–Cu(2)
C(1)–C(6)
1.916(4)
1.936(3)
2.402(4)
1.911(4)
1.957(3)
2.9789(13)
1.275(7)
1.961(4)
1.256(7)
1.942(4)
1.394(9)
Cu(1)–O(2)
Cu(1)–O(3)
Cu(1)–Cu(1)
Cu(2)–O(5)
Cu(2)–O(3)
O(1)–C(7)
O(3)–C(11)
O(4)–C(19)
O(6)–C(23)
C(1)–C(2)
1.908(4)
1.961(4)
2.9934(12)
1.915(4)
2.400(4)
1.272(6)
1.452(7)
1.271(7)
1.412(6)
1.389(9)
1.494(9)
C(1)–C(7)
O(2)–Cu(1)–O(1)
O(1)–Cu(1)–O(3)
O(1)–Cu(1)–O(3)
O(2)–Cu(1)–O(6)
O(3)–Cu(1)–O(6)
O(2)–Cu(1)–Cu(1)
O(3)–Cu(1)–Cu(1)
O(6)–Cu(1)–Cu(1)
O(4)–Cu(2)–O(6)
O(4)–Cu(2)–O(6)
O(6)–Cu(2)–O(6)
O(5)–Cu(2)–O(3)
O(4)–Cu(2)–Cu(2)
O(6)–Cu(2)–Cu(2)
O(3)–Cu(2)–Cu(2)
Cu(2)–O(6)–Cu(1)
93.63(17)
94.05(16)
167.95(18)
96.13(17)
82.12(14)
132.36(13)
40.12(11)
84.20(8)
172.76(16)
93.06(17)
80.11(16)
104.92(17)
132.59(12)
40.36(10)
84.09(8)
O(2)–Cu(1)–O(3)
O(2)–Cu(1)–O(3)
O(3)–Cu(1)–O(3)
O(1)–Cu(1)–O(6)
O(3)–Cu(1)–O(6)
O(1)–Cu(1)–Cu(1)
O(3)–Cu(1)–Cu(1)
O(4)–Cu(2)–O(5)
O(5)–Cu(2)–O(6)
O(5)–Cu(2)–O(6)
O(4)–Cu(2)–O(3)
O(6)–Cu(2)–O(3)
O(5)–Cu(2)–Cu(2)
O(6)–Cu(2)–Cu(2)
Cu(1)–O(3)–Cu(2)
Cu(2)–O(6)–Cu(2)
172.31(16)
93.01(16)
79.37(16)
105.78(18)
83.48(15)
132.28(12)
39.51(10)
93.21(18)
93.25(16)
170.14(18)
97.57(17)
83.91(15)
132.37(13)
40.01(11)
97.96(15)
99.64(16)
97.33(15)
The kinetic studies on the oxidation of 3,5-DTBCH2 were
carried out by the method of initial rates by monitoring the
increase in the characteristic quinone (3,5-DTBQ) absorption
band at 400.5 nm as a function of time. The reactivity studies
were performed in ethanol solution at 40 ◦C. Cu4(bnac)4(µ-
OEt)4 solutions (0.85 × 10−4 mol dm−3) were treated with
100 equiv. of 3,5-DTBCH2 in the presence of 20 equiv. of 4-
substituted pyridine derivatives. Fig. 3 shows the absorbance
versus wavelength spectra of Cu4(bnac)4(␮-OEt)4 in the
presence of 4Me2N-pyridine after 180 min (85%; TON of
85.2). The first apparent result over the first 100 min is a
significant difference on the reactivities in the presence of
various pyridines (Table 3).
In the absence of pyridines complex Cu4(bnac)4(␮-OEt)4
exhibits a negligibly low catecholase activity (TON of 8.2
after 100 min). The significant acceleration of the catechol
oxidation upon addition of pyridine as a Lewis base reveals
a significant push effect (Fig. 4). However, pyridine (without
catalyst) and 2,6-di-tert-butylpyridine in the presence of cat-
alyst had no significant effect, thus showing that the pyridine
wasnotactingsimplyasaBrønstedbase(Fig. 5). Thereaction
rate dependence on the Cu4(bnac)4(␮-OEt)4 concentration in
the presence of pyridine (Cu:py = 1:20) supports the one half
catalyst dependence in the kinetic rate equation indicating a
dissociative step from the tetramer to a dimeric species (Fig. 6
and Scheme 1).
Table 3
Hammett σ values and kinetic properties for Cu4(bnac)4(␮-OEt)4 in the
presence of substituted pyridines
Ligand
σ
Reaction
time (min)
V0 (106 s−1
)
Conversion
(%)
TON
–
–
100
–
8.2
66.4
67.8
45.8
23.8
22.4
16.9
8.2
66.4
67.8
45.8
23.8
22.4
16.9
4Me2N-py −0.83 100
1.21
1.27
0.75
0.43
0.38
0.26
H2N-py
4Ph–py
py
−0.66 100
−0.01 100
Fig. 3. UV–vis spectra of the oxidation of 3,5-DTBCH2 by O2 catalyzed by
0
100
0.50 100
0.66 100
[Cu4(bnac)4(␮-OEt)4]. Conditions: [Cu4(bnac)4(␮-OEt)4] = 0.85 × 10−4
4OAc–py
4CN-py
mol dm−3
,
[3,5-DTBCH2] = 8.50 × 10−3 mol dm−3 and [4Me2Npy] =
1.70 × 10−3 mol dm−3, under air at 40 ◦C in EtOH.

J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 235 (2005) 81–87
85
Fig. 4. Dependence of the reaction rates on the pyridine concentra-
tions for the oxidation reaction catalyzed by Cu4(bnac)4(␮-OEt)4. Condi-
tions: [Cu4(bnac)4(␮-OEt)4] = 0.85 × 10−4 mol dm−3 and [3,5-DTBCH2] =
8.50 × 10−3 M under air at 40 ◦C in EtOH.
Fig. 6. Dependence of the reaction rates on the Cu4(bnac)4(␮-OEt)4 con-
centrations for the oxidation reaction of 3,5-DTBCH2 in the presence of
20 equiv. pyridine. Conditions: [3,5-DTBCH2] = 8.50 × 10−3 mol dm−3 un-
der air at 40 ◦C in EtOH.
To determine the dependence of the rates on the substrate
concentration, solutions of the complex Cu4(bnac)4(␮-OEt)4
in the presence of excess of pyridine were treated with
increasing amounts of 3,5-DTBCH2. Under this exper-
imental condition, saturation kinetics was found for the
initial rates versus the 3,5-DTBCH2 concentrations (Fig. 7).
An analysis of the data based on the Michaelis–Menten
model, originally developed for enzyme kinetics, was
applied. The results evaluated from Lineweaver–Burk
plots are Vmax = 5.10 × 10−7 mol dm−3 s−1, KM = 8.80 ×
10−3 mol dm−3, kcat = 5.66 × 10−3 s−1 and k2(kcat/KM) =
0.64 mol−1 dm−3 s−1. The turnover rate of 20.4 h−1 is com-
parable to those values reported by Krebs and co-workers
[11] (4–214 h−1), but significantly lower than the value
reported by Monzani et al. [14] with kcat = 1188 h−1. In case
of our systems, the Cu · · · Cu distance is likely to be around
Fig. 5. The oxidation of 3,5-DTBCH2 catalyzed by Cu4(bnac)4(␮-OEt)4:
(×) in the presence of py (20 equiv.) without catalyst; (ꢀ) in the presence of
Cu4(bnac)4(␮-OEt)4 without pyridine; (ꢀ) in the presence of Cu4(bnac)4(␮-
OEt)4 with 2,6-di-tert-butylpyridine (20 equiv.); (ꢀ) in the presence of
Cu4(bnac)4(␮-OEt)4 with pyridine (20 equiv.) under air at 40 ◦C in EtOH.
˚
3.0 A, which allows a bridging catechol co-ordination
compatible with the distance between the two o-diphenol
oxygen atoms [10]. Fig. 8 is an other proof for the catechol
co-ordination. By adding non-oxidizable catechol such
as tetrachlorocatechol into the reaction mixture during
Scheme 1.

86
J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 235 (2005) 81–87
Fig. 7. Dependence of the reaction rates on the 3,5-DTBCH2 concen-
trations for the oxidation reaction catalyzed by Cu4(bnac)4(␮-OEt)4.
Inset: Lineweaver–Burk plots. The reactions were performed in ethanol
[Cu4(bnac)4(␮-OEt)4] = 0.85 × 10−4 mol dm−3
,
[py] = 1.70 × 10−3 mol
dm−3 and [3,5-DTBCH2] = 8.50 × 10−3 mol dm−3 under air at 40 ◦C.
Fig. 9. Dependence of the reaction rates on the dioxygen pressure.
Conditions: [Cu4(bnac)4(␮-OEt)4] = 0.85 × 10−4 mol dm−3, [py] = 1.70 ×
10−3 mol dm−3 and [3,5-DTBCH2] = 8.50 × 10−3 mol dm−3 at 40 ◦C in
EtOH.
the oxidation of 3,5-DTBCH2 the reaction was stopped,
supporting the inhibition effect of the Cl4CH2 [34–35].
Carrying out the kinetic experiments with Cu4(bnac)4(␮-
OEt)4 by varying the dioxygen pressure, first-order depen-
dence was found for the dioxygen concentration (Fig. 9).
As shown in Fig. 10, linear Hammett correlation with ρ
value of −0.45 is obtained for the series of 4-substituted
pyridines, demonstrating that electron-releasing groups en-
hance the catechol oxidation.
On the bases of the kinetic data the proposed mechanism
assumes a fast irreversible reduction of the in situ form-
ing CuII(bnac)2(␮-OEt)2(py)4 complex by 3,5-DTBCH2 to
Cu(I) · 2· · Cu(I) followed by the formation of the dicopper
dioxygencomplexCu(II)(O2)Cu(II)inafastpre-equilibrium,
which reacts then with 3,5-DTBCH2 in the rate-determining
step to the starting dicopper(I) species and the products 3,5-
DTBQ and H2O2, respectively.
Fig. 10. Plot of log(relative initial rate) vs. σ for the oxidation of 3,5-
DTBCH2 by Cu4(bnac)4(␮-OEt)4 in the presence of 4R-py under air at
40 ◦C.
4. Conclusion
It has been assumed that geometry around the copper ions,
intermetallic distance, lability of exogenous ligands and elec-
trochemical properties are the main key factors that determine
the catecholase-like activity of the complexes. In case of our
˚
system, the Cu · · · Cu distance is likely to be around 3.0 A
which allows a bridging catechol co-ordination compatible
with the distance between the two o-diphenol oxygen atoms
[10]. Ourpresentsystematicinvestigationshowsthattheelec-
tron density on the copper centers influenced by the nature of
axial ligands has a profound effect on the catecholase activity.
Fig. 8. Oxidation of catechols catalyzed by Cu4(bnac)4(␮-OEt)4. Condi-
tions: (ꢀ) [Cu4(bnac)4(␮-OEt)4] = 0.85 × 10−4 mol dm−3, [py] = 1.70 ×
10−3 mol dm−3 and [Cl4CH2] = 8.50 × 10−3 mol dm−3; (ꢀ) [Cu4(bnac)4
(␮-OEt)4] = 0.85 × 10−4 mol dm−3
,
[py] = 1.70 × 10−3 mol dm−3
, [3,5-
Acknowledgment
DTBCH2] = 8.50 × 10−3 mol dm−3 and [Cl4CH2] = 8.50 × 10−3 mol dm−3
after 600 s; (ꢀ) [Cu4(bnac)4(␮-OEt)4] = 0.85 × 10−4 mol dm−3, [py] =
1.70 × 10−3 mol dm−3 and [3,5-DTBCH2] = 8.50 × 10−3 mol dm−3 under
air at 40 ◦C in EtOH.
We thank the Hungarian National Research Fund (OTKA
T 043414) for financial support of this work.

J. Kaizer et al. / Journal of Molecular Catalysis A: Chemical 235 (2005) 81–87
87
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