Synthesis, X-ray structure and ascorbic oxidation properties of ternary α-amino acid Schiff base-bipy Cu(II) complexes as functional models for ascorbic oxidase

Article abstract of DOI:10.1016/j.poly.2013.01.020

Three ternary copper(II) complexes [Cu(5-bromo-salTyr)(bipy)]·1/ 2CH₃OH (1), [Cu(5-bromo-salLue)(bipy)]·CH₃OH (2) and [Cu(5-bromo-salTrp)(bipy)]·3CH₃OH (3) (where 5-bromo-salTyr, 5-bromo-salLue and 5-bromo-salTrp are tridentate Schiff base ligands derived from the condensation of 5-bromosalicylaldehyde with tyrosine, leucine and tryptophan, respectively, and bipy is 2,2′-bipyridine) have been prepared and characterized by elemental analysis, electronic, IR spectroscopies, magnetic measurement and cyclic voltammetry. The complexes, 1 and 3 have been also structurally characterized by X-ray diffraction technique. Crystal structures of 1 and 3 complexes displayed a distorted square-pyramidal geometry around Cu(II). The complexes are one-electron paramagnetic and show one d-d transition and one LMCT bands near 664 and 378 nm in aqueous methanol, respectively. The electrochemical studies of 1-3 in DMF medium exhibit an irreversible Cu(II)/Cu(I) redox couple in the potential range of -0.3 to 0.5 V and one additional irreversible anodic wave associated with oxidation of ligands about 0.8 V. These complexes are also catalytically active in oxidation of ascorbic acid in presence of dioxygen, models the ascorbate oxidation property of the Cu(II) sites in ascorbate oxidase. The catalytic cycle is effective for a [H₂A]:[complex] mole ratio of about 100 for 1-3.

Full text of DOI:10.1016/j.poly.2013.01.020

Polyhedron 53 (2013) 76–82
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, X-ray structure and ascorbic oxidation properties of ternary a-amino acid
Schiff base-bipy Cu(II) complexes as functional models for ascorbic oxidase
Zeinab Moradi-Shoeili , Zahra Amini , Davar M. Boghaei , Behrouz Notash b
a
a
a,
⇑
a
Department of Chemistry, Sharif University of Technology, P.O. Box 11155-3516, Tehran, Iran
Chemistry Department, Shahid Beheshti University, G.C., Evin, Tehran 1983963113, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Three ternary copper(II) complexes [Cu(5-bromo-salTyr)(bipy)]ꢀ½CH OH (1), [Cu(5-bromo-salLue)-
3
Received 4 September 2012
Accepted 23 January 2013
Available online 31 January 2013
(bipy)]ꢀCH
3
OH (2) and [Cu(5-bromo-salTrp)(bipy)]ꢀ3CH
3
OH (3) (where 5-bromo-salTyr, 5-bromo-salLue
and 5-bromo-salTrp are tridentate Schiff base ligands derived from the condensation of 5-bromosalicyl-
0
aldehyde with tyrosine, leucine and tryptophan, respectively, and bipy is 2,2 -bipyridine) have been pre-
pared and characterized by elemental analysis, electronic, IR spectroscopies, magnetic measurement and
cyclic voltammetry. The complexes, 1 and 3 have been also structurally characterized by X-ray diffraction
technique. Crystal structures of 1 and 3 complexes displayed a distorted square-pyramidal geometry
around Cu(II). The complexes are one-electron paramagnetic and show one d–d transition and one LMCT
bands near 664 and 378 nm in aqueous methanol, respectively. The electrochemical studies of 1–3 in
DMF medium exhibit an irreversible Cu(II)/Cu(I) redox couple in the potential range of ꢁ0.3 to 0.5 V
and one additional irreversible anodic wave associated with oxidation of ligands about 0.8 V. These
complexes are also catalytically active in oxidation of ascorbic acid in presence of dioxygen, models
the ascorbate oxidation property of the Cu(II) sites in ascorbate oxidase. The catalytic cycle is effective
Keywords:
Ascorbic acid
Ascorbate oxidase
a-Amino acid Schiff base
Copper
2
for a [H A]:[complex] mole ratio of about 100 for 1–3.
Ó 2013 Elsevier Ltd. All rights reserved.
1
. Introduction
Copper enzymes have always fascinated the biochemists be-
In particular, mechanistic studies on the oxidation of ascorbic
acid by molecular oxygen and Cu(II) complexes may provide a
route to understand their catalytic mechanisms and the nature of
the reactive intermediates involved in enzyme catalyzed processes
[9,10]. Several mechanism and theoretical investigations have
been reported for different binuclear copper oxidase models
[11,12] while the same for the mononuclear counterparts remain
surprisingly rare. The studies revealed that, the mononuclear
square pyramidal Cu(II) complexes possessing labile binding sites
are of great importance for modeling sites of copper oxidases and
their catalytic activity have been the subject of recent investiga-
tions [13–17]. Thus, extending further along this theme of re-
search, we set out to design functional mimics of an important
copper enzyme namely the AO.
cause of their critically important activities, such as transport, stor-
age of dioxygen and various oxidation reactions [1,2]. Chemical
approaches to study copper active sites and related model com-
plexes had been either biomimetic and/or bioinspired toward
structural–functional roles [3,4].
Ascorbate oxidase (AO) is a dimeric copper containing enzyme
which is found in plants and bacteria [5]. AO has four copper atoms
per functional unit, which are classified as types 1, 2, and 3 accord-
ing to their spectroscopic and redox properties [6]. Catalytic mech-
anism studies on the AO have shown the sequential one-electron
reduction of the enzyme. Electrons are provided by oxidation of
four molecules of possibly physiological substrate such as
L
-ascor-
Here in this contribution, we report the synthesis, crystal
structure and properties of a series of ternary mononuclear cop-
per(II) complexes (1–3) with [ONO] donor Schiff base ligands
bate which as a two electron donor, readily reduces the Cu(II) spe-
cies into their Cu(I) counterparts [7]. The reduced copper(I) sites
activate dioxygen and convert it to water in an electroprotic reac-
tion involving four electrons and four protons in AO [8].
0
and 2,2 -bipyridine (Scheme 1) that exhibit efficient ascorbic oxi-
dation activity under ambient conditions using aerial oxygen as
an oxidant. Catalytic activity of these complexes in the ascorbate
oxidation by molecular oxygen involves copper(I) intermediate
forming and this emulates the ascorbate oxidation property in
AO.
⇑
Corresponding author. Tel.: +98 21 66165306; fax: +98 21 66012983.
E-mail address: dboghaei@sharif.edu (D.M. Boghaei).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.01.020

Z. Moradi-Shoeili et al. / Polyhedron 53 (2013) 76–82
77
2
.3.2. Preparation of [Cu(5-bromo-salLue)(bipy)]ꢀCH
This compound was prepared using same procedure as used in
the case of compound 1 taking leucine (0.262 mg, 2 mmol) for
Schiff base synthesis. Anal. Calc. for (C24 26BrCuN ): C, 51.11;
H, 4.64; N, 7.45. Found: C, 50.67; H, 4.19; N, 7.46%. Yield: 87%.
3
OH (2)
H
3 4
O
ꢁ1
m.p. = 242 °C. IR (KBr, cm ) selected bands: 3425 (br), 1637s,
1
7
M
521w, 1459s, 1363w, 1310w, 1249w, 1171w, 822w, 798w,
67m, 636w, 559w, 401s. UV–Vis [MeOH/H O (4:1), kmax, nm ( °,
cm )]: 664(138), 377(4933), 277(25313), 239(28932).
2
e
ꢁ1
ꢁ1
2
.3.3. Preparation of [Cu(5-bromo-salTrp)(bipy)]ꢀ3CH
This compound was prepared using same procedure as for com-
pound 1 taking tryptophan (0.408 mg, 2 mmol) for Schiff base syn-
thesis. Anal. Calc. for (C31 33BrCuN ): C, 53.11; H, 4.74; N, 7.99.
Found: C, 53.42; H, 4.81; N, 8.1%. Yield: 62%. m.p. = 228 °C. IR (KBr,
3
OH (3)
Scheme 1. Schematic drawings of complexes 1–3.
H
4 6
O
2
. Experimental
ꢁ1
cm ) selected bands: 3418 (br), 1638s, 1516m, 1451s, 1365s,
1
4
6
316w, 1295w, 1168w, 830w, 810w, 767m, 739m, 640w, 556w,
2
.1. Chemical reagents
ꢁ1
ꢁ1
04s. UV–Vis [MeOH/H
2
O
(4:1),
k
max
,
nm
(e°,
M
cm )]:
65(135), 377(4563), 274(20684), 231(24346).
The chemicals for the synthesis were commercial products of
the highest available purity (from E. Merck, Fluka and Aldrich)
and were used without further purification. All solvents were dried
and distilled by standard methods prior to use [18].
2.4. Crystal structure determination and refinement
The X-ray diffraction measurements were made on a STOE
IPDS-II diffractometer with graphite monochromated Mo K radi-
ation. For [Cu(5-bromo-salTyr)(bipy)]ꢀCH OH (1), green block
shape and for [Cu(5-bromo-salTrp)(bipy)]ꢀ3CH OH (3), green nee-
2
.2. Measurements
a
3
Elemental analysis (C, H, N) data were performed on a Perkin El-
3
mer 2400 CHNS/O elemental analyzer. Infrared spectra (4000–
4
dle shape crystals were chosen using a polarizing microscope and
were mounted on a glass fiber which was used for data collection.
Cell constants and orientation matrices for data collection were ob-
tained by least-squares refinement of diffraction data from 12977
for 1, and 8071 unique reflections for 3. Data were collected to a
maximum 2h value of 58.36° for 1 and to a maximum 2h value of
ꢁ1
00 cm ) of solid samples were taken as a 1% dispersion in KBr
pellets using a Unicam Matson 1000 FT-IR spectrophotometer.
UV–Vis spectra were recorded by CARY 100 Bio VARIAN UV–Vis
spectrophotometer. Melting points of the products were moni-
tored on a BUCHI Melting Point Model B-540. A vibration sample
magnetometer (VSM) was used to measure the magnetic proper-
ties of the samples at room temperature. Ni was used as a standard.
The cyclic voltammetric(CV) measurements were made on a Auto-
lab/PGSTAT101 electrochemical analyzer utilizing the three-elec-
trode configuration of a Ag/AgCl as reference, a Pt auxiliary, and
Pt working electrodes. NOVA software was employed to carry out
the experiments and to acquire the data. The working electrode
was polished fresh before each experiment with alumina and son-
icated in deionised water. The tests were performed in the poten-
tial region ꢁ0.8 to +1 V at ambient temperature at a scan rate of
Table 1
Crystallographic and structure refinement data for compounds 1 and 3.
1
3
Empirical formula
2(C26
H
20BrCuN
3
C
3(CH
28
H
21BrCuN
4 3
O ,
O
4
),a CH
4
O
4
O)
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
1195.84
120(2)
0.71073
701.06
120(2)
0.71073
triclinic
monoclinic
P2 /c
ꢀ
2
0 mV/s. Recrystallized tetra(n-butyl)ammonium perchlorate
1
P1
ꢁ
3
Crystal size (mm
3
)
0.20 ꢂ 0.20 ꢂ 0.18
12.970(3)
15.914(3)
23.620(5)
90
97.87(3)
90
4829.3(17)
4
0.50 ꢂ 0.15 ꢂ 0.15
11.514(2)
11.697(2)
11.849(2)
84.63(3)
71.33(3)
88.26(3)
1505.2(5)
2
1.547
(
TBAP) (0.01 mol dm ) was used as supporting electrolyte and
a (Å)
b (Å)
c (Å)
DMF as solvent.
2
2
.3. Syntheses
a
(°)
b (°)
(°)
c
.3.1. Preparation of [Cu(5-bromo-salTyr)(bipy)]ꢀ½CH
3
OH (1)
3
V (Å )
To a mixture of tyrosine (0.362 g, 2 mmol) and KOH (0.112 g,
mmol) in 10 ml of hot MeOH was added dropwise a solution of
-bromosalicylaldehyde (0.402 g, 2 mmol) in 2 ml MeOH and stir-
Z
Dcalc (g cm 1
ꢁ
)
1.645
2
5
h Ranges for data collection (°)
F(000)
2.51–29.18
2416
2.601
2.41–29.15
718
2.103
ꢁ15 6 h 6 15
ꢁ15 6 k 6 16
ꢁ16 6 l 6 14
16822
red vigorously at 50 °C for 2 h. Then, a solution of copper (II) ace-
tate monohydrate (0.399 g, 2 mmol) dissolved in 10 ml MeOH
was added. The color changed from yellow to green after refluxing
the reaction mixture for 2 h. To the above reaction mixture, meth-
anolic solution of 2,2-bipyridine (0.312 mg, 2 mmol) was added
dropwise and refluxed for 4 h at 50 °C. The green precipitate was
Absorption coefficient (mmꢁ1)
Index ranges
ꢁ17 6 h 6 17
ꢁ
20 6 k 6 21
ꢁ32 6 l 6 29
Data collected
51651
Unique data (Rint
)
12977 (0.1536)
641/0
0.0881/0.1275
0.1475/0.1430
1.136
8071, (0.0728)
404/3
0.0635/0.1384
0.0935/0.1517
1.031
Parameters/restrains
filtered off, washed with MeOH and Et
Calc. for (C53 Cu ): C, 53.23; H, 3.71; N, 7.03. Found: C,
3.25; H, 3.68; N, 7.13%. Yield: 88%. m.p. = 198 °C. IR (KBr, cm
selected bands: 3435 (br), 1632s, 1515m, 1455s, 1374m, 1243m,
2
O and dried in vacuo. Anal.
b
Final R
1
/wR
2
(observed data)
b
H44Br
2
2 6 9
N O
Final R /wR2 (all data)
1
ꢁ1
2
5
)
Goodness-of-fit (GOF) on F (S)
Largest difference in peak and
0.827, ꢁ0.605
1.179, ꢁ1.395
ꢁ
3
hole (e Å
)
1
H
2
168m, 852m, 833m, 760s, 763m, 623s, 540w. UV–Vis [MeOH/
ꢁ1
ꢁ1
a
0
Z = 2.
2
O
(4:1),
k
max
,
nm
(e°,
M
cm )]: 664(144), 378(4715),
b
(w(F 2
2
2
w(F 2)2]1/2.
o
R
1
=
R
||F
o
| ꢁ |F
c
||/R
|F
o
|, wR
2
= [R
o
ꢁ F
c
) )/
R
80(24266), 237(29233).

7
8
Z. Moradi-Shoeili et al. / Polyhedron 53 (2013) 76–82
5
8.30° for 3, in a series of
x
scans in 1° oscillations and integrated
as the co-ligand, and also in order to maintain levels of basicity
in the medium and then relative proton affinities.
All these complexes have been characterized by elemental anal-
yses, IR, UV–Vis spectroscopies and cyclic voltammetry. Physico-
using the STOE X-AREA [19] software package. A numerical absorption
correction was applied using X-RED [20] and X-SHAPE [21] software.
The data were corrected for Lorentz and Polarizing effects. The
structures were solved by direct methods [22] and subsequent dif-
ference Fourier maps and then refined on F by a full-matrix least-
squares procedure using anisotropic displacement parameters
chemical data on the one-electron paramagnetic (
l
eff ꢃ 1.83
B
l )
2
mononuclear copper(II) complexes, 1–3 are summarized in Table 2.
The X-ray crystal structures of 1 and 3 have also been solved. The
spectral features are similar to those reported for ternary struc-
tures consisting of a tridentate ONO-donor Schiff base, a bidentate
N,N-donor heterocyclic base and the copper(II) center [26–28].
[
23]. The atomic factors were taken from the International Tables
for X-ray Crystallography [24]. All refinements were performed
using the X-STEP32 crystallographic software package [25]. A sum-
mary of crystal data, experimental details, and refinement results
is given in Table 1.
3.2. Description of the molecular structures of 1 and 3
3
. Results and discussion
Green block crystals of 1 were obtained by recrystallization of
3
.1. General aspects
dried solid of 1 in methanol over one week. For compound 3, green
needle crystals were grown by slow evaporation of methanolic
solution of compound 3 in five days. Crystallographic data and
parameters for 1 and 3 are summarized in Table 1. This table re-
The ternary complexes 1–3 have been synthesized from a reac-
tion of copper(II) acetate mono hydrate with Schiff bases of dian-
ionic ONO-donor -amino acid salicylaldiminates derived from
tyrosine (Tyr), leucine (Lue) and tryptophan (Trp), in refluxing
a
veals that compounds 1 and 3 crystallize in the monoclinic (space
ꢀ
group, P2
1
/c) and triclinic (space group, P1) crystal system,
0
methanol. In addition, the heterocyclic base, 2,2 -bipy was added
respectively.
Table 2
Physico-chemical data on [Cu(L)(bipy)] (1–3).
IRa (cmꢁ1
(C@O)
)
k
max, nm (e, M cm )b
ꢁ1
ꢁ1
l
(
l
B
)c
Cu(II)–Cu(I) coupled
L oxidation
Complex
eff
m
m(C@N)
d–d band
CT band
E
pc
E
pa1
i
pc/ipa
E
pa2
(1)
(2)
(3)
1632
1637
1638
1515
1521
1516
664 (144)
664 (138)
665 (135)
378 (4715)
377 (4933)
377 (4563)
1.69
1.96
1.83
ꢁ0.189
ꢁ0.219
ꢁ0.305
0.398
0.542
0.402
2.6
1.97
2.1
0.805
0.837
0.823
a
b
c
KBr phase.
In MeOH/H
2
O (4:1).
2
98 K.
d
In DMF.
Fig. 1. The labeled diagram of asymmetric unit of 1. Thermal ellipsoids are at 30% probability level.

Z. Moradi-Shoeili et al. / Polyhedron 53 (2013) 76–82
79
Table 3
in compound 3 copper(II) ion has five-coordination number in
which copper(II) ion is coordinated with one tridentate Schiff base
ligand and one bidentatebipyridine ligand. An ORTEP drawing of
asymmetric unit of 3 is presented in Fig. 3. Selected bond lengths
and angles with their standard deviations for 3 are listed in Table 4.
The coordination geometry for copper(II) can be describe as dis-
Selected bond distances (Å) and bond angles (°) for 1.
Cu(1)–O(1)
Cu(1)–N(3)
Cu(1)–O(2)
Cu(1)–N(2)
Cu(1)–N(1)
N(3)–C(7)
1.916(4)
1.929(5)
1.985(4)
2.005(5)
2.254(5)
1.288(7)
Cu(2)–O(5)
Cu(2)–N(6)
Cu(2)–O(6)
Cu(2)–N(5)
Cu(2)–N(4)
N(6)–C(33)
1.933(4)
1.934(5)
1.989(4)
2.011(5)
2.300(4)
1.292(7)
torted square-pyramidal, with trigonality index
s = 0.063 [29]. A
O(1)–Cu(1)–N(3)
O(1)–Cu(1)–O(2)
N(3)–Cu(1)–O(2)
O(1)–Cu(1)–N(2)
N(3)–Cu(1)–N(2)
O(2)–Cu(1)–N(2)
O(1)–Cu(1)–N(1)
N(3)–Cu(1)–N(1)
O(2)–Cu(1)–N(1)
N(2)–Cu(1)–N(1)
94.02(18)
161.98(17)
83.19(17)
93.21(18)
169.91(19)
87.76(17)
102.11(18)
107.87(19)
95.67(17)
77.4(2)
O(5)–Cu(2)–N(6)
O(5)–Cu(2)–O(6)
N(6)–Cu(2)–O(6)
O(5)–Cu(2)–N(5)
N(6)–Cu(2)–N(5)
O(6)–Cu(2)–N(5)
O(5)–Cu(2)–N(4)
N(6)–Cu(2)–N(4)
O(6)–Cu(2)–N(4)
N(5)–Cu(2)–N(4)
93.93(18)
162.19(18)
82.52(18)
89.90(18)
169.92(18)
91.00(17)
103.30(17)
111.41(17)
94.21(17)
76.64(17)
perspective view of the square-pyramidal coordination geometry
around Cu(II) atom is shown in Fig. 4. Similar to compound 1,
square plane is made up of one nitrogen and two oxygen atoms
of hydroxyl and carboxylic part of Schiff base and also one nitrogen
atom of bipyridine ligand. The axial site is occupied by another
nitrogen atom (N3) of bipyridine ligand (Fig. 4). The average Cu–
Nbipy distances is 2.125 Å which a little shorter than similar dis-
tance in compound 1. In compound 3, longer Cu–N bond distance
is observed in the case of Nbipy which occupy axial position in com-
parison with basal position (Table 4 and Fig. 4). Crystal packing
diagram of compound 3 is presented in Fig. S2. In the crystal pack-
ing of 3, there are several O–Hꢀ ꢀ ꢀO, N–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO primary
and secondary hydrogen bonding which stabilize crystal structure
of compound 3. A summary of the hydrogen bond parameters for 3
are listed in Table S2.
3
.2.1. Structure of [Cu(5-bromo-salTyr)(bipy)]ꢀCH
3
OH (1)
The structure of 1 consists of two crystallographically different
Cu(II) species in the asymmetric unit. Both Cu(II) ions have similar
five-coordination sphere in which copper(II) centers coordinated
with one tridentate Schiff base ligand and one bidentatebipyridine
ligand. An ORTEP drawing of the two Cu(II) centers in the asym-
metric unit is presented in Fig. 1. Selected bond lengths and angles
with their standard deviations for 1 are given in Table 3. The coor-
dination geometry for both Cu(II) are distorted square-pyramidal,
3.3. Electronic spectrophotometry
2
The electronic spectral data recorded in MeOH/H O (4:1), com-
pounds of 1–3 exhibit a similar absorptions, and displaying a d–d
bands in the electronic spectra near 665 nm and absorption bands
with trigonality index
s = 0.132 and 0.128 for Cu(1) and Cu(2),
respectively ( = 0 and 1 is defined for perfect square-pyramidal
s
ꢁ
about 378 nm, assigned to O (phenolate)–Cu(II) charge transfer
and trigonal–bipyramidal geometries, respectively) [29]. The coor-
dination environment around copper centers is shown in Fig. 2.
The square plane is made up of one nitrogen and two oxygen atoms
of hydroxyl and carboxylic part of Schiff base and also one nitrogen
atom of bipyridine ligand. The axial site is occupied by another
nitrogen atom of bipyridine ligand (Fig. 2). The average Cu–Nbipy
distances are 2.1295 and 2.1555 Å for Cu(1) and Cu(2), respectively
which are similar to the average of Cu–N bond distance found for
similar copper/phenanthroline system [30]. As shown in Table 3,
there are longer Cu–N bond distances in the case of Nbipy which oc-
cupy axial positions in comparison with basal positions (Cu(1)–
N(1), Cu(2)–N(4)). The bond lengths and angles for Schiff base
coordination sites are also in the normal ranges [30]. Crystal pack-
ing diagram of compound 1 is shown in Fig. S1. As shown in this
figure, there are several O–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO hydrogen bonding
which stabilize crystal structure of compound 1. A summary of
the hydrogen bond parameters for 1 are presented in Table S1.
[
31]. In addition, two more ligand based high energy transitions
⁄
were observed at 274 and 245 nm attributed to the
tions within the aromatic rings in the ligands.
p–p
transi-
3
.4. Redox properties
Cyclic voltammograms of the present complexes 1–3 were re-
corded using DMF as solvent and tetrabutylammonium perchlorate
(
ꢁ1
TBAP) as supporting electrolyte at a scan rate of 20 mV s in the
potential range +1 to ꢁ0.7 V versus Ag/AgCl reference. A represen-
tative voltammogram is displayed in Fig. 5 corresponding to [Cu(5-
bromo-salTyr)(bipy)]ꢀCH
3
OH (1). It shows only one irreversible
Cu(II)/Cu(I) redox couple (Epc = ꢁ0.189 V, Epa1 = 0.398 V). Also, one
additional irreversible oxidation wave is observed at potentials
greater than 0.8 V that tentatively assigned to oxidation of the
coordinated Schiff bases. A similar type of potential shift is earlier
observed in related ternary copper (II) Schiff base complexes [15].
The electrochemical results are summarized in Table 2.
3
.2.2. Structure of [Cu(5-bromo-salTrp)(bipy)]ꢀ3CH
The structure of 3 consists of one crystallographically indepen-
dent Cu(II) species in the asymmetric unit. Similar to compound 1,
3
OH (3)
Generally, the redox behavior of the complexes depend on a
number of factors such as the chelate ring size, axial ligation,
Fig. 2. View of the coordination environment around two crystallographically independent Cu(II) centers in 1.

8
0
Z. Moradi-Shoeili et al. / Polyhedron 53 (2013) 76–82
Fig. 3. The labeled diagram of asymmetric unit of 3. Thermal ellipsoids are at 30% probability level.
Table 4
Selected bond distances (Å) and bond angles (°) for 3.
Cu(1)–O(1)
1.926(3)
Cu(1)–N(4)
2.032(3)
Cu(1)–N(1)
1.944(3)
Cu(1)–N(3)
2.218(4)
Cu(1)–O(2)
1.973(3)
N(1)–C(7)
1.286(5)
O(1)–Cu(1)–N(1)
N(1)–Cu(1)–O(2)
N(1)–Cu(1)–N(4)
O(1)–Cu(1)–N(3)
O(2)–Cu(1)–N(3)
93.07(13)
83.39(12)
169.04(14)
99.48(14)
95.12(13)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–N(4)
O(2)–Cu(1)–N(4)
N(1)–Cu(1)–N(3)
N(4)–Cu(1)–N(3)
165.24(13)
90.96(13)
90.14(13)
111.93(13)
77.37(13)
Fig. 5. Cyclic voltammogram (0.003 mol 1 dm
Tyr)(bipy)]ꢀCH OH (1) in DMF (0.01 MTBAP as supporting electrolyte, scan rate:
0 mV/s).
ꢁ
ꢁ3
)
of [Cu(5-bromo-sal-
3
2
degree and distribution of unsaturation and substitution pattern
in the chelate ring [32]. Accordingly, we may conclude that there
is structurally and/or functionally similar environments around
the copper(II) ions of these complexes. And that, in turn, suggests
a similar electrochemical behavior and can lead to expect Cu(II)
ions can be oxidized or reduced at the same potential. This is
consistent with the single crystal X-ray diffraction results that
display similar distorted square-pyramidal geometry for Cu(II)
ions in 1 and 3.
Fig. 4. View of the coordination environment around Cu(II) center in 3.

Z. Moradi-Shoeili et al. / Polyhedron 53 (2013) 76–82
81
ing in absorption of CT band and also the reappearance of the d–d
band which demonstrates the gradual conversion of the reduced
species, Cu(I), to the oxidized form, Cu(II), using dioxygen as an
oxidant. The regeneration of the parent copper(II) species giving
an isosbestic point at 645 nm.
It is previously has been shown that the major probable product
of oxygen reduction in such catalytic cycle processes involving a
Cu-(II)/Cu(I) redox couple is hydrogen peroxide [13].
The catalytic cycle for 1–3 (Scheme 2) is found to be effective at
2
a [H A]:[complex] mole ratio of about 100. Catalytic activity of
these complexes in ascorbate oxidation by molecular oxygen
involving the reduced copper(I) intermediate models the ascorbate
oxidation property of the Cu(II) sites in AO.
4
. Conclusions
Three ternary copper(II) complexes containing tridentate ONO-
donor Schiff bases and a planar NN-donor heterocyclic ligand, bipy,
have been prepared and structurally characterized, to model the
copper sites of ascorbate oxidase. These complexes have been
found to be catalytically active in ascorbate oxidation by molecular
oxygen. The catalysts are effective with a mole ratio of ascorbic
acid to complex as approximately 100 that models the ascorbate
oxidation property of the active centers of AO.
Fig. 6. UV–Vis spectral changes of
1 (0.15 mM) (a–c) showing the stepwise
reduction of Cu(II) to Cu(I) upon addition of ascorbic acid (1.5 mM) and subsequent
conversion (d–f) of the copper(I) species obtained from the ascorbate reduction of 1
in aqueous MeOH and aerobic condition. The spectra were recorded with 1 min
time interval for a–c and about 30 min for d–f.
Acknowledgement
The authors are grateful to the Research Council of Sharif Uni-
versity of Technology for their financial support.
Br
HO
O
O
Appendix A. Supplementary data
HO
2
O
N
N
N
CuII
R
2 2
H O
HO
OH
A summary of the hydrogen bond parameters for 1 and 3 are
founded in the supporting information. CCDC 896111 and
O
O
2
(H A)
8
96112 contain the supplementary crystallographic data for 1
and 3, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
HO
O
O
2[CuI(5-bromo-salR)(bipy)]
O
2
HO
O
O
1
0.1016/j.poly.2013.01.020.
(A)
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