Synthesis, characterization, electrochemical studies and catecholase-like activity of a series of mononuclear Cu(II), homodinuclear Cu(II)Cu(II) and heterodinuclear Cu(II)Ni(II) complexes of a phenol-based compartmental ligand

Article abstract of DOI:10.1016/j.molcata.2005.06.064

A series of previously reported mononuclear Cu(II) and homodinuclear Cu(II)Cu(II) complexes, as well as, novel mononuclear Cu(II), homodinuclear Cu(II)Cu(II), and heterodinuclear Cu(II)Ni(II) complexes of a phenol-based dinucleating ligand with two differ

Full text of DOI:10.1016/j.molcata.2005.06.064

Journal of Molecular Catalysis A: Chemical 241 (2005) 1–7
Synthesis, characterization, electrochemical studies and catecholase-like
activity of a series of mononuclear Cu(II), homodinuclear Cu(II)Cu(II)
and heterodinuclear Cu(II)Ni(II) complexes of a phenol-based
compartmental ligand
Davar M. Boghaei∗, Mahdi Behzad, Abolfazl Bezaatpour
Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran
Received 12 October 2004; received in revised form 7 May 2005; accepted 23 June 2005
Available online 8 August 2005
Abstract
A series of previously reported mononuclear Cu(II) and homodinuclear Cu(II)Cu(II) complexes, as well as, novel mononuclear
Cu(II), homodinuclear Cu(II)Cu(II), and heterodinuclear Cu(II)Ni(II) complexes of a phenol-based dinucleating ligand with two differ-
ent N(amine)2O2 and N(imine)2O2 coordination sites, were synthesized and characterized by elemental analyses, infrared and electronic
absorption spectroscopies and conductivity measurements. The electrochemical behavior and catecholase-like activity of the complexes were
also studied using cyclic voltammetry and UV–vis spectrophotometry, respectively. Our results show that the dinuclear complexes are more
effective catalysts in the oxidation of catechol to the corresponding quinone than the mononuclear complexes. Furthermore, the homodinuclear
Cu(II)Cu(II) complexes show greater catalytic activity compared to the corresponding heterodinuclear Cu(II)Ni(II) complexes.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Dinuclear; Heterodinuclear; Compartmental; Catecholase; Catechol
1. Introduction
duced them as being suitable candidates for molecule-based
magnets [8–10]. There is also continuing interest in mecha-
nistic studies on the catalytic oxidation of catechols by tran-
sition metal complexes because this reaction plays key role in
the metabolism of aromatic compounds. Copper-containing
metallo-proteins and metallo-enzymes are involved in a vari-
ety of biological processes in living systems. Hemocyanin,
tyrosinase and catechol oxidase proteins have dinuclear cop-
per(II) centers at their active sites [11,12]. Catechol oxidases,
like tyrosinase, oxidize phenolic compounds to the corre-
sponding quinones in the presence of oxygen. During the
past decade or so, a very large number of binucleating lig-
ands and their complexes have been synthesized and studied.
Among many different types of dinucleating ligands, the
phenol-based compartmental ligands (Fig. 1) have attracted
particular attention [13–16].
The chemistry of dinucleating ligands and especially their
heterodinuclear complexes has unceasingly and deservedly
gained great attraction during the past decade [1–4]. The
presence of two metal ions in close proximity at the active
site of several metallo-proteins and metallo-enzymes has
simulated interest in the synthesis and study of dinuclear
complexes. In particular, the recent recognition of heterod-
inuclear cores at the active sites of purple acid phosphatase
[5] and human calcineurin (FeZn) [6], and human protein
phosphatase 1 (MnFe) [7] has encouraged the design of het-
erodinuclear complexes for functional and structural model
studies. Another interest in such bimetallic complexes lies
in the area of magnetochemistry. The magnetic interaction
between the metal ions of the dinuclear complexes has intro-
In the course of this study, Okawa and his co-workers
have reported the synthesis of homodinuclear and heterod-
inuclear complexes of a phenol-based compartmental lig-
∗
Corresponding author. Tel.: +98 21 6165306; fax: +98 21 6005718.
E-mail address: dboghaei@sharif.edu (D.M. Boghaei).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.06.064

2
D.M. Boghaei et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 1–7
eter. Conductivity measurements were performed using a
Jenway-4010 conductivity meter. All electrochemical experi-
ments were done using a setup comprised of a PC PII Pentium
300 MHz microcomputer equipped with a data acquisition
board (PCL-818PG, PC-Labcard Co.) and a custom made
potentiostat [18].
2.1. Typical procedure for the preparation of
mononuclear complexes [CuL1–4
]
2.1.1. Preparation of CuL2
The synthesis route for this complex and the other com-
plexes is shown in Fig. 2. H2Lꢀ (0.5 g, 1 mmol) was dissolved
in 30 mL of aqueous solution of KOH (0.11 g, 2 mmol) and
the reaction mixture was stirred at room temperature until
a clear yellow solution was obtained. To the resulting solu-
tion was added an aqueous solution of Cu(OAc)2·H2O (0.2 g,
1 mmol in 20 mL of water) to form a green precipitate. The
resulting precipitate was collected by suction filtration and
washed successively with water and ether and dried in vacuo.
The resulting solid was suspended in 30 mL of methanol
and to this mixture was added dropwise a solution of meso-
stilbenediamine (0.21 g, 1 mmol) in 20 mL of methanol and
the reaction mixture was refluxed for about 3 h. The resulting
solid was collected by suction filtration and washed succes-
sively with water and diethyl ether and dried in vacuo. The
complex was obtained as a green powder and the yield was
0.71 g, 92%. Analytically calculated for C34H30Br2CuN4O2
(%): C, 54.4; H, 4.0; Cu, 8.5; N, 7.5. Found (%): C, 54.3; H,
4.1; Cu, 8.4; N, 7.4. Selected IR data (ν, cm−1 KBr): 1623
(νC=N). UV–vis [λmax, nm (ε, M−1 cm−1)]: 604 (140), 378
(3700), and 368 (4750) in acetonitrile.
Fig. 1. Structure of phenol-based compartmental ligands.
and in which the N(amine)2O2 coordination site is pro-
vided by two phenolic oxygen atoms and the nitrogen atoms
from (CH3)NCH2CH2N(CH3) [13]. Karunakaran and Kan-
daswamy have also reported [15] the synthesis of related
macrocycles containing piperazine entity to provide the nitro-
gen atoms of the N(amine)2O2 coordination site. They have
reported the synthesis of homodinuclear Cu(II)Cu(II) com-
plexes but they did not study heterodinuclear complexes of
their ligands. Based on the above, we decided to synthesize
novel mononuclear Cu(II), homodinuclear Cu(II)Cu(II) and
heterodinuclear Cu(II)Ni(II) complexes. The catalytic reac-
tivity of these complexes, as well as four other previously
reported mononuclear and homodinuclear complexes, is also
studied in the aerobic oxidation of catechol to the correspond-
ing quinone.
2.1.2. CuL4
This complex was obtained as a green precipitate fol-
lowing the previously described procedure for CuL2 except
1,3-diamino-2-hydroxypropane was used instead of meso-
stilbenediamine. The yield was 0.60 g, 95%. Analytically
calculated for C23H24Br2CuN4O3 (%): C, 43.9; H, 3.8; Cu,
10.2; N, 8.9. Found (%): C, 43.8; H, 3.7; Cu, 10.3; N, 9.0.
2. Experimental
Selected IR data (ν, cm−1 KBr): 1621 (νC=N). UV–vis [λmax
,
nm (ε, M−1 cm−1)]: 610 (132), 378 (3900), and 367 (4880)
All the reactions were performed under aerobic con-
ditions unless otherwise noted. N,N’-Pyrazinebis(5-bromo-
3-formyl-2-hydroxybenzylamine) (H2Lꢀ), the mononuclear
in CH3CN.
2.1.3. Cu2L2
complexes [CuL1,3], the dinuclear complexes [14] [Cu2L1,3
]
To a hot suspension of the mononuclear complex CuL2
(0.39 g, 0.5 mmol) in 30 mL of methanol was added a
methanolic solution of copper(II) perchlorate hexahydrate
(0.10 g, 0.5 mmol). A deep green solution was obtained while
the amount of the solid mononuclear complex was being
decreased. The resulting solution was once filtered to sep-
arate any insoluble materials and the filtrate was evaporated
to dryness. The residue was re-dissolved in a minimum of
methanol and was left undisturbed until the desired product
was obtained. The resulting deep green precipitate was col-
lected by suction filtration and washed with water and diethyl
andmeso-stilbenediamine[17]weresynthesizedasdescribed
elsewhere. All the other materials were used as received.
Analyses for carbon, hydrogen and nitrogen were per-
formed using a Heraeus Elemental Analyzer CHN-O-Rapid
(Elementar-Analysesysteme, GmbH). Analyses for the metal
ions were conducted using a Varian AA-220 spectrophotome-
ter. IR spectra of KBr pellets of the ligands and the complexes
were recorded on an ABB Bomem MB series spectropho-
tometer in the range of 400–4000 cm−1. Electronic spectra
were recorded using a Cary 100 Bio Varian UV/Vis spectrom-

D.M. Boghaei et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 1–7
3
Fig. 2. Reaction pathway for the synthesis of the complexes.
ether successively and dried in vacuo. The yield was 0.13 g,
26%. Analytically calculated for C34H30Br2Cl2Cu2N4O10
(%): C, 40.5; H, 2.8; Cu, 12.6; N, 5.5. Found (%): C, 40.4; H,
2.7; Cu, 12.4; N, 5.6. Selected IR data (ν, cm−1 KBr): 1630
C22H22Br2Cl2CuN4NiO10 (%): C, 30.9; H, 2.6; Cu, 7.4;
N, 6.5; Ni, 6.9. Found (%): C, 31.0; H, 2.7; Cu, 7.5; N,
6.4; Ni, 6.7. Selected IR data (ν, cm−1 KBr): 1638 (νC=N),
1100 (νClO , sym.), 620 (νClO , diss.). UV–vis [λmax, nm
4
4
(νC=N), 1100 (νClO , sym.), 630 (νClO , diss.). UV–vis [λmax
,
(ε, M−1 cm−1)]: 594 (764), 384 (3600), and 368 (3990) in
acetonitrile. Molar conductance [Λm, S cm2 mol−1]: 220 in
acetonitrile.
4
4
nm (ε, M−1 cm−1)]: 597 (205), 399 (3400), and 368 (3750)
in acetonitrile. Molar conductance [Λm, S cm2 mol−1]: 215
in acetonitrile.
2.1.6. CuNiL2
2.1.4. Cu2L4
This complex was obtained as a brown precipitate fol-
lowing the previously described procedure for CuNiL1 but
the mononuclear complex CuL2 was used in the preparation
instead of CuL1. The yield was 0.14 g, 26%. Analytically cal-
culated for C34H30Br2Cl2CuN4NiO10 (%): C, 40.5; H, 3.0;
Cu, 6.3; N, 5.6; Ni, 5.8. Found (%): C, 40.4; H, 3.1; Cu,
6.2; N, 5.5; Ni, 5.9. Selected IR data (ν, cm−1 KBr): 1638
This complex was obtained as a green precipitate fol-
lowing the previously described procedure for Cu2L2 but
the mononuclear complex CuL4 was used in the prepara-
tion instead of CuL2. The yield was 0.14 g, 26%. Analyti-
cally calculated for C23H24Br2Cl2Cu2N4O11 (%): C, 31.0;
H, 2.7; Cu, 14.3; N, 6.3. Found (%): C, 31.1; H, 2.8; Cu,
14.5; N, 6.4. Selected IR data (ν, cm−1 KBr): 1638 (νC=N),
1100 (νClO , sym.), 628 (νClO , diss.). UV–vis [λmax, nm
(νC=N), 1092 (νClO , sym), 630 (νClO , diss.). UV–vis [λmax
,
4
4
nm (ε, M−1 cm−1)]: 594 (150), 404 (3400), and 368 (3740)
in acetonitrile. Molar conductance [Λm, S cm2 mol−1]: 215
in acetonitrile.
4
4
(ε, M−1 cm−1)]: 594 (764), 384 (3600), and 368 (3990) in
acetonitrile. Molar conductance [Λm, S cm2 mol−1]: 220 in
acetonitrile.
2.1.7. CuNiL3
2.1.5. CuNiL1
This complex was obtained as a brown precipitate fol-
lowing the previously described procedure for CuNiL1 but
the mononuclear complex CuL3 was used in the preparation
instead of CuL1. The yield was 0.14 g, 26%. Analytically cal-
culated for C23H24Br2Cl2CuN4NiO10 (%): C, 31.7; H, 2.8;
Cu, 7.4; N, 6.4; Ni, 6.8. Found (%): C, 31.6; H, 2.7; Cu,
This complex was obtained as a green precipitate follow-
ing the previously described procedure for Cu2L2 except
CuL1 and Ni(ClO4)2·6H2O were used in the prepara-
tion instead of CuL2 and Cu(ClO4)2·6H2O, respectively.
The yield was 0.15 g, 28%. Analytically calculated for

4
D.M. Boghaei et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 1–7
7.5; N, 6.5; Ni, 6.6. Selected IR data (ν, cm−1 KBr): 1638
N(amine)2O2 coordination site. Accordingly and according
to the similarity of the spectroscopic and electrochemical data
of our complexes with that of Cu(II)Ni(II) complex, reported
by Okawa and co-workers, we propose that these complexes
have Cu(II)Ni(II) structures (Fig. 2). Besides, it is obvious
that Ni(II) better resists in the square planner N(imine)2O2
coordination site. In the IR spectra of the ligand H2Lꢀ an
intense band centered at 1675 cm−1 is indicative of the pres-
ence of a carbonyl group which disappears upon the reaction
with the diamines in the synthesis of complexes. The disap-
pearance of this band and the appearance of another band
centered around 1620–1640 cm−1 is a good indication of
the Schiff base condensation of the diamines and the car-
bonyl group of the H2Lꢀ. This new band in the IR spectra of
(νC=N), 1100 (νClO , sym.), 630 (νClO , diss.). UV–vis [λmax
,
4
4
nm (ε, M−1 cm−1)]: 594 (764), 384 (3600), and 368 (3990)
in acetonitrile. Molar conductance [Λm, S cm2 mol−1]: 220
in acetonitrile.
2.1.8. CuNiL4
This complex was obtained as a brown precipitate fol-
lowing the previously described procedure for CuNiL1 but
the mononuclear complex CuL4 was used in the preparation
instead of CuL1. The yield was 0.14 g, 26%. Analytically cal-
culated for C23H24Br2Cl2CuN4NiO11 (%): C, 31.1; H, 2.7;
Cu, 7.2; N, 6.3; Ni, 6.7. Found (%): C, 31.2; H, 2.8; Cu,
7.3; N, 6.4; Ni, 6.5. Selected IR data (ν, cm−1 KBr): 1638
the mononuclear complexes is centered around 1620 cm−1
,
(νC=N), 1100 (νClO , sym.), 630 (νClO , diss.). UV–vis [λmax
,
which shifts to higher wave numbers upon coordination to
the second metal ions. Furthermore, the appearance of two
new bands around 1100 and 630 cm−1 is indicative of the
presence of un-coordinated perchlorate ions. The UV–vis
spectra of the complexes show three major bands. The intense
4
4
nm (ε, M−1 cm−1)]: 590 (160), 410 (3250), and 370 (3700)
in acetonitrile. Molar conductance [Λm, S cm2 mol−1]: 215
in acetonitrile.
*
bands centered around 370 nm is due to the ␲–␲ transi-
3. Results and discussions
tion associated with the azomethine group. Another intense
band is common for all of the complexes around 380 nm,
which could be attributed to the metal to ligand charge trans-
fer. The other weaker bands in the spectra of the complexes
are attributed to the d–d transitions of the metal ions. These
weaker bands in the UV–vis spectra of the mononuclear
and homodinuclear complexes, are centered around 600 nm,
which are due to the d–d transitions occurring in the cop-
per centers. In the UV–vis spectra of the heterodinuclear
complexes, this band is centered around 570 nm and is rep-
resentative of the d–d transitions in the Ni centers. Molar
conductance of the dinuclear complexes in acetonitrile is in
the range of 210–225 S cm2 mol−1 that is indicative of 2:1
electrolytic nature of these complexes.
3.1. Preparation and spectroscopic studies of the
complexes
The mononuclear complexes CuL1,3 and the dinuclear
complexes Cu2L1,3 have already been prepared by Kan-
daswamy and Karunakaran [14]. The other novel mononu-
clear complexes CuL2,4 as well as homodinuclear com-
plexes Cu2L2,4 and heterodinuclear complexes CuNiL1–4
were also prepared following the general procedures [13,14].
The mononuclear complexes were readily prepared by the
reaction of the ligand (H2Lꢀ) with copper(II) acetate fol-
lowed by the reaction with the diamines (Fig. 2). The reac-
tion of the mononuclear complexes with copper(II) and
nickel(II) perchlorates resulted in the synthesis of the cor-
responding dinuclear complexes (Fig. 2). Okawa and his
co-workers have reported the synthesis of Cu(II)M(II) com-
plexes of a phenol-based dinucleating ligand (CuM means
that Cu is in the N(amine)2O2 coordination site and M is
in the N(imine)2O2 coordination site) and their coordina-
tion position isomeric M(II)Cu(II) complexes. They have
studied thermodynamic stability of their complexes and
have shown that the Cu(II)M(II) complexes are more stable
compared to the coordination position isomeric M(II)Cu(II)
complexes. They have shown that the M(II)Cu(II) isomers
are electrochemically and thermodynamically converted to
their Cu(II)M(II) isomers [16]. They have actually shown
that upon heating their M(II)Cu(II) complexes in DMSO at
70 ◦C, The UV–vis spectrum shows characteristic absorp-
tions of Cu(II)M(II) coordination position isomers. Besides,
during the cyclic voltammetry of the M(II)Cu(II) complexes,
the cyclic voltammograms show the characteristic of the
Cu(II)M(II) complexes. This stability is due to the fact that
the Cu(II) metal ion is better stabilized in the more flexible
3.2. Electrochemistry
Cyclic voltammetry is an important tool for the character-
ization of dinuclear complexes. The cyclic voltammogram
of the mononuclear and dinuclear complexes show reduc-
tion potential waves at negative potentials because of the
hard nature of the phenoxide atoms in the ligands. The cyclic
voltammetry of the complexes in this work was studied in the
potential range from 0 to −1.5 V in acetonitrile at room tem-
perature and the data are summarized in Table 1. The cyclic
voltammograms of the homodinuclear complex Cu2L4 is also
shown in Fig. 3. The cyclic voltammograms of the mononu-
clear complexes show one quasi-reversible reduction wave,
which is due to the reduction of Cu(II) to Cu(I).
In the cyclic voltammograms of the homodinuclear com-
plexes two quasi-reversible reduction potential waves are
observed and these two redox potentials are assigned as fol-
lows:
CuIICuII ꢀ CuIICuI ꢀ CuICuI

D.M. Boghaei et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 1–7
5
Table 1
Electrochemical data for the complexes in acetonitrilea
Complexes
Epc
Epa
E11/2 = (Epc − Epa)/2
Epc
Epa
E21/2 = (Epc − Epa)/2
CuL2
CuL4
−0.72
−0.65
−0.69
−0.61
−0.78
−0.77
−0.74
−0.72
−0.46
−0.43
−0.46
−0.47
−0.65
−0.63
−0.62
−0.60
−0.59
−0.54
−0.57
−0.54
−0.71
−0.70
−0.69
−0.66
−
−
−
−
−
−
Cu2L2
Cu2L4
CuNiL1
CuNiL2
CuNiL3
CuNiL4
a
−1.30
−1.29
−
−1.18
−1.17
−
−1.24
−1.23
−
−
−
−
−
−
−
−
−
−
Unit: V vs. Ag/Ag+. Conditions: gold ultramicroelectrodes working, Pt auxiliary and Ag/Ag+ reference electrodes. Scan rate: 100 mV/s. Concentration of
the complexes: (10−3 M). At room temperature supporting electrolyte: tetra-n-butylammoniumhexafluoroborate (0.1 M).
The cyclic voltammograms of the heterodinuclear com-
plexes show one reduction potential wave in the applied
potential range, which is also assigned to the reductions at
the copper centers. The reduction in the nickel centers was
not observed in the applied potentials.
3.3. Catecholase-like activity studies
Although some deliberate studies [11,20,21] has been
directed towards the understanding of the factors affecting
the catecholase-like activity of copper-containing complexes;
the exploration of the oxidation chemistry of the dinuclear
Fig. 4. UV–visspectraofthe10 minintervaloxidationofcatecholbyCu2L4.
complexes is still necessary to fully understand parameters
affecting their catecholase-like activity. In our review in the
the complex concentrations. Typically, 20 mL of 10−3 molar
literature we found that little or no attention has been paid
solution of the complexes were treated with 50 equivalent of
to the study of compartmental heterodinuclear complexes as
catechol in the presence of air at room temperature. The initial
catalyst in the oxidation of catechols to the corresponding
quinones. Having this in our mind and in continuing our
interest in the oxidation of organic compounds under mild
conditions [22,23], we decided to study the catecholase-like
activity of our complexes. The product o-quinone is consid-
erably stable and has a strong absorption band at 390 nm.
A linear relationship between the concentration of the com-
plexes and the initial rate of the oxidation was observed for all
of the complexes, which indicates a first-order dependence on
rate for the catechol oxidation was determined by monitoring
the growth of the band at 390 nm of the product o-quinone at
5 min intervals (Fig. 4). A plot of log[A /(A∞ − At)] versus
∞
time is shown in Figs. 5–7, respectively, for the mononu-
clear, homodinuclear and heterodinuclear complexes. A plot
of log[A /(A∞ − At)] versus time is also shown in Fig. 8
∞
to compare the catalytic results for the mononuclear CuL4,
CuNiL4 and Cu2L4. Our results (Figs. 5–7) show that as the
ring size at the iminic compartment increases, an increase
in the catalytic activity of the complexes is also observed.
Fig. 5. Catecholase-like activity of mononuclear complexes: (a) CuL1; (b)
CuL2; (c) CuL3; (d) CuL4.
Fig. 3. Cyclic voltammogram of Cu2L4 (10−3 M), scan rate: 100 mV/s.

6
D.M. Boghaei et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 1–7
centers in close proximity is necessary to increase the cat-
alytic activity of these catalysts in the oxidation of cate-
chols. Comparison of the catalytic activity of the homod-
inuclear complexes with the corresponding heterodinuclear
complexes (Fig. 8) reveals that the presence of two easily
reduced metallic centers is necessary for obtaining the best
results.
This result is consistent with the previously reported
mechanism in which both metallic centers are reduced
from M(II) to M(I) upon reaction with catechols to form
quinones [19]. Cyclic voltammetry of the complexes shows
that in the homodinuclear complexes the two metallic
centers could be more easily reduced and therefore it
results in the better catalytic activity of the homodinuclear
complexes.
Fig. 6. Catecholase-like activity of heterodinuclear complexes: (a) CuNiL1;
(b) CuNiL2; (c) CuNiL3; (d) CuNiL4.
Acknowledgement
We are gratefully acknowledged to Dr. M.R. Ganjali and
Dr. S. Norouzi for obtaining cyclic voltammograms.
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