A chloro bridged Cu(II)-Cu(II) complex of a new aminophenol ligand: Magnetostructural, radical decay kinetic studies, highly efficient and aerial alcohol oxidation

Article abstract of DOI:10.1016/j.poly.2012.07.096

A new tripodal aminophenol-based ligand containing a pyridine unit was synthesized and characterized by IR and ¹H NMR spectroscopic techniques. A dimeric copper(II) complex of this ligand was prepared and characterized by X-ray crystallography, DFT calculations, spectroscopic techniques and magnetic susceptibility measurements. X-ray analysis revealed a complex in which one phenolate, and amine and imine nitrogens of the ligand are arranged in a distorted square pyramidal geometry (SP) around the copper ions. Two chloride bridges hold both copper atoms together to form the binuclear [L^AphCuCl]₂ complex. This complex exhibits nearly no superexchange coupling between the copper centers. The phenolate moieties of the copper complex were electrochemically oxidized to phenoxyl radical cations [L^AphCuCl]₂⁺. The decay kinetics of the mentioned radical were investigated using a simulation of the voltammogram data. In addition, highly efficient and eco-friendly oxidation of alcohols to aldehydes was achieved with molecular oxygen or air as the oxidant and the [L^AphCuCl]₂-Cs₂CO₃ system as a catalyst.

Full text of DOI:10.1016/j.poly.2012.07.096

Polyhedron 47 (2012) 94–103
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Polyhedron
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A chloro bridged Cu(II)–Cu(II) complex of a new aminophenol ligand:
Magnetostructural, radical decay kinetic studies, highly efficient and aerial alcohol
oxidation
⇑
S. Esmael Balaghi, Elham Safaei , Mohammad Rafiee, Mohammad Hossein Kowsari
Institute for Advanced Studies in Basic Sciences (IASBS), 45137-66731 Zanjan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A new tripodal aminophenol-based ligand containing a pyridine unit was synthesized and characterized
by IR and 1H NMR spectroscopic techniques. A dimeric copper(II) complex of this ligand was prepared
and characterized by X-ray crystallography, DFT calculations, spectroscopic techniques and magnetic sus-
ceptibility measurements. X-ray analysis revealed a complex in which one phenolate, and amine and
imine nitrogens of the ligand are arranged in a distorted square pyramidal geometry (SP) around the cop-
per ions. Two chloride bridges hold both copper atoms together to form the binuclear [LAphCuCl]2 com-
Received 23 May 2012
Accepted 19 July 2012
Available online 13 August 2012
Keywords:
Aminophenol
Aerial alcohol oxidation
Copper complexes
Cyclic voltammetry
plex. This complex exhibits nearly no superexchange coupling between the copper centers. The phenolate
ꢀ+
moieties of the copper complex were electrochemically oxidized to phenoxyl radical cations [LAphCuCl]2
.
The decay kinetics of the mentioned radical were investigated using a simulation of the voltammogram
data. In addition, highly efficient and eco-friendly oxidation of alcohols to aldehydes was achieved with
molecular oxygen or air as the oxidant and the [LAphCuCl]2–Cs2CO3 system as a catalyst.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
have synthesized Cu(II) complexes with tripodal aminophenol
ligands involving [N,O] coordination spheres as synthetic analogs
Many biological reactions are controlled by metalloenzymes
containing one or more metal atoms and with their coordinated li-
gands, such as histidine and tyrosine, holding them. The Lewis acid
properties of these metals allow them to complete a catalytic func-
tion that amino acids would not be able to catalyze [1]. Copper is
one of these metal ions and serves as a cofactor in many funda-
mental reactions mediated by copper enzymes. These catalytic
processing pathways include copper ions with two biological
accessible oxidation states (CuI and CuII), mostly coordinated by
amino acid residues and water [2,3]. Galactose oxidase (GOase),
the best characterized member of the radical-copper oxidase fam-
ily, has the ability to catalyze the oxidation of a wide range of pri-
mary alcohols to aldehydes. The active site of this enzyme contains
a Cu(II) ion, cysteine, water and redox active tyrosine moieties [3].
Hemocyanines and tyrosinases are type III multicopper proteins
and their active sites contain binuclear copper complexes. As well
as GOase, these two enzymes are responsible for catalyzing some
biological processes associated with redox reactions [4,5]. The
chemistry of the enzyme’s active site containing redox active tran-
sition-metal ions with pro-radical ligands is the basis of the syn-
thetic analog approach. For that purpose, bioinorganic chemists
of tyrosine and histidine moieties of amino acids [6–12]. Copper
model enzymes can be divided into mononuclear, binuclear and
multinuclear complexes. Significant efforts have also been made
to provide multinuclear copper complexes containing hydroxo,
oxo and chloro bridged complexes as models for active sites of
multi-copper enzymes and to evaluate their effective role in cata-
lytic enzymatic reactions [13]. Mononuclear copper complexes are
common as functional models in bioinorganic research, while
binuclear complexes have received little attention. Chloro-bridged
binuclear copper(II) complexes have received a special interest be-
cause of their diverse structural properties. On the other hand, the
magneto chemistry of these complexes is a subject of current inter-
est owing to their applications in molecular magnetism [14]. These
complexes are classified into three types according to their cop-
per(II)–copper(II) spin interaction: (a) non-interacting type, (b)
strongly interacting type and (c) weakly (superexchange) type be-
tween copper ions with large separation distances (3–5 Å), which
lead to antiferromagnetic or ferromagnetic properties of the com-
plexes [15]. Although there are some reports about oxygen or
chloro-bridged binuclear copper complexes as models for catechol
oxidase [16–19], there is no report on any of these complexes for
aerial alcohol oxidation. In this paper we report the synthesis,
characterization, magnetic and redox properties of the [LAphCuCl]2
complex of the newly synthesized redox active [N,O]-donor
⇑
Corresponding author.
E-mail address: safaei@iasbs.ac.ir (E. Safaei).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.096

S.E. Balaghi et al. / Polyhedron 47 (2012) 94–103
95
rate was set at 50 mV sꢀ1. The solutions were deoxygenated by bub-
bling nitrogen gas through them for 10 min.
The X-ray data for the reported complexes were collected on a
Bruker–Nonius Kappa CCD diffractometer at 100(2) K using graph-
ite-monochromated Mo K
a radiation (k = 0.71073 Å). Final cell
constants were obtained from a least-squares fit of integrated
reflections. Intensity data were corrected for Lorentz and polariza-
tion effects. The data sets were corrected for absorption (SADABS
,
Scheme 1. The structure of the HLAph ligand.
Bruker–Nonius 2004) [27]. The structures were solved by direct
method and refined by full-matrix least-squares techniques based
on F2
(SHELXTL software package) [28]. The GC analysis was carried
aminophenol ligand, 2,4-di-tert-butyl-6-(pyridine-2-ylmethylami-
no)phenol (HLAph) (Scheme 1), which provides the coordination
sphere around the copper centers. In addition, we describe a ‘‘green
chemistry approach’’ with respect to the highly efficient, selective
and eco-friendly oxidation of alcohols to aldehydes with molecular
oxygen as the oxidant and the [LAphCuCl]2 complex as the catalyst.
out using a VARIAN CP 3800 with dihydrogen carrier gas.
3. Experimental
3.1. Preparation of 3,5-di-tert-butylcyclohexa-3,5-diene-1,2-dione
(3,5-DTBQ)
3,5-Di-tert-butylcyclohexa-3,5-diene-1,2-dione (3,5-DTBQ) was
synthesized according to a modified literature procedure [29].
2. Materials and methods
Reagents and analytical grade materials were obtained from
commercial suppliers and used without further purification, except
those for electrochemical measurements. Elemental analyses
(C,H,N) were performed by the Elementar, Vario EL III. Fourier
transform infrared spectroscopy on KBr pellets was performed on
a FT IR Bruker Vector 22 instrument. NMR measurements were
done on a Bruker 400 instrument. UV–Vis absorbance digitized
spectra were collected using a CARY 100 Bio spectrophotometer.
Magnetic susceptibility data were measured from powder samples
of the solid material in the temperature range 2–290 K using a
SQUID susceptometer with a field of 1.0 T (MPMS-7, Quantum
Design, calibrated with a standard palladium reference sample,
error <2%). Multiple-field variable-temperature magnetization
measurements were done at 1, 4 and 7 T, also in the temperature
range 2–290 K with the magnetization equidistantly sampled on
a 1/T temperature scale.
The experimental data were corrected for underlying diamag-
netism by use of tabulated Pascal’s constants [20,21], as well as
for temperature-independent paramagnetism. The susceptibility
and magnetization data were simulated with our own package julX
for exchange coupled systems [22]. The simulations are based on
the usual spin-Hamiltonian operator (Eq. (1)) for binuclear com-
plexes with two spins S1 = S2 = 1/2.
3.2. Preparation of 2,4-di-tert-butyl-6-(pyridine-2-yl-
methylamino)phenol (HLAph
)
To a solution of 3,5-DTBQ (1 mmol, 0.22 g) in n-heptane (9 ml)
was added 2-(aminomethyl) pyridine (1 mmol, 0.108 ml), followed
by stirring for 30 min at room temperature. A yellow precipitate
was formed, collected by filtration and then washed with cold
n-heptane. The solid material was air-dried and crystallized in a
1:1 dichloromethane/n-hexane mixture (0.2 g, 65%). Anal. Calc.
(found) for C20H26N2O: C 77.38 (77.29), H 8.44 (8.26), N 9.02
(9.06). IR (KBr, cmꢀ1): 688, 751, 793, 871, 915, 959, 1030, 1143,
1193, 1246, 1306, 1362, 1473, 1586, 1623, 2869, 2902, 2956,
3059, 3347. 1H NMR (400 MHz, CDCl3, Me4Si) dH: 1.359(s, 9H),
1.489(s, 9H), 7.292(s, 2H), 7.354(q, 1H), 7.853(q, 2H), 8.264(t,
1H), 8.746(t, 1H), 8.899(s, 1H). 13C NMR (100 MHz, CDCl3, Me4Si)
dC: 29.42, 31.59, 34.62, 34.99, 76.71, 77.03, 77.34, 110.38, 121.23,
124.55, 125.09, 135.51, 136.73, 141.69, 149.74, 154.58, 155.98.
M.p.: 135–138 °C.
3.3. Synthesis of [LAphCuCl]2
To a solution of HLAph (1 mmol, 0.31 g) and triethylamine
(2 mmol, 0.28 ml) in ethanol (30 ml) was added CuCl2ꢁ2H2O
(1 mmol, 0.17 g), followed by stirring for 2 h at room temperature.
A purple precipitate was formed, collected by filtration and then
dried in a vacuum oven at 50 °C. Single-crystals suitable for
X-ray crystallography were obtained by slow diffusion of ether into
a solution of the complex in dichloromethane at room temperature
(0.81 g, 90%). Anal. Calc. (found) for C40H50Cl2Cu2N4O2: C 58.81
(58.59), H 6.17 (6.19), N 6.86 (6.83). IR (KBr, cmꢀ1): 762, 820,
864, 911, 1022, 1157, 1198, 1254, 1308, 1359, 1398, 1473, 1531,
1600, 2866, 2906, 2952, 3058, 3095. UV–Vis in CH2Cl2, kmax, nm
*
^
*
^
*
^
*
^
*
^
H ¼ ꢀ2J½S1 ꢁ S2ꢂ þ gbðS1 þ S2Þ ꢁ B
ð1Þ
where J is the spin coupling constant and g is the average of the
electronic g-matrix components (kept equal for both Cu sites). Diag-
onalization of the Hamiltonian was performed with the routine
ZHEEV from the LAPACK library [23] and the magnetic moments were
obtained from the first-order numerical derivative dE/dB of the
eigenvalues. The powder summations were done using a 16-point
Lebedev grid [24,25]. Intermolecular interactions were considered
(e
, Mꢀ1 cmꢀ1): 240(892), 574(157).
by using a Weiss temperature,
HW, as the perturbation of the tem-
perature scale, kT = k(T ꢀ W) for the calculation. Cyclic voltamme-
H
try was performed using an Autolab potentiostat/galvanostat 101.
The working electrode was a glassy carbon disk (2.0 mm diameter)
and a Pt wire was used as the counter electrode. The working elec-
trode potentials were measured versus a quasi-reference electrode
of silver wire (all electrodes from Azar Electrode). The Ag quasi-
reference electrode was calibrated using a ferrocene/ferrocenium
redox couple (0.41 V) as an external standard [26]. The ionic
strength of all the experimental solutions were maintained at
0.1 M by adding tetrabutylammonium perchlorate solution. Prior
3.4. Spectrophotometric investigation of mix ligand HLAph–bipyridine
and Cu2+ complexation
In an experiment, 2 ml of a CuCl2ꢁ2H2O solution in CH3CN
(4 ꢃ 10ꢀ6 M) was transferred into a cuvette. UV–Vis spectra were
recorded in the range 260–800 nm about 5 min after each addition
of 10
l
L of a solution containing a 1:1:1 ratio (1.60 ꢃ 10ꢀ1 M) of
the ligands HLAph/bipyridine/triethylamine. Changes in the absor-
bance of the copper chloride complex upon addition of the ligands
in solution were monitored at the LMCT maximum wavelength of
the complex.
to the measurement, the GC electrode was polished with 0.1
lm
alumina powder and washed with distilled water. The voltage scan

96
S.E. Balaghi et al. / Polyhedron 47 (2012) 94–103
3.5. Synthesis of [LAphCu(bpy)]
A solution of CuSO4 (1 mmol, 0.16 g) in 3 ml H2O was added to a
solution of HLAph (1 mmol, 0.22 g) and bipyridine (1 mmol, 0.156 g)
in 5 ml 50% ethanol. A purple precipitate was formed immediately.
The suspension was diluted with water, collected by filtration and
dried in a vacuum oven at 70 °C over 12 h (0.401 g, 80%). All efforts
to get crystals from the complex were unsuccessful. Anal. Calc.
(found) for C28H31CuN4O: C 66.84 (66.16), H 6.21 (6.13), N 11.14
(10.80). IR (KBr, cmꢀ1): 704, 755, 798, 827, 867, 912, 1035, 1087,
1153, 1202, 1258, 1358, 1469, 1553, 1595, 2868, 2906, 2955,
Scheme 3. Synthesis of the binuclear copper complex.
the metal. The m(C@N) band of the copper complex, compared with
the corresponding ligand, is shifted to lower energy (1600–
1630 cmꢀ1) by 10–20 cmꢀ1 which indicates coordination of the
imine nitrogen to the metal. The electronic spectrum of the [LAph-
CuCl]2 complex recorded in CH2Cl2 is presented in Fig. 1.
3065, 3411. UV–Vis in CH2Cl2, kmax, nm (e
, Mꢀ1 cmꢀ1): 238(714),
The intra-ligand absorption for the higher energy band
(<400 nm) is assigned to p–
p⁄ and n–p⁄ transitions (ILCT). The low-
287(713), 553(153).
er energy band (274 nm) is assigned to a phenolate-to-Cu(II)
charge transfer transition (LMCT). In order to explore the effects
of a secondary p-acceptor bipyridine (bpy) ligand on the electronic
3.6. Catalytic activity of the [LAphCuCl]2 complex for the oxidation of
alcohols
spectrum of complex, we decided to investigate the electronic
spectrum of the mixed ligand complex of [LAphCu(bpy)]. The men-
tioned complex was prepared in situ from a titration of the copper
salt with a mixture of HLAph, triethylamine and bipyridine in suit-
able ratio. The formation of [LAphCu(bpy)] was monitored by fol-
lowing the increase of the band at 553 nm, corresponding to the
LMCT maximum wavelength of the above mentioned complex.
The effect of bipyridine on the electronic spectrum of the complex
was observed by a blue shift compared to the transition band of
[LAphCuCl]2 at 574 nm (Fig. 1). This shift can be attributed to the
electron-withdrawing effect of bipyridine, stabilizing the pheno-
The oxidation was carried out in an oxygen atmosphere at room
temperature. In a typical experiment, alcohol (1 mmol), [LAphCuCl]2
(0.024 g, 5 mol%) and Cs2CO3 (0.650 g, 2 mmol) in 5 ml of air (oxy-
gen) saturated toluene were taken in a 15-ml two-necked round-
bottom flask which was equipped with an air or oxygen balloon.
The solution was magnetically stirred for some hours and the mix-
ture was filtered through a plug of silica, then diluted with toluene
(2 ml). The progress of the reaction was monitored by gas chroma-
tography. In a separate blank test, a 15-ml two-necked round-
bottom flask was charged with a solution of copper(II) chloride
or the ligand in a mixed medium of Cs2CO3 (0.650 g, 2 mmol) in
5 ml of air (oxygen) saturated toluene. Then the flask was equipped
with an oxygen balloon. The progress of the reaction was monitored
by gas chromatography.
late
p (HOMO) orbitals.
4.1. X-ray data collection and crystal structure determination of
[LAphCuCl]2
4. Discussion
4.1.1. Crystal structure of [LAphCuCl]2
The ligand HLAph was synthesized based on a simple condensa-
tion of 2-(aminomethyl) pyridine with 3,5-DTBQ to give the imin-
ophenol ligand. NMR and IR results show that an aminophenol
ligand has been produced. The reason for this criterion can be seen
clearly in Scheme 2. This reaction involves a [1,5] hydrogen sigma-
tropic shift between the carbon C1 and oxygen O5 atoms of the
iminophenol ligand (Scheme 2).
Crystals of [LAphCuCl]2, appropriate for X-ray analysis, were ob-
tained by slow diffusion of ether into a solution of the complex in
dichloromethane at 298 K. Dark purple crystals of [LAphCuCl]2 in
the monoclinic crystalline system, space group P21/c (a =
16.253(2), b = 9.1951(11), c = 13.4351(17) Å) were formed. Crystal
data for the [LAphCuCl]2 complex are given in Table 1. An ORTEP
view of the complex is shown in Fig. 2. Selected bond distances
and bond angles are listed in Table 2. All the bond lengths and an-
gles for [LAphCuCl]2 are presented in the supplementary material.
The symmetrical parts of the structure is formed by a binuclear
complex of the formula [LAphCuCl]2 and the two symmetrical parts
are linked by two chloride bridges (Fig. 2).
The binuclear copper complex was easily formed in good yield
by the reaction of a methanolic solution of the ligand with copper
chloride and triethylamine in a suitable ratio at room temperature
(Scheme 3).
In the IR spectrum of the ligand (HLAph), the mOH stretch is ob-
served in the range 2900–3000 cmꢀ1. The occurrence of this band
at such a relatively low wavenumber is characteristic of aminophe-
nol ligands and is an indication of extensive hydrogen bonding. A
strong band in the 1620–1640 cmꢀ1 region was assigned to
The distortion parameter
are the two largest L–M–L angles of the coordination sphere [30],
for this complex is 0.196 for each copper center (Table 2). Compar-
s
([(U1
ꢀ
U2)/60], in which U1 and U2
ing these
nal bipyramid (
s
values with the corresponding values for an ideal trigo-
= 1) and square pyramid ( = 0) indicate that this
m
(C@N). In the IR spectrum of the complex, the strong and sharp
band at a frequency around 3000 cmꢀ1 for the
OH stretch of the li-
gand disappeared, proving the coordination of the phenol group to
s
s
m
complex has a distorted square based pyramidal (SP) structure. In
this geometry for [LAphCuCl]2, the nitrogens N(1) and N(8), chloro
Scheme 2. [1,5] Hydrogen shift between the C(7) carbon and O(15) oxygen atoms in HLAph
.

S.E. Balaghi et al. / Polyhedron 47 (2012) 94–103
97
with the average value (3.499 Å) for chloro-bridged copper(II) di-
mers [32]. Regarding the Cu( -Cl2)Cu core, the distance ‘‘R0’’ be-
10000
8000
6000
4000
2000
0
l
tween the copper center and the apical chloro ligand Cu–Cl of
2.555(8) Å is larger than that value for the copper and basal chloro
ligand Cu–Cl of 2.320(9) Å. These two distances are in good agree-
ment with the reported range (2.2–2.554 Å) of Cu–Cl bond lengths
for five-coordinate copper(II) complexes [32]. The basal Cu–Cl–Cu
and apical Cu–Cl–Cu core angles are the same, 91.50(3)°, and both
Cl–Cu–Cl bond angles have a value of 88.51(3)°. Consequently, the
[LAphCuCl]
[LAphCuII(bpy)]
Cu(l-Cl2)Cu core has a symmetric geometry. The Cu–N distance of
2.044(2) Å and Cu–N(8) distance of 1.959(3) Å are relatively simi-
lar. The C(7)–N(8) bond length of 1.293 Å is shorter than a normal
C–N bond. According to the literature, C(7)–N(8) seems to be a
double bond, which confirms the sigmatropic shift during the li-
gand synthesis process [33].
4.2. Magnetic susceptibility measurements
230.0
330.0
430.0
530.0
630.0
730.0
830.0
λ(nm)
Magnetic susceptibility data for polycrystalline samples of
[LAphCuCl]2 complex were collected in the temperature range 2–
290 K with an applied magnetic field of 1 T. The effective magnetic
Fig. 1. UV–Vis spectra of [LAphCuCl]2 and [LAphCuII(bpy)].
moment per molecule (
nearly constant, 2.55
l
eff) for the [LAphCuCl]2 complex remains
l
B, in the temperature range 290–30 K
(Fig. 3), which is close to the value of the magnetic moment for
two uncoupled spins of S = 1/2. Below 30 K, this value starts to de-
crease smoothly with decreasing temperature and reaches a value
of leff = 2.06 lB at 1.88 K (Fig. 3), which is slightly lower than the
expected value for two uncoupled spin-only Cu(II) ions (S = 1/2,
Table 1
Crystal data and structure refinement for [LAphCuCl]2.
Identification code
[CuLAphCl]2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C40H50Cl2Cu2N4O2
816.82
100(2)
0.71073
monoclinic
P21/c
16.253(2)
9.1951(11)
13.4351(17)
90
g = 2.0).
The observed temperature dependent magnetic moment for
[LAphCuCl]2 confirms the dimeric nature of this complex. The fitting
of these data shows exchange coupling parameters of J = ꢀ1 cmꢀ1
and g = 2.10. It is observed that J is nearly zero, suggesting that
super-exchange coupling does not happen in [LAphCuCl]2. It is well
known that in binuclear copper(II) complexes, the singlet–triplet
(S–T) energy gap J correlates (Eqs. (2) and (3)) with the structural
b (Å)
c (Å)
a
(°)
b (°)
107.422(2)
90
1915.8
c
(°)
factors of the bridge angle
[12,28,30–32,34,35].
(
a)
and Cu. . .Cu distance (d)
Volume (Å3)
Z
2
Calculated density (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
2, 1.416
1.289
852
)
2J ¼ ET ꢀ ES
ð2Þ
ð3Þ
2
J ¼ 2j
ꢀ ðes
ꢀ
eaÞ =U
Crystal size (mm)
Theta range for data collection (°)
Limiting indices
0.16 ꢃ 0.14 ꢃ 0.02
2.58–30.82
ꢀ23 6 h 6 23
ꢀ13 6 k 6 13
ꢀ19 6 l 6 19
where 2
magnetic contributions, respectively.
The variation of , d and the ratio of
the Cu(3dx2ꢀy2)/Cl(3p) orbitals and therefore changes es
j
and (es
ꢀ
e
a)2/U terms describe the ferro- and antiferro-
Reflections collected/unique
Completeness
Refinement method
52862/6004 (Rint = 0.0590)
99.9% (h = 30.82)
a
a/R0 affects the overlap of
ꢀ
ea
Full-matrix least-squares on F2
6004/7/248
1.161
(the energy difference between the Cu(3dx2ꢀy2)/Cl(3px) and
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Cu(3dx2ꢀy2)/Cl(3py)) SOMO orbitals (Scheme 6). Based on the Hat-
field [36] theory, if a
/R0 is in the range of 32.6 to 34.8 Åꢀ1, the ex-
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0597, wR2 = 0.1306
R1 = 0.0821, wR2 = 0.1415
1.036 and ꢀ0.939
change interaction is ferromagnetic, and for values lower than 32.6
and higher than 34.8 Åꢀ1 the exchange interaction is antiferromag-
netic [37,38]. In the case of our chloro-bridged dimer, [LAphCu Cl]2,
Largest difference in peak and hole (e Åꢀ3
)
the value of the
34.8 Åꢀ1. It demonstrates that in this complex, the ferromagnetic
term 2 is nearly equal to the antiferromagnetic term (es
a)2/
U. Indeed this ratio ( /R0) is the crossover point between ferromag-
a
/R0 is 35.8 Åꢀ1 close to the border value of
bridging ligand Cl(1) and O(15) phenolate atoms of the ligand are
nearly coplanar, occupying the basal positions of the pyramid,
whereas the other chloro bridging ligand #Cl(1) occupies the apical
position. Consequently, the geometry of the complex consists of
two square pyramids (Scheme 4) sharing a base-to-apex edge with
parallel basal planes (SP II type) [31,32].
There are two other kinds of pyramidal arrangements [31]: SP I
type, square pyramids sharing a basal edge with the two bases
nearly perpendicular to one another, and SP III type, two square
pyramids that share a base-to apex edge, but with coplanar basal
planes (Scheme 5). The Cu–Cu distance of 3.495 Å is in agreement
j
ꢀ
e
a
netic and antiferromagnetic properties. In other word, our complex
shows no superexchange interaction between the copper centers.
This corresponds to the results for chloro-bridged binuclear cop-
per(II) complexes (SP II type) which usually show very small ferro-
magnetic [39–45] or antiferromagnetic coupling constants [46,47].
This behavior is attributed to the distortion around the copper ions,
resulting in the orthogonality between the magnetic orbitals
(dx2ꢀy2) positioned on the basal plane of each unit [31].

98
S.E. Balaghi et al. / Polyhedron 47 (2012) 94–103
Fig. 2. The structure of [LAphCuCl]2. Ellipsoids are plotted at the 40% probability level. Hydrogen atoms are omitted for clarity.
Table 2
Selected bond lengths (Å) and angles (°) for [LAphCuCl]2.
Cu23–O(15)
1.947(2)
Cu23–N(8)
1.959(3)
2.044(2)
2.3197(9)
2.5548(8)
2.5548(8)
1.331(5)
1.35023
Cu23–N23
Cu23–Cl23
Cu23–Cl23#1
Cl23–Cu23#1
N23–C(2)
Scheme 5. SP I and SP III geometries of binuclear dihalo-bridged Cu(II) complexes
[32].
N23–C23
C(2)–C(3)
1.39723
1.385(5)
1.382(5)
1.39523
1.444(5)
1.292(3)
1.38823
C(3)–C23
C23–C(5)
C(5)–C23
C23–C(7)
C(7)–N(8)
N(8)–C(9)
O(15)–Cu23–N(8)
O(15)–Cu23–N23
N(8)–Cu23–N23
O(15)–Cu23–Cl23
N(8)–Cu23–Cl23
N23–Cu23–Cl23
O(15)–Cu23–Cl23#1
N(8)–Cu23–Cl23#1
N23–Cu23–Cl23#1
Cl23–Cu23–Cl23#1
Cu23–Cl23–Cu23#1
82.45(10)
162.50(11)
80.07(11)
99.55(8)
150.76(7)
96.10(9)
98.0823
120.28(7)
90.08(7)
88.51(3)
91.49(3)
4.3. Computational structural study
Complementary to the X-ray structure determination of the
current synthesized dichloro-bridged Cu(II)–Cu(II) complex and
in order to gain more insight into the correlation between molec-
ular structure and spin multiplicity (2s + 1) of the complex, calcu-
lations based density functional theory (DFT) have been carried out
Fig. 3. variation of magnetic susceptibility (v) with variation in temperature for the
complex [LAphCuCl]2.
using the GAUSSIAN 09 suite of programs [48]. We used the X-ray
crystallographic information file (cif) as the initial molecular
geometry for all computations. Two separate molecular structural
optimization calculations have been performed at the B3LYP/6-
31G(d) level, one with multiplicity 1 and another with multiplicity
3. The calculated structural parameters of the optimized structures
are summarized in Table 3 and compared with the X-ray data for
Scheme 4. Geometry of the binuclear dichloro-bridged [LAphCuCl]2 complex (SP II
type).

S.E. Balaghi et al. / Polyhedron 47 (2012) 94–103
99
hydrogen atoms during of two similar runs as for the earlier calcu-
lations. Finally, we determined the energy gap between the HOMO
and LUMO orbitals of the dichloro-bridged Cu(II)–Cu(II) complex
from the structure obtained in the last calculations, reported in
Table 4. The low energy gap between the HOMO and LUMO
(ꢄ0.03 eV) is in good agreement with the small experimental
observed J (ꢀ1 cmꢀ1) for this complex.
4.4. Electrochemistry
The electrochemical behaviors of the ligand (LAph), metal ion
(Cu(acac)2) and [LAphCuCl]2 and [LAphCu(bpy)] complexes were
studied using cyclic voltammetry. The cyclic voltammogram (CV)
of the ligand (HLAph) revealed two anodic peaks (A1 and A2) in
the positive direction scan (Fig. 4). These oxidation peaks are
attributed to ligand centered redox processes in which the phenol
moiety of the ligand converts into related phenoxyl radicals.
In order to explore the effects of the secondary
p-acceptor
bipyridine ligand on the stability of the radical species, the electro-
chemical oxidation of the [LAphCu(bpy)] complex was investigated.
As seen in Fig. 4, comparing the oxidation potentials of [LAphCuCl]2
and [LAphCu(bpy)] with the ligand (756 mV), reveals a positive shift
in the peak potentials of the complexes of 110 and 403 mV for [Cu-
LAphCl]2 and [LAphCu(bpy)] respectively. It may be related to the
lower amount of negative charge on the phenolate group of the
complexes due to coordination to the copper center. [LAphCu(bpy)]
shows a positive shift compared to [LAphCuCl]2, which is attributed
Scheme 6. The two SOMO orbitals of the complex [LAphCuCl]2 [31].
Table 3
The structural parameters from the X-ray data and molecular geometry optimizations
by B3LYP method using 6-31G(d) basis sets for all atoms with two different
multiplicities.
to the hard oxidation of phenolate to the phenoxyl radical
[LAphCuCl]2 due to the
p-acceptor character of the bipyridine li-
ꢀ+
gand. A UV–Vis study of the phenolate-to-Cu(II) charge transfer
band of the [LAphCu(bpy)] complex showed a blue shift of the LMCT
band (Fig. 1). The higher energy of the phenolate-to-Cu(II) charge-
transfer band of this complex indicates that the Cu(II)bpy moiety
has a high tendency to pull electrons from the phenolate group to-
wards itself. Based on these results, one can say that the stability
constant of [LAphCu(bpy)] is more than the stability constant of
[LAphCuCl]2. The metal-centered voltammograms, which have been
observed in the negative potential range, reveal the CuII/CuI reduc-
tion of the complexes.
Method;
Cu–Cu
Cu–Cl bond, Cu–Cl–Cu Cl–Cu–Cl
a
/R0
s
multiplicity distance (Å) in-plane,
a
angle (°) angle (°) (° Åꢀ1
)
(Å)
out-of-plane
X-ray data, 3.495
(cif file)
DFT-based
2s + 1 = 1
DFT-based
2s + 1 = 3
2.320, 2.555 91.50
2.514, 2.448 90.15
2.508, 2.455 92.56
88.51
89.85
87.44
35.81 0.196
35.86 0.441
36.91 0.410
3.514
3.587
Fig. 5 shows that the reduction peak of the complexes shifts to
less negative potentials comparing to Cu(acac)2 due to the electron
donating ability of the ligands. The shift of the cathodic peak po-
tential for [LAphCu(bpy)] is considerably more than that for [LAph-
CuCl]2. For all of the complexes, the first anodic peak (A1) is
considered as the oxidation of the phenolate ligand to its radical
cation and its cathodic counterpart (C1) is considered as the reduc-
tion of this electrochemically generated radical.
It is shown in Fig. 6 that the CV of the ligand does not show a
considerable cathodic peak. This fact is indicative that the oxidized
intermediate formed at the surface of electrode is removed by a
chemical reaction in the timescale of the voltammetric experiment.
On the other hand, the CV of [LAphCu(bpy)] shows a cathodic peak
with a cathodic to anodic peak ratio nearly equal to but less than
unity. It is a good criteria for the stability of the electrogenerated
species on the timescale of the voltammetric experiments. The
timescale of a voltammetric experiment is determined by the var-
iation of the scan rate [49]. More detailed studies of the complexes
were performed at various scan rates (Fig. 6).
The magnitude of the cathodic currents and cathodic to anodic
peak current ratio increases with increasing scan rate. Such behav-
iors are good criteria for the EC mechanism to consist of an electron
transfer reaction (E) followed by a chemical reaction (C). According
to these results, electrochemical oxidation of [LAphCuCl]2 causes
the formation of a related radical that undergoes a degradation
reaction under these conditions [50]. The rate constant of the reac-
tion of the phenoxyl radical from the oxidation of the phenolate
Table 4
The total energy (in Hartree) and the energy gap (in eV) between the HOMO and
LUMO orbitals of the complex calculated by B3LYP method using 6-31G(d) basis sets
for two different multiplicities. In these calculations, all of the non-hydrogen atomic
sites are frozen.
Calculation type (frozen
non-hydrogen sites)
Energy (a.u.)
HOMO–LUMO
gap (eV)
B3LYP/6-31G(d)
2s + 1 = 1
B3LYP/6-31G(d)
2s + 1 = 3
ꢀ6124.96830267 0.02787
ꢀ6125.03784889
a
b
0.09392
0.09381
the current Cu(II) complex. The differences between the structural
parameters of the chloro-bridged Cu(II)–Cu(II) complex for ferro-
and antiferromagnetic conditions can be investigated. The
achieved parameters of both X-ray data (with Jexptl. = ꢀ1 cmꢀ1
)
and DFT calculations with multiplicity 1 correspond to an antifer-
romagnetic property for the complex, while the obtained parame-
ters of DFT calculations with multiplicity 3 match a ferromagnetic
property for the respected optimized structure of the complex.
Changing the spin multiplicity from 1 to 3, increased both the
Cu–Cu distance and the Cu–Cl–Cu (
the chloro-bridged complex.
In the next stage, keeping the distortions of real structure of the
complex, all atomic sites of the complex were frozen, except for the
a) angle of the main core of

100
S.E. Balaghi et al. / Polyhedron 47 (2012) 94–103
Fig. 4. Cyclic voltammograms of 1.0 mM (a) HLAph
,
(b) [LAphCuCl]2 and (c)
[LAphCuII(bpy)] in dichloromethane solution containing 0.1 M tetrabutylammonium
Fig. 6. Experimental (curves
a and c) and simulated (curves b and d) cyclic
voltammograms of 1.0 mM [LAphCuCl]2 scan rates are: 400 mV sꢀ1 for (a) and (b)
and 200 mV sꢀ1 for (c) and (d).
perchlorate at a glassy carbon electrode, scan rate 100 mV sꢀ1
.
Table 5
Alcohol oxidation results in toluene, catalyzed by copper [LAphCuCl]2 in the presence
of Cs2CO3 as a base and O2 as the oxidant.
Entry
Substrate
Time (h)
Conversion (%)
O2
8
Air
O2
89
Air
81
1
2
12
OH
8
12
96
86
OH
3
4
6
8
24
8
83
96
82
82
OH
OH
Fig. 5. Cyclic voltammograms of 1.0 mM (a) [LAphCuII(bpy)], (b) [LAphCuCl]2 and (c)
(Cu(acac)2) in dichloromethane, the other conditions are the same as for Fig. 4.
5
6
14
14
24
16
90
87
85
84
OH
OH
group was estimated at various scan rates. The simulation was car-
ried out based on the proposed EC mechanism and assuming semi-
infinite one-dimensional diffusion on a planar electrode. The
experimental parameters entered for the digital simulation consist
of the following: starting potential (Estart), switching potential
Cl
Cl
7
8
9
10
6
20
10
12
98
96
95
85
92
91
OH
Cl
(Eswitch), scan rate (v), half wave potential (E1/2) and analytical
OH
concentration of the species. The formal potentials were obtained
experimentally as the midpoint potential between the anodic
and cathodic peaks (Emid). All of these parameters were kept con-
NO2
8
stant and the observed rate constant of the chemical reaction, kobs
,
OH
OH
O2N
MeS
was allowed to change during the fitting processes. The fitting con-
sists of finding a rate constant for which the differences between
the digitally simulated and the experimental data reach a mini-
mum [51]. Fig. 6 shows that there is good agreement between
the simulated and experimental cyclic voltammograms. The
obtained rate constant of radical degradation was 0.6 sꢀ1 for [LAph-
CuCl]2, from the average of four independent simulations at
10
11
8
8
99
5
97
3
OH
24
24
12
12
12
92
87
OH

S.E. Balaghi et al. / Polyhedron 47 (2012) 94–103
101
Table 7
The result of blank tests with LAph and copper salt.
Entry Catalytic system
Solvent
Time
(h)
Conversion
(%)
1
2
3
4
5
Copper(II) chloride/Cs2CO3
Toluene 24
Toluene 24
28
24
66
17
63
Copper(II) acetate/Cs2CO3
Scheme 7. Catalytic activity of [LAphCuCl]2 in the aerobic oxidation of alcohols.
Copper(II) chloride/Cs2CO3/LAph
Toluene
Toluene 24
Toluene
8
L
Aph/Cs2CO3
Copper(II) chloride/Cs2CO3/
pyridine
8
Table 6
The effect of base and solvent on catalytic activity.
Entry
Base
Solvent
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
K2PO3
K2CO3
Et3N
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
1 eq Cs2CO3
2 eq Cs2CO3
3 eq Cs2CO3
Toluene
Toluene
Toluene
CH3CN
24
24
24
8
8
8
8
8
8
8
26
20
3
57
54
62
84
76
89
90
THF
1,2-Dichloroethane
n-Hexane
Toluene
Toluene
Toluene
10
various scan rates. The obtained rate constant of degradation is a
good criteria for the relative stability of the phenoxyl radical which
was obtained by electrochemical oxidation of the complex.
4.5. The catalytic activity of the [LAphCuCl]2 complex in the aerobic
oxidation of alcohols
In continuation of our studies on the application of [LAphCuCl]2,
we were interested in using this complex for the oxidation of ben-
zyl alcohols to the corresponding aldehydes. With optimized con-
ditions, we studied the efficiency of this complex for the oxidation
of various alcohols in toluene using cesium carbonate as a base and
air or oxygen as the oxidant at room temperature. The results for
the oxidation of a variety of benzyl alcohols are summarized in
Table 5. The catalytic activity of the [LAphCuCl]2 complex in the
aerobic oxidation of alcohols is shown in Scheme 7.
Fig. 7. Screening the reaction of benzyl alcohol under optimized conditions.
Having optimized the reaction conditions, we investigated the
effect of bases on the oxidation of benzyl alcohol. Using bases such
as K3PO4, triethylamine and CaCO3, the alcohol conversions were
lower than that in Cs2CO3 (Table 6, entries 1, 2, 3 and 9). The num-
ber of equivalents of Cs2CO3 was optimized (Table 6, entries) and
Scheme 8. The possible reaction pathway in the catalytic cycle of alcohol oxidation using the [LAphCuCl]2/Cs2CO3 catalyst.

102
S.E. Balaghi et al. / Polyhedron 47 (2012) 94–103
the best results (89% conversion) was achieved when 2 equivalents
of Cs2CO3 were used. The solvent effect was also investigated and
the results are summarized in Table 6. Employment of polar sol-
vents, such as acetonitrile and THF, yielded low conversions of ben-
zyl alcohol (>60%, Table 6, entries 4 and 5) compared to non-polar
solvents, such as n-hexane, 1,2-dichloroethane and toluene (<60%,
Table 6, entries 6, 7 and 9). According to the literature [40,41,44],
for the ‘‘copper complex–Cs2CO3’’ the higher activity in toluene
can be attributed to possible adduct formation between copper
and/or cesium cations and the aromatic solvent, as well as polarity
effects. These results lead us to suggest that Cs2CO3 acts not only as
a strong base for deprotonation of alcohol, but also as a heteroge-
neous support for copper to produce an stable [LAphCuCl]2–Cs2CO3
catalytic species [52]. Comparing the alcohol oxidation results in
the presence of Cs2CO3 with triethyl amine as a base, confirm this
claim. In this system, [LAphCuCl]2 is soluble in toluene while Cs2CO3
is not soluble. Therefore, the [LAphCuCl]2–Cs2CO3 catalyst can
possibly be considered as an intermediate between a homogenous
and heterogeneous catalyst (Scheme 8).
stabilization of the phenoxyl radicals was investigated. In addition,
a highly efficient and eco-friendly oxidation of alcohols to alde-
hydes was achieved with air or molecular oxygen as the oxidant
and the [LAphCuCl]2–Cs2CO3 system as a catalyst. This is the first re-
port on the aerial oxidation of alcohols using chloro bridged binu-
clear copper complexes.
Acknowledgments
The authors are grateful to the Institute for Advanced Studies in
Basic Sciences (IASBS). Special thanks are given to Prof. Babak Kar-
imi and Dr. Saman Alavi for valuable comments and manuscript
editing. E. Safaei gratefully acknowledges support from the Insti-
tute for Advanced Studies in Basic Sciences (IASBS) Research Coun-
cil under Grant No. G2012IASBS127. Thanks are due to Dr. Thomas
Weyhermüller, Dr. Eckhard Bill, Mr. Andreas Göbels and Mrs. Heike
Schucht (MPI-BAC) from Max-Planck-Institute for Bioinorganic
Chemistry for their valuable help.
All the alcohols could be oxidized to the corresponding alde-
hydes with good conversions (>80%, Table 5, entries 1–12), and
the formation of over-oxidized carboxylic acids was not observed.
The blank tests verified copper(II) chloride/Cs2CO3 and copper(II)
acetate/Cs2CO3 affords the aldehydes in low yield (Table 7). In
other words, this ‘‘ligand-free’’ copper catalyst does not show good
efficiency for the aerobic oxidation of alcohols. On the other hand,
the blank test with copper(II) chloride/Cs2CO3 and copper(II) ace-
tate/Cs2CO3 and free ligand improved the aldehyde yield (Table
7). Supposing the solubility effect of the ligand increased the alco-
hol conversion, the above test was repeated in the presence of pyr-
idine. According to the literature [11], pyridine causes increasing
solubility of the CuCl2 salt in a non-polar solvent by the formation
of the [Cu(py)2Cl2] coordinated complex. The result shows that
increasing the solubility of the copper salt in a non-polar solvent
causes an increased alcohol conversion (Table 7, entry 5). Compar-
ing these results with those for [LAphCuCl]2–Cs2CO3] as a catalyst
system led us to the fact that the structure of this coordination
copper complex has an important role in the oxidation of alcohols,
in which not only the copper but also the ligand is oxidized and
reduced during the catalytic cycle.
Alternatively, time screening of the reaction showed an initia-
tion time of 2 h (Fig. 7). It suggests the production of a catalytically
active species due to the possible electrostatic interaction between
the Cl anions of the [LAphCuCl]2 complex and the Cs cations, based
on literature reports [53]. The similarity of the results for alcohol
oxidation in the presence of O2 or air as the oxidant led us to the
consideration that the alcohol oxidation yield was independent
of the oxidant concentration. In other words, the oxidation rate is
zero order related to the oxidant concentration.
Appendix A. Supplementary data
CCDC 835103 contains the supplementary crystallographic data
for [LAphCuCl]2. 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, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.07.096.
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