Tuning of the redox potential and catalytic activity of a new Cu(ii) complex by: O -iminobenzosemiquinone as an electron-reservoir ligand

Article abstract of DOI:10.1039/c9nj06396j

The synthesis and characterization of a new Cu(ii) complex, LNIS2Cu^II (L^NIS = o-iminobenzosemiquinone), are reported. X-ray crystallography studies showed that two o-iminobenzosemiquinone radicals form a distorted square-planar geometry around the Cu(ii) center of LNIS2Cu^II. Magnetic measurements revealed the paramagnetic character of the complex caused by the presence of three unpaired electrons located on the o-iminobenzosemiquinonate ligands and the Cu^II center. Magnetochemical experiments, and EPR and DFT studies prove that the ground state of the complex is a doublet, which is consistent with the ferromagnetic coupling between Cu(ii) and o-iminobenzosemiquinone centers and stronger antiferromagnetic coupling between the iminobenzosemiquinone moieties. The ligand-centered redox reactions of the complex were studied by cyclic voltammetry. Aerobic oxidation of alcohols to aldehydes with TEMPO was studied in the presence of LNIS2Cu^II. Furthermore, LNIS2Cu^II was found to be an efficient catalyst in homo-coupling of terminal alkynes.

Full text of DOI:10.1039/c9nj06396j

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Tuning of the redox potential and catalytic activity of
a new Cu(II) complex by o-iminobenzosemiquinone
as an electron-reservoir ligand†
Cite this:DOI: 10.1039/c9nj06396j
a
a
b
Mina Nasibipour, Elham Safaei, * Grzegorz Wrzeszcz
and
b
Andrzej Wojtczak
NIS
2
II
NIS
The synthesis and characterization of a new Cu(II) complex, L
Cu (L
= o-iminobenzosemiquinone),
are reported. X-ray crystallography studies showed that two o-iminobenzosemiquinone radicals form a
NIS
II
distorted square-planar geometry around the Cu(II) center of L
2
Cu . Magnetic measurements revealed
the paramagnetic character of the complex caused by the presence of three unpaired electrons located
II
on the o-iminobenzosemiquinonate ligands and the Cu center. Magnetochemical experiments, and
EPR and DFT studies prove that the ground state of the complex is a doublet, which is consistent with
the ferromagnetic coupling between Cu(II) and o-iminobenzosemiquinone centers and stronger
antiferromagnetic coupling between the iminobenzosemiquinone moieties. The ligand-centered redox
reactions of the complex were studied by cyclic voltammetry. Aerobic oxidation of alcohols to
Received 26th December 2019,
Accepted 7th February 2020
DOI: 10.1039/c9nj06396j
NIS
II
NIS
II
aldehydes with TEMPO was studied in the presence of L
2 2
Cu . Furthermore, L Cu was found to be
rsc.li/njc
an efficient catalyst in homo-coupling of terminal alkynes.
Introduction
One of the most important areas of research in bio-
coordination chemistry is the study of the interaction of pro-
radical ligands with metal ions and investigation of their effects
on the reactivity of the metal center as well as the whole
1
complex. The main concept is that the cooperation between
metal and ligand in such complexes can modify the overall
properties of the system and improve their applicability as
models of different enzymes such as galactose oxidase
2
(
GOase). GOase is an enzyme with two tyrosinate moieties,
2
two histidine units, and H O molecules surrounding the Cu(II)
ion in a square-pyramidal geometry in its active site (Scheme 1).
The enzyme activity is related to three stable oxidation states of
3
A, Cu(II)-tyrosyl; B, Cu(II) tyrosine; and C, Cu(I) tyrosine:
II
ꢀ+
II
I
Cu –O Tyr 2 Cu –OTyr 2 Cu –OTyr
GOase performs the oxidation of primary alcohols to the
corresponding aldehydes through a Cu(II) phenoxyl-radical with
3a
Scheme 1 The GOase-active site and different redox states.
2 2 2
concomitant generation of H O and eventual reduction of O
4
to water. Due to the importance of insight into the structure
and electronic properties of the active site of the enzyme,
several models of galactose oxidase enzyme have been provided
a
Department of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454,
Iran. E-mail: e.safaei@shirazu.ac.ir
b
Faculty of Chemistry, Nicolaus Copernicus University in Torun, 87-100 Torun,
Poland
5
in the last few years.
The presence of metals with variable oxidation states and
proper ligands with variable oxidation states which permit
†
Electronic supplementary information (ESI) available. CCDC 1870228. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c9nj06396j
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ꢁ1
ꢁ1
The bands at n = 760 cm and n = 1600 cm showQC–H bending
and CQC, respectively.
NIS
II
Synthesis and characterization of L2 Cu
NAP
Treatment of H
2
L
with Cu powder in an equimolar ratio
under aerobic conditions in acetonitrile at room temperature
NIS
II
afforded needle crystals of L2 Cu (eqn (1)):
H L + O + Cu - CuL + H O
2 2
(1)
2
2
2
NAP
NIS
II
Elemental analysis confirms the 1 : 2 molar ratio of metal to
ligand in the complex. All the vibrations of the ligand are
present in the IR spectrum, confirming the presence of the
ligand in the complex. Disappearance of the strong nO–H
stretching band in the IR spectrum of the complex shows the
deprotonation of the ligand and coordination of the resultant
phenolate to the Cu(II) center.
Scheme 2 The structures of H
2
L
2
and its L Cu complex.
tuning the oxidation sates of the metal center (non-innocent
ligands) in coordination complexes are among the main require-
ments to achieve higher catalytic activity. Accordingly, the design
and synthesis of enzyme model complexes containing non-
6
In the IR spectrum of the complex, the phenolic n(C–O)
innocent ligands have attracted significant attention. In this
ꢁ1
stretching band appears at n = 1254 cm . The bands appearing
regard, several o-aminophenol ligands and their corresponding
metal complexes have been synthesized. The catalytic activities
of these ligand copper complexes in aerobic oxidation of alcohols
ꢁ
1
at n = 2958, 2940, and 2872 cm can be attributed to the
presence of 2,4-di tert-butylphenolate group in the ligand
structure. In the spectra of the complex a strong band appears
at n = 2226 indicating the presence of the –CN group and no
ligation from its N atom to Cu. Similar to the IR spectrum of the
ligand, the n(C–C), n (Q C–H) bending and n(CQC) stretching
7
have also been investigated.
In recent decades, alkyne building blocks have received
more attention in industrial, agricultural and molecular devices.
Conjugated 1,3-diynes obtained from acetylene coupling are one
ꢁ
1
ꢁ1
ꢁ1
8
bands appear at n = 1480 cm , n = 766 cm and n = 1601 cm ,
of the most important intermediates or materials.
ꢁ
1
respectively. The n = 3354 cm of the N–H group in the ligand
has disappeared in the complex structure which is attributed to
the conversion of the o-aminophenol ligand coordinated with
Cu(II) to other tautomeric deprotonated forms. In the IR spectrum
of the complex, we observe contributions from the C–O stretching
One of the oldest reactions for the preparation of bis acetylenes
is Glaser coupling, a homo coupling of terminal alkenes using
Cu(I) in a polar solvent and the presence of oxygen to reoxidize the
copper center in a catalytic cycle. In Hey coupling, a versatile
reaction in a wide range of solvents was provided using a Cu(I)–
TMEDA system. In the related Ellington reaction, a Cu(II) pyridine
system was applied for coupling of terminal alkynes.
Herein, we report the synthesis and characterization of a
new Cu(II) complex of redox-active o-aminophenolate H2L
in which NAP stands for the aminophenol form of the ligand
Scheme 2). Moreover, the catalytic activity of L
2
Cu (in which Green crystals were obtained from a methanol–dichloromethane
is o-iminobenzosemiquinone, a one electron oxidized form (1: 1) mixture. Crystallographic data are shown in Table 1.
of the coordinated L ) in the aerobic oxidation of a broad Selected bond distances and angles are listed in Table 2. The
ꢁ
1
ꢁ1
of semiquinonate at 1478 cm . The band seen at 3438 cm is
related to O–H of MeOH or H O, despite the analysis being done in
9–14
2
very dry conditions.
NAP
,
NIS
II
Crystal structure of L
2
Cu
NIS
II
(
L
NIS
NAP
NIS
2
II
range of alcohols to the corresponding aldehydes under mild solid state structure of L
conditions has been studied. Also, the complex was used as a of the L
Cu is shown in Fig. 1. The molecule
NIS
2
II
Cu complex has a C symmetry with the Cu ion
i
catalyst in the homo-coupling process of terminal alkynes.
located on the center of symmetry. Therefore, the asymmetric
unit consists of half of the molecule (half of the Cu ion and one
t
ligand). The C18 of the Bu group reveals the rotational disorder, with
two discrete populations of 57/43% occupancy. The coordination
sphere CuN O has a square-planar geometry with bi-dentate L
2 2
iminosemiquinoate coordinated via O and N atoms, while the nitrile
Results and discussion
Synthesis and characterization of H2L
NIS
NAP
NAP
2
The H L
ligand was synthesized via the condensation reaction substituent does not form any interaction with Cu(II) (Fig. 1).
NAP
of 2-aminobenzonitrile and 3,5-di-tert-butylcatechol as explained
Two chelate 5-membered rings formed by L
are flat. In
1
5
in the literature. The ligand was characterized by spectroscopic the coordination sphere, the Cu1–O1 and Cu1–N1 bond lengths
NAP
and elemental analysis. The ligand H L
exhibits characteristic are almost identical, being 1.9210(19) and 1.928(2) Å, respectively.
IR bands at n = 3421 cm and n = 3354 cm that belong to nO–H The N1–Cu1–O1 and N1–Cu1–O1[ꢁx, ꢁy, ꢁz] angles are 83.18(8)
ad nN–H vibrational stretches, respectively. The presence of a –CN and 96.82(8)1, respectively, with the difference reflecting the
group is confirmed by the stretching band at n = 2222 cm . The tert- proximity of both donor atoms within one ligand molecule.
butyl group bands appear at n = 2865 cm , n = 2904 and n = 2950 The valence geometry of the aminophenol ligand is typical for
2
ꢁ
1
ꢁ1
ꢁ
1
ꢁ
1
ꢁ
1
and the phenolic n(C–O) stretching band appears at n = 1272 cm
.
such systems (Table 2). However, the C1–O1 and C6–N1 bonds of
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NIS
2
II
Table 1 Crystallographic data for L
Cu
fragment (1.424(4), 1.448(4) and 1.416(4) Å, respectively), as
well as significantly shorter C2–C3 and C4–C5 (1.371(4) and
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
42 4 2
C H48CuN O
1
.361(4) Å, respectively) indicate that the ligand exists in the
704.38
Triclinic
P1%
NIS
o-iminobenzosemiquinoate radical mono anion L form. That
is consistent with the results of the metrical oxidation state
a = 8.5030(5) Å
b = 10.8824(8) Å
c = 11.7661(10) Å
a = 114.768(8)1
b = 90.404(6)1
g = 92.966(5)1
1
6
method used for calculation of the experimental MOS of the
aminophenolate ligand, which is ꢁ0.89(4).
Such a form was also detected, among others, for Cu com-
BIS
7b,17
plexes with L ligands,
or transition metal complexes with
3
16,18,19
Volume
Z
Temperature
Density (calculated)
Crystal size
986.78(14) Å
o-aminophenols.
The geometry of the nitrile substituent
1
NAP
in L
is typical, and the N2–C13–C12 angle is 178.4(4)1.
293(2) K
1.185 Mg m
ꢁ3
Conformation of the ligand can be described with two torsion
angles C1–C6–N1–C7 and C6–N1–C7–C8 in the bridging amine
fragment, with the values being ꢁ177.4(2) and 64.61(2)1, respectively.
In the observed conformation, the dihedral angle between two
phenyl moieties is 67.8(2)1. The dihedral angles between the amino-
3
0.518 ꢃ 0.298 ꢃ 0.130 mm
ꢁ1
Absorption coefficient
Reflections collected
Independent reflections
0.591 mm
6811
4344 [R(int) = 0.0512]
1.003
2
Goodness-of-fit on F
Final R indices [I 4 2s(I)]
R indices (all data)
R
R
1
= 0.0554, wR
= 0.0799, wR
2
= 0.1439
= 0.1741
2 2
phenyl C1–C6 and C7–C12 phenyl rings and the N O coordination
1
2
plane are 0.4(1) and 67.5(2)1, respectively. Two intramolecular
C16–H16Cꢂ ꢂ ꢂO1 and C17–H17Bꢂ ꢂ ꢂO1 interactions are found
with the Cꢂ ꢂ ꢂO distances being 2.972(4) and 3.123(4) Å, respec-
tively. All data for bond angles and lengths are shown in Tables
NIS
II
Table 2 Selected bond lengths [Å] and angles [1] for L
2
Cu
Cu1–O1
Cu1–N1
O1–C1
1.9210(19)
1.928(2)
1.298(3)
1.340(3)
1.424(4)
1.448(4)
1.371(4)
1.428(4)
1.361(4)
1.416(4)
96.82(8)
83.18(8)
96.82(8)
178.4(4)
NIS
II
S1 and S2 (ESI†). The monohydrate structure L
the monoclinic P2
2
Cu ꢂH
2
O in
1
/c space group was determined and included
C6–N1
C1–C2
C1–C6
C2–C3
in the PhD thesis http://gyan.iitg.ernet.in/handle/123456789/
1
9a
557.
In that report, the ligand was found in the same one-
electron oxidized form. However, it is not deposited in the CSD.
Therefore, the structure reported here is the only structure
available to the scientific community.
C3–C4
C4–C5
C5–C6
O1–Cu1–N1#1
O1–Cu1–N1
O1#1–Cu1–N1
N2–C13–C12
Magnetic measurements
NIS
II
Magnetic properties of the L
2
Cu complex were measured in
the temperature range of 1.9–300 K with 10 000 Oe magnetic field.
In a temperature range of 10–150 K, the product of magnetic
Symmetry transformations used to generate equivalent atoms: #1 ꢁ x,
y, ꢁz.
ꢁ
NIS
II
susceptibility and temperature (wꢂT) for the L Cu complex
2
ꢁ
1
remains nearly constant at 0.34 emu K mol (1.64 BM) (Fig. 2),
a value that corresponds to an isolated S = 1/2 ground spin state.
Below 10 K, the wꢂT slowly decreases reaching 0.30 emu K molꢁ
1
NIS
II
Fig. 1 Molecular structure of L
2
Cu . Thermal ellipsoids are set at 50%
probability.
NIS
II
Fig. 2 Magnetic susceptibility plot of L
2
Cu with best fitted parameters:
= ꢁ499 cmꢁ . The data are given by the dots and
1
1
.298(3) and 1.340(3) Å, respectively, are shorter than expected for
g = 1.893, J = 363, and J
1
the single bonds. Also, the consecutive bonds in the C2–C1–C6–C5 the line is a theoretical fit to the model described in the text.
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(1.55 BM), probably due to antiferromagnetic intermolecular Electrochemistry
interactions. With increasing temperature (150 to 300 K, Fig. 2),
NIS
II
Electrochemical properties of the L
2
Cu complex were
ꢁ1
the wꢂT reaches a value of 0.436 emu K mol (1.87 BM), which is
studied in CH Cl by cyclic voltammetry (CV) at 233 K. In the
2
2
ꢁ1
slightly larger than the value of 0.37 emu K mol (1.73 BM)
studied potential range, four quasi-reversible one-electron
redox processes were observed for the complex (Fig. 3 and
Table 3). Based on the reported results for similar complexes,
the negative potential peaks can be assigned to the ligand
based iminosemiquinone/amidophenoxide redox couples.
In addition, ligand-centered voltammograms are observed in
the positive potential range showing the iminobenzosemiqui-
none/iminobenzoquinone redox couple (Scheme 4).
fore, we can see four peak potentials for the two above-
mentioned ligand reduction and oxidation processes.
5,6d
expected for one uncoupled spin-only Cu(II) ion (S = 1/2).
Hence,
when the temperature increases, another spin state is populated
and it leads to an increase in the magnetic moment. These data
suggest the overall antiferromagnetic coupling between the
spins of two iminobenzosemiquinonate radicals and the Cu(II)
22
center. This result is consistent with the X-ray results that Cu(II)
NIS
is surrounded by two iminosemiquinone radicals (L
L
and
18,22
There-
0
NIS
), each having a spin of S = 1/2. Therefore, it is a three spin
complex with three possible spin compositions of mmm, mkm,
20
0
0
mmk in which the Stot = SCu + SLNIS + SLNIS , SLNIS* = SLNIS + SLNIS
and (Stot, S*) = (3/2, 1), (1/2, 1), (1/2, 0) (Scheme 3).
,
Electronic spectroscopy
According to Scheme 3, the experimental magnetic data have
NIS
II
The electronic spectrum of L
2 2 2
Cu in CH Cl exhibits a series
ˆ
ˆ ˆ
been fitted using the Heisenberg Hamiltonian: H = ꢁ2J(S1ꢂS2 +
of bands in the ultraviolet and visible regions (Fig. 4). The
absorption bands in 302 nm and 350 nm are attributed to the
p - p* intra-ligand charge transitions (ILCT). The electronic
absorption spectrum at 450 nm is related to the ligand to metal
charge transfer (LMCT) from phenolate orbitals to the d orbi-
tals of Cu(II). The broad band at lower energy (810 nm) refers to
ˆ ˆ ˆ ˆ
ˆ
ˆ
S1ꢂS3) ꢁ 2J S2ꢂS3, where S1 corresponds to spin of Cu, while S2
1
ˆ
and S3 correspond to two spins on the radicals. The magnetic
susceptibility data throughout the entire temperature range were
2
1
fitted with this model (Fig. 2) using the PHI program. Several
constraints were placed on the fit to avoid over-parameterization.
NIS
First, only a single g value for both L
radicals and the Cu(II)
5,6,18
the MLCT charge transfer.
center was included in the model and the fixed molecular field
0
ꢁ1
parameter was applied (zJ = ꢁ0.1 cm ). The J value is the
EPR analysis
NIS
coupling constant of Cu and L and J
1
is the coupling constant
ligands. Due to the opposing ferromagnetic
NIS
II
0
2 2 2
A frozen CH Cl solution of the neutral L Cu is EPR active
Fig. 5 and Table 4), due to the paramagnetic electronic ground
NIS
NIS
of two L and L
(
and antiferromagnetic effects, the model fit to the experimental
data is good for a relatively broad range of tested J and J1
parameters. The best fitted parameters are as follows: g = 1.893,
state (S = 1/2) caused by antiferromagnetic coupling between
two iminobenzosemiquinone ligands and Cu(II) unpaired elec-
trons. The observed four-line signals display hyperfine splitting
due to the I = 3/2 Cu center which is characteristic of mono-
meric Cu(II) complexes. Simulation of the experimental data
ꢁ
1
J = 363, and J
1
= ꢁ499 cm (Fig. 2). Fitting clearly indicates the
presence of the large antiferromagnetic coupling between the
ligand radicals and the ferromagnetic coupling between
the Cu(II) center and the L
NIS
L
results in the following parameters: g
x
ꢁ1
= 2.0365, g
y
= 2.0760,
NIS
ligand radicals. Similar results
4
z x y z
g = 2.1960, (A , A , A
) (ꢃ10 ; cm ) (5.70, 18.0, 191).
have been published for copper complexes with analogous
1
8
ligands. The set of models is consistent with the results of
theoretical calculations presented below. An acceptable fit to
the experimental data was also obtained for opposite models,
1
which were insensitive to the J parameter and therefore only J
ꢁ
1
was used in the fit. However, the best value of J = ꢁ204 cm
was obtained, which might indicate the antiferromagnetic
coupling between the copper center and ligand radicals. These
models were rejected as being contradictory to the DFT and
EPR results.
NIS
II
Fig. 3 Cyclic voltammograms of L
2
Cu . Conditions: 1 mM complex,
ꢁ1
0
.1 M NBu ClO , scan rates 50, 100, and 150 mV s , CH
4
4
2
Cl
2
, and 298 K.
NIS
2
II
+
Table 3 Redox potentials of L
Cu versus Fc /Fc
1
2
3
4
Compound
E
1/2/V
E
1/2/V
E
1/2/V
E
1/2/V
NIS
II
Scheme 3 Representation of variation in redox state in the L
2
Cu
NIS
2
II
complex.
L
Cu
ꢁ0.83
ꢁ0.44
0.33
0.77
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NIS
2
II
Scheme 4 Schematic representation of L
Cu complex oxidation state variation.
Theoretical analysis
The orbital shapes and energies are given in Fig. 6. Orbital
number 189 corresponds to the LUMO. The highest occupied
orbitals in this complex are presented by single occupied
molecular orbitals (SOMO). There are three paramagnetic centers
NIS
II
in the L
2
Cu complex; each of them comprises one unpaired
electron: one electron is located at the copper atom and two others
2
3
at radical-anion iminobenzosemiquinone ligands.
These
electrons occupy SOMOs 188a, 187a and 186a. The orbital
number 185 is the highest doubly occupied orbital (HDOMO),
which is obtained by a combination of corresponding a and b.
Orbitals 188a, 187a and 186a are so-called ‘‘magnetic’’ orbitals,
NIS
2
II
Fig. 4 Electronic spectrum of 2 mM CH
2
Cl
2
solutions of L
Cu . Inset determining the nature of exchange interactions (Fig. 6).
shows the zoom-in 400–500 nm region.
Exchange coupling of unpaired electrons in the paramagnetic
centers was estimated using the ‘‘broken symmetry’’ (BS) approach.
24
We have calculated the states with different orientation spins
of unpaired electrons – aaa, baa and aba, presented in Fig. 7
(where a is spin up, and b is spin down). Two types of exchange
interactions are possible in the complex containing three para-
magnetic centers: (1) between the copper atom and semiquinone
ligand ( J) and (2) between the two semiquinones ( J ) (Scheme 5).
1
The formula applied for the calculation of exchange inter-
actions in such systems with the use of the generalized spin-
2
5
projection approach developed by Yamaguchi is presented
below: (where E is the value of total energy of the corresponding
2
state, and S is the value of the spin-squared operator).
2
2
J = ꢁ(Eaaa ꢁ Eaba)/(hS iaaa ꢁ hS iaba)
2
2
2
J
1
= ꢁ(Eaaa ꢁ 2Eaab + Eaba)/(hS iaaa ꢁ 2hS iaab + hS iaba)
NIS
II
Fig. 5 EPR spectrum of L
2
Cu at frequency = 9.63649 GHz; power = 2 mW;
The results of estimation of exchange spin coupling constants
with the use of the 6-311++G(d,p) basis set and two functionals
B3LYP and TPSSh) are collected in Table 5. The calculations were
performed on the experimental geometry (SP-single point), opti-
mized for the complex (OPT). The obtained results point to the
presence of ferromagnetic coupling between each iminobenzo-
semiquinone radical and the copper center, and stronger anti-
ferromagnetic exchange coupling between the unpaired electrons
modulation frequency = 100 kHz; amplitude = 3.0 G; T = 30 K; 1 mM.
(
NIS
2
II
ꢁ1
4
Table 4 EPR data for L
Cu . The A
i
values [cm ] multiplied by 10
g
x
g
y
g
z
A
x
(Cu)
A
y
(Cu)
A
z
(Cu)
2
.0365
2.0760
2.1960
5.70
18.0
191
These parameters are a clear sign of the asymmetric coordina- of two iminobenzosemiquinone radicals (Table 5).
II
tion of ligands around the Cu center in a distorted square-
NIS
II
Investigation of the catalytic properties of the L
2
Cu complex
planar geometry. The presence of an unpaired electron at the
orbital of the Cu(II) ion was deduced from the trends of
d
x
2
ꢁy
2
A. Alcohol oxidation. Aerobic oxidation of alcohols has
5
,6
6i,26
NIS
II
g
z
4 g
x
, g
y
4 2.0023 and A
z
x
c A , A
y
.
been of interest to chemists.
2
In our research, the L Cu
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Fig. 6 The HDOMO, LUMO and ‘‘magnetic’’ (SOMO) orbital shapes and energies.
Fig. 7 Spin density distribution, cutoff = 0.012.
amount were optimized (Scheme 6). In the first step, the effect of
bases such as Cs CO , KOH and Na CO for oxidation of benzyl
alcohol was checked. In the presence of Cs CO , the highest
conversion of alcohol was observed. Optimization of the amount
of Cs CO showed that the best result was achieved with 2
equivalents of Cs CO
2
3
2
3
2
3
2
3
NIS NIS
NIS
Scheme 5 Schematic representation of two types of L /L and Cu/L
exchange interactions.
2
3
.
For investigating the solvent effect in this reaction, organic
solvents such as CH CN, CH Cl and toluene were applied and
3
2
2
our result clearly confirmed that the most favorable solvent for
this transformation is toluene (Table 6, entries 1–5). The amount
of catalyst was also optimized and the highest conversion per-
centage was observed when 3 mol% of complex was used
ꢁ1
Table 5 Exchange spin coupling constants [cm
]
SP
OPT
J
J
1
J
J
1
(
Table 6, entries 10–12). In the next stage, the impact of different
B3LYP
TPSSh
357
359
ꢁ447
ꢁ575
331
332
ꢁ379
ꢁ515
ꢁ3
amounts of TEMPO (5 ꢃ 10 mmol, 0.05 mmol, 0.075 mmol and
.1 mmol) was studied for the optimized condition (2 equivalents
Cs CO , toluene, and 3 equivalents catalyst). The best result was
0
2
3
catalytic activity for oxidation of alcohols to the corresponding achieved when the amount of TEMPO was 0.075 mmol (Table 7,
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II
aldehydes was investigated. Firstly, the L
2
Cu mediated entry 5). The comparison of the results shows that changing
oxidation of benzyl alcohol as a model compound was per- the amount of TEMPO was much more important than the
formed and the reaction conditions of solvent, base, and catalyst complex.
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Table 8 The result of blank tests for oxidation of benzyl alcohol with the
catalyst system
Time
(h)
Conversion
(%)
Entry
Catalytic system
NAP
1
2
3
4
5
H
2
L
3 mol%/Cs
2
CO
CO
3
8
8
8
8
8
28
24
20
—
33
Cu(OAc)
TEMPO 0.075 mmol/Cs
TEMPO 0.075 mmol/Cs
TEMPO 0.075 mmol/Cu(OAc)
mol%/Cs CO
2
3 mol%/Cs
2
3
CO
3
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2
II
2
Scheme 6 Optimized conditions for catalytic activity of L
Cu in aerobic
a
2
CO
3
oxidation of alcohol.
2
3
2
3
a
2
In the absence of O .
Table 6 The effect of various bases and solvents for the aerobic oxidation
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II
of benzyl alcohol by L
2
Cu
alcohols as substrates (Table 9). The results revealed that
L Cu /TEMPO has good efficiency only for benzyl alcohol
2
Catalyst Time Conversion
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II
Entry Base
Solvent
(mol%) (h)
(%)
and some of its derivatives and not for aliphatic and secondary
alcohols. In continuation, we did a comparison between the
1
2
3
4
5
6
7
8
9
1
1
1
2 eq. Cs
2 eq. KOH
2 eq. K CO
2 eq. Na CO
2
CO
3
Toluene
Toluene
Toluene
Toluene
2
2
2
2
2
2
2
2
2
2
3
5
8
8
8
8
8
8
8
8
8
8
8
8
26
14
12
12
28
16
16
12
10
13
29
30
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II
2
3
activity of L
2
Cu and other catalysts that had been proposed
2
3
in the literature (Table 10).
3 eq. Cs
2 eq. Cs
2
CO
CO
3
Toluene
2
3
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Dichloromethane
Toluene
No reaction was observed in the absence of an additional
base, suggesting that the deprotonation of alcohol to alcoholate
is necessary prior to the coordination of the substrate to the
2 eq. KOH
2 eq. K CO
2 eq. Na CO
2
3
2
3
0
1
2
2 eq. Cs
2 eq. Cs
2 eq. Cs
2
CO
CO
CO
3
Cu(II) center. Cs
cations, with high polarizability and size compared to other
alkaline metals, may mainly influence the solubility of Cs CO
in toluene and interaction of Cs CO with alcohols and conse-
It is worth mentioning that performing the mentioned quently, better deprotonation.
2 3
CO in toluene shows better activity. Cesium
2
2
3
3
Toluene
2
3
2
3
II
optimized reaction in the base-free condition led to a dramatic
decrease in reaction conversion.
There are three different mechanisms for Cu /TEMPO
’
mediated alcohol oxidation. Stahl s group has shown that the
In blank tests, replacement of the copper complex with Cu(II) hydrogen transfer to the nitroxyl ligand is the lowest-energy path-
acetate and the consequent decline in the aldehyde yield proves way, corresponding to the observed higher conversion percentage of
3
4
the important role of the ligand in the alcohol transformation. primary alcohols compared to the secondary ones (Scheme 7).
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II
Increase of the aldehyde yield by addition of free ligand to Cu(II)
A proposed mechanism for alcohol oxidation using L
2
Cu
acetate proves this claim. Comparison of two mentioned blank is shown in Scheme 8. After coordination of alcoholate to Cu(II),
test results (Table 8, entries 1 and 2) with the same complex of TEMPO as a co-catalyst performs the a H-abstraction from the
NIS
2
II
L
Cu as the catalyst (Table 9, entry 1), demonstrates the coordinated alcohol and formation of a ketyl radical in a
contribution of both the copper center and ligand due to the similar manner to the role of tyrosinate in the enzyme cycle
coordination process playing an important role in the oxidation (Scheme 1, B).
of alcohols. A blank test using only TEMPO as the catalyst was
It is suggested that an electron transfer to copper produces
performed. The contribution of TEMPO in alcohol oxidation aldehyde and Cu(I) species and electron and proton transfers
was only 20% (Table 8, entry 3). This means that TEMPO plays a from the Cu(I) complex and TEMPO–H return the system back
co-catalyst role in this reaction. In addition, oxidation of benzyl to the original Cu(II) complex and TEMPO. A blank test in the
2
alcohol using TEMPO and Cu(II) acetate as a catalyst provides absence of O , benzaldehyde was not produced which proves
only 33% aldehyde and after a long time the reactant has not the role of oxygen in the mechanism (Table 8, entry 4), although
converted. This result also shows the role of the complex in the the production of H O cannot be proven due to difficulty
2
2
mentioned oxidation reaction (Table 8, entry 5).
After optimizing the reaction conditions (3 mol% L
of hydrogen peroxide detection. It is worth mentioning that
Cu , the coordinated non-innocent ligands facilitate this Cu /Cu
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2
II
II
I
2
2 3
mmol Cs CO and 0.075 mmol TEMPO), we used other ping-pong mechanism.
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II
Table 7 The effect of tempo in the aerobic oxidation of benzyl alcohol by L
2
Cu
Entry
Base
Solvent
Catalyst (mol%)
Time (h)
Conversion (%)
ꢁ
3
1
2
3
4
5
6
TEMPO 5.0 ꢃ 10
2 eq. Cs
2 eq. Cs
2 eq. Cs
2
2
2
CO
CO
CO
3
3
3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
2
2
4
4
3
3
8
8
8
8
8
8
50
80
83
66
94
25
TEMPO 0.05 mmol
TEMPO 0.05 mmol
TEMPO 0.05 mmol
TEMPO 0.075 mmol
TEMPO 0.075 mmol
2 eq. KOH
2 eq. Cs
2
CO
3
—
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2
II
Table 9 Alcohol oxidation in toluene, catalyzed by L
Cu and TEMPO in the presence of Cs
2
CO
3
2
as base and O as oxidant
Substrate
Product
t (h) Conversion (%)
Substrate
Product
t (h) Conversion (%)
1
2
3
8
6
8
94
94
90
8
8
90
80
80
9
10
10
10
4
6
94
11
10
21
5
6
7
8
8
8
90
93
95
12
13
14
10
10
12
9
20
2
B. Homo coupling of alkynes. Alkynes are one of the most achieved conditions are shown in Table 10, entry 7. Homo-
important building blocks in organic synthesis, industry and coupling of various terminal alkynes that afforded the corres-
many natural compounds. Hence, scientists have paid much ponding symmetrical dienes in excellent yields was then examined
attention to the homo-coupling reaction of terminal alkynes. under the optimized conditions and summarized in Table 12. Also,
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II
The mentioned reaction catalyzed by copper complexes represents the comparison between the activity of L
2
Cu and some other
an effective strategy for the synthesis of the functionalized alkynes. catalysts for phenylacetylene homo-coupling reactions has been
35
Salkind and Fundyler have proposed a mechanism for the men- given in Table 13. Despite other reports, we have not used unsafe
tioned reaction, in which, Cu(I) ions will be coordinated to the pyridine or other amines such as the expensive base of DABCO etc.
alkyne triple bond in a side-on mode and help in C–H activation of as additives and harsh conditions.
this functional group in producing a Cu(II) acetylide complex, which
The suggested mechanism is shown in Scheme 10. The
traps the second alkyne. Coupling of these two coordinated alkyne coupling process is not effective in the absence of an additional
molecules generates the 1,3-diyne as a final product (Scheme 9). The base (Table 11, entry 8). We conclude that the role of the base is
oxidized copper species could be recovered and regenerated by the deprotonation of acetylene to acetylide and its coordination to
oxidation with the oxidant to complete the catalytic cycle.
In another part of this work, the Cu(II)-catalyzed homo through the oxidative addition of acetylide to the Cu(II) center.
coupling reaction of terminal alkynes under aerobic conditions Higher conversion values obtained in THF compared to the
the Cu(II) center. This means that the reaction does not proceed
was performed. The reaction was done under different reaction other solvents seem to be due to the better solubility of the
conditions (THF, acetonitrile, and toluene as solvents, KOH, complex in this solvent. In this solvent, KOH showed better
Cs
2
CO
3
, and Na
2
CO
3
as bases and time) (Table 11). The best activity than NaOH due to the higher basicity of the former.
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Table 10 Comparison between the activities of L
2
Cu with other catalysts in alcohol oxidation reaction
Catalyst (mol%)
Solvent/T (1C)
CH Cl /RT
Time (h)
3
Yield (%)
92
Additive
Ref. year
27, 2014
1
2
[Cu(CH
3
CN)
4
]PF
6
(4)
2
2
DMAP
DBED (0.1 mmol)
KO Bu (10 mol%)
TEMPO (10 mol%)
bpy
II
t
Cu Br
2
(5)
MeCN:H
2
O/RT
12
7
28, 1977
3
Cu(ClO
4
)
2
(4)
DMSO/RT
3
100
TMDP
29, 2006
TEMPO (6 mol%)
DABCO (10 mol%)
KOH
APIP
II
4
5
6
L
L
Cu (4)
MeCN/RT
Toluene/RT
5
4
20
100
100
97
7b, 2014
7a, 2013
30, 2010
BIS
II
Cu OAc (4)
](OTf) (5)
Cs
bpy
TEMPO (5 mol %)
2 3
CO , 2 eq.
[Cu(MeCN)
4
CH
3
CN/RT
7
8
([Cu(OOC(C
6
H
5
)Br)
))
Water/70
6
2
71
91
H
2
O
2
31, 2018
32, 2012
(C
10
H
9
N
3
)](ClO
4
CuBr
2
Acetonitrile:water/RT
Tetradentate N-donor ligand
TEMPO (5 mol%)
t-BuOK (5 mol%)
9
Bis[2-(p-tolyliminomethyl)]
CH
3
OH/60 1C
1
90
H
2
O
2
33, 2015
Phenolato Cu(II) (4)
NIS
1
0
CuL
2
(3)
Toluene/RT
8
95
TEMPO (0.075 mmol)
Cs CO (2 eq.)
This work
2
3
DMAP = N,N dimethylaminopyridine. TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl. DABCO = 1,4-diazabicyclo[2.2.2]octane.
changing the oxidation state of two o-iminobenzosemiquinone
ligands to o-aminophenol.
We think that by the reduction of Cu(II) to Cu(0), both
ligands will be disconnected. Therefore, the tuning of oxidation
state will be concentrated on the ligands instead of the central
metal. Hence, as against other catalysts mentioned in Table 13,
we have not used any additive in this reaction. The transformation
2 2 2
of O to H O in the last copper complex produces the first catalyst.
We do not have any proof of our claim about production of H
2 2
O
due to difficulty of hydrogen peroxide detection.
The stability of the complex under the reaction conditions
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II
was investigated by recording the UV-vis spectrum of L
2
Cu
solution in THF before the coupling reaction (without KOH and
phenylacetylene addition) and after the reaction (with KOH and
phenylacetylene addition) (Fig. S5, ESI†). It is considered that
the total feature of the complex spectrum has not considerably
changed and we can see a slight shift in the UV and visible parts
concomitant with a hyperchromicity in the first and hypochro-
micity in the latter, which means that the complex structure
shows nearly good stability in the presence of KOH.
Experimental
II
Materials and methods
Scheme 7 Stahl’s proposals for mechanism of Cu /nitroxyl mediated
alcohol oxidation.
All chemicals and solvents were purchased from Merck, Sigma-
Aldrich, Acros, and Fluka and used as received. Electronic spectra
were recorded on a Cary 5000 spectrophotometer. IR spectra were
The reaction was performed under an Ar atmosphere to inves- recorded in the solid state on an FT-IR Bruker Vector spectro-
tigate the importance of oxygen in this coupling process. photometer. Cyclic voltammetry (CV) was performed on a PAR-
As seen in the proposed mechanism (Scheme 10), in the first 263A potentiometer, equipped with a Ag wire reference electrode,
step, an acetylide ion will be coordinated to the Cu(II) centre. In a glassy carbon working electrode, and a Pt counter electrode
4 4 2 2
the next step, the addition of another acetylide ion and reductive with 0.1 M NBu ClO solution in CH Cl using liquid nitrogen
elimination process produce the final product concomitant with for decreasing the temperature. Ferrocene was used as an
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II
Scheme 8 Possible mechanistic scheme for L
2
Cu /nitroxyl mediated alcohol oxidation.
subtracted from the net data sets. EPR spectra were collected
using a Bruker EMX plus spectrometer operating with a premium
X-band (B9.5 GHz) microwave bridge. Low temperature measure-
2 2
ments of a frozen CH Cl solution were conducted using a Bruker
helium temperature-control system and a continuous flow cryostat.
Samples for X-band measurements were placed in 4 mm outer-
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II
Scheme 9 Catalytic activity of L
2
Cu in homo-coupling of phenylacetylene.
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2
II
Table 11 Optimization studies for the L
Cu -catalyzed homo coupling diameter sample tubes with a volume of B300 mL. EPR spectra were
reaction of phenylacetylene
simulated with the Easy Spin 3.1.7 software.
Entry Base
Solvent
Catalyst (mol%) t (h) Conversion (%)
Synthesis
1
2
3
4
5
6
7
8
KOH
KOH
THF
THF
THF
THF
Toluene
Acetonitrile
THF
1
2
2
2
2
2
3
3
5
5
80
100
NAP
15
H
2
L
. The ligand was synthesized according to the literature.
Cs
Na
2
CO
CO
3
4.5 100
To a mixture of 3,5-di-tert-butylcatechol (0.222 g, 1 mmol) and
.005 mL of triethylamine in n-hexane (4 mL), 2-aminobenzonitrile
0.118 g, 1 mmol) was added and the suspension was refluxed for
two days and then stirred at room temperature (30 1C) under air for
four days. The yellow colour solution turned to red upon stirring.
After that, the reaction mixture was kept for crystallization and then
2
3
6
6
6
70
50
50
0
(
KOH
KOH
KOH
—
2.5 100
2.5 20
THF
internal standard. The magnetic measurements were obtained re-crystallized thrice from a 1 : 1 CH Cl : MeOH solvent mixture for
2
2
NAP
with the use of a Quantum Design SQUID magnetometer, MPMS- purification. H
2
L
, a colourless crystalline solid, appeared. Yield:
XL-5. Measurements were performed with 10 mg L Cu poly- 2.18 g (67.7%). nmax(KBr)/cm : 3421 (O–H), 3354 (N–H), and 2222
crystalline samples. Diamagnetic contribution of the samples was (–CN) (Fig. S1, ESI†).
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II
ꢁ1
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Table 12 Substrate scope of the Cu-catalyzed C–C homo-coupling between different phenylacetylenes
Substrate
Product
Conversion (%)
1
1
1
00
00
00
1
1
00
00
100
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II
Reaction conditions: phenylacetylene derivatives (1 mmol), KOH (2 mmol), L
2
Cu (3 mol%), THF (3 mL), and time (2.5 h).
NIS
Table 13 Comparison between the activities of L
2
Cu and other catalysts in phenylacetylene C–C homo-coupling reactions
Catalyst (mol%)
CuI
Solvent/T (1C)
O/RT
Time (h)
6
Yield (%)
99
Additive
Et N (3 eq.)
Ref. (year)
36 (2014)
1
H
2
3
Pd(NH
3
)
2
(1)
TBAB (0.5 mmol)
2
3
CuBr (5)
CuI (5)
CH
Acetone/RT
3
CN/RT
2
3
80
91
Di-tert-butyldiaziridinone (0.9 mmol)
TMEDA (10 mol%)
37 (2013)
38 (2006)
Et
TBAF (5 eq.)
Et N (0.03 mmol)
PhCH NH (5 mol%)
3
N (3 mol%)
4
5
6
7
8
1
CuCl
CuCl
2
(5)
(3)
MeCN/120
Toluene/60
Solvent-free/RT
30
90
1.2
6
12
40 min
20 min
2.5
98
99
98
86
70
100
39 (2016)
40 (2013)
41 (2014)
10 (1960)
12 (1959)
This work
2
3
CuI (5)
CuI (4)
2
2
Pyridine
Pyridine
—
Cu(OAc)
2
a
NIS
0
CuL
2
(3)
THF/RT
a
KOH (2 eq.).DMAP = N,N dimethylaminopyridine. TBAB = tetrabutylammonium bromide. TMEDA = tetramethylethylenediamine. TBAF = tetra-n-
butylammonium fluoride.
NIS
2
II
NAP
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II
L
Cu . To a stirred solution of H
2
L
2
(0.325 g, 1 mmol) in Catalytic activity of the L Cu complex in alcohol oxidation
acetonitrile (10 mL), Cu powder (0.0635 g, 1 mmol) was added.
The resulting mixture was stirred for 5 h. Crystals suitable for X-ray
diffraction were obtained from a 1 : 1 CH Cl :MeOH solvent
Alcohol oxidation experiments were carried out in an oxygen
atmosphere at room temperature. In a typical experiment, 2 mL
NIS
II
2
2
of oxygen saturated toluene, alcohol (1 mmol), L
2
Cu (0.017 g,
(0.650 g, 2 mmol) and TEMPO (0.012 g,
.075 mmol) was stirred in a 5 mL two-necked flask equipped
mixture. Yield: 7.49 g (78%). Anal. calcd (found) for C42
C 70.83 (70.93), H 6.75 (6.81), and N 7.86 (7.86). nmax(KBr)/cm
253 (C–O), 2958, 2940 and 2872 (tert-butyl groups), and 2226
–CN) (Fig. S2, ESI†).
48 4 2
H N O Cu:
3
0
2 3
mol%), Cs CO
ꢁ
1
:
1
(
with an oxygen balloon for the required time. The reaction
progress was monitored by gas chromatography. Control reactions
2 2 3
were performed using Cu(OAc) and Cs CO (0.650 g, 2 mmol)
in 2 mL of oxygen saturated toluene under the same conditions
described above.
Calculations
Single point calculations and geometry optimizations were
4
2
performed using the Gaussian 09 program (Revision E.01),
4
3
44
the B3LYP /TPSSh functional, and the 6-311++G(d,p) basis
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II
4
5,46
Catalytic activity of the L2 Cu complex in homo-coupling of
phenylacetylene derivatives
set on all atoms. Broken-symmetry
were performed with the same functional and basis sets.
(BS) DFT calculations
Frequency calculations at the same level of theory confirmed In a typical reaction, a mixture of phenylacetylene (1 mmol),
NIS
that the optimized structures were located at a minimum on KOH (0.112 g, 2 mmol) and L
2
Cu (0.017 g, 3 mol%) was stirred
the potential energy surface. in an oxygen atmosphere at room temperature in THF (3 mL).
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II
Scheme 10 Plausible reaction mechanism of homo-coupling of phenylacetylene by L
2
Cu .
The reaction progress was monitored by TLC and gas chromato- No extinction correction was applied. Hydrogen atoms were
graphy. After disappearance of phenylacetylene, the product was located from the electron density maps and their positions were
separated by column chromatography (n-hexane/ethyl acetate constrained in the refinement. The structural data have been
1
mixture in 8 : 1 molar ratio) and characterized by H NMR deposited with Cambridge Crystallographic Data Centre, the
1
(Fig. S5–S10, ESI†). 1,4-Diphenyl buta-1,3-diyne:
H NMR deposition number being CCDC 1870228.†
(
400 MHz, DMSO-d
,4-di(pyridin-2-yl)buta-1,3-diyne: H NMR (400 MHz, DMSO-d ) d
6
6
) d 7.69–7.57 (m, 4H), 7.55–7.40 (m, 6H),
1
1
8
2
1
.66 (ddd, J = 4.86, 1.76, 0.98 Hz, 2H), 7.91 (td, J = 7.75, 1.80 Hz, Conclusion
H), 7.79 (dt, J = 7.85, 1.13 Hz, 2H), 7.52 (ddd, J = 7.65, 4.82,
NIS
1
In summary, a new Cu(II) complex (CuL ) of amino benzonitrile
based 0-aminophenol ligand was synthesized and characterized.
In the studied complex, the coordinating ligands were observed in
.23 Hz, 2H), 1,4-bis(p-fluorophenyl)buta-1,3-diyne: H NMR
(400 MHz, DMSO-d ) d 7.94–7.57 (m, 4H), 7.46–7.02 (m, 4H),
6
1
1
,4-bis( p-tolyl)buta-1,3-diyne: H NMR (400 MHz, DMSO d ) d 7.50
6
1ꢁ
the iminobenzosemiquinone [(ISQ) ] radical form. The complex
revealed a paramagnetic character caused by the presence of three
unpaired electrons located on the o-iminobenzosemiquinonate
(d, J = 7.68 Hz, 4H), 7.26 (d, J = 7.75 Hz, 4H), 2.35 (s, 6H), 1,4-bis-
1
(4-methoxyphenyl)buta-1,3-diyne: H NMR (400 MHz, DMSO-d
6
) d
7
.76–7.40 (m, 4H), 7.04–6.86 (m, 4H), 3.81 (s, 6H), 1,4-bis-
II
1
ligands and the Cu center. The complex possesses an S = 1/2
(2-chlorophenyl)buta-1,3-diyne: H NMR (400 MHz, DMSO-d
6
) d
ground state which results from strong antiferromagnetic inter-
actions between ligand radicals and weaker ferromagnetic
interactions between the copper(II) center and both radicals.
7
.79 (dd, J = 7.67, 1.67 Hz, 2H), 7.64 (dd, J = 8.16, 1.19 Hz, 2H), 7.53
47
(td, J = 7.75, 1.67 Hz, 2H), 7.44 (td, J = 7.59, 1.26 Hz, 2H).
II
The interaction of the redox-active Cu ion and non-innocent
X-ray analysis
NIS
L
ligands and the properties of non-innocent systems are well
The X-ray data were collected on an Oxford Sapphire CCD developed in this complex. We employed this complex as an
diffractometer using MoKa radiation l = 0.71073 Å, at 293(2) K, efficient catalyst for aerobic oxidation of aromatic alcohols
by the o–2y method. The structure has been solved by direct under mild conditions as a model for the galactose oxidase
methods and refined with the full-matrix least-squares method enzyme. Also, the complex was found to be an active catalyst for
2
48
on F with the use of the SHELX2014 program package. The C–H bond activation and homo-coupling of some terminal
1
6,49
analytical absorption correction was applied
(Table 1). alkynes. In both organic transformations, a low amount of the
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catalyst in safe media under mild conditions was employed to
obtain products in good to excellent yields. Comparison of the
alcohol oxidation and C–H activation activity of this catalyst
with those of other complexes shows a good to nice activity of
this complex. Some other reports of alcohol oxidation protocols,
shown in the Results and discussion part, suffer from usage of
high amounts of catalyst and TEMPO, and some expensive
bases such as DABCO, as well as C-H activation protocols, which
are performed under hard conditions, using additives or a high
amount of catalyst or bases.
6 (a) W. Kaim, Inorg. Chem., 2011, 50, 9752–9765; (b) K. P. Butin,
E. K. Beloglazkina and N. V. Zyk, Russ. Chem. Rev., 2005, 74,
531–553; (c) B. Butschke, K. L. Fillman, T. Bendikov, L. J. Shimon,
Y. Diskin-Posner, G. Leitus, S. I. Gorelsky, M. L. Neidig and
D. Milstein, Inorg. Chem., 2015, 54, 4909–4926; (d) C. N. Verani,
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K. Hildenbrand, E. Bill, T. Weyherm u¨ ller and K. Wieghardt,
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innocent O-aminophenol and Dithiolene Ligands with Transi-
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There are no conflicts to declare.
Acknowledgements
The authors are grateful to Shiraz University, and Nicolaus
Copernicus University for partial support under Center for Excellence
in Research and the BRAIN interdisciplinary research group (AW).
E. Safaei is thankful for the financial support of Iran National
Science Foundation (INSF)-Grant No. 91002093. Special thanks
go to Prof. Andrey Starikov, Prof. John F. Berry, Dr Ekhardt Bill,
Dr Zvonko Jagli ˇc i ´c , Dr Saeid Zakavi and Dr Touraj Karimpour for
their valuable help.
7
6
2
066–6074; (d) S. Ghorai and C. Mukherjee, Chem. – Asian J.,
014, 9, 3518–3524; (e) A. Rajput, A. Saha, S. K. Barman,
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