TEMPO-mediated aerobic oxidation of alcohols using copper(II) complex of bis(phenol) di-amine ligand as biomimetic model for Galactose oxidase enzyme

Article abstract of DOI:10.1016/j.poly.2015.11.003

Mononuclear copper complexes of four-dentate N₂O₂ bis(phenol) diamine ligands (H₂L^NEX X: C and OB in which C and OB are chloro and tert-butyl-methoxy substituents on phenol groups) have been synthesized and characterized by IR, UV-Vis, single crystal X-ray diffraction, magnetic susceptibility studies and cyclic voltammetry techniques. The CuL^NEX complexes show the square pyramid geometry of the coordination sphere with the copper centers surrounded by two nitrogen and oxygen atoms from the coordinating ligand and an axially bound water molecule. The effective magnetic moments of 1.7 and 1.8 BM confirm a monomer complex with copper(II) center. Electrochemical oxidation of these complexes yielded the corresponding Cu(II)-phenoxyl radical species. In addition, CuL^NEX complexes, have shown efficient catalytic activities for TEMPO-mediated oxidation of a set of alcohols to the corresponding aldehydes in the presence of molecular oxygen as oxidant at room temperature.

Full text of DOI:10.1016/j.poly.2015.11.003

Accepted Manuscript
TEMPO-mediated Aerobic Oxidation of Alcohols using Copper(II) Complex
of Bis(phenol) di-amine Ligand as Biomimetic model for Galactose oxidase
Enzyme
Elham Safaei, Leila Hajikhanmirzaei, Babak Karimi, Andrzej Wojtczak,
Patricia Cotič, Yong Lee
PII:
S0277-5387(15)00660-9
DOI:
http://dx.doi.org/10.1016/j.poly.2015.11.003
POLY 11647
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
16 June 2015
4 November 2015
Please cite this article as: E. Safaei, L. Hajikhanmirzaei, B. Karimi, A. Wojtczak, P. Cotič, Y. Lee, TEMPO-mediated
Aerobic Oxidation of Alcohols using Copper(II) Complex of Bis(phenol) di-amine Ligand as Biomimetic model
for Galactose oxidase Enzyme, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.11.003
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TEMPO-mediated Aerobic Oxidation of Alcohols using
Copper(II) Complex of Bis(phenol) di-amine Ligand as
Biomimetic model for Galactose oxidase Enzyme
a,b
b
b
Elham Safaei* , Leila Hajikhanmirzaei , Babak Karimi ,
c
d
e
Andrzej Wojtczak , Patricia Cotič , Yong Lee
a
Department of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454, Iran
b
Institute for Advanced Studies in Basic Sciences (IASBS), 45137-66731, Zanjan, Iran
c
Nicolaus Copernicus University, Department of Chemistry, 87-100, Torun, Poland
d
Institute of Mathematics, Physics and Mechanics, Jadranska 19. SI-1000, Ljubljana, Slovenia
e
Department of Chemistry, Changwon National University, Changwon 641-773, South Korea
Correspondence to: Elham Safaei
E-mail:safaei@iasbs.ac.ir
Abstract
NEX
Mononuclear copper complexes of four-dentate N
2
O
2
bis(phenol) diamine ligands (H L
2
X: C and OB in which C and OB are chloro and tert-butyl- methoxy substituents on phenol
groups) have been synthesized and characterized by IR, UV-Vis, single crystal X-ray
NEX
diffraction, magnetic susceptibility studies and cyclic voltammetry techniques. The CuL
complexes show the square pyramid geometry of the coordination sphere with the copper
centres surrounded by two nitrogen and oxygen atoms from the coordinating ligand and an
axially bound water molecule. The effective magnetic moments of 1.7 and 1.8 B.M confirm a
monomer complex with copper(II) center. Electrochemical oxidation of these complexes
NEX
yielded the corresponding Cu(II)–phenoxyl radical species. In addition, CuL
complexes,
have shown efficient catalytic activities for TEMPO–mediated oxidation of a set of alcohols
to the corresponding aldehydes in the presence of molecular oxygen as oxidant at room
temperature.
Keywords: GOase; Biomimetic copper complexes; Phenoxyl radical; Alcohol oxidation;
TEMPO.
1

1
. Introduction
Nature has invented many efficient enzymes able to assist in performing certain, necessary
reactions with high turnover rates. Many chemical transformations, which require dangerous
or expensive chemicals in the lab, also occur in nature under environmentally friendly
conditions by the enzyme catalysis. Oxidases are one of the most important enzymes that
catalyze oxidation of a substrate. These enzymes are subgroups of the oxidoreductase class
that catalyze an oxidation-reduction reaction involving reduction of molecular oxygen (O
2
) to
water (H O) or hydrogen peroxide(H ) [1,2]. Galactose oxidase (GOase) is a member of
2
2 2
O
the family of radical-coupled copper oxidases of fungal origin (Schemes 1, 2) that catalyze
two-electron oxidation of primary alcohols into aldehydes, with reduction of dioxygen into
hydrogen peroxide [3, 4].
Scheme 1. Active site for GOase and proposed mechanism for catalysis by GOase [4]
The structure of this biomolecule is well established (Scheme 2) [5, 6]. The crystal structure
of GOase contains a square-pyramidal copper ion with a tyrosinate ligand weakly bound in
the axial position, two histidine imidazole units, a second tyrosinate and either H
2
O or acetate
(
replacing the substrate) in the equatorial sites (Scheme 2) [7-14]. An axial tyrosine (Tyr475)
manages the proton transfer occurring during catalysis [8-13].
2

Scheme 2.Galactose oxidase (GOase)active site [14]
Active site of GOase is an important part of this enzyme as well as other enzymes, providing
structural, catalytic and electron transfer functions. Reproducing this active site in an enzyme
model in order to better understand the GOase structure and function and develop new
catalysts has been one of the most important challenges in the field of bio-inorganic
chemistry [15-19].
The catalytic oxidation of alcohols into corresponding carbonyl compounds is one of the
most fundamental reactions in organic chemistry [20–22]. In 1977, for the first time, a
ligand–assisted Cu catalytic system comprising copper with pyridine (py) or bipyridine (bipy)
ligands [23] was reported to catalyze the selective oxidation of alcohols to aldehydes using
O
2
as final oxidant. A few decades later, the catalytic activity of other N-containing ligands
such as 1,10–phenanthroline (phen) or TMEDA–assisted Cu systems was re-investigated
24]. The catalytic oxidation of alcohols to aldehydes based on the isolated Cu complexes has
[
been also reported by several groups [25-28]. The first synthesized Cu complex system,
which may be considered as the functional model of GOase, efficiently catalyzes the
oxidation of alcohols to aldehydes under O pressure at 25 ºC [29]. Other scientists have been
2
successfully imitating the active form of the natural enzyme GOase in numerous model
complexes [30-41]. The most well-known alcohol oxidation studies by aminophenol, imine
3

type ligands copper complexes have been reported by Wieghardt, Stacks, Timo and Fabbrini
groups [42-45]. Nitroxyl radicals are known to act as radical scavenging antioxidants. They
are extremely popular in various fields of science and technology [46]. Particularly, they have
found wide applications as oxidation catalysts [47, 48]. TEMPO (2,2,6,6-tetramethyl-
piperidinyloxyl) is a well-known nitroxyl radical which may oxidize a number of
functionalities [49–56]. Most of the studies on this stable radical have been reported for the
transformation of alcohols to the corresponding carbonyl compounds [54–59]. Only a few
groups have reported catalytic systems involving TEMPO in combination with transition
metals such as Ru [60] and Cu [62, 63] as co-catalysts for the oxidation of alcohols using
molecular oxygen or air under mild reaction conditions. The first copper/nitroxyl radical
catalyzing oxidation has been reported in 1984 by Semmelhack et al. [62], who managed to
oxidize allylic and benzylic alcohols at room temperature in DMF. Despite many reports on
oxidation of alcohols to aldehydes, reports on this organic transformation using copper(II)
complexes in the presence of TEMPO are scarce [62]. Stahl group [63] has summarizes their
contributions to the development and/or mechanistic characterization of a series of different
Cu-catalyzed aerobic oxidation reactions. One of the three mechanistic pathways for this
reaction involves redox cooperativity between Cu(II) and TEMPO cocatalyst (Scheme 3),
which enables highly versatile and efficient alcohol and amine oxidation reactions.
Scheme 3. Redox cooperativity with TEMPO co-catalyst [63]
4

2 2
The main focus of this project was to find new N O based ligand candidates which could
substitute classical bipy and phen with improved catalytic activity for the oxidation of
benzylic alcohols. Thus we paid attention to the synthesis of novel mononuclear copper
complexes of four-dentate N
2
O
2
bis(phenol) diamine ligands [64] and subsequently
investigate their performances as efficient catalytic system in combination with TEMPO in
the aerobic oxidation of benzylic alcohols (Scheme 4).
NEX
Scheme 4. Bis(phenol) diamine H
2
L
[64]
2
. Experimental
.1. Materials and physical measurements
2
Reagents or 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 Isfahan University of Technology. Fourier
transform infrared spectroscopy on KBr pellets was performed on a FT IR Bruker Vector 22
instrument. NMR measurements were performed on a Bruker 400 instrument. UV-Vis
absorbance digitized spectra were collected using a CARY 100 spectrophotometer. The
electronic absorption spectra of complexes have been measured in dichloromethane in the
2
00-800 nm range.
Crystals for the X-ray diffraction experiment were grown from the ethanol-dichloromethane
NEC
NEOB
solution. The X-ray intensities for the CuL
and CuL
complexes were collected with
5

an Oxford Sapphire CCD diffractometer using MoKα radiation λ =0.71073 Å, at 293(2) K,
by ω-2θ method. Structures were solved by direct methods and refined with the full-matrix
2
least-squares method on F with the use of SHELX97 [65] program package. Analytical
absorption corrections were applied (RED171 package of programs [66] Oxford Diffraction,
2
000). No extinction correction was applied. Hydrogen atoms were located from the electron
density maps and their positions were constrained in the refinement to keep the idealized
geometry for aromatic CH, methylene and methyl groups, with the corresponding C-H
distances of 0.97, 0.96 and 0.93 Å, as used in SHELXL-97 [65]). To keep the proper
geometry, water molecules were restrained during the refinement with H-O and H-H
NEC
distances of 0.84(1) and 1.4(1) Å, respectively. In CuL , the partial occupancy of 0.5 was
set for the O4W water molecule, due to the large atomic displacement parameters (ADP) for
O4W atom. On the other hand, for O1W the full occupancy was used, since it gave the better
fit to the observed diffraction data. However, the large ADP for its O1W atom indicate the
NEOB
possible positional disorder of that water. In CuL
, the void was found in the structure
and the difference electron density map indicated the presence of the disordered ethanol
molecule with the occupancy set to 0.25 based on the electron density level. Ethanol is
disordered in such a way, that both its C31 and C31a atoms are common for two alternative
positions, while the hydroxyl O32 occupies position of the methyl H atom in the alternative
molecule. Therefore, total occupancy of C31 is 0.50 and results from the superposition of
C31 and C31a from the molecule in the alternative position.
Magnetic susceptibility was measured from powder samples of solid material in the
temperature range 2-300 K by using a SQUID susceptometer (Quantum Design MPMS-XL-
5
) in a magnetic field of 1000 Oe.
Voltammetric measurements were made with a computer controlled electrochemical system
(
ECO Chemie, Ultrecht, The Netherlands) equipped with a PGSTA 30 model and driven by
6

2
GPES (ECO Chemie). A glassy carbon electrode with a surface area of 0.035 cm was used
as a working electrode and a platinum wire served as the counter electrode. The reference
electrode was an Ag wire as the quasi reference electrode. Ferrocene was added as an internal
standard after completion of a set of experiments, and potentials are referenced vs. the
+
NEC
NEOB
ferrocenium/ferrocene couple (Fc /Fc).Crystals of the CuL
and CuL
complexes
solutions.
suitable for the X-ray diffraction experiment were obtained from the EtOH-CH
2
Cl
2
2.2. Preparations
2
.2.1. Synthesis of Complexes
In a two-necked round-bottom flask, triethylamine (0.20 g, 2.00 mmol) was added to a
NEX
solution of H
2
L
2
(1.00 mmol) in ethanol (50 ml). Cu(OAc)2.H O (0.2 g, 1.00 mmol) was
added to this solution and the resulting mixture was refluxed for 2 h. Crystals suitable for X-
ray diffraction were obtained by the slow evaporation of dichloromethane and ethanol
mixture.
NEC
2
.2.1.1. Synthesis of CuL
Complex
Yield: 0.409 g (66%). Anal. Calcd. for C23
H
25Cl
4
CuN
2
O
4
(499.722 g/mol). C, 40.1; H, 4.0; N,
-1
4
.9%. Found: C, 40.66; H, 4.05; N, 5.27%. IR (KBr, cm ): 3445, 2896, 2354, 1633, 1452, 1317,
-
1
-1
1
170, 1029, 924, 865, 804, 748, 603, 447 UV-Vis in CH
1277), 622 (723).
.2.1.2. Synthesis of CuL
Yield: 0.426 g (65.5%). Anal. Calcd. for C33
2
Cl : λmax, nm (ε, M cm ): 432
2
(
NEOB
2
Complex
H49CuN
2
O
6
(534.2g/mol). C, 61.8; H, 5.1; N, 4.8%.
-
1
Found: C, 62.9; H, 5.24; N, 5.24%. IR (KBr, cm ):3445, 2937, 2356, 1616, 1459, 1423, 1310,
-1
-1
1
6
269, 1201, 1060, 863, 808, 559, 511, UV-Vis in CH
2
Cl : λmax, nm (ε, M cm ): 437 (2026),
2
46 (1860).
7

NEOB
2
.2.1.3. Catalytic activity of CuL
complex in oxidation of alcohols
NEOB
In a typical experiment, alcohol (1 mmol), CuL
(0.024 g, 5 mol%), TEMPO (0.024 g, 5
mol%) and Cs CO (0.650 g, 2 mmol) in 5 ml of oxygen saturated trifluorotoluene (TFT)
2
3
were taken in a 15-mL two-necked round-bottom flask was equipped with oxygen balloon.
The solution was magnetically stirred for some hours at room temperature and the mixture
was filtered through a plug of silica, and then diluted with TFT (2 mL). The progress of the
reaction was monitored by gas chromatography.
In a separate blank test, a 15-ml two-necked round-bottom flask was charged with the
2 3
solution of copper(II) acetate, TEMPO or ligand in mixed medium of Cs CO (0.650 g, 2
mmol) in 5 ml of oxygen saturated TFT. Then the flask was equipped with an oxygen balloon
at room temperature. The progress of the reaction was monitored by gas chromatography.
3. Results and Discussion
NEX
H
2
L
[X: C and OB ], were prepared from N,N' dimethylethylenediamine, formaldehyde
and 2,4-dichlorophenol or 2-tert-Butyl-4-methoxyphenol., in one step Mannich condensation.
The copper(II) complexes were synthesized by the following procedure (Scheme 5).
NEX
Scheme 5. The reaction pathway for the synthesis of CuL
In IR spectra of complexes, replacement of the strong and sharp νOH stretch band of the
-
1
phenols around 3300-3500 cm by a broad band, proved the coordination of phenol groups to
the metal.
8

In both complexes, the absorption maxima observed in the near-UV regions (below 300 nm)
are attributed to π→π* transitions of phenolate units. There are two absorption features in the
NEX
visible spectrum of CuL
complexes which are assigned to phenolate (π) -to- Cu(II) (dπ*)
charge transfer (LMCT) transitions (Fig.1, Table 1). It was observed that, when OMe electron
donating substituents are placed on the phenolate groups, the position of LMCT bands shifted
to higher wavelengths (lower energy).
Figure 1
-1
-1
NEX
Table 1. λmax/nm (ꢀ/M cm ) LMCT band of complexes CuL
(X: C, B and OB)
NEX
-1
-1
-1
-1
CuL
λmax/nm(ε /M cm )
λmax/nm(ε /M cm )
NEC
CuL
CuL
431 (1268)
460 (2285)
625 (715)
NEOB
665 (1864)
3
.1. Single crystal data collection and the crystal structure description
NEC
NEOB
Crystal and refinement data for the CuL
and CuL
are summarized in Table 2.
Selected bond lengths and angles are given in Tables 3.
NEC
NEOB
Table 2. Crystal data and structure refinement for CuL
and CuL
NEC
NEOB
Identification code
Empirical formula
Formula weight
Temperature; K
Wavelength; Å
CuL
CuN
CuL
C
18
H
21.5 Cl
4
2
O
3.75
28.5 2 4.25
C H43.5 Cu N O
531.21
293(2)
0.71073
545.69
293(2)
0.71073
Crystal system, space group
Unit cell dimensions; Å
Orthorhombic, Pca2(1)
a = 22.7101(6)
Triclinic, P-1
a = 10.7996(6)
b = 12.3188(3)
c = 16.0313(5)
b = 11.3898(10)
c = 13.1074(9)
α = 98.658(7)
β = 109.474(6)
γ = 95.963(6)
1482.06(19)
2, 1.223
3
Volume; Å
4484.9(2)
8, 1.573
3
Z, Calculated density; Mg/m
Absorption coefficient; mm
-
1
1.476
0.771
F(000)
2164
583
Crystal size; mm
Theta range for data collection,
Limiting indices
0.29 x 0.15 x 0.02
2.20 to 28.14
-22<=h<=29,
0.77 x 0.43 x 0.19
2.23 to 28.03
-14<=h<=14,
-14<=k<=14,
0
-15<=k<=16,
9

-20<=l<=17
-16<=l<=17
Reflections collected / unique
Completeness to θ = 25.00
Absorption correction
27516 / 9427 R(int)=0.0841
99.9 %
Analytical
9250 / 6257 R(int)=0.0460
99.9 %
Analytical
Max. and min. transmission
Refinement method
0.9663 and 0.6766
0.8665 and 0.5891
Full-matrix least-squares on
Full-matrix least-squares on
2
2
F
F
Data / restraints / parameters
Goodness-of-fit on F
9427 / 1 / 523
0.838
6256 / 3 / 334
1.045
2
Final R indices [I>2sigma(I)]
R indices (all data)
Absolute structure parameter
Largest diff. peak and hole; e.Å
R1=0.0534, wR2=0.1029
R1=0.1309, wR2=0.1181
0.308(17)
R1=0.05.36, wR2=0.1382
R1=0.0723, wR2=0.1534
-3
0.685 and -0.401
0.807 and -0.766
0
NEC
NEOB
Table 3. Selected bond lengths [Å] and angles [ ] for CuL
and CuL
NEC
NEC
NEOB
CuL
Molecule1
1.930(5)
1.952(5)
2.026(6)
2.056(6)
2.385(6)
CuL
Molecule2
1.923(5)
1.906(5)
2.004(6)
2.062(6)
2.550(5)
CuL
Cu1-O1
Cu1-O2
Cu1-N1
Cu1-N2
Cu1-O2W
angles [deg]
Cu2-O21
Cu2-O22
Cu2-N21
Cu2-N22
Cu2-O3W
Cu1-O1
Cu1-O2
Cu1-N1
Cu1-N2
1.884(2)
1.889(2)
2.042(2)
2.026(3)
O1-Cu1-O2
O1-Cu1-N1
O2-Cu1-N1
O1-Cu1-N2
O2-Cu1-N2
N1-Cu1-N2
O1-Cu1-O2W
O2-Cu1-O2W
N1-Cu1-O2W
N2-Cu1-O2W
89.7(2)
89.9(2)
169.4(2)
170.2(2)
92.1(2)
86.5(2)
93.2(2)
98.6(2)
92.0(2)
96.0(2)
O22-Cu2-O21
O21-Cu2-N21
O22-Cu2-N21
O21-Cu2-N22
O22-Cu2-N22
N21-Cu2-N22
O3W-Cu2-O21
O3W-Cu2-O22
O3W-Cu2-N21
O3W-Cu2-N22
89.6(2)
92.9(3)
174.0(3)
172.3(3)
90.8(2)
86.0(3)
93.9(2)
91.6(2)
93.7(2)
93.8(2)
O1-Cu1-O2
O1-Cu1-N1
O2-Cu1-N1
O1-Cu1-N2
O2-Cu1-N2
N2-Cu1-N1
90.15(9)
93.51(10)
168.84(10)
167.50(10)
92.50(9)
86.19(11)
NEC
3
.1.1. X-ray Crystal Structure of CuL
The complex crystallizes in the orthorhombic space group Pca2
1
. The asymmetric unit
NEC
NEC
contains one molecule of the [CuL
(H
2
O)] complex, second molecule CuL
with
weakly interacting water and two other water molecules. The complex molecules are
positioned in the crystal lattice with the equatorial CuN moieties almost parallel (dihedral
2
O
2
angle 5.0(3)°) but axial water ligands pointing to the opposite directions (Figure. 2).
Figure 2
1
0

2 3
In the first molecule, the coordination sphere CuN O has the square pyramid geometry with
the axially bound O2W water molecule. The Cu1-O1 and Cu1-O2 distances are
approximately 0.1 Å shorter than the Cu-N distances (Table 3). The axial Cu1-O2W bond of
2
.385(6) Å is significantly longer than bonds found in the equatorial CuN
2
2
O plane. The bond
angles in the base of the coordination pyramid reveal the largest deviations from the square
geometry as indicated by the O-Cu1-N angles involving atoms positioned trans. The angles
between the axial Cu1-O2W and bonds formed within the square plane vary from N1-Cu1-
O2W 92.0(2) ° to O2-Cu1-O2W 98.6(2)°.
In the second molecule, the axial interaction Cu2-O3W is significantly longer than in the first
molecule, with the distance Cu2-O3W of 2.550(5) Å. The weaker interaction to that ligand
seems to result in the shortening of the Cu2-O22 and Cu2-N21 bond distances within the
square-planar coordination sphere of Cu2 (Table 3), up to 0.046 Å, compared to those found
in the first molecule. Binding of the axial water molecules is structurally similar to the weak
coordination of the axial Tyr495 residue in the GOase [4] (Scheme 2). In molecule 2 the
largest deformation from the square geometry are found for the angles formed by equatorial
donor atoms positioned trans to each other. The angles N/O-Cu2-O3W in molecule 2 are
closer to 90 °, than those found in molecule 1, what might be related to the repulsions within
the coordination sphere of Cu1 with the shorter axial Cu-O bond but also to the
intramolecular strain caused by the differences in the ligand conformation. These differences
between two complex molecules are emphasized by the deviation of central Cu ion from the
equatorial best N
and Cu2, respectively.
The valence geometry of the H
2
O
2
plane towards the axial water, being 0.176(3) and 0.111(3) Å for Cu1
NEC
2
L
ligands is typical for such systems. The equatorial
NEC
arrangement of the H
2
L
ligand in the coordination sphere results in the twist of the central
ethylenediamine moiety, with the torsion angle N1-C8-C9-N2 being -55.1(9)°, a value typical
1
1

for analogous Fe or Mn complexes [64, 67]. In molecule 2 the corresponding N21-C28-C29-
N22 torsion angle is -21.8(19)°, what further emphasizes the differences within the
coordination sphere of Cu ions. As a result, the methyl groups bound to N in molecule 1 are
positioned trans, while in molecule 2 are cis relative to each other (Figure 2). The dihedral
NEC
angles between the phenolic rings of the L
ligands are 30.3° and 2.0°.
In molecule 1, the central chelate ring Cu1-N1-C8-C9-N2 has a conformation twisted on C8-
C9, the Cu1-O1-C1-C6-C7-N1 is an envelope on N1, while Cu1-O2-C16-C11-C10-N2 is an
envelope on C10. In molecule 2, the corresponding rings are envelopes on N21, N22 and
C27.
Analysis of the crystal packing revealed the series of H-bonds involving water OH as donors
NEC
and L
ligands Cl substituents and O atoms as acceptors (Table 4S, Supporting
Information).
.1.2. X-ray Crystal Structure of CuL
NEOB
3
The complex crystallizes in the triclinic space group P-1. The asymmetric part of the reported
NEOB
structure consists of the complex molecule CuL
and a disordered ethanol molecule
(
Figure 3). The absolute configuration R,R is found on N1, N2 atoms of the molecule in the
asymmetric unit. However, the centrosymmetric pace group P-1 results in the equimolar
presence of S,S-N1,N2 isomer in the structure. The coordination sphere CuN has a square
2
O
2
planar geometry with pairs of N and O atoms occupying cis positions due to the tetradentate
NEOB
coordination of the L
ligand. The Cu1-O1 and Cu1-O2 distances of 1.882(2) and
NEC
1
.889(2) Å, respectively, are significantly shorter than those found for CuL
complex. On
NEOB
the other hand, the Cu-N distances in CuL
are similar to those found for molecule 1 of
NEC
CuL , but reveal statistically significant differences when compared to molecule 2 of
NEC
CuL
(Table 3). Such geometry seems to result from the NEOB ligand conformation
NEC
described with the torsion angle N1-C8-C9-N2 -54.0(5) °, similar to that of CuL
molecule
1
2

1
or analogous Fe and Mn complexes [64, 67]. The largest angular deformation from the
square geometry is found for O1-Cu1-N2 of 167.5(1) °. Deviation of Cu1 from the N
plane is 0.010(2) Å.
2 2
O best
Figure 3
NEOB
The extended molecular conformation of L
is found, with the dihedral angle between
NEOB
best planes of the phenolic being 32.28(17) °. Three chelate rings are formed by L
ligand. Ring Cu1-N1-C8-C9-N2 is twisted on C8-C9, while both six-membered Cu1-O1-C1-
C6-C7-N1 and Cu1-O2-C16-C11-C10-N2 have a screw-boat conformation.
NEOB
The valence geometry of the L
ligands is typical for such systems. Due to the syn-clinal
NEOB
conformation of the central ethylenediamine moiety of L
, the N-Met groups are
positioned trans relative to each other, similar to the orientation found in Molecule 1 of
NEC
NEOB
CuL . The OMe groups of L
ligand are nearly co-planar with the phenolic rings, with
torsion angles C5-C4-O3-C21 -2.7(6) °and C12-C13-O4-C24 10.9(6) °.
The hydroxyl group of ethanol participates in the H-bond with O3[-x,-y,-z], with the O...O
distance being 2.900(4) Å. Analysis of the intermolecular interactions revealed the C-H…π
interactions between C22 and C23 methyl groups and the
being C22-
H22B…Cg(C11...C16)[-x,1-y,-z] 2.91 and C22-H22C…Cg(C11...C16)[-x,1-y,-z] 2.97 Å,
C23-H23A…Cg(C1...C6) [-x,2-y,-z] of 2.80 Å.
3
.2. Magnetic susceptibility measurement
NEC
NEOB
Temperature variation of measured susceptibility χ(T) for CuL
and CuL
is shown in
Fig 4. The presented data were corrected for the temperature-independent Larmor
diamagnetic susceptibility obtained from Pascal’s tables [68] and for the sample holder
contribution. The susceptibility of both samples increases with decreasing temperature,
following a Curie 1/ T law, which indicates paramagnetic behavior. Paramagnetic behavior is
1
3

also confirmed from the temperature independent effective magnetic moment of both
complexes (inset in Fig. 4). The effective magnetic moments of both complexes lie between
1
.70 and 1.80 BM for temperatures above 5 K. The values are slightly smaller than the spin-
only value of µeff = 1.9 BM for the isolated Cu(II) ion (S = 1/2) [69], but very close to the
values measured in other copper complexes (reported values in [70] were 1.77–1.83 BM).
This confirms the presence of one unpaired electron in the copper center of both complexes.
NEC
A very small downturn of the measured effective moment of CuL
at temperatures below 5
K could be attributed to a very weak antiferromagnetic interaction between the copper
monomers. This was confirmed by applying a Curie–Weiss law χ = C/(T-θ) to fit the data
above 20 K. The obtained Curie–Weiss temperature was θ = - 0.14 K (Table 4).
Figure 4
Table 4. Calculated parameters C and θ from the fit of the measured data. The effective magnetic
moment µeff per iron ion was calculated using a relation
C (emu K/mol)
0.403
θ (K)
0.380
0.033
µ
eff(BM)
1.8
NEC
CuL
CuL
NEOB
0.366
1.72
Table 4
3
.3. Electrochemistry
Cyclic voltammogram (CV) of complexes have been recorded in CH
.1 M [(n-Bu) N]ClO as a supporting electrolyte.
This research reveals that there are two quasi-reversible oxidation peaks for CuL
2
Cl
2
solutions containing
0
4
4
NEOB
complex in the positive potential range and there is one irreversible oxidation peaks for
NEC
CuL
complex. As the further oxidation of copper (II) redox process occurs at higher redox
potentials, these redox processes are probably attributed to the ligand centered redox
1
4

processes in which phenolate group yields phenoxyl radical in the complex, but there is no
direct evidence available yet. This observation can attributed to the shielding effect of t-Bu
groups for stabilization of phenoxyl radical groups. Typical cyclic voltammogram (CV) of
NEOB
CuL
, is presented in Figure 5.
Figure 5
NEC
cyclic voltammogram (CV) of CuL , is presented in Figure 1S. The metal-centred
voltammograms have been observed in the negative potential range, what corresponds to the
II
I
Cu /Cu reduction of copper complexes. These metal-centered voltammograms are
NEC
NEOB
electrochemically irreversible for CuL
complexes.
complex and quasi-reversible for CuL
3
.4. EPR spectroscopy results
Frozen solution EPR shows a single isotropic signal with a g value of 2.045 and 2.064 more
9
than the free electron g value attributed to single-electron copper (II) d configuration. A
NEOB
NEC
typical EPR spectrum of CuL
shown in Figure 2S.
has been shown in Figure 6. EPR of CuL
has been
Figure 6
NEX
3
.5. The catalytic activity of CuL
complex in the aerobic oxidation of alcohols
NEX
In continuation of our studies on the application of CuL
complexes, we report on
the aerobic oxidation of alcohols to corresponding aldehydes using this complex. To study
NEX
the catalytic oxidation of alcohols mediated by CuL , benzyl alcohol was chosen as a
model compound and the reaction conditions were optimized accordingly. Our experiments
NEC
showed that among the employed complexes, CuL
exhibited superior catalytic
1
5

NEOB
performance than CuL
(50% conversion) in catalyzing aerobic oxidation of benzyl
alcohol to benzaldehyde. This is most likely owed to the fact that the electron withdrawing
substituents at phenolate groups provided different electron densities on the Cu center in
this complex and should yield higher conversions of benzyl alcohol. Alternatively, time
NEX
screening of the CuL /TEMPO system alcohol oxidation showed the same initiation
NEOB
time for both complexes (Fig. 7). It considers that CuL
shows lower activity during the
reaction time. In addition after around 2 hrs a higher decrease in the rate of reactivity for
NEOB
CuL
was observed. It can possibly attributed to the lower stability of this complex
during the reaction time.
Figure 7
Next, we investigated the effect of bases for oxidation of benzyl alcohol. Using bases such as
KOH and NaHCO , the alcohol conversions were lower than that for Cs CO (Table 6,
entries 3-5). In the next stage, the amount of Cs CO was also optimized (Table 5, entries 1,
) and the best result was achieved when 2 equivalents of Cs CO were used.
3
2
3
2
3
2
2
3
The solvent effect was also investigated and the results have been summarized in Table 7. In
this context, various organic solvents such as CH CN, CH Cl etc. were studied and TFT
3
2
2
appeared to be the most favorable solvent for the described transformation (52%, Table 6,
entries 1).
1
6

Table 5. The effect of base on the alcohol oxidation
NEC
CuL
Entry
Base
Time (h)
Conversion (%)
1
2
3
4
5
6
4 mmol Cs
2
CO
3
6
58
2 mmol Cs
2
CO
3
6
12
12
8
52
25
20
10
9
4 mmol NaHCO
3
3
2 mmol NaHCO
2 mmol NaNO
3
4 mmol KOH
8
Table 6. The effect of solvent on the alcohol oxidation
Entry
Solvent
Time (h)
Conversion (%)
1
2
3
TFT
6
8
6
52
29
9
Toluene
3
CH CN
4
5
CH
2
Cl
2
+TFT
6
6
6
5
TFT+CH COOH
3
1
7

The effect of catalyst loading between 5.0 to 15.0 mol% was also studied within 12 hrs.
(
Table 7) and it was revealed that the maximum yield of benzaldehyde was obtained when
NEC
1
5.0 mol% of CuL
were employed. In addition, when the reaction condition employed for
NEC
catalytic experiment are 15.0 mol% of CuL , the impact of TEMPO loading was then
studied between 5-10 mol% under otherwise the same reaction condition indicated in Table 7.
The best result was obtained when the amount of TEMPO was chosen 5 mol% (Table 8). The
2 3
blank tests verified “ligand-free”, copper(II) acetate/Cs CO affords aldehyde in low yield
(
Table 9). Supposing the effect of the ligand on increasing the alcohol conversion, the blank
test with copper(II) acetate/Cs
2
CO
3
and free ligand improved the aldehyde yield. Comparing
NEC
these results with the same for [CuL
2 3
-Cs CO ] as catalyst system lead us to this fact that
the structure of this coordination copper complex and metal-ligand synergistic effects have an
important role in the oxidation of alcohols.
NEC
Table 7. The effect of CuL
equivalences on the alcohol oxidation
NEC
Entry
CuL
(mol%)
Time (h)
12
Conversion (%)
1
2
3
4
5
15
99.5
91
12
10
5
12
12
82
12
70
8
12
20
1
8

Table 8. The effect of TEMPO equivalences on the alcohol oxidation
Entry
TEMPO (mmol)
Time (h)
Conversion (%)
1
2
3
4
0.25
0.05
0.5
1
10
6
61
60
27
16
6
10
Table 9. The result of blank test for oxidation of benzyl alcohol with
catalyst system
Entry
Catalytic system
Time (h) Conversion (%)
1
2
3
4
Copper(II) acetate /Cs
2
CO
3
12
12
12
12
24
35
50
72
NEC
H
2
L
2 3
/Cs CO
2 3
Copper(II) acetate /Cs CO /
NEC
2
H L
NEC
CuL
/Cs
2
CO
3
To assess the general scope of the catalytic oxidation, several types of benzylic, and other,
different kinds of alcohols were chosen as substrates. Under optimized reaction conditions
NEC
(
2 3
TFT using 15 mol% of CuL , 2 mmol Cs CO and 0.05 mmol TEMPO), various
substituted benzaldehydes were obtained from their corresponding, readily available,
alcohols. According to the literature [71-73], in the “copper complex-Cs CO ” the higher
2
3
activity in TFT may be attributed to the possible adduct formation between copper and/or
cesium cations and aromatic solvent as well as to the polarity effect and higher solubility of
1
9

O
2
2 3
in this fluorinated solvent. These results led us to suggest that Cs CO most likely acts not
only as a strong base for deprotonating of alcohol but also as a heterogeneous support for
NEC
NEC
NEC
CuL
CuL
to produce an stable CuL -Cs
2
CO
3
catalytic species [74-76]. In this system,
NEC
is slightly soluble in TFT while Cs
2
CO
3
is not soluble. Therefore, CuL -Cs
2
CO
3
catalyst can be possibly considered as an actual intermediate between homogenous and
heterogeneous catalyst. The results for oxidations of variety of benzyl alcohols are
NEC
summarized in Table 10. Our studies show that CuL
is selective towards benzyl alcohol
while is totally inactive for aliphatic alcohols (Table 10). As can be seen, unsubstituted
benzylic alcohol is oxidized more efficiently than substituted ones (Table 10). These results
suggest that the influence of the substituent appears to be related to both electronic and steric
effects. This catalyst doesn't show any activity against secondary alcohols. The catalytic
NEC
activity of CuL
complex in the aerobic oxidation of alcohols is shown in Schemes 5 and 6.
The results of this study show that the carbonyl compound is the only product obtained with
comparatively good yields. Generally, over-oxidation of the primary alcohols to the
corresponding acids occurs with other catalysts, but in the present work the formation of
acids was not observed (Scheme 6).
2
0

NEC
Table 10. Alcohol oxidation results in TFT, catalysed by Copper [CuL ] and TEMPO in a presence
of Cs CO as base and O as oxidant
2
3
2
Entry
1
Substrate
Time (h)
12
Conversion (%)
100
Time (h)
24
Conversion (%)
100
2
8
92
12
99
3
4
12
8
91
93
24
12
96
96
OH
Cl
5
12
84
24
92
OH
6
7
8
9
12
12
12
12
75
73
64
79
24
24
24
24
91
89
84
80
Br
OH
OH
MeO
Me
1
1
0
1
12
12
16
9
24
24
24
10
1
2
8
4
12
5
2
1

1
2
3
. TFT (2.5 ml)
. CuLNEC (15 mol%)
. Cs CO (2 mmol, 0.65 g)
2
3
OH
O
H
R
1
4. RT/23 °C, Oxygen
R
R'
R'
5
. TEMPO(0.05 mmol)
mmol
NEC
Scheme 6. Catalytic activity of CuL in the aerobic oxidation of alcohols.
NEC
Since CuL
2 3
is not effective in the absence of additional base, we suggest that Cs CO is
responsible for the deprotonation of alcohols. Coordination of this alcoholate to copper(II)
complex produces an alkoxide complex. Intermolecular abstraction of hydrogen with
TEMPO leads to the formation of ketyl radical. In other word, nitroxyl radicals act as redox-
+
−
active organic co-catalyst to facilitate the 1 H /2e process associated with conversion of
an alkoxide ligand to a carbonyl compound. Stoichiometric reactivity studies showed that
both copper(II) and TEMPO participate in the alcohol oxidation step [62]. It suggest that an
electron transfer to copper produces aldehyde and Cu(I) species. and aerobic oxidation of
Cu(I) and TEMPO−H affords Cu(II) and TEMPO species back to the original Cu(II)
complex and cocatalyst. Based on Stahl studies, there are three different mechanisms for
oxidation of the alkoxide ligand (Scheme 7): bimolecular H-atom transfer, H-atom transfer to
an η2-nitroxyl ligand, and hydrogen transfer to an η1-nitroxyl ligand. Stahl group have
reported that the last exhibits the lowest-energy pathway for alcohol oxidation, rationalizes
the observed selectivity for primary over secondary alcohol oxidation (Scheme 7) [62].
2
2

II
Scheme 7. Stahl mechanistic Proposals for Cu /Nitroxyl mediated alcohol oxidation [62]
Considering experimental results and results of studies reported previously, our proposed
reaction mechanism for the system is shown in scheme 8.
NEC
II
L
Cu
O
RCH OH
2
N
LNECCuII
O
O
R
H
N
H O
2
H
N
LNECCuII
OH
O
O2
H
O
R
N
LNECCuI
C
OH
R
H
Scheme 8. The possible reaction pathway in the catalytic cycle of
NEC
2 3
alcohol oxidation using CuL /TEMPO/Cs CO catalytic system
4
. Conclusion
2
3

NEOB
NEC
In summary, two new copper(II) complexes (CuL
ligands were synthesized and characterized.
and CuL
) of aminophenol-based
Electrochemical oxidation of these complexes yielded the corresponding Cu(II)–phenoxyl
radical species.
In addition, highly efficient, and eco- friendly TEMPO- mediated oxidation of alcohol to
NEC
aldehyde was achieved with molecular oxygen as oxidant and non-precious catalyst CuL
-
Cs CO system. In conclusion, this system is found to be very efficient for a wide range of
2
3
benzylic alcohols. Advantages of this catalytic system are the use of low price copper
catalyst, molecular oxygen, high selectivity, low loading of TEMPO under mild reaction
conditions (room temperature). The disadvantage of the system is the lack of activity against
aliphatic primary and secondary alcohols and high loading of copper complex.
NEC
NEOB
Appendix A.The structural data for CuL
Cambridge Crystallographic Data Centre, the deposition number being CCDC 1041129 and
041130 respectively. 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.
and CuL
have been deposited with the
1
(
Acknowledgments
Authors are grateful to the Institute for Advanced Studies in Basic Sciences (IASBS),
Ljubljana, Changwon and Nicolaus Copernicus Universities for their valuable help. E. Safaei
gratefully acknowledges the support by the Institute for Advanced Studies in Basic Sciences
(
IASBS) Research Council under Grant No. G2014IASBS127. Yong-Ill Lee acknowledges
support by the Priority Research Centers Program (NRF 2010-0029634) through the National
Research Foundation of Korea (NRF).
2
4

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LTD, (1993) 6-10.
69] C. Kittel, Introduction to Solid State Physics (8th Ed), John Wiley & Sons (2005)55-57.
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