Homogeneous oxidation of alcohols in water catalyzed with Cu(II)-triphenyl acetate/bipyridyl complex

Article abstract of DOI:10.1016/j.poly.2017.06.030

A water-soluble mononuclear copper(II) complex, [Cu(OOCC(C₆H₅)₃)(C₁₀H₈N₂)(H₂O)](ClO₄)(CH₃OH) (triphenylacetic acid?=?HOOCC(C₆H₅)₃, 2-2′-bipyridyl (bipy)?=?C₁₀H₈N₂) was synthesized and used as a catalyst precursor in the oxidation of primary (cinnamyl alcohol, benzyl alcohol, cyclohexanol, and 1-heptanol) and secondary alcohols (1-phenylethanol, 3-pentanol, and 2-octanol) to corresponding aldehydes, ketones, and acids. The complex exhibited high catalytic activity toward benzyl alcohol to benzaldehyde as one product (97% conversion in 6?h) and cinnamyl alcohol to benzaldehyde (91.5%) and cinnamaldehyde (6.6%), in 6?h reaction time with less catalyst loading (1?mol%) at a moderate temperature (70?°C). Water was used as a solvent and H₂O₂ as an oxidant for alcohol oxidation. Thus, the Cu(II)/H₂O/H₂O₂ catalytic system could serve as an environmentally benign “green chemistry” alternative to oxidation methods in traditional organic solvents.

Full text of DOI:10.1016/j.poly.2017.06.030

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Polyhedron 134 (2017) 257–262
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Homogeneous oxidation of alcohols in water catalyzed with Cu(II)-
triphenyl acetate/bipyridyl complex
⇑
Hakan Ünver, Ibrahim Kani
Department of Chemistry, Faculty of Science, Anadolu University, Eskisehir 26210, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A
water-soluble mononuclear copper(II) complex, [Cu(OOCC(C6H5)3)(C10H8N2)(H2O)](ClO4)(CH3OH)
(triphenylacetic acid = HOOCC(C6H5)3, 2-20-bipyridyl (bipy) = C10H8N2) was synthesized and used as a
catalyst precursor in the oxidation of primary (cinnamyl alcohol, benzyl alcohol, cyclohexanol, and 1-
heptanol) and secondary alcohols (1-phenylethanol, 3-pentanol, and 2-octanol) to corresponding aldehy-
des, ketones, and acids. The complex exhibited high catalytic activity toward benzyl alcohol to benzalde-
hyde as one product (97% conversion in 6 h) and cinnamyl alcohol to benzaldehyde (91.5%) and
cinnamaldehyde (6.6%), in 6 h reaction time with less catalyst loading (1 mol%) at a moderate tempera-
ture (70 °C). Water was used as a solvent and H2O2 as an oxidant for alcohol oxidation. Thus, the Cu(II)/
H2O/H2O2 catalytic system could serve as an environmentally benign ‘‘green chemistry” alternative to
oxidation methods in traditional organic solvents.
Received 25 April 2017
Accepted 21 June 2017
Available online 29 June 2017
Keywords:
Catalysis
Copper
Complex
Oxidation
Alcohol
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
Sheldon Group. In addition to the use of expensive metal com-
plexes, higher temperature (100 °C), oxygen pressure (30 bar),
The catalytic oxidation of alcohols is a basic transformation
reaction in synthetic organic chemistry [1]. A variety of reagents
and catalytic systems have been developed to improve this oxida-
tion process in terms of economic and environmental reasons.
Unfortunately, these reagents are very active and involve stoichio-
metric amounts of traditional toxic metal–containing oxidizing
agents (i.e., CrO3 and KMnO4 [2]). That is why growing attention
has been paid to the replacement of these reagents with less toxic
oxidants such as H2O2 or O2. Another problem with these systems
is the use of hazardous organic solvents that cause unwanted
wastes, side products, and environmental problems. The catalytic
reactions in water could be favorable with lower cost, greater
safety, easier isolation of products and catalyst recovery, more
importantly, in terms of green chemistry, compared with those
performed in organic solvents; however, the effective catalytic sys-
tems for alcohol oxidation in water were scarcely explored [3].
Transition metal salts and metal–organic frameworks such as
Cu, Co, V, Pd, Ru, Rh, and Mo [4–10] have been used as catalyst pre-
cursors for the catalytic oxidation of alcohols with O2 or H2O2.
Reports on catalytic oxidation reactions using the Cu catalyst with
organic nitrogen-based ligands are mostly presented by other
groups [11–15]. The first water-soluble palladium(II) complex for
the catalytic oxidation of alcohols in water was reported by the
some external acid or base and organic solvents were required
for these catalytic systems [16]. Among the above mentioned
metal salts or metal–organic complexes, Cu is the most interesting
one because of the biomimetic functional model of the galactose
oxidase enzyme, which is a mononuclear copper compound [17].
It is known that the structure of the molecule organic framework
of metal complexes with different organic ligands should be highly
related to their catalytic activity. Although Cu(I)- and Cu(II)-con-
taining complexes with nitrogen donor–containing ligands were
studied as catalyst precursors for alcohol oxidations [18], carboxy-
late and neutral nitrogen–based ligand (i.e., bipyridyl and phenan-
throline) together with Cu(II) is not reported yet to the best of our
knowledge [19]. This study is the first report of this type on the
water-soluble Cu(II) complex, [Cu(OOCC(C6H5)3)(C10H8N2)(H2O)]
(ClO4)(CH3OH), explored to the oxidation of alcohols without the
use of any promoter in aqueous medium. Therefore, in this work,
our purpose is to extend the carboxylate group/nitrogen ligand sets
with Cu(II) and report on the solid-state structure of the copper(II)
complex based on these ligands.
2. Experiment
2.1. Materials–instrumentation–physical measurements
⇑
All chemicals were purchased from commercial sources and
used as received. IR spectra were measured with Jasco FT/IR-300
Corresponding author.
E-mail address: ibrahimkani@anadolu.edu.tr (I. Kani).
http://dx.doi.org/10.1016/j.poly.2017.06.030
0277-5387/Ó 2017 Elsevier Ltd. All rights reserved.

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H. Ünver, I. Kani / Polyhedron 134 (2017) 257–262
E Spectrophotometer using the KBr pellet in the range of 4000–
400 cmꢀ1. UV–Vis spectra of 1 were recorded on a Shimadzu UV-
2450 spectrophotometer. Crystallographic data were collected at
room temperature with a Bruker APEX II CCD using Mo-Ka radia-
tion and corrected for absorption with SADABS. The structure
was solved by direct methods. All non-hydrogen atoms were
refined anisotropically by full-matrix least squares on F2. The mag-
netic susceptibility measurements were performed using a Sher-
wood Scientific MXI model Gouy magnetic balance at room
temperature.
3. Results and discussion
3.1. Synthesis of the complex
The stoichiometric reaction of triphenyl acetic acid and bipy
with copper(II) perchloride in a methanol formed a blue solution,
from which suitable blue crystals of complex, [Cu(OOCC(C6H5)3)
(C10H8N2)(H2O)](ClO4)(CH3OH), were obtained through evapora-
tion of the solution in 3 days. Complex crystallized in a monoclinic
space group, P ꢀ 1, with 2 formula units in the unit cell as a
mononuclear complex cation, which is consistent with the molec-
ular structure found through elemental analysis. An ORTEP struc-
ture is presented in Fig. 1. Crystallographic data, bond distances
and angles relevant to the metal coordination sphere of the com-
plex are listed in Tables 1 and 2.
2.2. Synthesis of [Cu(OOCC(C6H5)3)(C10H8N2)(H2O)](ClO4)(CH3OH)
A
neutralized solution of triphenylacetic acid (150 mg,
Crystallographic analysis reveals that the Cu(II) ion is coordi-
nated with donor atoms O and N in a monomeric distorted
square-planar geometry. Cu(II) is surrounded by two nitrogen
atoms of chelating bipy ligand and by one oxygen atom of water
and one oxygen atom of the carboxylate group of the triphenylac-
etate ligand, giving an overall CuN2O2 binding set. Outside the
complex cation, one methanol molecule and one perchlorate ion
cyclize as counter ion. The CuAN distances are 1.986(2) Å and
1.993(2) Å. The carboxylate group coordinates in a monodentate
manner with a Cu1AO2 distance of 1.932(2) Å, which is shorter
than the water coordination Cu1AO7 distance of 1.968(2) Å. The
distance (3.240 Å) between the dangling oxygen atom of the car-
boxylate group and the copper ion is too large for the coordination
of the oxygen and the copper ion. The geometry around Cu(II) is a
distorted square-planar geometry with coordination angles within
the range of 81.8(1)–93.8(1) deviating from 90° and 173.0(1)° and
175.5(2)° deviating from 180°. The smaller angle (81.8 Å) corre-
sponds to that formed by the copper ion and the two N of the bipy
molecule, as it is common in copper–bipy complexes. The bipy
ligand is closer to planarity, as indicated by the N1AC5AC6AN2
torsion angle at ꢀ0.44°.
0.52 mmol, 5 mL methanol) with NaOH (1.04 mL, 0.5 M) was added
to a solution of copper(II) perchlorate, Cu(ClO4)2ꢁ6(H2O) (192.6 mg,
0.52 mmol) in MeOH (15 mL). After 1 h of stirring, the solution of
bipy (81.2 mg, 0.52 mmol) in MeOH (2 mL) was added to the mix-
ture. The final solution was refluxed in 5 h. The solution was fil-
tered off over Celite. Blue crystals were obtained through
evaporation of the solvent for 3 days. (205 mg; Yield: 55.4% m.p.:
283 °C; soluble in polar organic solvents, Anal. Cald. for C31H28
ClCuN2O8 (656.55 g/mol) C, 57.70; H, 4.04; N, 4.49. Found: C,
59.27; H, 4.83; N, 3.97%) (Significant IR bands (KBr,
cmꢀ1) (s,
strong; m, medium; w, weak): 3479w NAH, 1602m C@N;
1492m C@NAC@Csym
CAN; 731m CuAOACusym
-
m
m
m
1694m
1102m
m
COOasym
;
1444m
m
COOsym
;
m
m
;
;
m
OAH; 1085s
m
CAO; 768m
m
625m
m
CuAOACuasym) (UV–Vis kmax nm (CH3CN): 198, 245, 299.
Μagnetic moment (
l) = 1.4 B.M. at room temperature.
2.3. X-ray crystallography
Diffraction data for the complex collected with Bruker AXS
APEX CCD diffractometer equipped with a rotation anode at 296
(2) K, respectively using graphite monochromated Mo Ka radiation
In the unit cell structure, two mononuclear units are linked by
two OAH. . .O hydrogen bonds between OAH of the coordinated
water molecule and two oxygen atoms of the lattice perchlorate
ion, with an O3. . .O7 distance of 2.848 (4) Å (O3ꢀH3A. . .O7) and
an O3. . .O6 distance of 3.258 (4) Å (O3ꢀH3B. . .O6) (Fig. 2, Table 3).
at k = 0.71073 Å. The data reduction was performed with the Bru-
ker SMART program package [20]. The structures were solved by
direct methods and the non-hydrogen atoms were located through
subsequent difference Fourier syntheses [21]. Structure solution
was found with the SHELXS-97 package using the direct methods
and were refined SHELXL-97 [22] against F2 using first isotropic
and later anisotropic thermal parameters for all non-hydrogen
atoms. Hydrogen atoms were added to the structure model at
calculated positions. The molecular drawing was obtained using
MERCURY [23]. Geometric calculations were performed with PLATON
[24].
2.4. General procedure of the catalytic oxidation experiments
The oxidation reactions were performed in a 50 mL reaction
flask with a reflux condenser under vigorous stirring at 70 °C in a
temperature-controlled oil bath. In a typical experiment, the reac-
tion mixture was prepared as follows: 7.6 ꢂ 10ꢀ3 mmol of catalyst
into a 10 mL water solvent (0.76 mmol) of substrate alcohol (sub-
strate/cat. = 100) and an excess amount of H2O2 (30% in water,
1.95 mmol) in this order. The reactions were monitored by with-
drawing small aliquots at certain time intervals and analyzed on
a
GC with an HP-5 quartz capillary column (30 m ꢂ 0.32
mm ꢂ 0.25
l
m) and a flame ionization detector (FID). Each sample
was repeated twice. Identification of peaks was made by compar-
ing with chromatograms of authentic samples.
Fig. 1. Molecular structure of complex.

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H. Ünver, I. Kani / Polyhedron 134 (2017) 257–262
259
Table 1
Crystal data and structure refinement for complex.
Empirical formula
Formula weight
C31 H29 Cl Cu N2 O8
656.55
Temperature
Wavelength
296(2) K
0.71073 Å
ꢀ
Crystal system, space group
triclinic, P1
Unit cell dimensions
a = 8.5570(8) Å alpha = 91.701(4)°
b = 9.6193(9) Å beta = 93.799(4)°
c = 19.1802(18) Å gamma = 103.103
(4)o
Volume
1532.7(2) A3
Z, Calculated density
Absorption coefficient
F(000)
2, 1.423 Mg/m3
0.852 mmꢀ1
678
Crystal size
Theta range for data collection
Limiting indices
0.31 ꢂ 0.28 ꢂ 0.07 mm
2.13–28.42 (°)
ꢀ11 ꢃ h ꢃ 11, ꢀ12 ꢃ k ꢃ 12,
ꢀ25 ꢃ l ꢃ 25
Reflections collected/unique
Completeness to theta =
Absorption correction
38996/7618 [R(int) = 0.0277]
28.42 98.7%
None
Max. and min. transmission
Refinement method
0.9427 and 0.7780
Full-matrix least-squares on F2
7618/0/395
Data/restraints/parameters
Fig. 2. Unit cell of complex.
Goodness-of-fit (GOF) on F2
1.056
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0640, wR2 = 0.2001
R1 = 0.0714, wR2 = 0.2096
1.972 and ꢀ0.714
Table 3
Hydrogen bonding in complex.
Largest difference peak and hole
(e Aꢀ3
)
D–H. . .A
D–H (Å)
H. . .A (Å)
D. . .A (Å)
D–H. . .A (°)
O3–H3B. . .O2
O3–H3A. . .O6
C4–H4. . .O1
0.91
0.82
0.93
0.93
0.93
0.93
0.82
0.82
0.93
0.93
0.93
1.72
2.65
2.59
2.35
2.43
2.54
2.16
2.29
2.55
2.57
2.57
2.533(4)
3.258(8)
3.222(4)
2.781(4)
2.946(4)
3.056(5)
2.848(4)
3.004(9)
3.213(8)
3.298(10)
3.468(6)
146(6)
132(3)
125
108
115
115
142
146
129
Table 2
Selected bond lengths (Å) and angles
(°).
C10–H10. . .O2
C21–H21. . .O1
C30–H30. . .O3ii
O3–H3A. . .O7i
O8–H8A. . .O5ii
C23–H23. . .O6iii
C27–H27. . .O4iv
C27–H27. . .O7iv
Cu(1)-O(1)
Cu(1)-O(3)
Cu(1)-N(1)
Cu(1)-N(2)
N(1)-C(21)
N(1)-C(25)
N(2)-C(30)
N(2)-C(26)
O(1)-Cu(1)-O(3)
O(1)-Cu(1)-N(1)
O(3)-Cu(1)-N(1)
O(1)-Cu(1)-N(2)
O(3)-Cu(1)-N(2)
N(1)-Cu(1)-N(2)
1.932(2)
1.968(2)
1.986(3)
1.992(3)
1.331(4)
1.344(4)
1.338(4)
1.346(4)
92.87(10)
91.57(11)
175.52(10)
173.01(10)
93.82(11)
81.76(11)
136
163
Symmetry codes: (i) 1 ꢀ x, 1 ꢀ y, 1 ꢀ z; (ii) ꢀ1 + x, y, z; (iii) x, 1 + y, z; (iv) 1 ꢀ x, 2 ꢀ y,
1 – z.
ligand azomethine m(C@N) bond frequency shifted from 1589 to
1602 cmꢀ1, indicating that the coordination occurs via nitrogen
atom with a metal center. Strong bands at 768 and 731 cmꢀ1 cor-
respond to chelating bipy frequencies.
The UV–Vis spectrum of complex was measured in an acetoni-
trile solution and revealed three absorption bands at 198, 245, and
299 nm (Fig. SP2). The band at 198 nm is assigned to
The dangling oxygen atom (O2) of the carboxylate group forms an
intramolecular hydrogen bond with the hydrogen atom of the
coordinated water molecule (O2. . .O3 distance of 2.533 (4) Å).
p ?
p⁄ tran-
sitions of a carboxylate ligand. The other observed bands are attrib-
uted to bipy ligand (C@N) n ? p⁄ transitions. Any d ? d transitions
were not observed in the spectrum of the complex.
The magnetic property of complex in the solid state at room
temperature was examined by using a Gouy magnetic balance.
The complex exhibits the magnetic moment value of 1.40 which
correspond to one unpaired electron. This result is lower than
the spin only value (1.73 BM) indicate magnetic exchange interac-
tion between adjacent complex unit [26].
The shortest
p p distance between two parallel rings of bipy
ꢁ ꢁ ꢁ
ligands (N1C21-C25 and N2C26-C30) is 10.057 Å. Moreover, some
intermolecular contacts are observed between the carbon atoms of
bipy rings and the oxygen atoms of the perchlorate ion (Table 3).
The Cuꢁ ꢁ ꢁCu distance is 7.687 Å in the unit cell.
3.2. Spectroscopic study
The infrared spectra of the complex are consistent with the
determined X-ray structure (Fig. SP1). A broad band observed at
3479 cmꢀ1 is due to OAH stretching of the coordinated aqua
ligand. The band observed between 3034 and 3086 cmꢀ1 is attrib-
3.3. Catalytic studies
We have tested the copper(II) compound as homogeneous cat-
alyst precursor for the oxidation of primary and secondary alcohols
to the corresponding alcohols and ketones. The catalytic reactions
were performed under mild conditions (70 °C) in green solvent
water and using H2O2 (30% in water) as an oxidant.
Experimental results illustrated that complex is an effective cat-
alyst for the oxidation of alcohols. Among the studied alcohols
(Table 4, Fig. 3), complex is more active in primary alcohols than
uted to aromatic
m(CAH) vibrations of both neutral and anionic
ligands. The symmetric
m
sym(OCO) and asymmetric asym(OCO)
m
vibrational peaks of the carboxylate group were observed as strong
peaks at 1694 and 1444 cmꢀ1. The magnitude of separation
between the antisymmetric and the symmetric carboxylate
stretching of complex is 250 cmꢀ1, which indicates a monodentate
coordination in deference to Nakamoto’s method [25]. Free bipy

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H. Ünver, I. Kani / Polyhedron 134 (2017) 257–262
Table 4
Complex catalyzed oxidation of primary and secondary alcohols to carbonyl compounds.a,b
Entry
Substrate
Products, Conv. (%)
Total Conv.c (%)
TON/TOF (hꢀ1
)
1
19.5 (1 min)
16.2 (2 h)
6.6 (6 h)
11.6 (1 min)
39.2 (2 h)
84.9 (6 h)
31.1 (1 min)
55.4 (2 h)
91.5 (6 h)
31/1866
55/28
92/15
2
3
48.6 (2 h)
97.4 (6 h)
97.4 (6 h)
97/16
28.0 (6 h)
52.2 (24 h)
52.2 (24 h)
12.2 (6 h)
52/2
11/2
4
5
73.0 (6 h)
100 (24 h)
100 (24 h)
29.4 (6 h)
100/4
29/5
6
7
10.7 (24 h)
11/1
a
b
c
Conditions: alcohol (0.76 mmol), catalyst (7.6 ꢂ 10ꢀ3 mmol), H2O2 (19.5 mmol), water (10 mL), T = 70 °C.
Blank experiment was conducted for each substrate and negligible conversion was obtained (<2%).
Determined with GC.
Fig. 3. Comparison of alcohol oxidation with complex in water solvent.
in secondary ones. Entries 1, 2 and 3 illustrate that primary alco-
hols are more reactive than secondary alcohols and gave a higher
TON, up to 100. Notably, primary alcohols (entries 1–4) were not
converted into over-oxidation product carboxylic acids, whereas
secondary alcohols were oxidized to only the corresponding
ketones without CAC chain cleavage (entries 5–7).