Crystal structure of an alkoxide bridged dinuclear copper(II) complex: mild and selective oxidation of primary and secondary alcohols in water

Article abstract of DOI:10.1007/s11243-018-0252-2

A water-soluble dinuclear Cu(II) complex, [Cu₂(OOCC₆H₄Br)(OCH₃)(C₁₀H₈N₂)₂(ClO₄)₂] (4-bromobenzoic acid = HOOCC₆H₄Br; 2-2′-b

Full text of DOI:10.1007/s11243-018-0252-2

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0252-2
Crystal structure of an alkoxide bridged dinuclear copper(II) complex:
mild and selective oxidation of primary and secondary alcohols
in water
Hakan Ünver1
Received: 14 April 2018 / Accepted: 30 May 2018
© Springer International Publishing AG, part of Springer Nature 2018
Abstract
A water-soluble dinuclear Cu(II) complex, [Cu2(OOCC6H4Br)(OCH3)(C10H8N2)2(ClO4)2] (4-bromobenzoic acid=HOOC-
C6H4Br; 2-2′-bipyridyl=C10H8N2) was synthesized and characterized by X-ray crystallography. The complex was investi-
gated as a selective catalyst for the mild oxidation of primary and secondary alcohols in the presence of hydrogen peroxide
as oxidant at 70 ℃ in water. The complex proved to be an active catalyst for production of the corresponding aldehydes or
ketones. Thus, cinnamyl alcohol, benzyl alcohol, 1-phenyl ethanol and 2-butanol were quantitatively oxidized in 6 h, using
a low catalyst loading (1 mol%).
Introduction
basic auxiliary substances such as K2CO3, TEMPO, and
DMAP or co-solvents [8–15]. In contrast, reports of catalytic
alcohol oxidations conducted in water without any additives
are rare [16, 17]. There are several reports of water-soluble
complexes and their application for catalytic oxidation of
alcohols in water [18–22]. However, the use of expensive
and rare earth metals and their complexes including Pd, Ru,
Rh, etc. is associated with economical concerns. Copper
complexes are used efectively, especially in alcohol oxida-
tion reactions. However, these frequently require the use of
organic solvents, higher reaction temperatures, co-additives
or toxic oxidants [23–27]. In the last few years, our research
group has been interested in catalytic alcohol oxidations
conducted in water using hydrogen peroxide as an oxygen
source under mild conditions. In this context, we recently
reported on a water-soluble copper(II) complex including a
carboxylate ligand (triphenyl acetate) plus a neutral ligand
(2,2′-bipyridyl). This complex was successfully used to oxi-
dize primary and secondary alcohols to their carbonyl prod-
ucts using hydrogen peroxide [16]. We have also descried a
water-soluble copper(II) complex of 2,2′-dipyridylamine/4-
bromobenzoate ligands [17]. The catalytic activity of this
complex was investigated for the oxidation of alcohols and
alkenes with t-BuOOH or H2O2 in water. It was found to be
especially active for the oxidation of styrene and cyclohex-
ene with good selectivities (up to 100%).
The oxidation of alcohols is the key synthetic routes to
obtain their aldehydes, ketones or carboxylic acids [1, 2].
To date, many catalytic systems have been investigated for
these reactions, particularly with a view to the discovery of
environmentally friendly chemical processes. Conventional
oxidation reactions are conducted with oxidants based on
heavy metals, such as RuO4, SeO2, (C5H5NH)2(Cr2O7),
KMnO4, and CrO3, with associated risks of toxic waste and
environmental damage [3–5]. To overcome these drawbacks,
catalytic oxidations with hydrogen peroxide, molecular oxy-
gen or tert-butyl hydroperoxide (t-BuOOH) are attractive in
terms of both environmental and economic sustainability. In
terms of solvent choice, organic solvents have obvious dis-
advantages compared to water, which is the ideal for green
chemistry. Nevertheless, examples of catalytic alcohol oxi-
dations in water are rare [6, 7].
Many diferent transition metal complexes have been used
for catalytic alcohol oxidations in organic solvents, using
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0252-2) contains
supplementary material, which is available to authorized users.
* Hakan Ünver
hakanunver@anadolu.edu.tr
Following on from these fndings, in this paper we report
on the synthesis and characterization of a new water-soluble
dinuclear copper(II) complex, [Cu2(OOCC6H4Br)(OCH3)
1
Department of Chemistry, Faculty of Science, Anadolu
University, Eskişehir 26210, Turkey
Vol.:(0123456789)
1 3

Transition Metal Chemistry
(C10H8N2)2(ClO4)2], and an investigation of its catalytic
activity for the oxidation of primary and secondary alco-
hols in water without any additives under mild conditions.
non-hydrogen atoms. Hydrogen atoms were added at the
calculated positions. The molecular structures were drawn
using MERCURY [31]. The PLATON [32] program was
used for geometric analyses.
Experimental
General procedure for catalytic oxidations
Materials and methods
Catalytic oxidation experiments were carried out in a 50 mL
round-bottomed fask ftted with a refux condenser under
open air with a magnetic stirrer at 70 °C. In a typical experi-
ment, 5 mg of catalyst (5.3×10−3 mmol) was dissolved in
deionized water (10 mL). The substrate (0.53 mmol) (sub-
strate/catalyst = 100) plus hydrogen peroxide (2 mL of
30% in water, 1.95 mmol) was added and the mixture was
stirred. To monitor the progress of the reaction, ten drops of
reaction mixture were withdrawn at certain time intervals
and analyzed by GC with an HP-5 quartz capillary column
(30 m×0.32 mm×0.25 μm) with a fame ionization detector
(FID). Each analysis was repeated twice and the peak iden-
tifcation was done by comparing with authentic samples.
All reagents were commercially obtained (Sigma-Aldrich)
and used as received. The crystallographic data were col-
lected with a Bruker AXS difractometer. FTIR spectra were
recorded with a Perkin Elmer Spectrum 100 Spectrometer
using KBr discs in the range of 400–4000 cm−1. UV–Vis
spectra were recorded on a Shimadzu UV-2450 spectro-
photometer between 200 and 900 nm. Elemental analyses
were obtained with an Elementar Vario EL III microanalyzer
instrument. Magnetic susceptibility was measured with a
Sherwood Scientifc MX-I balance.
Synthesis of the complex
A solution of 4-bromobenzoic acid (105 mg, 0.52 mmol)
in methanol (10 mL, neutralized with 1.04 mL of 0.5 M
NaOH) was added dropwise to a solution of copper(II) per-
chlorate hexahydrate (192.6 mg, 0.52 mmol) in methanol
(10 mL). After stirring for 30 min, a solution of 2,2′-bipy-
ridine (81.2 mg, 0.52 mmol) in methanol (5 mL) was
added. The mixture was then refuxed for 12 h. The solu-
tion was then fltered. Blue crystals were collected after
5 days of solvent evaporation (215 mg; Yield: 48% m.p.:
235 °C). The complex is soluble in water and several polar
organic solvents (MeOH, EtOH, CH3CN), Anal. Calc. for
C28H23BrCl2Cu2N4O11 (869.39 g/mol) C, 38.68; H, 2.64;
N, 6.45%, Found: C, 38.61; H, 2.68; N, 6.50%. Signifcant
IR bands (KBr, ν cm−1) (s, strong; m, medium; w, weak):
Results and discussion
Structural analysis of the complex
The reaction between 4-bromobenzoic acid, 2,2′-bipyridine
and copper(II) perchlorate resulted in dark blue cubic crys-
tals of (Cu2(OOCC6H4Br)(OCH3)(C10H8N2)2(ClO4)2) after
slow evaporation of the methanol solvent over 5 days. The
crystals of the complex proved to be of the triclinic space
group, P−1. The molecular structure and molecular formula
of the complex are given in Fig. 1. The crystallographic data,
bond distances, bond angles and torsion angles are listed in
Tables 1 and 2.
According to the X-ray crystal structure, the complex
consists of a pair of Cu(II) centers, bridged by anionic car-
boxylate and methoxide ligands. In addition, each Cu(II)
center is coordinated by two nitrogen atoms of chelating
bipyridine ligands together with perchlorate anions, to form
a dimeric unit. All the Cu–N bond distances are similar
between 1.985 and 1.998 Å, which are also similar to the
literature values [33, 34]. The methoxide and carboxylate
groups are both coordinated in bidentate modes to the copper
atoms, acting as bridging ligands. The Cu–O bond lengths
of the carboxylate oxygens with Cu1 and Cu2, at 1.925 and
1.946 Å, are also similar to those of related complexes [35,
36]. The Cu–O bond lengths for the bridging methoxide
ligand are 1.929 and 1.917 Å for Cu1–O3 and Cu2–O3,
respectively [37, 38]. In the axial positions, the bond dis-
tances for the perchlorate ligands are Cu1–O4 = 2.402 Å
and Cu2–O12 = 2.399 Å, indicating that they are directly
3092–3030w νC–Harom.; 1617 m νC=N; 1678 m νCOOasym
;
1445 m νCOOsym; 1552 m νC=N–C=Csym; 1088 s νC–O;
770 m νC–N; 729 m νCu–O–Cusym; 621 m νCu–O–Cuasym
.
UV–Vis λmax nm (CH3CN): 241, 301, 311. Magnetic
moment (μ)=1.37 B.M. at room temperature.
X‑ray crystallography
Single crystal data collection was performed on a Bruker
AXS APEX CCD diffractometer equipped with an Mo
Kα radiation source at 293 (2) K. The data processing was
done with the Bruker SMART program package [28]. The
SHELXS- 97 [30] program was used for the structure solu-
tion by direct methods. Non-hydrogen atoms were located
through difference Fourier synthesis [29] and refined
with full-matrix least squares methods on F2 using frst
isotropic and later anisotropic thermal parameters for all
1 3

Transition Metal Chemistry
Fig. 1 Thermal ellipsoid plots
of the copper (II) complex at
the 40% probability level and its
molecular formula
O
O
Cl
O
N
N
O
Cu
O
O
O
O
O
O
N
CH3
Cl
Cu
O
N
Br
Table 1 Crystal data and structure refnement for complex
Table 2 Selected bond distances (Å) and bond angles and torsion
angles (°)
Empirical formula
Formula weight
C28 H23 Br Cl2 Cu2 N4 O11
869.39
Bond distances (Å)
Bond angles and torsion angles (°)
Temperature
Wavelength
Crystal system, space group
Unit cell dimensions
293 (2) K
0.71073 Å
Triclinic, P−1
Cu(1)–Cu(2) 3.127 (1)
Cu(1)–O(3) 1.929 (3)
Cu(1)–O(1) 1.926 (3)
Cu(1)–N(1) 1.997 (4)
Cu(1)–O(4) 2.402 (4)
Cu(1)–N(2) 1.997 (4)
Cu(2)–O(3) 1.916 (3)
Cu(2)–O(2) 1.945 (3)
Cu(2)–N(3) 1.986 (4)
Cu(2)–N(4) 1.994 (4)
Cu(2)–O(12) 2.399 (4)
O(3)–Cu(1)–O(4) 101.29 (16)
O(4)–Cu(1)–N(2) 84.35 (16)
N(1)–Cu(1)–O(4) 88.79 (17)
O(1)–Cu(1)–O(4) 92.83 (15)
N(4)–Cu(2)–O(12) 100.84 (16)
N(3)–Cu(2)–O(12) 95.77 (15)
O(3)–Cu(2)–O(12) 85.80 (14)
O(2)–Cu(2)–O(12) 92.97 (14)
O(12)–Cu(2)–N(3)–C(27) 81.49
O(4)–Cu(1)–N(2)–C(17) 88.18
a=9.4847 (17) Å alpha=100.568
(10)°
b=13.877 (2) Å beta=108.417
(10)°
c=13.992 (2) Å
gamma=106.929(10)°
Volume
1592.8 (5) A3
2, 1.813 Mg/m3
2.824 mm−1
Z, Calculated density
Absorption coefcient
F(000)
868
Crystal size
Theta range for data collection
Limiting indices
0.18×0.10×0.08 mm
1.61 to 28.44°
for Cu2. The reduction of angle values from 90° for
O4–Cu1–N2 and O4–Cu1–N1, at 84.36 and 88.78, respec-
tively, can be attributed to the steric efect of the bridged
methoxide ligand. The torsion angles of the bipy ligands for
O12–Cu2–N3–C27 and O4–Cu1–N2–C17 are 81.55° and
88.17°, respectively. The methoxy bridge angle between
Cu1 and Cu2 is 108.81°. Additionally, several intermolecu-
lar interactions can be identifed between the bipy carbons
and perchlorate oxygens.
−11≤h≤12, −18≤k≤18,
−18≤l≤17
Refections collected/unique
Completeness to theta=
Absorption correction
Refnement method
22,886/6946 [R(int)=0.0605]
28.44 86.4%
None
Full-matrix least-squares on F2
6946/0/435
Data/restraints/parameters
Goodness-of-ft on F2
1.011
Final R indices [I>2 sigma(I)]
R indices (all data)
Largest dif. peak and hole
R1=0.0624, wR2=0.1608
R1=0.1008, wR2=0.1847
1.513 and −1.008 e.A−3
Spectroscopic studies
The FTIR spectrum of the complex is given in Figure S1.
The peaks between 3030 and 3092 cm−1 are assigned to
the aromatic ʋ(C–H) vibrations of the ligands. Symmetric
and asymmetric vibration peaks of the carboxylate ligand
were observed at 1678 and 1445 cm−1, respectively. The
azomethine ʋ(C=N) vibration of the bipyridine ligand
shifted from 1589 to 1617 cm−1 upon complexation,
coordinated to the Cu(II) centers. The Cu1–Cu2 distance
is 3.127 Å.
The geometry around both Cu(II) centers is best described
as distorted square-pyramidal, with coordination angles
between 84.36°and 101.29° for Cu1 and 85.82°–100.83°
1 3

Transition Metal Chemistry
consistent with coordination of the N atom. Peaks
observed at 729 and 621 cm−1 are assigned to Cu–N bond
vibrations.
Catalytic studies
The homogenous catalytic oxidation of primary and sec-
ondary alcohols in the presence of this dinuclear copper(II)
complex under open air using H2O2 (30% in water) as an
oxidant was investigated at 70 ℃ in water as solvent. The
results are given in Table 3 and Fig. 2.
The UV–Vis spectrum of the complex was recorded in
acetonitrile solution between 200 and 800 nm. In total,
three absorption bands were observed at 241, 301 and
311 nm (Figure S2). The band at 241 can be assigned
to carboxylate-based π → π* intraligand transitions. The
bands at 301 and 311 nm arise from bipyridine ligand
n → π* transitions. No d → d transitions were observed
for the Cu centers.
According to the results, the copper complex was an
efective catalyst for the selected alcohol oxidations, giving
high TON values (up to 100). It is noteworthy that no traces
of carboxylic acid were detected during or after the reac-
tions; furthermore, the C=C bonds were not afected. The
complex was generally found to be more efective as a cata-
lyst for benzylic and cyclic alcohols rather than 1-heptanol
as a primary aliphatic alcohol. The total substrate conver-
sions ranged from 17 to 100% for a range of diferent alco-
hols with a 6 h reaction time (Table 3, Entries 2–7), whilst
the complete oxidation of cinnamyl alcohol required only
2 h (Table 3, Entry 1). The oxidation of cinnamyl alcohol
According to room temperature measurement of the
complex magnetic susceptibility with a Gouy balance,
the magnetic moment of the complex is 1.37 B.M. This
value is consistent with one unpaired electron, although
lower than the theoretical value of 1.73 B.M. due to the
magnetic interactions between the ligands and the metal
centers [39].
Table 3 Oxidation of various alcohols with copper (II) complex in water
Substrate
Product(s), conv. (%)
Yieldsa (%) TON/TOF
(h–1)
Entry
22 (1 min)
100 (2 h)
22/1320
100/50
1
11 (1 min)
88 (2 h)
11 (1 min)
12 (2 h)
100 (6 h)
53 (6 h)
100/16
53/9
2
3
71 (6 h)
71/12
4
5
100 (6 h)
100/17
100 (6 h)
17 (6 h)
100/17
17/3
6
7
Conditions: alcohol (0.57 mmol), catalyst (5.7 × 10−3 mmol), H2O2 (19.5 mmol), water (10 mL), T=70 ℃
Blank experiment was conducted without catalyst for each substrate and negligible conversion was obtained (<3%)
aDetermined with GC
1 3

Transition Metal Chemistry
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
Cyclopentanol
2-Butanol
1-Heptanol
1-Phenyl Ethanol
Cyclohexanol
Cinnamyl Alcohol
Benzyl Alcohol
0
1
5
15 30 45 60 90 120 150 180 210 240 270 300 330 360
Time (min.)
Fig. 2 Time-dependent catalytic oxidation of alcohols in water
gave both cinnamaldehyde (11%) and benzaldehyde (11%)
in the frst minute of the reaction and the total product for-
mation was found to be 22% with a TOF value of 1320.
The substrate was fully converted to aldehydes after 2 h
(TON=100), with overall 88% selectivity for benzaldehyde.
The oxidation of benzyl alcohol resulted in selective prod-
uct formation, with only benzaldehyde obtained in 100%
conversion for a 6-h reaction time; no other oxidation prod-
uct was observed (Entry 2). Cyclohexanol was oxidized to
cyclohexanone in a moderate yield of 53% after the same
time period (Entry 3). Cyclopentanol oxidation gave a higher
yield of product compared to cyclohexanol, with a 71% yield
of cylopentanone after 6 h (Entry 4). The unactivated pri-
mary aliphatic alcohol 1-heptanol gave the lowest product
conversion value of 17% as heptaldehyde (Entry 7). Accord-
ing to these fndings, the reactivities of aromatic alcohols
were much higher than aliphatic ones, in agreement with
previous studies [40].
of benzyl alcohol plus 1-phenyl ethanol mixture, 16.4% ben-
zaldehyde was obtained together with 62.4% acetophenone
(Entry 1). Hence, this catalytic system shows a tendency to
act on the secondary alcohol.
The oxidation of a mixture of a linear secondary alcohol
(2-butanol) and a cyclic alcohol (cyclopentanol) resulted in
lower conversions for both (Table 4, Entry 2), compared to
yields of 100 and 71% for 2-butanone and cyclopentanone
obtained from the initial experiments, on single substrates.
Overall, these experiments indicated that the competi-
tion between the mixtures of alcohols results in lower prod-
uct yields compared to the pure substrates under the same
conditions.
Conclusions
In summary, we have described the synthesis and charac-
terization of a dinuclear, water-soluble copper (II) complex.
The complex showed good homogenous catalytic activity for
the oxidation of both primary and secondary alcohols under
mild conditions (70 ℃) with hydrogen peroxide in water.
The selectivity of this catalytic system was tested between
a primary and a secondary alcohol, or between a cyclic alco-
hol and an aliphatic alcohol at 70 ℃ for 6 h reaction time,
with the results given in Table 4. In the catalytic oxidation
Table 4 Selective oxidation of
selected alcohols with complex
(II) complex in water
Entry
1
Alcohols
Products
Conv. (%)a/time (h)
Benzyl alcohol+1-phenyl Benzaldehyde+Acetophenone
ethanol
16.4 (6 h)+62.4 (6 h)
2
2-butanol+Cyclopentanol 2-butanone+Cyclopentanone
5.3 (6 h)+26.3 (6 h)
1 mmol each of the alcohols was tested with mmol complex 1, mmol H2O2 in water at 70 ℃
Conditions: alcohol (5.7 × 10−4 mmol), catalyst (5.7 × 10−6 mmol), H2O2 (19.5 mmol), water (10 mL),
T=70 ℃
aDetermined with GC
1 3

Transition Metal Chemistry
13. Samantha S, Das S, Samantha PK, Dutta S, Biswas P (2013) RSC
Adv 3:19455–19466
This system was found to give moderate to good yields for
benzylic and cyclic alcohols, whereas 1-heptanol as a rep-
resentative aliphatic alcohol gave poorer results. No over-
oxidation products could be detected in these reactions.
14. Jiang N, Ragauskas AJ (2005) Org Lett 7(17):3689–3692
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Chem 89:68–71
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17. Unver H, Kani I (2018) J Chem Sci 130:33
18. Brink GJ, Arends IWCE, Sheldon RA (2002) Adv Synth Catal
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Supplementary material
19. Bufn BP, Clarkson JP, Belitz NL, Kundu A (2005) J Mol Catal
A: Chem 225:111–116
CCDC 1830893 contains the supplementary crystallo-
graphic data for complex. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk.
20. Pagliaro M, Campestrini S, Crimina R (2005) Chem Soc Rev
34:837–845
21. Liu L, Yu M, Wayland BB, Fu X (2010) Chem Commun
46:6353–6355
22. Wang X, Wang C, Liu Y, Xiao J (2016) Green Chem 18:4605
23. Samanta S, Das S, Samanta PK, Dutta S, Biswas P (2013) RSC
Adv 3:19455–19466
24. Mannam S, Alamsetti SK (2007) Adv Synth Catal 349:2253–2258
25. Dijksman A, Arends IWCE, Sheldon RA (2001) Synlett
1:102–104
Acknowledgements The author thankfully acknowledges the Medici-
nal Plants and Medicine Research Centre of Anadolu University,
Eskişehir, Turkey for the use of X-ray Difractometer and Anadolu
University, Chemistry Department, Eskişehir, Turkey for the other
spectroscopic measurements.
26. Cecchetto A, Fontna F, Minisci F, Recupero F (2001) Tetrahedron
Lett 42:6651–6653
27. Jiang N, Ragauskas AJ (2006) J Org Chem 71:7087–7090
28. MART, Bruker AXS, (2000)
29. Sheldrick GM (2008) Acta Crystallogr. Sect. A 64:112
30. Sheldrick GM (1997) SHELXS-97, Program for Crystal Structure
Solution. University of Gottingen, Gottingen
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