Selective catalytic oxidation of aromatic substrates employing mononuclear copper(II) catalyst with H2O2

Article abstract of DOI:10.1016/j.poly.2018.11.007

A novel mononuclear complex [LCuCl₂(H₂O)]CH₂Cl₂ (L = 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine) was synthesized and characterized by elemental analysis, ultraviolet visible (UV–Vis) spectrum, and fourier transform infrared spectroscopy (FTIR). The single-crystal X-ray diffraction structure analysis of the complex revealed that the copper atom was octahedrally coordinated. The catalytic potential of this complex was examined by the oxidation reaction of various aromatic substrates, such as styrene, ethyl benzene, α-methyl styrene, and benzyl alcohol. The oxidation reactions were carried out in acetonitrile with H₂O₂ and catalytic amounts of [LCuCl₂(H₂O)] at 70 °C. The catalyst exhibited good catalytic activity in the oxidation of benzyl alcohol (97.50% conv. in 1 h; TONs = 341.25), α-methyl styrene (97.72% conv. in 4 h; TONs = 342.02) and methyl styrene (99.00% conv. in 8 h; TONs = 346.50). The major products were 96.07% benzoic acid from benzyl alcohol, 93.54% acetophenone from α-methyl styrene and 86.38% benzaldehyde from styrene, respectively. A plausible catalytic mechanism involving an active cation [LCu²⁺] and an epoxy intermediate was proposed based the results of time-dependent electrospray ionization mass spectrometry (ESI-MS) of the reaction mixture.

Full text of DOI:10.1016/j.poly.2018.11.007

Accepted Manuscript
Selective Catalytic Oxidation of Aromatic Substrates Employing Mononuclear
Copper(II) Catalyst with H2O2
Chengcheng Wu, Bin Liu, Xin Geng, Zhenyu Zhang, Shenghua Liu, Quanyuan
Hu
PII:
S0277-5387(18)30725-3
https://doi.org/10.1016/j.poly.2018.11.007
POLY 13554
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
12 July 2018
23 October 2018
3 November 2018
Please cite this article as: C. Wu, B. Liu, X. Geng, Z. Zhang, S. Liu, Q. Hu, Selective Catalytic Oxidation of Aromatic
Substrates Employing Mononuclear Copper(II) Catalyst with H2O2, Polyhedron (2018), doi: https://doi.org/
10.1016/j.poly.2018.11.007
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Selective Catalytic Oxidation of Aromatic Substrates Employing
Mononuclear Copper(II) Catalyst with H2O2
Chengcheng Wua, Bin Liua, Xin Genga, Zhenyu Zhanga, Shenghua Liub, Quanyuan Hua*
aHubei Collaboration Center for Advanced Organic Chemical Materials (Hubei University),
aMinistry of Education Key Laboratory for the Synthesis and Application of Organic Functional
Molecules (Hubei University), Wuhan 430062, China
bDepartment of Chemistry, Central China Normal University, Wuhan 430079, China
Abstract
A novel mononuclear complex [LCuCl2(H2O)]CH2Cl2 (L = 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)
pyridine) was synthesized and characterized by elemental analysis, ultraviolet visible (UV-vis)
spectrum, and fourier transform infrared spectroscopy (FTIR). The single-crystal X-ray diffraction
structure analysis of the complex revealed that the copper atom was octahedrally coordinated. The
catalytic potential of this complex was examined by the oxidation reaction of various aromatic
substrates, such as styrene, ethyl benzene, α-methyl styrene, and benzyl alcohol. The oxidation
reactions were carried out in acetonitrile with H2O2 and catalytic amounts of [LCuCl2(H2O)] at
70℃. The catalyst exhibited good catalytic activity in the oxidation of benzyl alcohol (97.50%
conv. in 1 h.; TONs = 341.25), methyl styrene (97.72% conv. in 4 h.; TONs = 342.02) and
methyl styrene (99.00% conv. in 8 h.; TONs = 346.50). The major products were 96.07% benzoic
acid from benzyl alcohol, 93.54% acetophenone from α-methyl styrene and 86.38% benzaldehyde
from styrene, respectively. A plausible catalytic mechanism involving an active cation [LCu2+] and
an epoxy intermediate was proposed based the results of time-dependent electrospray ionization
mass spectrometry (ESI-MS) of the reaction mixture.
Keywords: catalytic oxidation, copper(II) complex, aromatic substrates, single-crystal structure,
hydrogen peroxide
1. Introduction
Environment-friendly oxidations of organic compounds have ever been paid much attention.
Many efforts were focused on the activation and transportation of oxygen source with high
efficiency and metal complexes were generally utilized to overcome the challenge[1]. Transition
metals such as V[2], Fe[3], Mo[4], Co[5], Cu[6], Zn[7] et. al were put into use. Various
nitrogen-containing ligands were attempted. Although biomimetic catalytic systems use molecular
oxygen as oxidant, more reactive oxidants like peroxides and hypochlorites, have been operated at
room temperature frequently[8]. However, the activation of C-H bonds is still really difficult due to

its chemical inert property, which requires high temperature and pressure in most occasions[9].
Therefore, the development of bio-inspired catalysts worked in mild conditions is eagerly needed
in both industry and research.
Over the decades, great advances have been made in the understanding of the copper proteins
that transport oxygen and utilize dioxygen as oxidant to functionalize organic substrates, in which
hemocyanine and superoxide dismutase(SOD) were well known examples[10]. In the former case,
the active site is a copper center coordinated by three histidine ligands, which plays an important
role in transporting oxygen[11]. In the latter case, the SOD protects cells from peroxides damage.
The active center of SOD is a bimetallic (copper and zinc) core complex with imidazolate-bridged
ligand[12]. Obviously, the above two active sites contain common structural features, and
complexes with transition metal copper and N,N,N-tridentate ligands have been gradually
recognized[13]. Copper(II) complexes exhibit decent performance in photoelectric and biological
materials due to their unique properties [14]. Meanwhile, they have already been understood for
their catalytic effects[15]
.
As the biological systems have provided insights into the design of new catalysts that can
perform a desired chemical reaction like selective oxidation of alkanes and olefins[16]. In this paper,
a
biomimetic
catalyst
using
the
structural
properties
of
copper
and
2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine was designed and synthesized. The catalytic
performance of the catalyst was examined in the oxidation of various aromatic substrates with
H2O2 in acetonitrile.
2. Experiment
2.1. Materials and measurements
All chemicals were purchased from commercial sources and used as received. IR spectra were
measured with Jasco FT/IR-300 E Spectrophotometer using the KBr pellet in the range of
4000-400 cm-1. UV-vis spectra were recorded on a Shimadzu UV-2450 spectrophotometer.
ESI-MS measurements were conducted at a capillary temperature of 275 °C. Aliquots of the
solution were injected into the device at 0.3 mL/h. The mass spectrometer used for the
measurements was a Thermo Exactive Plus, and the data were collected in positive and negative
ion modes. The spectrometer was calibrated with the standard tune mix to give a precision of ca. 2
ppm in the region of 200 − 3000 m/z. The capillary voltage was 50 V, the tube lens voltage was
150 V, and the skimmer voltage was 25 V. the reaction solution was diluted with CH3CN for
ESI-MS at 275 °C (Positive mode) and 310 °C (Negative mode).
The ligand 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine (L) was synthesized from
2,6-pyridinedicarboxylic acid and pinacolone by Claisen Condensation[17], followed by
ring-closure reaction with hydrazine as described in the literature [18]
.
2.2 Synthesis of [LCuCl2(H2O)]CH2Cl2
Under nitrogen atmosphere, a solution of 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine (0.4g; 1
mmol) dissolved in 10 mL of dichloromethane was added dropwise to a solution of CuCl2·6H2O

(0.1g; 1 mmol) in ethanol (10 mL). The mixture was stirred at room temperature for one hour, and
then refluxed for another one hour. The green solid formed was isolated by filtration, and then
recrystallized from ethanol. (340.8 mg; yield: 85.2%; m.p.: 334.8°C; Λm: 133 mS L/mol.cm).
Anal. Calc. for C20H29Cl4CuN5O (560.83g/mol): C, 42.83; H, 5.21; N, 12.49. Found: C, 43.38;
H, 5.69; N, 12.17. IR (KBr, ν cm−1) (s, strong; m, medium; w, weak): 3444 s; 3195 s; 3122 m;
3053 m; 2966 s; 2961 m; 1615 s; 1579 s; 1462 s; 1368 w; 1292 s; 1235 m, 1163 w, 1086 w, 1023
-1
-1
m, 817 m, 709 w.
λ
ε
3
2.3 X-ray Crystallography
Diffraction data for the complex were collected with Bruker AXS APEX CCD diffractometer
equipped with a rotation anode at 100 K, using graphite monochromated Mo K radiation at λ =
0.71073 Å. Intensity data (2.972° < 2 < 51.328°) were corrected for Lorentz-polarization effects
and for absorption. Details of crystal and data collection parameters are summarized in Table1.
The data reduction was performed with the Bruker SMART program package [19]. The structures
were solved by direct methods and the non-hydrogen atoms were located through subsequent
difference Fourier syntheses. Structure solution was found with the SHELXS-97 package/Olex 2
using the direct methods and refined SHELXL-97 [20] 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[21]
.
Geometric calculations were performed with PLATON [22]
.
Table 1. Crystal data for [LCuCl2(H2O)]CH2Cl2
Formula
M
C20H29Cl4CuN5O
560.83
α (o)
β (o)
γ (o)
V (Å3)
69.8290(1)
89.433(2)
73.969(2)
2506.7(4)
100
-11≤ h ≤11
-18≤ k ≤18
-23≤ l ≤23
0.0955
Limiting
indices
Cry. system
Space group
a (Å)
triclinic
P-1
Rint
9.3140(9)
15.2617(15)
19.6349(19)
T (K)
R1[I>2 (I)] 0.0436
b (Å)
Z
4
wR2
0.0891
0.968
c (Å)
Dc(g/cm3)
No. of unique
reflection
1.486
Goodness of
fit (GOF)
on F2
µ(MoK)
(mm-1)
1.320
9302
2.4 General Procedure of Catalytic Oxidation Experiments
The reactions were carried out in acetonitrile at 70℃ with H2O2 as oxidant and the complex
LCuCl2(H2O) as catalyst. In a 100 mL Schlenk tube were mixed 10 mg of the catalyst, 3.5 mL
H2O2(30%) in 20 mL acetonitrile, and 0.4 – 0.7 mL of aromatic substrates (substrate/cat. = 350).
The reaction was carried out at 70ºC for 1 – 12 h under stirring. The reactions were monitored by
withdrawing small aliquots at certain time intervals and analyzed on a GC with an HP-5 quartz
capillary column (30 m × 0.32 mm × 0.25 μm) and a flame ionization detector (FID). To analyze

the reaction mixture during and after the reaction period, 2 – 3 drops of the reaction mixture were
taken and then mixed with acetonitrile 1:4 (v/v), in which each sample was repeated twice. The
identification of peaks was made by comparing chromatograms of authentic samples.
3. Results and Discussion
3.1. Synthesis and Characterization of the Compounds
A previous paper disclosed the synthesis of a family of mononuclear cobalt(II) and
manganese(II) complexes with L ligands [23]. Another contribution also confirmed that a parent
complex CuCl2(bmpp) (bmpp
=
2,6-bis[5-methyl-1H-pyrazol-3-yl]pyridine) formed
five-coordinated triclinic structure[24], in which the methyl groups replace the tert-butyl
substituents. However, to maximize yields and simplify protocols, wide investigations are needed.
Therefore, the synthesis of LCuCl2(H2O) complexes was done. As shown in Scheme 1, it was
possible to confirm the structure of the green solid complexes as LCuCl2(H2O).
Scheme 1. Synthetic route of LCuCl2(H2O)
In the infrared spectrum of LCuCl2(H2O) (Fig.1), the observed two strong peaks at 3444 cm−1
and 2966 cm-1 can be assigned to the stretching vibrations of O-H and C-H (the tert-butyl) bonds,
respectively. Besides, the shoulder peak around 3195 cm-1 is due to the N-H groups. The bands
centered at 1615cm−1, 1579cm−1, 1462 cm−1, are due to the cyclic-skeleton vibrations of aromatic
cycles.

100
95
90
85
80
75
70
65
60
LCuCl (H2O)
2
1368
1235
817
3444
1615
1579
3195
3122
3053
1292
1462
2966
3000
4000
3500
2500
2000
1500
1000
500
-1
Wavenumber/cm
Figure 1. FT-IR spectrum of the LCuCl2(H2O)
.
UV–vis spectra of the free ligand (L) and LCuCl2 H2O were recorded at 300 K in methanol
.
(Fig.2). A solid-state UV–vis spectrum of LCuCl2 H2O2 (500 nm - 800 nm) (insertion in Fig.2)
was also recorded using the reflectance technique. The spectrum of L shows two bands at 241 nm
and 307 nm, which are attributed to π-π* (241 nm) and n-π* (307 nm) transitions. Analogous
bands corresponding to transitions of the chromophores in the ligands are also observed in the
.
spectrum of LCuCl2 H2O2 at 250 nm which might be assigned to π-π* transitions of the B-band of
the aromatic rings, and 322 nm to the charge transfer from ligand to metal (LMCT)[25] (in
methanol). In the solid state UV–vis spectrum, a low intensity broad band ranged from 600-800
nm could be found (see insert part in Fig. 2), which is attributed to the d–d transition of the
copper(II) ions[26]
.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
L
LCuCl H O)
2(
2
1.5
1.0
0.5
0.0
LCuCl H O)
2(
2
500
600
700
800
Wavelength (nm)
300
400
500
600
700
800
Wavelength(nm)
Figure 2.
−
LCuCl2(H2O)
，
−
LCuCl2(H2O)

3.2 Crystal Structure Analysis
Single crystals of [LCuCl2(H2O)]CH2Cl2 were obtained by slow evaporation of its solution in
dichloromethane at room temperature. The crystallographic data and processing parameters are
summarized in Fig. 3.
Figure 3. Two independent molecules (a) and an overlay of two independent molecules in the
crystal of [LCuCl2(H2O)]CH2Cl2 (b) (some H atoms and CH2Cl2 molecules are omitted for clarity).
Thermal ellipsoids are drawn at the 30% level.
Selected interatomic distances (Å) and angles (°) are shown in Table 2.
Table 2. Selected bond lengths (Å) and angles (°) for compounds [LCuCl2(H2O)]CH2Cl2
Bond Length (Å)
Cu(1)-Cl(1)
Cu(1)-Cl(2)
Cu(1)-O(1)
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-N(4)
2.710(1)
2.936(1)
1.951(2)
1.969(3)
2.025(3)
2.038(3)
Cu(2)-Cl(3)
Cu(2)-Cl(4)
Cu(2)-O(2)
Cu(2)-N(6)
Cu(2)-N(7)
Cu(2)-N(9)
2.6545(9)
2.999(1)
1.950(2)
1.970(3)
2.029(3)
2.050(3)
Bond Angle (°)

Cl(1)-Cu(1)-Cl(2)
O(1)-Cu(1)-Cl(1)
O(1)-Cu(1)-Cl(2)
O(1)-Cu(1)-N(4)
O(1)-Cu(1)-N(1)
O(1)-Cu(1)-N(2)
N(4)-Cu(1)-Cl(1)
N(4)-Cu(1)-Cl(2)
N(1)-Cu(1)-Cl(1)
N(1)-Cu(1)-Cl(2)
N(1)-Cu(1)-N(4)
N(1)-Cu(1)-N(2)
N(2)-Cu(1)-Cl(1)
N(2)-Cu(1)-Cl(2)
N(2)-Cu(1)-N(4)
N(4)-N(5)-C(8)
N(5)-N(4)-C(6)
174.04(3)
90.39(7)
83.71(7)
99.65(10)
175.17(11)
103.08(10)
86.54(8)
95.25(8)
93.71(8)
92.22(8)
78.07(11)
79.16(11)
94.55(8)
86.01(8)
157.23(11)
111.9(3)
Cl(3)-Cu(2)-Cl(4)
O(2)-Cu(2)-Cl(3)
O(2)-Cu(2)-Cl(4)
O(2)-Cu(2)-N(9)
O(2)-Cu(2)-N(7)
O(2)-Cu(2)-N(6)
N(9)-Cu(2)-Cl(3)
N(9)-Cu(2)-Cl(4)
N(7)-Cu(2)-Cl(3)
N(7)-Cu(2)-Cl(4)
N(7)-Cu(2)-N(9)
N(6)-Cu(2)-Cl(3)
N(6)-Cu(2)-Cl(4)
N(6)-Cu(2)-N(9)
N(6)-Cu(2)-N(7)
N(7)-N(8)-C(27)
N(8)-N(7)-C(25)
169.60(3)
90.16(7)
81.25(7)
98.68(10)
104.21(10)
172.72(11)
88.41(8)
98.63(8)
94.42(8)
82.11(8)
156.91(11)
96.18(8)
92.79(8)
77.97(11)
78.94(11)
111.5(3)
105.5(3)
105.8(3)
The green crystal [LCuCl2(H2O)]CH2Cl2 is highly stable when exposure to air. X-ray analysis
confirms the six-coordinated structure of aqua complex LCuCl2(H2O). The μeff value of the copper
complex LCuCl2(H2O) (1.7 μB) indicates the slow-spin state. As depicted in
, the
Fig.3
asymmetric unit contains two independent molecules of [LCuCl2(H2O)]CH2Cl2. These two
molecules have similar conformation ( ) and dimensions ( ). A crystallographically
Fig. 3b
Table 2
imposed C2 axis passes through O(1), Cu(1), N(1), and C(3) atoms. The geometry around the
copper is a elongated octahedron with the axial positions occupied by the two chlorine atoms; the
bond length of O1-Cu1=1.951(2) Å (O2-Cu2 = 1.950(2) Å), N1-Cu1=1.969(3) Å (N6-Cu2 =
1.970(3) Å), N2-Cu1=2.025(3) Å (N7-Cu2 = 2.029(3) Å), and N4-Cu1=2.038(3) Å (N9-Cu2 =
2.050(3) Å) almost equals, but Cl2-Cu1=2.936 (1) Å (Cl4-Cu2 = 2.999 (1) Å) is longer than
others in the same molecule, possibly caused by Jahn-Teller effect[27]. The large angles of
N(4)–N(5)–C(8) (111.9(3)°) and N(7)-N(8)-C(27) (111.5(3)°) and small angles of N(5)–N(4)–C(6)
(105.5(3)°) and N(8)-N(7)-C(25) (105.8(3)°) in the pyrazole rings are consistent with the
coordination of the pyrazole arms while maintaining the NH proton [28]
.

. Hydrogen bond interactions (O-H···Cl: red dotted line; N-H···Cl: purple dotted line) (a)
Figure 4
and C-H···π interaction (violet dotted line) (b). (methyl groups and some H atoms are omitted for
clarity)
The intermolecular H-bonds, O-H···Cl (red dotted line) and N-H···Cl (purple dotted line) (
Fig.
,
) connect the neighbor units to form an infinite chain. For the C-H···π interaction (violet
4 Table 3
dotted line), the distances from C of the solvent molecule CH2Cl2 to the nearest substituted
pyrazole rings are 3.451(4) Å and 3.614(4) Å, respectively, indicating moderated interaction.
…
Distances and angles for Hydrogen bonds and C-
interactions
Angleb, º
154.9
Table 3
H

Hydrogen bond
O1-H···Cl2
O1-H···Cl1
O2-H···Cl3
O2-H···Cl4
N3-H···Cl2
N3-H···Cl1
N8-H···Cl4
N10-H···Cl3
C-H···π
Distancea, Å
3.117(2)
3.120(2)
3.115
156.9
147.8
3.095(2)
3.113(3)
3.150(3)
3.116(3)
3.144(3)
Distancec, Å
3.451(4)
3.641(4)
166.4
160.4
141.9
159.0
143.4
Angled, º
C39-H···π
157
C40-H···π
146
a
b
c
Distance between acceptor and donor,
Angle of acceptor-hydrogen-donor,
Centroid-to-centroid distance, d Angle of carbon-hydrogen-centroid.

3.3 Catalytic Studies
Based on our group and others reported work, the oxidation of alcohols and alkenes can be
catalyzed by mononuclear Cu(II) complexes under mild conditions [29]. The Cu(II)/H2O2/CH3CN
system shows high oxidative activity for styrene, -methyl styrene, and benzyl alcohol. The
oxidation reactions were carried out in acetonitrile using a molar ratio of aromatic substrate/H2O2
=1/8. Fig. 5 shows the comparison of the aromatic substrates conversion versus the reaction time
for [LCuCl2(H2O)]CH2Cl2, and all catalytic data are presented in Table 4. Under typical
conditions, [LCuCl2(H2O)]CH2Cl2 exhibited a unique activity in oxidation of aromatic substrates,
and over 95% total conversion was obtained within 4 h for styrene, -methyl styrene, and benzyl
alcohol. As for ethylbenzene, a low conversion of 47.09% was yielded in 8 h when using H2O2 as
oxidant.
100
90
80
methylstyrene
styrene
benzyl alcohol
70
60
50
40
30
20
10
0
ethylbenzene
0
60
120
180
240
300
360
420
480
Time(min)
Figure 5. The aromatic substrate conversion profiles versus the reaction time with
[LCuCl2(H2O)]CH2Cl2.
Table 4. Catalytic behaviors for oxidation of aromatic substrates
Total
Entry Substrate
Product Distribution, Yield/Select.(%)
TONs
Conv.(%)
(0.5h) 90.53 316.86
1
(0.5h) 35.64/39.25
(1h) 3.84/3.93
(0.5h) 54.89/60.75
(1h) 97.50
341.25
(1h) 93.66/96.07
(2h) 95.06
(4h) 97.72
332.75
342.02
2
(2h) 88.96/93.58
(4h) 91.41/93.54
(2h) 3.37/3.54
(4h) 2.84/2.90
(2h) 2.73/2.87
(4h) 3.47/3.55

(4h) 96.73
(8h) 99.00
338.56
346.50
3
4
(4h) 76.88/79.84
(8h) 85.52/86.38
(4h) 19.63/20.29
(8h) 12.21/12.34
(4h) 0.22/0.23
(8h) 1.27/1.28
(8h) 47.09
164.82
(8h) 47.09/100
Conversion is due to GC results.
Reaction conditions: 40 mmol of oxidant, 2.5 mmol of catalyst, subst./cat. = 350, T = 70℃,
TONs=number of moles of product per mole of catalyst
Benzyl alcohol converted relatively faster than the other alkenes tested, with total conversion of
97.50% in 1 h. Benzaldehyde was firstly produced, which was then oxidized to benzoic acid as the
final product with a selectivity of up to 96.07%.
The oxidation of α-methyl styrene by H2O2 underwent C=C cleavage. Acetophenone was the
major product with a yield of 91.41% and a selectivity of 93.54% in 4 h. 2-methylphenyl
epoxyethane was formed in very low yield (2.84 % in 4 h), which would further react with water
to produce 2-phenyl-1,2-propanediol (3.47% in 4 h ).
As for styrene, due to less steric hindrance than the methyl counterpart, more epoxy compounds
phenyl epoxyethane (19.63% in 4 h; phenyl ethanediol 0.22% in 4 h) were yielded. After a
runtime of 8 h, the amount of benzaldehyde increased from 76.88% to 85.52%, while that of
epoxy compound decreased from 19.63% to 12.21%. The reason why benzoic acid was not
detected lied in insufficient amount of H2O2.
Acetophenone was the sole product detected when ethylbenzene was oxidized. The conversion
of ethylbenzene was much lower compared with other three substrates. The reason could be
related to the catalytic process. Unlike styrene or benzyl alcohol, ethylbenzene could not
coordinate to copper center due to the absence of C=C or OH group. Therefore, the oxidation of
ethylbenzene might follow an intermolecular way, not an intramolecular oxygen-transfer from
copper atom of the catalyst, which resulted in a low reaction rate.
Blank experiments were carried out in order to demonstrate the advantage of the catalyst, for
which the results were shown in Table 5. Although the selectivity of acetophenone was similar
with or without catalyst (﹥93% in 4 h), poor conversions of α-methyl styrene were observed
.
without catalyst or with CuCl2 2H2O as catalyst (20.84% and 41.47% respectively in 4 h). When
the complex LCuCl2(H2O) was used, a significant high value of conversion was detected (97.72%
in 4 h), which strongly confirmed the importance of ligand in the catalytic procedure.
Table 5. Oxidation of -methyl styrene by H2O2 with different catalysts
Catalysts
Time/h
conversion
Selectivity/%
TONs

2
4
6
2
4
6
2
4
13.00
20.84
27.16
39.08
41.47
48.12
95.07
97.72
94.58
95.18
93.63
92.88
93.30
93.37
93.57
93.54
2.74
2.01
3.04
3.86
2.34
3.87
3.54
2.90
2.68
2.81
3.33
3.26
4.36
2.76
2.87
3.55
45.50
72.94
None
95.06
136.78
145.15
168.42
332.75
342.02
.
CuCl2 2H2O
LCuCl2(H2O)
In order to make clear the catalytic mechanism, experiments were carried out to track the
reaction process by ESI-MS. Samples were collected every 5 minutes interval from the beginning
of the catalytic reaction, with -methyl styrene as substrate. As showed in Fig.6, [LCuIICl]+ (m/z =
421.11) fragments were firstly observed, which indicated one Cl- dissociated from copper. After
10 minutes, [LCuII(HCOO)]+ (m/z = 431.14) fragments appeared, which indicated the existence of
[LCuII]2+ as an active intermediate. Imaginary fragments such as [LCuIICl(H2O2)]+, [LCuIII(O.)]2+,
[LCuII(H2O2)]2+, [LCuII(olefin)]2+, [LCuIII(O.)(olefin)]2+ or [LCuII(H2O2)(epoxy)]2+ were not
detected. One possible reason is that molecules such as H2O2, olefin and epoxy might be
dissociated from copper atom due to structural instability at the high temperature of analytic
condition. The supporting evidence is that the fragment [HL]+ (m/z = 324.22) resulted from
dissociation of the complex was detected, though 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl) pyridine(L)
is a stronger coordinating ligand compared to H2O2, olefin and epoxy compounds. Besides,
fragment of [CuIIL2]2+ (m/z = 354.67) was found.
Figure 6. Representation of the calculated peaks (red line) fit experimental species (black line)
present in the ESI-MS spectrum of the reaction solutions.
Although detailed mechanism of the copper catalyzed C=C bond cleavage reaction is unclear, it
is likely that the reaction involves an active cation [LCu2+] and an epoxy intermediate. A
mechanism involving radical cations and dioxetanes was ever proposed in oxidative C=C cleavage

of enamines with Cu complexes. [16b,30]. Taking the results of ESI-MS spectrum investigation into
consideration, we can propose the catalytic mechanism as depicted in Fig. 7.
H
R
R
- H2O
-HCHO
O
Ph
Ph
N
N Cu2+
N
N
H2O2
O
R
N Cu2+ O O
O
H
Ph
O O
N
H
Path a
H
R
R
Cl
Ph
HO
N
N
N
N Cu2+
N
- 2Cl-
- H2O
Ph
R
N
N
R
O
H2O
Path b
N Cu2+
O
H
N Cu2+
2+
N Cu OH2
+
Ph
N Cu
N
O
N
N
O
H
N
R
Cl
H
Ph
OH
HO
H2O2
N
LCuCl2(H2O)
Ph
R
Ph
R
Ph
O H
N
N
N
3+
- H2O
2+
3+
N Cu2+
O
N Cu
N
N Cu
N
O
N Cu
N
O
O
N
H
Figure 7. A proposed mechanism of Cu-catalyzed oxidative cleavage of C=C bond.
Herein, the catalytic cycle started from an active cation [LCu2+] which could be generated by
dissociation of H2O and two chloride anions from LCuCl2(H2O). An active copper-oxy radical
[Cu3+-O.] formed after [LCu2+] associated with H2O2 and then H2O cleavage, which associated
with an alkene molecule to generate a -metal-complex, followed by oxygen-shift to form
[LCu(II)(epoxy)] intermediate. The structure decomposed to form the epoxy product and the
active cation [LCu2+]. When another H2O2 associated to [LCu2+] (Path a), ketone and
formaldehyde would be emerged as final products. Two moles of H2O2 would be consumed for the
production of a molecule of ketone. In another case, when H2O associated to [LCu2+] (Path b),
diol product would be generated.
In general, we have performed catalytic oxidation of aromatic substrates for C=C and C-H
bonds with copper complex. The reactions employed rather mild conditions as compared with the
known methods in the oxidation of other alkenes using H2O2. Further investigations on the
mechanism and synthetic application are now in progress.
4. Conclusion
In this work, we reported the synthesis and characterization of a novel mononuclear Cu(II)
complex with 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine as tridentate ligand. Remarkably,
when H2O2 was used as oxidant, the complex showed very good catalytic activity in the oxidation
of α-methyl styrene, styrene and benzyl alcohol in acetonitrile solvent at 70oC.
Acknowledgment
This work was supported by the Natural Science Foundation of Hubei Province, China (Grant
No.2011CDB068). The authors would also like to thank the Analytical and Testing Center of
Hubei University for equipment access, and Prof./Dr. Qinghua Xia for critical reading and
valuable remarks on the manuscript.
CCDC <1853216> contains the supplementary crystallographic data for [LCuCl2(H2O)]CH2Cl2.

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.
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A novel mononuclear complex [LCuCl2(H2O)]CH2Cl2 (L = 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)
pyridine) was synthesized and structural characterized. The catalytic oxidation properties of the
complex was investigated. The reactions were carried out in acetonitrile with H2O2 and catalytic
amounts of [LCuCl2(H2O)] at 70oC. The catalyst exhibited good oxidative activity for benzyl
alcohol, methyl styrene and methyl styrene.