Carboxylic-supported copper complexes as catalyst for the green oxidative coupling of 2,6-dimethylphenol: Synthesis, characterization and structure

Article abstract of DOI:10.1016/j.crci.2012.11.011

Three new Cu(II) complexes with carboxylic ligand, namely {[Cu(qc) ₂(py)]·4H₂O}_∞ (1), [Cu(qc) ₂(4,4′-bpy)]_∞ (2) and [Cu(pc)(2,2′-bpy) (H₂O)]₂·H₂O (3) (Hqc = 3-hydroxy-2-quinoxalinecarboxylic acid, H₂pc = 4-hydroxyphthalic acid, py = pyrazine) have been synthesized and characterized. In both 1 and 2, each Cu(II) ion is coordinated by two quinoxalinecarboxylate moieties in the equatorial plane and two 4,4′-bpy or pyrazine units provide coordination in the axial positions, thus, resulting in a 1-D polymeric chain structure. Complex 3 has a dimeric structure in which two Cu(II) cations are bridged by two deprotonated pc^2- ligands and two 2,2′-bpy molecules. As heterogeneous catalysts, the title complexes showed high catalytic efficiency in the green oxidative polymerization of 2,6-dimethylphenol (DMP) to poly(1,4-phenylene ether) (PPE) in the presence of H₂O₂ as oxidant in water under mild conditions. Moreover, they allow reuse without significant loss of activity through four runs, which suggests that these catalysts are efficient, mild, and easily recyclable for the oxidative coupling of DMP. The preliminary study of the catalytic-structural correlations suggests that the coordination environment of the metal center plays an important role in the improvement of their catalytic efficiencies.

Full text of DOI:10.1016/j.crci.2012.11.011

C. R. Chimie 16 (2013) 451–461
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´
Full paper/Memoire
Carboxylic-supported copper complexes as catalyst for the green
oxidative coupling of 2,6-dimethylphenol: Synthesis, characterization
and structure
b
Qi Liu a, , Huai Guang Wu
*
a Jiangsu Key Laboratory of Agricultural Meteorology and the College of Computer and Software, Nanjing University of Information Science and Technology, 210044
Nanjing, People’s Republic of China
b Zhengzhou University of Light Industry, Zhengzhou, 450002 Henan, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history:
Three new Cu(II) complexes with carboxylic ligand, namely {[Cu(qc)2(py)]ꢀ4H2O} (1),
1
[Cu(qc)2(4,40-bpy)] (2) and [Cu(pc)(2,20-bpy)(H2O)]2ꢀH2O (3) (Hqc = 3-hydroxy-2-qui-
Received 20 June 2012
1
Accepted after revision 20 November 2012
Available online 5 January 2013
noxalinecarboxylic acid, H2pc = 4-hydroxyphthalic acid, py = pyrazine) have been
synthesized and characterized. In both 1 and 2, each Cu(II) ion is coordinated by two
quinoxalinecarboxylate moieties in the equatorial plane and two 4,40-bpy or pyrazine
units provide coordination in the axial positions, thus, resulting in a 1-D polymeric chain
structure. Complex 3 has a dimeric structure in which two Cu(II) cations are bridged by
two deprotonated pc2ꢁ ligands and two 2,20-bpy molecules. As heterogeneous catalysts,
the title complexes showed high catalytic efficiency in the green oxidative polymerization
of 2,6-dimethylphenol (DMP) to poly(1,4-phenylene ether) (PPE) in the presence of H2O2
as oxidant in water under mild conditions. Moreover, they allow reuse without significant
loss of activity through four runs, which suggests that these catalysts are efficient, mild,
and easily recyclable for the oxidative coupling of DMP. The preliminary study of the
catalytic–structural correlations suggests that the coordination environment of the metal
center plays an important role in the improvement of their catalytic efficiencies.
Keywords:
Copper(II) complexes
Oxidative coupling
Heterogeneous catalyst
Green chemistry
´
ß 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
Poly(1,4-phenylene ether) (PPE) as an engineering
plastic has been widely used in various areas, such as
In the past decades, metal–organic complexes, in which
the metal centers are interconnected by organic linkers to
display a variety of supramolecular architectures, have
attracted great attention due to their diverse structural
topologies and potential applications in areas of catalysis
and materials science [1,2]. In the specific case of catalysis,
the unique characteristic of the CuII/CuI redox couple
renders many of their complexes suited for various
catalysis reactions [3], especially for the green catalysis
process since copper is a cheap and minor toxic metal.
business equipment and the electrical and automotive
industries. The oxidative polymerization of 2,6-dimethyl-
phenol (DMP) to form PPE was first carried out by Hay and
his GE group in 1959 with a copper–pyridine complex as a
catalyst in a nitrobenzene solution [4]. Along with
the formation of PPE by C–O coupling, an undesirable
by-product, 3,30,5,50-tertramethyl-4,40-diphenoquinone
(DPQ), was also produced by the C–C coupling of two
monomeric phenols. Although the oxidative polymeriza-
tion of DMP in organic solvents has provided a convenient
method for manufacturing PPE in industry, both a solvent-
recovery process and an antiexplosive reactor are
required in industrial PPE production [5]. From the view
of green chemistry, the use of water as a reaction medium
is considered environmentally friendly and PPE can easily
* Corresponding author.
E-mail address: qrankl@163.com (Q. Liu).
1631-0748/$ – see front matter ß 2012 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2012.11.011

452
Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
be separated from water due to its insolubility in water
[6,7]. In 2004, Nishide et al. first used potassium
ferricyanide as an oxidant to synthesize PPE in alkaline
water. Unfortunately, the use of potassium ferricyanide is
still environmentally deleterious [8]. So far, the green
oxidative coupling of DMP in environmental friendly
solvents with clean oxidants is studied extremely rarely.
Therefore, this reaction is still a research field of great
challenge from the standpoint of ‘‘green chemistry and
technology’’. Meantime, it is extraordinarily desired that
the development of efficient catalysts suited for the
milder ‘‘green’’ conditions, and the investigation of
catalytic–structural correlations, which have great im-
portance for the exploitation of new catalysts. For the
purpose of replacing the traditional organic-medium
systems by environmental friendly processes, we have
prepared three new copper carboxylic adducts
{[Cu(qc)2(py)]ꢀ4H2O} (1), [Cu(qc)2(4,40-bpy)] (2) and
2.3. Synthesis of [Cu(qc)2(4,40-bpy)] (2)
1
.
To a stirred solution of 2 equiv. of Cu(ClO4)2 6H2O
(37.5 mg, 0.1 mmol) in H2O (5 mL) were added 2 equiv. of
4,40-bpy (15.6 mg, 0.1 mmol) in CH3OH (5 mL). After stirring
for 5 min, a solution of 3-hydroxy-2-quinoxalinecarboxylic
acid (19.0 mg, 0.1 mmol) in CH3OH (5 mL) was added, and
the mixture was then stirred for 10 min. Upon evaporation
of the solvent, the green block crystals were isolated in 50%
yield. Anal. calcd for C28H16CuN6O6: C, 56.43; H, 2.71; N,
14.10%. Found: C, 56.51; H, 2.79, N 13.98%; IR (KBr)/cm–1
3434(w), 1645(s), 1543(m), 1448(s), 1348(m), 1285(w),
1145(w), 1076(w), 879(w), 739(w), 552(m), 439(w).
:
2.4. Synthesis of [Cu(pc)(2,20-bpy)(H2O)]2ꢀH2O (3)
A mixture containing Cu(ClO4)2ꢀ6H2O (37.5 mg, 0.1
mmol), NaOH (4 mg, 0.1 mmol), 4-hydroxyphthalic acid
(9.1 mg, 0.05 mmol), 2,20-bpy (7.8 mg, 0.05 mmol) and
water (10 mL) was placed inside a Teflon-lined stainless
steel vessel (20 mL). It was heated to 120 8C for 48 h under
autogenous pressure and then cooled to room temperature
at a rate of 5 8C/h. The reaction solution was filtered and
left to stand undisturbed. Blue block single crystals
suitable for X-ray analysis were obtained after a period
of 1 week in a yield of 68%. Anal. calcd for C18H16CuN2O7: C,
49.60; H, 3.70; N, 6.43%. Found: C, 49.71; H, 3.58; N, 6.37%;
IR(KBr, cmꢁ1): 3507(m), 3083(m), 2680(w), 1551(s),
1389(s), 1306(m), 1266(m), 1161(w), 1066(w), 874(m),
729(m), 635(w), 452(w).
1
1
[Cu(pc)(2,20-bpy)(H2O)]2ꢀH2O (3) (Hqc = 3-hydroxy-2-
quinoxalinecarboxylic acid, H2pc = 4-hydroxyphthalic ac-
id, py = pyrazine) and explored their application as
catalysts for an aqueous-medium catalysis process of
the oxidative coupling of DMP by using hydrogen peroxide
as oxidant under mild conditions.
2. Experimental
2.1. General information and materials
All the solvents and chemicals (analytical grade) were
used without further purification. 3-Hydroxy-2-quinox-
alinecarboxylic acid (Hqc), 4-hydroxyphthalic acid (H2pc),
pyrazine (py), 4,40-bpy, 2,20-bpy and 2,6-dimethylphenol
(DMP) (99% pure) were used as obtained from J&K
Chemical Ltd. Elemental analyses (C, H, and N) were
performed on a Perkin-Elemer 240C analyzer. The IR
spectra were recorded in the range 4000–400 cmꢁ1 on a
Tensor 27 OPUS (Bruker) FT-IR spectrometer with KBr
pellets. The X-ray powder diffraction patterns (XRPD) were
recorded on a Rigaku D/Max-2500 diffractometer, operat-
ed at 40 kV and 100 mA, using a Cu-target tube and a
graphite monochromator. The intensity data were
2.5. General procedure for the catalytic oxidative coupling of
2,6-dimethylphenol
All of the polymerizations were carried out in a 10 mL,
three-necked, round-bottom reaction flask fitted with a
water condenser. One neck of the flask was equipped with a
mercury thermometer for measuring the reaction tempera-
ture. The crystal complex catalysts were ground well into
appropriate sizes to increase the surface area, but not too
small for a convenient filtration (the average particle size of
10–12 mm). DMP (122 mg, 1 mmol) was dissolved in water
recorded by continuous scan in a 2
u
/
u
mode from 38 to
(5 mL). The 1.5 mol % complex was added to the solution,
and the mixture was vigorously stirred under air at 40 8C.
Then, hydrogenperoxide(30% aqueous solution)was slowly
added into the mixture using a microinjector every 15 min
for two times to minimize H2O2 decomposition. After 4 h,
the reaction was stopped and the polymerproduct appeared
as an off-white powder after salting out by the addition of
sodium chloride (1.17 g, 0.02 mol). Then the mixture was
transferred into a separatory funnel, the organic materials
were extracted after the addition of a few milliliters of
CH2Cl2. This was repeated three times. The combined
organic extracts were dried with anhydrous MgSO4 and the
solvent, after filtration, was evaporated in vacuo. The
products were separated by preparative TLC performed
on dry silica gel plates with ethyl ether–petroleum ether
(1:3 v/v) as the developing solvents. PPE and DPQ were
collected and dried in vacuo. We also can get the polymeric
products by the simple filtration after salting out, followed
by the reprecipitation from chloroform to methanol.
808 with a step size of 0.028 and a scan speed of 88 minꢁ1
.
2.2. Synthesis of {[Cu(qc)2(py)]ꢀ4H2O} (1)
1
.
To a stirred solution of Cu(ClO4)2 6H2O (18.75 mg, 0.05
mmol) in H2O (2 mL) was added 1 equiv. of pyrazine (4 mg,
0.05 mmol) in CH3OH (5 mL). This was stirred for 5 min,
and then
a 5 mL CH3OH solution of 3-hydroxy-2-
quinoxalinecarboxylic acid (9.5 mg, 0.05 mmol) was
added to the reaction mixture and then filtered to give
a green solution. Slow evaporation of the solvent at
room temperature gave rise to green block single
crystals suitable for X-ray single-crystal analysis after
ca 5 days. Yield: ꢂ 40% (based on Cu). Anal. calcd for
C
22H20CuN6O10: C, 44.64; H, 3.41; N, 14.20%. Found: C,
44.58; H, 3.49; N, 14.27%; IR (KBr)/cm–1: 3464(m),
1683(s), 1609(m), 1374(m), 1326(m), 1221(w), 986(w),
882(w), 806(w), 776(m), 676(m), 446(w).

Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
453
Table 1
Crystal data and structure refinement for {[Cu(qc)2(py)]ꢀ4H2O} (1), [Cu(qc)2(4,40-bpy)] (2) and [Cu(pc)(2,20-bpy)(H2O)]2ꢀH2O (3).
1
1
1
2
3
Formula
C22H20CuN6O10
591.98
C28H16CuN6O6
596.01
C18H16CuN2O7
435.87
Formula weight
Crystal system
Space group
Triclinic
P-1
Orthorhombic
Fdd2
Orthorhombic
Pbcn
˚
a (A)
6.9911(4)
8.9006(4)
10.3196(5)
86.613(4)
71.990(4)
76.660(4)
594.14(5)
293(2)
11.1387(4)
12.2921(4)
35.3173(16)
90
20.5311(16)
11.1914(8)
15.4446(12)
90.00
˚
b (A)
˚
c (A)
a
b
g
(8)
(8)
(8)
90
90.00
90
90.00
3
˚
V (A )
T (K)
Z
4835.6(3)
293(2)
3548.7(5)
296(2)
1
8
8
m
(Mo K
a
) (cmꢁ1
)
0.991
1.637
1.277
Number of reflections collected [I > 2
d(I)]
3742
9997
12223
Number of independent reflections
2203
2201
3296
b
Final Ra, Rw
0.0387, 0.0986
0.0236, 0.0588
0.0417, 0.1011
P
P
a
R ¼ kFojꢁjFck= jFoj.
h
i
1=2
P
P
2
b
Rw
¼
ðkFojꢁjFckÞ2= wjFoj
.
The following is the standard procedure for the
applied using SADABS program [10]. All the structures
were solved by direct methods using the SHELXS program
of the SHELXTL package and refined with SHELXL [11].
Metal atoms in each complex were located from the E-
maps, and other non-hydrogen atoms were located in
successive difference Fourier syntheses and refined with
anisotropic thermal parameters on øFø2. The hydrogen
atoms were added theoretically, riding on the concerned
atoms and refined with fixed thermal factors. Crystal data
and structure refinement of complexes 1–3 are summa-
rized in Table 1. Selected bond lengths and bond angles are
listed in Table 2.
homogeneous oxidation: the crystal complex catalysts
were powdered to increase the surface area for the
catalysts homogeneous prior to use. Under air, the
substrate 2,6-dimethylphenol (0.122 g,
1 mmol) was
dissolved in the 1:1 (v/v, 5 mL) mixture of methanol–
toluene in a 10 mL flask, and a 0.5% molar ratio of catalyst/
substrate and a 5 molar ratio of NaOMe/catalyst were
added to above solution with a magnetic stirrer. Then,
hydrogen peroxide (30% aqueous solution) was slowly
added into the mixture using a syringe pump every 15 min
10 ml for five times to minimize H2O2 decomposition. After
2 h, the sample was concentrated in vacuo, and the
products were separated by preparative TLC performed
on dry silica gel plates. All reactions were run in duplicate,
and the data reported represent the average of these
reactions.
3. Results and discussion
3.1. Crystal structure of {[Cu(qc)2(py)]ꢀ4H2O} (1)
1
Poly(phenylene ether) (PPE): 1H NMR (CDCl3, TMS)
Crystallographic analysis reveals that 1 crystallizes in
the triclinic space group P-1 with the compositions of
one neutral asymmetric unit [Cu(qc)2(py)] and four
lattice water molecules, and assumes an infinite 1-D
chain array. As illustrated in Fig. 1, the Cu(II) ion, being
located at an inversion center, is six-coordinated by two
nitrogen atoms (N1, N1A) and two oxygen atoms (O2,
O2A) from different qcꢁ ligands and a pair of nitrogen
donors (N3, N3A) from two pyrazine molecules in a
nearly ideal octahedron with N1, N1A, O2, O2A compos-
ing the basal plane (the mean deviation from plane of
d
= 6.44 (s, 2H; HAr), 2.09 ppm (s, 6H; CH3); 13C NMR
(CDCl3, TMS)
d = 16.6–16.8, 114.1, 114.5, 124.4, 125.0,
129.0, 131.6, 132.7, 145.6, 146.4, 151.5, 154.5, 154.8 ppm;
IR (KBr): vC
-O-C
¼ 1186 cmꢁ1
.
Diphenoquinone (DPQ): 1H NMR (CDCl3, TMS)
d = 8.2 (s,
4H), 7.1 (s, 2H), 2.1 ppm (12H); 13C NMR (CDCl3, TMS)
= 17.07, 129.56, 135.67, 139.10, 187.21 ppm; IR (KBr):
vC¼O ¼ 1594 cmꢁ1
d
.
2.6. X-ray structure analyses
˚
0 A) and N3, N3A holding the axial positions. Although
X-ray single-crystal diffraction measurements for
complexes 1–3 were carried out on a Bruker Smart 1000
CCD diffractometer equipped with a graphite crystal
monochromator situated in the incident beam for data
collection at 293(2) K. The determinations of unit cell
the ligand is deprotonated, only the quinolinecarbox-
ylate group is involved in metal coordination. Each
deprotonated qcꢁ ligand serves as a two-connected node
to coordinate with the Cu(II) atom through its nitrogen of
quinoxaline and carboxylate in monodentate mode. The
bond angles around Cu(II) ion vary from 76.28 (7) to
parameters and data collections were performed with Mo-
˚
˚
Ka
radiation (
l
= 0.71073 A) and unit cell dimensions were
180.08. The Cu-O bond lengths are 1.9206 (17) A, and
obtained with least-square refinements. The program
SAINT [9] was used for integration of the diffraction
profiles. Semi-empirical absorption corrections were
the Cu-N bond lengths are in the range 2.094 (2)–2.400
˚
(2) A, which are comparable to those observed for
.
{[Cu(btp)(HBTC)2]3 0.5H2O}n and [Cu(btmb)(HBTC)]n

454
Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
Table 2
˚
Selected bond distances (A) and bond angles (8).
{[Cu(qc)2(py)]ꢀ4H2O} (1)
1
Bond distances
Cu1-O2
1.9214(18)
2.400(2)
Cu(1)-O(2)#1
Cu1-N3
1.9214(18)
2.095(2)
Cu1-N1
2.400(2)
2.095(2)
Cu(1)-N(1)#1
Cu(1)-N(3)#1
Bond angles
O(2)#1-Cu(1)-O(2)
O(2)#1-Cu(1)-N(3)
O(2)#1-Cu(1)-N(1)
N(3)-Cu(1)-N(1)
N(3)#1-Cu(1)-N(1)#1
180.0
O(2)#1-Cu(1)-N(3)#1
O2-Cu1-N3
89.95(8)
89.95(8)
76.26(7)
76.26(7)
89.86(8)
O(2)-Cu(1)-N(3)#1
N(3)#1-Cu(1)-N(3)
N(3)#1-Cu(1)-N(1)
O(2)-Cu(1)-N(1)#1
N(1)-Cu(1)-N(1)#1
90.05(8)
90.05(8)
103.74(7)
90.14(8)
90.14(8)
180.00(12)
89.86(8)
O(2)-Cu(1)-N(1)
O(2)#1-Cu(1)-N(1)#1
N(3)-Cu(1)-N(1)#1
103.74(7)
180.00(10)
[Cu(qc)2(4,40-bpy)] (2)
1
Bond distances
Cu1-O2
1.9165(18)
2.040(4)
Cu(1)-O(2)#1
Cu(1)-N(1)#1
1.9909(14)
2.3611(18)
Cu(1)-N(3)
Cu(1)-N(1)
2.031(4)
Cu(1)-N(4)#2
2.3611(18)
Bond angles
O(2)-Cu(1)-O(2)#1
O(2)-Cu(1)-N(4)#2
O(2)-Cu(1)-N(1)#1
N(4)#2-Cu(1)-N(1)#1
N(3)-Cu(1)-N(1)
176.57(11)
88.29(5)
103.11(6)
91.50(6)
88.50(6)
O(2)-Cu(1)-N(3)
91.71(5)
88.29(5)
76.98(6)
76.98(6)
91.50(6)
O(2)#1-Cu(1)-N(3)
N(3)-Cu(1)-N(4)#2
N(3)-Cu(1)-N(1)#1
O(2)#1-Cu(1)-N(1)
N(1)#1-Cu(1)-N(1)
91.71(5)
180.0
O(2)#1-Cu(1)-N(4)#2
O(2)#1-Cu(1)-N(1)#1
O(2)-Cu(1)-N(1)
88.50(6)
103.11(6)
177.01(12)
N(4)#2-Cu(1)-N(1)
[Cu(pc)(2,20-bpy)(H2O)]2ꢀH2O (3)
Bond distances
Cu1-O2
1.952(2)
Cu1-O1
Cu1-O6
1.960(2)
2.186(2)
Cu1-N2
1.992(3)
Cu1-N1
1.994(3)
Bond angles
O2-Cu1-O1
O2-Cu1-N1
O2-Cu1-O6
N1-Cu1-O6
91.75(10)
O2-Cu1-N2
93.23(11)
92.81(11)
99.18(10)
O1-Cu1-N2
N2-Cu1-N1
N2-Cu1-O6
162.76(11)
81.48(13)
97.39(11)
174.41(12)
89.15(10)
93.30(11)
O1-Cu1-N1
O1-Cu1-O6
(btp = 1,3-bis(1,2,4-triazol-1-yl)propane, btmb = 4,40-bis
(1,2,4-triazol-1-ylmethyl)biphenyl, and H3BTC = 1,3,5-
benzenetricarboxylic acid) [12]. The Cu(II) centers are
linked to each other by pyrazine ligand into an infinite
1-D chain running along the a direction with the intrachain
˚
CuꢀꢀꢀCu separation of 6.991 A (Fig. 2). Moreover, the 1-D
chains are linked by intermolecular hydrogen bonds
between lattice water moieties, and the oxygen atoms of
Fig. 1. Coordination environment around the Cu(II) centers in 1. All H atoms are omitted for clarity.

Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
455
Fig. 2. The 1-D chain structure extending along a axis in 1.
carboxylate and hydroxyl groups from the terminal qcꢁ
ligands to form 2-D ladder chains. These ladder chains are
further linked through the interchain hydrogen bonds into a
3-D supramolecular network.
3.2. Crystal structure of [Cu(qc)2(4,40-bpy)] (2)
1
Complex 2 crystallizes in the orthorhombic space group
Fdd2 with the monomer unit [Cu(qc)2(4,40-bpy)]. Each
Cu(II) atom exhibits a distorted octahedral environment,
the equatorial plane of which comprises two carboxylate
oxygen atoms and two quinoline nitrogen donors from two
distinct qcꢁ anions and two nitrogen atoms from two 4,40-
bpy ligands occupying the apical site (Fig. 3). The Cu-O/N
˚
distances are in the range 1.9901(16)–2.362(2) A, which
are in the normal range of those observed in copper
complexes [13]. Each completely deprotonated qcꢁ ligand
serves as a 2-connected node to coordinate with the Cu(II)
center. The [Cu(qc)2(4,40-bpy)] units are linked into
polymeric 1-D chains where 4,40-bpy connects the adja-
cent Cu(II) ions of the symmetry related units in an end-to-
end bridging mode (Fig. 4). Moreover, the CuꢀꢀꢀCu separa-
˚
tion of 11.139 A within a chain is significantly longer than
˚
that of 6.991 A in 1. It should be noted that significant
aromatic stacking interactions are also observed between
the adjacent chain arrays due to the interdigitated
disposition. The centroid-to-centroid distance of the
alternatively parallel benzene rings in the adjacent chains
˚
is 3.289 A. Two neighboring anti-parallel chains are held
together via such weak secondary interactions and
CꢁHꢀꢀꢀ
p supramolecular interactions to furnish a 3-D
supramolecular framework.
3.3. Crystal structure of [Cu(pc)(2,20-bpy)(H2O)]2ꢀH2O (3)
Different from complexes 1 and 2, in the crystal
structure of 3, the asymmetric unit consists of two Cu2+
ions, two 2,20-bpy ligands, two deprotonated pc2ꢁ ions,
two coordinated water molecules and one lattice water
molecule. Each Cu(II) ion is five-coordinated by two
nitrogen atoms of one 2,20-bpy ligand and three oxygen
atoms from two different deprotonated pc2ꢁ ligands and
Fig. 3. Coordination environment around the Cu(II) centers in 2. All H
atoms are omitted for clarity.

456
Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
Fig. 4. The 1-D chain structure extending along c axis in 2.
Fig. 5. Molecular structure of 3 with atom labeling of the asymmetric unit. All H atoms are omitted for clarity.
one coordinated water molecule, forming a distorted
trigonal bipyramidal geometry, where N1, N2 and O2A
locate at the equatorial positions while O6 and O1 occupy
the axial positions (Fig. 5). The d(Cu–Ocarboxylate) are in the
important in the synthesis of supramolecular architecture.
Thereexist O–HꢀꢀꢀO hydrogenbonding interactions between
lattice water molecules, coordinated water molecule and
carboxyl oxygen atoms with the distances in the range of
˚
˚
range of 1.952 (2)–1.960 (2) A, and d(Cu–Owater) is 2.186
2.630 (4)–3.40 (2) A, leading to the formation of a 2-D
˚
(2) A, and fall in the range of typical Cu–O (carboxylate and
supramolecular network.
water) distances [14]. The Cu–N bond lengths are 1.992 (3)
˚
˚
A and 1.994 (3) A. The distorted trigonal bipyramid CuO3N2
has the O(N)–Cu–O(N) angles ranging from 81.48 (13)8 to
174.41 (12)8, which are within the range expected for such
coordination. Each pc2ꢁ ligand connects the adjacent Cu(II)
centers to form a dimer in a bidentate bridging mode by one
of the two carboxylate groups with the CuꢀꢀꢀCu separation of
3.4. XRPD results
To confirm whether the crystal structure is truly
representative of the bulk materials for related catalytic
studies of complexes 1-3, XRPD experiments have been
carried out for 1-3. The XRPD experimental and computer-
simulated patterns of the corresponding complexes are
shown in Fig. 6. Although the experimental patterns have
few unindexed diffraction lines and some are slightly
broadened in comparison with those simulated from the
˚
3.128 A and another carboxylate group is free of coordina-
tion, while the 2,20-bpy ligand acts as a typical chelating
terminally coordinating the Cu(II) center. It is worthwhile to
note that hydrogen bonding interactions are usually

Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
457
(a)
(b)
Experimental
Simulated
Experimental
Simulated
10
20
30
2
40
(deg)
50
60
70
80
10
20
40
(deg5)0
60
70
80
30 2
θ
θ
Experimental
Simulated
(c)
10
20
40
(deg5)0
60
70
80
302
θ
Fig. 6. XRPD patterns of (a) for 1, (b) for 2 and (c) for 3.
single-crystal models, it can still be considered that the
bulk synthesized materials and the as-grown crystals are
homogeneous for 1-3.
Previous investigations have indicated that the oxida-
tive coupling of DMP in organic solvents proceeds at room
temperature, and higher temperature will result in more
by-products DPQ. However, in aqueous media, higher
temperature is necessary because the reaction does not
proceed at room temperature. Fig. 7 presents the effect of
reaction temperatures on the polymerization with 1,
showing a significant temperature effect on the reaction.
The DMP conversion increases significantly from 71% to
90% as the temperature increases from 30 to 60 8C, and the
best selectivity to PPE of 82.7% is achieved at 45 8C.
In order to study the effect of time on the polymeriza-
tion, the product analysis was done at regular intervals of
3.5. Catalytic oxidative coupling of DMP in water
The overall oxidative coupling process of DMP under
green reaction conditions is shown in Scheme 1. Initially,
complex 1 was selected as a catalyst model for optimiza-
tion of the reaction conditions. The influences of various
factors such as reaction temperature, reaction time, the
amounts of catalyst and oxidant as well as pH of the
solution have been investigated in detail.
O
n
CuII-catalyst
(PPE)
m
m H O
OH +
2
2
H2O
1/ 2 ( m-n )
+ 2 m H O
2
(DMP)
O
O
(DPQ)
Scheme 1. Oxidative polymerization of 2,6-dimethylphenol (DMP) in water.

458
Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
100
80
60
40
20
0
100
80
60
40
20
0
0
0.5
1
1.5
2
2.5
3
3.5
The amount of 1 (mmol %)
Fig. 9. Effect of the amount of 1 on the polymerization: (&) conversion;
(*) selectivity.
25
30
35
40
45
50
55
60
65
Temperature (oC)
decrease in activity at high catalyst concentrations can
arise from competitive formation of catalytically inactive
copper species. The resultant loss in catalyst concentration
will affect the parallel reactions that produce PPE and DPQ
equally, and this is consistent with the insensitivity of
selectivity to catalyst concentration. The influence of the
loading of oxidant is studied by using different volumes of
H2O2 from 0 to 0.03 mL (30 vol.%). The result indicates that
the addition of appropriate amount of H2O2 (0.02 mL) is
also necessary for obtaining high conversion and selectivi-
ty. At higher amounts of H2O2, the substrates almost
convert completely but the selectivity descends corre-
spondingly, which are similar with the previous results
[17].
It has been demonstrated that the pH of solution can
affect the yields and the distribution of products [18]. The
effect of pH on the DMP polymerization is thus investigat-
ed in Tris-HNO3 buffer solution in the range of pH 6.0–9.0
by maintaining other reaction parameters. As illustrated in
Table 3, it can be seen that for the catalytic system with 1,
DMP conversion first increases and then decreases with
increasing pH, and the maximum is at pH 7.5. The yield of
PPE increases with increasing pH and remains constant
above pH 7.5. It is worthy to note that the pH of the
solution exhibits great effect on the selectivity of PPE. The
selectivity of PPE is less than 82% in a neutral aqueous
solution, but about 88% in weakly acidic (pH 6.0) and
weakly alkaline (pH 8.5–9.0) solutions. Because of the high
conversion of DMP, good yield of PPE and the low loss of
H2O2 in neutral aqueous solution, optimal pH is around 7.0
Fig. 7. Effect of reaction temperature on the polymerization with 1: (&)
conversion; (*) selectivity.
100
80
60
40
20
0
2
3
4
5
6
7
8
9
Time (h)
Fig. 8. Effect of reaction time on the polymerization with 1: (&)
conversion; (*) selectivity.
time under similar reaction conditions. DMP conversion
increases quickly from about 77% to near 91% as the time
changes from 3 h to 8 h, while the selectivity to PPE
increases very slowly (Fig. 8). Further prolonging in
reaction time does not improve the conversion signifi-
cantly but decreases the selectivity slightly. The reason for
this may be that the by-product DPQ degrades the polymer
upon further processing at high temperatures [15]. In
addition, we find that the selectivity of PPE is not
correlated for the DMP conversion, which is close to an
earlier conclusion, since Viersen et al. have shown that
most DPQ is formed during the beginning of the reaction
[16].
Fig. 9 illustrates the effect of the amount of 1 on the
polymerization. The control reaction performed without
any catalyst shows that the copper complex is indispens-
able for the reaction system. The conversion increases with
the increased amounts of 1, but decreases for the larger
amounts. The maximum conversion is obtained at a 0.02
mmol amount of 1. However, the selectivity to PPE keeps
stable as the concentration of catalyst increases. The
Table 3
Effect of pH on the polymerization with 1a.
pH
Conversion (%)b
PPE (%)b
DPQ (%)b
Selectivityc
6.0
6.5
7.0
7.5
8.0
8.5
9.0
69
78
89
91
88
83
76
62
68
79
82
84
83
83
9
13
18
16
14
12
11
87.8
84.2
81.5
83.5
85.6
87.4
88.1
a
Standard conditions.
b
Conversions and isolated yields based on the DMP. All isolated
products were identified by 1H, 13C NMR and IR spectroscopic analyses.
c
Selectivity = ([PPE] ꢃ 100)/([PPE] + [DPQ]).

Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
459
Table 4
Results of the oxidative coupling of DMP with different catalysts performed under the optimized conditionsa.
Catalysts
Conversion (%)b
Yield (%)b
PPE
Selectivity (%)c
DPQ
1
91
93
75
82
94
26
22
29
9
11
8
87.2
90.9
94.4
62.7
61.1
64.4
30.0
2
3
100
71
6
Cu(ClO4)2 + Hqc + py
Cu(ClO4)2 + Hqc + 4,40-bpy
Cu(ClO4)2 + H2pc + 2,20-bpy
Cu(ClO4)2
15
14
16
21
68
69
25
a
Optimized conditions: DMP (1 mmol), H2O2 (0.02 mL) and catalyst (0.02 mmol) in 5 mL of water for 8 h at 50 8C.
Conversions and isolated yields based on the DMP, average of two runs.
Selectivity = ([PPE] ꢃ 100)/([PPE] + [DPQ]).
b
c
in practice. The pH effect on product distribution is not
Hqc or H2pc ligand improves the aqueous stabilities of
these complexes and also forms the opening coordination
geometry with less steric hindrance around the copper
center, which favors the coordination of DMP to Cu(II) to
form the proposed active copper species [24] and the
subsequent polymerization of DMP. Likewise, Reedijk et al.
have also shown that a series of CuII complexes incorpo-
rating structurally related N,O-containing ligands under-
went this polymerization more efficiently than copper(II)
salts [25].
entirely clear. It has been reported in the literature that
alkaline condition favored the formation of PPE, while
acidic solution led to the C–C coupling [19]. However, in
this work, both weakly acidic (pH < 6.5) and alkaline
aqueous solution (pH > 8) are suitable to the C–O coupling
reaction in our catalytic system. On the basis of the above
experimental results and previous literature [20], we think
the phenoxy radical associated with metal complex,
generated in the reaction process, is the main active
species. It is possible that pH of the solution affects the
state of metal complex associated with phenoxy radical,
consequently influencing the association of the copper
complex with phenoxy radical and induction to p-C atom.
With the optimized reaction conditions in hand, we
then evaluate catalytic efficiencies of the title complexes
for comparison. As shown in Table 4, dimer 3 gives
relatively high conversion and selectivity of PPE among the
three complexes, viz., the conversions and selectivities of
91% and 87.2% for 1, 93% and 90.9% for 2 as well as 100%
and 94.4% for 3. The dissimilarity is probably attributable
to the differences in the coordination environments of the
metal Cu(II) centers. In general, a lower coordination
number leads to a higher catalytic activity. Also, the
coordinated water molecules in the complexes are easy
leaving groups and can be readily replaced by the
substrate; thus, it is easier for the phenolate anion to
enter the metal coordination sphere when metal centers
are coordinated by water molecules, further enhancing the
catalytic activity. Consistent with our observation, Kol
et al. [21] observed that structural effects on reactivity
were pronounced in zirconium complexes for lactide
polymerization. Comparing with the Cu(II)–poly(N-viny-
limidazole), Cu(II)-polyamidoamines complex catalysts
giving about 95% PPE yield [22], the best result of the
copper-catalyzed synthesis of PPE in aqueous medium so
far, complex 3 has an obvious advantage in the oxidative
polymerization with the yield of PPE of 94%. However,
the contrasting reaction using Cu(ClO4)2 as catalyst only
gives trace amounts of PPE. We also notice that the title
complexes have obvious comparative advantage over the
Cu(ClO4)2/Hqc/py, Cu(ClO4)2/Hqc/4,40-bpy and Cu(ClO4)2/
H2pc/2,20-bpy catalysts under similar reaction conditions.
A potential explanation for this may be inferred from the
stabilizing effects [17,23] of the organic ligand and the
steric influence of the copper center. In 1-3, the bidentate
The potential benefits of the heterogeneous catalyst
include facilitation of catalyst separation from reagents
and reaction products, and simplification of methods for
catalyst recycle [26]. For a truly effective heterogeneous
catalyst, therefore, it is critical that recovery can be simple
and efficient, and that the recovered catalyst can retain its
original reactivity through multiple cycles. As 3 shows
higher catalytic activity, we have tested its recovery and
reuse. After the reaction was completed, the catalyst was
recovered by filtration for consecutive runs. The recovered
catalyst was used for a new reaction batch of DMP. Catalyst
3 exhibits excellent reusability for four times without
showing any significant deterioration of catalytic activity.
Then, we have carried out another control reaction,
because in the catalytic process the leached metal species
might catalyze efficiently the oxidative coupling reaction
instead of the heterogeneous catalysts [27]. To examine
this possibility, we filtered catalyst 3 after the oxidative
polymerization and allowed the filtered catalyst and the
filtrate to react with another aliquot of DMP, respectively,
as shown in the previous study [27]. The filtered catalyst
was subjected to the second run by charging with new
reagents and then the products were again formed with
about 85–92% of the original conversion. The filtrate
obtained from the first filtration was also used in a new
reaction with the addition of substrate. Only 6 to 9%
conversion of DMP was observed under similar conditions.
This result strongly suggests that the dominant reactive
species is the heterogeneous catalyst 3, not species such as
the leached metal. However, about 6–9% conversion by the
filtrate indicates that trace amounts of Cu metal ions might
be leached during the catalysis. Powder X-ray diffraction
(XRD) pattern of the filtered catalyst after the reaction has
revealed the same pattern as the original catalyst,
suggesting that the original structure of the filtered
catalyst has been retained during the reaction. Based on

460
Q. Liu, H.G. Wu / C. R. Chimie 16 (2013) 451–461
Table 5
should increase the catalytic activity of complex, it can be
expected that more studies could improve the catalytic
efficiencies via further structural modifications of copper
complex for practical application.
Oxidative coupling of DMP in watera or in methanol–toluene mixtureb
catalyzed by complexes 1-3.
Catalysts
Solvent
Conversion (%)
Selectivity (%)
1
H2O
91
68
87.2
89.3
90.9
91.5
94.4
96.3
CH3OH/toluene
H2O
Acknowledgments
2
3
93
CH3OH/toluene
H2O
67
This work is supported by Scientific Research Founda-
tion for Returned Scholars, Nanjing University of Informa-
tion Science and Technology (Grant No. S8110247001) and
A Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
100
74
CH3OH/toluene
a
Heterogeneous optimized conditions: DMP (1 mmol), H2O2 (0.02 mL)
and catalyst (0.02 mmol) in 5 mL of water for 8 h at 50 8C.
b
Homogeneous optimized conditions: DMP (1 mmol), NaOMe (0.14
mmol), H2O2 (0.05 mL) and catalyst (0.02 mmol) in a 1.5:1 MeOH/toluene
mixture for 3 h under the ambient temperature.
Appendix A. Supplementary material
these results, we have concluded that the heterogeneous
catalyst 3 could be recycled multiple times without a
significant loss of activity.
Supplementary material associated with this article can
be found at http://www.sciencedirect.com, at http://dx.doi.
org/10.1016/j.crci.2012.11.011.
For the purpose of fully understanding the intrinsic
advantage of heterogeneous vs. homogeneous catalysts,
the oxidation coupling of DMP is conducted in a 1.5:1 (v/v)
methanol–toluene mixed solution using complexes 1-3 as
homogeneous catalysts under the same reaction condi-
tions as the prior study (Table 5) [17a]. To our surprise, the
use of the title complexes acting as homogeneous catalysts
can effectively suppress diphenoquinone production and
improve the selectivity for PPE, while decreasing distinctly
the catalytic activities. For example, when utilizing 1 as
heterogeneous catalyst, the oxidation coupling of DMP
occurs with high 91% conversion and 87.2% selectivity for
PPE. Nevertheless, 1 assumes the markedly lower efficien-
cy (68% conversion) for the reaction in methanol–toluene
medium, albeit with slightly enhanced selectivity of 89.3%.
The superior activities of the complexes in the heteroge-
neous catalytic system may be attributed to full exposure
of metal sites, therefore giving an ultimately high degree of
metal-dispersion [28].
On the basis of the above experimental results and
previous literature [20,29], a plausible reaction mecha-
nism for the present oxidative coupling reaction was
proposed as follows: DMP is dissolved in the basic aqueous
phase to form the phenolate anion; these phenoxide
anions coordinate to Cu(II) centers, and then an electron
transfers from the coordinated phenoxide anions to Cu(II)
ions occur, leading to the formation of phenoxyl radicals.
The resultant Cu(I) ions were reoxidized to Cu(II) ions by
H2O2. The C–O coupling of these phenoxyl radicals yields
the linear polymer PPE; the C–C coupling of these phenoxyl
radicals results in the by-product DPQ.
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