Syntheses, crystal structures of a series of copper(II) complexes and their catalytic activities in the green oxidative coupling of 2,6-dimethylphenol

Article abstract of DOI:10.1016/j.jorganchem.2008.02.023

Three mixed-ligand Cu^II complexes bearing iminodiacetato (ida) and N-heterocyclic ligands, namely, [Cu₂(ida)₂(bbbm)(H₂O)₂] · H₂O (1), [Cu₂(ida)₂(btx)(H₂O)₂] · 2H₂O (2) and [Cu₂(ida)₂(pbbm)(H₂O)₂] · H₂O · 3CH₃OH (3) (bbbm = 1,1^′-(1,4-butanediyl)bis-1H-benzimidazole, btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene, pbbm = 1,1^′-(1,3-propanediyl)bis-1H-benzimidazole), in addition to three fcz-based Cu^II complexes, namely, {[Cu(fcz)₂(H₂O)₂] · 2NO₃}_n (4), {[Cu(fcz)₂(H₂O)] · SO₄ · DMF · 2CH₃OH · 2H₂O}_n (5) and {[Cu(fcz)₂Cl₂] · 2CH₃OH}_n (6) (fcz = 1-(2,4-difluorophenyl)-1,1-bis[(1H-1,2,4-triazol-l-yl) methyl]ethanol) have been prepared according to appropriate synthetic strategies with the aim of exploiting new and potent catalysts. Single crystal X-ray diffraction shows that 1 and 2 possess similar binuclear structures, 3 features a 2D pleated network, and 4 exhibits a 1D polymeric double-chain structure. Complexes 1-6 are tested as catalysts in the green catalysis process of the oxidative coupling of 2,6-dimethylphenol (DMP). Under the optimized reaction conditions, these complexes are catalytically active by showing high conversion of DMP and high selectivity of PPE. The preliminary study of the catalytic-structural correlations suggests that the coordination environment of the copper center have important influences on their catalytic activities.

Full text of DOI:10.1016/j.jorganchem.2008.02.023

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Journal of Organometallic Chemistry 693 (2008) 1927–1937
www.elsevier.com/locate/jorganchem
Syntheses, crystal structures of a series of copper(II) complexes
and their catalytic activities in the green oxidative coupling
of 2,6-dimethylphenol
a
a,b,
a
a
*
Erpeng Zhang , Hongwei Hou
, Huayun Han , Yaoting Fan
a
Department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR China
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China
b
Received 18 December 2007; received in revised form 18 February 2008; accepted 21 February 2008
Available online 4 March 2008
Abstract
II
Three mixed-ligand Cu complexes bearing iminodiacetato (ida) and N-heterocyclic ligands, namely, [Cu (ida) (bbbm)(H O) ] ꢀ H O
2
2
2
2
2
0
(
1), [Cu
2
(ida)
2
2
(btx)(H O)
2
] ꢀ 2H
2
O (2) and [Cu
2
(ida)
2
(pbbm)(H
2
O)
2
] ꢀ H
2
O ꢀ 3CH
3
OH (3) (bbbm = 1,1 -(1,4-butanediyl)bis-1H-benzimid-
0
azole, btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene, pbbm = 1,1 -(1,3-propanediyl)bis-1H-benzimidazole), in addition to three fcz-
II
based Cu
complexes, namely, {[Cu(fcz)
2
(H
2
O)
2
] ꢀ 2NO
3
}
n
(4), {[Cu(fcz)
2
(H
2
O)] ꢀ SO
4
ꢀ DMF ꢀ 2CH
3
2
OH ꢀ 2H O}
n
(5) and
{
[Cu(fcz) Cl ] ꢀ 2CH OH} (6) (fcz = 1-(2,4-difluorophenyl)-1,1-bis[(1H-1,2,4-triazol-l-yl) methyl]ethanol) have been prepared according
2
2
3
n
to appropriate synthetic strategies with the aim of exploiting new and potent catalysts. Single crystal X-ray diffraction shows that 1 and 2
possess similar binuclear structures, 3 features a 2D pleated network, and 4 exhibits a 1D polymeric double-chain structure. Complexes
1–6 are tested as catalysts in the green catalysis process of the oxidative coupling of 2,6-dimethylphenol (DMP). Under the optimized
reaction conditions, these complexes are catalytically active by showing high conversion of DMP and high selectivity of PPE. The pre-
liminary study of the catalytic–structural correlations suggests that the coordination environment of the copper center have important
influences on their catalytic activities.
Ó 2008 Published by Elsevier B.V.
Keywords: Copper(II) complexes; Crystal structures; Green catalysis; Oxidative coupling; Catalytic–structural correlation
1
. Introduction
As a typical catalytic application of copper complexes,
the oxidative coupling of 2,6-dimethylphenol (DMP) has
received considerable attention [4–7] since the main prod-
uct of this reaction, poly(1,4-phenylene ether) (PPE), is a
valuable engineering plastic [8] with excellent mechanical
properties and chemical resistance. Traditionally, the cou-
pling reaction is carried out homogeneously in organic sol-
vents or aqueous-organic biphasic solvents (in some cases)
[9–12]. However, along with ever-growing environmental
issues of chemical industry [13], the environmentally unde-
sirable processes, wherever in the laboratory scale synthesis
or in the manufacture of large-volume industrial chemicals,
should be replaced by ‘‘greener” catalytic ones [14] with
clean, safe, and inexpensive reaction solvents. Recently,
Kei Saito and co-workers reported the oxidative coupling
of DMP to form PPE with potassium ferricyanide as an
Over a period of years, considerable efforts have been
focused on the design and synthesis of the copper complex
which attract extensive interests in many fields such as
molecular magnetism [1], metalloproteins and enzymes
[
2]. In the specific case of catalysis, the unique characteristic
II
I
of the Cu /Cu redox couple renders many of their com-
plexes suited for various catalysis reactions [3], especially
for the green catalysis process since copper is a cheap
and minor toxic metal.
*
Corresponding author. Address: Department of Chemistry, Zheng-
zhou University, Zhengzhou 450052, PR China. Tel./fax: +86 371
67761744.
E-mail address: houhongw@zzu.edu.cn (H. Hou).
0
022-328X/$ - see front matter Ó 2008 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2008.02.023

1928
E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
oxidant in aqueous medium [15]. Unfortunately, the use of
potassium ferricyanide is still environmentally deleterious.
So far, the green oxidative coupling of DMP in environ-
mental friendly solvents with clean oxidants is studied
extremely rare. Therefore, this reaction is still a research
field of great challenge from the standpoint of ‘‘green
chemistry and technology”. Meantime, it is desired extraor-
dinarily that the development of efficient copper-containing
catalysts suited for the milder ‘‘green” conditions, and the
investigation of catalytic–structural correlations which
have great importance to exploiting new catalysts.
II
Hence, we prepared three mixed-ligand Cu complexes
II
1
–3 and three fcz-based Cu complexes 4–6 according to
appropriate synthetic strategies. For the purpose of replac-
ing the traditional organic-medium systems by the environ-
mental friendly processes, we explored an aqueous-medium
catalysis process of the oxidative coupling of DMP by
using hydrogen peroxide as oxidant and complexes 1–6
as catalysts. In order to understand the structural and
chemical factors that govern the activities of catalysts, the
catalytic–structural correlations have also been studied
through examining the effects of coordination environment
on the catalytic activities of the copper complexes.
Fig. 1. ORTEP drawings of two independent molecules in complex 1 with
thermal ellipsoids drawn at the 50% probability level. The hydrogen atoms
and uncoordinated solvent molecules are omitted for clarity.
Cu1, the imino nitrogen N1 and two carboxylate oxygen
atoms O1 and O3 from iminodiacetate take up the basal
positions along with the nitrogen N2 of the bridging bbbm
ligand, which lies across a crystallographic center of sym-
metry. An oxygen atom of coordinated water occupies
the apical position, leading to a square pyramidal geometry
around the Cu1 center. Unlike the Cu–N distances, the
Cu–O distances here are disparate due to Jahn–Teller dis-
2
. Results and discussion
2
.1. Synthesis
The self-assembly of Cu(ida) building blocks and poly-
II
dentate N-heterocyclic linkers has been proved to be a suc-
cessful strategy in the construction of coordination
polymeric frameworks [16]. According to this strategy,
tortion of the Cu ion. The axial Cu1–O5 distance
˚
(2.392(3) A) is much longer than the basal distances
˚
(1.965(3)–1.984(3) A). The atoms N1, O1, O3 and N2
II
the reaction of Cu with ida ligand and pbbm linker in
define a least-squares basal plane P(I), the Cu1 atom is dis-
˚
an aqueous-methanolic medium generated a 2D pleated
network 3 as expected. With bbbm or btx as the linker,
however, the corresponding product was binuclear com-
plex 1 or 2 irrespective of the reasonable variation of pH
or the reactants ratios. This suggests that the configuration
and flexibility of the bridging ligands may be the determin-
ing factors for the structure of resulted complex. In addi-
tion, the presence of different counterions has important
influence on the structure of cationic complex formed by
neutral ligands. Formerly, we synthesized two fcz-based
copper complexes by changing the counterions. With sul-
fate as the counterion, the 1D double chains coordination
polymer 5 was obtained [17]. With chlorine as the counter-
ion, the 2D network of complex 6 was formed [18]. Here,
we chose nitrate as the counterion and synthesized a new
placed by 0.1259 A from P(I) toward the apical oxygen
atom O5. The benzimidazole plane P(II) of bbbm ligand
defines a dihedral angle of 51.6° with P(I). Different from
the five-coordinated Cu1, Cu2 is four-coordinated by ida
and bbbm ligands. In the solid structure of 1, the binuclear
units are linked together through the hydrogen bonds
formed by water molecules and carboxylate oxygen atoms
of ida ligand, generating a 3D supramolecular structure as
shown in Fig. 2.
Complex [Cu (ida) (btx)(H O) ] ꢀ 2H O (2) crystallizes
2
2
2
2
2
in space group P2 /c with very similar structure to the
1
Cu1 unit in 1. As shown in Fig. 3, each Cu ion is located
in the basal center of a square pyramidal geometry formed
by the tridentate iminodiacetate, the bridging btx ligand
and the coordinated water. Different from that in complex
1, the dihedral angle between the basal plane P(I) (defined
by the atoms N1, O1, O3, and N4) and the triazole plane
P(II) of bridging btx ligand is 17.2°, which is comparable
to those of [Cu(ida)(Meim)(H O) ] ꢀ H O (Meim = N-
1
D double chains coordination polymer 4 through the
self-assembly of fcz ligand and copper ions.
2
.2. Description of crystal structures
2
2
2
methyl-imidazole)
(16.3°)
[19]
and
[Cu(ida)(im)
Crystallographic analysis reveals that complex [Cu2
(H O)] ꢀ H O (im = imidazole) (19.1°) [20]. As in 1, the
2
2
(
ida) (bbbm)(H O) ] ꢀ H O (1) crystallizes in space group
binuclear units of 2 are also linked together through inter-
molecular hydrogen bonds leading to a 3D network
structure (Fig. 4).
2
2
2
2
ꢀ
P1 and contains two kinds of crystallographically indepen-
dent Cu ions (Cu1 and Cu2) as shown in Fig. 1. Around

E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
1929
Fig. 2. View of the infinite 3D supramolecular structure of complex 1, generated through extensive hydrogen bonding. The hydrogen atoms and
uncoordinated water molecules are omitted for clarity.
molecule all coordinate to Cu1 forming a least-squares
˚
basal plane P(I) with the mean deviation of 0.0701 A.
˚
Cu1 is displaced by 0.1602 A from the plane P(I). Cu1
and Cu2 are bridged by the pbbm molecule leading to a
binuclear [Cu (ida) pbbm] unit (Fig. 5) which is similar
2
2
to that of 1 or 2. The dihedral angle between the benzimid-
azole plane P(II) of pbbm and P(I) (53.3°) is close to that of
II
1
. In addition, each Cu ion forms a somewhat longer
˚
bond (2.439(2) A) to the fifth donor atom O4A of another
Cu (ida) pbbm] unit, thus the square pyramidal coordina-
Fig. 3. ORTEP drawing of the binuclear structure of complex 2 with
thermal ellipsoids drawn at the 50% probability level. The hydrogen atoms
and uncoordinated water molecules are omitted for clarity.
[
2
2
tion geometry around copper center is completed and two
distinct carboxylate coordination modes of the same ida
ligand are observed. The C1 carboxylate group is mono-
dentate to Cu1 whereas the C4 carboxylate group forms
a didentate bridge. As a consequence, these [Cu2
Complex [Cu (ida) (pbbm)(H O) ] ꢀ H O ꢀ 3CH OH (3)
2
2
2
2
2
3
is an extended 2D network crystallizing in space group
P2
1
2
1
2
1
. As in 1 and 2, the atoms N1, O1 and O3 of the
(ida) pbbm] units are linked together through the longer
2
same iminodiacetate ligand and the N2 atom of the pbbm
Cu–O bonds leading to the infinite 2D pleated network
Fig. 4. View of the infinite 3D supramolecular structure of complex 2, generated through extensive hydrogen bonding. The hydrogen atoms and
uncoordinated water molecules are omitted for clarity.

1930
E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
II
II
of the Cu ion. The Cu ion locates strictly in the center
˚
of the equatorial plane P(I) (mean deviation = 0.0000 A)
defined by the atoms N1, N1A, N6B and N6C. Around
II
the Cu ion, two triazole rings define the small dihedral
angle of 21.1° with the equatorial plane P(I), which is com-
parable to the corresponding angle of complex 2. But the
other two triazole rings are nearly perpendicular to the
equatorial plane with the dihedral angle of 88.4°. The fcz
II
ligands link two Cu ions resulting in a bimetallic ring as
building block. The bimetallic rings are extended through
II
co-Cu to generate an infinite double chain. As shown in
Fig. 8, the nitrate anions are located in the vacancy among
the 1D chains and hold the cationic chains together to gen-
erate a 2D layered architecture.
2 2
Fig. 5. The coordination environment of Cu ions of [Cu (ida) (pbbm)]
group in complex 3 (ellipsoids at 50% probability). The hydrogen atoms
and uncoordinated solvent molecules are omitted for clarity.
The crystal structures of complexes 5 and 6 have been
described in our previous reports [17,18]. As shown in
Scheme 1, complex 5 exhibits a 1D double-chain structure
II
spread in a and b directions. As can be seen from Fig. 6, the
individual ring of the network in 3 is made up of 6 Cu(ida)
units connected together by two pbbm ligands leading to a
puckered ring structure. Such 6-metallic rings also can be
found in the reported mixed-ligand copper complexes
similar to that of complex 4. Each Cu ion is located in the
center of a distorted tetragonal pyramid. Four nitrogen
atoms from four fcz ligands occupy the basal sites, and
one oxygen atom from a terminally coordinated water mol-
ecule fills the apical position, which is different from that of
complex 4. Unlike 4 and 5, complex 6 possesses a 2D rhom-
bohedral grid structure, in which each copper center is 6-
coordinated by two chlorine ions and four nitrogen atoms
from four fcz ligands.
[
16], in which the honeycomb nets with interpenetrating
frameworks are constructed by Cu(ida) units and bipyri-
dine spacers.
Crystallographic analysis shows that complex
{
[Cu(fcz) (H O) ] ꢀ 2NO } (4) crystallizes in the triclinic
2
2
2
3 n
ꢀ
space group P1 and exhibits a 1D polymeric double-chain
structure as shown in Fig. 7. Each Cu ion is located in the
2.3. Catalytic oxidative coupling of DMP in water
II
center of an elongated octahedron. Four nitrogen atoms
from four fcz ligands occupy the equatorial sites, and two
oxygen atoms from two coordinated water fill the axial
The overall oxidative coupling process of DMP under
green reaction conditions is shown in Scheme 2. The C–O
coupling of DMP yields the linear polymer PPE, the C–C
coupling and subsequent oxidation of two DMP units
[21] result the by-product diphenoquinone (DPQ). Initially,
we used complex 1 as a represent catalyst to optimize the
˚
positions. The Cu–O bond distance (2.3967(12) A) is sub-
stantially longer than the basal Cu–N bond distances
˚
(
2.0188(12)–2.0407(12) A) due to Jahn–Teller distortion
Fig. 6. View of the 2D pleated network involving 6-metallic rings. The hydrogen atoms and uncoordinated solvent molecules are omitted for clarity.

E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
1931
Fig. 7. The double-chain structure of complex 4. The hydrogen atoms are omitted for clarity.
Fig. 8. The 2D supramolecular structure of complex 4, generated through extensive hydrogen bonding. The hydrogen atoms are omitted for clarity.
reaction conditions. As shown in Table 1, the influences of
various factors such as reaction temperature, reaction time,
the loading of catalyst, co-catalyst and oxidant were
explored.
sion increases significantly from 77% to 93% as the
temperature increases from 40 to 60 °C, and the best selec-
tivity of PPE of 88% is achieved at the temperature of
50 °C.
Previous investigations have indicated that the oxidative
coupling of DMP in organic solvents is usually carried out
at room temperature, and higher temperature will result in
more by-products DPQ. However, as to the aqueous med-
ium systems, higher temperature is necessary because the
reaction does not proceed at room temperature (Entry 1).
Thus several reaction temperatures (40, 50 and 60 °C) were
screened (Entries 2–4). It is observed that a significant tem-
perature effect on the reaction appeared. The DMP conver-
The effect of reaction time was also investigated. As the
time prolongs from 2 h to 8 h, the DMP conversion
increases quickly from about 50% to near 90%, while the
selectivity of PPE increases very slowly. Further prolong-
ing in reaction time does not improve the conversion signif-
icantly but decreases the selectivity slightly (Entry 8). The
reason for this may be that the by-product DPQ degrades
the polymer upon further processing at high temperatures
[22]. In addition, we find that the selectivity of PPE is not

1932
E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
F
F
F
F
N
N
N
N
C
N
N
C
N
N
N
N
N
N
N
N
N
N
N
OH
N
N
OH
Cu
Cu
OH
C
OH
C
H O
2
H O
2
N
N
N
N
N
F
F
F
F
{
[Cu(fcz)
2
(H
2
O)]·SO
4
·DMF·2CH
3
OH·2H
2
O}
n
(5)
F
N
N
N
N
F
N
N
HO
C
C
F
N
N
HO
N
N
Cl
N
N
N
N
F
Cu
F
Cl
N
N
OH
N
N
F
C
C
OH
N
N
N
N
F
N
N
F
{[Cu(fcz)
2
Cl
2
)]·2CH
3
OH}
n
(6)
Scheme 1. The structures of complexes 5 and 6.
CuII complex, NaOH
Water
m
OH + m H O2
H
O
H + 1/ 2 ( m-n )O
O + 2 m H O
2
2
n
(
DPQ)
(
PPE)
(
DMP)
Scheme 2. The oxidative coupling of 2,6-dimethylphenol in water.
correlated for the DMP conversion, which is closely to the
former conclusion [22], since Viersen et al. have shown that
most DPQ is formed during the beginning of the reaction
cess [4]. Thus, the effect of various NaOH-to-DMP ratios
on the reaction was surveyed (Entries 13–16). It is clear
that the DMP conversion increases dramatically with the
increase of NaOH-to-DMP ratio. The favorable effect on
the selectivity reaches the maximum of 88% at a 1:1 ratio
of NaOH-to-DMP. But the excess amount of NaOH
results in a decrease of the selectivity although the sub-
strates convert more completely (Entry 16). The aforemen-
[
23].
It has been demonstrated that this coupling process does
not proceed without the addition of base [15]. The role of
base is considered to be deprotonation of DMP into phe-
nolate species, which is crucial in the whole catalytic pro-

E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
1933
Table 1
Oxidative coupling of DMP in water catalyzed by complex 1
a
b
Entry
Temperature (°C)
Time (h)
Complex 1 (mol%)
NaOH/DMP
H
2
O
2
(lL)
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
a
RT
40
50
60
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
8
8
8
8
2
6
8
12
8
8
8
8
8
8
8
8
8
8
8
8
8
2
2
2
2
2
2
2
2
0
1
2
4
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
0.5
0.8
1
1.2
1
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
0
NR
77
89
93
52
76
89
96
Trace
80
89
90
80
84
89
94
76
85
88
89
98
69
88
83
80
86
88
83
1
1
1
1
1
1
1
1
1
1
2
2
81
88
89
77
86
88
85
81
85
88
88
82
1
1
1
1
10
20
30
40
Conversions and isolated yields based on the DMP, average of two runs.
Selectivity = ([PPE] ꢁ 100)/([PPE] + [DPQ]).
b
Table 2
a
Oxidative coupling of DMP catalyzed by complexes 1–6
b
b
c
Catalysts
Conversion (%)
Yield (%)
Selectivity (%)
PPE
3
PPE
DPQ
M
w
(/10 )
w n
M /M
1
2
3
88
77
83
69
60
74
9
11
8
88
84
90
10.5
9.6
11.3
2.0
1.8
2.1
CuCl
2
Trace
82
86
4
5
6
a
46
45
50
9
7
8
84
86
86
6.2
7.1
4.5
1.7
1.6
1.3
73
2 2
Optimized conditions: DMP (1 mmol), NaOH (1 mmol), H O (20 lL) and catalyst (0.02 mmol) in 5 mL of water for 8 h at 50 °C.
Conversions and isolated yields based on the DMP, average of two runs.
Selectivity = ([PPE] ꢁ 100)/([PPE] + [DPQ]).
b
c
tioned data, taken together with the previous finding of a
competing effect of hydroxide ions with phenolate anions
induced by the excess of basicity [24], allow a 1:1 ratio of
NaOH-to-DMP for optimization.
The influences of the loading of catalyst and oxidant have
also been explored. The control reaction performed without
any catalyst (Entry 9) shows that the copper complex is
indispensable for the reaction system. But the effect of the
amounts of catalyst is not striking and 2 mol% of complex
were also tested as catalysts for this oxidative coupling
reaction under the same conditions. As shown in Table 2,
complex 3 provides the results similar to that of 1 with
83% conversion and 90% selectivity. The performance of
complex 2 is slightly poorer than the above two catalysts
giving 77% conversion and 84% selectivity. Comparing
with the [Cu(tmeda)Cl ] catalyst giving 67% yield of PPE
2
[15], the best result of the copper-catalyzed synthesis of
PPE in aqueous medium so far, complexes 1 and 3 are
more efficient catalysts with the yields of PPE of 69% for
1 and 74% for 3, respectively. However, the contrasting
1
is sufficient. The results of entries 17–21 indicate that the
addition of appropriate amount of H O is also necessary
2
2
for obtaining higher conversion and selectivity. At higher
amounts of H O , the substrates almost convert completely
reaction using CuCl as catalyst only gives trace amounts
2
of PPE. We also notice that the Cu(NO ) /Meim catalyst
2
2
3 2
but the selectivity descends correspondingly. Similar results
have been found in our previous investigations [25].
After acquiring the optimized reaction conditions for
complex 1, the other two mixed-ligand complexes 2 and 3
which has been used successfully in organic systems, shows
a poor result in water with nearly no PPE obtained [12].
This suggests that complexes 1–3 have obvious compara-
tive advantage in the aqueous-medium oxidative coupling

1934
E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
of DMP. A potential explanation for this may be inferred
from the stabilizing effects [17,25] of the ida ligand and
the steric influence of the copper center. In case of com-
plexes 1–3, the tridendate mode of ida ligand not only
improves the aqueous stabilities of these complexes but
also forms the opening coordination geometry with less ste-
ric hindrance around the copper center which favors the
mixed-ligand copper complexes catalysts may represent a
starting point for the development of new kinds of catalysts
satisfying the need of ‘‘green” process.
3. Conclusion
In summary, with the aim of exploiting new and potent
catalysts, we have synthesized three mixed-ligand com-
plexes 1–3 by the self-assembly of Cu(ida) building block
with different auxiliary ligands, and three fcz-based com-
II
coordination of DMP to Cu to form the proposed active
copper species [26] and the subsequent polymerization of
DMP. Likewise, Reedijk and co-workers have also shown
II
II
that a series of Cu complexes incorporating structurally
plex 4–6 by the self-assembly of Cu ions with fcz ligand
related N,O-containing ligands underwent this polymeriza-
tion more efficiently than copper(II) salts [7d]. As to the
slightly lower DMP conversion obtained by complex 2,
the possible reason may associate with the features of dif-
ferent N-heterocyclic ligands presented in these complexes.
In order to further elucidate the influence of the coordi-
nation environment on the catalytic activities of copper
complexes, and understand the structural and chemical fac-
tors that govern the activities of catalysts in aqueous sys-
tems, three fcz-based complexes 4–6 with different
coordination geometries were studied as catalysts for the
oxidative coupling of DMP, the results were also reported
in Table 2. Under the same reaction conditions, complexes
in the presence of different counterions. Using complexes
1–6 as catalysts, a green aqueous-medium catalysis process
of the oxidative coupling of DMP has been explored with
great potentials in the aspect of ‘‘green chemistry and tech-
nology”. Recognizing that decreasing the steric hindrance
around the copper center should increase the catalytic
activity of complex, it can be expected that more studies
could improve the catalytic efficiencies via further struc-
tural modifications of copper complex for practical
application.
4. Experimental
4
and 5 with one or two coordinated water molecules
4.1. General
around each copper center give similar results with the con-
version of 86% and 82%, respectively. As premeditated,
complex 6 with coordinated chloride ions is found to be
the least active catalyst with only 73% conversion.
Although no remarkable differences exist among the con-
versions obtained by complexes 4–6, these results, to a cer-
tain extent, represent the influence of the coordination
environment on the catalytic activity of the copper com-
plex. Concretely, comparing with the strongly coordinated
chloride ions in complex 6, the coordinated water mole-
cules in 4 and 5 are easy leaving groups and can be readily
replaced by the oxidant or the substrate, thus it is easier,
for the phenolate anion, to enter the copper coordination
sphere when copper centers are coordinated by water mol-
ecules rather than chloride ions.
All reagents were obtained from commercial suppliers
and used without further purification. 2,6-dimethylphenol
(DMP) (99% pure) was used as obtained from J&K Chem-
ical Ltd. IR data were recorded on a Bruker Tensor 27
ꢂ1
spectrophotometer with KBr pellets in the 400–4000 cm
region. NMR spectra were recorded on a Bruker DPX-
400 spectrometer. Elemental analyses (C, H, and N) were
carried out on a FLASH EA 1112 elemental analyzer.
Molecular weights of the PPE were determined on a
Waters 150C gel permeation chromatography (GPC)
instrument. Polystyrene was used as calibration standards.
THF was used as an eluent at a flow rate of 1.0 ml/min.
Ligand 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene (btx)
was prepared according to our previous procedures [27].
0
On the basis of the above experimental results and pre-
vious literatures [4–7,15], we propose a plausible reaction
mechanism for the present oxidative coupling reaction.
DMP is dissolved in the basic aqueous phase to form the
phenolate anion; these phenolate anions coordinate to the
Ligands 1,1 -(1,3-propanediyl)bis-1H-benzimidazole (pbbm)
0
and 1,1 -(1,4-butanediyl)bis-1H-benzimidazole (bbbm) were
prepared according to the literature [28], except that benz-
imidazole was used instead of benzotriazole.
II
II
Cu center and subsequently are oxidized by Cu to the
phenoxyl radicals [15]. The C–O coupling of these phe-
noxyl radicals gives the polymer in the solid that precipi-
4.2. Synthesis of complex 1–6
Synthesis of [Cu (ida) (bbbm)(H O) ] ꢀ H O (1): An
2
2
2
2
2
I
tates from the water. The resultant Cu was reoxidized to
aqueous solution (2 mL) of sodium iminodiacetato
(8.8 mg, 0.05 mmol) was added into the 3 mL methanol
solution of CuCl ꢀ 2H O (8.5 mg, 0.05 mmol), then 3 mL
II
Cu by H O .
2
2
Although the optimal yields obtained by these com-
plexes in water are not high (45–74%) comparing with
those results obtained from organic systems, the present
system has the following significant advantages from the
standpoint of ‘‘green chemistry and technology”: (i) hydro-
gen peroxide and water are used to replace the environmen-
tal unfriendly oxidants and reaction solvents, (ii) the use of
2
2
methanol solution of bbbm (7.2 mg, 0.025 mmol) was
added drop wise to the above mixture. The resulting dark
blue solution was allowed to stand at room temperature.
After a few days, blue block-shaped crystals of complex 1
were obtained. The solid were collected by filtration and
dried in vacuo. The sample is stable in the air. Yield:

E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
1935
ꢂ1
6
6%. IR (cm , KBr): 3387 s, 3255 m, 2945 w, 1623 s,
992 m, 673 m. Anal. Calc. for C H CuCl F N O : C,
28 32 2 4 12 4
1
522 m, 1467 w, 1393 s, 1300 m, 1262 w, 1202 m, 1121
41.43; H, 3.95; N, 20.72. Found: C, 41.49; H, 3.90; N,
20.74%.
w, 1015 w, 910 m, 750 s. Anal. Calc. for C H Cu N O :
2
6
32
2
6
11
C, 42.68; H, 4.41; N, 11.49. Found: C, 43.02; H, 4.13; N,
1
1.38%.
4.3. General procedure for the catalytic oxidative coupling of
2,6-dimethylphenol
Synthesis of [Cu (ida) (btx)(H O)] ꢀ 2H O (2): The
2
2
2
2
procedure was similar to that of 1, except that btx was used
instead of bbbm. The resulting dark blue solution was
allowed to stand at room temperature. After a few days,
blue block-shaped crystals of complex 2 were obtained.
The solid were collected by filtration and dried in vacuo.
The sample is stable in the air. Yield: 69%. IR (cm
All of the crystal complex catalysts were ground well
into appropriate sizes prior to use. DMP (122 mg, 1 mmol)
was dissolved in water (5 mL) containing sodium hydrox-
ide (40 mg, 1 mmol) and sodium n-dodecyl sulfate (SDS)
(29 mg, 0.1 mmol) in a 10 mL flask. The 2 mol% complex
was added to the solution, and the mixture was vigorously
stirred under air at 50 °C. Then, hydrogen peroxide (30%
aqueous solution) was slowly added into the mixture using
a microinjector every 15 min for 2 times to minimize H O
ꢂ1
,
KBr): 3238 s, 3137 s, 1620 s, 1536 m, 1442 m, 1389 s,
301 m, 1131 s, 1006 m, 914 m, 738 m. Anal. Calc. for
C H Cu N O : C, 34.49; H, 3.90; N, 16.04. Found: C,
1
2
0
27
2
8
12
3
4.76; H, 3.69; N, 16.30%.
2
2
Synthesis of [Cu(ida) (pbbm)(H O) ] ꢀ 2H O (3): The
decomposition. After 8 h, the reaction was stopped and the
polymer product 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 separa-
tory funnel, the organic materials were extracted after the
addition of a few milliliters of CH Cl . This was repeated
2
2
2
2
procedure was also similar to that of 1, except that pbbm
was used instead of bbbm. The resulting dark blue solution
was allowed to stand at room temperature. A few weeks
later, blue block-shaped crystals of complex 3 had depos-
ited. The solid were collected by filtration and dried in
2
2
vacuo. The sample is stable in the air. Yield: 53%. IR
3 times. The combined organic extracts were dried with
ꢂ1
(
cm , KBr): 3372 m, 3139 m, 1611 s, 1510 m, 1461 w,
anhydrous MgSO and the solvent, after filtration, was
4
1
384 s, 1259 w, 1193 m, 1124 w, 1021 w, 908 m, 757 m.
evaporated in vacuo. The products were separated by pre-
parative 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 chloro-
form to methanol.
Anal. Calc. for C H Cu N O : C, 41.22; H, 5.44; N,
1
2
8
44
2
6
14
0.30. Found: C, 41.60; H, 5.06; N, 10.25%.
Synthesis of {[Cu(fcz) (H O) ] ꢀ 2NO } (4): A meth-
2
2
2
3 n
anol solution (3 mL) of fcz (0.2 mmol) was slowly added
into 3 mL methanol solution of Cu(NO ) ꢀ 3H O
3
2
2
(
0.1 mmol). The resulting azury solution was allowed to
1
stand at room temperature. A few weeks later, blue
block-shaped crystals of complex 4 had deposited. The
solid were collected by filtration and dried in vacuo. The
Poly(phenylene ether) (PPE): H NMR (400 MHz,
1
3
CDCl ) d: 2.10 (s, 6H), 6.45 (s, 2H).
C NMR
3
(100 MHz, CDCl ) d: 16.8, 114.5, 132.7, 145.6, 154.5. IR
3
ꢂ1
ꢂ1
sample is stable in the air. Yield: 50%. IR (cm , KBr):
(cm , KBr): 3429 m, 1607 s, 1471 s, 1306 s, 1189 s, 1022
3
439 s, 3130 s, 1620 s, 1503 m, 1277 m, 1132 s, 676 m. Anal.
s, 858 m.
Calc. for C H CuF N O : C, 37.31; H, 3.35; N, 23.44.
2
6
28
4
14 10
Found: C, 37.35; H, 3.39; N, 23.49%.
4.4. X-ray crystallographic analyses
Synthesis of {[Cu(fcz) (H O)] ꢀ SO ꢀ DMF ꢀ 2CH OH ꢀ
2
2
4
3
2
H O} (5): Complex 5 was synthesized according to our
The data of 1 and 4 were collected on a Bruker Aper
CCD diffractomer with graphite monochromated Mo Ka
radiation (k = 0.71073 A). The data of 2 and 3 were collected
2
n
previous procedure [17]. A mixture of methanol and DMF
solution (6 mL) of fcz (0.2 mmol) was slowly added into
˚
1
mL aqueous solution of CuSO ꢀ 5H O (0.1 mmol). The
on a Rigaku Saturn 724 CCD diffractomer with graphite
monochromated Mo Ka radiation (k = 0.71073 A). A single
4
2
˚
resulting clear solution was allowed to stand at room tem-
perature. A few weeks later, blue block-shaped crystals of
complex 5 were formed. The solid were collected by filtra-
tion and dried in vacuo. The sample is stable in the air.
Yield: 58%. IR (cm , KBr): 3445 s, 3131 s, 1620 s, 1500
m, 1279 m, 1130 s, 675 m. Anal. Calc. for C H CuF
4-
crystal suitable for X-ray diffraction was mounted on a
glass fiber. The data were collected at room temperature
and corrected for Lorenz-polarization effects. A correction
for secondary extinction was applied. The structures were
solved by direct methods and expanded using Fourier tech-
niques. The non-hydrogen atoms were refined anisotropi-
cally and hydrogen atoms were included but not refined.
The final cycle of full-matrix least-squares refinement was
based on observed reflections and variable parameters.
All calculations were performed using the SHELXL-97 crys-
tallographic software package [29], and refined by full-
ꢂ1
3
1
45
N O S: C, 38.61; H, 4.67; N, 18.89. Found: C, 38.52;
H, 4.67; N, 18.76%.
1
3
12
Synthesis of {[Cu(fcz) Cl ] ꢀ 2CH OH} (6): Complex
2
2
3
n
6
was synthesized according to the literature [18]. The pro-
cedure was similar to that of 4, except that CuCl ꢀ 2H O
2
2
was used instead of Cu(NO ) ꢀ 3H O. The azury block-
3
2
2
2
shaped crystals of complex 6 are also stable in the air. Yield:
matrix least squares methods based on F with isotropic
ꢂ1
6
3%. IR (cm , KBr): 3448 s, 3132 s, 1619 s, 1501 s, 1278 s,
thermal parameters for the non-hydrogen atoms. Table 3

1936
E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
Table 3
Crystal data and structure refinement for complexes 1–4
Complex
Formula
1
2
3
4
C
26
H
32Cu
2
N
6
O
11
C
20 2
H30Cu N
O
8 12
C
28
H44Cu
2
N
6
O
14
C
26 4 14 10
H28CuF N O
F
w
731.66
291(2)
0.71073
Triclinic
701.60
293(2)
0.71073
815.77
293(2)
0.71073
Orthorhombic
p212121
8.0276(16)
19.859(4)
21.265(4)
90
836.16
293(2)
0.71073
Triclinic
T (K)
˚
Wavelength (A)
Crystal system
Space group
Monoclinic
P2 /c
ꢀ
ꢀ
P1
1
P1
˚
a (A)
10.0069(19)
12.048(2)
13.382(3)
89.099(4)
87.413(4)
68.766(4)
1502.3(5)
2, 1.617
13.849(3)
11.039(2)
9.4429(19)
90
104.24(3)
90
1399.2(5)
2, 1.665
720
9.4225(7)
10.4660(8)
10.7979(8)
62.2480(10)
72.9370(10)
69.8560(10)
873.29(11)
1, 1.590
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
90
90
˚
3
V (A )
Z, D (g cm
F(000)
3390.1(12)
4, 1.492
1572
ꢂ3
c
)
752
427
h Range for data collection (°)
Reflection collected/unique (Rint
Data/restraints/parameters
2.37–27.50
13014/6767 (0.0187)
6767/9/430
1.019
0.0395, 0.0991
1.52–28.07
16916/3376 (0.0568)
3376/2/202
1.050
0.0396, 0.0972
1.92–24.99
33673/5972 (0.0473)
5972/0/442
1.094
0.0782, 0.2272
2.27–27.50
7651/3947 (0.0108)
3947/0/250
0.954
0.0299, 0.1072
)
2
Goodness-of-fit on F
a
b
2
Final R
1
, wR
a
R1 = jjF
o
j ꢂ jF
c
jj/jF j.
2 2
c
o
b
2
2 2 1/2
2
2
2
2
2
wR
2
= [w(jF
o
j ꢂ jF
j) /wjF
o
j ] , w = 1/[r (F
o o c
) + 0.0297P + 27.5680P], where P = (F + 2F )/3.
Table 4
˚
Selected bond lengths (A) and angles (°) for complexes 1–4
Complex 1
Cu1–O1
Cu1–N2
Cu2–N5
N4–Cu2–N5
O5–Cu1–N2
1.9653(19)
1.941(2)
1.962(2)
177.49(10)
96.68(11)
Cu1–O3
Cu2–O8
O1–Cu1–O3
O5–Cu1–O1
1.965(2)
1.975(2)
167.33(9)
85.32(9)
Cu1–O5
Cu2–O6
N1–Cu1–N2
O5–Cu1–N1
2.392(3)
1.970(2)
161.99(11)
101.08(11)
Cu1–N1
Cu2–N4
O6–Cu2–O8
O5–Cu1–O3
1.984(3)
1.993(2)
165.74(9)
91.93(9)
Complex 2
Cu1–O1
Cu1–N4
1.9533(18)
1.9593(19)
93.39(8)
Cu1–O3
N1–Cu1–N4
O5–Cu1–O1
1.9518(18)
167.73(8)
96.93(9)
Cu1–O5
O1–Cu1–O3
O5–Cu1–O3
2.257(2)
161.01(8)
100.43(8)
Cu1–N1
O5–Cu1–N1
1.968(2)
98.87(8)
O5–Cu1–N4
a
Complex 3
Cu1–O1
1.974(5)
1.964(6)
1.997(6)
175.5(3)
93.3(2)
Cu1–O3
Cu2–O5
Cu2–N5
O5–Cu2–O7
O4#1-Cu1-O3
1.990(5)
2.001(5)
1.962(6)
162.4(2)
99.6(2)
Cu1–O4#1
Cu2–O7
N1–Cu1–N2
2.439(6)
1.974(5)
174.3(3)
90.2(2)
Cu1–N1
1.985(6)
2.386(6)
163.0(2)
95.2(2)
Cu1–N2
Cu2–N6
N5–Cu2–N6
O4#1-Cu1–O1
Cu2–O6#2
O1–Cu1–O3
O4#1–Cu1–N2
O4#1–Cu1–N1
b
Complex 4
Cu1–N1
Cu1–O3
O3–Cu1–O3#1
a
2.0188(12)
2.3967(12)
180.000(1)
Cu1–N1#1
Cu1–O3#1
O3–Cu1–N1
2.0188(12)
2.3967(12)
93.37(5)
Cu1–N6#2
N1–Cu1–N1#1
N1–Cu1–O3#1
2.0407(12)
180.000(1)
86.63(5)
Cu1–N6#3
N6#2–Cu1–N6#3
2.0407(12)
180.00(8)
Symmetry transformations used to generate equivalent atoms: #1 x ꢂ 1/2,ꢂ y + 3/2, ꢂz + 2, #2 x + 1/2, ꢂy + 1/2,ꢂz + 2.
Symmetry transformations used to generate equivalent atoms: #1 ꢂ x + 1, ꢂy + 1, ꢂz + 2, #2 ꢂx + 1, ꢂy, ꢂz + 2, #3 x,y + 1.
b
shows crystallographic data and structure processing
Appendix A. Supplementary material
parameters of complexes 1–4. Selected bond lengths and
bond angles of complexes 1–4 are listed in Table 4.
CCDC 666231, 666232, 666233, and 666234 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.jorganchem.2008.02.023.
Acknowledgement
We thank the National Natural Science Foundation
(
Nos. 20671082 and 20371042), NCET and Ph.D. Founda-
tion of Ministry of Education of China for financial support.

E. Zhang et al. / Journal of Organometallic Chemistry 693 (2008) 1927–1937
1937
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