Three Cu(II) complexes based on mixed ligands: Their structures and catalytic behaviour

Article abstract of DOI:10.3184/174751912X13371887324682

The Cu(II) complexes, [Cu(2,5-pydc)(bmi)(H₂O)]_n, [Cu₂(H₂O)₂(2,6-pydc)₂(btx)] ·2H₂O and [Cu(btc)₂(bmi)₂]·1. 5H₂O, (bmi = 1-[(benzotriazol-yl)methyl)-1H-1,3-imidazole; 2,5-H ₂pydc = pyridine-2,5-dicarboxylic acid; btx = 1,4-bis(1,2,4-triazol- 1-ylmethyl)benzene; H₃btc = 1,3,5-benzenetricarboxylate) have been synthesised and their X-ray structures show that hydrogen bonds and π···π stacking interactions are employed in their construction. The complexes can effectively catalyse the oxidative coupling reaction of 2,6-di-tert-butylphenol in good yield and unique selectivity.

Full text of DOI:10.3184/174751912X13371887324682

JOURNAL OF CHEMICAL RESEARCH 2012
RESEARCH PAPER 413
JULY, 413–417
Three Cu(II) complexes based on mixed ligands: their structures and
catalytic behaviour
Jiyong Hua*, Chunli Liaob and Jin’an Zhaoa*
aDepartment of Chemistry and Chemical Engineering, Henan Univeristy of Urban Construction, Henan 467036, P. R. China
bDepartment of Bioengineering, Henan Univeristy of Urban Construction, Henan 467036, P. R. China
The Cu(II) complexes, [Cu(2,5-pydc)(bmi)(H2O)]n, [Cu2(H2O)2(2,6-pydc)2(btx)]·2H2O and [Cu(btc)2(bmi)2]·1.5H2O, (bmi =
1-[(benzotriazol-yl)methyl)-1H-1,3-imidazole; 2,5-H2pydc = pyridine-2,5-dicarboxylic acid; btx = 1,4-bis(1,2,4-triazol-1-
ylmethyl)benzene; H3btc = 1,3,5-benzenetricarboxylate) have been synthesised and their X-ray structures show that
hydrogen bonds and π···π stacking interactions are employed in their construction. The complexes can effectively
catalyse the oxidative coupling reaction of 2,6-di-tert-butylphenol in good yield and unique selectivity.
Keywords: Cu(II) complexes, crystal structure, mixed-ligand, catalytic reaction
C, 45.80; H, 3.16; N, 18.85. Found: C, 45.90; H, 2.90; N, 18.53%. IR
(cm−1): 3421m, 1670s, 1622s, 1524m, 1384s, 1337s, 1262m, 1095m,
1044m, 951w, 831m, 765s.
Inorganic–organic hybrid compounds have useful properties
applicable to catalysis, conductivity, luminescence and poros-
ity, in which their structural aspects play vital roles.1–3 As a
significant effect, anions can greatly regulate the structural
outcomes. In comparison with inorganic counterions, organic
anions such as aromatic polycarboxylates, which usually bear
multiform coordination fashions and structural features, may
more effectively tune the coordination architectures.4–6 Also,
nitrogen heterocycle synthons, together with aromatic polycar-
boxylates, are complementary in their coordination prefer-
ences and may act synergetically. Thus, the reactions of metal
ions with mixed ligands usually give rise to unique motifs,
combining functional properties which are greatly different
from those based on a single ligand.
The nature of the metal centre is also highly associated with
the properties of the complexes.7,8 Thus copper complexes are
functional materials that have wide potential applications, such
as in molecular magnetism, metalloproteins and enzymes and
particularly in catalysis of such transformations as C–C and
C–N bond-forming reactions, which have been found to be
very useful reactions in organic synthesis.9–12 As an important
C–C oxidation coupling product, diphenoquinone and its
derivatives are important intermediates for industrial synthesis
of a wide variety of special chemicals, such as pharmaceuti-
cals, dyes and agricultural chemicals.13,14 In an attempt to
examine the catalytic activity of mixed-ligand copper comple-
xes to catalyse such coupling reactions of sterically hindered
phenols, two five-coordinated and one four-coordinated Cu(II)
complexes have been isolated. Their catalytic behaviour shows
that these complexes, with different chemical environments,
can accelerate the coupling reaction and improve the yield.15
[Cu2(H2O)2(2,6-pydc)2(btx)]·2H2O (2): A reaction mixture of
Cu(NO3)2·2.5H2O (0.1 mmol, 0.0233 g), btx (0.1 mmol, 0.0120 g)
2,6-pydc (0.1 mmol, 0.0166 g), and water (10 mL), was placed in a
Teflon-lined stainless steel vessel, and then the pH was adjusted to 7
by addition of methanolic NaOCH3 solution. The mixture was sealed
and heated at 120 °C for two days, and then the reaction system was
cooled to room temperature. Blue rod-shaped crystals of 2 were
isolated in 61% yield (based on Cu). Anal. Calcd for C26H22Cu2N8O12:
C, 40.79; H, 2.90; N, 14.64. Found: C, 40.59; H, 3.21; N, 14.51%.
IR(cm−1): 3410s, 1675s, 1628s, 1540w, 1431w, 1383s, 1182w, 1140m,
1081w, 1007w, 915w, 729m, 687w.
[Cu(btc)2(bmi)2]·1.5H2O(3):AreactionmixtureofCu(NO3)2·2.5H2O
(0.1 mmol, 0.0233 g), H3btc (0.1 mmol, 0.0210 g) bmi (0.1 mmol,
0.0196 g) and water (10 mL), was placed in a Teflon-lined stainless
steel vessel, and then the pH was adjusted to 7 by addition of methan-
olic NaOCH3 solution. The mixture was sealed and heated at 130 °C
for two days, and then the reaction system was cooled to room temper-
ature. Purple block crystals of 3 were isolated in 48% yield (based on
Cu). Anal. Calcd for C38H29CuN10O13.5: C, 50.42; H, 3.23; N, 15.47.
Found: C, 50.16; H, 3.52; N, 15.12%. IR(cm−1): 3510s, 3138s, 1705s,
1618s, 1581s, 1521w, 1446m, 1370s, 1233s, 1168m, 1139s,1098s,
981w, 955w, 858w, 748s, 633m.
X-ray structure determination
A crystal suitable for X-ray determination was mounted on a glass
fibre. The data for 1, 2 and 3 were collected at room temperature on a
Rigaku Saturn 724 CCD diffractometer with graphite monochromated
Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by
direct methods and expanded with Fourier techniques. The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
included but not refined. The final cycle of full-matrix least-squares
refinement was based on observed reflections and variable parame-
ters. All calculations were performed with the SHELXL-97 crystallo-
graphic software package.17–19 Table 1 showed crystallographic crystal
data and processing parameters for all complexes, and Table 2 listed
their selected bond lengths and bond angles.
Experimental
All chemicals were reagent-grade quality and were purchased from
commercial sources and used without further purification. 1,4-bis(1,2,4-
triazol-1-ylmethyl)benzene (btx) and 1-[(benzotriazol-yl)methyl]-
1H-1,3-imidazole (bmi) were synthesised according to previously
published methods.16,17 IR data were recorded on a Bruker Tensor
27 spectrophotometer as KBr pellets in the 400–4000 cm–1 region.
Thermogravimetric experiments were performed using a TGA/SDTA
instrument. Elemental analyses (C, H and N) were carried out on a
Flash EA 1112 elemental analyser.
[Cu(2,5-pydc)(bmi)(H2O)]n (1): A reaction mixture of Cu(NO3)2·
2.5H2O (0.1 mmol, 0.0233 g), 2,5-pydc (0.1 mmol, 0.0166 g), bmi
(0.1 mmol, 0.0196 g) and water (10 mL), was placed in a Teflon-lined
stainless steel vessel, and then the pH was adjusted to 7 by addition of
methanolic NaOCH3 solution. The mixture was sealed and heated at
120 °C for three days, and then the reaction system was cooled
to room temperature. Blue rod-shaped crystals of 1 were isolated in
53% yield (based on Cu). Elemental analysis: Calcd for C17H14CuN6O5:
Catalytic reactions
The complexes were powdered to increase the surface area prior to
use, in which complex 1 acted as heterogeneous catalyst, whereas 2
and 3 acted as homogeneous catalysts. Taking complex 1 as a
representative catalyst, the reaction conditions were optimised. The
screened conditions indicated the optimal temperature to be 45 °C,
and 0.2 mL of the oxidant H2O2 to be used, with acetonitrile as reac-
tion medium, catalyst 0.02 mmol and the co-catalyst NaOCH3 1 mmol.
Also, the co-catalyst NaOCH3 should be added to the reaction system
with stirring for thirty minutes before the complex was added. After-
ward, H2O2 (200 µL of 30% aqueous solution) must be slowly added
into the mixture with a syringe pump in four portions at a time interval
of 25 minutes to minimise H2O2 decomposition. After 3 h, the sample
was concentrated in a vacuum, and the products were separated by
preparative TLC performed on dry silica gel plates with dichlo-
romethane–petroleum ether (1:4 v/v) as the developing solvents. The
* Correspondent. E-mail: hujiyong@hncj.edu.cn; zjinan@zzu.edu.cn

414 JOURNAL OF CHEMICAL RESEARCH 2012
products were collected and dried in vacuo. 3,3′,5,5′-tetra-tert-butyl-
4,4′-diphenoquinone (DPQ), IR(cm−1): 3440m, 3002w, 2958s, 2865m,
1605s, 1457m, 1386s, 1261w, 1090s, 1041m, 898m, 842w, 819w.
2,6-ditert-butylbenzoquinone (BQ), IR(cm−1): 3296w, 2960m, 2868m,
1655s, 1597m, 1367s, 1317m, 1244w, 1155m, 1075m, 922m, 877m,
826w.
CCDC 857751–857753 contains the supplementary crystallo-
graphic 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.
Table 2 Selected bond distances (Å) and angles (deg) for
complexes 1, 2, and 3
Complex 1
Cu(1)–O(1)
1.948(2) Cu(1)–O(3)#1
1.991(2) Cu(1)–N(6)
2.357(2) O(1)–Cu(1)–O(3)#1 168.09(10)
89.18(10) O(3)#1–Cu(1)–N(1) 90.87(10)
82.13(9) N(1)–Cu(1)–N(6)
94.91(9) N(1)–Cu(1)–O(5)
1.950(2)
2.033(2)
Cu(1)–N(1)
Cu(1)–O(5)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(6)
O(1)–Cu(1)–O(5)
160.25(10)
103.13(10)
Complex 2
Electronic Supplementary Information
Cu(1)–N(1)
1.911(3) Cu(1)–N(2)
2.004(3) Cu(1)–O(3)
1.945(3)
2.045(3)
Figures S1, S2, S3 and S4 have been deposited in the ESI
available through stl.publisher.ingentaconnect.com/content/
stl/jcr/supp-data.
Cu(1)–O(1)
Cu(1)–O(5)
2.224(4) N(1)–Cu(1)–N(2)
81.00(13) N(2)–Cu(1)–O(1)
79.39(13) N(2)–Cu(1)–O(3)
169.02(15)
98.78(13)
99.34(13)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–O(3)
Results and discussion
Complex 3
Crystal structure of 1: Complex 1 crystallises in the monocli-
nic system with space group P21/c. As shown in Fig. 1, the
asymmetric unit of 1 contains one 2,5-pydc2− molecule, one
bmi molecule and one water molecule. Each Cu(II) centre
coordinates two N atoms (Cu–N = 1.991 and 2.033 Å) and
three O atoms (Cu–O = 1.948–2.357Å), to give a distorted
square-pyramidal geometry around the metal centre. The
Cu–O and Cu–N bond lengths are comparable to those of other
Cu(II) complexes.21 The tri-coordinated 2,5-pydc2− molecule
binds two copper centres through two monodentate carb-
oxylate oxygen together with nitrogen atoms to form a 1D
chain running along the b-axis. Within the chain, the separa-
tion of Cu···Cu is 7.437 Å. This is different from the complex
reported from the reaction of Cu(II) and bmi under an inorga-
nic anion system. The reaction of Cu(NO3)2 and bmi at room
temperature gives rise to a 2D grid structure in which bmi acts
as a bidentate tecton bridging two Cu(II) centres to form a 2D
grid, whereas the bmi acts as a terminal molecule bonded to
the Cu(II) centre in 1.17
Cu(1)–O(4)
O(4)–Cu(1)–O(4)#1
O(4)#1–Cu(1)–N(1)
1.953(3) Cu(1)–N(1)
1.979(4)
180.000(1) O(4)–Cu(1)–N(1)
90.17(16)
89.83(16) N(1)–Cu(1)–N(1)#1 180.000(1)
O(4)#1–Cu(1)–N(1)#1 90.17(16) C(8)–O(4)–Cu(1)
108.7(3)
Symmetry transformation used to generate equivalent atoms:
#1 –x+1, y–1/2, –z+1/2; #2 –x+1, y+1/2, –z+1/2 for 1; #1 –x+1,
–y+1, –z+1 for 2; #1 –x+1, –y, –z+2 for 3.
(Fig. 2). The dihedral angle and the centroid-to-centroid sepa-
ration between benzene rings of benzotriazole and its neigh-
bouring triazole ring of benzotriazole are 14.3º and 3.934 Å,
respectively (Fig. 3). Moreover, the O···N separations are
3.027 Å for O5···N5′. Based on these weak π···π interactions
and hydrogen bonds, a 3D supramolecular architecture is
achieved.22 Consequently, the presence of the π···π interactions
and hydrogen bonds play an important role in molecular
organisation and target structure formation.
Crystal structure of 2: Single-crystal analysis shows that 2
also crystallises in the monoclinic system with space group
P21/c. Its asymmetric unit contains one copper centre, one
water molecule and half of a btx (Fig. 4). Each Cu(II) centre is
coordinated by two nitrogen atoms from btx and 2,6-pydc2−,
two carboxylate oxygen atoms of 2,6-pydc2− and one water
molecule. Two nitrogen atoms of the btx bridge two copper
In 1, the centroid-to-centroid separation between two near-
est pyridyl rings of two adjacent chains is 3.585 Å and the
pyridyl rings are parallel, which indicates that π···π interac-
tions are present. The C···O distances are 3.269 Å for C2···O2′.
The 1D chains are further stacked to form a 2D supramolecular
layer parallel to the b,c-plane based on these interactions
Table 1 Crystal data and structure refinement for complexes 1, 2, and 3
1
2
3
Formula
C17H14CuN6O5
445.88
Monoclinic
P21/c
15.460(3)
8.0382(16)
13.998(3)
90
97.26(3)
90
C26H22Cu2N8O12
765.60
Monoclinic
P21/c
4.9039(10)
10.302(2)
30.183(6)
90
93.55(3)
90
C38H26CuN10O13.50
902.24
Mr
Crystal system
Space group
a, Å
Triclinic
–
P1
7.6441(15)
11.897(2)
12.448(3)
100.62(3)
105.23(3)
103.45(3)
1025.4(4)
1
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
1725.6(6)
4
1521.9(5)
2
Z
Dcalcd/g cm–3
µ(MoKα)/ mm–1
F(000)/e
1.716
1.671
1.461
1.313
1.474
0.611
908
776
461
θ range/deg
Refl. Measured/
Refl. unique
Rint
Param. refined
R(F)/wR(F2)a ([I ≥ 2σ(I)])
R(F)/wR(F2)a (all reflexions)
GoF (F2)b
2.86–28.00
20639
2.09–27.96
18587
3.17–25.5
10641
4170
3649
3806
0.0470
267
0.0545/0.1047
0.0636/0.1096
1.165
0.0587
222
0.0695/0.1423
0.0903/0.1538
1.166
0.0672
299
0.0866/0.2012
0.1074/0.2174
1.124
a R1 = ΣFo − Fc/Fo. wR2 = [Σw(Fo  − Fc2)2/wFo 2]1/2. w=1/[σ2(Fo)2 + 0.0297P2 + 27.5680P], where P = (Fo2 + 2Fc2)/3. b GoF = [Σw(Fo2
2
2
− Fc2)2/(nobs − nparam)]1/2
.

JOURNAL OF CHEMICAL RESEARCH 2012 415
Fig. 1 The one-dimensional structure of 1, all hydrogen atoms and part labels are omitted for clarity.
Fig. 2 The 2D supramolecular layer by hydrogen bonds and π···π stacking interactions in 1.
Fig. 3 The 3D supramolecular structure, the labels and hydrogen atoms are omitted for clarity.

416 JOURNAL OF CHEMICAL RESEARCH 2012
Fig. 4 The dinuclear copper complex 2, all hydrogen atoms and part labels are omitted for clarity.
centres. The 2,6-pydc2– ligand has diverse binding modes, it
acts as a terminal tridentate ligand via two monodentate
carboxylate oxygen and one pyridyl nitrogen atoms. Thus, the
steric force of the chelating ligand prevents another bridging
ligand further linking the copper centres to form a more
complicated complex.
imidazole nitrogen atoms (Cu–N = 1.979 Å) of bmi, whereas
both H2btc– and bmi offer only one donor atom to the Cu(II)
centre.
Along the a-axis, the distance of two imidazole centroids is
3.354 Å with a dihedral angle of 0º. Also, the C···O interac-
tions with C11···O2′ 3.275 Å, C12···O3′ 3.178 Å, C18···O3′
3.424 Å, the mononuclear units are connected into a 1D supra-
molecular chain.26 Further, along the b-axis, the separation
between the centroid of the triazole ring and the centroid of the
phenyl ring of the benzitriazole group is 3.857 Å with a dihe-
dral angle of 0.39º. Together with the hydrogen bond (O5···N5
2.767 Å), a 2D supramolecular motif is generated parallel to
the a,b-plane. Along the c-axis, the separation between two the
phenyl centroids of H2btc– is 3.995 Å, with a dihedral angle
of 0º, achieves a 3D supramolecular architecture (Fig. S2
deposited in the ESI). The π···π interactions and hydrogen
bonds play crucial role in the formation of the complex.
Catalytic behaviour: An efficient catalyst should be able to
expand its coordination sphere and coordination number. The
principal reason for the ability of copper compounds to act
catalytically is the strong tendency of Cu(II) complexes to
adopt penta- or hexacoordinated geometries. Here, complexes
1, 2 are penta-coordinated and 3 is four-coordinated, and thus
these complexes were tested as catalysts for the oxidative
coupling reaction of 2,6-di-tert-butylphenol (DBP) with H2O2
as oxidant and NaOCH3 as co-catalyst at a suitable tempera-
ture. Taking 1 for example, several factors such as solvent
medium, reaction time, temperature as well as the amount of
oxidant have been investigated in detail by fixing the DBP at
1 mmol (0.206 g) in 2 mL reaction medium. The screened
conditions indicate the optimal temperature, amounts of H2O2,
Cu(II)-based catalyst and the co-catalyst are 45 °C, 0.2 mL,
0.02 mmol and 2 mmol, respectively. Moreover, the yield and
selectivity were markedly dependent on the solvent used, and
acetonitrile was screened as reaction medium, which is in line
with the previously reported research.27 2,6-di-tert-butylphe-
nol (DBP) was oxidised by Cu(II) complexes 1, 2 and 3 with
H2O2 as oxidant, giving rise predominantly to the coupled
product 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (DPQ)
and the by-product 2,6-di-tert-butylbenzoquinone (BQ). At the
optimal conditions, the yield of major product is 95.25% for 1
as catalyst, 93.65% for 2, and 92.04% for 3. Also, the conver-
sion degree of DBP is over 98% with the three catalysts. In
an attempt to oxidise DBP with H2O2 in the absence of the
catalyst or co-catalyst, less oxidation product was obtained
(30% or 64.3%, respectively), which is in accordance with
previous reports.28,29
Based on the C···O interactions of C8···O2′ = 3.286 Å, the
binuclear structure is further connected into a 2D supramo-
lecular architecture parallel to the a,b-plane. Furthermore, the
2D supramolecular layers are joined by the hydrogen bonds
(O5···O4′ = 2.740 Å, O5···O3′ = 2.688 Å, C12···O4′ = 3.488 Å,
C13···O6′ = 3.461 Å), and the C–H···π force between the C11
and the centroid (C8, C9, C10, C8′, C9′, C10′) with the separa-
tion of 3.5 Å, and with the angle C11–H11A···centroid of
123.86º, to form a 3D supramolecular motif (Fig. S1 deposited
in the ESI). Such C–H···π and hydrogen bonds play crucial
roles for the formation of 3D supramolecules. Similar hydro-
gen-bonding and C–H···π interactions are found in reported
papers.23,24 By comparison with other metal centres, such as
d10 ions, the Cu(II) centre has fewer such interactions incorpo-
rated in the complex under the organic anion system. Su and
co-workers reported two interpenetrating and one non-
interpenetrating Cu(II) complexes based on the btx synthon
with polyoxometalate as counter anion, which also indicates
that the anion can greatly affect the final architecture,
especially for polycarboxylate or polyoxometalate as anion.25
Crystal structure of 3: Small changes in the building units
can lead to a completely different structure. The replacement
of 2,5-pydc2– in 1 by H2btc– results in a mononuclear complex
3. As shown in Fig. 5, the fundamental building unit of 3
contains one Cu(II) centre, one H2btc– and one bmi. The four-
coordinated Cu(II) centre with planar coordination geometry
has the C2 configuration, which is completed by two carb-
oxylate oxygens (Cu–O = 1.953 Å) from two H2btc– and two
Compound 1 was tested for its stability in successive exper-
iments. The activity of the catalyst was found to lose only 12%
after the completion of the third run. The addition of base to
the reaction solvent makes an important contribution to the
DBP polymerisation. It has been demonstrated that the oxida-
tion of DBP is slow with the Cu(II) catalyst alone, but was
promoted significantly by adding a base such as NaOCH3
to the reaction mixture. The added base has a vital role in
generating the corresponding phenolate anion by dissociating
the DBP molecules, which was critical in the whole catalytic
process. The results indicate that complexes 1, 2 and 3 have
Fig. 5 The mononuclear copper structure of 3, part hydrogen
atoms and labels are omitted for clarity.

JOURNAL OF CHEMICAL RESEARCH 2012 417
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Conclusions
In this study, we have reported three new Cu(II) complexes
derived from mixed ligands, and shown that such complexes
are further connected to give 3D supramolecule architectures
based on hydrogen bonds and π···π interactions. The result of
the oxidative coupling reaction of 2,6-di-tert-bibutylphenol
(DBP) shows that the Cu(II) complexes, with potential vacant
coordination sites, give a good performance in the catalytic
system.
22 S. Rayati, A. Wojtczak and A. Kozakiewicz, Inorg. Chim. Acta, 2008, 361,
1530.
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We gratefully acknowledge the financial support by the
Talent Supporting Plan of He’nan Scientific and Technological
Innovation (No. 2010HASTIT016), and the He’nan key science
and technology research (Nos. 102102310079, 122102310061
and 112102310084).
Received 21 March 2012; accepted 11 April 2012
Paper 1201224 doi: 10.3184/174751912X13371887324682
Published online: 26 June 2012
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