New metal complexes with 5-(1H-imidazol-4-ylmethyl)aminoisophthalic acid: Syntheses, structures, electrochemistry and electrocatalysis

Article abstract of DOI:10.1016/j.ica.2009.05.048

Three new coordination polymers {[Cu(HL)(H₂O)]·H₂O}_n (1), [Ag(H₂L)]_n (2), and {[Co(HL)(phen)(H₂O)]·8H₂O}_n (3) [H₃L = 5-(1H-imidazol-4-ylmethyl)aminoisophtha

Full text of DOI:10.1016/j.ica.2009.05.048

Inorganica Chimica Acta 362 (2009) 4002–4008
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New metal complexes with 5-(1H-imidazol-4-ylmethyl)aminoisophthalic acid:
Syntheses, structures, electrochemistry and electrocatalysis
*
Jing Xu, Zhi Su, Man-Sheng Chen, Shui-Sheng Chen, Wei-Yin Sun
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of
Microstructures, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 April 2009
Received in revised form 19 May 2009
Accepted 20 May 2009
Available online 30 May 2009
Three new coordination polymers {[Cu(HL)(H2O)]ꢀH2O}n (1), [Ag(H2L)]n (2), and {[Co(HL)(phen)
(H2O)]ꢀ8H2O}n (3) [H3L = 5-(1H-imidazol-4-ylmethyl)aminoisophthalic acid, phen = 1,10-phenanthro-
line] have been synthesized under hydrothermal conditions. The results of X-ray diffraction analysis
revealed that complex 1 displays (3, 3)-connected 2D network with (4, 82) topology, while complexes
2 and 3 have infinite 1D chain structure, in which one of the two carboxylic groups of H2Lꢁ/HL2ꢁ is unco-
ordinated. The 2D layers of 1 or the 1D chains of 2 and 3 are further linked together by hydrogen bonds
Keywords:
Coordination polymers
Copper(II)
Silver(I)
Cobalt(II)
and p–p interactions to form 3D supramolecular frameworks. Moreover, the electrochemical properties
of complexes 1 and 2 have been studied by modified glassy carbon electrodes of 1 (Cu-GCE) and 2 (Ag-
GCE), and the results indicate that the Cu-GCE and Ag-GCE show one-electron redox peaks. In addition,
both Cu-GCE and Ag-GCE have good electrocatalytic activities toward the reduction of H2O2 in phosphate
buffer (pH 5.5) solution.
Electrochemistry
Electrocatalysis
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
compound 5-(1H-imidazol-4-ylmethyl)aminoisophthalic acid
(H3L) and its metal complexes are reported in this paper.
In the recent years, the design and synthesis of metal–organic
frameworks (MOFs) have attracted much attention from chemists
not only for their potential functions and possible applications in
gas storage, nonlinear optics, catalysis, magnetic materials and so
on, but also owing to their intriguing framework architectures and
topologies [1–3]. In particular, aromatic multicarboxylate ligands,
for example 1,3,5-benzenetricarboxylate, 1,3-benzenedicarboxyl-
ate, 1,4-benzenedicarboxylate, 1,2,4,5-benzenetetracarboxylate,
are well used in the construction of MOFs with interesting struc-
tures and special topologies due to their structural rigidity, chemical
stability and appropriate connectivity [4–7]. Meanwhile, the
imidazole-based ligands, such as 1,3,5-tris(imidazol-1-ylmethyl)-
2,4,6-trimethylbenzene, 1,3,5-tris(1-imidazolyl)benzene, 2,4,6-
tris[4-(imidazol-1-ylmethyl)phenyl]-1,3,5-triazine, were designed
and used to investigate the influence of bridging ligand on forma-
tion and structure of supramolecular architectures, and MOFs with
various structures including individual cages, one-dimensional (1D)
tubes, two-dimensional (2D) networks, three-dimensional (3D)
non-interpenetrating and interpenetrating frameworks have been
obtained [8,9]. With such background in mind, we have been focus-
ing our attention on design and synthesis of new carboxylic- and
imidazole-containing ligands to achieve new MOFs. One such
On the other hand, electrochemical reactions catalyzed by tran-
sition metal complexes have received considerable attention dur-
ing the past decades [10], and the electrochemistry and
electrocatalysis of copper complexes with different ligands have
well been studied [11]. Much efforts have been directed toward
the studies of copper complexes of chelating ligands with pyridine,
thioether, imidazole and imine groups, which not only result from
their fascinating structures but also from their potential applica-
tions as new materials [12]. However, only few electrochemical
properties of individual silver complexes have been reported
[13], although there are many reports on that of heterobimetallic
M–Ag complexes based on bidentate phosphine, 4,40-bipyidine,
Keggin and Wells–Dawson polyoxometalate clusters [14]. In this
paper, we report three new compounds {[Cu(HL)(H2O)]ꢀH2O}n (1),
[Ag(H2L)]n (2), and {[Co(HL)(phen)(H2O)]ꢀ8H2O}n (3) (phen = 1,10-
phenanthroline) with above mentioned H3L ligand, and their elec-
trochemical and electrocatalytic properties of the complexes have
been examined.
2. Experimental
2.1. Materials and measurements
All commercially available solvents and starting materials are
of reagent grade and were used as received without further
* Corresponding author. Tel.: +86 25 83593485; fax: +86 25 83314502.
E-mail address: sunwy@nju.edu.cn (W.-Y. Sun).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.05.048

J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008
4003
purification. Ligand H3L was readily prepared by reaction of imid-
tion (k = 0.71073 Å) using
x
ꢁ u scan technique. The data integra-
azole-4-carboxaldehyde with 5-aminoisophthalic acid followed by
reduction of sodium borohydride as reported for the synthesis of 4-
(1H-imidazol-4-yl)methylaminobenzoic acid [15]. FT-IR spectra
were recorded on a Bruker Vector22 FT-IR spectrophotometer
using KBr disks. Elemental analyses were taken on a Perkin–Elmer
240C elemental analyzer. Thermogravimetric analyses (TGA) were
performed on a TGA V5.1A Dupont 2100 instrument heating from
room temperature to 700 °C under N2 with a heating rate of 20 °C/
min. Powder X-ray diffraction (PXRD) measurements were per-
formed on a Bruker D8 Advance X-ray diffractometer using Cu
tion and empirical absorption corrections were carried out by SAINT
programs [16]. The structures were solved by direct methods using
SHELXS 97 [17a]. All the non-hydrogen atoms except those of disor-
dered solvent water molecules in 3 were refined anisotropically on
F2 by full-matrix least-squares methods [17b], and the oxygen
atoms of the uncoordinated water molecules in 3 were refined iso-
tropically. The hydrogen atoms except those of water molecules
were generated geometrically and refined isotropically using the
riding model. The hydrogen atoms of water molecules in 1 and
the coordinated one in 3 were located from Fourier map directly.
Details of the crystal parameters, data collection and refinements
for 1–3 are summarized in Table 1. Selected bond lengths and an-
gles for 1–3 are listed in Table 2.
Ka radiation (1.5418 Å), and the X-ray tube was operated at
35 kV and 20 mA. The data was collected in the 2h range of 5.00–
60.00° with a step size of 0.02°. All electrochemical measurements
were made on CHI 660A electrochemistry workstation.
Table 1
Crystallographic data for 1–3.
2.2. Synthesis of the complexes
Complex
1
2
3
2.2.1. Synthesis of complex {[Cu(HL)(H2O)]ꢀH2O}n (1)
Formula
Formula weight
Crystal system
space group
a (Å)
C12H13N3O6Cu
358.79
Monoclinic
P21/n
11.3869(1)
8.7250(7)
14.4431(13)
107.431(1)
1369.0(2)
4
C12H10N3O4Ag
368.10
Monoclinic
P21/n
8.7445(11)
13.6389(18)
10.4978(13)
102.345(2)
1223.1(3)
4
C24H35N5O13Co
660.37
Monoclinic
P21/n
12.067(5)
16.172(7)
16.358(6)
109.579(6)
3008(2)
4
A mixture of H3L (0.026 g, 0.10 mmol), Cu(NO3)2ꢀ6H2O (0.029 g,
0.10 mmol), NaOH (0.008 g, 0.20 mmol) and H2O (10 mL) was kept
in a 15 mL Teflon liner autoclave at 100 °C for 3 days. After the
mixture was cooled to room temperature, green platelet crystals
of complex 1 were collected with a yield of 34%. Anal. Calc. for
C12H13CuN3O6 (1): C, 40.17%; H, 3.65%; N, 11.71%. Found: C,
40.20%; H, 3.67%; N, 11.69%. IR (KBr pellet, cmꢁ1): 3285 (s), 3131
(ms), 1640 (ms), 1616 (ms), 1556 (s), 1502 (ms), 1475 (s), 1417
(s), 1349 (s), 1282 (ms), 786 (s), 722 (s), 620 (m).
b (Å)
c (Å)
b (°)
Volume (Å3)
Z
T (K)
293(2)
16.30
1.741
732
293(2)
16.66
1.999
728
293(2)
6.41
1.423
1316
l
(Mo K
a
) (cmꢁ1
)
Dcalcd (g/m3)
2.2.2. Synthesis of complex [Ag(H2L)]n (2)
F(0 0 0)
Reflections collected
A
mixture of H3L (0.026 g, 0.10 mmol), AgNO3 (0.017 g,
7163
6084
14586
0.10 mmol), NaOH (0.008 g, 0.20 mmol) and H2O (10 mL) was kept
in a 15 mL Teflon liner autoclave at 100 °C for 3 days. After the
mixture was cooled to room temperature, yellow block crystals
of complex 2 were collected with a yield of 42%. Anal. Calc. for
C12H10AgN3O4 (2): C, 39.24%; H, 2.75%; N, 11.45%. Found: C,
39.20%; H, 2.71%; N, 11.52%. IR (KBr pellet, cmꢁ1): 3386 (ms),
3120 (ms), 1686 (s), 1599 (s), 1559 (s), 1421 (ms), 1399 (s), 1386
(s), 1363 (s), 1333 (s), 1319 (s), 1296 (ms), 1261 (ms), 1087 (s),
787 (s), 720 (s), 623 (m).
Unique reflections
Goof
2677
1.006
0.0507
0.0372
2218
1.014
0.0401
0.0341
5287
1.023
0.0744
0.0948
0.3103
Rint
R1[I > 2
wR2[I > 2
r
(I)]a
(I)]b
r
0.0653
0.0770
a
R1
wR2 = |
=
R
||Fo| ꢁ |Fc||/
R|Fo|.
b
R
w(|Fo|2 ꢁ |Fc|2)|/
R .
|w(Fo)2|1/2
Table 2
Selected bond lengths [Å] and angles [°] for 1–3.
2.2.3. Synthesis of complex {[Co(HL)(phen)(H2O)]ꢀ8H2O}n (3)
A mixture of H3L (0.026 g, 0.10 mmol), Co(NO3)2ꢀ6H2O (0.029 g,
0.10 mmol), phen (0.020 g, 0.10 mmol), NaOH (0.008 g,
0.20 mmol), and H2O (10 mL) was kept in a 15 mL Teflon liner
autoclave at 100 °C for 3 days. After the mixture was cooled to
room temperature, red block crystals of complex 3 were collected
with a yield of 53%. Anal. Calc. for C24H35CoN5O13 (3): C, 43.63%; H,
5.34%; N, 10.61%. Found: C, 43.69%; H, 5.36%; N, 10.59%. IR (KBr
pellet, cmꢁ1): 3356 (m), 3286 (m), 1610 (s), 1561 (s), 1518 (s),
1424 (s), 1400 (s), 1355 (s), 1237 (s), 1142 (s), 1105 (s), 1081 (s),
850 (ms), 785 (s), 727 (m), 624 (m).
Complex 1
Cu(1)–O(1)#1
Cu(1)–O(4)#2
Cu(1)–N(1)
O(1)#1–Cu(1)–N(11)
N(11)–Cu(1)–O(4)#2
N(11)–Cu(1)–O(5)
O(1)#1–Cu(1)–N(1)
O(4)#2–Cu(1)–N(1)
1.917(2)
2.053(2)
2.241(2)
171.25(10)
95.73(10)
89.95(9)
90.89(9)
147.86(9)
Cu(1)–N(11)
Cu(1)–O(5)
1.937(2)
2.191(2)
O(1)#1–Cu(1)–O(4)#2
O(1)#1–Cu(1)–O(5)
O(4)#2–Cu(1)–O(5)
N(11)–Cu(1)–N(1)
O(5)–Cu(1)–N(1)
91.11(9)
92.04(8)
115.28(8)
80.42(10)
96.69(8)
Complex 2
Ag(1)–N(11)#1
N(11)#1–Ag(1)–O(1)
2.094(3)
178.88(11)
Ag(1)–O(1)
2.115(2)
Complex 3
Co(1)–O(2)#1
Co(1)–N(3)
Co(1)–N(2)
2.2.4. Complexes 1–3 modified glassy carbon (GC) electrode
The GC electrode was first carefully polished with alumina on
polishing paper and washed successively with double distilled
2.039(4)
Co(1)–N(11)
2.049(4)
2.109(4)
Co(1)–O(5)
2.136(4)
2.161(4)
Co(1)–N(1)
2.314(4)
water and acetone in an ultrasonic bath. About 25 lL of complex
O(2)#1–Co(1)–N(11)
N(11)–Co(1)–N(3)
N(11)–Co(1)–O(5)
O(2)#1–Co(1)–N(2)
N(3)–Co(1)–N(2)
O(2)#1–Co(1)–N(1)
N(3)–Co(1)–N(1)
N(2)–Co(1)–N(1)
96.67(17)
166.48(16)
96.37(17)
92.95(16)
78.10(16)
84.14(14)
92.30(15)
91.96(14)
O(2)#1–Co(1)–N(3)
O(2)#1–Co(1)–O(5)
N(3)–Co(1)–O(5)
N(11)–Co(1)–N(2)
O(5)–Co(1)–N(2)
N(11)–Co(1)–N(1)
O(5)–Co(1)–N(1)
92.35(16)
90.83(14)
93.53(16)
92.02(17)
93.93(14)
78.69(17)
172.45(13)
1 suspension (0.25 mg/mL) in acetone was cast on the surface of
GC electrode and dried in air to form a complex 1 modified elec-
trode (Cu-GCE). The complex 2 modified electrode (Ag-GCE) as
well as 3 (Co-GCE) was prepared in the same way.
2.3. X-ray crystallography
Symmetry codes for 1: #1: ꢁx, ꢁy + 1, ꢁz, #2: ꢁx + 1/2, y + 1/2, ꢁz + 1/2. Symmetry
codes for 2: #1: ꢁx + 3/2, y + 1/2, ꢁz + 1/2. Symmetry codes for 3: #1: x ꢁ 1/2,
ꢁy + 1/2, z ꢁ 1/2.
Crystallographic data of 1–3 were collected at 293 K on a Bruker
SMART CCD system equipped with monochromated Mo Ka radia-

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J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008
the amine-nitrogen chelate one Cu(II) atom (Scheme 1a). The imid-
3. Results and discussions
3.1. Description of the crystal structures
azole ring of the flexible arm is nearly perpendicular to the central
benzene ring since the dihedral angel between them is 89.02°.
The coordination mode of each metal atom coordinated by
three HL2ꢁ ligands and each HL2ꢁ ligand also connecting three
Cu(II) atoms as described above makes complex 1 a neutral 2D net-
work as illustrated in Fig. 1b. Both Cu(II) atom and HL2ꢁ ligand can
be considered as three-connected nodes in a ratio of 1:1 in 1.
Therefore, the complex 1 exhibits a (3, 3)-connected 2D network
with a (4, 82) topology which can be clearly seen from the simpli-
fied representation shown in Fig. 1c. Furthermore, the 2D networks
are linked together through the C–Hꢀ ꢀ ꢀO, O–Hꢀ ꢀ ꢀO and N–Hꢀ ꢀ ꢀO
hydrogen bonding interactions to result in formation of 3D frame-
work (Fig. S1). The hydrogen bonding data of complexes 1–3 are
3.1.1. Complex {[Cu(HL)(H2O)]ꢀH2O}n (1)
The title complex was synthesized by assembly reaction of H3L
ligand with Cu(NO3)2ꢀ6H2O using NaOH to neutralize the carbox-
ylic acid, and the complete deprotonation of the H3L to give HL2ꢁ
ligand was confirmed by the IR spectral data of complex 1 since
no IR bands in the range of 1760–1680 cmꢁ1 were observed in
the IR spectrum of 1 (see Section 2). The results of X-ray crystallo-
graphic analysis provided the direct evidence about the structure
of the complex and revealed that complex 1 crystallizes in mono-
clinic space group P21/n (Table 1). The asymmetric unit of 1 con-
sists of one Cu(II) atom, one HL2ꢁ ligand, one coordinated and
one non-coordinated water molecule. As displayed in Fig. 1a, each
Cu(II) atom with distorted square pyramid coordination geometry
is five-coordinated with a N2O3 donor set by two nitrogen atoms
from one HL2ꢁ ligand in a chelating mode, two oxygen atoms from
two carboxylate groups of two different HL2ꢁ ligands and one oxy-
gen atom from a coordinated water molecule. The Cu–O and Cu–N
bond lengths are in the range of 1.917(2)–2.191(2) Å and 1.937(2)–
2.241(2) Å, respectively, as listed in Table 2. A distance of 2.57 Å
between Cu1 and O3 indicates the existence of weak interaction be-
tween them. On the other hand, each HL2ꢁ ligand in 1 connects
three Cu(II) atoms through the Cu–O and Cu–N coordination inter-
actions. The coordination modes of the HL2ꢁ and H2Lꢁ ligands ap-
peared in complexes 1–3 are summarized in Scheme 1. Each
carboxylate group of the HL2ꢁ ligand in 1 coordinates with one me-
tal atom in monodentate fashion, and one imidazole- together with
summarized in Table S1. In addition, there are face-to-face
p–p
interactions between the imidazole ring planes with a centroid–
centroid separation of 3.61 Å which further consolidate the 3D
structure.
3.1.2. Complex [Ag(H2L)]n (2)
To investigate the impact of metal center with different coordi-
nation geometry on the structure of the assembly reaction product,
AgNO3 instead of Cu(NO3)2ꢀ6H2O, was used to react with H3L under
the same reaction conditions, it is interesting to find that the
deprotonation only occurred in one of the two carboxylic groups
of the H3L ligand and H2Lꢁ was generated to give complex 2. The
existence of non-deprotonated carboxylic group in 2 was con-
firmed by the observation of IR band at 1686 cmꢁ1 in the IR spec-
trum of 2 (see Section 2). The complex 2 crystallizes in the
monoclinic P21/n space group which is the same as that of 1 (Table
Fig. 1. (a) Coordination environment of Cu(II) in 1 with the ellipsoids drawn at the 30% probability level, uncoordinated water molecule and the hydrogen atoms were
omitted for clarity. (b) 2D layer structure of complex 1. (c) (4, 82) topological network in 1.

J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008
4005
1). The asymmetric unit of 2 contains one Ag(I) atom and one H2Lꢁ
ligand as depicted in Fig. 2a. The Ag(I) atom has slight distorted lin-
ear coordination geometry and each one is two-coordinated by one
oxygen and one nitrogen atom from two different H2Lꢁ ligands.
M
O
O
M
M
O
OH
M
O
O
O
O
O
O
O
O
NH
N
HN
N
NH
N
M
M
M
NH
NH
NH
(a)
(b)
(c)
Scheme 1. Coordination modes of HL2ꢁ and H2Lꢁ ligands in complexes 1–3.
Fig. 2. (a) Coordination environment of Ag(I) in 2 with the ellipsoids drawn at the 30% probability level, hydrogen atoms were omitted for clarity. (b) 1D infinite chain
structure of 2. (c) 2D structure of 2 linked by hydrogen bonds between the adjacent chains. (d) 3D structure of 2 linked by hydrogen bonds indicated by dashed line.

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J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008
The Ag–N distance of 2.094(3) Å and Ag–O one of 2.115(2) Å are
consistent with the reported values in the Ag-carboxylate and
Ag-imidazole complexes [18]. The deprotonated carboxylate group
in 2 coordinates with one Ag(I) in monodentate mode which is the
same as that in 1, and the imidazole-nitrogen coordinates with one
metal atom solely and the amine nitrogen does not take part in the
coordination which is different from that in 1 (Scheme 1b). Such
connections lead to the formation of a neutral 1D zigzag chain con-
each Co(II) center with distorted octahedral coordination geometry
is six-coordinated by one carboxylate oxygen atom from one HL2ꢁ
ligand with Co–O bond distance of 2.039(4) Å, two nitrogen atoms
from another HL2ꢁ ligand with Co1–N bond lengths of 2.314(4) and
2.049(4) Å, two nitrogen atoms from phen with Co1–N bond
lengths of 2.109(4) and 2.161(4) Å and one oxygen atom from a
coordinated water molecule (Table 2). On the other hand, one car-
boxylate group of the HL2ꢁ ligand adopts monodentate coordina-
tion mode to connect one Co(II) atom and the imidazole group
coordinates to the Co(II) atom with the amine nitrogen via a
chelating mode (Scheme 1c). Such a coordination mode makes
complex 3 an infinite 1D chain structure (Fig. 3b). There are face-
structed by the l
2-connection of the H2Lꢁ ligands and Ag(I) atoms
(Fig. 2b). The adjacent 1D chains are further connected into 2D
network through O(4)–H(1A)ꢀ ꢀ ꢀO(2) hydrogen bond with
O(4)ꢀ ꢀ ꢀO(2) distance of 2.582(3) Å and N(12)–H(12A)ꢀ ꢀ ꢀO(2) one
with N(12)ꢀ ꢀ ꢀO(2) distance of 2.799(4) Å (Fig. 2c). The 2D networks
are further joined together through N(1)–H(1A)ꢀ ꢀ ꢀO(3) hydrogen
bond to form a 3D structure (Fig. 2d, Table S1).
to-face
p–p interactions between the aromatic groups within the
chains since the benzene ring of HL2ꢁ and phen planes with dihe-
dral angle of 14.8° have a centroid–centroid distance of 3.64 Å,
furthermore, there are also
p–p interactions between the phen
3.1.3. Complex {[Co(HL)(phen)(H2O)]ꢀ8H2O}n (3)
planes from adjacent chains with dihedral angle of 0.03° and a cen-
troid–centroid distance of 3.70 Å as shown in Fig. 3c, in which the
The introduced aromatic co-ligand phen may increase the pos-
sibility of
p–p
interactions between the aromatic groups in the
p–p
interactions are indicated by dashed lines. It is noteworthy
resultant complex. The crystal structure of 3 is shown in Fig. 3a,
that there are abundant water molecules between the 1D chains
Fig. 3. (a) Coordination environment of Co(II) in 3 with the ellipsoids drawn at the 30% probability level, hydrogen atoms and solvent water molecules were omitted for
clarity. (b) Polyhedral representation of 1D infinite chain structure of 3. (c) Face-to-face interactions between the 1D chains of 3.
p–p

J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008
4007
the cathodic currents increase while the corresponding anodic cur-
rents decrease. The inset of Fig. 5a shows that catalytic current was
found to be close to linear with H2O2 concentration up to 12 mM.
The phenomenon of electroreduction of H2O2 at the surface of
Cu-GCE is similar to that of Ag-GCE (Fig. 5b).
20
10
Cu-GCE
Ag-GCE
0
3.4. Thermal analyses
-10
-20
-30
-40
To study the thermal stabilities of the coordination polymers,
thermal gravimetric analyses (TGA) of complexes 1–3 were per-
formed by heating to 700 °C under flowing N2. As shown in
Fig. S6, the TGA curve of 1 exhibits two weight loss steps in the
temperature range 25–700 °C. The first weight loss of 10.30% in
the temperature range 70–110 °C corresponds to the loss of two
water molecules (10.03% calculated). The second stage, which oc-
curs from 265 to 550 °C, is attributed to the elimination of the li-
gand, and the total weight loss of 78.75% indicates the
completely decomposition of the residue to give CuO (calculated:
77.83%). The decomposition of 2 begins at ca. 250 °C, and weight
loss occurs in a consecutive step and does not stop until heating
to 700 °C. Complex 3 lost eight lattice water molecules in the tem-
perature range of 25–100 °C with a weight loss of 22.50% (calcu-
lated 21.80%), and continue to lost one coordinated water
molecules in the temperature range of 110–140 °C with a weight
1.0
0.5
0.0
E / V (vs.SCE)
-0.5
-1.0
Fig. 4. Cyclic voltammograms of Ag-GCE and Cu-GCE in pH 5.5 phosphates buffer
solution in the potential range of 1.0 to ꢁ1.0 V. Scan rate: 100 mV/s.
(Fig. S2), and the 1D chains are linked together by strong C–Hꢀ ꢀ ꢀO,
N–Hꢀ ꢀ ꢀO hydrogen bonds to generate a 3D structure (Fig. S3).
3.2. Electrochemical properties
Compounds 1–3 are insoluble in water and common organic
solvents. Thus, the modified glassy carbon electrode (GCE) be-
comes the optimal choice to study their electrochemical property.
The electrochemical studies of Cu-GCE, Ag-GCE and Co-GCE were
carried out in aqueous phosphate buffer solution (pH 5.5). Fig. 4
shows the cyclic voltammograms of 1 and 2 in the potential range
of +1.0 to ꢁ1.0 V. At the modified Cu-GCE, a redox couple attrib-
uted to the Cu(II)/Cu(I) was observed [19,20], it showed a quasi-
reversible behavior in an aqueous medium [21]. The mean peak
potential E1/2 = (Epa + Epc)/2 was ꢁ33 mV vs. SCE for the Cu-GCE.
In fact, there is a sharp oxidation peak due to Cu(I) ꢁ eꢁ ? Cu(II)
while the reduction peak is very broad. This is most possibly due
to the instability of the reduced form of the complex with different
coordination geometry which can undergo a fast chemical oxida-
tion in an aqueous solution of pH 5.5 [19]. While at the modified
Ag-GCE, there exist a redox couple attributed to the Ag(I)/Ag
[22], and the cyclic voltammogram exhibited one oxidation and
one reduction peak, the mean peak potential E1/2 = (Epa + Epc)/2
was ꢁ120 mV vs. SCE for the Ag–GCE. However, at the modified
Co–GCE, there has been very weak electrochemical response in
aqueous phosphate buffer solution (pH 5.5) (Fig. S4).
350
-20
300
-25
-30
g
a
250
200
150
100
50
-35
-40
-45
-50
0
2
4
6
8
10
12
C (mM)
0
-50
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
E / V (vs.SCE)
(a)
200
180
160
140
120
100
80
Scan rates’ effect on the electrochemical behavior of the Ag-GCE
was investigated in the potential range of +1.0 to ꢁ1.0 V in pH 5.5
phosphates buffer aqueous solution, as shown in the Fig. S5a.
When the scan rate was varied from 50 to 450 mV/s, the peak
potentials change gradually: the cathodic peak potentials shifted
to negative direction, the peak-to-peak separation between the
corresponding cathodic and anodic peaks increased. The same phe-
nomenon was found for Cu-GCE (Fig. S5b).
f
a
60
40
20
3.3. Electrocatalytic reduction toward hydrogen peroxide
0
Hydrogen peroxide (H2O2) is one of the undesired by-products
of metabolite of dioxygen in biological system and the improve-
ment of its electroactivity is of general interest for applications
such as biosensors and fuel cells [23]. It is well known that direct
electroreduction of H2O2 requires a large overpotential. As shown
in Fig. 5a, the catalytic reduction of H2O2 by the Ag-GCE can be
seen clearly in the range of +0.50 to ꢁ1.0 V in pH 5.5 phosphates
buffer aqueous solution containing H2O2. With addition of H2O2,
-20
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
E / V (vs.SCE)
(b)
Fig. 5. Cyclic voltammograms of (a) Ag-GCE and (b) Cu-GCE in pH 5.5 phosphates
buffer solution containing H2O2 concentrations (from bottom to top) of 0.0, 0.9, 1.8,
3.6, 7.2, 10.8, 11.7 mM. Scan rate: 100 mV/s. Inset in (a) the variation of cathodic
peak current vs. H2O2 concentrations.

4008
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was observed at ca. 305 °C and does not stop until heating to
700 °C.
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4. Conclusion
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New Cu(II), Ag(I) and Co(II) coordination polymers have been
synthesized under hydrothermal conditions by reactions of the
corresponding metal salt with a imidazol- and carboxylic-contain-
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data showed that complexes 1–3 have high thermal stability and
the electrochemical studies of 1 and 2 indicated that the Cu-GCE
and Ag-GCE show good electrocatalytic activities toward the
reduction of H2O2 and may be applied as electrochemical sensors.
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Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 20731004 and 20721002)
and the National Basic Research Program of China (Grant No.
2007CB925103).
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crystallographic data for complexes 1, 2, and 3, respectively. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
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