Inorganica Chimica Acta 362 (2009) 4002–4008

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Inorganica Chimica Acta

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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 ofMicrostructures, 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 2009Received in revised form 19 May 2009Accepted 20 May 2009Available online 30 May 2009

Keywords:Coordination polymersCopper(II)Silver(I)Cobalt(II)ElectrochemistryElectrocatalysis

0020-1693/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.ica.2009.05.048

* Corresponding author. Tel.: +86 25 83593485; faxE-mail address: sunwy@nju.edu.cn (W.-Y. Sun).

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 analysisrevealed that complex 1 displays (3, 3)-connected 2D network with (4, 82) topology, while complexes2 and 3 have infinite 1D chain structure, in which one of the two carboxylic groups of H2L�/HL2� is unco-ordinated. The 2D layers of 1 or the 1D chains of 2 and 3 are further linked together by hydrogen bondsand p–p interactions to form 3D supramolecular frameworks. Moreover, the electrochemical propertiesof 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 phosphatebuffer (pH 5.5) solution.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

In the recent years, the design and synthesis of metal–organicframeworks (MOFs) have attracted much attention from chemistsnot only for their potential functions and possible applications ingas storage, nonlinear optics, catalysis, magnetic materials and soon, but also owing to their intriguing framework architectures andtopologies [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, chemicalstability and appropriate connectivity [4–7]. Meanwhile, theimidazole-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 designedand used to investigate the influence of bridging ligand on forma-tion and structure of supramolecular architectures, and MOFs withvarious structures including individual cages, one-dimensional (1D)tubes, two-dimensional (2D) networks, three-dimensional (3D)non-interpenetrating and interpenetrating frameworks have beenobtained [8,9]. With such background in mind, we have been focus-ing our attention on design and synthesis of new carboxylic- andimidazole-containing ligands to achieve new MOFs. One such

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compound 5-(1H-imidazol-4-ylmethyl)aminoisophthalic acid(H3L) and its metal complexes are reported in this paper.

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 andelectrocatalysis of copper complexes with different ligands havewell been studied [11]. Much efforts have been directed towardthe studies of copper complexes of chelating ligands with pyridine,thioether, imidazole and imine groups, which not only result fromtheir fascinating structures but also from their potential applica-tions as new materials [12]. However, only few electrochemicalproperties of individual silver complexes have been reported[13], although there are many reports on that of heterobimetallicM–Ag complexes based on bidentate phosphine, 4,40-bipyidine,Keggin and Wells–Dawson polyoxometalate clusters [14]. In thispaper, 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 havebeen examined.

2. Experimental

2.1. Materials and measurements

All commercially available solvents and starting materials areof reagent grade and were used as received without further

Table 1Crystallographic data for 1–3.

Complex 1 2 3

Formula C12H13N3O6Cu C12H10N3O4Ag C24H35N5O13CoFormula weight 358.79 368.10 660.37Crystal system Monoclinic Monoclinic Monoclinicspace group P21/n P21/n P21/na (Å) 11.3869(1) 8.7445(11) 12.067(5)b (Å) 8.7250(7) 13.6389(18) 16.172(7)c (Å) 14.4431(13) 10.4978(13) 16.358(6)b (�) 107.431(1) 102.345(2) 109.579(6)Volume (Å3) 1369.0(2) 1223.1(3) 3008(2)Z 4 4 4T (K) 293(2) 293(2) 293(2)l (Mo Ka) (cm�1) 16.30 16.66 6.41Dcalcd (g/m3) 1.741 1.999 1.423F(0 0 0) 732 728 1316Reflections collected 7163 6084 14586Unique reflections 2677 2218 5287Goof 1.006 1.014 1.023Rint 0.0507 0.0401 0.0744R1[I > 2r(I)]a 0.0372 0.0341 0.0948wR2[I > 2r(I)]b 0.0653 0.0770 0.3103

a R1 = R||Fo| � |Fc||/R|Fo|.b wR2 = |Rw(|Fo|2 � |Fc|2)|/R|w(Fo)2|1/2.

Table 2Selected bond lengths [Å] and angles [�] for 1–3.

Complex 1Cu(1)–O(1)#1 1.917(2) Cu(1)–N(11) 1.937(2)Cu(1)–O(4)#2 2.053(2) Cu(1)–O(5) 2.191(2)Cu(1)–N(1) 2.241(2)O(1)#1–Cu(1)–N(11) 171.25(10) O(1)#1–Cu(1)–O(4)#2 91.11(9)N(11)–Cu(1)–O(4)#2 95.73(10) O(1)#1–Cu(1)–O(5) 92.04(8)N(11)–Cu(1)–O(5) 89.95(9) O(4)#2–Cu(1)–O(5) 115.28(8)O(1)#1–Cu(1)–N(1) 90.89(9) N(11)–Cu(1)–N(1) 80.42(10)O(4)#2–Cu(1)–N(1) 147.86(9) O(5)–Cu(1)–N(1) 96.69(8)

Complex 2Ag(1)–N(11)#1 2.094(3) Ag(1)–O(1) 2.115(2)N(11)#1–Ag(1)–O(1) 178.88(11)

Complex 3Co(1)–O(2)#1 2.039(4) Co(1)–N(11) 2.049(4)Co(1)–N(3) 2.109(4) Co(1)–O(5) 2.136(4)Co(1)–N(2) 2.161(4) Co(1)–N(1) 2.314(4)O(2)#1–Co(1)–N(11) 96.67(17) O(2)#1–Co(1)–N(3) 92.35(16)N(11)–Co(1)–N(3) 166.48(16) O(2)#1–Co(1)–O(5) 90.83(14)N(11)–Co(1)–O(5) 96.37(17) N(3)–Co(1)–O(5) 93.53(16)O(2)#1–Co(1)–N(2) 92.95(16) N(11)–Co(1)–N(2) 92.02(17)N(3)–Co(1)–N(2) 78.10(16) O(5)–Co(1)–N(2) 93.93(14)O(2)#1–Co(1)–N(1) 84.14(14) N(11)–Co(1)–N(1) 78.69(17)N(3)–Co(1)–N(1) 92.30(15) O(5)–Co(1)–N(1) 172.45(13)N(2)–Co(1)–N(1) 91.96(14)

Symmetry codes for 1: #1: �x, �y + 1, �z, #2: �x + 1/2, y + 1/2, �z + 1/2. Symmetrycodes 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.

J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008 4003

purification. Ligand H3L was readily prepared by reaction of imid-azole-4-carboxaldehyde with 5-aminoisophthalic acid followed byreduction of sodium borohydride as reported for the synthesis of 4-(1H-imidazol-4-yl)methylaminobenzoic acid [15]. FT-IR spectrawere recorded on a Bruker Vector22 FT-IR spectrophotometerusing KBr disks. Elemental analyses were taken on a Perkin–Elmer240C elemental analyzer. Thermogravimetric analyses (TGA) wereperformed on a TGA V5.1A Dupont 2100 instrument heating fromroom 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 CuKa radiation (1.5418 Å), and the X-ray tube was operated at35 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 measurementswere made on CHI 660A electrochemistry workstation.

2.2. Synthesis of the complexes

2.2.1. Synthesis of complex {[Cu(HL)(H2O)]�H2O}n (1)A mixture of H3L (0.026 g, 0.10 mmol), Cu(NO3)2�6H2O (0.029 g,

0.10 mmol), NaOH (0.008 g, 0.20 mmol) and H2O (10 mL) was keptin a 15 mL Teflon liner autoclave at 100 �C for 3 days. After themixture was cooled to room temperature, green platelet crystalsof complex 1 were collected with a yield of 34%. Anal. Calc. forC12H13CuN3O6 (1): C, 40.17%; H, 3.65%; N, 11.71%. Found: C,40.20%; H, 3.67%; N, 11.69%. IR (KBr pellet, cm�1): 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).

2.2.2. Synthesis of complex [Ag(H2L)]n (2)A mixture of H3L (0.026 g, 0.10 mmol), AgNO3 (0.017 g,

0.10 mmol), NaOH (0.008 g, 0.20 mmol) and H2O (10 mL) was keptin a 15 mL Teflon liner autoclave at 100 �C for 3 days. After themixture was cooled to room temperature, yellow block crystalsof complex 2 were collected with a yield of 42%. Anal. Calc. forC12H10AgN3O4 (2): C, 39.24%; H, 2.75%; N, 11.45%. Found: C,39.20%; H, 2.71%; N, 11.52%. IR (KBr pellet, cm�1): 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).

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)2�6H2O (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 linerautoclave at 100 �C for 3 days. After the mixture was cooled toroom temperature, red block crystals of complex 3 were collectedwith 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 (KBrpellet, cm�1): 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).

2.2.4. Complexes 1–3 modified glassy carbon (GC) electrodeThe GC electrode was first carefully polished with alumina on

polishing paper and washed successively with double distilledwater and acetone in an ultrasonic bath. About 25 lL of complex1 suspension (0.25 mg/mL) in acetone was cast on the surface ofGC electrode and dried in air to form a complex 1 modified elec-trode (Cu-GCE). The complex 2 modified electrode (Ag-GCE) aswell as 3 (Co-GCE) was prepared in the same way.

2.3. X-ray crystallography

Crystallographic data of 1–3 were collected at 293 K on a BrukerSMART CCD system equipped with monochromated Mo Ka radia-

tion (k = 0.71073 Å) using x� u scan technique. The data integra-tion and empirical absorption corrections were carried out by SAINT

programs [16]. The structures were solved by direct methods usingSHELXS 97 [17a]. All the non-hydrogen atoms except those of disor-dered solvent water molecules in 3 were refined anisotropically onF2 by full-matrix least-squares methods [17b], and the oxygenatoms of the uncoordinated water molecules in 3 were refined iso-tropically. The hydrogen atoms except those of water moleculeswere generated geometrically and refined isotropically using theriding model. The hydrogen atoms of water molecules in 1 andthe coordinated one in 3 were located from Fourier map directly.Details of the crystal parameters, data collection and refinementsfor 1–3 are summarized in Table 1. Selected bond lengths and an-gles for 1–3 are listed in Table 2.

4004 J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008

3. Results and discussions

3.1. Description of the crystal structures

3.1.1. Complex {[Cu(HL)(H2O)]�H2O}n (1)The title complex was synthesized by assembly reaction of H3L

ligand with Cu(NO3)2�6H2O using NaOH to neutralize the carbox-ylic acid, and the complete deprotonation of the H3L to give HL2�

ligand was confirmed by the IR spectral data of complex 1 sinceno IR bands in the range of 1760–1680 cm�1 were observed inthe IR spectrum of 1 (see Section 2). The results of X-ray crystallo-graphic analysis provided the direct evidence about the structureof 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 HL2� ligand, one coordinated andone non-coordinated water molecule. As displayed in Fig. 1a, eachCu(II) atom with distorted square pyramid coordination geometryis five-coordinated with a N2O3 donor set by two nitrogen atomsfrom one HL2� ligand in a chelating mode, two oxygen atoms fromtwo carboxylate groups of two different HL2� ligands and one oxy-gen atom from a coordinated water molecule. The Cu–O and Cu–Nbond 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 Åbetween Cu1 and O3 indicates the existence of weak interaction be-tween them. On the other hand, each HL2� ligand in 1 connectsthree Cu(II) atoms through the Cu–O and Cu–N coordination inter-actions. The coordination modes of the HL2� and H2L� ligands ap-peared in complexes 1–3 are summarized in Scheme 1. Eachcarboxylate group of the HL2� ligand in 1 coordinates with one me-tal atom in monodentate fashion, and one imidazole- together with

Fig. 1. (a) Coordination environment of Cu(II) in 1 with the ellipsoids drawn at the 30omitted for clarity. (b) 2D layer structure of complex 1. (c) (4, 82) topological network i

the amine-nitrogen chelate one Cu(II) atom (Scheme 1a). The imid-azole ring of the flexible arm is nearly perpendicular to the centralbenzene ring since the dihedral angel between them is 89.02�.

The coordination mode of each metal atom coordinated bythree HL2� ligands and each HL2� ligand also connecting threeCu(II) atoms as described above makes complex 1 a neutral 2D net-work as illustrated in Fig. 1b. Both Cu(II) atom and HL2� ligand canbe considered as three-connected nodes in a ratio of 1:1 in 1.Therefore, the complex 1 exhibits a (3, 3)-connected 2D networkwith a (4, 82) topology which can be clearly seen from the simpli-fied representation shown in Fig. 1c. Furthermore, the 2D networksare linked together through the C–H� � �O, O–H� � �O and N–H� � �Ohydrogen bonding interactions to result in formation of 3D frame-work (Fig. S1). The hydrogen bonding data of complexes 1–3 aresummarized in Table S1. In addition, there are face-to-face p–pinteractions between the imidazole ring planes with a centroid–centroid separation of 3.61 Å which further consolidate the 3Dstructure.

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)2�6H2O, was used to react with H3L underthe same reaction conditions, it is interesting to find that thedeprotonation only occurred in one of the two carboxylic groupsof the H3L ligand and H2L� was generated to give complex 2. Theexistence of non-deprotonated carboxylic group in 2 was con-firmed by the observation of IR band at 1686 cm�1 in the IR spec-trum of 2 (see Section 2). The complex 2 crystallizes in themonoclinic P21/n space group which is the same as that of 1 (Table

% probability level, uncoordinated water molecule and the hydrogen atoms weren 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 H2L�

ligand as depicted in Fig. 2a. The Ag(I) atom has slight distorted lin-

NH

OO

OO

N

NH

M

MM

(a)

HN

O

O

N

M

M

Scheme 1. Coordination modes of HL2�

Fig. 2. (a) Coordination environment of Ag(I) in 2 with the ellipsoids drawn at the 30%structure of 2. (c) 2D structure of 2 linked by hydrogen bonds between the adjacent cha

ear coordination geometry and each one is two-coordinated by oneoxygen and one nitrogen atom from two different H2L� ligands.

OH

O

NH

(b)

NH

OO

OO

N

NH

M

M

(c)

and H2L� ligands in complexes 1–3.

probability level, hydrogen atoms were omitted for clarity. (b) 1D infinite chainins. (d) 3D structure of 2 linked by hydrogen bonds indicated by dashed line.

4006 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 areconsistent with the reported values in the Ag-carboxylate andAg-imidazole complexes [18]. The deprotonated carboxylate groupin 2 coordinates with one Ag(I) in monodentate mode which is thesame as that in 1, and the imidazole-nitrogen coordinates with onemetal atom solely and the amine nitrogen does not take part in thecoordination which is different from that in 1 (Scheme 1b). Suchconnections lead to the formation of a neutral 1D zigzag chain con-structed by the l2-connection of the H2L� ligands and Ag(I) atoms(Fig. 2b). The adjacent 1D chains are further connected into 2Dnetwork through O(4)–H(1A)� � �O(2) hydrogen bond withO(4)� � �O(2) distance of 2.582(3) Å and N(12)–H(12A)� � �O(2) onewith N(12)� � �O(2) distance of 2.799(4) Å (Fig. 2c). The 2D networksare further joined together through N(1)–H(1A)� � �O(3) hydrogenbond to form a 3D structure (Fig. 2d, Table S1).

3.1.3. Complex {[Co(HL)(phen)(H2O)]�8H2O}n (3)The introduced aromatic co-ligand phen may increase the pos-

sibility of p–p interactions between the aromatic groups in theresultant complex. The crystal structure of 3 is shown in Fig. 3a,

Fig. 3. (a) Coordination environment of Co(II) in 3 with the ellipsoids drawn at the 30%clarity. (b) Polyhedral representation of 1D infinite chain structure of 3. (c) Face-to-face

each Co(II) center with distorted octahedral coordination geometryis six-coordinated by one carboxylate oxygen atom from one HL2�

ligand with Co–O bond distance of 2.039(4) Å, two nitrogen atomsfrom another HL2� ligand with Co1–N bond lengths of 2.314(4) and2.049(4) Å, two nitrogen atoms from phen with Co1–N bondlengths of 2.109(4) and 2.161(4) Å and one oxygen atom from acoordinated water molecule (Table 2). On the other hand, one car-boxylate group of the HL2� ligand adopts monodentate coordina-tion mode to connect one Co(II) atom and the imidazole groupcoordinates to the Co(II) atom with the amine nitrogen via achelating mode (Scheme 1c). Such a coordination mode makescomplex 3 an infinite 1D chain structure (Fig. 3b). There are face-to-face p–p interactions between the aromatic groups within thechains since the benzene ring of HL2� 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 phenplanes 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 thep–p interactions are indicated by dashed lines. It is noteworthythat there are abundant water molecules between the 1D chains

probability level, hydrogen atoms and solvent water molecules were omitted forp–p interactions between the 1D chains of 3.

1.0 0.5 0.0 -0.5 -1.0

-40

-30

-20

-10

0

10

20

I / µ

A

E / V (vs.SCE)

Cu-GCEAg-GCE

Fig. 4. Cyclic voltammograms of Ag-GCE and Cu-GCE in pH 5.5 phosphates buffersolution in the potential range of 1.0 to �1.0 V. Scan rate: 100 mV/s.

0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

-50

0

50

100

150

200

250

300

350

I / µ

A

E / V (vs.SCE)

a

g

(a)

0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0-20

0

20

40

60

80

100

120

140

160

180

200

I / µ

A

E / V (vs.SCE)

a

f

(b)

0 2 4 6 8 10 12-50

-45

-40

-35

-30

-25

-20I

/ µA

C (mM)

Fig. 5. Cyclic voltammograms of (a) Ag-GCE and (b) Cu-GCE in pH 5.5 phosphatesbuffer 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 cathodicpeak current vs. H2O2 concentrations.

J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008 4007

(Fig. S2), and the 1D chains are linked together by strong C–H� � �O,N–H� � �O hydrogen bonds to generate a 3D structure (Fig. S3).

3.2. Electrochemical properties

Compounds 1–3 are insoluble in water and common organicsolvents. 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 werecarried out in aqueous phosphate buffer solution (pH 5.5). Fig. 4shows the cyclic voltammograms of 1 and 2 in the potential rangeof +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 peakpotential 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) � e�? Cu(II)while the reduction peak is very broad. This is most possibly dueto the instability of the reduced form of the complex with differentcoordination geometry which can undergo a fast chemical oxida-tion in an aqueous solution of pH 5.5 [19]. While at the modifiedAg-GCE, there exist a redox couple attributed to the Ag(I)/Ag[22], and the cyclic voltammogram exhibited one oxidation andone reduction peak, the mean peak potential E1/2 = (Epa + Epc)/2was �120 mV vs. SCE for the Ag–GCE. However, at the modifiedCo–GCE, there has been very weak electrochemical response inaqueous phosphate buffer solution (pH 5.5) (Fig. S4).

Scan rates’ effect on the electrochemical behavior of the Ag-GCEwas investigated in the potential range of +1.0 to �1.0 V in pH 5.5phosphates buffer aqueous solution, as shown in the Fig. S5a.When the scan rate was varied from 50 to 450 mV/s, the peakpotentials change gradually: the cathodic peak potentials shiftedto negative direction, the peak-to-peak separation between thecorresponding cathodic and anodic peaks increased. The same phe-nomenon was found for Cu-GCE (Fig. S5b).

3.3. Electrocatalytic reduction toward hydrogen peroxide

Hydrogen peroxide (H2O2) is one of the undesired by-productsof metabolite of dioxygen in biological system and the improve-ment of its electroactivity is of general interest for applicationssuch as biosensors and fuel cells [23]. It is well known that directelectroreduction of H2O2 requires a large overpotential. As shownin Fig. 5a, the catalytic reduction of H2O2 by the Ag-GCE can beseen clearly in the range of +0.50 to �1.0 V in pH 5.5 phosphatesbuffer aqueous solution containing H2O2. With addition of H2O2,

the cathodic currents increase while the corresponding anodic cur-rents decrease. The inset of Fig. 5a shows that catalytic current wasfound to be close to linear with H2O2 concentration up to 12 mM.The phenomenon of electroreduction of H2O2 at the surface ofCu-GCE is similar to that of Ag-GCE (Fig. 5b).

3.4. Thermal analyses

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 inFig. S6, the TGA curve of 1 exhibits two weight loss steps in thetemperature range 25–700 �C. The first weight loss of 10.30% inthe temperature range 70–110 �C corresponds to the loss of twowater 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 thecompletely decomposition of the residue to give CuO (calculated:77.83%). The decomposition of 2 begins at ca. 250 �C, and weightloss occurs in a consecutive step and does not stop until heatingto 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 watermolecules in the temperature range of 110–140 �C with a weight

4008 J. Xu et al. / Inorganica Chimica Acta 362 (2009) 4002–4008

loss of 2.86% (calculated 2.73%). The decomposition of the residuewas observed at ca. 305 �C and does not stop until heating to700 �C.

4. Conclusion

New Cu(II), Ag(I) and Co(II) coordination polymers have beensynthesized under hydrothermal conditions by reactions of thecorresponding metal salt with a imidazol- and carboxylic-contain-ing ligand 5-(1H-imidazol-4-ylmethyl)aminoisophthalic acid. TGAdata showed that complexes 1–3 have high thermal stability andthe electrochemical studies of 1 and 2 indicated that the Cu-GCEand Ag-GCE show good electrocatalytic activities toward thereduction of H2O2 and may be applied as electrochemical sensors.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant No. 20731004 and 20721002)and the National Basic Research Program of China (Grant No.2007CB925103).

Appendix A. Supplementary material

CCDC 727339, 727340, and 727341 contain the supplementarycrystallographic data for complexes 1, 2, and 3, respectively. Thesedata 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, inthe online version, at doi:10.1016/j.ica.2009.05.048.

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