Two Novel One-Dimensional α-Keggin-Based Coordination Polymers

Mar 9, 2009 - Institute of Functionalized Materials, Department of Chemistry, Liaoning Normal University, Dalian 116029, P. R. China, Institute of Pol...
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CRYSTAL GROWTH & DESIGN

Two Novel One-Dimensional r-Keggin-Based Coordination Polymers with Argentophilic {Ag3}3+/{Ag4}4+ Clusters

2009 VOL. 9, NO. 5 2110–2116

Limei Dai,†,‡,§ Wansheng You,*,† Enbo Wang,*,‡ Shuixing Wu,‡ Zhongmin Su,*,‡ Qinghua Du,† Yi Zhao,† and Yong Fang† Institute of Functionalized Materials, Department of Chemistry, Liaoning Normal UniVersity, Dalian 116029, P. R. China, Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal UniVersity, Changchun 130024, P. R. China, and Department of Chemistry, Heihe College, Heihe, Heilongjiang 164300, P. R. China ReceiVed June 25, 2008; ReVised Manuscript ReceiVed January 8, 2009

ABSTRACT: Two novel R-Keggin anion-based coordination polymers with argentophilic {Ag3}3+/{Ag4}4+ clusters, [{Ag3(bpy)4}{PMo12O40}] · 2H2O (1), [{Ag(bpy)}2{Ag4(bpy)6}{PMo11VO40}][{Ag(bpy)}2{PMo11VO40}] (2), have been hydrothemally synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and single-crystal X-ray diffraction. The crystal data for these are the following: C40H36Ag3Mo12N8O42P (1), triclinic P1j, a ) 10.5245(19) Å, b ) 12.058(2) Å, c ) 13.530(2) Å, R ) 87.015(2)°, β ) 72.146(2)°, γ ) 86.563(2)°, Z ) 1; C100H80Ag8Mo22N20O80P2V2 (2), triclinic P1j, a ) 13.3603(15) Å, b ) 16.3035(18) Å, c ) 16.7523(19) Å, R ) 89.896(2)°, β ) 84.000(2)°, γ ) 88.066(2)°, Z ) 1. Polymer 1 consists of an infinite zigzag one-dimensional (1D) chain constructed from [Ag3(bpy)4]3+ clusters and [PMo12O40]3- anions. Polymer 2 consists of isolated bisupporting [{Ag(bpy)}2{PMo11VO40}]2- anions and infinite 1D cationic chains [{Ag(bpy)}2{Ag4(bpy)6}{PMo11VO40}]2+ constructed from [{Ag(bpy)}2{PMo11VO40}]2- anions bridged via {Ag4(bpy)6}4+ clusters. Argentophilic {Ag3}3+ and {Ag4}4+ interactions exist in polymers 1 and 2. Ag · · · Ag interactions were proven using density functional theory, and the luminescent properties were investigated in polymers 1 and 2. The electrochemical behavior of the polymers modified carbon paste electrode (1-CPE, 2-CPE) and their electrocatalytic reductions of nitrite were investigated. Both 1-CPE and 2-CPE show good electrocatalytic activity toward the reduction of the nitrite and remarkable stability that can be ascribed to their insolubility. Introduction 1,2

The use of polyoxometalates (POMs) as discrete building blocks for the syntheses of extensive inorganic-organic hybrid polymers has attracted considerable attention. A great amount of effort has been given to the decoration of POMs with transition metals and/or organic species. Polymers based on silver ions and POMs have been the subject of great interest, which stems not only from their intriguing variety of architectures and topologies, but also from their applications in catalysis and sorption.3-7 Ag(I) is used as a metallic linkers due to its high affinity to N and O donors, its flexible coordination number and geometry in the polymers. A large number of silver (I)-POMs coordination polymers have been extensively reported, which shows their structural and coordinated diversity.8-10 In recent years, POMbased Ag(I) coordination polymers with argentophilic Ag · · · Ag interactions11 have also been constructed.12-16 For example, Cronin’s group reported a series of silver (I)-POMs coordination polymers with argentophilic Ag · · · Ag interactions. A typical example is [Ag(CH3CN)4]⊂{ [Ag(CH3CN)2]4[H3W12O40]} constructed from [H3W12O40]5- anions and argentophilic {Ag2}2+ clusters to give a covalently connected framework that contains microporous channels.3 Zhang et al. reported a three-dimensional (3D) high connected coordination polymer of R-metatungastes, Na4[Ag6(nicotinate)4][H2W12O40] · 12H2O, in which there are argentophilic {Ag2}2+ clusters, etc.16 We have been particularly interested in the modular assembly of novel complexes based on Keggin-type polyoxometalates.17-19 Meanwhile, design and synthesis of higher-nuclear Ag clusters would be significant in exploring the argentophilic Ag · · · Ag interactions. In this paper, * Corresponding author. Tel: +86-411-82159378. Fax: +86-411-82156858. E-mail: [email protected]. † Liaoning Normal University. ‡ Northeast Normal University. § Heihe College.

we report two novel one-dimensional coordination polymers based on Keggin-anions and {Ag}n (n ) 3,4) clusters with argentophilic interaction, [{Ag3(bpy)4}{PMo12O40}] · 2H2O (1), [{Ag(bpy)}2{Ag4(bpy)6}{PMo11VO40}][{Ag(bpy)}2{PMo11VO40}] (2) (bpy ) 2,2′bipyridine). To the best of our knowledge, it is the first example of polyoxometalate compounds with high-nuclear Ag(I) clusters. Experimental Section Materials and Methods. All chemicals were commercially purchased and used as supplied. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN Elemental analyzer. The IR spectrum was recorded in the range 4000-400 cm-1 on a TENSOR27 Bruker AXS spectrometer with a pressed KBr pellet. Thermogravimetric-differential thermogravimetric analysis (TG-DTA) was carried out on a Pyris Diamond TG/DTA instrument in flowing N2 with a heating rate of 10 °C · min-1. Luminescence spectrum for the solid samples was recorded with a HITACHI F-4600 fluorescence spectrophotometer. The compounds’ modified carbon paste electrodes (1-CPE and 2-CPE) were fabricated as follows: 0.5 g of graphite powder (purchased from Shanghai Chemical Plant and used as received) and 30 mg of 1 or 2 were mixed and ground together with an agate mortar and pestle to achieve a uniform and dry mixture. To the mixture, 0.3 mL of paraffine was added and stirred with a glass rod, and then the homogenized mixture was used to pack 3 mm inner diameter quartz tubes. The surface was wiped with weighing paper; electrical contact was established with copper rod through the back of the electrode. All electrochemical measurements were carried out on a CHI 600B electrochemical workstation at room temperature under nitrogen atmosphere. The working electrodes were the 1-CPE and 2-CPE. A platinum wire was used as the counter electrode and an Ag/AgCl (3 M KCl) was the reference electrode. Density functional theory (DFT) calculations reported here were performed with the ADF 2006.01 program.20 In our single-point calculations on simplified model structures extracted from single crystal data, local density approximation (LDA) was characterized by Vosko-Wilk-Nusair (VWN) exchangecorrelation potential21 and nonlocal gradient corrections were augmented as suggested by Becke22 for exchange functional and Perdew23 for correlation, respectively. Triple-ζ plus double-polarization (TZ2P) STO

10.1021/cg8006734 CCC: $40.75  2009 American Chemical Society Published on Web 03/09/2009

Two Novel Polymers with {Ag3}3+/{Ag4}4+ Clusters

Crystal Growth & Design, Vol. 9, No. 5, 2009 2111

Table 1. Theoretical Data for Polymers 1 and 2

a

Comparison of the local Ag coordination environments in 1 and 2 selected geometrical and electronic parameters. C: light gray, H: white, N: blue.

basis set was used for all electrons, and the zero order regular approximation (ZORA)24 was adopted to account for the relativistic effects of the most inert (core) electrons. Simplified structure models with coordination environment for interacted Ag atoms and calculated Ag · · · Ag Mulliken overlap population are listed in Table 1. Synthesis. (a). [{Ag3(bpy)4}{PMo12O40}] · 2H2O (1). A mixture of H3PMo12O40 · xH2O (0.46 g, 0.25 mmol),25 AgNO3 (0.085 g, 0.5 mmol), 2,2′-bpy (0.096 g, 0.5 mmol), and H2O (10 mL) was stirred for 30 min. The mixture was then transferred to a Teflon-lined stainless steel autoclave (25 mL) and kept at 200 °C for 3 days. After the autoclave was cooled to room temperature for 24 h, orange block-like crystals were obtained in a yield of 60% (based on Mo). Anal. Calcd for C40H36Ag3Mo12N8O42P: C, 17.12; H, 1.29, Ag, 11.53; Mo, 41.02; N, 3.99; O, 23.94; P, 1.10%. Found: C, 17.36; H, 1.28, Ag, 11.60; Mo, 40.78; N, 3.95; O, 23.94; P, 1.09%. (b). [{Ag(bpy)}2{Ag4(bpy)6}{PMo11VO40}][{Ag(bpy)}2{PMo11VO40}] (2). A mixture of H3PMo12O40 · xH2O (0.46 g, 0.25 mmol),25 V2O5 (0.045 g, 0.25 mmol), AgNO3 (0.085 g, 0.5 mmol), 2,2′-bpy (0.096 g, 0.5 mmol), and H2O (10 mL) was stirred for 30 min. The mixture was then transferred to a Teflon-lined stainless steel autoclave (25 mL) and kept at 180 °C for 2 days. After the autoclave was cooled to room temperature for 24 h, red block-like crystals were obtained in a yield of 25% (based on Mo). Anal. Calcd for C100H80Ag8Mo22N20O80P2V2: C, 20.08; H, 1.35, Ag, 14.43; Mo, 35.30; N, 4.69; O, 21.41; P, 1.04%; V, 1.70. Found: C, 19.98; H, 1.36, Ag, 14.48; Mo, 35.45; N, 4.47; O, 21.57; P, 1.03%; V, 1.66. X-ray Crystallographic Study. Crystal data for compound 1 and 2 were collected on a SMART APEX II-CCD single crystal X-ray diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) at 293 K. The structure for compounds 1 and 2 were solved by direct methods using the program SHELXS-97 and refined by fullmatrix least-squares methods on F2 using the SHELXL-97 program package.26 All of the non-hydrogen atoms were refined anisotropically. Positions of the hydrogen atoms attached to carbon atoms were fixed at their ideal positions, and those attached to oxygen atoms were not located. A summary of the crystallographic data and structural determination for 1 and 2 is provided in Table 2. Selected bond lengths and angles for 1 and 2 are listed in Table 3.

Results and Discussion Structural Descriptions of 1. Single-crystal X-ray diffraction analysis reveals that polymer 1 consists of one-dimensional (1D) infinite zigzag chains constructed from [Ag3(bpy)4] fragments and [PMo12O40]3- anions, as shown in Figure 1. The

b

Mo: gray, Ag: purple, O: red,

Table 2. Crystal Data and Structure Refinement for 1 and 2 compounds

1

2

empirical formula formula weight temperature (K) λ (Å) crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Dc (Mg/m3) µ (mm-1) F (000) reflections collected independent reflections Rint GOF on F2 R1, wR2 [I > 2σ(I)] R1, wR2 (all data)

C40H36Ag3Mo12N8O42P 2806.63 273(2) 0.71073 triclinic P1j 10.5245(19) 12.058(2) 13.530(2) 87.015(2) 72.146(2) 86.563(2) 1630.3(5) 1 2.859 3.232 1328 8185 5634 0.0233 1.100 0.0572, 0.1390 0.0731, 0.1452

C100H80Ag8Mo22N20O80P2V2 5979.30 273(2) 0.71073 triclinic P1j 13.3603(15) 16.3035(18) 16.7523(19) 89.896(2) 84.000(2) 88.066(2) 3626.9(7) 1 2.738 3.129 2836 18467 12646 0.0693 0.972 0.0738, 0.0899 0.1729, 0.1088

[PMo12O40]3- heteropolyanion exhibits the well-known R-Keggin-type structure,27,28 which is formed from 12 MoO6 octahedra and 1 PO4 tetrahedron. The central P atom is located at the inversion center, which indicates that the central P atom is surrounded by a cube of eight oxygen atoms with each oxygen site half-occupied. The P-O distances are in the range of 1.479(14)-1.611(15) Å, while the O-P-O angles vary from 105.1(7)° to 115.7(8)°. The Mo-O distances can be divided into three groups: Mo-Ot 1.612(9)-1.661(8) Å, Mo-Ob/c 1.803(12)-2.000(11) Å, and Mo-Oa 2.395(14)-2.492(14) Å. Two crystallographically unique Ag atoms exist in polymer 1. The Ag(1) atom is coordinated by three nitrogen atoms from two 2,2′-bpy ligands and one terminal oxygen atom from a Keggin unit, displaying a distorted tetrahedral coordinated geometry. The Ag(1)-O(22) distance is 2.516(8) Å, the Ag(1)-N distances are in the range of 2.290(10)-2.328(11) Å. The Ag(2) atom exhibits a linear coordination geometry coordinated by two nitrogen atoms from two bridging 2,2′-bpy ligands with a Ag(2)-N(4) distance of 2.155(10) Å. The two

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Table 3. Selected Bond Lengths (Å) and Angles (°) for Compounds 1 and 2a compound 1 Mo(1)-O(15) Mo(1)-O(16) Mo(1)-O(10) Mo(1)-O(11)#1 Mo(1)-O(18) Mo(1)-O(3) Mo(2)-O(22) Mo(2)-O(20) Mo(2)-O(12)#1 Mo(2)-O(13)#1 Mo(2)-O(18) Mo(2)-O(3) Mo(3)-O(7) Mo(3)-O(14)#1 Mo(3)-O(13) Mo(3)-O(6) Mo(3)-O(16) Mo(3)-O(4)#1 Mo(3)-O(2) Mo(4)-O(8) Mo(4)-O(9) Mo(4)-O(20) Mo(4)-O(10) Mo(4)-O(21) Mo(4)-O(3) Mo(5)-O(17) Mo(5)-O(6) Mo(5)-O(21) Mo(5)-O(12) Mo(5)-O(19) Mo(5)-O(2) Mo(5)-O(1) Mo(6)-O(5) Mo(6)-O(11) Mo(6)-O(19) Mo(6)-O(9) Mo(6)-O(14) Mo(6)-O(4) Mo(6)-O(1) Ag(1)-N(3) Ag(1)-N(2) Ag(1)-N(1) Ag(1)-O(22) Ag(1)-Ag(2) Ag(2)-N(4) Ag(2)-N(4)#2 Ag(2)-Ag(1)#2 O(2)-P(1)-O(3) O(15)-Mo(1)-O(16) N(3)-Ag(1)-N(2) N(3)-Ag(1)-O(22) N(4)-Ag(2)-N(4)#2 N(2)-Ag(1)-Ag(2)

compound 2 1.646(9) 1.841(12) 1.873(13) 1.937(11) 2.000(11) 2.438(13) 1.661(8) 1.975(12) 1.826(11) 1.969(10) 1.803(12) 2.446(15) 1.612(9) 1.814(12) 1.831(10) 1.966(11) 1.971(10) 2.421(13) 2.492(14) 1.645(9) 1.852(11) 1.855(12) 1.937(12) 1.958(10) 2.395(14) 1.644(9) 1.814(11) 1.836(9) 1.965(9) 1.982(10) 2.411(13) 2.453(15) 1.648(10) 1.825(10) 1.831(10) 1.949(9) 1.979(11) 2.414(14) 2.475(15) 2.290(10) 2.320(11) 2.328(11) 2.516(8) 2.9270(12) 2.155(10) 2.155(10) 2.9270(12)

105.2(7) 102.6(5) 130.4(4) 83.8(4) 180.0(5) 82.7(3)

Mo(1)-O(13) Mo(1)-O(39)#1 Mo(1)-O(34) Mo(1)-O(10) Mo(1)-O(42) Mo(1)-O(3) Mo(2)-O(24) Mo(2)-O(16) Mo(2)-O(43)#1 Mo(2)-O(39) Mo(2)-O(35) Mo(2)-O(3)#1 Mo(3)-O(17) Mo(3)-O(42) Mo(3)-O(19) Mo(3)-O(25) Mo(3)-O(16) Mo(3)-O(41) Mo(7)-O(8) Mo(7)-O(30) Mo(7)-O(36) Mo(7)-O(32) Mo(7)-O(38) Mo(7)-O(1) Mo(8)-O(22) Mo(8)-O(38)#2 Mo(8)-O(21) Mo(8)-O(23) Mo(8)-O(11) Mo(8)-O(1)#2 Mo(9)-O(29) Mo(9)-O(27) Mo(9)-O(31) Mo(9)-O(14) Mo(9)-O(32) Mo(9)-O(12) Ag(1)-N(3) Ag(1)-O(25) Ag(2)-N(5) Ag(2)-N(9) Ag(2)-Ag(4) Ag(3)-N(7) Ag(3)-O(32) Ag(4)-N(2) Ag(4)-O(15) Ag(4)-N(1)#3 Ag(4)-Ag(4)#3 O(4)-P(1)-O(3) O(12)-P(2)-O(1) N(4)-Ag(1)-N(3) N(5)-Ag(2)-N(9) N(7)-Ag(3)-N(8) N(2)-Ag(4)-N(1)#3

1.658(9) 1.836(10) 1.839(9) 1.957(11) 1.964(9) 2.475(16) 1.633(8) 1.849(10) 1.884(11) 1.941(11) 1.951(9) 2.434(17) 1.644(9) 1.806(9) 1.818(11) 1.977(9) 1.996(9) 2.433(15) 1.641(8) 1.802(11) 1.847(9) 1.958(9) 1.983(10) 2.454(19) 1.622(9) 1.844(10) 1.842(10) 1.951(9) 1.955(10) 2.396(19) 1.645(9) 1.846(11) 1.878(10) 1.882(9) 1.898(9) 2.425(16) 2.271(11) 2.373(9) 2.204(14) 2.289(16) 3.131(2) 2.285(13) 2.386(10) 2.305(15) 2.503(8) 2.507(14) 2.936(3) 107.6(8) 106.6(8) 71.9(5) 164.3(5) 72.8(5) 100.2(5)

a Symmetry transformations used to generate equivalent atoms: for 1: #1 -x + 1, -y, -z. #2 -x, -y + 1, -z + 1; for 2: #1 -x + 1, -y + 1, -z. #2 -x + 2, -y, -z + 1. #3 -x + 2, -y + 1, -z + 1.

pyridine rings of a rigid 2,2′-bpy molecule are distorted with a dihedral angle of 40.007° and two bridging 2,2′-bpy molecules join three Ag(I) ions into a linear trimeric structure. Such coordination of 2,2′-bpy in silver clusters have already been reported.29 The most unusual structural feature of the {Ag3}3+ clusters is that argentophilic Ag · · · Ag interactions exist. Spatial arrangement of 1 allows a high orbital overlap between the two d10 Ag centers and results in short Ag-Ag distances of 2.9270(12) Å.15c The formation of the Ag · · · Ag interactions compensates for the distortion of 2, 2′-bpy molecules to a great extent. The [PMo12O40]3- anions bridge the {Ag3}3+ clusters through two centrically symmetric terminal oxygen atoms to form 1D infinite zigzag chains. The adjacent hybrid chains are

Figure 1. Polyhedral and ball-stick representation of the onedimensional chain in polymer 1. All H atoms and water molecules are omitted for clarity.

Figure 2. Polyhedral and ball-stick representation of the twodimensional supramolecular layer in polymer 1. All H atoms and water molecules are omitted for clarity.

linked together based on extensive H-bonds (C8-H8 · · · O5 2.816 Å, C1-H1 · · · O9 2.314 Å, C19-H19 · · · O17 2.732 Å and C7-H7 · · · O11 2.531 Å etc.) to form an interesting twodimensional (2D) supramolecular layer (Figure 2). Furthermore, between adjacent layers constitute an extended 3D supramolecular network through extensive H-bonds interactions (C20-H20 · · · O212.601Å,C18-H18 · · · O132.608Å,C9-H9 · · · O19 2.679 Å, and C13-H13 · · · O16 2.617 Å, etc.). Structural Descriptions of 2. X-ray analysis reveals that compound 2 is composed of isolated bisupporting [{Ag(bpy)}2{PMo11VO40}]2- anions and infinite1D chain-like [{Ag(bpy)}2{Ag4(bpy)6}{PMo11VO40}]2+ cations, as can be seen in Figure 3. The [PMo11VO40]4- anions are a well-documented R-Keggin structure and all P-O and Mo-O bond lengths are in the normal ranges. Each Mo atom is disordered with one crystallographically independent metal position constrained as 11/12 Mo and 1/12 V.30 As shown in Figure 3a, in the isolated [{Ag(bpy)}2{PMo11VO40}]2- anions, each Keggin anion, acting as a tetradentate ligand, is coordinated to two {Ag(bpy)}+ fragments via its four surface bridging oxygen atoms. The Ag(1)-O(25) and Ag(1)O(16) distances are 2.373(9) Å and 2.462(9) Å, respectively. Meanwhile, the Ag(1) atom is coordinated by two nitrogen atoms from the 2,2′-bpy ligand with Ag(1)-N(4) and Ag(1)-N(3) distances of 2.248(12) Å and 2.271(11) Å, respectively. Therefore, the Ag(1) atom displays a trigonal pyramidal coordination geometry. It is interesting that adjacent [{Ag(bpy)}2{PMo11VO40}]2- anions are bolted together by the π-π interac-

Two Novel Polymers with {Ag3}3+/{Ag4}4+ Clusters

Figure 3. (a) Representation of the isolated bisupporting anions in polymer 2, showing the one-dimensional supramolecular chain via π-π interactions. (b) Polyhedral and ball-stick representation of the onedimensional cationic chain in polymer 2. All H atoms are omitted for clarity.

tion between 2,2′-bpy molecules (the closest distance 3.3266 Å) generating a regular 1D supramolecular chain along the b axis. As shown in Figure 3b, the [{Ag(bpy)}2{Ag4(bpy)6}{PMo11VO40}]2+ cationic chains are constructed from [{Ag(bpy)}2{PMo11VO40}]2- anions bridged through {Ag4(bpy)6}4+ clusters. There are three crystallographically unique Ag atoms. The Ag(3) atom is coordinated by two nitrogen atoms from a 2,2′-bpy ligand and one terminal oxygen from a Keggin anion in a nearly planar fashion (Ag(3)-N(8) 2.265(13) Å, Ag(3)-N(7) 2.285(13) Å, Ag(3)-O(32) 2.386(10) Å). Each Keggin anion is coordinated to two {Ag(bpy)}+ fragments to generate a [{Ag(bpy)}2{PMo11VO40}]2- fragment. The Ag(2) atom is coordinated by four nitrogen atoms from two chelating 2,2′bpy ligands (Ag(2)-N(10) 2.389(16) Å Ag(2)-N(9) 2.289(16) Å Ag(2)-N(6) 2.369(13) Å Ag(2)-N(5) 2.204(14) Å) and the Ag(4) atom is coordinated by two nitrogen atoms from bridging 2,2′-bpy molecules (Ag(4)-N(2) 2.305(15) Å Ag(4)-N(1)#3 2.507(14) Å). The two-pyridine rings of bridging 2,2′-bpy molecules are distorted with a dihedral angle of 38.548° so that they can bridge Ag(4) and Ag(4A) atoms. The Ag(4)-Ag(4A) argentophilic interactions with a distance of 2.936(3) Å are also observed. It is most interesting that Ag(2) fragments and Ag(4) fragments are only connected through argentophilic interactions with a distance of 3.131(2) Å. Thus, the {Ag4}4+ clusters are constructed via the bridging 2,2′-bpy ligands and the argentophilic interactions, exhibiting a broken line structure with a Ag(2)-Ag(4)-Ag(4A) bond angle of 166.315°. Spatial arrangement of 2 allows also a high orbital overlap between the two d10 Ag centers and results in the short Ag-Ag distances.15c The {Ag4(bpy)6}4+ and the [{Ag(bpy)}2{PMo11VO40}] segments are connected to each other into a 1D infinite chain along the b axis. It is most interesting that the infinite [{Ag(bpy)}2{Ag4(bpy)6}{PMo11VO40}]2+ cationic chains constitute a twodimensional supramolecular layer through π-π stacking interaction with a distance of 3.363 Å, (Figure 4). The 2D cationic layers and the isolated anionic layers are packed alternately along the a-axis, forming a 3D supramolecular structure through π-π stacking interaction with a distance of 3.5294 Å (Figure

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Figure 4. Array of The 1D cationic chains along the a-axis in polymer 2, showing the supramolecular π-π interactions between the neighboring pyridine rings.

5). We have performed comparative density functional theory (DFT) calculations to prove the Ag · · · Ag interaction. The results indicate a significant bonding interaction between the Ag centers. The Mulliken overlap integrals and the atomic partial charges are primarily correlated with the coordination environment of the individual Ag positions and the Ag · · · Ag distance. These parameters are summarized with the geometrical parameters of the Ag coordination environments in Table 1. The results can be explained by using a simple (electrostatic) crystal field model, according to which Ag(4d) electron density is repelled by the surrounding electronegative ligand positions and is stabilized between the Ag centers if significant Ag(4d)-Ag(4d) overlap is achieved.15a FT-IR Spectra and TG Analyses. In the IR spectra (Figure S2, Supporting Information), the vibration modes for υ (Mo-Oc-Mo), υ (Mo-Ob-Mo), υ (Mo-Od), and υ (P-O) occur at 797, 890, 969, and 1068 cm-1 for 1; 797, 877, 956 cm-11 attributed to υ (Mo(V)-O), and peaks at 1061 and 1074 cm-1 attributed to υ (P-O) for 2, while bands at 1161, 1439, 1604 cm-1 in the IR spectrum of 1 and 1320, 1445, 1624 cm-1 in the IR spectrum of 2 are characteristic absorption of 2,2′bpy ligands. The TG curves of 1 and 2 (Figure S3, Supporting Information) exhibit two continuous weight-loss steps between 30 and 800 °C. The total weight loss of 1 is about 23.98%, consistent with the calculated value of 23.54%, attributed to the loss of lattice, coordinated water molecules and bipyridine ligands; the total weight loss of 2 is about 25.28%, consistent with the calculated value of 26.12%, attributed to the loss of bipyridine ligands.31 Luminescent Property. Figure S4, Supporting Information presents the emission spectra of the title compounds in the solid state at room temperature. Compound 1 exhibits photoluminescence with an emission maximum at ca. 415 nm upon excitation at ca. 302 nm, whereas compound 2 presents an emission band at 406 nm at the same excitation condition, slight blue-shift in comparison to 1. This may be due to the stronger Ag · · · Ag interaction of 1 compared to that of 2 (according to DFT), and leading to the red-shift phenomena. However, the emission spectra of compounds 1 and 2 are red-shifted compared to that of 2,2′-bipy ligand.32 The explanation is that Ag · · · Ag interaction leads to this red-shift phenomena in 1 and 2. The excited

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Figure 5. Three-dimensional supramolecular framework of polymer 2 projected down the a-axis. All H atoms are omitted for clarity.

Figure 6. Cyclic voltammograms of the 1-CPE in 1 M H2SO4 solution at different scan rates.

states for 1 and 2 with short Ag · · · Ag contacts have been best described as ligand-to-metal-metal charge-transfer (LMMCT) in nature.33 Cyclic Voltammetry. Figures 6 and 8 show the typical cyclic voltammetric behavior of 1-CPE and 2-CPE in 1 M H2SO4 medium at different scan rates. Three reversible redox peaks were observed in the potential domain of +700 to -100 mV. The mean peak potentials E1/2 ) (Epa + Epc)/2 were -2.0 mV (I), +231.5 mV (II), and 379.5 mV (III) (scan rate: 200 mV s-1) for 1, +3.0 mV (I), +242.0 mV (II), and 395.0 mV (III) (scan rate: 200 mV s-1) for 2, respectively. Redox peaks I-I′, II-II′, III-III′ of 1 and 2 should be ascribed to three consecutive two-electron redox processes of Mo, respectively.34-36 The potential shifts of the three reversible redox peaks in 1-CPE and 2-CPE may result from their different components and structures. We have not observed redox peaks of V for compound 2, perhaps due to their weak signals embedded in the redox peaks of Mo.37 It is noteworthy that 1-CPE and 2-CPE possess a high stability, when the potential range is located in +700 to -100 mV, the peak current remains almost unchanged over 500 cycles at a scan rate of 100 mV · s-1. After 1-CPE and 2-CPE were stored at room temperature for one month, the peak current decreased only 5% and 8%, respectively. The electrocatalytic reductions of nitrite in 1 M H2SO4 aqueous solution were investigated at the 1-CPE and 2-CPE (Figures 7 and 9). It is well-known that direct electroreduction of nitrite ions requires

Figure 7. Cyclic voltammograms of the 1-CPE in 1 M H2SO4 solution containing 0.0, 4.0, 16.0 mM NaNO2 and a bare CPE in 5.0 mM NaNO2 + 1 M H2SO4 solution. Potentials vs. Ag/AgCl. Scan rate: 50 mV s-1.

Figure 8. Cyclic voltammograms of the 2-CPE in the 1 M H2SO4 solution at different scan rates.

a large overpotential at most electrode surfaces, and no obvious response was observed at a bare CPE. Our investigations indicate that 1-CPE and 2-CPE have good electrocatalytic activity toward the reduction of nitrite. With the addition of nitrite, all reduction

Two Novel Polymers with {Ag3}3+/{Ag4}4+ Clusters

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Figure 9. Cyclic voltammograms of the 2-CPE in 1 M H2SO4 solution containing 0.0, 4.0, 16.0 mM NaNO2 and a bare CPE in 5.0 mM NaNO2 + 1 M H2SO4 solution. Potentials vs. Ag/AgCl. Scan rate: 50 mV s-1.

peak currents increased while the corresponding oxidation peak currents dramatically decreased, suggesting that the reduction of nitrite is mediated by the reduced species of Keggin-ions in compounds 1 and 2. The high electrocatalytic activity is due to the chain-like structural feature of 1 and 2 that stabilizes the Keggin-ions in compounds 1 and 2.

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Conclusions In summary, polymers 1 and 2 have been obtained by using modular Keggin clusters as building blocks and silver ions as bridging units. Polymer 1 contains an infinite zigzag 1D chain constructed from [Ag3(bpy)4] clusters and [PMo12O40]3- anions. Polymer 2 consisted of isolated bisupporting [{Ag(bpy)}2{PMo11VO40}]2- anions and infinite 1D chain [{Ag(bpy)}2{Ag4(bpy)6}{PMo11VO40}]2+ constructed from {Ag4(bpy)6}4+ clusters bridging [{Ag(bpy)}2{PMo11VO40}]2- anions. Argentophilic {Ag3}3+ and {Ag4}4+ clusters exist in polymers 1 and 2, respectively. Furthermore, Ag(2) and Ag(4) are connected only through the argentophilic Ag · · · Ag interactions in polymer 2. To the best of our knowledge, it is the first example of polyoxometalate compounds with high-nuclear Ag(I) clusters. Their modified carbon paste electrodes (1-CPE, 2-CPE) exhibit excellent electrocatalytic activity toward the reduction of nitrite and remarkable stability. Acknowledgment. Authors thank Dr. Kan YH for his kind computational support using the ADF2006.01 program. This work was supported by National Science Foundation of China (No. 20773057), Liaoning Provincial Educational Commission (Project No. 605L207), and Scientific Research Fund of Heilongjiang Provincial Education Department (No. 11523035). Supporting Information Available: X-ray crystallographic files in CIF format. IR spectra, TG curves, and luminescence spectra of 1 and 2. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC reference numbers 691522 and 691523 for 1 and 2. Supporting information is available free of charge via the Internet at http://pubs.acs.org.

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