Polyhedron 29 (2010) 1243–1250

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Polyhedron

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Syntheses, structures, photoluminescences of silver(I) coordination polymerswith 2-aminopyrazine and varied dicarboxylate ligands

Di Sun, Geng-Geng Luo, Na Zhang, Qin-Juan Xu, Rong-Bin Huang *, Lan-Sun ZhengState Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

a r t i c l e i n f o

Article history:Received 18 August 2009Accepted 30 December 2009Available online 14 January 2010

Keywords:Silver(I)2-AminopyrazineCoordination polymerCrystal structurePhotoluminescence

0277-5387/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.poly.2009.12.038

* Corresponding author.E-mail address: rbhuang@xmu.edu.cn (R.-B. Huang

a b s t r a c t

Four new silver(I) coordination polymers, namely [Ag(NH2pyz)(ox)0.5]n (1), [Ag(NH2pyz)(adp)0.5�2H2O]n

(2), [Ag2(NH2pyz)2(bdc)�H2O]n (3) and [Ag2(NH2pyz)2.5(ndc)]n (4) [NH2pyz = 2-aminopyrazine, ox = oxa-late anion, adp = adipate anion, bdc = 1,4-benzenedicarboxylate anion, ndc = 1,4-naphthalenedicarboxy-late anion] have been synthesized by solution phase ultrasonic reactions of Ag2O with heterocyclicNH2pyz and various dicarboxylates under ammoniacal conditions. The complexes were characterizedby elemental analyses, IR spectra and single-crystal X-ray diffraction. Complex 1 is a three-dimensional(3D) framework with an a-ThSi2 topology. Complex 2 features a 2D 44-sql net involving infinite 1D dou-ble Ag-NH2pyz chains and flexible adp anion spacers. Complex 3 is a 3D framework in which 1D singleAg-NH2pyz chains are pillared by bdc anions to form a 2D 63-hcb network, adjacent 2D networks arepacked into a 3D framework through bridging O atoms of bdc anions. Complex 4 is a 2D structure builtfrom infinite 1D stair-like chains containing finite Ag4(NH2pyz)5 subunits. The results show that thestructural diversity of the complexes result from the nature of the dicarboxylate ligands. The photolumi-nescence properties of the complexes were also investigated in the solid state at room temperature.

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1. Introduction

The rational design and synthesis of novel, discrete and poly-meric metal–organic hybrid frameworks remain a popular researchfocus in recent years [1–6] due to their fascinating coordinationarchitectures and their potential applications in various fields,including gas storage, conductivity, catalysis, photophysics, photo-chemistry, fluorescence, magnetism and biological properties [7–12]. Although the synthesis of such coordination polymers (CPs)is mainly based on the self-assembly of suitable metal ions andthe structural characterization of organic building blocks, it is alsohighly influenced by numerous other factors such as the solventsystem, temperature, counter ions with different bulk or coordina-tion ability, template, metal/ligand ratio and even pH value [13–26]. Besides these aspects, non-covalent interactions such ashydrogen bonding, p�p interactions, metal–metal interactionsplay important roles in the construction of supramolecular struc-tures [27,28]. Despite being possible to get some control on thestructures by a judicious choice of the metal ions, circumspectconsideration of the geometry of the organic ligands and the orien-tations of the donor groups, prediction and control of the supramo-lecular assembly of molecules remains a compelling challenge.

On the other hand, the employment of mixed ligands during theself-assembly process has gradually become a powerful approach,

ll rights reserved.

).

which is expected to result in frameworks with more diverse struc-tural motifs compared to using a single type of ligand. In our recentstudy, 2-aminopyrimidine and its derivatives have been success-fully used to construct a series of Ag(I) complexes with dinuclear,tetranuclear, 1D, 2D and 3D structures [29–31], and in order toget high-dimensional structures with interesting photolumines-cence properties, we have successfully introduced ancillary dicar-boxylate ligands into the previous silver/aminopyrimidinesystem. As is already known, both exo-bidentate N-donor ligands,such as 4,40-bipyridine and pyrazine, and O-donor ligands, suchas di-, tri- and tetra-carboxylates, are excellent synthons that havebeen employed for the design of functional solid materials [32].Although many reports about the usage of pyrazine as a synthonto construct Ag-containing hybrid frameworks have been pre-sented in recent literatures [33–41], Ag(I) complexes with NH2pyzand/or other ancillary ligands have received rare attention [42],which may be due to a certain instability of NH2pyz as well asthe weak coordination ability of the amino group towards theAg(I) centers. Following a similar strategy to that of the silver/2-aminopyrimidine/dicarboxylate system, we have focused ourattention on the silver/NH2pyz/dicarboxylate system to systemati-cally investigate the impact of the dicarboxylate ligands on thestructure and have obtained four new Ag(I) CPs: [Ag(NH2pyz)(ox)0.5]n (1), [Ag(NH2pyz)(adp)0.5�2H2O]n (2), [Ag2(NH2pyz)2(bdc)�H2O]n (3), [Ag2(NH2pyz)2.5(ndc)]n (4) [NH2pyz = 2-aminopyrazine,ox = oxalate anion, adp = adipate anion, bdc = 1,4-benzenedicar-boxylate anion, ndc = 1,4-naphthalenedicarboxylate anion]. The

1244 D. Sun et al. / Polyhedron 29 (2010) 1243–1250

results show that the structural diversity of the complexes resultsfrom the coordination modes (Scheme 1), length and flexibility ofthe dicarboxylate ligands [43,44].

2. Experimental

2.1. Materials and methods

All reagents and solvents were obtained commercially and usedwithout further purification. Infrared spectra were recorded on aNicolet AVATAT FT-IR360 spectrometer in the frequency range4000–400 cm�1 using KBr pellets. The elemental analyses (C, H,N) were determined on a CE instruments EA 1110 analyzer. Photo-luminescence measurements were performed on a Hitachi F-4500fluorescence spectrophotometer with solid powder on a 1 cmquartz round plate.

2.2. Synthesis of [Ag(NH2pyz)(ox)0.5]n (1)

A mixture of Ag2O (115 mg, 0.5 mmol), NH2pyz (94 mg,1 mmol) and H2ox (90 mg, 1 mmol) were stirred in CH3OH–H2Omixed solvent (10 ml, v/v: 1/1). Then an aqueous NH3 solution(25%) was dropped into the mixture to give a clear solution underultrasonic treatment (160 W, 40 kHz). The resultant solution wasallowed to evaporate slowly in darkness at room temperature forseveral days to give colorless crystals of 1 (yield, 54%). The crystalswere washed with a small volume of cold CH3OH and diethyl ether.Anal. Calc. for AgC5H5N3O2: C, 24.31; H, 2.04; N, 17.01. Found: C,24.27; H, 2.01; N, 16.98%. IR (KBr) m (cm�1): 3392 (m), 3290 (m),2958 (m), 2921 (m), 1633 (s), 1608 (s), 1560 (s), 1473 (s), 1348(m), 1316 (m), 1216 (m), 767 (m), 649 (w), 512 (w), 487 (m).

2.3. Synthesis of [Ag(NH2pyz)(adp)0.5�2H2O]n (2)

The synthesis of 2 was similar to that of 1, but with H2adp(148 mg, 1 mmol) in place of H2ox. Colorless crystals of 2 were ob-tained in 58% yield. Anal. Calc. for AgC7H13N3O4: C, 27.03; H, 4.21;N, 13.51. Found: C, 27.11; H, 4.16; N, 13.48%. IR (KBr) m (cm�1):3400 (m), 3300 (m), 2953 (m), 2914 (m), 1625 (s), 1572 (s), 1468(s), 1387 (m), 1364 (m), 1225 (m), 1172 (m), 1025(w), 767 (m),523 (w), 466 (m).

2.4. Synthesis of [Ag2(NH2pyz)2(bdc)�H2O]n (3)

The synthesis of 3 was similar to that of 1, but with H2bdc(166 mg, 1 mmol) in place of H2ox. Colorless crystals of 3 were ob-tained in 65% yield. Anal. Calc. for Ag2C16H14N6O5: C, 32.79; H, 2.41;N, 14.34. Found: C, 32.70; H, 2.45; N, 14.38%. IR (KBr) m (cm�1): =3400 (m), 3298 (m), 1638 (s), 1565 (s), 1469 (s), 1415 (m), 1367(m), 1235 (m), 792 (m), 524 (w), 458 (m).

Scheme 1. Coordination modes of

2.5. Synthesis of [Ag2(NH2pyz)2.5(ndc)]n (4)

The synthesis of 4 was similar to that of 1, but with H2ndc(216 mg, 1 mmol) of H2ox. Colorless crystals of 4 were obtainedin 55% yield. Anal. Calc. for Ag2C22H18N7.5O4: C, 39.61; H, 2.72; N,15.75. Found: C, 39.70; H, 2.68; N, 15.72%. IR (KBr) m (cm�1):3389 (m), 3295 (m), 2946 (m), 2934 (m), 1615 (s), 1565 (s), 1445(s), 1467 (m), 1359 (m), 1235 (m), 795 (m), 674 (w), 520 (w),459 (m).

2.6. X-ray crystallography

Single crystals of the complexes 1–4 with appropriate dimen-sions were mounted on a glass fiber and used for data collection.Data were collected on a Bruker-AXS CCD diffractometer equippedwith a graphite-monochromated Mo Ka radiation source(k = 0.71073 Å) for 1–4. All absorption corrections were performedwith the SADABS program [45]. All the structures were solved by di-rect methods using SHELXS-97 [46] and refined by full-matrix least-squares techniques using SHELXL-97 [47]. All the non-hydrogenatoms were treated anisotropically. The positions of hydrogenatoms were generated geometrically. The crystallographic detailsof 1–4 are summarized in Table 1. Selected bond lengths and bondangles of 1–4 are displayed in Table 2. The hydrogen bond geome-tries are shown in Table S1 (in the supporting information).

3. Results and discussion

3.1. Syntheses and IR

The syntheses of complexes 1–4 were carried out in the dark-ness to avoid photodecomposition and are summarized in SchemeS1, in the supporting information. The formation of the productswas not significantly affected by changes of the reaction mole ratioof organic ligands to metal ions, and the resultant crystals areinsoluble in water and common organic solvents. The infraredspectra and elemental analyses of 1–4 are fully consistent withtheir formations. Their IR spectra exhibit absorptions in the range�3300 to �3400 cm�1, corresponding to the N–H stretching vibra-tions of the amino group. Strong characteristic bands of the carbox-ylic groups are observed in the range �1630 to �1560 cm�1 for theasymmetric vibrations and �1480 to �1350 cm�1 for the symmet-ric vibrations. The absence of any characteristic bands around1700 cm�1, attributed to carboxylic groups, indicates the completedeprotonation of all the carboxylate groups in 1–4, as a result ofthe reaction with the Ag(I) ions.

3.2. Crystal structure of [Ag(NH2pyz)(ox)0.5]n (1)

The single-crystal X-ray diffraction analysis reveals that com-plex 1 crystallizes in the orthorhombic space group Fdd2. Each

N-donor and O-donor ligands.

Table 1Crystallographic data for complexes 1–4.

Complex 1 2 3 4

Formula AgC5H5N3O2 AgC7H13N3O4 Ag2C16H14N6O5 Ag2C22H18N7.5O4

Mr 246.98 311.06 586.06 667.18Crystal system orthorhombic triclinic monoclinic monoclinicSpace group Fdd2 P�1 P21/n C2/ca (Å) 13.170(3) 7.242 (4) 12.939(3) 10.1966(2)b (Å) 26.176(5) 8.246(4) 9.943(2) 16.9504(3)c (Å) 7.799(2) 9.774(5) 14.603(3) 26.4607(5)a (�) 90 94.03(1) 90 90b (�) 90 105.527(8) 97.10(3) 90.392(2)c (�) 90 110.504(9) 90 90Z 16 2 4 4V (Å3) 2688.4(9) 518.0(5) 1864.1(6) 4573.3(2)Dcalc (g cm�3) 2.441 1.994 2.088 1.932l (mm�1) 2.943 1.945 2.144 1.760F (0 0 0) 1904 310 1144 2612Number of unique reflections 1323 1800 3266 4020Number of observed reflections [I > 2r(I)] 1253 1482 2479 3788Parameters 101 138 263 325Goodness-of-fit (GOF) on F2 1.059 1.020 1.000 1.413Final R indices [I > 2r(I)]a,b R1 = 0.0404, R1 = 0.0461, R1 = 0.0309 R1 = 0.0407,

wR2 = 0.0989 wR2 = 0.1113 wR2 = 0.0741 wR2 = 0.0886R indices (all data) R1 = 0.0416, R1 = 0.0567, R1 = 0.0450, R1 = 0.0439,

wR2 = 0.1007 wR2 = 0.1161 wR2 = 0.0808 wR2 = 0.0895Largest difference peak and hole (e �3) 0.850 and �0.945 1.187 and �0.570 0.695 and �0.700 0.599 and �1.081

a R1 =P

||Fo| - |Fc||/P

|Fo|.b wR2 = [

Pw(F2

o � F2c )2/

Pw(F2

o )2]0.5.

Table 2Selected bond lengths (Å) and angles (�) for 1–4.

Complex 1Ag1–N1i 2.227(4) Ag1–O1 2.490(4)Ag1–N2 2.254(4) N2–Ag1–O1 97.55(13)N1i–Ag1–N2 139.87(15) N1i–Ag1–O1 121.86(14)

Complex 2Ag1–N2 2.239(5) Ag1–O2ii 2.576(4)Ag1–N3i 2.258(5) Ag1–O1 2.537(5)N2–Ag1–N3i 163.29(17) O1–Ag1–O2ii 158.92(17)N2–Ag1–O1 97.11(16) N2–Ag1–Ag1ii 104.54(12)N3i–Ag1–O1 89.91(16) N3i–Ag1–Ag1ii 89.99(13)N2–Ag1–O2ii 88.93(15) O1–Ag1–Ag1ii 94.13(12)N3i–Ag1–O2ii 89.90(16) O2ii–Ag1–Ag1ii 64.79(11)

Complex 3Ag1–N2 2.280(3) Ag2–N3ii 2.300(3)Ag1–O2 2.304(3) Ag2–N4 2.334(3)Ag1–N5 2.363(3) Ag2–O1 2.489(3)Ag1–O1i 2.587 (3) Ag2–O2iii 2.639(3)N2–Ag1–O2 129.56(12) O2–Ag1–O1i 72.46(11)N2–Ag1–N5 120.09(13) N5–Ag1–O1i 101.02(10)O2–Ag1–N5 109.00(13) N3ii–Ag2–N4 121.22(12)N2–Ag1–O1i 106.82(11) N3ii–Ag2–O1 133.21(11)N4–Ag2–O1 102.99(11)

Complex 4Ag1–N4 2.197(4) O1–Ag2 2.458(4)Ag1–N2 2.243(5) Ag2–N3ii 2.181(4)Ag1–O1 2.479(4) Ag2–N8 2.233(4)Ag1–O2i 2.515(4) Ag2–O2i 2.497(4)N4–Ag1–N2 170.22(16) N3ii–Ag2–N8 158.75(17)N4–Ag1–O1 105.08(14) N3ii–Ag2–O1 103.36(15)N2–Ag1–O1 81.97(16) N8–Ag2–O1 89.53(15)N4–Ag1–O2i 98.20(14) N3ii–Ag2–O2i 113.11(15)N2–Ag1–O2i 89.62(15) N8–Ag2–O2i 85.30(16)O1–Ag1–O2i 80.12(12) O1–Ag2–O2i 80.87(12)

Symmetry codes: (i) 1.25 � x, �0.25 + y, �0.25 + z for 1.(i) x + 1, y, z; (ii) �x + 2, �y + 2, �z for 2.(i) x � 1/2, �y + 1/2, z � 1/2; (ii) x + 1, y, z; (iii) x + 1/2, �y + 1/2, z + 1/2 for 3.(i) x + 1/2, y � 1/2, z; (ii) �x + 5/2, �y + 1/2, �z for 4.

D. Sun et al. / Polyhedron 29 (2010) 1243–1250 1245

asymmetric unit contains one Ag(I) ion, one NH2pyz ligand andhalf of an ox anion. A twofold axis runs through the midpoint ofthe C–C bond of the ox anion. As shown in Fig. 1a, the four-coordi-

nated Ag(I) ion, which adopts a distorted tetrahedral geometrywith bond angles spanning from 97.55� to 139.87�, is coordinatedby two N atoms from two different l2-NH2pyz ligands and two Oatoms from one ox anion. The distortion of the tetrahedron canbe indicated by the calculated value of the s4 parameter introducedby Houser and co-workers [48] to describe the geometry of a four-coordinate metal system, which is 0.69 in 1 (for a perfect tetrahe-dral geometry, s4 = 1). The two carboxylate groups of the ox anionare coordinated to the Ag(I) ions with a g-O,O0-chelating mode. TheAg1–O1 bond length (2.490(4) Å) is compatible with those in O-containing Ag(I) complexes [49], but the Ag1–O2 bond length istoo long at 2.713(4) Å to suggest anything other than a secondaryinteraction. The Ag–N bond lengths [Ag1–N2 = 2.254(4), Ag1–N1i = 2.227(4) Å] are within the normal range expected for Ag(I)–pyrazine complexes [50].

In addition, as shown in Fig. 1b, NH2pyz acts as a bidentate N,N0–donor to link Ag(I) ions to form 1D zig–zag chains along the baxis. Within the polymeric chain, the N–Ag–N angle is significantlybent at an angle of 139.9(2)�, deviating from 180� for a linear two-coordinated Ag(I) center due to the existence of oxygen atoms ofan adjacent ox ligand. When viewed along the a axis, the 3D frame-work of 1 is comprised of alternating layers A and B. In detail, the Aand B layers are comprised of NH2pyz ligands and Ag2(ox) units,respectively, and linked with each other through hydrogen bonds(Fig. 1d). Each amino group acts as a bi-donor to form hydrogenbonds with two oxygen acceptors belonging to two ox anions[N3� � �O2iii = 2.907(6), N3� � �O1iv = 2.895(7) Å]. The details of thehydrogen bonding for 1 are shown in Table S1 (supportinginformation).

To further understand the structure of the complex, topologicalanalysis by reducing the multidimensional structure to a simplenode-and-spacer net was performed on 1. Taking the geometricalcenter of the Ag(I) as a network node, it is linked by one ox andtwo NH2pyz ligands; hence, each Ag(I) can be described as a 3-con-nected node. Based on the above analysis, we found that 1 is athree-connected 3D framework comprised of the ‘‘shortest circuit”(NH2pyz)6Ag10(ox)4 macrocyclic motif. The parallel single helixruns through the structure along the c axis, as is marked by arrow-heads in Fig. 1c. This framework is topologically equivalent to the

Fig. 1. (a) The coordination environment of the Ag(I) center and the linkage modes of the ligands in 1 with 30% thermal ellipsoid probability; hydrogen atoms are omitted forclarity (symmetry code: (i) �x + 5/4, y � 1/4, z � 1/4). (b) Space-filling mode of the 1D zig–zag chain in 1 viewed along the c axis. (c) The 3D framework with a-ThSi2 topology.(d) 3D framework viewed along the a axis comprised of layers A and B.

1246 D. Sun et al. / Polyhedron 29 (2010) 1243–1250

extended network formed by the silicon atoms in a-ThSi2 [51] andsimilar to the previously reported [Ag2(pyrazine)3](BF4)2 [52],which is the most famous example of one of the three simplest3D nets built up with three-connected centers only [53] (symmetrycodes: (i) �x + 5/4, y�1/4, z�1/4; (iii) �x + 5/4, y + 1/4, z + 1/4; (iv)x + 1/2, y, z + 1/2).

3.3. Crystal structure of [Ag(NH2pyz)(adp)0.5�2H2O]n (2)

To study the influence of the length and flexibility of thedicarboxylate on the types of formed network, the synthesis ofcomplex 2 using a longer dicarboxylate was carried out. The re-sults of the crystallographic analysis reveals that complex 2 crys-tallizes in the triclinic space group P1. Each asymmetrical unitcontains one Ag(I) ion, one NH2pyz ligand, half of an adp anionand two uncoordinated water molecules. As shown in Fig. 2a,Ag1, which adopts a rare square-planar coordination fashion(only �2% of all silver complexes adopt this stereochemistry)[54] without consideration of Ag� � �Ag interactions, is coordinatedby two N atoms from two different NH2pyz ligands and two Oatoms from two different adp anions [Ag1–N2 = 2.239(5), Ag1–O1 = 2.537(5) Å; N2–Ag1–N3i = 163.29(17), N3i–Ag1–O2ii =89.90(16)�]. The distortion of the square-planar geometry canbe indicated by the calculated value of the s4 parameter, whichis 0.27 (for perfect square-planar geometry, s4 = 0). The square-planar Ag(I) center has axial sites that are blocked by pyrazinylrings and lattice water molecules. This kind of coordination modeof Ag(I) is analogous to the previously reported complexes [Ag(tsa)pyz] and [Ag(nsa)pyz] (tsa = p-toluenesulfonate, nsa = 1-nap-thalenesulfonate) [55].

In addition, as shown in Fig. 2b, NH2pyz acts as a bidentate N,N0-donor to link Ag(I) ions into 1D chains along the a axis. Withinthe polymer chain the N–Ag–N bond angle is slightly bent at an an-gle of 163.3(2)�. Two adjacent 1D chains are bridged by carboxylategroups to form a double chain structure parallel to the a-direction.Secondary forces, such as Ag� � �Ag and p� � �p interactions, reinforce

the double chain. The separation of Ag(1)� � �Ag(1)ii is 3.114(2) Å,which is longer than the Ag� � �Ag distance in silver metal, but obvi-ously shorter than the twice the van der Waals radius of Ag(I) [56].Additionally, the distance between the centroids of the pyrazinylrings (3.741(4) Å) indicates a weak p� � �p stacking interaction.(symmetry codes: (i) 1 + x, y, z; (ii) 2 � x, 2 � y, �z).

Adp is a more flexible ligand than ox and can bend and rotatefreely to offer a suitable conformation for coordination to theAg(I) ion. As illustrated in Fig. 2c, the adp anions adopt a curledconformation [57–62] to clip the dinuclear Ag(I) unit through car-boxylate groups in the bidentate syn–syn mode and extend the 1DAg–NH2pyz double chains into a 2D grid with a window of7.24 � 10.49 Å. Taking the Ag(I) ions as four-connected nodes,the NH2pyz and adp ligands as linkers, the grid can be consideredas a 44-sql network (Fig. 2d). Lattice water molecules fill the gap ofadjacent grids and extend the 2D sheets into a 3D network viahydrogen bonding (average N� � �O = 2.87 and O� � �O = 2.69 Å) builtby free water, amino groups of NH2pyz and oxygen atoms of car-boxylate groups.

3.4. Crystal structure of [Ag2(NH2pyz)2(bdc)�H2O]n (3)

When rigid H2bdc was used in the reaction, complex 3 was iso-lated. The asymmetric unit of 3 consists of two Ag(I) ions, twoNH2pyz ligands, one bdc (sitting on an inversion center) and onelattice water molecule. A view of the local coordination geometriesaround Ag(I) ions is depicted in Fig. 3a, and selected bond lengthsand angles are listed in Table 2. Each Ag(I) ion features a tetrahe-dral geometry and is coordinated by two N atoms (Ag1: N2 andN5; Ag2: N3 and N4) belonging to two different NH2pyz ligands,and two O atoms (Ag1 and Ag2: O1 and O2) belonging to two dif-ferent bdc anions. The average Ag–N and Ag–O bond lengths are2.32 and 2.50 Å, respectively. The s4 parameters, 0.78 for Ag1and 0.74 for Ag2, indicate the silver environments are distorted tet-rahedral geometries. The shortest Ag� � �Ag distance between mole-cules is 4.08 Å and no direct Ag� � �Ag interaction is found.

Fig. 2. (a) The coordination environment of the Ag(I) center and the linkage modes of the ligands in 2 with 30% thermal ellipsoid probability; hydrogen atoms are omitted forclarity (symmetry codes: (i) 1 + x, y, z; (ii) 2 � x, 2 � y, �z). (b) Ball-and-stick diagram of the double chain structure. (c) The 2D network in the bc plane. (d) 44-sql network.

Fig. 3. (a) The coordination environments of the Ag(I) centers and the linkage modes of ligands in 3 with 30% thermal ellipsoid probability; hydrogen atoms are omitted forclarity (symmetry codes: (i) �1/2 + x, 1/2 � y, �1/2 + z; (ii) 1 + x, y, z; (vi) 1/2 + x, �1/2 � y, 1/2 + z). (b) Ball-and-stick diagram of a pair of parallel linear coordination polymerchains. (c) Schematic representation of the 2D layer. (d) View of the 2D network with 63-hcb topology.

D. Sun et al. / Polyhedron 29 (2010) 1243–1250 1247

Similar to 1, NH2pyz in 3 also acts as a bidentate N,N0-donor tolink Ag(I) to form 1D zig–zag chains along the a axis (Fig. 3b). Asshown in Fig. 3c, the O atoms of bdc anions act as bridges to link1D zig–zag chains into a 2D corrugated network. Consideration ofthe Ag(I) centers as three-connected nodes, the 2D network can be

simplified to a 63-hcb network. The adjacent 2D sheets are packedinto a 3D framework through bridging bdc anions. Moreover, N–H� � �O and O–H� � �O hydrogen bonds with an average D� � �A distanceof 2.82 Å (D = donor of hydrogen, A = acceptor of hydrogen), play animportant role in stabilizing the 3D framework (Table S1).

1248 D. Sun et al. / Polyhedron 29 (2010) 1243–1250

3.5. Crystal structure of [Ag2(NH2pyz)2.5(ndc)]n (4)

Complex 4 crystallizes in the monoclinic space group C2/c. Theasymmetrical unit of 4 consists of two Ag(I) ions, two and one-halfNH2pyz and one ndc anion. A perspective view of 4 is depicted inFig. 4a, and selected bond lengths and angles are listed in Table2. The coordination environments of the Ag(I) centers are analo-gous to those in complex 3, each Ag(I) center adopts a distortedtetrahedral geometry (s4 = 0.60 and 0.63 for Ag1 and Ag2, respec-tively) and they are coordinated by two N atoms (Ag1: N2 andN4; Ag2: N3 and N8) from two different NH2pyz and two O atoms(Ag1 and Ag2: O1 and O2) from two different ndc anions. N7 of oneof the NH2pyz ligands is disordered over two orientations withequal site occupancy due to a twofold axis passing through it.The average Ag–N and Ag–O bond lengths are 2.21 and 2.49 Å,respectively.

Compared to the infinite Ag-NH2pyz chains in complexes 1–3,the infinite 1D stair-like chain structure in 4 is built by finiteAg4(NH2pyz)5 linear subunits through bridging O atoms of ndc an-ions, shown in Fig. 4b. It is worth noting that the middle NH2pyzligand within a finite Ag4(NH2pyz)5 subunit is almost perpendicu-lar to its neighboring NH2pyz ligands, with a dihedral angle of84.6�. The stair-like chains are further linked by ndc anions witha l4-g2:g2 coordination fashion to form a 2D network (Fig. 4c).In addition to one coordinated oxygen atom of each carboxylicgroup of ndc, another oxygen atom acts as an acceptor of hydrogento build a 1D hydrogen bonded chain with the amino group ofNH2pyz, in which R4

4(8) eight-member rings and ndc anions arealternating [N1� � �O3v = 2.851(6), N1� � �O4ii = 2.935(6), N5� � �O3i =2.963(6), N5� � �O4vi = 2.836(6) Å]. The shortest distance betweencentroids of adjacent pyrazine rings is 3.366(3) Å which indicatesthe existence of a strong p� � �p stacking interaction. The Cpyrazinyl–H� � �pndc and Namino–H� � �ppyrazinyl interactions [C8� � �Cg1 =3.579(6) Å, C8–H8A� � �Cg1 = 171�; N7� � �Cg2iii = 3.328(10) Å, N7–H7C� � �Cg2iii = 166�; Cg1 and Cg2 are the centroids of the C12/C13/C14/C15/C16/C17 and N4/C5/C6/N6/C7/C8 rings respectively]incorporate the hydrogen bonds and p� � �p stacking interaction tostabilize the resulting 3D framework. (Symmetry codes: (i) x + 1/

Fig. 4. (a) The coordination environments of the Ag(I) centers and the linkage modes ofclarity (symmetry codes: (i) x + 1/2, y � 1/2, z; (ii) �x + 5/2, �y + 1/2, �z; (iii) x � 1/2, yincorporating finite Ag4(NH2pyz)5 linear subunits. (c) The 2D network in the bc plane.

2, y � 1/2, z; (ii) �x + 5/2, �y + 1/2, �z; (iii) �x + 2, y, �z + 1/2; (v)�x + 3/2, �y + 1/2, �z; (vi) x�1/2, y�1/2, z).

3.6. Photoluminescence properties

Recently, inorganic–organic hybrid materials have been investi-gated for potential applications, such as light-emitting diodes(LEDs) [63]. Such complexes can be classified according to the nat-ure of their lowest excited states which may be of metal-centered(MC), ligand-to-metal charge transfer (LMCT), metal-to-ligandcharge transfer (MLCT), metal-to-metal charge transfer (MMCT), li-gand-to-ligand charge transfer (LLCT), intraligand (IL) and intrali-gand charge transfer (ILCT) types [64–66]. The vast majority ofAg(I) complexes are known to emit photoluminescence at lowtemperature [67,68] and only a few Ag(I) complexes display in-tense photoluminescence at room temperature [69–71]. In our pre-vious work, we explored a series of polynuclear d9/d10 heterometalcomplexes with strong blue photoluminescence [72,73]. Herein,we reported the photoluminescence properties of this new Ag/2-aminopyrazine/dicarboxylate family in the solid state. The solidstate photoluminescence data for the free ligands and complexes1–4 at room temperature are shown in Table 3 and Fig. 5. Theemission bands of the dicarboxylate ligands can be assigned tothe p* ? n transition, as previously reported [74], which is veryweak compared to that of the p* ? p transition of NH2pyz, sothe carboxylate ligands almost give no contribution to the fluores-cent emission of the as-synthesized CPs [75]. Complexes 1–4 exhi-bit blue photoluminescence in the solid state, with a similaremission maxima range from 413 to 421 nm upon excitation at325–350 nm at room temperature. Such fluorescent emissionsmay be tentatively assigned to the intraligand transition of coordi-nated NH2pyz, since a similar emission at 394 nm was also ob-served for the NH2pyz ligand. The shifted emissions andenhancement of luminescence of 1–4 were attributed to ligandcoordination to the metal center, which effectively increases therigidity of the ligand and reduces the loss of energy by radiation-less decay. These complexes may be excellent candidates for pho-

ligands in 4 with 30% thermal ellipsoid probability; hydrogen atoms are omitted for+ 1/2, z; (iv) �x + 2, y, �z + 1/2) (b) View showing the infinite stair-like 1D chain

Table 3Comparison of emission peaks between complexes 1–4 and the free ligands.

Ligand (nm) NH2pyz H2ox H2adp H2bdc H2ndc Complex (nm) 1 2 3 4

kex 330 280 330 330 330 kex 330 325 330 350kem 394 342 378 384 480 kem 421 419 413 418

Fig. 5. Solid-state emission spectra for 1–4 at room temperature.

D. Sun et al. / Polyhedron 29 (2010) 1243–1250 1249

toactive materials and for thermally stable and solvent-resistantblue fluorescent material.

4. Conclusions

This work presents a series of new Ag(I) CPs generated from themixed-ligand system of 2-aminopyrazine and dicarboxylates un-der similar synthetic conditions. It is clear that the distinct frame-work structures spanning from 2D to 3D are ascribed to thedifferent coordination modes, length and flexibility of the dicar-boxylate ligands. The carboxylate group is not only used as aneffective organic spacer but also as a counteranion in the construc-tion of the CPs. The rigid bridging pyrazinyl ligand utilizes two ter-minal N atoms to coordinate with Ag(I) ions, just like a spacer, togenerate 1D chain building blocks in 1–4. The dicarboxylates actas bidentate ‘‘pillars” to extend the 1D chains into higher-dimen-sional coordination networks. A systematic investigation was real-ized in this work by using varied dicarboxylate ligands, and CPswith different structures and topologies were obtained. Moreover,the photoluminescence properties of these complexes have beenexamined.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (No. 20721001) and 973 Project(Grant 2007CB815301) from MSTC.

Appendix A. Supplementary data

CCDC 734707, 734708, 734709 and 734710 contain the supple-mentary crystallographic data for 1–4. These data can be obtainedfree of charge via http://www.ccdc.cam.ac.uk/conts/retriev-ing.html, or from the Cambridge Crystallographic Data Centre, 12Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;

or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associatedwith this article can be found, in the online version, at doi:10.1016/j.poly.2009.12.038.

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