Coordination Polymers Involving 1,2-Phenylenediacetic Acid via the

Jan 7, 2010 - thermal methods afforded microcrystalline products unsui- table for single crystal X-ray diffraction analysis. Consider- ing that the in...
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DOI: 10.1021/cg901545k

Formation of Three New Silver(I) Coordination Polymers Involving 1,2-Phenylenediacetic Acid via the Modulation of Dipyridyl-Containing Ligands

2010, Vol. 10 1443–1450

Guo-Ping Yang, Yao-Yu Wang,* Ping Liu, Ai-Yun Fu, Ya-Nan Zhang, Jun-Cheng Jin, and Qi-Zhen Shi Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China Received December 9, 2009

ABSTRACT: Three new silver(I) metal-organic frameworks (MOFs), namely, [Ag2(pda)(bipy)2]n 3 4nH2O (1), [Ag2(pda)(dpe)2(H2O)]n 3 4nH2O (2), and [Ag(Hpda)(bpp)]n (3), were prepared by hydrothermal reactions of silver(I) acetate with H2pda (H2pda=1,2-phenylenediacetic acid) in the presence of different pyridyl-containing ligands (bipy=4,40 -bipyridine, dpe=1,2di(4-pyridyl)-ethylene, and bpp=1,2-bis(4-pyridyl)propane) and structurally characterized by single crystal X-ray diffraction. Complex 1 shows a two-dimensional (2D) four-connected network with a long topological vertex Schl€ afli symbol (4.62.4.63.64.6). Of particular interest, a new unreported octameric water cluster, (H2O)8, which is comprised of a cyclic water tetramer and four dangling water monomers, exists in 1. And the pseudorotaxane motifs are formed by these octameric clusters and the [Ag8(pda)2(bipy)4] loops. Complex 2 features a one-dimensional (1D) ladder-like chain structure, and the 2D networks, assembling by the combination of the pentameric water cluster, (H2O)5, and pda dianions, are supported by the 1D polymeric chains to form an infinite three-dimensional (3D) supramolecular framework. Complex 3 is a highly puckered 2D (6,3) sheet and further stacks via hydrogen bonding interlocks notated as R22 (18) to afford a unique 3D 66 topology with a long topological vertex Schl€afli symbol of (6.62.6.62.6.62) but which is not a typical diamondoid network. The diverse arrangements of the complexes show the remarkable sensitivity of the AgI-H2pda system to the different dipyridyl ligands; that is to say, the subtle modulation of dipyridyl-containing ligand has a significant effect on the overall MOFs and the coordination modes of H2pda ligands. In addition, the properties of thermogravimetric analysis-differential scanning calorimetry, X-ray powder diffraction, and photoluminescent behaviors of the complexes have been also discussed.

Introduction The rational design and well-controlled construction of metal-organic frameworks (MOFs) is a very fertile research focus in recent years, mostly motivated by the potential applications as functional materials via supramolecular chemistry and crystal engineering of the constituent building blocks using the tools of the modern synthetic chemistry.1 Indeed, many MOFs have been reported to exhibit intriguing properties including molecular recognition, magnetism, gas storage, ion exchange, and so on.2,3 Thus far, it is well-known that the organic ligands play important roles in the design and syntheses of desirable MOFs. The changes in functional-group, flexibility, length, and symmetry of the ligands can result in a remarkable class of materials bearing diverse architectures and functions. Among them, the rigid organic polycarboxylates are often employed as bridging ligands to afford a variety of novel functional complexes due to their versatile coordination modes.4,5 It is very surprised that very little attention was focused on the flexible aromatic dicarboxylate ligands; however, when they react with different metal ions, they can adopt different conformations to meet the space requirement and lead to unexpected, unpredictable, and intriguing frameworks. Therefore, the construction of target complexes with special properties must be very appealing to synthetic chemists by utilizing the ligands. Moreover, the combination of metal *Corresponding author. E-mail: [email protected]. r 2010 American Chemical Society

ions and different dicarboxylates or rigid and flexible pyridylcontaining bridging ligands, such as bipy (4,40 -bipyridine), dpe (1,2-di(4-pyridyl)ethylene), bpp (1,2-bis(4-pyridyl)propane), or both can allow the formation of MOFs possessing fascinating architectures and novel topology.6 According to that mentioned above, recently we have begun to utilize a multifunctional flexible tetra-carboxylicate acid, namely, 3,30 ,4,40 -benzophenonetetracarboxylic acid, to isolate a series of new unusual MOFs.7 To extend our previous research work and fully understand the coordination chemistry of the flexible ligands, another flexible aromatic dicarboxylate, 1,2-phenylenediacetic acid (H2pda) as the starting building block, the MOFs of which have been largely uninvestigated so far,8 was introduced based on the following considerations: (1) as symmetrical and flexible ligands, C(sp3)-C(sp2, on the carboxylate group) bonds can rotate freely, so the H2pda ligand has three possible steric conformations (Scheme 1a-c), which may provide various connection modes with metal ions and thus afford abundant structural motifs; (2) it can also provide potential identified sites of hydrogen bonding and π 3 3 3 π stacking interactions to form complicated structures with higher dimensions; (3) its deprotonation may be affected by different pH values, which will again have a crucial influence on network topology. Herein, we report the syntheses, crystal structures, and characterizations of three new silver(I) MOFs, namely, [Ag2(pda)(bipy)2]n 3 4nH2O (1), [Ag2(pda)(dpe)2(H2O)]n 3 4nH2O (2), and [Ag(Hpda)(bpp)]n (3), using subtle modification of the length of pyridyl-containing bridging ligands. The results Published on Web 01/07/2010

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Scheme 1. The Steric Conformations of H2pda Ligand and the Coordination Modes Observed in 1-3

Table 1. Crystallographic Data for Complexes 1-3 complex empirical formula

indicate that the modulation of the dipyridyl-containing ligands has a significant effect on the overall MOFs and the coordination modes of H2pda ligands. Moreover, thermogravimetric analysis-differential scanning calorimetry (TGADSC), X-ray powder diffraction (XRPD), and photoluminescent behaviors of the complexes are also discussed. Experimental Section Materials and Physical Measurements. All reagents used in the syntheses were of analytical grade, and 1,2-phenylenediacetic acid was bought from Alfa Aesar. Elemental analyses for carbon, hydrogen, and nitrogen atoms were determined with a PE 2400 Elemental analyzer. The infrared spectra of KBr pellets were recorded with a BRUKER EQUINOX-55 spectrometer in the range of 4000-400 cm-1. Luminescence spectra for the solid samples were recorded with a Hitachi F-4500 fluorescence spectrophotometer. Thermogravimetric analyses were performed on a NETZSCH STA 449C microanalyzer in a N2 atmosphere at a heating rate of 10 min-1 between ambient temperature and 1000 °C. The XRPD pattern was recorded with a Pigaku D/Max 3III diffractometer. Preparation of Complexes 1-3. [Ag2(pda)(bipy)2]n 3 4nH2O (1). A mixture of AgOAc (16.7 mg, 0.1 mmol), H2pda (9.7 mg, 0.05 mmol), bipy (15.6 mg, 0.1 mmol), and H2O of 15 mL was adjusted to pH = 6 with 0.5 M NaOH solution and then placed in a sealed Teflon-lined stainless steel vessel, heated at 130 °C for 3 days. Then the reaction system was cooled to room temperature over 24 h. Colorless needle crystals of 1 were obtained. Yield: 49%. Elemental analysis (%): calcd for C30H32N4O8Ag2 (792.34) C 45.48, H 4.07, N 7.07; found C 45.06, H 3.70, N 6.98%. IR (KBr, cm-1): 3361s, 3037 m, 1589s, 1531 m, 1486w, 1399s, 1217 m, 1094w, 1071w, 1042w, 994w, 944w, 912w, 803 m, 619 m, 569w, 495w. [Ag2(pda)(dpe)2(H2O)]n 3 4nH2O (2). Complex 2 was synthesized in a way similar to that described for complex 1, except that bipy was replaced by dpe (1.0 mmol, 18.2 mg). Yield: 42%. Elemental analysis (%): calcd for C34H38N4O9Ag2 (862.42) C 47.35, H 4.44, N 6.50; found C 46.85, H 4.87, N 6.75%. IR (KBr, cm-1): 3423s, 2923 m, 1595s, 1496 m, 1382s, 1212 m, 1069w, 1004w, 993w, 971w, 825 m, 708w, 620w, 548 m. [Ag(Hpda)(bpp)]n (3). Complex 3 was synthesized in a similar way as that described for complex 1, except that bipy was replaced by bpp (1.0 mmol, 19.8 mg) and H2pda (19.4 mg, 0.05 mmol) at pH = 5. Yield: 31%. Elemental analysis (%): calcd for C23H23N2O4Ag (449.30) C 55.33, H 4.64, N 5.61; found C 55.05, H 4.92, N 6.01%. IR (KBr, cm-1): 3452s, 3065s, 3019 m, 2925 m, 2826 m, 1936 m, 1718s, 1605s, 1497 m, 1438 m, 1347 m, 1259 m, 1193w, 898 m, 804w, 758 m, 709w, 509 m. X-ray Crystallography. Single crystal X-ray diffraction analyses of 1-3 were carried out on a Bruker SMART APEX II CCD diffractometer equipped with a graphite monochromated Mo KR

formula weight crystal system space group a/A˚ b/A˚ c/A˚ R/° β/° γ/° V/A˚3 Z T/K dcalcd/g 3 cm-3 μ/mm-1 F(000) θ range (°) reflections collected Rint goodness-of-fit on F2 R1a, wR2b [I > 2σ(I)] R1, wR2 (all data)

1

2

3

C30H32N4O8Ag2 792.34 monoclinic P21/c 10.686(2) 18.519(4) 15.891(4) 90 106.038(4) 90 3022.1(12) 4 273(2) 1.741 1.353 1592 1.73-25.10 14968 0.0658 0.913 0.0664, 0.1530 0.1240, 0.1703

C34H38N4O9Ag2 862.42 triclinic P1 11.3125(13) 11.5420(13) 13.9032(16) 84.266 82.614(2) 75.128(2) 1735.7(3) 2 296(2) 1.650 1.178 872 1.48-25.10 8777 0.0255 1.042 0.0457, 0.1130 0.0711, 0.1305

C23H23N2O4Ag 499.30 monoclinic P21/n 11.3734(1) 8.8235(14) 21.111(3) 90 94.015(3) 90 2113.4(6) 4 296(2) 1.569 0.986 1016 1.93-25.09 10335 0.0559 1.032 0.0502, 0.1125 0.0976, 0.1436

R1 = (Σ||Fo| - |Fc|)/Σ|Fo|. b wR2 = [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2.

a

radiation (λ = 0.71073 A˚) by using j/ω scan technique at 273(2) K for 1, 296(2) K for 2, and 3. The structures were solved by direct methods with SHELXS-97. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by the use of geometrical restraints. A full-matrix leastsquares refinement on F2 was carried out by using SHELXS-979 and SHELXL-97.10 Hydrogen atoms of water molecules were located by Fourier map, and then refined by riding mode. A summary of the crystallographic data and structure refinement details are given in Table 1, and selected bond lengths and angles are listed in Table 2. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center. CCDC reference numbers: 734033-734035.

Results and Discussion Syntheses and General Characterization. The first attempts to react silver(I) salts with H2pda ligand directly by hydrothermal methods afforded microcrystalline products unsuitable for single crystal X-ray diffraction analysis. Considering that the introduction of pyridyl-containing ancillary ligands might direct the frameworks of the formed metalorganic complexes, the dipyridyl ligands with different lengths, such as bipy, dpe, and bpp, as the auxiliary ligands were chosen in the system (Scheme 2). By fine-tuning the reaction conditions such as the reaction temperature, pH values, and the metal-to-ligand ratio, three complexes were isolated in good crystalline forms. The results reveal that the dipyridyl ligands play very crucial roles in the process of forming MOFs. This “mixed-ligands” method is a very effective method to build MOFs and has aroused much attention in the design of novel MOFs.11 The IR spectra of the three complexes were performed by KBr pellets in the range of 4000 - 400 cm-1. The spectra show characteristic absorption bands mainly attributable to the carboxylate groups stretching vibrations. The samples all display a strong absorption in the range of ∼1610-1580 cm-1, which may be assigned to the asymmetric stretching mode, and the strong bands at about ∼1440-1380 cm-1 are attributed to the symmetric stretching mode. The spectrum of complex 3 exhibits a strong band at ∼1718 cm-1, assignable to the protonated carboxylate group of Hpda ligand,5e

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Table 2. Selected Bond Lengths (A˚) and Angles (°) of Complexes 1-3a Complex 1 Ag(1)-N(1) Ag(2)-N(4)#3 Ag(1)-Ag(2) N(2)#1-Ag(1)-N(1) N(1)-Ag(1)-O(3)#2 N(4)#3-Ag(2)-N(3) N(4)#3-Ag(2)-Ag(1)

2.199(6) 2.206(6) 3.121(1) 166.6(2) 95.4(2) 161.2(2) 104.59(17)

Ag(1)-O(1) Ag(2)-O(5) Ag(2)-N(4)#5 N(3)-Ag(1)-N(1) N(3)-Ag(1)-Ag(1)#4 N(2)-Ag(2)-N(4)#5

2.564(4) 2.597(4) 2.136(4) 159.62(16) 82.76(11) 166.49(18)

Ag(1)-O(1) Ag(1)-Ag(1)#6 N(2)-Ag(1)-N(1) N(1)-Ag(1)-Ag(1)#6

2.512(5) 2.9796(10) 160.88(17) 89.37(12)

Ag(1)-N(2)#1 Ag(2)-N(3)

2.186(6) 2.220(6)

Ag(1)-O(3)#2 Ag(2)-O(1)

2.566(6) 2.500(5)

N(2)#1-Ag(1)-O(3)#2 N(1)-Ag(1)-Ag(2) N(4)#3-Ag(2)-O(1) N(3)-Ag(2)-Ag(1)

92.7(2) 86.01(18) 111.2(2) 90.21(18)

N(2)#1-Ag(1)-Ag(2) O(3)#2-Ag(1)-Ag(2) N(3)-Ag(2)-O(1) O(1)-Ag(2)-Ag(1)

101.84(18) 106.64(13) 86.2(2) 60.16(14)

Complex 2 Ag(1)-N(1) Ag(2)-N(2)

2.204(4) 2.130(4)

Ag(1)-N(3) Ag(1)-Ag(1)#4

2.195(4) 3.1417(9)

N(3)-Ag(1)-O(1) N(1)-Ag(1)-Ag(1)#4 N(2)-Ag(2)-O(5)

88.12(14) 105.41(10) 96.69(16)

N(1)-Ag(1)-O(1) O(1)-Ag(1)-Ag(1)#4 N(4)#2-Ag(2)-O(5)

112.23(14) 61.19(8) 96.79(16)

Ag(1)-N(2)

2.169(5)

Ag(1)-N(1)

2.170(5)

N(2)-Ag(1)-O(1) N(2)-Ag(1)-Ag(1)#6

97.50(17) 98.85(12)

N(1)-Ag(1)-O(1) O(1)-Ag(1)-Ag(1)#6

94.10(16) 116.66(12)

Complex 3

a Symmetry codes: #1 x - 1, -y þ 1/2, z - 1/2; #2 -x þ 1, y þ 1/2, -z þ 3/2; #3 x þ 1, -y þ 1/2, z þ 1/2; #4 -x þ 2, -y þ 1, -z; #5 x - 1, y þ 2, z þ 1; #6 -x þ 1, -y, -z þ 2.

Scheme 2. The Synthetic Procedures of Complexes 1-3

Figure 1. The coordination environment of the silver(I) ion in 1. Hydrogen atoms and guest water molecules are omitted for clarity. Symmetry codes: A, 1 - x, 0.5 þ y, 1.5 - z; B, -1 þ x, 0.5 - y, -0.5 þ z; C, 1 þ x, 0.5 - y, 0.5 þ z.

which is consistent with the result of the X-ray diffraction analysis. Crystal Structure of [Ag2(pda)(bipy)2]n 3 4nH2O (1). Singlecrystal analysis reveals that complex 1 crystallizes in the monoclinic space group P21/c and exhibits a 2D doublelayered grid network. The asymmetric unit of 1 contains two silver(I) ions, one pda ligand, two bipy ligands, and four lattice water molecules. As shown in Figure 1, there are the two kinds of coordination environments around silver(I) ions. Each silver(I) ion adopts a three-coordinated T-shaped geometry including argentophilic Ag 3 3 3 Ag interactions (3.121 A˚),12 which is shorter than the van der Waals radii (3.44 A˚), and is rather close to the Ag 3 3 3 Ag distance in silver metal (2.89 A˚).13 All silver(I) ions take coordination with one oxygen atom from one pda ligand and two nitrogen atoms from two discrete bipy ligands. The bond lengths of Ag-N are in the range from 2.186 to 2.220 A˚ and Ag-O are 2.500

and 2.566 A˚, respectively, which are consistent with those values reported for Ag-carboxylate and Ag-pyridyl complexes.14 The pda ligand shows a trans bis-monodentate mode (Scheme 1d) to bridge the silver(I) ions to form a 1D chain in the assembly of 1. Then these 1D chains are further crosslinked by bridging bipy ligands to generate a 2D doublelayered grid network with 10.216  11.423 A˚2 windows, as depicted in Figure 2a. From the viewpoint of network topology, all the silver(I) centers can be regarded as fourconnected nodes (Figure S1, Supporting Information); therefore, the 2D double-layered network can be simplified as a unique net with a long topological vertex Schl€ afli symbol of (4.62.4.63.64.6), as shown in Figure 2b. Of further interest, four lattice water molecules (O5, O6, or its symmetry related atoms) form a cyclic water tetramer through hydrogen bonding interactions (Table S1, Supporting Information), locating in the windows, with each water monomer acting as both single hydrogen bonding donor and acceptor. When the free water molecules were removed from 1, the effective void space is calculated by the PLATON

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Figure 2. (a) The 2D double-layered grid structural motif of 1 as a space-filling representation. (b) Schematic view of the four-connected network topology with a vertex symbol of (4.62.4.63.64.6); the nodes represent the silver(I) ions.

program as being 15.6% of the crystal volume (470.7 A˚3 out of the 3022.3 A˚3 unit cell volume).15 The remaining four hydrogen atoms of the cyclic water tetramer are 0.386 and 0.705 A˚ beyond the ring plane, showing an unusual uudd form but not udud form (Figure S2, Supporting Information), that has just been observed experimentally.16 The average O 3 3 3 O distance within the tetramer is 2.784 A˚, which is very close to the corresponding value of 2.743 A˚ calculated in the discrete water tetramer.17 Moreover, other four free water (O7, O8, or its symmetry related atoms) molecules are bonded to the water corner of the tetramer, affording an octameric water cluster, (H2O)8, which is comprised of a cyclic water tetramer and four dangling water monomers (Figure 3a). Then the pseudorotaxane motifs are formed by these octameric clusters and the [Ag8(pda)2(bipy)4] loops, which is relatively scarcely observed in coordination frameworks1c,g (Figure 3b). And remarkably, the octameric cluster described here is different from the previously reported examples, such as opened cube, cubane, cyclic ring, extended book-shaped ring, and so on.18 To the best of our knowledge, the (H2O)8 cluster exhibits a novel arrangement that has not been reported so far. Then the “guest” water cluster, (H2O)8, just like a “glue” agglutinates and supports the “host” 2D double-layered net to form a 3D supramolecular framework by abundant hydrogen bonding interactions involving carboxylic oxygen atoms and crystallization water molecules (Figure 3c). Crystal Structure of [Ag2(pda)(dpe)2(H2O)]n 3 4nH2O (2). When a longer rigid ligand dpe was employed instead of bipy ligand in the experiment, a 1D ladder-like complex 2 was obtained. The fundamental building unit of 2 consists of two silver(I) ions, one pda ligand, two dpe ligands, one coordinated water molecule, and four lattice water molecules. As shown in Figure 4a, the first silver(I) ion (Ag1) is located in a distorted tetrahedral environment formed by one oxygen atom from one pda, two nitrogen atoms from two different dpe ligands, and one symmetry-related silver(I) ions (Ag 3 3 3 Ag interactions: 3.141 A˚); the second silver(I) center (Ag2) adopts a T-shaped coordination geometry and is three-coordinated by two nitrogen atoms from two discrete dpe ligands and one coordinated water molecule, and the distance between Ag2 centers is 4.016 A˚, far longer than 3.44 A˚, the sum of van der Waals radii, implying the absence of the Ag 3 3 3 Ag interactions. In 2, the pda ligand features a trans monodentate mode (Scheme 1e) and the other deprotonated carboxylate group

Figure 3. (a) The octomeric water cluster and its immediate environment as found in 1. Symmetry codes: A, 2 - x, -y, 1 - z; B, 2 - x, -y, 2 - z; C, x, y, 1 þ z; D, 1 þ x, y, 1 þ z; E, 1 - x, -y, 1 - z; F, 1 þ x, y, z; G, 1 - x, -y, 2 - z. (b) The pseudorotaxane motif formed by octameric cluster and the [Ag8(pda)2(bipy)4] loop (left) and its schematic view (right). (c) The perspective view of the 3D supramolecular framework of 3. Hydrogen atoms are omitted for clarity.

does not take coordination with silver(I) ions, which is different from that of 1. Therefore, the 1D laddered chains with the appending antiparallel pda ligands are formed

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Figure 4. (a) The coordination environment of the silver(I) ion in 2. All hydrogen atoms and free water molecules are omitted for clarity. Symmetry codes: A, 2 - x, 1 - y, -z; B, -1 þ x, 2 þ y, 1 þ z. (b) View of the 1D laddered chains with the appending antiparallel pda ligands. (c) View of the 2D layered network of 2 through π 3 3 3 π stacking interactions; the pda ligands are omitted for clarity.

by the bridging bidentate bpe ligands in complex 1 (Figures 4b and S3, Supporting Information). Then the neighboring 1D chains are connected to a 2D layered network through π 3 3 3 π stacking interactions (Figure 4b), where the centroid-to-face distances of the phenyl moieties of bpe ligands are 3.649 and 3.744 A˚ (Figure S4, Supporting Information). Further analysis of the crystal packing shows that a pentameric water cluster is formed by four lattice water molecules and one clathrated water molecule (Figure 5a). The average O 3 3 3 O distance within the (H2O)5 cluster is 2.766 A˚, which is very close to the value of complex 1. The water pentamers were often found to exist in large water clusters, liquid water, and the surfaces of proteins.19 However, its solid structural studies are quite limited in comparison with those of the more common water clusters. As we know, the water cluster of 2 is the first reported as like that of 1. In particular, a 2D network is assembled by the combination of the (H2O)5 cluster and pda dianions based on hydrogen bonding interactions (Figure S5, Supporting Information). As shown in Figure 5b, within the 2D layer, each individual (H2O)5 cluster is hydrogen bonded to four pda dianions (O 3 3 3 O: 2.914, 2.763, 2.810, 2.786, 2.779, and 2.837 A˚) as a four-connected spacer. Finally, these 2D nets are further supported by the 1D polymeric pillars to form an infinite 3D supramolecular framework (Figure S6, Supporting Information). In this case, these water clusters may play a complementary role for stabilizing and strengthening the overall structure of the complex 2 by hydrogen bonding interlocks like that of complex 1.

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Figure 5. (a) Self-assembly of the pentameric water cluster and its immediate environment as found in 2. Symmetry codes: A, 1 - x, 2 - y, 1 - z; B, 1 - x, 1 - y, 1 - z; C, -1 þ x, y, 1 þ z; D, 1 - x, 2 y, -z; E, 1 - x, 1 - y, -z. (b) View of the 2D network formed by the water pentamers and pda dianions, showing the hydrogen-bonding motif.

Figure 6. The coordination environment of the silver(I) ion in 3. Hydrogen atoms are omitted for clarity. Symmetry codes: A, 1 - x, -y, 2 - z.

Crystal Structure of [Ag(Hpda)(bpp)]n (3). Complex 3 exhibits a 2D wave-like layered structure. The asymmetric unit of 3 contains one independent silver(I) ion, one unique Hpda ligand, and one bpp ligand. As shown in Figure 6, silver(I) center (Ag1) has three-coordinated geometry defined by one oxygen atom from one Hpda unit and two nitrogen atoms from two individual bpp ligands. But the Ag-Ag interactions exist in 3, the distance of which is 2.980 A˚ (Ag1 3 3 3 Ag1A) and is very close to the Ag 3 3 3 Ag distance in silver metal (2.89 A˚).13 The bond lengths of Ag-N and Ag-O in 3 are typical values for silver(I) complexes.14 The Hpda ligand shows a trans monodentate mode (Scheme 1f) and the other carboxylate is protonated in 3, which is different from those of complexes 1 and 2. Generally, the flexible pyridyl-containing bpp ligand can adopt

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Figure 7. (a) The 2D wave-like network polymer of complex 3 (Hpda ligands have been omitted for clarity). (b) Schematic view of the (6,3) topology network of 3. (c) The hydrogen bonding interlocks of Hpda ligands. (d) The unusual 66 topology network by hydrogen bonding interactions; the hydrogen-bonding links (dashed bonds) are shown at the bottom left only for clarity.

four different conformations (TT, GG, TG, and GG0 , T = trans and G=gauche, Figure S7, Supporting Information)20 with respect to relative orientations of the methylene groups. In 3, the bpp ligands with a TT conformation providing a longer N-to-N separation with the distance of 9.339 A˚ link the adjacent silver(I) ions to form a 2D wave-like network (Figure 7a). Topologically, if we define bpp ligands as the single spacers of connection to silver(I) centers, this type of network can be referred to as a unique example of a (6,3) topological net, as shown in Figure 7b. Furthermore, the neighboring 2D corrugated layers are combined to form a 3D supramolecular framework by hydrogen bonding interlocks (O3 3 3 3 O1: 2.518 A˚) of Hpda ligands, which can be notated as R22 (18) (Figure 7b). Then the overall 3D network is simplified as an unusual 66 topology with a long topological vertex Schl€afli symbol (6.62.6.62.6.62) considering the hydrogen bonding interactions (Figure 7d).21 Interestingly, this is the same short Schl€ afli symbol as that of diamond, Lonsdaleite, and a number of other four-connected nets;22 however, the topology is extremely different from them. There are three six-membered circuits with the chair conformation and other three twisted nonchair formation in complex 3, which is not observed experimentally to date,23 and other reported metal-organic coordination polymers with typical 66 topology just contain two or four or six sixcircuits with the chair conformation in the crystal structures. Discussion of the Structural Diversities of the Complexes. It has been demonstrated that the structural diversities of the complexes are undoubtedly related to the secondary ligand-directed inclusion.24 The combined effect of the different ligands is a key point to influence the final complexes, which depends on the competition and adaptability of the

components. That is to say, both kinds of organic ligands have to adjust themselves to meet the coordination geometry of metal center ions and the lower energetic arrangement in the assembly process. For 1, the rigid bipy ligands bridge the 1D [Ag2(pda)]n chains to form a 2D double-layered grid network with a long vertex symbol of (4.62.4.63.64.6). But for 2 and 3, with the increasing distance of two nitrogen atoms of dipyridyl ligands, although the reactors were kept constant in the synthetic process, the corresponding structures are largely different from the 1D ladder-like chain structure to the 2D (6,3) layered network. More recently, Li and his co-workers8a isolated two 1D zigzag chain and one 0D monomer complexes constructed from H2pda ligand in the presence of nitrogen-containing chelating ligands (2,20 -bipyridine, 1,10-phenanthroline). These results show that the subtle modulation of dipyridyl-containing ligand play an important role in the process of forming MOFs, and the successful preparations of these complexes will provide a valuable approach for the construction of other coordination polymers by introduction of other kinds of organic ligands. Thermal Analyses, X-ray Power Diffraction Analyses, and Photoluminescent Properties. Thermogravimetric (TG) analyses have been studied in the N2 atmosphere for complexes 1 and 2 between 20 and 1000 °C (Figure S8, Supporting Information). The results indicate that 1 showed an initial weight loss of 10.54% from ∼25 to ∼130 °C (The DSC curve shows an endothermic peak in range of ∼70-110 °C), corresponding to the removal of four free water molecules per formula unit (calculated: 9.09%), and then the main framework of 1 was stable up to ∼310 °C, followed by the weight loss of the organic ligands after the temperature

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In 2, a longer rigid ligand dpe was taken into account, and a 1D ladder-like chain structure was formed. When a flexible bpp ligand was used, 3 exhibits a unique 3D nondiamondoid 66 topological network with a long vertex Schl€afli symbol (6.62.6.62.6.62) based on a puckered 2D (6,3) sheet via hydrogen bonding interlocks. Notably, two novel water clusters, (H2O)8 and (H2O)5, are fused in complexes 1 and 2, which have not been reported in documents so far. Our research demonstrates that the H2pda ligand could be a potential building block with the combination of different coligands to construct coordination polymers with unusual properties, and the accessory ligand inclusion has a significant effect on the construction of the overall MOFs. Figure 8. Solid-state emission spectrum of H2pda ligand and complexes 1-3 at room temperature.

(The DSC curve shows a sharp exothermic peak at ∼350 °C). For 2, the TG-DSC curve is similar to that of 1: the weight loss of 10.25% below ∼215 °C (calculated: 10.44%) is attributed to loss of four lattice water and one coordination water molecules (The DSC curve shows an endothermic peak at ∼80 °C and an exothermic peak at ∼273 °C), and then the framework of complex 2 begins to collapse. Moreover, the purity of complexes was carried out by X-ray power diffraction analyses, in which the experimental patterns of 1-3 are almost consistent with their simulated patterns (Figures S9-S11, Supporting Information), although there are some minor different peaks in the experimental patterns, such as the positions, intensities, etc., compared with that of the simulated. And as shown in Figure S9, the diffraction spectra of 1 after heating at 265 °C are almost same as that of the assynthesized complex, which indicates that 1 remained the intact host framework after removal of the guest water molecules.25 But when complex 1 was heated at 315 °C, the main diffraction peaks were missed or absent in the spectra pattern, which was in good agreement with the TG-DSC curve of 1. The mixed inorganic-organic hybrid coordination polymers with d10 metal ions (AgI, CuI, ZnII, CdII, HgII) have been investigated for their fluorescence properties and potential applications.26 Therefore, in the present work, the luminescent properties of complexes 1-3 and free H2pda ligand were studied in the solid state (λex = 300 nm). As shown in Figure 8, very similar emission bands (For 1, λem= 451 and 412 nm; 2, λem = 456 nm and 409 nm; 3, λem = 396 and 450 nm) were observed in the three samples, while the emission of the free H2pda ligand with λem = 414 nm is observed. The lower energy bands would be attributed to the ligand-to-metal charge transfer (LMCT). The shoulder bands of the emission spectrum in the complexes would be assigned to the intraligand emission from dipyridyl ligands. Generally speaking, silver(I) complexes might emit weak photoluminescence at low temperature, and, consequently, the complexes reported herein represent unusual examples of room-temperature luminescent Ag-containing polymers.27 Conclusions In summary, we have successfully synthesized three new silver(I) coordination polymers constructed from the H2pda ligand by fine-tuning modification of dipyridyl-containing bridging ligands. For 1, a rigid rodlike bipy ligand was selected as an accessory ligand, and a 2D grid network was obtained.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20771090) and TRAPOYT, and Specialized Research Found for the Doctoral Program of Higher Education (No. 20050697005). Supporting Information Available: X-ray crystallographic files in CIF format of complexes 1, 2, and 3. Hydrogen bonding lengths and angles of complexes 1, 2, 3, additional figures of the structures, and TG-DSC curves, XRPD patterns of complexes 1, 2, and 3. This material is available free of charge via the Internet at http://pubs. acs.org.

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