Syntheses, Structures, and Properties of SilverâOrganic Frameworks

Article pubs.acs.org/crystal

Syntheses, Structures, and Properties of Silver−Organic Frameworks Constructed with 1,1′-Biphenyl-2,2′,6,6′-tetracarboxylic Acid Bo Li,† Shuang-Quan Zang,*,† Can Ji,† Hong-Wei Hou,† and Thomas C. W. Mak†,‡ †

Department of Chemistry, Zhengzhou University, Zhengzhou 450001, People’s Republic of China Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: The reactions of 1,1′-biphenyl-2,2′,6,6′-tetracarboxylic acid (H4bptc), Ncontaining auxiliary ligand, with different silver(I) salts under hydrothermal or solvent evaporation conditions yielded five unusual complexes with distinct structural features: {[Ag4(bptc)(bpy)4(H2O)]·(H2O)12}n (1), {[Ag1.5(bptc)0.5(bpy)1.5]·(NH4)0.5·(H2O)3}n (2), {[Ag2(H2bptc)(bpy)2]·(H2O)2}n (3), {[Ag2(H2bptc)(bpy)2]·(HClO4)}n (4), and {[Ag(H2bptc)0.5(bpe)]·(H2O)2}n (5) (bpy = 4,4′-bipyridine, bpe = 1,2-bis(4-pyridyl)ethene). Complexes 1−3 and 5 exhibit two-dimensional (2D) layer, and such layers are further united together to generate three-dimensional supramolecular structure through hydrogen bonding and π···π interactions, respectively. In complexes 1 and 2, butterfly shaped 16membered water ring and infinite water tape are obtained, respectively. Both compounds 4 and 5 are 2-fold interpenetrated 3D frameworks. Complex 4 shows channels along the c axis that are occupied by the perchloric acid molecules. Different from robust 4, the 3D supramolecular architecture of 5 is based on the formation of hydrogen bonding between (H2O)4 cluster and the adjacent layers. In addition, the thermogravimetric analysis, powder X-ray diffraction (XRD), and photoluminescent behavior of the complexes have also been investigated.

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INTRODUCTION The construction of inorganic−organic coordination polymeric complexes has attracted great attention from chemists due to their potential as functional materials.1 A great number of coordination polymers with various structures have been reported to date; nevertheless, it is still a great challenge to predict the exact structure of assembly products in crystal engineering. In fact, the formation of coordination polymers is influenced by factors such as solvent,2 the pH value of the reaction,3 molar ratio of the molecular components,4 solvent,5 steric requirement of the counterions,6 reaction temperature,7 together with the coordination nature of metal ions8 and organic ligands,9 although the influencing information on supramolecular assemblies is not yet well understood. As is well-known, silver(I) ion principally exhibits linear, seesaw, trigonal, square-pyramidal, trigonal-bipyramidal, squareplanar, tetrahedral, and octahedral coordination geometries and has high affinity for hard donor atoms such as nitrogen or oxygen atoms,10 and is apt to form short Ag···Ag contacts, which have been provedn to be two of the most important factors contributing to the formation of such complexes and special properties.11 In our previous studies, we reported the syntheses and structures of silver complexes with adjacent dense polycarboxylate ligands.12 The results showed that these complexes have novel structures due to their multiple coordination modes for adjusting microstructures and producing frameworks of various dimensions and connections. © 2012 American Chemical Society

To expand our system, and, at the same time, to further investigate the influence factor on formation of supramolecular architectures, we designed and prepared a new polycarboxylate ligand: 1,1′-biphenyl-2,2′,6,6′-tetracarboxylic acid (H4bptc). As a member of polycarboxylate ligands, H4bptc has a rich variety of coordination modes with four carboxylic groups, which may be completely or partially deprotonated upon the pH and help to construct novel coordination polymers with various dimensions and connections; on the other hand, it is a flexible ligand because two phenyl rings can rotate around the C−C single bond, which can be used to construct intriguing coordination polymers because of the noncoplanarity of two phenyl rings. Unfortunately, it is difficult to control the symmetry of the resulting frameworks due to the silver(I) ion’s rich coordination modes. Although many H4bptc bridged MOFs have been reported,13 those on a silver ion are relatively rare, probably because of their low solubility, which makes structural analyses difficult.14 On the basis of our previous studies, herein, we focus on the self-assembly of silver(I) and carboxylate incorporating bpy or bpe as auxiliary ligand and obtained five new metal− organic frameworks, {[Ag4(bptc)(bpy)4(H2O)]·(H2O)12}n (1), { [ A g 1 . 5 ( b p t c ) 0 . 5 ( b p y ) 1 . 5 ]· ( N H 4 ) 0 . 5·( H 2 O ) 3 } n ( 2 ) , {[Ag 2 (H 2 bptc)(bpy) 2 ]·(H 2 O) 2 } n (3), {[Ag 2 (H 2 bptc)(bpy)2]·(HClO4)}n (4), and {[Ag(H2bptc)0.5(bpe)]·(H2O)2}n Received: November 22, 2011 Revised: December 31, 2011 Published: January 24, 2012 1443

dx.doi.org/10.1021/cg2015447 | Cryst. Growth Des. 2012, 12, 1443−1451

Crystal Growth & Design

Article

Syntheses. H4bptc was synthesized according to the literature.15 Other starting materials were of reagent quality and were obtained from commercial sources without further purification. Synthesis of {[Ag4(bptc)(bpy)4(H2O)]·(H2O)12}n (1). Excess aqueous NH3 solution was slowly added dropwise to a suspension of Ag2O (0.023 g, 0.1 mmol) in MeOH/H2O (6 mL, 5:1 v/v), and the mixture was stirred for 15 min. H4bptc (0.017 g, 0.05 mmol) and 4,4′bipyridine (0.015 g, 0.1 mmol) were then slowly added, and stirring was continued for another 30 min. The resultant colorless solution was allowed to stand in the dark at room temperature for a week to give colorless prisms of 1 (yield, 75% based on silver). Anal. Calcd for C56H64Ag4N8O21 (1616.63): C, 41.61; H, 3.99; N, 6.93. Found: C, 41.67; H, 4.08; N, 6.99. IR/cm−1 (KBr): 3440 (s), 1600 (m), 1413 (w), 1366 (m), 1222 (w), 1119 (w), 805 (w), 696 (w), 617 (w). Synthesis of {[Ag1.5(bptc)0.5(bpy)1.5]·(NH4)0.5·(H2O)3}n (2). The procedure is also similar to the synthesis of 1 except that AgNO3 (0.017 g, 0.1 mmol) was used instead of Ag2O. Yield: 73% based on silver. Anal. Calcd for C46H46Ag3N7O14 (1244.51): C, 44.39; H, 3.73; N, 7.88. Found: C, 43.38; H, 3.75; N, 7.81. IR/cm−1 (KBr): 3404 (m), 3038 (w), 1598 (s), 1405 (s), 1216 (w), 803 (m), 616 (w), 569 (w), 495 (w). Synthesis of {[Ag2(H2bptc)(bpy)2]·(H2O)2}n (3). 4,4′-Bipyridine (0.015 g, 0.1 mmol) was added to an aqueous solution (7 mL) of AgNO3 (0.017 g, 0.1 mmol) and H4bptc (0.017 g, 0.05 mmol), and the pH was adjusted to ca. 5 with dropwise addition of dilute KOH. Next, the resultant solution was placed in a Teflon-lined stainless steel vessel and heated at 160 °C for 3 days, and then slowly cooled to room temperature. Yield: 61% based on silver. Anal. Calcd for C36H28Ag2N4O10 (892.36): C, 48.45; H, 3.16; N, 6.28. Found: C, 48.50; H, 3.11; N, 6.21. IR/cm−1 (KBr): 3440 (s), 1718 (m), 1603 (s), 1416 (m), 814 (m), 765 (w), 638 (w). Synthesis of {[Ag2(H2bptc)(bpy)2]·(HClO4)}n (4). 4,4′-Bipyridine (0.015 g, 0.1 mmol) was added to an aqueous solution (7 mL) of AgClO4 (0.021 g, 0.1 mmol) and H4bptc (0.017 g, 0.05 mmol). Colorless block-like crystals of 4 were obtained in 45% yield based on silver. Anal. Calcd for C36H24Ag2ClN4O12 (955.78): C, 45.24; H, 2.53; N, 5.86. Found: C, 45.23; H, 2.59; N, 5.91. IR/cm−1 (KBr): 3423 (br), 1714 (s), 1598 (m), 1411 (s), 1384 (w), 1090 (w), 804 (w), 626 (w). Synthesis of {[Ag(H2bptc)0.5(bpe)]·(H2O)2}n (5). The procedure is also similar to the synthesis of 3 except that bpe (0.018 g, 0.1 mmol) was used instead of bpy. Yield: 50% based on silver. Anal. Calcd for

(5). As shown in Scheme 1, the bptc4− and H2bptc2− ligands in 1−5 exhibit a variety of coordination fashions in coordinating with Ag(I) ions. Scheme 1. Coordination Modes of the bptc4− and H2bptc2− in Complexes 1−5

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All reagents and solvents employed in the present work were of analytical grade as obtained from commercial sources without further purification. Elemental analysies for C, H, and N were performed on a PerkinElmer 240 elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range from 4000 to 400 cm−1 on a Bruker VECTOR 22 spectrometer. Thermal analysis was performed on a SDT 2960 thermal analyzer from room temperature to 800 °C with a heating rate of 20 °C/min under nitrogen flow. Powder X-ray diffraction (PXRD) data for 1−5 were collected on a Rigaku D/Max-2500PC diffractometer with Cu Kα radiation (λ = 1.5406 Å) over the 2θ range of 5−50° at room temperature. Luminescence spectra for the solid samples were recorded on a Hitachi 850 fluorescence spectrophotometer.

Table 1. Crystallographic Data for Complexes 1−5 formula Mr cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V/Å3 Z Dc/g cm−3 F(000) reflns collected independent reflns Rint GOF on F2 R1, wR2 [I > 2σ(I)] R1, wR2 (all data) a

1

2

3

4

5

C56H64Ag4N8O21 1616.63 triclinic P1̅ 14.4608(5) 15.5768(6) 16.2543(7) 77.430(3) 87.842(3) 66.798(4) 3279.8(2) 2 1.637 1624 24 541 11 533 0.0281 1.062 0.0499, 0.1348 0.0752, 0.1472

C46H46Ag3N7O14 1244.51 monoclinic P2/c 14.8314(11) 9.7047(17) 22.344(2) 90 131.552(6) 90 2406.8(5) 2 1.717 1248 9776 4230 0.0253 1.009 0.0653, 0.1332 0.0870, 0.1442

C36H28Ag2N4O10 892.36 monoclinic P2/c 13.6268(13) 11.3907(9) 22.824(3) 90 110.823(8) 90 3311.3(6) 4 1.790 1784 13 081 5828 0.1083 0.939 0.0821, 0.1043 0.2022, 0.1311

C36H24Ag2ClN4O12 955.78 monoclinic P2/c 13.1642(5) 9.9022(5) 16.0294(11) 90 124.520(4) 90 1721.60(16) 2 1.844 950 6766 3028 0.0250 1.042 0.0604, 0.1524 0.0759, 0.1611

C20H18AgN2O6 490.23 monoclinic C2/c 22.927(5) 14.592(3) 13.588(3) 90 117.98(3) 90 4014.5(15) 8 1.622 1976 13 779 3528 0.0698 1.094 0.0835, 0.1888 0.1213, 0.2178

R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2. 1444



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Table 2. Selected Bond Distances and Angles for Complexes 1−5a 1 Ag1−N1 Ag1−O2 Ag2−O2 Ag3−O6 Ag4−N4#2 N1−Ag1−N3 N1−Ag1−O2 N7−Ag2−N5 N8#2−Ag3−N6#1 N8#2−Ag3−O5 N2#1−Ag4−N4#2

2.206(4) 2.498(4) 2.447(3) 2.536(4) 2.211(4) 147.71(2) 114.89(1) 144.22(2) 152.30(2) 85.81(2) 143.80(2)

Ag1−N3 Ag2−N7 Ag3−N8#2 Ag3−O5 Ag4−O5 N1−Ag1−O7 N3−Ag1−O2 N7−Ag2−O2 N8#2−Ag3−O6 N6#1−Ag3−O5 N2#1−Ag4−O5

2.209(4) 2.196(4) 2.175(4) 2.585(4) 2.412(4) 94.64(1) 86.26(1) 116.15(1) 117.05(2) 116.97(1) 109.83(1)

Ag1−O7 Ag2−N5 Ag3−N6#1 Ag4−N2#1 N3−Ag1−O7 O7−Ag1−O2 N5−Ag2−O2 N6#1−Ag3−O6 O6−Ag3−O5 N4#2−Ag4−O5

2.482(4) 2.223(4) 2.195(4) 2.194(4) 106.46(1) 98.68(1) 96.48(1) 90.37(2) 51.07(1) 92.31(1)

2 Ag1−N2 Ag2−N3 N2−Ag1−N2#1 O2−Ag1−O2#1 N3−Ag2−N1

2.221(4) 2.157(5) 164.6(3) 98.4(2) 168.7(2)

Ag1−N1 N1−Ag1−N2#1

2.157(8) 169.0(3)

Ag1−O2

2.502(5)

N2−Ag1−O2 N2 −Ag1−O2#1

107.62(2) 82.66(2)

Ag2−N1 N2−Ag1−O2#1 N2#1−Ag1 −O2#1

2.163(5) 82.66(2) 107.62(2)

3 Ag1−N2#1 N1−Ag1−Ag1#2

2.170(8) 95.4(2)

Ag1−Ag1#2 N2#1−Ag1−Ag1#2

2.185(5)

N(1)#1−Ag(1)−N(1)

3.196(2) 93.5(2)

4 Ag(1)−N(1) N(2)#3−Ag(2)−N(2)

2.191(5) 180.0(1)

Ag(2)−N(2)

168.7(3)

5 Ag1−N1 N2#2−Ag1−N1

2.152(7) 166.0(3)

Ag1−N2#2 N2#2−Ag1−Ag1#3

2.147(7) 84.6(2)

Ag1−Ag1#3 N1−Ag1−Ag1#3

3.344(2) 105.5(2)

a Symmetry codes: #1, x, y, z − 1; #2, x + 1, y, z for 1; #2, −x + 2, y, −z + 5/2 for 2; #1, x, y + 1, z; #2, −x + 2, y, −z + 3/2 for 3; #1, −x, y, −z + 3/2; #3, −x + 2, −y + 2, −z + 2 for 4; #2, x, y, z + 1; #3, −x + 1/2, −y + 1/2, −z + 1 for 5.

Figure 1. (a) Metal coordination and atom labeling in complex 1. (b) 2D layer parallel to the ac plane. Blue dotted lines represent interchain π···π stacking. (c) 3D supramolecular network for 1 built from the hydrogen bonding between the water molecules and adjacent layers. Bpy ligands are marked as rods. (d) Perspective view of 16-membered water ring and two tetramers illustrating the hydrogen-bonding scheme. Symmetry codes: #1, x, y, z − 1; #2, x + 1, y, z; #3, x, y − 1, z; #4, −x + 2, −y, −z + 1.

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Figure 2. (a) Metal coordination and atom labeling in complex 2. All hydrogen atoms and NH4+ ions are omitted for clarity. (b) 2D layer parallel to the ac plane. Teal dotted lines represent interchain π···π stacking. (c) 3D supramolecular network for 2. (d) Hydrogen-bonding motifs selfassembled from bptc4−, water, and NH4+ ions. Symmetry codes: #1, −x − 1, y, −z + 3/2; #2, −x + 2, y, −z + 5/2; #3, −x + 1, −y, −z + 2; #4, −x, − y, −z + 2; #5, −x, y, −z + 3/2; #6, x − 1, y, z; #7, x − 1, −y + 1, z + 1/2; #8, −x + 1, −y + 1, −z + 1; #9, x, −y + 1, z − 1/2; #10, x − 1, −y + 1, z − 1/2.

tetrahedron can be indicated by the calculated value of the τ4 parameter introduced by Houser20 to describe the geometry of a four-coordinated transition metal system (for perfect tetrahedral geometry, τ4 = 1), while Ag2, Ag3, and Ag4 are all coordinated in T-shaped geometries and are ligated by two nitrogen atoms from two different bpy and one carboxylic oxygen atom. Within this unit, the Ag1···Ag2 distance spanned by O2 is 3.432(8) Å, implying the existence of ligand-supported argentophilic interactions.21 However, the distances of Ag2− O13W (2.763(4) Å), Ag3−O5 (2.585(4) Å), and Ag4−O4 (2.608(4) Å) are beyond the range of 2.32−2.52 Å for silver(I) carboxylate22 but still shorter than the sum of van der Waals radii (3.24 Å) of the Ag(I) cation and oxygen atom,23 suggesting the existence of significant Ag···O interactions. Taking weak Ag−O interactions into account, the coordination mode of bptc4− can be described as μ4-(η2-O,O′),(η2-O′,O″), (η2-O′′′,O′′′′),O′′′ (mode I in Scheme 1). Adjacent Ag atoms are interconnected by bpy ligands to give an infinite Ag-bpy chain, which is connected by the weak argentophilic interaction (Ag1···Ag2 3.432(8) Å) to lead to a double chain. Each double chain is further stabilized by intrachain π···π stacking interactions in a face-to-face fashion with centroid-to-centroid distances of 3.651(2), 3.695(2), 3.730(2), and 3.776(2) Å, respectively. Two such neighboring double chains are associated together by bptc4− anions, and a layer structure of 1 is thus formed (Figure 1b). As shown in Figure 1c, an overall 3D supramolecular framework results from the linkage of neighboring layers through one water molecule (O13W). It is interesting to note that other water molecules exist in the space between layers to furnish a (H2O)24 unit. The unit (H2O)24 has a butterfly shaped 16-membered ring (O1W, O4W, O5W, O6W, O7W, O9W, O10W, and O12W and their symmetry-related molecules) fused with two four-membered rings (O3W, O4W, O5W, and

C20H18AgN2O6 (490.23): C, 49.00; H, 3.70; N, 5.71. Found: C, 49.08; H, 3.65; N, 5.79. IR/cm−1 (KBr): 3406 (s), 1710 (m), 1578 (s), 1353 (m), 1326 (m), 1228 (w), 818 (w), 709 (w), 644 (w). X-ray Crystallography. Single-crystal X-ray diffraction data of complexes 1−5 were collected on a Bruker SMART APEX CCD diffractometer16 equipped with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å) at room temperature using the ω-scan technique. Empirical absorption corrections were applied to the intensities using the SADABS program.17 The structures were solved using the program SHELXS-9718 and refined with the program SHELXL-97.19 All nonhydrogen atoms were subjected to anisotropic refinement. The hydrogen atoms of the organic ligands were included in the structure factor calculation at idealized positions using a riding model and refined isotropically. The hydrogen atoms of the coordinated and solvent water molecules were located from difference Fourier maps, and then restrained at fixed positions and refined isotropically. The crystallographic data and selected bond distances and angles for 1−5 are listed in Tables 1 and 2, respectively.

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RESULTS AND DISCUSSION

Crystal Structure of {[Ag4(bptc)(bpy)4(H2O)]·(H2O)12}n (1). Compound 1 is constructed from a layer substructure, which is extended by one water molecule (O13W) into a threedimensional supramolecular framework with 16-membered water ring trapped between the layers. As shown in Figure 1a, the asymmetric unit of 1 contains four silver atoms, one bptc4− ligand, four bpy ligands, one coordinated water molecule, and 12 free water molecules. Selected bond lengths and angles are listed in Table 2. In the asymmetry unit, four unique Ag(I) cations display two types of coordination geometries (neglecting the weak Ag···O and Ag···Ag interactions). Ag1 is coordinated by two oxygen atoms from two bptc4− anions and two nitrogen atoms from two bpy ligands. Ag1 adopts distorted tetrahedral coordination geometries with the bond angles spanning from 86.28(1)° to 147.7(1)°, with the τ4 = 0.69 for Ag1. The distortion of the 1446



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Figure 3. (a) The coordination environment of Ag(I) atoms in 3. (b) View of the Ag1-bpy double chain A along the b axis. (c) View of the Ag2-bpy double chain B along the b axis. (d) 2D layer parallel to the bc plane. (e) The 3D supramolecular framework of 3. Teal dotted lines represent intrachain and interchain π···π stacking. Symmetry codes: #1, x, y + 1, z; #2, −x + 2, y, −z + 3/2; #3, −x + 1, y, −z + 1/2.

tetrahedral coordination geometry with bond angles spanning from 82.66(2)° and 164.6(3)°, with the τ4 = 0.62 for Ag1. For Ag2, two nitrogen atoms are incorporated into the coordination sphere. The Ag−N bond lengths fall in the range 2.157(3) and 2.163(3) Å. Bpy ligands bridge crystallographically unique Ag(I) centers into an infinite Ag-bpy chain, with the bptc4− ligand dangling on the side. However, there are also weak Ag···O interactions between the Ag(I) cations and carboxyl oxygen atoms and one water molecule (Ag2−O2#4 2.947(7) Å, Ag2−O1W#3 3.110(8) Å). Thus, bptc4− exhibits a μ3-(η2O,O′),O′,O′ coordination mode (mode II in Scheme 1) and connects three Ag(I) cations simultaneously. Next, these versatile Ag···O weak interactions assemble these alternate chains into the 2D layer substructure of 2 (Figure 2b). Within the layer, each pair of bpy ligands is aligned in a face-to-face stacking mode, and three types of π···π stacking interactions are detected, with the centroid-to-centroid distances of 3.655(3), 3.747(3), and 3.894(3) Å, respectively. Notably, the lattice water molecules reside in the interlayer void space and link each other by hydrogen bonds to produce a water tape involving NH4+ ions as well as the carboxyl oxygen atoms. In the water tape, O1W acts as donors to hydrogen bond to O1 and O4 (O1W−H1WB···O1 and O1W−H1WA···O4), respectively, and as an acceptor hydrogen bonded by O2W (O2W− H2WA···O1W). O2W acts as donors to hydrogen bond to O1 and O1W (O2W−H2WB···O1 and O2W−H2WA···O1W), and as acceptors hydrogen bonded by N4 and O3W (N4− H4C···O2W and O3W−H3WB···O2W), respectively. O3W acts as donors to hydrogen bond to O2W and N4 (O3W− H3WB···O2W and O3W−H3WA···N4). The geometric parameters of hydrogen bonds are summarized in Table S2. Such water tapes assemble the 2D sheets into the overall

O11W), and further hydrogen bonded to two water monomers (O2W and O8W) through O3W and O9W, respectively. The geometric parameters of the 16-membered water ring, tetramers, and monomers are summarized in Table S1. A closer look at the cyclic (H2O)4 cluster that we present here reveals it adopts the up−up−down−down (uudd) configuration with four free hydrogen atoms in a uudd fashion. Although the uudd configuration is energetically less stable as compared to the energy minimum up−down−up−down (udud) configuration,24 it has been observed in the crystalline solids of metal complexes.25 It is worth pointing out that the three-dimensional supramolecular framework is also reinforced by the cyclic 16-membered water ring. Two 2D water/ice layers containing large water rings have been observed in solid states,26 but discrete large water ring encapsulated in the metal−organic frameworks is limited. The study of water clusters is an important and challenging topic of contemporary supramolecular chemistry because it may help to get a better insight into the mechanism of proton transfer in biological systems. Without doubt, the above studies about water molecular self-assemblies are essential to understand how hydrogen-bonded channels can be purposely generated, because the formation of noncovalent bonding interactions is difficult to control. Crystal Structure of {[Ag1.5(bptc)0.5(bpy)1.5]·(NH4)0.5·(H2O)3}n (2). As shown in Figure 2a, the structure of 2 contains one and a half crystallographically unique Ag(I) ions, half bptc4− ligand, one and a half bpy, half NH4+ ion, and three lattice water molecules. Ag1 is coordinated by two nitrogen atoms (N2 and N2#1) from two different bpy ligands and two oxygen atoms (O2 and O2#1) from two bptc4− anions. Ag1 adopts a distorted 1447



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complicated three-dimensional supramolecular framework. As shown in Figure 2d, the water tape contains two six-membered rings and one five-membered ring, and this kind of water tape can be defined as R64(12)R63(10)R64(12).27 In contrast to the abundant research on structural characterization of a wide variety of water−chloride associates in different crystalline materials,28 much less attention has been focused on the identification and description of hybrid hydrogen-bonded water assemblies with NH4+ ions. Crystal Structure of {[Ag2(H2bptc)(bpy)2]·(H2O)2}n (3). As shown in Figure 3a, the asymmetric unit of 3 contains two silver atoms, one-half deprotonated H2bptc2−, two bpy ligands, and two coordinated water molecules. Both crystallographically independent Ag1 and Ag2 atoms adopt linear geometry that is ligated by two nitrogen atoms from two different bpy. Adjacent Ag atoms are interconnected by bpy ligands to give an infinite Ag-bpy chain, and further associated together through Ag···Ag interaction (Ag1···Ag1#2 3.196(2) Å, Ag2···Ag2#3 3.326(2) Å) to generate double chains A and B (Figure 3b,c). Within chain A, each pair of bpy ligands is aligned in a face-to-face stacking mode, and two types of π···π stacking interactions are detected with centroid-to-centroid distances of 3.671(4) and 3.550(3) Å, respectively. Similar to chain A, chain B is also reinforced by the formation of π···π stacking interactions with centroid-tocentroid distances of 3.657(4) and 3.847(4) Å, respectively. Such double chains A and B are stacked alternatively in an ABAB fashion through H2bptc2− ligands bridging adjacent double chains, resulting in a layer structure (Figure 3d). Similar to 2, H2bptc2− does not coordinate with Ag(I) cations but only has coordinative interactions with Ag(I) cations, exhibiting μ2(η2-O,O′),O″ coordination mode (mode III in Scheme 1) with Ag···O distances being 2.735(8)−3.121(9) Å. Finally, the 2D layer structures are further extended into a 3D supramolecular architecture via π···π stacking interactions with a centroid-tocentroid distance of 3.937(4) Å (Figure 3e). Crystal Structure of {[Ag2(H2bptc)(bpy)2]·(HClO4)}n (4). As shown in Figure 4a, the asymmetric unit consists of two silver ions, one partially deprotonated H2bptc2−, one bpy molecule, and one free perchloric acid. A medium-broad peak at 3423 cm−1 suggests that the perchlorate anion is most likely present in the protonated form as HClO4. Selected bond lengths and angles are listed in Table 2. Both Ag1 and Ag2 are coordinated by two nitrogen atoms from bpy ligands with linear shaped coordination geometries. Alternate Ag1 and Ag2 atoms are connected by bpy to generate an infinite Ag-bpy chain extending along the a axis. Similar to the above-mentioned, half deprotonated H2bptc2− does not coordinate with Ag(I) cations but only has coordinative interactions with Ag(I) cations with Ag···O distances being 2.687(6) and 2.809(5) Å, suggesting the existence of significant Ag···O interactions. Thus, H2bptc2− features a μ3-(η2O,O′),O″,O′′′ bridging mode (mode IV in Scheme 1) to extend the Ag-bpy into intricate three-dimensional supramolecular architecture with the free perchloric acid filling into the voids, as shown in Figure 4b. To better understand the 3D structure of 4, topological analysis was carried out. If the H2bptc2− anion can be viewed as a node that is a 3-connected node because it links one Ag1 and two Ag2 atoms, Ag1 is a 3-connected node, and Ag2 is a 4-connected node, respectively. Such connectivity repeats infinitely to give the 3D framework of 4 as schematically shown in Figure 4b. According to the simplification principle,29 the resulting structure of 4 is a (3,3,4)-connected net with its Schläfli symbol of {62·8}{62·8}{62·82·102}. It is noteworthy that

Figure 4. (a) The coordination environment of Ag(I) atoms in 4 with hydrogen atoms omitted for clarity. (b) (left) The 3D network in 4. (right) The topological representation of the 3D structure of 4 (pink, Ag1 atom; green, Ag2 atom; blue, the H2bptc ligand). (c) (left) View of the 2-fold interpenetration of 4 containing 1D open channels encapsulating perchlorate molecules. (right) Topological representation of 4. Symmetry codes: #1, −x, y, −z + 3/2; #2, −x + 1, y, −z + 1/ 2; #3, −x + 2, −y + 2, −z + 2; #4, x + 1, −y + 1, z + 1/2; #5, −x + 1, y + 1, −z + 3/2.

there are open channels in the single 3D net of 4 (Figure 4c), which allows another identical net to penetrate; the entire structure of 4 is a 2-fold interpenetrated 3D framework, which shows channels along the c axis that are occupied by the free perchloric acid molecules. Crystal Structure of {[Ag(H2bptc)0.5(bpe)]·(H2O)2}n (5). When we introduced bpe into the silver/H4bptc system, a 2fold interpenetration was also obtained. As shown in Figure 5a, the asymmetric unit consists of one silver ion, half deprotonated H2bptc2−, one bpe molecule, and two free water molecules. O1W and O2W were refined with an occupancy factor of 0.5. Selected bond lengths and angles are listed in Table 2. Ag1 is coordinated in a linear geometry and is surrounded by two nitrogen atoms from bpy ligands (Ag1−N1 2.152(7) Å; Ag1−N2#2 2.147(7) Å; N1−Ag2−N2#2 166.0(3) °). As shown in Figure 5b, alternate Ag1 atoms are connected by bpe to form an infinite 1D chain, which is further associated together through Ag···Ag distance 3.34(2) Å. Each double chain is further consolidated by weak intrachain π···π stacking interactions with centroid-to-centroid distances being 4.005 Å. H2bptc2− does not coordinate with Ag(I) cations but only has coordinative interactions with Ag(I) cations with Ag···O distances being 2.763(7) and 2.782(6) Å. Thus, H2bptc2− 1448



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Figure 5. (a) Metal coordination and atom labeling in complex 5. (b) 2D layer parallel to the ac plane. Teal dotted lines represent interchain π···π stacking. (c) 3D supramolecular network for 5. (d) A space-filling view of the 2-fold interpenetration framework. Symmetry codes: #1, −x + 1, y, −z + 3/2; #2, x, y, z + 1; #3, −x + 1/2, −y + 1/2, −z + 1.

were changed, complex 4 was produced. When bpe was used instead of bpy in 3, compound 5 was obtained, which proved that the secondary ligands also have a significant effect on the construction of the frameworks. Coordination of the bptc4− or H2bptc2− Anion. According to previous studies, many coordination compounds composed of 1,1′-biphenyl-2,2′,6,6′-tetracarboxylic acid (H4bptc) have been reported.13 However, in the compounds based on ZnII, NiII, CoII, and AgI ions, the partly or fully deprotonated H4bptc anions mainly exhibit monodentate, bidentate bridging, and bidentate chelate coordination modes. The bptc4− or H2bptc2− anions in complexes 1−5 display a variety of coordination fashions, as illustrated in Scheme 1. In complex 1, the carboxylate groups of bptc4− anion display tridentate (μ3-η1,η2), bidentate (μ2-η2), and bis-monodentate bridging coordination modes, respectively. In this mode, bptc4− anion linked adjacent Ag-bpy chains to furnish a layer structure, which are further linked through intermolecular hydrogen bonding to lead to a three-dimensional supramolecular architecture. In 2, the bptc4− anion connects three silver(I) ions, where the carboxylate groups display bis-bidentate (μ2-η2) coordination modes. In complexes 1 and 2, butterfly shaped 16membered water ring and infinite water tape are obtained, respectively, which proved that the structural variations of both host MOFs and guest water aggregates are undoubtedly associated with the different coordination modes of bptc4− anions. In complex 3, the carboxylate groups show monodentate bridging and chelate coordination modes. In this mode, H2bptc2− anion also linked adjacent Ag-bpy chains to furnish a layer structure, which are further linked through π···π stacking to lead to a three-dimensional supramolecular architecture. In 4, the carboxylate groups show monodentate coordination

exhibits a semichelating mode (mode V in Scheme 1) and connects two Ag(I) cations simultaneously. Next, the versatile Ag···O weak interactions assemble these 1D double chains into the two-dimensional network substructure. In addition, hydrogen bonding between solvent water molecules as well as between solvent water molecules and the carboxylic O atoms (O1W−O2W 2.66(2) Å, O2W−O1 2.791(1) Å, O3W−O4 2.825(1) Å, O3W−O1W 2.683(2) Å) is also observed, and thus results in a 3D supramolecular architecture with a hole (ca. 12.02 × 10.24 Å2), as shown in Figure 5c. Notably, the presence of the large voids in this supramolecular framework facilitates the formation of interpenetrations. The channels are filled by another identical framework in a typical 2-fold interpenetration fashion (Figure 5d). Influence of Synthetic Conditions on the Structures of Compounds. To obtain isolate crystals of Ag/bptc/bpy family from solutions under ambient conditions, aqueous ammonia solution is used considering that it not only acts as base but can react with Ag(I) ions forming[Ag(NH3)2]+ species, which in turn can slowly release free Ag(I) cations based on the chemical equilibrium between them. At the same time, the Ag(I) source was changed from Ag2O (1) to AgNO3 (2). To further investigate the coordination modes of the H4bptc anions and the resulting frameworks, we carried out numerous parallel experiments by adjusting the pH values from 4 to 6, changing the reaction temperatures from 120 to 160 °C, and varying the counterions. However, only crystals of compounds 3−5 were obtained. When pH value and reaction temperature were changed, complex 3 was obtained. The carboxylate ligands were fully deprotonated (2) and partly deprotonated (3), which finally affect the construction of the final product structures. When the counterions and pH value 1449



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modes. In 5, the carboxylate groups show bidentate chelate coordination modes. From the above description, we can see that the diversiform coordination fashions of the bptc4− or H2bptc2− anions have a significant effect on the framework structures of the coordination polymers. Thermal Analyses and PXRD Patterns. Thermogravimetric (TG) analyses of complexes 1−5 have been conducted under a N2 atmosphere between 30 and 800 °C (Figure S1). For complex 1, a gradual weight loss between 30 and 132 °C is attributed to the release of water molecules (observed, 14.47%; calculated, 14.41%). Decomposition of the anhydrous residue was observed from 158 °C. For complex 2, weight loss corresponding to the release of water molecules occurred from 30 to 103 °C (observed, 8.68%; calculated, 8.75%), and then the host framework started to decompose from 167 °C. The TG curve of 3 showed that this complex lost its water molecules between 30 and 183 °C (observed, 4.03%; calculated, 3.95%). Rapid weight loss was observed from 235 °C, which indicated collapse of the whole structure. As for anhydrous complex 4, no obvious weight loss was observed until the temperature reached 216 °C, indicating that this metal−organic framework material has modest thermal stability. In the case of complex 5, weight loss of water molecules took place in the range of 30−109 °C (observed, 7.34%; calculated, 7.28%), and the anhydrous component began to decompose at 109 °C. The synthesized products of 1−5 have been characterized by powder X-ray diffraction (PXRD) (Figures S2−S6). The observed PXRD patterns correspond well with the results simulated from the single crystal data, indicating the high purity of the synthesized samples. The difference in reflection intensities between the simulated and observed patterns was due to the variation in preferred orientation of the powder samples during the collection of the observed PXRD data. Photoluminescent Properties. The solid-state photoluminescent properties of complexes 1−5 have been investigated in the solid state at room temperature. The emission and excitation peaks of 1−5 are shown in Figures S7−S11. Here, emission bands are observed at 469 nm (λex = 379 nm) for 1, 467 nm (λex = 380 nm) for 2, 469 nm (λex = 400 nm) for 3, 549 nm (λex = 365 nm) for 4, and 442 nm (λex = 387 nm) for 5, respectively. For excitation wavelength between 280 and 480 nm, there is no obvious emission observed for free H4bptc under the same experimental conditions, while free bpy and bpe ligands present very weak photoluminescence emission.30 Therefore, the fluorescent emissions in the coordination polymers may be proposed to originate from the coordination of bptc4− and H2bptc2− to the silver(I) atoms. Generally speaking, silver(I) complexes might emit weak photoluminescence at low temperature due to the intense spin−orbital coupling of Ag(I).31 Consequently, complexes 1−5 present unusual examples of room-temperature luminescent Agcontaining polymers.

ions, water molecules, as well as to the carboxyl oxygen atoms in 2 provides novel structural aspects of water and new insights into water with implications in biological environments. The investigations in this article not only illustrate that structural and compositional diversity of coordination polymers can be achieved by changing the pH values, the reaction temperatures, and the neutral ligands, but also provide that H4bptc is a good candidate for construction of silver−organic coordination polymers. This interesting observation prompts us to further develop the rational synthetic strategy to obtain new crystalline materials via an appropriate external stimulus. Other investigations of factors such as anionic templates with different conformations that affect the assembly of the silver−organic framework are currently underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S11 including TG curves, the powder X-ray patterns, solid-state excitation and emission spectrum, and Xray data in CIF files (that has been also deposited as CCDC 854766−854770 in the Cambridge Crystallographic Data Centre). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS We gratefully acknowledge financial support by the National Natural Science Foundation of China (No. 20901070) and Zhengzhou University (People’s Republic of China).

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REFERENCES

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CONCLUSIONS Five silver(I) complexes, assembled from H4bptc, have been hydrothermally or solvent evaporation prepared and structurally characterized. By the weak noncovalent interactions including π−π stacking and hydrogen bonding, complexes 1− 3 and 5 extend into the 3D supramolecular networks, while complexes 4 and 5 form 2-fold interpenetrated 3D frameworks. The rare butterfly shaped 16-membered ring fused with two four-membered rings in 1 and 1D water tape involving NH4+ 1450



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Syntheses, Structures, and Properties of SilverâOrganic Frameworks

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