Ligand and pH-Controlled ZnII Bilayer Coordination

Jul 18, 2007 - layers are further pillared by (btc)4- ligands into a two-dimensional (2D) bilayer network. Complex 2 is an unusual 2D double-...
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CRYSTAL GROWTH & DESIGN

Ligand and pH-Controlled ZnII Bilayer Coordination Polymers Based on Biphenyl-3,3′,4,4′-tetracarboxylate Wang,†,‡

Gou,†

Hu,*,†

Ji-Jiang Lei Huai-Ming Zhong-Xi Gang-Lin Xue,† Meng-Lin Yang,† and Qi-Zhen Shi†

Han,†

Dong-Sheng

2007 VOL. 7, NO. 8 1514-1521

Li,‡

Department of Chemistry/Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Northwest UniVersity, Xi’an 710069, China, and Department of Chemistry and Chemical Engineering/Shaanxi Key Laboratory of Chemical Reaction Engineering, Yanan UniVersity, Yan’an 716000, China ReceiVed April 4, 2007; ReVised Manuscript ReceiVed May 30, 2007

ABSTRACT: Four novel ZnII coordination polymers, [Zn(btc)0.5(H2O)]n (1), {[Zn(btc)0.5(4,4′-bpy)0.5(H2O)]‚1.5H2O}n (2), {[Zn2(H2btc)2(4,4′-bpy)4]‚H2O}n (3), [Zn(H2btc)(bpe)]n (4) [H4btc ) biphenyl-3,3′,4,4′-tetracarboxylic acid, 4,4′-bpy ) 4,4′-bipyridine, and bpe ) 1,2-bis(4-pyridyl)ethane], have been synthesized by hydrothermal reactions. Single-crystal X-ray structural analysis reveals that the four polymers exhibit different novel bilayer architectures. Complex 1 possesses a bilayer structure in which two helical layers are further pillared by (btc)4- ligands into a two-dimensional (2D) bilayer network. Complex 2 is an unusual 2D doublelayered supramolecular motif generated by hydrogen bonding interactions of two single-layered networks. Complex 3 displays a 2D supramolecular bilayer network formed by the one-dimensional {[Zn2(H2btc)2(4,4′-bpy)4]‚H2O}n polymer. Complex 4 features a 2D double-layered framework in which two puckering single layers are linked by µ2-carboxylato groups of (H2btc)2- anions. The diverse structures illustrate rational adjustment of the second ligand, and the pH value is a good method to further design bilayer metalorganic compounds with novel structures and properties. In addition, the thermal stabilities and photoluminescence properties of 1-4 were also studied. Introduction The self-assembled construction of coordination polymers is of current interest in the field of supramolecular chemistry and crystal engineering, not only because of their potential applications in gas storage, molecular sieves, ion-exchange, catalysis, magnetism, nonlinear optics, and molecular sensing, but also because of their intriguing variety of architectures and topologies, such as rectangular grids, brick walls, herringbones, ladders, rings, boxes, diamondoids, and honeycombs.1,2 In contrast to the variety of structures, a new structural motif called a molecular bilayer appeared,3 but such examples are relatively scarce. Usually, bilayer architectures have been fabricated by the assembly of T-shaped,4 non-T-shaped,5 and rectangular building blocks.6 Having reviewed the literature, we noticed that special bilayer structural motifs such as non-T-shaped layers linked by carboxylate ligands and hydrogen bonds are surprisingly rare. Furthermore, it is well-known that self-assembly processes is highly affected by several factors such as the ligand’s nature, medium, template, metal-ligand ratio, pH value, and counterion.7 Therefore, an investigation for understanding the relationships between the structures of complexes and the nature of ligands, as well as other factors, is still important. At the same time, to date, no systematic investigation of the relationship between the self-assembly of zinc bilayer complexes and different factors has been carried out. In the construction of novel metal-organic frameworks (MOFs), polycarboxylate ligands, such as 1,2-benzenedicarboxylate, 1,3,5-benzenetricarboxylate, and 1,2,4,5-benzenetetracarboxylate, have been extensively employed in the preparation of such metal-organic complexes in possession of multidimensional networks and interesting properties.8 In contrast, the biphenyl-3,3′,4,4′-tetracarboxylic acid (H4btc) ligand * To whom correspondence should be addressed. Tel/Fax: +86-2988303331. E-mail: [email protected]. † Northwest University. ‡ Yanan University.

Chart 1.

Structure of H4btc

(Chart 1), as a member of multidentate O-donor ligands, is rarely used.6e However, its following structure features inspire our research interests: (a) it has four carboxyl groups that may be completely or partially deprotonated, depending on the pH; (b) it is a flexible ligand: two phenyl rings can be rotated around the C-C single bond. To investigate the influence of the second ligand [rigid 4,4′bipyridine (bpy) and flexible 1,2-bis(4-pyridyl)ethane (bpe)] and pH value on the assembly processes of the (btc)4- ligand and metal ions as well as the framework structures of their complexes, we synthesized four zinc(II) complexes, namely, [Zn(btc)0.5(H2O)]n (1), {[Zn(btc)0.5(4,4′-bpy)0.5(H2O)]‚1.5H2O}n (2), {[Zn2(H2btc)2(4,4′-bpy)4]‚H2O}n (3), and [Zn(H2btc)(bpe)]n (4) [4,4′-bpy ) 4,4′-bipyridine, and bpe ) 1,2-bis(4-pyridyl)ethane]. To the best of our knowledge, these four zinc(II) bilayer coordination polymers based on biphenyl-3,3′,4,4′-tetracarboxylate are first synthesized. This work may provide useful information for the further design of bilayer metal-organic compounds with novel structures and properties. Experimental Section The reagents and solvents employed were commercially available and used as received without further purification. Elemental analyses (C, H, N) were determined with a Vario EL III elemental analyzer.

10.1021/cg0703240 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007

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Crystal Growth & Design, Vol. 7, No. 8, 2007 1515

Table 1. Crystal Data and Structural Refinement Parameters for Complexes 1-4 complex

1

2

3

4

empirical formula formula weight crystal system space group a/Å b/Å c/Å a/° b/° γ/° V/Å3 Z F calc (g cm-3) µ (mm-1) F(000) λ (Mo-KR)/Å reflections collected unique reflections parameters S on F2 R1, wR2 [I > 2σ(I)] R1, wR2 (all data) ∆ F min and max (e Å-3)

C8H5O5Zn 246.49 triclinic Pıj 4.7735(10) 6.7584(14) 12.087(3) 85.662(3) 88.264(3) 82.804(3) 385.67(14) 2 2.123 3.172 246 0.71073 1912 1340 136 1.039 0.0340, 0.0864 0.0432, 0.0894 0.547and-0.490

C13H12NO6.50Zn 351.61 monoclinic P2/n 13.3835(10) 6.9617(5) 15.6162(11) 90 110.1930(10) 90 1365.56(17) 4 1.710 1.830 716 0.71073 6583 2416 210 1.027 0.0296, 0.0702 0.0398, 0.0767 0.333 and -0.280

C72H50N8O17Zn2 1429.94 monoclinic P21/c 12.0568(19) 24.049(4) 10.6693(17) 90 105.964(2) 90 2974.3(8) 2 1.597 0.894 1468 0.71073 21343 5274 448 1.043 0.0383, 0.0864 0.0573, 0.0963 0.474 and -0.454

C28H20N2O8Zn 577.83 triclinic Pıj 9.2671(17) 10.8252(19) 12.390(2) 97.306(2) 106.614(2) 93.547(2) 1175.1(4) 2 1.633 1.105 592 0.71073 8474 4133 353 0.996 0.0674, 0.1123 0.1429, 0.1395 0.551 and -0.505

The FT-IR spectra were recorded from KBr pellets in the range 4000400 cm-1 on a Bruker EQUINOX-55 spectrometer. Thermogravimetric analyses (TGA) were performed under nitrogen with a heating rate of 10 °C‚min-1 using a NETZSCH STA 449C thermogravimetric analyzer. The UV-vis spectral measurements were carried out on a Shimadzu UV-2550 spectrophotometer. Fluorescence spectra were performed on a Hitachi F-4500 fluorescence spectrophotometer at room temperature. Synthesis of [Zn(btc)0.5 (H2O)]n (1). A mixture of Zn(NO3)2‚6H2O (0.2 mmol), biphenyl-3,3′,4,4′-tetracarboxylic dianhydride (0.1 mmol), NaOH (0.4 mmol), and 4,4′-bpy (0.1 mmol) in water (10 mL) was stirred for 30 min in air (the pH value of the mixture was 7), then sealed in a 25 mL Telfon-lined stainless steel container, which was heated to 160 °C for 72 h. After the sample was cooled to room temperature at a rate of 2 °C/h, the colorless crystals were obtained in ca. 46% yield based on Zn. Anal. found (calcd) for C8H5O5Zn: C, 38.96 (38.98); H, 2.02 (2.04) %; FT-IR (KBr, cm-1): 3405 (m), 1612 (w), 1607 (w), 1547 (s), 1491 (m), 1400(m), 1164 (m), 1067 (w), 839 (m), 780 (m), 669 (m), 615 (w), 579 (m), 461 (w). Synthesis of {[Zn(btc)0.5(4,4′-bpy)0.5(H2O)]‚1.5H2O}n(2). The preparation of 2 was similar to that of 1 except that 0.1 mmol of 4,4′-bpy was changed to 0.2 mmol (the pH value of the mixture was 7). The colorless crystals were obtained in ca. 45% yield based on Zn. Anal. found (calcd) for C13H12 NO6.50Zn: C, 44.37 (44.40); H 3.42 (3.44); N, 3.95 (3.98) %; FT-IR (KBr, cm-1): 3543 (m), 1604 (m), 1544 (s), 1508 (w), 1405 (m), 1387 (m), 1223 (w), 1099 (m), 872 (m), 782 (m), 715 (m), 642 (m), 462 (w). Synthesis of {[Zn2(H2btc)2(4,4′-bpy)4]‚H2O}n (3). The preparation of 3 was similar to that of 2 except that 0.4 mmol of NaOH was changed to 0.2 mmol (the pH value of the mixture was 5). The colorless crystals were obtained in ca. 47% yield based on Zn. Anal. found (calcd) for C72H50N8 O17Zn2: C, 60.45 (60.47); H, 3.46 (3.52); N, 7.82 (7.84) %; FT-IR (KBr, cm-1): 3405 (m), 1715 (s), 1613 (m), 1545 (m), 1490 (s), 1396 (s), 1225 (m), 1164 (m), 1098 (m), 816 (m), 787 (m), 644 (m), 460 (w).

Scheme 1.

Synthesis of [Zn(H2btc)(bpe)]n (4). The preparation of 4 was similar to that of 3 except that bpe (0.20 mmol) was used instead of 4,4′-bpy (the pH value of the mixture was 5). Colorless crystals were obtained in ca. 48% yield based on Zn. Anal. found (calcd) for C28H20N2O8Zn: C, 58.16 (58.20); H, 3.47 (3.49); N, 4.83 (4.85) %; FT-IR (KBr, cm-1): 3422 (m), 1620 (m), 1574 (s), 1487 (m), 1434 (m), 1404 (s), 1370 (m), 1231 (m), 848 (m), 794 (m), 746 (w), 672 (w), 559 (m), 522 (w), 448 (w). X-ray Crystallography. Intensity data were collected on a Bruker Smart APEX II CCD diffractometer with graphite-monochromated Mo-KR radiation (λ ) 0.71073 Å) at room temperature. Empirical absorption corrections were applied by using the SADABS program. The structures were solved by direct methods and refined by the fullmatrix least-squares based on F2 using SHELXTL-97 program.9 All nonhydrogen atoms were refined anisotropically, and the hydrogen atoms of organic ligands were generated geometrically. Crystal data and structural refinement parameters for 1-4 are summarized in Table 1 and selected bond distances and bond angles are listed in Table S1, Supporting Information. CCDC 632095 (1), 632096 (2), 632097 (3), and 632098 (4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.can.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Centre, 12 Union Road, Cambridge CB2 1EZ, U.K., fax (+44) 1223-336033, or deposit @ccdc.cam.ac.uk).

Results and Discussion Syntheses. The hydrothermal reactions of Zn(NO3)2‚6H2O and biphenyl-3,3′,4,4′-tetracarboxylic dianhydride with bipyridyl ligands (rigid 4,4′-bpy, flexible bpe) and NaOH gave complexes 1-4, which formed under similar reaction conditions but with a different second ligand and pH value (Scheme 1). It is noteworthy that 4,4′-bpy does not appear in 1. However, without

Syntheses of Complexes 1-4

Figure 1. The coordination environment of ZnII in 1.

1516 Crystal Growth & Design, Vol. 7, No. 8, 2007 Scheme 2.

Wang et al.

(a-c) Coordination Modes of (btc)4- in 1-4

4,4′-bpy, the directed reaction of biphenyl-3,3′,4,4′-tetracarboxylic dianhydride and Zn(NO3)2‚6H2O under the same conditions generated unidentified white powder products, which suggests that 4,4′-bpy plays an important template role in the formation of 1. Similar situations also occurred in other systems.10 The successful isolation of 1 prompted us to extend our study to other complexes based on biphenyl-3,3′,4,4′tetracarboxylate. Thus, we changed 0.1 mmol of 4,4′-bpy in 1 to 0.2 mmol in an attempt to assembly the two-dimensional (2D) bilayer in 1 into a three-dimensional (3D) network. Unexpectedly, another type 2D supramolecular bilayer motif 2 can be obtained as a single phase as perfect crystals. Although detailed studies are still required to better understand this phenomenon, we propose that coordination modes of the (btc)4ligand may have a very significant effect on the final structure. It is well-known that the structural geometry could be controlled and modulated by selecting an appropriate pH value and second ligand.10,11 In an attempt to obtain the other novel bilayer architectures, 2D supramolecular bilayer complex 3 was obtained under identical conditions when the amounts of NaOH in 2 were changed to 0.2 mmol. When flexible bpe was used instead of rigid 4,4′-bpy in 3, structurally different 2D bilayer frameworks were formed in 4 under similar reaction conditions. Unfortunately, under the same reaction conditions as the preparation of polymers 2, the reactions of Zn(NO3)2‚6H2O and biphenyl-3,3′,4,4′-tetracarboxylic dianhydride with bpe only generated white powder products whose composition cannot be identified. These results indicate that the formation of 1-4 under similar reaction conditions depends primarily on the chemical nature of the bridging ligands and pH value of reaction system. Crystal Structures. [Zn(btc)0.5 (H2O)]n (1). Single-crystal X-ray structural analysis shows that 1 is a bilayer structure in which two helical layers are linked by (btc)4- ligands to generate a 2D pillared bilayer framework. Each ZnII is primarily coordinated by three oxygen atoms [Zn(1)-O(1), 1.983(2) Å; Zn(1)-O(3B), 1.955(3) Å; Zn(1)-O(4B), 1.959(3) Å] from three (btc)4- ligands and one oxygen atom [Zn(1)-O(5), 1.930(3) Å] from the coordinated water molecule to furnish a distorted tetrahedral geometry (Figure 1). The (btc)4- ligands in 1 acts as a hexadentate ligand; two 3,3′-carboxyl groups adopt a trans conformation and bis(bidentate) bridging mode, while the other two 4,4′-carboxyl groups exhibit a bis(monodentate) bridging mode (Scheme 2a); the pair of phenyl rings of the ligand is nearly coplanar. On the basis of the connection mode, the ZnII atoms are bridged by the V-shaped phthalic groups of (btc)4-

ligands to form the left-handed helical chains with a pitch of 6.758(2) Å (Figure 2a). As shown in Figure 2a, adjacent samehanded helical chains are further connected by the oxygen atoms (O4) of the 3,3′-carboxyl groups in two different (btc)4- ligands to generate a 2D helical layer. Interestingly, the 2D helical layers are linked by (btc)4- pillars to generate a novel 2D bilayer framework with a distance of 9.992(2) Å (based on Zn‚‚‚Zn) between the two single layers (Figure 2b). When viewed down along the crystallographic b-axis, approximately rectangular channels with dimensions of 6.548 × 5.417 Å [based on C(2)‚‚‚C(7)] are evident in 1 (Figure 2c). As far as we know, such 2D bilayer motif with single helical layers is still very rare in systems of metal-organic complexes.12 The adjacent 2D bilayers are further linked to each other by O-H‚‚‚O hydrogen bonds [O(5)‚‚‚O(2), 2.686(4) Å] to complete the final 3D supramolecular architecture (Figure S1, Supporting Information). {[Zn(btc)0.5(4, 4′-bpy)0.5(H2O)]‚1.5H2O}n(2). Complex 2 features an unusual 2D bilayer architecture generated by hydrogen-bonding interactions of two single-layered networks. As shown in Figure 3, each ZnII atom is coordinated by one nitrogen atom from a 4,4′-bpy ligand, two oxygen atoms from two carboxylic groups of two (btc)4- ligands, and one aqua ligand. Its coordination geometry can be described as a distorted tetrahedral geometry. Zn-O bond lengths fall in the range 1.9518(19)-2.022(2) Å, and Zn-N bond lengths are 2.029(2) Å. These values are within the normal experimental limitation (Table S1, Supporting Information). Unlike in compound 1, all carboxylic groups of the (btc)4- ligands in 2 adopt a monodentate mode to connect with four Zn atoms. In addition, the conformation of 3,3′-carboxyl groups and linking mode of phenyl rings of the ligand are similar to that 1 (Scheme 2b). In such a way, the dimeric Zn2 units [Zn‚‚‚Zn, 4.559(3) Å] are linked by (btc)4- ligands to form a one-dimensional (1D) [Zn2(btc)(H2O)2]n ringlike chain. The adjacent 1D chain is further linked by 4,4′-bpy ligands into a 2D single-layered network with coordinated H2O molecules attached to one side of the network (Figure 4a). Another outstanding feature is that these adjacent two 2D layers in a face-to-face manner are further packed into a doublelayered structure by O-H‚‚‚O hydrogen bonds [O(5)‚‚‚O(2), 2.638(3) Å] between the two single layers (Figure 4b,c). Although the single-layered network contains a 13.38 × 11.16 Å (based on Zn‚‚‚Zn) window, the effective channels in the actual crystal structure of 2 are further reduced due to the

ZnII Bilayer Coordination Polymers

Crystal Growth & Design, Vol. 7, No. 8, 2007 1517

Figure 2. (a) Left: view of a single left-handed net in 1, showing the 2D helical layer. Right: space-filling diagram of one helical chain in the 2D helical layer. (b) Pillared bilayer motif in 1 viewed along the c-axis. (c) Side view along the b-axis showing bilayer motif in 1 (different color Zn atoms lying in two layers).

Figure 3. The coordination environment of a dinuclear ZnII unit in 2.

significant offset stacking of adjacent two 2D single layers. Free water molecules are located in the channels of the bilayers and are hydrogen bonded to 3,3′-carboxyl groups or coordinated water molecule oxygen atoms of the host network [O‚‚‚O, 2.700(3)-2.983(4) Å]. Thus, the adjacent 2D bilayers are further linked to each other by these hydrogen bonds to complete the final 3D supramolecular architecture (Figure S2, Supporting Information). This is, to our knowledge, the first example of bilayer architecture assembled by hydrogen-bonding interactions. {[Zn2(H2btc)2(4,4′-bpy)4]‚H2O}n (3). Single-crystal X-ray diffraction reveals that 3 is a 2D supramolecular bilayer network based on hydrogen bonding interactions of the adjacent 1D double chain {[Zn2(H2btc)2(4,4′-bpy)4]‚H2O}n polymer. Each

ZnII atom is coordinated by four oxygen atoms from three different (H2btc)2- anions and two nitrogen atoms from two monodentate (not bridging bidentate) 4,4′-bpy ligands to give a distorted octahedral geometry (Figure 5a). The four Zn-O distances fall in the range 1.997(2)-2.410(2) Å, and Zn-N bond lengths are 2.162(2) and 2.265(2) Å, similar to those in other zinc-tetracarboxylate coordination polymers.13 Unlike in 1 and 2, the (H2btc)2- anions in 3 act as a tetradentate ligand; one carboxylic group adopts a bidentate chelating mode, while another one exhibits a bidentate bridging mode, connecting three ZnII atoms. In addition, the 3,3′-carboxyl groups in 3 adopt a cis conformation (Scheme 2c); the dihedral angle between the pair of phenyl rings of the ligand is 37.2°. On the basis of the connection mode, the dimeric Zn2 units are linked by (H2btc)2anions to form an extended 1D double chain along the b-axis (Figure 5b). The Zn‚‚‚Zn distance is 4.315(2) Å, which is shorter than that in 2, revealing that pH value has a great effect on the bilayer framework. Interestingly, the adjacent 1D chains are parallel to each other and are further united together through O-H‚‚‚N hydrogen bonds [O(6)‚‚‚N(2), 2.6034(1) Å; O(3)‚‚‚ N(4), 2.6654(1) Å] to give a 2D supramolecular bilayer motif (Figure 6). [Zn(H2btc)(bpe)]n (4). Compound 4 is a 2D bilayer coordination polymer containing ring dimeric Zn2 units. In the 2D network, each ZnII atom is coordinated by four carboxylate oxygen atoms from three different (H2btc)2- anions and two nitrogen atoms from two bpe ligands to form a distorted octahedral geometry (Figure 7a). In the framework, Zn-O bond

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Figure 4. (a) Left: view of a single net in 2, showing the 2D grid layer with a 13.38 × 11.16 Å window and coordinated H2O molecules pointing vertically up from the layer. Right: schematic illustration of a single net in 2. (b) The 2D supramolecular bilayer motif in 2 assembled by hydrogen bonds of adjacent nets viewed along the b-axis. (c) Side view along the c-axis showing a bilayer motif in 2.

Figure 5. (a) The coordination environment of a dinuclear ZnII unit in 3. (b) The 1D chain (along b) in 3. 4,4′-bpy and H2O molecules have been omitted for clarity.

and Zn-N distances are in the range 2.033(4)-2.237(4) and 2.159(5) Å-2.283 (5) Å, respectively. Two zinc atoms are

bridged by a pair of carboxylate groups ends from two different (H2btc)2- anions into a dinuclear zinc unit with a Zn‚‚‚Zn distance of 4.362(2) Å, which is larger than that in 3, revealing that the second ligand has a great influence on the bilayer framework. The dimeric Zn2 units are linked by four exotetradentate (H2btc)2- anions (4,4′-COOH groups do not take part in coordination) to two adjacent dinuclear zinc units, thus generating a 1D [Zn2(H2btc)2]n double chain (Figure 7b), in which the conformation and linking mode of (H2btc)2- anions are similar to that of 3 and the dihedral angle between the pair of phenyl rings is 39.8°. All the bpe ligands adopt a trans conformation and further link adjacent double chains into a unique 2D bilayer net as shown in Figure 8a. The 2D bilayer framework can also be considered as being constructed by two puckering layers of [Zn(H2btc) (bpe)] linked by µ2-carboxylato groups of (H2btc)2- anions. The two adjacent 4,4′-COOH groups are hydrogen bonded to each other [O(4)‚‚‚O(5), 2.5817(1) Å; O(3)‚‚‚O(6), 2.6021(1) Å]. Thus, the 2D bilayer framework is further united together through both hydrogen bonds and coordinate covalent bonds to give a final 2D structure (Figure 8b). Strikingly, when viewed along the c-axis, the 2D structure of 4 (Figure 8b) is similar to the 2D supramolecular structure of 3 (Figure 6b). This comparison indicates that the 2D supramolecular structure of 3 can be converted into the 2D bilayer of 4 through the replacement of the rigid 4,4′-bpy ligands by flexible bpe ligands.

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Figure 6. (a) The 2D supramolecular H-bonds motif in 3 (viewed along the b-axis) formed by O-H‚‚‚N interactions of adjacent chains. (b) Side view along the c-axis showing 2D supramolecular bilayer motif in 3 (different color Zn atoms lying in two layers).

Figure 7. (a) The coordination environment of a dinuclear ZnII unit in 4. (b) The 1D chain (along c) in 4. bpe and H2O molecules have been omitted for clarity.

It is obvious from the above descriptions that the second ligand (rigid 4,4′-bpy and flexible bpe) and pH value have significant effects on the final structures of the resulting complexes. As demonstrated by a comparison of complexes 1 and 2, the second ligand (rigid 4,4′-bpy) causes the distinctness of the coordination modes of (btc)4- ligands and finally results in the formation of different structures. In addition, the lengths of the second ligand have a significant effect on the structures, as evidenced by the fact that the structures of 3 are significantly different from that of 4, even though the coordination modes of (btc)4- ligands is the same. Furthermore, the comparison of 1 and 2 with 3 and 4 shows that the increase in pH value results

in a higher connectivity level of (btc)4- ligands, which in turn affects the formation of the final structures. Thermal Analysis. To examine thermal stability of these frameworks, thermal gravimetric analysis (TGA) measurements were carried out. Compound 1 lost its coordinated water molecules below 215 °C, the weight loss found of 7.5% was consistent with that calculated (7.3%). After the loss of water molecules, the 2D framework was stable up to 400 °C and then began to decompose upon further heating. The TG curve of 2 showed an initial weight loss of 11.9% below 165 °C corresponding to the removal of the solvent and coordination water molecules (calcd. 12.7%), and then the compound was stable up to 210 °C, followed by another weight loss after that temperature. Compound 3 showed a weight loss of 1.28% below 100 °C, which closely matches the weight loss of 1.26% for removing one guest water molecule per formula unit from the framework of 3. In the case of 4, the 2D framework was stable up to 267 °C and then began to decompose upon further heating. (Figure S3, Supporting Information, shows the TG curves for 1-4.) Luminescent Properties. The solid-state photoluminescent spectra of complexes 1-4 are depicted in Figure 9. Complex 1 shows an intense emission peaks at 424 nm, which means a red shift of ca. 36 nm relative to that of the free biphenyl3,3′,4,4′-tetracarboxylic dianhydride ligand (Figure S4, Supporting Information). We tentatively assign it to the intraligand fluorescence. Similar to 1, complex 2 also exhibits an intense fluorescent emission peak at 442 nm. Since the free 4,4′-bpy ligand does not show any luminescence in the range 400-800 nm,14 the strongest peak at 442 nm may also be interpreted as the intraligand fluorescence.13,15 The UV-vis spectra of the two complexes and biphenyl-3,3′,4,4′-tetracarboxylic dianhydride ligand are measured in the solid state at room temperature. Since the absorption spectra of complexes 1 and 2 are similar to that

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Figure 8. (a) Left: 2D bilayer motif in 4 viewed along the c-axis. Right: schematic illustration of the 2D bilayer in 4. (c) Side view along the b-axis showing bilayer motif in 4 (different color Zn atoms lying in two layers).

Conclusion Four new coordination polymers were synthesized by the hydrothermal self-assembly of H4btc, ZnII, and bipyridyl ligands (rigid 4,4′-bpy, flexible bpe) as the molecular building blocks. Complexes 1 and 2 display luminescent properties resulting from intraligand fluorescence and may be potential candidates for blue luminescent materials. Our research results demonstrate the structural diversity in MOFs that can be achieved by adjustments of the second ligands and pH value in the present system. Moreover, the ligand and pH-directed assembly may provide useful information for the further design of bilayer metal-organic compounds with novel structures and properties. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 20573083) and the Natural Science Foundation of Shaanxi Province (Grant No. 2004B09).

Figure 9. The emission spectra of 1-4 at room temperature (λ 360 nm).

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Supporting Information Available: X-ray crystallographic files in CIF format, selected bond distances and bond angles for 1-4, solid state emission spectra of the free biphenyl-3,3′,4,4′-tetracarboxylic dianhydride, and TGA curves for 1-4 are available free of charge via the Internet at http://pubs.acs.org.

References of biphenyl-3,3′,4,4′-tetracarboxylic dianhydride (Figure S5, Supporting Information), their absorption bands can be assigned to the intraligand π-π* transitions of the ligand. Thus, the above two complexes may be an excellent candidate for blue-lightemitted materials, whereas for complexes 3 and 4, the fluorescence is obviously decreased, and only two weak emission peaks were observed at 446 and 418 nm, respectively. Apparently, this phenomenon is associated with the structures of the complexes. In complexes 1 and 2, the pair phenyl rings of the ligands are nearly coplanar, which is favorable for the reduction of the energy of the π-π* transition to some extent, while for complexes 3 and 4, the dihedral angle between the pair of phenyl rings of the ligand are 37.2° and 39.8°, respectively, in which a larger distortion of two phenyl rings is disadvantageous to the luminescence.

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