ZnII Coordination Poylmers Based on 2,3,6,7 ... - ACS Publications

DOI: 10.1021/cg900659r

ZnII Coordination Poylmers Based on 2,3,6,7-Anthracenetetracarboxylic Acid: Synthesis, Structures, and Luminescence Properties

2009, Vol. 9 4840–4846

Ze Chang, Ai-Shun Zhang, Tong-Liang Hu, and Xian-He Bu* Department of Chemistry and Tianjin Key Lab on Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, China Received June 12, 2009; Revised Manuscript Received July 29, 2009

ABSTRACT: In our efforts to investigate polydentate ligands bearing bulky backbones, we used a bulky ligand, 2,3,6,7anthracenetetracarboxylic acid (H4ata), to react with ZnII ions under different conditions, yielding four new coordination polymers, {[Zn(ata)0.5(DMF)(H2O)] 3 DMF 3 H2O}n (1), {[Zn(ata)0.5(4,40 -bipy)0.5(DMF)] 3 DMF}n (2), {[Zn(ata)0.5(azpy)0.5(DMF)] 3 DMF}n (3), and {[Zn(ata)0.5(4,40 -bipy)(DMAC)] 3 (DMA)0.5}n (4) (DMF = N,N-dimethylformamide, 4,40 -bipy = 4,40 -bipyridine, azpy=4,40 -azopyridine, DMAC=N,N-dimethylacetamide, DMA=dimethylamine), which were characterized by IR and single crystal X-ray diffraction. Complex 1 forms a two-dimensional (2D) network with (4,4) topology assembled by ata4ligands and ZnII ions. Complexes 2 and 3 are three-dimensional (3D) frameworks with {83}2{85;10} topology containing infinite 2D networks pillared by 4,40 -bipy and azpy, respectively. Complex 4 takes a 2D structure with (4,4) net containing infinite onedimensional chains constructed by ZnII ions with 4,40 -bipy and ata4- ligands. These results show that the reaction of 2,3,6,7anthracenetetracarboxylic acid with ZnII in DMF tends to yield a layer structure which could be pillared by bipyridine-like ligands into 3D frameworks, and the final structure of the coordination polymer is greatly influenced by the solvent. Furthermore, the luminescence properties of 1-4 were studied in the solid state at room temperature.

Introduction In recent years, coordination polymers have been extensively studied due to their versatile architecture and potential applications for ion-exchange, gas storage, separation, catalysis, and so on.1 One of the most attractive targets of the research in this field is the controllable assembly of coordination polymers with desired structure and properties, and many strategies have already been developed based on previous works.2,3 Among all the rational strategies to construct threedimensional (3D) coordination polymers, the “pillar-layer” method is one of the most effective ways to synthesize new structures for useful properties such as hydrophilic/hydrophobic character, hydrogen bonding, and open metal site can be easily obtained by the introduction of functional ligands. Also, the structure characteristics such as pores and channels might be controlled and adjusted by the use of different pillars.3,4 As a kind of versatile ligand, carboxylic acids have been widely used in the construction of “pillar-layer” structures as they could serve either as a layer component or pillar linkers. Bipyridine ligands, which are another kind of useful building block, were usually introduced into the architectures as pillar building blocks. Though the strategy is simple and clear, the synthesis of “pillar-layer” coordination polymers with carboxylic acid and bipyridine as mixed ligands is still a relatively complex process because the final products are often unpredictable. Generally, many factors can affect the structures of the final products such as counteranions,5 the pH values of the reaction solutions,6 temperature,7 molar ratio between reactants,8 and solvent system.9 Pyromellitic acid (H4bta) has been used as a good ligand for the construction of coordination polymers.10 H4bta can be partially or fully deprotonated to generate different anions at *Corresponding author. E-mail: [email protected]. Fax: þ86-2223502458. Tel: þ86-22-23502809. pubs.acs.org/crystal

Published on Web 08/25/2009

different pH values. Therefore, it can potentially afford various coordination modes in multicoordinated ways with metal ions to form a series of coordination polymers with different structures and useful properties. As an expansion of H4bta, 2,3,6,7-anthracenetetracarboxylic acid (H4ata) has been reported for many years but has not been used in coordination chemistry. With a structure similar to H4bta, H4ata was expected to have the same advantages in the construction of coordination polymers. Also, as reported before, the introduction of anthracene based ligands into coordination polymers might lead to interesting luminescence properties of the final product.11,12 On the basis of our previous research of anthracene based coordination polymers, we are interested in the coordination chemistry of H4ata. We found that this ligand is effective in promoting “pillar-layer” structure coordination polymers due to its coordination interactions with ZnII ions, which show new structures and useful luminescence properties. Herein, we report the syntheses and crystal structures of four ZnII coordination polymers based on H4ata, namely, {[Zn(ata)0.5(DMF)(H2O)] 3 DMF 3 H2O}n (1), {[Zn(ata)0.5(4,40 -bipy)0.5(DMF)] 3 DMF}n (2), {[Zn(ata)0.5(azpy)0.5(DMF)] 3 DMF}n (3), and {[Zn(ata)0.5(4,40 -bipy)(DMAC)] 3 (DMA)0.5}n (4) (DMF = N,N-dimethylformamide, 4,40 -bipy = 4,40 -bipyridine, azpy = 4,40 -azopyridine, DMAC=N,N-dimethylacetamide, DMA=dimethylamine). Furthermore, the luminescence properties of 1-4 were studied in the solid state at room temperature. Experimental Section Materials and Physical measurements. All the chemicals used for synthesis are of analytical grade and commercially available. H4ata and azpy were synthesized according to the reported methods.13,14 Elemental analyses (C, H, and N) were performed on a PerkinElemer240C analyzer. IR spectra were measured on a Tensor 27 OPUS (Bruker) FT-IR spectrometer with KBr pellets. The luminescence r 2009 American Chemical Society

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Crystal Growth & Design, Vol. 9, No. 11, 2009

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Table 1. Crystal Data and Structure Refinement Parameters for Complexes 1-4 empirical formula formula weight temperature/K crystal system space group a /A˚ b /A˚ c /A˚ β /° V (A˚3) Z Dc (g 3 cm-3) F(000) θ range /° reflns collected independent reflns goodness-of-fit R1a (I > 2σ (I )) wR2b (I > 2σ (I )) a

1

2

3

4

C15H21N2O8Zn 422.73 293(2) monoclinic P21/c 7.7210(15) 10.433(2) 23.876(6) 106.52(3) 1843.9(7) 4 1.523 876 3.28-27.47 16778 4178 1.022 0.0674 0.0964



C96H76N18O24Zn4 2127.31 293(2) monoclinic C2/m 14.705(3) 14.636(3) 11.656(2) 97.80(3) 2485.4(8) 1 1.421 1090 3.10-27.48 13145 2955 1.095 0.0715 0.1871

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

Chart 1

spectra for the powdered solid samples were measured on a Varian Cary Eclipse fluorescence spectrophotometer and all the measurements were carried out under the same experimental conditions. The X-ray powder diffraction spectra (XRPD) were recorded on a Rigaku D/Max-2500 diffractometer at 40 kV, 100 mA for a Cutarget tube and a graphite monochromator. Simulation of the XRPD pattern was carried out by the single-crystal data and diffraction-crystal module of the Mercury (Hg) program available free of charge via the Internet at http://www.iucr.org. Synthesis. Single crystals of complexes 1-4 suitable for X-ray analysis were obtained by a similar method, so only the synthesis of 1 is described in detail. {[Zn(ata)0.5(DMF)(H2O)] 3 DMF 3 H2O}n (1). H4ata (0.1 mmol) was dissolved in DMF (5 mL) in a beaker and then heated to 80 °C. Zn(NO3)2 3 6H2O (0.25 mmol) was added to the solution with stirring. After reacting for ca. 5 min, the hot solution was filtered and the filtrate was allowed to slowly evaporate at room temperature. Yellow block crystals were obtained in about two weeks. Yield 30% based on H4ata. FT-IR (KBr pellets, cm-1): 3401w, 2929w, 1655s, 1587s, 1462m, 1434m, 1377s, 1250w, 1105w, 1047w, 934w, 891w, 803m, 688w, 666w, 604w, 472w. Anal. Calcd for C15H21N2O8Zn: C, 42.62; H, 5.00; N, 6.63%. Found: C, 42.21; H, 5.29; N, 6.10%. {[Zn(ata)0.5(4,40 -bipy)0.5(DMF)] 3 DMF}n (2). 4,40 -Bipy (0.1 mmol) was added after the addition of Zn(NO3)2 3 6H2O. Yield 30% based on H4ata. FT-IR (KBr pellets, cm-1): 3520w, 3442w, 3094w, 1656s, 1610s, 1598s, 1462m, 1376s, 1253w, 1223w, 1070m, 941w, 892w, 805s, 728w, 642w, 476w. Anal. Calcd for C20H21N3O6Zn 3 0.5H2O: C, 50.69; H, 4.68; N, 8.86%. Found: C, 50.24; H, 4.69; N, 8.71%. {[Zn(ata)0.5(azpy)0.5(DMF)] 3 DMF}n (3). Azpy (0.1 mmol) was added after the addition of Zn(NO3)2 3 6H2O. Yield 20% based on H4ata. FT-IR (KBr pellets, cm-1): 3607w, 3522w, 3102m, 1662s, 1606s, 1459m, 1422s, 1391s, 1363s, 1247w, 1226w, 1091w, 1047w, 1027w, 942w, 899m, 848m, 800m, 696w, 668w, 572w, 460w. Anal. Calcd for C20H21N4O6Zn 3 0.5H2O: C, 49.24; H, 4.54; N, 11.48%. Found: C, 48.95; H, 4.50; N, 11.14%.

{[Zn(ata)0.5(4,40 -bipy)(DMAC)] 3 (DMA)0.5}n (4). DMAC was used as solvent and 4,40 -bipy (0.1 mmol) was added after the addition of Zn(NO3)2 3 6H2O. Yield 30% based on H4ata. FT-IR (KBr pellets, cm-1): 3520w, 3442w, 3074w, 1608s, 1491m, 1455m, 1383s, 1313m, 1289m, 1223m, 1073m, 1044w, 1010w, 925w, 877w, 807m, 786m, 734w, 716w, 670w, 642w, 628w, 590w, 481w, 441w. Anal. Calcd for C94H95N13O24Zn4: C, 55.00; H, 4.66; N, 8.87%. Found: C, 54.81; H, 4.86; N, 8.80%. X-ray Data Collection and Structure Determinations. X-ray single-crystal diffraction data for complexes 1 and 4 were collected on a Rigaku SCX-mini diffractometer at 293(2) K with Mo KR radiation (λ=0.71073 A˚) by ω scan mode, and diffraction data for complexes 2 and 3 were collected on a Rigaku Saturn70 diffractometer at 293(2) K with Mo KR radiation (λ=0.71073 A˚) by ω scan mode. The program SAINT15 was used for integration of the diffraction profiles. All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by fullmatrix least-squares methods with SHELXL (semiempirical absorption corrections were applied using SADABS program).16 Metal atoms in each complex were located from the E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. The hydrogen atoms of the ligands were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. For 2 and 4, the solvent molecules in the compounds were disordered and located from difference maps. The disorder was treated by performing half occupancies with disordered atoms of the guest molecules. Detailed crystallographic data are summarized in Table 1. The selected bond lengths and angles are given in Table 2.

Results and Discussion Synthesis. The reactions of Zn(NO3)2 3 6H2O with H4ata in different conditions yielded four new coordination polymers. Reaction of H4ata and Zn(NO3)2 3 6H2O in DMF resulted in a layer structure in which DMF and water molecules occupied partial coordination sites of zinc ions. When 4,40 -bipy or azpy was added as the starting material, the O atoms from coordinated water molecules were substituted by N atoms from pyridine and the layer structure was then pillared and expanded into 3D frameworks. Also, the solvent deeply affects the final structure of coordination polymers. When Zn(NO3)2 3 6H2O was allowed to react with H4ata and 4,40 -bipy in DMF and DMAC, respectively, the structures of the resulting architectures were totally different. A 3D framework was obtained in DMF while in DMAC the structure was composed of 2D sheets. All the

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Table 2. Selected Bond Lengths [A˚] and Bond Angles [deg] for Complexes 1-4a 1 Zn(1)-O(1) Zn(1)-O(3)#1 O(3)#1-Zn(1)-O(1) O(3)#1-Zn(1)-O(1W) O(1)-Zn(1)-O(1W)

1.958(3) 1.958(3) 136.09(13) 98.97(13) 109.74(14)

Zn(1)-O(5) Zn(1)-O(1W) O(3)#1-Zn(1)-O(5) O(1)-Zn(1)-O(5) O(1W)-Zn(1)-O(5)

2.019(3) 2.000(3) 105.06(13) 102.58(13) 98.81(15)

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

2.062(2) 2.112(2) 2.287(3) 102.67(9) 101.51(9) 94.86(10) 146.52(10) 102.66(9)

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

2.044(2) 2.022(2) 97.77(10) 89.03(9) 155.41(10) 104.01(10) 59.69(9)

3 Zn(1)-N(1) Zn(1)-O(1) O(1)-Zn(1)-O(4)#3 O(1)-Zn(1)-O(5) O(4)#3-Zn(1)-O(5)

2.036(3) 1.947(2) 100.17(9) 104.01(9) 118.87(8)

Zn(1)-O(4)#3 Zn(1)-O(5) O(1)-Zn(1)-N(1) O(4)#3-Zn(1)-N(1) O(5)-Zn(1)-N(1)

1.9634(18) 1.9684(19) 114.02(10) 106.14(9) 113.15(10)

4 Zn(1)-O(1) Zn(1)-O(3) O(1)#4-Zn(1)-O(1) O(1)#4-Zn(1)-O(3) O(1)-Zn(1)-O(3) O(1)#4-Zn(1)-O(1W) O(1)-Zn(1)-O(1W) O(3)-Zn(1)-O(1W) O(1)#4-Zn(1)-N(1) O(1)-Zn(1)-N(1)

2.064(3) 2.134(5) 98.43(18) 89.48(12) 89.48(12) 92.24(12) 92.24(12) 177.37(17) 88.55(15) 172.98(14)

Zn(1)-O(1W) Zn(1)-N(1) O(3)-Zn(1)-N(1) O(1W)-Zn(1)-N(1) O(1)#4-Zn(1)-N(1)#4 O(1)-Zn(1)-N(1)#4 O(3)-Zn(1)-N(1)#4 O(1W)-Zn(1)-N(1)#4 N(1)-Zn(1)-N(1)#4

2.168(4) 2.208(4) 89.83(14) 88.22(14) 172.98(14) 88.55(15) 89.83(14) 88.22(14) 84.5(2)

a Symmetry transformations used to generate equivalent atoms: #1 -x þ 2, y - 1/2, -z þ 3/2; #2 x, -y þ 3/2, z þ 1/2; #3 x, -y þ 3/2, z - 1/2; #4 x, -y þ 1, z.

crystals of 1-4 were obtained by slow evaporation of the solution at room temperature. If the solution was heated to accelerate the reaction, the quality of the obtained crystals uaually were not good enough for structural analysis or only precipitation was obtained. Compound 1 tends to lose solvent and decompose in air while 2-4 are stable, so only 2-4 were characterized by XRPD and TG. All the peaks displayed in the measured X-ray powder diffraction patterns for the samples of 2-4 can closely match those in the simulated patterns generated from single-crystal diffraction data, indicating a single phase for each compound was formed. Descriptions of Crystal Structures. {[Zn(ata)0.5(DMF)(H2O)] 3 DMF 3 H2O}n (1). X-ray crystallographic analysis revealed that 1 crystallizes in monoclinic space group P21/c. The asymmetric unit in 1 contains one ZnII ion, half ata4-, one coordinated DMF, one coordinated water molecule, one free DMF molecule, and one free water molecule. As shown in Figure 1, H4ata in 1 was fully deprotonated and displayed μ4-bridging fashion. Each deprotonated carboxyl of ata4coordinated to one ZnII ion with monodentate mode. The angle between carboxylate plane and anthracene plane is 50.5° and 41.5° for O1-C1-O2 and O3-C9-O4, respectively. Each ZnII ion in 1 was four-coordinated with four O atoms from two individual asymmetric μ4-ata4- (O1, O3), one DMF molecule (O5) and one water molecule (O1w), respectively. The Zn-O bond distances are normal for Zn1-O1=1.958(3) A˚, Zn1-O3=1.958(3) A˚, Zn1-O5=2.019(3) A˚, and Zn1-O1w= 2.000(3) A˚.

In 1, ZnII ions were joined by ata4- ligands to form infinite 2D layers with (4,4) grid with the ZnII ions presented as twoconnecting nodes and ata4- ligands served as four connected nodes, respectively (Figure 1b). Anthracene planes of ata4ligands were nearly coplanar, while coordinated DMF and water molecules distributed in both sides of the 2D sheets alternately. The distance between adjacent layers was about 7.5 A˚ and free DMF and water molecules were located between the layers by hydrogen bonds. As shown in Figure 1c, free water molecule O2w served both as donor (O2w-H2wA 3 3 3 O2: 2.848(5) A˚, 170.2°; O2w-H2wB 3 3 3 O4: 2.795(5) A˚, 172.8°) and acceptor (O1w-H1wB 3 3 3 O2w: 2.651(5) A˚, 174.7°), respectively. The coordinated water molecules O1w served only as donor in the interactions between O1w and the free DMF (O1w-H1wA 3 3 3 O6: 2.662(5) A˚, 173.3°). With strong hydrogen bonds between layers serving as pillars, 2D infinite sheets were connected into a 3D supramolecular structure. Free DMF molecules between layers might influence the formation of the structure (Figure 1d). {[Zn(ata)0.5(4,40 -bipy)0.5(DMF)] 3 DMF}n (2). The asymmetric unit of 2 was composed of one ZnII ion, half ata4-, half 4,40 -bipy, one coordinated DMF, and one free DMF molecule. As shown in Figure 2, the ZnII ion is five coordinated by a carboxylate from one ata4- with chelating mode (O1 and O2), a carboxylate from another ata4- with monodentate mode (O4), one O atom from coordinated DMF (O5), and one N atom from 4,40 -bipy (N1). The Zn-O (2.022(2) A˚ to 2.287(3) A˚) and Zn-N (2.062(2) A˚) distances are both within normal range.17 H4ata ligands in 2 were fully deprotonated

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Figure 1. (a) Coordination environment of ZnII ions in 1; free solvent molecules are omitted for clarity. (b) View from the a direction showing the 2D (4,4) net layer constructed by ata4- and Zn polyhedra. (c) Hydrogen bonds between layer structure and free solvent molecules. (d) View in parallel with the 2D layers with free DMF shown in space filling mode occupying the space between layers. Hydrogen bonds are shown as green dashed line.

Figure 2. (a) Coordination environment of ZnII ions in 2, free solvent molecules are omitted for clarity. (b) Side view of the pillar-layer structures. Different layers were shown in distinct color. (c) Schematic drawing of the space between layers. The yellow sphere represents the disordered DMF molecules. (d) Schematic drawing of the 3,4-connected net in which the ZnII ions were represented by light blue spheres and ata4- ligands represented by yellow spheres. The purple spokes represent 4,40 -bipy.

with the carboxylates at 2 and 6 site bonded with ZnII using two O atoms while the other two using only one O atom. The angle between carboxylate plane and anthracene plane is

67.2° for the chelating ones and 22.4° for the other two. The difference between the two angles is much bigger than that of 1, indicating the flexibility of the ligand when coordinating

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with metal ions. Different from the planar layers in 1, wavy layers were constructed by μ4-ata4- ligands and ZnII ions in 2. The angle between adjacent anthracene planes is 62.8° and the distance between adjacent layers was about 8.5 A˚ (with van der Waals radius), slightly longer than that of 1. With 4,40 -bipy as the pillar, the wavy layers were further connected into a 3D framework (Figure 2b) and the space between layers was occupied by coordinated and free DMF molecules. The coordinated DMF molecules were arranged along the c direction and the free ones were located in the center of the cage surrounded by anthracene backbones, 4,40 -bipy ligands, and coordinated DMF molecules (Figure 2c). In each cage there were two DMF molecules from two individual asymmetrical units and these solvent molecules were mildly compressed in the cage and two sites disordered. Topology analysis of the framework gave a 3,4-connected net with stoichiometry (3-c)2(4-c) while ZnII ions as 3-connected nodes and ata4- ligands as 4-connected nodes. The total Schlafli symbol was given as {83}2{85; 10} (Figure 2d). The construction of the structure could be described in view of the fact that the layer structures in 1 were modified using 4,40 -bipy ligands by substituting coordinated water molecules with it. The layer structures then adopted a wavy motif to adapt the changes brought by pillar ligands. {[Zn(ata)0.5(azpy)0.5(DMF)] 3 DMF}n (3). The structure of 3 is similar to that of 2 except that 4,40 -bipy ligands were replaced by azpy ligands (Figure 3). In 3, Zn1 with distorted tetrahedral coordination geometry is four-coordinated by three O atoms (O1, O4 from two different ata4- ligands and O5 from the coordinated DMF) and one N atom (N1 from azpy). The Zn-O distances are 1.947(2), 1.9634(18), and 1.9684(19) A˚, and the Zn-N one is 2.036(3) A˚, respectively. Though the organization of ZnII ions and ata4- ligands in 3 is similar to that in 2, the substitution of azpy to 4,40 -bipy had just changed the conformation and coordination mode of the carboxylate ligand and further influenced the dimensions and the angle of unit cell. In 3, four carboxylate of one ata4ligand coordinated to four distinct ZnII ions using a monodentate mode rather than a chelating one. The angle between carboxylate plane and anthracene plane is 65.5° for O1-C1-O2 and 23.9° for O3-C9-O4, slightly different from that of 2. The dimensions and the beta angle of unit cell of 3 are enlarged for the length of azpy being longer than that of 4,40 -bipy. The angle between adjacent anthracene planes which coordinated to the same metal center is 74.5° and the distance between adjacent layers was about 8.1 A˚ (with van der Waals radius). Though the space between layers is compressed in the direction perpendicular to the bc plane, the free DMF molecules sandwiched were not disordered for the b axis of unit cell is enlarged. {[Zn(ata)0.5(4,40 -bipy)(DMAC)] 3 (DMA)0.5}n (4). When DMAC was used as solvent in the same reaction condition of 2, a new coordination polymer 4 was obtained. The result of X-ray crystallographic analysis revealed that 4 crystallized in the monoclinic space group C2/m. As displayed in Figure 4a, the ZnII ions in 4 were six coordinated by two O atoms from two individual ata4- ligands (O1, O1A) (A=x, -y þ 1, z), one O atom from coordinated DMAC (O3), one O atom from coordinated water (O1w), and two N atoms from two different 4,40 -bipy (N1, N1A) (A=x, -y þ 1, z). ZnII ion was located in the center of the distorted octahedral coordination environment, O1, O1A, N1, and N1A (A = x, -y þ 1, z) comprised the equatorial plane, and O3 and O1w occupied the apical positions. The Zn-O distances range

Chang et al.

Figure 3. (a) Coordination environment of ZnII ions in 3; free solvent molecules are omited for clarity. (b) View perpendicular to the layers to show the connectivity of layers and pillared azpy. ZnII ions showed as polyhedral. Coordinated DMF molecules are omitted for clarity.

from 2.064(3) to 2.168(4) A˚, and the Zn-N distance is 2.208(4) A˚. The ata4- ligands in 4 were fully deprotonated and coordinated with four metal centers using monodentate mode. Every carboxylate bonded with one metal center and the angle between the carboxylate plane and the anthracene plane is 45.7°. Different from 1-3, one ata4- in 4 was connected to another two ata4- ligands by carboxylate at 2,3 and 6,7 positions with ZnII ions as bridges to result in infinite 1D chains in which the anthracene planes were coplanar. The 4,40 -bipy ligands in 4 were also bridged by ZnII ions to form zigzag chains and the angle between adjacent 4,40 -bipy planes is 85.2°. These two kinds of chains were parallel and alternatively organized into planar 2D layers with trigonal windows (Figure 4b). The layer presented a distorted (4,4) grid, while ata4- ligands and ZnII ions were treated as two kinds of four connected nodes. In the unit cell, 2D layers packed in A-B mode and the space left by the packing of two neighboring layers is filled by the coordinated DMAC molecules from adjacent layers and free DMA molecules decomposed from DMAC. The structure analysis of complexes 1-3 indicated a rule that when H4ata was allowed to coordinate with ZnII ions in

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Figure 5. Emission spectra of 1-4 in the solid state at room temperature.

Figure 4. (a) Coordination environment of ZnII ions in 4, coordinated DMAC and free DMA molecules omitted for clarity. (b) A view of two kinds of 1D infinite chains composed of metal centers with ata4- ligands and 4,40 -bipy, respectively. (c) A view of the A-B packing style layers. Coordinated DMAC and free DMA molecules which occupied the pores among layers omitted for clarity.

DMF, layer structures are preferred. Furthermore, the layer structures are flexible and could be expanded into 3D architectures by replacing the coordinated solvent molecules with pillar bipyridine ligands such as 4,40 -bipy and azpy. It should be noticed that the solvent used in the reaction deeply influenced the final structure obtained. For example, when DMAC was used as solvent while other factors such as temperature, concentration, and ratio of reactants were keep the same with 2, the pillar-layer structure was not available in 4. Also, in complexes 1-3, free DMF molecules found between the layers might be an important factor that influences the assembly process.

Thermal Stabilities of the Complexes. Thermogravimetric analyses (TG) were carried out for complexes 2-4, and the results are shown in Figure S2 (see Supporting Information). Complex 2 is stable up to 70 °C and shows a weight loss of 14.06% below 230 °C. That result is slightly lower than the calculated value for the loss of DMF (15.73%) and might be attributed to the decomposition of DMF. A slow loss of weight without a stable range was observed before the decomposition of the residue occurred at 430 °C. The decomposition of 3 did not start before 90 °C and the framework collapsed in two steps. In the first step, in the range from 90 to 330 °C, 30.40% of weight was lost corresponding to the loss of both free and coordinated DMF (calcd 30.52%). The second step is from 330 to 480 °C for the total loss of 82.43% weight. For 4, a weight loss of 23.45% was observed in the temperature range of 70-165 °C, which corresponds to the loss of coordinated DMAC, coordinated water, and the free DMA (calcd 23.39%), and then the complex decomposed fast as the temperature increased. The framework finally collapsed at 420 °C. Photoluminescent Property. Luminescent compounds are of great current interest for various potential applications.18 The synthesis of coordination polymers with carefully chosen ligands and metal ions can be an efficient method for the construction of novel luminescence materials.12,19 Accordingly, the emission spectra of complexes 1-4 were measured in the solid state at room temperature (see Figure 5). It is clear that there is one emission peak at 436 nm for 1 (λex = 350 nm), two emission peaks at 447 and 478 nm for 2 (λex = 350 nm), three emission peaks at 431, 486, and 525 nm for 3 (λex=383 nm), and two emission peaks at 447 and 478 nm for 4 (λex = 350 nm), respectively. These emissions are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature for the reason that ZnII ions are difficult to oxidize or to reduce to their d10 configuration. Comparing the emission peak of H4ata ligand at 485 nm, the emission peaks at 478 nm for 2 and 4 and 486 nm for 3 can be tentatively assigned to intraligand transfer π*-π transitions, namely, ligand-to-ligand charge transfer (LLCT), similar to the reported results on coordination polymers with a anthracene backbone.20 As 1 contains ata4ligand only, it was expected to have only one emission associated with the ligand. To our surprise, a blue shift of about 50 nm was observed compared with the emission of

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the ligand. This phenomenon might be assigned to a charge transfer (CT) process between the host anthracene unit (electron acceptor) and the guest molecules (electron donor).12 Therefore, emissions observed at 447 nm for 2 and 4 and 431 nm for 3 might be assigned to a combination of the CT process and intraligand emission states, as reported for other ZnII complexes with N-donor ligands.21 The 525 nm peak for 3 originated from the emission of azpy. For 2 and 4, the emission peaks were just at the same wavelength though the relative intensity of the two peaks is different. As the components of these two complexes are similar to each other, the differences might be attributed to the structural diversity along with a different ratio of ata4- ligands to 4,40 -bipy in the complexes.

Chang et al.

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Conclusion II

Four Zn coordination polymers with 2,3,6,7-anthracenetetracarboxylic acid (H4ata) have been synthesized and characterized. The coordination chemistry of H4ata with ZnII ion was investigated. The study of the synthesis and structures of the complexes presents the evidence that when H4ata coordinated with ZnII ions in DMF solvent, layer structures are preferred. Two “pillar-layer” coordination polymers, 2 and 3, with 4,40 -bipy and azpy as pillar ligands respectively, have been successfully obtained. Also, the importance of solvent in the assembly process was proven by the distinct structures obtained when the solvent was changed to DMAC. In addition, photoluminescence properties of complexes 1-4 were studied in the solid state at room temperature. All of them display blue emissions with different densities.

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Acknowledgment. This work was financially supported by the 973 Program of China (2007CB815305), the NNSF of China (50673043 and 20773068), NSF of Tianjin, China (07JCZDJC00500),

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Supporting Information Available: X-ray crystallographic data for complexes 1-4 in CIF format and Figures S1-S2. This information is available free of charge via the Internet at http://pubs.acs.org/.

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ZnII Coordination Poylmers Based on 2,3,6,7 ... - ACS Publications

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