Syntheses, Structures, and Photoluminescence of a Series of d10

Nov 3, 2009 - Syntheses, Structures, and Photoluminescence of a Series of d10 Coordination Polymers with R-Isophthalate (R = −OH, −CH3, and −C(C...
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DOI: 10.1021/cg900825y

Syntheses, Structures, and Photoluminescence of a Series of d10 Coordination Polymers with R-Isophthalate (R = -OH, -CH3, and -C(CH3)3)

2009, Vol. 9 5334–5342

Lu-Fang Ma,†,‡ Li-Ya Wang,*,† Jiang-Liang Hu,† Yao-Yu Wang,*,‡ and Guo-Ping Yang‡ †

College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, P. R. China, and ‡Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry, Northwest University, Xi’an 710069, P. R. China Received July 17, 2009; Revised Manuscript Received October 13, 2009

ABSTRACT: Eight new d10 metal-organic coordination polymers, [Zn(mip)(bpa)]n (1), [Zn(mip)(bpp)]n (2), [Cd(mip)(bpp)]n (3), [Zn2(tbip)2(bpa)(H2O)]n (4), [Zn2(tbip)2(dpe)(H2O)]n (5), {[Zn(tbip)(bpp)] 3 H2O}n (6), {[Cd(tbip)(bpp)(H2O)] 3 3H2O}n (7), and {[Cd(tbip)(H2O)(bipy)0.5] 3 H2O}n (8) were prepared through hydrothermal reactions of H2mip or H2tbip (H2mip = 5-methylisophthalic acid and H2tbip = 5-tert-butylisophthalic acid) with different pyridyl-containing ligands (bpa = 1,2-bis(4-pyridyl)ethane, bpp = 1,3-bis(4-pyridyl)propane, dpe = 1,2-di(4-pyridyl)ethene and bipy = 4,40 -bipyridine), respectively. Except for complexes 2 and 6, all the other six compounds were reported for the first time. These complexes were structurally characterized by elemental analysis, IR spectroscopy, and X-ray single-crystal diffraction. Complex 1 is a 3-fold interpenetrating three-dimensional (3D) network with CdSO4 topology constructed from alternately left-handed and right-handed helical channels. Complexes 2 and 3 are 4fold interpenetrated diamondoid-like networks. Polymers 4 and 5 are isostructural, showing 2-fold 3D interpenetrating R-Po networks constructed from binuclear zinc nodes. Complex 6 possesses a two-dimensional (2D) corrugated network. Complex 7 has a one-dimensional (1D) tube-like chain along the a direction and is further linked by hydrogen bonding and π-π stacking interactions to form a 3D supramolecular network. Complex 8 features a 2D layer and further forms a 3D supramolecular framework by hydrogen bonding. These results show that the influence of steric hindrance of an organic ligand on the structures of d10 coordination polymers is realized through changing the substituted groups of the dicarboxylate derivatives. Furthermore, thermal stabilities and photoluminescent properties of the complexes were also studied in the solid state.

Introduction The construction of metal-organic frameworks (MOFs) has become an exciting and expanding approach to novel materials. The interest is stimulated not only by the MOFs’ structural diversities but also by their extensive potential applications in such areas as separation, molecular recognition, ion exchange, gas sorption and storage, nonlinear optics, magnetics and catalysis.1-7 In addition, hydrothermal synthesis at mild temperatures (100-200 C) under autogeous pressure has proven to be a powerful approach in the preparation of low-soluble organic-inorganic hybrid materials. Many coordination polymers with special properties have been synthesized by using hydrothermal reactions in the past decades, but the mechanism about the influence of the structures is vacant and unpredictable. Recently, many factors, such as the coordination geometry of the metal ions, solvent systems, counterions, and metal-to-ligand ratios, have also been found to influence the network and the topology of coordination polymers.8-10 In principle, selection and synthesis of the organic bridging ligands play a crucial role in the design and construction of desirable frameworks, because changes in flexibility, length, and symmetry of organic ligands can result in materials bearing diverse architectures and functions. Thus, the prospect of tuning the properties of MOFs through a systematic change of organic ligands provides an impetus for further research on metal-organic supramolecular architectures. *To whom correspondence should be addressed. E-mail: wlya@lynu. edu.cn. pubs.acs.org/crystal

Published on Web 11/03/2009

As typical aromatic polycarboxylate ligands, benzene-1,3dicarboxylic acid (isophthalic acid, H2isop) and its derivatives with similar coordinated groups, that is, those with a 120 angle between two carboxylic groups, show versatile coordination modes and have been used widely to construct coordination polymers.11-14 In this context, we choose dipyridyl ligands incorporated with isophthalic acid (H2ip), 5-nitroisophthalic acid (H2nip), 5-tert-butylisophthalic acid (H2tbip) and 5-methylisophthalic acid (H2mip), 5-hydroxyisophthalic acid (H2hip), respectively, as the building blocks to construct various coordination polymers. These polycarboxyl compounds are chemically pertinent and differentiated by the 5-position substituents. It is anticipated that these modified linkers may afford different supramolecular assemblies in view of their steric and/or electronic effect. In our previous work, we have shown the role of secondary ligands, the pH value, and temperature effects on the structure of dicarboxylate-based coordination polymer.15 As a continuation of our research on the dicarboxylate-based coordination polymer, in this work, we systematically investigate the steric effect of the substituted groups of an organic ligand on the structure of dicarboxylate-based coordination polymers. Eight new Zn and Cd(II) coordination polymers have been synthesized from H2tbip or H2mip in combination with the dipyridyl ligands 1,2-bis(4-pyridyl)ethane (bpa), 1,3-bis(4-pyridyl)propane (bpp), 1,2-di(4-pyridyl)ethane (dpe), and 4,40 -bipyridine (bipy), and their structures have been characterized by X-ray analysis. These were then compared to the Zn (Cd) complexes, [Zn(hip)(bpa)]n (9), {[Zn(hip)(bpp)] 3 H2O}n (10), {[Cd(hip)(H2O)(bpp)] 3 2H2O}n (11), and {[Zn(hip)(dpe)] 3 (dpe)0.5}n (12), constructed from 5-hydroxylisophthalic acid reported by r 2009 American Chemical Society

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Table 1. Crystallographic Data for Complexes compound formula fw temperature crystal system space group unit cell dimensions (A˚, )

1 C21H18N2O4Zn 427.74 291(2) orthorhombic Pnna a = 9.032(1) b = 16.145(2) c = 14.791(2)

3 C22H20Cd N2O4 488.80 291(2) monoclinic P21/n a = 11.194(9) b = 11.372(9) c = 16.159(2) β = 102.143(1)

4 C36H38N2O9Zn2 773.42 291(2) monoclinic P21/n a = 9.5977(7) b = 17.493(13) c = 21.206(15) β = 101.787(10)

5 C36H36N2O9Zn2 771.41 291(2) monoclinic P21/n a = 9.587(5) b = 17.457(9) c = 21.086(11) β = 102.083(6)

7 C25H31CdN2O8 599.92 291(2) monoclinic P2/m a = 12.27(2) b = 10.297(2) c = 12.831(2) β = 91.002(2)

V (A˚3) Z F (g cm-3) F(000) GOF R1, wR2 [I > 2σ(I)] R1, wR2 (all data) residuals (e A˚-3)

2157.0(4) 4 1.317 880 1.080 0.035, 0.110 0.044, 0.116 0.87, -0.24

2011.0(3) 4 1.614 984 1.091 0.021, 0.061 0.024, 0.063 1.17, -0.37

3485.4(4) 4 1.474 1600 1.051 0.025, 0.063 0.031, 0.065 0.33, -0.25

3451(3) 4 1.485 1592 1.026 0.039, 0.096 0.055, 0.104 0.37, -0.78

1621.0(5) 2 1.229 614 1.073 0.072, 0.201 0.093, 0.217 0.97, -0.69

Cao et al.,13b,c and differences in their structures and physical behaviors were found to result from the nature of the noncoordinated groups of the dicarboxylate ligands. Discussions of their subtleties are addressed below. Experimental Section Materials and Physical Measurements. All reagents used in the syntheses were of analytical grade. Elemental analyses for carbon, hydrogen, and nitrogen atoms were performed on a Vario EL III elemental analyzer. The infrared spectra (4000-600 cm-1) were recorded by using KBr pellet on an Avatar 360 E. S. P. IR spectrometer. Thermogravimetric analyses were carried out on a STA449C integration thermal analyzer. Fluorescent analyses were performed on an Hitachi F-4500 analyzer. The powder X-ray diffraction (PXRD) patterns were recorded with a Rigaku D/Max 3III diffractometer. Preparation of the Title Complexes. [Zn(mip)(bpa)]n (1). A mixture of H2mip (0.1 mmol, 17.7 mg), bpa (0.1 mmol, 18.4 mg), Zn(OAc)2 3 2H2O (22.0 mg, 0.1 mmol), KOH (0.1 mmol, 5.6 mg), and H2O (15 mL) was placed in a Teflon-lined stainless steel vessel, heated to 170 C for 4 days, and then cooled to room temperature over 24 h. Colorless block crystals of 1 were obtained. Yield: 38% based on Zn (16.25 mg). Elemental analysis (%): calcd for C21H18N2O4Zn C 58.96, H 4.24, N 6.55; found C 58.88, H 4.29, N 6.52. IR (cm-1): 3434 m, 2922 m, 1616 s νas(COO-), 1583 m νs(COO-), 1432 m, 1357 s, 1027 m, 775 m. [Cd(mip)(bpp)]n (3). A mixture of H2mip (0.1 mmol, 17.7 mg), bpp (0.1 mmol, 19.8 mg), Cd(OAc)2 3 2H2O (26.7 mg, 0.1 mmol), KOH (0.1 mmol, 5.6 mg), and H2O (15 mL) was placed in a Teflon-lined stainless steel vessel, heated to 170 C for 4 days, and then cooled to room temperature over 24 h. Colorless block crystals of 3 were obtained. Yield: 45% based on Cd (21.99 mg). Elemental analysis (%): calcd for C22H20Cd N2O4 C 54.06, H 4.12, N 5.73; found C 54.01, H 4.17, N 5.63. IR (cm-1): 3438 m, 2925 m, 1613 s νas(COO-), 1562 m νs(COO-), 1426 m, 1368 s, 1069 m, 777 m. [Zn2(tbip)2(bpa)(H2O)]n (4). A mixture of H2tbip (0.1 mmol, 23.1 mg), bpa (0.1 mmol, 18.3 mg), Zn(OAc)2 3 2H2O (22.0 mg, 0.1 mmol), KOH (0.1 mmol, 5.6 mg), and H2O (15 mL) was placed in a Teflon-lined stainless steel vessel, heated to 170 C for 4 days, and then cooled to room temperature over 24 h. Colorless block crystals of 4 were obtained. Yield: 28% based on Zn (10.83 mg). Elemental analysis (%): calcd for C36H38N2O9Zn2 C 55.90, H 4.95, N 3.62; found C 55.78, H 4.98, N 3.53. IR (cm-1): 3457 m, 2965 m, 1618 s νas(COO-), 1574 m νs(COO-), 1432 m, 1355 s, 1070 m, 776 m. [Zn2(tbip)2(dpe)(H2O)]n (5). 5 was synthesized in a procedure analogous to that of 4 except that bpa was replaced by dpe. Colorless block crystals of 5 were obtained. Yield: 32% based on Zn (14.66 mg). Elemental analysis (%): calcd for C36H36N2O9Zn2 C 56.05, H 4.70, N 3.63; found C 56.14, H 4.66, N 3.57. IR (cm-1): 3404 m, 2962 m, 1607 s νas(COO-), 1571 m νs(COO-), 1439 m, 1366 s, 1331 s, 1066 m, 780 m.

8 C17H20CdNO6 446.74 291(2) triclinic P1 a = 9.850(2) b = 10.050(3) c = 11.128(5) R = 98.413(3) β = 102.76(3) γ = 118.571(2) 901.7(5) 2 1.645 450 1.056 0.023, 0.055 0.024, 0.056 0.74, -0.43

{[Cd(tbip)(bpp)(H2O)] 3 3H2O}n (7). 7 was synthesized in a procedure analogous to that of 6 except that the Zn salt was replaced by Cd(OAc)2. Colorless crystals of 7 were obtained. Yield: 36% based on Cd (21.60 mg). Elemental analysis (%): calcd for C25H31CdN2O8 C 50.05, H 5.21, N 4.67; found C 50.12, H 5.26, N 4.78. IR (cm-1): 3426 m, 2953 m, 1613 s νas(COO-), 1560 m νs(COO-), 1427 m, 1371 s, 1016 m, 732 m. {[Cd(tbip)(H2O)(bipy)0.5] 3 H2O}n (8). 8 was synthesized in a procedure analogous to that of 7 except that the bpp was replaced by bipy. Colorless crystals of 8 were obtained. Yield: 41% based on Cd (18.32 mg). Elemental analysis (%): calcd for C17H20CdNO6 C 45.70, H 4.51, N 3.14; found C 45.59, H 4.56, N 3.23. IR (cm-1): 3388 m, 2951 m, 1613 s νas(COO-), 1563 s νs(COO-), 1428 m, 1369 s, 1226 m, 777 m. X-ray Crystallography. Single crystal X-ray diffraction analyses of the title compounds were carried out on a Bruker SMART APEX II CCD diffractometer equipped with a graphite monochromated MoKR radiation (λ = 0.71073 A˚) by using a φ/ω scan technique at room temperature. The structures were solved by direct methods with SHELXS-97. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by use of geometrical restraints. A full-matrix least-squares refinement on F2 was carried out using SHELXL-97. The crystallographic data and selected bond lengths and angles for complexes are listed in Table 1 and Tables S1-S6, Supporting Information.

Results and Discussion Structural Descriptions. [Zn(mip)(bpa)]n (1). Single crystal X-ray structural analysis showed that complex 1 crystallized in the orthorhombic Pnna space group. The asymmetric unit consists of one Zn(II) ion, one mip, and one bpa molecule. As shown in Figure 1a, each four-coordinated Zn(II) ion is bound by two N atoms from two bpa molecules and two oxygen atoms from two mip ligands. The Zn-O bond length, 1.945(2) A˚, is within the range reported for tetrahedral environments.17 The Zn-N bond length, 2.069(2) A˚, is also similar to those found in other tetrahedral zinc complexes of bpa.16 The coordination geometry of the zinc ion is a distorted tetrahedron with angles ranging from 96.46(14) (N(1)-Zn(1)-N(1A)) to 131.17(14) (O(1)-Zn(1)-O(1A)). Each mip in 1 adopts the bis-monodentated coordination mode (Scheme 1a) to connect two adjacent zinc(II) ions and form left-handed and right-handed 1D chains (Figure 1b). The pitch of the helical chain is the same as the length of the b axis. The bpa ligands then link the 1D chains to form an overall 3D framework composed of helical (Zn2þ-mip-Zn2þmip-Zn2þ-bpa-Zn2þ-bpa-Zn2þ)n chains of opposite handness. When viewed along the c axis, the left-handed helical

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Figure 1. (a) Coordination environment of the zinc atom in 1. The hydrogen atoms are omitted for clarity. Symmetry codes: A, x, 1.5 - y, 0.5 z. (b) The 3D framework viewed along the b axis and two types of helical chains constructed from zinc and mip. (c) The 3D framework and the CdSO4 topology viewed along the c axis and structural topology of 1 displaying 3-fold interpenetration.

channels and right-handed helical channels exist alternately in the framework of 1. Topologically, the architecture of 1 can be simplified as a CdSO4 network (Figure 1c). Then in order to minimize the presence of large cavities and to stabilize the framework during the assembly process, other two identical networks are filled in the cavities giving a 3D 3fold interpenetrating network, as shown in Figure 1c. [Zn(mip)(bpp)]n (2). Each Zn(II) ion is four-coordinated with a tetrahedral environment by two N atoms from two bpp molecules and two oxygen atoms from two mip ligands. The mip ligand has a bis-monodentate bridging mode (Scheme 1a). Bis-monodentate mip and exobidentate bpp bridge four-coordinated Zn(II) ions to form a 4-fold interpenetrating 3D framework with the diamondoid-like

topology, and more details are discussed in our recent Communication.16 [Cd(mip)(bpp)]n (3). The crystal structure of 3 was solved in the space group P21/n, and Figure 2a shows the arrangement about the Cd(II) center. Each Cd(II) center has a distorted octahedral geometry. The six atoms coordinated to the Cd(II) ion are composed of two nitrogen atoms from two bpp and four oxygen atoms from two different mip. The Cd-O and Cd-N bond lengths are in the range of 2.2364(18)2.637(2) and 2.273(2)-2.300(2) A˚, respectively. Different from 1 and 2, mip acts as a chelating bis-bidentate ligand (Scheme 1b). Remarkably, mip and exobidentate bpp ligands bridge Cd(II) ions to form a 3D framework which is referred to as a unique example of diamondoid-like 66

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Figure 2. (a) Coordination environment of zinc atom in 3. Symmetry codes: A, 0.5 þ x, 1.5 - y, 0.5 þ z; B, -0.5 þ x, -0.5 - y, -0.5 þ z. (b) Perspective view of the three-dimensional network. (c) Perspective view of the 4-fold interpenetrated diamond networks in 3 with bridging ligands omitted for clarity.

Scheme 1. Coordination Modes of H2tbip and H2mip Observed in 1-8

topology when the Cd ions act as four-connected nodes (Figure 2b). The distances of Cd 3 3 3 Cd nodes linked by mip and bpp ligands are 9.67 and 13.04 A˚, respectively. The large distances between the metal atoms result in large cavities within the network, and thus three identical frameworks fill this space to generate a 4-fold interpenetrating network (Figure 2c).

[Zn2(tbip)2(bpa)(H2O)]n (4) and [Zn2(tbip)2(dpe)(H2O)]n (5). As listed in Table 1, complexes 4 and 5 crystallize in the same monoclinic space group P21/n and have similar cell parameters, and the crystal structure analysis revealed that they do indeed have the same structure. Thus, only the structure of 4 is described here. The structure of 4 consists of a 3D network containing noncentrosymmetric dinuclear zinc units as nodes. As depicted in Figure 3a, there are two types of coordination environments around the zinc ions in the dinuclear unit. Zn1 is in a tetrahedral coordination sphere, which is defined by four oxygen atoms from four different tbip. Zn2 is five coordinated [O3N2] and resides in a trigonal-bipyramidal coordination environment: three oxygen atoms, O1 and O3, from two different tbip ligands, O9 from a coordinated water molecule and two nitrogen atoms from two bpa ligands. The Zn-O and Zn-N bond lengths are in the range of 1.9340(14)-2.1459(13) and 2.0899(16)-2.1618(17) A˚, respectively. The Zn 3 3 3 Zn distance bridged by the two carboxylate groups is 3.49 A˚. As shown in Scheme 1c,d, the tbip ligands exhibit two kinds of coordination modes: one has a bis-monodentate bridging mode and the other has a pentadentate bridging mode. As a result, the dinuclear zinc units are connected by tbip to form a 2D carboxylate layer (Figure 3b). The adjacent 2D layers are then pillared by bpa to form a 3D framework . If we define the dimeric zinc units as the secondary building units (SBUs), the 3D network of 4 can be viewed as a uniform 6-connected R-Po topology. The large cavities in the structure allow two such 3D networks to weave into a 2-fold interpenetrating structure as depicted in Figure 3c.

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Figure 3. (a) Coordination environment of the dinuclear zinc unit in 4. Symmetry codes: A, x, y, 1 þ z; B, -0.5 - x, -0.5 þ y, 0.5 - z; C, 0.5 - x, -0.5 þ y, 0.5 - z. (b) The dinuclear zinc clusters connected by H2tbip ligands to form a 2D layer. (c) Structural topology of 4 displaying 2-fold interpenetration.

{[Zn(tbip)(bpp)] 3 H2O}n (6). Each Zn(II) ion is four-coordinate with a tetrahedral coordination geometry of two N atoms from two bpp molecules and two oxygen atoms from two tbip ligands. Each tbip in 6 adopts the bis-monodentate coordination mode (Scheme 1c) to connect adjacent zinc ions into 1D chains. The 1D chains are then linked by bpp molecules to form a 2D corrugated network, and more details are discussed in our recent Communication.16 {[Cd(tbip)(bpp)(H2O)] 3 3H2O}n (7). The asymmetric unit of 7 consists of one Cd(II) ion, one tbip, one bpp, one coordinated water molecule, and three lattice water molecules. As shown in Figure 4a, each Cd(II) center has a distorted pentagonal bipyramidal geometry. Two chelating

Ma et al.

carboxylate groups from different tbip ligands and one pyridyl nitrogen atom from a bpp ligand comprise the equatorial basal plane, with one water molecule and one pyridyl nitrogen atom from the other bpp ligand occupying the apical sites. The Cd-O and Cd-N bond distances are in the range of 2.307(4)-2.631(5) and 2.325(7)-2.333(8) A˚, respectively. Pairs of symmetry-related Cd atoms are bridged by pairs of bpp ligands to construct dinuclear units with Cd 3 3 3 Cd distances of 12.52 A˚. These dinuclear units are then interconnected through the tbip anions, which display a chelating bis-bidentate coordination mode (Scheme 1e), to yield 1D tubelike chains along the a direction (Figure 4b). Intermolecular O-H 3 3 3 O hydrogen bonding interactions (O(3)-H(1W) 3 3 3 O(1)#1, 2.736(6) A˚, 151.7, #1 = -x þ 1, y, -z þ 1) between the coordinated water molecules (O3) and the COO- group (O1) of tbip lead to the formation of a 2D layer, as shown in Figure 4c. Interestingly, when these hydrogen bonds are taken into account, each CdO3 center acts as a 5-connected trigonal bipyramidal node to connect five ligands, which serves as the planar 4-connected nodes. Thus, an unusual (4,5)-connected network (Figure 4c) with the Schl€ afli symbol of (44 3 62)(44 3 66) is constituted. In addition, intermolecular π-π interactions (the angle is 0 between the aromatic rings) 10e between adjacent layers further stabilize the crystal structure to give a 3D supramolecular network. The interplanar distance between neighboring parallel pyridine groups of bpp is ca. 3.86 A˚, and the corresponding centroid-to-centroid distance is ca. 3.88 A˚. {[Cd(tbip)(H2O)(bipy)0.5] 3 H2O}n (8). The asymmetric unit of 8 consists of one Cd(II) ion, one tbip, half a bipy, one coordinated water, and one free water molecule. As shown in Figure 5a, the Cd(II) ion is seven-coordinated to five carboxylate oxygen atoms of three tbip ligands, one nitrogen atom of one bpp, and one water molecule. The Cd-O bond distances are in the range of 2.220(2)-2.5908(19) A˚ and the Cd-N bond distance is 2.270(2) A˚. The coordination mode of tbip is shown Scheme 1f. In 8, two carboxylate groups bridge two Cd(II) ions to form a [Cd(μ-tbip)]2 binuclear structure containing a four-membered Cd2O2 ring in which the Cd 3 3 3 Cd distance is 3.923 A˚. Pairs of dinuclear [Cd(μ-tbip)]2 units are then linked by two tbip ligands to form 22-membered rings, and a [Cd(tbip)(H2O)]¥ chain is thus formed by an alternating arrangement of the 4-membered and 22-membered rings (Figure 5b). Within the 22membered ring, the Cd 3 3 3 Cd separation is 10.166 A˚. The 1D chains are subsequently connected by the bipy ligands through the axial positions of both Cd centers in the subunits to form a 2D layer structure with the Schl€ afli symbol of (42 3 6)(42 3 63 3 8), as shown in Figure 5c. There are also intramolecular hydrogen bonds between carboxylate groups and free water molecules (O 3 3 3 O distances 2.860(4), 2.890(4) A˚) and between free water molecules and coordinated water molecules (O 3 3 3 O distance 2.786(3)). These strong hydrogen bonding interactions between 2D layers lead to the formation of a 3D supramolecular framework. Influence of the Steric Hindrance of Dicarboxylates on the Final Structures. Our aim was to investigate the effects of the steric hindrance effect of substituted group in the bridging ligand on the self-assembly of supramolecules and coordination polymers. H2tbip and H2mip were selected as starting materials in our study, and the Zn (Cd) complexes constructed from hip reported by Cao in the literature are also compared here.13b,c An overview of the coordination polymer structures for 1-12 is schematically depicted in

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Figure 4. (a) Perspective view of the coordination environment of the Cd atom in 7. Symmetry codes: A, 2 - x, -y, 2 - z; B, x, -1 þ y, z. (b) 1D tube-like chain along the a axis in 7. (c) Perspective view of the 2D supramolecular network in 7. The hydrogen-bonding interactions are indicated by green dashed lines.

Scheme 2. From these results, the choice of different steric hindrance of coexistent groups in dicarboxylate derivatives is clearly critical in determining the binding fashions of the dicarboxylate ligand, as well as the resultant extended networks. The different groups result in changes in dimensionality and interpenetration for the coordination polymers. First, although complexes 1, 4, and 9 all contain zinc ions and the same secondary bpa ligand, the replacement of the hydroxyl group by methyl or tert-butyl group results in a change from the 5-fold interpenetrating framework of 9 to the 3-fold interpenetrating framework of 4, and then to the 2fold interpenetrating framework of 1. A similar change also occurs in 5 and 12. Second, complexes 2, 6, and 10 all contain zinc ions, and all anions display the same coordination modes with each carboxylate group acting as bis-monodentate bridging mode. The replacement of the hydroxyl group by a methyl or tert-butyl group results in a change from a 4-fold interpenetrating diamondoid framework to a 2-fold

interpenetrating diamondoid framework and further to a 2D corrugated layer. A change of dimensionality also occurs in complexes 3, 7, and 11. Aside from the variation in the degree of interpenetration, there is also an effect on the structure and composition of the coordination polymer. Thus, although the carboxylate coordination sites of tbip, mip, and hip are very similar, their coordination chemistries are obviously different, presumably due to the different steric bulk of the substituted group in the bridging ligand. In addition, the structural comparison among the above complexes indicates that the small steric bulk of hip and mip favors the formation of high-dimensional and highly interpenetrating products. PXRD and Thermal Analysis. In order to check the phase purity of these compounds, the PXRD patterns of title compounds were checked at room temperature. As shown in Figure S1, Supporting Information, the peak positions of the simulated and experimental XRPD patterns are in

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Figure 5. (a) Coordination environment of Cd(II) ion in 8. Symmetry codes: A, 3 - x, 2 - y, 1 - z; B, 1 þ x, 1 þ y, z. (b) 1D chain structure. (c) 2D layered framework of 8.

agreement with each other, demonstrating the good phase purity of the compounds. The thermal stability of these new crystalline materials was investigated by thermogravimetric (TG) analysis experiments (see Figure S2, Supporting Information). The weight loss curve indicated that 1 was stable up to ca. 352 C, whereupon expulsion of its organic components occurred. The final mass remnant of 21.4% for 1 at ∼800 C likely represented deposition of ZnO (19.0% calcd). Complex 3 exhibited no mass loss until ∼305 C. Its mass remnant of 25.5% corresponding to CdO (26.3% calcd). The TG analysis of 4 and 5 showed that the first weight loss of 5.4% (calcd. 4.7%) for 4, 4.8% (calcd. 4.7%) for 5 observed from 30 to 165 C for 4 and 340 C for 5, corresponded to the loss of one coordinated water molecule or one lattice water molecule per formula unit. The TG curves for compounds 7 and 8 showed that an initial weight loss in the temperature

range 30-368 C for 7 and 30-233 C for 8, which was due to the loss of the lattice water and coordinated water molecules (weight loss of 13.7% (calcd. 14.4%) for 7 and 7.2% (calcd. 8.1%) for 8. The further weight loss represented the decomposition of the material. Luminescent Properties. As the d10 metal-organic polymer, their luminescent properties are studied in the solid state at room temperature. The emission spectra of the title complexes were investigated in the solid state at room temperature, as depicted in Figure 6. Upon excitation at ca. 325 nm for 1, 3 and 310 nm for 4, 5, 7, 8, these complexes exhibit intense fluorescence emission bands at ca. 434 nm for 1, 457 nm for 3, 361 and 380 nm for 4, 372 nm for 5, 355 nm for 7, and 362 and 380 nm for 8. In order to understand the nature of such emission bands, the luminescent properties of free ligands were also measured, upon excitation

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Scheme 2. Synthesis of Complexes 1-12

prepared and structurally characterized. From a comparison of the results constructed from H2mip, H2tbip, and H2hip, we conclude that variations in the molecular self-assembly are influenced by the steric hindrance of substituted group in the bridging ligands. The small steric bulk of organic ligand favors the formation of high-dimensional and highly interpenetrating products and vice versa. These coordination polymers exhibit intense fluorescence emissions and may be candidates for fluorescent materials. On the basis of this work and our previous work, further syntheses, structural diffraction studies, and physical characterization of compounds of other metals, including rare earth metal centers, with these ligands are also under way in our laboratory.

Figure 6. Solid-state emission spectra of polymers (1 (red); 3 (green); 4 (blue); 5 (pink); 7 (dark yellow); 8 (olive)) at room temperature.

at ca. 280 nm for H2mip and 294 nm for H2tbip, which show the similar emissions at ca. 350 nm for H2mip and 324 nm for H2tbip. In comparison to the free carboxylate ligands, most of the emission maxima of complexes 1, 3, 4, 5, 7, 8 are changed, which are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal transfer (LMCT) in nature, since the ZnII and CdII ions are difficult to oxidize or reduce. Thus, they may be assigned to intraligand (π-π*) fluorescent emission.18 Although more detailed theoretical and spectroscopic studies may be necessary for better understanding of the luminescent mechanism, the strong fluorescence emissions of those complexes make them potentially useful photoactive materials. Conclusions 10

Eight d metal coordination polymers, with different dimensionality and degrees of interpenetration, and assembled from two benzene-1,3-dicarboxylic acid derivatives (H2tbip, H2mip) and a series of bipyridyl ligands, have been

Acknowledgment. This work was supported by the Natural Science Foundation of China (Nos. 20471026 and 20771090) and Henan tackle key problem of science and technology (Nos. 072102270030 and 072102270034) and the Foundation of Education Committee of Henan province (Nos. 2006150017 and 2008A150018). Supporting Information Available: X-ray crystallographic files in CIF format, patterns of PXRD and thermal analysis of 1, 3, 4, 5, 7, 8, and selected bond distances and bond angles for 1, 3, 4, 5, 7, 8. This information is available free of charge via the Internet at http:// pubs.acs.org/.

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