Different Molecular Frameworks of Zinc(II) and Cadmium(II

A series of novel Zn(Cd)-double betaine coordination polymers with different ... Networks with Topological Diversity: Gas Adsorption, Thermal Stabilit...
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CRYSTAL GROWTH & DESIGN

Different Molecular Frameworks of Zinc(II) and Cadmium(II) Coordination Polymers Constructed by Flexible Double Betaine Ligands

2006 VOL. 6, NO. 2 444-450

A-Qing Wu,†,‡ Yan Li,†,‡ Fa-Kun Zheng,*,† Guo-Cong Guo,*,† and Jin-Shun Huang† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and Graduate School, The Chinese Academy of Sciences, Beijing 100039, P. R. China ReceiVed August 3, 2005; ReVised Manuscript ReceiVed NoVember 11, 2005

ABSTRACT: A series of six novel Zn(Cd)-double betaine coordination polymers have been obtained by the mild reactions of the respective metal nitrate with three flexible double betaine ligands. Single-crystal X-ray structural analyses reveal that the six complexes possess different molecular frameworks derived from various coordination modes and flexibilities of double betaine ligands, which indicates that the nature of the ligands has a great influence on the structural topologies of the metal-organic molecular architectures. Extensive hydrogen bonds play important roles in crystal packing and supramolecular frameworks. It is noteworthy that the different one-dimensional water chains with “arm-chair” and “zigzag” characteristics are presented in the complexes Zn(L2)(H2O)(NO3)](NO3)‚ 2H2O (3) and [Cd(L2)(H2O)(NO3)]NO3‚3H2O (4) [L2 ) 1,3-bis(pyridinio-4-acetato)-propane], respectively . Introduction

Scheme 1

During the last two decades, the rational design and syntheses of novel coordination polymers have achieved considerable progress in the field of supramolecular chemistry and crystal engineering,1,2 not only because of their intriguing structural motifs but also because of their potential applications in catalysis, molecular adsorption, magnetism, nonlinear optics, luminescence, and molecular sensing.3 It is well-known that organic ligands play crucial roles in the design and construction of desirable frameworks, because changes in flexibility, length, and symmetry of organic ligands can result in a remarkable class of materials bearing diverse architectures and functions.4 Thus, the prospect of tuning the properties of metal-organic frameworks through a systematic change of organic ligands provides an impetus for further research on metal-organic supramolecular architectures. However, the aim of obtaining the desirable architectures of coordination polymers constructed by organic ligands and metal ions is still a long-term challenge to chemists due to the difficult prediction of either the compositions or the structures of the reaction products, especially those from flexible ligands because of their flexibility and conformational freedom in the assembly process.5 Furthermore, multiple noncovalent forces, such as hydrogen bonds, π-π stacking, and host-guest ionic interactions, play important roles in supramolecular assembly of metal ions and organic ligands.6,7 So, a great deal of work should be done to extend the knowledge of relevant structural types and establish proper synthetic strategies to design desirable architectures with physical properties. In this study, we elaborately designed three double betaine ligands with different flexibilities (Scheme 1), namely, L1 ) 1, 3-bis(pyridinio-4-carboxylato)-propane, L2 ) 1,3-bis(pyridinio4-acetato)-propane, and L3 ) 1,4-bis(pyridinio-4-carboxylato)1,4-dimethylbenzene. The reactions of M(NO3)2‚xH2O (M ) Zn or Cd) and L ligands in water solution under mild conditions successfully afforded six novel Zn(II) and Cd(II) coordination

polymers, [Zn(L1)(H2O)2](NO3)2 (1), [Cd(L1)(NO3)2]‚H2O (2), [Zn(L2)(H2O)(NO3)](NO3)‚2H2O (3), [Cd(L2)(H2O)(NO3)]NO3‚ 3H2O (4), and [Zn(L3)(H2O)4](NO3)2‚H2O (5), and [Cd(L3)(H2O)4(NO3)]NO3‚H2O (6),8 in which a variety of different topological structures and coordination modes of ligands were observed (Scheme S1, Supporting Information). Interestingly, we found synchronously that Zn(II) and Cd(II) complexes with the same ligand exhibit completely different topological structures.

* To whom correspondence should be addressed. Fax: +86 591 83714946. E-mail: [email protected]. † State Key Laboratory of Structural Chemistry, the Chinese Academy of Sciences. ‡ Graduate School, the Chinese Academy of Sciences.

Experimental Procedures Materials and Measurement. Elemental analyses (C, H, N) were determined on an Elementar Vario ELIII analyzer. IR spectra were measured as KBr pellets on a Nicolet Magna 750 FT IR spectrometer in the range of 400-4000 cm-1. Thermogravimetric analyses were carried out on a NETZSCH STA 449C unit at a heating rate of 15 °C/min under a nitrogen atmosphere. Syntheses of L1, L2, and L3. The flexible double betaine ligands L1, L2, and L3 were prepared according to the literature.8,9 Anal. Calc. for L1 (C15H14N2O4, %): C, 62.93; H, 4.93; N, 6.29. Found (%): C, 62.34; H, 4.78; N, 6.77. Selective IR data (KBr, cm-1): 1642, 1624 (s, νas(COO)), 1567, 1510, 1470, 1453 (m, νCdC(phenyl and pyridyl rings)), 1368 (s, νs(COO)), 795, 785 (s, δC-H), 699 (m, δCdC). Anal. Calc. for L2 (C17H18N2O4, %): C, 64.96; H, 5.77; N, 8.91. Found (%): C, 64.52; H, 6.05; N, 8.67. Selective IR data (KBr, cm-1): 1635 (s, νas(COO)), 1571, 1515, 1472, 1435 (m, νC dC), 1377 (s, νs(COO)), 745 (w, δC-H), 689 (m, δCdC). Anal. Calc. for L3 (C20H16N2O4, %): C, 68.96; H, 4.63;

10.1021/cg050378e CCC: $33.50 © 2006 American Chemical Society Published on Web 12/27/2005

Zn(II) and Cd(II) Coordination Polymers

Crystal Growth & Design, Vol. 6, No. 2, 2006 445

Table 1. Crystal Data and Structure Refinements for Complexes 1-5 complexes

1

2

3

4

5

formula Mr space group a (Å) b (Å) c (Å) β (°) V (Å3) Z F (g/cm3) µ (mm-1) λ (Å) T (K) R1, wR2 (obs) goodness-of-fit

C15H18N4O12Zn 511.70 C2 12.236(8) 6.2011(18) 14.155(5) 111.19(4) 1001.5(8) 2 1.697 1.301 0.71073 293(2) 0.0413, 0.1124 1.038

C15H16CdN4O11 540.72 P21/n 13.5979(15) 8.1208(5) 18.1611(19) 111.895(5) 1860.8(3) 4 1.930 1.246 0.71073 293(2) 0.0197, 0.0503 1.066

C17H24N4O13Zn 557.77 Fdd2 18.6008(11) 62.936(4) 7.5918(5)

C17H26CdN4O14 622.82 P21/n 15.721(4) 4.7898(11) 31.371(7) 90.439(3) 2362.2(9) 4 1.751 1.003 0.71073 293(2) 0.0398, 0.1037 1.053

C20H26N4O5Zn 627.82 P21/n 17.353(5) 7.0076(19) 21.571(6) 95.870(4) 2609.3(13) 4 1.579 1.022 0.71073 293(2) 0.0683, 0.1742 1.009

N, 8.04. Found (%): C, 68.82; H, 4.68; N, 8.13. Selective IR (KBr, cm-1): 3112, 3035, 2997 (w, νC-H(phenyl and pyridyl rings)); 1633 (s, νas(COO)); 1567, 1519, 1458, 1432 (m, νC-C(phenyl and pyridyl rings)); 1356 (s, νs(COO)); 781, 756 (s, δC-H); 693 (s, δCdC). Syntheses of the Coordination Complexes. Five new complexes 1-5 were obtained by the same method as that employed for the synthesis of complex 6.8 An aqueous solution (10 mL) containing M(NO3)2‚xH2O (x ) 4 (Cd) or 6 (Zn)) (0.5 mmol) and L ligands (0.5 mmol) was stirred at about 80 °C for several hours and then filtered. The filtrate was kept at room temperature for about 20 days to give blocklike crystals suitable for X-ray analyses. All complexes are stable in air and soluble in water but insoluble in ethanol. Yield: 66.7% (based on L1) for 1. Anal. Calc. (C15H18N4O12Zn, %): C, 35.21; H, 3.54; N, 10.95. Found (%): C, 34.50; H, 3.92; N, 10.55. Selective IR data (KBr, cm-1): 3668-2552 (wide peak) 1635 (s, νas(COO)), 1574 (s), 1520 (w), 1469, 1402 (m, νCdC(phenyl and pyridyl rings)), 1314 (s, νs(COO)), 785 (s, δC-H), 692 (m, δCdC). Yield: 57.3% (based on L1) for 2. Anal. Calc. (C15H16CdN4O11, %): C, 33.32; H, 2.98; N, 10.36. Found (%): C, 33.66; H, 2.62; N, 10.98. Selective IR data (KBr, cm-1): 3604 (m), 3058 (m), 1615 (s, νas(COO)), 1560 (s), 1467 (s, νCdC(phenyl and pyridyl rings)), 1411, 1397 (s, νs(COO)), 782 (s, δC-H), 756 (m), 691 (m, δCdC). Yield: 48.5% (based on L2) for 3. Anal. Calc. (C17H24N4O13Zn, %): C, 36.61; H, 4.33; N, 10.05. Found (%): C, 35.98; H, 4.68; N, 10.63. Selective IR data (KBr, cm-1): 3419 (m), 1639 (s, νas(COO)), 1462 (w, νCdC(phenyl and pyridyl rings)), 1384, 1355 (s, νs(COO)), 1313 (w), 813 (w, δC-H), 696 (w, δCdC). Yield: 51.4% (based on L2) for 4. Anal. Calc. (C17H26CdN4O14, %): C, 32.79; H, 4.20; N, 9.00. Found (%): C, 32.97; H, 4.45; N, 8.33. Selective IR data (KBr, cm-1): 3442 (m), 3055 (m), 1647 (s, νas(COO)), 1469 (w), 1418 (m, νCdC(phenyl and pyridyl rings)), 1383 (s, νs(COO)), 765 (w, δC-H), 698 (m, δCdC). Yield: 72.6% (based on L3) for 5. Anal. Calc. (C20H26N4O15Zn, %): C, 38.26; H, 4.17; N, 8.92. Found (%): C, 38.82; H, 4.68; N, 8.13. Selective IR data (KBr, cm-1): 3439 (m), 1640 (m, νas(COO)), 1567 (m), 1461 (w, νCdC(phenyl and pyridyl rings)), 1357 (s, νs(COO)), 786 (w), 759 (w, δC-H), 691 (w, δCdC). X-ray Crystallographic Studies. The crystal structures of the five new complexes were studied by single-crystal X-ray diffraction analyses. Data collections were performed at 293(2) K on a Rigaku AFC7R diffractometer for 1 with graphite-monochromatic Mo KR radiation (λ ) 0.71073 Å)10 and on a Rigaku Mercury CCD for 2-5 with graphite monochromatic Mo KR radiation (λ ) 0.71073 Å).11 The structures were solved by direct methods, which revealed the positions of the metal atoms using Siemens SHELXTL Version 5.0 package of crystallographic software.12 The remaining non-hydrogen atoms were located by successive different Fourier syntheses. Hydrogen atoms were added according to theoretical models with the hydrogen atoms of all water molecules of complexes 1 and 3-5 being not included. Hydrogen atoms of water molecule in 2 were located from the difference Fourier syntheses and refined isotropically with the O-H distances fixing on 0.93 Å. The structures were refined using full-matrix least-squares refinement on F2. All non-hydrogen atoms were refined anisotropically. Pertinent crystal data and structure refinement results for the five new complexes 1-5 are listed in Table 1. The disordered Zn2, O3W, and O4W atoms in 5 were treated with variable site occupancy factors, which converged to 0.345, 0.732, and 0.587.

8887.4(10) 16 1.667 1.183 0.71073 293(2) 0.0470, 0.1229 1.048

Results and Discussion Synthetic Strategy. The double betaine ligands contain two betaine functions in the same molecule and accordingly bear two anionic carboxylate groups and two positively charged quaternary ammonium moieties, resulting in much more varied coordination modes.9,13-17 Previous studies have shown that the variation of double betaine ligands regarding the type of bridging units, the flexibility of molecular backbone and conformational preference, as well as the metal ions employed and their counterions, has a profound influence on the polymeric structures obtained.15-19 On the other hand, the Group 2B zinc triad of d-metal dications ranging from the smallest Zn(II) (sixcoordinate ionic radius 88 pm) of borderline hardness, through cadmium (ionic radius 109 pm) to the soft mercury(II) (ionic radius 116 pm) presents a useful series for studying possible variations in coordination preferences and distortions,20 which is helpful to discuss the influence of the metal ion radius on the polymeric structures obtained. On the basis of these ideas, we synthesized six novel Zn(II) and Cd(II) complexes with three elaborately designed double betaine ligands. Descriptions of Crystal Structures. [Zn(L1)(H2O)2](NO3)2 (1). Single-crystal X-ray diffraction analysis revealed that 1 consists of one-dimensional (1D) chain cations and lattice nitrate ions and crystallizes in the space group C2. As shown in Figure 1a, the Zn(II) atom is located in the position of 2-fold axis and bonded to two carboxylate O atoms from two different L1 ligands, each donating a terminal O atom, and the other two coordination sites are occupied by symmetrically related water molecules, forming a slightly distorted tetrahedral coordination geometry. The bond distances of Zn1-O2 (1.919(4) Å) and Zn1-O1W (1.962(4) Å) are typical for Zn-O coordination. The Zn(II) atoms are linked by L1 ligands with the syn-syn monodentate coordination mode of both carboxylate groups, which is similar to the other Zn-double betaine complexes,17 generating parallel infinite wavelike chains extending along the [1 0 1] direction. As a sequence of the syn-syn monodentate coordination mode of carboxylate groups in L1, the carboxylato C1-O2(coordinated) distance (1.265(7) Å) is slightly longer than the other C1-O1(uncoordinated) distance (1.241(6) Å). The Zn atoms are bridged by the L1 ligands to form an ideal linear relationship with the Zn‚‚‚Zn‚‚‚Zn angle of 180°. The two pyridine rings in the L1 ligand are not parallel to each other with a dihedral angle of 61.37(7)°. The adjacent chains are linked by hydrogen bonding of an uncoordinated O1 atom and coordinated O1W (O1W‚‚‚O1#A ) 2.664(3) Å, symmetry code #A; 1 - x, -1 + y, 1 - z) into a two-dimensional (2D) layer parallel to the (1 0 1h) plane, as shown in Figure 1b. The lattice nitrate ions are

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Figure 1. (a) 1D chain structure of 1 bridged by the L1 ligand with the syn-syn monodentate coordination mode. (b) 2D layered structure of 1 formed by O1W‚‚‚O1 hydrogen bonds with the lattice nitrate ions being fixed to the layer by the O1W‚‚‚O6 hydrogen bonds.

fixed by the O1W‚‚‚O6#B (symmetry code #B; 0.5 + x, -0.5 + y, z) hydrogen bonds with a distance of 2.678(3) Å. Kurtz powder second harmonic generation (SHG) measurements for 1 were used to verify its chiral space group as well as to judge its potential application as a second-order NLO material.21 A Nd:YAG laser was used to generate fundamental 1064 nm light with a pulse width of 8 ns and a pulse frequency of 10 Hz. Microcrystalline potassium dihydrogen phosphate (KDP) served as the standard. The results show that 1 exhibits a powder SHG efficiency about 0.5 times as large as that of KDP, which confirms its structural noncentrosymmetry. The thermogravimetric analyses of 1 (Figure S4, Supporting Information) show that 1 can be thermally stable up to 165 °C. The dehydration of two coordination water molecules occurred in range 165-215 °C with a weight loss of 7.80% (calcd. 7.04%). The decomposition process of the L1 ligand occurred from 255 to 277 °C with a weight loss of 54.60% (calcd. 55.95%), and heating to 450 °C gave black inorganic residuals. [Cd(L1)(NO3)2]‚H2O (2). The crystal structure of 2 also comprises 1D metal-carboxylate chains and lattice water molecules. As illustrated in Figure 2a, the Cd(II) atom is coordinated by four chelating carboxylate O atoms from two different L1 ligands and three nitrate O atoms from two nitrate ligands. The polyhedron of hepta-coordinated Cd(II) atoms can thus be described as a distorted monocapped trigonal prism with the two trigonal surfaces being defined by two nitrate O5 and O9 atoms and a carboxylate O1 atom (Cd1-O5 ) 2.3067(13), Cd1-O9 ) 2.5896(16), and Cd1-O1 ) 2.3455(12) Å) for one surface and two carboxylate O3#A′ and O4#A′ atoms and one nitrate O8 atom for another surface (Cd1-O3#A′ ) 2.3626(12), Cd1-O4#A′ ) 2.3957(12) and Cd1-O8 ) 2.3427(13) Å, symmetry code #A′: x + 1/2, -y - 1/2, z - 1/2). The O2 atom occupies the capping position (Cd1-O2 ) 2.4503(12) Å). The Cd(II) atoms are linked by the chelating-chelating coordination mode of both carboxylate groups in L1 ligands to generate infinite zigzag chains extending along the [1 0 1h] direction. The chelating-chelating coordination mode of the L1 ligand in 2 is similar to those reported by T. C. W. Mak et al.13b,14,17 Different from 1, the Cd atoms bridged by the L1 ligands are not located in a line with the angle of Cd‚‚‚Cd‚‚‚Cd of 101.54°. The dihedral angle of two pyridine rings in the L1

Figure 2. (a) 1D zigzag chain structure of 2 linked by the L1 ligand adopting a chelating-chelating coordination mode. (b) 2D layered structure of 2 linked by O1W‚‚‚O8 and O1W‚‚‚O5 hydrogen-bonding interactions.

ligand is 50.10(6)°, which is smaller than that of 1. The lattice water molecules connect the nitrate oxygen atoms of adjacent chains by hydrogen bonds of O1W‚‚‚O5#B′ (O1W‚‚‚O5#B′ ) 2.885(2) Å, O1W-HW2‚‚‚O5#B′ ) 168(3)°, symmetry code #B′: 1 - x, 1 -y, -z) and O1W‚‚‚O8 (O1W‚‚‚O8 ) 2.912(2) Å, O1W-HW1‚‚‚O8 ) 155(3)°), which makes the chains further to form a 2D structure parallel to the (1 0 1) plane, as shown in Figure 2b. [Zn(L2)(NO3)(H2O)](NO3)‚2H2O (3). The structure of 3 is a three-dimensional (3D) network constructed by the L2 ligand with lattice water molecules and nitrate ions located in the channels of a 3D network. As shown in Figure 3a, the Zn(II) atom is coordinated by three carboxylate O atoms from three different L2 ligands, two chelating nitrate O atoms from one nitrate ligand, and one water O atom, forming a distorted octahedron with the equatorial plane consisting of O2, O4, O6, and O7 atoms and the axial positions being occupied by O3 and O3W atoms. The Zn(II) atoms are linked by one syn,antiO,O′ carboxylate group of L2 ligands to form an infinite Zn‚‚‚Zn helix metalo-chain along the c direction, as shown in Figure 3c. Each metalo-chain is connected by four adjacent metalochains through two carboxylate groups of L2 ligands in the (syn,anti)-syn coordination mode to form a 3D network, as shown in Figure 3b. Such a coordination mode of the L2 ligand in 3 is similar to that in the reported double betaine complexes, whereas most of them exhibit the chain and layered polymeric structures.13a,15,16 The dihedral angle between the two pyridine rings in L2 ligand is 54.78(6)°.

Zn(II) and Cd(II) Coordination Polymers

Crystal Growth & Design, Vol. 6, No. 2, 2006 447

Figure 3. (a) The coordination environment around Zn(II) in 3 with a (syn,anti)-syn coordination mode of L2 ligand. (b) The 3D network of 3 constructed by the L2 ligand with the 1D water chains located in the 3D network. (c) Zn(II) atoms are linked by one syn,anti carboxylate group of L2 ligands to form an infinite Zn‚‚‚Zn wavelike metalo-chain along the c direction. (d) The 1D “arm-chair” water chains formed by O1W‚‚‚O2W hydrogen bonds extending along the a direction. Table 2. Geometrical Parameters of Hydrogen Bonds in 3a atoms involved

distance (Å)

atoms involved

distance (Å)

O1W‚‚‚O11 O1W‚‚‚O2Wi O1W‚‚‚O2Wii O2W‚‚‚O8iii

2.792(2) 2.783(3) 2.877(3) 2.867(3)

O3W‚‚‚O4iv O3W‚‚‚O9v O3W‚‚‚O3Wv

2.899(2) 2.788(2) 2.922(3)

a Symmetry transformations used to generate equivalent atoms: (i) x 0.5, y, 0.5 + z; (ii) x - 0.25, 0.75 - y, 0.25 + z; (iii) -x, 0.5 - y, z - 0.5; (iv) 0.25 + x, 0.75 - y, z - 0.25; (v) -x - 0.5, 0.5 - y, z.

Extensive hydrogen-bonding interactions exist in a 3D network of 3. The geometrical parameters pertaining to the hydrogen bonds are collected in Table 2. An interesting feature is that there exists a 1D hydrogen-bonded water-chain linked alternately by O1W and O2W atoms (Figure 3d), which resembles the “arm-chair” motif proposed by Nagle.22 The 1D water-chain structures constitute a potentially important form of water that is poorly understood.23 Many fundamental biological processes appear to depend on the unique properties of water chains.24 However, the nature for requiring the structural constraints in stabilizing 1D water chains has not been fully illustrated.25 The 1D water chains in 3 are stabilized by strong O1W‚‚‚O11 and O2W‚‚‚O8 hydrogen bonds between water molecules and coordinated nitrate ions of the 3D network. Complex 3 possesses chiral space group Fdd2, but second harmonic generation measurements have not been done due to its intrinsic absorbance coming from its brownish color. [Cd(L2)(NO3)(H2O)]NO3‚3H2O (4). The crystal structure of 4 comprises 2D metal-carboxylate cation layers, lattice water molecules, and nitrate ions. The Cd(II) atom is coordinated by four bridging carboxylate O atoms from four different L2 ligands, one nitrate O atom from two nitrate ligands and one water O atom. Hence, the coordination polyhedron of the hexacoordinated Cd(II) atom can be viewed as a slightly distorted octahedron with the equatorial plane consisting of O2, O4, O7, and O1W atoms and the axial positions being occupied by O1 and O3 atoms. The Cd(II) atom is displaced by 0.1929(12) Å from the equatorial plane of the octahedron in the direction of

the O1 atom. The Cd(II) atoms are bridged by two carboxylate groups from two different L2 ligands both in the syn,anti coordination mode, which presented in the double betaine complexes reported previously,9,13,16 to form an infinite Cd‚‚‚Cd chain along the b direction. The adjacent chains are interconnected by the flexible backbones of L2 ligands to form a 2D layer parallel to the bc plane, as shown in Figure 4a. The two rings in the L2 ligand are nearly perpendicular to each other with a dihedral angle of 89.79(9)°, which is much larger than that in 3. Extensive hydrogen-bonding interactions also exist in 4, as shown in Figure 4b. The geometrical parameters pertaining to the hydrogen bonds are collected in Table 3. The 2D layers are linked by hydrogen-bonding interactions to form the 3D network with water molecules occupying the 3D channels. Interestingly, there are three slightly different 1D water chains by the hydrogen-bonding interactions, as shown in Figure 4c. In the first water chains (Figure 4c-I), the O2W water molecules are hydrogen bonded to symmetrically related water molecules to form a 1D “zigzag” helix chain extending along the b direction. In the second water chains (Figure 4c-II), O4W water molecules are hydrogen bonded to symmetrically related waters to form a 1-D “zigzag” helix chain. Simultaneously, the O4W water chains are anchored to the two adjacent layers by the O4W‚‚‚O6 hydrogen bonds. However, the situation of the third water chains differs from the former chains (Figure 4c-III). The O3W water molecules are linked to each other to form 1D “zigzag” helix water chains by hydrogen bonds built with the symmetrically related O3W. Then, the water chains are further hydrogen bonded to the lattice nitrate ions and anchored to the two adjacent layers by the O9‚‚‚O1W and O8‚‚‚O1W hydrogen bonds. The 1D “zigzag” water chains in 4 are different from that in 3, with the latter exhibiting the “arm-chair” character. [Zn(L3)(H2O)4](NO3)2‚H2O (5). The crystal structure of 5 consists of 1D cations chains, lattice nitrate ions, and water molecules, as depicted in Figure 5a. Two crystallographically independent Zn(II) atoms sit at symmetric centers. The coordination polyhedron of hexa-coordinated Zn(II) centers can be

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Figure 5. (a) 1D chain structure of 5 linked by the L3 ligand adopting a syn-syn coordination mode. (b) 2D layered structure of 5 linked by the O2W‚‚‚O5W hydrogen bonds. (c) 3D network of 5 constructed by hydrogen bonds. Table 4. Geometrical Parameters of Hydrogen Bonds in 5a atoms involved O1W‚‚‚O2W1 O1W‚‚‚O41 O1W‚‚‚O232 O2W‚‚‚O5W O4WA‚‚‚O123

distance (Å) 2.990(2) 2.906(2) 2.929(3) 2.732(2) 2.740(7)

atoms involved O4W‚‚‚O14 O4W‚‚‚O123 O5W‚‚‚O131 O5W‚‚‚O225 O2W‚‚‚O5W6

distance (Å) 2.875(3) 2.889(3) 2.797(2) 2.883(3) 2.849(2)

a Symmetry transformations used to generate equivalent atoms: (1) 1 x, 1 - y, -z; (2) 1.5 - x, 0.5 + y, 0.5 - z; (3) 2.5 - x, 0.5 + y, 0.5 - z; (4) 3 - x, 1 - y, 1 - z; (5) 1.5 - x, 1.5 + y, 0.5 - z; (6) 1 - x, 2 - y, -z.

Figure 4. (a) 2D layered structure of 4 linked by the L2 ligand with the (syn,anti)-(syn,anti) coordination mode with the water molecules and nitrate ions being omitted. (b) The 3D structure of 4 linked by the hydrogen bonds viewed along the b direction with the water molecules occupying the 3D channels. (c) Three slightly different 1D water chains in 4. Table 3. Geometrical Parameters of Hydrogen Bonds in 4a atoms involved O2W‚‚‚O2Wa O2W‚‚‚O2Wb O4W‚‚‚O4Wc O4W‚‚‚O4Wd O4W‚‚‚O6 O3W‚‚‚O3We

distance (Å)

atoms involved

distance (Å)

2.749(7) 2.789(6) 2.794(3) 2.794(3) 2.934(4) 2.767(4)

O3W‚‚‚O3Wf

2.767(4) 2.873(5) 2.999(5) 2.767(6) 2.931(4)

O3W‚‚‚O10 O1W‚‚‚O8g O1W‚‚‚O9g O1W‚‚‚O1h

a Symmetry transformations used to generate equivalent atoms: (a) -x, -y, -z; (b) -x, 1 - y, -z; (c) 0.5 - x, -0.5 + y, 0.5 - z; (d) 0.5 - x, 0.5 + y, 0.5 - z; (e) -0.5 - x, -0.5 + y, 0.5 - z; (f) -0.5 - x, 0.5 + y, 0.5 - z; (g) 0.5 + x, 0.5 - y, -0.5 + z; (h) x, -1 + y, z.

viewed as an ideal octahedron that is completed by four water (O1W and O2W for Zn1 and O3W and O4W for Zn2) and two carboxylate O atoms (O4 for Zn1 and O1 for Zn2) from two different L3 ligands. The Zn1 and Zn(2) centers are bridged by the L3 ligands by a syn-syn coordination mode to form 1D chains extending along the [2 0 1] direction. Despite the same coordination mode of two carboxylate groups in 1 and 5, the

two Zn atoms are bound to the L3 ligand in the trans form in 5 other than in the cis form in 1. The two rings in L3 ligand are nearly parallel to each other with a dihedral angle of 3.77(8)°. Similar to 1, the Zn atoms are bridged by the L3 ligands to form an ideal linear relationship with the Zn1‚‚‚Zn2‚‚‚Zn1 angle of 180°. The adjacent chains are linked by hydrogen-bonding interactions of coordinated water O2W and lattice water O5W in a 2D layer parallel to the (1 0 2h) plane, as shown in Figure 5b. The 2D layers are linked further by the hydrogen-bonding interactions, which are formed by lattice water molecules, nitrate O atoms, and coordinated water molecules, to result in a 3D network (see Figure 5c). The geometrical parameters pertaining to the hydrogen bonds of 5 are collected in Table 4. [CdL3(H2O)4(NO3)]NO3‚H2O (6). The detailed structure of 6 has been reported by us,8 in which each molecular unit contains one discrete [Cd(H2O)4L(NO3)]+ cation, one lattice nitrate anion, and one lattice water molecule (Figure 6). In L3 ligand of 6, only one carboxylate group participates in coordinaton and exhibits a syn coordination mode, which is first found in the double betaine complexes. The mononuclear [Cd(H2O)4L(NO3)] units are connected through intermolecular hydrogen bonds and π-π stacking reactions to generate a 3D network. Although all complexes have the same metal-double betaine ratio of 1:1, the exhibiting topological structures are completely different regardless of the same ligands or metal centers used

Zn(II) and Cd(II) Coordination Polymers

Crystal Growth & Design, Vol. 6, No. 2, 2006 449 Table 5. The νAs(COO), νS(COO), and ∆ Values in IR Spectra of Three Ligands and Complexes 1-6

Figure 6. Molecular structure of 6 with syn coordination mode of the L3 ligand. All hydrogen atoms, lattic water molecules, and nitrate ions were omitted for clarity.

Scheme 2 Different Topological Structures of 1-6

compds

νas(COO) (cm-1)

νs(COO) (cm-1)

∆ (cm-1)

L1 1 2 L2 3

1642, 1624 1635 1615 1635 1639

4 L3 5 6a

1647 1633 1640 1638

1368 1314 1411, 1397 1377 1384 1355 1383 1356 1357 1354

274, 256 321 204, 218 258 255 284 264 277 283 284

a

(Scheme 2). 1 is of [ZnL1] infinite wavelike chains with an ideal linear relationship with the Zn‚‚‚Zn‚‚‚Zn angle of 180° (Scheme 2b), while 2 is of [CdL1] infinite zigzag chains with the angle of Cd‚‚‚Cd‚‚‚Cd of 101.54 ° (Scheme 2c). The twist degrees of two pyridine rings in L1 ligand in the two complexes are different with 61.37(7)° for 1 and 50.10(6)° for 2. Similarly, structural topologies of 3 and 4 with the same L2 ligand are also completely distinct with the former exhibiting a 3D complicated network (Scheme 2e), while the latter possessing a 2D network (Scheme 2d). 5 and 68 with the same L3 ligand exhibit also different structural topologies with the same 1D ZnL3 chains of 5 as 1 and isolated mononuclear CdL3 units of 6 (Scheme 2a). The different topological structures of the Zn(Cd) complexes with different double betaines lie mainly in ligand flexibilities. The complexes with L1 or L3 ligands have lower dimensional structures, whereas those with L2 ligands exhibit higher dimensional structures, which might be attributed to the much greater flexibility of the L2 ligand compared to that of the L1 or L3 ligands. The introduction of the two methylene groups between each pair of carboxylate groups and pyridinyl rings makes it easier for the L2 ligand to connect the metal atoms along different directions into higher dimensional structures. While for L3 ligands, the introduction of the phenyl group between two methylene groups promotes its complexes to form lower dimensional structures compared to the complexes with the L1 ligand. The flexibility effects of double betaine ligands on the structural dimensionality have also been discussed in other double betaine complexes.9 To further probe into the

coordination modes syn-syn chelating-chelating syn,anti syn (syn,anti)-(syn,anti) syn-syn syn

Ref 8.

effects of flexibility of double betaine, similar reactions were attempted for the corresponding Hg(II) complexes with the three double betaine ligands. Regrettably, we only determined the crystal structure of a Hg(II) complex with the L3 ligand, in which the discrete mononuclear complex molecules are linked by π-π stacking interactions to form a 1D chain structure.26 On the other hand, the structural differences with the same L ligands may mainly be attributed to the radii of the metal atoms with the Cd(II) atom having a larger atomic radius than the Zn(II) atom, resulting in different coordination environments and ligating tendencies.3d Thus, in competitive coordination of larger nitrate ions and smaller water molecules, the Cd(II) ions prefer to bind the nitrate ion relative to the Zn(II) ions, which can be shown in 1 and 2 or in 5 and 6. For example, in 1 water molecules complete four coordination of the Zn(II) center, while nitrate ions are located in the lattice by hydrogen-bonding interactions. Contrarily, in 2 the water molecules fill in the lattice, while nitrate ligands take part in coordination of the Cd(II) centers. IR Spectroscopy. It is well-known that the different ∆ values between the asymmetrical and symmetrical stretching frequencies in the carboxylate complexes, νasym(COO) and νsym(COO), have close relationships to the carboxylate coordination modes.27 Generally, the ∆ values in the monodentate carboxylate complexes are larger than those in the free carboxylate ions, whereas the ∆ values in the chelating carboxylate complexes are in contrast smaller. However, the ∆ values in the η2-bridging carboxylate complexes remain almost invariable. The IR spectra of three double betaine ligands and five new Zn(Cd) complexes can be referred to in Figures S1-3, Supporting Information. The νas(COO), νs(COO), and ∆ values in IR spectra of three ligands and six complexes are collected in Table 5, which are in agreement with the structural determinations. Conclusions In summary, six Zn(Cd)-double betaine coordination polymers with different molecular frameworks have been synthesized by the mild reactions of Zn(Cd) nitrate with three elaborately designed double betaine ligands, which possess different flexibilities, and a variety of coordination modes of double betaine have been observed. This research not only shows the significant influences of the coordination modes and flexibilities of double betaine ligands on the structural topology but also illustrates an effect of metal atoms with different atomic radii and ligating tendencies on the polymeric components, which provides valuable instructions for the rational syntheses of coordination polymers with desired molecular architectures. Extensive hydrogen bonds play an important role in crystal packing and noncovalent frameworks.

450 Crystal Growth & Design, Vol. 6, No. 2, 2006

Acknowledgment. We gratefully acknowledge the financial support of the NSF for Distinguished Young Scientist of China (20425104) and the NSF of Fujian Province (A0420002, E0510029, 2005I017). Supporting Information Available: IR spectra for the three double betaine ligands and complexes 1-5, the TG curve of complex 1, the XRPD diagrams of complexes 1-3, and X-ray crystallographic files in CIF for complexes 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

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