Syntheses, Structures, and Physical Properties of Camphorate

Apr 27, 2010 - bimb =1,2-bis(imidazol-1-ylmethyl)-benzene, 1,3-bimb=1,3-bis(imidazol-1-ylmethyl) ...... Y. B.; Lee, Y.; Vog, T.; Jacobson, A. J. J. Am...
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DOI: 10.1021/cg1000107

Syntheses, Structures, and Physical Properties of Camphorate Coordination Polymers Controlled by Semirigid Auxiliary Ligands with Variable Coordination Positions and Conformations

2010, Vol. 10 2596–2605

Xiao-Qiang Liang, Dong-Ping Li, Cheng-Hui Li, Xin-Hui Zhou, Yi-Zhi Li, Jing-Lin Zuo,* and Xiao-Zeng You* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China Received January 4, 2010; Revised Manuscript Received March 28, 2010

ABSTRACT: A series of new coordination polymers, {[Zn(1,2-bimb)(D-ca)] 3 2H2O}n (1), [Zn(1,3-bimb)(D-ca)]n (2), [Cd(1,3bimb)(D-ca)]n (3), [Zn(1,4-bimb)(D-ca)]n (4), and {[Cd2(dpys)(D-ca)2(H2O)2] 3 H2O}n (5), (D-H2ca = D-camphoric acid, 1,2bimb =1,2-bis(imidazol-1-ylmethyl)-benzene, 1,3-bimb=1,3-bis(imidazol-1-ylmethyl) -benzene, 1,4-bimb=1,4-bis(imidazol1-ylmethyl)-benzene, dpys=4,40 -dipyridylsulfide), have been prepared under hydrothermal conditions and characterized by elemental analyses, IR, vibrational circular dichroism, thermogravimetric and X-ray structural analyses. Their structures and properties can be tuned by variable coordination positions and conformations of the auxiliary ligands. Complex 1 features a three-dimensional (3D) supramolecular framework with a one-dimensional (1D)-nanosized channel. Complexes 2 and 3 show acentric 1D double-stranded chains. The chiral two-dimensional sheet with (4,4) topology is found in compound 4. Compound 5 displays a 1D ladderlike chain. Photoluminescent properties of 1 and 5 are studied. Compounds 2-5 display a second-order nonlinear optical effect and ferroelectric behaviors.

Introduction Enormous effort has been dedicated to the construction and investigation of noncentrosymmetric or chiral metalorganic coordination polymers owing to their potential applications in the fields of chiral catalysis and resolution, ferroelectrics, nonlinear optics, and magnetism.1-5 Generally, metal-organic coordination polymers crystallized in the chiral and acentric space group may generate physical properties of second harmonic generation (SHG), ferroelectrics, pyroelectricity, and piezoelectricity, which are possiblly useful in electric-optical devices, information storages, nonlinear optical (NLO) devices, and light modulators. Therefore, the rational design and preparation of noncentrosymmetric or chiral coordination polymers with tailorable architectures and specific physical properties have become an increasingly remarkable subject. An effective strategy is the judicious selection of chiral ligands. Some versatile ligands containing N- or O-donor atoms have been extensively employed in the construction of chiral polymers.6-8 One of typical chiral ligands is camphoric acid.9 The semirigid ligands with multinitrogen heterocycle, such as 1,n-bis(imidazol-1-ylmethyl)-benzene (n=2, 3, 4), 1,4-bis(imidazol-1-ylmethyl) naphthalene, 1,n-bis(triazol-1-ylmethyl)benzene (n=2, 3, 4), 9,10-bis(triazol-1-ylmethyl)anthracene, 1,4-bis(n-pyridylmethyl)piperazine (n=3, 4), and 4,40 -dipyridylsulfide, are also widely used in coordination polymers because of variable coordination positions, conformations, and structural diversity.10-15 Moreover, chiral coordination polymers based on naturally chiral polycarboxylate groups and polypyridyl groups have been studied recently to prepare three-dimensional (3D) chiral frameworks.9a,c,h-k However, *Corresponding authors. Tel.: þ86 25 83593893; fax: þ86 25 83314502; e-mail: [email protected] (J.-L.Z); [email protected] (X.-Z.Y.). pubs.acs.org/crystal

Published on Web 04/27/2010

chiral coordination polymers assembled from naturally chiral polycarboxylate groups and semirigid ligands with multinitrogen heterocycle remain largely unexplored so far. We report herein the reactions of metal ions, chiral camphoric acid and 1,n-bis(imidazol-1-ylmethyl)-benzene (n=2, 3, 4), or 4,40 -dipyridylsulfide. Five new coordination polymers, {[Zn(1,2-bimb)(D-ca)] 3 2H2O}n (1), [Zn(1,3-bimb)(D-ca)]n (2), [Cd(1,3-bimb)(D-ca)]n (3), [Zn(1,4-bimb)(D-ca)]n (4), and {[Cd2(dpys)(D-ca)2(H2O)2] 3 H2O}n (5) (D-H2ca=D-camphoric acid, 1,2-bimb =1,2-bis(imidazol-1-ylmethyl)-benzene, 1,3-bimb= 1,3-bis(imidazol-1-ylmethyl) -benzene, 1,4-bimb = 1,4-bis(imidazol-1-ylmethyl)-benzene, dpys=4,40 -dipyridylsulfide), have been obtained and structurally characterized (Scheme 1). The photoluminescent, nonlinear optic and ferroelectric properties of these complexes have been studied in the solid state. Additionally, the positional and conformational role of the semirigid ligands for metal complexes have been discussed in detail. Experimental Section General Procedures. The bridging 1,n-bis(imidazol-1-ylmethyl)benzene (1,n-bimb) ligands were synthesized according to the literature method.16 All other chemicals were of reagent quality and obtained from commercial sources without further purification. Elemental analyses for C, H, and N were performed on a CHN-O-Rapid analyzer and an Elementar Vario MICRO analyzer. The IR spectra were obtained on a Bruker Vector 22 FT-IR spectrometer with KBr discs in the 4000-400 cm-1 range. The solid-state vibrational circular dichroism (VCD) was obtained on a Bruker PMA50 spectrometer by using KBr pellets in the 1800-1200 cm-1 range. Thermogravimetric analyses (TGA) were collected on a Perkin-Elmer Pyris 1 TGA analyzer from room temperature to 750 °C with a heating rate of 20 °C/ min under nitrogen. The X-ray powder diffraction (XRPD) analysis was performed by a Philips X-pert X-ray diffractometer at a scanning rate of 4° min-1 in the 2θ range from 5° to 60°, with graphite monochromatized Cu KR radiation (λ = 1.5418 A˚). Photoluminescence r 2010 American Chemical Society

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Table 1. Crystallographic Data and Details of Refinements for Complexes 1-5 complexes

1

2

3

4

5

C24 H28CdN4O4 548.90 orthorhombic Pna21 14.466(4) 10.691(3) 15.601(4) 90 90 90 2412.7(10) 4 1.511 0.942 2.31-26.00 12274 4566 304 1120 291(2) 0.0540, 0.1065 0.0720, 0.1107 1.001 0.478, -1.019 0.03(4)

C24H28N4O4Zn 501.87 monoclinic P21 6.6272(6) 17.3245(17) 9.9414(9) 90 97.3380(10) 90 1132.05(18) 2 1.472 1.124 2.07-26.00 6176 3695 301 524 291(2) 0.0544, 0.1157 0.0633, 0.1176 1.068 0.522, -0.676 0.06(2)

C30H40Cd2N2O11S 861.50 orthorhombic Cmc21 18.8555(15) 31.189(2) 12.0975(15) 90 90 90 7114.3(11) 8 1.609 1.311 2.10-26.00 19174 6901 482 3472 291(2) 0.0568, 0.1274 0.0763, 0.1314 1.015 1.040,-1.128 0.03(4)

)

)

C24H28N4O4Zn empirical formula C24H32N4O6Zn 537.91 501.87 Mr crystal system monoclinic orthorhombic Pna21 space group P21/n a (A˚) 10.793(2) 14.760(5) b (A˚) 22.999(5) 10.497(4) c (A˚) 12.253(3) 15.085(5) R (°) 90 90 β (°) 99.139(5) 90 γ (°) 90 90 3002.9(12) 2337.2(14) V (A˚3) Z 4 4 1.190 1.426 Dc (mg/m-3) -1 0.856 1.089 μ (mm ) θ range [ ° ] 1.77-26.00 2.36-25.99 collected reflections 16018 12131 unique reflections 5892 3884 parameters 350 306 F(000) 1128 1048 T (K) 291(2) 291(2) 0.0580, 0.1072 0.0524, 0.1141 R1a, wR2b [I > 2σ(I )] 0.0884, 0.1122 0.0570, 0.1148 R1a, wR2b [all data] GOF 1.024 1.063 0.457, -0.570 0.378, -0.531 largest peak and hole (e A˚-3) Flack parameter -0.02(2) P P P 2 2 2 P a b R1 = Fo| - |Fc / |Fo|. wR2 = [ w(|Fo | - |Fc |) / w(|Fo2|)2]1/2.

Scheme 1. Molecular Structure of the Ligands Used in This Paper

analyses for the solid samples were recorded on an AMINCO Bowman Series 2 luminescence spectrometer. The second-order nonlinear optical intensity was estimated by measuring a powder sample relative to urea. A pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm was used to generate a SHG signal from powder samples. The backscattered SHG light was collected by a spherical concave mirror and passed through a filter that transmits only 532 nm radiation. Electric hysteresis loop of a pellet of powders was measured by the Premier II ferroelectric tester at room temperature.

Preparations. {[Zn(1,2-bimb)(D-ca)] 3 2H2O}n (1). A mixture of Zn(NO3)2 3 6H2O (30 mg, 0.10 mmol), D-H2ca (20 mg, 0.10 mmol), 1,2-bimb (13 mg, 0.05 mmol), NaOH (2 mg, 0.05 mmol), and H2O (10 mL) was placed in a Teflon-lined stainless steel container and heated at 140 °C for 75 h. After the reaction mixture was slowly cooled to room temperature, colorless crystals of 1 were obtained. Yield: 42%. Anal. Calc. for C24H32N4O6Zn: C, 53.59; H, 6.00; N, 10.42. Found: C, 53.89; H, 5.97; N, 10.07%. IR (KBr, cm-1): 3428(br), 3130(m), 2964(m), 1665(s), 1591(vs), 1458(m), 1383(s),

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Table 2. Selected Bond Lengths (A˚) and Angles (°) for 1-5a Complex 1 N(2)-Zn(1) N(3)-Zn(1) O(4)-Zn(1)-O(1a) O(1a)-Zn(1)-N(2) O(1a)-Zn(1)-N(3)

2.000(3) 2.005(3) 104.6(1) 116.4(1) 107.3(1)

O(4)-Zn(1) Zn(1)-O(1a) O(4)-Zn(1)-N(2) O(4)-Zn(1)-N(3) N(2)-Zn(1)-N(3)

1.914(2) 1.925(2) 120.7(1) 96.0(1) 109.6 (1)

Complex 2 N(1)-Zn(1) O(1)-Zn(1) Zn(1)-O(3a) N(1)-Zn(1)-N(4a) N(4a)-Zn(1)-O(3a) N(4a)-Zn(1)-O(1) N(1)-Zn(1)-O(2) O(3a)-Zn(1)-O(2)

1.903(6) 2.069(4) 2.061(5) 103.4(3) 110.4(2) 96.9(2) 97.9(2) 92.2(2)

Zn(1)-N(4a) O(2)-Zn(1) N(1)-Zn(1)-O(3a) N(1)-Zn(1)-O(1) O(3a)-Zn(1)-O(1) N(4a)-Zn(1)-O(2) O(1)-Zn(1)-O(2)

1.977(4) 2.426(4) 93.7(2) 101.1(2) 145.0(2) 147.5(2) 54.6(2)

Complex 3 Cd(1)-N(1) Cd(1)-O(2) Cd(1)-O(1) N(1)-Cd(1)-N(2b) N(2b-Cd(1)-O(2) N(2b)-Cd(1)-O(3a) N(1)-Cd(1)-O(1) O(2)-Cd(1)-O(1) N(1)-Cd(1)-O(4a) O(2)-Cd(1)-O(4a) O(1)-Cd(1)-O(4a)

2.212(5) 2.327(4) 2.394(4) 100.1(2) 96.3(2) 142.3(2) 144.9(2) 54.8(2) 111.1(2) 152.7(2) 98.0(2)

N(1)-Zn(1) O(2)-Zn(1) O(3a)-Zn(1)-O(2) O(2)-Zn(1)-N(4) O(2)-Zn(1)-N(1)

2.035(6) 1.983(4) 109.5(2) 87.9(2) 135.1(2)

N(1)-Cd(1) Cd(1)-O(4) Cd(1)-O(3) N(2)-Cd(2) Cd(2)-O(7) Cd(2)-O(8) O(1)-Cd(1)-O(4) O(4)-Cd(1)-O(1W) O(4)-Cd(1)-N(1) O(1)-Cd(1)-O(3) O(1W)-Cd(1)-O(3) O(1)-Cd(1)-O(2) O(1W)-Cd(1)-O(2) O(3)-Cd(1)-O(2) N(2)-Cd(2)-O(7) N(2)-Cd(2)-O(2W) O(7)-Cd(2)-O(2W) O(6)-Cd(2)-O(8) O(2W)-Cd(2)-O(8) O(6)-Cd(2)-O(5) O(2W)-Cd(2)-O(5)

2.298(7) 2.275(5) 2.407(7) 2.229(7) 2.259(5) 2.389(6) 111.5(2) 103.4(2) 94.4(2) 99.6(2) 81.3(2) 53.9(2) 88.3(2) 116.3(2) 118.4(2) 90.6(2) 90.9(2) 111.4(2) 141.4(2) 55.4(2) 124.3(2)

Cd(1)-N(2b) Cd(1)-O(3a) Cd(1)-O(4a) N(1)-Cd(1)-O(2) N(1)-Cd(1)-O(3a) O(2)-Cd(1)-O(3a) N(2b)-Cd(1)-O(1) O(3a)-Cd(1)-O(1) N(2b)-Cd(1)-O(4a) O(3a)-Cd(1)-O(4a)

2.225(6) 2.363(5) 2.415(5) 94.6(2) 87.1(2) 120.2(2) 100.0(2) 94.2(1) 88.1(2) 55.2(2)

Complex 4 N(4)-Zn(1) Zn(1)-O(3a) O(3a)-Zn(1)-N(4) O(3a)-Zn(1)-N(1) N(4)-Zn(1)-N(1)

2.074(6) 1.944(5) 80.5(2) 115.3(2) 101.9(2)

Complex 5 Cd(1)-O(1) Cd(1)-O(1W) Cd(1)-O(2) Cd(2)-O(6) Cd(2)-O(2W) Cd(2)-O(5) O(1)-Cd(1)-O(1W) O(1)-Cd(1)-N(1) O(1W)-Cd(1)-N(1) O(4)-Cd(1)-O(3) N(1)-Cd(1)-O(3) O(4)-Cd(1)-O(2) N(1) -Cd(1)-O(2) N(2)-Cd(2)-O(6) O(6)-Cd(2)-O(7) O(6)-Cd(2)-O(2W) N(2)-Cd(2)-O(8) O(7)-Cd(2)-O(8) N(2)-Cd(2)-O(5) O(7)-Cd(2)-O(5) O(8)-Cd(2)-O(5)

2.275(6) 2.284(5) 2.433(6) 2.236(6) 2.321(5) 2.476(6) 138.5(2) 107.2(2) 91.4(2) 56.4(2) 146.5(2) 164.2 (2) 96.0(2) 141.0(2) 100.6(2) 90.3(2) 91.3(2) 54.9(2) 93.1(2) 133.4(2) 94.1(2)

a Symmetry codes: (a) x - 1/2, -y þ 1/2, z þ 1/2 for 1; (a) -x þ 1/2, y þ 1/2, z þ 1/2 for 2; (a) -x þ 3/2, y - 1/2, z - 1/2; (b) -x þ 3/2, y þ 1/2, z þ 1/2 for 3; (a) x, y, z - 1 for 4.

1357(s), 1287(m), 1240(w), 1104(m), 1089(m), 952(w), 789(s), 739(w), 659(w). [Zn(1,3-bimb)(D-ca)]n (2). Compound 2 was prepared by similar method of 1 except for using 1,3-bimb instead of 1,2-bimb. Yield: 56%. Anal. Calc. for C24H28N4O4Zn: C, 57.43; H, 5.62; N, 11.17. Found: C, 57.22; H, 5.77; N, 11.27%. IR (KBr, cm-1): 3130(m), 2966(m), 1593(s), 1520(s), 1446(m), 1383(vs), 1358(s), 1271(w), 1233(w), 1112(m), 1088(s), 947(m), 810(w), 730(m), 658(m), 631(w).

[Cd(1,3-bimb)(D-ca)]n (3). Compound 3 was prepared by similar method of 2 except for using Cd(NO3)2 3 4H2O instead of Zn(NO3)2 3 6H2O. Yield: 58%. Anal. Calc. for C24H28N4O4Cd: C, 52.51; H, 5.14; N, 10.21. Found: C, 52.29; H, 5.29; N, 10.37%. IR (KBr, cm-1): 3136(m), 2962(m), 1544(vs), 1456(m), 1395(s), 1363(s), 1286(w), 1232(m), 1111(m), 1084(s), 1027(w), 938(w), 811(m), 730(m), 656(m), 630(w). [Zn(1,4-bimb)(D-ca)]n (4). Compound 4 was prepared by similar method of 1 except for using 1,4-bimb instead of 1,2-bimb. Yield: 56%. Anal. Calc. for C24H28N4O4Zn: C, 57.43; H, 5.62; N, 11.17. Found: C, 57.77; H, 5.31; N, 11.12%. IR (KBr, cm-1): 3098(m), 2961(m), 1598(vs), 1531(s), 1521(m), 1460(m), 1445(m), 1425(m), 1383(m), 1371(m), 1355(m), 1338(s), 1303(m), 1281(m), 1247(m), 1236(w), 1118(m), 1104(s), 950(w), 848(w), 789(w), 739(m), 657(m). {[Cd2(dpys)(D-ca)2(H2O)2] 3 H2O}n (5). A mixture of Cd(NO3)2 3 4H2O (30 mg, 0.10 mmol), D-H2ca. (20 mg, 0.10 mmol), dpys (9.4 mg, 0.05 mmol), NaOH (4 mg, 0.10 mmol), MeCN (2 mL), and H2O (4 mL) was placed in a Teflon-lined stainless steel container, heated at 140 °C for 75 h and slowly cooled to room temperature. The crystals were collected, washed by distilled water and dried in vacuum. Yield: 36%. Anal. Calc. for C30H40Cd2N2O11S: C, 41.84; H, 4.68; N, 3.25. Found: C, 42.13; H, 4.84; N, 2.99%. IR (KBr, cm-1): 3398(br), 2967(m), 1581(vs), 1547(s), 1480(m), 1462(m), 1422(m), 1401(s), 1368(m), 1318(w), 1294(w), 1220(w), 1127(w), 1065(w), 1019(w), 926 (w), 817(m), 722(m), 630(w). X-ray Structure Determinations. The crystal structures of 1-5 were collected on a Bruker SMART APEX CCD diffractometer17 using graphite monochromatized Mo KR radiation (λ=0.71073 A˚) at 291(2) K. The raw data were reduced and corrected for Lorentz and polarization effect using the SAINT program and for absorption using the SADABS program. The structures were solved by direct methods and refined with the full-matrix least-squares technique using SHELXTL version5.1.18 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. Hydrogen atoms were localized in their calculated positions and refined using a riding model. The crystallographic data are summarized in Table 1. Selected bond lengths and bond angles are presented in Table 2. The corresponding H-bond data are shown in Table S1, Supporting Information.

Results and Discussion Description of Crystal Structures. Structure of {[Zn(1,2bimb)(D-ca)] 3 2H2O}n (1). Compound 1 crystallizes in the monoclinic space group, P21/n. In the asymmetry unit of 1, there is one Zn(II) atom, one 1,2-bimb ligand, one D-ca2ligand, and two solvated water molecules. As illustrated in Figure 1a, each Zn(II) atom is four-coordinated by two oxygen atoms from D-ca2 ligands and two nitrogen atoms from 1,2-bimb ligands, forming a slightly distorted [ZnN2O2] tetrahedral geometry. The 1,2-bimb ligand adopts an I-shape conformation with the dihedral angles of 83.9° and 85.2° between the phenyl and each imidazolyl ring, respectively. Each Zn(II) center is linked by D-ca2- and 1,2-bimb ligands to generate a two-dimensional (2D) layer motif, in which the Zn 3 3 3 Zn distances separated by D-ca2- and 1,2-bimb are 9.420 and 13.522 A˚, respectively (Figure 1b). Meanwhile, each layer interacts with neighboring layers via π-π interactions between the imidazolyl rings (centroid-centroid separation of 3.661 A˚), further extending the 2D layers into a 3D supramolecular framework (Figure 1c). It is noteworthy that the 3D supramolecular framework has onedimensional (1D)-nanosized channel with dimensions of ca. 10.2  8.4 A˚ (Figure 1d). Additionally, there are versatile strong hydrogen bonding interactions (the d(D 3 3 3 A) distance is about 2.900 A˚) formed by guest water molecules, and the carboxylate oxygen atom and the guest water molecule in the cavity (Figure 1e). After the removal of these guest solvents, the volume of the effective void, calculated using PLATON is 28.6% of the unit-cell volumes in 1.

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Figure 1. (a) View of the coordination environment of Zn(II) in complex 1; thermal ellipsoids are drawn at the 50% probability level. Lattice water molecules and hydrogen atoms have been omitted for clarity. (b) Two-dimensional layer. (c) The 3D supramolecular architecture via π-π stacking interactions. (d) The 3D architecture with 1D nanosized channels. (e) The 3D framework’s vacancy occupied by the guest water molecules and the existence of hydrogen-bonding interactions (black, C; blue, N; red, O; cyan, Zn; light gray, H).

Structures of [Zn(1,3-bimb)(D-ca)]n (2) and [Cd(1,3-bimb)(D-ca)]n (3). Compounds 2 and 3 are isotypics and crystallize in the noncentrosymmetric space group Pna21. Therefore, only the crystal structure of compound 3 is discussed in detail. In 3, the asymmetric unit is made up of one Cd(II) atom, one D-ca2- ligand, and one 1,3-bimb ligand. As shown

in Figure 2a, each Cd(II) center sits in a distorted octahedral geometry in which the equatorial plane is completed by one nitrogen atom (N2b) and three carboxylate oxygen atoms (O2, O3a, and O4a) and the axial position is occupied by the other nitrogen and carboxylate oxygen atoms (N1 and O1). The bond lengths of Cd-O (carboxylate) are not equivalent

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Figure 2. (a) View of the coordination environment of Cd(II) in complex 3; thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. (b) The 1D acentric double-stranded chain (black, C; blue, N; red, O; green, Cd).

and range from 2.327(4) to 2.415(5) A˚, which are in accordance with similar reported complexes.9 The 1,3-bimb ligand exhibits a Z-shape conformation with the dihedral angles between the phenyl and each imidazolyl ring being 77.9° and 88.3°, respectively. Each Cd(II) center is connected by D-ca2- and 1,3-bimb ligands into a double-stranded chain with the neighboring Cd 3 3 3 Cd distance of 9.545 A˚ (Figure 2b). Structures of [Zn(1,4-bimb)(D-ca)]n (4). Compound 4 crystallizes in the chiral space group P21. The asymmetric unit of 4 consists of one Zn(II) atom, one 1,4-bimb ligand, and one 2D-ca ligand. As shown in Figure 3a, each Zn(II) atom adopts a slightly distorted tetrahedral arrangement and is coordinated to two nitrogen atoms from 1,4-bimb ligands and two carboxylate oxygen atoms from D-ca2- ligands. The 1,4-bimb ligand employs an I-shape conformation, and the dihedral angles between the phenyl and each imidazolyl ring are 66.4° and 82.2°, respectively. The D-ca2- ligands connect Zn atoms into a 1D [Zn(D-ca)]n chain. These chains are further linked by the 1,4-bimb ligands to afford a 2D network with (4,4) topology (Figure 3b,c). The Zn 3 3 3 Zn distances spanned by D-ca2- and 1,4-bimb ligands are 9.941 and 13.928 A˚, respectively. Compared with 1, the 3D supramolecular framework produced by π-π interactions and 1D-nanosized channel is not found in 4. Structure of {[Cd2(dpys)(D-ca)2(H2O)2] 3 H2O}n (5). Compound 5 crystallizes in the noncentrosymmetric space group Cmc21. The asymmetric unit of 5 contains two Cd(II) atoms, one dpys ligand, two D-ca2- ligands, two coordinated aqua molecules, and one solvated water molecule. Two Cd(II) atoms show the same coordination environment. As illustrated in Figure 4a, the Cd atoms are coordinated by four carboxylate oxygen atoms from D-ca2- ligands, one pyridyl nitrogen atom from dpys ligand, and one coordinated water molecule, showing a distorted octahedral sphere with the Cd-O lengths of 2.259(5)-2.476(6) A˚ and the Cd-N lengths of 2.229(7)-2.298(7) A˚. The dpys ligand exhibits a semistaggered

Figure 3. (a) View of the coordination environment of Zn(II) in complex 4; thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. (b) Twodimensional layer (black, C; blue, N; red, O; cyan, Zn). (c) Schematic description of the (4,4)-connected 2D network.

conformation, and the dihedral angle between the pyridyl ring planes and the angle of C3-S1-C8 are 48.06° and 105.1(4)°, respectively (Figure 4b). The carboxylate-oxygen atoms from 2D-ca ligands join two Cd(II) atoms, yielding a 1D polymeric chain. Each chain is further linked to give a 1D ladderlike polymer via nitrogen atoms from dpys ligands (Figure 4c). It is interesting that the O-H 3 3 3 O hydrogen bonds from coordinated aqua molecules, solvated water molecules, and carboxylate oxygen atoms connect these 1D chains to give a 2D wavelike supramolecular network (Figure 4d). Every layer interacts with neighboring layers through hydrogen bonding interactions from coordinated aqua molecules and carboxylate oxygen atoms, further extending the 2D layers into a 3D supramolecular framework (Figure 4e). As illustrated in Scheme 2, D-camphoric acid in complexes 1-5 show diversified coordination modes. In complexes 1 and 4, carboxylate groups of D-camphoric acid take

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Figure 4. View of the coordination environments of Cd(II) in complex 5; thermal ellipsoids are drawn at the 50% probability level (Symmetry codes: (a) 1-x, y, z; (b) 2-x, y, z). Lattice water molecule and hydrogen atoms have been omitted for clarity. (b) The conformation of the dpys ligand. (c) The 1D chiral ladderlike chain. (d) The 2D layered structure via hydrogen-bonding interactions. (e) The 3D supramolecular architecture via hydrogen-bonding interactions (black, C; blue, N; red, O; yellow, S; green, Cd, gray, H).

monodentate coordination mode owing to the requirement of zinc centers holding tetrahedral geometries. However, they show bidentate coordination mode in 3 and 5 to satisfy octahedral geometries of cadmium centers. Interestingly, in

2, carboxylate groups of D-camphoric acid adopt both monodentate and bidentate coordination modes, probably resulting from the tetragonal pyramidal structure of zinc center. These coordination fashions have been also observed

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Scheme 2. Three Coordination Modes of the D-Camphoric Acid Observed in Compounds 1-5

Scheme 3. 1,3-bimb and dpys Ligands Show Different Conformations

Figure 5. The IR absorption spectra (bottom) and VCD (top) of 4 in the solid state at room temperature.

in other similar compounds.9g,j,k The results suggest that the coordination behaviors of carboxylate groups of D-camphoric acid depend on the different coordination environments of metal centers. XRPD and Thermogravimetric Analyses (TGA). In order to check the phase purity of the products, X-ray powder diffraction (XRPD) experiments have been carried out for all complexes. The peak positions of the experimental and simulated XRPD patterns are in agreement with each other, which demonstrate that the crystal structures are truly representative of the bulk materials (Figures S1S5, Supporting Information). The differences in intensity may be owing to the preferred orientation of the powder samples. TGA curves are made under flowing N2 for crystalline samples of 1-5 in the temperature range of 25-750 °C (Figure S6, Supporting Information). Compound 1 experienced a weight loss of 7.09% below 110 °C corresponding to the release of the guest water molecules (calcd. 6.70%), ensued by a series of weight losses beyond that temperature. It can be seen from the TGA curves of 2 and 3 that the network remained intact until it is heated to 200 and 230 °C, and then it began to collapse. For compound 4, the decomposition of the compound occurs at ca. 300 °C, implying higher thermal stability of the frameworks. Complex 5 loses two coordinated and one guest water molecules between 25 and 240 °C with a weight loss of 6.93% (calcd. 6.27%). Vibrational Circular Dichroism (VCD). VCD spectroscopy extends the range of CD measurements into the infrared region and provides a useful method to analyze structural information of chiral molecules, especially chiral coordination polymers.19 As shown in Figure 5, the solid-state IR and

VCD spectra of complex 4 have been studied. In the IR spectra, complex 4 shows the characteristic bands of the carboxylate groups for stretch vibrations at 1598, 1460, 1445 cm-1. The bands at 1236, 1247, 1281, 1303, 1338, 1355, 1371, 1383, 1425, 1521, 1531 cm-1 for 4 are assigned to skeleton vibrations of the aromatic ring. The experimental VCD bands at 1558-1638 and 1439-1478 cm-1 for 4 correspond to C-O stretch vibrations. The bands at 1222-1427 and 1491-1535 cm-1 correspond to skeleton vibrations of the aromatic rings. IR absorption peaks correspond to VCD features, which could be either positive or negative (Cotton effect). The distinct VCD signals confirm that the bulk crystals of complex 4 are chiral.

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Figure 6. Electric hysteresis loop of a pellet of powders of compounds 2(a), 3(b), 4(c), and 5(d) (the different curves stand for different hysteresis loops at different voltage measurements).

Opto-Electronic Properties. Photoluminescence. The photoluminescent properties of compounds 1-5, and the free 1,2bimb and dpys ligands are investigated in the solid state at room temperature. When excitated at 287 nm, the 1,2-bimb ligand exhibits one strong emission maximum at 342 nm (Figure S7, Supporting Information). No obvious photoluminescence was observed for 2-4 and the dpys ligand under the same experiment conditions. Complex 1 shows an emission band centered at 454 nm when excitated at 400 nm (Figure S8, Supporting Information). Complex 5 shows an intense fluorescent emission peak at 529 nm with an excitation length of 394 nm. Compared with compound 1, the red shift of the emission occurring in 5 probably results from the change of organic ligands. Second Harmonic Generation Efficiency. According to the methods proposed by Kurtz and Perry,20 the second harmonic generation (SHG) efficiency of compounds 2-5 were measured by using the microcrystalline samples. Preliminary experimental results indicate that compounds 2, 3, 4, and 5 exhibit SHG intensities about 0.3, 0.1, 0.3, and 0.8 times that of urea, respectively. It is noteworthy that the SHG efficiency of compound 5 is stronger than that of other compounds. The enhancement of the SHG efficiency may be ascribed to the interactions of strong hydrogen bonding and the presence of push-pull effect centers.21

Ferroelectricity. A compound that crystallizes in one of the 10 polar point groups (C1, Cs, C2, C2v, C3, C3v, C4, C4v, C6, C6v) may have the potential to display ferroelectric behavior. Compounds 2-5 crystallize noncentrosymmetric and chiral space group Pna21, Pna21, P21, Cmc21, respectively, which fall into the polar point groups C2v or C2. Thus, their ferroelectric features were investigated (Figure 6). Experimental results demonstrate that compounds 2 and 3 display ferroelectric behaviors featuring electric hysteresis loops with a remnant polarization (Pr) of ca. 0.022 μC 3 cm-2 for 2 and 0.082 μC 3 cm-2 for 3, and a coercive field (Ec) of ca. 6.46 kV 3 cm-1 for 2 and 8.44 kV 3 cm-1 for 3, respectively. Saturation of the spontaneous polarizations (Ps) of 2 and 3 are about 0.095 and 0.223 μC 3 cm-2, respectively. There is electric hysteresis loop in compound 4, which represent typical ferroelectric feature with a remnant polarization (Pr) of ca. 0.052 μC 3 cm-2 and a coercive field (Ec) of ca. 5.83 kV 3 cm-1. Saturation of the spontaneous polarizations (Ps) of 4 occurs about 0.143 μC 3 cm-2. The measurement of electric hysteresis loops indicates that 5 displays a ferroelectric characteristic with a remnant polarization (Pr) of ca. 0.154 μC 3 cm-2, a coercive field (Ec) of ca. 27.51 kV 3 cm-1, and saturation of the spontaneous polarizations (Ps) of ca. 0.406 μC 3 cm-2. It is

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noteworthy that compounds 2 and 3 are isotypics, and a remnant polarization (Pr) of compound 2 is smaller than that of compound 3. The different ferroelectric feature may be associated with the polarizability of metal ions. The ferroelectric feature of Cd-coordination polymer is larger than that of Zn-coordination polymer, which is probably due to soft metal ions possessing the higher polarizability than hard metal ions.22 Role of the Conformation and Position of the Semirigid Ligands. According to the positions of two imidazole groups relative to the central benzene ring plane, semirigid 1,n-bimb (bimb = bis(imidazol-1-ylmethyl)-benzene) ligands can adopt different conformations; for example, the 1,3-bimb ligand can adopt V-shape, Z-shape, L-shape, and I-shape conformations. The conformations of the 1,3-bimb ligand are defined as follows: V-shape is two imidazole groups located in the same side of the central benzene ring plane; Z-shape is two imidazole groups distributing on the two sides of the central benzene ring plane; L-shape is one imidazole group located above or below the central benzene ring plane and the other imidazole ring being close to the benzene ring plane; I-shape is two imidazole rings being close to the benzene ring plane.23 The dpys also shows different conformations in the light of the relation of the dihedral angle formed by the pyridyl rings and a magic angle (C-S-C). When the dihedral angle is almost identical to a magic angle and perpendicular, it shows eclipsed and staggered conformations which are extreme forms, respectively. Contrarily, other conditions often show semistaggered conformation (Scheme 3).24 Furthermore, the coordination groups are in different positions in the multidentate dpys and 1,n-bimb ligands (1,1-, 1,2-, 1,3-, or 1,4-positions). The conformation and position can be described as “steric orientation effect” in the paper. The steric orientation effect of auxiliary ligands has an influence on structural features of complexes 1-5. The 1,2-bimb ligand with I-shape conformation gives rise to π-π interactions between the imidazolyl rings, which induces the generation of 3D supramolecular framework with 1D-nanosized channel for compound 1. The matched effects of the 1,3-bimb ligand with Z-shape conformation and 2D-ca ligand drive the formation of 1-D double-stranded chains for compounds 2 and 3. Without π-π interactions between the imidazolyl rings, the 1,4-bimb ligand with I-shape conformation only prompts the construction of the 2D sheet with (4,4) topology for compound 4. The dpys ligand with semistaggered conformation self-organizes a 1D ladderlike chain for compound 5. The steric orientation effect of auxiliary ligands may affect ferroelectric behavior. The experimental results show that complexes 2 and 4 display different ferroelectric behaviors, while no ferroelectric behavior is observed for compound 1. As we know, ferroelectric behavior is associated with dipoles which are vectors and rely on steric orientation. The different steric orientations show the different dipoles, which further leads to the different ferroelectric behaviors. Conclusions and Perspectives In this paper, five new metal-organic coordination polymers have been synthesized by using chiral camphoric acid and four N-donor auxiliary ligands with a different coordination positions and conformations. The structures and properties of the resulting coordination polymers have been tuned through the modification of the auxiliary organic ligand.

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The results demonstrate that the steric orientation effect of the auxiliary organic ligand may affect the structures and properties of coordination polymers. Further studies on the steric orientation effect of the auxiliary organic ligand are still under way in our laboratory. Acknowledgment. This work was supported by the National Basic Research Program of China (2006CB806104 and 2007CB925100), and the National Natural Science Foundation of China (20725104 and 20721002). Supporting Information Available: The XRPD patterns and TGA curves of 1-5, emission spectra of the free 1,2-bimb ligand, 1 and 5. X-ray crystallographic file in CIF format for 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

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