Effect of Different Imidazole Ancillary Ligands on Supramolecular

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DOI: 10.1021/cg101261f

Effect of Different Imidazole Ancillary Ligands on Supramolecular Architectures of a Series of Zn(II) and Cd(II) Complexes with a Bent Dicarboxylate Ligand

2011, Vol. 11 480–487

Chang-Chun Ji,† Ling Qin,† Yi-Zhi Li,† Zi-Jian Guo,† and He-Gen Zheng*,†,‡ †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China, and ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China Received September 26, 2010; Revised Manuscript Received December 8, 2010

ABSTRACT: Five new complexes, namely, {[Zn(L)(L1)] 3 (H2O)(DMF)2}n (1), {Cd(L)(L1)0.5}n (2), {[Zn2(L)2(L2)2] 3 DMF}n (3), {[Zn(L)(L3)] 3 (H2O)5}n (4), and {[Cd2(L)2(L3)1.5(H2O)]2 3 H2O}n (5), have been synthesized by the solvothermal reaction of 4,40 -(hexafluoroisopropylidene)bis(benzoic acid) (H2L) with different transition metal ions in the presence of ancillary ligands 1,4-bis(imidazol-1-ylmethyl)benzene (L1), 4,40 -bis(imidazol-1-ylmethyl)-biphenyl (L2), and 4,40 -bis(benzimidazol-1-ylmethyl)biphenyl (L3). Complex 1 displays a two-dimensional (2D) framework containing two kinds of ring units. In complex 2, one-dimensional (1D) Cd chains are assembled by L2- and L1 ligands into a non-interpenetrated three-dimensional structure. Complex 3 possesses a 2D framework which is generated by joining both P and M helices by L2- ligands. The structure of 4 is a three-dimensional (3D) framework with {42.6.83} net containing meso-helices, while 5 is a 2D structure with {414.6} net. The photoluminescent properties of 1-5 have been studied in the solid state at room temperature.

Introduction In recent decades, the chemists have devoted themselves to the development of new crystalline materials with a variety of properties, functions, and potential applications such as gas sorption,1 luminescence,2 molecular magnetism,3 nonlinear optics,4 catalysis,5 and ion-exchange.6 The factors influencing the construction of metal-organic frameworks (MOFs) are complicated. The general thought is that the selections of solvent system, temperature,7 organic ligands,8 different metal ions,9 and counterions.10 In particular, the organic ligands have significant influences on the desirable MOFs because of the different spacer lengths, flexibility, steric hindrance effects, conformational preferences, and so on.11 Thus, the prospect of controlling the properties and structures of MOFs by selection of proper organic ligands has attracted more interest in research on metal-organic supramolecular architectures.12 The ligand 4,40 -(hexafluoroisopropylidene)bis(benzoic acid) (H2L) as a semirigid dicarboxylate ligand has been investigated widely in recent years. Because of its bent geometry, it can induce porous framework with many characters such as selective adsorption, catalysis, magnetism, fluorescence, and so on.13 The ancillary ligands containing N-donor such as 4,40 -bipyridine (bipy) have been used widely with H2L together to construct the desired structures. For example, 4,40 -bipyridine as an N-donor ligand is beneficial to the syntheses of extended MOFs and can generate high dimensional structures owing to its simple bridging mode and strong coordination ability, while the utilization of imidazole ligands as coligands to react with H2L has not been reported. Flexible imidazole ligands have also been widely used to construct MOFs. The flexible nature of spacers allows the ligands to bend and rotate when they coordinate to metal centers, which often causes the structural *To whom correspondence should be addressed. E-mail: zhenghg@nju. edu.cn. Fax: 86-25-83314502. pubs.acs.org/crystal

Published on Web 01/04/2011

diversity.14 To examine the influence of the flexibility, spacer length, and steric hindrance of the imidazole ligands on the assembly of supramolecular entities, three imidazole ligands, namely, 1,4-bis(imidazol-1-ylmethyl)benzene (L1), 4,40 -bis(imidazol-1-ylmethyl)-biphenyl (L2), and 4,40 -bis(benzimidazol-1-ylmethyl)biphenyl (L3), were used to react with the Zn(II) and Cd(II) as ancillary ligands (Scheme 1). Five new complexes are obtained, namely, {[Zn(L)(L1)] 3 (H2O)(DMF)2}n (1), {Cd(L)(L1)0.5}n (2), {[Zn2(L)2(L2)2] 3 DMF}n (3), {[Zn(L)(L3)] 3 (H2O)5}n (4), and {[Cd2(L)2(L3)1.5(H2O)]2 3 H2O}n (5); the details of their syntheses, structures, and physical properties are reported below. Experimental Section Reagents and Physical Measurements. H2L ligand was purchased and used as received. L1, L2, and L3 were prepared by the methods reported in the literature.15 All other chemicals were of reagent grade quality from commercial sources and were used without further purification. The IR absorption spectra of the compounds were recorded in the range of 400-4000 cm-1 by means of a Nicolet (Impact 410) spectrometer with KBr pellets (5 mg of sample in 500 mg of KBr). C, H, and N analyses were carried out with a Perkin-Elmer 240C elemental analyzer. Luminescent spectra were recorded with a SHIMAZU VF-320 X-ray fluorescence spectrophotometer at room temperature (25 °C). Powder X-ray diffraction (PXRD) measurements were performed on a Philips X0 pert MPD Pro X-ray diffractometer using CuKR radiation (λ = 0.15418 nm), in which the X-ray tube was operated at 40 kV and 40 mA. The assynthesized samples were characterized by thermogravimetric analysis (TGA) on a Perkin-Elmer thermogravimetric analyzer Pyris 1 TGA up to 1023 K using a heating rate of 10 K min-1 under a N2 atmosphere. Synthesis of {[Zn(L)(L1)] 3 (H2O)(DMF)2}n (1). A mixture of H2L, L1, Zn(NO3)2 3 6H2O (a molar ratio of 1:1:1), and the solvent of H2O/DMF (1:2, v/v, 8 mL) was sealed in a 15 mL PTFE-lined stainless-steel acid digestion bomb and heated at 85 °C for 48 h and then was cooled to give large quantities of colorless block crystals of 1. The pure crystals of 1 were isolated by filtration, washed with r 2011 American Chemical Society

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Scheme 1. Dicarboxylate Ligand H2L and N-Donor Ligands L1, L2, and L3

DMF and water, and dried in air. Yield of the reaction was ca. 46% based on H2L. Anal. Calcd for C31H22O4N4F6Zn: C, 53.66%, H 3.20%, N 8.07%; found C, 53.54%, H 3.09%, N 8.17%. IR (KBr, cm-1): 3421(s), 1623(s), 1570(m), 1525(m), 1443(m), 1368(s), 1291(m), 1253(s), 1211(m), 1171(s), 1137(m), 1111(m), 1094(w), 1020(w), 971(w), 945(w), 930(w), 863(w), 844(w), 781(m), 748(m), 725(m), 692(m), 656(w), 620(m), 590(w), 563(w), 509(w). Synthesis of {Cd(L)(L1)0.5}n (2). An equivalent of H2L, L1, and Cd(NO3)2 3 4H2O dissolved in the solvent of H2O/DMF (1:2, v/v, 8 mL) was sealed in a 15 mL PTFE-lined stainless-steel acid digestion bomb and heated at 100 °C for 72 h and then was cooled to give large quantities of colorless foliate crystals of 2. The pure crystals of 2 were isolated by filtration, washed with DMF and water, and dried in air. Yield of the reaction was ca. 32% based on H2L. Anal. Calcd for C24H15O4N2F6Cd: C, 46.36%, H 2.43%, N 4.51%; found C, 46.25%, H 2.49%, N 4.56%. IR (KBr, cm-1): 3419(s), 1612(s), 1594(s), 1536(s), 1453(w), 1416(s), 1352(m), 1291(s), 1241(s), 1207(s), 1175(m), 1154(m), 1135(m), 1113(m), 1083(m), 1021(m), 970(m), 959(m), 942(m), 930(w), 873(s), 855(s), 780(s), 748(m), 725(s), 690(w), 657(m), 624(w), 539(w), 505(m). Synthesis of {[Zn2(L)2(L2)2] 3 DMF}n (3). A mixture of H2L, L2, Zn(NO3)2 3 6H2O (a molar ratio of 1:1:1) and the solvent of H2O/ DMF (5:3, v/v, 8 mL) was sealed in a 15 mL PTFE-lined stainlesssteel acid digestion bomb and heated at 120 °C for 48 h and then was cooled to give large quantities of colorless block crystals of 3. The pure crystals of 3 were isolated by filtration, washed with DMF and water, and dried in air. Yield of the reaction was ca. 55% based on H2L. Anal. Calcd for C77H59O9N9F12Zn2: C, 57.33%, H 3.69%, N 7.81%; found C, 57.25%, H 3.73%, N 7.78%. IR (KBr, cm-1): 3406(s), 1676(m), 1620(s), 1571(m), 1560(m), 1524(m), 1502(m), 1442(m), 1366(s), 1317(w), 1291(m), 1253(s), 1240(s), 1210(m), 1171(s), 1137(m), 1113(m), 1091(m), 1021(w), 1006(w), 970(w), 944(w), 929(w), 843(m), 815(w), 780(m), 748(m), 725(s), 690(w), 655(m), 635(w), 573(w), 506(w). Synthesis of {[Zn(L)(L3)] 3 (H2O)5}n (4). An equivalent of H2L, L3, and Zn(NO3)2 3 6H2O dissolved in the solvent of H2O/DMF (1:1, v/v, 8 mL) was sealed in a 15 mL PTFE-lined stainless-steel acid digestion bomb and heated at 85 °C for 48 h and then was cooled to give large quantities of colorless block crystals of 4. The pure crystals of 4 were isolated by filtration, washed with DMF and water, and dried in air. Yield of the reaction was ca. 25% based on L3. Anal. Calcd for C45H40O9N4F6Zn: C, 56.29%, H 4.20%, N 5.83%; found C, 56.24%, H 4.31%, N 5.76%. IR (KBr, cm-1): 3420(s), 1613(s), 1556(m), 1505(m), 1464(w), 1445(w), 1380(s), 1293(m), 1249(s), 1209(s), 1170(s), 1136(m), 1010(w), 968(w), 941(w), 929(w), 845(m), 804(w), 775(w), 745(m), 723(s), 688(s), 645(w), 608(w), 566(w), 546(w), 502(w). Synthesis of {[Cd2(L)2(L3)1.5(H2O)]2 3 H2O}n (5). A mixture of H2L, L3, Cd(NO3)2 3 4H2O (a molar ratio of 1:1:1), and the solvent H2O/DMF/EtOH (5:2:1, v/v/v, 8 mL) was sealed in a 15 mL PTFElined stainless-steel acid digestion bomb and heated at 80 °C for 48 h and then was cooled to give large quantities of colorless foliate

crystals of 5. The pure crystals of 5 were isolated by filtration, washed with DMF, EtOH, and water, and dried in air. Yield of the reaction was ca. 60% based on L3. Anal. Calcd for C152H104O19N12F24Cd2: C, 59.21%, H 3.40%, N 5.45%; found C, 59.24%, H 3.45%, N 5.42%. IR (KBr, cm-1): 3414(s), 1656(w), 1592(s), 1537(s), 1502(s), 1462(w), 1441(w), 1392(s), 1292(m), 1254(s), 1224(s), 1210(s), 1169(m), 1137(w), 1021(m), 1007(w), 970(w), 945(w), 929(w), 911(w), 856(m), 818(w), 777(m), 747(s), 725(s), 689(w), 645(w), 608(w), 565(w), 543(w), 506(w). X-ray Crystallographic Measurements. Single crystals of 1-5 were prepared by the methods described in the synthetic procedures. X-ray crystallographic data of 1-5 were collected at room temperature using epoxy-coated crystals mounted on glass fiber. All measurements were made on a Bruker Apex Smart CCD diffractometer with graphite-monochromated MoKR radiation (λ = 0.71073 A˚). The structures of 1-5 were solved by direct methods, and the non-hydrogen atoms were located from the trial structure and then refined anisotropically with SHELXTL using a full-matrix least-squares procedures based on F2 values.16 The hydrogen atom positions were fixed geometrically at calculated distances and allowed to ride on the parent atoms. The distribution of peaks in the channels of 1 and 4 was chemically featureless to refine using conventional discrete-atom models. To resolve these issues, the contribution of the electron density by the remaining water molecule was removed by the SQUEEZE routine in PLATON.17 The numbers of solvent water molecules in 1 and 4 were obtained by element analyses. The relevant crystallographic data are presented in Table 1, while the selected bond lengths and angles are given in Supporting Information, Table S1.

Results and Discussion Synthesis. The complexes were synthesized by the reaction of M(NO3)2 (M = Zn and Cd), H2L, and different imidazole ligands. Solvothermal synthesis was applied to this system for multidimensional coordination complexes. Many parallel experiments proved that the quality of crystals depended on the temperatures and the solvents, while the reaction mole ratio of the ligands and metal salts do not influence the formation of products significantly. Though high temperature can induce to the multidimensional complexes, it will lead to impure products at the same time. So we selected the proper temperature by repeating parallel experiments. Complexes 1, 4, and 5 were obtained at low temperature under 90 °C for 48 h, while 2 was obtained at a higher temperature of 100 °C for 72 h, and 3 got the best product at 120 °C for 48 h. In addition, the ratio of water and DMF affected the quality of the crystals. The best ratios decided by parallel experiments are used to get complexes 1-4. In particular, complex 5 would not be obtained without the existence of

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Table 1. Crystallographic Data and Structure Refinement Details for Complexes 1-5 complex

1

2

3

4

5

formula formula weight crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) Z V (A˚3) Dcalcd (g cm-3) μ(Mo KR) (mm-1) F(000) theta min-max (deg) tot., uniq data R(int) observed data [I > 2σ(I)] Nref, Npar R1, wR2 (all data) Sa min and max resd dens (e 3 A˚-3)

C31H22F6N4O4Zn 693.90 monoclinic P21/n 13.001(3) 26.347(6) 13.131(3) 90.00 117.311(4) 90.00 4 3996.5(16) 1.153 0.675 1408 1.83, 26.00 21385, 7842 0.041 4634 7842, 415 0.0437, 0.0997 1.069a -0.532, 0.322

C24H15F6N2O4Cd 621.78 monoclinic P21/n 7.4302(9) 25.077(3) 13.2325(16) 90.00 102.193(2) 90.00 4 2410.0(5) 1.714 0.987 1228 1.62, 26.00 12829, 4703 0.055 3293 4703, 328 0.0492, 0.1584 1.000a -0.453, 0.970

C77H59F12N9O9Zn2 1613.07 monoclinic P21/c 14.253(2) 29.965(5) 18.340(3) 90.00 112.8700 90.00 4 7217(2) 1.485 0.761 3296 1.69, 26.00 38676, 14144 0.048 9667 14144, 984 0.0558, 0.1222 1.037a -0.468, 0.403

C45H30F6N4O4Zn 870.10 monoclinic C2/c 23.685(3) 14.647(2) 29.080(4) 90.00 101.867(2) 90.00 8 9873(2) 1.171 0.560 3552 1.72, 26.00 26454, 9674 0.036 7184 9674, 541 0.0493, 0.1363 1.048a -0.768, 0.759

C152H104F24N12O19Cd4 3310.09 triclinic P1 13.9275(15) 16.4116(17) 17.7029(19) 110.472(2) 96.531(2) 109.872(2) 1 3438.6(6) 1.597 0.716 1660 1.77, 25.00 17208, 11885 0.031 7745 11885, 937 0.0435, 0.0744 1.001a -0.880, 0.793

a

R1 = Σ||Fo| - |Fc||/|Σ|Fo|. wR2 = {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2, where w = 1/[σ2(Fo2) þ (aP)2 þ bP], P = (Fo2 þ 2Fc2)/3.

Figure 1. An ORTEP drawing of 1 showing 30% ellipsoid probability (hydrogen atoms are omitted for clarity). Symmetry codes: #1 = -x, 2 - y, -z; #2 = 1.5 - x, 0.5 þ y, 0.5 - z; #3 = 1.5 - x, -0.5 þ y, 0.5 - z.

ethanol. The resultant crystals are stable in air and insoluble in water or common organic solvents. Crystal Structures. Structure of {[Zn(L)(L1)] 3 (H2O)(DMF)2}n (1). X-ray diffraction reveals that only one crystallographically independent Zn(II) center is contained in the fundamental asymmetric unit (Figure 1). Zn(II) is in the center of a distorted tetrahedral geometry, defined by two nitrogen atoms from two separate L1 ligands, and two oxygen atoms from two different acid ligands. The Zn-O distances are in the range of 1.968(2)-1.972(2) A˚, and Zn-N distances vary in the range of 2.001(3)-2.008(2) A˚. They are reasonable compared to those values in reported work.18 As shown in Figure 2a, the two carboxylate groups of L2- ligand adopt the monodentate-bridging coordination mode (μ1:η1) to link two zinc(II) atoms to form a 1D zigzag chain. Two cis-L1 ligands, the dihedral angles between imidazole and phenyl are 71.791(4)° and 86.333(4)°, join two Zn atoms to form a ring. And all the neighboring 1D chains can be connected by the L1 ligands rings to form a 2D layer (Figure 2b). The 2D layer can be viewed as two types of rings arranged alternately. As illustrated in Figure 2c, the A ring contains four L2- ligands, two L1 ligands, and six Zn atoms. And the B ring is the L1 ligands ring mentioned before, which is smaller than A ring. The adjacent 2D layers can extend to a 3D supramolecular structure by the π-π interactions between neigh-

Figure 2. (a) The 1D zigzag chain constructed by L2- ligands and Zn ions along the b direction. (b) View of the 2D layer formed by joining all zigzag chains through L1 ligands perpendicular to the ac plane. (c) The illustration of 2D layer containing two types of rings (hydrogen atoms and solvent molecules are omitted for clarity).

boring imidazole rings and phenyl rings (Figure 3a). The L2ligands have no contributions to π-π interactions. Figure 3b shows that there are two types of π-π interactions existing in adjacent B rings: one is between the imidazole rings N1, N2, and C18-C20 of symmetry related pairs of ligands (red and green). The planes of two rings are parallel and the interplanar distance is 4.183(2) A˚ with the slipping angles β=γ=32.6°. Another is between the phenyl rings C22-C27 of symmetry related pairs of ligands (green and pink). The two planes are parallel, too. The interplanar distance is 4.525(2) A˚ with the slipping angles β = γ=40.9°. It is obvious that two types of π-π interactions are weak. Structure of {Cd(L)(L1)0.5}n (2). X-ray analysis reveals that the asymmetric unit of 2 consists of one crystallographically independent cadmium(II) cations, one deprotonated L2- ligand, and half of a L1 ligand. As shown in

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Figure 4, the L2- ligand adopts the coordination mode in which one bonding carboxylate group is a monodentate bridge and the other is a bidentate-monodentate bridge (μ2:η2:η1), that is, bidentate through O1 and O2 toward Cd1 and monodentate across O1 toward another Cd1. Different in complex 1, the L1 ligand is trans-configuration with a dihedral angle between imidazole and phenyl of 61.933(9)°. The Cd center is six-coordinated by five oxygen atoms belonging to four separated L2- ligands as well as one nitrogen atom from the L1 ligand and presents a slightly distorted octahedron with Cd-O distances ranging from 2.217(4) to 2.517(4) A˚, which can be compared with values reported.19 All Cd atoms are connected to form a 1D chain by carboxyl bridges (Figure 5a). Then the L1 ligands joined all infinite 1D chains into a 2D layer (Figure 5b). And as can be seen in Figure 5c, the 3D framework is finally obtained by linking 2D layers through acid ligands. Structure of {[Zn2(L)2(L2)2] 3 DMF}n (3). There are two tetra-coordinated crystallographically unique zinc(II) ions in the fundamental building unit of complex 3. As illustrated in Figure 6, the Zn1 center presents a slightly distorted tetrahedron, defined by two oxygen atoms from two different L2ligands and two nitrogen atoms from two different L2

Figure 3. (a) View of the 3D supramolecular structure of 1 formed by π-π interactions between adjacent L1 rings. (b) The detailed illustration of π-π interactions between neighboring imidazole rings and phenyl rings (hydrogen atoms and the other part of the structure are omitted for clarity; different colors represent the three rings belonging to three neighboring 2D layers).

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ligands. The Zn-O bond distances are 1.953(2) and 2.039(3) A˚, respectively. And the Zn-N bond distances are 2.005(3) and 2.037(3) A˚, respectively. Zn2 center adopts a tetrahedral coordination environment formed by two oxygen atoms from L2- ligands and two nitrogen atoms from L2 ligands with the Zn-O distances ranging from 1.951(2) to 1.958(3) A˚ and the Zn-N distances varying from 2.004(3) to 2.016(3) A˚. They are reasonable compared with the values in reported work.20 It is obvious that the L2 ligands have two different configurations when coordinated with zinc atoms contributing to its flexibility. One is named L type shown by the lavender color, in which the biphenyl is nearly planar and the angle between two phenyl rings is 5.571(9)°. The dihedral angles between imidazole and phenyl are 112.259(4)° and 120.487(3)°, respectively. Another one named B type in blue showed no coplanar mode, in which the angle between two phenyl rings is 46.228(1)°. The dihedral angles between imidazole and phenyl are 80.747(4)° and 97.436(4)°, respectively. The two types of L2 ligands connected adjacent zinc atoms to form a 1D helix with different chirality as described in Figure 7a. The 1D left-handed (M)/ right handed (P) helical infinite chain is around the crystallographic 21 axis in the a direction with a long pitch of 14.253(2) A˚. The L2- occupied the remaining coordination sites of ZnII within the chain as bridging ligands with the monodentate-bridging coordination mode, that is, μ1:η1 bridge coordination fashion. Because left-handed and right-handed helical chains coexist in the crystal structure, the whole crystal is racemic and does not exhibit chirality. The occurrence of helical structure may contribute to the flexibility of the L2 ligand. As shown in Figure 7b, the P helical chains are marked by pink and red colors, respectively. The pink chains and red chains are in different directions when viewed down the a axis. Such classification is used to distinguish the M helical chains by aqua and green colors, too. Along the c direction, neighboring helical chains polycatenated to each other and formed a 2D layer with many cavities filled by DMF molecules (Figure 7c). While in the b direction, the adjacent 2D layers give rise to a 3D supramolecular structure by π-π interactions between the rings C5-C10 and C11-C16 of symmetry-related pairs of ligands. The dihedral angle R defined by the stacked rings is 8.6° and the interplanar distance is 4.203(2) A˚. The slipping angles (β = 30.2° and γ = 24.7°) revealed a weak stacking

Figure 4. An ORTEP drawing of 2 showing 30% ellipsoid probability (hydrogen atoms are omitted for clarity. All carbon and fluorine atoms are not labeled for clarity). Symmetry codes: #1 = 0.5 þ x, 1.5 - y, -0.5 þ z; #2 = 1.5 - x, -0.5 þ y, 1.5 - z; #3 = 1 - x, 1 - y, 1 - z; #4 = 2 x, 1 - y, -z.

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Figure 5. (a) The 1D Cd chain constructed by oxygen and carboxyl bridges with Cd ions along the a direction. (b) View of the 2D layer formed by joining all Cd chains through L1 ligands. (c) The L2- ligands connected adjacent 2D layers to give rise to the 3D framework of 2 (hydrogen atoms are omitted for clarity).

Figure 6. An ORTEP drawing of 3 showing 30% ellipsoid probability (hydrogen atoms are omitted for clarity, L2- ligands are shown in sticks mode for clarity. All carbon and fluorine atoms are not labeled for clarity). The L2 ligands are classified as two types by different configurations (blue: B type, lavender: L type). Symmetry codes: #1 = -1 þ x, y, z; #2 = 1 þ x, y, z.

interaction (Figure 7d). All the helical chains are arranged in “-P-M-P-M-” model both in the b and c directions as illustrated in Figure 7b. Structure of {[Zn(L)(L3)] 3 (H2O)5}n (4). In complex 4, each asymmetric unit consists of one Zn (II) atom, one L2- ligand, one L3 ligand, and five lattice water molecules. As shown in Figure 8, the Zn(II) center is four-coordinated by two carboxylate oxygen atoms from two different L2- ligands and by two nitrogen atoms from two different L3 ligands in a severely distorted tetrahedral geometry. The distances of the Zn-O bonds range from 1.937(2) to 1.939(2) A˚, which are close to those in reported work before.21 And the distances of the Zn-N bonds are 2.016(2) and 2.021(2) A˚. Similar to

Figure 7. (a) The illustration of right-handed helix (P helix) and left-handed helix (M helix) constructed by L2 ligands along the a direction. (b) View of the 3D supramolecular structure formed by polycatenation (along the c direction) and π-π interactions (along the b direction) of neighboring helices. (c) The illustration of polycatenation of two helices and the cavity filled by DMF molecule. (d) The illustration of π-π interactions between L2 ligands in neighboring helices along the b direction (hydrogen atoms are omitted for clarity).

complex 3, the L2- ligand adopts the monodentate (μ1:η1) coordination mode. According to different configurations caused by the flexibility of ligand, the L3 ligands are classified to two types named B (blue) and R (red), respectively. The biphenyl in trans-configurational B type L3 ligand

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(in blue color) is coplanar with the angle of 0° between two phenyl rings. And the dihedral angle between imidazole and phenyl is 107.339(3)°. The biphenyl in cis-configurational R type L3 ligand (in red color) is not coplanar with the angle of 36.808(9)° between two phenyl rings. The dihedral angles between imidazole and phenyl are 42.187(4)° and 70.643(3)°, respectively. A notable feature for 4 is the presence of a mesohelical chain (Figure 9a).22 The B and R imidazole ligands connect adjacent Zn ions alternately to generate a mesohelical chain with a long pitch of 41.108(4) A˚ which has not been reported widely before. And the adjacent infinite mesohelical chains are arranged one by one in the ac plane. As can be seen in Figure 9b, the L2- ligands join neighboring Zn atoms to form a 1D zigzag chain without regard to the interactions with L3 ligands. And the adjacent zigzag chains are arranged like a cross shape. The final 3D framework of 4 is obtained by the interconnection with both meso-helical chains and zigzag chains (Figure 9c). A better insight into the nature of 4 can be achieved by regarding the Zn atom as a four-connected node, the L2- ligand and L3 ligand as linear

Figure 8. An ORTEP drawing of 4 showing 30% ellipsoid probability (hydrogen atoms are omitted for clarity. All carbon and fluorine atoms are not labeled for clarity). The L3 ligands are classified as two types by different configurations (blue: B type, red: R type). Symmetry codes: #1 = 0.5 þ x, 0.5 þ y, z; #2 = 1 - x, y, 0.5 - z; #3 = 0.5 - x, 1.5 - y, 1 - z; #4 = -0.5 þ x, -0.5 þ y, z.

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linkers to reduce multidimensional structures to simple node-and-linker nets. The simplified structure of 4 is a {42.6.83} net topology (Figure 10). Structure of {[Cd2(L)2(L3)1.5(H2O)]2 3 H2O}n (5). The structure of complex 5 is a two-dimensional framework in which the asymmetric unit contains two cadmium(II) atoms, two L2- ligand, one and a half L3 ligands, one coordinated water, and one lattice water. As illustrated in Figure 11, the Cd1 center presents a slightly distorted octahedron, defined by three oxygen atoms from two different L2- ligands, one oxygen atom from coordinated water, and two nitrogen atoms from two L3 ligands. The Cd-O bond distances vary in the range of 2.304(3)-2.371(3) A˚, and the Cd-N bond distances vary in the range of 2.260(3)-2.279(3) A˚. Cd2 adopts a distorted octahedral coordination environment, too. Different from Cd1, it is formed by five oxygen atoms from three L2- ligands and one nitrogen atom from L3 ligand. The Cd-O distances range from 2.267(3) to 2.400(3) A˚, and the Cd-N distance is 2.211(3) A˚. All values are in the reasonable range.23 Quite similar to complex 4, the L3 ligands are classified to two types named B (blue) and R (red), respectively. The biphenyl in B type L3 ligand, which is

Figure 10. Topological illustration for the 4-connected network of 4. Golden nodes represent Zn atoms, pink rods represent L2ligands, and blue rods represent L3 ligands.

Figure 9. (a) The 1D meso-helix chains constructed by L3 and Zn ions along the a axis (left); the arranging mode of adjacent helical chains in the ac plane (right). (The L2- ligands are omitted for clarity.) (b) View of the 1D chain formed by bridging all Zn atoms through L2- ligands (left); the neighboring chains extended along cross directions (right). (The L3 ligands are omitted for clarity.) (c) The L2- chains connect adjacent helical chains to give rise to the 3D framework of 4 (hydrogen atoms and lattice water molecules are omitted for clarity).

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Figure 11. An ORTEP drawing of 5 showing 30% ellipsoid probability (hydrogen atoms are omitted for clarity, L2- ligands are shown in sticks mode for clarity. All carbon and fluorine atoms are not labeled for clarity). The L3 ligands are classified as two types by different configurations (blue: B type, red: R type). Symmetry codes: #1 = x, 1 þ y, z; #2 = -1 þ x, y, z; #3 = 2 - x, -y, -z; #4 = -x, -y, -z; #5 = 1 þ x, y, z.

trans-configuration indicated by blue color, is coplanar with the angle between two phenyl rings very close to 0°. And the dihedral angle between imidazole and phenyl is 67.510(7)°, while the angle between two phenyl rings in the R type L3 ligand, which is cis-configuration shown by red color, is 36.546(1)°. The dihedral angles between imidazole and phenyl are 35.272(7)° and 71.061(6)°, respectively. The Figure 11 also shows us that there are two coordination modes of L2ligands, which is clear in Figure 12a, too. In one type of L2ligands, one carboxylate group exhibits a bismonodentate μ2-syn, syn-bridge mode (μ2:η1:η1) to bridge Cd1 and Cd2 centers, and another carboxylate group shows chelate-bidentate fashion to chelate one central Cd ion. Two L2ligands of this type connect two Cd1 atoms to form a M2L2 ring (A ring). In another type of L2- ligands, two carboxylate groups coordinate to metal atoms both in chelating mode. The two types of L2- ligands alternately connect the neighboring Cd2 atoms and construct M6L4 macro cycles (shown by pink color). The A rings and B macro cycles are arranged along the a axis side by side, and produce a 1D chain of acid ligands and metal atoms. All the 1D chains can extend to the 2D plane by L3 ligands in both intrachain and interchain modes (Figure 12d). As illustrated in Figure 12b, the B type L3 ligands act as intrachain bridges to connect Cd1 atoms belonging to every other A ring and thread the middle A ring. As shown in Figure 12c, the R type L3 ligands act as interchain bridges to connect Cd1 and Cd2 atoms belonging to adjacent 1D chains, respectively. From a topological perspective, the SBU [Cd1Cd2CO2] acts as a 5-connected node, and complex 5 represents {414.6} topology by using L2- and L3 ligands as linkers (Figure 13). Thermal Analysis and PXRD Results. To characterize the complexes more fully in terms of thermal stability, their thermal behaviors were studied by TGA (Supporting Information, Figure S4). For complex 1, a weight loss is observed

Ji et al.

Figure 12. (a) The 1D chain containing two different units (A: M2L2 ring; B: M6L4 macro cycle). (The L3 ligands are omitted for clarity.) (b) The B type L3 ligands act as intrachain bridges to connect Cd1 atoms belonging to every other A ring across the middle one. (The L2- ligands joining Cd2 atoms and R type L3 ligands are omitted for clarity.) (c) The R type L3 ligands act as interchain bridges to connect Cd1 and Cd2 atoms belonging to adjacent 1D chains. (The L2- ligands joining Cd2 atoms and B type L3 ligands are omitted for clarity.) (d) The L2- chains are connected by L3 ligands in both intrachain and interchain modes to give rise to the 2D structure of 5 along the b axis (hydrogen atoms and lattice water molecules are omitted for clarity).

Figure 13. Overall {414.6} topology of complex 5. Golden node represents cluster fragment [Cd1Cd2CO2] units.

from 70 to 214 °C, which is attributed to the loss of the lattice water, and the two DMF molecules, with a weight loss of 21.25% (calcd 21.24%), and then the structure is decomposed since 320 °C. For complex 2, no weight loss is observed until it reaches 435 °C, at which the decomposition of the 2 occurred. Complex 3 is less stable with the weight loss of one lattice DMF molecule 4.46% (calcd 4.53%) before 309 °C, and then the framework is decomposed since 416 °C. In the case of 4, a weight loss is observed from 132 °C, which is attributed to the release of the five lattice waters with a weight loss of 9.36% (calcd 9.38%), and then the network collapses at 385 °C. The TGA study of complex 5 shows a little weight of 1.63% (calcd 1.65%) from 254 °C, which

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belongs to the loss of the three lattice water, and then the framework collapses at 310 °C. To confirm whether the crystal structures are truly representative of the bulk materials, XRD experiments were carried out for 1-5. The XRD experimental and computer-simulated patterns of the corresponding complexes are shown in the Supporting Information, Figures S5-S9, and they show that the bulk synthesized materials and the measured single crystals are the same. Photoluminescent Properties. Luminescent compounds are of great interest due to their various applications in chemical sensors and photochemistry. The luminescent properties of free ligands L, L1, L2, and L3, with complexes 1, 2, 3, 4, and 5 have been investigated in the solid state at room temperature, as depicted in the Supporting Information (Figures S1-S3). Intense emissions of the free L, L1, L2, and L3 ligands were observed with wavelengths at 395 nm in L, 400 nm in L1, 425 nm in L2, and 395 nm in L3, which could be attributed to the π*-π transitions. The emission characters of complexes 1, 2, 3, and 4 are similar to that of the free L, L1, L2, and L3 ligands. The emission peaks are at 395 nm in 1, 385 nm in 2, 415 nm in 3, 390 nm in 4, and 385 nm in 5, among which, complexes 1, 2, 4, 5 show a small blue shift compared with that of L, L1, L3 ligands. While complex 3 shows a small red shift compared with that of L and L2 ligands, they all probably attributed to the π*-π intraligand fluorescence due to their emission characters close to the emission bands of ligands.24

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Conclusions In this work, five new complexes have been successfully synthesized under solvothermal conditions. The different structures of 1-5 are mainly caused by the different ancillary ligands L1, L2, and L3. The results show that the flexible ligand can take different modes to meet the coordination nature of the metal cations. Besides, spacer length and steric hindrance of the imidazole ligands also can affect the assembly of supramolecular structure. Furthermore, the flexibility of ancillary ligands are inclined to generate ring or cycle structures and the probability of forming helical chains are enlarged with an increase of the spacer length. The consequent diversity of topologies is also increased as we expected.

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Acknowledgment. This work was supported by grants from the Natural Science Foundation of China (Nos. 91022011; 20971065; 20721002) and National Basic Research Program of China (2010CB923303; 2007CB925103).

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Supporting Information Available: X-ray crystallographic information files (CIF) are available for complexes 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

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