Ligand-Directed and pH-Controlled Assembly of Chiral 3d−3d

Jun 18, 2010 - Use of unsymmetric ligand 1,2,4-benzenetricarboxylic acid (1,2,4-H3BTC) and controlling the reaction pH value enabled isolation ... Vie...
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DOI: 10.1021/cg100418a

Ligand-Directed and pH-Controlled Assembly of Chiral 3d-3d Heterometallic Metal-Organic Frameworks

2010, Vol. 10 3515–3521

Zhi Su,† Jian Fan,† Taka-aki Okamura,‡ Wei-Yin Sun,*,† and Norikazu Ueyama‡ †

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China, and ‡Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Received March 29, 2010; Revised Manuscript Received June 13, 2010

ABSTRACT: Use of unsymmetric ligand 1,2,4-benzenetricarboxylic acid (1,2,4-H3BTC) and controlling the reaction pH value enabled isolation of two novel chiral 3d-3d heterometallic complexes [Zn2Co(tib)3(H2O)5][Zn6(tib)2(1,2,4-BTC)6] 3 12.7H2O (1) and [ZnCo(tib)(1,2,4-BTC)(H2O)2]Cl 3 3H2O (2) [tib =1,3,5-tris(1-imidazolyl)benzene]. Comparative study revealed that the use of symmetric ligand 1,3,5-benzenetricarboxylic acid (1,3,5-H3BTC) resulted one achiral 3d-3d heterometallic complex [ZnCo(tib)(1,3,5-BTC)Cl] (3) with 3D structure. Complex 1 is a 2D network with cationic and anionic layers arranged alternately, while 2 is a 3D framework with rare 1D helical water chains. Furthermore, complex 1 displays ferroelectric behavior with a remnant electric polarization (Pr) of ∼0.177 μC/cm2 and an electric coercive field (Ec) of ∼17.68 kV/cm.

Introduction The chiral metal-organic frameworks (MOFs) have attracted current attention not only because of their intriguing and beautiful structure but also because of their potential application in enantioselective processes, molecular recognition, replication, and catalysis.1,2 Supramolecular chirality of MOFs results either from the spatial organization of achiral building blocks or from the chirality of components.3,4 Although the recent success with MOFs has provided new routes toward the synthesis of chiral structure with achiral precursors, yet the chiralization process concerning how supramolecular interactions lead to the generation of chirality that are not present in the components is unclear, thus unpredictable.5 On the other hand, the desire for novel chiral materials has motivated researchers to use unsymmetric ligands as templates to introduce chirality into the supramolecular entities. Recently, interest in heterometallic complexes is rapidly expanding because the specific combination of different metal ions with a particular connection pattern via organic connectors may trigger the interesting electronic communication between metal ions and ligands within the framework, which allows tailoring the chemical and physical properties in a wide range. So far much interest has focused on the 3d-4f (transition-lanthanide) heterometallic complexes; however, it is very much surprising that significantly less effort has been devoted to the parallel exploitation of the 3d-3d (transition-transition) heterometallic complexes.6 Challenges still remain in the design and synthesis of 3d-3d heterometallic complexes since the 3d metal ions show the slight difference in their coordination nature (coordination ability, coordination geometry, and so on) as compared to that between the 3d-4f ones.7 The selfassembly of heterometallic complex necessitates the preferential selection of two or more types of metal ions from a mixture while avoiding formation of the alternative homometallic complexes.

In this work, imidazole-containing ligand, 1,3,5-tris(1-imidazolyl)benzene (tib), and carboxylate-containing one, 1,2,4benzenetricarboxylic acid (1,2,4-H3BTC) or 1,3,5-benzenetricarboxylic acid (1,3,5-H3BTC), were applied as the mixed linkers, which would be favorable for the formation of heteroleptic complexes by fulfilling the subtle and implicit coordination requirements around different 3d metal centers with N atoms or carboxylate groups. Two novel chiral 3d-3d heterometallic MOFs, [Zn2Co(tib)3(H2O)5][Zn6(tib)2(1,2,4BTC)6] 3 12.7H2O (1) and [ZnCo(tib)(1,2,4-BTC)(H2O)2]Cl 3 3H2O (2), were hydrothermally synthesized by controlling the reaction pH values. Comparative studies on the structuredirecting effects of both unsymmetric and symmetric ligands were carried out as well since such studies can manifest the important factor regulating the chiralization process of the resulting framework. So the reaction with the symmetric 1,3,5benzenetricarboxylic acid was performed, and one achiral heterometallic complex [ZnCo(tib)(1,3,5-BTC)Cl] (3) was successfully synthesized. The as-synthesized compounds 1-3 were characterized and formulated by inductively coupled plasma (ICP), elemental analysis (EA), infrared (IR) and single crystal X-ray diffraction. Furthermore, the ferroelectric property of 1 has been explored. Experimental Section

*To whom correspondence should be addressed. Fax: þ86-25-83314502. E-mail: [email protected].

All commercially available chemicals and solvents are of reagent grade and were used as received without further purification. The tib ligand was synthesized according to the method reported previously.8 Elemental analyses for C, H and N were performed on a PerkinElmer 240C Elemental Analyzer at the analysis center of Nanjing University. Thermogravimetric analyses (TGA) were carried out on a simultaneous SDT 2960 thermal analyzer under nitrogen with a heating rate of 10 °C min-1 from room temperature to 700 °C. FTIR spectra were recorded in the range of 400-4000 cm-1 on a Bruker Vector22 FT-IR spectrophotometer using KBr pellets. The content of Zn(II) and Co(II) in 1-3 was determined by measurements of inductively coupled plasma (ICP) on a J-A1100 (Jarrell-Ash, USA) ICP spectrometer. Powder X-ray diffraction (PXRD) patterns were measured on a Shimadzu XRD-6000 X-ray diffractometer with Cu KR (λ = 1.5418 A˚) radiation at room temperature. The electric

r 2010 American Chemical Society

Published on Web 06/18/2010

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

1

2

3

C129H113.40CoN30O53.70Zn8 3524.99 200 0.50(2) hexagonal P63 19.910(3) 19.910(3) 20.117(3) 90 90 120 6906.3(19) 2 1.695 3588 3.11 - 25.00 41758 6050 1.023 0.0827 0.2111

C24H25ClCoN6O11Zn 733.19 293(2) 0.49(3) orthorhombic P212121 7.1599(9) 39.159(5) 9.7127(12) 90 90 90 2723.2(6) 4 1.769 1460 2.16 - 25.24 14118 3180 0.964 0.0630 0.1390

C24H14ClCoN6O6Zn 642.16 293(2) monoclinic P21/c 8.6817(9) 16.8803(18) 15.6226(16) 90 95.541(2) 90 2278.8(4) 4 1.872 1288 1.78 - 25.25 11454 3216 0.994 0.0464 0.1204

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

hysteresis loops were recorded on a Ferroelectric Tester Precision Premier II made by Radiant Technologies, Inc. Synthesis of [Zn2Co(tib)3(H2O)5][Zn6(tib)2(1,2,4-BTC)6] 3 12.7H2O (1). A mixture of tib (27.6 mg, 0.1 mmol), 1,2,4-H3BTC (21.0 mg, 0.1 mmol), ZnCl2 (13.6 mg, 0.1 mmol), and CoCl2 3 6H2O (23.7 mg, 0.1 mmol) in 10 mL of H2O was stirred for 10 min in air; 0.5 M NaOH was added dropwise to pH ∼8-9, and then the mixture was turned into a 16 mL Teflon-lined stainless steel container and heated at 180 °C for 3 days. Purple column crystals of 1 were obtained by filtration and washed by water and ethanol several times with yield of 34%. The result of ICP revealed that the molar ratio of Zn(II)/ Co(II) is 8:1. Anal. Calcd for C129H113.40CoN30O53.70Zn8 (%): C 43.92, H 3.22, N 11.92. Found: C 43.89, H 3.19, N 11.98. IR (KBr, cm-1): 3410(s, br), 1615(s), 1489(s), 1376(s), 1346(s), 1290(m), 1110(m), 1077(m), 1017(s), 821(m), 788(m), 763(m), 732(m), 651(m). Synthesis of [ZnCo(tib)(1,2,4-BTC)(H2O)2]Cl 3 3H2O (2). The title complex was prepared by similar procedures used for preparation of 1 except that the pH value of reaction solution was adjusted to ∼5-6 instead of 8 -9 for 1. Red platelet crystals of 2 were obtained with yield of 47%. The result of ICP revealed that the molar ratio of Zn(II)/Co(II) is 1:1. Anal. Calcd for C24H27ClCoN6O11Zn (%): C 39.20, H 3.70, N 11.43. Found: C 39.24, H 3.76, N 11.81. IR (KBr, cm-1): 3439(s,br), 1618(s), 1572(s), 1515(s), 1394(s), 1318(m), 1243(s), 1110(m), 1074(s), 1018(m), 950(m), 854(m), 763(m), 749(m), 650(m). Synthesis of [ZnCo(tib)(1,3,5-BTC)Cl] (3). The complex 3 was obtained by similar procedures used for preparation of 1 with reaction pH of ∼8-9 except that 1,3,5-H3BTC (21.0 mg, 0.1 mmol) was used instead of 1,2,4-H3BTC. The reaction of tib (27.6 mg, 0.1 mmol), 1,3,5-H3BTC (21.0 mg, 0.1 mmol), ZnCl2 (13.6 mg, 0.1 mmol), and CoCl2 3 6H2O (23.7 mg, 0.1 mmol) in H2O (10 mL) was also carried out at pH ∼5-6 as for preparation of 2. The same product of blue block crystals of 3 was obtained by reaction pH of ∼8-9 and 5-6 with typical yield of 57%. The result of ICP revealed that the molar ratio of Zn(II)/Co(II) is 1:1. Anal. Calcd for C24H14ClCoN6O6Zn (%): C 44.89, H 2.20, N 13.09. Found: C 44.93, H 2.19, N 13.07. IR (KBr, cm-1): 1625(s), 1544(s), 1517(s), 1429(m), 1361(s), 1338(m), 1265(m), 1108(m), 1072(s), 1013(m), 947(m), 871(m), 720(m), 673(m), 645(m). X-ray Crystallography. The crystallographic data for 1 was collected on a Rigaku RAXIS-RAPID imaging plate diffractometer at -73 °C, with graphite-monochromated Mo KR radiation (λ = 0.71075 A˚). The structure was solved by direct methods with SIR929 and expanded using Fourier techniques.10 All non-hydrogen atoms were refined anisotropically by the full-matrix least-squares method

Table 2. Selected Bond Lengths [A˚] and Bond Angles [deg] for Complexes 1-3a 1 Zn1-N32 Zn2-N52 Zn3-O3 Zn3-N112 Zn4-O9 Zn4-N132 Co1-O13 O14-Zn1-N32 O15-Zn2-N52 O3-Zn3-N112 O3-Zn3-O5i O3-Zn3-O7ii O9-Zn4-N132 O1iii-Zn4-O9 O9-Zn4-O11iii O13-Co1-N12iii O13iii-Co1-N12 N12-Co1-N12iii

1.975(8) 2.002(7) 1.966(6) 1.982(7) 1.989(6) 1.999(7) 2.179(9)

Zn1-O14 Zn2-O15 Zn3-O5i Zn3-O7ii Zn4-O1iii Zn4-O11iii Co1-N12 N32-Zn1-N32iv N52-Zn2-N52v O5i-Zn3-N112 O7ii-Zn3-N112 O5i-Zn3-O7ii O1iii-Zn4-N132 O11iii-Zn4-N132 O1iii-Zn4-O11iii O13-Co1-N12 O13-Co1-O13iii

111.7(3) 110.0(3) 98.4(3) 114.5(3) 107.2(3) 98.4(3) 108.1(3) 112.8(3) 85.8(4) 88.7(3) 98.5(4)

1.94(2) 1.917(15) 1.919(7) 1.944(7) 1.946(8) 1.939(7) 2.063(9) 107.1(3) 108.9(4) 122.0(3) 117.3(3) 97.6(3) 116.1(3) 123.5(3) 97.9(3) 171.0(4) 86.4(4)

2 Zn1-O3 Zn1-O7 Co1-O1 Co1-O8 O3-Zn1-O7 O3-Zn1-N12 O3-Zn1-O5vi O1-Co1-O8 O1-Co1-N52vii O1-Co1-N32viii

1.958(6) 1.986(6) 1.976(7) 2.036(6)

Zn1-N12 Zn1-O5vi Co1-N52vii Co1-N32viii

97.1(3) 111.4(3) 108.2(3) 104.3(3) 128.0(3) 97.0(3)

2.009(7) 1.984(6) 2.002(8) 1.989(6)

O5vi-Zn1-O7 O7-Zn1-N12 O5vi-Zn1-N12 O8-Co1-N52vii O8-Co1-N32viii N32viii-Co1-N52vii

102.2(3) 101.8(3) 130.0(3) 104.9(3) 106.0(3) 114.7(3)

Zn1-N12 Zn1-O2ix Co1-O5ix Co1-N32xii

2.003(4) 1.991(3) 1.935(3) 1.992(4)

3 Zn1-Cl1 Zn1-O1 Co1-O3 Co1-N52xi

2.2116(15) 1.949(3) 1.923(3) 2.013(4)

Cl1-Zn1-O1 Cl1-Zn1-N12 Cl1-Zn1-O2ix O3-Co1-N52xi O3-Co1-O5x O3-Co1-N32xii

108.93(10) 111.41(13) 115.45(12) 113.66(16) 110.44(15) 106.35(15)

O1-Zn1-N12 O1-Zn1-O2ix O2ix-Zn1-N12 O5x-Co1-N52xi N32xii-Co1-N52xi O5x-Co1-N32xii

112.69(16) 111.12(14) 96.93(14) 93.93(15) 111.27(16) 121.11(16)

a Symmetry transformations used to generate equivalent atoms: (i) -y, x - y, z; (ii) -x þ y, 1 - x, z; (iii) 1 - y, 1 þ x - y, z; (iv) 1 - y, x - y, z; (v) 2 - y, 1 þ x - y, z; (vi) 3/2 - x, -y, -1/2 þ z; (vii) 5/2 - x, -y, -3/2 þ z; (viii) 2 - x, -1/2 þ y, 1/2 - z; (ix) 1 - x, 1 - y, -z; (x) 1 þ x, y, z; (xi) 1 þ x, -1 þ y, z; (xii) -x, -1/2 þ y, 1/2 - z.

on F2. Hydrogen atoms in the structure of 1 were generated geometrically. All calculations were carried out on SGI workstation using the teXsan crystallographic software package of Molecular Structure Corporation.11 The crystallographic data collections for 2 and 3 were carried out on a Bruker Smart Apex CCD area-detector diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) at 293(2) K using the ω-scan technique. The diffraction data were integrated by using the SAINT program,12 which were also used for the intensity corrections for the Lorentz and polarization effects. Semiempirical absorption correction was applied using the SADABS program.13 The structures were solved by direct methods and all the non-hydrogen atoms were refined anisotropically on F2 by the full-matrix least-squares technique using the SHELXL-97 crystallographic software package.14 Hydrogen atoms in 2 and 3 were generated geometrically. Details of the crystal parameters, data collection and refinements for 1-3 are summarized in Table 1. Selected bond lengths and angles for 1-3 are listed in Table 2. Hydrogen bonding distances and angles for 1 are summarized in Table S1, Supporting Information. Further details are provided in the Supporting Information.

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Figure 1. (a) ORTEP drawing of [Zn2Co(tib)3(H2O)5]6þ (left) and [Zn6(tib)2(BTC)6]6- (right) in 1 with the ellipsoids drawn at 30% probability level, the hydrogen atoms were omitted for clarity. (b) The ABAB packing diagram of the cationic (left) and anionic parts in 1. (c) The simplified topological representation of complex 1 with ABCD packing diagram (the red triangle, the 3-fold axis; the green hexagon, the 6-fold axis).

Results and Discussion Synthesis, Powder X-ray Diffraction (PXRD), and Thermal Stability of the Complexes. Complexes 1-3 with heterometallic centers were synthesized by using both of ZnCl2 and CoCl2 3 6H2O. The reactions of tib and 1,2,4-H3BTC/1,3,5H3BTC with only ZnCl2 or CoCl2 3 6H2O were also carried out under pH values of 8-9 and 5-6, respectively. The results of PXRD (Figure S1, Supporting Information) indicate that the products with the same metal centers under different pH values are the same but are different from complexes 1-3, implying that the heterometallic centers in 1-3 are important for the framework structure and may also be the source of the chirality of 1 and 2. The purities of complexes 1-3 were confirmed by investigation of PXRD in which the simulated and as-synthesized spectra are almost the same (Figure S2, Supporting Information). The thermogravimetric analysis (TGA) was performed in order to verify the thermal stability of complexes 1-3 (Figure S3, Supporting Information). A total weight loss of 9.25% was observed for complex 1 in the temperature range

of 40-215 °C, which is ascribed to the loss of both coordinated and free water molecules (calcd 9.04%), and the residue is stable up to 310 °C. For 2, there are two steps for the loss of solvent molecules: the first loss in the temperature range of 40-120 °C corresponding to the loss of three free water molecules (obsd 7.20%, calcd 7.25%) and the second loss from 120 to 160 °C corresponding to the two coordinated ones (obsd 4.90%, calcd 4.91%). The residue is stable up to 300 °C. There is no obviously loss weight for 3 up to the decomposition happened at 430 °C. Crystal Structure of [Zn2Co(tib)3(H2O)5][Zn6(tib)2(1,2,4BTC)6] 3 12.7H2O (1). Complex 1 synthesized with pH value of 8 - 9 crystallizes in hexagonal chiral space group P63 and contains two separated two-dimensional (2D) fragments, [Zn2Co(tib)3(H2O)5]6þ as the cationic part and [Zn6(tib)2(1,2,4BTC)6]6- as the anionic part (Figure 1a). Within the [Zn2Co(tib)3(H2O)5]6þ unit, each Zn(II) atom (Zn1 and Zn2) is fourcoordinated by three nitrogen atoms from three different tib ligands and one terminal water molecule, and each Co1 atom is six-coordinated by three nitrogen atoms from three distinct

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tib ligands and three terminal water molecules with coordination bond lengths and angles in normal range (Figure 1a, Table 2). On the other hand, each tib ligand in turn links two Zn(II) and one Co(II) atoms to generate an infinite 2D network with typical 63-hcb topology, where the tib ligand is treated as a 3-connector (Figure S4, Supporting Information). In 1, the cationic part adopts an ABAB packing pattern along c axis, and the Zn1 and Co1 atom are located at the 3-fold axis and the Zn2 atom are positioned at the 6-fold axis (Figure 1b). Within the [Zn6(tib)2(1,2,4-BTC)6]6- part, each Zn(II) atom is four-coordinated by three carboxylate oxygen atoms from three different 1,2,4-BTC3- ligands and one nitrogen atom from one tib ligand (Figure 1a). As shown in Figure S5, Supporting Information, the tib and 1,2,4BTC3- ligands are connected by Zn3 and Zn4 to form a 2D double-layered structure, where one layer is defined by Zn3 atoms and the other defined by Zn4 atoms. Three Zn(II) atoms and three 1,2,4-BTC3- units are linked together to generate a C3 symmetric chiral macrocycle with its cavity covered by the tib ligand (Figures S5 and S6, Supporting Information). The anionic part also adopts an ABAB packing pattern along the c axis (Figure 1b). Thus complex 1 is a 2D network in an ABCD packing pattern with the cationic and anionic layers in an alternate arrangement (Figure 1c). The noncoordinated water molecules are filled in the voids between the layers, which link the 2D layers into threedimensional (3D) framework via hydrogen bonds thus consolidate the ultimate structure (Table S1 and Figure S7, Supporting Information). Crystal Structure of [ZnCo(tib)(1,2,4-BTC)(H2O)2]Cl 3 3H2O (2). It is interesting to note that complex 2 shows completely different structure although the reaction conditions for preparation of 1 and 2 are the same except that the pH value for synthesis of 2 is between 5 and 6. Complex 2 crystallizes in orthorhombic chiral space group P212121, which is obviously different the hexagonal P63 one that appeared in 1.15 As shown in Figure 2a, each Zn1 atom in 2 is four-coordinated by one nitrogen atom from tib, two oxygen atoms from two 1,2,4-BTC3- ligands and one terminal water molecule. Each Co1 atom is four-coordinated by one oxygen atom from 1,2,4-BTC3- ligand, two nitrogen atoms from two tib ligands and one terminal water molecule, which is obviously different from that of six-coordinated Co(II) in complex 1. The tib units are linked by Co(II) atoms to form an infinite onedimensional (1D) left-handed helix with a pitch of 7.16 A˚ (Figure 2b-d). The similar helical structure has been observed in [Ag(tib)(PPh3)]CF3SO3 with the connection of tib units and Ag(I) atoms.16 In 2, the helix propagates along a axis generating a cylindrical channel with the diametric dimension of 3.0 A˚, where the chloride anions are accommodated (Figure S8, Supporting Information). The 1,2,4BTC3- ligands are connected by Zn(II) atoms to afford a 1D zigzag chain along c axis (Figure 2b). Each helix is surrounded by two helices and two zigzag chains with the interpenetration of its grooves (Figure 2b). The interconnection of the helices and zigzag chains via metal centers generates a 3D chiral structure of 2 as illustrated in Figure 2b. In 2, the right-handed helix was observed along c direction, which is constructed by the noncoordinated water molecules (O7, O9, O10) and the carboxylate oxygen (O4) atom (Figures 2e and S7). It is of particular interest to study the water chain structure since many fundamental biological processes are depending on the unique properties of water chain, and so far only limited examples with 1D helical water

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Figure 2. (a)ORTEP drawing of 2 with the ellipsoids drawn at 30% probability level; the hydrogen atoms and free water molecules are omitted for clarity. (b) The 3D cationic framework (middle) and 1D chains formed by the tib (up) and 1,2,4-BTC3- ligands (bottom) with metal atoms in 2. (c) The topological representation of complex 2 [red, 1,2,4-BTC3-; green, tib; violet, Co(II); turquoise, Zn(II)]. (d) The homochiral helical chain formed by tib and metal centers along a direction. (e) 1D helical water chain along c direction.

chain have been reported.17 The average O 3 3 3 O distances in the helical chain is 2.72 A˚, which is compatible with that of Ih- and Ic-type ice (2.74 A˚, where Ih and Ic refer to hexagonal and cubic ice, respectively), indicating the existence of strong hydrogen-bonding within the chain.18 Furthermore, the topological analysis was carried for complicate structure of 2 and according to the simplified principle, each Zn(II), Co(II), tib, and 1,2,4-BTC3- can be considered as 3-connector/node by omitting the terminal water molecules (Figure 2c).19 However, the Zn1 and 1,2,4-BTC3-, as well as the Co1 and tib, are the same and the crystallographic deviations are very small,

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thus the framework of 2 is treated as binodal instead of tetranodal.20 Thus, the framework of 2 is a chiral binodal 3-connected net with the Point (Schl€ afli) symbol of (122 3 14)(123), which is obviously different from the chiral 3-connected (10,3)-a topological nets.21 In the (10,3)-a net, the 1D helical chains connected each other to form the resulting 3D framework, while in complex 2, the 1D helical chains and the 1D achiral chains are arranged alternately (Figure 2c). Crystal Structure of [ZnCo(tib)(1,3,5-BTC)Cl] (3). In order to investigate the role of unsymmetric ligand during the chiralization process, the reaction with symmetric ligand 1,3,5-H3BTC was carried out, and a new heterometallic complex [ZnCo(tib)(1,3,5-BTC)Cl] (3) was obtained under wide range pH conditions. As expected, complex 3 crystallizes in centrosymmetric space group P21/c (Table 1). There is no coordinated or noncoordinated water molecule in 3 although it was synthesized in water, which suggests the tight packing of the backbone.22 There are one Co(II) and one Zn(II) in the asymmetric unit of 3, and each metal center is tetrahedrally coordinated (Figure 3a). The chloride anion is terminally coordinated to Zn(II) atom (Figure 3a), while in 2 the chloride located in the voids of framework. The tib ligands are joined together by Co(II) atoms to form a 1D zigzag chain along b axis and the 1,3,5-BTC3- ligands are dimerized by two Zn(II) atoms with the Zn 3 3 3 Zn distance of 3.72 A˚, then such dimers are connected by Co(II) atoms leading to the formation of a 1D ladder structure along a axis (Figure 3b). The ladders are further interconnected by the tib ligands to form a 3D framework (Figure 3b). Similarly, the topological analysis was applied for 3, in which the binuclear Zn(II), Co1, tib and 1,3,5-BTC3- can be regarded as 4-, 4-, 3-, and 3-connected nodes/connectors, respectively. Thus, the resulting structure of 3 is a 3D tetra-nodal (3,4)-connected net with the Point (Schl€ afli) symbol of (6 3 82)2(62 3 83 3 10)(62 3 84)2(62 3 8)2 (Figure 3c). Structural Comparison. Complexes 1 and 2 crystallize in chiral space groups, while 3 crystallizes in a centrosymmetric one, which may be ascribed to the symmetry of the carboxylate ligands. The unsymmetric ligand of 1,2,4-BTC3- in the reaction system of tib, Co(II) and Zn(II) led to the formation of chiral frameworks, while the symmetric one of 1,3,5BTC3- led to the achiral one. Another remarkable difference among complexes 1-3 is the coordination geometries of central Zn(II) and Co(II). In 1, the Zn(II) is four-coordinated with distorted tetrahedral geometry, while the Co(II) is six-coordinated with distorted octahedral geometry. However, the Zn(II) and Co(II) in 2 and 3 are fourcoordinated with distorted tetrahedral geometry. The structural difference between 1 and 2 is resulted from the different pH value of the reaction system.23 In addition, complex 1 is a zwitterion, with cationic and anionic parts, while 2 is a cationic 3D framework with chloride counteranions filled in the voids of helix. Furthermore, it is noteworthy that the chiral blocks of two complexes are completely different, namely 1 belongs to the spatial chirality while 2 is 1D helical chirality. Ferroelectric Property of Complex 1. Since complex 1 crystallizes in a chiral space groups (P63), which is associated with the point groups of C6, one of the ten polar point groups (C1, C2, Cm, C2v, C3, C3v, C4, C4v, C6, C6v), required for ferroelectric behavior, the dielectrical hysteresis loop of complex 1 was recorded at room temperature by using powdered samples in pellets.24 Experimental results indicate that 1 does display ferroelectric behavior. Figure 4a shows the polarization (P) versus applied field (E) plots at room temperature with

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Figure 3. (a) ORTEP drawing of 3 with the ellipsoids drawn at 30% probability level; the hydrogen atoms are omitted for clarity. (b) The 3D framework (middle) and 1D chains formed by the tib (up) and 1,3,5-BTC3- ligands (bottom) with metal atoms in 3. (c) The topological representation of 3 [red, 1,3,5-BTC3-; green, tib; violet, Co(II); turquoise, binuclear Zn(II)].

applying field up to (75 kV/cm. The resulting curve shows an electric hysteresis loop with a remnant electric polarization (Pr) of ∼0.177 μC/cm2 and an electric coercive field (Ec) of ca. 17.68 kV/cm, which is comparable with that of powder sample of MOF [(Tp)2Fe2(CN)6Ni3((1S,2S)-chxn)6](ClO4)4 3 2H2O [Tp = hydrotris(pyrazolyl)borate; (1S,2S)-chnx = (1S,2S)-(þ)-1,2-diaminocyclohexane] (Pr = 0.10 μC/cm2)24d and much larger than that in the previously reported cyanobridged ferroelectric compound (Pr ≈ 0.041 μC/cm2).24a,25

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Figure 4. (a) Polarization versus applied electric field curve (electric hysteresis loop) of 1. (b) Electric coercive field (Ec) versus applied field plot (right) and remnant polarization (Pr) versus applied field plot (left) for 1.

The leakage current is not larger than 10-7 A/cm2, and no abrupt increase was found under the applied field of 100 kV/ cm, which further proved that complex 1 has ferroelectric property (Figure S9, Supporting Information).26 The Pr versus E and the Ec versus E plots at room temperature are shown in Figure 4b. The Pr and Ec values increase when applying a stronger electric field, thus suggesting that the observed P-E hysteresis loop is in the process of increasing the Pr and Ec values. Conclusions In conclusion, two chiral 3d-3d heterometallic MOFs were obtained under different pH conditions. The use of mixed linkers based on different donor groups is a powerful strategy for synthesizing the heterometallic complexes. Supramolecular chirality of these two frameworks is induced by the use of unsymmetric ligand 1,2,4-benzenetricarboxylate ligand, which is confirmed by the comparative study. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant nos. 20731004 and 20721002) and the National Basic Research Program of China (Grant nos. 2007CB925103 and 2010CB923303).

Supporting Information Available: X-ray crystallographic file in CIF format, hydrogen bonding data (Table S1), PXRD (Figures S1 and S2), TGA (Figure S3),crystal structure and topology (Figures S4-S8), and leakage current vs applied field (Figure S9). This material is available free of charge via the Internet at http://pubs. acs.org.

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