Synthesis and Characterization of 3d-3d Homo-and Heterometallic

Nov 12, 2009 - Telephone: +86-25-83593485. Fax: +86-25-83314502. E-mail: [email protected]. Cite this:Cryst. Growth Des. 9, 12, 5190-5196 ...
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DOI: 10.1021/cg900613p

Synthesis and Characterization of 3d-3d Homo- and Heterometallic Coordination Polymers with Mixed Ligands

2009, Vol. 9 5190–5196

Zhi Su, Zheng-Shuai Bai, Jian Fan, Jing Xu, and Wei-Yin Sun* 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 Received June 6, 2009; Revised Manuscript Received September 17, 2009

ABSTRACT: Two new 3d-3d homo- and heterometallic coordination polymers with mixed organic ligands, namely, [Zn2(IBC)(BTC)(H2O)] 3 H2O (1) and [CdCo(IBC)(BTC)(H2O)] (2) [HIBC = 3,5-di(1H-imidazol-1-yl)benzoic acid, H3BTC = 1,3,5-benzenetricarboxylic acid], have been synthesized and fully characterized by X-ray diffraction, IR, elemental analysis, thermogravimetric analysis, photoluminescence, and magnetic measurements. Complex 1 with homonuclear Zn(II) is a rare threedimensional (3D) (3,6)-connected rtl (rutile) framework with [Zn2O2] and [Zn2(OCO)2] two different kinds of binuclear Zn(II) secondary building units (SBUs). While complex 2 with heteronuclear Cd(II) and Co(II) atoms is a 3D framework based on [Cd2O2] and [Co2O2] SBUs. In 2, the SBUs are extended into an infinite eight-connected rare hex-type (hexagonal-type) network. The binuclear [Co2O2] SBU in 2 shows antiferromagnetic interactions. Both 1 and 2 were constructed by the same ligands, however with completely different structures and topologies, the results revealed that the coordination geometries of the metal centers have a subtle influence on the structure of the complexes.

Introduction A great number of metal-organic frameworks (MOFs) with diverse structures and interesting properties have been reported in the past decades.1 Among them, the reported two(2D) and three-dimensional (3D) MOFs have been mainly focused on the homometallic coordination polymers, whereas the chemistry as well as the synthetic strategy toward the heterometallic coordination polymers has received less attention to date,2 and the limited reported heterometallic coordination polymers mainly contain 3d-4f (transition-lanthanide) metal ions, not only because of their intriguing structural diversity and fascinating topologies resulting from the variable and flexible coordination behavior of the lanthanide ions,3 but also for their magnetic, luminescent, nonlinear optical (NLO), and catalytic properties.4-8 However, in the cases of 3d-3d (transition-transition) heterometallic polymers, the design and synthesis are still a challenge to chemists, since the difference of coordination nature (coordination ability, coordination geometry, and so on) between the 3d-3d metal ions is much less than that between the 3d-4f ones. Therefore, the energetic competitions are the key component in the selfassembly process of 3d-3d heterometallic coordination networks.9 In addition, it is also a challenge in obtaining phase pure heterometallic networks using different 3d-3d metal salts. Therefore, the ligand design strategy is very important in the construction of 3d-3d heterometallic polymers. Ligands with different coordination groups, coordination abilities, and/or coordination modes might be desirable to satisfy the central metal ions with different coordination nature as mentioned above. Taking into account of this, asymmetric ligand with different types of Lewis donors, namely, 3,5-di(1H-imidazol-1-yl)benzoic acid (HIBC), was used in this study together with 1,3,5-benzenetricarboxylic acid (H3BTC) as coligand to *Corresponding author. Mailing address: Coordination Chemistry Institute, Nanjing University, Nanjing 210093, China. Telephone: þ86-2583593485. Fax: þ86-25-83314502. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 11/12/2009

build 3d-3d heterometallic coordination polymers for the existence of both N and O donor atoms and the presence of the conjugated aromatic rings and rigid skeletons.10,11 Herein, we report the syntheses, crystal structures, and properties of two novel 3d-3d homo- and heterometallic coordination polymers, namely, [Zn2(IBC)(BTC)(H2O)] 3 H2O (1) and [CdCo(IBC)(BTC)(H2O)] (2) [HIBC = 3,5di(1H-imidazol-1-yl)benzoic acid, H3BTC = 1,3,5-benzenetricarboxylic acid]. The compounds were characterized by elemental, X-ray crystallographic, and thermogravimetric analyses. The topological analysis, magnetism, and photoluminescence properties of the complexes were investigated. Experimental Section All commercially available chemicals and solvents are of reagent grade and used as received without further purification. The compound HIBC was synthesized based on the reported procedures.10e,12 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. FT-IR spectra were recorded in the range of 400-4000 cm-1 on a Bruker Vector22 FT-IR spectrophotometer using KBr pellets. The content of Cd(II) and Co(II) in 2 was determined by measurements of inductively coupled plasma (ICP) on a J-A1100 (Jarrell-Ash, USA) ICP spectrometer. The magnetic measurements in the temperature range of 1.8 to 300 K were carried out on a Quantum Design MPMS7 SQUID magnetometer in a filed of 2000 Oe. Diamagnetic corrections were made with Pascal’s constants. The solid-state absorption spectra of 1 and 2 were recorded on a Shimadzu UV3600 UV-vis-NIR spectrophotometer. The crystalline 1, 2, and ligands were grinded into powder, and the luminescence spectra for the powder solid samples were measured at room temperature on an Aminco Bowman Series2 spectrophotometer with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra the pass width is 5 nm. All measurements were carried out under the same experimental conditions. Preparation of [Zn2(IBC)(BTC)(H2O)] 3 H2O (1). A mixture of ZnCl2 (27.2 mg, 0.2 mmol), H3BTC (21.0 mg, 0.1 mmol), HIBC r 2009 American Chemical Society

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Crystal Growth & Design, Vol. 9, No. 12, 2009 Table 1. Crystallographic Data for Complexes 1 and 2

compound

1

2

empirical formula formula weight temperature/K crystal system space group a/A˚ b/A˚ c/A˚ R/(°) β/(°) γ/(°) V/A˚3 Z Dcalc/(g 3 cm-3) F(000) θ range/(°) reflections collected independent reflections goodness-of-fit on F2 R[I > 2σ(I)]a wR2[I > 2σ(I)]b

C22H16N4O10Zn2 627.13 293(2) triclinic P1 8.1541(9) 10.4047(11) 14.2934(15) 100.014(3) 93.657(2) 105.699(2) 1141.8(2) 2 1.824 632 2.07-25.10 5791 4003 0.964 0.0416 0.1052

C22H14CdCoN4O9 649.70 293(2) triclinic P1 8.827(2) 9.906(3) 13.939(4) 100.010(4) 102.604(4) 110.800(3) 1068.9(5) 2 2.019 642 1.56-25.10 5334 3760 1.039 0.0633 0.1636

a 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.

(25.4 mg, 0.1 mmol), and NaOH (16.0 mg, 0.4 mmol) in 10 mL of H2O was stirred for 10 min in air, and then transferred to a 16 mL Teflon lined stainless steel container and heated at 210 °C for 3 days. After the mixture was cooled to room temperature, colorless block crystals of 1 were obtained (yield: 37%). Anal. calcd for C22H16N4O10Zn2 (%): C 42.13, H 2.57, N 8.93. Found: C 42.21, H 2.49, N 8.99. IR (KBr, cm-1): 3404(s, br), 1622(s), 1578(s), 1519(m), 1431(m), 1373(m), 1348(s), 1195(w), 1107(m), 952(m), 792(w), 760(m), 726(m), 648(w), 584(w). Preparation of [CdCo(IBC)(BTC)(H2O)] (2). The synthetic procedure was similar to that described for the preparation of 1 except using CdCl2 3 2.5H2O (22.8 mg, 0.1 mmol) and CoCl2 3 6H2O (23.7 mg, 0.1 mmol) instead of ZnCl2. Black block crystals of 2 were obtained (yield 63%). The result of ICP revealed that the molar ratio of Cd(II): Co(II) is 1:1. Anal. Calcd for C22H14CdCoN4O9 (%): C 40.67, H 2.17, N 8.62. Found: C 40.69, H 2.11, N 8.58. IR (KBr, cm-1): 3441(vs, br), 1623(s), 1586(s), 1554(s), 1521(m), 1437(m), 1375(m), 1345(s), 1248(m), 1113(m), 1075(m), 1012(m), 938(w), 819(w), 760(m), 648(w), 601(w), 531(w). Crystallography. The crystallographic data collections for 1 and 2 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,13 which were also used for the intensity corrections for the Lorentz and polarization effects. Semiempirical absorption correction was applied using the SADABS program.14 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.15 Hydrogen atoms of IBC- and BTC3- in 1 and 2 were generated geometrically and the hydrogen atoms of water molecules were located directly. The crystallographic details and selected bond lengths and bond angles are provided in Tables 1 and 2, respectively.

Results and Discussion Synthesis and Thermal Stability of the Complexes. Complex 1 was successfully prepared by reaction of the mixed organic ligands with Zn(II) salt under hydrothermal conditions, however, no crystals were obtained by the same method for the reactions of the ligands with Cd(II) or Co(II) salts, respectively. It is interesting that compound 2 with heterometallic centers was obtained by using mixed Cd(II) and Co(II) salts rather than the Cd(II) or Co(II) salt alone. In complex 2, the Cd(II) and Co(II) centers are ligated by more

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complicated coordination modes of BTC3- and IBC- than μ4-bridge of BTC3- and IBC- in 1 (Scheme 1). NaOH was used to neutralize the carboxylic acids, and the complete deprotonation of the carboxylic groups were confirmed by IR spectral data, since no IR bands in the range of 1760-1680 cm-1 were observed in the IR spectra of 1 and 2 (see Experimental Section), as well as by the results of crystallographic analysis (vide infra). TGA were performed to verify the thermal stability of the complexes. As depicted in Figure S1, Supporting Information, a total weight loss of 6.22% was observed for complex 1 in the temperature range of 80-200 °C, which is ascribed to the loss of both coordinated and free water molecules (calcd 5.75%), and the residue is stable up to about 400 °C. For 2, a weight loss of 3.40% was observed in the temperature range of 280-310 °C, which corresponds to the release of the coordinated water molecule (calcd 2.78%), further weight loss was observed at about 430 °C. The results revealed that complexes 1 and 2 have high thermal stability. Crystal Structure of [Zn2(IBC)(BTC)(H2O)] 3 H2O (1). The results of crystallographic analysis provide the direct evidence of the structure of complex 1. As illustrated in Figure 1a, there are two independent Zn(II) centers with different coordination environments in the asymmetric unit of 1. The Zn1 with trigonal bipyramidal coordination geometry is five-coordinated by three carboxylate oxygen atoms (O3, O8A, and O8B) from three different BTC3- ligands, one oxygen atom (O9) from a terminal water molecule, and one imidazole nitrogen atom (N1) from an IBC- ligand. The Zn1-O bond lengths are in the range of 1.974(2)-2.5337(23) A˚ and the Zn1-N one is 1.987(3) A˚ (Table 2). The O3, O8A, and N1 atoms form the equatorial plane and the apical positions are occupied by O8B and O9 with O9-Zn1-O8B bond angle of 173.600(95)°. Comparably, Zn2 atom is tetrahedrally four-coordinated by two carboxylate oxygen atoms (O1A and O2A) and one imidazole nitrogen atom (N4) from three different IBC- ligands, and one carboxylate oxygen atom (O6A) from a BTC3- ligand. The average Zn2-O bond length is 1.951 A˚ and the Zn2-N bond distance is 1.972(3) A˚. On the other hand, each BTC3- ligand in 1 links four Zn(II) atoms using its three carboxylate groups, two adopting μ1-η0:η1 monodentate and one adopting μ2-η2:η0bridging modes, and the IBC- one also links four Zn(II) atoms using its two imidazole groups and a μ2-η1:η1-bridging carboxylate group as shown in Scheme 1a. It is noteworthy that there are two different binuclear Zn(II) secondary building units (SBUs) in 1, namely, [Zn2O2] SBU formed by two Zn1 and two μ2-η2:η0-bridging carboxylate groups of two different BTC3- with a Zn1 3 3 3 Zn1 separation of 3.55 A˚ and [Zn2(OCO)2] SBU formed by two Zn2 and two μ2-η1:η1bridging carboxylate groups of two different IBC- with a Zn2 3 3 3 Zn2 separation of 4.09 A˚. The BTC3- ligands link the [Zn2O2] SBUs, and the IBC- ones connect the [Zn2(OCO)2] SBUs to give 1D chains, respectively, as shown in Figure 1b,c. Such 1D chains are further interconnected by the IBC- and BTC3- ligands to form an ultimate 3D framework as shown in Figure 1d. Much effort has been devoted to the study of framework connectivity and topological analysis, which has been demonstrated to be a useful and simple method to analyze the extended frameworks, especially for the complicated 3D networks.16,17 As far as it is known, the coordination networks with local connectivity higher than six are rare because of the limited coordination numbers of the transition metal

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Table 2. Selected Bond Lengths (nm) and Bond Angles (°) for Complexes 1 and 2a 1 Zn(1)-O(3) Zn(1)-N(1) Zn(1)-O(8)#2 Zn(2)-O(6) Zn(2)-N(4)#4 O(3)-Zn(1)-O(8)#1 O(8)#1-Zn(1)-N(1) O(8)#1-Zn(1)-O(9) O(8)#2-Zn(1)-N(1) O(8)#2-Zn(1)-O(9) O(6)-Zn(2)-O(2)#3 O(2)#3-Zn(2)-N(4)#4 O(2)#3-Zn(2)-O(1)#5

1.974(2) 1.987(3) 2.5337(23) 1.901(2) 1.972(3) 115.12(9) 110.34(10) 96.71(11) 82.108(94) 173.600(95) 121.72(11) 92.65(11) 103.70(10)

Cd(1)-N(3) Cd(1)-O(2)#2 Cd(1)-O(3)#3 Cd(1)-O(5)#1 Co(1)-O(8)#4 Co(1)-O(1)#1 N(3)-Cd(1)-O(6)#1 N(3)-Cd(1)-O(2)#2 O(6)#1-Cd(1)-O(3)#3 O(2)#2-Cd(1)-O(3)#3 O(2)#2-Cd(1)-O(4) O(3)#3-Cd(1)-O(4) N(3)-Cd(1)-O(3) O(4)-Cd(1)-O(3) N(3)-Cd(1)-O(5)#1 O(6)#1-Cd(1)-O(5)#1 O(3)-Cd(1)-O(5)#1 O(8)#4-Co(1)-N(1) O(1)#1-Co(1)-N(1) N(1)-Co(1)-O(9) O(8)#4-Co(1)-O(1)#5 O(1)#1-Co(1)-O(1)#5

2.250(5) 2.342(5) 2.386(5) 2.496(5) 1.919(4) 1.985(4) 149.96(18) 86.84(18) 85.99(17) 166.72(15) 82.85(17) 109.00(16) 88.83(17) 53.30(15) 95.25(17) 54.74(16) 164.40(16) 112.2(2) 114.9(2) 93.6(2) 84.77(18) 75.50(17)

Zn(1)-O(8)#1 Zn(1)-O(9) Zn(2)-O(1)#5 Zn(2)-O(2)#3

1.984(2) 2.064(3) 2.002(2) 1.951(2)

O(3)-Zn(1)-N(1) O(3)-Zn(1)-O(9) N(1)-Zn(1)-O(9) O(3)-Zn(1)-O(8)#2 O(8)#2-Zn(1)-O(8)#1 O(6)-Zn(2)-N(4)#4 O(6)-Zn(2)-O(1)#5 N(4)#4-Zn(2)-O(1)#5

129.03(11) 94.97(10) 101.57(11) 86.580(88) 77.040(85) 129.58(12) 98.33(10) 108.58(12)

Cd(1)-O(6)#1 Cd(1)-O(4) Cd(1)-O(3) Co(1)-N(1) Co(1)-O(9) Co(1)-O(1)#5 O(6)#1-Cd(1)-O(2)#2 N(3)-Cd(1)-O(3)#3 N(3)-Cd(1)-O(4) O(6)#1-Cd(1)-O(4) O(6)#1-Cd(1)-O(3) O(2)#2-Cd(1)-O(3) O(3)#3-Cd(1)-O(3) O(2)#2-Cd(1)-O(5)#1 O(3)#3-Cd(1)-O(5)#1 O(4)-Cd(1)-O(5)#1 O(8)#4-Co(1)-O(1)#1 O(8)#4-Co(1)-O(9) O(1)#1-Co(1)-O(9) N(1)-Co(1)-O(1)#5 O(9)-Co(1)-O(1)#5

2.265(5) 2.402(5) 2.465(4) 2.035(5) 2.085(5) 2.317(4) 91.24(17) 89.07(17) 132.90(18) 76.33(16) 119.65(16) 110.50(15) 82.02(15) 84.80(16) 83.00(15) 129.07(15) 127.7(2) 100.5(2) 97.97(18) 88.05(19) 173.33(17)

2

a Symmetry transformations used to generate equivalent atoms: #1 x, y þ 1, z; #2 2 - x, 1 - y, -z; #3 x þ 1, y - 1, z - 1; #4 x þ 1, y, z - 1; #5 -x þ 1, -y þ 1, -z for 1; #1 x, y - 1, z; #2 -x þ 1, -y þ 2, -z þ 1; #3 -x þ 2, -y þ 2, -z þ 1; #4 x - 2, y - 2, z - 1; #5 -x, -y þ 1, -z for 2.

Scheme 1. The Coordination Modes of BTC3- and IBC- Ligands in Complexes 1 (a) and 2 (b)

ions as well as the steric hindrance of the most commonly used organic ligands, and only some examples involving high coordination number lanthanide metal centers and polynuclear metal-cluster building blocks with uninodal eight-connected nodes have been reported.18 A better insight into the complicating 3D framework 1 can be accessed by topological analysis. As discussed above,

there are [Zn2O2] and [Zn2(OCO)2] SBUs in 1 and if each SBU is considered as a node, the [Zn2O2] SBU connects four BTC3- and two IBC- ligands and the [Zn2(OCO)2] one links two BTC3- and four IBC- ligands (Figure S2a,b, Supporting Information). Therefore, the [Zn2O2] and [Zn2(OCO)2] SBUs can be considered as 6-connectors, respectively. Meanwhile, each IBC- and BTC3- ligand in turn links three SBUs, respectively. Thus, each ligand can be treated as a 3-connector. The simplified 3D framework of 1 is shown in Figure 1e. Furthermore, two [Zn2O2] and [Zn2(OCO)2] SBUs centered 6-connected nodes as well as the IBC- and BTC3- centered 3-connected nodes are almost the same, and the crystallographic deviations are very small, thus, the framework can be treated as binodal rather than tetranodal. According to the simplification principle,19 such special framework features a binodal (3,6)-connected net with its Schl€ afli symbol (4 3 62)2(42 3 610 3 83). The analysis of the vertex symbol and coordination sequence revealed that complex 1 is related to rare rtl (rutile) topology.20 Crystal Structure of [CdCo(IBC)(BTC)(H2O)] (2). It is interesting that complex 2 with a heterometallic center was obtained by mixed CdCl2 3 2.5H2O and CoCl2 3 6H2O instead of ZnCl2 used in 1. As shown in Figure 2a, there are one cobalt(II) and one cadmium(II), one BTC3- and one IBC-, and one coordinated water molecule in the asymmetric unit of 2. The Co(II) atom is five-coordinated by two carboxylate oxygen atoms (O1A and O1B) and a imidazole nitrogen

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Figure 2. (a) The coordination environment of Cd(II) and Co(II) atoms in 2 with 30% probability displacement ellipsoids. Hydrogen atoms are omitted for clarity. (b) 1D ladder-like chain formed by BTC3with Cd(II) and Co(II) atoms. (c) Side (left) and top (right) views of the 2D sheet constructed by IBC- and the center Cd(II) and Co(II) atoms. The green and pink polyhedrons represent Cd(II) and Co(II) atoms, respectively. (d) The 3D packing diagram of 2 (green: IBC-; red: BTC3-). Figure 1. (a) The coordination environment of Zn(II) atoms in 1 with 30% probability displacement ellipsoids. Hydrogen atoms and free water molecules were omitted for clarity. (b) The polyhedral view of the 1D chain formed by BTC3- and Zn(II) in 1. (c) The 1D chain formed by IBC- and Zn(II) in 1. (d) 3D framework of 1 (green: IBC-; red: BTC3-). (e) The rare rtl topology of 1.

atom (N1) from three different IBC- ligands, one carboxylate oxygen atom (O8A) from a BTC3- ligand, and one terminal water molecule (O9) to complete the distorted

trigonal-bipyramidal coordination geometry. The Co1-O bond lengths are in the range of 1.919(4)-2.317(4) A˚ and the Co1-N one is 2.035(5) A˚ (Table 2). The O1A, O8, and N1 atoms form the equatorial plane and the apical positions are occupied by O9 and O1B with a O9-Co1-O1B bond angle of 173.33(17)°. The Cd(II) atom is seven-coordinated by six carboxylate oxygen atoms (O2A, O3A, O3B, O4A, O5, and O6) from four different BTC3- ligands and a imidazole nitrogen atom (N3) from a IBC- ligand to complete a distorted capped-octahedral coordination geometry.

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The Cd1-O bond lengths range from 2.265(5) to 2.496(5) A˚ and the Cd1-N bond distance is 2.250(5) A˚ (Table 2). Comparing the asymmetric units of 1 and 2, it can be clearly seen that the coordination geometries of Co(II) and Cd(II) atoms in 2 are different from those of Zn(II) in 1, which leads to the formation of different 3D frameworks. It is noteworthy that the BTC3- and IBC- ligands adopt distinct coordination modes in 2 compared to those in complex 1 to satisfy the geometric requirements of Co(II) and Cd(II) centers (Scheme 1). In complex 2, each BTC3- ligand connects four metal atoms [three Cd(II) and one Co(II)] to form an infinite 1D ladderlike chain with the carboxylate groups, adopting μ1-η0:η1 monodentate, μ1-η1:η1-chelating, and μ2-η2:η1-bridging coordination modes, as shown in Scheme 1b and Figure 2b. On the other hand, each IBCligand links five metal atoms [two Cd(II) and three Co(II)] to give a 2D network using its two imidazole groups and one carboxylate group adopting a μ3-η2:η1-bridging mode, as depicted in Scheme 1b and Figure 2c. The 2D sheets are further linked together by the ladder-like chains formed by BTC3- ligands to generate a 3D framework (Figure 2d). There are two binuclear centers in complex 2, namely, [Co2O2] and [Cd2O2] bridged by μ2-O1 atoms of IBC- and μ2-O3 atoms of BTC3-, respectively, with the Co1 3 3 3 Co1 and Cd1 3 3 3 Cd1 separations of 3.41 and 3.66 A˚, respectively. From the topological view of complex 2, the 3D structure is built from infinite 1D chains, which are interconnected by BTC3- and IBC- ligands. The infinite chain consists of [Co2O2] and [Cd2O2] SBUs linked by carboxylate groups of BTC3- and IBC- ligands. The infinite chain is stacked in parallel along the [001] direction and interconnected by six neighboring chains, so the vertex of the framework of 2 should be 8-connected (Figure 3a).21 Consequently, the topology of 2 can be regarded as rare hex-type (hexagonaltype) (connecting number is 8) with its Schl€ afli symbol (36 3 418 3 53 3 6), according to the nomenclature defined by Yaghi and O’Keeffe10a (Figure 3b). However, the corresponding complex connectivity results in the lower symmetry of the framework of 2 (P1) compared to an ideal hex framework (P6/mmm) (Figure S3, Supporting Information).18a Similar to the bcg net with uninodal eight-coordinated nodes and Schl€afli symbols (36 3 414 3 57 3 6), however, the difference between the hex and bcg net (Figures S3 and S4, Supporting Information) is the packing diagram of the planes formed by eight-connected nodes. In bcg (Figure S4, Supporting Information), the planes adopted an -ABAB- packing diagram, while an -AAAA- packing diagram in hex (Figure S3, Supporting Information). Another striking feature in complex 2 is the heterometallic centers of Co(II) and Cd(II) with a ratio of 1:1, according the results of crystallographic analysis and ICP. As it is known, a series of 3D heterometallic 3d-4f MOFs were synthesized with designed organic ligands, and only a few heterometallic 3d-3d polymers were reported.9 The complex 2 is the first heterometallic 3d-3d example with the rare hex-topology, which would enrich the heterometallic coordination polymers and will bring us a new synthetic approach to access the heterometallic polymers. The different structures of complexes 1 and 2 may be caused by the coordination preferences of the different metal ions. The four- and five-coordinated Zn(II) atoms in 1 have tetrahedral and trigonal bipyramidal coordination geometries, respectively, while the Co(II) and Cd(II) atoms in 2 adopted five-coordinated trigonal bipyramidal and

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Figure 3. (a) The infinite chain in 2 containing [Cd2O2] and [Co2O2] SBUs (pink: Co; green: Cd) (left). Perspective (middle) and schematic (right) views of the eight-connected node in the framework 2. (b) Schematic view of the hex topology of 2.

seven-coordinated capped-octahedral geometries, respectively. The coordination environments of Zn1 in 1 and Co1 in 2 are similar with one imidazole nitrogen and three carboxylate oxygen atoms and one water molecule due to their similar coordination nature.22 However, the radius of Cd(II) is larger than that of Zn(II), resulting in higher coordination numbers than that of Zn(II), and inducing the different coordination modes of IBC- and BTC3- in 2, and leading to the different structures of 1 and 2.23 Absorption and Photoluminescence Properties of Complexes 1 and 2. The mixed inorganic-organic hybrid coordination polymers, especially for d10 metal ions, have been investigated for potential photoactive materials.24 Therefore, in the present work, the absorption and photoluminescence properties of complexes 1 and 2 were studied in the solid state at room temperature. Both 1 and 2 display intense absorption bands with maxima around 260 and 360 (shoulder) nm, which can be assigned to the spin-allowed π-π* intraligand transitions.25 The absorption maxima at ca. 550 and 605 nm detected in 2 are due to the d-d transitions.26 The results of photoluminescent study showed that emissions were observed for complex 1 and free H3BTC ligand as shown in Figure S5, Supporting Information, however, no obvious luminescence was observed for complex 2 as well as the HIBC ligand under the experimental conditions. Intense emission was observed at 410 nm for 1 with excitation at 358 nm, which may be tentatively assigned

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(H3BTC) ligands. In addition, photoluminescence in the solid state at room temperature and magnetic properties of complexes were studied. The results revealed that the coordination mode of the carboxylate groups and the coordination geometry of central metals play an important role in the structure construction. Complex 1 is homometallic polymer with rare rtl topology. The synthesis of complex 2 with 3d-3d heterometallic centers and rare hex-topology brings us a new synthetic approach to access heterometallic polymers.

Figure 4. Temperature dependences of magnetic susceptibility χM and χMT for 2. The solid lines represent the fitted curve. 3-

to the intraligand fluorescence of BTC , since a similar emission band at 395 nm was detected for the H3BTC ligand (Figure S5, Supporting Information). The red-shifted emission of 1 was considered to mainly originate from the influence of the coordination of metal atom to the ligand.11,21b,27 Magnetic Property of Complex 2. The temperature dependence of magnetic susceptibilities of 2 was studied from 300 to 1.8 K with a 2 kOe applied magnetic field. The χM and χMT vs T curves for 2 are shown in Figure 4. The χMT value at 300 K is 6.057 emu K mol-1 for 2, which is much higher than the two isolated spin-only Co(II) atoms of 3.75 emu K mol-1 due to spin-orbital coupling of Co(II) in 2. Along with the temperature decrease, the χMT values decrease slowly and then more rapidly below 35 K to reach a value of 3.880 emu K mol-1 for 2 at 1.8 K. The behavior of the χM versus T and the shape of the χMT versus T curve are the characteristic of the occurrence of weak antiferromagnetic interactions between the adjacent Co(II) centers. As it is known, the magnetic analysis for the Co(II)containing compounds is rather complicated because of its spin-orbital coupling, and some approximate methods are often applied to analyze the magnetic interactions between Co(II) ions.28 In this work, the main magnetic interactions may be considered to occur between adjacent Co(II) ions bridged by the O atom of the carboxylate group, and the exchange interactions between the Co(II) ions bridged by IBC- must be very weak because of the long Co 3 3 3 Co separations about 9.91 A˚. An attempt was made to fit the magnetic susceptibility data evaluating that the magnetic coupling (J) between the neighboring Co(II) centers linked by O atoms and the interbinuclear interactions (zJ’) through the IBC- and BTC3- ligands (Scheme S1, Supporting Information). The spin Hamiltonian is expressed as follows: 2 P 2  ¼ -2J S^1 S^2 ¼ -JðS^2 S^ Þ29 H T

i ¼1

The best fit was obtained with values of J = -0.23473 cm-1, g = 2.45032, and zJ0 = -0.18108 cm-1. The agreement factor R, defined as Σ[(χMT)obsd - (χMT)calcd]2/Σ(χMT)2, is equal to 2.0  10-5. The negative J and zJ0 indicate the antiferromagnetic coupling interactions within and between the [Co2O2] binuclear unit, and the interaction between the [Co2O2] binuclear unit is rather weak. Conclusions Two novel 3d-3d homo- and heterometallic coordination polymers was synthesized with mixed 3,5-di(1H-imidazol1-yl)benzoic acid (HIBC) and 1,3,5-benzenetricarboxylic acid

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, TGA curves of complexes 1 and 2 (Figure S1), crystal structure and topology (Figures S2-S4), photoluminescence spectra for 1 (Figure S5), and spin topology for 2 (Scheme S1). This information is available free of charge via the Internet at http:// pubs.acs.org.

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