Structural diversity of six metal–organic frameworks from a rigid bisimidazole ligand and their adsorption of organic dyes

Six metal–organic frameworks, namely, [Cd2(odc)2(2,6-bin)2]$(CH3)2NH (1), [Cd2(tdc)2(2,6-bin)2(H2O)2] (2), [Cd2(bzdc)2(2,6-bin)2]$4DMF (3), [Cd2(hfdc)2(2,6-bin)2]$H2O (4), [Cd3(tpo)2(2,6-bin)3(H2O)4]$2DMF$2H2O (5) and [Zn3(btb)2(2,6-bin)]$8DMF (6) {2,6-bin 1⁄4 2,6-bisimidazoylnaphthalene, H2odc 1⁄4 4,40oxybisbenzoic acid, H2tdc 1⁄4 thiophene-2,5-dicarboxyl acid, H2bzdc 1⁄4 benzophenone-4,40-dicarboxylic acid, H2hfdc 1⁄4 2,20-bis(4-carboxyphenyl)hexafluoropropane, H3tpo 1⁄4 tris-(4-carboxylphenyl)phosphine oxide and H3btb 1⁄4 4,40,400-benzene-1,3,5-triyl-tribenzoic acid} have been synthesized and structurally characterized. Compound 1 exhibits a three-fold interpenetration of 4T25 network. Compound 2 presents a five-fold interpenetration of dia network. Compound 3 displays a 2D / 2D three-fold interpenetration of sql layers. Compound 4 features a 2D / 3D parallel polycatenation of sql layers. Compound 5 exhibits an unusual 3,4,6-connected self-catenated network of 3,4,6T206 topology. Compound 6 shows a rare 3,8-connected self-catenated network of 3,8T72 topology. Their adsorption behaviors to organic dyes have been studied.


Introduction
Organic dyes have been widely used in medicines and the textile, paper, and printing industries, and the polluted water from dyes poses a signicant threat to the environment and human health. 1 Among various techniques for the removal of water pollutants, adsorption is the most attractive method owing to its simplicity and efficiency. 2 Consequently, numerous adsorption materials have been extensively investigated, including carbon nanotubes, clay, zeolites, sol-gel adsorbents and polymer resins. 3 However, these adsorbents oen face problems like low adsorption capacity, slow adsorption kinetics, complex preparation process, and chemical or thermal instability. The development of an effective adsorbent that possesses good adsorption ability and selectivity toward dye removal is still a challenge.

Experimental section
All the starting materials were of analytic grade and used as received without further purication. Elemental analyses (C, H, N) were performed with a Perkin-Elmer 240c elemental analyzer. TGA was performed on a Perkin-Elmer TG-7 analyzer heated from 30 to 800 C under nitrogen. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D2 Phaser. UV-vis adsorption spectra were collected on a Shimadzu UV-3101PC spectrophotometer to monitor the adsorption progress.

X-ray crystallography
Single-crystal XRD data for compounds 1-6 were recorded on a Bruker Apex II diffractometer with graphite monochromatized Mo Ka radiation (l ¼ 0.71073Å) at 285(2) K. Absorption corrections were applied using the multiscan technique. All the structures were solved by Direct Method of  and rened by the full-matrix least-squares techniques by using the SHELXL-97 (ref. 8) program within WINGX. No-hydrogen atoms were rened with anisotropic temperature parameters. The hydrogen atoms of the organic ligands were rened as rigid groups. For the high vibration, the disordered Ow1 in compound 4 was rened with a total occupancy of 1 and handled by isotropic renement. The hydrogen atoms of water molecules in compounds 2, 4, 5 are not added. It should be noted that the guest molecules in 6 are highly disordered and could not be modeled properly so the diffused electron densities resulting from them were removed by the SQUEEZE routine in PLATON. 9 The detailed crystallographic data and structure renement parameters for 1-6 are summarized in Table 1.

Results and discussion
Structure description of 1 Compound 1 crystallizes in a monoclinic space group P2 1 /n. In the asymmetric unit, there exists one crystallographically unique Cd(II) atom, one odc 2À anion, two half 2,6-bin ligands and one free (CH 3 ) 2 NH. As shown in Fig. 1a, the Cd(II) atom is ligated by four oxygen atoms from two odc 2À anions (Cd-O, 2.2854 (16)   bond angles are within the normal range (Table S1 in the ESI †). The odc 2À anion connects two Cd(II) atoms through two chelate carboxlylates. The 2,6-bin ligand acts as a linker to bridge two Cd(II) atoms. In compound 1, every two odc 2À anions are connected by Cd(II) atoms to give a 1D helical chain with a pitch of 24.54Å, corresponding to the double length of b axis. Two such chains interweave together to assemble a double helix (Fig. 1c). These helixes are bridged by the 2,6-bin to build a 3D framework (Fig. 1b). Based on the concept of topology, 10 the framework can be simplied into a 4-connected 4T25 network, with the point symbol of (6 5 $8) (Fig. 1d). The potential voids are lled via mutual interpenetration of two independent equivalent networks, giving a rise to a 3-fold interpenetrating network (Fig. 1e). The 4T25 features self-catenation, 11 which is detected as catenation of strong 10-rings and 14-rings of the same net, when odc 2À ligand is presented by 2-coordinated node to avoid edge-crossings ( Fig. S7 †). The catenation in the net originates from interweaving of the helical chains [Cd(odc)]. The same topology was found before only in one compound, [Zn(tpdc)(bpmp)] (H 2 tpdc ¼ 1,1 0 :3 0 ,1 00 -terphenyl-4,4 00 -dicarboxylic acid; bpmp ¼ 1,4-bis(3-pyridylmethyl)piperazine). 12

Structure description of 2
Compound 2 crystallizes in a monoclinic space group P2 1 /c. In the asymmetric unit, there exists one crystallographically unique Cd(II) atom, one tdc 2À anion, two half 2,6-bin ligands and one coordinated water molecule. As shown in Fig. 2a  network, the potential voids are lled via mutual interpenetration of the other four independent equivalent frameworks, generating a ve-fold interpenetrating architecture (Fig. 2d).

Structure description of 3
Compound 3 crystallizes in a monoclinic space group P2 1 /c. In the asymmetric unit, there exists one crystallographically unique Cd(II) atom, one bzdc 2À anion, two half 2,6-bin ligands and two uncoordinated DMF molecules. As shown in Fig. 3a, the Cd(II) atom is coordinated by four oxygen atoms from two bzdc 2À anions (Cd-O, 2.2787(18)-2.4132(18)Å) and two nitrogen atoms from two 2,6-bin ligands (Cd-N, 2.238(2)-2.256(2)Å). The Cd(II) to O/N distances and bond angles are within the normal range (Table S3 in the ESI †). The bzdc 2À anion connects two Cd(II) atoms through two chelate carboxylates. The 2,6-bin ligands act as linkers to join two Cd(II) atoms. In compound 3, the Cd(II) atoms are bridged by organic ligands to assemble a 2D undulated framework of sql topology (Fig. 3b). The neighboring Cd(II) atoms are separated by distances of 15.59Å, 16.28Å (through 2,6-bin) and 14.86Å (via bzdc 2À ). The void space in the single framework is so large that three identical frameworks having one middle plane catenate each other to form a 2D / 2D polycatenane of frequently observed pattern, which can be described by 8L23 extended ring net (Fig. 3d). 13 For the mean planes of these three layers are parallel and coincident, we can see that along the a axis there still exist 1D open channels, which are occupied by the uncoordinated DMF molecules (Fig. 3e). The void space accounts for approximately 32.7% of the whole crystal volume as obtained by PLATON analysis.

Structure description of 4
Compound 4 crystallizes in a triclinic space group P 1. In the asymmetric unit, there exists one crystallographically unique Cd(II) atom, one hfdc 2À anion and two half 2,6-bin ligands. As shown in Fig. 4a, the Cd(II) atom is coordinated by three oxygen atoms from two hfdc 2À anions (Cd-O, 2.2175(17)-2.403(3)Å) and two nitrogen atoms from two 2,6-bin ligands (Cd-N, 2.235(2)-2.245(2)Å). The Cd(II) to O/N distances and bond angles are within the normal range (Table S4 in the ESI †). The hfdc 2À anion connects two Cd(II) atoms by one monodentate and one chelate carboxylates. The 2,6-bin ligands act as linkers to bridge two Cd(II) atoms. In compound 4, the Cd(II) atoms are joined by organic ligands to assemble a framework of sql topology (Fig. 4b). Within the framework, the neighboring Cd(II) atoms are separated by distances of 15.53Å, 15.57Å (through 2,6-bin) and 14.68Å (by hfdc 2À ). In order to minimize the space vacuum and stabilize the framework, each undulated layer catenates with two others from above and below. Although the catenated nets in 4 also have parallel mean planes, different from 3, these mean planes are not coincident and the neighbors are separated by distances of 4.53Å, thus giving a 2D / 3D polycatenane. This polycatenated system can be described by extended ring net with topology hex frequent for 2D coordination polymers (Fig. 4c and d). 13 Structure description of 5 Compound 5 crystallizes in a triclinic space group P 1. The asymmetric unit contains one and a half Cd(II) atoms, one tpo 3À anion, one and a half 2,6-bin, two coordination water molecules, one lattice water molecule and one lattice DMF molecule. As shown in Fig. 5a, there are two crystallographically Cd(II) atoms in compound 5. Both Cd1 and Cd2 are six-coordinated in distorted octahedral coordination geometries, but their coordination environments are entirely different. Cd1 is coordinated by two nitrogen atoms from two 2,6-bin ligands (Cd-N, 2.262 (3) (Table S5 in the ESI †). The tpo 3À anion links four Cd(II) atoms by two monodentate and one monodentatebridging carboxylates. Cd1 and its symmetry-related Cd1A are bridged by two m 2 -O3 atoms from the monodentate-bridging carboxylates into a binuclear Cd 2 unit, with a Cd1/Cd1A distance of 3.65Å and a Cd1-O3-Cd1A angle of 99.68 . Each binuclear unit is surrounded by eight organic ligands (four 2,6bin and four tpo 3À ), each Cd2 atom coordinates four organic ligands (two 2,6-bin and two tpo 3À ), and each tpo 3À anion connects one Cd2 atom and two binuclear units. Topologically, the tpo 3À anions can be considered as 3-connected nodes, Cd2 atoms can be viewed as 4-connected nodes, and the binuclear units can be reduced to 6-connected nodes (Fig. 5b). Therefore, the whole 3D framework can be simplied as a (3,4,6)- connected net of 3,4,6T206 topology (Fig. 5c). To the best of our knowledge, there is only one coordination polymer of this topology: {[Co 3 (L)(BPY) 1.5 ]$H 2 O} n (CSD RefCode EWUHUB: H 3 L ¼ 4-methyoxy-3,5-bis(4-carboxyphenyl)benzoic acid, BPY ¼ 4,4 0bipyridine). 6c The 3,4,6T206 net has a peculiar property of selfcatenation. This property originates from mixing ligands of different types into one framework. The tricarboxilate tpo 3À ligand forms 2D subnet (partial composition [Cd 3 (tpo) 2 ]) of 3,4c bex topology, which is very common for 2D coordination polymers. 14 The 2D subnets pack parallel to each other, and 2,6bin ligands steach them into 3D framework passing through strong 6-rings of the bex layers (Fig. 5d).
Thermal analysis. To characterize the thermal stabilities of compounds 1-6, their thermal behaviors were investigated by TGA (Fig. S8 in the ESI †). The experiments were performed on samples consisting of numerous single crystals powder of 1-6 under nitrogen atmosphere with a heating rate of 10 C min À1 . For 1, the weight loss in the range of 25-270 C is ascribed to release of dimethylamine molecules (obsd 3.56%, calcd 3.46%). The decomposition of the framework occurs at ca. 380 C. Compound 2 is stable up to ca. 240 C. For 3, the weight loss in the range of 130-320 C is attributed to release of DMF molecules (obsd 16.9%, calcd 18.6%). The decomposition of the framework occurs at ca. 350 C. Compound 4 is stable up to ca. 360 C. For 5, the weight loss in the range of 25-230 C is attributed to release of DMF and water molecules (obsd 11.3%, calcd 11.6%). The framework is stable up to ca. 300 C. For 6, the weight loss in the range of 25-240 C is attributed to the release of DMF molecules (obsd 30.5%, calcd 30.6%). The framework is stable up to ca. 340 C.

Adsorption capability towards methyl orange
The capture abilities of 1-6 towards to methyl orange (MO) were evaluated. The adsorption experiments were run as follows: 5 mg of host material was immersed to 10 mL of MO aqueous solutions (4 Â 10 À5 mol L À1 ), respectively. The clear solution aer centrifuging was measured by UV/vis spectra to monitor the MO concentration at different time intervals. As shown in Fig. 7, for compounds 1-5, within 10-30 minutes, the adsorption efficiencies of MO reached 99.3% for 1, 97.9% for 3, 93.0% for 4 and almost 100% for 2 and 5. Obviously, these compounds exhibit highly efficient adsorption to MO. Comparatively, compound 6 presents interesting adsorption behavior to MO: at 20 minutes, the adsorption of MO reached a maximum of 60.3%; while, at 30 minutes, the value of adsorption decreased to 16.3%, which means the desorption of MO from compound 6; till to 60 minutes, the value of adsorption was stable at 55.4%.
From the structures above, we can nd that only compounds 3 and 6 exhibit open frameworks, but the channels with minor window size are hard to trap MO molecules. Thus, the adsorption of 1-6 occurs mainly due to the surface interactions. In compound 6, the Zn 3 cluster is surrounded by six bis(monodentate) carboxylates and the axial position of metal clusters are occupied by 2,6-bin ligands, which blockages the accessment of MO molecules. Thus, the adsorption is mainly ascribed by the p-p conjugations between benzene rings from host framework and MO molecules. 16 Such weak interactions induce the spontaneous desorption of MO molecules in the process of adsorption. While, in compounds 1-5, besides the p-p conjugation effects, the bigger ionic radius of Cd(II) is favorable to the electrostatic interaction between central metal ions and anionic MO molecules. 17 Such synergistic effect denitely improves the adsorption ability of host materials.
In order to conrm the mechanism of MO adsorption for 1-6, the adsorption-desorption cycle tests were evaluated. The release tests were carried out with methanol as the elution solvent. The MO-uptaked 1-6 were dispersed in 10 ml methanol by sonication for 5 min and centrifuged. Aer 5 times, the combined solution was measured by UV/vis spectra to monitor the MO concentration. The results revealed that the adsorbed MO molecules can be released almost completely (Fig. S9-S14, ESI †). Obviously, the weak surface interactions due to the p-p conjugation effects favor the release of MO molecules from host frameworks in methanol. The PXRD patterns of MO-released 1-6 indicate the stability of crystalline sample (Fig. S1-S6 in ESI †). Meanwhile, the MO-released 1-6 still show good reusability of MO adsorption (Fig. S9-S14, ESI †). The adsorption capacities of 1-6 towards cationic dyes methylene blue (MB) and rhodamine B (RhB) were also evaluated. As shown in Fig. S15-S20 (ESI), † the UV/vis adsorption spectra showed that these compounds almost do not exhibit adsorption of MB and RhB, in spite of their larger rigid aromatic cycle comparing to MO molecule. The results further demonstrated that the electrophilicity of central metal ions plays an important role for the adsorption of anionic dye.

Conclusions
In this study, based on the mixed-ligand strategy, six new MOFs have been synthesized an structurally characterized. Compound 1 exhibits a three-fold interpenetration of 4T25 network. Compound 2 presents a ve-fold interpenetration of dia network. Compound 3 displays a 2D / 2D three-fold interpenetration of sql layers. Compound 4 features a 2D / 3D parallel polycatenation of sql layers. Compound 5 exhibits an unusual 3,4,6-connected self-catenated network of 3,4,6T206 topology. Compound 6 shows a rare 3,8-connected selfcatenated network of 3,8T72 topology. The result of this study demonstrates that the rational selection of organic ligands with specic rigidity and length is an effective approach to the construction of entangled motifs. Almost all of these compounds exhibit selective adsorption to anionic methyl orange, which demonstrates that the surface physicochemical interactions including electrostatic interactions and p-p conjugation effects play a signicant synergistic effect in the adsorbing dyes. These compounds not only ll the aesthetic diversity of coordinative network chemistry, but also may provide a potential way to the selective design of versatile network-based materials.

Conflicts of interest
There are no conicts to declare.