Syntheses, structural diversity, magnetic properties and dye absorption of various Co Ĳ II ) MOFs based on a semi-flexible 4-(3,5-dicarboxylatobenzyloxy)benzoic acid †

Five novel Co Ĳ II ) metal – organic frameworks (MOFs) constructed from semi-flexible 4-(3,5-dicarboxylatobenzyloxy)benzoic acid (H 3 L), namely {[Co 1.5 Ĳ HL) Ĳ 4,4 ′ -bidpe) 2 Ĳ H 2 O)] · 3H 2 O} n ( 1 ), {[Co 3 Ĳ L) 2 Ĳ 4,4 ′ -bibp) 3 Ĳ μ 2 -O) 2 ] · 2H 2 O} n ( 2 ), {[Co Ĳ HL) Ĳ 1,3-bitl)] · (1,4-Diox)} n ( 3 ), [Co 2 Ĳ HL) 2 Ĳ 3,5-bipd) 2 ] n ( 4 ), and {[Co Ĳ HL) Ĳ tib)] · 0.5H 2 O · NMP} n ( 5 ) (4,4 ′ -bidpe = 4,4 ′ -bis Ĳ imidazolyl)diphenyl ether, 4,4 ′ -bibp = 4,4 ′ -bis Ĳ imidazol-1-yl)biphenyl, 1,3-bitl = 1,3-bis Ĳ 1-imidazoly)toluene, 3,5-bipd = 3,5-bis Ĳ 1-imidazoly)pyridine, and tib = 1,3,5-tris Ĳ 1-imidazolyl)benzene), were synthesized under solvothermal conditions and further characterized by elemental analysis, IR spectra, powder X-ray diffraction (PXRD), thermogravimetric (TG) analysis and single-crystal X-ray diffraction. Different architectural topologies have been generated by adjusting the N-donor ligands. Single-crystal X-ray diffraction analysis reveals that complex 1 shows a rare 1D → 2D polyrotaxane network. Complex 2 possesses an unprecedented 2-nodal (3,10)-connected 3D framework with a Schläfli symbol of (4 3 ) 2 Ĳ 4 6 · 6 32 · 8 3 ) Ĳ 4 3 ) 2 . When the 2-connected points (H 3 L and 1,3-bitl ligands) are not calculated, complex 3 shows a hcb uninodal 3-connected 2D network with the Schläfli symbol (6 3 ), which further constructs a 3D supramolecular structure through O – H ⋯ O hydrogen bonds; while the 2-connected points are taken into account, complex 3 exhibits an unprecedented 3-nodal (2,2,4)-connected network. Complex 4 presents an unprecedented 2-nodal (3,5)-connected 3D framework with a (4 · 6 2 ) Ĳ 4 · 6 6 · 8 3 ) topology, while complex 5 exhibits another unprecedented 2-nodal (3,5)-connected 2D framework with a (4 2 · 6 7 · 8) Ĳ 4 2 · 6) Schläfli symbol and shows 2D → 3D supramolecular structure through O – H ⋯ O hydrogen bonds. Meanwhile, the magnetic properties of complexes 2 and 4 are discussed. Moreover, the dye adsorption and mechanism studies indicate that the pore size, the uncoordinated O atoms in carboxyl groups and the uncoordinated carboxyl groups of the MOFs have significant effects on the dye adsorption capacity.


Introduction
In recent years, the construction and research of metal-organic frameworks (MOFs) have attracted significant research attention in materials science due to their diverse topological structures 1-3 and their promising applications in luminescence, 4-6 magnetism, 7,8 gas storage, 9,10 ion exchange, 11 molecular sensing and separation, 12 catalysis, 13,14 dye adsorption 15,16 and so on. 17 Particularly, the removal of organic dyes (e.g. methylene blue, rhodamine B, acriflavine hydrochloride, Congo red, brilliant green and methyl orange) from wastewater is becoming a significant application for MOFs depending on the pore shape, and this aspect attracts considerable attention of the majority of chemists. 18 Dyes are very difficult to degrade because they are highly stable to light and oxidation. 19 For example, Li's group has reported that Co-based MOFs assembled using 5-(pyridine-4-yl)isophthalic ligands show a good dye adsorption capacity based on appropriate pores. 19a Tamara L. Church's group synthesized an amino-functionalized MOF (amino-MIL-101ĲAl)), which shows an excellent MO (methyl orange) adsorption capacity. 20 Therefore, we can conclude that both pore size and functional groups can affect the adsorption capacity. However, it is still a challenging work to assemble the porous MOFs influenced by host-guest interactions. Meanwhile, there are still quite a few examples that the uncoordinated oxygen atoms or uncoordinated carboxyl groups of MOFs act as functional groups in dye removal.
It is well known that the structures of the organic ligands play a critical role in the pore size of the coordination polymers, which further affects the adsorption ability. Therefore, we selected semi-flexible triscarboxylates (4-(3,5dicarboxylatobenzyloxy)benzoic acid) with five different imidazole bridging linkers as N-donor ligands in the mixed-ligand system based on the following features: i) the free rotation of -CH 2 -Ocan promote the flexibility of the triscarboxylate ligands to meet the requirements of coordination geometries of CoĲII) ions for constructing the final structure, ii) the suitable size of carboxylate ligands makes it a feasible candidate to generate MOFs with an appropriate window, iii) the three carboxyl groups of the carboxylate ligands could exhibit more abundant coordination mode while coordinating with CoĲII) ions, and iv) the location of three carboxyl groups makes it easier to generate uncoordinated oxygen atoms or uncoordinated carboxyl groups. Meanwhile, the different configurations and flexibility of five auxiliary N-donor ligands can be used to construct a series of intriguing frameworks with different pore sizes.

Synthesis and characterization
In this work, the synthesis of complexes 1-5 was constructed from H 3 L and the related CoĲII) salt in the presence of five N-donor (4,4′-bidpe, 4,4′-bibp, 1,3-bitl, 3,5-bipd and tib) bridging linkers under solvothermal conditions. These complexes, 1-5, are stable in the solid state upon extended exposure to air. Meanwhile, all of the complexes 1-5 have poor solubility in water and common organic solvent, but can be slightly soluble in very high polarity solvents.
Single-crystal X-ray diffraction shows that compound 1 crystallizes in the triclinic space group P1. In the asymmetric unit, there exist one and a half CoĲII) centers (two types of CoĲII) ions (Co1 and Co2); occupancy ratio: one for Co2 and a half for Co1), one HL 2− anion, two 4,4′bidpe ligands and one coordinated water molecule. As   shown in Fig. 1a, the Co1 atom is six-coordinated, coordinated by four N atoms from four independent 4,4′-bidpe ligands and two O atoms from two coordination water molecules, displaying a slightly distorted octahedral coordination geometry. Meanwhile, the Co2 atom is fourcoordinated by two O atoms from two independent HL 2− ligands and two N atoms from two different 4,4′-bidpe ligands, forming a slightly distorted tetrahedral coordination geometry. The bond lengths of Co-O and Co-N vary between 1.941Ĳ2)-2.118Ĳ2) Å and 2.012Ĳ3)-2.159Ĳ3) Å, respectively.
Structure description of [Co 2 ĲHL) 2 Ĳ3,5-bipd) 2 ] n (4) In order to investigate the effect of auxiliary ligand configuration on the final structure, the 3,5-bipd ligand was employed in the mixed system. And another framework 4 was obtained. It crystallizes in the triclinic space group P1, and the asymmetric unit consists of two different types of CoĲII) ions (Co1 and Co2), two HL 2− ligands and two 3,5-bipd ligands. As shown in Fig. 4a, the environment around Co1 can be described as a slightly distorted octahedral geometry, coordinated by three O atoms from two different HL 2− ligands and three N atoms from three independent 3,5-bipd ligands. Interestingly, Co2 shows the same coordination environment as Co1. The Co-O bond distances are in the range of 2.016Ĳ3)-2.387Ĳ4) Å, and the Co-N bond distances are in the range of 2.079Ĳ3)-2.262Ĳ3) Å, respectively.

Structural comparison and discussion
As is well known, the flexibility of the carboxylate ligands, the central metals and the configuration of N-donor ligands play a key role in the construction of the structures of MOFs. As shown in Scheme 2 and Fig. S4, † H 3 L exhibits versatile coordination modes and different dihedral angles between the two phenyl rings in H 3 L, which further results in a series of novel topologies. Although complexes 1, 3 and 5 exhibit the same coordination mode (μ 2 -(κ 1 -κ 0 )-(κ 1 -κ 0 )), the dihedral angle of these complexes are different (42.17Ĳ3)°for 1, 68.72Ĳ2)°f or 3 and 61.19Ĳ2)°for 5). Meanwhile, compared with 3 and 5, the two coordinated carboxyl groups are obtained from two different phenyl rings in 1. In complex 2, the H 3 L ligands show a completely deprotonated mode (μ 4 -(κ 1 -κ 1 )-(κ 1 -κ 0 )-(κ 1κ 0 )) with a 22.90Ĳ1)°dihedral angle. In complex 4, the H 3 L ligands show a different partially deprotonated coordination mode (μ 2 -(κ 1 -κ 1 )-(κ 1 -κ 0 )), and the dihedral angle between two phenyl rings is the biggest one (75.98Ĳ3)°). Interestingly, both the biggest (4) and the smallest (2) dihedral angle of the H 3 L ligands show a 3D framework through connecting with N-donor ligands. Therefore, we considered that the flexibility of H 3 L ligands could deeply influence the final architecture.
As is well known, the different lengths and configurations of N-donor ligands greatly influence the final structure. 27 Except for 1 being a semi-flexible-auxiliary ligand, the N-donor ligands can be seen as rigid-auxiliary ligands in complexes 2-5. Therefore, only complex 1 shows a rare 1D → 2D polyrotaxane network, and complexes 2-5 show an unprecedented 3D framework, a 2D network, a 3D unprecedented framework and a 2D unprecedented network, respectively. Meanwhile, influenced by the configuration of the N-donor ligands, complexes 1, 3, 4 and 5 have one uncoordinated carboxyl groups, while complex 2 have uncoordinated oxygen atoms. Moreover, the pore sizes of complexes 1-5 are different due to the configuration of the N-donor ligands. Thus, complexes 1-5 can be used to investigate the adsorption capacity, which is affected by their different structures.

Thermal analyses
To estimate the thermal stabilities of compounds 1-5, their thermal behaviors were investigated by thermogravimetric analyses (TGA) under a N 2 atmosphere at a heating rate of 10°C min −1 (Fig. S5 †). The TG curves of 1 and 2 exhibit a good resemblance. The first weight loss was from room temperature to 240°C for 1 (obsd: 6.59%, calcd: 6.67%) and from 100-240°C for 2 (obsd: 2.02%, calcd: 2.08%), corresponding to the removal of the lattice H 2 O and/or coordinated H 2 O molecules. The second weight loss corresponds to the loss of the organic ligands at ca. 335°C for 1 and ca. 270°C for 2. The remaining weight corresponds to the formation of CoO (obsd: 32.65% calcd: 10.42% for 1; obsd: 42.49%, calcd: 13.02% for 2). There is no lattice solvent molecule in 3 and 4, therefore, the TG curves exhibit plateaus before 305°C for 3 and 360°C for 4, and then the pyrolysis of the framework took place. The remaining weight corresponds to the formation of CoO (obsd: 29.19%, calcd: 10.94% for 3; obsd: 28.26%, calcd: 12.84% for 4). For complex 5, the first weight loss in the temperature range of 180-240°C is consistent with the removal of the lattice NMP and H 2 O molecules (obsd: 12.42%, calcd: 12.82%). The second weight loss corresponds to the loss of the organic ligands at ca. 350°C. The remaining weight corresponds to the formation of CoO (obsd: 37.31%, calcd: 9.90%). The larger CoO remaining weight (obsd) may be caused by the incomplete decomposition of compounds 1-5 under 800°C.

UV-visible spectra of complexes 1-5
Based on the different colors of complexes 1-5, their solid state UV-vis adsorption spectra were measured at room tem-perature (Fig. S6 †). The spectra of complexes 1-5 show two wide absorption bands in the range of 235-420 nm and 430-640 nm, respectively. And the absorption band from 235-420 nm should be considered as π-π* transitions of the ligands. 28 Another absorption band from 430-625 nm can be ascribed to the spin-allowed d-d electronic transitions of the d 7 (Co 2+ ) cation. 29 The different colors of complexes 1-5 are caused by the absorption band and absorption intensity.

Magnetic properties of 2 and 4
The magnetic susceptibility measurements were taken in the temperature range of 2-300 K under an applied field of 1000 Oe. For complex 2, as shown in Fig. 6, the χ M T value is 9.98 at 300 K, which is almost twice higher than the expected value for three high-spin CoĲII) ions (5.63 cm 3 K mol −1 , S = 3/2, g = 2.0). When the temperature is lowered, the χ M T value decreased gradually and reached the value of 0.68 cm 3 K mol −1 at 2 K. Meanwhile, the χ M value increases continuously along with the decrease in temperature. These results clearly manifest that the presence of antiferromagnetic coupling between octahedral CoĲII) ions and the structure of the [Co 3 Ĳμ 2 -O) 2 N 6 ĲCOO) 2 ] trinuclear cluster may be responsible for this phenomenon. [30][31][32] Meanwhile, the temperature dependence of the reciprocal susceptibilities (χ M −1 ) was plotted and it obeys well the Curie-Weiss law above 15 K, with C = 11.11 cm 3 K mol −1 and θ = −32.28 K. The negative θ values reveals the dominant antiferromagnetic interactions between the CoĲII) ions or the presence of spin-orbit coupling. 33 For complex 4, the χ M T value is 2.31 at 300 K, which is larger than that for magnetically isolated spin-only CoĲII) ions (1.88 cm 3 K mol −1 , S = 3/2, g = 2.0). As the sample was cooling down, the χ M T value falls slowly until about 65 K and decreases sharply to 1.49 cm 3 K mol −1 at 2 K (Fig. 6). This result reveals the presence of antiferromagnetic interaction between neighboring CoĲII) ions. Meanwhile, the magnetic susceptibilities (χ M −1 ) also perfectly obey the Curie-Weiss law in the temperature range of 2-300 K, with C = 2.34 cm 3 K mol −1 and θ = −2.63 K. Both the negative θ and the decrease of χ M T can be attributed to the antiferromagnetic coupling interactions between the neighboring CoĲII) ions. 34

Dyes adsorption and mechanism
Based on the different channels and pores of the titled MOFs, the investigation of guest capture in solution to explore the porosity and its potential application seems to be promising. In our work, three organic dyes (Fig. S7, † acriflavine hydrochloride (AH), methyl orange (MO) and Congo red (GR)) with different functional groups (-CH 3 or -NH 2 ) and molecular sizes were selected to investigate their influence on dye adsorption. 35,36 5 mg of MOFs were dispersed in a 5 mL aqueous solution of AH, MO or GR at a concentration of 20 ppm. Upon adsorption, the solution was tested at certain time intervals using UV-vis absorption spectroscopy.
As shown in Fig. S8, † MO and GR have similar structures except the -NH 2 groups. However, GR can be adsorbed by all the titled MOFs, and MO can only be absorbed by MOF-2. Thus, we considered that the adsorption mechanism between MO and MOF-2 is different. Moreover, the IR spectrum of MOF-2 did not change before and after MO absorption (Fig.  S10 †). Therefore, the possible reason for the MO absorption of MOF-2 may be caused by the physical forces between the MO and MOF-2.
Meanwhile, for practical applications, the reversibility of the adsorption--release process was investigated under the saturated solution of NaCl in DMF (Fig. S12 †). 37,43 Unfortunately, the results reveal that all complexes 1-5 cannot desorb GR, and the desorption capacities for GR and MO are also lower than 7.4% and 23.7%, respectively. In order to investigate the stability of MOFs 1-5, the PXRD patterns of the recycled powders were obtained, as shown in Fig. S13. † The PXRD results revealed that the five CoĲII) MOFs were stable in the dye adsorption process.

Conclusions
In summary, five MOFs were synthesized based on a semi-flexible 4-(3,5-dicarboxylatobenzyloxy)benzoic acid and five different imidazole bridging linkers (4,4′-bidpe, 4,4′bibp, 1,3-bitl, 3,5-bipd and tib) under solvothermal conditions, with the final packing structures exhibiting different architectures from a 2D polyrotaxane network to an unprecedented 3D framework. These results reveal that the configuration of the bridging N-donor linkers has significant effects on the H 3 L coordination modes, the pore size and the final structures. Variable-temperature mag-netic studies indicate that complexes 2 and 4 exhibit antiferromagnetic couplings between neighboring CoĲII) ions. The dye adsorption and mechanism studies indicate that the pore size, the uncoordinated O atoms in carboxyl groups and the uncoordinated carboxyl groups of the MOFs have significant effects on the dye adsorption capacity.

Conflicts of interest
There are no conflicts to declare.