Modulation of Magnetic Behavior and Hg2+ ... - ACS Publications

Article Cite This: Chem. Mater. 2018, 30, 7979−7987

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Modulation of Magnetic Behavior and Hg2+ Removal by SolventAssisted Linker Exchange Based on a Water-Stable 3D MOF Zhichao Shao,† Chao Huang,†,‡ Jian Dang,† Qiong Wu,† Yeye Liu,† Jie Ding,*,† and Hongwei Hou*,† †

The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou 450007, P. R. China

‡

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S Supporting Information *

ABSTRACT: SC−SC solvent-assisted linker exchange (SALE) in MOF materials is of high interest due to the potential applications. In this work, we reported an efficient transformation via SALE in a water-stable 3D MOF ({[Ni1.5(L)(4,4′-azobpy)(H2O)]·6.5H2O}n (1)) (H3L = 1-aminobenzene-3,4,5-tricarboxylic acid). The reaction progress of SALE was monitored and proved by single crystal XRD, PXRD and UV−vis absorption analyses, and the structural integrity of progeny MOFs {[Ni1.5(L)(4,4′-bpy)(H2O)]·6H2O}n (2), {[Ni1.5(L)(bpe)(H2O)]·8.5H2O}n (3), and {[Ni1.5(L)(NH2-bpy)(H2O)]·7.5H2O}n (4) were maintained very well, which is rarely in reported MOF materials. Theoretical calculations and the analyses of core−shell crystals give us better understanding of the exchange process. Interestingly, MOFs 2 and 3 obtained by SALE methods showed the different magnetic behavior comparing to parent MOF 1. Especially, because of the functional −NH2 group, progeny MOF 4 displayed the good capability to remove Hg2+ ions with an adsorption capacity of 93.693 mg/g. This work provides a new way to develop multifunctional MOF materials.

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INTRODUCTION Recently, MOF materials with tunable properties have been of considerable interest for different applications, involving ferroelectric, gas adsorption, luminescence, magnetism, and catalysis.1−9 Especially, such materials can be remarkably modulated by the single-crystal-to-single-crystal (SC−SC) transformation to provide excellent platform for the precise study of structure−property relationship.10−16 In the study of SC−SC process, guest molecule modification and metal ion exchange are common to trigger the difference of material properties.17−24 Except for these, solvent-assisted ligand exchange (SALE) is an effective way for the insertion of longer pillars into a MOF system,25,26 and the morphology of the crystal is almost maintained during the reaction,27−30 which can support an attractive strategy for the organic linker functionalization without changing the special porous structure in a MOF system. When the SC−SC SALE strategy can be carried out, the role of MOF materials will be regulated by differently useful subgeneration structures (Figure 1). Commonly, in the reported MOF systems, the magnetic behavior depends on the regulation of the central metal ion nodes. In contrast, it is rare that the magnetic behavior can be modulated by the organic linker modification. When the delectron configuration of central metal ion is deeply affected by the organic linker, there will be the possibility to generate the changed magnetic behavior in a MOF system. The SALE strategy can provide such a possibility to tune the magnetic behavior with the unchanged topological structure in MOFs. © 2018 American Chemical Society

Figure 1. Diagram of solvent-assisted linker exchange.

However, the synergistic effect of functional organic linker and special porous structure in a MOF system gives a robust platform to construct a wonderful absorber. For example, the functionalized MOFs decorated with neutral groups, such as thiol, thioether, hydroxyl, azine, and sulfur groups, were exploited for the removal of different heavy mental ions(Pb2+, UO2+, Cd2+, and Hg2+) from aqueous solution.9,31,32 The SALE strategy can avoid the damage of usefully porous structure in MOF during postfunctionalization, and the functional link can be facilely arranged at the suitable place under the single crystal state, which will enhances the potential for material applications. Although drastic solid-state structural Received: August 27, 2018 Revised: October 21, 2018 Published: October 23, 2018 7979

DOI: 10.1021/acs.chemmater.8b03621 Chem. Mater. 2018, 30, 7979−7987

Article

Chemistry of Materials

Figure 2. (a) Coordination environment around Ni2+. (b) View of bamboo-like structure. (c) View of 2D network of 1. (d) The 3D network.

ligand is concentrated at the edges of the single crystal and decreases in concentration with single crystal depth, resulting in a core−shell arrangement. The core and shell were analyzed respectively by single crystal X-ray diffraction (SCXRD). Combined with the theoretical calculation of the bond energy, the reaction mechanism was expounded. Interestingly, although the daughter MOFs {[Ni1.5(L)(4,4′-bpy) (H2O)]· 6H2O}n (2), and {[Ni1.5(L)(bpe)(H2O)]·8.5H2O}n (3) have similar structures, this transformation causes subtle geometry changes of local environment in the SBUs, which have modified the magnetic behavior. More importantly, when the linker with −NH2 group was instead of the initial 4,4′-azobpy, progeny MOF {[Ni1.5(L)(NH2-bpy)(H2O)]·7.5H2O}n (4) could effectively remove heavy metal ion (Hg2+) in water. This work provides a new way to develop variable functionality of MOF materials.

rearrangement in this method is the main hinder to limited its application, causing the collapse of structures and the loss of single crystallinity, the selection of suitable parent MOFs is possibly a useful solution for the SC−SC SALE.33 Therefore, the SALE exploration of new water-stable MOF is imperative,34 which not only can provide better evidence to study the mechanism of the SALE phenomenon, but also can offer the ways to modify function application and reassembly of components.35,36 In this work, we have successfully used SC-SC SALE to incorporate different linkers into a water-stable 3D structure {[Ni1.5(L)(4,4′-azobpy)(H2O)2]·6.5H2O}n (1) and characterized the process of the SC−SC transformation in detail. The transformation processes are fully supported by single crystal and powder X-ray diffraction studies as well as UV−vis spectra. At the beginning of the transformation process, the exchanged 7980



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EXPERIMENTAL SECTION

Materials and Physical Measurements. All the solvents and reagents were purchased from commercial source without further purification. The data of elemental analyses (C, N, and H) were collected by a FLASH EA 1112 elemental analyzer. Thermogravimetric analyses (TGA) were performed on a Netzsch STA 449C thermal analyzer at a heating rate of 10 °C min−1 in air. UV−vis spectra were measured by JASCO-750 UV−vis spectrophotometer. The Fourier Transform Infra-Red (FT-IR) spectra were carried out on a Bruker Tensor 27 spectrophotometer in the range of 400−4000 cm−1. NH3 adsorption experiment is carried out by Autosorb-iQ automated gas sorption analyzer. Variable-temperature magnetic susceptibilities were measured by a MPMS-7 SQUID magnetometer. Diamagnetic corrections were performed with Pascal’s constants. Powder X-ray Diffraction (PXRD) patterns were obtained by Cu Kα1 radiation on a PANalytical X’Pert PRO diffractometer. Preparation of Complexes. Synthesis of {[Ni1.5(L)(4,4′-azobpy)(H2O)]·6.5H2O}n (1). A mixture of Ni(CH3COO)2·4H2O (0.025g, 0.1 mmol), 4,4′-azopyridine (4,4′-azobpy) (0.0092g, 0.05 mmol), acetonitrile (2 mL), and H2O (3 mL) was stirred sealed in a 10 mL glass vial at 100 °C for 1 h. Then, the mixture of 1-aminobenzene3,4,5-tricarboxylic acid (L) (0.022g, 0.1 mmol) and H2O (1 mL) was stirred and adjusted to pH = 9 with 2 M NaOH solution, sealed in the system at 100 °C for 1 day, followed by slow cooling to room temperature. Yellow crystals of 1 were obtained in a yield of 84% based on H3L. Anal. Calcd. (%) for C38H26Ni3N10O28, C, 34.12; H, 10.48; N, 6.96. Found: C, 33.99; H, 4.59; N, 10.75. IR (KBr, cm−1): 3540m, 1593s, 1391m, 1313m, 1217m, 1091s, 835w, 774w, 660w. Synthesis of 2−4. Single crystals of 1 were dried (72 mg, 0.08 mmol) and immersed in a 2-dram screw cap vial with H2O/CH3CN (4 mL/1 mL) of 4,4′-bpy (50 mg, 0.32 mmol) or 4,4′-vinylenedipyridine (bpe) (59 mg, 0.32 mmol) or NH2-bpy (56 mg, 0.32 mmol). The vial was capped and placed in an isothermal oven at 80 °C. The progress of the linker exchange reaction was monitored by test of single crystal and ultraviolet spectrophotometer. The linker exchange was completed after 48 h for 2, 96 h for 3 and 10 days for 4. Anal. Calcd. (%) for C38H64N6Ni3O30 (2), C, 37.81; H, 5.3; N, 6.96. Found: C, 37.56; H, 4.53; N, 7.04. IR (KBr, cm−1): 3424m, 1614m, 1556m, 1461m, 1389s, 1303s, 1107w, 835w, 732w, 655w. Anal. Calcd. (%) C42H64N6Ni3O30 (3), C, 38.53; H, 4.89; N, 6.42. Found: C, 38.37; H, 4.96; N, 6.56. IR (KBr, cm−1): 3451m, 1610m, 1588m, 1470m, 1389s, 1216s, 1168w, 818w, 731w, 638w. Anal. Calcd. (%) C19H29N4Ni1.5O14 (4), C, 36.44; H, 4.63; N, 8.95. Found: C, 36.92; H, 4.08; N, 8.66. IR (KBr, cm−1): 3411m, 1621m, 1565m, 1462m, 1378s, 1202s, 1134w, 811w, 737w, 643w. Single Crystal X-ray Crystallography. The crystallographic data of 1−4 were collected on a Bruker D8 VENTURE diffractometer with Mo Kα radiation (λ = 0.71073 Å). The integration of the diffraction data, as well as the intensity corrections for the Lorentz and polarization effects, were performed using the SAINT program.37 Semiempirical absorption correction was performed using SADABS program.38 The structures were solved by direct methods and refined with a full matrix least-squares technique based on F2 with the SHELXL-1997 crystallographic software package.39 The hydrogen atoms except for those of water molecules were generated geometrically and refined isotropically using the riding model. Crystallographic data and structure processing parameters are summarized in Tables S2 and S3 of the Supporting Information (SI). Selected bond lengths and bond angles of 1−4 are listed in Table S4. Adsorption Studies. All the adsorption experiments were carried out using 8 mg of MOF 1−4 and 10 mL of Hg2+ standard solution. The Hg2+ solution was prepared by dissolving Hg(ClO4)2 in deionized water and diluted to desired concentration. To study the adsorption capacity, adsorption experiments were performed at pH = 3.0 (100 ppm of Hg2+ solution) under continuous stirring. The Hg2+ concentrations of solid samples were measured after 5 h by RA-915 M Zeeman effect mercury analyzer.

Article

RESULTS AND DISCUSSION

Crystal Structures. Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in monoclinic space group C2/c. As shown in Figure 2a, the asymmetric unit contains one and a half Ni2+, one L3−, one 4,4′-azopyridine, one coordinated H2O molecule and seven guest H2O molecules. Ni1 ions are six coordinated and display distorted octahedral coordination geometry, the equatorial plane was occupied by three O atoms (O1, O2, O3) from L3− and one N1 atom from 4,4′azopyridine. The O7 atom is from H2O molecule and the N5 atom form the axial site. For Ni2, the coordination environment is also a distorted octahedron surrounded by four O atoms (O5, O5a, O6, O6a) from L3− and two N atoms (N4, N4a) from two 4,4′-azopyridine molecules. The Ni−O bond lengths fall in the range of 2.048(3)-2.401(5) Å. The length of Ni−N falls in the range of 2.117−2.210(5) Å. The O−Ni−O bond angle is about in the range of 62.41−180°(the maximum value is 180.00(11) for O(5)−Ni(2)−O(5)#3, the minimum value is 62.41(9) for O(3)#2-Ni(1)−O(2)#2.). The N−Ni−N angle is 91.87° and 180°. These values are similar to the values found in other nickel complexes.40,41 The L3− in 1 is completely deprotonated, along with three carboxylic groups acting as μ1-η1:η0, μ1-η2:η0, and μ2-η1:η1 modes in a clockwise direction, respectively. Ni1 and Ni2 are connected with a carboxylic group by μ2-η1:η1. The distance of Ni1 and Ni2 is 5.3714 Å. At the same time, two Ni1 and two Ni2 atoms are linked together by carboxylate groups to obtain a [Ni4(CO2)6] unit presenting a parallelogram of 7.3781 Å × 10.024 Å, the quadrilateral constitute a one-dimensional bamboo-like structure by extending continuously (Figure 2b). The 1D chains are interconnected in a parallel manner and united each other via the Ni−N connections to afford a 2D layer net. In addition, interactions of 4,4′-azopyridine contribute to the formation of a 3D framework. In order to understand the structure more intuitively, the connection method of 1 is further analyzed. Ligand L, metal unit [Ni− OCO−Ni] and 4,4′-azopyridine can be considered as 3connecting nodes, 4-connected nodes and 2-connecting lines, respectively. Then the structure of 1 turns into a complicated 3D (2,3,4)-connected network. Accordingly, the overall Schläfli symbol becomes {64·82} {42·63·8} {42·63·8} (Figure S1d). The structures of 2-4 are similar to 1, in which 4,4′-azobpy was replaced by 4,4′-bpy, bpe and NH2-bpy(Figure 3). It is worth noting that the pore size changes to 11.26 Å in 2, and increases to 13.473 Å in 3. The result of NH3 sorption by TPD (Temperature-Programmed Desorption) experiment further confirmed that the hole sizes of the materials have changed obviously (Figure S2). In addition, the dihedral angle of Ni1 and Ni2 with the connected carboxyl group reduces from 14.062° in 1 to 7.347° in 2. For 3, the dihedral angle increases to 16.307°(Figure S3). The change of the angle is due to the transformation of linkers between the layers, which changes the orbit-coupled interaction between electron carriers, laying the foundation for the subsequent magnetic research. Study of SALE Transformation. In previous reports, PXRD patterns were the most favorable means to monitor the exchange process in the absence of single crystal data.42 In this work, transformation process was also confirmed by PXRD data, which revealed that the crystal structures of 2 and 3 still could be detected by SCXRD after SALE. And structures of 2 and 3 were initially constructed on the basis of their analogues 7981


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4,4′-azobpy coming from decomposition of 1, chemical stability and thermal stability of 1 were studied. First, 1 was soaked in different solvents (DMF, CH3CN, THF, CH2Cl2, CH3COCH3, H2O, CH3CN), mixture of H2O/CH3CN = 4:1 and different aqueous solution (pH = 1−14) for 48 h at 80 °C. 1 still kept high-quality single crystal state, and the unit-cell parameters were measured by single-crystal X-ray diffraction. The integrity of the framework was further checked out by PXRD patterns (Figures 6, S6 and S7). Second, the TG analysis of 1 was carried out to verify the thermal stability (Figure S8). The first mass loss of 17.59% from 57 °C to 224.5 °C could be attributed to the release of guest H2O molecules (calcd 18%). The overall framework of 1 begins to decompose from 286.4 °C, corresponding to the decomposition of ligands. The remaining weight is in accordance with the formation of NiO. As a result, 1 has good chemical stability and thermal stability, it cannot be broken down in the process of SALE. Compared with PXRD and UV−vis spectra, the high-quality single crystal data of the daughter MOFs as the most effective and direct analysis parameters is seldom involved and has not been used to explore the intermediate state and exchange reaction process in the previous reports. The process of structural transformation is beneficial to understand intuitively the reaction mechanism and the performance differences. Thus, we have obtained high-quality crystals of 2−4 and intermediate states to confirm the nature of the linkerexchanged materials. The results show that 4,4′-azobpy is completely replaced by 4,4′-bpy in 2 or bpe in 3, and partially exchanged by 4,4′-bpy or bpe in intermediate states (core− shell), in which the shell is MOF 2 or 3 and the core is MOF 1 (Figure 5). Crystal color changed from yellow to green, which were observed under an optical microscope periodically (Figure 4), this phenomenon was also characterized by the diffuse reflectance spectrum (Figure S9). As shown in Tables S2 and Table S3, the good crystallographic parameters of 2, 3 and intermediate states show that the displacement reactions have not destroyed the original crystal shape and single crystallinity.

Figure 3. Changes in structure before and after exchange reaction.

containing 4,4′-azobpy. It is worth noting that the observed low angle reflection in MOF samples describes the space between the 2D sheets formed by L and is correlated to the length of the pillaring linker.43 As shown in Figure 4, with the

Figure 4. Experimental PXRD patterns of 1 → 2 SALE process.

increase of exchange time, the low angle peak (2θ = 7.0°) weakened, and a new peak (2θ = 8.4°) appeared and enhanced gradually. Finally, the 7.0° peak is replaced by the 8.4° peak in the case of 2. This phenomenon is not existed in 3 since 4,4′azobpy and bpe are of similar length (Figure S4). In 2, the low angle peak shifts to higher 2θ when compared to the PXRD pattern of 1, which accord with the reduction of spacing between 2D sheets during the linker-exchange. To further state the linker-exchange process, UV−vis spectra were used to monitor the concentration of 4,4′-azobpy in exchange system. The characteristic absorption peak of 4,4′azobpy is at 450 nm. As shown in Figure S5, with the increase of exchange time, the absorption peak at 450 nm appears and gradually increases. The concentration of 4,4′-azobpy reached the maximum after 4 days. We have successfully obtained single crystals of 4,4′-azobpy from the supernatant liquid and determined the single crystal structure. As expected, 4,4′azobpy was efficiently replaced out of 1. In order to rule out

Figure 5. Single crystal testing of the intermediate state.

Study of Exchange Mechanism. Since the metal ions in 1−3 are located in different chemical environments, the coordination stabilities between Ni2+ ions and the linkers are different, which provide the possibility for the occurrence of SALE reactions. We define the coordination energy ΔE as eq 1: ΔE = (E Ni 2 + + E′) − E

(1)

2+

where E, ENi , and E′ are the energies of the complete compound [Ni(L)2/3(linker)2/3(H2O)2/3], a free metal ion, and the remaining specie [L2/3(linker)2/3(H2O)2/3]2−, respectively. According to eq 1, the higher the coordination energy 7982


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single crystal-daughter MOF. Importantly, such a SC-SC SALE is a heterogeneous process and different from the commonly homogeneous dissolution−recrystallization. In a dissolution− recrystallization process, the crystals of a complex dissolve slightly in some certain solution. Subsequently, new tiny crystals were grown perpendicularly on the surface of the original crystals owing to the self-assembly of dissolved components.47−49 In the work, the good chemical stability of MOF 1 was an unique advantage to avoid the possible dissolution−recrystallization process. The phenomenon of SC−SC was shown clearly by the single crystal photos and the monitoring of exchange process. Such an efficient of SALE phenomenon has not been exhibited in the previous reports. Magnetic Behavior. As structural and functional carriers, secondary building units (SBUs) play vital roles in properties of MOFs.50,51 As shown in Figure 1, SALE transformations are accompanied by the breaking and reforming of chemical bonds, which alters the local environment of the SBUs, so this change can directly modify the magnetic properties including the coupling constant, magnetic anisotropy, magnetic ordering, spin-crossover behavior, and slow magnetic relaxation.52−54 In order to verify this, variable temperature magnetic susceptibilities of 1−3 were collected at a 1 kOe dc field (Figure 7). The

Figure 6. Experimental PXRD patterns of 1 for water stability.

ΔE is, the stronger the bonding between the metal ion and the linkers becomes. The coordination energies were calculated via DFT calculations, and the geometries were optimized by the B3LYP functional and the 6-31+G (d, p) basis set44 (Table S1). Because there are two coordination modes in the MOFs, ΔE should be the superposition of ΔE (mode I) and ΔE (mode II). The coordination energies are in the order of ΔE (2) > ΔE (3) > ΔE (1) with values of 23.32 kJ/mol for 1, 46.06 kJ/mol for 2, and 31.06 kJ/mol for 3. This was also confirmed by the thermogravimetric analysis data of 1−3. The framework collapsed temperature are in the order of 2 (370 °C) > 3 (358 °C) >1 (286.4 °C) (Figure S8). Since Ni2+ exhibit same coordination modes, no extra energy is needed to overcome configuration changes in the SALE process. Therefore, we surmise that the coordination stability should dominate the exchange process. The formation of stronger coordination bond is the driving force for the spontaneous exchange reaction.45 The larger ΔE is, the shorter the reaction time is, and the easier the reaction occurs. In addition, single crystal images of intermediate state showed that the exchanged ligand is concentrated at the edges of the crystal, resulting in a core−shell arrangement at the beginning of the exchange reaction. With the increase of reaction time, the exchange process was conducted from the edge to the center. The core and shell were analyzed respectively by single crystal X-ray diffraction, and results accord with our expectations. Referring the instances of reported SC−SC, a theoretical exchange process was inferred as follows:45,46,13,14 since the channel of original MOF is occupied by lattice waters, the linker molecules cannot entry channel freely, which lead the linkers to only contact with the edge of the MOF skeleton, and further replace the original linkers of the edge. However, the vibration of the frame and spatial change will be accompanied by the occurring of exchange reaction, which provides an opportunity for the linkers’ diffusion in the exchanged MOF channel. Compared to 2, the linker length in 3 was similar to that in 1, only a slight increase from 9.006 to 9.340 Å. This vibrations phenomenon was relatively weaker and the free space will not be created violently. As the daughter MOF generated preliminarily at the crystal edge, the linkers in the solution would enter the channel of daughter MOF for sequential exchange. The exchange−diffusion synergistic effect is a major cause for the formation of core−shell structures. Finally, the original linkers are replaced completely to form a new green

Figure 7. Temperature dependence of χMT for 1−3.

χMT value of 1 at 300 K is 3.73 cm3 K mol−1, which is a little bit higher than the spin only value of 3.0 cm3 K mol−1 for three spin-only Ni2+ ions with S = 1 and g = 2.00. Upon cooling, the χMT value decreases to a minimum of 3.61 cm3 K mol−1 at 40 K, increases to a maximum of 4.14 cm3 K mol−1 at 12.5 K, and finally decreases to 2.99 cm3 K mol−1 at 2 K. The decrease of χMT above 40 K suggests a dominant AF interaction. The magnetic data in the range of 20−300 K obey the Curie−Weiss law with C = 3.69 cm3 K mol−1 and θ = −0.53 K (Figure S10). The minimum at 40 K indicates a ferrimagnetic-like behavior, which is supported by the fact that maximum χMT of low temperature is higher than that of room temperature.55,56 The M − H curve at 2 K manifests a featureless increase with its final saturation value up to 6.06 Nβ at the highest field of 80 kOe (Figure S11, left), which is close to the theoretical value of 6 Nβ for a Ni3 unit. Field-cooled (FC) and zerofield-cooled (ZFC) magnetizations were performed under an applied field of 50 Oe (Figure S12 left), and the bifurcation at 16.2 K indicates the onset of long-range magnetic ordering. 7983


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Chemistry of Materials For 2, the χMT value at 300 K is 3.22 cm3 K mol−1, which is close to the value expected, 3.0 cm3 K mol−1, and no significant decrease was observed in the process of cooling. However, the χMT value increases to a maximum of 3.74 cm3 K mol−1 at 8.5 K and then finally goes down to a value of 3.12 cm3 K mol−1at 2 K. This is indicative of the existence of ferrimagnetic-like behavior and the M − H curve also confirmed this behavior (Figure S11, right). FC magnetization of 2 is field dependent (Figure S13), which is compatible with the ferrimagnetism.57 Compared with 1, Neil temperature change to 8.5 K, and small final saturation value at 2 K also shows the electronic coupling decreases. On the contrary, the χMT value of 3 at 300 K is 3.82 cm3 K mol−1 because of their spin−orbit coupling interaction about this phenomenon. As the temperature decreases, the value of χMT continuously decreases and reaches a minimum of 2.70 cm3 K mol−1 at 2 K. Magnetic phase transition occurs at 18.3 K, and the feature may be caused by antiferromagnetic exchange interactions. The plot of χM−1 versus T for 1 is consistent with the Curie−Weiss law in the temperature range of 2−300 K, with C = 3.81 cm3 K mol−1 K and θ = −0.57 K (Figure S10). The large negative value of the Weiss constant indicates a dominating antiferromagnetic coupling. As seen, magnetic behavior has been modified because the SALE transformation causes subtle geometry changes of local environments in the SBUs. According to reported literatures, magnetic track of MOF materials is mainly composed of the combination of ligand orbits and metal ion orbits (t2g and eg). Orbital overlap can cause the antiferromagnetic coupling effect, and the orthogonality of the orbits is easy to cause the ferromagnetic coupling effect.58 First, for 1 → 2, ferrimagneticlike behavior was strengthened at low temperature. The magnetic interactions are transmitted through the carboxylate bridges along (Ni−OCO−Ni), and the magnetic orbitals are composed of Ni2+ dx2−y2 and dz2 orbitals. Through the analysis of the structure,we found there is big difference dihedral angle between Ni2+ and the connected carboxyl group (7.347° for 2 and 14.062° for 1, as shown in Figure S3). The reduction of dihedral angle led to the orthogonality of magnetism orbit, which is easier to cause the ferromagnetic interaction. However, for 1 → 3, a more weak interaction was observed. Structure analysis shows that the dihedral angles increase from 14.062° to 16.307°, which led to the further orbit overlap. Ni2+ ions in 3 show more strong single-ion anisotropy than 1, which triggered the enhanced role of antiferromagnetic.59 On the whole, we infer that the transformation of magnetic behavior results from the contribution of the overlap orbit. Removing Heavy Metal Ions Hg2+. On the basis of 1 → 2, the NH2-bpy was introduced instead of 4,4′-bpy to synthesize 4 with amino functionalization, because the amino-based MOFs have a good performance of Hg2+ removal in previous reports. Before the experiment of removing heavy metal ions, the structure of 4 was verified. It can be seen that 4 and 2 are very similar except for the amino group from the crystallographic data and PXRD (Figure S14). The results of thermogravimetric analysis also showed that 4 has good thermal stability similar to 2 (Figure S15), which laid the foundation for removing heavy metal ions in water. The adsorption experiments were carried out at 25 °C using 8 mg of MOF and 10 mL of Hg2+ standard solution to investigate the adsorption behaviors. The Hg2+ solution was prepared by dissolving Hg(ClO4)2 in deionized water and diluted to get desired concentration. To study the adsorption ability, adsorption experiments were performed at the obtained

optimum pH value (pH = 3) for 100 ppm of Hg2+ solution under continuous stirring. The Hg2+ concentration of crystal samples was determined after 5 h. As we expected, 4 showed a significant ability to remove Hg2+ with an adsorption capacity of 93.693 mg/g after 5 h. As a comparison, the adsorption amounts of 1−3 are 0.182 mg/g, 0.199 mg/g and 0.208 mg/g, respectively (Figure 8).

Figure 8. Adsorption properties of 1−4 for Hg2+ in water.

To further investigate the mechanism of Hg2+ sorption, FTIR spectra of 4 and 4-Hg (the 4 sample after Hg adsorption) were studied. In Figure 9a, the typical stretch mode of −NH2

Figure 9. (a)FT-IR spectra of 4 and 4-Hg. (b) XPS survey spectra of 4 and 4-Hg. 7984


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Chemistry of Materials shows a large shift from 3446 to 3366 cm−1 in 4-Hg (the 4 sample after Hg adsorption), indicating the strong binding interactions between Hg2+ and −NH2 groups. There are strong interactions between Hg2+ and −NH2 groups that could limit N−C stretching vibrations, giving an obvious change at 1240 cm−1. These data show that the high adsorption efficiency of 4 for Hg2+ may result from the strong Hg−N interactions, which also have been demonstrated by XPS studies. As shown in Figure 9b, the appearance of Hg 4f peaks verifies that Hg2+ is undoubtedly loaded on 4. A more detailed structure of Hg species could be obtained in the high resolution XPS spectrum of Hg 4f. In Figure S16, there are two peaks at 100.8 and 104.8 eV, corresponding to Hg 4f7 and Hg 4f5. Compared with the Hg2+ purified binding energies at 99.9 eV for Hg 4f7 and 103.9 eV for Hg 4f5, respectively, a remarkable shift of 0.9 eV can be observed in 4-Hg, which reveals the formation of strong affinities between Hg2+ and 4. These results are consistent with the FT-IR analyses. After the adsorption test, 4-Hg was isolated and washed with water to remove the residual Hg2+ on the exterior of 4-Hg. Then the sample of 4-Hg was dried and examined by Energy-dispersive X-ray spectroscopy (EDS). As shown in the Figure S17, the existence of Hg was confirmed and Hg elements are uniformly distributed in 4-Hg. In addition, according to the reported literature, metal ions have to detach large share of hydration water before they enter the smaller channels of adsorbents.60,61 Therefore, we infer that the specie of mercury is Hg2+ in adsorption experiments. The 1100 cm−1 peaks of IR spectrum for perchlorate was found (Figure S18), which shows the anion was also accompanied by Hg2+ ions into the material center. After the adsorption test, the solid samples of 4 were recovered by centrifugation, PXRD results show that the structure of 4 is complete and 4 is recyclable as an effective material for removing Hg2+ (Figure S19). This result confirmed that the introduction of amino groups effectively endowed parent MOF with the property of adsorbing heavy metal ions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chao Huang: 0000-0001-8576-8024 Hongwei Hou: 0000-0003-4762-0920 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation (Nos. 21371155, 21671174, and 21701201) and the Natural Science Foundation of Henan province (182300410008).

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REFERENCES

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CONCLUSIONS In summary, we have discovered and characterized an efficient transformation of magnetic behavior and removing ability of heavy metal ions via SC−SC SALE, which are fully supported by single crystal and powder X-ray diffraction studies as well as UV−vis spectra. Water-stable MOF 1 can be used as a useful precursor, and link with different functional groups can simply introduce functions into MOF materials. These results highlight the importance of the SC−SC transformation in the study of dynamic molecular systems and underscore the feasibility of SALE as efficient strategy to develop adjustable MOF materials.

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Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b03621. Additional experimental details and supporting figures(PDF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) Crystallographic data (CIF) 7985


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