Modulation of Magnetic Behavior and Hg2+ Removal by Solvent

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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03621 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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Chemistry of Materials

Modulation of magnetic behavior and Hg2+ removal by solvent-assisted linker exchange based on a water-stable 3D MOF Zhichao Shaoa, Chao Huanga,b, Jian Danga, Qiong Wua, Yeye Liua, Jie Ding*a and Hongwei Hou*a †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

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)(NH2bpy)(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.

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-singlecrystal (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.1724 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 sub-generation structures (Fig. 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 d-electron 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. On the other hand, 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 sulphur 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 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 waterstable 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

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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 characterised 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 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 behaviour. 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.

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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.

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 1h. Then, the mixture of 1aminobenzene-3,4,5-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.

Fig. 1 The diagram of solvent-assisted linker exchange. 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 SHELXL1997 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 Table S2 and S3. 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,


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adsorption experiments were performed at pH = 3.0 (100 ppm Hg2+ solution) under continuous stirring. The Hg2+ concentrations of solid samples were measured after 5h by RA-915M Zeeman effect mercury analyzer.

angle is about in the range of 62.41-180o(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 91.87o and 180o. 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 (Fig. 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 analysed. Ligand L, metal unit [Ni–OCO–Ni] and 4,4’-azopyridine can be considered as 3-connecting 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} (Fig. S1d).

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

RESULTS AND DISCUSSION Crystal structures. Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in monoclinic space group C2/c. As shown in Fig. 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

The structures of 2-4 are similar to 1, in which 4,4’-azobpy was replaced by 4,4'-bpy, bpe and NH2-bpy(Fig. 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 (Fig.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° (Fig.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. The 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 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 Fig. 4, with the 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 (Fig. 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.

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accordance with the formation of NiO. As a result, 1 has good chemical stability and thermal stability, it can’t 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

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 Fig. S5, with the increase of

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

Fig. 3. The changes in structure before and after exchange reaction. exchange time, the absorption peak at 450 nm appears and gradually increases. The concentration of 4,4’-azobpy reached the maximum after four 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 4,4’-azobpy coming from decomposition of 1, chemical stability and thermal stability of 1 were studied. Firstly, 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 Xray diffraction. The integrity of the framework was further checked out by PXRD patterns (Fig. S6, Fig. S7 and Fig. 6). Secondly, the TG analysis of 1 was carried out to verify the thermal stability (Fig. S8). The first mass loss of 17.59% from 57 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

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 highquality crystals of 2-4 and intermediate states to confirm the nature of the linker-exchanged 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 (Fig. 5). Crystal color changed from yellow to green, which were observed under an optical microscope periodically (Fig. 4), this phenomenon was also characterized by the diffuse reflectance spectrum (Fig. S9). As shown in Table S2 and Table S3, the good crystallographic parameters of 2, 3 and intermediate states show that the displacement reactions haven't destroyed the original crystal shape and single crystallinity. The 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 provides the possibility for the occurrence of SALE reactions. We define the coordination energy ∆E as equation (1) ∆E = (ENi2++ E’) - E

(1)

where E, ENi2+, 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 equation (1), the higher the coordination energy ∆E is, the stronger the bonding between the metal ion and the linkers is. The coordination energies were calculated via DFT calculations, and the geometries were optimized by


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Fig. 5. Single crystal testing of the intermediate state

Fig. 6 Experimental PXRD patterns of 1 for water stability. the B3LYP functional and the 6-31+G (d, p) basis set44 (Table S1). Because there are two coordination modes in the MOFs, ∆E should be the superposition of ∆E (mode I) and ∆E (mode II). The coordination energies are in the order of ∆E (2)> ∆E (3)> ∆E (1) with values of 23.32 kJ/mol for 1, 46.06 kJ/mol for 2, 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) (Fig. 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 ∆E 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 can’t 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 exchangediffusion 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 single crystaldaughter MOF. Importantly, such a SC-SC SALE is a heterogeneous process and different from the commonly homogeneous dissolution-recrystallization. In a dissolutionrecrystallization 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 Fig. 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 (Fig. 7). The 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 (Fig. 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 (Fig. 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 (Fig. S12 left), and the bifurcation at 16.2 K indicates the onset of longrange magnetic ordering. 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 (Fig. S11, right). FC magnetization of 2 is fielddependent (Fig. S13), which is compatible with the ferrimagnetism.57 Compared with 1, Neil temperature change to 8.5K, 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 CurieWeiss law in the temperature range of 2-300 K, with C = 3.81 cm3 K mol−1 K and θ = -0.57 K (Fig. S10). The large negative value of the Weiss constant indicates a dominating antiferromagnetic coupling.

Fig. 7. Temperature dependence of χMT for 1-3. As we 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 Firstly, for 1→2, ferrimagnetic-like behavior was strengthened at low temperature. The magnetic interactions are transmitted through the carboxylate bridges along (NiOCO-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 Fig. 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

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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.

Fig. 8. The adsorption properties of 1-4 for Hg2+ in water. 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 (Fig.S14). The results of thermogravimetric analysis also showed that 4 has good thermal stability similar to 2 (Fig.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 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 (Fig. 8). To further investigate the mechanism of Hg2+ sorption, FT-IR spectra of 4 and 4-Hg (the 4 sample after Hg adsorption) were studied. In Fig. 9a, the typical stretch mode of -NH2 shows a large shift from 3446 cm-1 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 Fig. 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 Fig. S16, there are two peaks at 100.8 eV


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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.

ASSOCIATED CONTENT Supporting Information. Additional experimental details and supporting figures. Crystallographic data for 1-4 (CIF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

ORCID Hongwei Hou: 0000-0003-4762-0920

Notes Fig. 9. (a)FT-IR spectra of 4 and 4-Hg. (b) XPS survey spectra of 4 and 4-Hg. 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 4Hg was dried and examined by Energy-dispersive X-ray spectroscopy (EDS). As shown in the Fig. 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 (Fig. 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+ (Fig.S19). This result confirmed that the introduction of amino groups effectively endowed parent MOF with the property of adsorbing heavy metal ions.

The authors declare no competing financial interest. The Xray crystallographic coordinates for structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers 1584041, 1584043, 1584044, 1859335. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre.

ACKNOWLEDGMENT 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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SYNOPSIS TOC: An efficient transformation of magnetic behavior and Hg2+ removal via SC-SC solvent-assisted linker exchange was discovered and characterized, and 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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Modulation of Magnetic Behavior and Hg2+ Removal by Solvent

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