Hydrothermal Synthesis, Structure and Properties of a 3D Pillar-layered Metal-organic Framework Based on Amino-arenedisulfonate Ligand①

GUAN Lei LUO Guan-Hua WANG Ying

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Hydrothermal Synthesis, Structure and Properties of a 3D Pillar-layered Metal-organic Framework Based on Amino-arenedisulfonate Ligand①

GUAN Lei②LUO Guan-Hua WANG Ying

(113001)

One novel metal-organic framework (MOF), [Ba()(H2O)1.5]n(1, H2= aniline-2,5-disulfonic acid), has been synthesized by hydrothermal method. Each barium atom is eleven-coordinated into a distorted monocapped pentagonal antiprismatic arrangement. Compound 1 shows an interesting 3D pillar-layered structure constructed from 2D inorganic layers [Ba(SO3)2(H2O)1.5]nand organic pillars of phenyl moieties of2-linkages. The inorganic layers are supported by the organic pillars, generating a novel 3D open framework structure with {3, 46, 55, 65, 74}2{3}{5} topology. The result of fluorescence measurement can reveal that the decayed emission band centered at 492 nm may be caused by the interactions of the ligands and the metal ions. Compound 1 exhibits selective toward the adsorption of CO2over N2at 273 K.

hydrothermal synthesis, metal-organic framework, pillar-layered structure, coordination polymer;

1 INTRODUCTION

Metal-organic frameworks, as an emerging class of porous inorganic-organic hybrid materials, have attracted considerable interest in materials science due to their diverse porous structures[1-5]. Many potential applications of MOFs arise due to the porous nature of the structure, though the variability of the coordination around the metal ions can play an important role[6-9]. In addition, it is conceivable that different physical and chemical properties can arise out of the functional groups as well. Thus, supramolecular design of MOFs allows a number of organic ligands, with two or more functional groups, to be linked to metal ions giving myriad variations and tunable properties in synthesized structures[10, 11].

The most commonly used organic linkers are the aromatic polycarboxylate, due to their structural rigidity, strong bonding interactions, and rich diversity of coordination modes utilized for exten- sion of the metal ions into high-dimensional structures[12-15]. However, recently there is growing interest in the employment of their sulfonic analogous for the construction of novel MOF structures. Sulfonate is an excellent group for constructing MOFs, which can enhance the number of possible geometrical combination between the O-donors and can allow for bridging the metal ions with new coordination modes. Hence, the coexi- stence of multisulfonate groups in the same ligand may result in many interesting structures with special applications[16, 17].

A trifunctional (two sulfonate and an amino groups) ligand, H2, has been chosen to construct MOFs owing to the following two considerations: (a) parabisulfonate groups can prefer to bridge metal ions, constructing MOFs with diverse structures, and bisulfonate groups prefer to coordinate to main group metals, especially alkaline earth metals; and (b) the amino group can be regarded as a modified group, which can decorate the MOF to obtain many of the properties. Herein, we report on the synthesis and characterization of a novel 3D pillar-layered MOF with {3, 46, 55, 65, 74}2{3}{5} topology based on H2. Additionally, the luminescent and gas adsorption properties of compound 1 were also studied.

Scheme 1. Molecular structure of H2ligand

2 EXPERIMENTAL

2.1 Materials and general measurements

All reagents were purchased from commercial sources and used without further purification. X-ray single-crystal diffraction data were collected at 296 K from a single crystal mounted atop a glass fiber with a Bruker Apex-II diffractometer using graphite-monochromated Mo(= 0.71073 ?) radiation. Elemental analysis was performed on a Perkin-Elmer 240C instrument. The FT-IR spectrum was recorded on a Nicolette FTIR spectrometer using KBr pellets in the range of 4000～400 cm-1. Thermogravimetric analysis was carried out on a NETZSCH STA 449C unit at a heating rate of 10°C·min-1under a nitrogen atmosphere. Photoluminescence analysis was performed on a Perkin Elemer LS55 fluorescence spectrometer. N2and CO2absorption measurements were performed using an Autosorb IQ2 instrument. Prior to the measurement of the isotherms, the samples were desolvated for 24 h under high vacuum conditions at 130°C.

2.2 Synthesis of compound 1

Ba(NO3)2(0.261 g, 1 mmol) and H2(0.253 g, 1 mmol) were dissolved in 20 mL H2O. After stirring for 2 h, the mixture was sealed in a 25 mL Teflon-lined stainless-steel vessel, and then heated to 120℃ for 36 h. After cooling to room temperature in 12 h, the resulting colorless crystals were washed with distilled water to give the pure sample. The colorless single crystals were obtained in ca. 72% yield based on Ba(II).Elemental Anal. Calcd.(%) forC12H16Ba2N2O15S4: C, 34.65; H, 1.92; N, 3.37. Found(%): C, 34.81; H, 1.96; N, 3.55.Infrared spectrum (cm-1):(O–H) = 3068,(C=C) = 1535,(OH) = 1478, 1405,(C–N) = 1282,(SO32–) = 1212, 1160, 1124, 1046, 1016, 911,(C–H) = 850, 837,(N–H) = 724, 655,(C–S) = 609,(Ba–O) = 561[18, 19].

2.3 X-ray crystallographic measurement

Data collection for compound 1 was carried out on a Bruker Smart CCD diffractometer equipped with graphite-monochromated Moradiation (= 0.71073 ?) at 296 K. Data reduction was performed with SAINT[20], and empirical absorption corrections were applied by the SADABS program. The struc- ture was solved by direct methods using the SHELXS program and refined with the SHELXL program[21]. Heavy atoms and other non-hydrogen atoms were directly obtained from a difference Fourier map. Final refinements were performed by full-matrix least-squares methods with anisotropic thermal parameters for all non-hydrogen atoms on2. C-bonded H atoms were placed geometrically and refined as riding model. O-bonded H atoms were placed in idealized positions and constrained to ride on their parent atoms. The atomic occupancy of the coordinated water molecule (O8, H8B and H8C) is 0.5. Selected bond distances and bond angles are given in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for Compound 1

Symmetry codes: (A) ?+1, ?+1, ?+2; (B), ?+1/2,+1/2; (C) ?+1,?1/2, ?+3/2

C12H16Ba2N2O15S4,= 831.19, colorless blocks, crystal size 0.26mm × 0.23mm × 0.22mm, mono- clinic,21/,= 10.559(4),= 12.087(5),= 9.002(4) ?,= 97.783(6)°,1138.3(8) ?3,= 296 K,= 2,= 3.88 mm?1),min= 0.432,max= 0.482, 5612 reflections measured, 1988 unique (int= 0.043), 1784 observed (2()), (sin/)max= 0.595 ??1, parameters = 164, restraints = 2,0.055(observed refl.),0.149(all refl.),1.11, (Δmax)=1.89 and (Δmin) = –1.76 e·??3.

3 RESULTS AND DISCUSSION

3.1 Structure description

Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinicsystem,21/space group. The asymmetric unit of compound 1 contains one barium ion, one2-ligand, and one and a half of coordinated water molecules, as shown in Fig. 1a. Each barium atom is eleven- coordinated, showing a distorted monocapped pentagonal antiprism geometry, as shown in Fig. 1b. Its coordination sites are occupied by sulfonate oxygen atoms (O(1), O(2), O(2)A, O(3), O(4)C, O(5) and O(6)D) and terminal water molecules (O(7), O(7)B, O(8) and O(8)B). The Ba–O bond lengths fall in the range of 2.683(8)～3.084(16) ?. The O–Ba–O bond angles are in the range of 31.4(4)～177.7(3)°(Table 1). These distances and angles are comparable to those of barium-sulfonatecomplexes (see Table 1)[22, 23]. The ligand shows only one kind of coordination mode in compound 1 (Fig. 1c). It bridges six barium ions, leaving the amino group uncoordinated, where the two sulfonate groups adopt1-1:2:1and1-1:1coordination modes to coordinate with six different barium ions, respect- tively. In addition, the chelating and bridging coordination modes of thesulfonate groups of the ligand we reported are different from those reported in references[22, 23]. As reported, thesulfonate groups displaydiverse coordination modes, which together with the versatile coordination ability of the bridging ligand makes it adaptable to metal ions of different sizes and leads tovarious topologies[22, 23].

Fig. 1. (a) Molecular structure of compound 1. The asymmetric structure unit of compound 1 with atomic labeling scheme. All hydrogen atoms are omitted for clarity; (b) Ba1 ion exhibits a monocapped pentagonal antiprism geometry; (c) Coordination mode of2-ligand

(Symmetry codes: (A) ?+1, –+1, –+2; (B),+1/2,+1/2; (C) ?+1,?1/2, ?+3/2; (D) –+1, –+1, –+2; (E)+1,,)

Hydrogen bonding interactions are present in the crystal structure of compound 1. Hydrogen bonding interactions are listed in Table 2. These interactions involve the coordinated water molecules, the amino nitrogen atoms, and the sulfonate oxygens on the ligands. Collectively, the hydrogen bonds can contri- bute to the overall stability of the crystal lattices in 1.

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°) for Compound 1

Compound 1 displays a 3D structure constructed by the 2D inorganic layers [Ba(SO3)2(H2O)1.5]nand the pillars of organic phenyl moieties of2-linkages (Fig. 2a and 2b). The 2D inorganic layers are supported by2-ligands as pillars to form a 3D pillared-layered structure. Such structure is charac- teristic of the metalsulfonate coordination poly- mers[24]. The separation between the neighboring layers is 16.05 ?. The 2D inorganic layer features one-dimensional chains bridged by sulfonate groups, as shown in Fig. 2c. The barium ions are bridged by water molecules to form one-dimensional chain structures [Ba(H2O)1.5]n. The distance is 6.06 ? between the adjacent one-dimensional chains. The two sulfonate groups of2-ligands can adopt the η1:μ1μ2μ1and η1:μ1μ1μ1coordination modes to bridge the one-dimensional chains through coordination actions with barium ions. This type of 3D pillared- layered structure constructed from2-ligands has never been observed. From the above description, we can see that two factors can play important roles in forming the 3D pillared-layered structure: (i) the2-ligand can provide two sulfonate groups at the para-position of the benzene ring; (ii) the rigid bridging linker2-ligand can play a supporting role through coordination actions. Up to now, a large number of pillarlayered structures have been reported, such as {(Btc)2(Tib)2(H2O)4].(H2O)2}n(Btc = 1,3, 5-benzenetricarboxylate, Tib = 1,3,5-tris (imidazol-1- ylmethyl)benzene)[25], [Cd(HCOO)(4-tba)]n(4-Htba = 4-(1,2,4-triazole)benzoic acid)[26]and [Sr(H2)- (H2O)2]·3H2O (H4= N,N?-piperazine-bis(methyl- enephosphonic acid))[27].However, the pillar-layered structures constructed from the ligands containing sulfonate are quite rare, compared with the car- boxylate or phosphonate[25-27].

The crystal structure can be simplified to the network by topological approach for better under- standing of the nature of this intricate framework[28]. Each2-ligand is considered as 6-connected node due to the bridging of six neighboring barium ions, and each barium center acts as a 6-connected node due to the coordination of the six surrounding2-ligands. Both oxygen atoms (O(7) and O(8)) of water molecules can be considered as linkers. Based on this simplification, the2-ligand, barium atom and water molecule act as 6, 6 and 2-connected nodes in a ratio of 1:1:2 in this structure, respectively (Fig. 3a～3c). Hence, compound 1 exhibits a 3D open framework with Schl?fli symbol of {3, 46, 55, 65, 74}2{3}{5}, as shown in Fig. 3d. The amino group of the ligands is uncoordinated with barium ions, thusthe 3D open framework is decorated by the amino groups. It may have some potential applications[28-30].

Fig. 2. Schematic representation of (a) the 3D pillar-layered structure, (b) the 2D inorganic layered structure, and (c) one-dimensional chains [Ba(H2O)1.5]nand sulfonate group

Fig. 3. Schematic representation of (a) the ligand-based 6-connected nodes, (b) the 6-connected Ba1 nodes, (c) the 2-connected water nodes, and (d) the 3D open framework structure

3.2 Luminescent property

The luminescent properties of the free ligand and compound 1 have been determined in the solid state at room temperature. As shown in Fig. 4, the free ligand exhibits photoluminescence emission at 394 nm upon excitation at 315 nm, which may be attributed to*-n or*-transitions[31]. Compound 1 exhibits green luminescence emission at 492 nm by using an excitation wavelength of 315 nm. It can mean the red-shift of 98 nm relative to the free ligand. The red-shift can mainly originate from the ligand- to-metal charge transfer transition or the change of crystalline structure. Similar red-shifts are observed in other MOFs[31]. UV-Vis diffuse reflectance spectrum is another evident for ligand-to-metal charge transfer effect[31]. As reported in the literatures, the emission spectra show various degree of red-shift, which could be ascribed to the structure diversities arising from the diverse conformation and the different positions of coordination atoms. Then, the structure diversities further change the charge transi tion energy. The crystalline solid of compound 1 can display the photoluminescence property. It implies that it is a promising candidate for hybrid photo-active material with potential applications such as light emitting diode[32].

Fig. 4. Solid-state emission spectra of the ligand and compound 1 at room temperature

Fig. 5. TGA curve for compound 1

3.3 Thermal analysis

The thermogravimetric diagram of compound 1 is shownin Fig. 5. The phase of the sample remains stable up to 273°C. The dehydration step is completed at 311°C with mass reduction of 3.26% (calcd. 3.25%), indicating the loss of coordination water molecules. Surprisingly, this process is completed at around 311°C, which is much higher than that expectedfor coordinated water mole- cules[33]. This high temperature of dehydration shows strong bridging actions of water molecules with the barium ions. On further heating, the ligands are gradually decomposed in the temperature range of 311～900°C. The total weight loss is ca. 48.59%, with the final residues of the mixture of barium oxide and barium sulfate[33].

3.4 Gas adsorption

To check the adsorption properties of compound 1 to host molecules, N2and CO2adsorptions were carried out at 273 K, respectively. As shown in Fig. 6, compound 1 can take up a relatively significant amount of CO2(73.8 cm3/g) but a negligible amount of N2(4.7 cm3/g) at 273 K and 1 atm. It indicates that compound 1 has a good capacity for CO2/N2separation[34]. The adsorption of CO2by compound 1 may be attributed to the conjugated delocalization system and the smaller kinetic diameter of CO2compared to that of N2(CO2, 3.3 and N2, 3.6 ?)[35], which may strengthen the electric interaction between the guest small molecules and the host framework. It is well known that N-containing organic heterocycles can enhance CO2selectivity. Thus, the incorporation of such heterocyclics into the microporous MOFs has the potential to increase the sorption capacity and the selectivity for CO2. The characteristics for the adsorption of CO2on heterocyclic microporous polymers are considered to be attributed to the lone pair electrons of heteroatoms, which can appear to play an important role because they can provide alkalinity sites through dipole- dipole interactions, thus enhancing the CO2adsorp- tion properties[36].

Fig. 6. N2and CO2adsorption isothermsat 273 K for compound 1

4 CONCLUSION

In summary, a novel MOF [Ba()(H2O)1.5]nhas been synthesized under hydrothermal conditions by using H2as the ligand. It shows a 3D pillar-layered structure with unique {3, 46, 55, 65, 74}2{3}{5} topology based on two kinds of topologically none- quivalent 6-connected nodes and one kind of 2- connected node. The2-ligand acts as an unprece- dented hexadentate bridging ligand. It exhibits blue photoluminescence in the solid state, and demon- strates selective toward the adsorption of CO2over N2at 273 K.

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5 March 2018;

12 June 2018 (CCDC 1475311)

① This project was supported by the Liaoning Provincial Education Department (No. L2015299) and Innovative training program for College Students (Nos. 201710148000118, 201710148000147)

. E-mail: glsyncoord@163.com

10.14102/j.cnki.0254-5861.2011-1998