Three coordination compounds based on tris(1-imidazolyl)benzene: Hydrothermal synthesis, crystal structure and adsorption performances toward organic dyes

Accepted Manuscript Three coordination compounds based on Tris(1-imidazolyl)benzene: Hydrothermal synthesis, crystal structure and adsorption performances towards organic dyes Jun-Jiao Li, Chong-Chen Wang, Jie Guo, Jing-Rui Cui, Peng Wang, Chen Zhao PII: DOI: Reference:

S0277-5387(17)30652-6 https://doi.org/10.1016/j.poly.2017.10.011 POLY 12874

To appear in:

Polyhedron

Received Date: Accepted Date:

31 July 2017 4 October 2017

Please cite this article as: J-J. Li, C-C. Wang, J. Guo, J-R. Cui, P. Wang, C. Zhao, Three coordination compounds based on Tris(1-imidazolyl)benzene: Hydrothermal synthesis, crystal structure and adsorption performances towards organic dyes, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.10.011

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Three coordination compounds based on Tris(1-imidazolyl)benzene: Hydrothermal synthesis, crystal structure and adsorption performances towards organic dyes Jun-Jiao Li, Chong-Chen Wang∗, Jie Guo, Jing-Rui Cui, Peng Wang, Chen Zhao Beijing Key Laboratory of Functional Materials for Building Structure and Environment Remediation/Sino-Dutch R&D Centre for Future Wastewater Treatment Technologies, Beijing University of Civil Engineering and Architecture, Beijing 100044, P. R. of China

Abstract: Three coordination compounds, zero-dimensional Co(tib)(ADC)2 (BUC-60), one-dimensional Zn3(tib)2Cl6 (BUC-61) and three-dimensional [Cu2(tib)2(MoO4)Cl]Cl (BUC-62), were obtained from the reaction of 1,3,5-tris(1-imidazolyl)benzene (tib), 1,3-adamantanedicarboxylic acid (H2adc), phosphomolybdic acid hydrate (H3PO4·12MoO3) and the corresponding metal salts under hydrothermal conditions. The as-prepared samples were characterized by single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, CHN elemental analyses, thermal gravity analyses and photoluminescence spectroscopy. BUC-60 and BUC-61 exhibit good adsorption performances towards congo red (CR) and mordant blue 13 (MB13). The maximum adsorption capacities of BUC-60 and BUC-61 toward CR are 1949 and 1992 mg g-1, respectively, along with those towards MB13 being 564 and 209 mg g-1, respectively. In addition, BUC-60 can selectively capture anionic dyes molecules from a dye matrix.

Keywords: Coordination compound, 1,3,5-tris(1-imidazolyl)benzene, adsorption, organic dyes, separation ∗

Corresponding author. Tel.: +86 10 61209186; fax: +86 10 61209186.

E-mail address: [email protected]

1. Introduction Coordination polymers (CPs) or metal-organic frameworks (MOFs), as a new type of porous materials, have attracted increasing attention worldwide [1,2], due to their versatile applications, like catalysis [3], adsorption [4], electrical applications [5], luminescence [6] and magnetism [7], resulting from their large functional surface area, high porosity, tunable pore size and geometries [8-12]. In particular, CPs and MOFs have exhibited great potential in applications with liquid phase adsorption, which has attracted more and more interest from scientists [13-15]. For example, Matzger and coworkers studied a stable MOF (MIL-100) which is completely water stable and possesses large adsorption capacities towards the pharmaceuticals furosemide and sulfasalazine from water. Wang and coworkers reported a novel organic-inorganic hybrid compound, (4-Hap)4[Mo8O26] (4-ap = 4-aminopyridine), that shows ultra-high uptake of organic dyes and exhibits different dye adsorption capacities towards methylene blue (MB) and methyl orange (MO), along with rapid and highly selective adsorption of MB from a MB/MO matrix [16]. Subsequently, Wang and coworkers prepared a silver coordination polymer, which could quickly and efficiently separate the MO and MB from their mixture [17]. In addition, CPs and MOFs are also promising in sample pretreatment, especially in solid-phase extraction (SPE) or solid-phase microextraction (SPME), by virtue of their tunable pore size, permanent nanoscale porosity, high surface area and good thermal and water stabilities [18]. Wang and coworkers prepared a zinc(II) coordination polymer which could act as a novel sorbent for SPE to trace benzo[a]pyrene in edible oils [19]. Zhang and coworkers synthesized a stable Zn(II) pyrazolate-carboxylate framework with excellent sensitivity and selectivity for non-polar benzene homologues [20]. Ouyang and coworkers found that a MIL-101(Cr)-coated SPME fiber could be applied for the determination of volatile compounds [21]. Inspired by all the above and our previous works [16,17,22-24], our group is paying more attention to prepare suitable CPs or MOFs adsorbents to conduct high capacity adsorption and high-efficiency separation of organic pollutant matrices. By

taking these aspects into account, the tridentate ligand 1,3,5-tris(1-imidazolyl)benzene (tib), 1,3-adamantanedicarboxylic acid (H2adc) and phosphomolybdic acid hydrate (H3PO4·12MoO3) (as illustrated in Scheme 1) were utilized to prepare three coordination compounds, namely [Co(tib)(Hadc)2] (BUC-60), [Zn3(tib)2Cl6] (BUC-61) and [Cu2(tib)2(MoO4)Cl]Cl (BUC–62). The structure analyses based on single crystal X-ray diffraction data revealed that BUC-60, BUC-61 and BUC-62 possess 0D, 1D and 3D structures, respectively. In addition, their thermal stabilities, luminescence and adsorption properties have been investigated.

Scheme1. Structural formulae of tib, H2adc and H3PO4·12MoO3.

2. Experimental 2.1 Materials and measurements All chemicals were commercially available reagent grade and used without further purification. CHN Elemental analyses were performed using an Elementar Vario EL-III instrument. FTIR spectra were recorded on a Nicolet-6700 Fourier Transform infrared spectrophotometer in the region ranging from 4000 to 400 cm-1. Thermogravimetric analyses were performed from 70 to 800 oC in a nitrogen gas stream at a heating rate of 10 oC min-1 on a DTU-3c thermal analyzer using α-Al2O3 as a reference. Luminescence spectra of the solid powder samples were recorded on a Hitachi F-7000 spectrophotometer at room temperature.

2.2 Synthesis of BUC-60 BUC-60 was hydrothermally synthesized by mixing H2adc (0.3 mmol, 0.0673 g), CoSO4·7H2O (0.3 mmol, 0.0843 g), tib (0.3 mmol, 0.0828 g) and deionized water (10

ml) with a molar ratio of 1:1:1:1852 in a 25 mL Teflon-lined stainless steel Parr bomb under autogenous pressure, heating at 160 oC for 72 h. After the hydrothermal reaction, the Parr bomb was slowly cooled down to room temperature. Purple rod crystals were produced (yield: 72% based on CoSO4·7H2O). Anal. Calcd. for BUC-60, C39H42N6O8Co: C, 42.1; N, 10.7; H, 5.4%. Found: C, 42.3; N, 10.7; H, 5.5%. IR (KBr)/cm-1: 3335, 3181, 3149, 3120, 3095, 2927, 2914, 2859, 1731, 1678, 1621, 1603, 1509, 1530, 1477, 1360, 1333, 1320, 1274, 1257, 1213, 1157, 1105, 1013, 902, 876, 862, 737, 678, 650, 577, 554, 532, 480, 420.

2.3 Synthesis of BUC-61 A mixture of H2adc (0.3 mmol, 0.0673 g), ZnCl2 (0.3 mmol, 0.0409 g) and tib (0.3 mmol, 0. 0828 g) was sealed in a 25 mL Teflon-lined stainless steel Parr bomb containing deionized H2O (10 mL), heated at 140 oC for 72 h and then cooled down to room temperature. Small white block-like crystals of BUC-61 (yield 90 % based on ZnCl2) were isolated and washed with deionized water and ethanol. Anal. Calcd. for BUC-61, C30H24N12Zn3Cl6 : C, 37.4; N, 17.5; H, 2.5%. Found: C, 37.6; N, 17.4; H, 2.6%. IR (KBr)/cm-1: 3371, 3130, 1671, 1622, 1546, 1514, 1385, 1359, 1317, 1272, 1258, 1181, 1125, 1078, 1048, 1012, 973, 934, 848, 817, 758, 664, 611.

2.4 Synthesis of BUC-62 Blue rod-like crystals of BUC-62 (yield: 81% based on CuCl2) were synthesized from a mixture of H3PO4·12MoO3 (0.25 mmol, 0.4563 g), CuCl2·2H2O (0.2 mmol, 0.0341 g) and tib (0.4 mmol, 0. 1104 g) with a molar ratio of 5:4:8 under the same condition as BUC-61. Anal. Calcd. for BUC-62, C30H24N12O4Cu 2MoCl2: C, 39.5; N,18.5; H, 2.6%. Found: C, 39.6; N, 18.5; H, 2.8%. IR (KBr)/cm-1: 3433, 3135, 3120, 3045, 1619, 1505, 1306, 1280, 1257, 1131, 1109, 1082, 1074, 1016, 950, 941, 926, 906, 889, 874, 850, 813, 801, 760, 691, 677, 652.

2.5 X-ray crystallography X-ray single-crystal data collection for BUC-60, BUC-61 and BUC-62 was

performed with a Bruker Smart 1000 CCD area detector diffractometer with graphite-monochromatized MoKa radiation (λ = 0.71073 Å) using the φ-ω mode at 298(2) K. SMART software [25] was used for data collection and SAINT software [26] for data extraction. Empirical absorption corrections were performed with the SADABS program [27]. The structures were solved by direct methods (SHELXS-97) [28] and refined by full-matrix-least squares techniques on F2 with anisotropic thermal parameters for all of the non-hydrogen atoms (SHELXL-97) [28]. All hydrogen atoms were located by Fourier difference synthesis and geometrical analysis. These hydrogen atoms were allowed to ride on their respective parent atoms. All structural calculations were carried out using the SHELX-97 program package [28]. Crystallographic data and structural refinements for BUC-60, BUC-61 and BUC-62 are summarized in Table 1. Selected bond lengths and angles for all the coordination compounds are listed in Table 2. Table 1 Details of the X-ray data collection and refinement for BUC-60, BUC-61 and BUC-62 Formula

BUC-60

BUC-61

BUC-62

C 39H 42N 6O8Co

C30H24N 12Zn3Cl6

C30H24N12O4Cl2Cu2Mo

M

781.72

961.42

910.53

Crystal system

Monoclinic

Monoclinic

Orthorhombic

Space group

P2(1)/c

P2(1)/c

Pna2(1)

a (Å)

10.8559(8)

9.0272(9)

16.9137(17)

b (Å)

27.781(2)

14.9895(14)

15.7126(16)

c (Å)

11.9622(9)

26.767(3)

12.9472(14)

α (°)

90

90

90

β (°)

105.590(2)

97.718(2)

90

90

90

90

γ (°) 3

V (Å )

3475.0(5)

3589.1(6)

3440.8(6)

Z

4

4

4

µ(Mo, Kα) (mm-1)

0.560

2.479

1.795

Total reflections

17035

17485

13429

Unique

6111

6311

5833

1636

1920

1816

Goodness-of-fit on F

1.022

1.028

1.105

Rint

0.0619

0.0721

0.0876

R0

0.0497

0.0557

0.1156

ωR2

0.1046

0.1319

0.2720

R1(all data)

0.0967

0.0882

0.1300

ωR2(all data)

0.1211

0.1459

0.2853

F(000) 2

Largest diff. peak and hole (e/Å3)

0456, -0.383

1.185, -0.635

Absolute structure parameter

7.540, -1.328 0.06 (3)

Table 2 Selected bonds and angles for BUC-60, BUC-61 and BUC-62 [Å and o]. BUC-60 Bond lengths (Å) Co(1)-O(5)

1.941(3)

Co(1)-O(1)

1.980(2)

2.026(3)

Co(1)-N(2)

2.042(3)

O(5)-Co(1)-O(1)

110.47(11)

O(5)-Co(1)-N(4)

111.12(11)

O(1)-Co(1)-N(4)

110.06(11)

O(5)-Co(1)-N(2)

93.31(11)

O(1)-Co(1)-N(2)

116.09(10)

N(4)-Co(1)-N(2)

114.68(11)

Zn(1)-N(8)

2.000(5)

Zn(1)-N(2)

2.002(5)

Zn(1)-Cl(2)

2.2263(18)

Zn(1)-Cl(1)

2.2661(19)

Zn(2)-N(10)

2.015(5)

Zn(2)-N(4)

2.031(5)

Zn(2)-Cl(3)

2.2296(17)

Zn(2)-Cl(4)

2.232(2)

Zn(3)-N(6)

2.017(5)

Zn(3)-N(11)

2.021(5)

Zn(3)-Cl(6)

2.2184(19)

Zn(3)-Cl(5)

2.2580(18)

N(8)-Zn(1)-N(2)

109.3(2)

N(8)-Zn(1)-Cl(2)

113.39(17)

N(2)-Zn(1)-Cl(2)

106.40(16)

N(8)-Zn(1)-Cl(1)

106.40(16)

N(2)-Zn(1)-Cl(1)

109.48(17)

Cl(2)-Zn(1)-Cl(1)

111.83(8)

N(10)-Zn(2)-N(4)

113.8(2)

N(10)-Zn(2)-Cl(3)

106.47(15)

N(4)-Zn(2)-Cl(3)

116.07(17)

N(10)-Zn(2)-Cl(4)

105.65(17)

N(4)-Zn(2)-Cl(4)

101.15(17)

Cl(3)-Zn(2)-Cl(4)

113.35(7)

N(6)-Zn(3)-N(11)

113.7(2)

N(6)-Zn(3)-Cl(6)

107.22(16)

N(11)-Zn(3)-Cl(6)

108.87(16)

N(6)-Zn(3)-Cl(5)

102.68(16)

N(11)-Zn(3)-Cl(5)

108.95(16)

Cl(6)-Zn(3)-Cl(5)

115.43(8)

Mo(1)-O(2)

1.744(10)

Mo(1)-O(4)

1.780(11)

Mo(1)-O(3)

1.784(12)

Mo(1)-O(1)

1.814(11)

Cu(1)-N(7)

1.899(11)

Cu(1)-N(2)

1.913(13)

Cu(1)-O(1)

1.933(10)

Cu(1)-O(3)#1

1.950(11)

Cu(1)-Cl(2)

2.731(5)

Cu(2)-N(10)

1.987(11)

Cu(2)-N(4)

1.992(11)

Cu(2)-N(6)

2.069(14)

2.096(14)

Cu(2)-O(4)#3

2.377(11)

109.6(5)

O(2)-Mo(1)-O(3)

110.7(5)

Co(1)-N(4) o

Bond angles ( )

BUC-61 Bond lengths (Å)

Bond angles (o)

BUC-62 Bond lengths (Å)

Cu(2)-N(12)#2 o

Bond angles ( ) O(2)-Mo(1)-O(4)

O(4)-Mo(1)-O(3)

111.5(5)

O(2)-Mo(1)-O(1)

108.8(5)

O(3)-Mo(1)-O(1)

106.2(5)

N(7)-Cu(1)-N(2)

178.2(5)

N(7)-Cu(1)-O(1)

85.4(4)

N(2)-Cu(1)-O(1)

93.6(5)

N(7)-Cu(1)-O(3)#1

94.9(5)

N(2)-Cu(1)-O(3)#1

85.7(5)

O(1)-Cu(1)-O(3)#1

162.9(5)

N(7)-Cu(1)-Cl(2)

92.2(3)

N(2)-Cu(1)-Cl(2)

89.3(4)

O(1)-Cu(1)-Cl(2)

95.6(4)

O(3)#1-Cu(1)-Cl(2)

101.5(4)

N(10)-Cu(2)-N(4)

174.8(5)

N(10)-Cu(2)-N(6)

89.9(5)

N(4)-Cu(2)-N(6)

90.6(5)

N(10)-Cu(2)-N(12)#2

90.2(6)

N(4)-Cu(2)-N(12)#2

88.9(5)

N(6)-Cu(2)-N(12)#2

176.3(6)

N(10)-Cu(2)-O(4)#3

91.9(4)

N(4)-Cu(2)-O(4)#3

93.2(4)

N(6)-Cu(2)-O(4)#3

N(12)#2-Cu(2)-O(4)#3

90.7(5)

Symmetry transformations used to generate equivalent atoms: #1 -x+1, -y+3, z+1/2; #2 -x+1/2, y+1/2, z-1/2; #3 x, y-1, z; #4 -x+1/2, y-1/2, z+1/2; #5 -x+1/2, y+3/2, z+1/2 #6 -x+1, -y+3, z-1/2 #7 x, y+1, z

#8 -x+1/2, y-3/2, z-1/2

2.6 Batch adsorption In order to investigate the adsorption abilities of BUC-60, BUC-61 and BUC-62, three typical organic dyes, anionic congo red (CR), anionic mordant blue 13 (MB13) and cationic methylene blue (MB), were selected as models to conduct the adsorption experiments. Fifty milligrams of the CPs and MOFs adsorbent powders, with a particle size less than 0.08 mm, were added to 200 mL CR (100 mg L-1), MB13 (20 mg L-1) or MB (10 mg L-1) aqueous solutions in a 300 mL breaker and were vibrated for the desired contact time in a water bath shaker with a speed of 150 r min-1 at room temperature. Five milliliter aliquots were extracted, in which the adsorbent particles were separated by centrifugation (ZONKIA SC-3610) at 5000 rpm for 10 min. A Laspec Alpha-1860 spectrometer was used to determine the CR, MB13 and MB concentration changes from their maximum absorbance at 493, 551 and 672 nm, respectively. The maximum adsorption capacity experiments were also conducted under different experiment conditions; 20 milligrams (0.020 g) of BUC-60 and BUC-61 were mixed with CR and MB13 solutions (200 ml of 200 mg L-1). The other experiments conditions were the same as above.

2.7 Selective adsorption towards different dyes

To test the selective adsorption abilities of the above-stated CPs and MOFs as adsorbents, 50 mg powder adsorbents were added to 200 mL of a CR (100 mg L-1)/MB (20 mg L-1) matrix. The mixtures were vibrated at 150 r min-1 at room temperature, controlled by a water bath shaker. At a certain time intervals, UV-vis absorption spectroscopy was used to record the maximum absorbance to calculate the residual dye concentration of the aqueous solutions.

3. Result and discussion All three coordination compounds are stable and insoluble in water and common organic solvents, including but not limited to methanol, ethanol, ether and N,N-dimethylformamide (DMF). It is worth to noting that we failed to synthesize BUC-61 without the addition of H2adc, implying H2adc might play a role of a catalyst and pH regulator [22].

3.1 Structural description of BUC-60 In the 0D discrete structure of BUC-60 (Fig. 1a), the Co(II) atoms, in a tetrahedral geometry (Fig. 1b), are four-coordinated by two oxygen atoms from two partly deprotonated Hadc- ligands and two nitrogen atoms from two tib ligands. The Co-O and Co-N bond distances are comparable to those in the counterparts reported previously [29,30]. Each tib ligand, acting as a bidentate bridging ligand, links two Co(II) atoms, which makes the third imidazole group in the tib ligand to be terminal (Fig. 1c), similar to the previously reported [Cu(tib)2(N3)2]·2H2O [31]. The partly deprotonated Hadc- ligands act as both monodentate ligands to complete the tetrahedral geometry and counterions to balance the charge of the cationic [Co(tib)]2+ moiety. The terminal groups of both the tib and Hadc- ligands result in the zero-dimensional discrete unit of [Co(tib)(Hadc)2].

Fig. 1 (a) Packing view of the framework for BUC-60. (b) Highlight of the coordination polyhedron for the Co(II) atom.

3.2 Structural description of BUC-61 The coordination polymer BUC-61 is built up of 1D neutral [Zn3(tib)2Cl6] chains, as illustrated in Fig. 2a, in which the Zn(II) ion is tetrahedrally coordinated by two Clligands and two nitrogen atoms from two tib ligands. The Zn-N and Zn-Cl bond distances are similar to the normal values in the reported counterparts [32,33]. It can be clearly seen that the Zn1, Zn2 and Zn3 centers in BUC-61 are almost identical. Each tib ligand, as a tridentate ligand, joins Zn(II) centers via the tridentate mode into a 1D helix chain running parallel to the a-axis (Fig. 2b), which is very different from the coordination mode of the tib ligand in BUC-60.

Fig. 2 (a) The asymmetric unit of BUC-61 and the coordination environments around the Zn(II) atoms. H atoms are omitted for clarity. (b) The 1D helix chain in BUC-61.

3.3 Structural description of BUC-62 In the three-dimensional structure of the metal-organic framework BUC-62, as shown in Fig. 3a, the Cu1 atom is five-coordinated in a distorted square-pyramidal geometry with the Addison tau parameter τ = 0.26 (Fig. 3c), by two oxygen atoms (O1 and O3) from two (MoO4)2- units in the axial direction, two pyridyl nitrogen atoms (N7 and N2) from two tib ligands and a Cl- ligand in the equatorial plane. The Cu2 atom is five-coordinated in a distorted square-pyramidal geometry with τ = 0.03 (Fig. 3c) by one oxygen atom from a (MoO4)2- anion and four nitrogen atoms from four different tib ligands. The Cu-O, Cu-N and Cu-Cl bond lengths are comparable to related CPs and MOFs [34,35]. The Cu2-centered coodination square pyramid is slightly distorted, with the bond lengths 2.377(11) Å for the Cu-O bond and 1.987(11)-2.069(14) Å for the Cu-N bonds, and the bond angles approximate to 90o or 180o. The (MoO4)2- units, as a tridentate inorganic linker, join three Cu(II) centers via

the atoms O1, O3 and O4 to form an infinite [Cu2(MoO4)]n chain along the c-axis, depicted in Fig. 3b. The tib ligands act as tridentate ligands to join the Cu(II) ions into a 3D structure (Fig. 4). The 3D framework of BUC-62 is also described as coordination polymeric layers, [Cu2(tib)2]n4n+ (Fig. 4), bridged by (MoO4)2- ligands through the Cu1-N2 and Cu2-N4 coordinated bonds.

Fig. 3 (a) The asymmetric unit of BUC-62 and the coordination environments around the Cu(II) atoms. H atoms are omitted for clarity. (b) CuCl-MoO4 chains bridged by tib ligands (Cl atoms are not shown). (c) Highlight of the coordination polyhedrons for the Cu1 and Cu2 atoms.

Fig. 4 (a) The [Cu 2(tib)2]n4n+ layer of BUC-62. (b) 3D framework of the MOF BUC-62 along the b-axis.

The tib ligand plays very different roles in BUC-60, BUC-61 and BUC-62. In detail, the tib ligand acts as a bidentate linker in BUC-60, and as a tridentate linker in BUC-61 and 62. In addition, the partly deprotonated ligand Hadc- in BUC-60 acts as a counter-ion to balance the cationic charge of [Co(tib)]2+; while in BUC-61 and 62, Cl- and (MoO4)2- act as counter-ions, respectively. Furthermore, the tib ligand in the CP BUC-61 acts as a tridentate bridging linker to join two Zn(II) atoms to form an approximately rectangular metallamacrocycle and to join the third Zn(II) atom into a 1D infinite chain; however, in MOF BUC-62, the tib ligands act as tridentate bridging ligands to join the Cu(II) atoms to form a three-dimension framework.

3.4 Thermal properties The thermal stabilities of the three coordination compounds were examined from 70 to 800 oC under a nitrogen gas stream at a heating rate of 10 oC min-1, as shown in Fig. 5 and Table 3. Taking BUC-60 as an example, the thermal decomposition process of BUC-60 can be divided into two stages. The first weight loss of 35.3% (calcd. 35.3%) in the temperature range 375-435 oC corresponds well to the removal of the tib ligand. The second weight loss of about 53.8% (calcd. 55.1%) occurs in the temperature range 435-465 oC and can be assigned to the removal of the Hadc- ligand.

The final residue of 10.9% (calcd: 9.6 %) is presumably assigned to CoO, resulting from the rich nitrogen gas environment.

Fig. 5 The TGA curves of BUC-60, BUC-61 and BUC-62.

Table 3 The TGA weight loss of BUC-60, BUC-60 and BUC-62 First weight loss

Second weight loss

Residue

T/oC

Found

calcd.

Comp.

T/oC

Found

calcd.

Comp.

Found

calcd.

Comp.

BUC-60

375-435

35.3%

35.3%

tib

435-465

53.8%

55.1%

Hadc-

10.9%

9.6%

CoO

BUC-61

330-410

21.2%

22.2%

Cl-

410-655

58.7

54.6

tib

21.2%

25.3%

ZnO

BUC-62

220-365

7.9%

7.8%

Cl-

365-570

58.6

60.6

tib

33.4%

33.4%

CuO & MoO4

3.5 Photoluminescence properties Zn(II)-based coordination polymers have been widely investigated for photoluminescent properties [36-38]. Hence, in the present study, the emission spectra of BUC-61 along with the free tib ligand were examined in the solid state at room

temperature. The maximum emission at a wavelength of 405 nm with an excitation wavelength of 360 nm can be assigned to the π*-π transition of the free tib ligand [33,39-43]. As shown in Fig. 6, an intense band was found at 412nm (λex = 360 nm) for BUC-61, which is similar to the free tib ligand. It is difficult to oxidize or reduce the Zn(П) ion, with the d 10 configuration [41], which is the reason that the emission of BUC-61 is neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature [44-46]. Therefore, it may be tentatively be ascribed to an intraligand transition of tib or a ligand-to-ligand charge transition (LLCT) [33,41]. The small red shift of the emission maximum between BUC-61 and the tib ligand is mainly due to the influence of the coordination of the ligand to the metal atom [47].

Fig. 6 Luminescent spectra of BUC-61 at room temperature.

3.6 Adsorption experiments The adsorption performances of BUC-60, BUC-61 and BUC-62 towards some typical organic dyes, namely anionic CR, anionic MB13 and cationic MB, were carried out in a batch system. The adsorption experiments results revealed that both BUC-60 and BUC-61 exhibit good adsorption performances towards CR and MB13. As illustrated in Fig. 7, 91.4 and 90.0% CR (the initial concentration being 100 mg

L-1) could be removed after 30 min contact with BUC-60 and BUC-61 as adsorbents, also, after 300 min contact, 59.2 and 94.4% MB13 (initial concentration: 20 mg L-1) could be adsorbed by BUC-60 and BUC-61, respectively. Both BUC-60 and BUC-61 demonstrated poor adsorption activities to the cationic MB dye. It is also worth noting that BUC-62 exhibited weak uptake performances to both anionic dyes (CR and MB13) and the cationic dye (MB). As listed in Table 4, both BUC-60 and BUC-61 present high adsorption capacities towards CR and MB13, much higher than the uptake capacities of activated carbon. The possible mechanism involved in the adsorption process of BUC-60 and BUC-61 towards to CR and MB13 is electrostatic interactions between the anionic sulfonic groups of CR and MB13 and the positively charged Co 2+ and Zn2+ ions of BUC-60 and BUC-61, respectively [48]. Moreover, the π-π interactions between the aromatic rings of CR and MB13 and the aromatic imidazole rings of the coordination compounds also should be considered in the adsorption processes of CR and MB13 on BUC-60 and BUC-61, respectively [22,23,49]. Also, BUC-60 can selectively capture anionic dyes molecules from a dye matrix, namely CR and MB (200 mL, CCR =100 mg L-1 and CMB=20 mg L-1).

Fig. 7 UV-vis absorption spectra of CR with BUC-60 (a), BUC-61 (d), BUC-62 (g); MB13 with BUC-60 (b), BUC-61 (e), BUC-62 (h); MB with BUC-60 (c), BUC-61 (f), BUC-62 (i) as adsorbents.

Table 4 The adsorption capacities of BUC-60, BUC-61 and activated carbon towards CR and MB13

Adsorption capacity (mg g-1)

BUC-60

BUC-61

Activate carbon

CR MB13

1949 564

1992 209

35 34

4. Conclusions In summary, three new coordination compounds based on the tib ligand have been successfully synthesized and structurally characterized. The structure analyses indicate that BUC-60, BUC-61 and BUC- 62 possessed 0D, 1D and 3D structures, respectively. All the coordination compounds are thermally stable. BUC-61 exhibits luminescent activity resulting from an intraligand transition of the tib ligand or a ligand-to-ligand charge transition (LLCT). Both BUC-60 and BUC-61 display good capability for adsorption toward the anionic dyes CR and MB13, which may be contributed to electrostatic interactions and π-π stacking interactions. BUC-60 could also efficiently separate different organic dyes from their mixture in simulated wastewater.

Appendix A. Supplementary data CCDC 1491060, 1482298 and 1539469 contain the supplementary crystallographic data for BUC-60, BUC-61 and BUC-62, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].

Acknowledgements

We give thanks for the financial support from the National Natural Science Foundation of China (51578034), Project of Construction of Innovative Teams and Teacher Career Development for Universities and Colleges Under Beijing Municipality (IDHT20170508), the Beijing Natural Science Foundation & Scientific Research Key Program of Beijing Municipal Commission of Education (KZ201410016018), Beijing Talent Project (2016023) and BUCEA Post Graduate Innovation Project (PG2017012).

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Three coordination compounds based on Tris(1-imidazolyl)benzene: Hydrothermal synthesis, crystal structure and adsorption performances towards organic dyes

Three coordination compounds were hydrothermally synthesized from 1,3,5-tris(1-imidazolyl)benzene, of which BUC-60 and BUC-61 show good adsorption performances towards congo red and mordant blue 13.