Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 109–115

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Tuning zinc(II) coordination architectures by rigid long bis(triazole) and different carboxylates: Synthesis, structures and fluorescence properties Xiao-xiao Wang a, Zuo-xi Li b, Baoyi Yu c, Kristof Van Hecke c, Guang-hua Cui a,⇑ a

College of Chemical Engineering, North China University of Science and Technology, 46 West Xinhua Road, Tangshan 063009, Hebei, PR China Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, PR China c Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 S3, B-9000 Ghent, Belgium b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A new rigid bis(triazole) ligand was

synthesized.  Three new Zn(II) coordination

polymers were synthesized.  IR, XPRD, TGA technique for the

polymers.  X-ray single-crystal structure

analyses and discussion for the polymers.  Luminescence properties of three complexes were investigated.

a r t i c l e

i n f o

Article history: Received 10 December 2014 Received in revised form 17 March 2015 Accepted 16 April 2015 Available online 24 April 2015 Keywords: Bis(triazole) ligand Coordination polymer Fluorescence Hydrothermal Topology Zinc(II)

a b s t r a c t Three metal–organic coordination polymers containing rigid bis(triazole) ligand, namely, [Zn1.5(btb) (nbta)(H2O)]n (1), {[Zn(btb)(3-nph)](H2O)}n (2) and [Zn(btb)(4-nph)]n (3) (btb = 4,40 -bis(1,2,4-triazolyl1-yl)-biphenyl, 3-H2nph = 3-nitrophthalic acid, H3nbta = 5-nitro-1,2,3-benzenetricarboxylic acid, and 4-H2nph = 4-nitrophthalic acid) were synthesized under hydrothermal conditions and structurally characterized by X-ray single-crystal diffraction. Complex 1 possesses an interesting 3D coordination framework with a rarely binodal (4,4)-connected frl topological structure. Complexes 2 and 3 exhibit similiar 2D (4,4) grid layers with different point symbol (44  64) in 2 and (44  62) in 3. Furthermore, thermal stability of these compounds has been discussed. Complexes 1–3 exhibit strong solid-state fluorescence at room temperature in solid state. Ó 2015 Elsevier B.V. All rights reserved.

Introduction The design and construction of metal–organic coordination polymers(MOCPs) has become a very attractive research field, not only because of their fascinating structural diversities but also ⇑ Corresponding author. Tel.: +86 0315 2592169; fax: +86 0315 2592170. E-mail address: [email protected] (G.-h. Cui). http://dx.doi.org/10.1016/j.saa.2015.04.048 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

owe to their potential application in luminescence, gas storage, ion exchange, magnetism, and so on [1–4]. Thus, structural design or modification of the coordination polymers has become a very active field in crystal engineering [5–8]. From a synthetic point of view, the judicious selection of appropriate organic ligands is proved to be one of the most effective ways to manipulate the versatile structures of MOCPs [9–11]. Among multitudinous organic ligands, triazole-containing ligands, in particular, have proven to

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be pleasurable candidates for the construction of novel MOCPs due to their flexible and diverse coordination modes. Until now, Wang and our group reported a series MOCPs based on triazole or their derivatives and organic carboxylates [12–15]. However, the MOCPs constructed from a rod-like rigid 4,40 -bis(1,2,4-triazolyl1-yl)-biphenyl (btb) ligand have been rarely reported, when screening of the Cambridge Structural Database (version 5.35, May 2014) for fragments containing the btb ligand [16]. The rigid bis(triazole) ligands are promising candidates of rod-like building blocks owing to two terminal sp2 nitrogen atoms [17]. Further, this btb ligand may exhibits versatile photoluminescence properties due to the conjugated backbones, which could offers an interesting approach to preparing optical materials [18]. To further understand the coordination chemistry of the bis(triazole), and to explore new materials with beautiful architectures and functional properties, in this work, we chose the rigid btb as main linker and 5-nitro-1,2,3-benzenetricarboxylic acid (H3nbta), 3-nitrophthalic acid (3-H2nph) and 4-nitrophthalic acid (4H2nph) as auxiliary ligands, three new zinc(II) MOCPs with intriguing structures were hydrothermally synthesized and characterized by elemental analysis, infrared spectrum (IR), thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD). [Zn1.5(btb)(nbta)(H2O)]n (1) is a rarely 3D binodal (4,4)-connected frl topological structure, {[Zn(btb)(3-nph)](H2O)}n (2) and [Zn(btb)(4-nph)]n (3) exhibit similiar 2D (4,4) sql network with different point symbol (44  64) in 2 and (44  62) in 3. The crystal structures and topological analyses of these polymers, along with the effect of the carboxylate ligands on the ultimate frameworks, are presented and discussed. 1–3 possess strong photoluminescence properties in solid state at room temperature, which reveal their potential application as luminescence materials. Experimental Materials and measurements All reagents and solvents were obtained from commercial sources and used without further purification. Elemental analysis (C, H, and N) was performed on a Perkin-Elmer 240C Elemental Analyzer. IR spectra were recorded from KBr pellets in the range of 4000–400 cm1 on an Avatar 360 (Nicolet) spectrophotometer. Thermogravimetric analysis (TGA) was carried out on a NETZSCH TG 209 thermal analyzer from room temperature to 800 °C with a heating rate of 10 °C/min under N2 atmosphere. Luminescence spectra for the powdered solid samples were measured at room temperature on a Hitachi F-7000 fluorescence spectrophotometer. X-ray powder diffraction (XRPD) measurements were made on a Rigaku D/Max-2500PC X-ray diffractometer using Cu-Ka radiation (k = 0.1542 nm) and x  2h scan mode at 293 K. Preparation of the btb and complexes 1, 2 and 3 Synthesis of btb A mixture of 1,2,4-1H-triazole (60.0 mmol, 4.144 g), CuI (2.5 mmol, 0.475 g), 1,10-phenanthroline (15.0 mmol, 2.703 g), 4,40 -dibromobiphenyl (12.0 mmol, 3.744 g) and K2CO3 (96.0 mmol, 12.67 g) was suspended in 60 mL of dimethylformamide. The reaction mixture was refluxed for 72 h at 160 °C in oil bath and then cooled to room temperature. The solvent was removal in vacuo and the crude product was poured with excess ice water to get rid of excess 1,2,4-1H-triazole. After filtration, the filter cakes were placed in a soxhlet extractor to undergo extraction in boiling dichloromethane for 8 h, which affords 4,40 -bis(1,2,4-triazolyl-1yl)-biphenyl (btb) as off-white powder. Yield: 75.3%. Anal. Calcd. for C16H12N6 (%): C, 66.61; H, 4.20; N, 29.15. Found (%): C, 66.39;

H, 4.36; N, 29.36. 1HNMR (400 MHz DMSO) d/ppm: 7.98–7.99 (8H, multiplet, aromatic protons), 8.29 (2H, singlet, N@CHAN), 9.4 (2H, singlet, N@CHAN). IR (KBr, cm1): 3017 (m), 2856 (m), 1689 (s), 1577 (s), 1516 (m), 1441 (m), 1258 (s), 1176 (w), 1136 (w), 907 (w), 855 (w), 751 (m), 677(w), 568(w), 536(m). Synthesis of [Zn1.5(btb)(nbta)(H2O)]n (1) An aqueous solution of H3nbta (0.3 mmol, 76 mg) and Zn(NO3)26H2O (0.5 mmol, 199 mg) was added slowly to the suspension of btb (0.5 mmol, 199 mg) in 4 mL of acetonitrile under vigorous stirring at room temperature, and the pH was maintained at 7 by the addition of a 1 mol/L KOH solution. The slurry was kept under magnetic stirring for 30 min at room temperature, heated at 140 °C for 3 days in a Teflon-lined vessel (25 mL). After the mixture cooled to room temperature at a rate of 5 °C/h. Colorless block single crystals of 1, suitable for X-ray diffraction, were collected by filtration washed with deionized water. Yield: 65.1% (handpicking crystals, based on Zn(NO3)26H2O). Anal. Calcd. for C25H16N7O9Zn1.5 (%): C, 45.9; H, 2.5; N, 15.0. Found (%): C, 45.7; H, 2.7; N, 14.8. IR (KBr, cm1): 3418 (s), 3112 (w), 2935 (w), 1650 (w), 1625 (s), 1523 (m), 1424 (w), 1327 (s), 1273 (m), 1147 (m), 1055 (w), 977 (m), 826 (m), 729 (w), 652 (w), 514 (w). Synthesis of {[Zn(btb)(3-nph)](H2O)}n (2) A mixture of 3-H2nph (0.3 mmol, 63 mg), Zn(NO3)26H2O (0.5 mmol, 199 mg), btb (0.2 mmol, 58 mg) and 12 mL distilled water was stirred for 30 min. The mixture was transferred to a 25 mL Teflon-lined reactor and kept under autogenous pressure at 140 °C for 3 days. After the reactor was slowly cooled to room temperature at a rate of 5 °C/h, the colorless block single crystals of 2 were obtained in a yield of about 50.6 % based on Zn(NO3)26H2O. Anal. Calcd. for C24H17N7O7Zn (%): C 49.7, H 3.0, N 16.9; Found (%): C, 49.8; H, 2.7; N, 17.1. IR (KBr, cm1): 3418 (m), 3096 (w), 1598 (w), 1525 (s), 1460 (w), 1352 (s), 1280 (m), 1228 (w), 1149 (m), 1056 (m), 975 (s), 827 (m), 785 (w), 752 (w), 673 (w), 536 (w). Synthesis of [Zn(btb)(4-nph)]n (3) The preparation of complex 3 was similar to that of 2 except that 3-H2nph (0.3 mmol, 63 mg) was used instead of 4-H2nph. Yield: 62.6% (based on Zn(NO3)26H2O). Anal. Calcd. for C24H15N7O6Zn (%): C, 51.3; H, 3.0; N, 17.5; Found (%): C 51.5, H 2.9, N 17.7. IR (KBr, cm1): 2948 (w), 2157 (w), 1648 (m), 1555 (s), 1526 (m), 1372 (s), 1182 (m), 1028 (w), 984 (s), 917 (w), 756 (w), 663 (w), 546 (w), 488 (m).

Table 1 Crystal and refinement data for complexes 1–3. Complex

1

2

3

Chemical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalcd (g/cm3) l (mm1) GOF on F2 R1a [I > 2r (I)] wR2b (all data)

C25H16N7O9Zn1.5 656.50 Triclinic Pı¯ 7.981(3) 8.812(4) 18.181(7) 84.357(4) 84.342(3) 82.028(4) 3595.6(6) 2 1.737 1.516 1.182 0.0241 0.0652

C24H17N7O7Zn 580.82 Monoclinic P21/n 12.781(5) 10.962(4) 16.853(6) 90 100.443(4) 90 6371.4(15) 4 1.661 1.122 0.917 0.0452 0.1097

C24H15N7O6Zn 562.80 Monoclinic P21/n 12.7911(3) 10.9623(3) 16.3076(5) 90 97.328(3) 90 7144.1(14) 2 1.648 1.142 0.997 0.0497 0.1360

R1 = R||Fo|  |Fc||/R|Fo|; wR2 = R [w F2o  F2c 2]/R[w F2o 2]1/2.

X.-x. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 109–115 Table 2 Selected bond lengths [Å] and angles [°] for complexes 1–3. Parameter

Value

Parameter

Value

1 Zn(1)–O(5)#1 Zn(1)–N(6)#3 Zn(2)–O(4) Zn(2)–O(1W) O(5)#1–Zn(1)–O(3)#2 O(3)#2–Zn(1)–N(6)#3

1.9365(15) 2.0152(16) 2.1669(14) 2.0951(14) 116.15(6) 114.46(6)

1.9631(14) 2.0416(14) 2.0897(17) 1.273(2) 120.65(7) 88.18(6)

88.18(6) 180.0

Zn(1)–O(3)#2 Zn(1)–O(2) Zn(2)–N(3) O(3)–C(24) O(5)#1–Zn(1)–N(6)#3 N(3)#4–Zn(2)– O(1W)#4 N(3)–Zn(2)–O(1W) N(3)#4–Zn(2)–O(4)

92.17(6) 94.81(6)

O(1W)–Zn(2)–O(4)#4 N(3)#4–Zn(2)–O(4)#4

85.19(6) 92.17(6)

87.83(6) 85.19(6)

O(1W)–Zn(2)–O(4) O(4)#4–Zn(2)–O(4)

94.80(5) 180.0

2 Zn(1)–O(4) Zn(1)–N(6)#2 O(4)–Zn(1)–O(1)#1 O(1)#1–Zn(1)–N(6)#2 O(1)#1–Zn(1)–N(3)

1.973(2) 2.028(3) 113.16(10) 108.56(11) 98.87(11)

Zn(1)–O(1)#1 Zn(1)–N(3) O(4)–Zn(1)–N(6)#2 O(4)–Zn(1)–N(3) N(6)#2–Zn(1)–N(3)

1.982(2) 2.067(3) 123.50(11) 109.51(10) 99.55(11)

3 Zn(1)–O(5)#1 Zn(1)–N(4) O(5)#1–Zn(1)–O(4) O(4)–Zn(1)–N(4) O(4)–Zn(1)–N(1)#2

1.990(3) 2.037(3) 111.93(11) 109.27(12) 96.87(12)

Zn(1)–O(4) Zn(1)–N(1)#2 O(5)#1–Zn(1)–N(4) O(5)#1–Zn(1)–N(1)#2 N(4)–Zn(1)–N(1)#2

2.005(3) 2.077(3) 126.34(12) 109.09(11) 98.35(12)

N(3)#4–Zn(2)–O(1W) O(1W)#4–Zn(2)– O(1W) N(3)#4–Zn(2)–O(4W) O(1W)#4–Zn(2)– O(4)#4 N(3)–Zn(2)–O(4)#4 O(1W)#4–Zn(2)–O(4)

111

condensation procedure, where 4,40 -dibromobiphenyl reacts with 1,2,4-1H-triazole in the presence of CuI catalyst, 1,10-phenathroline, and K2CO3. Although the cation size of the base does have an effect on the yield of coupling reactions of the aryl halides and 1,2,4-1H-triazole, high yield was also obtained by employed DMF as the reacting solvent. It is well known that the base K2CO3 is much cheaper than Cs2CO3. More importantly, the chelating ligand 1,10-phenathroline is commercially available, and the price is also low. The as-synthesized ligand btb is characterized by 1HNMR and IR.

91.82(6) 87.83(6)

Symmetry codes for 1: #1: x, y  1, z; #2: x + 1, y  1, z + 1; #3: x + 1, y, z; #4: x, y, z + 1. For 2: #1: x + 3/2, y + 1/2, z + 3/2; #2: x + 1/2, y + 1/2, z + 1/2. For 3: #1: x + 3/ 2, y + 1/2, z + 3/2; #2: x + 3/2, y + 3/2, z + 1/2.

X-ray crystallography Crystallographic data for complexes 1–3 were collected on a Bruker Smart CCD diffractometer with Mo Ka radiation (k = 0.71073 Å) by using an x  2h scan mode. Semi-empirical absorption corrections were applied using the SADABS program [19]. The structures were solved by direct methods and refined on F2 by full-matrix least-squares using the SHELXL-97 program package [20]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of all water molecules could be located from a difference Fourier map, while the other hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters riding on the corresponding parent atoms. Crystal parameters and details of the final refinement parameters are shown in Table 1. The selected bond lengths and bond angles for 1–3 are listed in Table 2. Results and discussion Design and synthesis of btb ligand The N-arylation of 1,2,4-1H-triazole with aryl bromides or iodides has been found to be greatly accelerated by combining catalytic CuO with some chelating ligands in the presence of Cs2CO3 (Scheme 1) [21]. Herein the 1,2,4-1H-triazole-arylated ligand btb was synthesized in good yield using a modified Ullmann

Crystal structures Crystal structure of [Zn1.5(btb)(nbta)(H2O)]n (1) Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the triclinic space group Pı¯. The asymmetric unit of 1 contains one and a half Zn(II) ion, one btb ligand, one completely deprotonated nbta3 ligand and one coordinated water molecule. As shown in Fig. 1a, Zn1 is coordinated by three oxygen atoms from different nbta3 ligands and one nitrogen atom from btb ligand to give the square planar geometry with the s4 parameter being 0 [22], and at Zn2 the coordination environment is a distorted octahedron, in which the basal plane contains four oxygen donor (O4, O4#4, O1W and O1W#4, symmetry code: #4: x, y, z + 1) from two monodentate carboxylate groups and two coordinated water molecules. The apical position contains two nitrogen atoms (N3, N3#4) from different btb ligands. The Zn–N distances from 2.0152(16) Å to 2.0897(17) Å, and the Zn–O lengths are in the range of 1.9321(13)–2.1669(14) Å, which are all comparable to those observed in related Zn(II) polymer structures [23]. In compound 1, the nbta3 ligand is completely deprotonated and coordinates to four Zn centers (three Zn1 centers, one Zn2 center) to form a Zn4(nbta)3 unit. The Zn4(nbta)3 units are conjoined into a 2D grid-like layer along the a- and b-axis with the through-ligand Zn  Zn distances of 4.4500(3)–7.5024(4) Å. Each Zn2 links two water molecules to further modify the 2D structure (Fig. 1b). Notably, the btb ligands display cis-conformation with Ndonor  N-Csp3  Csp3 torsion angle of 1.43° and act as l2-bridging linker bridging adjacent Zn1 or Zn2 atoms to assemble into two parallel [Zn2(btb)2] units. All the Zn  Zn distances through btb ligands are 17.5776(8) Å. Furthermore, the [Zn2(btb)2] units link the parallel 2D grid-like layers Zn4(nbta)3 by sharing zinc common centers into a 3D framework (Fig. 1c) with the layers separation of 16.55 Å. A topological analysis of this network by the TOPOS 4.0 program [24], the 3D net can be simplified by considering the Zn1 atoms, Zn2 atoms, and nbta3 ligands as 4-connected nodes while the neutral btb ligands act as connector. As shown in Fig. 1d, the Zn1 atoms connect directly to three nbta3 ligand nodes and one Zn2 atom through a btb tether, while the Zn2 atoms connect to two Zn1 atoms through btb linkers and to two nbta3 ligand nodes. Thus, each nbta3 ligand node connects to three Zn1 atoms and one Zn2 atom. Hence, the rare binodal frl ‘‘Ferey ladder’’ topology was obtained with a Schläfli symbol of (42  64)(64  82). This frl topology has been seen previously only in the Cd(II)-MOF {[Cd3(acon)2(dpa)2(H2O)2]4H2O} (acon = transaconitate, dpa = 4,40 -dipyridylamine) [25]. Solvent accessible voids within the polymeric framework occupy 3.3% of the unit cell volume, as calculated by the PLATON program [26].

Scheme 1. Molecular scheme of ligand btb.

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Fig. 2. (a) The coordination environment of Zn(II) ion in 2. Hydrogen atoms are omitted for clarity (30% ellipsoid probability). Symmetry codes: #1: x + 3/2, y + 1/ 2, z + 3/2; #2: x + 1/2, y + 1/2, z + ½. (b) The 1D zigzag chain formed by Zn(II) and 4-nph2 ligands in 2. (c) View of the 2D layer in 2.

Fig. 1. (a) The coordination environment of Zn(II) ion in 1. Hydrogen atoms are omitted for clarity (30% ellipsoid probability). Symmetry codes: #1: x, y  1, z; #2: x + 1, y  1, z + 1; #3: x + 1, y, z; #4: x, y, z + 1. (b) The 2D network connected by Zn(II) and nbta3 anions. (c) View of the 3D framework of 1. (d) Corresponding binodal frl topology for 1.

Crystal structure of {[Zn(btb)(3-nph)](H2O)}n (2) Single-crystal X-ray diffraction analysis reveals that complex 2 is a 2D network and crystallizes in the monoclinic space group

P21/n. The asymmetric unit of 2 consists of only one independent Zn(II) ion, one neutral btb, one 3-nph anion and one lattice water molecule. As shown in Fig. 2a, The Zn(II) ion is located in a highly distorted tetrahedral geometry (s4 = 0.67), where the metal atom is coordinated by two nitrogen atom from two btb ligands with Zn–N distances of 2.028(3) Å and 2.067(3) Å, two oxygen atoms from two 3-nph2 ligands, with Zn–O distances of 1.973(2) Å and 1.982(2) Å, respectively. The ligand btb adopts l2-bridging coordination mode bridging two Zn(II) ions to generate a 1D zigzag chain, where the non-bonding distance of Zn  Zn is 17.5940(8) Å. In each btb ligand the dihedral angle between two triazole planes of 35.35(7)°. The 3-nph2 acting as bis-monodentate ligands bridge two adjacent Zn(II) ions to build the other 1D zigzag chain, in which the two nitro group substituents of the neighboring carboxylate ligands display opposite directions (Fig. 2b). The two kinds of 1D chains crossed each

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between btb and 3-nph2 ligands further reinforce the 2D network. Intriguingly, a void volume of 151.1 Å3 is also left and the pore volume ratio was 6.5%. Crystal structure of [Zn(btb)(4-nph)]n (3) Complex 3 crystallizes in the monoclinic space group P21/n. There are one Zn(II) ion, one btb and one 4-nph2 ligand in the asymmetric unit of complex 3. Fig. 3a illustrates the coordination environment of the metal center, as similar with complex 2, which is a highly distorted tetrahedral geometry with the s4 of 0.66. In complex 3, each Zn(II) ion is coordinated by two oxygen atoms (O4, O5#1, symmetry code: #1: x + 3/2, y + 1/2, z + 3/2) deriving from two 4-nph2 anions and two nitrogen atoms (N1#2, N4, symmetry code: #2: x + 3/2, y + 3/2, z + 1/2) originating from two btb ligands. The Zn–N distances are 2.037(3) Å and 2.077(3) Å, and the Zn–O lengths are 1.990(3) Å and 2.005(3) Å. The bond angles at the Zn center range from 96.87(12)° to 126.34(12)°. Similar to complex 2, the btb ligands take l2-bridging coordination mode and exhibit anti-conformation. The adjacent Zn(II) ions are bridged by the btb ligands to produce a 1D zigzag chain and the non-bonding distance of Zn  Zn is 17.6250(7) Å. The fully deprotonated 4-nph2 ligands coordinate to neighboring Zn atom performing bis-monodentate coordination mode to form a 1D zigzag chain traveling along the crystallographic c-direction with a through-ligand Zn  Zn contact distance of 6.1204(6) Å. Two types of 1D chains are combined to generate a 2D (4,4) sql network (Fig. 3b). For the organic ligand 4-nph2 in complex 3, although its coordination mode is the same as that in complex 2, the nitro group substituents in the different sites. Subtle variation of carboxylate ligand, 4-nph2 has weaker steric hindrance than 3nph2, which makes slightly effect on the final 2D structure. The 2D layers are further extended into a 3D supramolecular framework by three kinds of hydrogen bonding interactions between btb and carboxylate groups (H1  O2#3 = 2.52 Å, C1– H1  O2#3 = 137°, H16  O3#4 = 2.26 Å, C16–H16  O3#4 = 168°, H16  O6#4 = 2.52 Å, C16–H16  O6#4 = 117°, symmetry codes: #3: x + 1/2, y + 1/2, z + 3/2; #4: x + 1, y + 1, z + 1) (Fig. 3c). Effect of organic carboxylate ligands on the structures of the MOCPs In this work, we select rigid 4,40 -bis(1,2,4-triazolyl-1-yl)-biphenyl and three different aromatic carboxylates with nitro group to investigate their influence on the assembly of the bis(triazole)based MOCPs. The differences of the carboxylic anions and the conformations of btb lead to the distinction of the resultant structures. According to the above structural descriptions, the btb N-containing ligands uniformly behave as l2-bridging linkers to connect the metal centers in 1–3, but display different conformations (Fig. 4)

Fig. 3. (a) The coordination environment of Zn(II) ion in 3. Hydrogen atoms are omitted for clarity (30% ellipsoid probability). Symmetry codes: #1: x + 3/2, y + 1/ 2, z + 3/2; #2: x + 3/2, y + 3/2, z + ½. (b) The 2D (4,4) sql network in 3. (c) A schematic representation of the 3D supramolecular framework formed by H-bonds (dashed lines) in 3.

other resulting in a 2D (4,4) grid structure. As depicted in Fig. 2c, there exist a kind of cycle with the dimension of 6.15  17.59 Å, which is constructed by two btb ligands, two nbta3 ligands and two Zn(II) ions. Each Zn(II) ion is linked by adjacent another four Zn(II) ions, which can be regarded as a 4-connected node through btb and 3-nph2. So the topology of the 2D structure of 2 is described as 4-connected sql network and the point symbol is (44  64). Moreover, hydrogen bonding interactions C–H  O

Fig. 4. The conformation of btb in complexes 1–3.

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well as one nitro group in 3 position but acts as l2-bridging linker, a 2D 4-connected sql layer with a point symbol of (44  64) for 2 was formed. 3-nph2 and 4-nph2 display the same coordination mode. When the 4-nph2 with two carboxyl groups and one nitro group in 4 position was employed, the other 2D (44  62) sql network for 3 was constructed, which is further extended into a 3D supramolecular framework by hydrogen bond interactions. The above results clearly suggest that the non-coordinated nitro group as electron-withdrawing component may have some effect on the electron density of the carboxylic acid ligands and the subtle difference in secondary carboxylic acid ligand has a great influence on the final architecture of the MOCPs. IR and XRPD The IR spectra exhibit the main characteristic bands of btb, water molecules and carboxylate ligands for the title complexes. There is no absorption peak around 1700 cm1 for –COOH observed, indicating that all carboxyl groups of the organic moieties are deprotonated [27]. Characteristic bands at 1523 cm1 for 1, 1525 cm1 for 2, and 1526 cm1 for 3 are assigned to the mC@N stretching vibration of 1,2,4-triazole. The strong characteristic bands at 1625 cm1, 1424 cm1 for complex 1, 1598 cm1, 1352 cm1 for complex 2 and 1648 cm1, 1372 cm1 for complex 3 can be considered as the asymmetric and symmetric vibrations of the carboxyl groups, respectively. The Dt[tas(COO)–ts(COO)] are 201 cm1, 246 cm1 and 276 cm1, indicating monodentate coordination of the carboxylate group to the metal center [28]. The strong broad band, with its center around 3450 cm1 for 1 and 2, relates to the O–H stretching vibration modes of hydrogen bonds. X-ray powder dffraction (XRPD) was used to check the purity of complexes 1–3. As shown in Fig. 5, The bulk of the crystalline samples of complexes 1–3 were confirmed by a good match between the experimental and simulated X-ray powder diffraction (XRPD) patterns, indicating that the bulk synthesized material and the as-grown crystals are homogeneous. Thermal and photoluminescence properties Thermogravimetric experiments (TGA) were carried out to study their thermal stabilities, which is an important parameter for the title complexes (Fig. 6). The TGA curves of 1 and 2 show two obvious weight loss steps. The first weight loss of 3.0% for 1 in the region of 185–230 °C is equivalent to the elimination of the coordinated water molecules. Pyrolysis of the residual

Fig. 5. (a–c) The X-ray powder diffraction patterns calculated from the singlecrystal data and that obtained from the experiments for polymers 1–3, respectively.

and distinct Ndonor  N–Csp3  Csp3 torsion angles (1.42° for 1, 129.41° for 2, and 117.29° for 3), which lead to generating different non-bonding ZnZn distances through the bis(triazole) ligands (17.5776(8) Å for 1, 17.5940(8) Å for 2, 17.6250(7) Å for 3). In complexes 1–3, the carboxyl groups show different coordination modes, when the co-ligand was the nbta3 anion which shows l4-bridging mode, a 3D binodal frl framework for 1 was obtained. If nbta3 was replaced by 3-nph2 with two carboxyl groups as

Fig. 6. The TGA curves of polymers 1–3 were measured in N2 atmosphere.

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115

Acknowledgments The project was supported by the National Natural Science Foundation of China (51474086), Natural Science Foundation – Steel and Iron Foundation of Hebei Province (B2015209299) and the Hercules Foundation (project AUGE/11/029 ‘‘3D-SPACE: 3D Structural Platform Aiming for Chemical Excellence’’) for funding. Appendix A. Supplementary data CCDC 1033021-1033023 contain the supplementary crystallographic data for the complexes 1-3. 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 CB21EZ, UK; fax: +44 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.saa.2015.04.048. Fig. 7. Emission spectra of complexes 1–3 and the free btb ligand.

References substance occurs at 320 °C with a series of consecutive weight losses and stops at 510 °C, which implies the decomposition of organic ligands btb and nbta3 (calcd. 84.9%). The residue weight of 11.7% for 1 is due to the Zn and O components in ZnO (calcd. 12.4%). The TGA curve of complex 2 indicates the losses of the lattice water molecules with 2.3% (calcd. 3.1%) from 140 °C to 165 °C. The second weight loss of 82.2% is attributed to the decomposition of the ligands btb and 3-nph2 (calcd. 82.9%), which begins at 305 °C and completes by 604 °C. The percent of ZnO as a final product is about 14.8%, which is in accordance with the expected value 14.0%. There is only one weight loss stage in 3 and the weight loss corresponding to the release of the organic components is observed from 347 °C to 549 °C. Finally, the remnant is 15.1%, which should be ZnO (calcd. 14.5%). The samples for solid-state luminescence of complexes 1–3 and the free ligand btb have been investigated at room temperature due to their potential photoactive applications (Fig. 7) [29–30]. The free btb ligand displays a broad and strong emission at 439 nm upon excitation at 380 nm, which is probably attributed to n ? p⁄ or p ? p⁄ transition [31]. It is clear that there are emission bands at 383 nm, 410 nm, and 399 nm for complexes 1–3, respectively, with all the excitation at 350 nm. In comparison to free btb ligand, the blue-shift could be attributed to the coordination of the ligands to Zn(II) ions, which increase the rigidity of the ligands and reduce the loss of energy via radiationless decay of the intra-ligand emission excited state so that enhanced the luminescence efficiency [32]. Therefore, the blue-shift of emission of 1–3 (56 nm for 1, 29 nm for 2, 40 nm for 3) may be owing to the reduced conjugation of the ligand upon coordination. Conclusion In summary, three zinc(II) coordination polymers constructed from the btb assisted with carboxylate ligands have been successfully synthesized and characterized. Structure analysis demonstrate that the multidimensional coordination/supramolecular framework can be modulated by distinct carboxylate co-ligands in Zn-btb systems. The result shows that complexes 1–3 display blue-shift compared with free btb ligand and emit purple luminescence.

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Tuning zinc(II) coordination architectures by rigid long bis(triazole) and different carboxylates: Synthesis, structures and fluorescence properties.

Three metal-organic coordination polymers containing rigid bis(triazole) ligand, namely, [Zn1.5(btb)(nbta)(H2O)]n (1), {[Zn(btb)(3-nph)]·(H2O)}n (2) a...
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