Four Zinc(II) coordination polymers with dicarboxylate and Tri(4-pyridylphenyl)amine ligand: Syntheses, crystal structures and physical properties

Journal of Molecular Structure 1199 (2020) 127005

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Four Zinc(II) coordination polymers with dicarboxylate and Tri(4pyridylphenyl)amine ligand: Syntheses, crystal structures and physical properties Min Yu a, Fang Xuan a, Jian Li a, b, Guang-Xiang Liu a, c, * a b c

Nanjing Key Laboratory of Advanced Functional Materials, Nanjing Xiaozhuang University, Nanjing, 211171, PR China College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, PR China State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 June 2019 Received in revised form 10 August 2019 Accepted 29 August 2019 Available online 8 September 2019

Four novel zinc(II) coordination polymers (CPs), [Zn(TPPA)(phda)]n (1), [Zn(TPPA)(glut)$H2O]n (2), [Zn2(TPPA)2(brtp)2$2H2O]n (3) and [Zn4(TPPA)2(sdba)4$3H2O]n (4) (H2phda ¼ 1,3-phenylenediacetic acid, H2glut ¼ glutaric acid, H2brtp ¼ 2-bromoterephthalic acid and H2sdba ¼ 4,40 -sulfonyldibenzoic acid), have been successfully synthesized by varying the carboxylate anions in the presence of tri(4pyridylphenyl)amine (TPPA) ligand. The CPs were structurally characterized by single-crystal X-ray diffraction and furthered IR spectroscopy, elemental analyses and thermogravimetric analyses. Structural analyses reveal that complex 1 exhibits a noninterpenetrating three-dimensional (3D) framework with 65.8-dmp topology. Complex 2 consists of two-dimensional (2D) (4,4) grid networks with the point symbol of 44.62. Complex 3 has a 2D two-fold interpenetrating layer structure, while complex 4 features a 3D two-fold interpenetrating 4-connected 65.8-cds net. A comparison of all complexes suggested that the structural diversity of the complexes could be tuned by altering the carboxylate ligand. Four complexes indicate high thermal stabilities and different photoluminescence behavior in the solid state. Moreover, the nonlinear optical and ferroelectric properties of 1 have also been investigated. © 2019 Elsevier B.V. All rights reserved.

Keywords: Zinc(II) coordination polymer Dicarboxylate Triangular ligand Luminescence

1. Introduction Coordination polymers (CPs) have received much attention not only for their charming structures and distinctive topologies [1e3], but also for their numerous potential applications as functional materials in magnetism [4], electric conductivity [5], gas adsorption and separation [6], heterogeneous catalysis [7], light emitting [8], chemical sensing [9], nonlinear optics [10] and luminescence [11]. The structural types of CPs are susceptible to many factors, involving the nature of metal ions, organic ligands, counterions, ratio of metal and ligand, and coordination and cocrystallization of solvents [12e16]. Therefore, it has become an important topic to employ the appropriate factors to design and construct CPs with desired properties. Among these factors, the coordination geometry of metal ions and the structural characteristics of polydentate

* Corresponding author. Nanjing Key Laboratory of Advanced Functional Materials, Nanjing Xiaozhuang University, Nanjing, 211171, PR China. E-mail address: [email protected] (G.-X. Liu). https://doi.org/10.1016/j.molstruc.2019.127005 0022-2860/© 2019 Elsevier B.V. All rights reserved.

organic ligands play a paramount role. Systematic studies of diverse ligands, leading to different structures in the formation of coordination polymers, are thus important and of intense interest [17e19]. Owing to their natural characteristics, the tripodal multidentate ligands are prominent for constructing porous CPs with aesthetic topological structure [20e22]. Compared with benzene- and triazine-centered triangular ligands, the N-centered triangular ligands more easily meet the geometric requirement of metal ions [23]. Recently, a triangular N-centered ligand, namely, tris(4-(1Himidazole-1-yl)phenyl)amine (TIPA, Scheme 1), has been investigated intensely in the fields of coordination polymers [24e27]. Dozens of coordination polymers, with interesting properties and aesthetic topology structures, have been reported, because three nitrogen atoms of TIPA ligand can adjust coordination orientation along the CeN bonds connecting phenyl and imidazolyl [28]. When three imidazolyl groups of TIPA were replaced by three pyridyl groups (TPPA, Scheme 1), the conformational and geometrical flexibility weakened because nitrogen coordination atoms located on the axes of phenyl-pyridyl arms. TPPA ligand may have different

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Scheme 1. TIPA, TPPA and four different dicarboxylate ligands.

coordination modes when coordinating to metal ions, which may induce diverse structures [29e31]. On the other hand, Zn(II) complexes have attracted extensive interest in recent years in that they not only exhibit appealing structures but also possess photoluminescent properties [32e36]. The d10 Zn(II) ion is particularly suited for the construction of coordination polymers: The spherical d10 configuration is associated with a flexible coordination environment so that geometries of these complexes can vary from tetrahedral to octahedral and severe distortions in the ideal polyhedron occur easily. Herein, we focus on the possibility of reproducing the structural motifs in order to extend this series of coordination polymers by varying the length and conformational flexibility of dicarboxylic acids in order to study and tune the optical properties of the prepared CPs. In the paper, we report the synthesis, structural characterization, physical properties of four novel zinc(II) coordination polymers, [Zn(TPPA)(phda)]n (1), [Zn(TPPA)(glut)$H2O]n (2), [Zn2(TPPA)2(brtp)2$2H2O]n (3) and [Zn4(TPPA)2(sdba)4$3H2O]n (4) (H2phda ¼ 1,3-phenylenediacetic acid, H2glut ¼ glutaric acid, H2brtp ¼ 2-bromoterephthalic acid, H2sdba ¼ 4,40 -sulfonyldibenzoic acid and TPPA ¼ tri(4pyridylphenyl)amine). 2. Experimental

spectrophotometer in the range of 4000e400 cm1. The luminescent spectra for the powdered solid samples were measured at ambient temperature on a Horiba FluoroMax-4P-TCSPC fluorescence spectrophotometer. The backscattered SHG light was collected by a spherical concave mirror and passed through a filter that transmits only 532 nm radiation. The ferroelectric property of the solid-state sample was measured by the Premier II ferroelectric tester at room temperature.

2.2. Syntheses of hybrid complexes 2.2.1. [Zn(TPPA)(phda)]n (1) A mixture containing ZnI2 (31.9 mg, 0.1 mmol), H2phda (19.4 mg, 0.1 mmol), and TPPA (47.6 mg, 0.1 mmol) in DMF/CH3OH/ H2O (1:1:2) solution (10 ml) was sealed in a Teflon-lined stainless steel container and heated at 120  C for 3 days. After being cooled down to room temperature, colorless block crystals of 1 were obtained in 62% yield based on TPPA. Anal. Calcd. for C43H33N4O4Zn: C, 70.25; H, 4.52; N, 7.62. Found: C, 70.13; H, 4.52; N, 7.58%. IR (KBr, cm1): 3472 (m), 3088 (w), 2921 (w), 1609 (m), 1583 (m), 1542 (s), 1523 (s), 1501 (m), 1429 (m), 1387 (s), 1307 (w), 1224 (m), 1169 (w), 1091 (w), 1071 (m), 1021 (s), 928 (w), 853 (m), 824 (m), 794 (m), 751 (m), 682 (m), 626 (m).

2.1. Materials and analyses All the reagents and solvents for syntheses and analyses were commercially available and employed as received without further purification. The TPPA ligand was prepared according to the reported method [37]. Elemental analyses (C, H and N) were performed on a Vario EL III elemental analyzer. Thermal gravimetric analyses (TGA) were performed on a Netzsch STA-409PC instrument in flowing N2 with a heating rate of 10  C min1. Infrared spectra were recorded on KBr discs using a Nicolet Avatar 360

2.2.2. [Zn(TPPA)(glut)·H2O]n (2) The preparation of 2 was similar to that of 1 except that H2glut was used instead of H2phda. Pale yellow pillar crystals of 2 were collected in a 57% yield. Anal. Calc. for C38H32N4O5Zn: C, 66.14; H, 4.67; N, 8.12. Found: C, 65.97; H, 4.69; N, 8.10%. IR data (KBr, cm1): 3461 (w), 3069 (m), 2803 (w), 1607 (m), 1562 (s), 1423 (s), 1384 (s), 1321 (w), 1308 (w), 1205 (w), 1162 (w), 1061 (w), 1011 (m), 962 (w), 909 (m), 863 (w), 811 (m), 793 (s), 741 (w), 682 (m), 632 (w).

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2.2.3. [Zn2(TPPA)2(brtp)2·2H2O]n (3) A mixture containing Zn(NO3)2$6H2O (29.7 mg, 0.1 mmol), H2brtp (24.5 mg, 0.1 mmol), TPPA (47.6 mg, 0.1 mmol) and NaOH (8.0 mg, 0.2 mmol) in 8 ml deionized water was sealed in a Teflonlined stainless steel container and heated at 160  C for 3 days. After being cooled down to room temperature, colorless block crystals of 3 were obtained in 62% yield based on TPPA. Anal. Calc. for C82H58Br2N8O10Zn2: C, 61.33; H, 3.64; N, 6.98. Found: C, 61.22; H, 3.65; N, 6.97%. IR data (KBr, cm1): 3465 (w), 3079 (w), 3042 (w), 1587 (s), 1537 (m), 1438 (m), 1382 (s), 1337 (m), 1226 (m), 1087 (s), 934 (w), 914 (w), 817 (m), 661 (m), 650 (m), 581 (m), 544 (w). 2.2.4. [Zn4(TPPA)2(sdba)4·3H2O]n (4) The same synthetic procedure was followed to synthesize 4 as that of 3 except H2brtp was replaced by H2sdba (30.6 mg, 0.1 mmol). Colorless block crystals of 4 were collected in a 35% yield. Anal. Calc. for C122H86N8O27S4Zn4: C, 58.95; H, 3.49; N, 4.51. Found: C, 58.83; H, 3.50; N, 4.50%. IR data (KBr, cm1): 3492 (m), 3072 (m), 1553 (s), 1497 (m), 1417 (s), 1389 (s), 1361 (s), 1322 (m), 1217 (w), 1165 (w), 1112 (w), 1081 (m), 972 (w), 848 (m), 811 (m), 795 (w), 717 (w), 672 (m), 645 (m), 582 (w), 511 (w). 2.3. Single-crystal X-ray diffraction Single crystal X-ray diffraction data for complexes 1e4 were recorded on a Bruker Smart Apex II CCD diffractometer using Mo Ka radiation (l ¼ 0.71073 Å). The diffraction data were integrated by using the SAINT program [38], which was also used for the intensity corrections for the Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the SADABS program [39]. The structures were solved by direct methods using the SHELXS-2016 program [40] and refined by full-matrix least-squares on F2 using the SHELXL-2016 program [41]. All of the non-hydrogen atoms were refined anisotropically. The hydrogen atoms of the ligands were placed in the geometrically calculated positions. The detailed crystallographic data and structure refinement parameters for 1e4 are summarized in Table 1. Selected bond lengths (Å) and bond angles (º) for 1e4 are listed in Table 2. 3. Results and discussion 3.1. Crystal structure descriptions Single-crystal X-ray diffraction study reveals that complex 1 crystallizes in orthorhombic system acentric space group Pna21 with a Flack parameter of 0.18(3). The asymmetric unit contains an independent Zn(II) ion, a unique TPPA ligand and an individual phda2 anion as depicted in Fig. 1a. The central Zn(II) ion is fourcoordinated by two carboxyl oxygen atoms from two different phda2 anions and two pyridyl nitrogen atoms from two different TPPA ligands in a distorted tetrahedral geometry. The bond angles around the central Zn(II) atom vary from 95.0(3) to 127.0(3)º and The ZneO(N) distances vary from 1.993(9) to 2.072(6) Å, which are comparable to those reported for other zinceoxygen and zincenitrogen donor compounds [42]. The Zn(II) ions are bridged by the phda2 anions adopting the bis(monodentate) bridging mode to form 21 helix chain along the c axis with the Zn/Zn separation of 6.687 Å, as shown in Fig. 1b. All the helix chains are homochiral with the left-hand feature, so the 3D metal-organic framework belongs to the chiral molecule proved by single X-ray diffraction. The 1D helix chains are further linked through the TPPA ligands to generate a three-dimensional (3D) framework with 65.8dmp topology. Although the flexibility of the organic ligands leads to the formation of a 1D helical chain structure, the entire 3D architecture of complex 1 is chiral which is rare.

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Table 1 Crystal data and structure refinements for 1e4. Complex

1

2

Empirical formula Molecular weight Crystal system Space group a (Å) b (Å) c (Å) a (º) b (º) g (º) V (Å3) Z T (K) Dcalc (g cm3) m (mm1) F(000) q range (º) Reflections collected Rint Data/restraints/parameters Goodness-of-fit on F2 R1[I > 2s(I)]a wR2[I > 2s(I)]b Largest diff. peak and hole (e Å3)

C43H33N4O4Zn 735.10 Orthorhombic Pna21 10.1540(8) 31.523(3) 11.2226(9) 90 90 90 3592.1(5) 4 293(2) 1.359 0.733 1524 2.78e25.50 51008 0.0717 6676/626/554 1.123 0.0902 0.1931 0.583/-0.407

C38H32N4O5Zn 690.04 Monoclinic P21/c 14.799(15) 17.96(2) 12.237(12) 90 99.54(4) 90 3207(6) 4 293(2) 1.429 0.818 1432 2.79e25.01 50305 0.0956 5706/0/433 1.035 0.0517 0.0909 0.467/-0.511

Complex

3

4

Empirical formula Molecular weight Crystal system Space group a (Å) b (Å) c (Å) a (º) b (º) g (º) V (Å3) Z T (K) Dcalc (g cm3) m (mm1) F(000) q range (º) Reflections collected Rint Data/restraints/parameters Goodness-of-fit on F2 R1[I > 2s(I)]a wR2[I > 2s(I)]b Largest diff. peak and hole (e Å3)

C82H58Br2N8O10Zn2 1605.92 Triclinic P
ı 11.1754(13) 15.2076(17) 22.997(3) 86.824(3) 80.137(3) 87.280(3) 3841.9(8) 3 300(2) 1.388 1.727 1632 2.79e25.50 47389 0.0374 14282/18/947 1.038 0.0537 0.1478 0.905/-0.795

C122H86N8O27S4Zn4 2485.70 Monoclinic C2/c 18.204(3) 17.378(3) 37.207(7) 90 90.438(5) 90 11770(4) 4 300(2) 1.403 0.954 2680 3.12e27.66 93645 0.0992 13606/0/780 1.074 0.0570 0.1223 0.485/-0.606

a b

R1 ¼ SjjFoj - jFcjj/SjFoj. wR2 ¼ jSw(jFoj2 - jFcj2)j/Sjw(Fo)2j1/2.

In order to investigate the influence of the carboxylate ligands on the structure of the complexes, reaction of H2glut with ZnI2 in the present of TPPA were carried out, and 2 was obtained. Complex 2 crystallizes in the monoclinic symmetry space group P21/c. As illustrated in Fig. 2a, the structure of 2 contains one crystallographic Zn(II) ion, one unique glut2 anion, one individual TPPA ligand and one free water molecule. As shown in Fig. 2a. The central Zn(II) ion is four-coordinated by two carboxyl oxygen atoms from two different glut2 anions and two pyridyl nitrogen atoms from two different TPPA ligands in a distorted tetrahedral geometry. The bond angles around the central Zn(II) atom vary from 94.37(13) to 116.27(12)º. The ZneO bond lengths are 1.931(3) and 1.946(3) Å, and the ZneN bond lengths are 2.018(3) and 2.038(3) Å, respectively. Two adjacent Zn(II) ions are firstly cohered together by a pair of glut2 anions with the bidentate bridging and monodentate

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Table 2 Selected bond lengths (Å) and angles (º) for 1e4. 1 Zn(1)eN(4) Zn(1)eO(3)#1 O(2)-Zn(1)-N(4) O(3)#1-Zn(1)-N(4) 2 Zn(1)eO(1) Zn(1)eO(3)#1 O(1)-Zn(1)-O(3)#1 O(1)-Zn(1)-N(2) O(3)#1-Zn(1)-N(2) 3 Zn(1)eO(1) Zn(1)eO(5) Zn(2)eO(3) Zn(2)eO(7)#2 Zn(2)eN(6) O(1)-Zn(1)-O(5) O(1)-Zn(1)-N(2) O(5)-Zn(1)-N(2) O(3)-Zn(2)-O(7)#2 O(3)-Zn(2)-N(6) O(7)#2-Zn(2)-N(6) O(3)-Zn(2)-N(7)#3 O(7)#2-Zn(2)-N(7)#3 4 Zn(1)eO(6) Zn(1)eO(7)#1 Zn(1)eO(9) Zn(1)eN(4) Zn(1)eO(12)#1 N(4)-Zn(1)-O(9) N(4)-Zn(1)-O(6) O(9)-Zn(1)-O(6) N(4)-Zn(1)-O(7)#1 O(9)-Zn(1)-O(7)#1 O(6)-Zn(1)-O(7)#1 N(4)-Zn(1)-O(12)#2 O(9)-Zn(1)-O(12)#2 O(6)-Zn(1)-O(12)#2 O(7)#1-Zn(1)-O(12)#2

2.038(7) 1.993(9) 127.0(3) 100.5(5)

Zn(1)eN(2)#2 Zn(1)eO(2) O(3)#1-Zn(1)-N(2)#2 N(4)-Zn(1)-N(2)#2

2.072(6) 1.944(9) 95.0(3) 103.3(3)

1.931(3) 1.946(3) 94.37(13) 110.03(12) 116.27(12)

Zn(1)eN(2) Zn(1)eN(3)#2 O(1)-Zn(1)-N(3)#2 O(3)#1-Zn(1)-N(3)#2 N(2)-Zn(1)-N(3)#2

2.018(3) 2.038(3) 114.80(14) 111.42(11) 109.43(13)

1.984(3) 1.992(3) 1.955(3) 2.008(3) 2.039(4) 100.26(12) 111.30(14) 105.35(13) 132.08(15) 104.72(15) 112.03(15) 94.59(14) 101.77(15)

Zn(1)eN(2) Zn(1)eN(3)#1 Zn(2)eN(7)#3 Zn(2)eO(8)#2

2.014(3) 2.019(3) 2.071(4) 2.423(4)

O(1)-Zn(1)-N(3)#1 O(5)-Zn(1)-N(3)#1 N(2)-Zn(1)-N(3)#1 N(6)-Zn(2)-N(7)#3 O(3)-Zn(2)-O(8)#2 O(7)#2-Zn(2)-O(8)#2 N(6)-Zn(2)-O(8)#2 N(7)#3-Zn(2)-O(8)#2

102.60(13) 110.46(14) 124.38(14) 108.19(15) 92.52(14) 57.62(14) 91.75(15) 156.28(15)

2.042(2) 2.045(3) 2.029(3) 2.018(3) 2.047(3) 110.96(12) 97.69(11) 87.96(12) 92.65(12) 156.39(11) 88.55(12) 100.06(12) 88.47(12) 162.01(11) 87.68(12)

Zn(2)eO(10) Zn(2)eO(8)#1 Zn(2)eO(5) Zn(2)eO(11)#1 Zn(2)eN(2)#2 O(8)#1-Zn(2)-N(2)#2 O(8)#1-Zn(2)-O(5) N(2)#2-Zn(2)-O(5) O(8)#1-Zn(2)-O(10) N(2)#2-Zn(2)-O(10) O(5)-Zn(2)-O(10) O(8)#1-Zn(2)-O(11)#1 N(2)#2-Zn(2)-O(11)#1 O(5)-Zn(2)-O(11)#1 O(10)-Zn(2)-O(11)#1

2.059(3) 2.017(3) 2.029(2) 2.063(3) 2.023(3) 98.33(12) 90.65(11) 102.37(11) 161.57(12) 99.80(12) 88.74(12) 87.39(12) 100.87(11) 156.72(11) 85.95(13)

Symmetry transformations used to generate equivalent atom: #1: xþ1, -yþ1, zþ1/2; #2: xþ3/2, yþ1/2, z-1/2 for 1; #1: xþ2, -y, -zþ2; #2: xþ1, -yþ1/2, z-1/2 for 2; #1: xþ1, y1, z; #2: x-1, y-1, z; #3: x-1, yþ1, z for 3; #1: xþ1/2, yþ1/2, z; #2: xþ1/2, -yþ1/2, zþ1/2 for 4.

coordination modes to form a dimeric unit with the Zn/Zn separation of 5.232 Å. Adjacent binuclear units are cross-linked by TPPA coligands along two different directions to result in a 2D layer motif containing rhombic grids with a side length of 18.349 Å, as shown in Fig. 2b. Individual layers stack along the c direction with a eABABe sequence (Fig. 2c and d) and are further cohered together by O(1 W)eH1WB$$$O(4) and O(1 W)eH1WA$$$O(2) hydrogenbonding interactions. Additionally, there exists a larger number of parallel benzene rings for this packing. Cofacial arrangement 3.812 Å apart between the layers means stronger intermolecular pep interactions synergistically stabilizing its 3D supramolecular architecture. The use of H2brtp ligand results in a 2D two-fold interpenetrating topology of complex 3. The structure analysis reveals that the asymmetric unit consists of two crystallographically independent Zn(II) ions, two individual brtp2 anions, two unique TPPA ligands and two lattice water molecules. As illustrated in Fig. 3a, Zn1 ion displays a distorted tetrahedral geometry, being surrounded by two carboxylic oxygen atoms from two different brtp2 anions. While Zn2 ion is five-coordinated with a slightly distorted square pyramidal coordination geometry with a t value of 0.400, by two nitrogen atoms from two different TPPA ligands and three carboxylic oxygen atoms from two distinct brtp2 anions. The ZneO bond lengths are in the range of 1.984(3) - 2.423(4) Å, the ZneN bond lengths are between 2.014(3) and 2.071(4) Å, and the

coordination angles around Zn ion span from 57.62(14) to 156.28(15)º. The ZneN and ZneO bond lengths and the bond angles around Zn(II) atoms in 3 are comparable with other Zn(II) coordination polymers [42]. The brtp2 anions adopt two kinds of coordination modes (m2h1:h1-and m2-h2:h1-). In mode I, two carboxylate groups have a dihedral angle of 4.71 and 58.28 towards the plane of the corresponding linking phenyl rings, respectively. Compared with mode I, two carboxylate groups have a dihedral angle of 10.11 and 37.79 towards the plane of the corresponding linking phenyl rings in mode II, respectively. Each Zn(II) ion is connected to two different brtp2 anions and each brtp2 anion is connected to two Zn(II) ions, building up a 1D chain, which are further connected by TPPA ligands to form a 2D puckered sheet (Fig. 3b). There is a large window in the puckered sheet with dimensions of 19.67  22.96 Å2. Within the sheet, the Zn(II) ions are not all coplanar, rather, half fall in one plane, half in the other parallel plane. The puckered nature of the sheets with large square windows may offer a good chance to generate a 2D þ 2D / 2D interpenetration bi-layer structure with a {44.62} topology (Fig. 3c). More interestingly, the neighboring 2D sheets are staggered parallel with each other, where the lattice water molecules are sandwiched between the layers. The 2D layers are sustained by OeH/O hydrogen-bonding between the oxygen atoms of the carboxylate and lattice water molecules and CeH$$$p interactions

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Fig. 1. (a) Perspective view of the coordination environments of the Zn(II) ion in 1 with thermal ellipsoids at 30% probability. The symmetry codes: #1: xþ1, -yþ1, zþ1/2; #2: xþ3/2, yþ1/2, z-1/2. (b) 1D helical chain [Zn(phda)]n of 1. (c) View of the 3D structure constructed by [Zn(phda)]n helical chains and TPPA ligands. (d) Schematic representations of a simplified 3D network for 1 with dmp topology.

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Fig. 2. (a) ORTEP view of 2 showing the local coordination environment of the Zn(II) ion. #1: xþ2, -y, -zþ2; #2: xþ1, -yþ1/2, z-1/2. (b) Perspective view of the 2D layer structure based on the dimeric unit. (d) Schematic representation of the 2D (4,4) framework.

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Fig. 3. (a) Perspective view of the coordination environments of the Zn(II) ion in 3 with thermal ellipsoids at 30% probability. The symmetry codes: #1: xþ1, y-1, z; #2: x-1, y-1, z; #3: x-1, yþ1, z. (b) View of the 2D [Zn(brtp)(TPPA)]n network of 2. (c) The 2D þ 2D / 2D interpenetration network in 3. (d) 3D supramolecular structure based on CeH$$$p and OeH/O interactions in 3.

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Fig. 4. (a) Perspective view of the coordination environments of the Zn(II) ion in 4 with thermal ellipsoids at 30% probability. The symmetry codes: #1: xþ1/2, yþ1/2, z; #2: xþ1/2, -yþ1/2, zþ1/2. (b) One-dimensional chain structure of 4. (c) The 3D metal-organic framework constructed from 1D double chains and TPPA ligands. (d) A schematic diagram of the CdSO4 topology. (e) A view of the 2-fold interpenetrating net.

among the TPPA ligands (Fig. 3d). Obviously, this packing mode decreases the molecular repulsion and stabilizes the whole structure of 3. Complex 4 crystallizes in the monoclinic symmetry space group C2/c. As illustrated in Fig. 4a, the structure of 4 contains two unique Zn(II) ions, two unique sdba2 anions, one TPPA ligand, as well as one and a half lattice water molecules with the metal-based building unit comprising a binuclear Zn(II)-tetracarboxylate

paddlewheel cluster. In this binuclear unit (Fig. 3b), each Zn(II) ion resides in a square pyramidal coordination geometry with the apical position occupied by one nitrogen atom from the TPPA ligand, while the basal plane consists of four carboxylate oxygen atoms. The intradimer Zn/Zn separation is 2.9577(5) Å, within the normal range found in other reported bimetallic paddlewheel units of type [M2(O2CR)4] [43e45]. In 4, each paddlewheel unit connects to two neighboring paddlewheel clusters via bridging sdba2

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Fig. 5. TGA curves of complexes 1e4.

anions, which adopt a bridging tetradentate mode to form a 1D double chain (Fig. 4b). The TPPA ligand connects the two paddle wheels of two different double chains with a separation of 18.016 Å. These chains are further connected by bridging TPPA linkers to give rise to a complicated 3D network with the small solvent-accessible void space (equal to 13.3% of the cell volume) (Fig. 4c). The two pyridine rings in the TPPA ligand twist from the phenyl ring plane by a torsion angle of 23.46 and 37.01, respectively. Better insight into the elegant framework of 4 can be accessed by the topology method. In this analysis, the metal center can be viewed as a 4connect node and meanwhile the sdba2 and TPPA bridges can be viewed as linkers. In this way, this net can be simplified as CdSO4 topology (Fig. 4d). Moreover, the occurrence of 2-fold interpenetration is also observed, as exhibited in Fig. 4e. From the structural descriptions above, we can see that complexes 1e4 exhibit structural diversity from 2D to 3D net frameworks due to the different coordination modes of the ligand and diverse coordination numbers of Zn(II) ion. The coordination numbers of Zn(II) ions in complexes 1e4 are 4, 4/5, 4 and 5 respectively. In 1, phda2 ligands link Zn(II) ions to chain structures, which is further extended by TPPA ligands to 3D framework. In 2, glut2 ligands link Zn(II) ions to dimers, and TPPA as bridge ligands, help to construct 2D supramolecular structures of complex 2. In 3, brtp2 ligands link Zn(II) ions to chain structures, which is further bridged by TPPA ligands to 2D lattice layers. In 4, sdba2 ligands link Zn(II) ions to 1D double chains, and TPPA as bridge ligands, help to construct 3D framework of complex 4. In conclusion, it is obviously found that the length and conformational flexibility of dicarboxylic acids lead to the main differences of structures of 1e4; that as well as coordination geometries of Zn(II) ion are also important for different networks of the titled complexes. 3.2. FT-IR spectroscopy The IR spectra of 1e4 show the absence of the characteristic bands at around 1700 cm1 attributed to the protonated carboxylate group indicates that the complete deprotonation of four carboxylate ligands upon reaction with metal ion. The characteristic bands of carboxyl groups are shown in the range of 1540e1610 cm1 for antisymmetric stretching and 13601460 cm1 for symmetric stretching. The separations (D) between

Fig. 6. (a) Solid-state emission spectra of complexes 1e4 at room temperature; (b) CIE chromaticity diagrams of complexes 1e4.

nasym(CO2) and nsym(CO2) bands indicate the presence of different coordination modes. Complex 1 shows a pairs of nasym and nsym frequencies at 1583, 1387 (Dn ¼ 196) cm1 corresponding to the carbonyl functionality of dicarboxylate ligand indicating a symmetric monodentate coordination mode. Complex 2 shows two pairs of nasym and nsym frequencies at 1607, 1423 (Dn ¼ 184) and 1562, 1384 (Dn ¼ 178) cm1 for the carbonyl functionality indicating two coordination modes as observed in the crystal structure. Complex 3 shows two pairs of nasym and nsym frequencies at 1587, 1362 (Dn ¼ 215) and 1537, 1387 (Dn ¼ 150) cm1 for the carbonyl functionality indicating two coordination modes as observed in the crystal structure. Complex 4 shows a pairs of nasym and nsym frequencies at 1553, 1387 (Dn ¼ 166) cm1 corresponding to the carbonyl functionality of dicarboxylate ligand indicating a bidentate coordination mode. The bands in the region 640-1310 cm1 are attributed to the eCHe in-plane or out-of-plane bend, ring breathing, and ring deformation absorptions of benzene ring,

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M. Yu et al. / Journal of Molecular Structure 1199 (2020) 127005

Fig. 7. Electric hysteresis loops of complex 1 at room temperature.

respectively. Weak absorptions observed at 3069-3088 cm1 can be attributed to nCeH of benzene ring. The bands arising from the OeH stretching modes of the coordinated and uncoordinated water molecules are observed in the range of 3400e3500 cm1 [46]. 3.3. Thermogravimetric analyses To characterize the thermal stabilities of four complexes, we carried out TGA analyses (Fig. 5). The experiments were performed on samples consisting of numerous single crystals of 1e4 in a nitrogen atmosphere with a heating rate of 10  C min1. The TGA curve of 1 reveals that no obvious weight loss was observed until the temperature rose to 240  C. The anhydrous compound decomposes from 240 to 700  C, indicating the release of organic components. The TGA curve of 2 shows that the first weight loss between 40 and 110  C corresponds to the dehydration process (obsd 2.70%; calcd. 2.60%), then the framework is stable up to 275  C. A rapid weight loss can be detected from 270 to 700  C. That is attributed to the complete decomposition of the organic ligands. The TGA curve of 3 shows that the first weight loss of 2.30% between 40 and 100  C corresponds to the loss of two lattice water molecule per formula unit, afterwards it is stable up to 300  C. The framework collapsed in the temperature range 300e700  C before the final formation of a metal oxide. For 4, the first weight loss of 2.10% (calcd. 2.17%) occurs in the range of 40e110  C, indicating the loss of three lattice water molecules. Then, the framework of 4 decomposes gradually above 230  C. 3.4. Photoluminescent properties Photoluminescence properties of Zn(II) complexes have attracted intense interest due to their potential applications in photochemistry, chemical sensors, and electroluminescent display [47e49]. The photoluminescent properties of 1e4 and TPPA ligand were investigated in solid state at room temperature (Fig. 6a). The TPPA ligand exhibits emission band with a maximum at 521 nm upon excitation at 430 nm, which may be assigned to p*/n or p*/p transitions of the ligands [50e52]. As shown in Fig. 6a, the emission peaks of complexes occur at 539 nm (lex ¼ 450 nm) for 1, 551 nm (lex ¼ 450 nm) for 2, 540 nm (lex ¼ 450 nm) for 3 and 568 nm (lex ¼ 450 nm) for 4. Under the same experimental conditions, the emission intensities of free

H2phda, H2glut, H2brtp and H2sbda are weaker than that of TPPA ligand, so it is considered that it has no significant contribution to the fluorescent emission of the complexes with the presence of TPPA ligand. Comparison to free TPPA ligand, their emission peaks for 1e4 are red-shifted by ca. 18 nm, 30 nm, 19 nm and 47 nm, respectively. These emissions are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature, since Zn(II) ion is difficult to oxidize or reduce due to their d10 configuration [53e55]. The photoluminescent of 1e4 may originate from the intraligand p*/p or p*/n transition since similar emissions were also observed for the ligands themselves. The emission discrepancy of these complexes is probably due to the differences of organic ligands and coordination environments of central metal ions, which have a close relationship to the photoluminescence behavior [56,57]. Their chromaticity coordinates (1e4) are (0.3939, 0.5898), (0.4183, 0.5437), (0.3786, 0.5341) and (0.4401, 0.5344) (Fig. 6b). As we know that the luminescence characteristic of CPs is closely related to their structures [58]. The size of the metal, the structure of the secondary building units (SBUs) and the orientation of the linkers all affect the emission properties of the material [59e62]. Complexes 1e4 contain the same linker (TPPA) in different configurations, allowing comparative study of their photoluminescence diversity. The different visual fluorescence maybe attributed to the various structures of four complexes (1 (3D), 2 (2D), 3 (2D) and 4 (3D)). The enhancement of their emission intensity compared with that of the TPPA ligand can be properly attributed to the increased rigidity of the TPPA ligand when bound to Zn(II) ions, which will effectively reduce the loss of energy. 3.5. SHG response As mentioned in the introduction, CPs show interesting nonlinear optical properties (NLO) and second harmonic generation (SHG) responses. SHG is completely dependent on crystal symmetry and SHG materials have been extensively applied to high technique fields such as laser devices and optical communications. Generally, an SHG-active material must require the absence of a symmetric center. Lin, Xiong, Chen and coworkers illustrated that CPs with noncentrosymmetric structures can be rationally designed and prepared by making use of metal ions with specific geometries and high directional coordination bonds [63e69]. Thus, rationally designing and synthesizing CPs with SHG responses has enabled us to understand the relationships between structures and properties. Considering that complex 1 crystallizes in the acentric space group (Pna21), its nonlinear optical properties were studied. The strength of the second harmonic generation (SHG) efficiency of complex 1 was tested by measuring the microcrystalline powder samples with 60e90 mm in diameters. Preliminary examinations indicate that complex 1 is SHG-active and the SHG efficiency is approximately 0.8 times that of urea, which indicates that complex 1 has potential application in optical material. The modest powder SHG response of complex 1 may be attributed to a comparatively short donoreacceptor system, which is essential for second-order optical nonlinearity. Recently, a prevalent research has focused on developing ferroelectric materials based on coordination polymers (CPs) and have reported some such materials [70e74]. However, the reported ferroelectric materials are mostly built upon chiral organic tectons, whereas the proper use of achiral ligands by spontaneous are scarce [75e77]. Herein, we describe the preliminary investigation of the possible ferroelectric property of complex 1. Complex 1 crystallizes in the acentric space group Pna21, which belongs to the polar point group C2v, which falls in one of the 10 polar point groups (C1, Cs, C2, C2v, C4, C4v, C3, C3v, C6, C6v) required for ferroelectric materials.

M. Yu et al. / Journal of Molecular Structure 1199 (2020) 127005

The electric hysteresis loop of complex 1 reveals its ferroelectric behavior. Fig. 7 clearly shows that 1 holds typical characteristics of ferroelectric materials, with a remnant polarization (Pr) of ca. 0.08 mC cm2 and coercive field (Ec) of 18.0 kV cm1. Saturation of the spontaneous polarization (Ps) occurs at ca. 0.21 mC cm2, which is close to that for a typical ferroelectric compound (e.g., NaKC4H4O6$H2O, Rochelle salt; usually Ps ¼ 0.25 mC cm2), but significantly smaller than that observed in KDP (KH2PO4, ca. 5 mC cm2) [78,79]. Besides polarization of a large organic positive ion, the large Ps value may also be due to the large polarizability and mobility of the relatively free charge carrier (phda2). The introduction of organic ligands could reduce the electrostatic attraction between positive and negative charges and increase the polarizability and mobility of the charge carriers. In addition, the orderedisorder of the hydrogen bonds may also contribute to the large remnant and saturation spontaneous polarization [80e82]. The dielectric constant (εr) and dielectric loss (tand) for complex 1 are 5.25 and 14.2%, respectively [83].

[9]

[10]

[11] [12]

[13]

[14]

[15]

4. Conclusion We have successfully synthesized four zinc(II) coordination polymers under solvothermal conditions. Complex 1 exhibits a noninterpenetrating three-dimensional (3D) framework with 65.8dmp topology. Complex 2 consists of two-dimensional (2D) (4,4) grid networks with the point symbol of 44.62. Complex 3 has a 2D two-fold interpenetrating layer structure, while complex 4 features a 3D two-fold interpenetrating 4-connected 65.8-cds net. A careful structural comparison reveals that the similarity of the Zn(II) coordination geometry and TPPA ligand binding mode, but the significant discrepancy of their network, which may be attributed to the length and conformational flexibility of dicarboxylic acids. The photoluminescent of 1e4 may originate from the intraligand p*/p or p*/n transition for the TPPA ligand. The different visual fluorescence maybe attributed to the various structures of four complexes. Moreover, complex 1 has also modest power SHG activity and ferroelectric properties.

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[19] [20]

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Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21671107), the Key Project of Chinese Ministry of Education (No. 210102) and the Qing Lan Project of Jiangsu Provincial Department of Education.

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