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Hydrothermal synthesis of three zinc(II) coordination polymers from 0D to 2D: Synthesis, structure, luminescence properties and effect of auxiliary ligand on their structural architectures
Journal of Molecular Structure 1195 (2019) 252e258
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Hydrothermal synthesis of three zinc(II) coordination polymers from 0D to 2D: Synthesis, structure, luminescence properties and effect of auxiliary ligand on their structural architectures Juan Song* , Bo-Feng Duan, Jiu-Fu Lu, Rui Wu, Quan-Chao Du Shaanxi Key Laboratory of Catalysis, College of Chemical &Environment Science, Shaanxi University of Technology, Han Zhong, 723001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 April 2019 Accepted 9 May 2019 Available online 31 May 2019
Three new coordination compounds, [Zn2(bppc)2(bdc)(H2O)2]$4H2O(1) [Zn4(bppc)2(o-bdc)2(ox)(H2O)2]n$2nH2O (2), [Zn2(bppc)(ox)1.5(H2O)2]n$3.5nH2O (3) (Hbppc ¼ 2,6-bis(pyrazin-2-yl)pyridine-4acid, H2(o-bdc) ¼ 1,2-benzenedicarboxylic acid, carboxylate, H2bdc ¼ 1,4-benzenedicarboxylic H2ox ¼ oxalic acid), with different structural dimensionality (0D, 1D and 2D, respectively) have been hydrothermally synthesized and structurally characterized. Single crystal X-ray analysis revealed that compound 1 shows a dimer structure, compound 2 and 3 show one-dimensional ring chain structure and two-dimensional structure respectively. The structure differences of 1e3 show that the aromatic acid as auxiliary ligand have important influence on the final structures. Additionally, the luminescent properties of 1e3 have been investigated with fluorescent spectra in the solid state, and 1e3 display a strong fluorescent emission at room temperature and have potential applications as fluorescent-emitting materials. © 2019 Elsevier B.V. All rights reserved.
Keywords: Crystal structure Hydrothermal synthesis Coordination polymer Luminescence
1. Introduction During the past two decades, coordination polymers have attracted the attention of chemists, physicists, and materials scientists because of their coordination-driven architectures including different dimensionality networks [1e3] and tremendous potential applications in gas storage and separation [4,5], non-linear optics [6], catalysis [7], sensing [8], luminescence [9], magnetism [10], drug delivery [11] and so on. Although, considerable efforts have been made to rational design and syntheses coordination polymers, the realization of controllable structures with desired physical properties are still a great challenge [12]. It is well known that, there are various factors affecting the coordination self-assembly process and the structures of the coordination polymers, such as the structural characteristics and the coordination ability of organic ligands, the coordination geometry of metal ions, the choice of the auxiliary ligand, and the reaction conditions (counter anion, pH values, reaction temperature, types of solvents and so on) [13,14]. Among these factors, the selection of organic ligand is important to constructe functional coordination polymers.
* Corresponding author. E-mail addresses: [email protected], [email protected] (J. Song). https://doi.org/10.1016/j.molstruc.2019.05.030 0022-2860/© 2019 Elsevier B.V. All rights reserved.
Up to now, the rigid N-donor ligands (polypyridine or pyrazine) are often widely used as bridging and chelating blocks to construct coordination polymers due to their versatile coordination modes and the ability to adopt different conformations according to the geometric needs of the different metal ions [15,16]. In this regard, the rigid N-donor ligands have proven to be good candidates for construction of novel coordination polymers. Taking all the considerations mentioned above in count, we chose 2,6-bis(pyrazin-2yl)pyridine-4-carboxylic acid (Hbppc) as a versatile ligand base on the following considerations: (i) It possesses one pyridyl, two pyrazinyl and one carboxyl groups which can show various coordination modes in the formation of coordination polymers; (ii) The carboxyl and N-heterocyclic groups of Hbppc can act as hydrogenbond acceptors or donors, and the extended conjugated system can easily form p-p interactions. All of these factors have greatly influences on constructing supramolecular framework; (iii) To the best of our knowledge, the coordination chemistry and structural properties of based on Hbppc has not been reported except our group [17]. Besides N-donor ligands, carboxylate ligands are another class of excellent ligands acting as organic linkers or terminal ligands to adjust the coordination mode of rigid blocks and the resulting structure of coordination polymers [18,19]. Naturally, mixed N-
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donor and carboxylate ligands have been proved successfully to construct various coordination polymers [20]. With the purpose of understanding the coordination chemistry of pyridine or polypyrazine ligand and constructing novel coordination polymers with fascinating structures, we used 2,6-bis(pyrazin-2-yl)pyridine-4carboxylic acid as primary ligand and three multicarboxylate as auxiliary ligands to react with transition-metal ions Zn(II) through the hydrothermal reaction, isolating three new complexes, namely [Zn2(bppc)2(bdc)(H2O)2]$4H2O(1) [Zn4(bppc)2(o-bdc)2(ox)(H2O)2]n$2nH2O (2), [Zn2(bppc)(ox)1.5(H2O)2]n$3.5nH2O (3). Here we reported their syntheses, crystal structures, effect of auxiliary ligand on their structural architectures and the state luminescent properties.
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diffractometer with graphite-monochromated Mo-Ka radiation (l ¼ 0.71073 Å) at room temperature. Empirical absorption corrections were applied using the SADABS program. The structures were solved by direct methods and refined by full-matrix least-squares based on F2 using SHELXTL-97 program [21]. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms of organic ligands were generated geometrically. Crystallographic data and experimental details of structural analyses for complexes are summarized in Table 1. Selected bond length and angle parameters are listed in Table 2. 3. Results and discussion 3.1. Description of the structures
2. Experimental procedure 2.1. Materials and physical measurements Hbppc ligand was prepared by myself. Other chemicals and reagents were purchased commercially and used without further purification. Elemental analyses (C, H, N) were determined with a Vario EL III elemental analyzer. Infrared spectra were recorded on a Bruker EQUINOX55 spectrometer as KBr pellets in the range of 4000～400 cm1. Fluorescence spectra were performed on a Hitachi F-4500 fluorescence spectrophotometer at room temperature. Thermal gravimetry analyses (TGA) were carried out with a Universal V2.6 DTA system at a heating rate of 10 C/min in a nitrogen atmosphere. 2.2. Synthesis of compounds 2.2.1. Synthesis of [Zn2(bppc)2(bdc) (H2O)2]·4H2O(1) A mixture of Hbppc (27.9 mg, 0.1 mmol), ZnCl2 (27.3 mg, 0.2 mmol), H2bdc (16.6 mg, 0.1 mmol) in deionized water (10 mL) was stirred and then the mixture was sealed in a 25 mL Teflon-lined stainless-steel vessel and heated to 160 C for 3 d. Then the reaction system was cooled to room temperature during 48 h to give rise to colorless block crystals of 1 in ca. 50% yield based on Zn, which were collected by filtration and washed with deionized water. Elemental analysis (%):calcd for C36H44N10O20Zn2 (Mr ¼ 1067.55): C 40.50, H 4.15, N 13.12. found: C 40.38, H 4.03, N 12.95. IR(cm1): 3416(s), 2026(w), 1618(m), 1563(m), 1503(w), 1402(m), 1333(w), 1168(w), 1036(w), 849(w), 788(w), 743(w), 616(w). 2.2.2. Synthesis of [Zn4(bppc)2(o-bdc)2 (ox)(H2O)2]n·2nH2O (2) The preparation of 2 was similar to that of 1 except that H2bdc was replaced by H2(o-bdc)(16.6 mg, 0.1 mmol) and H2ox(12.6 mg, 0.1 mmol). Colorless strip crystals of 2 were obtained. Yield:55% based on zinc. Elemental analysis (%): calcd for C46H40N10O24Zn4 (Mr ¼ 1378.36): C 40.50, H 4.15, N 13.12. found: C 40.31, H 4.13,N 12.95. IR(cm1):3417(s), 2026(w), 1629(s), 1558(m), 1483(w), 1404(m), 1739(m), 1338(w), 1319(w), 1171(w), 1037(w), 848(w), 794(w), 767(w), 713(w). 2.2.3. Synthesis of [Zn2(bppc)(ox)1.5(H2O)2]n·3.5nH2O (3) The preparation of 3 was similar to that of 1 except that H2bdc was replaced by H2ox(12.6 mg, 0.1 mmol). Colorless strip crystals of 3 were obtained. Yield:65% based on zinc. Elemental analysis (%): calcd for C17H18N5O13Zn2 (Mr ¼ 631.10):C 32.35, H 2.87, N 11.10. found:C 32.17, H 2.79,N 11.08. IR(cm1):3416(s), 2026(w), 1619(m), 1455(w), 1379(w), 1320(w), 1167(w), 1035(w), 793(w), 617(w). 2.3. Determination of crystal structures Intensity data were collected on a Bruker Smart APEX II CCD
3.1.1. [Zn2(bppc)2(bdc) (H2O)2]·4H2O(1) Single-crystal X-ray analysis reveals that 1 crystallizes in triclinic system, space group P-1. Compound 1 displays a dimer structure, as shown in Fig. 1a. The asymmetric unit of 1 contains one crystallographically independent ZnII cations, one (bppc)-, half a (bdc)2anions, one coordination water and four lattice water molecules. Zn1 is coordinated by three nitrogen atoms from one (bppc)- anion (N1,N3,N4) and one oxygen atoms (O3) from (bdc)2- anion and (O5,O6) from two coordination water molecules, showing a distorted octahedral coordination geometry. The Zn1A has the same coordination geometry with Zn1. In 1, the (bppc)- anion coordinates to the ZnII cation with a tridentate chelating coordination mode (Scheme 1a), the (m-bdc)2- anion bridge the two adjacent ZnII cation with a m2-h1:h1 bridging coordination mode to form a dimer [Zn2(bppc)2(bdc)(H2O)4] structure. 3.1.2. [Zn4(bppc)2(o-bdc)2 (ox)(H2O)2]n·2nH2O (2) Compound 2 is a one-dimensional chain structure, as shown in Fig. 2a. The asymmetric unit of 2 consists of two ZnII cations, one (bppc)- ligand, one (o-bdc)2- anion, half an (ox)2- anion, one coordination water and one lattice water molecules. The Zn1 is pentacoordinated by three nitrogen atoms (N1,N3,N4) from one (bppc)anion, one oxygen atom O3 from one (o-bdc)2anion and one oxygen atoms O5 from one coordination water molecules with a distorted triangular bipyramid geometry. Zn2 is hexa-coordinated
Table 1 Crystallographic data and structure refinement for 1e3. Compounds
1
2
3
Formula
C36 H44 N10 O20 Zn2 1067.55 296(2) Triclinic P-1 8.3430(11) 9.2810(14) 15.3490(17) 99.326(5) 94.442(4) 108.357(7) 1102.6(3) 1 1.608 550 1.024 5399/3774
C46 H40 N10 O24 Zn4 1378.36 296(2) Monoclinic P2(1)/c 7.2220(7) 15.6790(14) 22.564(2) 90 96.056(2) 90 2540.8(4) 2 1.802 1396 1.033 12433/4504
C17 H19 N5 O13.50 Zn2 640.11 296(2) Monoclinic P2(1)/c 10.3794(8) 24.6377(18) 10.3056(8) 90 115.5940(10) 90 2376.8(3) 4 1.789 1296 1.042 11788/4179
fw T/K Crystal system Space group a/Å b/Å c/Å a/ b/ g/ V/Å3 Z Dc/g.cm3 F(000) GOF on F2 Reflection/ unique 0.0372, 0.0923 R1,wR2 [I > 2 (I)] 0.0717, 0.14025 0.0381, 0.0833 R1,wR2 (all data) 0.1392, 0.1677 0.0542, 0.0904 0.0500, 0.0989 P P P P R ¼ (jjFojdjFcjj)/ jFoj, wR ¼ [ (jjFoj2djFcjj2)2/ w(F2o)]1/2.
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Table 2 Selected bond lengths (Å) and angles ( ) for 1e4. 1 Zn(1)-O(3) Zn(1)-O(6) Zn(1)-N(4) O(3)-Zn(1)-N(3) N(3)-Zn(1)-O(6) N(3)-Zn(1)-N(1) O(3)-Zn(1)-N(4) O(6)-Zn(1)-N(4) O(3)-Zn(1)-O(5) O(6)-Zn(1)-O(5) N(4)-Zn(1)-O(5) 2 Zn(1)-O(3) Zn(1)-N(3) Zn(1)-N(4) Zn(2)-O(5)#1 Zn(2)-O(7) Zn(2)-O(6)#1 O(3)-Zn(1)-O(9) O(9)-Zn(1)-N(3) O(9)-Zn(1)-N(1) O(3)-Zn(1)-N(4) N(3)-Zn(1)-N(4) O(1)-Zn(2)-O(5)#1 O(5)#1-Zn(2)-O(8)#2 O(5)#1-Zn(2)-O(7) O(1)-Zn(2)-O(10) O(8)#2-Zn(2)-O(10) O(1)-Zn(2)-O(6)#1 O(8)#2-Zn(2)-O(6)#1 O(10)-Zn(2)-O(6)#1 3 Zn(1)-O(3) Zn(1)-N(3) Zn(1)-N(1) Zn(2)-O(1) Zn(2)-O(8)#1 Zn(2)-O(7) O(3)-Zn(1)-O(9) O(9)-Zn(1)-N(3) O(9)-Zn(1)-O(5) O(3)-Zn(1)-N(1) N(3)-Zn(1)-N(1) O(3)-Zn(1)-N(4) N(3)-Zn(1)-N(4) N(1)-Zn(1)-N(4) O(1)-Zn(2)-O(8)#1 O(1)-Zn(2)-O(4)#2 O(8)#1-Zn(2)-O(4)#2 O(10)-Zn(2)-O(7) O(4)#2-Zn(2)-O(7) O(10)-Zn(2)-O(6)#2 O(4)#2-Zn(2)-O(6)#2
1.956(5) 2.155(5) 2.217(5) 164.1(2) 83.6(2) 75.4(2) 89.5(2) 91.8(2) 91.3(2) 169.16(18) 87.1(2)
Zn(1)-N(3) Zn(1)-N(1) Zn(1)-O(5) O(3)-Zn(1)-O(6) O(3)-Zn(1)-N(1) O(6)-Zn(1)-N(1) N(3)-Zn(1)-N(4) N(1)-Zn(1)-N(4) N(3)-Zn(1)-O(5) N(1)-Zn(1)-O(5)
2.115(5) 2.157(6) 2.272(5) 99.5(2) 120.0(2) 89.9(2) 74.8(2) 149.7(2) 85.68(19) 85.7(2)
1.944(2) 2.068(3) 2.171(3) 2.080(2) 2.088(3) 2.401(3) 94.10(11) 116.42(12) 100.82(11) 99.43(10) 74.86(10) 95.33(10) 143.59(10) 101.60(10) 93.06(11) 85.98(11) 152.66(10) 86.49(10) 89.08(12)
Zn(1)-O(9) Zn(1)-N(1) Zn(2)-O(1) Zn(2)-O(8)#2 Zn(2)-O(10)
1.978(3) 2.156(3) 2.009(2) 2.088(2) 2.108(3)
O(3)-Zn(1)-N(3) O(3)-Zn(1)-N(1) N(3)-Zn(1)-N(1) O(9)-Zn(1)-N(4) N(1)-Zn(1)-N(4) O(1)-Zn(2)-O(8)#2 O(1)-Zn(2)-O(7) O(8)#2-Zn(2)-O(7) O(5)#1-Zn(2)-O(10) O(7)-Zn(2)-O(10) O(5)#1-Zn(2)-O(6)#1 O(7)-Zn(2)-O(6)#1
149.34(11) 101.46(11) 76.79(11) 97.72(11) 150.84(11) 120.85(10) 95.97(11) 80.09(10) 88.29(11) 165.90(11) 57.47(10) 87.86(11)
Zn(1)-O(9) Zn(1)-O(5) Zn(1)-N(4) Zn(2)-O(10) Zn(2)-O(4)#2 Zn(2)-O(6)#2 O(3)-Zn(1)-N(3) O(3)-Zn(1)-O(5) N(3)-Zn(1)-O(5) O(9)-Zn(1)-N(1) O(5)-Zn(1)-N(1) O(9)-Zn(1)-N(4) O(5)-Zn(1)-N(4) O(1)-Zn(2)-O(10) O(10)-Zn(2)-O(8)#1 O(10)-Zn(2)-O(4)#2 O(1)-Zn(2)-O(7) O(8)#1-Zn(2)-O(7) O(1)-Zn(2)-O(6)#2 O(8)#1-Zn(2)-O(6)#2 O(7)-Zn(2)-O(6)#2
2.049(3) 2.151(3) 2.214(3) 2.064(3) 2.095(3) 2.163(3) 167.54(11) 79.64(10) 90.21(10) 91.69(12) 97.91(11) 89.45(12) 85.48(10) 97.10(12) 90.44(11) 100.87(12) 89.07(10) 79.06(10) 170.88(11) 90.63(10) 88.83(10)
2.026(2) 2.097(3) 2.201(3) 2.038(2) 2.088(3) 2.134(3) 92.38(11) 98.68(11) 168.36(11) 99.20(11) 74.89(11) 110.41(11) 75.60(11) 150.30(11) 97.68(11) 92.44(10) 163.80(11) 168.47(11) 88.53(10) 86.53(11) 78.64(9)
Symmetry transformations used to generate equivalent atoms: For 1:#1 -xþ2,-yþ2,-z. For 2: #1 -xþ2,-yþ1,-zþ1; #2 -xþ2,-yþ2,-zþ1. For 3: #1 -xþ1,-y,-zþ1; #2 -x,y-1/2,zþ1/2; #3 -x,yþ1/2,-zþ1/2; #4 -xþ1,-yþ1,-zþ1.
by one oxygen atom O1A from (bppc)- anions, two oxygen atoms(O5, O6) from (o-bdc)2anion, two oxygen atoms(O7, O8A) from (ox)2anion and one oxygen atom O10 from one coordination water molecules with a distorted octahedral geometry. The N1, N3, N4, and O3 atoms comprise the equatorial plane and the O5 and O6 atoms occupy the axial positions. The O1A,O5, O6 and O8A atoms comprise the equatorial plane and the O7 and O10 atoms occupy the axial positions. In compound 2, two (bppc)- anions, two (o-bdc)2anions and four ZnII cations form a 29-membered rings (Fig. 2b). The adjacent tetranuclear rings were bridged by the (ox)2anions with a m2h1:h1:h1:h1 coordination mode to form one-dimensional ring chain structure (Fig. 2c).
3.1.3. [Zn2(bppc)(ox)1.5(H2O)2]n·3.5nH2O (3) The single-crystal X-ray diffraction analysis reveals that 3 crystallizes in monoclinic system P21/c space group. The asymmetric unit of 3 contains two crystallographically independent ZnII cations, one (bppc)-, one and a half (ox)2- anions, two coordination water and three and a half lattice water molecules. As shown in Fig. 3a, the Zn1 atom is coordinated by three nitrogen atoms (N1,N3,N4) from one (bppc)- ligand, two oxygen atoms(O3, O5) from one (ox)2anion and one oxygen atom O9 from one coordination water molecules showing a distorted octahedral coordination geometry. The O5 and O9 atoms occupy the axial positions and the N1, N3, N4and O3 atoms comprise the equatorial plane. The Zn2 atom is coordinated by one oxygen O1 atom from one (bppc)- anion ligand, four carboxylic oxygen atoms (O4A,O6A, O7, O8A)of (ox)2- anion
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Fig. 1. (a) Coordination environment of Zn(II) ion in 1, in which the hydrogen atoms are omitted for clarity; (b) View of the dimer [Zn2(bppc)2(bdc)(H2O)4] structure.
coordinates to the Zn1II cation with a tridentate chelating coordination mode, the carboxylate section of the (bppc)- ligand coordinates to the Zn2II cation by bridging coordination mode (Scheme 1b) and the (ox)2- ligand coordinates to the ZnII cation with a m2-h1:h1:h1:h1coordination mode. The Zn1II and Zn2II cations were connected by four (bppc)- and six (ox)2- ligands forming 53-membered rings (Fig. 3b). The rings are further joined by (ox)2ligands to form 1D ring chain(Fig. 3c), and the (ox)2- ligands further extend the 1D ring chains into a 2D layer structure (Fig. 3d).
3.2. Comparative structural study Scheme. 1. Coordination modes of the (bppc)- ligand in 1e3.
and one oxygen atom O10 of coordination water molecule also showing a distorted octahedral coordination geometry. In 3, The pyridyl and pyrazinyl sections of the (bppc)- ligand
Compounds 1e3 were prepared as single-phase crystalline products by hydrothermal reactions of Hbppc ligand and ZnCl2 together with corresponding O-donor assistant ligands. The structural research remind us to study the effects of auxiliary ligands on the final structures. As far as the zinc atoms coordination: The commonly coordination numbers for this cation are 4 and 5, while 6 is much less common. In the structures of 1 and 3 all the zinc
Fig. 2. (a) Coordination environment of Zn(II) ion in 2, and the hydrogen atoms are omitted for clarity; (b) The 29-membered ring constructed by (bppc)- anions, (o-bdc)2anions and ZnII cations; (c) The one-dimensional ring chain structure of 2.
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atoms present CN ¼ 6, compound 2 shows zinc atoms with two different coordination numbers (5 and 6). Compounds 1e3 show three structures with different dimensionality (dimeric units, 0D; ring chain, 1D; layers, 2D) with the same (bppc)- ligands and different auxiliary ligand. In the compound 1, the auxiliary (bdc)2ligand bridge the two adjacent ZnII cation with a m2-h1:h1 bridging coordination mode and was sandwiched between the two ZnII units, meanwhile, the carboxyl oxygen of (bppc)- ligand did not participate in the coordination, which makes the structure has not been further extended and showing a dimer structure. Compound 2 contains two kinds of carboxylic acid auxiliary ligand. The (o-bdc)2ligand bridge the two adjacent ZnII cation to form a 29-membered rings and the (ox)2- ligand further bridge the rings to form onedimensional ring chain structure. In compound 3, the (ox)2- auxiliary ligand coordinates to the ZnII cation with a m2h1:h1:h1:h1coordination mode, and the (ox)2- ligand not only participates in the formation of the ring but also expands the ring chain to two-dimensional structure. 3.3. FT-IR spectra The IR spectra of free ligand and compounds 1e3 were performed as KBr pellets in the range 4000e400 cm1(Fig. S1). The strong bands at 3416 cm1 for 1 and 3, 3417 cm1 for 2 are the stretching vibration of the O-H group. The peaks of 1618 cm1, 1629 cm1and 1619 cm1 are assigned to the skeleton vibration of C]N in 1e3, respectively, which display certain shifts in contrast with 1598 cm1 in the ligand. It is thus assumed that nitrogen atoms in the ligand coordinate to metal atoms. The absence of any strong bands around 1700 cm1 indicates that the carboxylate groups of ligand are completely deprotonated. In the IR spectrum of 1, the asymmetric and symmetric stretching vibrations of the COO groups appear at 1563 cm1and 1402 cm1 separately. The difference in the value between them is less than 200 cm1, which
indicates that the carboxylate groups coordinate to the metal ions only in bidentate-chelating mode. In the IR spectre of 2 and 3, The difference between nas(COO) and ns(COO) (D ¼ 154 cm1 and 225 cm1 for 2, 164 cm1 and 241 cm1 for 3) suggests that the carboxylate groups coordinate to the metal ions in both monodentate-bridging mode and bidentate-chelating modes [22]. These IR spectra are in good agreement with the result of X-ray structural analysis.
3.4. Thermal behavious In order to characterize the thermal stability of compounds 1e3, their thermal decomposition behavior was investigated as shown in Fig. 4. For 1, there exists a weight loss of 10.16% occurring below 200 C, which correspond to the continuous loss of six water molecules and can be compared with the calculated value of 10.04%. Its framework keeps stable to 365 C, and then the framework collapses accompanying the release of the (bdc)2- and (bppc)ligands. For 2, there exists a weight loss of 5.03% occurring below 200 C, which correspond to the continuous loss of four water molecules and can be compared with the calculated value of 5.22%.The second stage weight loss can be detected from 309 to 410 C attributed to the departure of (o-bdc)2- and (ox)2ligands (Calcd. 30.18%; Found: 29.95%). Above 525 C, the compound begins to lose (bppc)- ligand and then starts to decompose. Compound 3 first loses its lattice water molecule below 110 C, and the weight loss found of 8.89% is consistent with that calculated (8.56%). The second stage weight loss can be detected from 110 to 180 C attributed to the departure of coordinated water molecules (Calcd. 5.70%; Found: 5.59%). Above 390 C, the compound begins to lose their ligands and then starts to decompose.
Fig. 3. (a) Coordination environment of Zn(II) ion in 3, in which the hydrogen atoms are omitted for clarity; (b) The 53-membered ring constructed by (bppc)- ligands, (ox)2- ligands and ZnII cations; (c)The one-dimensional ring chain structure of 3. (d) 2D layer structure of 3.
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Fig. 4. TGA curves for complexes 1e3.
3.5. Photoluminescence properties Taking into account the excellent luminescent properties of d10 Zn(II) metal coordination polymers, the solid-state luminescence spectrum of free ligand (Hbppc) and 1e3 were studied at ambient temperature(Fig. 5). The enhancement of fluorescence intensity in 1e3 may be attributed to ligand chelation to the Zn(II) centers, which effectively increases the rigidity of the coordination polymers and reduces the loss of energy by nonradiative decay of the intraligand (p/p*)excited state [23]. Intense emission bands are observed at 420 and 466 nm (lex ¼ 290 nm) for Hbppc, which can be assigned to the ligand-centered charge transitions, that is, the p* / n and p* / p transitions. The emissions are also observed at 504 nm for 1, 499 nm for 2 and 502 nm for 3 respectively upon excitation at 280 nm. As previously reported [24], the solid aromatic carboxylate ligands H2bdc and H2ox are nearly nonfluorescent in the range of 400～600 nm at ambient temperature. Compared with the free Hbppc, the main peaks in 1e3 are redshifted, which may result from the coordination of the ligands to the metal centers [25]. Notably, a lower energy emission was also detected at 388 nm for 3 which can be attributed to the intraligand p* / p transition.
donor ligands and different O-donor ligands. Compounds 1e3 show intriguing 0D, 1D and 2D structures. Their structural diversities indicate that the structural diversity could be modulated by the species of auxiliary O-donor ligands. In addition, these three Zn(II) compounds also exhibit intense luminescence in the solid state at room temperature. Further research works about Zn(II) coordination polymers based on mixed Hbppc and O-donor ligands are under way by our group. Acknowledgment The authors thank financial assistance from the Key Scientific Research Project of Education Department of Shaanxi Province (17JS027) and Introducing Talents Foundation of Shaanxi University of Technology (no. SLGKYQD2-11). This work is also supported by team of syngas catalytic conversion of Shaanxi university of Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.05.030.
4. Conclusions
References
In conclusion, three Zn(II)-based coordination polymers with different structures have been synthesized based on the rigid N-
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Fig. 5. Emission spectra of the free Hbppc ligand and complexes 1e3.
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Hydrothermal synthesis of three zinc(II) coordination polymers from 0D to 2D: Synthesis, structure, luminescence properties and effect of auxiliary ligand on their structural architectures Juan Song* , Bo-Feng Duan, Jiu-Fu Lu, Rui Wu, Quan-Chao Du Shaanxi Key Laboratory of Catalysis, College of Chemical &Environment Science, Shaanxi University of Technology, Han Zhong, 723001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 April 2019 Accepted 9 May 2019 Available online 31 May 2019
Three new coordination compounds, [Zn2(bppc)2(bdc)(H2O)2]$4H2O(1) [Zn4(bppc)2(o-bdc)2(ox)(H2O)2]n$2nH2O (2), [Zn2(bppc)(ox)1.5(H2O)2]n$3.5nH2O (3) (Hbppc ¼ 2,6-bis(pyrazin-2-yl)pyridine-4acid, H2(o-bdc) ¼ 1,2-benzenedicarboxylic acid, carboxylate, H2bdc ¼ 1,4-benzenedicarboxylic H2ox ¼ oxalic acid), with different structural dimensionality (0D, 1D and 2D, respectively) have been hydrothermally synthesized and structurally characterized. Single crystal X-ray analysis revealed that compound 1 shows a dimer structure, compound 2 and 3 show one-dimensional ring chain structure and two-dimensional structure respectively. The structure differences of 1e3 show that the aromatic acid as auxiliary ligand have important influence on the final structures. Additionally, the luminescent properties of 1e3 have been investigated with fluorescent spectra in the solid state, and 1e3 display a strong fluorescent emission at room temperature and have potential applications as fluorescent-emitting materials. © 2019 Elsevier B.V. All rights reserved.
Keywords: Crystal structure Hydrothermal synthesis Coordination polymer Luminescence
1. Introduction During the past two decades, coordination polymers have attracted the attention of chemists, physicists, and materials scientists because of their coordination-driven architectures including different dimensionality networks [1e3] and tremendous potential applications in gas storage and separation [4,5], non-linear optics [6], catalysis [7], sensing [8], luminescence [9], magnetism [10], drug delivery [11] and so on. Although, considerable efforts have been made to rational design and syntheses coordination polymers, the realization of controllable structures with desired physical properties are still a great challenge [12]. It is well known that, there are various factors affecting the coordination self-assembly process and the structures of the coordination polymers, such as the structural characteristics and the coordination ability of organic ligands, the coordination geometry of metal ions, the choice of the auxiliary ligand, and the reaction conditions (counter anion, pH values, reaction temperature, types of solvents and so on) [13,14]. Among these factors, the selection of organic ligand is important to constructe functional coordination polymers.
* Corresponding author. E-mail addresses: [email protected], [email protected] (J. Song). https://doi.org/10.1016/j.molstruc.2019.05.030 0022-2860/© 2019 Elsevier B.V. All rights reserved.
Up to now, the rigid N-donor ligands (polypyridine or pyrazine) are often widely used as bridging and chelating blocks to construct coordination polymers due to their versatile coordination modes and the ability to adopt different conformations according to the geometric needs of the different metal ions [15,16]. In this regard, the rigid N-donor ligands have proven to be good candidates for construction of novel coordination polymers. Taking all the considerations mentioned above in count, we chose 2,6-bis(pyrazin-2yl)pyridine-4-carboxylic acid (Hbppc) as a versatile ligand base on the following considerations: (i) It possesses one pyridyl, two pyrazinyl and one carboxyl groups which can show various coordination modes in the formation of coordination polymers; (ii) The carboxyl and N-heterocyclic groups of Hbppc can act as hydrogenbond acceptors or donors, and the extended conjugated system can easily form p-p interactions. All of these factors have greatly influences on constructing supramolecular framework; (iii) To the best of our knowledge, the coordination chemistry and structural properties of based on Hbppc has not been reported except our group [17]. Besides N-donor ligands, carboxylate ligands are another class of excellent ligands acting as organic linkers or terminal ligands to adjust the coordination mode of rigid blocks and the resulting structure of coordination polymers [18,19]. Naturally, mixed N-
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donor and carboxylate ligands have been proved successfully to construct various coordination polymers [20]. With the purpose of understanding the coordination chemistry of pyridine or polypyrazine ligand and constructing novel coordination polymers with fascinating structures, we used 2,6-bis(pyrazin-2-yl)pyridine-4carboxylic acid as primary ligand and three multicarboxylate as auxiliary ligands to react with transition-metal ions Zn(II) through the hydrothermal reaction, isolating three new complexes, namely [Zn2(bppc)2(bdc)(H2O)2]$4H2O(1) [Zn4(bppc)2(o-bdc)2(ox)(H2O)2]n$2nH2O (2), [Zn2(bppc)(ox)1.5(H2O)2]n$3.5nH2O (3). Here we reported their syntheses, crystal structures, effect of auxiliary ligand on their structural architectures and the state luminescent properties.
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diffractometer with graphite-monochromated Mo-Ka radiation (l ¼ 0.71073 Å) at room temperature. Empirical absorption corrections were applied using the SADABS program. The structures were solved by direct methods and refined by full-matrix least-squares based on F2 using SHELXTL-97 program [21]. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms of organic ligands were generated geometrically. Crystallographic data and experimental details of structural analyses for complexes are summarized in Table 1. Selected bond length and angle parameters are listed in Table 2. 3. Results and discussion 3.1. Description of the structures
2. Experimental procedure 2.1. Materials and physical measurements Hbppc ligand was prepared by myself. Other chemicals and reagents were purchased commercially and used without further purification. Elemental analyses (C, H, N) were determined with a Vario EL III elemental analyzer. Infrared spectra were recorded on a Bruker EQUINOX55 spectrometer as KBr pellets in the range of 4000～400 cm1. Fluorescence spectra were performed on a Hitachi F-4500 fluorescence spectrophotometer at room temperature. Thermal gravimetry analyses (TGA) were carried out with a Universal V2.6 DTA system at a heating rate of 10 C/min in a nitrogen atmosphere. 2.2. Synthesis of compounds 2.2.1. Synthesis of [Zn2(bppc)2(bdc) (H2O)2]·4H2O(1) A mixture of Hbppc (27.9 mg, 0.1 mmol), ZnCl2 (27.3 mg, 0.2 mmol), H2bdc (16.6 mg, 0.1 mmol) in deionized water (10 mL) was stirred and then the mixture was sealed in a 25 mL Teflon-lined stainless-steel vessel and heated to 160 C for 3 d. Then the reaction system was cooled to room temperature during 48 h to give rise to colorless block crystals of 1 in ca. 50% yield based on Zn, which were collected by filtration and washed with deionized water. Elemental analysis (%):calcd for C36H44N10O20Zn2 (Mr ¼ 1067.55): C 40.50, H 4.15, N 13.12. found: C 40.38, H 4.03, N 12.95. IR(cm1): 3416(s), 2026(w), 1618(m), 1563(m), 1503(w), 1402(m), 1333(w), 1168(w), 1036(w), 849(w), 788(w), 743(w), 616(w). 2.2.2. Synthesis of [Zn4(bppc)2(o-bdc)2 (ox)(H2O)2]n·2nH2O (2) The preparation of 2 was similar to that of 1 except that H2bdc was replaced by H2(o-bdc)(16.6 mg, 0.1 mmol) and H2ox(12.6 mg, 0.1 mmol). Colorless strip crystals of 2 were obtained. Yield:55% based on zinc. Elemental analysis (%): calcd for C46H40N10O24Zn4 (Mr ¼ 1378.36): C 40.50, H 4.15, N 13.12. found: C 40.31, H 4.13,N 12.95. IR(cm1):3417(s), 2026(w), 1629(s), 1558(m), 1483(w), 1404(m), 1739(m), 1338(w), 1319(w), 1171(w), 1037(w), 848(w), 794(w), 767(w), 713(w). 2.2.3. Synthesis of [Zn2(bppc)(ox)1.5(H2O)2]n·3.5nH2O (3) The preparation of 3 was similar to that of 1 except that H2bdc was replaced by H2ox(12.6 mg, 0.1 mmol). Colorless strip crystals of 3 were obtained. Yield:65% based on zinc. Elemental analysis (%): calcd for C17H18N5O13Zn2 (Mr ¼ 631.10):C 32.35, H 2.87, N 11.10. found:C 32.17, H 2.79,N 11.08. IR(cm1):3416(s), 2026(w), 1619(m), 1455(w), 1379(w), 1320(w), 1167(w), 1035(w), 793(w), 617(w). 2.3. Determination of crystal structures Intensity data were collected on a Bruker Smart APEX II CCD
3.1.1. [Zn2(bppc)2(bdc) (H2O)2]·4H2O(1) Single-crystal X-ray analysis reveals that 1 crystallizes in triclinic system, space group P-1. Compound 1 displays a dimer structure, as shown in Fig. 1a. The asymmetric unit of 1 contains one crystallographically independent ZnII cations, one (bppc)-, half a (bdc)2anions, one coordination water and four lattice water molecules. Zn1 is coordinated by three nitrogen atoms from one (bppc)- anion (N1,N3,N4) and one oxygen atoms (O3) from (bdc)2- anion and (O5,O6) from two coordination water molecules, showing a distorted octahedral coordination geometry. The Zn1A has the same coordination geometry with Zn1. In 1, the (bppc)- anion coordinates to the ZnII cation with a tridentate chelating coordination mode (Scheme 1a), the (m-bdc)2- anion bridge the two adjacent ZnII cation with a m2-h1:h1 bridging coordination mode to form a dimer [Zn2(bppc)2(bdc)(H2O)4] structure. 3.1.2. [Zn4(bppc)2(o-bdc)2 (ox)(H2O)2]n·2nH2O (2) Compound 2 is a one-dimensional chain structure, as shown in Fig. 2a. The asymmetric unit of 2 consists of two ZnII cations, one (bppc)- ligand, one (o-bdc)2- anion, half an (ox)2- anion, one coordination water and one lattice water molecules. The Zn1 is pentacoordinated by three nitrogen atoms (N1,N3,N4) from one (bppc)anion, one oxygen atom O3 from one (o-bdc)2anion and one oxygen atoms O5 from one coordination water molecules with a distorted triangular bipyramid geometry. Zn2 is hexa-coordinated
Table 1 Crystallographic data and structure refinement for 1e3. Compounds
1
2
3
Formula
C36 H44 N10 O20 Zn2 1067.55 296(2) Triclinic P-1 8.3430(11) 9.2810(14) 15.3490(17) 99.326(5) 94.442(4) 108.357(7) 1102.6(3) 1 1.608 550 1.024 5399/3774
C46 H40 N10 O24 Zn4 1378.36 296(2) Monoclinic P2(1)/c 7.2220(7) 15.6790(14) 22.564(2) 90 96.056(2) 90 2540.8(4) 2 1.802 1396 1.033 12433/4504
C17 H19 N5 O13.50 Zn2 640.11 296(2) Monoclinic P2(1)/c 10.3794(8) 24.6377(18) 10.3056(8) 90 115.5940(10) 90 2376.8(3) 4 1.789 1296 1.042 11788/4179
fw T/K Crystal system Space group a/Å b/Å c/Å a/ b/ g/ V/Å3 Z Dc/g.cm3 F(000) GOF on F2 Reflection/ unique 0.0372, 0.0923 R1,wR2 [I > 2 (I)] 0.0717, 0.14025 0.0381, 0.0833 R1,wR2 (all data) 0.1392, 0.1677 0.0542, 0.0904 0.0500, 0.0989 P P P P R ¼ (jjFojdjFcjj)/ jFoj, wR ¼ [ (jjFoj2djFcjj2)2/ w(F2o)]1/2.
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Table 2 Selected bond lengths (Å) and angles ( ) for 1e4. 1 Zn(1)-O(3) Zn(1)-O(6) Zn(1)-N(4) O(3)-Zn(1)-N(3) N(3)-Zn(1)-O(6) N(3)-Zn(1)-N(1) O(3)-Zn(1)-N(4) O(6)-Zn(1)-N(4) O(3)-Zn(1)-O(5) O(6)-Zn(1)-O(5) N(4)-Zn(1)-O(5) 2 Zn(1)-O(3) Zn(1)-N(3) Zn(1)-N(4) Zn(2)-O(5)#1 Zn(2)-O(7) Zn(2)-O(6)#1 O(3)-Zn(1)-O(9) O(9)-Zn(1)-N(3) O(9)-Zn(1)-N(1) O(3)-Zn(1)-N(4) N(3)-Zn(1)-N(4) O(1)-Zn(2)-O(5)#1 O(5)#1-Zn(2)-O(8)#2 O(5)#1-Zn(2)-O(7) O(1)-Zn(2)-O(10) O(8)#2-Zn(2)-O(10) O(1)-Zn(2)-O(6)#1 O(8)#2-Zn(2)-O(6)#1 O(10)-Zn(2)-O(6)#1 3 Zn(1)-O(3) Zn(1)-N(3) Zn(1)-N(1) Zn(2)-O(1) Zn(2)-O(8)#1 Zn(2)-O(7) O(3)-Zn(1)-O(9) O(9)-Zn(1)-N(3) O(9)-Zn(1)-O(5) O(3)-Zn(1)-N(1) N(3)-Zn(1)-N(1) O(3)-Zn(1)-N(4) N(3)-Zn(1)-N(4) N(1)-Zn(1)-N(4) O(1)-Zn(2)-O(8)#1 O(1)-Zn(2)-O(4)#2 O(8)#1-Zn(2)-O(4)#2 O(10)-Zn(2)-O(7) O(4)#2-Zn(2)-O(7) O(10)-Zn(2)-O(6)#2 O(4)#2-Zn(2)-O(6)#2
1.956(5) 2.155(5) 2.217(5) 164.1(2) 83.6(2) 75.4(2) 89.5(2) 91.8(2) 91.3(2) 169.16(18) 87.1(2)
Zn(1)-N(3) Zn(1)-N(1) Zn(1)-O(5) O(3)-Zn(1)-O(6) O(3)-Zn(1)-N(1) O(6)-Zn(1)-N(1) N(3)-Zn(1)-N(4) N(1)-Zn(1)-N(4) N(3)-Zn(1)-O(5) N(1)-Zn(1)-O(5)
2.115(5) 2.157(6) 2.272(5) 99.5(2) 120.0(2) 89.9(2) 74.8(2) 149.7(2) 85.68(19) 85.7(2)
1.944(2) 2.068(3) 2.171(3) 2.080(2) 2.088(3) 2.401(3) 94.10(11) 116.42(12) 100.82(11) 99.43(10) 74.86(10) 95.33(10) 143.59(10) 101.60(10) 93.06(11) 85.98(11) 152.66(10) 86.49(10) 89.08(12)
Zn(1)-O(9) Zn(1)-N(1) Zn(2)-O(1) Zn(2)-O(8)#2 Zn(2)-O(10)
1.978(3) 2.156(3) 2.009(2) 2.088(2) 2.108(3)
O(3)-Zn(1)-N(3) O(3)-Zn(1)-N(1) N(3)-Zn(1)-N(1) O(9)-Zn(1)-N(4) N(1)-Zn(1)-N(4) O(1)-Zn(2)-O(8)#2 O(1)-Zn(2)-O(7) O(8)#2-Zn(2)-O(7) O(5)#1-Zn(2)-O(10) O(7)-Zn(2)-O(10) O(5)#1-Zn(2)-O(6)#1 O(7)-Zn(2)-O(6)#1
149.34(11) 101.46(11) 76.79(11) 97.72(11) 150.84(11) 120.85(10) 95.97(11) 80.09(10) 88.29(11) 165.90(11) 57.47(10) 87.86(11)
Zn(1)-O(9) Zn(1)-O(5) Zn(1)-N(4) Zn(2)-O(10) Zn(2)-O(4)#2 Zn(2)-O(6)#2 O(3)-Zn(1)-N(3) O(3)-Zn(1)-O(5) N(3)-Zn(1)-O(5) O(9)-Zn(1)-N(1) O(5)-Zn(1)-N(1) O(9)-Zn(1)-N(4) O(5)-Zn(1)-N(4) O(1)-Zn(2)-O(10) O(10)-Zn(2)-O(8)#1 O(10)-Zn(2)-O(4)#2 O(1)-Zn(2)-O(7) O(8)#1-Zn(2)-O(7) O(1)-Zn(2)-O(6)#2 O(8)#1-Zn(2)-O(6)#2 O(7)-Zn(2)-O(6)#2
2.049(3) 2.151(3) 2.214(3) 2.064(3) 2.095(3) 2.163(3) 167.54(11) 79.64(10) 90.21(10) 91.69(12) 97.91(11) 89.45(12) 85.48(10) 97.10(12) 90.44(11) 100.87(12) 89.07(10) 79.06(10) 170.88(11) 90.63(10) 88.83(10)
2.026(2) 2.097(3) 2.201(3) 2.038(2) 2.088(3) 2.134(3) 92.38(11) 98.68(11) 168.36(11) 99.20(11) 74.89(11) 110.41(11) 75.60(11) 150.30(11) 97.68(11) 92.44(10) 163.80(11) 168.47(11) 88.53(10) 86.53(11) 78.64(9)
Symmetry transformations used to generate equivalent atoms: For 1:#1 -xþ2,-yþ2,-z. For 2: #1 -xþ2,-yþ1,-zþ1; #2 -xþ2,-yþ2,-zþ1. For 3: #1 -xþ1,-y,-zþ1; #2 -x,y-1/2,zþ1/2; #3 -x,yþ1/2,-zþ1/2; #4 -xþ1,-yþ1,-zþ1.
by one oxygen atom O1A from (bppc)- anions, two oxygen atoms(O5, O6) from (o-bdc)2anion, two oxygen atoms(O7, O8A) from (ox)2anion and one oxygen atom O10 from one coordination water molecules with a distorted octahedral geometry. The N1, N3, N4, and O3 atoms comprise the equatorial plane and the O5 and O6 atoms occupy the axial positions. The O1A,O5, O6 and O8A atoms comprise the equatorial plane and the O7 and O10 atoms occupy the axial positions. In compound 2, two (bppc)- anions, two (o-bdc)2anions and four ZnII cations form a 29-membered rings (Fig. 2b). The adjacent tetranuclear rings were bridged by the (ox)2anions with a m2h1:h1:h1:h1 coordination mode to form one-dimensional ring chain structure (Fig. 2c).
3.1.3. [Zn2(bppc)(ox)1.5(H2O)2]n·3.5nH2O (3) The single-crystal X-ray diffraction analysis reveals that 3 crystallizes in monoclinic system P21/c space group. The asymmetric unit of 3 contains two crystallographically independent ZnII cations, one (bppc)-, one and a half (ox)2- anions, two coordination water and three and a half lattice water molecules. As shown in Fig. 3a, the Zn1 atom is coordinated by three nitrogen atoms (N1,N3,N4) from one (bppc)- ligand, two oxygen atoms(O3, O5) from one (ox)2anion and one oxygen atom O9 from one coordination water molecules showing a distorted octahedral coordination geometry. The O5 and O9 atoms occupy the axial positions and the N1, N3, N4and O3 atoms comprise the equatorial plane. The Zn2 atom is coordinated by one oxygen O1 atom from one (bppc)- anion ligand, four carboxylic oxygen atoms (O4A,O6A, O7, O8A)of (ox)2- anion
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Fig. 1. (a) Coordination environment of Zn(II) ion in 1, in which the hydrogen atoms are omitted for clarity; (b) View of the dimer [Zn2(bppc)2(bdc)(H2O)4] structure.
coordinates to the Zn1II cation with a tridentate chelating coordination mode, the carboxylate section of the (bppc)- ligand coordinates to the Zn2II cation by bridging coordination mode (Scheme 1b) and the (ox)2- ligand coordinates to the ZnII cation with a m2-h1:h1:h1:h1coordination mode. The Zn1II and Zn2II cations were connected by four (bppc)- and six (ox)2- ligands forming 53-membered rings (Fig. 3b). The rings are further joined by (ox)2ligands to form 1D ring chain(Fig. 3c), and the (ox)2- ligands further extend the 1D ring chains into a 2D layer structure (Fig. 3d).
3.2. Comparative structural study Scheme. 1. Coordination modes of the (bppc)- ligand in 1e3.
and one oxygen atom O10 of coordination water molecule also showing a distorted octahedral coordination geometry. In 3, The pyridyl and pyrazinyl sections of the (bppc)- ligand
Compounds 1e3 were prepared as single-phase crystalline products by hydrothermal reactions of Hbppc ligand and ZnCl2 together with corresponding O-donor assistant ligands. The structural research remind us to study the effects of auxiliary ligands on the final structures. As far as the zinc atoms coordination: The commonly coordination numbers for this cation are 4 and 5, while 6 is much less common. In the structures of 1 and 3 all the zinc
Fig. 2. (a) Coordination environment of Zn(II) ion in 2, and the hydrogen atoms are omitted for clarity; (b) The 29-membered ring constructed by (bppc)- anions, (o-bdc)2anions and ZnII cations; (c) The one-dimensional ring chain structure of 2.
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atoms present CN ¼ 6, compound 2 shows zinc atoms with two different coordination numbers (5 and 6). Compounds 1e3 show three structures with different dimensionality (dimeric units, 0D; ring chain, 1D; layers, 2D) with the same (bppc)- ligands and different auxiliary ligand. In the compound 1, the auxiliary (bdc)2ligand bridge the two adjacent ZnII cation with a m2-h1:h1 bridging coordination mode and was sandwiched between the two ZnII units, meanwhile, the carboxyl oxygen of (bppc)- ligand did not participate in the coordination, which makes the structure has not been further extended and showing a dimer structure. Compound 2 contains two kinds of carboxylic acid auxiliary ligand. The (o-bdc)2ligand bridge the two adjacent ZnII cation to form a 29-membered rings and the (ox)2- ligand further bridge the rings to form onedimensional ring chain structure. In compound 3, the (ox)2- auxiliary ligand coordinates to the ZnII cation with a m2h1:h1:h1:h1coordination mode, and the (ox)2- ligand not only participates in the formation of the ring but also expands the ring chain to two-dimensional structure. 3.3. FT-IR spectra The IR spectra of free ligand and compounds 1e3 were performed as KBr pellets in the range 4000e400 cm1(Fig. S1). The strong bands at 3416 cm1 for 1 and 3, 3417 cm1 for 2 are the stretching vibration of the O-H group. The peaks of 1618 cm1, 1629 cm1and 1619 cm1 are assigned to the skeleton vibration of C]N in 1e3, respectively, which display certain shifts in contrast with 1598 cm1 in the ligand. It is thus assumed that nitrogen atoms in the ligand coordinate to metal atoms. The absence of any strong bands around 1700 cm1 indicates that the carboxylate groups of ligand are completely deprotonated. In the IR spectrum of 1, the asymmetric and symmetric stretching vibrations of the COO groups appear at 1563 cm1and 1402 cm1 separately. The difference in the value between them is less than 200 cm1, which
indicates that the carboxylate groups coordinate to the metal ions only in bidentate-chelating mode. In the IR spectre of 2 and 3, The difference between nas(COO) and ns(COO) (D ¼ 154 cm1 and 225 cm1 for 2, 164 cm1 and 241 cm1 for 3) suggests that the carboxylate groups coordinate to the metal ions in both monodentate-bridging mode and bidentate-chelating modes [22]. These IR spectra are in good agreement with the result of X-ray structural analysis.
3.4. Thermal behavious In order to characterize the thermal stability of compounds 1e3, their thermal decomposition behavior was investigated as shown in Fig. 4. For 1, there exists a weight loss of 10.16% occurring below 200 C, which correspond to the continuous loss of six water molecules and can be compared with the calculated value of 10.04%. Its framework keeps stable to 365 C, and then the framework collapses accompanying the release of the (bdc)2- and (bppc)ligands. For 2, there exists a weight loss of 5.03% occurring below 200 C, which correspond to the continuous loss of four water molecules and can be compared with the calculated value of 5.22%.The second stage weight loss can be detected from 309 to 410 C attributed to the departure of (o-bdc)2- and (ox)2ligands (Calcd. 30.18%; Found: 29.95%). Above 525 C, the compound begins to lose (bppc)- ligand and then starts to decompose. Compound 3 first loses its lattice water molecule below 110 C, and the weight loss found of 8.89% is consistent with that calculated (8.56%). The second stage weight loss can be detected from 110 to 180 C attributed to the departure of coordinated water molecules (Calcd. 5.70%; Found: 5.59%). Above 390 C, the compound begins to lose their ligands and then starts to decompose.
Fig. 3. (a) Coordination environment of Zn(II) ion in 3, in which the hydrogen atoms are omitted for clarity; (b) The 53-membered ring constructed by (bppc)- ligands, (ox)2- ligands and ZnII cations; (c)The one-dimensional ring chain structure of 3. (d) 2D layer structure of 3.
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Fig. 4. TGA curves for complexes 1e3.
3.5. Photoluminescence properties Taking into account the excellent luminescent properties of d10 Zn(II) metal coordination polymers, the solid-state luminescence spectrum of free ligand (Hbppc) and 1e3 were studied at ambient temperature(Fig. 5). The enhancement of fluorescence intensity in 1e3 may be attributed to ligand chelation to the Zn(II) centers, which effectively increases the rigidity of the coordination polymers and reduces the loss of energy by nonradiative decay of the intraligand (p/p*)excited state [23]. Intense emission bands are observed at 420 and 466 nm (lex ¼ 290 nm) for Hbppc, which can be assigned to the ligand-centered charge transitions, that is, the p* / n and p* / p transitions. The emissions are also observed at 504 nm for 1, 499 nm for 2 and 502 nm for 3 respectively upon excitation at 280 nm. As previously reported [24], the solid aromatic carboxylate ligands H2bdc and H2ox are nearly nonfluorescent in the range of 400～600 nm at ambient temperature. Compared with the free Hbppc, the main peaks in 1e3 are redshifted, which may result from the coordination of the ligands to the metal centers [25]. Notably, a lower energy emission was also detected at 388 nm for 3 which can be attributed to the intraligand p* / p transition.
donor ligands and different O-donor ligands. Compounds 1e3 show intriguing 0D, 1D and 2D structures. Their structural diversities indicate that the structural diversity could be modulated by the species of auxiliary O-donor ligands. In addition, these three Zn(II) compounds also exhibit intense luminescence in the solid state at room temperature. Further research works about Zn(II) coordination polymers based on mixed Hbppc and O-donor ligands are under way by our group. Acknowledgment The authors thank financial assistance from the Key Scientific Research Project of Education Department of Shaanxi Province (17JS027) and Introducing Talents Foundation of Shaanxi University of Technology (no. SLGKYQD2-11). This work is also supported by team of syngas catalytic conversion of Shaanxi university of Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.05.030.
4. Conclusions
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