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Synthesis and characterization of different zinc(II) oxide nano-structures from two new zinc(II)–Quinoxaline coordination polymers
Journal of Molecular Structure 1095 (2015) 8–14
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of different zinc(II) oxide nano-structures from two new zinc(II)–Quinoxaline coordination polymers Fatemeh Molaei a, Fahime Bigdeli a, Ali Morsali b,⇑, Sang Woo Joo c,⇑, Giuseppe Bruno d, Hadi Amiri Rudbari e a
Department of Chemistry, Payame Noor University, Abhar, Zanjan, Iran Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea d Dipartimento di Chimica Inorganica, Vill. S. Agata, Salita Sperone 31, Università di Messina, 98166 Messina, Italy e Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran b c
h i g h l i g h t s Two new one-dimensional zinc(II) coordination polymers have been synthesized by slow evaporation method. Nano crystalline ZnO have successfully been synthesized via techniques of thermolyses. The final product was characterized using XRD and SEM methods. The same precursors to prepare ZnO nanostructure with different techniques gave us various morphologies.
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Article history: Received 25 October 2014 Received in revised form 2 February 2015 Accepted 31 March 2015 Available online 9 April 2015 Keywords: Zinc(II) coordination polymers Quinoxaline Nanostructures Zinc(II) oxide
a b s t r a c t Two new zinc(II) coordination polymers, [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2), Quinoxaline = Benzopyrazine, have been synthesized and characterized by IR spectroscopy. Compounds 1 and 2 were structurally characterized by single crystal X-ray diffraction and are one-dimensional coordination polymers with coordination environment of octahedral and tetrahedral respectively. Nanostructures of zinc(II) oxide were obtained by thermolyses of compound 1 in oleic acid, calcination of compound 1 at 500 °C under air atmosphere and sol–gel processes. Also, nanopowders of zinc(II) oxide were obtained by calcination of compound 2 at 450 and 550 °C. The nanomaterials were characterized by scanning electron microscopy and X-ray powder diffraction (XRD). The thermal stability of compounds 1 and 2 both their bulk were studied by thermo-gravimetric (TGA) and differential thermal analyses (DTA). This study demonstrates the coordination polymers may be suitable precursors for the preparation of nanoscale materials. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Coordination polymers or ‘‘inorganic and organic hybrid polymers’’ are infinite structures in zero, one, two and three dimensions. The structure of this sort of polymer is constructed from coordination bonds and some weak interactions such as hydrogen bonds, and p–p stacking, [1]. The synthesis of coordination polymers with different metal ions and ligands have led to a wide range of potential applications as e.g. molecular wires, electrical conductors, molecular magnets, in host–guest chemistry and in catalysis [2–17]. Their ⇑ Corresponding authors. E-mail addresses: [email protected], [email protected] (A. Morsali), [email protected] (S.W. Joo). http://dx.doi.org/10.1016/j.molstruc.2015.03.070 0022-2860/Ó 2015 Elsevier B.V. All rights reserved.
specific applications such as NLO (Nonlinear Optics) [18], catalysis [19,20], gas storage [21–23], conductivity [24–26], and magnetic [27–34] are the main reasons that their synthesis has been grown during the last decades. The Zn(II) ions as metal centers have been used extensively for the synthesis of various organic–inorganic networks [35–39]. In contrast to inorganic nanomaterials, syntheses of nanostructured metal coordination polymers seems to be surprisingly sparse, and to date most investigations on coordination polymers have been studied only in the solid state and studies of their properties were limited to investigations at the macroscopic scale [40]. Over the past decade, synthesis of nanostructured ZnO materials with controlled sizes, morphologies, structures, and properties has widely studied and variety of ZnO nanostructures, including nanorods, nanowires, nanoflowers, nanotubes, nanosheets and
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N
Table 3 Hydrogen bonds for compound [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1).
N Fig. 1. Depiction of Quinoxaline ligand.
DAH A
d(DAH)
d(H A)
d(D A)
<(DHA)
O(1W)AH(1WA) N(5) O(1W)AH(1WB) O(5)#1 O(1W)AH(1WB) O(6)#1 O(2W)AH(2WA) O(3W)#2 O(2W)AH(2WB) O(3W)#3 O(3W)AH(3WA) N(2) O(3W)AH(3WB) O(1)#4 O(3W)AH(3WB) O(3)#4
0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85
1.93 2.36 2.39 1.91 1.87 1.98 2.27 2.25
2.760(3) 3.117(3) 3.192(3) 2.755(2) 2.718(2) 2.792(3) 3.000(2) 3.001(3)
164 148 158 170 173 160 144 147
Table 1 Crystal data and structure refinement for [Zn(Quinoxaline)(NO3)2(H2O)2]n QuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2). Identification code
1
2
Empirical formula Formula weight T (K) Wavelength Crystal system Space group
C16H18N6O9Zn 503.73 120(2) 0.71073 Triclinic P1
C16H12Br2N4Zn 485.49 293(2) 0.71073 Triclinic P1
Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Density (calculated) (Mg/m3) Absorption coefficient (mm 1) F(0 0 0) Crystal size h Range for data collection (°) Index ranges
7.2324(7) 10.0471(10) 14.0671(15) 98.087(2) 95.692(2) 102.678(2) 978.35(17) 2 1.710 1.322 516 0.31 0.28 0.23 2.11–29.00 96h69 13 6 k 6 13 19 6 l 6 19 Reflections collected 14,393 Independent reflections 5181 (Rint = 0.0332) Refinement method Full-matrix leastsquares on F2 Data/restraints/parameters 5181/0/289 Goodness-of-fit (GOF) on F2 1.010 Final R indices [I > 2r(I)] R1 = 0.0422 and wR2 = 0.0852 R indices (all data) R1 = 0.0571 and wR2 = 0.0907 Largest diff. peak and hole (e Å 3) 0.687 and 0.617
8.2312(3) 8.6568(2) 12.4310(4) 80.1210(10) 86.224(2) 73.9530(10) 838.49(5) 2 1.923 6.233 472 0.26 0.13 0.11 1.66–27.00 10 6 h 6 10 11 6 k 6 11 15 6 l 6 15 28,782 3648 (Rint = 0.0325) Full-matrix leastsquares on F2 3648/0/208 1.058 R1 = 0.0225 and wR2 = 0.0555 R1 = 0.0303 and wR2 = 0.0589 0.629 and 0.623
Table 2 Bond lengths [Å] and angles [°] for [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2). Compound 1 Zn(1)AO(2W) Zn(1)AO(4) Zn(1)AO(1W) Zn(1)AN(1) Zn(1)AO(1) Zn(1)AO(2) O(2W)AZn(1)AO(4) O(2W)AZn(1)AO(1W) O(4)AZn(1)AO(1W) O(2W)AZn(1)AN(1) O(4)AZn(1)AN(1)
2.0394(17) 2.0597(18) 2.0638(17) 2.119(2) 2.2121(18) 2.2749(19) 91.92(7) 168.54(7) 87.56(7) 100.15(8) 106.39(8)
O(1W)AZn(1)AN(1) O(2W)AZn(1)AO(1) O(4)AZn(1)AO(1) O(1W)AZn(1)AO(1) N(1)AZn(1)AO(1) O(2W)AZn(1)AO(2) O(4)AZn(1)AO(2) O(1W)AZn(1)AO(2) N(1)AZn(1)AO(2) O(1)AZn(1)AO(2)
90.97(8) 87.75(7) 153.34(7) 87.60(7) 99.89(7) 83.00(7) 95.98(7) 85.66(7) 157.22(7) 57.51(6)
Compound 2 Br(1)AZn(1) Br(2)AZn(1) Zn(1)AN(1) Zn(1)AN(3) N(1)AZn(1)AN(3)
2.3598(3) 2.3588(3) 2.0710(17) 2.0760(18) 119.36(7)
N(1)AZn(1)ABr(2) N(3)AZn(1)ABr(2) N(1)AZn(1)ABr(1) N(3)AZn(1)ABr(1) Br(2)AZn(1)ABr(1)
104.41(5) 103.81(5) 103.88(5) 104.77(5) 121.782(13)
Fig. 2. ORTEP diagram and representation of ZnII environment (a) in compound [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and (b) in compound [Zn(Quinoxaline)2(Br)2]n (2).
nanorings have been reported [41–46]. ZnO is a well-known semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy of 60 meV at room temperature [47,48] and it has a wide range of applications such as solar cells, luminescent, electrical and acoustic devices, gas and chemical sensors, coatings, catalysts, micro lasers, memory arrays and biomedical applications [49,50]. Here we report two new Zn(II) coordination polymers as new precursors for Zn(II) oxide nano spherical particles. In this paper we describe the preparation of two new zinc(II) coordination polymers, [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1)
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Fig. 3. Crystal packing of compound 1, O(1W)AH(1WA) N(5), O(3W)AH(3WA) N(2), O(1W)AH(1WB) O(5)#1, O(1W)AH(1WB) O(6)#1 [x 1, y, z], O(2W)AH(2WA) O(3W)#2 [ x + 1, y + 1, z + 1], O(2W)AH(2WB) O(3W)#3 [x, y + 1, z], O(3W)AH(3WB) O(1)#4, O(3W)AH(3WB) O(3)#4 [ x, y + 1, z + 1] bonds are shown by dash lines.
and [Zn(Quinoxaline)2(Br)2]n (2), Quinoxaline = Benzopyrazine (Fig. 1), and its use for preparation of ZnO nanostructures. Experimental Materials and physical techniques All reagents and solvents for the synthesis and analysis were purchased from Merck Company and used without further purification. IR spectra were recorded using Perkin–Elmer 597 and Nicolet 510P spectrophotometers. Melting points were measured on an Electrothermal 9100 apparatus. X-ray powder diffraction (XRD) measurements were performed using an X’pert diffractometer of Philips company with monochromatized Cu ka radiation. Single crystal measurements were made using a Bruker APEX area-detector diffractometer. The intensity data were collected using graphite monochromated Mo Ka radiation (k = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least-squares techniques on F2. Structures solution and refinement were accomplished using SHELXL-97 program packages [51]. The molecular structure plots were prepared using Mercury software [52]. TGA and DTA curves were recorded using a PL-STA1500 device manufactured by Thermal Sciences. The samples were characterized with a scanning electron microscope (SEM) (Philips XL 30) with gold coating. Synthesis of [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) To a magnetically stirred acetonitrilic solution of 1 mmol Quinoxaline (0.130 g) was added dropwise a 2 mmol (0.138 g) of NaNO2 and 1 mmol (0.297 g) Zn(NO3)26H2O in acetonitrile at room temperature over 30 min. The reaction mixture was stirred for 1 h at room temperature. Single crystals suitable for X-ray analysis were obtained by slow evaporation of this solution at room temperature (m.p. 96 °C), which were filtered off, washed with acetone and dried in air.
Synthesis of [Zn(Quinoxaline)2(Br)2]n (2) To a magnetically stirred acetonitrilic solution of 2 mmol KBr (0.238 g) was added dropwise a 1 mmol (0.130 g) of Quinoxaline and 1 mmol (0.297 g) Zn(NO3)26H2O in acetonitrile at room temperature over 30 min. The reaction mixture was stirred for 3 h at room temperature. Single crystals suitable for X-ray analysis were obtained by slow evaporation of this solution at room temperature (m.p. 260 °C), which were filtered off, washed with acetone and dried in air. Synthesis of ZnO nanostructures from compound 1 Zinc(II) oxide nanostructures was synthesized from compound 1 through simple calcination. Amount of compound 1 (50 mg) was heated to 500 °C for 4 h. After cooling, white precipitate was obtained. The solid was washed with EtOH and dried under nitrogen atmosphere. and in the other technique, The precursor 1 (1 mmol) were dissolved in oleic acid (8 mL) to form a greenish black solution. This solution was heated to 200 °C for 1 h in air. At the end of the reaction, a black precipitate was formed. A small amount of toluene and a large excess of EtOH were added to the reaction solution and ZnO nano structures were separated by centrifugation. The dark solid was washed with EtOH and dried in air. In Sol–gel technique, mixture of 1 mmol of compound 1, 1 g polyvinyl alcohol and 30 mL EtOH was stirred for 1 h, in the mild temperature, finally was obtained by slow evaporation of this solution, a compound such as the gel, after drying, it was heated to 500 °C for 4 h. Synthesis of ZnO nanopowders from compound 2 Zinc(II) oxide nanopowders was synthesized from compound 2 through simple calcination. Amount of compound 2 (50 mg) was heated to 450 °C for 4 h. After cooling, black precipitate was obtained. The solid was washed with EtOH and dried under nitrogen atmosphere.
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Fig. 4. (a) View of supramolecular framework in 2 with CAH N hydrogen bonds (blue) and CAH Br (red) in dashed lines. (b) A fragment of the one-dimensional supramolecular compound 2 along crystallographic a, c axis (Zn = black, Br = orange, N = blue, C = gray). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Results and discussion Reaction between the organic nitrogen donor-based Quinoxaline ligand and mixtures of zinc(II) nitrate with sodium nitrite or potassium bromide yielded crystalline materials formulated as [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2), respectively. The IR spectra display characteristic absorption bands for the Quinoxaline ligand in compounds 1 and 2. IR (selected bands for compound 1; in cm 1): 3409(br), 2969(m), 1642(s), 1395(vs), 1266(m), 1029(m), 806(s), 683(m) cm 1 and IR (selected bands for compound 2; in cm 1): 3421(br), 3036(w), 1610(s), 1504(vs), 1466(s), 1359(vs), 1284(m), 1266(m), 1214(vs), 1146(s), 1130(vs), 964(vs), 873(s), 765(vs), 626(m). The relatively weak absorption bands at around 3000 cm 1 are due to the CAH modes involving the aromatic ring hydrogen atoms of the Quinoxaline ligand. The absorption bands with variable intensity in the frequency range 1400–1650 cm 1 correspond to ring vibrations of the pyrazine moiety of the Quinoxaline ligand. Also, the IR spectrum of compound 1 shows the characteristic stretching frequency of the bridging H2O group observed at 3409 cm 1 and characteristic band of the nitrate groups appears at 1395 cm 1.
Single crystal X-ray diffraction analysis (Tables 1–3) of compounds 1 and 2 was carried out and the ORTEP diagrams of the title complexes are shown in Fig. 2. Single X-ray crystal analysis reveals that [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2) complexes crystallize in Triclinic with space group Pı¯ and shows compounds 1 and 2 are one-dimensional polymers. The zinc(II) atoms of compound 1 have been coordinated by six atoms and have octahedral coordination sphere as ZnNO5. The asymmetric unit of compound 1 contains one Zn(II) atom, one Quinoxaline ligand, two nitrate anions and two coordinated water molecules. One Quinoxaline ligand in compound 1 coordinate to any zinc(II) atom by nitrogen atom of pyrazine group and other Quinoxaline ligand coordinate by hydrogen bonding interaction between N(5) atom of Quinoxaline ligand and O(1W)AH(1WA) atom of water molecule (Fig. 3). In compound 2 also zinc(II) atoms have been coordinated by four atoms and have tetrahedral coordination sphere as ZnN2Br2. Each zinc atom is coordinated by nitrogen atom of Quinoxaline ligand and two bromide anions. Hydrogen bonding interactions are present between the CAH N and CAH Br atoms of chains compound 2 (Fig. 4). In order to examine the thermal stability of the three compounds, thermal gravimetric (TG) and differential thermal analyses (DTA)
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Fig. 6. Thermal behavior of [Zn(Quinoxaline)2(Br)2]n (2). Fig. 5. Thermal behavior of [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1).
were carried out for compounds 1 and 2 between 30 and 800 °C. The TG curve of compound 1 indicates that the compound does not melt and is stable up to 50 °C at which temperature it begins to decompose (Fig. 5). Removal of the Quinoxaline ligand and decomposition of the nitrate anions takes place in the range between 50 and 360 °C with two endotherms effects at 100 and 170 °C with a mass loss of 82.0%, ultimately giving a brown, amorphous solid that appears to be ZnO. The TG curve of compound 2 indicates that this compound is stable up to 80 °C, at which temperature it begins to decompose (Fig. 6). Removal of the Quinoxaline ligand and decomposition of the bromide anions takes place in three steps between 80 and 531 °C and mass loss of 87.5% with one endothermic at 93 °C and two exothermic at 296 and
323 °C. Mass loss calculations show that the final decomposition product is ZnO. ZnO nanostructures can be synthesized from compound 1 and 2 by calcination, sol–gel and thermolysis methods. The morphology and size of the as-prepared ZnO sample were further investigated using Scanning Electron Microscopy (SEM). ZnO nanoparticles from compound 1 obtained by calcination at 500 °C with an average diameter of about 85 nm and sol–gel technique with an average diameter of about 55 nm (Figs. 7 and 8). Also, nanoflowers ZnO obtained by thermolysis of compound 1 at 200 °C (Fig. 9). ZnO nanopowders have been prepared by direct calcination of compound 2 at 450 and 550 °C (Fig. 10). To investigate the composition and phase information of the final products, X-ray diffraction (XRD) was carried out. Fig. 11 shows X-ray powder diffraction pattern of ZnO nanostructures obtained from calcination, sol–gel and
Fig. 7. The SEM image of ZnO nanoparticles produced by calcination of compound 1 at 500 °C in air (up) and its particle size distribution histogram (down).
Fig. 8. The SEM image of ZnO nanoparticles produced after of compound 1 by Sol– gel of method (up) and its particle size distribution histogram (down).
F. Molaei et al. / Journal of Molecular Structure 1095 (2015) 8–14
Fig. 9. The SEM image of ZnO nanoflowers produced after thermolyses of compound 1.
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Fig. 11. X-ray powder diffraction pattern of compound 1 and 2 after calcination, sol–gel and thermolysis methods.
Conclusions Two new one-dimensional zinc(II) coordination polymers, [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2) have been synthesized by slow evaporation method. This polymers grows in one-dimension network by coordination bonds and non-covalence bonds such as hydrogen bonding. The coordination number of Zn(II) ions in compounds 1 and 2 are six and four, respectively. Nano crystalline ZnO has successfully been synthesized via techniques of thermolyses of compound 1 in oleic acid, calcination and sol–gel processing and calcination of compound 2. The final product was characterized using XRD and SEM methods. The same precursors to prepare ZnO nanostructure with different techniques gave us various morphologies. Supplementary material Crystallographic data for the structures reported in the paper has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no, CCDC 908962 and 935545 for compounds 1 and 2, respectively. Copies of the data can be obtained on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: + 44 1223/336033; e-mail: [email protected]]. Acknowledgements The authors also thank Payame Noor and Tarbiat Modares Universities for all the supports provided.
Fig. 10. SEM image of ZnO nanopowder prepared by calcination of compound 2 at 450 °C (up) and 550 °C (down) in air.
thermolysis of compound 1 and 2. The obtained pattern matches with the standard pattern of hexagonal zinc(II) oxide with the lattice parameters (a = 3.24982 Å, C = 5.20661 and z = 2) which are same with the reported values, (JCPDS card number 36-1451). There is no unknown peak in XRD pattern, so purities are detected in the XRD pattern and sharp diffraction peaks indicate good crystallinity of ZnO nanoparticles. The broadening of the peaks indicated that the particles were of nanometer scale.
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Synthesis and characterization of different zinc(II) oxide nano-structures from two new zinc(II)–Quinoxaline coordination polymers Fatemeh Molaei a, Fahime Bigdeli a, Ali Morsali b,⇑, Sang Woo Joo c,⇑, Giuseppe Bruno d, Hadi Amiri Rudbari e a
Department of Chemistry, Payame Noor University, Abhar, Zanjan, Iran Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea d Dipartimento di Chimica Inorganica, Vill. S. Agata, Salita Sperone 31, Università di Messina, 98166 Messina, Italy e Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran b c
h i g h l i g h t s Two new one-dimensional zinc(II) coordination polymers have been synthesized by slow evaporation method. Nano crystalline ZnO have successfully been synthesized via techniques of thermolyses. The final product was characterized using XRD and SEM methods. The same precursors to prepare ZnO nanostructure with different techniques gave us various morphologies.
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Article history: Received 25 October 2014 Received in revised form 2 February 2015 Accepted 31 March 2015 Available online 9 April 2015 Keywords: Zinc(II) coordination polymers Quinoxaline Nanostructures Zinc(II) oxide
a b s t r a c t Two new zinc(II) coordination polymers, [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2), Quinoxaline = Benzopyrazine, have been synthesized and characterized by IR spectroscopy. Compounds 1 and 2 were structurally characterized by single crystal X-ray diffraction and are one-dimensional coordination polymers with coordination environment of octahedral and tetrahedral respectively. Nanostructures of zinc(II) oxide were obtained by thermolyses of compound 1 in oleic acid, calcination of compound 1 at 500 °C under air atmosphere and sol–gel processes. Also, nanopowders of zinc(II) oxide were obtained by calcination of compound 2 at 450 and 550 °C. The nanomaterials were characterized by scanning electron microscopy and X-ray powder diffraction (XRD). The thermal stability of compounds 1 and 2 both their bulk were studied by thermo-gravimetric (TGA) and differential thermal analyses (DTA). This study demonstrates the coordination polymers may be suitable precursors for the preparation of nanoscale materials. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Coordination polymers or ‘‘inorganic and organic hybrid polymers’’ are infinite structures in zero, one, two and three dimensions. The structure of this sort of polymer is constructed from coordination bonds and some weak interactions such as hydrogen bonds, and p–p stacking, [1]. The synthesis of coordination polymers with different metal ions and ligands have led to a wide range of potential applications as e.g. molecular wires, electrical conductors, molecular magnets, in host–guest chemistry and in catalysis [2–17]. Their ⇑ Corresponding authors. E-mail addresses: [email protected], [email protected] (A. Morsali), [email protected] (S.W. Joo). http://dx.doi.org/10.1016/j.molstruc.2015.03.070 0022-2860/Ó 2015 Elsevier B.V. All rights reserved.
specific applications such as NLO (Nonlinear Optics) [18], catalysis [19,20], gas storage [21–23], conductivity [24–26], and magnetic [27–34] are the main reasons that their synthesis has been grown during the last decades. The Zn(II) ions as metal centers have been used extensively for the synthesis of various organic–inorganic networks [35–39]. In contrast to inorganic nanomaterials, syntheses of nanostructured metal coordination polymers seems to be surprisingly sparse, and to date most investigations on coordination polymers have been studied only in the solid state and studies of their properties were limited to investigations at the macroscopic scale [40]. Over the past decade, synthesis of nanostructured ZnO materials with controlled sizes, morphologies, structures, and properties has widely studied and variety of ZnO nanostructures, including nanorods, nanowires, nanoflowers, nanotubes, nanosheets and
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N
Table 3 Hydrogen bonds for compound [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1).
N Fig. 1. Depiction of Quinoxaline ligand.
DAH A
d(DAH)
d(H A)
d(D A)
<(DHA)
O(1W)AH(1WA) N(5) O(1W)AH(1WB) O(5)#1 O(1W)AH(1WB) O(6)#1 O(2W)AH(2WA) O(3W)#2 O(2W)AH(2WB) O(3W)#3 O(3W)AH(3WA) N(2) O(3W)AH(3WB) O(1)#4 O(3W)AH(3WB) O(3)#4
0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85
1.93 2.36 2.39 1.91 1.87 1.98 2.27 2.25
2.760(3) 3.117(3) 3.192(3) 2.755(2) 2.718(2) 2.792(3) 3.000(2) 3.001(3)
164 148 158 170 173 160 144 147
Table 1 Crystal data and structure refinement for [Zn(Quinoxaline)(NO3)2(H2O)2]n QuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2). Identification code
1
2
Empirical formula Formula weight T (K) Wavelength Crystal system Space group
C16H18N6O9Zn 503.73 120(2) 0.71073 Triclinic P1
C16H12Br2N4Zn 485.49 293(2) 0.71073 Triclinic P1
Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Density (calculated) (Mg/m3) Absorption coefficient (mm 1) F(0 0 0) Crystal size h Range for data collection (°) Index ranges
7.2324(7) 10.0471(10) 14.0671(15) 98.087(2) 95.692(2) 102.678(2) 978.35(17) 2 1.710 1.322 516 0.31 0.28 0.23 2.11–29.00 96h69 13 6 k 6 13 19 6 l 6 19 Reflections collected 14,393 Independent reflections 5181 (Rint = 0.0332) Refinement method Full-matrix leastsquares on F2 Data/restraints/parameters 5181/0/289 Goodness-of-fit (GOF) on F2 1.010 Final R indices [I > 2r(I)] R1 = 0.0422 and wR2 = 0.0852 R indices (all data) R1 = 0.0571 and wR2 = 0.0907 Largest diff. peak and hole (e Å 3) 0.687 and 0.617
8.2312(3) 8.6568(2) 12.4310(4) 80.1210(10) 86.224(2) 73.9530(10) 838.49(5) 2 1.923 6.233 472 0.26 0.13 0.11 1.66–27.00 10 6 h 6 10 11 6 k 6 11 15 6 l 6 15 28,782 3648 (Rint = 0.0325) Full-matrix leastsquares on F2 3648/0/208 1.058 R1 = 0.0225 and wR2 = 0.0555 R1 = 0.0303 and wR2 = 0.0589 0.629 and 0.623
Table 2 Bond lengths [Å] and angles [°] for [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2). Compound 1 Zn(1)AO(2W) Zn(1)AO(4) Zn(1)AO(1W) Zn(1)AN(1) Zn(1)AO(1) Zn(1)AO(2) O(2W)AZn(1)AO(4) O(2W)AZn(1)AO(1W) O(4)AZn(1)AO(1W) O(2W)AZn(1)AN(1) O(4)AZn(1)AN(1)
2.0394(17) 2.0597(18) 2.0638(17) 2.119(2) 2.2121(18) 2.2749(19) 91.92(7) 168.54(7) 87.56(7) 100.15(8) 106.39(8)
O(1W)AZn(1)AN(1) O(2W)AZn(1)AO(1) O(4)AZn(1)AO(1) O(1W)AZn(1)AO(1) N(1)AZn(1)AO(1) O(2W)AZn(1)AO(2) O(4)AZn(1)AO(2) O(1W)AZn(1)AO(2) N(1)AZn(1)AO(2) O(1)AZn(1)AO(2)
90.97(8) 87.75(7) 153.34(7) 87.60(7) 99.89(7) 83.00(7) 95.98(7) 85.66(7) 157.22(7) 57.51(6)
Compound 2 Br(1)AZn(1) Br(2)AZn(1) Zn(1)AN(1) Zn(1)AN(3) N(1)AZn(1)AN(3)
2.3598(3) 2.3588(3) 2.0710(17) 2.0760(18) 119.36(7)
N(1)AZn(1)ABr(2) N(3)AZn(1)ABr(2) N(1)AZn(1)ABr(1) N(3)AZn(1)ABr(1) Br(2)AZn(1)ABr(1)
104.41(5) 103.81(5) 103.88(5) 104.77(5) 121.782(13)
Fig. 2. ORTEP diagram and representation of ZnII environment (a) in compound [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and (b) in compound [Zn(Quinoxaline)2(Br)2]n (2).
nanorings have been reported [41–46]. ZnO is a well-known semiconductor with a wide direct band gap (3.37 eV) and a large exciton binding energy of 60 meV at room temperature [47,48] and it has a wide range of applications such as solar cells, luminescent, electrical and acoustic devices, gas and chemical sensors, coatings, catalysts, micro lasers, memory arrays and biomedical applications [49,50]. Here we report two new Zn(II) coordination polymers as new precursors for Zn(II) oxide nano spherical particles. In this paper we describe the preparation of two new zinc(II) coordination polymers, [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1)
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Fig. 3. Crystal packing of compound 1, O(1W)AH(1WA) N(5), O(3W)AH(3WA) N(2), O(1W)AH(1WB) O(5)#1, O(1W)AH(1WB) O(6)#1 [x 1, y, z], O(2W)AH(2WA) O(3W)#2 [ x + 1, y + 1, z + 1], O(2W)AH(2WB) O(3W)#3 [x, y + 1, z], O(3W)AH(3WB) O(1)#4, O(3W)AH(3WB) O(3)#4 [ x, y + 1, z + 1] bonds are shown by dash lines.
and [Zn(Quinoxaline)2(Br)2]n (2), Quinoxaline = Benzopyrazine (Fig. 1), and its use for preparation of ZnO nanostructures. Experimental Materials and physical techniques All reagents and solvents for the synthesis and analysis were purchased from Merck Company and used without further purification. IR spectra were recorded using Perkin–Elmer 597 and Nicolet 510P spectrophotometers. Melting points were measured on an Electrothermal 9100 apparatus. X-ray powder diffraction (XRD) measurements were performed using an X’pert diffractometer of Philips company with monochromatized Cu ka radiation. Single crystal measurements were made using a Bruker APEX area-detector diffractometer. The intensity data were collected using graphite monochromated Mo Ka radiation (k = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least-squares techniques on F2. Structures solution and refinement were accomplished using SHELXL-97 program packages [51]. The molecular structure plots were prepared using Mercury software [52]. TGA and DTA curves were recorded using a PL-STA1500 device manufactured by Thermal Sciences. The samples were characterized with a scanning electron microscope (SEM) (Philips XL 30) with gold coating. Synthesis of [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) To a magnetically stirred acetonitrilic solution of 1 mmol Quinoxaline (0.130 g) was added dropwise a 2 mmol (0.138 g) of NaNO2 and 1 mmol (0.297 g) Zn(NO3)26H2O in acetonitrile at room temperature over 30 min. The reaction mixture was stirred for 1 h at room temperature. Single crystals suitable for X-ray analysis were obtained by slow evaporation of this solution at room temperature (m.p. 96 °C), which were filtered off, washed with acetone and dried in air.
Synthesis of [Zn(Quinoxaline)2(Br)2]n (2) To a magnetically stirred acetonitrilic solution of 2 mmol KBr (0.238 g) was added dropwise a 1 mmol (0.130 g) of Quinoxaline and 1 mmol (0.297 g) Zn(NO3)26H2O in acetonitrile at room temperature over 30 min. The reaction mixture was stirred for 3 h at room temperature. Single crystals suitable for X-ray analysis were obtained by slow evaporation of this solution at room temperature (m.p. 260 °C), which were filtered off, washed with acetone and dried in air. Synthesis of ZnO nanostructures from compound 1 Zinc(II) oxide nanostructures was synthesized from compound 1 through simple calcination. Amount of compound 1 (50 mg) was heated to 500 °C for 4 h. After cooling, white precipitate was obtained. The solid was washed with EtOH and dried under nitrogen atmosphere. and in the other technique, The precursor 1 (1 mmol) were dissolved in oleic acid (8 mL) to form a greenish black solution. This solution was heated to 200 °C for 1 h in air. At the end of the reaction, a black precipitate was formed. A small amount of toluene and a large excess of EtOH were added to the reaction solution and ZnO nano structures were separated by centrifugation. The dark solid was washed with EtOH and dried in air. In Sol–gel technique, mixture of 1 mmol of compound 1, 1 g polyvinyl alcohol and 30 mL EtOH was stirred for 1 h, in the mild temperature, finally was obtained by slow evaporation of this solution, a compound such as the gel, after drying, it was heated to 500 °C for 4 h. Synthesis of ZnO nanopowders from compound 2 Zinc(II) oxide nanopowders was synthesized from compound 2 through simple calcination. Amount of compound 2 (50 mg) was heated to 450 °C for 4 h. After cooling, black precipitate was obtained. The solid was washed with EtOH and dried under nitrogen atmosphere.
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Fig. 4. (a) View of supramolecular framework in 2 with CAH N hydrogen bonds (blue) and CAH Br (red) in dashed lines. (b) A fragment of the one-dimensional supramolecular compound 2 along crystallographic a, c axis (Zn = black, Br = orange, N = blue, C = gray). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Results and discussion Reaction between the organic nitrogen donor-based Quinoxaline ligand and mixtures of zinc(II) nitrate with sodium nitrite or potassium bromide yielded crystalline materials formulated as [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2), respectively. The IR spectra display characteristic absorption bands for the Quinoxaline ligand in compounds 1 and 2. IR (selected bands for compound 1; in cm 1): 3409(br), 2969(m), 1642(s), 1395(vs), 1266(m), 1029(m), 806(s), 683(m) cm 1 and IR (selected bands for compound 2; in cm 1): 3421(br), 3036(w), 1610(s), 1504(vs), 1466(s), 1359(vs), 1284(m), 1266(m), 1214(vs), 1146(s), 1130(vs), 964(vs), 873(s), 765(vs), 626(m). The relatively weak absorption bands at around 3000 cm 1 are due to the CAH modes involving the aromatic ring hydrogen atoms of the Quinoxaline ligand. The absorption bands with variable intensity in the frequency range 1400–1650 cm 1 correspond to ring vibrations of the pyrazine moiety of the Quinoxaline ligand. Also, the IR spectrum of compound 1 shows the characteristic stretching frequency of the bridging H2O group observed at 3409 cm 1 and characteristic band of the nitrate groups appears at 1395 cm 1.
Single crystal X-ray diffraction analysis (Tables 1–3) of compounds 1 and 2 was carried out and the ORTEP diagrams of the title complexes are shown in Fig. 2. Single X-ray crystal analysis reveals that [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2) complexes crystallize in Triclinic with space group Pı¯ and shows compounds 1 and 2 are one-dimensional polymers. The zinc(II) atoms of compound 1 have been coordinated by six atoms and have octahedral coordination sphere as ZnNO5. The asymmetric unit of compound 1 contains one Zn(II) atom, one Quinoxaline ligand, two nitrate anions and two coordinated water molecules. One Quinoxaline ligand in compound 1 coordinate to any zinc(II) atom by nitrogen atom of pyrazine group and other Quinoxaline ligand coordinate by hydrogen bonding interaction between N(5) atom of Quinoxaline ligand and O(1W)AH(1WA) atom of water molecule (Fig. 3). In compound 2 also zinc(II) atoms have been coordinated by four atoms and have tetrahedral coordination sphere as ZnN2Br2. Each zinc atom is coordinated by nitrogen atom of Quinoxaline ligand and two bromide anions. Hydrogen bonding interactions are present between the CAH N and CAH Br atoms of chains compound 2 (Fig. 4). In order to examine the thermal stability of the three compounds, thermal gravimetric (TG) and differential thermal analyses (DTA)
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Fig. 6. Thermal behavior of [Zn(Quinoxaline)2(Br)2]n (2). Fig. 5. Thermal behavior of [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1).
were carried out for compounds 1 and 2 between 30 and 800 °C. The TG curve of compound 1 indicates that the compound does not melt and is stable up to 50 °C at which temperature it begins to decompose (Fig. 5). Removal of the Quinoxaline ligand and decomposition of the nitrate anions takes place in the range between 50 and 360 °C with two endotherms effects at 100 and 170 °C with a mass loss of 82.0%, ultimately giving a brown, amorphous solid that appears to be ZnO. The TG curve of compound 2 indicates that this compound is stable up to 80 °C, at which temperature it begins to decompose (Fig. 6). Removal of the Quinoxaline ligand and decomposition of the bromide anions takes place in three steps between 80 and 531 °C and mass loss of 87.5% with one endothermic at 93 °C and two exothermic at 296 and
323 °C. Mass loss calculations show that the final decomposition product is ZnO. ZnO nanostructures can be synthesized from compound 1 and 2 by calcination, sol–gel and thermolysis methods. The morphology and size of the as-prepared ZnO sample were further investigated using Scanning Electron Microscopy (SEM). ZnO nanoparticles from compound 1 obtained by calcination at 500 °C with an average diameter of about 85 nm and sol–gel technique with an average diameter of about 55 nm (Figs. 7 and 8). Also, nanoflowers ZnO obtained by thermolysis of compound 1 at 200 °C (Fig. 9). ZnO nanopowders have been prepared by direct calcination of compound 2 at 450 and 550 °C (Fig. 10). To investigate the composition and phase information of the final products, X-ray diffraction (XRD) was carried out. Fig. 11 shows X-ray powder diffraction pattern of ZnO nanostructures obtained from calcination, sol–gel and
Fig. 7. The SEM image of ZnO nanoparticles produced by calcination of compound 1 at 500 °C in air (up) and its particle size distribution histogram (down).
Fig. 8. The SEM image of ZnO nanoparticles produced after of compound 1 by Sol– gel of method (up) and its particle size distribution histogram (down).
F. Molaei et al. / Journal of Molecular Structure 1095 (2015) 8–14
Fig. 9. The SEM image of ZnO nanoflowers produced after thermolyses of compound 1.
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Fig. 11. X-ray powder diffraction pattern of compound 1 and 2 after calcination, sol–gel and thermolysis methods.
Conclusions Two new one-dimensional zinc(II) coordination polymers, [Zn(Quinoxaline)(NO3)2(H2O)2]nQuinoxalineH2O (1) and [Zn(Quinoxaline)2(Br)2]n (2) have been synthesized by slow evaporation method. This polymers grows in one-dimension network by coordination bonds and non-covalence bonds such as hydrogen bonding. The coordination number of Zn(II) ions in compounds 1 and 2 are six and four, respectively. Nano crystalline ZnO has successfully been synthesized via techniques of thermolyses of compound 1 in oleic acid, calcination and sol–gel processing and calcination of compound 2. The final product was characterized using XRD and SEM methods. The same precursors to prepare ZnO nanostructure with different techniques gave us various morphologies. Supplementary material Crystallographic data for the structures reported in the paper has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no, CCDC 908962 and 935545 for compounds 1 and 2, respectively. Copies of the data can be obtained on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: + 44 1223/336033; e-mail: [email protected]]. Acknowledgements The authors also thank Payame Noor and Tarbiat Modares Universities for all the supports provided.
Fig. 10. SEM image of ZnO nanopowder prepared by calcination of compound 2 at 450 °C (up) and 550 °C (down) in air.
thermolysis of compound 1 and 2. The obtained pattern matches with the standard pattern of hexagonal zinc(II) oxide with the lattice parameters (a = 3.24982 Å, C = 5.20661 and z = 2) which are same with the reported values, (JCPDS card number 36-1451). There is no unknown peak in XRD pattern, so purities are detected in the XRD pattern and sharp diffraction peaks indicate good crystallinity of ZnO nanoparticles. The broadening of the peaks indicated that the particles were of nanometer scale.
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