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Sonochemical synthesis of a new nano-plate lead(II) coordination polymer constructed of maleic acid
Inorganica Chimica Acta 363 (2010) 2506–2511
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Sonochemical synthesis of a new nano-plate lead(II) coordination polymer constructed of maleic acid Leila Aboutorabi a, Ali Morsali b,* a b
Department of Chemistry, Payame Noor University, Abhar, Zanjan, Islamic Republic of Iran Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 27 February 2010 Received in revised form 6 April 2010 Accepted 13 April 2010 Available online 20 April 2010 Keywords: Nano-plate Lead(II) oxide Maleic acid Crystal structure
a b s t r a c t A new nano-sized lead(II) coordination polymer of maleic acid (H2Mal), [Pb(l7-Mal)]n (1), has been synthesized by sonochemical method and characterized by scanning electron microscopy, X-ray powder diffraction, elemental analyses and IR spectroscopy. The compound 1 was structurally characterized by single-crystal X-ray diffraction. Thermal stability of nano and bulk samples of compound 1 were studied and compared with each other. After calcination of nano-sized compound 1 at 600 °C, pure phase microsized lead(II) oxide has been produced. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Within the field of coordination polymers, efforts to use metal ions and organic spacers simultaneously have recently been developed [1–8]. During the past decades, a number of these compounds, with interesting polymeric motifs, have been successfully designed and synthesized. Some of them exhibit encouraging potential for application in catalysis, nonlinear optics, gas separation, magnetic properties, and molecular recognition [5]. Lead(II)–carboxylate systems containing mono-, di-, and polycarboxylate [9–12] have been studied extensively in the construction of metal–organic networks. In contrast to inorganic nanomaterials, specific syntheses of nano-structured coordination polymers seems to be surprisingly sparse, and to date most investigations on coordination polymers have been carried out only in the solid state and studies of their properties were limited to investigations at the macroscopic scale. The potential use of coordination polymers as materials for nanotechnological applications would seem to be very extensive as nanometer-scaled materials often exhibit the new interesting size-dependent physical and chemical properties that can not be observed in their bulk analogous. Although considerable effort has been dedicated to the controlled synthesis of nanoscale particles of metals, oxides, sulfides, and ceramic materials, little attention was focused to date on nanoparticles of supramolecular compounds such as coordination polymers [13]. Several different synthetic approaches have been * Corresponding author. Fax: +98 2188009730. E-mail addresses: [email protected], [email protected] (A. Morsali). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.04.024
offered for the preparation of coordination polymers. Some of them are (1) slow diffusion of the reactants into a polymeric matrix, (2) diffusion from the gas phase, (3) evaporation of the solvent at ambient or reduced temperatures, (4) precipitation or recrystallisation from a mixture of solvents, (5) temperature controlled cooling and (6) hydrothermal synthesis. In this paper we describe a simple synthetic sonochemical preparation of nano-structures of a new coordination polymer, [Pb(l7-Mal)]n (1), {(H2Mal = maleic acid}, and on their conversion into nano-structured lead(II) oxide by calcination at moderately elevated temperature. Sonochemistry is the research area in which molecules undergo a chemical reaction due to the application of powerful ultrasound radiation (20 kHz–10 MHz) [14]. Ultrasound induces chemical changes due to cavitation phenomena involving the formation, growth, and instantaneously implosive collapse of bubbles in a liquid, which can generate local hot spots having a temperature of roughly 5000 °C, pressures of about 500 atm, and a lifetime of a few microseconds [15]. These extreme conditions can drive chemical reactions which have been developed to fabricate a variety of nano compounds [14]. In recent years many kinds of nanomaterials have been prepared by this method [15–25]. 2. Experimental All reagents and solvents for the synthesis and analysis were commercially available and were used as received. A multiwave ultrasonic generator (Sonicator_3000; Misonix, Inc., Farmingdale, NY, USA) was used. IR spectra were recorded using Perkin–Elmer 597 and Nicolet 510P spectrophotometers. Microanalyses were
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carried out using a Heraeus CHN-O-Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. The thermal behavior was measured with a PL-STA 1500 apparatus. X-ray powder diffraction (XRD) measurements were performed using a Philips diffractometer manufactured by X’pert with monochromatized Cu Ka radiation and simulated XRD powder patterns based on single crystal data were prepared using the MERCURY [26]. Single crystal diffraction measurements were made at 293(2) K using a Nonius kappa-CCD diffractometer. The intensity data were collected using graphite monochromated Mo Ka radiation. The structures were solved by direct methods and refined by full-matrix least-squares techniques on F2. The samples were characterized with a scanning electron microscope with
gold coating. The molecular structure plots were prepared using MERCURY [26] and ORTEPIII [27]. To prepare the nano-plates of [Pb(l7-Mal)]n (1), a 50 ml of a 0.01 M solution of lead(II) nitrate dehydrate in MeOH was positioned in a high-density ultrasonic probe, and 50 ml of a 0.01 M maleic acid in MeOH was added dropwisely to that solution. After the end of the titration the solution remained in the bath for a selected aging time at a selected temperature. The obtained precipitates were filtered off, washed with water and then dried in air. Product: d.p. >300 °C, yield: 0.257 g (80%). Anal. Calc. for C4H2O4Pb: C, 14.95; H, 0.63. Found: C, 14.90; H, 0.70%. IR (KBr, cm 1) selected bands: 539(w), 617(m), 1292(m), 1411(vs), 1504(vs), 1540(vs) and 1638(m).
MeOH
calcination nano-structure of compound 1
by ultrasound Pb(NO3)2 + H2Mal
PbO water
by the branched tube method
single crystals of compound 1
Scheme 1. Materials produced and synthetic methods.
Fig. 1. The IR spectra of (a) bulk materials as synthesized of compound [Pb(l7-Mal)]n (1) and (b) nano-plats of compound [Pb(l7-Mal)]n (1) prepared by sonochemical method.
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Fig. 2. The XRD patterns of (a) simulated from single-crystal X-ray data of compound [Pb(l7-Mal)]n (1) and (b) nano-plates of compound [Pb(l7-Mal)]n (1).
To isolate single crystals of [Pb(l7-Mal)]n (1), maleic acid (0.341 g, 3 mmol) and lead(II) nitrate dehydrate (1 g, 3 mmol) were placed in the arm to be heated. Water was carefully added to fill both arms, and then the arm to be heated was placed in a bath at 60 °C. After 10 days, colorless crystals were deposited in the cooler arm which were filtered off, washed with water and air dried.
Product: (0.165 g, yield 51%), m.p. >300 °C. Anal. Calc. for C4H2O4Pb: C, 14.95; H, 0.63. Found: C, 14.98; H, 0.65%. IR (KBr, cm 1) selected bands: 539(w), 617(m), 1311(m), 1403(vs), 1504(vs), 1535(vs) and 1643(w). 3. Results and discussion The reaction between maleic acid (H2Mal) and lead(II) nitrate using two different routes provided crystalline materials of the general formula [Pb(l7-Mal)]n (1). Scheme 1 gives an overview of the methods used for the synthesis of [Pb(l7-Mal)]n (1) using the two different routes. The elemental analysis and IR spectra of the nano-plates and of the single crystalline material are indistinguishable. The relatively weak IR absorption bands around 2965 cm 1 are due to the C–H modes involving the aliphatic hydrogen atoms (Fig. 1). The symmetric and asymmetric vibrations of the carboxylate group are observed at 1404 and 1532 cm 1 as sharp absorption bands. The D(masym–msym) value of 128 cm 1 indicates that the carboxylate groups of ‘‘Mal2 ” anions coordinate to the lead(II) centres in a bridging mode [28]. Fig. 2 shows the simulated XRD pattern from single-crystal Xray data (see below) of compound 1 (Fig. 2a), in comparison with
Fig. 3. SEM photographs of [Pb(l7-Mal)]n (1) nano-flower formed by nano-plate with different scale-bars (concentration of Pb+2 and Mal2 ions are 0.01 M in MeOH).
Fig. 4. SEM photographs of [Pb(l7-Mal)]n (1) nano-structure (concentration of Pb+2 and Mal2 ions are 0.01 M in water).
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L. Aboutorabi, A. Morsali / Inorganica Chimica Acta 363 (2010) 2506–2511 Table 1 Crystal data and structure refinement [Pb(l7-Mal)]n (1). Identification code Empirical formula Formula weight T (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Density (calculated) (Mg/m3) Absorption coefficient (mm 1) F(0 0 0) Crystal size (mm3) Theta range for data collection (°) Index ranges
Fig. 5. A fragment of the two-dimensional network in compound [Pb(l7-Mal)]n (1), H atoms are omitted for clarity.
Reflections collected Independent reflections Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R [I > 2r(I)] R indices (all data) Largest difference peak, hole (e Å
Compound 1 C4H2O4Pb 321.25 293(2) 0.71073 monoclinic P21/c 9.8910(10) 6.9610(10) 8.2710(10) 111.13(5) 531.18(11) 4 4.017 31.686 560 0.25 0.25 0.08 2.21–30.12 13 6 h 6 10 96k69 11 6 l 6 11 8341 1563 phi and omega scans full-matrix least-squares on F2 1563/0/83 1.054 Rl = 0.0311, wR2 = 0.0809 R1 = 0.0359, wR2 = 0.0840 4.062, 2.368
3
)
Table 2 Bond lengths (Å) and angles (°) for [Pb(l7-Mal)]n (1). Pb1–O1 Pb1–O2 Pb1–O4 Pb1–O3 Pb1–O4i Pb1–O2ii Pb1–O3i
x, y + 1/2,
z + 1/2. ii:
x,
120
y,
z.
TG DTA
100
81.17(2) 52.90(2) 79.98(2) 73.37(2) 72.97(2) 122.69(2) 141.82(2) 70.88(1) 95.94(2) 119.93(2) 80.52(2) 121.85(1) 126.23(2) 48.93(2) 136.42(1)
60 50 40
80
30
60
20
40
Δ t(°C)
the XRD pattern of a typical sample of compound 1 prepared by the sonochemical process (Fig. 2b). Acceptable matches, with slight differences in 2h, were observed between the simulated and experimental powder X-ray diffraction patterns (Fig. 2b). This indicates that the compound obtained by sonochemical process as nanoplaces is identical to that obtained by single crystal diffraction. The significant broadening of the peaks indicates that the particles are of nanometer dimensions. Estimated by the Sherrer formula, D = 0.891 k/bcos h, where D is the average grain size, k the X-ray wavelength (0.15405 nm), and h and b the diffraction angle and full-width at half maximum of an observed peak, respectively, the average size of the particles is 60 nm, which is in agreement with that observed by scanning electron microscopy, as shown in Fig. 3. Fig. 3 indicates the original morphology of the nano-plate flower-like structure with the diameter varying between 40 and 70 nm. The morphology of compound 1 prepared by the sonochemical method (Fig. 3) is very interesting and it is composed of nano flow-
Symmetry operations: i:
Weight of loss(%)
Fig. 6. Schematic representation of Pb(II) environment: in compound [Pb(l7-Mal)]n (1) (i: x, y + 1/2, z + 1/2. ii: x, y, z). Ellipsoids 30% probability.
O2–Pb1–O4i O2–Pb1–O1 O4i–Pb1–O1 O2–Pb1–O3 O4i–Pb1–O3 O1–Pb1–O3 O2–Pb1–O2ii O4i–Pb1–O2ii O1–Pb1–O2ii O3–Pb1–O2ii O2–Pb1–O4i O4–Pb1–O4i O1–Pb1–O4 O3–Pb1–O4 O2–Pb1–O4i
2.52(1) 2.41 (1) 2.73(2) 2.61(1) 2.44(2) 2.69(1) 2.84(1)
10
20
0
0
0
200
400
600
-10 800
Temperature(°C) Fig. 7. Thermal behavior of compound [Pb(l7-Mal)]n (1) at nano-size.
er-like formed by nano-plates with sizes of about 40–70 nm. The different concentrations of lead(II) nitrate and ligand Mal2
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Fig. 8. XRD pattern PbO prepared by calcination of compound [Pb(l7-Mal)]n (1).
solution (both of 0.001 and 0.01 M in MeOH and water) were tested, however, appropriate nano-size of compound 1 obtained under concentration of 0.01 M in MeOH. According to Fig. 4, the particles obtained under concentration of 0.01 M in water have large sizes and the morphology of compound is formed from mixture of nano-plate and nano-wires. The IR spectrum and XRD pattern of a typical sample of compound 1 prepared by the sonochemical process under concentration of 0.01 M in water is also same with the crystalline sample. This point shows that the reaction in two different solvents produces the same product but the morphology and size of compounds are difference. Determination of the structure of compound 1 by X-ray crystallography shows the complex in the solid state (Figs. 5, 6 and S1) to be a 2D polymeric network. A view of the coordination environment around the lead(II) ion in compound 1 is shown in Fig. 6, and Table 1 lists selected bond distances and angles. The title complex crystallizes in monoclinic with space group P21/c. Single-crystal X-ray diffraction analysis of reveals that the lead(II) ion in compound 1 is coordinated by seven oxygen atoms of ‘‘Mal2 ” anions, resulting in a seven-coordinate complex with a PbO7 chromophore. The one of carboxylate moiety of the ‘‘Mal2 ” ligand acts as both bidentate, and bridging group (totally tridentate) in a l-1,3 mode, where two oxygen atoms of the carboxylate group bidentately coordinate to a lead(II) ion, creating a four-membered chelate ring, and one of them also bridges two adjacent lead(II) ions. The second of carboxylate moiety of the ‘‘Mal2 ” ligand acts as both bidentate, and bridging group (totally tetradentate) in a l2,4 mode, yielding a two-dimensional network. The intrachain distance between two neighboring lead(II) ions is 4.221 Å. The lead(II) ion in 1 is seven-coordinate and displays hemidirected geometry with a stereo-chemically active lone pair as expected (Fig. 6). Additionally, the O2–Pb–O4i angle of 136.42(14)° suggests that there is a hole in the coordination sphere of 1 due to a lone pair-bond pair repulsion. The observed shortening of this Pb–O bond [bond distances for Pb1–O2 and Pb1–O2ii of 2.412(4) and 2.686(4) Å (Table 2)] supports the presence of this feature [29]. To examine the thermal stability of the nano-sized plates and the single crystals of compound 1, thermal gravimetric (TG) and differential thermal analyses (DTA) were carried out between 20 and 600 °C under nitrogen flow (Figs. 7 and S2). Compound 1 at bulk material does not melt and is stable up to 335 °C at which temperature it begins to decompose. Decomposition of the ‘‘Mal2 ” ligand takes place between 335 and 425 °C with three exothermic effects at 350, 392 and 425 °C. The solid residue formed at around
425 °C is suggested to be PbO (observed 68.70, calcd: 69.45%). Nano-sized plates of compound 1 are less stable and start to decompose at 240 °C. Detectable decomposition of the nanoparticles of 1 thus starts about 95° earlier than that of its bulk counterparts, probably due to the much higher surface to volume ratio of the nano-sized particles, as more heat is needed to annihilate the lattices of the single crystals. The DTA curve displays three exothermic effects at 350, 392 and 425 °C for the single crystals of compound 1 (Fig. S2). The difference between the two maximum intensities in the DTA curves also indicate, in agreement with the TGA results, the lower stability of the nano-plates when compared with their single crystal counterparts (Fig. 7). Micro-crystals PbO have been generated by thermal decomposition of nano-sized compound 1. The final product upon calcination of compound 1 is, based on their XRD patterns (Fig. 8), PbO. The phase purity of the as-prepared tetragonal PbO micro-crystals are completely obvious and all diffraction peaks are perfectly indexed to the orthorhombic PbO structure with the PbO lattices with the parameters of a = 5.8931(4) Å, b = 5.4904(4) Å, c = 4.7528(4) Å, Z = 4 in Pbcm for PbO (JCPDS card file No. 77-1971). No characteristic peaks of impurities are detected in the XRD pattern. Fig. 9 shows SEM images of PbO micro-crystals, produced by calcination of nano-sized compound 1. However, the precursor
Fig. 9. SEM photograph of PbO microparticles produced by calcinations of nanostructured compound [Pb(l7-Mal)]n (1).
L. Aboutorabi, A. Morsali / Inorganica Chimica Acta 363 (2010) 2506–2511
with morphology of nano-plates gives micro-crystals PbO and the morphology of the PbO particles is quite different from compound 1. This point may be due to complete decomposition and breakup of the compound with change of morphology of nano-structure.
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ta_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.ica.2010.04.024. References
4. Conclusion A new Pb(II) coordination polymer, {[Pb(l7-Mal)]n (1), H2Mal = maleic acid} has been synthesized using a thermal gradient approach and by sonochemical irradiation. Compound 1 was structurally characterized by single-crystal X-ray diffraction. The crystal structure of compound 1 consists of a two-dimensional polymer and shows the coordination number in the Pb(II) ions is seven. Reduction of the particle size of the coordination polymers of 1 to a few dozen nanometers results in lower thermal stability when compared to the single crystalline samples. Calcination under air produces micro-sized particles of PbO. It is interesting to note that this precursor is one of the few samples prepared by sonication as an alternative synthetic procedure to form nano-sized particles of a coordination polymer. This method of preparation may have some advantages such as: it takes place with shorter reaction times, produces better yields and it also is likely to produce nano-sized particles of the coordination polymer. Acknowledgements This work was supported by the Payame Noor and Tarbiat Modares Universities. Appendix A. Supplementary material CCDC 660710 contains the supplementary crystallographic data for 1. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
M.J. Zaworotko, B. Moulton, Chem. Rev. 101 (2001) 2619. O.M. Yaghi, G. Li, H. Li, Nature 378 (1995) 703. A. Morsali, M.Y. Masoomi, Coord. Chem. Rev. 253 (2009) 1882. J.A. Ramsden, W. Weng, A.M. Arif, J.A. Gladysz, J. Am. Chem. Soc. 114 (1992) 5890. A. Aumuller, P. Erk, G. Klebe, S. Hunig, J.U. von Schutz, H.-P. Werner, Angew. Chem., Int. Ed. 98 (1986) 759. M.A.M. Abu-Youssef, A. Escuer, D. Gatteschi, M.A.S. Goher, F.A. Mautner, R. Vicente, Inorg. Chem. 38 (1999) 5716. M. Aoyagi, K. Biradha, M.J. Fujita, Am. Chem. Soc. 121 (1999) 7457. M. Fujita, Y.J. Kwon, S. Washizu, K.J. Ogura, J. Am. Chem. Soc. 116 (1994) 1151. H.P. Xiao, J.X. Yuan, Acta Crystallogr., Sect. E 60 (2004) 501. Z.R. Ranjbar, A. Morsali, J. Mol. Struct. 936 (2009) 206. M.J.S. Fard-Jahromi, A. Morsali, Ultrason. Sonochem. 17 (2010) 435. H. Sadeghzadeh, A. Morsali, CrystEngCommun 12 (2009) 370. S. Kawaguchi, Coord. Chem. Rev. 70 (1986) 51. K.S. Suslick, S.B. Choe, A.A. Cichowlas, M.W. Grinstaff, Nature 353 (1991) 414. M. Sugimoto, J. Magn. Chem. Mater 133 (1994) 460. M. Martos, J. Morales, L. Sanchez, R. Ayouchi, D. Leinen, F. Martin, J.R. Ramos Barrado, Electrochim. Acta 46 (2001) 2939. M.V. Landau, L. Vradman, M. Herskowitz, Y. Koltypin, J. Catal. 201 (2001) 22. H. Wang, Y.N. Lu, J.J. Zhu, H.Y. Chen, Inorg. Chem. 42 (2003) 6404. J.H. Zhang, Z. Chen, Z.L. Wang, N.B. Ming, J. Mater. Res. 18 (2003) 1804. M.A. Alavi, A. Morsali, Ultrason. Sonochem. 17 (2010) 132. A. Askarinejad, A. Morsali, Ultrason. Sonochem. 16 (2009) 124. M.A. Alavi, A. Morsali, Ultrason. Sonochem. 17 (2010) 441. A. Askarinejad, A. Morsali, Chem. Eng. J. 150 (2009) 569. M.A. Alavi, A. Morsali, Ultrason. Sonochem. 15 (2008) 833. N. Soltanzadeh, A. Morsali, Ultrason. Sonochem. 17 (2010) 139. MERCURY 1.4.1, Copyright Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK, 2001–2005. Bruner Advanced X-ray Solutions SHELXTL (Version 6.14), Bruker AXS Inc., Madison, Wisconsin, USA, 2000–2003. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed., John Wiley and Sons, New York, 1997. L. Shimoni-Livny, J.P. Glusker, C.W. Bock, Inorg. Chem. 37 (1998) 1853.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Sonochemical synthesis of a new nano-plate lead(II) coordination polymer constructed of maleic acid Leila Aboutorabi a, Ali Morsali b,* a b
Department of Chemistry, Payame Noor University, Abhar, Zanjan, Islamic Republic of Iran Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 27 February 2010 Received in revised form 6 April 2010 Accepted 13 April 2010 Available online 20 April 2010 Keywords: Nano-plate Lead(II) oxide Maleic acid Crystal structure
a b s t r a c t A new nano-sized lead(II) coordination polymer of maleic acid (H2Mal), [Pb(l7-Mal)]n (1), has been synthesized by sonochemical method and characterized by scanning electron microscopy, X-ray powder diffraction, elemental analyses and IR spectroscopy. The compound 1 was structurally characterized by single-crystal X-ray diffraction. Thermal stability of nano and bulk samples of compound 1 were studied and compared with each other. After calcination of nano-sized compound 1 at 600 °C, pure phase microsized lead(II) oxide has been produced. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Within the field of coordination polymers, efforts to use metal ions and organic spacers simultaneously have recently been developed [1–8]. During the past decades, a number of these compounds, with interesting polymeric motifs, have been successfully designed and synthesized. Some of them exhibit encouraging potential for application in catalysis, nonlinear optics, gas separation, magnetic properties, and molecular recognition [5]. Lead(II)–carboxylate systems containing mono-, di-, and polycarboxylate [9–12] have been studied extensively in the construction of metal–organic networks. In contrast to inorganic nanomaterials, specific syntheses of nano-structured coordination polymers seems to be surprisingly sparse, and to date most investigations on coordination polymers have been carried out only in the solid state and studies of their properties were limited to investigations at the macroscopic scale. The potential use of coordination polymers as materials for nanotechnological applications would seem to be very extensive as nanometer-scaled materials often exhibit the new interesting size-dependent physical and chemical properties that can not be observed in their bulk analogous. Although considerable effort has been dedicated to the controlled synthesis of nanoscale particles of metals, oxides, sulfides, and ceramic materials, little attention was focused to date on nanoparticles of supramolecular compounds such as coordination polymers [13]. Several different synthetic approaches have been * Corresponding author. Fax: +98 2188009730. E-mail addresses: [email protected], [email protected] (A. Morsali). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.04.024
offered for the preparation of coordination polymers. Some of them are (1) slow diffusion of the reactants into a polymeric matrix, (2) diffusion from the gas phase, (3) evaporation of the solvent at ambient or reduced temperatures, (4) precipitation or recrystallisation from a mixture of solvents, (5) temperature controlled cooling and (6) hydrothermal synthesis. In this paper we describe a simple synthetic sonochemical preparation of nano-structures of a new coordination polymer, [Pb(l7-Mal)]n (1), {(H2Mal = maleic acid}, and on their conversion into nano-structured lead(II) oxide by calcination at moderately elevated temperature. Sonochemistry is the research area in which molecules undergo a chemical reaction due to the application of powerful ultrasound radiation (20 kHz–10 MHz) [14]. Ultrasound induces chemical changes due to cavitation phenomena involving the formation, growth, and instantaneously implosive collapse of bubbles in a liquid, which can generate local hot spots having a temperature of roughly 5000 °C, pressures of about 500 atm, and a lifetime of a few microseconds [15]. These extreme conditions can drive chemical reactions which have been developed to fabricate a variety of nano compounds [14]. In recent years many kinds of nanomaterials have been prepared by this method [15–25]. 2. Experimental All reagents and solvents for the synthesis and analysis were commercially available and were used as received. A multiwave ultrasonic generator (Sonicator_3000; Misonix, Inc., Farmingdale, NY, USA) was used. IR spectra were recorded using Perkin–Elmer 597 and Nicolet 510P spectrophotometers. Microanalyses were
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carried out using a Heraeus CHN-O-Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. The thermal behavior was measured with a PL-STA 1500 apparatus. X-ray powder diffraction (XRD) measurements were performed using a Philips diffractometer manufactured by X’pert with monochromatized Cu Ka radiation and simulated XRD powder patterns based on single crystal data were prepared using the MERCURY [26]. Single crystal diffraction measurements were made at 293(2) K using a Nonius kappa-CCD diffractometer. The intensity data were collected using graphite monochromated Mo Ka radiation. The structures were solved by direct methods and refined by full-matrix least-squares techniques on F2. The samples were characterized with a scanning electron microscope with
gold coating. The molecular structure plots were prepared using MERCURY [26] and ORTEPIII [27]. To prepare the nano-plates of [Pb(l7-Mal)]n (1), a 50 ml of a 0.01 M solution of lead(II) nitrate dehydrate in MeOH was positioned in a high-density ultrasonic probe, and 50 ml of a 0.01 M maleic acid in MeOH was added dropwisely to that solution. After the end of the titration the solution remained in the bath for a selected aging time at a selected temperature. The obtained precipitates were filtered off, washed with water and then dried in air. Product: d.p. >300 °C, yield: 0.257 g (80%). Anal. Calc. for C4H2O4Pb: C, 14.95; H, 0.63. Found: C, 14.90; H, 0.70%. IR (KBr, cm 1) selected bands: 539(w), 617(m), 1292(m), 1411(vs), 1504(vs), 1540(vs) and 1638(m).
MeOH
calcination nano-structure of compound 1
by ultrasound Pb(NO3)2 + H2Mal
PbO water
by the branched tube method
single crystals of compound 1
Scheme 1. Materials produced and synthetic methods.
Fig. 1. The IR spectra of (a) bulk materials as synthesized of compound [Pb(l7-Mal)]n (1) and (b) nano-plats of compound [Pb(l7-Mal)]n (1) prepared by sonochemical method.
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Fig. 2. The XRD patterns of (a) simulated from single-crystal X-ray data of compound [Pb(l7-Mal)]n (1) and (b) nano-plates of compound [Pb(l7-Mal)]n (1).
To isolate single crystals of [Pb(l7-Mal)]n (1), maleic acid (0.341 g, 3 mmol) and lead(II) nitrate dehydrate (1 g, 3 mmol) were placed in the arm to be heated. Water was carefully added to fill both arms, and then the arm to be heated was placed in a bath at 60 °C. After 10 days, colorless crystals were deposited in the cooler arm which were filtered off, washed with water and air dried.
Product: (0.165 g, yield 51%), m.p. >300 °C. Anal. Calc. for C4H2O4Pb: C, 14.95; H, 0.63. Found: C, 14.98; H, 0.65%. IR (KBr, cm 1) selected bands: 539(w), 617(m), 1311(m), 1403(vs), 1504(vs), 1535(vs) and 1643(w). 3. Results and discussion The reaction between maleic acid (H2Mal) and lead(II) nitrate using two different routes provided crystalline materials of the general formula [Pb(l7-Mal)]n (1). Scheme 1 gives an overview of the methods used for the synthesis of [Pb(l7-Mal)]n (1) using the two different routes. The elemental analysis and IR spectra of the nano-plates and of the single crystalline material are indistinguishable. The relatively weak IR absorption bands around 2965 cm 1 are due to the C–H modes involving the aliphatic hydrogen atoms (Fig. 1). The symmetric and asymmetric vibrations of the carboxylate group are observed at 1404 and 1532 cm 1 as sharp absorption bands. The D(masym–msym) value of 128 cm 1 indicates that the carboxylate groups of ‘‘Mal2 ” anions coordinate to the lead(II) centres in a bridging mode [28]. Fig. 2 shows the simulated XRD pattern from single-crystal Xray data (see below) of compound 1 (Fig. 2a), in comparison with
Fig. 3. SEM photographs of [Pb(l7-Mal)]n (1) nano-flower formed by nano-plate with different scale-bars (concentration of Pb+2 and Mal2 ions are 0.01 M in MeOH).
Fig. 4. SEM photographs of [Pb(l7-Mal)]n (1) nano-structure (concentration of Pb+2 and Mal2 ions are 0.01 M in water).
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L. Aboutorabi, A. Morsali / Inorganica Chimica Acta 363 (2010) 2506–2511 Table 1 Crystal data and structure refinement [Pb(l7-Mal)]n (1). Identification code Empirical formula Formula weight T (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Density (calculated) (Mg/m3) Absorption coefficient (mm 1) F(0 0 0) Crystal size (mm3) Theta range for data collection (°) Index ranges
Fig. 5. A fragment of the two-dimensional network in compound [Pb(l7-Mal)]n (1), H atoms are omitted for clarity.
Reflections collected Independent reflections Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R [I > 2r(I)] R indices (all data) Largest difference peak, hole (e Å
Compound 1 C4H2O4Pb 321.25 293(2) 0.71073 monoclinic P21/c 9.8910(10) 6.9610(10) 8.2710(10) 111.13(5) 531.18(11) 4 4.017 31.686 560 0.25 0.25 0.08 2.21–30.12 13 6 h 6 10 96k69 11 6 l 6 11 8341 1563 phi and omega scans full-matrix least-squares on F2 1563/0/83 1.054 Rl = 0.0311, wR2 = 0.0809 R1 = 0.0359, wR2 = 0.0840 4.062, 2.368
3
)
Table 2 Bond lengths (Å) and angles (°) for [Pb(l7-Mal)]n (1). Pb1–O1 Pb1–O2 Pb1–O4 Pb1–O3 Pb1–O4i Pb1–O2ii Pb1–O3i
x, y + 1/2,
z + 1/2. ii:
x,
120
y,
z.
TG DTA
100
81.17(2) 52.90(2) 79.98(2) 73.37(2) 72.97(2) 122.69(2) 141.82(2) 70.88(1) 95.94(2) 119.93(2) 80.52(2) 121.85(1) 126.23(2) 48.93(2) 136.42(1)
60 50 40
80
30
60
20
40
Δ t(°C)
the XRD pattern of a typical sample of compound 1 prepared by the sonochemical process (Fig. 2b). Acceptable matches, with slight differences in 2h, were observed between the simulated and experimental powder X-ray diffraction patterns (Fig. 2b). This indicates that the compound obtained by sonochemical process as nanoplaces is identical to that obtained by single crystal diffraction. The significant broadening of the peaks indicates that the particles are of nanometer dimensions. Estimated by the Sherrer formula, D = 0.891 k/bcos h, where D is the average grain size, k the X-ray wavelength (0.15405 nm), and h and b the diffraction angle and full-width at half maximum of an observed peak, respectively, the average size of the particles is 60 nm, which is in agreement with that observed by scanning electron microscopy, as shown in Fig. 3. Fig. 3 indicates the original morphology of the nano-plate flower-like structure with the diameter varying between 40 and 70 nm. The morphology of compound 1 prepared by the sonochemical method (Fig. 3) is very interesting and it is composed of nano flow-
Symmetry operations: i:
Weight of loss(%)
Fig. 6. Schematic representation of Pb(II) environment: in compound [Pb(l7-Mal)]n (1) (i: x, y + 1/2, z + 1/2. ii: x, y, z). Ellipsoids 30% probability.
O2–Pb1–O4i O2–Pb1–O1 O4i–Pb1–O1 O2–Pb1–O3 O4i–Pb1–O3 O1–Pb1–O3 O2–Pb1–O2ii O4i–Pb1–O2ii O1–Pb1–O2ii O3–Pb1–O2ii O2–Pb1–O4i O4–Pb1–O4i O1–Pb1–O4 O3–Pb1–O4 O2–Pb1–O4i
2.52(1) 2.41 (1) 2.73(2) 2.61(1) 2.44(2) 2.69(1) 2.84(1)
10
20
0
0
0
200
400
600
-10 800
Temperature(°C) Fig. 7. Thermal behavior of compound [Pb(l7-Mal)]n (1) at nano-size.
er-like formed by nano-plates with sizes of about 40–70 nm. The different concentrations of lead(II) nitrate and ligand Mal2
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Fig. 8. XRD pattern PbO prepared by calcination of compound [Pb(l7-Mal)]n (1).
solution (both of 0.001 and 0.01 M in MeOH and water) were tested, however, appropriate nano-size of compound 1 obtained under concentration of 0.01 M in MeOH. According to Fig. 4, the particles obtained under concentration of 0.01 M in water have large sizes and the morphology of compound is formed from mixture of nano-plate and nano-wires. The IR spectrum and XRD pattern of a typical sample of compound 1 prepared by the sonochemical process under concentration of 0.01 M in water is also same with the crystalline sample. This point shows that the reaction in two different solvents produces the same product but the morphology and size of compounds are difference. Determination of the structure of compound 1 by X-ray crystallography shows the complex in the solid state (Figs. 5, 6 and S1) to be a 2D polymeric network. A view of the coordination environment around the lead(II) ion in compound 1 is shown in Fig. 6, and Table 1 lists selected bond distances and angles. The title complex crystallizes in monoclinic with space group P21/c. Single-crystal X-ray diffraction analysis of reveals that the lead(II) ion in compound 1 is coordinated by seven oxygen atoms of ‘‘Mal2 ” anions, resulting in a seven-coordinate complex with a PbO7 chromophore. The one of carboxylate moiety of the ‘‘Mal2 ” ligand acts as both bidentate, and bridging group (totally tridentate) in a l-1,3 mode, where two oxygen atoms of the carboxylate group bidentately coordinate to a lead(II) ion, creating a four-membered chelate ring, and one of them also bridges two adjacent lead(II) ions. The second of carboxylate moiety of the ‘‘Mal2 ” ligand acts as both bidentate, and bridging group (totally tetradentate) in a l2,4 mode, yielding a two-dimensional network. The intrachain distance between two neighboring lead(II) ions is 4.221 Å. The lead(II) ion in 1 is seven-coordinate and displays hemidirected geometry with a stereo-chemically active lone pair as expected (Fig. 6). Additionally, the O2–Pb–O4i angle of 136.42(14)° suggests that there is a hole in the coordination sphere of 1 due to a lone pair-bond pair repulsion. The observed shortening of this Pb–O bond [bond distances for Pb1–O2 and Pb1–O2ii of 2.412(4) and 2.686(4) Å (Table 2)] supports the presence of this feature [29]. To examine the thermal stability of the nano-sized plates and the single crystals of compound 1, thermal gravimetric (TG) and differential thermal analyses (DTA) were carried out between 20 and 600 °C under nitrogen flow (Figs. 7 and S2). Compound 1 at bulk material does not melt and is stable up to 335 °C at which temperature it begins to decompose. Decomposition of the ‘‘Mal2 ” ligand takes place between 335 and 425 °C with three exothermic effects at 350, 392 and 425 °C. The solid residue formed at around
425 °C is suggested to be PbO (observed 68.70, calcd: 69.45%). Nano-sized plates of compound 1 are less stable and start to decompose at 240 °C. Detectable decomposition of the nanoparticles of 1 thus starts about 95° earlier than that of its bulk counterparts, probably due to the much higher surface to volume ratio of the nano-sized particles, as more heat is needed to annihilate the lattices of the single crystals. The DTA curve displays three exothermic effects at 350, 392 and 425 °C for the single crystals of compound 1 (Fig. S2). The difference between the two maximum intensities in the DTA curves also indicate, in agreement with the TGA results, the lower stability of the nano-plates when compared with their single crystal counterparts (Fig. 7). Micro-crystals PbO have been generated by thermal decomposition of nano-sized compound 1. The final product upon calcination of compound 1 is, based on their XRD patterns (Fig. 8), PbO. The phase purity of the as-prepared tetragonal PbO micro-crystals are completely obvious and all diffraction peaks are perfectly indexed to the orthorhombic PbO structure with the PbO lattices with the parameters of a = 5.8931(4) Å, b = 5.4904(4) Å, c = 4.7528(4) Å, Z = 4 in Pbcm for PbO (JCPDS card file No. 77-1971). No characteristic peaks of impurities are detected in the XRD pattern. Fig. 9 shows SEM images of PbO micro-crystals, produced by calcination of nano-sized compound 1. However, the precursor
Fig. 9. SEM photograph of PbO microparticles produced by calcinations of nanostructured compound [Pb(l7-Mal)]n (1).
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with morphology of nano-plates gives micro-crystals PbO and the morphology of the PbO particles is quite different from compound 1. This point may be due to complete decomposition and breakup of the compound with change of morphology of nano-structure.
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ta_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.ica.2010.04.024. References
4. Conclusion A new Pb(II) coordination polymer, {[Pb(l7-Mal)]n (1), H2Mal = maleic acid} has been synthesized using a thermal gradient approach and by sonochemical irradiation. Compound 1 was structurally characterized by single-crystal X-ray diffraction. The crystal structure of compound 1 consists of a two-dimensional polymer and shows the coordination number in the Pb(II) ions is seven. Reduction of the particle size of the coordination polymers of 1 to a few dozen nanometers results in lower thermal stability when compared to the single crystalline samples. Calcination under air produces micro-sized particles of PbO. It is interesting to note that this precursor is one of the few samples prepared by sonication as an alternative synthetic procedure to form nano-sized particles of a coordination polymer. This method of preparation may have some advantages such as: it takes place with shorter reaction times, produces better yields and it also is likely to produce nano-sized particles of the coordination polymer. Acknowledgements This work was supported by the Payame Noor and Tarbiat Modares Universities. Appendix A. Supplementary material CCDC 660710 contains the supplementary crystallographic data for 1. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-
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