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Hydrothermal and sonochemical synthesis of a nano-sized 2D lead(II) coordination polymer: A precursor for nano-structured PbO and PbBr2
Journal of Molecular Structure 929 (2009) 187–192
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Hydrothermal and sonochemical synthesis of a nano-sized 2D lead(II) coordination polymer: A precursor for nano-structured PbO and PbBr2 Alireza Aslani a, Ali Morsali a,*, Veysel T. Yilmaz b, Canan Kazak c a
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14155-4838 Tehran, Islamic Republic of Iran Department of Chemistry, Faculty of Arts and Sciences, Uludag University, 16059 Bursa, Turkey c Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayis University 55019 Kurupelit, Samsun, Turkey b
a r t i c l e
i n f o
Article history: Received 29 October 2008 Received in revised form 18 April 2009 Accepted 20 April 2009 Available online 3 May 2009 Keywords: Nano-structure Lead(II) bromide Lead(II) oxide Coordination polymer
a b s t r a c t A new nano-structured lead(II) coordination polymer, [Pb(l-bpp)(l-Br)2]n (PBB) [bpp = 1,3-di(4-pyridyl)propane], has been synthesized by the reaction of a mixture lead(II) acetate and KBr with bpp by both hydrothermal and sonochemical methods. Reaction conditions, such as temperature, time, concentration and initial reagents play important roles in the size, morphology and crystal growth of the final products. The nano-sized PbBr2 and PbO were prepared from the calcination of the nano-structured PBB at vacuum and air atmosphere. The structure of PBB was determined by X-ray crystallography, while nano-structural materials were characterized by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). Thermal stability of bulk and nano-sized particles of PBB was studied and compared with each other. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Nanoparticles are a class of materials with properties distinctively different from their bulk and molecular counterparts and find use in a variety of different areas, such as electronic, magnetic and optoelectronic, biomedical, pharmaceutical, cosmetic, energy, environmental, catalytic, and materials applications. Because of the potential of this technology, there has been a worldwide increase in investment in nanotechnology research and development [1,2]. Although extensive effort has been done for the synthesis of metals, oxides, sulfides, and ceramic materials with nanoscale dimension, little attention has given to date on supramolecular compounds such as coordination polymers. Design and synthesis of metal–organic coordination polymers are of great interest due to the special properties of these compounds and their potential applications in sorption, electrical conductivity and catalysis [3–8]. In the past decade, their fascinating properties have prompted studies on the architectures of many such metal–organic coordination polymers. The structure of coordination polymers may be influenced by such factors as the typical coordination of the metal ions, the structural characteristics of polydentate organic ligands, the metal to ligand ratio, the counter ions and many other contributions. Alteration of any of these factors can lead to the formation of new structures or extended frameworks [7,8].
* Corresponding author. Tel.: +98 2182884416; fax: +98 2188009730. E-mail address: [email protected] (A. Morsali). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.04.026
Several different synthetic approaches have been 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. As a part of our work on different coordination polymers, in this paper, we describe hydrothermal and sonochemical preparations of nanoparticles of a new coordination polymer, namely [Pb(l-bpp)(l-Br)2]n (PBB) [bpp = 1,3-di(4-pyridyl)propane]. Sonochemistry is the research area in which molecules undergo a reaction due to the application of powerful ultrasound radiation from 20 to 10 MHz [9–13]. Furthermore, the characterizations of the nano-sized PbBr2 and PbO obtained from the calcination of PBB at vacuum and atmosphere of air were also reported. 2. Experimental 2.1. Measurements 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), equipped with a converter/transducer and titanium oscillator (horn), 12.5 mm in diameter, operating at 24 kHz with a maximum power output of 600 W, was used for the ultrasonic irradiation. The ultrasonic generator automatically adjusts the
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power level. IR spectra were recorded using Perkin–Elmer 597 and Nicolet 510P spectrophotometers. Microanalyses were carried out using a Heraeus CHN-O-Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. The DTA and TGA data was obtained using a PL-STA 1500 apparatus and platinum crucibles with a heating rate of 5 °C min1 in a static atmosphere of nitrogen. X-ray powder diffraction (XRPD) measurements were performed using a Philips diffractometer manufactured by X’pert with monochromatized CuKa radiation. Simulated XRPD patterns were calculated using Mercury [14] based on the single crystal data. Particle sizes of selected samples were estimated using the Sherrer method. For characterization with a scanning electron microscope, samples were gold coated. The single crystal diffraction measurement was performed on a Stoe-IPDS-2 diffractometer equipped with a graphite monochromated Mo-Ka radiation. The structure was solved by direct methods and refined by full-matrix least-squares techniques on F2 with the SHELXL-97 program [15]. Details of crystal data, data collection, structure solution and refinement are given in Table 1.
calcd. for C13H14Br2N2Pb: C, 27.60, H, 2.47; N, 4.95%). IR (selected bands; in cm1): 526.0(m), 826.0(s), 998.0(s), 1065.0(m), 1215.0(m), 1413.0(m), 1592.0(vs), 2900.0(w), 3011.0(w). The different concentrations of metal and ligand solution (0.02, 0.05 and 0.08 M) with the same aging time (1 h) were tested at different power of ultrasonic irradiation (60, 120 and 180 kHz). To isolate single crystals of PBB, bpp (0.5 mmol, 0.098 g), lead(II) acetate (0.189 g, 0.5 mmol) and potassium bromide (0.166 g, 1 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill both arms. The tube was sealed and the ligand-containing arm immersed in an oil bath at 60 °C while the other arm was kept at ambient temperature. After 6 days, colorless crystals that deposited in the cooler arm were isolated, filtered off, washed with acetone and ether and air dried. D.p. = 330 °C. Analysis found: C, 27.40, H, 2.40; N, 4.80%; calcd. for C13H14Br2N2Pb: C, 27.60, H, 2.47; N, 4.95%. IR (selected bands; in cm1): 526.0(m), 826.0(s), 998.0(s), 1066.0(m), 1215.0(m), 1415.0(m), 1593.0(vs), 2880.0(w), 3012.0(w).
2.2. Synthesis
3. Results and discussion
In order to prepare PBB at nano-scale, 10 ml methanolic solution of lead(II) acetate (0.3 M) and KBr (0.6 M) in a vessel was positioned in a high-density ultrasonic probe, operating at 24 kHz with a maximum power output of 600 W. Into this solution 10 ml methanolic solution of the bpp ligand (0.3 M) was added dropwise. The precipitates were filtered off, washed with methanol and then dried in air. D.p. = 230 °C. Analysis found: C, 27.70, H, 2.70; N, 4.30%; calcd. for C13H14Br2N2Pb: C, 27.60, H, 2.47; N, 4.95%. IR (selected bands; in cm1): 527.0(m), 826.0(s), 998.0(s), 1066.0(m), 1215.0(m), 1414.0(m), 1593.0(vs), 2900.0(w), 3010.0(w). In the solvothermal procedure, 0.189 g (0.5 mmol) lead(II) acetate, 0.166 g (1 mmol) KBr and 0.098 g (0.5 mmol) bpp were dissolved in 15 ml EtOH, H2O or a mixture of H2O and EtOH. The solution was charged into a Teflon-lined stainless steel autoclave and heated at 150 °C for 48 h. After the autoclave was cooled to room temperature, the product was filtered and dried and characterized. D.p. = 230 °C. Analysis found: C, 27.50, H, 2.60; N, 4.70%;
Reaction of bpp with a mixture of lead(II) acetate and KBr led to the formation of a new 2D lead(II) coordination polymer [Pb(lbpp)(l-Br)2]n (PBB). Nanoparticles of PBB were obtained by both solvothermal in an ethanolic solution and ultrasonic irradiation in a methanolic solution, while single crystals of PBB was obtained using a heat gradient applied to a solution of the reagents (the ‘‘branched tube method”) [16]. Scheme 1 gives an overview of the methods used for the synthesis of PBB using the three different routes. The elemental analysis and IR spectra of the nanoparticles and of the single crystalline material are indistinguishable. The relatively weak IR absorption bands around 3025–3035 cm1 are due to the C–H modes involving the aromatic ring hydrogen atoms. The absorption bands with a variable intensity in the frequency range 1404–1600 cm1 correspond to vibrations of the bpp rings. Single crystal X-ray diffraction analysis of the compound PBB was carried out. The ORTEP diagrams and packing of the title complexes are shown in Fig. 1. Compound PBB is two-dimensional neutral metallopolymer and consists of the lead(II) ions bridged by both bpp and bromide ligands, thus forming two-dimensional infinite framework. On the other hand, the structures may be considered coordination polymers of lead(II) consisting of one-dimensional linear chains, running parallel to the c axis, with a building block of [PbBr2]. Two Br– anions doubly bridge two lead(II) ions and the bromide ions act as bridging group (totally bidentate) in a l-1, 2 mode, yielding the Pb2Br2 rhombs, forming an environment of PbN2Br4. The intrachain Pb Pb distances within the [PbBr2]n chains is 4.407 Å. The individual polymeric chains are almost parallel to each other and further bridged by bidentate bridging bpp ligands, resulting in two-dimensional framework as shown in Fig. 1b. The coordination around the Pb atoms in compound PBB is distorted octahedron and repeating units have an inversion center. However, the compound PBB shows similar structural motifs and packing characteristics with its analogues [18] and there are no any p–p interactions involving the bpp ligands and the 2D-layers are held by van der Waals interactions. The X–Pb–Y angles in the compound PBB suggest that there is no hole in the coordination sphere of the lead ion due to a stereochemically active lone pair. The lead ion thus has a holodirected geometry as is commonly observed for lead(II) with intermediate coordination numbers (6–8) in the presence of hard-donor ligands [17]. The presence of four anionic Br- and two bulky bpp ligands increases steric crowding around lead(II) and results in strong interligand repulsions. This may be the reason of the disappearance of
Table 1 Crystallographic data for PBB. Empirical formula Mr T (K) Radiation, k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b(°) V (Å3) Z Dc (g/cm3) l (mm1) F(0 0 0) Crystal size (mm) h range (°) Index range (h, k, l) Reflections collected Independent reflections (Rint) Absorption correction Data/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] wR indices (all data) Largest diff. peak and hole (e Å3)
C13H14Br2N2Pb 565.27 293 0.71073 Monoclinic P21/m 4.4074(4) 15.3460(9) 11.2417(10) 96.873(7) 754.88(11) 2 2.487 16.452 516 0.48 0.25 0.07 1.82/25.99 5/5 , 18/18, 13/13 11026 1952 (0.1031) Numerical 1554/95 0.901 0.0317 .0647 0.978 and 0.937
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by ultrasound
in argon
nanoparticles of compound PBB
NP PbBr2
calcination Pb(OAc) 2 + KBr + bpp
by hydrothermal nanoparticles of compound PBB in air by heat gradient
NP PbO
single crystals of compound PBB
Scheme 1. Materials produced and synthetic methods.
Fig. 1. (a) Coordination environment of lead(II) in PBB and (b) a fragment of the two-dimensional polymer in PBB.
the gap in the coordination polyhedron, thereby resulting in less common holodirected geometry. This is in accord with holodirected eight-coordinate lead(II) ions in [Pb(4,4-bpy)(NO3)(SCN)]n containing the bulky 4,40 -bipyridine ligand together with two different anionic species [18] and with holodirected seven-coordinate lead(II) ions in [Pb(bpe)(SCN)2]n containing the bulky bpe ligand together with two thiocyanate anions [19]. However, the holodirected geometry with six coordinate is rarely reported [17]. The inactivity of the lone pair in the coordination sphere of Pb atoms in compound PBB may probably due to the ability of both bpp and bromide ligands for bridging and forming two-dimensional coordination polymer that may causes the gap is vanished. This situation has also been observed in the other analogues of this compound such as [PbLBr2]n (L is 4,40 -bpy/bpa/bpe) [20]. Fig. 2 shows the XRPD pattern of compound PBB simulated from its single crystal X-ray data, the XRPD pattern of PBB prepared by the hydrothermal route and typical PBB prepared by the sonochemical process. Acceptable matches were observed between the simulated and experimental powder X-ray diffraction patterns. This indicates that the compounds obtained by both
hydrothermal and sonochemical process have single crystalline phases, almost identical to that obtained by the branched tube method. The significant broadening of the peaks indicates that the particles are of nanometer dimensions. Estimated by the Sherrer formula, D = 0.891k/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 45 nm, which is in agreement with that observed by scanning electron microscopy, as shown in Figs. 3 and 4. The morphology of compound PBB prepared by the hydrothermal (Fig. 3) and sonochemical methods (Fig. 4) is the same and they are composed of particles with sizes of about 40 nm. To examine the thermal stability of the nano-sized particles and the single crystals of PBB, thermal analyses (TG/DTA) were carried out between 30 and 610 °C in a static atmosphere of nitrogen (Figs. 5 and 6). Compound PBB as a bulk phase is very stable and does not decompose up to temperature of 325 °C, at which temperature decomposition starts. Between 325 and 400 °C removal of bpp occurs with a mass loss of 33.40% (calcd. 34.67%). At higher temper-
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Fig. 2. The XRPD patterns PBB: (a) simulated from single crystal X-ray data of PBB, (b) nano-structured of PBB prepared by the hydrothermal method and (c) nano-structured of PBB prepared by sonochemical method.
Fig. 3. SEM image of nanoparticles of PBB as produced by hydrothermal.
Fig. 5. TGA curves of PBB: (a) single crystals and (b) as nanoparticles.
Fig. 6. DTA curves of PBB: (a) as single crystals and (b) as nanoparticles. Fig. 4. SEM image of nanoparticles of PBB as produced by ultrasound.
atures, the residue, presumably PbBr2, was decomposed and mass loss calculations show that the final decomposition product is PbO (Fig. 5a). Compared to the bulky material, nano-sized particles of compound PBB are much less stable and start to decompose at 195 °C. The TG curve (Fig. 5b) exhibits two distinct decomposition stages between 195–300 and 405–560 °C with a total mass loss of 60.90% (calcd. 60.60%). Detectable decomposition of the nanoparti-
cles of PBB thus begins about 130 degree earlier than that of its bulk counterparts, probably due to the much higher surface to volume ratio of the nano-sized particles. Moreover, the DTA curves display an endothermic peak at 320 °C and two distinct exothermic peaks at 415 and 520 °C for the single crystals of compound PBB (Fig. 5a), whereas only an endothermic peak at 310 °C for the nano-sized particles of this compound (Fig. 5b). Both TG and DTA results indicate that although both nano-sized and bulk materials
A. Aslani et al. / Journal of Molecular Structure 929 (2009) 187–192
have the same composition, they exhibit a different thermal decomposition behavior. Thermal decomposition of the nano-sized particles of PBB in argon and air produced PbBr2 and PbO nanoparticles, respectively, as established by their powder XRPD patterns (Fig. 7). The obtained patterns match with the standard patterns of tetragonal PbO and orthorhombic PbBr2 [10,21]. The phase purity of the prepared tetragonal PbO and orthorhombic PbBr2 nanoparticles are completely obvious and all diffraction peaks are perfectly indexed to the tetragonal PbO and orthorhombic PbBr2 structures with the lattice parameters of a = 3.9729, b = 3.9729, c = 5.0217 Å, Z = 2 and S.G = P4/nmm (JCPDS card file No. 05-0561) for PbO, and a = 8.062, b = 9.5393, c = 4.7348 Å, Z = 4 and S.G = Pnam (JCPDS card file No. 31-0679) for PbBr2. No characteristic peaks of impurities are detected. The broadening of the peaks indicated that the particles were of nanometer scale and estimation by the Sherrer formula, D = 0.891k/bcos h, shows that the average sizes of the particles are ca. 45 and 80 nm for PbBr2 and PbO nanoparticles, respectively. Figs. 8 and 9 show the SEM images of the PbBr2 and PbO nanoparticles obtained from calcination of compound PBB under argon and air atomsphere, respectively. The morphology of the nanoparticles of PbBr2 and PbO is very similar to that of compound PBB (see Figs. 3 and 4). This point may be due to the direct removal of the bpp ligand without changing of morphology under the calcinations in argon and air.
191
Fig. 8. SEM photograph of PbBr2 nanoparticles produced by calcination of compound PBB under argon atmosphere.
4. Conclusion A new Pb(II) coordination polymer, {[Pb(l-bpp)(l-Br)2]n (PBB), bpp = 1,3-di(4-pyridyl)propane} has been synthesized using a thermal gradient approach, by hydrothermal method and sonochemical irradiation. Compound PBB was structurally characterized by single-crystal X-ray diffraction. The crystal structure of compound PBB consists of a two-dimensional polymer in which the PbII ions is octahedrally coordinated by two bpp and four bromo ligands. TG studies indicate that reduction of the particle size of the coordination polymers of PBB to a few dozen nanometers results in lower thermal stability when compared to the single crystalline samples. The calcination of PBB under air and argon atmospheres produces nano-sized particles of PbO and PbBr2 with same morphologies. This study demonstrates the coordination polymers may be suitable precursors for the preparation of nanoscale materials. To the best of our knowledge, this is the first report of a synthesis of nano-sized particles of PbO and PbBr2 from a lead(II) coordination polymer. Furthermore, it is interesting to note that this precursor is one of the few samples prepared by sonication as an alternative
Fig. 9. SEM photograph of PbO nanopowders produced by calcination of compound PBB in air atmosphere.
synthetic procedure to form nano-sized particles of a coordination polymer. This method of preparation may have some advantages such as: it takes shorter reaction times, produces better yields and it also is likely to produce nano-sized particles of the coordination polymer. From this perspective, further systematic studies of other coordination polymers with different metal ions are ongoing in our laboratory, which may offer new insights into metal–organic coordination polymers and nanochemistry.
Fig. 7. XRPD patterns PbO nanoparticles (a) and PbBr2 nanoparticles (b) prepared by calcinations of compound PBB.
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Acknowledgement Support of this investigation by Iran National Science Foundation, INSF is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2009.04.026. References [1] S. Leininger, B. Olenyuk, P.J. Stang, Chem. Rev. 100 (2000) 853. [2] S. Karasawa, Y. Sano, T. Akita, N. Koga, T. Itoh, H. Iwamura, P. Rabu, M. Drillon, J. Am. Chem. Soc. 120 (1998) 10080. [3] A.J. Cheethan, Science 264 (1994) 794. and references therein. [4] R.C. Finn, J. Zubieta, Inorg. Chem. 40 (2001) 2466. [5] G. Alberti, U. Costantiono, J.L. Atwood, J.E.D. Davies, D.D. MacNicol, F. Vogtle (Eds.), Comprehensive Supramolecular Chemistry, vol. 7, Elsevier Science, New York, 1996, pp. 1–24 (Chapter 1). [6] J.M. Thomas, Chem. Commun. (2001) 675.
[7] S.L. Suib, Chem. Rev. 93 (1993) 803. [8] G.K.H. Shimizu, G.D. Enright, C.I. Ratcliffe, K.F. Preston, J.L. Reid, J.A. Ripmeester, Chem. Commun. (1999) 1485. [9] K.S. Suslick, S.B. Choe, A.A. Cichowlas, M.W. Grinstaff, Nature 353 (1991) 414. [10] M. Martos, J. Morales, L. Sanchez, R. Ayouchi, D. Leinen, F. Martin, J.R. Ramos Barrado, Electrochim. Acta 46 (2001) 2939. [11] M. Sugimoto, J. Magn, Chem. Mater. 133 (1994) 460. [12] M.V. Landau, L. Vradman, M. Herskowitz, Y. Koltypin, J. Catal. 201 (2001) 22. [13] H. Wang, Y.N. Lu, J.J. Zhu, H.Y. Chen, Inorg. Chem. 42 (2003) 6404. [14] Mercury 1.4.1, Copyright Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK, 2001–2005. [15] G.M. Sheldrick, Acta Cryst. A64 (2008) 112. [16] I. Wharf, T. Gramstad, R. Makhija, M. Onyszchuk, Can. J. Chem. 54 (1976) 3430. [17] L. Shimoni-Livny, J.P. Glusker, C.W. Bock, Inorg. Chem. 37 (1998) 1853. [18] A. Morsali, A.R. Mahjoub, Chem. Lett. (2004) 64. [19] A. Morsali, V.T. Yilmaz, C. Kazak, L.-G. Zhu, Helv. Chim. Acta 88 (2005) 2513. [20] A. Aslani, A. Morsali, V.T. Yilmaz, O. Büyükgüngör, Inorg. Chim. Acta 362 (2009) 1506. [21] Z.W. Zhao, K. Konstantinov, L. Yuan, H.K. Liu, S.X. Dou, J. Nanosci. Nanotech. 4 (2004) 861.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Hydrothermal and sonochemical synthesis of a nano-sized 2D lead(II) coordination polymer: A precursor for nano-structured PbO and PbBr2 Alireza Aslani a, Ali Morsali a,*, Veysel T. Yilmaz b, Canan Kazak c a
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14155-4838 Tehran, Islamic Republic of Iran Department of Chemistry, Faculty of Arts and Sciences, Uludag University, 16059 Bursa, Turkey c Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayis University 55019 Kurupelit, Samsun, Turkey b
a r t i c l e
i n f o
Article history: Received 29 October 2008 Received in revised form 18 April 2009 Accepted 20 April 2009 Available online 3 May 2009 Keywords: Nano-structure Lead(II) bromide Lead(II) oxide Coordination polymer
a b s t r a c t A new nano-structured lead(II) coordination polymer, [Pb(l-bpp)(l-Br)2]n (PBB) [bpp = 1,3-di(4-pyridyl)propane], has been synthesized by the reaction of a mixture lead(II) acetate and KBr with bpp by both hydrothermal and sonochemical methods. Reaction conditions, such as temperature, time, concentration and initial reagents play important roles in the size, morphology and crystal growth of the final products. The nano-sized PbBr2 and PbO were prepared from the calcination of the nano-structured PBB at vacuum and air atmosphere. The structure of PBB was determined by X-ray crystallography, while nano-structural materials were characterized by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). Thermal stability of bulk and nano-sized particles of PBB was studied and compared with each other. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Nanoparticles are a class of materials with properties distinctively different from their bulk and molecular counterparts and find use in a variety of different areas, such as electronic, magnetic and optoelectronic, biomedical, pharmaceutical, cosmetic, energy, environmental, catalytic, and materials applications. Because of the potential of this technology, there has been a worldwide increase in investment in nanotechnology research and development [1,2]. Although extensive effort has been done for the synthesis of metals, oxides, sulfides, and ceramic materials with nanoscale dimension, little attention has given to date on supramolecular compounds such as coordination polymers. Design and synthesis of metal–organic coordination polymers are of great interest due to the special properties of these compounds and their potential applications in sorption, electrical conductivity and catalysis [3–8]. In the past decade, their fascinating properties have prompted studies on the architectures of many such metal–organic coordination polymers. The structure of coordination polymers may be influenced by such factors as the typical coordination of the metal ions, the structural characteristics of polydentate organic ligands, the metal to ligand ratio, the counter ions and many other contributions. Alteration of any of these factors can lead to the formation of new structures or extended frameworks [7,8].
* Corresponding author. Tel.: +98 2182884416; fax: +98 2188009730. E-mail address: [email protected] (A. Morsali). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.04.026
Several different synthetic approaches have been 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. As a part of our work on different coordination polymers, in this paper, we describe hydrothermal and sonochemical preparations of nanoparticles of a new coordination polymer, namely [Pb(l-bpp)(l-Br)2]n (PBB) [bpp = 1,3-di(4-pyridyl)propane]. Sonochemistry is the research area in which molecules undergo a reaction due to the application of powerful ultrasound radiation from 20 to 10 MHz [9–13]. Furthermore, the characterizations of the nano-sized PbBr2 and PbO obtained from the calcination of PBB at vacuum and atmosphere of air were also reported. 2. Experimental 2.1. Measurements 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), equipped with a converter/transducer and titanium oscillator (horn), 12.5 mm in diameter, operating at 24 kHz with a maximum power output of 600 W, was used for the ultrasonic irradiation. The ultrasonic generator automatically adjusts the
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power level. IR spectra were recorded using Perkin–Elmer 597 and Nicolet 510P spectrophotometers. Microanalyses were carried out using a Heraeus CHN-O-Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. The DTA and TGA data was obtained using a PL-STA 1500 apparatus and platinum crucibles with a heating rate of 5 °C min1 in a static atmosphere of nitrogen. X-ray powder diffraction (XRPD) measurements were performed using a Philips diffractometer manufactured by X’pert with monochromatized CuKa radiation. Simulated XRPD patterns were calculated using Mercury [14] based on the single crystal data. Particle sizes of selected samples were estimated using the Sherrer method. For characterization with a scanning electron microscope, samples were gold coated. The single crystal diffraction measurement was performed on a Stoe-IPDS-2 diffractometer equipped with a graphite monochromated Mo-Ka radiation. The structure was solved by direct methods and refined by full-matrix least-squares techniques on F2 with the SHELXL-97 program [15]. Details of crystal data, data collection, structure solution and refinement are given in Table 1.
calcd. for C13H14Br2N2Pb: C, 27.60, H, 2.47; N, 4.95%). IR (selected bands; in cm1): 526.0(m), 826.0(s), 998.0(s), 1065.0(m), 1215.0(m), 1413.0(m), 1592.0(vs), 2900.0(w), 3011.0(w). The different concentrations of metal and ligand solution (0.02, 0.05 and 0.08 M) with the same aging time (1 h) were tested at different power of ultrasonic irradiation (60, 120 and 180 kHz). To isolate single crystals of PBB, bpp (0.5 mmol, 0.098 g), lead(II) acetate (0.189 g, 0.5 mmol) and potassium bromide (0.166 g, 1 mmol) were placed in the main arm of a branched tube. Methanol was carefully added to fill both arms. The tube was sealed and the ligand-containing arm immersed in an oil bath at 60 °C while the other arm was kept at ambient temperature. After 6 days, colorless crystals that deposited in the cooler arm were isolated, filtered off, washed with acetone and ether and air dried. D.p. = 330 °C. Analysis found: C, 27.40, H, 2.40; N, 4.80%; calcd. for C13H14Br2N2Pb: C, 27.60, H, 2.47; N, 4.95%. IR (selected bands; in cm1): 526.0(m), 826.0(s), 998.0(s), 1066.0(m), 1215.0(m), 1415.0(m), 1593.0(vs), 2880.0(w), 3012.0(w).
2.2. Synthesis
3. Results and discussion
In order to prepare PBB at nano-scale, 10 ml methanolic solution of lead(II) acetate (0.3 M) and KBr (0.6 M) in a vessel was positioned in a high-density ultrasonic probe, operating at 24 kHz with a maximum power output of 600 W. Into this solution 10 ml methanolic solution of the bpp ligand (0.3 M) was added dropwise. The precipitates were filtered off, washed with methanol and then dried in air. D.p. = 230 °C. Analysis found: C, 27.70, H, 2.70; N, 4.30%; calcd. for C13H14Br2N2Pb: C, 27.60, H, 2.47; N, 4.95%. IR (selected bands; in cm1): 527.0(m), 826.0(s), 998.0(s), 1066.0(m), 1215.0(m), 1414.0(m), 1593.0(vs), 2900.0(w), 3010.0(w). In the solvothermal procedure, 0.189 g (0.5 mmol) lead(II) acetate, 0.166 g (1 mmol) KBr and 0.098 g (0.5 mmol) bpp were dissolved in 15 ml EtOH, H2O or a mixture of H2O and EtOH. The solution was charged into a Teflon-lined stainless steel autoclave and heated at 150 °C for 48 h. After the autoclave was cooled to room temperature, the product was filtered and dried and characterized. D.p. = 230 °C. Analysis found: C, 27.50, H, 2.60; N, 4.70%;
Reaction of bpp with a mixture of lead(II) acetate and KBr led to the formation of a new 2D lead(II) coordination polymer [Pb(lbpp)(l-Br)2]n (PBB). Nanoparticles of PBB were obtained by both solvothermal in an ethanolic solution and ultrasonic irradiation in a methanolic solution, while single crystals of PBB was obtained using a heat gradient applied to a solution of the reagents (the ‘‘branched tube method”) [16]. Scheme 1 gives an overview of the methods used for the synthesis of PBB using the three different routes. The elemental analysis and IR spectra of the nanoparticles and of the single crystalline material are indistinguishable. The relatively weak IR absorption bands around 3025–3035 cm1 are due to the C–H modes involving the aromatic ring hydrogen atoms. The absorption bands with a variable intensity in the frequency range 1404–1600 cm1 correspond to vibrations of the bpp rings. Single crystal X-ray diffraction analysis of the compound PBB was carried out. The ORTEP diagrams and packing of the title complexes are shown in Fig. 1. Compound PBB is two-dimensional neutral metallopolymer and consists of the lead(II) ions bridged by both bpp and bromide ligands, thus forming two-dimensional infinite framework. On the other hand, the structures may be considered coordination polymers of lead(II) consisting of one-dimensional linear chains, running parallel to the c axis, with a building block of [PbBr2]. Two Br– anions doubly bridge two lead(II) ions and the bromide ions act as bridging group (totally bidentate) in a l-1, 2 mode, yielding the Pb2Br2 rhombs, forming an environment of PbN2Br4. The intrachain Pb Pb distances within the [PbBr2]n chains is 4.407 Å. The individual polymeric chains are almost parallel to each other and further bridged by bidentate bridging bpp ligands, resulting in two-dimensional framework as shown in Fig. 1b. The coordination around the Pb atoms in compound PBB is distorted octahedron and repeating units have an inversion center. However, the compound PBB shows similar structural motifs and packing characteristics with its analogues [18] and there are no any p–p interactions involving the bpp ligands and the 2D-layers are held by van der Waals interactions. The X–Pb–Y angles in the compound PBB suggest that there is no hole in the coordination sphere of the lead ion due to a stereochemically active lone pair. The lead ion thus has a holodirected geometry as is commonly observed for lead(II) with intermediate coordination numbers (6–8) in the presence of hard-donor ligands [17]. The presence of four anionic Br- and two bulky bpp ligands increases steric crowding around lead(II) and results in strong interligand repulsions. This may be the reason of the disappearance of
Table 1 Crystallographic data for PBB. Empirical formula Mr T (K) Radiation, k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b(°) V (Å3) Z Dc (g/cm3) l (mm1) F(0 0 0) Crystal size (mm) h range (°) Index range (h, k, l) Reflections collected Independent reflections (Rint) Absorption correction Data/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] wR indices (all data) Largest diff. peak and hole (e Å3)
C13H14Br2N2Pb 565.27 293 0.71073 Monoclinic P21/m 4.4074(4) 15.3460(9) 11.2417(10) 96.873(7) 754.88(11) 2 2.487 16.452 516 0.48 0.25 0.07 1.82/25.99 5/5 , 18/18, 13/13 11026 1952 (0.1031) Numerical 1554/95 0.901 0.0317 .0647 0.978 and 0.937
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by ultrasound
in argon
nanoparticles of compound PBB
NP PbBr2
calcination Pb(OAc) 2 + KBr + bpp
by hydrothermal nanoparticles of compound PBB in air by heat gradient
NP PbO
single crystals of compound PBB
Scheme 1. Materials produced and synthetic methods.
Fig. 1. (a) Coordination environment of lead(II) in PBB and (b) a fragment of the two-dimensional polymer in PBB.
the gap in the coordination polyhedron, thereby resulting in less common holodirected geometry. This is in accord with holodirected eight-coordinate lead(II) ions in [Pb(4,4-bpy)(NO3)(SCN)]n containing the bulky 4,40 -bipyridine ligand together with two different anionic species [18] and with holodirected seven-coordinate lead(II) ions in [Pb(bpe)(SCN)2]n containing the bulky bpe ligand together with two thiocyanate anions [19]. However, the holodirected geometry with six coordinate is rarely reported [17]. The inactivity of the lone pair in the coordination sphere of Pb atoms in compound PBB may probably due to the ability of both bpp and bromide ligands for bridging and forming two-dimensional coordination polymer that may causes the gap is vanished. This situation has also been observed in the other analogues of this compound such as [PbLBr2]n (L is 4,40 -bpy/bpa/bpe) [20]. Fig. 2 shows the XRPD pattern of compound PBB simulated from its single crystal X-ray data, the XRPD pattern of PBB prepared by the hydrothermal route and typical PBB prepared by the sonochemical process. Acceptable matches were observed between the simulated and experimental powder X-ray diffraction patterns. This indicates that the compounds obtained by both
hydrothermal and sonochemical process have single crystalline phases, almost identical to that obtained by the branched tube method. The significant broadening of the peaks indicates that the particles are of nanometer dimensions. Estimated by the Sherrer formula, D = 0.891k/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 45 nm, which is in agreement with that observed by scanning electron microscopy, as shown in Figs. 3 and 4. The morphology of compound PBB prepared by the hydrothermal (Fig. 3) and sonochemical methods (Fig. 4) is the same and they are composed of particles with sizes of about 40 nm. To examine the thermal stability of the nano-sized particles and the single crystals of PBB, thermal analyses (TG/DTA) were carried out between 30 and 610 °C in a static atmosphere of nitrogen (Figs. 5 and 6). Compound PBB as a bulk phase is very stable and does not decompose up to temperature of 325 °C, at which temperature decomposition starts. Between 325 and 400 °C removal of bpp occurs with a mass loss of 33.40% (calcd. 34.67%). At higher temper-
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Fig. 2. The XRPD patterns PBB: (a) simulated from single crystal X-ray data of PBB, (b) nano-structured of PBB prepared by the hydrothermal method and (c) nano-structured of PBB prepared by sonochemical method.
Fig. 3. SEM image of nanoparticles of PBB as produced by hydrothermal.
Fig. 5. TGA curves of PBB: (a) single crystals and (b) as nanoparticles.
Fig. 6. DTA curves of PBB: (a) as single crystals and (b) as nanoparticles. Fig. 4. SEM image of nanoparticles of PBB as produced by ultrasound.
atures, the residue, presumably PbBr2, was decomposed and mass loss calculations show that the final decomposition product is PbO (Fig. 5a). Compared to the bulky material, nano-sized particles of compound PBB are much less stable and start to decompose at 195 °C. The TG curve (Fig. 5b) exhibits two distinct decomposition stages between 195–300 and 405–560 °C with a total mass loss of 60.90% (calcd. 60.60%). Detectable decomposition of the nanoparti-
cles of PBB thus begins about 130 degree earlier than that of its bulk counterparts, probably due to the much higher surface to volume ratio of the nano-sized particles. Moreover, the DTA curves display an endothermic peak at 320 °C and two distinct exothermic peaks at 415 and 520 °C for the single crystals of compound PBB (Fig. 5a), whereas only an endothermic peak at 310 °C for the nano-sized particles of this compound (Fig. 5b). Both TG and DTA results indicate that although both nano-sized and bulk materials
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have the same composition, they exhibit a different thermal decomposition behavior. Thermal decomposition of the nano-sized particles of PBB in argon and air produced PbBr2 and PbO nanoparticles, respectively, as established by their powder XRPD patterns (Fig. 7). The obtained patterns match with the standard patterns of tetragonal PbO and orthorhombic PbBr2 [10,21]. The phase purity of the prepared tetragonal PbO and orthorhombic PbBr2 nanoparticles are completely obvious and all diffraction peaks are perfectly indexed to the tetragonal PbO and orthorhombic PbBr2 structures with the lattice parameters of a = 3.9729, b = 3.9729, c = 5.0217 Å, Z = 2 and S.G = P4/nmm (JCPDS card file No. 05-0561) for PbO, and a = 8.062, b = 9.5393, c = 4.7348 Å, Z = 4 and S.G = Pnam (JCPDS card file No. 31-0679) for PbBr2. No characteristic peaks of impurities are detected. The broadening of the peaks indicated that the particles were of nanometer scale and estimation by the Sherrer formula, D = 0.891k/bcos h, shows that the average sizes of the particles are ca. 45 and 80 nm for PbBr2 and PbO nanoparticles, respectively. Figs. 8 and 9 show the SEM images of the PbBr2 and PbO nanoparticles obtained from calcination of compound PBB under argon and air atomsphere, respectively. The morphology of the nanoparticles of PbBr2 and PbO is very similar to that of compound PBB (see Figs. 3 and 4). This point may be due to the direct removal of the bpp ligand without changing of morphology under the calcinations in argon and air.
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Fig. 8. SEM photograph of PbBr2 nanoparticles produced by calcination of compound PBB under argon atmosphere.
4. Conclusion A new Pb(II) coordination polymer, {[Pb(l-bpp)(l-Br)2]n (PBB), bpp = 1,3-di(4-pyridyl)propane} has been synthesized using a thermal gradient approach, by hydrothermal method and sonochemical irradiation. Compound PBB was structurally characterized by single-crystal X-ray diffraction. The crystal structure of compound PBB consists of a two-dimensional polymer in which the PbII ions is octahedrally coordinated by two bpp and four bromo ligands. TG studies indicate that reduction of the particle size of the coordination polymers of PBB to a few dozen nanometers results in lower thermal stability when compared to the single crystalline samples. The calcination of PBB under air and argon atmospheres produces nano-sized particles of PbO and PbBr2 with same morphologies. This study demonstrates the coordination polymers may be suitable precursors for the preparation of nanoscale materials. To the best of our knowledge, this is the first report of a synthesis of nano-sized particles of PbO and PbBr2 from a lead(II) coordination polymer. Furthermore, it is interesting to note that this precursor is one of the few samples prepared by sonication as an alternative
Fig. 9. SEM photograph of PbO nanopowders produced by calcination of compound PBB in air atmosphere.
synthetic procedure to form nano-sized particles of a coordination polymer. This method of preparation may have some advantages such as: it takes shorter reaction times, produces better yields and it also is likely to produce nano-sized particles of the coordination polymer. From this perspective, further systematic studies of other coordination polymers with different metal ions are ongoing in our laboratory, which may offer new insights into metal–organic coordination polymers and nanochemistry.
Fig. 7. XRPD patterns PbO nanoparticles (a) and PbBr2 nanoparticles (b) prepared by calcinations of compound PBB.
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