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Mechanochemical synthesis and characterization of kinetically and thermodynamically stable polymorphs of a lead(II) coordination polymer
Accepted Manuscript Mechanochemical synthesis and characterization of kinetically and thermodynamically stable polymorphs of a lead(II) coordination polymer Kamran Akhbari, Ali Morsali PII: DOI: Reference:
S0020-1693(15)00065-1 http://dx.doi.org/10.1016/j.ica.2015.01.037 ICA 16401
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
9 December 2014 21 January 2015 30 January 2015
Please cite this article as: K. Akhbari, A. Morsali, Mechanochemical synthesis and characterization of kinetically and thermodynamically stable polymorphs of a lead(II) coordination polymer, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.01.037
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Mechanochemical Synthesis and Characterization of Kinetically and Thermodynamically Stable Polymorphs of a Lead(II) Coordination Polymer
Kamran Akhbari, Ali Morsali* Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 141554838, Tehran, Islamic Republic of Iran
Abstract 1α polymorph of [Pb(µ-Cl)2(µ-bipy)]∞ (bipy = 4,4'-bipyridine) was synthesized by branch tube method. It was found that one of the ways to synthesize of 1β is grinding of 1α. Mechanochemical reactions of compounds [Pb(NO2)2(4,4'-bipy)]n (2), [Pb(NO3)(SCN)(4,4'bipy)]n (3), [Pb2(NO3)4(4,4'-bipy)]n (4) and [Pb2(SCN)4(4,4'-bipy)]n (5) with KCl, are other ways to synthesize of 1β by solid-state reactions. Synthesis of compounds 2-5 from 1α and 1β, was not possible but synthesis of [Pb(Br)2(4,4'-bipy)]n (6) and [Pb(I)2(4,4'-bipy)]n (7) from 1α and 1β was done successfully. In addition, the morphology of initial precursor has direct influence on morphology of resulting calcined material. Keywords: Polymorph, Mechanochemical, Lead(II), Coordination Polymer. *Corresponding author. Fax: +982182884416. Email: [email protected]
1
1. Introduction Areas of significant and long-standing interest in crystal engineering have included solidstate structural transformations and polymorphism [1]. The ability of a solid material to exist in multiple forms or crystal structures is known as polymorphs. Polymorphism can potentially be found in any crystalline material including polymers, minerals and metals and is related to allotropy, which refers to chemical elements. When polymorphism exists as a result of difference in crystal packing, it is called packing polymorphism. Polymorphs have different stabilities and may spontaneously convert from a metastable form (unstable form) to the stable form at a particular temperature. They also exhibit different melting points, X-ray crystal structure and diffraction patterns. Various conditions in the crystallization process are the main reason responsible for the development of different polymorphic forms. These conditions include: 1) Solvent effects (the packing of crystal may be different in polar and non-polar solvents), 2) Certain impurities inhibiting growth pattern and favors the growth of a metastable polymorphs, 3) The level of super saturation from which material is crystallized (in which generally the higher the concentration above the solubility, the more likelihood of metastable formation), 4) Temperature at which crystallization is carried out, 5) Geometry of covalent bonds (differences leading to conformational polymorphism), 6) Change in stirring conditions, 7) pH change and 8) pressure. In the field of supramolecular and coordination polymers, Vittal et al. designed and performed interesting works on studying polymorphism. They crystallized the polymer {[AgL][ClO4]}∞ (L = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane) in two polymorphs which consisted of coordination polymers {[AgL]+}∞ in sine-wave and zigzag conformations, respectively [2]. They also prepared [Zn2(bpeb)(obc)2]⋅2H2O and [Zn2(bpeb)(obc)2]⋅5H2O (bpeb = 1,4-bis[2-(4-pyridyl)ethenyl]benzene and H2obc = 4,4'-oxybisbenzoic acid), which are 2
different solvates, by solvent exchange in a single-crystal-to-single-crystal (SCSC) manner in MeOH [3]. They successfully utilized the metal-organic salt K2(SDC) (H2SDC=4,4stilbenedicarboxylic acid) to investigate the reversible cleavage of a cyclobutane ring. The two polymorphs of K2SDC undergo reversible cyclobutane formation by UV light and cleavage by heat in cycles [4]. Recently, they reported two polymorphic forms of a ladder coordination polymer [Zn2(µ-O2C-p-Tol)2(O2C-p-Tol)2(bpe)2] (HO2C-p-Tol = para-toluic acid and bpe = trans-1,2-bis(4-pyridyl)ethene) which have parallel and crisscross alignments of the C=C bond pairs. Both polymorphs undergo 100% [2+2] cycloaddition reaction but the one with a parallel orientation of C=C bonds (thermodynamic product) retain its single crystallinity after the reaction [5]. It should be also mentioned that other researchers have some unique works on studying polymorphism [6-9]. For example, Englert, et al. studied two consecutive reversible phase transitions which occur in a single crystal of the 2D coordination compound [Pb(µ-Cl)2(µbipy)]∞ (1), (bipy=4,4'-bipyridine). These transformations relate to the highly distorted lowtemperature structure of 1α, via the intermediate room-temperature phase of 1β, to the arrangement of maximum symmetry in 1γ [10]. Although there is a recent surge of activities in the good old mechanical reactions due to environmentally benign way of making new and existing compounds [11], in this work we want to introduce another way to synthesize of 1α by branch tube method [12] and show the new aspect from mechanochemical reaction in synthesis of 1β polymorph.
2. Experimental Section Materials and Physical Techniques 3
All reagents for the synthesis and analysis were commercially available and used as received. Microanalyses were carried out using a Heraeus CHN-O- Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. IR spectra were recorded using Perkin-Elmer 597 and Nicolet 510P spectrophotometers. The thermal behavior was measured with a PL-STA 1500 apparatus with the rate of 10 ºC.min-1 between 20 and 500 °C (for compounds 1α, 1β, 1γ, 6 and 7) and 20 and 600 °C (for compounds 2-5) in a static atmosphere of air. X-ray powder diffraction (XRD) measurements were performed using a Philips diffractometer of X’pert company with monochromated Cu-kα radiation. The samples were characterized with a scanning electron microscope with gold coating.
Synthesis of [Pb(µ-Cl)2(4,4'-bipy)]n (1α) by branch tube method. Lead(II) acetate trihydrate (2 mmol, 0.758 g), 4,4'-bipyridine (2 mmol, 0.312 g) and KCl (4 mmol, 0.298 g) were loaded into one arm of a branch tube and both of the arms were filled slowly by methanol. The chemical-bearing arm was immersed in an oil bath kept at 60 °C. Crystals were formed on the inside surface of the arm kept at ambient temperature after a few days. Figures S1a and 1b show the IR spectrum and PXRD pattern of it, respectively. d.p. = 205 °C, Yield: 0.721 g (83% based on final product), Anal. calc. for C10H8C12N2Pb: C, 27.66; H, 1.86; N, 6.45, found; C, 28.11; H, 1.79, N, 6.35%.
3. Results and Discussion For the first time, 1α polymorph was synthesized by Nordell et al. under hydrothermal conditions [13]. Englert, et al. showed that when 1β polymorph is exposed to temperatures below 271 K, it adopts a highly distorted structure of low symmetry and high space filling [10]. We succeeded in 4
synthesis of 1α polymorph by branch tube method which is stable at room temperature conditions (Figures S1 and 1). The underlying topology of the 2D network is maintaining in all three phases of 1 (Figure S2 in the SI). 1α (with Pȋ space group), 1β (with C2/m space group) and 1γ (with Cmmm space group) polymorphs have triclinic, monoclinic and orthorhombic crystal systems, respectively. Thus 1α is the kinetically stable polymorph and by temperature increase, the more stable thermodynamic phases of 1β and 1γ with higher symmetry are forming. The TG analysis also approved that the thermal stability of 1β is more than 1α (Figure S3 in the SI). For synthesis of [Pb(NO2)2(4,4'-bipy)]n (2), [Pb(NO3)(SCN)(4,4'-bipy)]n (3), [Pb2(NO3)4(4,4'-bipy)]n (4) and [Pb2(SCN)4(4,4'-bipy)]n (5), we evaluated solid-state reactions of compound 1α with NaNO2, NaNO3/KSCN, NaNO3 and KSCN, respectively. But our analyses indicated that only 1β polymorph was obtained from these mechanochemical reactions (Figure 1). Grinding of compound 1α for 20 minutes without any additive resulted in formation of 1β, too (Figure 1). Because the activation energy which is needed for this conversion is provided by grinding of it, 1α was converted to 1β and the salts which was mentioned above, did not affect on formation of 1β (Figure 2). On the other word, one of the ways to synthesis of 1β is grinding of 1α. Because our attempts to synthesis of compounds [Pb(NO3)(SCN)(4,4'-bipy)]n (3), [Pb2(NO3)4(4,4'-bipy)]n (4) and [Pb2(SCN)4(4,4'-bipy)]n (5) from 1α precursor by mechanochemical reaction was not successful, we synthesized them by their previous reported procedures [14] (Figures S1 and S4 in the SI). [Pb(NO2)2(4,4'-bipy)]n (2) is a new compound which we could not prepare good single crystals of it. Compounds 3-5 form three-dimensional coordination polymers (Figure S5 in the SI). Except of compound 4 (Figure S6 in the SI) which has the similar thermal stability in comparison with 1β (Figure S3 in the SI), two other compounds (3 and 5) have lower thermal stability in comparison to it (Figure S6 in the SI). Solid-state reactions of compounds 2-5 with 5
KCl, only resulted in formation of 1β polymorph (Figure S7). Further many reactions have been reported to occur by grinding with alkali halides, too [15]. In addition, the reverse reactions were not possible and the final resulting compound after solid-state reaction of 1β polymorph with NaNO2, NaNO3/KSCN, NaNO3 and KSCN salts was the initial 1β polymorph (Figure ( S8). Thus in addition to grinding of compound 1α , mechanochemical reaction of compounds 2-5 with KCl, is another way to synthesis of 1β by solid solid-state reactions (Figure 3). Compounds [Pb(Br)2(4,4'-bipy)]n (6) [16] and [Pb(I)2(4,4'-bipy)]n (7) [17] were reported previously. Both of them are isotypic to the crystal structure of the analogous room-temperature 1β polymorph and form two-dimensional two coordination polymers (Figure S9 in the SI). We used the two polymorphs of 1α and 1β in order to synthesis of 6 and 7 by mechanochemical reaction of them with KBr and KI, respectively (SI). Unlike the two sets of previous reactions to synthesis of compounds 2-5 from 1α and 1β , which were not successful, synthesis of 6 and 7 from 1α and 1β was done successfully (Figure S10). Thus pure phase of compounds 6 and 7 were prepared (Figure 4). Thermo gravimetric analyses showed that compounds 6 and 7 have similar thermal stability up to 270 °C (Figure S11 in the SI) and their thermal stability are more than 1α and 1β polymorphs (Figure S3 in the SI). Since the Pb2+ is a soft metal ion, it prefers to form coordination bonds with Br¯ and I¯ rather than Cl¯ ion [18], thus compounds 6 and 7 with higher thermal stability were formed. As was expected due to more stability of compounds 6 and 7 in comparison to 1α and 1β , the reverse reactions were not possible and compounds 6 and 7 did not convert to 1α and 1β by mechanochemical reaction (Figure ( 5). In order to study the stability of 1α and 1β polymorphs, after structural transformations, we have performed the thermal treatments which were described by Englert, et al again [10]. We found 6
out that: i) 1β which was obtained after thermal treatment of 1α at 322 K was not stable at room temperature and converted back to 1α (Figure S12c in the SI), ii) 1α which was obtained after thermal treatment of 1β at 271 K was stable at room temperature conditions and did not convert back to 1β (Figure S12d in the SI) and iii) 1γ which was obtained after thermal treatment of 1β at 423 K was not stable and converted back to 1β at room temperature conditions (Figure S12c in the SI). In order to prepare nanostructures of 1α and 1β by a top-down top approach, we evaluated the thermal treatment of them in oleic acid (OA) at 373 K. The XRD patterns of the samples after this process in OA showed the formation and existence of 1β polymorph for both of them ((Figure S13c,d in the SI). SEM images (Figure 6a,b) of the residues obtained from thermal treatment of 1α and 1β in OA, showed the formation of 1β microrods and mixture of 1β nanorods/agglome nanorods/agglomerated structure, respectively. In the case of 1α , partial of thermal energy was spent to convert it to 1β , thus no agglomeration was observed, but in the case of 1β , mixture of 1β nanorods/agglomerated structure was formed. Review of the related literature demonstrates that supramolecular polymers may be suitable precursors for the preparation of desirable nanoscale materials such as metals and metal oxides with interesting advantages [19]. Thus in order to investigate the relation between the morphology of initial precursors and the morphology of resulting materials, calcination of 1β samples which were prepared by thermal treatment in OA, were done at 593 K in static atmosphere of air. The XRD patterns (Figure S13e,f in the SI) of the resulting materials matched with the standard patterns of cubic Pb and orthorhombic PbCl2, thus we had mixture of Pb and PbCl2. SEM images (Figure 6c,d) of the residues obtained from calcination of 1β samples, show that the mixture of Pb/PbCl2 which was obtained from calcination of 1β microrods is less agglomerated than that was obtained from calcination of 1β nanorods/agglomerated structure. 7
Thus we can conclude that the morphology of initial precursor has direct influence on morphology of resulting material from calcination of it.
3. Conclusions In summary we showed that by branch tube method, we can synthesize kinetically stable 1α polymorph of [Pb(µ-Cl)2(µ-bipy)]∞. In addition to grinding of compound 1α, mechanochemical reaction of compounds 2-5 with KCl, is another way to synthesis of 1β by solid-state reactions. Compounds 6 and 7 can be synthesized from 1α and 1β. Since the Pb2+ is a soft metal ion, it prefers to form coordination bonds with Br¯ and I¯ rather than Cl¯ ion, thus compounds 6 and 7 with higher thermal stability were formed. Due to more stability of compounds 6 and 7 in comparison to 1α and 1β, the reverse reactions were not possible. It seems that solid-state mechanochemical reaction as a green synthetic procedure can be used for synthesis of polymorphs and other supramolecular polymers. Mechanochemical synthesis of other supramolecular polymers such as coordination polymers and metal-organic frameworks is undergoing in our laboratory and its results will be reported in future. On the other hand, SEM images of the residue obtained from thermal treatment of 1α and 1β in OA, showed the formation of 1β microrods and mixture of 1β nanorods/agglomerated structure, respectively. In the case of 1α, partial of thermal energy was spent to convert it to 1β, thus no agglomeration was observed, but in the case of 1β, mixture of 1β nanorods/agglomerated structure was formed. Finally by calcination of the two 1β samples, we can conclude that the morphology of initial precursor has direct influence on morphology of resulting material from calcination of it.
8
Acknowledgements Support of this investigation by Tarbiat Modares University and National Elite Foundation is gratefully acknowledged. References [1] a) C. B. Aakeröy, N. R. Champness, C. Janiak, CrystEngComm, 12 (2010) 22; b) K. Akhbari, A. Morsali, CrystEngComm. 14 (2012) 1618; c) K. Akhbari, A. Morsali, Inorg. Chem. 52 (2013) 2787. [2] S. Muthu, J. H. K. Yip, J. J. Vittal, J. Chem. Soc., Dalton Trans., (2001) 3577. [3] I-H Park, S. S. Lee, J J. Vittal, Chem. Eur. J. 19 (2013) 2695. [4] G. K. Kole, T. Kojima, M. Kawano, J. J. Vittal, Angew. Chem. Int. Ed. 53 (2014) 2143. [5] A. Chanthapally, W. T. Oh, J. J. Vittal, Chem. Commun., 50 (2014) 451. [6] A. Cingolani, S. Galli, N. Masciocchi, L. Pandolfo, C. Pettinari, A. Sironi, J. Am. Chem. Soc., 127 (2005) 6144. [7] I. Kalf, P. Mathieu, U. Englert, New J. Chem., 34 (2010) 2491. [8] S. A. Barnett, A. J. Blake, N. R. Champness, C. Wilson, Dalton Trans., (2005) 3852. [9] K. Fukuhara, S-i Noro, K. Sugimoto, T. Akutagawa, K. Kubo, T. Nakamura, Inorg. Chem., 52 (2013) 4229. [10] C. Hu, U. Englert, Angew. Chem. Int. Ed. 45 (2006) 3457.
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[11] a) A. L. Garay, A. Pichon, S. L. James. Chem. Soc. Rev., 36 (2007) 846; b) T. Friščić, J. Mater. Chem., 20 (2010) 7599; c) S. L. James, C. J. Adams, C. Bolm, D. Braga, P. Collier, T. Friščić, F. Grepioni, K. D. M. Harris, G. Hyett, W. Jones, A. Krebs, J. Mack, L. Maini, A. G. Orpen, I. P. Parkin, W. C. Shearouse, J. W. Steed, D. C. Waddell, Chem. Soc. Rev., 41 (2012) 413. [12] G. Mahmoudi, A. Morsali, A.D. Hunter and M. Zeller, CrystEngComm, 9 (2007) 704. [13] K. J. Nordell, K. N. Schultz, K. A. Higgins, M. D. Smith, Polyhedron 23 (2004) 2161. [14] A. Morsali, A. Mahjoub, Polyhedron 23 (2004) 2427. [15] a) M. Nagarathinam, A. Chanthapally, S. H. Lapidus, P. W. Stephens, J. J. Vittal, Chem. Commun., 48 (2012) 2585, b) J. F. Fernández-Bertran, Pure Appl. Chem., 71 (1999) 581; c) N. Shan, F. Toda, W. Jones, Chem. Commun., (2002) 2372; d) D. Braga, L. Maini, S. L. Giaffreda, F. Grepioni, M. R. Chierotti, R. Gobetto, Chem. Eur. J., 10 (2004) 3261. [16] a) Y. Cui, J. Ren, G. Chen, W. Yu, Y. Qian, Acta Crystallogr., Sect.C: Cryst.Struct.Commun., 56 (2000) e552; b) A. Aslani, A. Morsali, V. T. Yilmaz, O. Büyükgüngör, Inorg. Chim. Acta., 362 (2009) 1506. [17] Y.-J. Shi, Y. Xu, Y. Zhang, B. Huang, D.-R. Zhu, C.-M. Jin, H.-G. Zhu, Z. Yu, X.-T. Chen, X.-Z. You, Chem. Lett. (2001) 678. [18] H. Mayr, M. Breugst, A. R. Ofial, Angew. Chem. Int. Ed. 50 (2011) 6470. [19] M. Y. Masoomi, A. Morsali, Coord. Chem. Rev. 256 (2012) 2921.
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Figure 1. The XRD patterns of (a) simulated from single crystal X-ray data of compound [Pb(µCl)2(4,4’-bipy)]n (1α), (b) 1α polymorph prepared by branch tube method, (c) simulated from single crystal X-ray data of compound [Pb(µ-Cl)2(4,4’-bipy)]n (1β), (d-g) 1β polymorph prepared from solid-state reaction of compound 1α with NaNO2, NaNO3/KSCN, NaNO3 and KSCN, respectively, and h) 1β polymorph prepared from grinding of compound 1α for 20 minutes. 11
[Pb(Cl)2(4,4'-bipy)]n
[Pb(Cl)2(4,4'-bipy)]n
-
R 2 SS N O th wi
R O -3 S S N th wi
SSR with NO3 -/SCN -
[Pb(Cl)2(4,4'-bipy)]n g wi SSR din n th i SC Gr N[Pb(Cl)2(4,4'-bipy)]n [Pb(Cl)2(4,4'-bipy)]n
[Pb(Cl)2(4,4'-bipy)]n
Figure 2. A schematic diagram illustrating the structural conversion of 1α to 1β by mechanochemical reaction with NaNO2, NaNO3/KSCN, NaNO3, KSCN and grinding of compound 1α without any additive additive.
12
[Pb(NO2)2(4,4'-bipy)]n (2) wi SSR th Cl -
[Pb(NO3)(SCN)(4,4'-bipy)]n (3) SSRClh wit
[Pb(Cl)2(4,4'-bipy)]n S wit SR hC l[Pb2(SCN)4(4,4'-bipy)]n (5)
SSRClh wit [Pb2(NO3)4(4,4'-bipy)]n (4)
Figure 3. A schematic diagram illustrating the irriversible solid-state structural transformations of 2-5 3D coordination polymers to 1β 2D coordination polymer by mechanochemical reaction.
13
[Pb(Br)2(4,4'-bipy)]n (6) w i SSR th Br -
SSRBrh wit
[Pb(Cl)2(4,4'-bipy)]n
[Pb(Cl)2(4,4'-bipy)]n S wi SR th I
R SS h Iw it
[Pb(I)2(4,4'-bipy)]n (7) Figure 4. Schematic representation of the irreversible solid-state structural conversion of 1α and 1β to compounds 6 and 7 by mechanochemical reaction.
14
Figure 5. The XRD patterns of (a) simulated from single crystal X-ray data of compound [Pb(Br)2(4,4'-bipy)]n (6), (b) the resulting compound obtained after solid-state reaction of 6 with KCl, (c) simulated from single crystal X-ray data of compound [Pb(I)2(4,4'-bipy)]n (7) and (d) the resulting compound obtained after solid-state reaction of 7 with KCl.
15
Figure 6. SEM images of a) 1β prepared from thermal treatment of 1α in oleic acid at 373 K, b) 1β after thermal treatment in oleic acid at 373 K, c) mixture of Pb and PbCl2 which was obtained from calcination of 1β microrods and d) mixture of Pb and PbCl2 which was obtained from calcination of 1β nanorods/agglomerated structure.
16
Graphical abstract
In addition to grinding 1α, mechanochemical reaction is another way to synthesis 1β. Mechanochemical reaction of [Pb(Br)2(4,4'bipy)]n and [Pb(I)2(4,4'-bipy)]n with KCl was not resulted in formation of 1 polymorphs.
17
Research highlights:
Branch tube method was used for synthesis of kinetically stable polymorph. Thermodynamically stable polymorph was obtained by grinding kinetically stable one. Mechanochemical reaction is a green procedure for synthesis of polymorphs.
18
S0020-1693(15)00065-1 http://dx.doi.org/10.1016/j.ica.2015.01.037 ICA 16401
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
9 December 2014 21 January 2015 30 January 2015
Please cite this article as: K. Akhbari, A. Morsali, Mechanochemical synthesis and characterization of kinetically and thermodynamically stable polymorphs of a lead(II) coordination polymer, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.01.037
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Mechanochemical Synthesis and Characterization of Kinetically and Thermodynamically Stable Polymorphs of a Lead(II) Coordination Polymer
Kamran Akhbari, Ali Morsali* Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 141554838, Tehran, Islamic Republic of Iran
Abstract 1α polymorph of [Pb(µ-Cl)2(µ-bipy)]∞ (bipy = 4,4'-bipyridine) was synthesized by branch tube method. It was found that one of the ways to synthesize of 1β is grinding of 1α. Mechanochemical reactions of compounds [Pb(NO2)2(4,4'-bipy)]n (2), [Pb(NO3)(SCN)(4,4'bipy)]n (3), [Pb2(NO3)4(4,4'-bipy)]n (4) and [Pb2(SCN)4(4,4'-bipy)]n (5) with KCl, are other ways to synthesize of 1β by solid-state reactions. Synthesis of compounds 2-5 from 1α and 1β, was not possible but synthesis of [Pb(Br)2(4,4'-bipy)]n (6) and [Pb(I)2(4,4'-bipy)]n (7) from 1α and 1β was done successfully. In addition, the morphology of initial precursor has direct influence on morphology of resulting calcined material. Keywords: Polymorph, Mechanochemical, Lead(II), Coordination Polymer. *Corresponding author. Fax: +982182884416. Email: [email protected]
1
1. Introduction Areas of significant and long-standing interest in crystal engineering have included solidstate structural transformations and polymorphism [1]. The ability of a solid material to exist in multiple forms or crystal structures is known as polymorphs. Polymorphism can potentially be found in any crystalline material including polymers, minerals and metals and is related to allotropy, which refers to chemical elements. When polymorphism exists as a result of difference in crystal packing, it is called packing polymorphism. Polymorphs have different stabilities and may spontaneously convert from a metastable form (unstable form) to the stable form at a particular temperature. They also exhibit different melting points, X-ray crystal structure and diffraction patterns. Various conditions in the crystallization process are the main reason responsible for the development of different polymorphic forms. These conditions include: 1) Solvent effects (the packing of crystal may be different in polar and non-polar solvents), 2) Certain impurities inhibiting growth pattern and favors the growth of a metastable polymorphs, 3) The level of super saturation from which material is crystallized (in which generally the higher the concentration above the solubility, the more likelihood of metastable formation), 4) Temperature at which crystallization is carried out, 5) Geometry of covalent bonds (differences leading to conformational polymorphism), 6) Change in stirring conditions, 7) pH change and 8) pressure. In the field of supramolecular and coordination polymers, Vittal et al. designed and performed interesting works on studying polymorphism. They crystallized the polymer {[AgL][ClO4]}∞ (L = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane) in two polymorphs which consisted of coordination polymers {[AgL]+}∞ in sine-wave and zigzag conformations, respectively [2]. They also prepared [Zn2(bpeb)(obc)2]⋅2H2O and [Zn2(bpeb)(obc)2]⋅5H2O (bpeb = 1,4-bis[2-(4-pyridyl)ethenyl]benzene and H2obc = 4,4'-oxybisbenzoic acid), which are 2
different solvates, by solvent exchange in a single-crystal-to-single-crystal (SCSC) manner in MeOH [3]. They successfully utilized the metal-organic salt K2(SDC) (H2SDC=4,4stilbenedicarboxylic acid) to investigate the reversible cleavage of a cyclobutane ring. The two polymorphs of K2SDC undergo reversible cyclobutane formation by UV light and cleavage by heat in cycles [4]. Recently, they reported two polymorphic forms of a ladder coordination polymer [Zn2(µ-O2C-p-Tol)2(O2C-p-Tol)2(bpe)2] (HO2C-p-Tol = para-toluic acid and bpe = trans-1,2-bis(4-pyridyl)ethene) which have parallel and crisscross alignments of the C=C bond pairs. Both polymorphs undergo 100% [2+2] cycloaddition reaction but the one with a parallel orientation of C=C bonds (thermodynamic product) retain its single crystallinity after the reaction [5]. It should be also mentioned that other researchers have some unique works on studying polymorphism [6-9]. For example, Englert, et al. studied two consecutive reversible phase transitions which occur in a single crystal of the 2D coordination compound [Pb(µ-Cl)2(µbipy)]∞ (1), (bipy=4,4'-bipyridine). These transformations relate to the highly distorted lowtemperature structure of 1α, via the intermediate room-temperature phase of 1β, to the arrangement of maximum symmetry in 1γ [10]. Although there is a recent surge of activities in the good old mechanical reactions due to environmentally benign way of making new and existing compounds [11], in this work we want to introduce another way to synthesize of 1α by branch tube method [12] and show the new aspect from mechanochemical reaction in synthesis of 1β polymorph.
2. Experimental Section Materials and Physical Techniques 3
All reagents for the synthesis and analysis were commercially available and used as received. Microanalyses were carried out using a Heraeus CHN-O- Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. IR spectra were recorded using Perkin-Elmer 597 and Nicolet 510P spectrophotometers. The thermal behavior was measured with a PL-STA 1500 apparatus with the rate of 10 ºC.min-1 between 20 and 500 °C (for compounds 1α, 1β, 1γ, 6 and 7) and 20 and 600 °C (for compounds 2-5) in a static atmosphere of air. X-ray powder diffraction (XRD) measurements were performed using a Philips diffractometer of X’pert company with monochromated Cu-kα radiation. The samples were characterized with a scanning electron microscope with gold coating.
Synthesis of [Pb(µ-Cl)2(4,4'-bipy)]n (1α) by branch tube method. Lead(II) acetate trihydrate (2 mmol, 0.758 g), 4,4'-bipyridine (2 mmol, 0.312 g) and KCl (4 mmol, 0.298 g) were loaded into one arm of a branch tube and both of the arms were filled slowly by methanol. The chemical-bearing arm was immersed in an oil bath kept at 60 °C. Crystals were formed on the inside surface of the arm kept at ambient temperature after a few days. Figures S1a and 1b show the IR spectrum and PXRD pattern of it, respectively. d.p. = 205 °C, Yield: 0.721 g (83% based on final product), Anal. calc. for C10H8C12N2Pb: C, 27.66; H, 1.86; N, 6.45, found; C, 28.11; H, 1.79, N, 6.35%.
3. Results and Discussion For the first time, 1α polymorph was synthesized by Nordell et al. under hydrothermal conditions [13]. Englert, et al. showed that when 1β polymorph is exposed to temperatures below 271 K, it adopts a highly distorted structure of low symmetry and high space filling [10]. We succeeded in 4
synthesis of 1α polymorph by branch tube method which is stable at room temperature conditions (Figures S1 and 1). The underlying topology of the 2D network is maintaining in all three phases of 1 (Figure S2 in the SI). 1α (with Pȋ space group), 1β (with C2/m space group) and 1γ (with Cmmm space group) polymorphs have triclinic, monoclinic and orthorhombic crystal systems, respectively. Thus 1α is the kinetically stable polymorph and by temperature increase, the more stable thermodynamic phases of 1β and 1γ with higher symmetry are forming. The TG analysis also approved that the thermal stability of 1β is more than 1α (Figure S3 in the SI). For synthesis of [Pb(NO2)2(4,4'-bipy)]n (2), [Pb(NO3)(SCN)(4,4'-bipy)]n (3), [Pb2(NO3)4(4,4'-bipy)]n (4) and [Pb2(SCN)4(4,4'-bipy)]n (5), we evaluated solid-state reactions of compound 1α with NaNO2, NaNO3/KSCN, NaNO3 and KSCN, respectively. But our analyses indicated that only 1β polymorph was obtained from these mechanochemical reactions (Figure 1). Grinding of compound 1α for 20 minutes without any additive resulted in formation of 1β, too (Figure 1). Because the activation energy which is needed for this conversion is provided by grinding of it, 1α was converted to 1β and the salts which was mentioned above, did not affect on formation of 1β (Figure 2). On the other word, one of the ways to synthesis of 1β is grinding of 1α. Because our attempts to synthesis of compounds [Pb(NO3)(SCN)(4,4'-bipy)]n (3), [Pb2(NO3)4(4,4'-bipy)]n (4) and [Pb2(SCN)4(4,4'-bipy)]n (5) from 1α precursor by mechanochemical reaction was not successful, we synthesized them by their previous reported procedures [14] (Figures S1 and S4 in the SI). [Pb(NO2)2(4,4'-bipy)]n (2) is a new compound which we could not prepare good single crystals of it. Compounds 3-5 form three-dimensional coordination polymers (Figure S5 in the SI). Except of compound 4 (Figure S6 in the SI) which has the similar thermal stability in comparison with 1β (Figure S3 in the SI), two other compounds (3 and 5) have lower thermal stability in comparison to it (Figure S6 in the SI). Solid-state reactions of compounds 2-5 with 5
KCl, only resulted in formation of 1β polymorph (Figure S7). Further many reactions have been reported to occur by grinding with alkali halides, too [15]. In addition, the reverse reactions were not possible and the final resulting compound after solid-state reaction of 1β polymorph with NaNO2, NaNO3/KSCN, NaNO3 and KSCN salts was the initial 1β polymorph (Figure ( S8). Thus in addition to grinding of compound 1α , mechanochemical reaction of compounds 2-5 with KCl, is another way to synthesis of 1β by solid solid-state reactions (Figure 3). Compounds [Pb(Br)2(4,4'-bipy)]n (6) [16] and [Pb(I)2(4,4'-bipy)]n (7) [17] were reported previously. Both of them are isotypic to the crystal structure of the analogous room-temperature 1β polymorph and form two-dimensional two coordination polymers (Figure S9 in the SI). We used the two polymorphs of 1α and 1β in order to synthesis of 6 and 7 by mechanochemical reaction of them with KBr and KI, respectively (SI). Unlike the two sets of previous reactions to synthesis of compounds 2-5 from 1α and 1β , which were not successful, synthesis of 6 and 7 from 1α and 1β was done successfully (Figure S10). Thus pure phase of compounds 6 and 7 were prepared (Figure 4). Thermo gravimetric analyses showed that compounds 6 and 7 have similar thermal stability up to 270 °C (Figure S11 in the SI) and their thermal stability are more than 1α and 1β polymorphs (Figure S3 in the SI). Since the Pb2+ is a soft metal ion, it prefers to form coordination bonds with Br¯ and I¯ rather than Cl¯ ion [18], thus compounds 6 and 7 with higher thermal stability were formed. As was expected due to more stability of compounds 6 and 7 in comparison to 1α and 1β , the reverse reactions were not possible and compounds 6 and 7 did not convert to 1α and 1β by mechanochemical reaction (Figure ( 5). In order to study the stability of 1α and 1β polymorphs, after structural transformations, we have performed the thermal treatments which were described by Englert, et al again [10]. We found 6
out that: i) 1β which was obtained after thermal treatment of 1α at 322 K was not stable at room temperature and converted back to 1α (Figure S12c in the SI), ii) 1α which was obtained after thermal treatment of 1β at 271 K was stable at room temperature conditions and did not convert back to 1β (Figure S12d in the SI) and iii) 1γ which was obtained after thermal treatment of 1β at 423 K was not stable and converted back to 1β at room temperature conditions (Figure S12c in the SI). In order to prepare nanostructures of 1α and 1β by a top-down top approach, we evaluated the thermal treatment of them in oleic acid (OA) at 373 K. The XRD patterns of the samples after this process in OA showed the formation and existence of 1β polymorph for both of them ((Figure S13c,d in the SI). SEM images (Figure 6a,b) of the residues obtained from thermal treatment of 1α and 1β in OA, showed the formation of 1β microrods and mixture of 1β nanorods/agglome nanorods/agglomerated structure, respectively. In the case of 1α , partial of thermal energy was spent to convert it to 1β , thus no agglomeration was observed, but in the case of 1β , mixture of 1β nanorods/agglomerated structure was formed. Review of the related literature demonstrates that supramolecular polymers may be suitable precursors for the preparation of desirable nanoscale materials such as metals and metal oxides with interesting advantages [19]. Thus in order to investigate the relation between the morphology of initial precursors and the morphology of resulting materials, calcination of 1β samples which were prepared by thermal treatment in OA, were done at 593 K in static atmosphere of air. The XRD patterns (Figure S13e,f in the SI) of the resulting materials matched with the standard patterns of cubic Pb and orthorhombic PbCl2, thus we had mixture of Pb and PbCl2. SEM images (Figure 6c,d) of the residues obtained from calcination of 1β samples, show that the mixture of Pb/PbCl2 which was obtained from calcination of 1β microrods is less agglomerated than that was obtained from calcination of 1β nanorods/agglomerated structure. 7
Thus we can conclude that the morphology of initial precursor has direct influence on morphology of resulting material from calcination of it.
3. Conclusions In summary we showed that by branch tube method, we can synthesize kinetically stable 1α polymorph of [Pb(µ-Cl)2(µ-bipy)]∞. In addition to grinding of compound 1α, mechanochemical reaction of compounds 2-5 with KCl, is another way to synthesis of 1β by solid-state reactions. Compounds 6 and 7 can be synthesized from 1α and 1β. Since the Pb2+ is a soft metal ion, it prefers to form coordination bonds with Br¯ and I¯ rather than Cl¯ ion, thus compounds 6 and 7 with higher thermal stability were formed. Due to more stability of compounds 6 and 7 in comparison to 1α and 1β, the reverse reactions were not possible. It seems that solid-state mechanochemical reaction as a green synthetic procedure can be used for synthesis of polymorphs and other supramolecular polymers. Mechanochemical synthesis of other supramolecular polymers such as coordination polymers and metal-organic frameworks is undergoing in our laboratory and its results will be reported in future. On the other hand, SEM images of the residue obtained from thermal treatment of 1α and 1β in OA, showed the formation of 1β microrods and mixture of 1β nanorods/agglomerated structure, respectively. In the case of 1α, partial of thermal energy was spent to convert it to 1β, thus no agglomeration was observed, but in the case of 1β, mixture of 1β nanorods/agglomerated structure was formed. Finally by calcination of the two 1β samples, we can conclude that the morphology of initial precursor has direct influence on morphology of resulting material from calcination of it.
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Acknowledgements Support of this investigation by Tarbiat Modares University and National Elite Foundation is gratefully acknowledged. References [1] a) C. B. Aakeröy, N. R. Champness, C. Janiak, CrystEngComm, 12 (2010) 22; b) K. Akhbari, A. Morsali, CrystEngComm. 14 (2012) 1618; c) K. Akhbari, A. Morsali, Inorg. Chem. 52 (2013) 2787. [2] S. Muthu, J. H. K. Yip, J. J. Vittal, J. Chem. Soc., Dalton Trans., (2001) 3577. [3] I-H Park, S. S. Lee, J J. Vittal, Chem. Eur. J. 19 (2013) 2695. [4] G. K. Kole, T. Kojima, M. Kawano, J. J. Vittal, Angew. Chem. Int. Ed. 53 (2014) 2143. [5] A. Chanthapally, W. T. Oh, J. J. Vittal, Chem. Commun., 50 (2014) 451. [6] A. Cingolani, S. Galli, N. Masciocchi, L. Pandolfo, C. Pettinari, A. Sironi, J. Am. Chem. Soc., 127 (2005) 6144. [7] I. Kalf, P. Mathieu, U. Englert, New J. Chem., 34 (2010) 2491. [8] S. A. Barnett, A. J. Blake, N. R. Champness, C. Wilson, Dalton Trans., (2005) 3852. [9] K. Fukuhara, S-i Noro, K. Sugimoto, T. Akutagawa, K. Kubo, T. Nakamura, Inorg. Chem., 52 (2013) 4229. [10] C. Hu, U. Englert, Angew. Chem. Int. Ed. 45 (2006) 3457.
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[11] a) A. L. Garay, A. Pichon, S. L. James. Chem. Soc. Rev., 36 (2007) 846; b) T. Friščić, J. Mater. Chem., 20 (2010) 7599; c) S. L. James, C. J. Adams, C. Bolm, D. Braga, P. Collier, T. Friščić, F. Grepioni, K. D. M. Harris, G. Hyett, W. Jones, A. Krebs, J. Mack, L. Maini, A. G. Orpen, I. P. Parkin, W. C. Shearouse, J. W. Steed, D. C. Waddell, Chem. Soc. Rev., 41 (2012) 413. [12] G. Mahmoudi, A. Morsali, A.D. Hunter and M. Zeller, CrystEngComm, 9 (2007) 704. [13] K. J. Nordell, K. N. Schultz, K. A. Higgins, M. D. Smith, Polyhedron 23 (2004) 2161. [14] A. Morsali, A. Mahjoub, Polyhedron 23 (2004) 2427. [15] a) M. Nagarathinam, A. Chanthapally, S. H. Lapidus, P. W. Stephens, J. J. Vittal, Chem. Commun., 48 (2012) 2585, b) J. F. Fernández-Bertran, Pure Appl. Chem., 71 (1999) 581; c) N. Shan, F. Toda, W. Jones, Chem. Commun., (2002) 2372; d) D. Braga, L. Maini, S. L. Giaffreda, F. Grepioni, M. R. Chierotti, R. Gobetto, Chem. Eur. J., 10 (2004) 3261. [16] a) Y. Cui, J. Ren, G. Chen, W. Yu, Y. Qian, Acta Crystallogr., Sect.C: Cryst.Struct.Commun., 56 (2000) e552; b) A. Aslani, A. Morsali, V. T. Yilmaz, O. Büyükgüngör, Inorg. Chim. Acta., 362 (2009) 1506. [17] Y.-J. Shi, Y. Xu, Y. Zhang, B. Huang, D.-R. Zhu, C.-M. Jin, H.-G. Zhu, Z. Yu, X.-T. Chen, X.-Z. You, Chem. Lett. (2001) 678. [18] H. Mayr, M. Breugst, A. R. Ofial, Angew. Chem. Int. Ed. 50 (2011) 6470. [19] M. Y. Masoomi, A. Morsali, Coord. Chem. Rev. 256 (2012) 2921.
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Figure 1. The XRD patterns of (a) simulated from single crystal X-ray data of compound [Pb(µCl)2(4,4’-bipy)]n (1α), (b) 1α polymorph prepared by branch tube method, (c) simulated from single crystal X-ray data of compound [Pb(µ-Cl)2(4,4’-bipy)]n (1β), (d-g) 1β polymorph prepared from solid-state reaction of compound 1α with NaNO2, NaNO3/KSCN, NaNO3 and KSCN, respectively, and h) 1β polymorph prepared from grinding of compound 1α for 20 minutes. 11
[Pb(Cl)2(4,4'-bipy)]n
[Pb(Cl)2(4,4'-bipy)]n
-
R 2 SS N O th wi
R O -3 S S N th wi
SSR with NO3 -/SCN -
[Pb(Cl)2(4,4'-bipy)]n g wi SSR din n th i SC Gr N[Pb(Cl)2(4,4'-bipy)]n [Pb(Cl)2(4,4'-bipy)]n
[Pb(Cl)2(4,4'-bipy)]n
Figure 2. A schematic diagram illustrating the structural conversion of 1α to 1β by mechanochemical reaction with NaNO2, NaNO3/KSCN, NaNO3, KSCN and grinding of compound 1α without any additive additive.
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[Pb(NO2)2(4,4'-bipy)]n (2) wi SSR th Cl -
[Pb(NO3)(SCN)(4,4'-bipy)]n (3) SSRClh wit
[Pb(Cl)2(4,4'-bipy)]n S wit SR hC l[Pb2(SCN)4(4,4'-bipy)]n (5)
SSRClh wit [Pb2(NO3)4(4,4'-bipy)]n (4)
Figure 3. A schematic diagram illustrating the irriversible solid-state structural transformations of 2-5 3D coordination polymers to 1β 2D coordination polymer by mechanochemical reaction.
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[Pb(Br)2(4,4'-bipy)]n (6) w i SSR th Br -
SSRBrh wit
[Pb(Cl)2(4,4'-bipy)]n
[Pb(Cl)2(4,4'-bipy)]n S wi SR th I
R SS h Iw it
[Pb(I)2(4,4'-bipy)]n (7) Figure 4. Schematic representation of the irreversible solid-state structural conversion of 1α and 1β to compounds 6 and 7 by mechanochemical reaction.
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Figure 5. The XRD patterns of (a) simulated from single crystal X-ray data of compound [Pb(Br)2(4,4'-bipy)]n (6), (b) the resulting compound obtained after solid-state reaction of 6 with KCl, (c) simulated from single crystal X-ray data of compound [Pb(I)2(4,4'-bipy)]n (7) and (d) the resulting compound obtained after solid-state reaction of 7 with KCl.
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Figure 6. SEM images of a) 1β prepared from thermal treatment of 1α in oleic acid at 373 K, b) 1β after thermal treatment in oleic acid at 373 K, c) mixture of Pb and PbCl2 which was obtained from calcination of 1β microrods and d) mixture of Pb and PbCl2 which was obtained from calcination of 1β nanorods/agglomerated structure.
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Graphical abstract
In addition to grinding 1α, mechanochemical reaction is another way to synthesis 1β. Mechanochemical reaction of [Pb(Br)2(4,4'bipy)]n and [Pb(I)2(4,4'-bipy)]n with KCl was not resulted in formation of 1 polymorphs.
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Research highlights:
Branch tube method was used for synthesis of kinetically stable polymorph. Thermodynamically stable polymorph was obtained by grinding kinetically stable one. Mechanochemical reaction is a green procedure for synthesis of polymorphs.
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