DOI: 10.1021/cg9005218
Study on the Reaction of Polymeric Zinc Ferrocenyl Carboxylate with Pb(II) or Cd(II)
2009, Vol. 9 4504–4513
Jinpeng Li, Linke Li, Hongwei Hou,* and Yaoting Fan Department of Chemistry, Zhengzhou University, Henan 450052, P.R. China Received May 14, 2009; Revised Manuscript Received July 22, 2009
ABSTRACT: In this paper, the reaction of polymeric zinc ferrocenyl carboxylate, {[Zn(OOCClH3C6Fc)2(H2O)3](CH3OH)}n (1; FcC6H3ClCOONa = sodium 2-chloro-4-ferrocenylbenzoic), with Pb(NO3)2 or Cd(NO3)2 was investigated. By immersion of selected big single crystals of 1 into aqueous solutions of Pb(NO3)2 and Cd(NO3)2, respectively, two central metal ion exchange induced single-crystal to single-crystal (SCTSC) transformation products, {[Zn0.74Pb0.26(OOCClH3C6Fc)2(H2O)3](H2O)}n (2) and {[Zn0.82Cd0.18(OOCClH3C6Fc)2(H2O)3](H2O)}n (3), could be obtained. In contrast to above-mentioned SCTSC products, 1 can dissolve well in THF solution, and the reaction of the solution containing 1 with Pb(NO3)2 or Cd(NO3)2 results in a 1-D Pb(II) polymer, [Pb(μ2-η2-OOCClH3C6Fc)2(CH3OH)2]n (4), and a mononuclear Cd(II) complex, {[Cd(η2-OOCClH3C6Fc)2(H2O)3](CH3OH)2} (5). Our results indicate that metal ion exchange can be implemented through heterogeneous or homogeneous pathways and suggest an easy means to remove lead and cadmium ions. At the same time, it also set the stage for using analogous metal carboxylates for selective exchange of other cations.
Introduction Organic-inorganic hybrid materials have received much attention owing to their potential applications in several technological areas such as adsorption and separation processes, ion exchange, catalysis, and sensor technology.1-4 One of the salient features of the hybrid materials is that they often contain substitution-active metal sites in their framework, and the metal centers are incorporated in the interstitial or porous coordination network. This character makes such materials better for ion exchange.5 Other coordination-driven metal ions will be capable of entering the interstices or pores (cavities) and reacting with substitution-active metal sites, and then it might be applied to cation exchange materials,6 which can effectively wipe off some highly toxic heavy metals. The central metal ion exchange process is, however, seldom observable by single-crystal X-ray diffraction studies because the possible drastic structural changes during the process cause loss of crystallinity. So retaining single crystallinity7 even after the occurrence of exchange reactions is very important for the development and study of central metal ion exchange materials. Recent studies8 show, through exchange of central metal ion, the framework integrity of some porous metal phosphonate, carboxylate, and sulfonate complexes can be maintained. Hence, exchange of central metal ion in porous complexes can be utilized to design some new metal ion exchange materials. This is a promising strategy for the synthesis and design of solid materials, which could be achieved by immersing some selectable stable big crystals into the aqueous solution of a certain metal salt and generating the replacement of cations. In order to further investigate the coordination-driven central metal ion exchange reaction, we designed and synthesized a polymeric zinc ferrocenyl carboxylate {[Zn(OOCClH3C6Fc)2(H2O)3](CH3OH)}n (1, FcC6H3ClCOONa = sodium 2-chloro-4-ferrocenylbenzoic). Through single-crystal to *To whom correspondence should be addressed. E-mail: houhongw@ zzu.edu.cn. Tel and Fax: þ86-371-67761744. pubs.acs.org/crystal
Published on Web 08/07/2009
single-crystal (SCTSC) transformation, {[Zn0.74Pb0.26(OOCClH3C6Fc)2(H2O)3](H2O)}n (2) and {[Zn0.82Cd0.18(OOCClH4C6Fc)2(H2O)3](H2O)}n (3) were obtained. In addition, 1 can dissolve well in THF solution, and the reaction of the solution containing 1 with Pb(NO3)2 or Cd(NO3)2 results in a 1-D Pb(II) polymer, [Pb(μ2-η2-OOCClH3C6Fc)2(CH3OH)2]n (4), and a mononuclear Cd(II) complex, {[Cd(η2-OOCClH3C6Fc)2(H2O)3](CH3OH)2} (5) (Scheme 1). Experimental Section General Information and Materials. 2-Chloro-4-ferrocenylbenzoic acid and its corresponding sodium salts were prepared according to literature methods.9 All other chemicals were of reagent grade quality obtained from commercial sources and used without further purification. Carbon, hydrogen, and nitrogen analyses were carried out on a FLASH EA 1112 elemental analyzer. Atom adsorption analysis was conducted on a Hitch Z-8000 atom adsorption analyzer. IR data were recorded on a BRUKER TENSOR 27 spectrophotometer with KBr pellets in the 400-4000 cm-1 region. Measurements of the pH values were carried out on a pH meter of model 6071 (Jenco Electronics, Ltd., Shanghai, China). Powder X-ray diffraction (PXRD) patterns were recorded using Cu KR1 radiation on a PANalytical X’prtPro diffractometer. Field emission scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) were conducted on a JSM-6490 scanning electron microscope. Measurements of the ICP data were carried out on an inductively coupled plasma emission spectrometer of model IRIS advantage (Thomas Jankowski Associates (TJA), Phillipsburg, New Jersey, USA). Atomic force microscopy (AFM) images were taken with a silicon nitride scanning tip (Nanoprobe, Veeco Metrology Group, Santa Barbara, CA). Preparation of {[Zn(OOCClH3C6Fc)2(H2O)3](CH3OH)}n, 1. FcC6H4ClCOONa (36.7 mg, 0.10 mmol) in methanol (5 mL) was added dropwise to the mixture of methanol (3 mL) and water (1 mL) of Zn(OAc)2 3 2H2O (11.3 mg, 0.05 mmol). The resultant orange solution was allowed to stand at room temperature in the dark. Good quality red crystals were obtained several days later. Yield 38% (18.2 mg). Elemental Anal. Calcd for C35H34Cl2Fe2O8Zn (830.59): C, 50.57; H, 4.09. Found: C, 50.32; H, 4.86. IR (cm-1, KBr): 3421s, 1602s, 1399s, 1168m, 1103m, 1044m, 835s, 758, 489s. Preparation of {[Zn0.74Pb0.26(OOCClH3C6Fc)2(H2O)3](H2O)}n, 2. Several big single crystals of polymer 1 (0.010 g) were immersed in r 2009 American Chemical Society
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Scheme 1. The Synthesis of Complexes 1-5a
a
FcC6H4ClCOO- ligand is abbreviated as FcPhClCO2 for clarity only in this scheme.
Table 1. Crystal Data and Structure Refinement for Complexes 1-5 complex 1 2 3 4 5 formula C35H34ZnFe2O8Cl2C68H48Pb0.52Cl4Fe4Zn1.48O16C68H48Cd0.36Cl4Fe4Zn1.64O16C36H32PbFe2Cl2O6C35H30CdCl2Fe2O8 fw 830.59 1690.75 1633.93 950.41 873.59 temp (K) 291(2) 293(2) 293(2) 293(2) 291(2) wavelength (A˚) 0.71073 0.71073 0.71073 0.71073 0.71073 cryst syst monoclinic monoclinic monoclinic triclinic orthorhombic Pnma space group C2/c C2/c C2/c P1 a (A˚) 45.375(9) 45.302(9) 45.298(9) 8.2338(16) 8.4181(13) b (A˚) 9.981(2) 9.953(2) 9.951(2) 14.404(3) 45.214(7) c (A˚) 8.0212(16) 7.991(16) 7.987(16) 14.754(3) 9.7430(15) R (deg) 90 90 90 80.99(3) 90 β (deg) 94.78(3) 94.74(3) 94.79(3) 84.24(3) 90 γ (deg) 90 90 90 78.00(3) 90 3620.0(13) 3591.1(12) 3588.0(12) 1686.3(6) 3708.3(10) V (A˚3) Z 4 2 2 2 4 1.524 1.564 1.512 1.872 1.565 Dc (g 3 cm-3) F(000) 1696 1686 1645 928 1752 θ range for data collection (deg)0.90-25.50 3.05-27.49 2.45-27.64 3.16-27.48 2.49-27.50 reflns collected/unique 5704/3261 21173/4111 21336/4136 20497/7686 20731/4316 data/restraints/params 3261/6/232 4111/0/228 4136/0/218 7686/8/444 4316/1/217 1.107 0.983 0.999 1.064 1.186 GOF on F2 0.0563, 0.1601 0.0815, 0.2510 0.0556, 0.1826 0.0307, 0.0683 0.0723, 0.2023 final R1a, wR2b a
R1 = ||Fo| - |Fc||/|Fo|. b wR2 = [w(|Fo2| - |Fc2|)2/(w|Fo2|2]1/2; w = 1/[σ2(Fo)2 þ 0.0297P2 þ 27.5680P], where P = (Fo2 þ 2Fc2)/3.
5 mL of a 1 mg/mL aqueous solution of Pb(NO3)2. After 5 days, red exchanged crystals were obtained at room temperature. The crystals are stable in air. Elemental Anal. Calcd for C34H24Cl2Fe2Zn0.74Pb0.26O8 (845.38): C, 48.26; H, 2.84. Found: C, 48.17; H, 2.88. IR (cm-1, KBr): 3655w, 3422m, 1707s, 1603s, 1567m, 1535m, 1396s, 1169ws, 1104w, 1030w, 877m, 492m. Anal. Calcd for 2 (ICP data): Pb, 6.37; Zn, 5.72; Fe, 13.21. Found: Pb, 5.27; Zn, 5.09; Fe, 11.75. Preparation of {[Zn0.82Cd0.18(OOCClH3C6Fc)2(H2O)3](H2O)}n, 3. According to the synthetic method of 2, compound 3 was prepared through the use of aqueous solution of Cd(NO3)2 3 4H2O instead of aqueous solution of Pb(NO3)2. After 5 days, red exchanged crystals were obtained at room temperature. The crystals are stable in air. Elemental Anal. Calcd for C34H24Cl2Fe2Zn0.82Cd0.18O8 (816.97): C, 49.94; H, 2.94. Found: C, 48.97; H, 3.01. IR (cm-1, KBr): 3098m, 1706s, 1602s, 1536m, 1396s, 1169w, 1105w, 1046w, 836m, 498m. Anal. Calcd for 3 (ICP data): Cd, 2.48; Zn, 6.56; Fe, 13.67. Found: Cd, 2.10; Zn, 5.98; Fe, 12.29. Preparation of [Pb(μ2-η2-OOCClH3C6Fc)2(CH3OH)2]n, 4. The crystals of polymer 1 (41.5 mg, 0.05 mmol) were dissolved in THF solution (4 mL). A methanol solution (5 mL) of Pb(NO3)2 (33.1 mg,
0.1 mmol) was added dropwise to the above clear solution. The resultant orange solution was allowed to stand at room temperature in the dark. Good quality red crystals of 4 were obtained 1 week later. Yield: 53% (42.1 mg). Elemental Anal. Calcd for C36H32O6Cl2Fe2Pb (950.41): C, 45.45; H, 4.05. Found: C, 45.21; H, 3.96. IR (cm-1, KBr): 3432m, 2365m, 1600s, 1518m, 1384s, 1104m, 852m, 488m. Preparation of {[Cd(η2-OOCClH3C6Fc)2(H2O)3](CH3OH)2}, 5. The crystals of polymer 1 (41.5 mg, 0.05 mmol) were dissolved in THF solution (4 mL). A methanol (5 mL) and water (1 mL) mixed solution of Cd(NO3)2 3 4H2O (30.8 mg, 0.1 mmol) was added dropwise to the above clear solution. The resultant orange solution was allowed to stand at room temperature in the dark. Good quality red crystals of 5 were obtained 3 weeks later. Yield: 36% (34.2 mg). Elemental Anal. Calcd for C35H30CdCl2Fe2O8 (873.59): C, 48.08; H, 3.43. Found: C, 47.80; H, 3.22. IR (cm-1, KBr): 3442s, 3096m, 2927m, 1599s, 1401s, 1282m, 1168m, 1103m, 1043m, 848s, 759m, 706m, 670m, 487s. Crystallographic Studies. The diffraction intensity data of 1, 2, 3, and 4 were measured at room temperature on a Rigaku RAXIS-IV or SATURN-724 diffractometer using graphite-monochromated
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Figure 1. (a) The structure of {[Zn(OOCClH4C6Fc)2(H2O)3](CH3OH)}n, 1. (b) View of 3-D packing diagram of polymer 1 down the c axis. The lattice methanol molecules and hydrogen atoms are omitted for clarity. Mo KR radiation (λ = 0.71073 A˚). Data of 5 was collected at room temperature on a Bruker Aper II CCD diffractometer using graphitemonochromated Mo KR radiation (λ = 0.71073 A˚). The structures were solved by direct methods and expanded with Fourier techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included but not refined. The final cycle of full-matrix least-squares refinement was based on observed reflections and variable parameters. All calculations were performed with the SHELXL-97 crystallographic software package.10 Table 1 shows crystallographic data and processing parameters for all complexes. Ion Exchange Test. The effect of the initial concentration of Pb(II) on lead ion exchange was also examined. Ten milligrams of
crystalline 1 was immersed in 5 mL of an aqueous solution of Pb(NO3)2, the concentration of which ranged from 0.2 to 1 mg/mL, in a glass tube. The tubes were allowed to stand for 5 days at room temperature. Then the resultant crystalline materials were filtered, washed with deionized water, and dissolved in HNO3 solution. The final concentrations of Pb(II) in the resultant crystalline materials were measured using a Hitch Z-8000 atom adsorption analyzer. In order to determine exchange capacities of complex 1 toward Pb(II), Cd(II), Cu(II), Ni(II), Co(II), Mn(II), and Cr(II), 10 mg of crystalline 1 was immersed in 8 mL of 0.05 mg/mL metallic nitrate aqueous solution, and the mixture was allowed to stay for 5 days at room temperature. Then the resultant suspensions were filtered. The initial and final concentrations of all tested elements in solution
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Figure 2. The percentage of exchanged zinc ions of crystalline 1 in different solution concentrations of lead nitrate. were measured using a Hitch Z-8000 atom adsorption analyzer. The amounts of exchanged metal ions were calculated from differences between the concentration of the metal ions in the filtered solutions and that in the initial solution. The pH values of the solution before and after each batch experiment were measured.
Results and Discussion X-ray Structure of {[Zn(OOCClH3C6Fc)2(H2O)3](CH3OH)}n (1). Crystallographic analysis reveals that 1 crystallizes in the monoclinic space group C2/c and exhibits a onedimensional (1-D) chain structure. Each six-coordinated Zn(II) ion is in a regular octahedron environment with six oxygen atoms from two FcC6H3ClCOO- units and four coordinated water molecules, respectively (Figure 1a). The Zn-O bond lengths are in the range of 2.057(3)-2.208(2) A˚. The bond angles around Zn(II) ions vary from 87.28(16)° to 180.00(1)°. Two of four water molecules as bridging water molecules link Zn(II) ions in an infinite 1-D chain. The adjacent Zn 3 3 3 Zn distance is 4.011(2) A˚. Through aromatic π-π stacking interactions between two adjacent ferrocenyl rings (edge-to-face CH-π interaction, 2.741 A˚),11 adjacent 1-D chains are further linked into an infinite 3-D supramolecular network (Figure 1b). The 1-D chains and interstices can be easily distinguished from this figure. The adjacent Zn 3 3 3 Zn distance between two adjacent chains is 9.981(2) A˚, from which we can clearly see that the interstices are formed between two adjacent chains. The metal centers are incorporated in the interstitial coordination network that provides flow tunnels within the solid. Central Metal Ion Exchange Induced SCTSC Transformation. Central metal ion exchange properties of organicinorganic hybrid materials are closely associated with the metal ions’ coordination ability.8b Due to the strong coordination ability of lead or cadmium ions to unsaturated ferrocenyl carboxylate,12 lead or cadmium could partly replace the central zinc ions in polymer 1. Notably, polymer 1 cannot dissolve in H2O, CH3COCH3, and CH3CN, but can slightly dissolve in C2H5OH and can dissolve in THF, hot CH3OH, high-polar solvents DMF and DMSO, etc. We first investigated the influence of concentrations of Pb(NO3)2 (from 0.2 to 1 mg/mL) on the percentage of exchanged zinc ions. As shown in Figure 2, the percentage of exchanged zinc ions in concentrated solutions of Pb(NO3)2 is slightly greater than those in dilute solutions. For example, the percentage is 26.22% in 1 mg/mL solution of
Figure 3. Photographs of single crystal of 1 in the central metal ion exchange process.
Pb(NO3)2, whereas the percentages are 22.89% in 0.4 mg/mL solution of Pb(NO3)2 and 20.22% in 0.2 mg/mL solution of Pb(NO3)2. The rising trend of the percentage gradually becomes slow, which indicates that the percentage of exchanged zinc ions would be finite. Subsequently, Pb-exchanged induced product {[Zn0.74Pb0.26(OOCClH3C6Fc)2(H2O)3](H2O)}n, 2, was obtained by immersing the big crystals of as-synthesized 1 into an aqueous solution of 1 mg/mL lead nitrate for 5 days. From a photograph of the crystal (Figure 3), we can clearly see that 2 entirely maintains single crystallinity. X-ray diffraction confirmed that a SCTSC transformation had occurred, resulting in a Pb-Zn complex, and the complex retained the same structure as polymer 1. As shown in Figure 4, analogous powder X-ray diffraction analysis and IR spectra also verify that the exchanged product 2 retains 1’s structure. Complex 2 was carefully analyzed using an atomic adsorption spectrophotometer, and the content of zinc atom is 73.13%, while that of lead is 26.87%. These results agree well with elemental analysis and ICP (Tables S-1 and S-2 of the Supporting Information). Similar to 2, Cd-exchanged product {[Zn0.82Cd0.18(OOCClH3C6Fc)2(H2O)3](H2O)}n, 3, could also be prepared by immersing the big crystals of assynthesized 1 into an aqueous solution of 1 mg/mL cadmium nitrate for 5 days. Complex 3 also inherited the single crystallinity and crystal structure of 1. Analogous powder X-ray diffraction analysis and IR spectra (Figure 4) also verify that 3 retains 1’s structure. The contents of cadmium and zinc are 18.11% and 81.89%, respectively, which were
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Figure 4. (a) IR spectra of 1 and its Pb(II)- and Cd(II)-exchanged products (2 and 3). (b) PXRD patterns of 1 and its Pb(II)- and Cd(II)exchanged products (2 and 3).
measured by atomic adsorption and confirmed by element analysis and ICP (Tables S-1 and S-2 of the Supporting Information). To further clarify the central metal ion exchange, energydispersive X-ray spectroscopy (EDS) measurements on 2 and 3 were carried out. Figure 5a shows the SEM and EDS results of the crystal surface of 2. The detected surface atom ratios for carbon, oxygen, iron, chlorine, lead, and zinc are 71.42%, 15.46%, 5.62%, 5.05%, 0.65%, and 1.67%, respectively. After this crystal is cut (Figure 5b), the interior atom ratios of the crystal for carbon, oxygen, iron, chlorine, lead, and zinc are 72.93%, 16.38%, 4.47%, 3.95%, 0.53%, and 1.74%, respectively. Obviously, the percentage of exchanged zinc ions on the surface of 2 is slightly greater than that in the interior, and the mean value of EDS results accord with atomic adsorption spectrophotometer, element analysis and ICP. Analogous results also occur in 3 (Figure 6). These results are similar to our previous report,8c where there is a diminishing concentration gradient of Fe(III) from the crystal surface to the interior. It indicates that there is some
resistance to both external mass transfer and intraparticle diffusion. In addition, microtopographic atomic force microscopy (AFM) is a technique that is used for further identifying SCTSC transformation in a dynamic regime.13 Usually, a dissolution-reprecipitation process14 will occur in the course of ion exchange, and the formation of a new crystalline phase on the surface of the old one starts, which continues until the ion exchange is complete. Meanwhile, the height of microcrystallites (or peaks or holes14) becomes more pronounced, and complexes will rapidly lose their crystallinity turning opaque.15 This loss of singularity, which indicates a significant restructuring of the crystal in conjunction with changes in the crystallographic symmetry of the complex, is, to us, inconsistent with the proposed SCTSC process. If the crystal surface profile or height of microcrystallites has no obvious change, we think that it will be a SCTSC process. The AFM technique is a good method that can be utilized to image the crystal surface profile to reveal nanoscopic features on the surface.
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Figure 5. SEM and EDS for (a) the surface and (b) the interior (after a single crystal is cut) of 2.
Figure 6. SEM and EDS for (a) the surface and (b) the interior (after a single crystal is cut) of 3.
From Figure 7, we can see that the crystal surface profiles and peak heights of AFM images of 1 and 2 show no obvious change, which indicates a SCTSC process. During
the course of ion exchange, we cannot find a dissolutionreprecipitation process. Similar phenomena also occur between 1 and 3.
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Figure 7. Images of the single crystal surface of (a) 1 and (b) 2 and profiles of the single crystal surface of (c) 1 and (d) 2 (note the different scale for the Z-axis representing peaks height of crystal surface).
In order to assess metal ion exchange capacities of polymer 1 toward Pb(II), Cd(II), Cu(II), Ni(II), Co(II), Mn(II), and Cr(II), 10 mg of crystalline 1 was immersed in 8 mL of 0.05 mg/mL metallic nitrate aqueous solution, and the mixture was allowed to stand for 5 days at room temperature. The initial and final concentrations of all tested elements in solution were measured using a Hitch Z-8000 atom adsorption analyzer. The distribution coefficient (Kd) was determined according to the following equation:16 ðC0 -Cf Þ V Kd ¼ Cf ms where C0 and Cf are the initial and final metal ion concentrations in aqueous solution (mM), V is the solution volume (mL), and ms is the mass of sorbent (g). Figure 8 displays the exchange capacities of 1 for different metallic nitrates. Polymer 1 exhibits high ion exchange capacity for millimolar concentrations of Pb(II), with log Kd,Pb(II) values of 4.63. Polymer 1 was also found to exchange Pb(II) selectively over Cd(II), Cu(II), Ni(II), Co(II), Mn(II), and Cr(II), with log Kd,M(II) values of 3.79 for Cd, 3.38 for Cu, 2.36 for Ni, 2.18 for Co, 2.43 for Mn, and 1.22 for Cr. Meanwhile, Na(I) and K(I) were not exchanged to any appreciable extent. Based on the log Kd values and different exchange capacities, polymer 1 had an affinity for those metallic nitrates in decreasing order as follows: Pb(II) > Cd(II) > Cu(II) > Mn(II) > Ni(II) > Co(II) > Cr(II) . Na(I) and K(I). It should be pointed out that the d10s2 configuration of Pb(II) ion owns
Figure 8. Ion-exchange capability of crystalline 1 in an aqueous solution at room temperature.
the greatest degree of ion polarization among all those ions, so Pb(II) ions show stronger coordination capacity with oxygen atoms. In addition, we also found that the maximum exchange of metal ions was demonstrated at pH values of 4.1-5.5. Under low pH values (pH < 2), the crystalline polymer 1 will be ineffective. When all these metal ions (the concentration for each metal ion is 0.05 mg/mL) coexist in a mixed solution, polymer 1 selectively exchanges large amounts of only Pb(II) (100%), Cd(II) (92.34%), and Cu(II) (76.37%). From the above analysis and experiment, we find that 1 exhibits higher ion exchange capacity for lead and cadmium ions than other metal ions.
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Figure 10. The structure of complex {[Cd(η2-OOCClH4C6Fc)2(H2O)3](CH3OH)2}, 5.
Figure 9. The structure of complex [Pb(μ2-η2-OOCClH4C6Fc)2(CH3OH)2]n, 4.
The Reaction of a Solution Containing 1 with Pb(NO3)2 or Cd(NO3)2. In contrast to above-mentioned SCTSC, the reaction of the THF/CH3OH mixed solution containing 1 with Pb(NO3)2 or Cd(NO3)2 in THF/CH3OH mixed solution was also investigated. Polymer 1 can dissolve well in THF solution, and the reaction of the solution containing 1 with Pb(NO3)2 in CH3OH solution gives bright red crystals of [Pb(μ2-η2-OOCClH3C6Fc)2(CH3OH)2]n, 4. Single-crystal X-ray analysis reveals that in polymer 4, each FcC6H3ClCOOligand displays a tridentate mode and bridges two adjacent Pb(II) ions as a bridging tridentate ligand (Figure 9). The Pb(II) ion is eight-coordinated and binds to four oxygen atoms from two FcC6H3ClCOO- units, two bridging oxygen atoms arising from two different FcC6H3ClCOO- units, and two oxygen atoms from two methanol molecules. The eight oxygen atoms in 4 form two head-to-head four-square pyramids, the lead atom occupying the peak position. The Pb 3 3 3 Pb distances in 4 are 4.332(3) and 4.236(3) A˚. The O-Pb-O bond angles range from 50.71(3)° to 154.29(9)°. Analysis of the crystal packing of 4 reveals that an infinite 2-D supramolecular network is formed by the intramolecular π-π stacking interactions between two adjacent benzimidazole and ferrocenyl rings with an interplanar separation of 3.578 A˚ (center-tocenter separation 3.627 A˚).17 The bond lengths between the Pb and O atoms [2.476(3)-2.774(3) A˚] are within the range for Pb-O covalent bonds (ranging from 2.46 to 2.96 A˚).18 The bridging carboxylate oxygen atoms link all Pb(II) ions to form a 1-D chain structure. The 1-D polymer 4 is similar to the reported Pb-carboxlate polymers, in which carboxlyate ligands are FcCHdCHCOO-, FcC(CH)dCHCOO-, and m-FcC6H4COO-.12 Generally, such structure is suggested to be thermodynamically favored over other species. The reaction of 1 with Cd(NO3)2 in THF/CH3OH mixed solution produces a mononuclear Cd(II) complex with formula {[Cd(η2-OOCClH3C6Fc)2(H2O)3](CH3OH)2}, 5. The central Cd(II) ion is seven-coordinated with four oxygen atoms from two FcC6H3ClCOO- groups in bidentate chelate mode and three oxygen atoms from terminal coordinated aqueous molecules (Figure 10). The seven oxygen atoms in 5 form an axially
distorted, quinquangular bipyramidal arrangement. The five atoms O1-O2-O1A-O2A-O5 in complex 5 form an equatorial plane (the mean derivation is 0.0073), the two oxygen atoms from two H2O molecules occupy the axial positions, and the axial angle O3-Cd1-O4 is 166.6°. The Cd-O lengths are in the range of 2.305(9)-2.476(16) A˚. The bond angles around Cd(II) ion vary from 53.60(11) to 170.43(16)°. These weak but significant intermolecular interactions play an important role in stabilizing the molecular structure. To probe into the reaction processes in THF/CH3OH mixed solution, control experiments were carried out. When we utilize FcC6H3ClCOONa to react with the Pb(NO3)2 or Cd(NO3)2 in THF/CH3OH or CH3OH solvents, complexes 4 and 5 can also be obtained. Furthermore, complex 4 can be obtained from the reaction of 5 with Pb(NO3)2 in THF/ CH3OH mixed solution at ambient temperature. When we utilize FcC6H3ClCOONa to react with the Cd(II) and Zn(II) ions [Cd(II)/Zn(II) g 1/1] in THF/CH3OH or CH3OH solvents, only a polycrystalline powdery substance was obtained. Analysis of the polycrystalline powder revealed that the structure is essentially identical to that of 5 (Figure S3 of the Supporting Information). Careful inspection of the polycrystalline powder, implemented on an atomic adsorption spectrophotometer, indicates that the content of the cadmium ion is nearly 100% in contrast to zinc ion. Similary, when we utilize the FcC6H3ClCOONa to react with two correlative metal ions [Pb(II)/Cd(II) or Pb(II)/Zn(II) g 1/1] or three correlative metal ions [Pb(II)/Cd(II)/Zn(II) g 1/1/1], the structures of those polycrystalline powders are essentially identical to that of 4. In our previous paper, we have demonstrated the skeletons of such polymers could be maintained in DMF or THF solution by measuring their molecular weight.19 Hence, we think that in the assembly of 4 or 5, one possible reason is that the free carboxylate oxygen atoms of 1 bind with Pb(II) or Cd(II) ion, giving rise to a Pb(II) or Cd(II) adduct of 1. As Pb(II)-O or Cd(II)-O bonds in the adducts become stronger, the Zn(II)-O bonds become weaker until they break because Pb(II) or Cd(II) exhibits stronger coordination capability than Zn(II).12 Usually, by the reactions of metal complexes as metalloligands with different transition metals, heterometallic complexes are prepared.20 Another possible reason is that in the assembly of 4 or 5, the competition of the metal ion with ferrocenyl carboxylate and a dissolving, recrystallizating process occurs, which determines the final crystalline products. Thermogravimetric Analysis (TGA). The TG-DTA of polymers 1-5 were determined in the range 20-750 °C in air (Figures S4-S8 of the Supporting Information). TG data shows that 1 loses weight from 76 to 105 °C corresponding to
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the losses of crystallized and coordinated solvent molecules (observed 9.94%, calculated 10.35%), and then it keeps losing weight from 405 to 609 °C corresponding to the decomposition of FcC6H3ClCOO- ligands. Finally, a plateau region is observed from 610 to 700 °C. A brown amorphous residue of ZnO þ Fe2O3 (observed 29.98%, calculated 31.03%) remains. There are two weak exothermic peaks at 315 and 434 °C, and there is a strong exothermic peak at 576 °C on the DTA curve. Compound 2 first loses the solvent molecules and coordinated solvent molecules from 66 to 117 °C (observed 8.01%, calculated 8.52%) and then goes through complicated multiple weight loss steps in the temperature range of 329-662 °C corresponding to the decomposition of FcC6H3ClCOO- ligands. In the end, a brown amorphous residue of PbO þ ZnO þ Fe2O3 (observed 33.46%, calculated 32.88%) remains. There is a strong exothermic peak at 530 °C on the DTA curve of polymer 2. A strong exothermic peak at 609 °C can be observed on the DTA curve of polymer 3; it keeps losing weight from 45 to 124 °C corresponding to the losses of crystallized and coordinated solvent molecules (observed 8.51%, calculated 8.81%), and then a plateau region is observed from 636 to 750 °C, leaving a brown amorphous residue of CdO þ ZnO þ Fe2O3 (observed 30.18%, calculated 30.54%). We find from the TG curve that 4 loses weight from 91 to 114 °C corresponding to the loss of coordinated solvent molecules (observed 6.11%, calculated 6.73%). The decomposition process was completed at 730 °C giving brown amorphous residue (PbO þ Fe2O3) as the final decomposition product (observed 40.55%, calculated: 40.29%). There are two strong endothermic peaks at 514 and 566 °C exothermic on the DTA curve of 4. On the DTA curve of 5, one weak endothermic peak at 303 °C and one strong exothermic peak at 640 °C can be found. The weight loss from 30 to 149 °C corresponds to the departure of the H2O and CH3OH (observed 6.07%, calculated: 6.73%), and the weight loss from 301 to 674 °C corresponds to the decomposition of FcC6H3ClCOO- ligands. For complex 5, a brown amorphous residue of CdO and Fe2O3 remains (observed 16.77%, calculated 16.83%). Conclusion In short, a new central metal ion exchange material for the exchange of hazardous metal ions was demonstrated through central metal ion exchange reaction. In crystalline state, 1 undergoes a partial central metal ion exchange reaction, and two SCTSC transformation products (2 and 3) are obtained. If 1 is dissolved in THF solution, the reaction of the solution containing 1 with Pb(NO3)2 or Cd(NO3)2 results in 4 or 5, which illuminates that the lead or cadium ions have stronger coordination capacity than the zinc ion. The foregoing results indicate that the metal ferrocenyl carboxylate might be regarded as a new class of excellent central metal ion exchange materials because of their insolubility, thermal stabilization, and tailor-made architectures. Further investigation using analogous metal carboxylate for selective exchange of other cations is in progress. Acknowledgment. This work was supported by the National Natural Science Foundation (Nos. 20671082 and 20801048), Program for New Century Excellent Talents of Ministry of Education of China (NCET), the Ph.D. Programs Foundation of Ministry of Education of China,
Li et al.
and the Outstanding Talented Persons Foundation of Henan Province. Supporting Information Available: ICP test results, atomic adsorption test results, IR spectra and PXRD patterns of 5 and the different proportions of powdery contents, and TG-DSC analysis. This material is available free of charge via the Internet at http:// pubs.acs.org.
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