Rational Construction of Porous Polymeric Cadmium Ferrocene-1,1′-disulfonates for Transition Metal Ion Exchange and Sorption Liwei Mi,† Hongwei Hou,*,† Zhiyong Song,† Huayun Han,† Hong Xu,† Yaoting Fan,† and Seik-Weng Ng‡
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2553–2561
Department of Chemistry, Zhengzhou UniVersity, Henan 450052, P. R. China, and Department of Chemistry, UniVersity of Malaya, 50603 Kuala Lumpur, Malaysia ReceiVed May 21, 2007; ReVised Manuscript ReceiVed August 24, 2007
ABSTRACT: Porous pillared bilayer open coordination polymer {[Cd(bpp)2(O3SFcSO3)] · (CH3OH)2 · (H2O)6}n 1 (Fc ) ferrocene) and 3D porous coordination polymer {[Cd(bpy)2(O3SFcSO3)] · (CH3OH)4}n 2 have been assembled from Cd(NO3)2, ferrocene-1,1′disulfonate, and bridging ligands 1,3-bis(4-pyridyl)propane (bpp) or 4,4′-bipyridine (bpy). Both of them show very special adsorption properties to copper salts. Along with the increase in the solution concentration of Cu(NO3)2, the percentage of adsorbed copper ions falls, but the percentage of exchanged central cadmium ions and the amount of adsorbed copper ions rises. In dilute solution, there mainly exists metal ion sorption, whereas both ion sorption and exchange are in charge of the whole progress in strong solution. Meanwhile, such materials could be used as ideal metal ion adsorbent toward some other metal cations, including of Pb2+, Zn2+, Mn2+, Co2+, Ni2+. However, when all these metal ions coexist in a mixture solution, the two ferrocenyl complexes selectively adsorb large amounts of only Pb2+ (for 1, 88.54%; for 2, 75.80%) and Cu2+ (for 1, 90.94%; for 2, 79.45%). Ion sorption and central cadmium ion exchange might be considered as being dominated by the coordination ability of metal ions to free functional groups, ionic radii of adsorbed metal ions, and the solution concentration of adsorbed metal salts. On the basis of the recognition of ion exchange, the center metal ion-exchange product of 2, {[Cd0.5Cu0.5(bpy)2(O3SFcSO3)] · (CH3OH)4}n 3, could be obtained by ionexchange-induced single crystal to single crystal transformation. Introduction 1
MOFs with porous structures are an interesting target for chemical synthesis and crystal engineering because of their great potential application in environmental and industrial processes. These porous materials were constructed by various rigid organic bidentate ligands containing pyridine rings,2 dicarboxylate,3 phosphonate,4 and sulfonate5 as spacers and metal cores as connectors. A significant feature of these frameworks is their large well-defined pore shape and pore size. The character has significant applications in ion exchange,6 gas storage, and separation.7 Ion exchange of these materials is achieved directly by suspending such frameworks in a solution containing metal salts. The search for similar ion-exchange materials with improved properties has been performed in recent years.8 For example, Lehn and his co-workers have studied the anion inclusion and exchange both in and out of the inner cavities of multicomponent cylindrical nanoarchitectures.9 Literatures concerning ion-exchange properties of open framework chalcogenides have also been reported by Kanatzidis.10 Heterodinuclear metal complexes could be easily prepared through ionexchange reaction.11 Furthermore, recent documents have also referred to the research on the ion-exchange properties of the two-dimensional barium sulfonate network12 and other layered phosphates13 and disulfonamide complexes.14 Many MOFs with porous structures also exhibit excellent potential application in gas sorption and seperation.15 Recent advances in the area of functional zeolite materials show that zeolites can be used as metal ion adsorbents.16 Zeolite’s sorption characteristics to transition metal cations demonstrate that the table porous materials have potential applications as highly selective ion adsorbents.17 Moreover, the sorption properties of * E-mail:
[email protected]. † Zhengzhou University. ‡ University of Malaya.
a zeolite analogue coordination polymer with amine and group I metal salts has further testified the ion sorption behavior of MOFs.18 Some other literatures have touched on the use of MOFs with porous structures in binding metals ions.19 Thus, we believe that MOFs with porous structures can also be utilized as metal ions adsorbents. If porous MOFs served as metal ion adsorbents, the ion sorption might involve physical sorption and chemical sorption. In the progress of chemical sorption, metal ions must interact with ligands, and the interaction closely associates with ionexchange properties and may result in ion exchange,20 i.e., ion sorption of porous MOFs always take place simultaneously along with ion exchange. These performances accord well with the requirements of potential ion-removal materials. The open framework chalcogenide (NH4)4In12Se20 has also been used for heavy metal ion capture and ion exchange.21 Our strategy for creating room-temperature metal ion adsorbents and exchangers utilizes inorganic polymeric cadmium ferrocene-1,1′-disulfonates because MOFs constructed by aryl sulfonates22 can shape porous structures, which have multiple dimensions and variant topologies with free functional groups. In virtue of the unsaturated sulfonate groups, metal sulfonates might have secondary coordination ability to recognize metal ions. On the basis of these recognition, we created a scheme of two nanoporous cadmium ferrocene-1,1′-disulfonates {[Cd(bpp)2(O3SFcSO3)] · (CH3OH)2 · (H2O)6}n 1 and {[Cd(bpy)2(O3SFcSO3)] · (CH3OH)4}n 2. They show highly selectivity to various divalent metal ions at room temperature, and metal ion sorption take place simultaneously with ion exchange. The solution concentration of metal ions affects the ion sorption and exchange behavior. A possible mechanism of ion sorption and exchange in porous MOFs is proposed. According to this mechanism, the center metal ion-exchange product {[Cd0.5Cu0.5(bpy)2(O3SFcSO3)] · (CH3OH)4}n 3 was
10.1021/cg070468e CCC: $37.00 2007 American Chemical Society Published on Web 11/15/2007
2554 Crystal Growth & Design, Vol. 7, No. 12, 2007
Mi et al. Scheme 1
Table 1. Crystal Data and Structure Refinement for Polymers 1–3
formula fw T (K) wavelength (Å) color cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm-3) abs coeff (mm-1) F(000) cryst size (mm3) θ range for data collection (deg) no. of reflns collected/unique data/restraints/params GOF on F2 final R indices [I > 2σ(I)]
1
2
3
C38H56CdFeN4O14S2 1025.24 291(2) 0.71073 yellow triclinic P1j 11.257(2) 11.830(2) 10.914(2) 111.80(3) 107.36(3) 110.67(3) 1097.7(4) 1 1.551 0.978 530 0.20 × 0.18 × 0.16 2.15–25.00 3425/3425 3425/0/275 1.094 R1 ) 0.0567 wR2 ) 0.1564
C34H40CdFeN4O10S2 897.07 291(2) 0.71073 yellow orthorhombic P2(1)2(1)2 11.831(3) 13.490(4) 11.700(3) 90 90 90 1867.4(8) 2 1.595 1.130 916 0.40 × 0.35 × 0.30 2.29–27.50 10775/4136 4136/0/243 1.014 R1 ) 0.0821 wR2 ) 0.2003
C34H40Cd0.5Cu0.5FeN4O10S2 872.64 293(2) 0.71073 blue orthorhombic P2(1)2(1)2 11.8429(9) 13.4829(9) 11.6921(11) 90 90 90 1867.0(3) 2 1.552 1.685 897 0.40 × 0.35 × 0.28 2.29–27.50 16128/4282 4282/26/239 1.078 R1 ) 0.0547 wR2 ) 0.1576
obtained via ion-exchange-induced single crystal to single crystal transformation (see Scheme 1). Experimental Section General Methods. Ferrocene-1,1′-disulfonate was prepared according to the literature procedure.23 Other chemicals were of A. R. Grade and used without further purification. IR spectra were recorded on a Fourier Bruker Tensor-27 spectrophotometer with pressed KBr pellets in the 400–4000 cm-1 region. Elemental analyses (Carbon, hydrogen, and nitrogen) were carried out on a Carlo-Erba 1106 Elemental Analyzer. Synthesis of {[Cd(bpp)2(O3SFcSO3)] · (CH3OH)2 · (H2O)6}n 1. A 5 ml solution of Cd(NO3)2 · 4H2O (30.8 mg, 0.1 mmol) in methanol was added to a 5 mL methanol solution of Na2Fc(SO3)2 (39.0 mg, 0.1 mmol), followed by dropwise addition of a 5 mL methanol solution of bridging ligand bpp (39.6 mg, 0.2 mmol). The resulting reaction mixture was stirred for about 10 min and then filtered to give a clear red solution. Slow evaporation of the solvent at room temperature gave yellow single crystals suitable for X-ray single-crystal analysis in high yield (91%) after 2 weeks. Anal. Calcd for C38H56CdFeN4O14S2 (%): C, 44.48; H, 5.46; N, 5.46. Found: C, 44.38; H, 5.35; N, 5.52. IR (KBr) (cm-1): 2945w, 1614s, 1559w, 1503w, 1428s, 1225m, 1184s, 1044s, 1016m, 815m, 649s, 613w, 517w, 493s. Synthesis of {[Cd(bpy)2(O3SFcSO3)] · (CH3OH)4}n 2. The synthetic procedure of 2 is identical to that of 1 (bpy: 31.2 mg, 0.2 mmol). Yellow crystals were deposited within two days (33% yield). Anal. Calcd for C34H40CdFeN4O10S2 (%): C, 45.48; H, 4.46; N, 6.24. Found: C, 45.23; H, 4.25; N, 6.42. IR (KBr) (cm-1): 3438s, 3104w, 1607s, 1537w, 1493w, 1418s, 1226s, 1166s, 1040s, 1010m, 811s, 648s, 489s. Synthesis of {[Cd0.43Cu0.57(bpy)2(O3SFcSO3)] · (CH3OH)4}n 3. A solution of Cu(NO3)2 · 3H2O (24.2 mg, 0.1 mmol) in 5 mL of methanol
was treated with big single crystals of 2 (23.70 mg, 0.1 mmol). After being set aside for 30 days, blue transparent crystals were obtained at room temperature. The crystals are easy to effloresce in the air. IR (KBr) (cm-1): 3450s, 1605s, 1535w, 1492s, 1410m, 1385s, 1203s, 809s, 659s, 494s. Crystallography. The measurement of polymer 1 was made on a Rigaku RAXIS-IV image plate area detector. The measurements of polymers 2 and 3 were made on a MART APEX II CCD diffractometer. All measurements use graphite-monochromatic Mo-KR radiation (λ ) 0.71073Å). The data were collected at a temperature of 18 ( 1°C using the ω–2θ scan technique and corrected for Lorenz-polarization effects. A correction for secondary extinction was applied. The final cycle of full-matrix least-squares refinement was based on observed reflections and variable parameters. All calculations were performed using the SHELXL-97 Program.24 Crystal data and structure refinement of the three structures are summarized in Table 1. Main bond lengths and bond angles of 1–3 are listed in Tables 2–4. Gas Sorption Measurement. Sorption isotherm studies of 1 and 2 were performed using a Quantachrome Nova-1000e sorption instrument. As-synthesized crystals of known weight, which were placed in a cylindrical quartz tube (height, 235.5 mm; diameter, 6 mm), were heated at 150°C at 1 × 10-5 Torr for 1.5 h to remove all guest molecules. The N2 gas (UHP grade) was then added incrementally, and the isotherms were recorded at each equilibrium pressure by the static volumetric method. Langmuir surface area and pore volume were estimated using the multipoint Brunauer–Emmett–Teller (BET) equation. Metal Ion Sorption and Exchange Measurement. The sorption behaviors to divalent metal salts from aqueous solutions were performed using the batchwise method at room temperature. The concentration of the metal ion in solution was determined by atomic sorption spectrometry. The mole number of cations adsorbed and cadmium ions
Porous Polymeric Cadmium Ferrocene-1,1′-Disulfonates Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1a Cd(1)–N(1) Cd(1)–N(2)#2 Cd(1)–O(1)#1 N(1)–Cd(1)–N(1)#1 N(1)#1–Cd(1)–N(2)#2 N(1)#1–Cd(1)–N(2)#3 N(1)–Cd(1)–O(1)#1 N(2)#2–Cd(1)–O(1)#1 N(1)–Cd(1)–O(1) N(2)#2–Cd(1)–O(1) O(1)#1–Cd(1)–O(1)
2.351(4) 2.357(4) 2.361(4) 180.0(3) 85.63(15) 94.37(15) 92.64(15) 86.86(15) 87.36(15) 93.14(15) 180.000(1)
Cd(1)–N(1)#1 Cd(1)–N(2)#3 Cd(1)–O(1) N(1)–Cd(1)N(2)#2 N(1)–Cd(1)N(2)#3 N(2)#2–Cd(1)N(2)#3 N(1)#1–Cd(1)O(1)#1 N(2)#3–Cd(1)O(1)#1 N(1)#1–Cd(1)–O(1) N(2)#3–Cd(1)–O(1)
2.351(4) 2.357(4) 2.361(4) 94.37(15) 85.63(15) 180.000(1) 87.36(15) 93.14(15) 92.64(15) 86.86(15)
a Symmetry transformations used to generate equivalent atoms: #1 -x - 1, -y + 1, -z + 1; #2 x, y + 1, z; #3 -x - 1, -y, -z + 1.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 2a Cd(1)–N(2) 2.321(8) Cd(1)–N(3)#1 2.326(7) Cd(1)–O(1)#2 2.364(6) Cd(1)–O(1) 2.364(6) Cd(1)–N(1)#2 2.375(6) Cd(1)–N(1) 2.376(6) N(2)–Cd(1)–N(3)#1 180.000(1) N(2)–Cd(1)–O(1)#2 89.68(18) N(3)#1–Cd(1)–O(1)#2 90.32(18) N(2)–Cd(1)–O(1) 89.68(18) N(3)#1–Cd(1)–O(1) 90.32(18) O(1)#2–Cd(1)–O(1) 179.4(4) N(2)–Cd(1)–N(1)#2 89.00(15) N(3)#1–Cd(1)–N(1)#2 91.00(15) O(1)#2–Cd(1)–N(1)#2 98.7(3) O(1)–Cd(1)–N(1)#2 81.3(3) N(2)–Cd(1)–N(1) 89.00(15) N(3)#1–Cd(1)–N(1) 91.00(15) O(1)–Cd(1)–N(1) 98.7(3) N(1)#2–Cd(1)–N(1) 178.0(3) O(1)#2–Cd(1)–N(1) 81.3(3) a
Symmetry transformations used to generate equivalent atoms: #1 x, y, z + 1; #2 -x, -y, z. Table 4. Selected Bond Lengths (Å) and Angles (deg) for 3 (M ) Cd/Cu)a M(1)–N(2) M(1)–O(1)#2 M(1)–N(1)#2 N(2)–M(1)–N(3)#1 N(3)#1–M(1)–O(1)#2 N(3)#1–M(1)–O(1) N(2)–M(1)–N(1)#2 O(1)#2–M(1)–N(1)#2 N(2)–M(1)–N(1) O(1)–M(1)–N(1) O(1)#2–M(1)–N(1)
2.318(6) 2.364(5) 2.392(5) 180.000(2) 89.44(14) 89.44(14) 90.74(12) 98.8(2) 89.26(12) 98.8(2) 81.2(2)
M(1)–N(3)#1 M(1)–O(1) M(1)–N(1) N(2)–M(1)–O(1)#2 N(2)–M(1)–O(1) O(1)#2–M(1)–O(1) N(3)#1–M(1)–N(1)#2 O(1)–M(1)–N(1)#2 N(3)#1–M(1)–N(1) N(1)#2–M(1)–N(1)
2.322(6) 2.364(5) 2.392(5) 90.56(14) 90.56(14) 178.9(3) 89.26(12) 81.2(2) 90.74(12) 178.5(2)
a Symmetry transformations used to generate equivalent atoms: #1 x, y, 1 + z; #2 2 - x, 2 - y, z.
exchanged was also determined on the Z28000 graphite oven atomic sorption spectrophotometer.
Results and Discussion X-ray Crystallography of 1. As shown in Figure 1a, structural unit of polymer 1 exhibits a petal-shaped geometry. The central Cd(II) ion lies in an elongated octahedral coordination environment [CdN4O2] formed by four nitrogen atoms from four bpp groups and two oxide donors from two monodentate SO3- groups, respectively. In the block unit, the Cd–N bands range from 2.351(4) to 2.357(4) Å and the distance of Cd–O is 3.361(4) Å. Four N atoms around the central Cd(II) ion from four bpp bridging ligands are coplanar. The main bond angles are O1–Cd–O1a 180°, N1–Cd1–N2a 94.37(15)°, and N1a–Cd1– N2a 85.63(15)°, respectively. The two-dimensional nanoporous network framework of 1 is depicted in Figure 1b and based on [Cd(bpp)2(SO3FcSO3)] building blocks. The nanometer-sized pores can be easily distinguished from this figure. In this structure, the cadmium atoms locate in the center of the octahedrons, which comprise four nitrogen atoms and two oxygen atoms. However, the sulphur atoms stand in the peak of the tetrahedrons, which are constructed by a sulphur atom and three conjoining oxygen
Crystal Growth & Design, Vol. 7, No. 12, 2007 2555
atoms from sulfonate. Two adjacent Cd(II) cations are bridged by two bpp, forming a double-stranded chain of loops extending along the crystallographic b axis, in which each bpp molecule adopts a trans–gauche conformation and the aromatic rings in opposite are nearly parallel. The size of the loops is measured by the Cd · · · Cd distance (11.830 Å) and the distance between the middle carbon atoms of the two diametrically opposing · · · CH2CH2CH2 · · · spacers is 8.388 Å. Along the chains formed by ligand -O3SFcSO3- and the central Cd(II) ion, the separation between the adjacent cadmium ions is 11.072Å. One chain formed by bridge ligands bpp and cadmium ions and the other formed by ligands -O3SFcSO3- and central Cd(II) ions cross each other, leading to a two dimensional network structure. Among the framework, there are regular parallelogrammic nanosized lattices (11.830×11.072Å2) formed by four adjacent cadmium atoms. The groups of -O3SFcSO3- act as bifunctional spacers to coordinate the CdN4 complex cations, generating unusual neutral 2D nanoporous network construction. {[Cd(bpy)2(O3SFcSO3)] · (CH3OH)4}n 2. Different from 1, a single-crystal X-ray diffraction study revealed that polymer 2 is a 3D threefold interpenetrated network with molecular formula [Cd(bpy)2(O3SFcSO3)]n, which crystallizes in the space group P21212. As illustrated in Figure 2a, each Cd(II) cation lays on a crystallographic threefold axis and adopts a slightly distorted octahedral environment by coordinating to four pyridyl groups of four bpy bridging ligands and two -O3SFcSO3ligands. The Cd–N bond distances fall in the range of 2.321–2.376 Å, and the Cd-O bond distance is 2.364Å. The main bond angles around the center Cd(II) ions are between 89.00(15) and 179.4(4)°. In coordination polymer 2, there are two types of 4,4′bipyridine ligands, which are distinguished by their dihedral angles of two pyridine rings (59.2 and 61.5°), and these two pyridine linkers joined with the Cd atoms to form a 2D (4,4) grid network (Figure 2b). The shortest distances between the two adjacent paralleling linkers in this structure are 11.83 and 13.49 Å, respectively. Simultaneously, -O3SFcSO3- anions set in the middle of two Cd(II) nodes and the space of Cd · · · Cd is 13.49 Å. All these gave rise to a 3D threefold interpenetrated network. It should be noted that the shortest distance between two -O3SFcSO3- is 9.455 Å and the nanocubic channels could be measured as 13.490 × 11.700 × 11.831 Å3. Metal Ion Sorption and Exchange Behavior. Generally, stable porous coordination networks with large void volumes have been widely employed as gas adsorbents. In this work, the total void volumes, Vvoid, within the crystals of 1 and 2 without solvent guests are, respectively, 25.0 and 53.0% per unit volume, which were calculated by PLATON. Sorption isotherms of N2 gas of polymers 1 and 2 at 77 K are illustrated in Figure 3. As shown in Figure 5, 2 can adsorb above five times more nitrogen than 1 per unit, which is close to the total potential solvent accessible void volume of per unit cell of 1 (274.2 Å3) and 2 (988.2 Å3) calculated by PLATON. Porous MOFs constructed by aryl sulfonates can also be employed as metal ion adsorbents because they exhibit the analogous structures with zeolites.25 Hence, we utilized 5.0 mg of powdery 1 to measure the percentage of metal ions absorbed of divalent copper salts, lead salts, cobalt salts, nickel salts, manganese salts, and zinc salts in 5 mL of 0.05 mg/mL solutions, using the Z28000 graphite oven atomic sorption spectrophotometer. Figure 4 displays the different sorption behaviors of 1 to these divalent metal salts. Powdery 1 shows the different recognition of different metal ions, even the same metal ions with different anionic sites. For instance, the adsorbed
2556 Crystal Growth & Design, Vol. 7, No. 12, 2007
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Figure 1. (a) Building unit of polymer [Cd(bpp)2(O3SFcSO3)]n 1. (b) 2D structure of polymer 1 with petal-shaped units (hydrogen atoms and solvent molecules were deleted for clarity).
percentage of copper ions with different anionic sites is from 96 to 99% and the adsorbed percentage of lead ions with different anionic parts is between 94 and 96%. Clearly, anionic parts of copper salts or lead salts weakly affect the adsorbed percentage of metal ions. But there are gigantic differences for zinc salts, cobalt salts, nickel salts, and manganese salts with different anionic sites. The adsorbed percentages of zinc ions are 14.98, 50.92, 56.91, and 74.48 (%) with the negative ions Cl-, NO3-, OAc-, and SO42-, respectively. For the negative ion Cl-, the adsorbed percentage of zinc ions is much lower than those of the others. They interact very weakly with the uncoordinated oxygen atoms from -O3SFcSO3- because the aryl sulfonate groups are thought of as poor ligands.26 The adsorbed percentages of cobalt salts, manganese salts, and nickel salts are between 11.94 and 33.12, 13.71 and 31.33, and 52.31 and 61.07 (%), respectively. The relative uptake values follow the sequence Cu2+ > Pb2+ > Ni2+ > Co2+ ∼ Mn2+ > Zn2+. This agrees well with the previous report, which indicated that the coordination strength of divalent transition metal ions toward the sulfonate group increases in the order of Zn2+ ∼ Co2+
