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DOI: 10.1021/cg100078b

Microporous La(III) Metal-Organic Framework Using a Semirigid Tricarboxylic Ligand: Synthesis, Single-Crystal to Single-Crystal Sorption Properties, and Gas Adsorption Studies

2010, Vol. 10 3410–3417

Prem Lama,† Arshad Aijaz,† Subhadip Neogi,‡ Leonard J. Barbour,‡ and Parimal K. Bharadwaj*,† †

Department of Chemistry, Indian Institute of Technology, Kanpur, 208016, India, and Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa

‡

Received January 20, 2010; Revised Manuscript Received June 7, 2010

ABSTRACT: Two La(III) coordination polymers, {[La(cpia)(2H2O)] 3 4H2O}n (1) and {[La(cpia)(H2O)(2DMF)}n (2) (cpiaH3 = 5-(4-carboxy-phenoxy)-isophthalic acid, DMF = N,N0 -dimethylformamide), have been synthesized under hydro- and solvothermal conditions, respectively. The lattice water molecules in compound 1 could be partially replaced at RT with different solvent molecules such as ethanol, acetone, and pyridine leading to three new daughter crystals {[La(cpia)(2H2O)] 3 2H2O 3 C2H6O}n (1a), {[La(cpia)(2H2O)] 3 2H2O 3 C3H6O}n (1b), and {[La(cpia)(2H2O)] 3 2H2O 3 C5H5N}n (1c) in a single-crystal to single-crystal (SC-SC) manner. All five compounds were further characterized by IR spectroscopy, elemental analysis, X-ray powder diffraction, and thermogravimetry. Each polymer forms a carboxylate-bridged three-dimensional structure with each metal adopting LaO9 geometry. Thermogravimetric analysis (TGA) shows that compound 1 loses water molecules beginning at ∼80 °C and continues until 300 °C, and it is thermally stable up to 400 °C. All the daughter compounds 1a-1c lose solvent molecules in a stepwise manner and become fully desolvated at 320 °C. Like 1, all the daughter crystals are also stable up to 400 °C and beyond that it starts to decompose. On the other hand, 2 starts to lose weight continuously beginning at ∼80 °C and breaks down without showing any plateau. The sorption measurements performed on desolvated 1 show that N2 molecules are adsorbed at the surface only at 77 K; however, appreciable amounts of CO2 are readily and reversibly incorporated at 273 K.

Introduction Synthesis of metal-organic framework (MOF) structures have been a subject of intense research activity in many laboratories, because these materials show potential applications in the areas of sensors, gas adsorption, molecular sieves, ion exchange, size-selective separation, nonlinear optics, heterogeneous catalysis, and host-guest induced separation.1 Polycarboxylate linkers are among the most important and widely used bridging ligands for the construction of stable MOFs with permanent porosity.2 Because of this high thermal stability of polycaboxylate complexes, it is possible to remove solvent molecules from the lattice without destruction of the original framework.3 If the crystallinity is maintained upon guest removal/inclusion, then it becomes possible4 to locate the guests and to determine their orientation in the voids, and so on. A variety of elongated dicarboxylate ligands have been used to construct a family of stable and highly porous isoreticular MOFs, whereas tricarboxylate ligands have been used by several groups to construct mesoporous MOFs.5 Yaghi and co-workers have reported extremely porous isoreticular MOFs built from linear dicarboxylate groups with varied dimensions.6 In recent years, an attractive target has been the construction of rare-earth metal complexes due to their potential applications as magnetic and luminescence materials, in selective gas adsorption.7 For transition metal ions, the directional properties of the d orbitals can dictate the mode of coordination. In contrast, design and control over lanthanide-based frameworks are more difficult owing to their high and variable coordination numbers, the f orbitals being *To whom correspondence should be addressed. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 06/24/2010

buried and not showing strong interactions with the ligand donors.8 The high affinity of lanthanides for O donor atoms makes carboxylates excellent candidates as bridging ligands for preparing stable frameworks. A large number of MOFs with a wide range of structural variations are known with both transition and rare earth metal ions and carboxylate donors connected to rigid aromatic groups.9 We have focused on the synthesis of new MOFs using a semirigid 5-(4-carboxyphenoxy)-isophthalic acid as a bridging ligand that incorporates both rigidity and flexibility in the construction of coordination polymers. Herein, we report the synthesis of two new La(III) complexes, {La(cpia)(2H2O) 3 4H2O}n (1) and {[La(cpia)(H2O)(2DMF)}n (2) under hydro- and solvothermal conditions, respectively. The complex 1 shows reversible adsorption-desorption of water, ethanol, pyridine, and acetone molecules residing in the channels in a single-crystal to single-crystal (SC-SC) manner. Adsorption of N2 and CO2 has been probed with desolvated 1. Experimental Section Materials and Methods. The lanthanum salt 5-hydroxyisophthalic acid and 4-fluorobenzonitrile were acquired from Aldrich and used as received. All solvents and K2CO3 were procured from S. D. Fine Chemicals, India. All solvents were purified prior to use. The hydro/ solvothermal synthesis was performed in Teflon-lined stainless steel autoclaves. Infrared (IR) spectra were obtained (KBr disk, 400-4000 cm-1) on a Perkin-Elmer model 1320; microanalyses for the compounds were carried out using a CE-440 elemental analyzer (Exeter Analytical Inc.). X-ray powder patterns were acquired (CuKR radiation at a scan rate of 3o/min, 293 K) on a PHILIPS PW100 diffractometer. Thermogravimetric analyses (TGA) were recorded (heating rate of 5 °C/min) on a Mettler Toledo thermal analyzer. r 2010 American Chemical Society

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Crystal Growth & Design, Vol. 10, No. 8, 2010

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Table 1. Crystal and Structure Refinement Data for 1, 1a-1c, 10 , and 2 compound formula fw T, K system space group a, A˚ b, A˚ c, A˚ R (°) β (°) γ (°) U, A˚3 Z Fcalc, g/cm3 μ, mm-1 refl collected independent refl GOF final Rin [I > 2σ(I)] Rin all data, F2 refine

1

1a

1b

1c

10

2

C15H13O13La 540.16 100 triclinic P1 9.376(5) 9.940(4) 11.377(7) 92.979(6) 97.836(5) 110.330(3) 979.3(9) 2 1.832 2.247 6235 3680 1.169 R1 = 0.0644, wR2 = 0.1402 R1 = 0.0936, wR2 = 0.1875



C20H18NO11La 587.26 100 triclinic P1 9.404(3) 9.957(5) 11.459(8) 93.381(5) 97.716(4) 110.242(5) 991.2(9) 2 1.968 2.223 8842 4203 1.244 R1 = 0.0528, wR2 = 0.1147 R1 = 0.0734, wR2 = 0.1679


C21H23N2O10La 602.32 100 triclinic P1 8.849(4) 11.171(5) 12.667(7) 73.686(5) 85.005(8) 74.930(2) 1160.3(10) 2 1.724 1.900 7469 4773 1.276 R1 = 0.0551, wR2 = 0.1228 R1 = 0.0828, wR2 = 0.1984

Gas Adsorption Measurements. All gas sorption isotherms were measured using a Micromeritics ASAP2020 volumetric gas adsorption instrument. The as-synthesized compound was transferred to preweighed analysis tubes, which were capped with a transeal to prevent intrusion of oxygen and atmospheric moisture during transfers and weighing. The samples were evacuated under dynamic vacuum at 180 °C until the outgas rate was less than 2 mTorr/min. The evacuated analysis tube containing degassed sample was then transferred carefully to an electronic balance and weighed to determine the mass of the sample. After determining the mass of the degassed samples, the analysis tube was transferred back to the analysis port of the gas adsorption instrument. The outgas rate was again confirmed to be less than 2 mTorr/min. The specific surface areas of the compound have been obtained applying the BET (N2 isotherms) and Langmuir (CO2 isotherms) analyses. The temperatures of the measurements for CO2 and N2 were 273 and 77 K, respectively. Synthesis. Synthesis of 5-(4-Carboxy-phenoxy)-isophthalic Acid (cpiaH3). This ligand was synthesized as previously reported.10 Synthesis of {La(cpia)(2H2O) 3 4H2O}n (1). A mixture of cpiaH3 (0.04 g, 0.13 mmol), La(NO3)3 3 6H2O (0.08 g, 0.18 mmol), and NaOH (1M, 0.15 mL) in 3 mL of water was sealed in a Teflon-lined autoclave and heated under autogenous pressure to 180 °C for three days. After the mixture was cooled to room temperature at a rate of 0.1 °C/min, colorless crystals of 1 were collected, washed with water and methanol, and dried at room temperature to give 68% yield based on La(NO3)3.6H2O. Anal. Calcd. for 1: C, 33.35%; H, 2.43%. Found: C, 32.96%; H, 3.52%. IR (cm-1): 3386(w,br), 3073(s), 2924(s), 1620(m), 1596(m), 1543(m), 1462(s), 1390(m), 1248(m), 1212(m), 1167(s), 976(m), 863(m), 800(m), 781(s), 717(s). Synthesis of {[La(cpia)(H2O)(2DMF)}n (2). A mixture of cpiaH3 (0.04 g, 0.13 mmol) and La(NO3)3 3 6H2O (0.08 g, 0.18 mmol) in 2.5 mL of DMF were sealed in a Teflon-lined autoclave and heated under autogenous pressure to 110 °C for two days. After the mixture was cooled to room temperature at a rate of 0.1 °C/min, colorless block-shaped crystals of 2 were collected, washed with DMF and methanol, and dried at room temperature to give 62% yield based on La(NO 3)3 3 6H2 O. Anal. Calcd. for 2: C, 41.87%; H, 3.84%; N, 4.65%. Found: C, 41.85%; H, 3.89%; N, 4.58%. IR (cm-1 ): 3419(w,br), 3068(s), 2928(s), 1650(m), 1614(m), 1581(m), 1549(m), 1403(m), 1254(m), 1215(m), 1157(s), 1105(s), 972(m), 894(m), 790(s), 713(s), 673(s). Single-Crystal X-ray Studies. Single-crystal X-ray data on 1, 10 , 2 and the daughter crystals 1a-1c were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite monochromated Mo KR radiation (λ = 0.71073 A˚). The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from the International Tables for X-ray Crystallography. The data integration and reduction were

carried out with SAINT11 software. Empirical absorption correction was applied to the collected reflections with SADABS12 and the space group was determined using XPREP.13 The structure was solved by the direct methods using SHELXTL-9714 and refined on F2 by full-matrix least-squares using the SHELXL-9715 program package. In compound 1, atom O(4) was refined isotropically and all other non-hydrogen atoms were refined anisotropically. In compound 2, atoms C(1) and C(13) were refined isotropically and all other non-hydrogen atoms were refined anisotropically. In compound 1a, atoms O(4), O(8), C(1), C(2), C(5), C(16) were refined isotropically, and all other non-hydrogen atoms were refined anisotropically. In compound 1b, atoms O(2), OW(4), C(16), C(17), C(18) were refined isotropically, and all other non-hydrogen atoms were refined anisotropically. In complex 1c, atoms OW(4), N(1), C(1), C(16), C(17), C(19), and C(20) were refined isotropically, and all other non-hydrogen atoms were refined anisotropically. In compound 10 , atom O(2) was refined isotropically, and all other non-hydrogen atoms were refined anisotropically. The hydrogen atoms of the coordinated water molecules of all compounds were located by difference Fourier synthesis. The hydrogen atoms of the noncoordinated water molecules, except OW(3) in the case of compounds 1, 1a-1c, and 10 could not be located in difference Fourier maps. The H atoms have been refined as follows: the hydrogen atoms attached to carbon atoms were positioned geometrically and treated as riding atoms using SHELXL default parameters. The H atoms of water molecules were located from difference Fourier maps and refined freely keeping the O-H bond distances constrained to ∼0.90 A˚ with the DFIX command. Several DFIX and DANG commands were used to fix the bond distances of solvent molecules. Data collection, lattice parameters, and structure solution parameters are collected in Table 1, while selective bond distances and angles are given in Table S1 (Supporting Information).

Results and Discussion Once isolated, all compounds were found to be stable in air and insoluble in common organic solvents as well as water. Single-crystal studies reveal that the complex 1 crystallizes in the triclinic space group P1 with one lanthanum ion, one cpia3-, two metal bound, and four lattice water molecules in the asymmetric unit. Each of the three-carboxylate groups of the ligand cpia3- binds differently viz. chelating, syn-syn, and chelating and bridging (Figure 1a). Each La(III) ion is surrounded by an O9 coordination environment (Figure 2) provided by two aqua molecules (La-O = 2.573-2.609 A˚) and seven oxygen atoms from two chelating and three bridging cpia3- ligands (La-O = 2.388-2.841 A˚). Two LaO9 polyhedra form an edge-sharing

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dimeric unit that is propagated by the bridging carboxylate along the crystallographic a-axis (Figure 3). These chains are further linked by other carboxylate groups of cpia3- ligands to generate a three-dimensional (3D) framework (Figure 4). There are some reports16 of MOFs built with LaO9 polyhedra with polycarboxylic acids. In 1, the La 3 3 3 La distance along the chain in the direction of the a-axis are alternatively long (La(1) 3 3 3 La(10 ) = 5.155 A˚) and short (La(1) 3 3 3 La(10 ) = 4.435 A˚) (Figure 5). The ligand is not planar; rather, the phenyl rings are twisted at the O atom (O5) at an angle of 116.8°. In order to simplify the complicated connectivity of ligands and metal centers, the network topology of 1 was analyzed by considering the La2 dimeric unit as a node and generated the topology to obtain a (4,8) connected net as shown in Figure 6. In this topology, La(III) is a 8-connected node while ligand’s substituted arene core is a 4-connected node. Instead of a 3-connected node due to the tripodal nature of the ligand, a 4-connection arises because one carboxylate group acts as a bridge between two La(III) centers. The total potential solvent accessible void volume of 1 calculated by PLATON17 is found to be 32.1% of the unit cell volume. Compound 2 crystallizes in the triclinic space group P1 with one La(III), one cpia3-, one coordinated aqua molecule, and two coordinated DMF molecules in the asymmetric unit. The binding modes of carboxylates of ligand cpia3- observed in this polymer are shown in Figure 1b. Unlike in 1, the LaO9

Figure 1. Different coordination modes observed for the cpiaH3 ligand. Color code: carbon, dark gray; hydrogen, light gray; oxygen, red; lanthanum, pink. (a) Chelating, (b) syn-syn, (c) bridging and chelating.

Lama et al.

polyhedron in 2 is formed with two DMF molecules (La-ODMF, 2.522-2.560 A˚), one water molecule (La-O, 2.615 A˚), and six O atoms from two chelating and two bridging cpia3- ligands (La-O, 2.455-2.645 A˚) (Figure 7). Two metal ions with two bridging carboxylate groups form a dimeric unit with a La 3 3 3 La separation of 5.673 A˚. Each of these units connects six different ligand moieties, four DMF molecules, and two water molecules. The 2D framework is further stabilized by O-H 3 3 3 O, C-H 3 3 3 π interactions to generate a 3D structure (Figure 8). In this case, taking the La2 dimeric unit as a node, the topology of 2 was generated which gave rise to a (3,6) connected net (Figure 9). The PXRD patterns of the as-synthesized compounds 1 and 2 are similar to those simulated from the single-crystal structures, showing the phase purity of the bulk sample.18 The thermal stabilities of 1 and 2 were examined by thermogravimetric analysis (TGA). TGA of 1 reveals18 a weight loss of 20.3% (expected = 20.2%) in the temperature range of 80300 °C, corresponding to two coordinated H2O and four H2O molecules residing in the open channels, and no decomposition takes place up to 400 °C. Upon further heating, the obvious weight loss above 400 °C corresponds to the decomposition of 1. Compound 2 shows a continuous weight loss of 27.3% (expected = 27.4%), corresponding to the removal of one coordinated H2O and two coordinated DMF molecules

Figure 3. Diagram showing the projection of polyhedra on the ab plane.

Figure 2. (a) A view showing the coordination environment around the La(III) ion, (b) view of LaO9 polyhedra.

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Figure 4. (a) Three-dimensional packing diagram of 1 view along the crystallographic c-axis. Hydrogen and guest molecules are omitted for clarity, (b) space-filling model of complex 1 on the ac plane without embedded water molecules.

Figure 5. A view showing a metal chain running along the a direction.

Figure 6. View of the 3D topology in 1.

Figure 7. Dimeric unit in the crystal structure of 2 (H atoms have been omitted for clarity).

up to ∼320 °C and decomposes without showing any plateau.18 The IR spectra of all the compounds show18 a

broad peak in the range of 3419-3380 cm-1, indicating the presence of water molecules and showing strong absorption bands between 1390 and 1620 cm-1 that are diagnostic of coordinated carboxylates.19 For 2, the peak at 1650 cm-1 indicates the presence of CdO stretching for metal coordinated DMF molecules.19d,e Single-Crystal to Single-Crystal Transformations. Interestingly, 1 shows a notable ability to partially substitute the lattice water molecules maintaining its single crystallinity when it is immersed in solvents such as ethanol, acetone, and pyridine. To probe SC-SC transformation with different solvent molecules, a mother crystal (1) of suitable size was selected and used throughout at room temperature (RT). When the mother crystal was immersed in ethanol for two days, it transformed into {[La(cpia)(2H2O)] 3 2H2O 3 C2H6O}n (1a). The structural determination of 1a reveals insignificant changes in the lattice parameters and the space group remains unchanged. However, two water molecules out of four present in the lattice have been replaced by a single ethanol molecule with different H-bonding interactions. TGA of 1a shows18 a total weight loss of 21.3% (expected = 21.4%),

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Figure 8. Three-dimensional packing diagram of 2 view along the crystallographic a-axis (dash lines are showing H-bonding and C-H 3 3 3 π between the 2D layers; brown: H-bonding and light green: C-H 3 3 3 π).

Figure 9. Schematic representation of observed (3,6) connected net in 2.

corresponding to two coordinated H2O, one ethanol, and two H2O molecules residing in the channels and no decom-

position up to 400 °C. Similarly, when the mother crystal was immersed in acetone or pyridine for two days two new

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daughter products {[La(cpia)(2H2O)] 3 2H2O 3 C3H6O}n (1b) and {[La(cpia)(2H2O)] 3 2H2O 3 C5H5N}n (1c) are obtained, respectively. The crystal system and space group remain unaltered in both cases. The bond lengths and angles involving the metal ion as well as all bond lengths and angles in the ligand moiety remain almost unaltered. Compound 1b reveals a total weight loss of 23.2% (expected = 23.3%), corresponding to two coordinated H2O, one acetone, and two H2O molecules embedded within the channels of 1b up to 320 °C, after which no weight loss occurs until 400 °C. Compound 1c loses metal bound water and lattice solvent molecules cleanly up to ∼320 °C with a weight loss of 25.8% (expected = 25.7%).18 In 1b, the presence of the CdO stretching peak at 1703 cm-1 in the IR spectrum also confirms the presence of acetone molecules.19e As for the other compounds, 1c decomposes only after 400 °C. When the daughter crystals 1a-1c are immersed in water for 40 h, crystals 1a0 -1c0 (i.e., 10 ) are obtained. X-ray structural analysis reveals that in each case, the original mother crystal (compound 1) is regenerated. Thus, these substitution reactions appear to be reversible (Scheme 1), but exchange of solvent molecules in the lattice is not possible between the daughter crystals, even after three days. It should also be noted here that none of the daughter compounds (1a-1c) can be obtained by direct solvothermal synthesis. During the exchange process, transparency of the single crystal is retained. The possibility of dissolution of 1 in the successive liquids, followed by crystallization or renucleation at the surface and growth of the new phase, is excluded by the photographs taken (Figure 10) of the mother crystal and when it transforms to 1a, 1b, or 1c. No changes in size, morphology, color, or transparency are observed. The 3D architectures of 1a, 1b, and 1c are displayed,18 showing that the ethanol, acetone, and pyridine molecules are packed within the channels, respectively. The PXRD patterns18 of 1 and all the exchanged compounds 1a, 1b, and 1c are almost identical to one another, confirming that the framework remains intact through the exchange processes. It is a wellknown phenomenon that guest solvent molecules located in


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the channels can often be removed from a framework structure without causing framework collapse, and furthermore, they sometimes can be reinserted. To explore this possibility, 1 was heated at 110 °C under a vacuum for 2 h, but the crystal became opaque although the framework remains intact as confirmed by the power X-ray diffraction analysis.3a,18 Gas Adsorption Studies. The gas adsorption properties of the desolvated 1 toward N2 (surface area = 16.3 A˚2, kinetic diameter = 3.64 A˚) at 77 K and CO2 (surface area = 17.9 A˚2, kinetic diameter = 3.3 A˚20) at 273 K were studied in order to determine their textural properties and their potential use for gas separation and storage purposes. At 273 K, sorption experiment of desolvated 1 shows CO2 sorption (Figure 11) without any hysteresis, which accounts for a moderate adsorption capacity of ∼23 cm3 g-1 at p/po 0.030 and a specific Langmuir surface area of ∼147 m2 g-1. To our surprise, no nitrogen diffusion is observed at 77 K (BET surface of

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