Variation of Structures of Coordination Polymers of Ca(II), Sr(II), and Ba(II) with a Tripodal Ligand: Synthesis, Structural, and Gas Adsorption Studies Subhadip Neogi,† Jorge A. R. Navarro,*,‡ and Parimal K. Bharadwaj*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1554–1558
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur UP 208016, India, and Departamento de Quı´mica Inorga´nica, UniVersidad de Granada, AVenue FuentenueVa S/N, 18071 Granada, Spain ReceiVed August 30, 2007; ReVised Manuscript ReceiVed January 15, 2008
ABSTRACT: The podand ligand tris-{2-(4-carboxyphenoxy)ethyl}amine (ptaH3) readily reacts with Ca(II), Sr(II), and Ba(II) nitrates in a water-THF mixture at room temperature to afford {[Ca(ptaH)(H2O)] · 6H2O}n, 1, {[Sr(ptaH)(H2O)3] · 6H2O}n, 2, and {[Ba(ptaH)(H2O)] · H2O}n, 3. The crystallographic studies performed on these systems revealed that their structures are based on infinite chains of interlinked metallocycles which give rise to open frameworks with the pores being filled with hydration water molecules. The XRPD experiments performed after solvent removal upon heating are consistent with framework stability for 1 and a structural change for 2 and 3. The solid-gas adsorption measurements performed on 1 agree with the typical behavior of an ultramicroporous material in which the N2 molecules are not able to diffuse through its porous framework at 77 K; however appreciable amounts of CO2 are readily incorporated at 293 K. By contrast, compounds 2 and 3 do not adsorb either N2 molecules at 77 K or CO2 at 293 K, which is indicative of framework collapse upon solvent removal. Introduction The synthesis of porous metal-organic framework (MOF) structures for sorption of gas molecules has been a subject1–5 of considerable current interest. Overall structures of these coordination polymers, including tunability of the pore size and shape, depend both on the ligand topology and coordination characteristics of the metal ion(s) used. Occasionally, the pores are not empty but contain solvent molecules as guests. Some of the guest molecules may be bound to metal, and the rest are supramolecularly bonded inside the pores. While the MOF normally collapse upon guest removal by heating, a robust structure can withstand the heat treatment with minimum damage to afford a porous material. Porous solids with narrow channels or accessible coordinatively unsaturated metal sites offer excellent systems for gas adsorption because of the unusually high adsorbate-adsorbent interactions.6,7 In this regard, ultramicroporous open frameworks (pore size < 0.5 nm)8 lead to selective gas adsorption processes and eventually ordered adsorbate phases. We have designed a podand ligand9 (ptaH3) containing a carboxylate group at each terminal connected to the bridgehead nitrogen through a long linker. Carboxylate linkages have been found to be able to stabilize network structures against collapse upon guest removal.10 The disposition of the carboxylate groups in the ptaH3 system facilitate the formation of coordination polymers as found with a number of lanthanide as well as transition metal ions.9 So far the transition metal ions have been used extensively to form open-framework structures for gas sorption studies. We have directed our attention toward alkaline earth metals because they bind to carboxylate quite strongly and lighter alkaline earth metal ions can improve the gas-storage properties significantly.11,12 In the present paper, we describe synthesis, structural characterization and gas adsorption proper* To whom correspondence should be addressed. E-mail:
[email protected]. † Indian Institute of Technology Kanpur. ‡ Universidad de Granada.
ties of new porous Ca(II), Sr(II), and Ba(II) coordination polymers using the podand ptaH3. Experimental Section Materials. The metal salts were obtained from Aldrich and were used as received. All other chemicals were procured from S. D. Fine Chemicals, India. All solvents were purified prior to use. Physical Measurements. Spectroscopic data were collected as follows: IR (KBr disk, 400-4000 cm-1), Perkin-Elmer Model 1320; X-ray powder patterns (Cu KR radiation at a scan rate of 3°/min, 293 K), Siefert ISODEBYEFLEX-2002 X-ray generator or PHILIPS PW100 diffractometer; thermogravimetric analysis (heating rate of 5 °C/min under a nitrogen atmosphere), Mettler Toledo Star System. Microanalysis data for the compounds were obtained from CDRI, Lucknow, India. Sorption isotherms were measured in a Micromeritics Tristar 3000 volumetric instrument under continuous adsorption conditions. Prior to measurement, powder samples were heated at 130 °C for 2 h and outgassed to 10-3 Torr using a Micromeritics Flowprep. The specific surface area of the compounds has been obtained by application of the BET (N2 isotherms) and Langmuir (CO2 isotherms) analyses. Synthesis. The ligand tris-{2-(4-carboxyphenoxy)ethyl}amine (ptaH3) was prepared following a method as described previously.9c {[Ca(ptaH)(H2O)] · 6H2O}n, 1. The tripodal ligand ptaH3(0.13 g, 0.25 mmol) was dissolved in a THF (10 mL) solution containing pyridine (2 mL). To this, an aqueous solution of Ca(NO3)2 · 4H2O (0.15 g, 0.38 mmol) was added dropwise, and the reaction mixture stirred for 2 h at RT. The filtrate on slow evaporation at RT, afforded colorless rectangular crystals after 48 h in ∼80% yield. The same compound can be isolated in ∼60% yield when 0.25 mmol of the ligand was reacted with 0.38 mmol of CaCO3 hydrothermally (5 mL of H2O) under an autogenous pressure at 180 °C for 72 h. Anal. Calcd for C27H39NO16Ca: C, 48.14; H, 5.83; N, 2.08%. Found: C, 49.13; H, 5.72; N, 2.09%. {[Sr(ptaH)(H2O)3] · 6H2O}n, 2. Compound 2 was prepared at room temperature, adopting a procedure similar to that described for the synthesis of 1 using Sr(NO3)2 · 4H2O in place of Ca(NO3)2 · 4H2O. The desired product was isolated as colorless needle-shaped crystals after 3 days in ∼75% yield. Compound 2 was also isolated in ∼60% yield when 0.25 mmol of the ligand was allowed to react with 0.38 mmol of SrCO3 hydrothermally (5 mL of H2O) under an autogenous pressure at 180 °C for 72 h. Anal. Calcd for C27H43NO18Sr: C, 42.83; H, 5.72; N, 1.85%. Found: C, 43.93; H, 5.09; N, 1.22%. {[Ba(ptaH)(H2O)] · 2H2O}n, 3. Compound 3 was synthesized as colorless rectangular parallelepiped crystals in ∼60% yield, following
10.1021/cg700822y CCC: $40.75 2008 American Chemical Society Published on Web 04/10/2008
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Table 1. Crystal and Structure Refinement Data for 1-3 empirical formula fw temp radiation, wavelength cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (Mg/m3) µ (mm-1) F(000) reflns collected independent reflns refinement method GOF Final R indices [I > 2σ(I)] R indices (all data)
C27H39NO16Ca (1) 673.67 100(1) K Mo KR, 0.71073 Å triclinic P1j 10.900(5) 12.738(5) 12.821(5) 104.64(5) 114.11(5) 94.67(5) 1537.3(11) 2 1.455 0.282 712 10 313 4953 full-matrix least-squares on F2 1.117 R1 ) 0.083, wR2 ) 0.163 R1 ) 0.125, wR2 ) 0.217
C27H43NO18Sr (2) 757.24 100(1) K Mo KR, 0.71073 Å triclinic P1j 11.329(5) 12.372(5) 13.340(4) 110.228(5) 94.941(4) 103.332(5) 1678.8(12) 2 1.498 1.685 788 25 059 6023 full-matrix least-squares on F2 1.026 R1 ) 0.045, wR2 ) 0.104 R1 ) 0.069, wR2 ) 0.114w
C27H31NO12Ba (3) 698.86 93(1) K Mo KR, 0.71073 Å triclinic P1j 10.053(5) 11.411(4) 13.223(5) 67.482(3) 80.635(5) 86.437(5) 1382.6(11) 2 1.676 1.503 702 9223 3676 full-matrix least-squares on F2 1.321 R1 ) 0.093, wR2 ) 0.174 R1 ) 0.143, wR2 ) 0.218
Scheme 1. Diagram Showing Different Binding Mode of the Podand with the Metal Ions in Three Different Metal-Organic Framework Structures
the method adopted for the synthesis of 1 with Ba(NO3)2instead of Ca(NO3)2 · 4H2O. The same compound can be prepared hydrothermally in 55% yield using BaO and the ligand. Anal. Calcd for C27H31NO12Ba: C, 46.40; H, 4.47; N, 2.00%. Found: C, 45.81; H, 4.88; N, 2.16%. X-ray Structural Studies. Single crystal X-ray data on 1 and 2 were collected at 100 K, while those of 3 were collected at 293 K on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from International Tables for X-ray Crystallography. The data integration and reduction were processed with SAINT13 software. An empirical absorption correction was applied to the collected reflections with SADABS14 using XPREP.15 The structure was solved by the direct method using SHELXTL16 and refined on F2 by full-matrix least-squares technique using the SHELXL97 program package. Non-hydrogen atoms were refined anisotropically. All hydrogen atoms were located in successive difference Fourier maps, and they were treated as riding atoms using SHELXL default parameters. The crystal and refinement data are collected in Table 1.
Results and Discussion The compounds once isolated are found to be stable in air and not soluble in water and common organic solvents (acetonitrile, DMF, etc.), which points to polymeric nature of the materials. High yields of the products indicate that they are thermodynamically stable under the prevailing reaction conditions. The IR spectra of all the three compounds show strong absorption bands between 1350 and 1550 cm-1 that are diagnostic17 of coordinated carboxylates. The asymmetric unit of 1 consists of one Ca(II) ion, one ptaH2- ligand, and seven H2O molecules. Each metal ion is bonded to four carboxylates from four different partially deprotonated ligand units, while each ligand entity is bound to four Ca(II) showing different binding modes (Scheme 1). The
H atom attached to the protonated carboxylate could not be located in the difference map. The coordination environment about the Ca(II) ions is distorted octahedral geometry with ligation from five carboxylate O atoms and one H2O molecule (Ow1). A wide variation is observed in the Ca-O(carboxylate) bond distances,18 although they are within the range observed10 in other octahedral Ca(II)-carboxylate structures. One of the carboxylate groups of the podand exhibits a bis-bridging coordination mode with Ca · · · Ca separations of 3.84(8) Å. The MOF propagates in threedimension via bridgehead N atom of the podand. Here, two Ca(II) ions are separated by either 10.09 or 11.17 Å via two different arms of ptaH2-, while the distance between the bridgehead N atoms is 17.91 Å showing the different types of cavities occupied by hexameric or octameric water clusters (Figure 1). The binding of all the available carboxylate groups to Ca(II) ions makes the structure quite robust (Figure 2). Thermogravimetric analysis of 1 shows that water removal starts above 60 °C, and a loss of 18%, corresponding to all the water molecules (calcd 18.72%) takes place above 150 °C. The decomposition of 1 is achieved only above ∼250 °C. The FTIR spectrum of 1shows a broadband centered around 3400 cm-1, attributable19 to the O-H stretching frequency of the water cluster. This broadband vanishes upon heating the compounds at 120 °C for 3 h under low pressure (0.1 mm), suggesting escape of the water molecules from the lattice. This also makes one vacant coordination site on the metal. However, deliberate exposure of a dehydrated sample of 1 does not lead to readsorption of water as monitored by IR spectroscopy. The X-ray powder patterns of 1 before and after water removal show good agreement of the peaks (Figure 3), demonstrating that the porous
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Neogi et al.
Figure 1. Perspective view of the structure of 1 where two types of water clusters occupy the voids. O atoms belonging to water clusters are green.
Figure 3. PXRD curves showing the presence of permanent microporasity in 1: (a) simulated and (b) experimental (blue, as synthesized; red, after desolvation).
Figure 2. View of the overall structure of 1 without water clusters. All H atoms are omitted for clarity.
network is preserved. Calculation of the solvent-accessible volume20 reveals that 1 contains ∼17.5% void volume. It should be noted, however, that the XRPD traces slightly shift to higher 2θ angles and broaden upon dehydration, which should be indicative of a lower crystallinity, and that the available void volume for the evacuated material should be lower than the one calculated for hydrated starting material. The structure of 2 is a 2D coordination polymer in which each Sr(II) is bonded to three carboxylates from three different partially deprotonated ptaH2- units and three water molecules forming monocapped octahedral coordination geometry. The binding mode of the ligand to Sr(II) is illustrated in Scheme 1. Each ligand adopts a bent molecular structure and joins two Sr(II) centers using the long aromatic linker with internal distance of either 11.32 or 12.61 Å. Unlike in 1, only two strands of ptaH2- are coordinated to metals, with one bridging carboxylate (Sr · · · Sr ) 4.08(5) Å), and they propagate the 1D polymeric structure along the a axis. The third strand, where the carboxylate remains protonated, is H-bonded to a metalbound water molecule (Ow8); the latter is also H-bonded to one lattice-water molecule (Ow9), forming a water dimer that
Figure 4. View of the hydrogen-bonded metallocycles in 2 extending along the crystallographic a axis. Only those water molecules that are involved in interchain H-bonding are shown.
connects (Figure S12) adjacent metallocyclic chain to form overall 2D structure. However, the carboxylate proton attached to this strand could not be located in the difference map. The metal-bound water molecules (Ow5 and Ow7) are supramolecularly bonded to five other lattice-water molecules, forming a discrete (H2O)7 cluster that occupies the cavity (Figure 4) with strong water-water and water-MOF interactions.18 Thermogravimetric analysis of 2 shows that weight loss begins at 50 °C, and a loss of 22% corresponding to all of the water (calculated 21.41%) takes place above 140 °C . Complete decomposition is achieved beyond 270 °C. The band for the O-H stretching frequency18 pertaining to the water cluster appears around 3400 cm-1 in the FT-IR spectrum and vanishes upon heating of the compound at 120 °C under 0.1 mm pressure for 3 h because of the escape of the water molecules from the lattice. However unlike 1, the powder X-ray diffraction patterns of 2before and after water removal show significant differences
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Figure 5. Perspective view of the continuous metallocycles in 3. Only the metal-bound water molecule is shown.
(Figure S7), suggesting collapse of the structure upon water removal. Readsorption of water does not take place when a dehydrated sample of 2 is exposed to water vapor for several days. The H-bonded 3D structure of 2 gives about 37.8% as the solvent-accessible void volume. In 3, Ba(II) exhibits tetra-coordination with ligation from carboxylate O of the ligand (Scheme 1) and water molecules. As in the two previous cases, the H atom attached to one of the protonated carboxylates in ptaH2-could not be located in the difference map. None of the carboxylate group binds the large Ba(II) ion in the µ2-mode, and unlike 1 and 2, there is no bridging carboxylate in 3. The overall structure looks like an array of metallocycles extending approximately along the crystallographic c axis (Figure 5). The Ba-O(carboxylate) bond distances span the range of 1.873(6)-1.957(2) Å, and the Ba-O(H2O) bond distance is 2.026(3) Å. Both distances are slightly longer than the literature values.12 The other O atoms (O5, O8) are quite far away and are not considered to be bonded to Ba(II). The lone metal-bound water molecule exhibits O-H stretching as a broadband centered around 3500 cm-1 that vanishes upon heating of the compound at low pressure (0.1 mm) at 120 °C for 2 h. Thermogravimetric analysis of 3 shows that weight loss begins at 50 °C and is completed by 100 °C, corresponding to one water molecule. Complete decomposition takes place only after 250 °C. XRPD studies show that the framework collapses upon water removal. Gas Adsorption Measurements. The gas adsorption properties of anhydrous 1, 2, and 3 toward N2 at 77 K and CO2 at 293 K have been studied to determine their textural properties and their potential use for gas separation and storage purposes.21 The N2 adsorption isotherm of material 1 is indicative of adsorption taking place at the external surface of the particles only (BET surface of 8 m2 g-1). However, this material is able to adsorb appreciable amounts of CO2 at room temperature, exhibiting a type I sorption isotherm, which accounts for a specific Langmuir surface area of 110 m2 g-1. The different behavior of CO2 adsorption at 293 K compared to N2 at 77 K can be related to the typical behavior of a material with small pore size opening, in the range of ultramicropores (
