A Family of Flexible Lanthanide Bipyridinedicarboxylate Metal

Article pubs.acs.org/crystal

A Family of Flexible Lanthanide Bipyridinedicarboxylate Metal− Organic Frameworks Showing Reversible Single-Crystal to SingleCrystal Transformations Mikaela Gustafsson,*,† Jie Su,† Huijuan Yue,‡ Qingxia Yao,† and Xiaodong Zou*,† †

Berzelii Centre EXSELENT on Porous Materials and Inorganic and Structural Chemistry, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden ‡ State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 130012, Changchun, China S Supporting Information *

ABSTRACT: A family of flexible lanthanide metal−organic frameworks, [Ln2(bpydc)3(H2O)3]·nDMF (denoted as SUMOF-6Ln; Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er, H2bpydc =2,2′-bipyridine-5,5′-dicarboxylic acid), was synthesized and characterized. SUMOF-6-Ln has a monoclinic space group P21/c. The three-dimensional framework contains chains of LnOn (n = 7−8) polyhedra connected through the bpydc linkers forming 1D rhombic channels along the c-axis. SUMOF-6-Ln showed reversible breathing phenomenon upon desorption/adsorption of the solvent, with up to 27% changes of the unit cell dimensions and 23% changes of the unit cell volume. Single crystal X-ray diffraction (XRD) revealed that the desolvation and resolvation of SUMOF-6-Ln occurred via single-crystal to single-crystal transformations. The thermal behavior of SUMOF-6-Sm was also examined. SUMOF-6-Eu and SUMOF-6-Tb showed solid-state luminescent properties.

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INTRODUCTION Among the vast group of porous materials, metal−organic frameworks (MOFs) or coordination polymers have recently gained large interests due to their exceptionally high surface areas and tunable pore size and functionality.1 One unique feature of MOFs is their structure flexibility, that is, the ability to adjust their structures to its surrounding environment without changing the coordination and topology. Many MOF structures remained their porosity and long-range ordering under an external stimulus, such as temperature, pressure, a gas or a liquid,2 although most MOF structures collapsed. A MOF can response upon an external stimulus and undergo structural changes. The pores or channels can either shrink or swell when subject to removal or uptake of guest molecules.3 It is important to study the structural response of a MOF to an external stimulus in order to explore potential applications of the material, for example as sensors, gas storage and separation, and heterogeneous catalysis. It is therefore desired to detect and study structural transformations in MOF materials during gas adsorption or catalytic reactions. Numerous examples of © 2012 American Chemical Society

single-crystal to single-crystal transformations were reported in MOF materials.2c−e,4 One of our ongoing research projects is to construct MOFs using 2,2′-bipyridine-5,5′-dicarboxylic acid (bpydc) as a linker. Postsynthesis modifications could then be performed at the bipyridyl groups of the linker in the MOF. There are a few examples of MOFs constructed by the bpydc linker; [Nd2(bpydc)3(H2O)4],5 [La2(bpydc)3(H2O)4]·4H2O,6 [M(bpydc)(H2O)·H2O] (M = Zn, Co, and Ni)7 and [(bpydc)PtCl2]3(Ln(H2O)3)2·5H2O (Ln:Gd and Y).8 Other related lanthanide-based MOFs, built by biphenyldicarboxylate (bpdc) linker, [M(bpdc)1.5(H2O)·0.5DMF] (M = Tb, Ho, Er and Y), have also been reported.9 Here, we present the synthesis, structure and breathing behavior of a family of new homeotypic lanthanide-based MOFs, [Ln2(bpydc)3(H2O)3]·nDMF (SUMOF-6-Ln; Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er). To the best of our Received: March 21, 2012 Revised: May 8, 2012 Published: May 9, 2012 3243

dx.doi.org/10.1021/cg300379u | Cryst. Growth Des. 2012, 12, 3243−3249

Crystal Growth & Design

Article

Table 1. Crystallographic Data and Structure Refinement Details for SUMOF-6-Sm(as), SUMOF-6-Sm(dry), SUMOF-6-Gd(as), and SUMOF-6-Gd(dry) SUMOF-6-Sm(as) formula formula weight T (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) μ (mm−1) F(000) crystal size (mm3) crystal color θ range (deg) index ranges completeness reflns collected reflns unique reflns [I > 2σ(I)] Rint goodness-of-fit final R indices[I > 2σ(I)] R indices (all data)

SUMOF-6-Sm(dry)

SUMOF-6-Gd(dry)

C36H18Sm2N6O15 1075.26 293(2) monoclinic P21/c 27.0951(8) 11.3510(8) 17.0184(6) 90 98.218(3) 90 5180.4(4) 4 1.379 2.303 2080 0.20 × 0.12 × 0.08 colorless 3.19−26.37 −33 ≤ h ≤ 32, −14 ≤ k ≤ 7, −21 ≤ l ≤ 21 0.996 19322 10542 4618 0.0729 0.976 R1 = 0.0676, R2 = 0.1466

C36H18Gd2N6O15 1089.06 293(2) monoclinic P21/c 25.6487(14) 15.1875(19) 17.0378(12) 90 97.910(6) 90 6573.8(10) 4 1.100 2.046 2096 0.18 × 0.16 × 0.13 colorless 4.05−24.41 −29 ≤ h ≤ 29, −17 ≤ k ≤ 17, −18 ≤ l ≤ 19 0.967 69801 10473 5228 0.2269 0.972 R1 = 0.0697, R2 = 0.1487

C36H18Gd2N6O15 1089.06 293(2) monoclinic P21/c 27.0154(19) 11.5213(18) 16.995(3) 90 98.137(10) 90 5236.4(12) 4 1.381 2.569 2096 0.12 × 0.08 × 0.04 colorless 3.97−26.37 −33 ≤ h ≤ 33, −14 ≤ k ≤ 13, −21 ≤ l ≤ 21 0.987 65016 10498 4915 0.1798 1.006 R1 = 0.0846, R2 = 0.1900

R1 = 0.1812, R2 = 0.1857

R1 = 0.1513, R2 = 0.1620

R1 = 0.1561, R2 = 0.1698

R1 = 0.1800, R2 = 0.2190

Characterization. X-ray powder diffraction (XRPD) of SUMOF6-Ln (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) were performed on a PANalytical X'Pert PRO diffractometer equipped with a Pixel detector and using Cu Kα1 radiation (λ = 1.5406 Å). Fourier transform infrared (FT-IR) spectroscopy was performed on a Varian 670-IR spectrometer. A FT-IR spectrum of SUMOF-6-Sm(dry) is shown in Figure S2, Supporting Information. Thermogravimetric analysis (TGA) was performed under nitrogen flow between room temperature and 550 °C with a heating rate of 2 °C/min using a highresolution thermogravimetric analyzer (PERKIN ELMER TGA 7). Fluorescence spectroscopy data were recorded on a Varian Cary Eclipse Fluorescence spectrophotometer. Elemental analyses of C, H, and N were performed on a Carlo Erba Flash 1112 elemental analyzer at the Medac Ltd., Chobham, U.K. The content of guest molecules in SUMOF-6-Sm that was exposed to air (denoted as SUMOF-6Sm(dry)) was determined by TGA and elemental CHN analysis. The chemical formula deduced from the elemental analysis is [Sm2(bpydc)3(H2O)3]·2.1DMF for SUMOF-6-Sm(dry) (found (wt %) C 41.51, H 3.10, N 9.31; calcd (wt %) C 41.10, H 3.16, N 9.19). Single-Crystal X-ray Diffraction. Single-crystal X-ray diffraction data of the as-synthesized SUMOF-6-Sm (SUMOF-6-Sm(as)), SUMOF-6-Sm(dry), the as-synthesized SUMOF-6-Gd (SUMOF-6Gd(as)) and SUMOF-6-Gd exposed to air (SUMOF-6-Gd(dry)) were recorded at room temperature on an Oxford Diffraction Xcalibur 3 diffractometer, with Mo Kα radiation (λ = 0.71073 Å). The rodshaped crystals of SUMOF-6-Sm(as) and SUMOF-6-Gd(as) were selected and mounted in a capillary (0.5 mm) with a small amount of mother liquid to prevent solvent loss from the crystal during data collection. Data reduction and empirical absorption correction were performed using the CrysAlisPro program. The structures were solved by direct methods using SHELXS-97 program.10 All non-hydrogen atoms could be located directly from the difference Fourier maps.

knowledge, the structure of SUMOF-6-Ln has not been reported. The single-crystal to single-crystal transformations of SUMOF-6-Ln upon desolvation and resolvation were studied. The solid-state luminescent properties of the SUMOF-6-Eu and SUMOF-6-Tb are also investigated.

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SUMOF-6-Gd(as)

C36H18Sm2N6O15 1075.26 293(2) monoclinic P21/c 25.5273(15) 15.5117(14) 17.1005(13) 90 98.298(5) 90 6700.4(9) 4 1.066 1.781 2080 0.17 × 0.16 × 0.06 colorless 3.14−26.37 −17 ≤ h ≤ 31, −17 ≤ k ≤ 19, −21 ≤ l ≤ 19 0.998 27893 13675 4653 0.1187 0.961 R1 = 0.0726, R2 = 0.1674

EXPERIMENTAL SECTION

Synthesis of SUMOF-6-Ln MOFs (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er). All chemicals were purchased from Aldrich and used without further purification. The SUMOF-6-Ln MOFs, with a general chemical formula [Ln2(bpydc)3(H2O)3]·guest, were synthesized by solvothermal synthesis from a mixture of lanthanide nitrate salt, 2,2′bipyridine-5,5′-dicarboxylic acid (H2bpydc) and dimethylformamide (DMF) at 120 °C for one day. A typical synthesis procedure of SUMOF-6-Sm is as follows. A mixture of Sm(NO3)3·6H2O (0.122 g, 0.27 mmol), H2bpydc (0.100 g, 0.41 mmol) and DMF (10 mL) was stirred at room temperature for one hour. The mixture was then placed in an autoclave and heated in a preheated oven at 120 °C for one day. The molar ratio of Ln/H2bpydc/DMF was 2:3:957. Rodshaped crystals (Figure S1, Supporting Information) were collected and washed with DMF. The crystals were sensitive to air, started to crack when exposed in air, and were therefore kept in DMF. Other SUMOF-6-Ln (Ln = Nd, Eu, Gd, Tb, Dy, Ho, and Er) MOFs were prepared by the same procedure as described for SUMOF-6-Sm, except for the different lanthanide sources; Nd(NO3)3·6H2O, Eu(NO 3 ) 3 ·5H 2 O, Gd(NO 3 ) 3 ·6H 2 O, Tb(NO 3 ) 3 ·6H 2 O, Dy(NO3)3·5H2O, Ho(NO3)3·5H2O, and Er(NO3)3·5H2O. Optimization of various synthesis parameters was performed for SUMOF-6-Sm. The effects of synthesis temperature (85, 120, 140, and 160 °C), heating time and solvent type (DMF, DEF, DMF/heptane, and DMF/THF) on the crystallinity of SUMOF-6-Sm were studied. 3244



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Framework hydrogen atoms were placed geometrically and constrained using the riding model to the parent atoms. Final structure refinement was done using the SHELXL-97 program by minimizing the sum of squared deviations of F2 using a full-matrix technique. Because of the large pore sizes and possible disorders of the guest DMF and H2O molecules, it was not possible to locate the guest molecules in the channels. The PLATON/SQUEEZE program11 was used to remove the scattering contribution from the disordered guest molecules and to produce solvent-free diffraction intensities, which were used in the final structure refinement. Because of the large fractional pore volume, the intensity and resolution of the diffraction data were rather poor as indicated by the low I/σ, high Rint-value and low 2θ angles. These also resulted in relatively high R-values in the final refinements. Crystallographic data and details of the structure refinements are presented in Table 1.

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RESULTS AND DISCUSSION Synthesis of SUMOF-6-Ln. XRPD patterns of the assynthesized SUMOF-6-Ln (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) MOFs synthesized at 120 °C are shown in Figure 1a. The peaks appear at similar 2θ angles for the different SUMOF6-Ln MOFs, indicating that the SUMOF-6-Ln MOFs are homeotypic. At a lower synthesis temperature (85 °C), no solid was formed. This was because H2bpydc did not dissolve at this temperature. At higher synthesis temperatures (140 and 160 °C), the crystallinity of SUMOF-6-Sm became poorer. The crystallinity of SUMOF-6-Sm also decreased when other solvents than DMF, such as DEF, DMF/heptane and DMF/ THF were used. The best SUMOF-6-Sm crystals with the sharpest diffraction peaks in the XRPD patterns were obtained in DMF at 120 °C for one day. Longer synthesis time did not improve the crystallinity of SUMOF-6-Sm. Structural Description and Structural Transformation. The as-synthesized SUMOF-6-Sm(as) crystallizes in a monoclinic space group P21/c with the unit cell a = 25.5273(15) Å, b = 15.5117(14) Å, c = 17.1005(13) Å, β = 98.298(5)°, and V = 6700.4(9) Å3. The asymmetric unit of SUMOF-6-Sm(as) contains two Sm ions, three bpydc ions and three water molecules coordinated to the Sm ions. The coordination of the two Sm atoms is illustrated in Figure 2a. Sm1 is sevencoordinated to six oxygen atoms from six different bpydc linkers and one water molecule, and forms a monocapped trigonal prism. Sm2 is eight-coordinated forming a tetragonal antiprism and binds to six oxygen atoms from six different bpydc linkers and two oxygen atoms from water or DMF molecules. Each carboxylate group connects two different Sm ions. The SmOn polyhedra are connected through alternating two and four carboxyl groups forming a 1D inorganic chain along the [001] direction. The 1D inorganic chains are linked by bpydc groups in the [110] and [1−10] directions to form a 3D framework (Figure 2b and c) containing 1D channels (Figure 3a). The free diameter of the channel is about 19.0 × 5.3 Å. The total solvent-accessible volume of the framework was estimated to be 52.6% using the PLATON/VOID routine. Single crystals SUMOF-6-Sm(dry) were obtained by exposing the as-synthesized crystals of SUMOF-6-Sm(as) to air. Single crystal X-ray diffraction revealed that SUMOF-6Sm(dry) has the same space group P21/c as SUMOF-6-Sm(as), but with different unit cell parameters a = 27.0951(8) Å, b = 11.3510(8) Å, c = 17.0184(6) Å, β = 98.218(3)°, and V = 5180.4(4) Å3. The transformation from SUMOF-6-Sm(as) to SUMOF-6-Sm(dry) was very fast and occurred within a few minutes. SUMOF-6-Sm(dry) showed the same topology with

Figure 1. X-ray powder diffraction (XRPD) patterns of SUMOF-6-Ln (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) MOFs synthesized at 120 °C (λ = 0.71073 Å). (a) As-synthesized SUMOF-6-Ln, (b) dried SUMOF-6-Ln, and (c) dried SUMOF-6-Ln soaked in DMF overnight.

the same coordination and connectivity as SUMOF-6-Sm(as). Sm1 was seven-coordinated while Sm2 was eight-coordinated with a tetragonal antiprism coordination (Table S1, Supporting Information). As given in Supporting Information Table S1, the Sm− O(carboxylate) bond lengths in SUMOF-6-Sm(as) range from 2.337(9) to 2.438(9) Å. The Sm−O(water) bond lengths are longer than those of the Sm−O(carboxylate), within the range of 2.558(10) to 2.607(10) Å. The bond lengths of Sm− O(carboxylate) in SUMOF-6-Sm(dry) are in the range of 3245



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Figure 2. Structure of SUMOF-6-Sm(as) showing (a) the coordination of two symmetry-independent Sm ions and (b−c) the three-dimensional framework viewed along (b) [100] and (c) [010]. Chains of SmOn polyhedra run along the [001] direction and are connected through bpydc linkers along [110] and [1−10] directions, respectively. SmOn polyhedra are shown in purple; oxygen, carbon and nitrogen atoms in red, black, and blue, respectively. Hydrogen atoms and guest molecules are omitted for clarity.

Figure 3. Comparison between the structures of SUMOF-6-Sm(as) and SUMOF-6-Sm(dry) viewed along the [001] direction. The shrinkage of the 1D channels upon solvent removal caused a large shortening of the b-axis and a slight elongation of the a-axis. The unit cell volume decreased by 23%. SmOn polyhedra are shown in purple; oxygen, carbon and nitrogen atoms in red, black and blue, respectively. Hydrogen atoms and guest molecules are omitted for clarity.

SmOn chains along the c-axis are rigid. Consequently, the unit cell volume decreased significantly, by 23% from 6700.4(9) to 5180.4(4) Å3. The free diameters of the channels were changed from 19.0 × 5.3 to 20.8 × 1.4 Å. The total solvent-accessible volume of the framework also decreased remarkably, from 52.6% to 38.0% after removal of the guest solvents, as estimated using the PLATON/VOID routine. Single-crystal X-ray diffraction showed that SUMOF-6Gd(as) and SUMOF-6-Gd(dry) are homeotypic with SUMOF-6-Sm(as) and SUMOF-6-Sm(dry), respectively. SUMOF-6-Gd(as) underwent similar structural transformation as SUMOF-6-Sm(as) upon the removal of guest molecules (Table 1), with the largest change of the b parameter from 15.1875(19) to 11.5213(18) Å (24%), followed by the change of the a parameter (5%, from 25.6487(14) to 27.0154(19) Å). The Gd−O bond lengths of SUMOF-6-Gd(as) and SUMOF-6Gd(dry) are slightly shorter than the Sm−O bond lengths, as listed in Table S1 (Supporting Information). This is due to the

2.315(8) to 2.458(7) Å, similar to those in SUMOF-6-Sm(as). The Sm−O(water) bond lengths in SUMOF-6-Sm(dry) are also longer than those of the Sm−O(carboxylate), in the range of 2.528(7)−2.739(11) Å. It is worth noting that one Sm− O(water) bond length (Sm2−O12) increased significantly when SUMOF-6-Sm(as) was exposed to air, from 2.607(10)− 2.739(11) Å, indicating that the coordination bond between Sm2 and the water molecule O12 was weakened. The three-dimensional framework of SUMOF-6-Sm underwent a structural transformation upon the removal of the guest molecules in the pores (Table 1 and Figure 3). The unit cell parameters a and b delimiting the channel dimensions changed significantly, from 25.5273(15) to 27.0951(8) Å for the a parameter and 15.5117(14) to 11.3510(8) Å for the b parameter, with the largest change occurred for the b parameter (27%). The c parameter, on the other hand, remained almost the same, from 17.1005(13) to 17.0184(6) Å. This indicates that the lattice flexibility is mainly in the ab-plane and the 1D 3246



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smaller ionic radius of the Gd(III) ion compared to that of the Sm(III) ion. The total solvent-accessible volume of the framework was estimated to be 53.0% for SUMOF-6-Gd(as) and 39.9% for SUMOF-6-Gd(dry). Crystallographic information of SUMOF-6-Sm(as), SUMOF-6-Sm(dry), SUMOF-6Gd(as), and SUMOF-6-Gd(dry) obtained from single crystal X-ray diffraction is presented in Table 1. To compare the shrinkage of the framework due to the removal of guest molecules, we adopt the lozenge structural representation and calculation method using the formula tgα = b/(a·sin β), as described by Férey and co-workers.12 The edges of lozenge represent the distances between two inorganic chains through the bpydc linkers. The angle 2α describes the angle between two bpydc linkers, which can be determined from the unit cell parameters. The change in angle α symbolizes the framework deformation upon removal of guest molecules. When DMF and water molecules have occupied the 1D channels, the framework is fully expanded with a large α value (31.5° for SUMOF-6-Sm(as) and 30.9° for SUMOF-6Gd(as)). After the sample being exposed to air for a few minutes, the solvents in the channels were evaporated and a lower α value (22.8° for SUMOF-6-Sm(dry) and 23.3° for SUMOF-6-Gd(dry)) was obtained. Both the pores and void volumes of SUMOF-6-Ln shrank dramatically when dried in air. Structural Reversibility. As a single-crystal to single-crystal transformation of SUMOF-6-Sm(as) to SUMOF-6-Sm(dry) was observed upon desolvation, we were interested in whether the transformation was reversible, i.e. SUMOF-6-Sm(dry) transforms back to the original structure of SUMOF-6-Sm(as) after readsorbing the solvent molecules. The as-synthesized SUMOF-6-Sm was first dried in air at room temperature (SUMOF-6-Sm(dry)), then at 50 °C (SUMOF-6-Sm(50C)) or 200 °C (SUMOF-6-Sm(200C)) for three hours under nitrogen atmosphere, and finally soaked in DMF overnight to SUMOF6-Sm(50C dmf) or SUMOF-6-Sm(200C dmf). The samples were studied by XRPD, and compared with the simulated XRPD patterns, as shown in Figure 4. The 110 and 200 peaks in the XRPD patterns simulated from the refined structures of SUMOF-6-Sm(as) and SUMOF-6-Sm(dry) show significant shifts in 2θ from each other. The peak positions in the XRPD patterns of SUMOF-6-Sm dried in air and at 50 °C are very similar, and agree very well to those in the XRPD pattern simulated from the structure model of SUMOF-6-Sm(dry) (Figure 4). This indicates that the unit cell parameters of SUMOF-6-Sm(50C) is the same as those of SUMOF-6-Sm(dry). However, the relative peak intensities changed for SUMOF-6-Sm(dry) and SUMOF-6-Sm(50C). This change can be due to minor loss of the solvent. The peak intensity distribution in the XRPD pattern of SUMOF-6Sm(50C) is more similar to that of the XRPD pattern simulated from the structure model of SUMOF-6-Sm(dry) (Figure 4). The sample SUMOF-6-Sm(50C) was transformed to SUMOF-6-Sm(50C dmf) after being kept in DMF overnight. The peak positions were shifted and the XRPD pattern resembles the simulated XRPD pattern of SUMOF-6Sm(as) (Figure 4). This indicates that the structure of SUMOF-6-Sm(dry) changed back to its original structure of SUMOF-6-Sm(as), and the structural transformation between SUMOF-6-Sm(as) and SUMOF-6-Sm(dry) was reversible upon desolvation and resolvation. XRPD from the sample SUMOF-6-Sm(200C) showed that the structure decomposed and the sample became amorphous. The structure of the

Figure 4. XRPD patterns of simulated SUMOF-6-Sm(as) (in black, from the refined SUMOF-6-Sm(as) structure), simulated SUMOF-6Sm(dry) (in purple, from the refined SUMOF-6-Sm(dry) structure), experimental SUMOF-6-Sm(dry) (in blue), SUMOF-6-Sm(50C) (in green) and SUMOF-6-Sm(50C dmf) (in red, SUMOF-6-Sm(50C) soaked in DMF overnight). After the solvent loss, the 110 peak moved to a higher 2θ angle, from 6.68° for SUMOF-6-Sm(as) to 8.45° for SUMOF-6-Sm(dry) and SUMOF-6-Sm(50C), while the 200 peak moved to a slightly lower angle, from 6.99° to 6.58°. After the solvent uptake, the 110 and 200 peaks of SUMOF-6-Sm(50C dmf) returned back to the original positions, 6.60° and 6.93°, respectively.

SUMOF-6-Sm(as) could not be recovered after the sample SUMOF-6-Sm(200C) had been soaked in DMF overnight. The Sm and carboxylate coordination bonds were broken at this high temperature and could not be reformed when the sample was soaked in DMF. To check whether the reversible structural transformation was via single-crystal to single-crystal, single crystal X-ray diffraction experiments were performed. After being exposed in air for a few minutes, a SUMOF-6-Sm(as) single crystal transformed to a SUMOF-6-Sm(dry) single crystal. We noticed that the crystals cracked when dried in air with a quick loss of the solvent. For collecting high quality single crystal X-ray diffraction data, the crystal was placed in a capillary together with some DMF solvent to slow down the desolvation process. The desolvated SUMOF-6-Sm(dry) single crystal transformed back to a SUMOF-6-Sm(resolvated) single crystal after being kept in DMF at room temperature for one week. Single crystal X-ray diffraction confirmed that the framework structure of SUMOF-6-Sm(resolvated) was the same as that of the assynthesized sample SUMOF-6-Sm(as). XRPD showed that the other SUMOF-6-Ln compounds have the same breathing behavior during the desolvation and resolvation (Figure 1); the unit cell shrank upon desolvation and recovered after resolvation. Thermal Analysis. The TG curve of SUMOF-6-Sm(dry) (Figure 5) indicates that about two uncoordinated DMF molecules were released between room temperature and 200 °C. The three coordinated water molecules were removed from the Sm(III) ions between 200 and 400 °C. The bpydc linkers started to release at 400 °C. Photoluminescence Study. The solid-state luminescent properties of SUMOF-6-Eu and SUMOF-6-Tb were investigated at room temperature. As shown in Figure 6a, upon excitation at 327 nm, SUMOF-6-Eu exhibits four typical emission bands from 570 to 710 nm, which are assigned to the 5 D0 → 7FJ (J = 1, 2, 3, 4) transitions of the Eu(III) ions, respectively. The most intense emission at 613 nm is attributed to the electric dipole induced 5D0 → 7F2 transition, which is hypersensitive to the coordination environment of the Eu(III) 3247



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→ 7F2 is the strongest. In the emission spectrum of SUMOF-6Eu, the 5D0 → 7F2 is much stronger than the 5D0 → 7F1, indicating that the Eu(III) ion is not located at the inversion center and with a low coordination symmetry, which is in agreement with the structure of SUMOF-6-Eu. For SUMOF-6Tb, the emission spectrum excited at 333 nm exhibits several peaks between 450 and 675 nm (Figure 6b), which are associated with the 5D0 → 7FJ (J = 6, 5, 4, 3) transitions of Tb(III) ions. The most intense emission is located at the 5D0 → 7F5 transition (545 nm). These transitions are in good agreement with earlier reported luminescence studies of terbium-based MOFs.13

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CONCLUSIONS A family of flexible lanthanide metal−organic frameworks, [Ln2(bpydc)3(H2O)3]·nDMF (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er), was synthesized. The three-dimensional framework contains chains of LnOn (n = 7, 8) polyhedra connected through the bpydc linkers forming 1D rhombic channels along the c-axis. X-ray powder diffraction (XRPD) showed that the structure of SUMOF-6-Ln (Ln = Sm and Gd) MOFs was flexible and could “breath” upon desorption/adsorption of the solvent, with a change of the unit cell volume by up to 23%. XRPD showed that the desolvation and resolvation process of SUMOF-6-Sm was reversible. Single crystal X-ray diffraction revealed that the desolvation and resolvation of SUMOF-6-Sm occurred via single-crystal to single-crystal transformation. SUMOF-6-Eu and SUMOF-6-Tb also showed solid-state luminescent properties.

Figure 5. TG curve of SUMOF-6-Sm(dry) showing the type and number of molecules released at different temperatures, which correspond well to the chemical formula deduced from the structure refinement and CHN analysis. First, two DMF molecules were lost between room temperature and 200 °C, followed by the release of the three coordinated H2O molecules up to 400 °C. The H2bpydc linkers were lost between 400 and 500 °C, resulting in Sm2O3.

ion. It is well-known that the intensity ratio of 5D0 → 7F2 to 5D0 → 7F1 is strongly dependent on the local symmetry of the Eu(III) ions. In a site with inversion symmetry the 5D0 → 7F1 is dominating, while in a site without inversion symmetry the 5D0

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ASSOCIATED CONTENT

S Supporting Information *

Selected bond lengths of SUMOF-6-Sm(as), SUMOF-6Sm(dry), SUMOF-6-Gd(as), and SUMOF-6-Gd(dry), optical microscopy images and FT-IR spectrum of SUMOF-6-Sm(as). X-ray crystallographic information files (CIF) are available for SUMOF-6-Sm(as), SUMOF-6-Sm(dry), SUMOF-6-Gd(as) and SUMOF-6-Gd(dry) (CCDC numbers: 871112−871115). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +46 8 16 23 89 (X.Z.). Fax: +46 8 15 21 87 (X.Z.). Email: [email protected] (X.Z.). Web: http://www.mmk.su.se/ ∼zou (X.Z.). Email: [email protected] (M.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the Swedish Research Council (VR), the Swedish Governmental Agency for Innovation Systems (VINNOVA) and the Göran-Gustafsson Foundation for Nature Sciences and Medical Research. Dr. Charlotte Bonneau is acknowledged for her initiating work in the project. Dr. Jie Su thanks the Wenner-gren Foundation for a postdoctoral grant.

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REFERENCES

(1) (a) Kitagawa, S.; Kitaura, R.; Noro, S.-I. Angew. Chem., Int. Ed. 2004, 43, 2334−2375. (b) Rowsell, J. L. C.; Yaghi, O. M. Microporous

Figure 6. Photoluminescence spectra of (a) SUMOF-6-Eu excited at 327 nm and (b) SUMOF-6-Tb excited at 333 nm. 3248



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Mesoporous Mater. 2004, 73, 3−14. (c) Ferey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F. Acc. Chem. Res. 2005, 38, 217−225. (2) (a) Férey, G. Chem. Soc. Rev. 2008, 37, 191−214. (b) Uemura, K.; Matsuda, R.; Kitagawa, S. J. Solid State Chem. 2005, 178, 2420− 2429. (c) Vougo-Zanda, M.; Huang, J.; Anokhina, E.; Wang, X.; Jacobson, A. J. Inorg. Chem. 2008, 47, 11535−11542. (d) Wu, C.-D.; Lin, W. B. Angew. Chem., Int. Ed. 2005, 44, 1958−1961. (e) Serre, C.; Millange, F.; Thouvenot, C.; Noguès, M.; Marsolier, G.; Louër, D.; Férey, G. J. Am. Chem. Soc. 2002, 124, 13519−13526. (3) (a) Maji, T. P.; Uemura, K.; Chang, H. C.; Matsuda, R.; Kitagawa, S. Angew. Chem., Int. Ed. 2004, 43, 3269−3272. (b) Serre, C.; Millange, F.; Thouvenot, C.; Noguès, M.; Marsolier, G.; Louër, D.; Férey, G. J. Am. Chem. Soc. 2002, 124, 13519−13626. (c) Surblé, S.; Serre, C.; Mellot-Draznieks, C.; Millange, F.; Férey, G. Chem. Commun. 2006, 284−286. (4) (a) Suh, M. P.; Cheon, Y. R. Aust. J. Chem. 2006, 59, 605−612. (b) Gustafsson, M.; Bartoszewicz, A.; Martín-Matute, B.; Sun, J. S.; Grins, J.; Zhao, T.; Li, Z. G.; Zhu, G. S.; Zou, X. D. Chem. Mater. 2010, 22, 3316−3322. (c) Allan, P. K.; Xiao, B.; Teat, S. J.; Knight, J. W.; Morris, R. E. J. Am. Chem. Soc. 2010, 132, 3605−3611. (d) Su, Z.; Chen, M.; Okamura, T.-A.; Chen, M.-S.; Chen, S.-S.; Sun, W.-Y. Inorg. Chem. 2011, 50, 985−991. (e) Chandler, B. D.; Enright, G. D.; Udachi, K. A.; Pawsey, S.; Ripmeester, J. A.; Cramb, D. T.; Shimizu, G. K. H. Nat. Mater. 2008, 7, 229−235. (f) Liu, D.; Lang, J.-P.; Abrahams, B. F. J. Am. Chem. Soc. 2011, 133, 11042−11045. (g) Sarma, D.; Natarajan, S. Cryst. Growth Des. 2011, 11, 5415−5423. (h) Park, H. J.; Lim, D.-W.; Yang, W. S.; Oh, T.-R.; Suh, M. P. Chem.Eur. J. 2011, 17, 7251−7260. (i) Bernini, M. C.; Gándara, F.; Iglesias, M.; Snejko, N.; Gutiérrez-Puebla, E.; Brusau, E. V.; Narda, G. E.; Monge, M. Á . Chem.Eur. J. 2009, 15, 4896−4905. (j) Suh, M. P.; Cheon, Y. E.; Lee, E. Y. Chem.Eur. J. 2007, 13, 4208−4215. (k) Wang, C.; Zheng, M.; Lin, W. B. J. Phys. Chem. Lett. 2011, 2, 1701−1709. (l) Yao, Q. X.; Sun, J. L.; Li, K.; Su, J.; Peskov, M. V.; Zou, X. D. Dalton Trans. 2012, 41, 3953−3955. (5) Schoknecht, B.; Kempe, R. Z. Anorg. Allg. Chem. 2004, 630, 1377−1379. (6) Guo, G.-S.; Wang, C.-C.; Liu, Y.-C.; Wang, Z.-H.; Liu, Y.; Guo, H.-Y. Chin. Inorg. Chem. 2010, 26, 1583−1589. (7) Huh, S.; Jung, S.; Kim, Y.; Kim, S.-J.; Park, S. Dalton Trans. 2010, 39, 1261−1265. (8) (a) Szeto, K. C.; Prestipino, C.; Lamberti, C.; Zecchina, A.; Bordiga, S.; Bjørgen, M.; Tilset, M.; Lillerud, K. P. Chem. Mater. 2007, 19, 211−220. (b) Szeto, K. C.; Kongshaug, K. O.; Jakobsen, S.; Tilset, M.; Lillerud, K. P. Dalton Trans. 2008, 2054−2060. (9) Guo, X. D.; Zhu, G. S.; Fang, Q. R.; Xue, M.; Tian, G.; Sun, J.; Li, X. T.; Qiu, S. L. Inorg. Chem. 2005, 44, 3850−3855. (10) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (11) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7. (12) Millange, F.; Serre, C.; Guillou, N.; Férey, G.; Walton, R. I. Angew. Chem., Int. Ed. 2008, 47, 4100−4105. (13) (a) Guo, X. D.; Zhu, G. S.; Fang, Q. R.; Xue, M.; Tian, G.; Sun, J.; Li, X. T.; Qiu, S. L. Inorg. Chem. 2005, 44, 3850−3855. (b) Li, Z. G.; Zhu, G. S.; Guo, X. D.; Zhao, X. J.; Jin, Z.; Qiu, S. L. Inorg. Chem. 2007, 46, 5174−5178.

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A Family of Flexible Lanthanide Bipyridinedicarboxylate Metal

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