Journal of Materials Chemistry A

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1 Journal of Materials Chemistry A PAPER Cite this: J. Mater. Chem. A, 2013, 1, 5061 Received 9th January 2013 Accepted 18th February 2013 DOI: /c3ta00115f Flexible lanthanide MOFs as highly selective and reusable liquid MeOH sorbents Constantinos G. Efthymiou, a Eleni J. Kyprianidou, a Constantinos J. Milios, b Manolis J. Manos* c and Anastasios J. Tasiopoulos* a A series of new 3-D Ln 3+ metal organic frameworks (MOFs), which are based on the semi-rigid ligand H 3 CIP [H 3 CIP ¼ 5-(4-carboxybenzylideneamino)isophthalic acid], [Ln 2 (CIP) 2 (DMF) 4 x (H 2 O) x ] (Ln 3+ ¼ La 3+,Ce 3+, Pr 3+,Sm 3+,Eu 3+,Gd 3+,Tb 3+,Dy 3+,Ho 3+ ; x ¼ 0 2) are reported. Magnetic susceptibility data for selected compounds indicated the presence of antiferromagnetic interactions, while the photoluminescence properties of the MOFs revealed emission peaks that are red-shifted compared to the emission (405 nm) of the uncoordinated H 3 CIP ligand, whereas the Eu 3+ and Tb 3+ analogues showed emission peaks typical of these lanthanide ions. Single-Crystal-to-Single-Crystal (SCSC) coordinating solvent exchange experiments with acetone and methanol for the Ce 3+ analogue afforded compounds that are isostructural with the pristine material and contain one acetone and one water or 1.25 methanol and 0.75 water ligated molecules per Ce 3+ respectively. The capability of activated Ce 3+ MOF to absorb liquid MeOH was evaluated by means of 1 H NMR spectroscopy. The results of the sorption experiments indicated a maximum absorption capacity of 96(2) mg g 1 and fast kinetics, while the sorbent is reusable and is also capable of highly selective sorption of MeOH over EtOH. Overall this work emphasizes the multifunctional nature of lanthanide MOFs and provides insight into the liquid-phase sorption of small organic molecules by such materials demonstrating their high potential as sorbents. Introduction The research on metal organic frameworks (MOFs) has seen an enormous amount of activity over the past decade, not only because of their interesting structural features but also due to their extraordinary physical properties. 1 7 Among the known MOFs, the subcategory of lanthanide metal organic frameworks (LnMOFs) deserves special attention, because LnMOFs represent examples of multifunctional materials combining a number of attractive properties such as magnetism, porosity, photoluminescence (PL) and catalytic activity The uniqueness of LnMOFs arises from the nature of the Ln 3+ ions that are simultaneously magnetic and PL centers and bind weakly to neutral solvent molecules. 27 Particularly the weak connection of the solvent molecules to the Ln 3+ ion is one of the key factors for several intriguing properties that LnMOFs may exhibit. 11,13,18,20 a Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus. atasio@ucy.ac.cy; Fax: ; Tel: b Department of Chemistry, University of Crete, Voutes 71003, Herakleion, Greece c Department of Chemistry, University of Ioannina, Ioannina, Greece. emanos@cc.uoi.gr Electronic supplementary information (ESI) available: PXRD, structural gures, thermal analysis data and graphs, experimental details for the sorption experiments and single crystal X-ray crystallographic data for all new compounds. CCDC For ESI and crystallographic data in CIF or other electronic format see DOI: /c3ta00115f For instance, the easy release of the solvent ligands from LnMOFs without the collapse of their structures could induce an excellent liquid-phase sorption capacity for various organic molecules that may occupy not only the pore space but also the free coordinating sites of the Ln 3+ ion. The latter property of LnMOFs, which is largely unexplored, can be useful in the separation of organic molecules and remediation of organic waste. Although LnMOFs may show an innate capability for liquidphase absorption of organic molecules due to the intrinsic attributes of the Ln 3+ ions (i.e. weak connection of solvent molecules), enhancement of this property can be achieved by introducing exibility into the structures of these materials, because exibility allows dramatic structural alterations during the absorption processes (e.g. expansion of the pore space, changes in the con gurations of the ligands, etc.) without deterioration of the crystalline frameworks. 5,28 One strategy that has been employed for this purpose involves the use of semirigid polytopic ligands. 5 We have thus synthesized and reported the semirigid H 3 CIP [H 3 CIP ¼ 5-(4-carboxybenzylideneamino)- isophthalic acid] tricarboxylic acid (Fig. 1) and a Nd 3+ MOF (UCY-2) with this ligand. 28 UCY-2 showed an unusual capability for single-crystal-to-single-crystal (SCSC) coordinating solvent exchange transformations, which results from its exible structure (due to the presence of the exible CIP 3 ligands) and the facile removal of the coordinating solvent molecules This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1,

2 Journal of Materials Chemistry A Paper Fig. 1 Representation of the H 3 CIP ligand. Colour code: O, red; N, blue; C, grey. The H atoms have been omitted for clarity. from the Ln 3+ ion. The solvent exchange properties of UCY-2 allowed the incorporation of a variety of organic compounds into this MOF with the inserted molecules terminally ligated to the Ln 3+ ions (while in some cases additional molecules are hosted as guests in the pores). Here, we report the synthesis, crystal structures, thermal stability, PL and magnetism studies of a series of analogues of UCY-2, with the general formula [Ln 2 (CIP) 2 (DMF) 4 x (H 2 O) x ] (UCY-4 UCY-12; UCY-4:Ln¼ La 3+, x ¼ 0; UCY-5:Ln¼ Ce 3+, x ¼ 0; UCY-6:Ln¼ Pr 3+, x ¼ 0; UCY-7:Ln¼ Sm 3+, x ¼ 1; UCY-8:Ln¼ Eu 3+, x ¼ 2; UCY-9: Ln¼ Gd 3+, x ¼ 1; UCY-10: Ln¼ Tb 3+, x ¼ 2; UCY-11: Ln¼ Dy 3+, x ¼ 2; UCY-12: Ln¼ Ho 3+, x ¼ 1). Furthermore, we describe the SCSC coordinating solvent exchange of UCY-5 with acetone and MeOH and also provide detailed 1 H NMR data for the liquid-phase sorption of MeOH by the solventfree material demonstrating, for the rst time through systematic studies, the potential of LnMOFs as highly efficient, selective and reusable liquid MeOH sorbents. Experimental section Materials All procedures were performed under aerobic conditions. Solvents and reagents were obtained from commercial sources and used as received. H 3 CIP$2EtOH$H 2 O was prepared as described elsewhere. 28 Syntheses UCY-4 UCY-12: solid Ln(NO 3 ) 3 (0.36 mmol) was added to a solution of H 3 CIP$2EtOH$H 2 O (0.150 g, 0.35 mmol) in DMF (5 ml) in a 20 ml glass vial. The vial was tightly capped; the mixture was sonicated for 3 minutes, and then heated without stirring at 100 C for 20 hours. During this period, colorless plate-like crystals of the MOF were formed, isolated by ltration, washed several times with DMF and diethyl ether and dried under vacuum. Yield: 40 70%. Compound UCY-4 was analyzed as UCY-4$4H 2 O. Anal. calc. for C 44 H 52 La 2 N 6 O 20 :C, 41.85; H, 4.15; N, Found: C, 41.95; H, 4.39; N, 6.52%. Selected IR data (KBr pellet, cm 1 ): 3410(s), 2967(w), 2798(w), 1670(s), 1598(s), 1352(s). Compound UCY-5 was analyzed as UCY-5$6H 2 O. Anal. calc. for C 44 H 56 Ce 2 N 6 O 22 : C, 40.62; H, 4.34; N, Found: C, 40.63; H, 4.20; N, 6.12%. Selected IR data (KBr pellet, cm 1 ): 3412(s), 2964(w), 2794(w), 1672(s), 1595(s), 1355(s). Compound UCY-6 was analyzed as UCY-6$5.5H 2 O. Anal. calc. for C 44 H 55 N 6 O 21.5 Pr 2 : C, 40.85; H, 4.29; N, Found: C, 40.97; H, 4.24; N, 6.65%. Selected IR data (KBr pellet, cm 1 ): 3423(s), 2963(w), 2792(w), 1669(s), 1598(s), 1352(s). Compound UCY-7 was analyzed as UCY-7$3H 2 O. Anal. calc. for C 41 H 45 N 5 O 19 Sm 2 : C, 40.61; H, 3.74; N, Found: C, 40.48; H, 3.85; N, 5.62%. Selected IR data (KBr pellet, cm 1 ): 3425(s), 2973(w), 2800(w), 1670(s), 1593(s), 1349(s). Compound UCY-8 was analyzed as UCY-8$2DMF$4.5H 2 O. Anal. calc. for C 44 H 57 Eu 2 N 6 O 22.5 : C, 39.62; H, 4.31; N, Found: C, 39.57; H, 4.25; N, 6.53%. Selected IR data (KBr pellet, cm 1 ): 3423(s), 2960(w), 2798(w), 1672(s), 1578(s), 1352(s). Compound UCY-9 was analyzed as UCY-9$DMF$4.5H 2 O. Anal. calc. for C 44 H 55 Gd 2 N 6 O 21.5 : C, 39.84; H, 4.18; N, Found: C, 39.72; H, 4.32; N, 6.45%. Selected IR data (KBr pellet, cm 1 ): 3410(s), 2964(w), 2798(w), 1668(s), 1594(s), 1356(s). Compound UCY-10 was analyzed as UCY-10$5.5H 2 O. Anal. calc. for C 38 H 45 N 4 O 21.5 Tb 2 : C, 37.42; H, 3.72; N, Found: C, 37.56; H, 3.82; N, 4.48%. Selected IR data (KBr pellet, cm 1 ): 3416(s), 2969(w), 2797(w), 1670(s), 1597(s), 1354(s). Compound UCY-11 was analyzed as UCY-11$DMF$4H 2 O. Anal. calc. for C 41 H 49 Dy 2 N 5 O 21 : C, 38.69; H, 3.88; N, Found: C, 38.87; H, 4.07; N, 5.32%. Selected IR data (KBr pellet, cm 1 ): 3410(s), 2968(w), 2799(w), 1678(s), 1593(s), 1358(s). Compound UCY-12 was analyzed as UCY-12$DMF$7H 2 O. Anal. calc. for C 44 H 60 Ho 2 N 6 O 24 : C, 38.11; H, 4.36; N, Found: C, 38.24; H, 4.20; N, 5.85%. Selected IR data (KBr pellet, cm 1 ): 3410(s), 2964(w), 2798(w), 1672(s), 1594(s), 1353(s). The purity of all products was also con rmed by the comparison of the experimental powder X-ray diffraction patterns to those calculated from the single crystal X-ray data (see Fig. S1 in the ESI ). UCY-5/X (X ¼ MeOH, acetone): single crystals of UCY-5$6H 2 O (0.026 g, 0.02 mmol) and X (5 ml) were mixed in a 23 ml Te onlined stainless steel autoclave. The autoclave was sealed and placed in an oven operated at 50 C, remained undisturbed at this temperature for 2 days and then was allowed to cool at room temperature. The crystals of the exchanged compound were isolated by ltration and dried in the air. Compound UCY- 5/MeOH was analyzed as UCY-5/MeOH$5H 2 O. Anal. calc. for C 34.5 H 39 Ce 2 N 2 O 21 : C, 37.74; H, 3.58; N, Found: C, 37.92; H, 3.72; N, 2.42%. Selected IR data (KBr pellet, cm 1 ): 3412(s), 2925(w), 1618(s), 1577(s), 1386(s). Compound UCY-5/acetone was analyzed as UCY-5/acetone$4H 2 O. Anal. calc. for C 38 H 40 Ce 2 N 2 O 20 : C, 40.57; H, 3.58; N, Found: C, 40.33; H, 3.74; N, 2.42%. Selected IR data (KBr pellet, cm 1 ): 3402(s), 2974(w), 1594(s), 1350(s). UCY-5/dry: single crystals of UCY-5/acetone were dried in a vacuum oven operating at 60 C for 12 h, affording the solventfree product. MeOH sorption experiments The MeOH sorption experiments were performed as described below: in an equimolar solution of MeOH and toluene (it was used as a reference for the calculation of the amount of sorbed 5062 J. Mater. Chem. A, 2013, 1, This journal is ª The Royal Society of Chemistry 2013

3 Paper MeOH, see ESI ) incdcl 3 was added solid UCY-5/dry. The mixture was kept under magnetic stirring for 12 h. The resulting slurry was ltered and the ltrate was analyzed for its MeOH content by 1 H NMR spectroscopy. Competitive MeOH EtOH experiments were performed similarly to the above-described manner with the difference that mixtures of MeOH EtOH were used instead of MeOH solutions. For the kinetic experiments, again UCY-5/drywasaddedasasolidina mixture of MeOH (0.22 mmol) and toluene (0.22 mmol) in CDCl 3 and the resulting slurry was kept under magnetic stirring. Small volumes of aliquots were taken at designated times, centrifuged and then the supernatant liquid was measured by 1 H NMR spectroscopy to determine its MeOH content and the amount of the sorbed solvent was calculated. All liquid sorption experiments were repeated three times and the determined values were averaged. More details for the sorption experiments and the 1 H NMR measurements are provided in the ESI. Physical measurements Elemental analyses (C, H, N) were performed using the in-house facilities of the University of Cyprus, Chemistry Department. IR spectra were recorded on KBr pellets in the cm 1 range using a Shimadzu Prestige-21 spectrometer. PXRD diffraction patterns were recorded on a Shimadzu 6000 Series X-ray diffractometer (CuKa radiation, l ¼ Å). Photoluminescence (PL) spectra were obtained on an Edinburgh Xe900 spectro uorometer. Thermal stability studies were performed with a Shimadzu TGA 50 thermogravimetric analyzer. Magnetic susceptibility measurements were obtained with the use of a Quantum Design SQUID magnetometer (MPMS-XL). Direct current (DC) measurements were conducted from 5 to 300 K in an applied eld of 0.1 T. The measurements were performed on nely ground polycrystalline samples. Experimental data were corrected for the sample holder and for the diamagnetic contribution of the samples calculated from Pascal constants. 1 H NMR spectroscopy was carried out on an Avance Bruker NMR spectrometer (300 MHz). Single crystal X-ray crystallography Single crystal X-ray diffraction data were collected on an Oxford- Diffraction Supernova diffractometer, equipped with a CCD area detector utilizing Mo-Ka (l ¼ Å) radiation. Suitable crystals were attached to glass bers using paratone-n oil and transferred to a goniostat where they were cooled for data collection. Empirical absorption corrections (multi-scan based on symmetry-related measurements) were applied using CrysAlis RED so ware. 29 The structures were solved by direct methods with SIR and re ned on F 2 using full-matrix least squares with SHELXL So ware packages used: CrysAlis CCD for data collection, 29 CrysAlis RED for cell re nement and data reduction, 29 WINGX for geometric calculations, 32 and DIAMOND 33 for molecular graphics. The non-h atoms were treated anisotropically, whereas the H atoms were placed in calculated, ideal positions and re ned as riding on their respective carbon atoms. Electron density contributions from disordered guest molecules were handled using the SQUEEZE procedure from the PLATON so ware suite. 34 In order to limit the disorder of the terminal ligands, various restraints (DFIX, ISOR, DELU) have been applied in the re nement of the crystal structures. Selected crystal data for all compounds are given in Table 1 and Tables S1 and S2 (in ESI ). Full details can be found in the CIF les provided in the ESI. Results and discussion Journal of Materials Chemistry A Syntheses and structures of UCY-4 UCY-12 We have recently initiated a research program that involves the synthesis of new exible LnMOFs with potential SCSC transformation properties and capability of liquid-phase absorption of various organic molecules. Our approach towards exible MOFs consists of the use of semi-rigid ligands. Thus, we have prepared the tricarboxylic ligand H 3 CIP that contains a semirigid imine (CH]N) linkage between its phenyl-carboxylate moieties, which induce some (but not unlimited) rotational freedom to the phenyl rings of this organic molecule. The rst LnMOF synthesized with this ligand was a Nd 3+ compound (UCY-2). 28 In this work, we report a series of analogues of UCY-2 with different lanthanide ions that were isolated from reactions Table 1 Selected crystal data for the compounds a reported in this work Compound a (Å) b (Å) c (Å) V (Å 3 ) R 1 b (%) UCY (2) (5) (5) (4) 4.73 UCY (2) (6) (7) (4) 5.75 UCY-5/MeOH (4) (2) (2) 4929(2) 5.76 UCY-5/acetone (2) (5) (9) (7) 4.09 UCY (2) (9) (2) (8) 6.74 UCY (2) (7) (6) (4) 4.91 UCY (2) (2) (2) (9) 7.18 UCY (2) (2) (8) (6) 7.50 UCY (2) (2) (9) (7) 4.45 UCY (8) (7) (5) (4) 8.55 UCY (2) (7) (2) (6) 8.64 a All compounds crystallize in the monoclinic space group C2/c. b R 1 ¼ P kf o F c k/ P F o. This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1,

4 Journal of Materials Chemistry A of Ln(NO 3 ) 3 (Ln 3+ ¼ La 3+,Ce 3+,Pr 3+,Sm 3+,Eu 3+,Gd 3+,Tb 3+, Dy 3+,Ho 3+ ) and H 3 CIP in DMF at 100 C. All compounds are structurally related being isostructural to UCY-2 and thus only the structure of UCY-4 will be discussed in detail. UCY-4 crystallizes in the monoclinic space group C2/c and contains the dinuclear building block [La 2 (COO) 6 (DMF) 4 ] as the secondary building unit (SBU), Fig. 2. Each SBU consists of two ninecoordinated La 3+ ions and six COO from six different CIP 3 ligands. Four of the COO groups bridge the two La 3+ centers in either syn,syn-h 1 :h 1 :m 2 or h 1 :h 2 :m 2 fashion. The remaining two COO groups act as chelating ligands, with each of them coordinating to a single La 3+ ion. The coordination sphere of each La 3+ is completed by two oxygen atoms from disordered DMF terminal ligands. The SBUs extend in nitely creating a three dimensional (3,6)- connected net with the dinuclear unit and the ligands representing the 6-c and 3-c nodes respectively (Fig. S2 ). The point symbol for this net is {4 2.6} 2 { } and is called u-net. 28 The solvent-accessible volume of UCY-4 calculated by PLA- TON 34 is Å 3 corresponding to 23.2% of the unit cell volume. The 3D-structure contains small cavities with diameters 3 4 Å (Fig. 3) as found by PLATON (taking into account the van der Waals radii of the atoms). A representation of the pore network of UCY-2 with the structure visualization program MERCURY 35 reveals that the pores communicate through narrow passages that are only 2 Å wide (Fig. S3 ). At this point, it is worth comparing the structures of the various Ln 3+ -CIP analogues. UCY-2 and UCY-4 UCY-12 crystallize in the same space group, but of course differ in their unit cell parameters (Table 1). Although there are several reports for isostructural LnMOFs indicating a linear decrease of the cell Fig. 2 Representation of the (a) dinuclear SBU and (b) connectivity of three SBUs in the structure of UCY-4. Paper Fig. 3 Space-filling representation of the 3D-structure of UCY-4 viewed down the c-axis. parameters with decreasing Ln 3+ ionic radius, 18,36 the plots of these parameters vs. the lanthanide ionic radii for UCY-2 and UCY-4 UCY-12 do not follow a speci c trend (Fig. S4 ). This is not unexpected taking into account the variations in terminal ligation and the lattice solvents in these compounds that can signi cantly affect the unit cell dimensions. Speci cally, the terminal ligands in these MOFs are DMF and H 2 O(Eu 3+,Tb 3+ and Dy 3+ analogues), exclusively DMF molecules (La 3+,Ce 3+,Pr 3+ analogues), DMF and mixed DMF H 2 O solvents (Nd 3+,Sm 3+ analogues) or mixed DMF H 2 O solvents (Gd 3+ and Ho 3+ analogues). Furthermore, as revealed by the TGA and elemental analyses data, compounds UCY-2, UCY-4 UCY-7 and UCY-10 contain 2 6 lattice H 2 Oper formula unit, whereas the pores of UCY-8, UCY-9, UCY-11 and UCY-12 host 1 2 DMFand4 7 H 2 Operformulaunit. Thermal stability studies The thermal stability of UCY-4 UCY-12 was investigated by means of the thermogravimetric analysis (TGA) technique. The TG/DTG (1 st derivative) data for compounds UCY-4 UCY-12 show weight losses from 20 C to temperatures C assigned to the removal of lattice (H 2 O or/and DMF) and coordinated solvent molecules. At higher temperatures, the compounds exhibit weight losses attributed to the elimination of the CIP 3 ligands. The decomposition of the compounds is completed at temperatures between 600 and 700 C. A detailed description of the thermal analyses data for UCY-4 UCY-12 and the TG/DTG diagrams (Fig. S5 S13 ) are provided in the ESI. Magnetic properties Direct current (DC) magnetic susceptibility measurements were performed on polycrystalline samples of complexes UCY-6 and UCY-9 UCY-12 in the K range under an applied eld of 0.1 T. The results are plotted as the c M T product vs. T in Fig. 4, while in Table 2, the theoretical and the experimentally found c M T values at room temperature for the complexes are given for comparison. From a quick look at the graphs it becomes apparent that in all complexes the c M T product decreases with decreasing temperature. However, this on its own is not conclusive regarding the magnetic interactions within the dinuclear units, since for lanthanides the decrease of the c M T value upon cooling may be due to either antiferromagnetic interactions and/or depopulation of the Stark sublevels J. Mater. Chem. A, 2013, 1, This journal is ª The Royal Society of Chemistry 2013

5 Paper Fig. 4 DC magnetic susceptibility data for UCY-6 and UCY-9 UCY-12 in the Krangeand anappliedfieldof0.1 T. Thesolid line represents thefitting of the data for the Gd 3+ analogue (UCY-9). Table 2 Theoretical and experimentally found c M T values (cm 3 mol 1 K) for UCY-6, 9, 10, 11 and 12 (at 300 K) a Complex g j 27 c M T theor. c M T exper. [Pr 2 ](UCY-6) [Gd 2 ](UCY-9) [Tb 2 ](UCY-10) [Dy 2 ](UCY-11) [Ho 2 ](UCY-12) a Assuming the corresponding g j values for each Ln center. We were able to successfully simulate the data for the complex UCY-9 adopting a 1-J model and Hamiltonian (1). Using the program MAGPACK 37 and by employing the Hamiltonian Ĥ ¼ 2J 1 (Ŝ1Ŝ2) (1) Journal of Materials Chemistry A uncoordinated H 3 CIP ligand likely due to the electron-withdrawing effect of the lanthanide ions, Fig The emission spectra of compounds UCY-5 (as well as those of UCY-6, UCY-7, UCY-9, UCY-12) and UCY-4 exhibit signi cantly red-shi ed peaks at 432 and 447 nm respectively (Fig. 5) ascribed also to the electron-withdrawing capability of the lanthanide ions. The study of the PL properties of the Eu 3+ and Tb 3+ analogues (UCY-8 and UCY-10) revealed that the H 3 CIP ligand can efficiently sensitize these visible-light-emitting lanthanide ions. Speci cally, the emission spectrum of UCY-8 contains typical Eu 3+ peaks upon excitation at 271 nm, Fig. 6a. Thus, there are four peaks at 563, 592, 616 and 697 nm and a shoulder at 647 nm, which are assigned to the 5 D 0 / 7 F J (J ¼ 0 4) transitions of the Eu 3+ ion. The most intense peak is due to the electric dipole induced 5 D 0 / 7 F 2 transition, which is largely affected by the coordination environment of the Eu 3+ ion. The medium intensity peak at 592 nm is caused by the magnetic dipole induced 5 D 0 / 7 F 1 transition, which is not sensitive to the environment of Eu 3+.The ratio of the intensities 5 D 0 / 7 F 2 / 5 D 0 / 7 F 1 is 3.8, much higher than 0.67 being the typical value for a centrosymmetric Eu 3+ ion. 21 Therefore, the PL data for UCY-8 are in agreement with the crystal structure of this MOF indicating a non-centrosymmetric environment for the Eu 3+ center. On the other side, the emission spectrum of the Tb 3+ MOF UCY-10 (l ex ¼ 307 nm) shows three major peaks at 489, 543 and 584 nm assigned to the 5 D 4 / 7 F 6, 5 D 4 / 7 F 5 and 5 D 4 / 7 F 4 transitions of the Tb 3+ ion, whereas the peak corresponding to the 5 D 4 / 7 F 3 transition is too weak to be observed (Fig. 6b). Liquid MeOH sorption properties Prompted by the presence of highly disordered and thus easily removable coordinating solvent molecules in UCY-4 UCY-12 in combination with the relatively open structure and stability in air and various solvents of these MOFs, we decided to investigate the possibility to employ these materials as sorbents for small organic molecules in the liquid-phase. The Ce 3+ analogue we obtained a good quality t of the experimental data with parameters J ¼ cm 1 and g ¼ 2.00, which prove the existence of a diamagnetic ground state for each magnetically isolated {Gd 2 } dimer. The magnetic susceptibility data for all other complexes were tted at the temperature region K to a Curie Weiss law yielding q values of 27.9 K for UCY-6, 2.8 K for UCY-10, 2.3 K for UCY-11 and 6.9 K for UCY-12. The obtained results are in excellent agreement with previously reported values for dinuclear lanthanide complexes, 21 suggesting the presence of antiferromagnetic exchange interactions within the dinuclear units, even though one may not exclude the effect of the depopulation of the Stark sub-levels on the q value. Photoluminescence properties The solid state emission spectra of the MOFs were recorded at room temperature with an excitation wavelength of 345 nm. UCY-2 and UCY-11 show an emission peak at 415 nm, which is slightly red-shi ed compared to that (405 nm) of the Fig. 5 Room temperature PL emission spectra of UCY-4, UCY-5, UCY-11 and H 3 CIP obtained with an excitation wavelength of 345 nm. The spectrum of UCY-2 that is similar to that of UCY-11 and those of UCY-6, UCY-7, UCY-9 and UCY-12 that are similar to that of UCY-5 are not shown for clarity. This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1,

6 Journal of Materials Chemistry A Paper Fig. 7 Equilibrium data for the MeOH sorption by UCY-5 (contact time 12 h, initial MeOH concentrations in the range ppm). The solid line represents the fitting of the data with the LF model. Fig. 6 Room temperature PL excitation and emission spectra of (a) UCY-8 and (b) UCY-10. (UCY-5) was chosen for these investigations, since Ce 3+ is the less expensive lanthanide ion and thus, more attractive for practical applications. We have concentrated on the study of MeOH absorption by UCY-5, because such a procedure could be of interest for the removal of MeOH impurities in a series of industrial processes. Although numerous papers dealing with the gas-phase adsorption of MeOH by MOFs have been published, 39 studies on the liquid-phase MeOH sorption properties of such materials are very limited. 40 In order to prepare the solvent-free UCY-5(UCY-5/dry) and thus activate this MOF, the material was treated with acetone (in order to exchange the coordinating and lattice solvents by acetone) and then heated under vacuum at 60 C releasing the acetone solvent molecules of the exchanged material. UCY-5/dry was then tested and evaluated as a MeOH sorbent by performing batch studies. 41 The MeOH sorption equilibrium data are plotted in Fig. 7 and the corresponding 1 H NMR data are shown in the ESI (Fig. S14 ). The best description of the data (R 2 ¼ 0.98) is provided by the Langmuir Freundlich (LF) model 41b ðbc e Þ 1 n q ¼ q m 1 þðbc e Þ 1 n where q (mg g 1 ) is the amount of MeOH per gram of the MOF material sorbed at the equilibrium concentration C e (ppm of MeOH remaining in solution), q m is the maximum sorption capacity of the sorbent, b (L mg 1 ) is the Langmuir constant related to the free energy of the sorption and n is a Freundlich constant. 41 The results of the tting gave the parameters q m ¼ 96(2) mg g 1, b ¼ (3) L mg 1 and n ¼ 0.5(1). The maximum exchange capacity q m found is consistent with the absorption of 1.5 equivalents of MeOH per Ce 3+ ion of UCY-5/dry. UCY-5/dry that is used in liquid MeOH sorption experiments (and thus converted to UCY-5/MeOH) can be easily regenerated by treating it with acetone under re ux conditions and heating the resulted UCY-5/acetone under vacuum at 60 C. The activated MOF, prepared from the regenerated UCY-5/acetone, showed an identical sorption capacity with that of the original UCY-5/dry (i.e. that prepared from the as-synthesized UCY-5). This sorption capability is retained even a er (at least) 5 cycles of regeneration/reuse experiments. Thus, the sorbent is reusable, something that is crucial for practical applications. Furthermore, UCY-5/MeOH (or UCY-5/acetone) materials prepared from original and regenerated UCY-5/dry samples display almost identical PXRD patterns (Fig. S15 and S16 in the ESI ), which also indicates the reusability of UCY-5/dry. The kinetics of the MeOH sorption by UCY-5/dry was also investigated (Fig. 8 and Fig. S17 in the ESI ) for a MeOH initial concentration corresponding to 1 equivalent of MeOH per formula unit of UCY-5/dry. It can be seen that this process is very fast at the beginning, since within only 5 minutes UCY-5/ dry absorbs 65% of the MeOH content of the solution. Then, the absorption process slows down and a er 1 hour reaches 78% sorption and no signi cant further removal of MeOH can be seen a er 4 h. Another 9% of MeOH removal can be observed a er 12 h, while a er 24 h there is 96% absorption of the initial MeOH amount. No signi cant additional absorption can be seen a er 48 h and nally a er 72 h the removal of MeOH was found to be almost quantitative (99%). The kinetics of MeOH sorption was also studied for higher initial MeOH concentrations (corresponding to 2 equivalents of MeOH per formula unit of UCY-5/dry) and in these cases, the maximum MeOH absorption can be reached in 12 h (Fig. S18 in the ESI ). Finally, competitive MeOH EtOH sorption experiments were carried out. Note that MeOH EtOH separation is very difficult to be achieved because of the close characteristics of these alcohol 5066 J. Mater. Chem. A, 2013, 1, This journal is ª The Royal Society of Chemistry 2013

7 Paper Journal of Materials Chemistry A Fig. 8 The kinetics for the MeOH sorption by UCY-5/dry for an initial MeOH concentration of 1778 ppm. The solid line is only a guide for the eye. compounds (e.g. similar molecular sizes and boiling points) and such a process is of interest for removal of MeOH impurities from alcoholic beverages as well as for puri cation of bioethanol. 39b Again, most studies were focused on the separation of gas MeOH EtOH by MOFs. 39 Here we provide some of the rst data about such separation in the liquid phase. Speci cally, UCY-5/dry was treated with an equimolar mixture of MeOH EtOH and the progress of the absorption was monitored by 1 H NMR spectroscopy (Fig. S19 in the ESI ). We have observed that a er a few minutes of sorbent solution contact UCY-5/dry absorbed only MeOH (50% of the initial amount) and the absorption was continuous and nally a er 72 h reached 80% (Fig. 9a). Remarkably, UCY-5/dry absorbs no EtOH even a er 24 h. Only a er 48 h UCY-5/dry shows a small sorption of EtOH (15% of the initial EtOH amount) and nally a er 72 h the material absorbs 21% of the initial quantity of EtOH (Fig. 9a). We have also tested the selectivity of UCY-5/dry for a mixture of MeOH : EtOH with a molar ratio 1 : 2, i.e. for a mixture with a relatively large excess of EtOH. Although the material shows higher EtOH sorption in comparison with the previous experiments (experiments with a mixture MeOH : EtOH 1 : 1), still EtOH is sorbed to a signi cantly smaller extent than MeOH (Fig. 9b and Fig. S20 in the ESI ). Thus, a er 72 h the material absorbed 64% and 20% of the initial MeOH and EtOH amounts respectively (Fig. 9b). These results indicate high separation efficiency of UCY-5/dry for MeOH vs. EtOH, which may be due to the narrow pores and channels of UCY-5/dry favouring the diffusion of the smaller MeOH. It is more likely this excellent MeOH selectivity of UCY-5/dry is a purely kinetic effect rather than a thermodynamic one, because actually the exchange of DMF by MeOH results in larger changes in the crystal structures of Ln 3+ CIP 3 MOFs (i.e. in their unit cell volumes) than the corresponding exchange process with EtOH. 28 Fig. 9 Representation of the % MeOH EtOH sorption capacity of UCY-5, which was measured for mixtures of MeOH EtOH with a ratio of (a) 1 : 1 and (b) 1 : 2, versus time. The lines are only a guide for the eye. exchanged products. The elucidation of the crystal structures of UCY-5/MeOH and UCY-5/acetone revealed that the exchanged compounds have related crystal structures to pristine UCY-5 SCSC transformations To get insight into the structure of the MeOH and acetoneexchanged UCY-5 (UCY-5/MeOH and UCY-5/acetone), heterogeneous solvent-exchange reactions of single crystals of UCY-5 with MeOH or acetone at 50 C were performed and X-ray diffraction data were collected on single crystals of the Fig. 10 Representation of the SCSC solvent exchange of UCY-5 with MeOH and acetone. For emphasis, the inserted organic molecules are represented with large balls. The 0.25 MeOH disordered with 0.75 H 2 O per Ce 3+ as well as the one of the two disordered positions of the carbonylic oxygen of acetone ligands are not shown for clarity. This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1,

8 Journal of Materials Chemistry A (Table 1) and contain one MeOH and mixed H 2 O MeOH (0.75/ 0.25) (UCY-5/MeOH) or one acetone and water (UCY-5/acetone) terminal solvents per Ce 3+ ion (Fig. 10). Note that the content of MeOH found from the crystal structure of UCY-5/MeOH (1.25 equivalents per Ce 3+ ion) is in very good agreement with that (1.5 equivalents) determined by the liquid-phase MeOH sorption experiments. As also observed for UCY-2, the replacement of the terminal solvents of UCY-5 by MeOH H 2 O and acetone H 2 O resulted in 12 and 9% reduction of its unit cell volume respectively, a fact that indicates the framework exibility of this MOF. Conclusions In conclusion, we have synthesized a number of new Ln 3+ MOFs (UCY-4 UCY-12) based on the tricarboxylic ligand H 3 CIP, which display exible three-dimensional framework structures with a u-net topology. Magnetic susceptibility data revealed the presence of dominant antiferromagnetic interactions for the compounds, whereas all MOFs are photoluminescent with emissions either red-shi ed compared to that of the H 3 CIP ligand or lanthanide-based (for the Eu 3+ and Tb 3+ analogues). These MOFs exhibit a facile topotactic exchange of their coordinating terminal solvents with acetone and methanol in a SCSC fashion, as demonstrated for the case of UCY-5. The solvent-free form of UCY-5 (UCY-5/dry) showed an exceptional capability to absorb liquid MeOH with a capacity of 96(2) mg g 1. The kinetics of the MeOH sorption is very fast, whereas UCY-5/dry shows a high preference for MeOH over EtOH as revealed by the sorption experiments with mixtures of these alcohols. Furthermore, UCY-5/dry can be regenerated and reused as a MeOH sorbent several times with no loss of its capacity. Thus, for the rst time the ability of LnMOFs to behave as highly efficient sorbents for small organic molecules in the liquid phase is demonstrated through systematic investigations. Furthermore, our combined liquid-phase MeOH sorption and SCSC transformation studies con rmed that LnMOFs have the capability to absorb organic molecules not only as guests but also as ligands for the lanthanide ions. It is anticipated that such materials may be proved as superior sorbents for organic molecules with metal coordination capability due to the high stability of their solvent-free forms and the strong tendency of the unsaturated Ln 3+ ions to coordinate with various organic ligands. On-going efforts are underway to investigate the sorption properties of various LnMOFs for a variety of small and relatively bulky liquid organic molecules and these results will be presented in the near future. Acknowledgements This work was supported by the Cyprus Research Promotion Foundation Grants DIDAKTUP/DISEK/0308/22 and NEKYP/ 0308/02 which are co-funded by the Republic of Cyprus and the European Regional Development Fund. Notes and references Paper 1(a) M. Eddaoudi, D. B. Moler, H. L. Li, B. L. Chen, T. M. Reineke, M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319; (b) D. Bradshaw, J. B. Claridge, E. J. Cussen, T. J. Prior and M. J. Rosseinsky, Acc. Chem. Res., 2005, 38, 273; (c) D. Zhao, D. J. Timmons, D. Yuan and H.-C. Zhou, Acc. Chem. Res., 2011, 44, 123; (d) D. J. Tranchemontagne, J. L. Mendoza-Cortés, M. O'Keeffe and O. M. Yaghi, Chem. Soc. Rev., 2009, 38, (a) G. Ferey, Chem. Soc. Rev., 2008, 37, 191; (b) R. J. Kuppler, D. J. Timmons, Q.-R. Fang, J.-R. Li, T. A. Makal, M. D. Young, D. Yuan, D. Zhao, W. Zhuang and H.-C. Zhou, Coord. Chem. Rev., 2009, 253, (a) S. T. Zheng, T. Wu, J. A. Zhang, M. Chow, R. A. Nieto, P. Y. Feng and X. H. Bu, Angew. Chem., Int. 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