Microporous Rare Earth Coordination Polymers: Effect of Lanthanide

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Chem. Mater. 2002, 14, 2616-2622

Microporous Rare Earth Coordination Polymers: Effect of Lanthanide Contraction on Crystal Architecture and Porosity A. Dimos,† D. Tsaousis,† A. Michaelides,*,† S. Skoulika,*,† S. Golhen,‡ L. Ouahab,‡ C. Didierjean,§ and A. Aubry§ Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece, Laboratoire de Chimie du Solide Inorganique et Moleculaire UMR 6511, Institut de Chimie de Rennes, Universite de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France, and Laboratoire de Cristallographie et Modelisation des Materiaux Mineraux et Biologiques (LCM3B), Universite Henri Poincare Nancy I, ESA 7036, BP 239, 54506 Vandoeuvre les Nancy Cedex, France Received December 10, 2001. Revised Manuscript Received April 8, 2002

A series of microporous coordination polymers of adipic (H2ad) and pimelic acids (H2pim) with rare earth metals have been synthesized and structurally characterized. In the case of adipic acid, two distinct structure types were isolated: structure type I with formula [Ln2(ad)3(H2O)4]6H2O, Ln ) Ce3+, Pr3+, Nd3+, and strucure type II with formula [Ln2(ad)3(H2O)4]xH2O, Ln ) Nd3+, Sm3+, Gd3+, Er3+, Yb3+. Type I compounds are isostructural to lanthanum adipate (Inorg. Chem. 1998, 37, 3407) and contain interconnected channels filled with hydrogen-bonded water molecules. The structure of type II polymers consists of interpenetrated (4,4) metal-organic networks. Channels along one crystallographic direction are created by the interpenetration. The minimum width of the channels depends on the nature of the metal incorporated in the framework. The passage from type I to type II structure is ascribed to the well-known effect of lanthanide contraction. In the case of H2pim, one compound of formula [La(pim)(Hpim)(H2O)]H2O was isolated. Channels, filled with water molecules, are formed along one crystallographic direction. In all compounds, the water molecules can be reversibly removed.

Introduction The construction of metal-organic analogues of inorganic zeolites is a subject of intense current interest.1,2 It is generally expected that, owing to their molecular nature, this class of framework materials will offer interesting applications in separation science3 and catalysis.4 The metal-ligand bonds that hold the crystalline framework are relatively weak, and removal of * To whom correspondence should be addressed. † University of Ioannina. ‡ Universite de Rennes I. § Universite Henri Poincare Nancy I. (1) (a) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. Acc. Chem. Res. 1998, 31, 474. (b) Janiak, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 1431. (c) Kitagawa, S.; Kondo, M. Bull. Chem. Soc. Jpn. 1998, 71, 1739. (d) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. Engl. 1999, 38, 2638. (e) Zaworotko, M. J. Angew. Chem., Int. Ed. Engl. 2000, 39, 3052. (f) Aoyama, Y. In Topics in Current Chemistry, Design of Organic Solids; Weber, E., Ed.; Springer: New York, 1998. (2) (a) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1990, 112, 1546. (b) Carlucci, L.; Ciani, G.; Proserpio, D. M.; Sironi, A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1895. (c)Yaghi, O. M.; Li, H. J. Am. Chem. Soc. 1996, 118, 295. (d) Hennigar, T. L.; MacQuarrie, D. C.; Losier, P.; Rogers, R. D.; Zaworotko, M. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 972. (e) Gardner, G. B.; Venkataraman, D.; Moore, J.; Lee, S. Nature 1995, 374, 792. (3) (a) Kondo, M.; Yoshitomi, T.; Seki, K.; Matsuzaka, H.; Kitagawa, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 1725. (b) Yaghi, O. M.; Li, G.; Li, H. Nature 1995, 378, 703. (c) Novo, S.-i.; Kitagawa, S.; Kondo, K.; Seki, K. Angew. Chem., Int. Ed. Engl. 2000, 39, 2082. (d) Li, D.; Kaneko, K. J. Phys. Chem. B 2000, 104, 8940. (e) Chen, B.; Eddaoudi, M.; Reineke, T. M.; Kampf, J. W.; O’Keefe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 11559. (f) Tabarei, L. C.; Navarro, J. A. R.; Salas, J. M. J. Am. Chem. Soc. 2001, 123, 383.

the guest molecules from the channels is frequently accompanied by collapse of the structure.5 Despite this drawback, examples of robust metal-organic microporous materials have appeared in the literature in recent years, but their number is still limited.3,6 Most of the aforementioned metal-organic frameworks (MOFs) have been obtained by using low coordination divalent transition metals. Recently, we7 and others8 have used rare earth cations and dicarboxylic acids to obtain robust microporous coordination polymers. Indeed, the high coordination number of the cations in conjuction with the bridging or chelating bridging ability of the carboxylate group seems to prevent collapse of the structure upon removal of the (4) (a) Eddaoudi, M.; Li, H.; Reineke, T.; Fehr, M.; Kelley, D.; Groy, T. L.; Yaghi, O. M. Top. Catal. 1999, 9, 105. (b) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 116, 1151. (c) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jin, Y.; Kim, K. Nature 2000, 404, 982. (5) (a) Subramanian, S.; Zaworotko, M. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2127. (b) Abrahams, B. F.; Hoskins, B. F.; Michail, D. M.; Robson, R. Nature 1994, 369, 727. (6) (a) Li, H.; Eddaoudi, M.; O’Keefe, M.; Yaghi, O. M. Nature 1999, 402, 276. (b) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148. (c) Lin, K.-J. Angew. Chem., Int. Ed. Engl. 1999, 38, 2732. (d) Goodgame, D. M. L.; Grachvogel, D. A.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1999, 38, 153. (e) Livage, C.; Egger, C.; Ferey, G. Chem. Mater. 1999, 11, 1546. (f) Biradha, K.; Hongo, Y.; Fujita, M. Angew. Chem., Int. Ed. Engl. 2000, 39, 3843. (g) Kiang, Y. H.; Gardner, G. B.; Lee, S.; Xu, Z.; Lobkovsky, E. B. J. Am. Chem. Soc. 1999, 121, 8204. (7) Kiritsis, V.; Michaelides, A.; Skoulika, S.; Gohlen, S.; Ouahab, L. Inorg. Chem. 1998, 37, 3407.

10.1021/cm011302i CCC: $22.00 © 2002 American Chemical Society Published on Web 05/18/2002

Microporous Rare Earth Coordination Polymers

guest. Furthermore, rare earth coordination compounds may present interesting luminescence8a,9 and magnetic properties.8d,10 In our previous work,7 we used La3+ and adipic acid to construct a 3-D microporous coordination polymer of formula [La2(ad)3(H2O)4]6H2O with ad ) adipato (C6H8O42-), 1, posessing large intersecting channels running in the three directions of space. The structure was stabilized by an extensive hydrogen-bonded network formed by both enclathrated and coordinated water molecules. Thus, it was of interest to examine whether this architecture is maintained across the lanthanide series. Indeed, the ionic radii of the rare earth cations decrease with increasing atomic number (lanthanide contraction). As a result, the increasingly important ligand-ligand repulsion when passing from the lanthanum to the last member of the series might lead to other types of structure and physical properties. Furthermore, we supposed that the strength of the hydrogen-bonding network is probably related to the nature of the cation, and an imminent modification of structure would probably be reflected on the geometry of the network. Finally, to investigate whether other long-chain dicarboxylic acids can generate robust microporous coordination polymers, we used pimelic acid (hereafter denoted as H2pim) as ligand and La3+ as metal center. Experimental Section Preparations. [Ce2(ad)3(H2O)4]6H2O (2) was prepared with the same technique as that used for gadolinium adipate (see below) but at pH ) 6. Large colorless crystals, isostructural to lanthanum adipate, were formed after 10 months. [Pr2(ad)3(H2O)4]6H2O (3) was prepared with the same technique as that used for neodymium adipate (see below). Green crystals of complex morphology (0.2 × 0.1 × 0.08 mm3) isostructural to lanthanum adipate, were formed after about 4 months. [Nd2(ad)3(H2O)4]6H2O, 4, and [Nd2(ad)3(H2O)4]H2O, 5. Suitable single crystals were prepared by using a double diffusion silica gel technique. A silicate solution (d ) 1.06 g‚cm-3) was added, under stirring, to an aqueous solution prepared by mixing 35 mL of 0.01 M adipic acid and 5 mL of 1 N HNO3. When the pH reached the value 6.1, the gelling solution was poured in a test tube and allowed to stand for 2 days. Then, a “neutral” (not containing adipic acid) gelling solution was carefully added at the top of the gel and allowed to stand for 3 days. The “neutral” gelling solution was prepared by mixing 35 mL of distilled water with 5 mL of 1 N HNO3 and brought to pH 6.1 with a silicate solution (d ) 1.06 g‚cm-3). Finally, an aqueous solution of 0.07 M NdCl3 was added at the top of the “neutral” gel. Some prismatic light blue crystals of 5, of approximate dimensions 0.4 × 0.3 × 0.1 mm3, were formed after about 4 months. The composition of the crystals was established by X-ray analysis. After some of these crystals were (8) Reineke, T. M.; Eddaoudi, M.; Fehr, M.; Kelley, D.; Yaghi, O. M. J. Am. Chem. Soc. 1999, 121, 1651. (b) Pan, L.; Huang, X.; Li, J.; Wu, Y.; Zheng, N. Angew. Chem., Int. Ed. Engl. 2000, 39, 527. (c) Pan, L.; Woodlock, E. B.; Wang, X.; Zheng, C. Inorg. Chem. 2000, 39, 4174. (d) Ma, B.-Q.; Zhang, D.-S.; Gao, S.; Jin, T.-Z.; Yan, C.-H.; Xu, G.-X. Angew. Chem., Int. Ed. Engl. 2000, 39, 3644. (e) Reineke, T. M.; Eddaoudi, M.; O’Keefe, M.; Yaghi, O. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 2590. (f) Serpaggi, F.; Ferey, G. J. Mater. Chem. 1998, 8, 2737. (g) Reineke, T. M.; Eddaoudi, M.; Moler, D.; O’Keefe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 4843. (9) Ma, L.; Evans, O. R.; Foxman, B. M.; Lin, W. Inorg. Chem. 1999, 38, 5837. (10) (a) Lengendziewicz, J.; Borzechowska, M.; Oczko, G.; Meyer, G. New J. Chem. 2000, 24, 53. (b) Panagiotopoulos, A.; Zafiropoulos, T. F.; Perlepes, S. P.; Bakalbassis, E.; Masson-Ramade, J.; Kahn, O.; Terzis, A.; Raptopoulou, C. Inorg. Chem. 1995, 34, 4918.

Chem. Mater., Vol. 14, No. 6, 2002 2617 removed from the gel-solution interface, new crystals of the same color but different morphology (compound 4) were formed within 2 months (0.20 × 0.10 × 0.09 mm3). X-ray analysis showed that they were isostructural to lanthanum adipate.7 All attempts to obtain at pH 6 either compound 4 or 5, in powder form, resulted in the formation of another, unidentified crystalline phase. [Sm2(ad)3(H2O)4]1.5H2O (6) was prepared with the same technique as that used for neodymium adipate, but at pH ) 8.1. Prismatic colorless crystals (0.4 × 0.3 × 0.1 mm3) were formed after about 5 months and are isostructural to gadolinium adipate. This phase was also prepared in powder form by mixing, under stirring, 45 mL of aqueous 0.1 M Sm(NO3)3 (pH ) 6) with 10 mL of 0.1 M adipic acid solution (pH ) 6). The experimental XRPD pattern was in agreement with that simulated from the single-crystal data. [Gd2(ad)3(H2O)4]xH2O (7). A sodium silicate solution (Merck, d ) 1.06 g‚cm-3) was slowly added, under vigorous stirring, to an aqueous solution prepared by mixing 16 mL of distilled water, 2 mL of 0.1 M adipic acid, and 2 mL of 1 M HNO3. When the pH reached the value of 7.9, the resulting gelling solution was poured in a test tube and allowed to stand for about 2 days. An aqueous solution of 0.07 M GdCl3 was then carefully added at the top of the gel. Colorless transparent prismatic crystals of approximate dimensions 0.25 × 0.15 × 0.05 mm3 were formed within 10 days. Their composition was established by X-ray crystallography. No lattice water could be found in the crystal (x ) 0). This phase was also prepared in powder form by mixing 45 mL of a solution of 0.1 M GdCl3 (pH ) 6) with 10 mL of a solution of 0.1 M adipic acid (pH ) 6). The precipitate was washed with water and left to dry in air. The water content (x ≈ 1.4) was determined by thermogravimetric measurements. The experimental X-ray pattern was in good agreement with the simulated pattern reproduced from the Fc values of the crystal structure. Anal. Calcd (%): C, 25.59; H, 4.12. Found (%): C, 25.32; H, 3.74. [Er2(ad)3(H2O)4]xH2O (8) was prepared in powder form by mixing, under stirring, 30 mL of an aqueous solution of 0.02 M Er2(SO4)3 (pH ) 6) with 13.5 mL of an aqueous solution of 0.1 M adipic acid (pH ) 6). The precipitate was abundantly washed with water and dried in air. The XRPD spectrum showed that this phase is isostructural to gadolinium adipate. [Yb2(ad)3(H2O)4]xH2O (9) was prepared in powder form, by mixing, under stirring, 45 mL of an aqueous solution of 0.1 M YbCl3 (pH ) 6) with 45 mL of an aqueous solution of 0.1 M adipic acid (pH ) 6). The precipitate was abundantly washed with water and dried in air. The XRPD spectra showed that this phase is isostructural to gadolinium adipate. [La(pim)(Hpim)(H2O)]H2O (10). Single crystals were prepared by using a double-diffusion silica gel technique. A “neutral” gelling solution was prepared by adding a silicate solution (d ) 1.06 g‚cm-3) to an aqueous solution of 0.1 M HNO3 until pH 6. The solution was carefully poured in a glass U tube and allowed to stand for about 1 day. Then, 10 mL of 0.05 M pimelic acid (pH ) 6) was added to one side of the gel column and 10 mL of 0.05 M La(NO3)3 was added to the other side. Some crystals of approximate dimensions 0.4 × 0.1 × 0.03 mm3, suitable for X-ray analysis, were formed after about 4 months. Their composition was established by X-ray crystallography. This crystalline phase was also prepared in powder form by mixing 25 mL of 0.5 M pimelic acid at pH ) 6.2 with 25 mL of 0.5 M La(NO3)3. The resulting mixture was stirred for about 1.5 h, and then the precipitate formed was separated by filtration, washed thoroughly with water, and air-dried for some days. The experimental XRPD pattern is in good agreement with the simulated one. Anal. Calcd (%): C, 34.08; H, 4.87. Found (%): C, 32.87; H, 5.19. Physical Measurements. Simultaneous TG-DTA analysis was carried out, in air, at a scan speed of 5 °C, on a NETZSCH STA 449C apparatus. X-ray powder diffraction (XRPD) data were collected on a Bruker D8 Advanced System diffractome-

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Table 1. Coordination Bond Lengths (Å) for Compounds 2, 3, and 4 M- - -O

Ce2(C6O4H8)3(H2O)10

Pr2(C6O4H8)3(H2O)10

Nd2(C6O4H8)3(H2O)10

M- - -O(1w) M- - -O(2w) M- - -O(1)a M- - -O(1) M- - -O(2) M- - -O(3)b M- - -O(4) M- - -O(5)c M- - -O(6)

2.514(2) 2.493(3) 2.513(2) 2.779(2) 2.563(2) 2.467(2) 2.578(2) 2.494(2) 2.610(2)

2.497(3) 2.478(3) 2.495(2) 2.771(2) 2.545(2) 2.441(2) 2.560(2) 2.474(2) 2.589(2)

2.481(3) 2.463(3) 2.474(3) 2.761(3) 2.524(3) 2.415(3) 2.533(4) 2.460(3) 2.572(3)

a-c Symmetry codes: (a) -x, 1 - y, -z; (b) 1 - x, -y, -z; (c) x, 1 + y, z.

Table 2. Coordination Bond Lengths (Å) for Compounds 5, 6, and 7 M- - -O

Nd2(C6O4H8)3(H2O)5

Sm2(C6O4H8)3(H2O)5.5

Gd2(C6O4H8)3(H2O)4

M- - -O(1w) M- - -O(2w) M- - -O(1) M- - -O(2) M- - -O(3)a M- - -O(3)b M- - -O(4)b M- - -O(5) M- - -O(6)

2.449(3) 2.428(3) 2.474(3) 2.564(3) 2.442(3) 2.636(3) 2.524(3) 2.481(3) 2.500(3)

2.427(5) 2.401(5) 2.444(5) 2.535(5) 2.408(5) 2.622(5) 2.504(5) 2.449(5) 2.470(5)

2.401(8) 2.399(8) 2.425(8) 2.520(8) 2.386(8) 2.611(8) 2.480(8) 2.418(8) 2.456(7)

a,b

Symmetry codes: (a) x, y, z - 1; (b) 0.5 - x, 0.5 - y, 1 - z.

ter. Simulation of the XRPD patterns was carried out with the CaRIne software.11 Single-Crystal Structure Analyses. Data were collected at room temperature using Bruker P4 and CAD4 diffractometers with monochromatic Mo Ka radiation. All structures were solved by direct methods using SHELXS-86 and SHELXL97 software.12,13 All non-hydrogen atoms were refined anisotropically. Coordination bond lengths for compounds 2-4 are given in Table 1, and those for compounds 5-7 are given in Table 2. Bond lengths and angles and the hydrogen-bonding geometry for compound 10 are shown in Table 3. Drawings were made using SCHAKAL-97 software.14

Results and Discussion Single crystals of composition [Ln2(ad)3(H2O)4]6H2O, Ln ) Ce3+ (2), Pr3+ (3), Nd3+ (4), were found by X-ray (11) Boudias, C.; Monceau, D. CaRIne Crystallography 3.1; Universite de Compiegne: France, 1998. (12) (a) Sheldrick, G. M. SHELXS 86: Structure Solving Program; University of Gottingen: 1986. (b) Sheldrick, G. M. SHELXL 97: Crystal Structure Refinement; University of Gottingen: 1997. (13) Crystal data for 2, Ce2(C6O4H8)3(H2O)10: triclinic, P-1, a ) 9.234(1) Å, b ) 9.766(2) Å, c ) 10.690(1) Å, R ) 67.83(1)°, β ) 84.48(1)°, γ ) 61.85(1)°, V ) 783.2(2) Å3, final R1 value 0.0195 for 2594 independent reflections (I > 2σ(I)). Crystal data for 3, Pr2(C6O4H8)3(H2O)10: triclinic, P-1, a ) 9.242(1) Å, b ) 9.764(1) Å, c ) 10.682(1) Å, R ) 67.75(1)°, β ) 84.58(1)°, γ ) 61.79(1)°, V ) 781.9(1) Å3, final R1 value 0.0213 for 2555 independent reflections (I > 2σ(I)). Crystal data for 4: Nd2(C6O4H8)3(H2O)10: triclinic, P-1, a ) 9.236(2) Å, b ) 9.743(1) Å, c ) 10.668(2) Å, R ) 67.60(1)°, β ) 84.70(1)°, γ ) 61.77(2)°, V ) 777.4(2) Å3, final R1 value 0.0229 for 2462 independent reflections (I > 2σ(I)). Crystal data for 5, Nd2(C6O4H8)3(H2O)5: monoclinic, C2/c, a ) 23.657(2) Å, b ) 14.184(2) Å, c ) 8.881(1) Å, β ) 108.81(1)°, V ) 2867.3(6) Å3, final R1 value 0.0277 for 2308 independent reflections (I > 2σ(I)). Crystal data for 6, Sm2(C6O4H8)3(H2O)5.3: monoclinic, C2/c, a ) 23.511(4) Å, b ) 14.094(2) Å, c ) 8.847(1) Å, β ) 105.60(1)°, V ) 2823.6(7) Å3, final R1 value 0.0349 for 1867 independent reflections (I > 2σ(I)). Crystal data for 7, Gd2(C6O4H8)3(H2O)4: monoclinic, C2/c, a ) 23.347(2) Å, b ) 14.099(1) Å, c ) 8.803(1) Å, β ) 106.03(1)°, V ) 2785.1(3) Å3, final R1 value 0.0373 for 1372 independent reflections (I > 2σ(I)). Crystal data for 10, La(C7O4H10)(C7O4H11)(H2O)2: monoclinic, P21/n, a ) 9.144(2) Å, b ) 8.729(3) Å, c ) 22.460(3) Å, β ) 92.09(3)°, V ) 1791.4(8) Å3, final R1 value 0.0438 for 1664 independent reflections (I > 2σ(I)).

Table 3. Coordination and Hydrogen Bond Lengths (Å) for Compound 10 La- - -O(5) La- - -O(7)a La- - -O(8) La- - -O(1) La- - -O(12)

2.462(7) 2.487(6) 2.565(6) 2.591(7) 2.597(8)

La- - -O(6) La- - -O(3) La- - -O(2) La- - -O(5)b La- - -O(7)

O(2)sH- - -O(4)c O(2w)sH- - -O(1w)d O(1w)sH- - -O(6)e

Hydrogen Bonds 2.458(1) O(2w)sH- - -O(1w) 2.799(1) O(1w)sH- - -O(1w)e 2.852(1)

2.603(7) 2.641(7) 2.708(8) 2.711(7) 2.761(7) 2.770(1) 2.876(1)

a-e Symmetry codes: (a) -x, 2 - y, -z; (b) -x, 1 - y, -z; (c) -x, -y, -z; (d) 0.5 - x, -0.5 + y, 0.5 - z; (e) -x, 2 - y, 1 - z.

structure analysis to be isostructural to 1. Details about the structure of 1 (hereafter denoted as structure type I) may be found in ref 7. Single crystals of composition [Ln2(ad)3(H2O)4]xH2O were grown for Ln3+ ) Nd3+ (5), Sm3+ (6), and Gd3+ (7). Moreover, the XRPD patterns revealed that this structure type (hereafter denoted as structure type II) holds also for Er3+ (8) and Yb3+ (9). For purposes of comparison, the coordination spheres for both types of structure are shown in Figure 1. In both cases, the coordination number is nine, but in structure type I there are five organic ligands around the metal while there are only four in structure type II. Presumably this is a manifestation of the well-known effect of lanthanide contraction and has some dramatic effects on crystal architecture. Indeed, in structure type I, the presence of five organic ligands permits extension of the metal organic network in the third dimension of space (Figure 2). This is not possible in the second type of structure, which is made up of two-dimensional metal-organic networks with large squarelike cavities (Figure 3). We may remark that the metal centers are grouped in pairs via bridging carboxylate oxygen atoms. Each pair of metals is four-connected to four neighboring pairs, and the network thus formed is topologically equivalent to a (4,4) network. Each cavity is interwoven by another network in the so-called inclined interpenetration mode (Figure 4), producing an interlocked 3-D structure.15 Interestingly, the large angle of interpenetration (∼60 °C) creates channels parallel to the c axis (Figure 5). The minimum width of the channels (d), measured as the distance between the nearest opposite carbon atoms, depends on the nature of the metal cation (Nd3+, 5.66 Å; Sm3+, 5.52 Å; Gd3+, 5.33 Å). We remark that d decreases in an approximately linear manner with the ionic radius16 of the metal. An extrapolation to the lower radii of Er3+ and Yb3+ shows that d for these compounds should be around 5 Å. Thus, there is an enlargement of about 0.7 Å in the width of the channels. This is probably significant for the adsorption properties of these compounds. In this respect, we observed by X-ray analysis that in the case of the Gd3+ polymer the channels are essentially empty whereas residual electron density compatible with the presence of disordered water molecules was found in the corresponding Nd3+ and Sm3+ polymers. (14) Keller, E. SCHAKAL97. A Fortran Program for the Graphical Representation of Molecular and Crystallographic Models; Kristallographishes Institut der Universitat, Freiburg: Germany, 1997. (15) Batten, S. R.; Robson, R. Angew. Chem., Int. Ed. Engl. 1998, 37, 1460. (16) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.

Microporous Rare Earth Coordination Polymers

Figure 1. Coordination mode of the adipate dianions for both structure types: (a) structure type I, Pr2(C6O4H8)3(H2O)10; (b) structure type II, Gd2(C6O4H8)3(H2O)4.

In the introductory section of this work, we invoked the possible role of the hydrogen bond network in the passage from structure type I to structure type II. The topology of the hydrogen-bonded water molecules in structure type I is given in Figure 6. Of interest here is the variation of the strength of the hydrogen-bonded network across the lanthanide series. To do this, we will assume that the well-known length-strength relationship (i.e. the shorter the contact, the stronger the interaction) is valid. This is not unreasonable, since all

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these networks are isostructural. All hydrogen bonds in compounds 1-4 are shown in Table 4. For a given compound and for each bond, we calculated the difference in bond length (∆d) between that of the compound under examination and that of 1. The results are shown in Figure 7. It is obvious that when going from Ce3+ to Nd3+ there is a very slight progressive elongation of most of the H bonds, but the energetic effect should certainly be negligible. Therefore, the passage to the second structure type (type II) observed from Nd3+ up to Yb3+ is to be essentially ascribed to the lanthanide contraction effect. TG-DTA analysis performed, in air, on a powder sample of gadolinium adipate (see Experimental Section) shows an endothermic weight loss of 2.91% between ambient temperature and 100 °C. The broadness of the peak suggests that only lattice water is removed in this step (calcd 2.98%) without any phase transformation.17 This is also consistent with the crystal structure analysis results, which showed no lattice water in the channels. In a second step, completed at about 160 °C, we observe an endothermic weight loss of 7.55% corresponding to removal of all coordinated water molecules (calcd 8.53%). The XRPD patterns showed that a phase transformation occurs after complete dehydration, leading to an unidentified crystalline phase. This transformation is reversible because the XRPD patterns obtained after rehydration are identical with those of the initial compounds (see Supporting Information). The single crystals obtained from the interaction of pimelic acid with La3+ were of composition [La(pim)(Hpim)(H2O)]H2O, 10. The La3+ cation is 10-fold coordinated by 10 oxygen donor atoms belonging to three hydrogen pimelate (Hpim-) anions, four pimelate (pim2-) dianions, and one water molecule. The Hpim- ligands act in the monodentate and bidentate-bridging modes while the pim2- ligands act in the chelate-bridging mode. A consequence of these modes of coordination is the formation of carboxylate-bridged helicoidal metal chains parallel to the b axis of the crystal (Figure 8). The metal-metal distances are 4.324 and 4.500 Å. The metal chains are connected through the Hpim- ligands running along the one diagonal of the ac plane and the pim2- ligands running along the other diagonal of the same plane (Figure 9). The starlike distribution of ligands around the metal centers creates channels of variable width along the b axis. Well-ordered lattice

Figure 2. Projection of the structure of Pr2(C6O4H8)3(H2O)10 along b. For clarity only half of the non-centrosymmetric ligand is shown. This ligand (marked with an asterisk) is directed appriximately upward and downward, ensuring the 3-D character of the network.

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Figure 3. Projection of a 2-D network of Gd2(C6O4H8)3(H2O)4. The structure is made up of interpenetrated 2-D networks.

Figure 5. Projection of the structure of Sm2(C6O4H8)3(H2O)4 along c. A channel containing disordered water molecules (nonbonded open circles) is created by the inclined interpenetration. d represents the minimum width of the channel.

Figure 4. Inclined interpenetration of 2-D networks of Gd2(C6O4H8)3(H2O)4. Four such networks are shown. The circles represent the midpoint between two bridged Gd3+ cations. The topological equivalence of the present network with a (4,4) network is obvious.

water molecules (OW1) linked by hydrogen bonds reside in the channels. Additional H-bonds connect the guest molecules to the coordinated water molecules (Figure 9). The conformation of the Hpim- ligands is twisted while that of pim2- is extended, as in the case of the similar (but not isostructural) compound aqua(glutarato)(hydrogen glutarato) lanthanum(III) monohydrate.18 Furthermore, the distance 2.45 Å between O2 and O4 (Table 3) suggests formation of a very short hydrogen bond, as in the above-mentioned complex of La3+ with glutaric acid.18 A simultaneous TG-DTA analysis performed in air on a powder sample of lanthanum pimelate showed that complete dehydration (7.44% weight loss, calcd 7.29%) occurs between 50 and 125 °C. Another weight loss (12.26%), probably due to partial decomposition of the ligand, was observed between 160 and 270 °C, followed (17) (a) Kiang, Y.-H.; Gardner, G. B.; Lee, S.; Asgaonkar, A.; Venkataraman, D. J. Am. Chem. Soc. 1996, 118, 6946. (b) Jung, C.S.; Park, S. H.; Kim, K. M.; Jang, H. G. Inorg. Chem. 1998, 37, 5781. (18) Benmerad, B.; Guehria-Laidoudi, A.; Balegroune, F.; Birkedal, H.; Chapuis, G. Acta Crystallogr. 2000, C56, 789.

Figure 6. Topology of the hydrogen-bonded water molecules in the structure of Pr2(C6O4H8)3(H2O)10. Filled circles represent the metal cations. Table 4. Hydrogen Bond Lengths (Å) for 1 and the Difference, ∆d, in Hydrogen Bond Length between Those of Compounds 2-4 and That of 1 no.

type of H-bond DsH- - -A

1 d(D- - -A)7

1 2 3 4 5 6 7 8 9 10 11

O(1w)sH- - -O(6)a O(2w)sH- - -O(3w)b O(2w)sH- - -O(4)c O(4w)sH- - -O(2)d O(4w)sH- - -O(4w)e O(5w)sH- - -O(5w)f O(3w)sH- - -O(5)g O(3w)sH- - -O(4w)e O(1w)sH- - -O(3w)b O(5w)sH- - -O(10) O(4w)sH- - -O(5w)h

2.721 2.732 2.748 2.748 2.846 2.848 2.857 2.865 2.882 2.899 2.907

2

∆d(D- - -A) 3

4

-0.010 0. -0.014 0.012 0.010 0.011 0.002 0.020 0.029 0.008 0.009 0.009 -0.004 0.003 -0.005 -0.002 -0.007 -0.007 0.021 0.036 0.042 -0.002 0.033 0.035 -0.005 -0.014 -0.019 -0.005 0.030 0.013 0. -0.004 0.022

a-h Symmetry codes: (a) -x, 1 - y, -z; (b) x, 1 + y, z; (c) 1 - x, 1 - y, -z; (d) x, -1 + y, z; (e) -x, -y, -1 - z; (f) 1 - x, -y, 1 - z; (g) -x, -y, -z; (h) 1 - x, -y, -z.

by significant weight loss beyond 325 °C. No attempt was made to identify the products of decomposition.

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Figure 7. Graphical representation, for all HB types present in structure type I, of the difference ∆d between the hydrogen bond lengths of compounds 2-4 and that of 1. A tendency for progressive, although slight, elongation of the bond lengths when going from La3+ to Nd3+ is obvious.

Figure 8. Coordination modes of Hpim- and pim2- in the structure of 10. The hydrogen bonds are indicated by dashed lines.

Figure 9. Projection of the structure of 10 along b. The channel formed contains lattice water molecules. The dashed lines represent hydrogen bonds.

A sample heated separately in air at 105 °C and subjected to TG-DTA analysis showed 4.26% weight loss up to 125 °C, corresponding to removal of coordinated

water molecules (calcd 3.78%). Complete dehydration was observed by heating at 125 °C for some hours. The XRPD patterns showed that the structure is retained, even when heating the samples at 150 °C. As we have already mentioned, we attribute the change in crystal architecture in rare earth adipates to the well-known effect of lanthanide contraction. This effect was also invoked recently by Li et al.,8b to explain analogous results observed in a series of compounds of rare earth metals with 3,5-pyrazoledicarboxylic acid. The isostructurality observed within a series of rare earth compounds also offers the key for the fine-tuning of physical properties. We presented here the example of the type II adipate structure in which the width of the channel depends on the metal incorporated in the framework. Further tuning of this property may probably be achieved by formation of solid solutions. These effects are well-known in inorganic zeolites,19 but as far as we know this is the first time they are reported in coordination solids. Steric separations based on micropore size are well-known applications of molecularsieve zeolites (including drying).20 In future work we intend to study more closely the adsorption of water as a function of the pore width. Furthermore, magnetic10 or luminescence8a,9 properties may be systematically studied through formation of isostructural compounds or solid solutions. The creation of channels by interpenetration also merits some comments. There are some examples in the literature in which, despite the interpenetration, large channels still remain throughout the structure.2e,21 In the present case, however, the channels are created by the inclined interpenetration of the (4,4) sheets, and to the best of our knowledge, there is only one previous example in the literature.22 (19) Johnson, G. M.; Mead, P. J.; Dahn, S. E.; Weller, M. T. J. Phys. Chem. B 2000, 104, 1454. (20) Yang, R. T. Gas Separation by Adsorption Processes; Imperial College Press: London, 1997. (21) Evans. O. R.; Lin, W. Chem. Mater. 2001, 13, 2705.

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The presence of a non-ionized carboxylate group in lanthanum pimelate reflects the crucial role of the pH of the milieu from which the crystal precipitates.8b In this case, more organic ligands than in adipates are needed to neutralize the trivalent cation and the coordination number rises to 10. Consequently, a comparison of this structure with that of adipates is pointless. However, the comparison with aqua(glutarato)(hydrogen glutarato) lanthanum(III) monohydrate,18 which has a similar structure, shows that the pimelate has a more “open” structure, as evidenced by the significantly lower density (1.687 versus 2.060). Furthermore, the enclathrated and coordinated water molecules are removed without collapse of the structure, (22) Carlucci, L.; Ciani, G.; Proserpio, D. M. New J. Chem. 1998, 1319.

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while this is not possible in the case of the glutarate compound. We believe that these facts may be correlated with the longer methylene chain of pimelic acid. Acknowledgment. This work was partly supported by the Research Commitee of the University of Ioannina. We thank Dr. T. Vaimakis for recording the TGDTA diagrams, Dr. T. Bakas for the powder X-ray data, and the authorities of the region of Epirus for the purchase of the P4 diffractometer. Supporting Information Available: Tables of crystallographic data for compounds 2-7 and 10. TG-DTA diagrams for 7 and 10 and X-ray patterns (simulated and experimental) for 7 and 10 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM011302I