Construction of Lanthanide Metal−Organic Frameworks by Flexible

Construction of Lanthanide Metal-Organic Frameworks by Flexible Aliphatic Dicarboxylate Ligands Plus a Rigid m-Phthalic Acid Ligand De-Xin Hu, Feng Luo, Yun-Xia Che, and Ji-Min Zheng*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1733-1737

Department of Chemistry, Nankai UniVersity, Tianjin, 300071, P. R. China ReceiVed NoVember 29, 2006; ReVised Manuscript ReceiVed April 16, 2007

ABSTRACT: Six rare earth coordination polymers, namely, [La2(H2O)4(Hip)2(L1)2]‚4H2O (1) (H2L1 ) succinate acid, H2ip ) m-phthalic acid), Ln(H2O)(ip)0.5(L2) (Ln ) Gd (2), Dy (3), Y (4), H2L2 ) glutaric acid), [Sm2(H2O)2(ip)(L2)2]‚3H2O (5), and Er(H2O)2(ip)(L3)0.5 (6) (H2L3 ) adipic acid), have been synthesized hydrothermally from the self-assembly of the earth ions (Ln3+) with flexible aliphatic dicarboxylate ligands (H2L1, H2L2, H2L3) and the rigid m-phthalic acid ligand (H2ip). Compounds 2, 3, and 4 show the common R-Po nets built on six-connected dinuclear Ln2 units (Ln2 ) Gd2, Dy2, and Y2). The structure of compound 5 can be viewed from the tetrahedral sra-nets based on Sm-O-C rods. In compounds 1 and 6, the La-O-C rods and Er-O-C rods are in turn linked by organic spacers to give two dimensional (2D) square nets, which can be further extended to three-dimensional (3D) supramolecular frameworks via hydrogen-bonding contacts. Furthermore, the thermogravimetric analyses of all the compounds were carried out. Introduction The crystal engineering of metal-organic frameworks (MOFs) is becoming an increasingly popular field of research in view of the potential applications and unusual topologies of these new materials.1-3 Much work has focused on the rational design of multidimensional infinite architectures by controlling the favored geometry of ligands and metals, in which the construction of transition-metal-carboxylate polymers is a successful paradigm.4-7 Unfortunately, in contrast to the fruitful production of MOFs with d-block transition metal ions, the design and control over high-dimensional lanthanide-based frameworks is currently a formidable task owing to their high and variable coordination numbers and flexible coordination environments.8,9 On the other hand, lanthanide ions, with their high and variable coordination numbers and flexible coordination environments, are good candidates to provide unique opportunities for the discovery of unusual network topologies,10-12 thus leading us to this interesting and challenging field. It is believed that lanthanide ions have high affinity and prefer to bind to hard donor atoms (such as O-donor ligands). On the basis of this inherent nature, many three-dimensional (3D) lanthanide polymeric complexes with multicarboxylate ligands have been reported.13 So far, much work is focused on using single rigid multicarboxylate ligands (namely, R, e.g., 1,4-benzenedicarboxylic acid, 1,3,5-benzenetriacetic acid, and 1,2,4,5-benzenetetracarboxylic acid) or flexible multicarboxylate ligands (namely, F, e.g., H2L1, H2L2, H2L3) to prepare lanthanide-containing MOFs.14 By contrast, based on a query to the Cambridge Structural Database (CSD), we find that the synthesis of multidimensional lanthanide-containing MOFs by using hybrid multicarboxylate ligands of R+F is less developed,15 although there are some outstanding explorations aimed at constructing lanthanide-containing MOFs by utilization of polycarboxylate ligands with characteristics of both flexibility and rigidity, such as benzene-1,4-dioxylacetate and benzene-1,3-dioxylacetate.16 As we known, the introduction of an F ligand into the system of an R ligand plus lanthanide ions and vice versa provides * To whom correspondence should be addressed. Fax: 86-22-23508056. Tel: 86-22-23507950. E-mail: [email protected].

competition among oxygen atoms from F or R for lanthanide ions, thus leading to the formation of unforeseen lanthanidecontaining MOFs, which are the so-called third generation coordination polymers with dynamic frameworks and striking functions.13a Herein, we report our work aimed at enriching the field of (R+F)-based lanthanide-containing MOFs: six lanthanide-containing MOFs displaying diverse structural motifs were successfully produced by the aggregation of lanthanide ions, an R ligand of m-phthalic acid, and a series of F ligands (H2L1, H2L2, H2L3). Subsequently, the six framework polymers were primarily evaluated by using thermogravimetric analysis (TGA). Experimental Section Materials and Physical Measurements. Commercially available reagents are used as received without further purification. Elemental analyses (C, H) were performed on a Perkin-Elmer 2400 analyzer. Infrared spectra were obtained from KBr pellets on a Nicolet MagnaIR 560 Infrared spectrometer in the 4000-400 cm-1 region. Fluorescence measurements were performed on an Aminco-Bowman Series AB2 luminescence spectrometer. The crystal samples were characterized by TGA on a thermoflex analyzer (Rigaku) up to 800 °C using a heating rate of 5 °C min-1 in N2. Synthesis of [La2(H2O)4(Hip)2(L1)2]‚4H2O (1). A mixture of La(NO3)3‚6H2O (216 mg, 0.5 mmol), succinate acid (118 mg, 1.0 mmol), and isophthalic acid (133 mg, 0.8 mmol) was added to 8 mL of deionized water, and its pH was controlled in the range 4-5 with a 0.4 mol L-1 aqueous solution of potassium hydroxide. After the mixture was stirred for 10 min, it was placed in a 25 mL Teflon-lined reactor and heated at 160 °C in an oven for 3 days and then slowly cooled to room temperature. Well-shaped, light, colorless single crystals of complex 1 suitable for X-ray four-circle diffraction analysis were obtained (yield ca. 45%, based on La). Elemental analysis for 1: C24H34La2O24 (Mr ) 984.33). Calcd: C, 29.29; H, 3.48%. Found: C, 29.27; H, 3.51%. IR (KBr, cm-1): 3360(s), 2618(s), 1698(vs), 1611(s), 1544(vs), 1418(vs), 1328(m), 1239(s), 1156(m), 948(s), 800(m), 696(s). Synthesis of Ln(H2O)(ip)0.5(L2) (Ln ) Gd(2), Dy(3), Y(4)). Since polymers 2, 3, and 4 are isostructural, we thus present the preparation of the gadolinium complex. A procedure identical to that for 1 was followed to prepare 2, 3, 4 except that La(NO3)3‚6H2O and succinate acid were replaced by Ln(NO3)3‚6H2O (Ln ) Gd, Dy, and Y, 0.5 mmol) and glutaric acid (130 mg, 1.0 mmol) (yield ca. 52, 65, and 62%, based on Gd, Dy, and Y, respectively). Elemental analysis for 2: C9H10GdO7 (Mr ) 387.42). Calcd: C, 27.90; H, 2.60%. Found: C, 27.88; H, 2.57%.

10.1021/cg0608615 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007

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Table 1. Crystallographic Data and Structure Refinement Details for 1-6 formula fw T (K) cryst syst space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) Z Dc (g cm-3) µ (mm-1) F(000) reflns collected reflns unique goodness-of-fit R1 [I > 2σ(I)] wR2 (all data)

1 C24H34La2O24 984.33 298(2) triclinic P1h 9.5593(19) 12.298(3) 15.836(3) 71.08(3) 84.47(3) 70.51(3) 2 1.969 2.636 968 16280 7527 1.013 0.0508 0.1735

2 C9H10GdO7 387.42 294(2) monoclinic C2/c 17.4826(19) 14.2661(16) 8.3567(13) 90 95.276(2) 90 8 2.480 6.413 1472 6378 2449 1.038 0.0232 0.0556

3 C9H10DyO7 392.67 294(2) monoclinic C2/c 17.541(2) 14.2888(18) 8.3715(10) 90 95.332(2) 90 8 2.497 7.175 1488 6361 2479 1.036 0.0265 0.0688

Scheme 1. Coordinated Mode of Carboxyl Groups Involved in This Research

IR (KBr, cm-1): 3300(s), 3184(s), 1928(w), 1611(w), 1587(m), 1541(vs), 1436(s), 1319(s), 1295(m), 1190(s), 1008(m), 962(s), 827(s), 747(s), 627(s). Synthesis of [Sm2(H2O)2(ip)(L2)2]‚3H2O (5). A procedure identical to that for 2 was followed to prepare 5 except that Gd(NO3)3‚6H2O was replaced by Sm(NO3)3‚6H2O (222 mg, 0.5 mmol) (yield ca. 63%, based on Sm). Elemental analysis for 5: C18H26O17Sm2 (Mr ) 815.09). Calcd: C, 26.52; H, 3.22%. Found: C, 26.55; H, 3.24%. IR (KBr, cm-1): 3122(s), 1604(s), 1542(vs), 1456(vs), 1412(vs), 1350(s), 1274(s), 1196(m), 1058(m), 920(m), 812(m), 751(s), 650(m). Synthesis of Er(H2O)2(ip)(L3)0.5 (6). A procedure identical to that for 1 was followed to prepare 6 except that La(NO3)3‚6H2O and succinate acid were replaced by Er(NO3)3‚6H2O (231 mg, 0.5 mmol) and adipic acid (150 mg, 1.0 mmol) (yield ca. 59%, based on Er). Elemental analysis for 6: C11H12ErO8 (Mr ) 439.47). Calcd: C, 30.06; H, 2.75%. Found: C, 30.03; H, 2.78%. IR (KBr, cm-1): 3266(s), 1915(w), 1606(vs), 1519(vs), 1449(vs), 1287(s), 1140(s), 998(m), 959(m), 835(m), 742(s), 662(s), 528(m). X-ray Structural Studies. Suitable single crystals of 1-6 were selected and mounted in air onto thin glass fibers. Accurate unit cell parameters were determined by a least-squares fit of 2θ values, and intensity data were measured on a Bruker Smart CCD or Rigaku Raxis Rapid IP diffractiometer with Mo KR radiation (λ ) 0.71073 Å) at room temperature. The intensities were corrected for Lorentz and polarization effects as well as for empirical absorption based on a multiscan technique;17a all structures were solved by direct methods and refined by full-matrix least-squares fitting on F2 by SHELX-97.17b All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms except those for water molecules in all the compounds were located at geometrically calculated positions and refined with isotropic thermal parameters. Hydrogen atoms for water molecules in all the compounds were located from difference Fourier maps and refined with isotropic thermal parameters. Crystallographic data for the six compounds are summarized in Table 1.

4 C9H10O7Y 319.08 298(2) monoclinic C2/c 17.507(4) 14.196(3) 8.3366(17) 90 95.40(3) 90 8 2.055 5.677 1272 7842 2447 0.978 0.0303 0.0700

5 C18H26O17Sm2 815.09 294(2) monoclinic C2/c 7.9862(9) 14.9936(17) 19.864(2) 90 93.875(2) 90 4 2.281 4.985 1576 7255 2880 1.296 0.0411 0.0862

6 C11H12ErO8 439.47 293(2) triclinic P1h 6.7577(14) 10.003(2) 10.490(2) 92.79(3) 107.95(3) 108.78(3) 2 2.317 6.697 420 6229 2850 1.050 0.0225 0.0550

unit contains two La(III) ions, two Hip- ligands, two L1 ligands, four coordinated water molecules, and four free water molecules. The two La(III) ions show a coordination number (CN) of 10, which is completed by six L1 O atoms, two Hip- O atoms, and two water O atoms (O15, O16 for La1, and O17, O18 for La2). The La-O bond distances are in the range of 2.480-2.791 Å for La-Ocarboxylate and 2.611-2.617 Å for La-Owater, all of which are comparable to those reported for other La-oxygen donor complexes.18 The primal La1 and La2 are linked together by sharing one edge (O9, O13) with the nearest La-La distance of 4.401 Å. This connectivity is repeated infinitely along the ac plane to create La-O-C rods of edge shared LaO10 bicapped square antiprisms (Figure 1 and Supporting Information Figure S1). The rods are linked by the -C2H4- units of L1, which connect each rod to two neighboring rods in the ac plane, thus generating a 2D net structure with the Hip- ligands out of the net plane, together with an inter-rod distance of 9.635 Å (Figure 2). The carboxyl groups of L1 and Hip- adopt tridentate and chelate coordinated modes (IV, II), respectively. Furthermore, if the H-bond interactions (O4-H4A‚‚‚O12/ 2.620Å, O8-H8‚ ‚‚O19/2.610Å) are taken into account, then the special 2D net is extended to the 3D supramolecular frameworks with 15.1% void volume occupied by free water molecules.19 Ln(H2O)(ip)0.5(L2) (Ln ) Gd(2), Dy(3), Y(4)). X-ray diffraction studies suggest that polymers 2, 3, and 4 are isostructural, and thus we will restrict our presentation and discussion to the gadolinium complex. In this Gd-containing MOF, two ip2-, three L2, and one terminal water molecule (H2O(7)) provide eight O atoms for a Gd(III) ion to furnish the GdO8 polyhedron. The Gd-O bond lengths obviously vary from 2.24 to 2.52 Å, which is comparable to that observed in other Gd-containing polymers.14c,16,20 Two crystallographically equivalent Gd atoms are bridged by one ip2- carboxyl group and two L2 carboxyl groups to generate a dinuclear Gd2 fragment with a Gd-Gd distance of 3.853 Å, which joins two ip2- and four L2 to furnish the six-connected secondary building unit (SBU).

Results and Discussion Description of Crystal Structures. The coordinated modes of carboxyl groups are listed in Scheme 1, where modes I-IV are common, but mode V is rare. [La2(H2O)4(Hip)2(L1)2]‚4H2O (1). An X-ray diffraction study performed on complex 1 reveals that each asymmetric

Figure 1. The wires/sticks presentation of the La-O-C rod.

Construction of Lanthanide MOFs

Crystal Growth & Design, Vol. 7, No. 9, 2007 1735

eration of the geometry of this node, this 3D framework is classified to be an R-Po net with 41263 topology (Figure 4). [Sm2(H2O)2(ip)(L2)2]‚3H2O (5). Interestingly, under the same reaction conditions as mentioned above for polymers 2, 3, and 4, the structure of the Sm-displaced polymer is rather different. The main reason may be that CN ) 9 for Sm(III) ions, which causes the structural diversity. In the present framework, each Sm atom is nine-coordinated by six L2 O atoms, and two ip2- O atoms, as well as one terminal water O atom. The Sm-O bond lengths are in the range of 2.398-2.664 Å, similar to the Sm-O bond lengths in other Sm-containing polymers.14c,16,20a,21 The coordinated modes of carboxyl groups are modes II and IV. As illustrated in Figure 6, this 3D framework structure is constructed from Sm-O-C rods composed of edge-shared SmO9 polyhedra (Figure 5 and Supporting Information S2). The Figure 2. View of the 2D net built from the La-O-C rods with the ip2- out of the net plane.

Figure 5. The wires/sticks presentation of the Sm-O-C rod.

Figure 3. The six-connected dinuclear Gd2 unit acts as a six-connected node.

Consequentially, a 3D Gd-containing framework structure is built, where each SBU connects six neighboring SBUs in six directions (Figure 4). In contrast to the L1 ligand in 1, the L2 ligand in 2 is more flexible, and the resulting coordinated modes of carboxyl groups are bidentate and tridentate (III, IV). In addition, the completely deprotonated ip2- ligand does not remain silent but acts in the bi(bidentate) mode to bridge four metal centers. For clarity, we use the topological method to analyze this 3D framework: the six-connected SBU is viewed to be the sixconnected node (Figure 3); furthermore, based on the consid-

adjacent Sm-Sm distance is 4.124 Å. Each rod connects to four neighboring rods by both -C3H6- units of L2 and -C6H6- linkers of ip2-. The separated distance between neighboring rods is 6.408 Å. The resulting frameworks hold 9.1% void space, in which free water molecules reside. From a topological view, the structure can be simplified by connecting the carboxylate carbon atoms at the vertices of the rods to form a tetrahedral sra-net (Figure 6). To the best of our knowledge, the sra-net built on M-O-C rods (M ) transition metal) is productive;22 however, the sra-net built on Ln-O-C rods (Ln ) rare earth metal) is less developed. Er(µ-OH2)2(ip)(L3)0.5 (6). The structure of 6 is also constructed from Er-O-C rods composed of edge-sharing ErO9 polyhedra (Figure 7 and Supporting Information S3). Each Er atom is ligated by four L3 O atoms, three ip2- O atoms, and two terminal water O atoms with Er-O bond lengths in the range of 2.227-2.638 Å, which are comparable to those observed in other Er-containing polymers.14b,23 The coordinated

Figure 4. Left: 3D Gd-containing frameworks. Right: schematic description of this 3D six-connected R-Po framework by the node-to-node method.

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Figure 6. Left: 3D Sm-containing frameworks. Right: schematic description of this 3D four-connected sra-net by connecting the carboxylate carbon atoms at the vertices of the rods.22

Figure 7. The wires/sticks presentation of the Er-O-C rod.

Figure 8. View of the 2D net built from the Er-O-C rods.

modes of carboxyl groups in this structure are modes I, II, and V. As mentioned above in the structure, each Er-O-C rod running along [100] combines with two neighboring rods via both -C4H8- units of L3 and -C6H6- linkers of ip2-, thus resulting in a 2D sheet structure along [101] (Figure 8). In addition, these 2D sheets are further arranged to construct 3D supramolecular frameworks through O2‚‚‚O7/2.653 Å contacts. Note from these structures of the abovementioned compounds we find in this system that the rare earth ions favor F oxygen atoms over R oxygen atoms and the dimension of these compounds are somewhat determined by the carboxyl groups of R: in polymer 1, the partly deprotonated Hip- with one carboxylate showing the chelate mode II induces the formation of the 2D net; similarly, the carboxyl groups of ip2- adopting

Figure 9. TG curves for 1 (black), 2 (red), 5 (blue), and 6 (green).

the chelate II and monodentate I mode correspond to the 2D net of compound 6, whereas in polymers 2-5, the bi(bidentate) or bi(chelate) modes of carboxyl groups of ip2- generate the 3D frameworks. However, in the present lanthanide metalorganic coordination compounds, the average bridging Ln-O bond lengths and the CN of lanthanide ions show a regular order, which is derived from the lanthanide contraction. Thermal Analyses. Thermogravimetric analysis (TGA) was carried out in the interest of studying the thermal behaviors of the open-framework polymer materials. The experiments were performed on samples consisting of numerous single crystals of each compound under N2 atmosphere with a heating rate of 5 °C/min. TGA of 1 shows the first step weight loss (14.6%) at 56-175 °C, corresponding to the loss of free water molecules and coordinated water molecules (calcd: 14.5%); there is no weight loss from 175 to 284 °C, and after that, polymer 1 begins to chemically decompose. Since compounds 2, 3, and 4 are isostructural, TG analysis was carried out only on 2. In the TG plot, there are two main weight losses: the first, from 136 to 205 °C, corresponds to the release of coordinated water molecules (calcd: 4.6%, observed: 4.4%), and the second weight loss is due to decomposition of the compound. As for compound 5, the initial weight loss in the range 74-208 °C corresponds to the loss of the lattice water and coordinated water molecules (calcd: 11.0%, observed:11.4%), and then the remaining compound is stable up to 400 °C. As for 6, the release of two coordinated water molecules is observed (calcd: 8.2%, observed: 7.4%) in the temperature range 115-200 °C, and after 400 °C, the remaining compound begins to decompose (Figure 9). Hence, we can see that the decomposition of the (R+F)-based lanthanide-containing compounds will be up to 400 °C.

Construction of Lanthanide MOFs

Conclusion Through this original study, we have synthesized six lanthanide-containing MOFs by rationally selecting mixed ligands of F+R. Compounds 1 and 6 are 2D net structures and are further extended to 3D supramolecular frameworks via H-bond interactions, built from La-O-C and Er-O-C rods, respectively. However, in polymer 5, the combination of Sm-O-C rods with organic linkers gives 3D sra-frameworks. Polymers 2, 3, and 4 are isostructural and show common R-Po nets built from dinuclear Ln2 units (Ln ) Gd(2), Dy(3), Y(4)). To the best of our knowledge, analogous lanthanide-containing MOFs constructed from mixed ligands of F+R are not found; obviously, our reports are the first case. Hence, we believe that the preliminary results presented here may provide a promising pathway to the rational design of diverse lanthanide-containing MOFs frameworks, a goal that we are pursuing actively. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50572040). Supporting Information Available: Crystallographic information files and polyhedron presentation of the La-O-C rod of polymer 1, the Sm-O-C rod of polymer 5, and the Er-O-C rod of polymer 6. This material is available free of charge via the Internet at http:// pubs.acs.org.

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