Hydrothermal Syntheses and Characterization of Novel 3D Open

CRYSTAL GROWTH & DESIGN

Hydrothermal Syntheses and Characterization of Novel 3D Open-Framework and 2D Grid Lanthanide Fumarates: Ln2(fum)3(H2fum)(H2O)2 (Ln ) Ce or Nd), [Sm2(fum)3(H2O)4](H2O)3, and [Yb2(fum)3(H2O)4](H2O)2

2006 VOL. 6, NO. 4 933-939

Guoqi Zhang, Guoqiang Yang,* and Jin Shi Ma CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China ReceiVed September 22, 2005; ReVised Manuscript ReceiVed February 8, 2006

ABSTRACT: Four novel open-framework or rectangular grid coordination polymers of lanthanide fumarates, where the lanthanides CeIII, NdIII, SmIII and YbIII were used, were readily synthesized and structurally characterized. The direct hydrothermal reaction of fumaric acid and appropriate lanthanide oxides provided a highly efficient approach to the syntheses of multidimensional framework solids. Two types of three-dimensional (3D) open frameworks, Ln2(fum)3(H2fum)(H2O)2 (Ln ) Ce or Nd; 1 and 2) and [Sm2(fum)3(H2O)4](H2O)3 (3), and one two-dimensional (2D) rectangular grid polymer, [Yb2(fum)3(H2O)4](H2O)2 (4), have been isolated from the hydrothermal reaction. The solids 1 and 2 are isostructural and consist of three types of ligand binding modes contributing to link the metal-O polyhedral chains to 3D open-framework architectures. Compound 3 has a 3D open-framework structure remarkably different from those of 1 and 2, owing to distinct ligand bridging patterns. The framework is microporous and incorporates some crystalline water molecules therein by hydrogen-bonding interactions. In contrast, the structure of compound 4 features a 2D rectangular grid-like framework composed of large cavities (10.5 × 9.4 Å2). Furthermore, the thermal behaviors and PXRD patterns of four compounds correlated with the corresponding structural features have been also investigated. Introduction The synthesis of inorganic-organic hybrid framework solids with porous structures from small molecular building blocks has attracted high concern for some years and is still of current interest,1,2 owing to the observation of the large number of potential applications in catalysis, gas sorption and desorption, fluorescent sensing, optoelectronic devices, and molecular magnetism.3 The wide combination of simple organic linkers (polycarboxylates, polyphosphonates, etc.) with inorganic metals involving transition metals and rare earths may lead to a rich variation and modulation of both open-framework structures and physical properties of the resultant solid materials.4-6 Hydrothermal synthesis has recently become a useful tool for the fabrication of novel metal-carboxylate frameworks.7 It is of obvious advantage in that the products obtained by hydrothermal methods generally present relatively compact crystal packing and reduced metal-aqua coordination, and hence condensed metal-carboxylate frameworks. In the field, most of the previous studies have focused on the syntheses of coordination polymers containing transition-metal ions but few lanthanide ions, due to the predictability of the coordination geometry of transition metal ions in contrast with lanthanide ions.8 Hybrids incorporating rare earths and aliphatic, unsaturated organic acids such as acetylenedicarboxylic or fumaric acid may be promising multidimensional solid materials, according to the limited amounts of literature reported only recently.6,9,10 While a few transition metal fumarate frameworks were synthesized under hydrothermal conditions, which usually displayed interesting structures and special properties,9 rare earth fumarate hybrids have been rarely obtained to date.11 Transition metal fumarate complexes usually exhibit versatile and interesting coordination modes and framework topologies, since the dicarboxylate groups of fumaric acid can act as both monodentate and multidentate ligands. By the use of fumaric * To whom correspondence should be addressed. E-mail: gqyang@ iccas.ac.cn.

acid, the one-dimensional (1D) coordination polymer of Cu(II),9a the 1D and 3D polymer frameworks of Cu(II) and mixed imine ligands,9c the 2D grid network of Cd(II),9d the 3D homometallic molecular ferrimagnet of Ni(II),9b and the interpenetrated diamondoid net of Ni(II) and sheets of Ni(II) or Co(II) with mixed imine ligands9e have been previously observed. Attracted by the performance of the diversity on topologic structures of such transition metal fumarates and the observed open frameworks with special luminescent and magnetic properties of other lanthanide carboxylates,8,10-13 we recently have been interested in studies of the hydrothermal syntheses and properties of lanthanide fumarate frameworks. Herein we report on the synthesis, characterization, and X-ray diffraction analysis of four novel lanthanide fumarates, Ln2(fum)3(H2fum)(H2O)2 (Ln ) Ce or Nd; 1 and 2), [Sm2(fum)3(H2O)4](H2O)3 (3), and [Yb2(fum)3(H2O)4](H2O)2 (4). Three 3D polymeric solids with two types of open frameworks and one 2D rectangular grid polymer were obtained. All four framework polymers were characterized by FT-IR spectroscopy, elemental analysis, TGA-DSC, and powder X-ray diffraction analysis. Experimental Section General. All starting materials were purchased from Aldrich Chemicals and used without further purification. Typically, the preparation of the coordination polymer was performed by combining fumaric acid and the corresponding lanthanide oxide under hydrothermal conditions in a Teflon-lined vessel (12 mL). All preparations were repeated at least three times and found to be fully reproducible. All the products were highly stable in air and insoluble in water and common organic solvents at ambient conditions. Characterization of the samples was performed by using X-ray diffraction, combined thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), FT-IR spectra, and elemental analysis. Powder X-ray diffraction (PXRD) data were collected using monochromated Cu KR radiation (λ ) 1.540 56 Å) on a Rigaku D/max2500 diffractometer. TGA-DSC analysis was obtained on a NETZSCH STA 409PC instrument in the temperature range of 30-800 °C (10 °C/min) with flowing N2(g). A BIO-RAD FT-165 IR spectrometer was used to gather FT-IR spectra using KBr pellets. Elemental analyses were done with a Carlo Erba-1106 instrument.

10.1021/cg050488l CCC: $33.50 © 2006 American Chemical Society Published on Web 03/09/2006

934 Crystal Growth & Design, Vol. 6, No. 4, 2006

Zhang et al.

Table 1. Crystal and Structure Refinement Data for Compounds 1-4

empirical formula formula wt crystal size, mm3 crystal color crystal system space group T, K λ, Å a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z F(000) dcalcd, g/cm3 µ, mm-1 reflns collected unique reflns variables R1a, wR2b, % goodness of fit a

1

2

3

4

C16H14O18Ce2 774.51 0.35× 0.28 × 0.17 light yellow monoclinic P21/c 293(2) 0.710 73 8.3950(4) 14.6394(8) 8.7713(5) 90.00 103.0970(10) 90.00 1049.93(10) 2 740 2.450 4.377 5988 2060 173 1.68, 4.11 1.110

C8H7O9Nd 391.37 0.29× 0.13 × 0.08 light purple monoclinic P21/c 293(2) 0.710 73 8.3358(17) 14.536(3) 8.7214(17) 90.00 103.06(3) 90.00 1029.4(4) 4 744 2.519 5.086 9429 2329 171 2.06, 5.93 1.090

C12H6O19Sm2 744.39 0.28× 0.21 × 0.11 colorless monoclinic P21/n 293(2) 0.710 73 9.5162(19) 14.817(3) 14.844(3) 90.00 91.51(3) 90.00 2092.3(7) 4 1395 2.363 5.647 4664 4170 300 2.53, 6.69 0.995

C12H18O18Yb2 796.34 0.31× 0.26 × 0.22 colorless monoclinic C2/c 293(2) 0.710 73 15.8946(14) 9.3986(8) 14.0599(11) 90.00 91.9440 (10) 90.00 2099.2(3) 4 1496 2.520 8.943 5920 2062 187 1.89, 4.78 1.046

R ) (Fo - Fc)/(Fo). b Rw ) [w(Fo2 - Fc2)2/(w(Fo2)2)]1/2.

Ce2(fumarate)3(H2fumarate)(H2O)2, (1). Fumaric acid (0.174 g, 1.5 mmol), Ce2O3 (0.164 g, 0.5 mmol), and water (6 mL) were added to a Teflon-lined vessel, which was sealed and held at 150 °C for 16 h. The reaction vessel was allowed to slowly cool to room temperature. Some light yellow blocklike single crystals were found among amounts of amorphous pale-yellow powder. The crystals were isolated manually and then washed with deionized water and ethanol and dried in air. Yield: 38.7 mg, 10% based on CeIII. Further attempts to improve the yield were carried out by increasing the ratio of ligand to metal; for instant, when the ligand-metal ratio rises up to 5:1, about 30% yield of single crystals were separated, while the amorphous powder remains as a main component. Analytical data for the light yellow crystals: IR (KBr pellet, cm-1): 3487, 3418, 3067, 2851, 2357, 1682, 1543, 1369, 1319, 1207, 988, 907, 798, 687, 567. Anal. Calcd (%) for C16H14O18Ce2: C, 24.81; H, 1.82. Found: C, 24.77; H, 1.84. Nd2(fumarate)3(H2fumarate)(H2O)2, (2). Fumaric acid (0.174 g, 1.5 mmol), Nd2O3 (0.166 g, 0.5 mmol), and water (6 mL) were added to a Teflon-lined vessel, which was sealed and held at 150 °C for 16 h. The reaction vessel was allowed to slowly cool to room temperature. Large amounts of light-purple blocklike single crystals were obtained as a pure phase. The crystals were then washed with deionized water and ethanol and dried in air. Yield: 102 mg, 35% based on fumaric acid. IR (KBr pellet, cm-1): 3491, 3418, 3074, 2855, 2368, 1689, 1531, 1389, 1323, 1181, 976, 895, 806, 691, 579. Anal. Calcd (%) for C16H14O18Nd2: C, 24.55; H, 1.80. Found: C, 23.70; H, 1.62. [Sm2(fumarate)3(H2O)4](H2O)3, (3). Fumaric acid (0.174 g, 1.5 mmol), Sm2O3 (0.176 g, 0.5 mmol), and water (6 mL) were added to a Teflon-lined vessel, which was sealed and held at 150 °C for 12 h. After the reaction vessel was slowly cooled to room temperature, colorless blocklike crystals were observed and collected by filtration, then washed with deionized water and ethanol and dried in air. Yield: 206 mg, 56% based on SmIII. IR (KBr pellet, cm-1): 3425, 2342, 1690, 1535, 1396, 1200, 972, 895, 798, 691, 575. Anal. Calcd (%) for C12H20O19Sm2: C, 18.74; H, 2.62. Found: C, 18.75; H, 2.16. [Yb2(fumarate)3(H2O)4](H2O)2, (4). Fumaric acid (0.174 g, 1.5 mmol), Yb2O3 (0.254 g, 0.5 mmol), and water (6 mL) were added to a Teflon-lined vessel, which was sealed and held at 150 °C for 12 h. After the reaction vessel was slowly cooled to room temperature, colorless blocklike crystals of the product were observed and collected by filtration, then washed with deionized water and ethanol and dried in air. Yield: 247 mg, 62% based on YbIII. IR (KBr pellet, cm-1): 3368, 3179, 2399, 1558, 1408, 1215, 972, 895, 798, 656, 544, 447. Anal. Calcd (%) for C12H18O18Yb2: C, 18.10; H, 2.28. Found: C, 17.55; H, 2.00. Single-Crystal X-ray Diffraction. Suitable single crystals of 1-4 were selected and mounted in air onto thin glass fibers. Accurate unit cell parameters were determined by a least-squares fit of 2θ values

measured for 200 strong reflections, and intensity data sets were measured on a Bruker Smart 1000 CCD or Rigaku Raxis Rapid IP diffractometer with Mo KR radiation (λ ) 0.710 73 Å) at room temperature. The intensities were corrected for Lorentz and polarization effects, but no corrections for extinction were made. All structures were solved by direct methods. The non-hydrogen atoms were located in successive difference Fourier synthesis. The final refinement was performed by full matrix least-squares methods with anisotropic thermal parameters for non-hydrogen atoms on F2. Crystallographic data and experimental details for structure analyses are summarized in Table 1.

Results and Discussion Syntheses. Although the use of various dicarboxylic acids as bridges between lanthanide centers has been extensively studied with terephthalate, malonate, adipate, etc.,8,10-12 examples involving fumarate as bridging ligand are scarcely reported,11 probably due to the lack of an effective synthetic method and the poor solubility of the products in all common organic solvents. Inspired by the advantage of previous results for the hydrothermal synthesis of metal-carboxylate frameworks,5 we herein developed a new method for the high-yield synthesis of lanthanide fumarates. The direct combination of fumaric acid and the corresponding lanthanide oxides in an appropriate reaction ratio under hydrothermal conditions has led to the quantitative isolation of pure crystalline samples (14). The four compounds are highly stable in air and insoluble in water and common organic solvents. They were well characterized by FT-IR spectroscopy, elemental analysis, and powder X-ray diffraction. Accordingly, single-crystal X-ray structure analysis confirmed their structures in the solid state. Structural Analyses. Since the crystals of 1 and 2 are isomorphous according to the X-ray structure analysis, only the structure of 1 is depicted in detail. The crystal structure of complex 1 is polymeric with a repeating structure unit of formula Ce2(fum)3(H2fum)(H2O)2. The coordination mode of Ce(III) is shown in Figure 1. Each Ce(III) cation has the same ninecoordinate environment and is surrounded by eight oxygen atoms from seven distinct fumarate moieties including one protonated fumarate unit and one water molecule. Overall there are three types of Ce-O bridging modes in 1, as presented in Figure 2. 1-L1 and 1-L3 exhibit full monodentate and µ2-oxobridged chelating patterns, respectively, whereas 1-L2 shows a

Novel 3D Open-Framework and 2D Grid Ln Fumarates

Crystal Growth & Design, Vol. 6, No. 4, 2006 935

Figure 1. ORTEP drawing of 1 showing the coordination environments of CeO8(H2O) polyhedra and the polyoxo-bridging inorganic chain with edge-sharing mode. Hydrogen atoms are omitted for clarity.

Figure 2. Three distinct ligand conformations (1-L1, 1-L2, and 1-L3) observed in 1.

double monodentate coordination mode with the protonated fumarate unit. This is uncommon in the reported lanthanidedicarboxylate frameworks.10-14 The average Ce-O bond distance of CeO8(H2O) polyhedra is 2.54 Å; the shortest Ce-O separation is 2.419(2) Å, resulting from the Ce(1)-O(1) bond of bridging carboxylate, and the longest is 2.777(2) Å of Ce(1)O(5) from the edge-sharing Ce-O bond. Other distances of CeO(fum) vary in the range of 2.419-2.573 Å, comparable to the usual Ce-O(carboxylate) bond reported.15 Also, the Ce(1)O(9) bond from coordinated water shows the second longest separation of 2.595(2) Å. The CeO8(H2O) polyhedra centers are edge-sharing through three COO- bridges of the fumarate ligand, which thus create one-dimensional infinite chains (Figures 1 and 3). The adjacent cerium(III) centers have a general separation of 4.717 Å. This artificial one-dimensional inorganic cerium(III) oxide chain should be unusual and only similar to the reported Eu(III)-O chains.10a,12 Furthermore, the 1D infinite chains are linked together with type 1-L1 ligands to form a 2D layered structure along the crystallographic [100] direction. This 2D structure acts as a subnetwork, and the shortest interlayer distance of Ce‚ ‚‚Ce is 8.775 Å (calculated between the two cerium atom centers). Figure 3 represents the polyhedra and ball-stick model of the layered subnetwork. This type of organic-inorganic layered structure is very different from the reported transition metal fumarate frameworks, as well as the widely investigated

Figure 3. Perspective view of 1 down the [100] direction, showing the 2D layered framework structure. Cerium(III) polyhedra, carbon atoms, and oxygen atoms are represented in green, black, and red, respectively.

lanthanide adipate frameworks,9,12,15a but somewhat similar to the europium(III) acetylenedicarboxylate framework observed recently.10a Finally, the 2D layered structure is further constructed into a 3D open framework with the two types of ligand 1-L2 and 1-L3, as shown in Figure 4. The structure of 3 is composed of a repeating molecular composition [Sm2(fum)3(H2O)4](H2O)3, in which SmIII ions have two types of coordination environments, though both are surrounded by nine oxygen atoms, as shown in Figure 5. The SmIII centers are both linked to seven oxygen atoms from five


Zhang et al. Table 2. Selected Bond Lengths (Å) for Compounds 1-4a 1 Ce(1)-O(1) Ce(1)-O(3) Ce(1)-O(2)#1 Ce(1)-O(7) Ce(1)-O(5)

2.4188(17) 2.4716(17) 2.5344(18) 2.5732(18) 2.7774(18)

Nd(1)-O(1) Nd(1)-O(3) Nd(1)-O(2)#1 Nd(1)-O(7) Nd(1)-O(5)

2.385(2) 2.436(2) 2.498(2) 2.542(2) 2.756(2)

Ce(1)-O(4)#1 Ce(1)-O(5)#1 Ce(1)-O(6) Ce(1)-O(9)

2.4665(16) 2.4860(16) 2.5413(17) 2.5949(17)

Nd(1)-O(4)#1 Nd(1)-O(5)#1 Nd(1)-O(6) Nd(1)-O(9)

2.433(2) 2.461(2) 2.508(2) 2.562(2)

Sm(1)-O(2) Sm(1)-O(4) Sm(1)-O(8) Sm(1)-O(1w) Sm(2)-O(6) Sm(2)-O(8) Sm(2)-O(10) Sm(2)-O(4) Sm(2)-O(4w)

2.472(3) 2.628(3) 2.474(3) 2.477(3) 2.356(3) 2.638(3) 2.474(3) 2.422(3) 2.422(3)

2

3

Figure 4. Perspective view of 1 down the [001] direction showing the 3D open-framework structure. Cerium(III) polyhedra, carbon atoms, and oxygen atoms are represented in green, black, and red, respectively

Sm(1)-O(1) Sm(1)-O(3) Sm(1)-O(5) Sm(1)-O(12) Sm(1)-O(2w) Sm(2)-O(7) Sm(2)-O(9) Sm(2)-O(11)#2 Sm(2)-O(3w)

2.547(3) 2.494(3) 2.442(3) 2.367(3) 2.446(3) 2.514(3) 2.541(3) 2.446(3) 2.457(3) 4

Yb(1)-O(10) Yb(1)-O(9) Yb(1)-O(6) Yb(1)-O(3)

2.239(3) 2.264(3) 2.323(16) 2.370(3)

Yb(1)-O(1) Yb(1)-O(4)#3 Yb(1)-O(5) Yb(1)-O(4)

2.249(3) 2.267(2) 2.364(13) 2.421(2)

a Symmetry transformations used to generate equivalent atoms for 1-4: #1, x, -y - 1/2, z - 1/2; #2, x - 1/2, -y - 1/2, z - 1/2; #3, -x + 1/2, -y + 1/ , -z. 2

Figure 5. ORTEP drawing of 3 showing the coordination environments of SmO7(H2O)2 polyhedra with two unsymmetrical SmIII centers. Hydrogen atoms are omitted for clarity.

fumarate ligands and two coordinated water molecules. The Sm-O bond distances in two SmIII centers are obviously discrepant (Table 2). The shortest and longest Sm(1)-O lengths are 2.367(3) and 2.628(3) Å, respectively, while they are 2.356(3) and 2.638(3) Å for Sm(2)-O bond lengths, respectively. Therefore, the two centers of the SmO7(H2O)2 polyhedra are not symmetric, and they are bridged with three fumarate ligands, two in oxygen-sharing mode and one in carboxylatebridging pattern. Thus, the two SmIII centers have a separation of 4.178 Å. With exception of the coordinated water molecules, each asymmetric unit also contains three crystalline water molecules. There are three types of bridging coordination modes in 3, each with distinct crystal engineering functions. Two of them (3-L1 and 3-L2) are of mixed bidentate bridging and chelating modes, while the third type (3-L3) is of a bis-chelating mode (Figure 6). The edge-sharing SmO7(H2O)2 polyhedra centers are further connected through another carboxylate bridge to form a 1D infinite sinusoid-like inorganic chain. This chain is similar neither to the fully edge-sharing inorganic chain in the structure of 1 and 2 nor to the recently reported examples on EuIII-carboxylate frameworks.10a, 16 Again, all three types of ligands serve to link the 1D inorganic chains and propagate along three dimensions of space to produce the resultant 3D

open-framework architecture as shown in Figure 7. This framework is microporous and has small 1D void channels. The dimensions of the pore are small with an estimated free aperture of ∼3.77 × 8.99 Å2. The small channels are partially occupied by both the disordered free water molecules and the coordinated water groups, which well stabilize the open-framework structure of 3. The structure of 4 also features a polymeric metal-organic framework with the fumarate as chelating and bridging ligands and YbIII as the connecting nodes. The framework of 4 contains a molecular composition of Yb2(fum)3(H2O)4](H2O)2. Each YbIII center is surrounded by eight oxygen atoms, two of which are from water molecules and the other six belong to four fumarate ligands. This is markedly distinct from the nine-coordinate pattern in the structures of 1-3 (Figure 8). There are two crystallographically independent fumarate dianions in the structure, unlike the cases of 1-3 (Figure 9). One (4-L1) of them is of mixed bridging and chelating modes, which connects two YbIII centers with edge-sharing mode and bridges the discrete YbIII dimers to form a 1D sheet parallel to the bc plane (Figure 10a). The closest carbon-carbon separation between two linking ligands is 3.937 Å, and the dimeric Yb‚‚‚Yb is separated by 3.907 Å. The second type (4-L2) of binding ligand is centrosymmetric in a bis-chelating mode, which is in fact perpendicular to the bc plane, pillaring the sheets between them (Figure 10b). Consequently, a rectangular grid-like network containing large cavities of 10.5 × 9.4 Å2 are observed in the structure of 4. Additionally, each asymmetric unit involves one crystalline water molecule, interacting with two coordinated water molecules by hydrogen bonds, which partially occupy the large cavities and stabilize the 2D grid framework. This result provides another example of utilizing the linear unsaturated ligand to construct a novel porous structure.11 It is worth noting that the use of various lanthanide oxides in this work has dramatically influenced the solid-state architectures of the resultant products. For 1-3, the metal ions are



Figure 6. Three distinct ligand conformations (3-L1, 3-L2, and 3-L3) observed in 3.

Figure 7. Perspective view of 3 down the [100] direction, showing the 3D open-framework structure composed of 1D void channels. Samarium(III) polyhedra, carbon atoms, and oxygen atoms are represented in green, black, and red, respectively. The hydrogen atoms and crystalline water molecules are omitted for clarity.

all nine-coordinate, and there are three distinct types of ligand binding modes in the structures. In the structure of 4, the metal ion is eight-coordinate, and there are only two distinct ligand conformations. In addition, in the structures of 1 and 2, seven fumarate ligands provide eight oxygen atoms to coordinate with the metal ion and extend the metal-organic subnetworks to 3D structures. In the structure of 3, there are five fumarate ligands coordinating to the metal ion with seven oxygen atoms, which also results in a 3D architecture. However, four ligands coordinate to the metal ion with six oxygen atoms in the structure of 4, and a 2D framework forms. This phenomenon is presumably a manifestation of the well-known effect of lanthanide contraction and has some dramatic effects on crystal architectures considering the similar lanthanide-organic hybrid frameworks reported in the literature.15a,17 Indeed, in the structure types of 1-3, the presence of more ligands around the metal center permits expansion of the coordination framework in the third dimension of space, while this is not possible in the structure type of 4. Thermal Analyses and PXRD Studies. Combined TGADSC analysis was carried out in the interest of studying the

Figure 8. The ORTEP structure (30% probability level) showing the coordination environments of YbIII-O polyhedra in 4.

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 10 °C/min. The crystals of 1 appeared much thermostable since the first endothermic weight loss of 19.61% corresponding to the departure of both the coordinated water molecules and the protonated fumarate ligands (calcd 19.64%) was observed between 232 and 330 °C (see Supporting Information). Then the remaining compound remained nearly intact until 390 °C, beyond which the compound decomposed. This thermal behavior is obviously different from the previous examples of lanthanide-carboxylate complexes, as well as our other compounds (2-4),10-12,16 probably due to the peculiar coordination environment and intermolecular forces and hence high stability for this compound. Though the solid 2 is of identical coordination architecture to 1, they show different thermograms. Solid 2 exhibited the first endothermic weight loss of ca. 2.22% in the temperature range of 60-166 °C, assigned to the liberation of about one-half of the water molecules (calcd 2.32%), which was similar to some reported compounds involving water molecules.11-16,18 Again, in the temperature range of 226-309 °C, the compound suffered the second endothermic weight loss of ca. 16.82% from the leaving


Zhang et al.

Figure 11. PXRD patterns from the data of simulated (a), experimental (b), and partially dehydrated sample (c) of compound 3.

Figure 9. Two distinct ligand binding conformations (4-L1 and 4-L2) observed in 4.

Figure 12. PXRD patterns from the data of simulated (a), experimental (b), and partially dehydrated sample (c) of compound 4. Table 3. The Relevant O-H‚‚‚O Hydrogen Bond Data for 1 and 2a D-H‚‚‚A

Figure 10. Perspective view of 4 (a) down the [100] direction showing a 1D sheet parallel to the bc plane and (b) down the [001] direction showing the 2D rectanglar grid structure. YbIII polyhedra, carbon atoms, and oxygen atoms are represented in green, black, and red, respectively

of the remaining water molecules and protonated fumarate ligands (calcd 17.14%). The compound was then stable up to 390 °C, and after that the decomposition occurred. Why were the water molecules combined with the protonated ligands lost together at high temperature in complex 1, but about one-half of water molecules lost at a rather lower temperature and the remaining leave at the higher temperature together with the protonated ligand in complex 2? We proposed that it was related to the subtle differences in the crystal structures of 1 and 2 though they were isomorphous. Previously we have mentioned that the metal centers (Ce or Nd) are surrounded by one protonated fumarate ligand and one water molecule in both 1 and 2; the small molecular components are in fact fixed with intermolecular hydrogen bonds besides coordination bonds.

d(H‚‚‚A), Å

d(D‚‚‚A), Å

∠(DHA), deg

O(8)-H(8)‚‚‚O(2)#1 O(9)-H(9A)‚‚‚O(4)#2 O(9)-H(9B)‚‚‚O(6)#3

Complex 1 1.86(2) 2.084(14) 2.065(18)

2.662(3) 2.910(2) 2.815(2)

166.1 161(2) 146(2)

O(8)-H(8)‚‚‚O(2)#1 O(9)-H(9A)‚‚‚O(4)#2 O(9)-H(9B)‚‚‚O(6)#3

Complex 2 1.87(2) 2.209(2) 2.061(2)

2.660(3) 2.904(2) 2.804(2)

153(5) 155.1(2) 153.4(2)

a Symmetry transformations used to generate equivalent atoms: #1, x, -y - 1/2, z - 1/2; #2, -x, y + 1/2, -z + 1/2; #3, -x, -y, -z.

Indeed, the differences in the hydrogen-bonding interactions can interpret the distinct thermal behaviors for two compounds. Table 3 illustrates the hydrogen bond parameters for 1 and 2. It is shown that the OH group of the fumarate ligand and water molecule participate in the hydrogen bonds. The ligand is hydrogen bonded with similar short H‚‚‚O separation of 1.86(2) Å in both 1 and 2, while the hydrogen bonds resulting from water molecules are significantly different. In both crystals, the water molecule is fixed with two O-H‚‚‚O bonds. The H‚‚‚O separations are 2.084(14) and 2.065(18) Å for 1, while the two H‚‚‚O distances are 2.061(2) and 2.209(2) Å for 2. Therefore, the relatively weak hydrogen bonds in 2 should be responsible for the easy loss of about one-half of water molecules upon heating. The solid 3 consists of both uncoordinated and coordinated water molecules; hence the thermogram of 3 displayed a two-


step endothermic weight loss in the temperature range of 117212 °C (obsd 7.02%) and 230-372 °C (obsd 9.01%), corresponding to the loss of three (calcd 7.00%) and four (calcd 9.33%) water molecules per formula unit; then the residuals also suffered decomposition after 390 °C. Compound 4 underwent an almost sequential endothermic weight loss of ca. 13.29% between 124 and 297 °C, assigned to the departure of the coordinated and crystalline water molecules (calcd 13.61%). Finally, the compound began to decompose after 390 °C. The PXRD patterns of polymer frameworks 1 and 2 reveal that both compounds are pure single phase and are also of completely identical molecular structures, consistent with the simulated results from the single-crystal X-ray data (see Supporting Information). From the structural features of polymers 3 and 4 depicted above, there are a large amount of water molecules incorporated in the frameworks (seven and eight water molecules per repeating unit, respectively). As a result, investigating the influence of the enclathrated water on the framework structure is helpful for the better understanding of the framework stability and design of new porous materials for potential gas (or solvents) sorption applications. Heating the crystal samples of 3 and 4 at 140 °C for 12 h resulted in the partial departure of water molecules as determined by elemental analysis.19 Figures 11 and 12 represent the PXRD patterns for two samples, as seen from which the dehydrated samples of two compounds retain still strong diffraction peaks, indicating the good crystallinity in the two dehydrated products. However, the diffraction patterns are slightly different from those of the initial crystals, probably owing to some changes of the coordination environments of the lanthanides or the formation of new and more contractive framework structures after suffering from the loss of crystalline and coordinated water molecules. Attempts to obtain the precise structures from the dehydrated crystals were unsuccessful, attributed to their poor crystal quality after water molecules were removed from the lattice. Conclusion In summary, a new series of open-framework or rectangular grid materials of lanthanides based on fumaric acid were isolated as single crystals under mild hydrothermal conditions in high yield. The X-ray structure analysis revealed the dependence for the diversity of the resultant metal-organic frameworks on the introduction of various lanthanide ions and the well-known effect of lanthanide contraction. Combined TGA-DSC analysis showed the different thermal stability for the polymeric solid materials with various framework topologies. Particularly, the high weight loss temperature of the cerium-fumarate framework indicated the unusual coordination environment therein. The aqua-containing framework structures of 3 and 4 retained their crystallinity after the removal of partial water molecules from their PXRD patterns. Acknowledgment. Financial support from NSFC (Grants 50221201, 20173066, and 20333080) and the major state basic research development program (Grant G2000078100) of China is gratefully acknowledged. We thank Prof. Y. Li and H.-W. Ma for X-ray diffraction analysis. Supporting Information Available: X-ray crystallographic files for compounds 1-4 in CIF format, TGA-DSC curves for compounds 1-4, and PXRD patterns for compounds 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.


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CG050488L

Hydrothermal Syntheses and Characterization of Novel 3D Open

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