A Series of Novel LnâSuccinateâOxalate Coordination Polymers

A Series of Novel Ln-Succinate-Oxalate Coordination Polymers: Synthesis, Structure, Thermal Stability, and Fluorescent Properties Xing-Jing Zhang,†,‡ Yong-Heng Xing,*,† Jing Han,† Xiao-Qing Zeng,§ Mao-Fa Ge,§ and Shu-Yun Niu†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3680–3688

College of Chemistry and Chemical Engineering, Liaoning Normal UniVersity, Huanghe Road 850#, Dalian, China, 116029, College of Chemistry, Jilin Normal UniVersity, Shida Road 1301#, Siping, China, 136000, and Institute of Chemistry, the Chinese Academy of Sciences, Beijing, 100080, P. R. China ReceiVed March 19, 2008; ReVised Manuscript ReceiVed July 8, 2008

ABSTRACT: A series of novel coordination polymers [Ln(suc)(ox)0.5(H2O)] (Ln ) Sm (1) and Tb (2)); [Pr(suc)(ox)0.5(H2O)2] · 2H2O (3); and [KLn(suc)0.5(ox)1.5(H2O)] · H2O (Ln ) Nd (4), Eu (5), Tb (6) and Gd(7)) have been synthesized by the reaction of nitrate salts of Ln(III) with succinic acid and oxalic acid under hydrothermal conditions and were characterized by elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction. Structural analyses reveal that all seven complexes have 3D net structures, and in these complexes, they were crystallized in triclinic space group P1j for complexes 1 and 2, orthorhombic space group Fddd for complex 3, and orthorhombic space group Pbam for complex 4-7, respectively. In particular, for 4-7, K ions were introduced into the structures of complexes and made the structures more novel and complicated. Thus the complexes have the same ligands but different structures. The results show that coordination modes of the linear succinic acid and oxalic acid ligands are versatile and can adopt different conformations according to different lanthanide ions. The thermogravimetric analysis of 1-7 and photoluminescent properties of 2, 5 and 6 are discussed in detail. Introduction The design and synthesis of novel polymeric metal-organic complexes have attracted intense interest owing to the realization of their potential for use as magnetism, zeolite-like catalysis activity, gas storage, ion exchange, molecular recognition, and optical properties, etc.1 In this aspect, considerable progress has been made on the theoretical prediction and network-based approaches for controlling the topology and geometries of the networks to produce useful functional materials.2 Therefore, an enormous amount of research is being focused on constructing novel coordination polymers by choosing versatile organic ligands and functional metal ions.3 As functional metal centers, there has been an upsurge in the use of rare earth metals for their fantastic coordination properties and special chemical charactertics arising from 4f electrons and the properties to form isostructural complexes.4 As we know, rare earth ions have high affinity for hard donor atoms and ligands containing oxygen or hybrid oxygen-nitrogen atoms, especially multicarboxylate ligands, which are usually employed in the architectures for lanthanide coordination polymers.5 And the use of flexible multicarboxylate ligands as building blocks in the construction of coordination frameworks is attractive because the flexibility and conformational freedoms of the ligands may offer various possibilities for construction of frameworks with unique structures and useful properties.6 As flexible dicarboxylate ligand, succinic acid is one of flexible carboxylate-containing ligands, and a large number of coordination polymers of Ln-succinate7 have also been synthesized, such as [Sc2(C4H4O4)2.5(OH)],7a [Eu2(C4H4O4)3(H2O)2] and [Tb2(C4H4O4)3(H2O)2] · H2O,7c [Pr(H2O)]2[O2C(CH2)2CO2]3 · H2O,7e [Sm2(C4H4O4)3(H2O)2] · 0.5H2O,7f etc. Among the complexes reported above, all contain just one type of carboxylate compound as ligand with different coordination modes, to form various structures (2D and 3D * Corresponding author. E-mail: [email protected]. † Liaoning Normal University. ‡ Jilin Normal University. § The Chinese Academy of Sciences.

dimension). In order to further understand their synthetic regularity and various coordination modes, as well as influence on carboxylate group from different ligands to structures, it is necessary to explore and design a series of a new family of complexes with more complicated structures for improving desirable photoluminescent properties. Taking account of the above, our group has been focusing our attention on the construction of novel coordination frameworks by using flexible dicarboxylate (succinic acid, maleic acid, glutaric acid, adipate, etc.) as the first ligands and other aromatic carboxylates (benzoic acid; 1,2-benzene dicarboxylic acid (oH2BDC), 1,3-benzene dicarboxylic acid (m-H2BDC), 1,4benzene dicarboxylic acid (p-H2BDC); 2-nitrobenzoic acid, 3-nitrobenzoic acid, 4-nitrobenzoic acid; 3-methoxybenzoic acid (m-MOBA), 2,3-dimethoxybenzoic acid (2,3-DMOBA); aminobenzoic acid; hydroxylbenzoic acid, etc.) as the second ligands. And several series of lanthanide coordination polymers with two types of carboxylates as mixed-ligands were studied, such as (i) a series of Ln-suc-bc coordination polymer ([Ln2(suc)0.5(bc)3(H2O)2] [Ln ) Tb, Eu, Sm, Pr; H2suc ) succinic acid; Hbc ) benzoic acid]8); (ii) a series of Ln-malox coordination polymer ([Ln(mal)(ox)0.5(H2O)2] · 2H2O [Ln ) Pr, Nd, La; H2mal ) maleic acid; H2ox ) oxalic acid]); (iii) a series of Ln-suc-ox coordination polymer. Herein we first present the synthesis of seven new three-dimensional porous Ln-sucox rare earth coordination polymers, [Ln(suc)(ox)0.5(H2O)] (Ln ) Sm (1) and Tb (2)); [Pr(suc)(ox)0.5(H2O)2] · 2H2O (3); [KLn(suc)0.5(ox)1.5(H2O)] · H2O (Ln ) Nd (4), Eu (5), Tb (6) and Gd (7)). The thermal stability and luminescent properties have been investigated too. Experimental Section All chemicals purchased were of reagent grade or better and were used without further purification. Lanthanide nitrate salts were prepared via dissolving lanthanide oxides with 6 M HNO3 while adding a bit of H2O2 for Tb4O7 and then evaporating at 100 °C until the crystal film formed. C and H analyses were made on a Perkin-Elmer 240C

10.1021/cg800294c CCC: $40.75  2008 American Chemical Society Published on Web 09/05/2008

R ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) [∑w(Fo2 - Fc2)2/[∑w(Fo2)2]1/2; [Fo > 4σ(Fo)]. b Based on all data. a

7 6

C5H6O10KTb 424.12 orthorhombic Pbam 8.876(3) 16.367(5) 7.224(2) 90 90 90 1049.4(5) 4 2.684 0.016 × 0.027 × 0.417 800 7.180 2.49-28.22 6245 1327 90 1.370 and -1.900 1.098 0.0253 (0.0258) 0.0674 (0.0679)

5

C5H6O10KEu 417.16 orthorhombic Pbam 8.916 (2) 16.499(3) 7.260(2) 90 90 90 1067.9(4) 4 2.595 0.018 × 0.025 × 0.325 792 6.306 3.36-27.47 9992 1322 94 0.501 and -2.742 1.229 0.0238(0.0243) 0.0600 (0.0603)

4

Figure 1. Experimental and heated (250 °C) X-ray powder diffraction diagram of complex 1 compared to the calculated one.

C5H6O10KNd 409.44 orthorhombic Pbam 8.974(2) 16.717(3) 7.310(2) 90 90 90 1096.5(4) 4 2.480 0.018 × 0.022 × 0.322 780 5.157 3.33-27.48 9713 1334 94 1.078 and -1.905 1.191 0.0239 (0.0248) 0.0625 (0.0628)

3

C5H12O10Pr 373.06 orthorhombic Fddd 9.722(2) 15.535(3) 28.003(6) 90 90 90 4229.2(15) 16 2.344 0.141 × 0.221 × 0.344 2896 4.650 3.00-27.48 9846 1216 82 0.684 and -0.503 1.071 0.0203 (0.0249) 0.0453 (0.0472) C5H6O7Tb 337.02 triclinic P1j 6.656(1) 7.700(1) 8.039 (1) 101.679(2) 102.290(2) 101.454(2) 381.45(11) 2 2.934 0.023 × 0.027 × 0.437 314 9.274 2.68-28.98 2377 1748 118 2.967 and -2.905 1.224 0.0410 (0.0419) 0.1095 (0.1104) C5H6O7Sm 328.46 triclinic P1j 6.759(1) 7.674 (2) 8.051(2) 101.92(3) 103.14(3) 101.44(3) 384.47(13) 2 2.837 0.435 × 0.02 × 0.023 308 7.641 3.20-27.46 3792 1736 130 0.483 and -0.904 1.218 0.0136 (0.0141)b 0.0325 (0.0326)b formula M (g mol-1) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalc crystal size (mm) F(000) µ(Mo KR) /mm-1 θ (deg) reflections collected independent reflections (I > 2σ(I)) parameters ∆(F) (e Å-3) goodness of fit Ra wR2a

Table 1. Crystallographic Data for Complexes 1-7

2 1

Crystal Growth & Design, Vol. 8, No. 10, 2008 3681

C5H6O10KGd 424.122.45 orthorhombic Pbam 8.898 (2) 16.434(3) 7.245 (1) 90 90 90 1059.5(4) 4 2.648 0.017 × 0.037 × 0.317 796 6.696 3.37-27.46 9918 1309 94 0.706 and -2.051 1.146 0.0204 (0.0234) 0.0446 (0.0456)

Ln-Succinate-Oxalate Coordination Polymers

Figure 2. Experimental and heated (250 °C) X-ray powder diffraction diagram of complex 3 compared to the calculated one. automatic analyzer at the analysis center of Liaoning Normal University. Content of lanthanide was analyzed on a Plasma-Spec(I)-AES model ICP spectrometer. Infrared (IR) spectra were recorded on JASCO FT/ IR-480 PLUS Fourier transform spectrophotometer with pressed KBr pellets in the range 200-4000 cm-1. Thermogravimetric anaylses (TGA) were performed under N2 atmosphere at 1 atm with a heating rate of 10 °C/min on a Perkin-Elmer Diamond TG/DTA. The luminescence spectra were reported on a JASCO FP-6500 spectrofluorimeter (solid) in the range of 200-850 nm. X-ray powder diffraction (XRD) data were collected on a Bruker Advance-D8 with Cu KR radiation, in the range 5° < 2θ < 60°, with a step size of 0.02° (2θ) and an acquisition time of 2 s per step. Synthesis of [Sm(suc)(ox)0.5(H2O)] (1). The complex was prepared by hydrothermal reaction. Sm(NO3)3 · 6H2O (0.30 g, 0.66 mmol), succinic acid (H2suc, 0.12 g, 1 mmol), oxalic acid (H2ox · 2H2O, 0.06 g, 0.50 mmol), and H2O (15 mL) were mixed in a 25 mL beaker. The pH value was adjusted to 4-6 with l mol · L-1 potassium hydroxide aqueous solution. After stirring for 2 h, the mixture was sealed in the bomb and heated at 180 °C for four days, then cooled at 10 °C/3 h to 100 °C, followed by slow cooling to room temperature. After filtration, the product was washed with distilled water and then dried at room temperature. Light yellow block crystals suitable for X-ray diffraction

3682 Crystal Growth & Design, Vol. 8, No. 10, 2008

Zhang et al. Scheme 1. The Coordination Mode of Succinic Acid in Complex 1

Figure 3. Experimental and heated (250 °C) X-ray powder diffraction diagram of complex 4 compared to the calculated one. Table 2. Selected Bond Lengths (A) for Complexes 1, 3 and 4a Complex 1 Sm-O3#1 Sm-O7#3 Sm-O6 Sm-O2

2.309(2) 2.403(2) 2.440(2) 2.505(2)

Pr-O4#1 Pr-O2#3 Pr-O3 Pr-O1 Pr-O4#3

2.446(2) 2.512(2) 2.553(3) 2.571(3) 2.879(3)

Nd-O5 Nd-O2 Nd-O3

2.384(4) 2.491(3) 2.547(5)

Sm-O1#2 Sm-O5 Sm-O4 Sm-O1

2.4025(19) 2.424(2) 2.4557(18) 2.5485(19)

Complex 3 Pr-O4#2 Pr-O2 Pr-O3#3 Pr-O1#3 Pr-O4

2.446(2) 2.512(2) 2.553(3) 2.571(3) 2.879(3)

Complex 4 Nd-O1 Nd-O4 Nd-O5#2

2.473(3) 2.483(3) 2.581(4)

a Symmetry transformations used to generate equivalent atoms: #1, -x + 1, -y, -z + 2; #2, -x, -y, -z + 1; #3, x - 1, y, z for 1; #1, x + 1/4, y + 1/4, -z + 1; #2, -x + 1, -y + 1, -z + 1; #3, -x + 5/4, -y + 5/4, z for 3; #2, -x, -y + 2, -z + 1 for 4.

Figure 4. Coordination environments of Sm in complex 1 with nonhydrogen atoms by diamond. Symmetry codes follow: (A) 1 - x, -y, 2 - z; (B) -x, -y - 1 - z; (C) x - 1, y, z. analysis were obtained in ca. 52.32% yield based on Sm(III). Elemental analysis results for C5H6O7Sm (Mr ) 328.46), calcd: C, 18.23; H, 1.86; Sm, 45.76. Found: C, 18.27; H: 1.83; Sm, 45.78. IR data (KBr pellet, ν [cm-1]): 3602, 3488, 2978, 2932, 1780, 1615, 1547, 1428, 1315, 1219, 1181, 1001, 954, 912, 890, 794, 758, 682, 536, 488, 384, 329.

Synthesis of [Tb(suc)(ox)0.5(H2O)] (2). Complex 2 was synthesized by a method similar to that of complex 1 except that Sm(NO3)3 · 6H2O was replaced by Tb(NO3)3 · 6H2O, and the pH value was adjusted with solution of 1 M KOH to 4-6 and the reaction was heated at 160 °C for 4 days. White crystals were obtained in a 50.37% yield based on Tb(III). Elemental analysis results for C5H6O7Tb (Mr ) 337.02), calcd: C, 17.84; H, 1.82; Tb, 47.13. Found: C, 17.80; H: 1.78; Tb, 47.16. IR data (KBr pellet, ν [cm-1]): 3460, 2973, 2930, 1714, 1622, 1548, 1435, 1316, 1294, 1220, 1183, 1069, 994, 974, 959, 913, 890, 796, 761, 722, 685, 599, 548, 489, 394, 329. Synthesis of [Pr(suc)(ox)0.5(H2O)2] · 2H2O (3). Complex 3 was synthesized by a method similar to that of complex 1, using Pr(NO3)3 · 6H2O instead of Sm(NO3)3 · 6H2O as the starting material. Light-green block crystals were obtained in a 49.48% yield based on Pr(III). Elemental analysis results for C5H12O10Pr (Mr ) 373.06). calcd: C, 16.04; H, 3.25; Pr, 37.79. Found: C, 16.08; H: 3.22; Pr, 37.77. IR data (KBr pellet, ν [cm-1]): 3488, 2928, 1703, 1617, 1543, 1416, 1315, 1218, 988, 949, 793, 756, 679, 647, 592, 627, 503, 422, 402, 379, 342, 323, 304. Synthesis of [KNd(suc)0.5(ox)1.5(H2O)] · H2O (4). Nd(NO3)3 · 6H2O (0.30 g, 0.66 mmol), succinic acid (H2suc, 0.06 g, 0.50 mmol), oxalic acid (H2ox · 2H2O, 0.18 g, 1.5 mmol), KOH (0.22 g, 4mmol) and H2O (15 mL) were mixed in a 25 mL beaker. After stirring for 2 h, the mixture was sealed in the bomb and heated at 180 °C for four days, then cooled at 10 °C/3 h to 100 °C, followed by slowly cooling to room temperature. Light-puple block crystals were obtained in a 21.54% yield based on Nd(III). Elemental analysis results for C5H6O10KNd (Mr ) 409.44), calcd: C, 14.61; H, 1.48; Nd, 35.25. Found: C, 14.65; H: 1.47; Nd, 35.23. IR data (KBr pellet, ν [cm-1]): 3607, 3501, 2932, 1750, 1614, 1545, 1418, 1315, 1218, 951, 793, 757, 680, 647, 531, 499, 382. Synthesis of [KEu(suc)0.5(ox)1.5(H2O)] · H2O (5). Complex 5 was synthesized by a method similar to that of complex 4, using Eu(NO3)3 · 6H2O instead of Nd(NO3)3 · 6H2O as the starting material. White block crystals were obtained in a 22.34% yield based on Eu(III). Elemental analysis results for C5H6O10KEu (Mr ) 417.16), calcd: C, 14.35; H, 1.42; Eu, 36.45. Found: C, 14.38; H: 1.44; Eu, 36.43. IR data (KBr pellet, ν [cm-1]): 3481, 2929, 1710, 1620, 1429, 1316, 1220, 955, 912, 890, 795, 759, 683, 537, 489, 388. Synthesis of [KTb(suc)0.5(ox)1.5(H2O)] · H2O (6). Complex 6 was synthesized by a method similar to that of complex 4, using Tb(NO3)3 · 6H2O instead of Nd(NO3)3 · 6H2O as the starting material. Pale-white block crystals were obtained in a 19.63% yield based on Tb(III). Elemental analysis results for C5H6O10KTb (Mr ) 424.12), calcd: C, 14.19; H, 1.43; Tb, 37.45. Found: C, 14.15; H: 1.41; Tb, 37.47. IR data (KBr pellet, ν [cm-1]): 3593, 3398, 2973, 2950, 1713, 1633, 1434, 1315, 1220, 1183, 1069, 994, 974, 959, 913, 890, 796, 761, 722, 685, 599, 547, 489, 404, 330. Synthesis of [KGd(suc)0.5(ox)1.5(H2O)] · H2O (7). Complex 7 was synthesized by a method similar to that of complex 4, using Gd(NO3)3 · 6H2O instead of Nd(NO3)3 · 6H2O as the starting material. White block crystals were obtained in a 25.36% yield based on Gd(III). Elemental analysis results for C5H6O10KGd (Mr ) 422.45), calcd: C, 14.19; H, 1.43; Gd, 37.25. Found: C, 14.20; H: 1.42; Gd, 37.22. IR data (KBr pellet, ν [cm-1]): 3473, 2930, 1712, 1619, 1549, 1432, 1382, 1316, 1220, 1182, 1122, 1069, 994, 958, 913, 890, 795, 760, 685, 600, 539, 511, 490, 390, 336, 326. Crystal Structure Determinations. Suitable single crystals of seven complexes were mounted on glass fibers for X-ray measurement. Reflection data were collected at room temperature on a Bruker AXS SMART APEX II CCD diffractometer with graphite monochromatized Mo KR radiation (λ ) 0.71073 Å). All absorption corrections were performed using the SADABS program.9a Crystal structures were solved by the direct method. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were fixed at calculated positions and refined by using a riding mode. All calculations were performed using the



Figure 5. A one-dimensional chain structure formed Sm and its corresponding centrosymmetric atoms through the terminal carboxylate bridging interactions. Hydrogen atoms are omitted for clarity.

Figure 6. One-dimensional chain inorganic building units linked to each other by the gauche succinic acids into a 2D layer structure on the [101] plane. Hydrogen atoms, parts of oxalic acid, and water molecules are omitted for clarity. SHELX-97 program.9b Crystal data and details of the data collection and the structure refinement are given in Table 1.

Results and Discussion Syntheses. Hydrothermal syntheses favor crystallization of complexes 1-7. In our first attempts, complexes 1-3 obtained only a small quantity of microcrystals unsuitable for singlecrystal X-ray diffraction under the hydrothermal conditions. Then, we found that changing the pH value of the reaction mixture may be helpful for crystal growth. In view of this, we adjusted gradually the pH values of the reaction system. First of all, we attempted to adjust them with ethylenediamine, but no good results were obtained. Then, as expected, at pH 5-6 adjusted by 1 mol L-1 KOH well-formed single crystals of 1-3 suitable for single-crystal X-ray diffraction were obtained in satisfying yields under hydrothermal conditions. For complexes 4-7, KOH was added into the reaction system directly in the form of solid instead of the solution, and the mol ratio of mixed ligands in 4-7 (msuc:mox ) 1:3) is different from that of 1-3 (msuc:mox ) 2:1). It shows that the pH value and the mol ratio of mixed ligands play an important role during the process of the reaction. In addition, it can be found that using inorganic

alkali is more favorable to the reaction than organic alkali, and using starting materials with a different mol ratio of succinic acid/oxalic acid has an influence on the composition of complexes. X-ray Powder Diffraction Study. The X-ray powder diffraction data and the heated X-ray powder diffraction data (250 °C) of complexes 1, 3 and 4 were measured. The results show that the X-ray powder diffraction data of complexes 1, 3 and 4 are in agreement with that calculated on the basis of their structural data (Figures 1, 2, 3), that is, complexes have been obtained successfully as pure crystalline phases. The heated (250 °C) diffraction peaks of complexes 1, 3 and 4 in X-ray diffraction patterns are very similar in intensity and position to those appearing in the XPRD of complexes 1, 3 and 4. It is shown that complexes are stable below 250 °C. Single-Crystal X-ray Structures of 1-7. The X-ray structure analyses reveal that complexes 1-2 and 4-7 are isomorphous, and 3 is different. Complexes 1-3 are lanthanide-multicarboxylate coordination polymer, while, 4-7 are lanthanide-alkali-multicarboxylate coordination polymer; here, complexes 1, 3 and 4 are taken as an example to depict the three-dimensional structure in detail. The selected bond lengths of complexes 1, 3, and 4


Figure 7. Packing structure along the [010] direction of complex 1.

Figure 8. Local coordination environments of Pr ion in complex 3 with non-hydrogen atoms drawn by diamond. The blue dashed bonds are the two long Pr-O bonds. Symmetry codes follow: (A) x + 1/4, y + 1/4, -z + 1; (B) -x + 1, -y + 1, -z + 1; (C) -x + 5/4, -y + 5/4, z.

are listed in Table 2. The crystal structure of complex 1 reveals that it is a three-dimensional framework, crystallizing in triclinic space group P1j. An asymmetric unit [Sm(suc)(ox)0.5(H2O)]

Zhang et al.

contains one eight-coordinated samarium ion, one succinic acid, half an oxalic acid ligand, and one coordinated water molecule to form a distorted tricapped trigonal-prismatic geometry. The coordination modes of samarium ion are shown in Figure 4. Sm is coordinated with eight oxygen atoms from one chelating bidentate carboxyl group (O1 and O2), three dimonodentate carboxyl groups from succinic acids (O1B, O3A, and O7C), two dimonodentate carboxyl groups from oxalic acid (O4 and O5), and one coordinated water molecule (O6). The distances of Sm-Osuc (from succinic acid), Sm-Oox (from oxalic acid), and Sm-Owt (from coordinated water) are in the ranges of 2.309(2)-2.548(2), 2.424(2)-2.456(2), and 2.440(2) Å, respectively, all of which are comparable to those reported for other samarium-oxygen donor complexes.7f,8 For succinic acid ligand, the coordination modes of both carboxylate groups are as follows (Scheme 1): (i) one of carboxylate groups adopts a µ2-η1-η1-bridging (namely one oxygen atom of the carboxylate group connects one samarium ion, the other one connects also one samarium ion and the carboxylate group coordinates to two metal ions) coordination mode; (ii) the other one adopts a µ2η2-η1-bridging (namely one oxygen atom of the carboxylate group connects two samarium ions, the other one connects one samarium ion and the carboxylate group coordinates to two metal ions) coordination mode. And it is remarkable that the succinic acid ligands assume a gauche conformation (the torsion angle of C1-C2-C4-C3 is 77.27(3)°), which is obviously different from that of reported complexes, such as the torsion angle of succinic acid in complex [Tb2(suc)0.5(bc)3(OH)2]8 (H2suc ) succinic acid, Hbc ) benzoic acid) is 180 °. To deeply understand the structures of frameworks and coordinated carboxylate molecular conformation, it would be important to explore the connection ways of the metal centers and carboxylate ligands. In complex 1, lanthanide metal center atoms (Sm) and the corresponding centrosymmetric atoms are linked through the carboxylate bridging interactions, and the polyhedra are edged-shared to form dimer polyhedra [(SmO8)2] and to connect each other by carbon atoms to generate metal-oxygen chains extending infinitely along the [001] direction (Figure 5), in which the adjacent Sm · · · Sm contact is 4.121(4) and 4.595(6) Å. Along the [100] direction, the chains are linked by the gauche succinic acids into a 2D layer structure (Figure 6). The oxalic acids are located on a center of inversion and act as double bidentate (tetradentate) ligands in a linear chain that connects two Sm atoms in two different layers to form a 3D framework (as shown in Figure 7). The structure of complex 2 is similar to that of 1, as shown in Supporting Information.

Figure 9. Lanthanide metal center atom (Pr) and its corresponding centrosymmetric atoms linked by carboxyl groups of the succinic acid ligands to lead to an infinite inorganic rod-shaped structure.



Figure 10. Rod-shaped inorganic building units linked to each other by the carbon atoms of the succinate anions on the [110] plane. Hydrogen atoms, parts of oxalic acid, and water molecules are omitted for clarity.

Figure 11. The views of the 3D porous framework viewed along the x axis direction. Color scheme: Pr, green; C, purple; O of suc, ox, and the coordination water, red; the lattice water, cyan. H atoms are omitted for clarity.

For elicitation of the complex [Pr(mal)(ox)0.5(H2O)2] · 2H2O (H2mal ) maleic acid; H2ox ) oxalic acid) that our group has synthesized, a new four carbon linear fatty acid ligand (succinic acid (H2suc)) has been used to react with Pr(NO3)3 · 6H2O and oxalic acid by varying slightly the reaction conditions as those used for preparing [Pr(mal)(ox)0.5(H2O)2] · 2H2O, and one new complex [Pr(suc)(ox)0.5 (H2O)2] · 2H2O (3) was obtained. In complex 3, the praseodymium atom is coordinated with eight oxygen atoms from four dimonodentate carboxyl groups (O1, O1C, O4A, and O4B) from succinic acids, two dimonodentate carboxyl groups (O2 and O2C) from oxalic acid, and two coordination water

Figure 12. The coordination modes of Nd(III) (a) and K (b) ions in complex 4. Symmetry code: (A) x, y, 1 - z; (B) x, y, 2 - z; (C) -x, 2 - y, z).

molecules (O3 and O3C). It is remarkable that there are two oxygen atoms, O4 and O4C, from succinic acid ligand (Figure


Zhang et al.

Scheme 2. The Coordination Mode of Oxalic Acid in Complex 4

8), with Pr-O distances of 2.879(3) A, variously longer than other Pr-O bond lengths. It is indicated that there is a weak interaction between Pr and O4 (or O4C) atom. The average distances of Pr-Osuc (from succinic acid), Pr-Oox (from oxalic acid) and Pr-Owt (from water molecule) are 2.509(3), 2.512(2), and 2.553(3) Å, respectively. The average length of Pr-Osuc is similar to those of related complexes. For example, {[Pr(H2O)]2[O2C(CH2)2CO2]3 · H2O}n (2.512 Å). 7e The average length of Pr-Oox (from oxalic acid) is also similartothoseofreportedcomplexes,suchas[Pr(H2O)3]2(C2O4)[O(CH2CO2)2]2 (2.515(3) Å). 10 However, the average distance of Pr-Owt (2.550(3) Å) is longer than related lan-

Figure 13. The structure formed lanthanide metal center atoms (Nd) and its corresponding centrosymmetric atoms with succinic acids along the x axis and with oxalic acids along the y axis.

Figure 14. Packing structure along the z axis of 4. Guest water molecules are omitted for clarity.

Figure 15. The TG-DTG-DTA curves of complex 1. Table 3. Thermal Decomposition Data for Complexes 1-7 DTG peak stage temp (°C) I II III I II III I II III I II III IV I II III I II III I II III

346.0 437.0 797.0 273.0 429.0 806.0 124.0 428.0 784.0 116 435 613 789 256 412 799 272 433 810 344 445 803

mass loss (%) obsd

calcd

5.54 30.77 13.48 4.60 29.88 12.64 4.76 29.23 27.61 4.34 21.29 5.34 14.17 8.65 31.07 13.93 8.94 30.28 13.98 7.79 23.27 20.94

5.48 31.05 13.40 5.34 30.27 13.06 4.82 30.02 27.64 4.40 20.52 5.37 14.17 8.63 31.64 13.90 8.49 31.12 13.67 8.52 24.14 20.83

probable composition of removed groups 1 H2O 0.5 succinic acid and 1 CO2 1 H2 O 0.5 succinic acid and 1 CO2 1 H2 O 0.5 succinic acid and 1 CO2 1 H2 O 1 H2O and 1.5 CO2 0.5 CO2 0.5 succinic acid 2 H2 O 1.5 oxalic acid 0.5 succinic acid 2 H2O 1.5 oxalic acid 0.5 succinic acid 2 H2O 0.5 succinic acid and 2 CO2

complex 1

1 CO2 2 1 CO2 3 2 CO2 4

5

6

7 1 CO2

thanide complexes. For example, {[Pr(H2O)]2[O2C(CH2)2CO2]3}n and {[Pr(H2O)]2[O2C(CH2)2CO2]3 · H2O}n (2.490(3) and 2.493(5) Å, respectively). 11,7e Lanthanide metal center atom (Pr) and its corresponding centrosymmetric atom link through two chelating/bridging bidentate carboxyl groups of succinic acid ligands to form an infinite inorganic rod-shaped building unit, as seen in Figure 9. These rod-shaped building units were linked to each other through the carbon atoms of the succinate anions on the [110] plane to form a two-dimensional layer structure (Figure 10). Two coordination waters and one oxalic acid coordinated to each Pr atom stretch out of the Pr-suc layer, with the water pair on one side and the oxalic acid on the other side, but their orientations are opposite for the neighboring Pr atom. Along the z direction, the Pr-suc layers are pillared by the oxalic acid ligand, thus resulting in a 3D framework (Figure 11), where the interlayer Pr · · · Pr contact via oxalic acid link is ca. 6.4 A. The 3D framework has channels, in which they are occupied by lattice water molecules, that is, four per formula. However, as the coordination water molecules protrude into the interlayer region, in fact they block the channels along the x direction, and the openness of channels along the two diagonal directions is ca. 3 × 7 Å estimated with the exclusion of the vdW radii of the surface atoms. The lattice water molecules are supported


Figure 16. Room-temperature solid-state photoluminescence spectra of (a) complexes 2 (bottom) and 6 (top); (b) complex 5.

by the coordination water and COO groups of the framework via H-bonds, with O · · · O distances of ca. 2.82 Å. The lattice water molecules also form H-bonded dimer with O · · · O distances of 2.74 Å. To our knowledge, the coordination polymer of Ln-multicarboxylate-alkali system is rare, in which alkali metals participate in coordination and as supporting framework atoms. So, in order to obtain more novel and complicated structures and evaluate the effects of alkali metals to the structure within the framework formation of their complexes, K ions were introduced into the structures of complexes. As expected, complexes 4-7 were obtained. Elemental analysis and thermogravimetric analyses studies performed on the complexes 4-7 reveal that they are extremely similar in structure. Here complex 4 is taken as an example to present and discuss the structure in detail. The structure of complex 4 reveals that it is a threedimensional framework, crystallizing in the orthorhombic space group Pbam. An asymmetric unit [KNd(suc)0.5(ox)1.5(H2O)] · H2O contains one nine-coordinated neodymium ion and one seven-coordinated potassium ion, half a succinic acid and 1.5 oxalic acid ligands, one coordination water and one free water molecule. The coordination modes of neodymium and potassium atoms are showed in Figure 12. Nd is coordinated with nine oxygen atoms from one chelating bidentate carboxyl group (O3 and O5C) and one dimonodentate carboxyl group (O5) from succinic acid, six dimonodentate carboxyl groups (O1, O1A, O2, O2A, O4, and O4A) from oxalic acid (shown as Figure 12a). Seven oxygen atoms coordinated with K atom are from six dimonodentate carboxyl groups (O1, O1B, O2, O2B, O4 and O4B) from oxalic acid, one coordination water molecule (O6) (Figure 12b). The distances of Nd-Osuc (from succinic acid), Nd-Oox (from oxalic acid), and K-O are in the ranges of 2.384(4) -2.547(5), 2.483(3) -2.491(3), and 2.720(3)-2.850(3) Å, respectively, all of which are comparable to those reported for other neodymium-potassium-oxygen complexes.7,12 It is well-known that the oxalic acid ligand has possibly many types of coordination modes, but to our knowledge, the


coordination modes (Scheme 2) of oxalic acid in complex 4 have not reported as yet. Complex 4 is the first example with the coordination mode, that is, µ4-η2-η2-bridging (Scheme 2) (namely one oxygen atom of the carboxylate group connects two metal atoms (K and Nd), the other one connects also two metal atoms (K and Nd) and the carboxylate group coordinates to four metal atoms (two K and one Nd or two Nd and one K)) coordination modes. Compared with the structures of complexes 5-7, it can be found that ways of the metal atoms connected by carboxylate groups are the same as those of complex 4, shown in Figure S1 in the Supporting Information, therefore, their 3D structures are also similar each other. To distinctly investigate the structures for frameworks, it would be important to explore the connection modes of the metal centers and organic ligands. As shown in Figure 13, lanthanide metal center atoms (Nd) and the corresponding centrosymmetric atoms are linked through the succinic acids to generate a 1D infinitely extending chain structure along the [100] direction; furthermore, along the y + x and y - x directions, the chains are linked by oxalate such that there are two tropisms into a 2D layer structure and the two tropistic oxalates appear alternately. The packing structure of complex 4 is shown in Figure 14. Thermal Properties. The thermal stability of complexes in air was examined by the TG, DTG and DTA techniques in the temperature range of 20-1000 °C. Figure 15 shows the TG-DTG-DTA curves for complex 1 at a heating rate of 10 °C/min under N2 atmosphere. The thermoanalytical data for complexes 1-7 are listed in Table 3. As shown in Figure 15, the thermal decomposition process of complex 1 can be divided into three stages. The first weight loss of 5.54% between 215 and 376 °C corresponds to the release of one free water molecule. The second weight loss of 30.77% was observed in the temperature range of 378-704 °C, which is attributed to the release of 0.5 succinic acid molecule and one CO2 molecule (31.05%, theoretical weight loss). In the third stage, in the range of 736-900 °C, it accounts for 13.48% of weight loss, which is attributed to the release of one CO2 molecule (13.40%, theoretical weight loss). Photoluminescent Properties. The luminescent behaviors of complexes 2, 5 and 6 were investigated in the solid state at room temperature. As shown in Figure 16, when excited at 285 nm for complexes 2 and 6 and 396 nm for 5, they emit green luminescence (2 and 6) and red light (5) at room temperature, respectively. The emission peaks of the complexes correspond to the transitions from 5D4f7Fn (n ) 6, 5, 4, and 3) transitions at 490, 544, 583, and 621 nm for the Tb(III) ion in 2 and 6 and 5 D0f7Fn (n ) 1, 2, 3, and 4) transitions at 590, 618, 653, and 702 nm for the Eu(III) ion in 5. Among these emission lines, the most striking green luminescence (5D4f7F5) for complex 2 and 6 and red emissions (5D0f7F2) for complex 5 were observed in their emission spectra. Conclusions In summary, we have successfully assembled succinic acid (H2suc), oxalic acid (H2ox) and Ln(III) (Ln ) Sm, Pr, Eu, Tb, Nd, Gd) salts into a series of 3D coordination frameworks, [Ln(suc)(ox)0.5(H2O)] (Ln ) Sm (1) and Tb (2)); [Pr(suc)(ox)0.5(H2O)2] · 2H2O (3); [KLn(suc)0.5(ox)1.5(H2O)] · H2O (Ln ) Nd (4), Eu (5), Tb (6) and Gd (7)). In the reaction process, it is found that the synthetic conditions (such as pH value, mol ratio of mixed ligands, etc.) play a crucial role in controlling the structure of the complexes. When the quantity of succinic acid (H2suc) is more than that of oxalic acid (mol ratio is more than


2:1), it favors formation of coordination polymer like complex 1-3; when the quantity of succinic acid (H2suc) is less than that of oxalic acid (mol ratio is more than 1:3), it favors formation of coordination polymer like complex 4-7. The results show that it is essential to construct complicated framework coordination polymer using shorter linker polymulticarboxyl ligands (oxalic acid). The thermogravimetric analysis of 1-7 and photoluminescent properties of complexes 2, 5 and 6 were discussed. Different metal ions coordinated to the ligands have an effect on the courses of thermal decomposition of complexes, so the course of thermal decomposition of complexes is appreciably different. Complexes 2 and 6 emit green and complex 5 does red luminescence at room temperature, respectively, and they could be anticipated as potential fluorescent materials. Acknowledgment. We wish to express our sincere thanks to National Natural Science Foundation of China (Grant No. 20771051), SRF for ROCS, SEM, and Education Foundation of Liaoning Province in China (Grant 2007T093) for financial assistance. Supporting Information Available: Tables listing selected bond lengths and bond angles and figures showing coordination modes of oxalic acid, coordination environments of the Ln and K, chain structure, layer structure, packing structure, and TG-DTG-DTA curves of complexes 2-7. This material is available free of charge via the Internet at http://pubs.acs.org. Tables of atomic coordinates, isotropic thermal parameters, and complete bond distances and angles have been deposited with the Cambridge Crystallographic Data Center. Copies of this information may be obtained free of charge, by quoting the publication citation and deposition numbers CCDC 657481 (1), 671822 (2), 657482 (3), 657484 (4), 657483 (5), 671823 (6), and 657485 (7) from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, U.K. (fax +44-1223-336033; e-mail [email protected]; http://www.ccdc.cam.ac.uk).

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CG800294C

A Series of Novel LnâSuccinateâOxalate Coordination Polymers

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