One-, Two-, and Three-Dimensional Lanthanide Complexes

The progressive structural variation from the 1D zigzag chains (1−2) to 2D networks (3−4) and to 3D frameworks (5, 6, and 7) is attributed to the ...
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One-, Two-, and Three-Dimensional Lanthanide Complexes Constructed from Pyridine-2,6-dicarboxylic Acid and Oxalic Acid Ligands Mao-Sheng Liu,† Qiong-Yan Yu,† Yue-Peng Cai,*,† Cheng-Yong Su,‡ Xiao-Ming Lin,† Xiu-Xia Zhou,† and Ji-Wen Cai*,‡

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 4083–4091

Key Laboratory of Technology on Electrochemical Energy Storage and Power Generation in Guangdong UniVersities, School of Chemistry and EnVironment, South China Normal UniVersity, Guangzhou, 510006, China, and School of Chemistry and Chemical Engineering, School of Pharmaceutical Sciences, Sun Yat-Sen UniVersity, Guangzhou, 510080, China ReceiVed May 18, 2008; ReVised Manuscript ReceiVed July 23, 2008

ABSTRACT: The reaction of pyridine-2,6-dicarboxylic acid (H2PDA) and oxalic acid (H2ox) with Ln2O3/Ce(NO3)3 under hydrothermal conditions generated a series of new one-, two-, and three-dimensional (1D, 2D, and 3D) coordination polymers, namely, {[Ln(PDA)(ox)0.5(H2O)3] · H2O}n (Ln ) La(1) and Ce(2)), {[Ln2(PDA)(ox)2 · 4(H2O)]}n (Ln ) Nd(3) and Sm(4)), {[Ln2(PDA)(ox)2 · 7(H2O)]}n (Ln ) Eu(5), Tb(6), and Er(7)) (PDA2- ) pyridine-2,6-dicarboxylate anion, ox2- ) oxalate anion). The complexes were characterized by X-ray single-crystal determination, spectroscopy and fluorescent analyses. Compounds 1 and 2 (type I structure) are 1D zigzag chain structures bridged by bis-bidentate oxalate. The 1D chains are further linked by the significant intermolecular π · · · π interactions between pyridine rings of coordinating PDA2- groups to form a 2D supramolecular framework network. In compounds 3 and 4 (type II structure), the square motifs formed by four Ln(III) ions, serving as the building blocks, are assembled into a highly ordered 2D (4,4) grid. Similarly, via weak π · · · π interactions between two central pyridine rings from two adjacent sheets, the 2D layers are further stacked up into the final 3D structures. Complexes 5, 6, and 7 (type III structure) are isomorphous and have the same 3D network structures fabricated through two alternately arranging hexanuclear Ln3+ (Ln ) Eu(5), Tb(6) and Er(7)) units, and the guest water molecules are trapped in the cavity. The progressive structural variation from the 1D zigzag chains (1-2) to 2D networks (3-4) and to 3D frameworks (5, 6, and 7) is attributed to the lanthanide contraction effect and the different coordination modes of PDA2-. Two new coordination modes of pyridine-2,6-dicarboxylic acid are observed, which proved that pyridine-2,6-dicarboxylic acid could be used as an effective bridging ligand to construct lanthanide-based coordination polymers. The fluorescent properties of compounds 3, 5, and 6 have also been investigated. Introduction The rational design and synthesis of rare-earth organic frameworks (REOF) have attracted extensive interest in the field of supramolecular chemistry and crystal engineering. Although the flexibility of the coordination sphere of lanthanide ions makes the design difficult compared with the predictability of the coordination geometry of the transition metals.1 The pliability or coordinative ambivalence, coupled with the tendency of lanthanides to adopt high coordination numbers and the rigidity of the organic ligands employed, makes the f-block metal ions attractive in giving a variety of intriguing structures dictated by the different size of the lanthanide ions and can result in fascinating and interesting molecular topologies and crystal packing motifs.2,3 These compounds have potential applications as functional materials with optical, magnetism, catalysis, electrical conductivity properties, etc.4-6 Consequently, studies in these aspects are expanding rapidly, and a lot of investigations on the lanthanide coordination polymers have been published recently.6-9 On the other hand, it is well-known that carboxylate ligands play an important role in constructing novel metal-organic frameworks (MOFs) based on lanthanides in coordination chemistry. They usually adopt not only diverse coordination binding modes such as terminal monodentate, chelating to one * To whom correspondence should be addressed. (Y.-P.C.) Phone: +86-2033033475. Fax: +86-20-85215865. E-mail: [email protected]. (J.-W.C.) +86020-84115178. E-mail: [email protected]. † South China Normal University. ‡ Sun Yat-Sen University.

metal center, bridging bidentate in syn-syn, syn-anti, or antianti configuration to two metal centers, but also supramolecular contacts such as hydrogen bonding, π · · · π interactions, etc.10 Thus, many spectacular REOFs have been documented, such as one-dimensional (1D) chains and ladders,11 two-dimensional (2D) grids,12 three-dimensional (3D) microporous networks, interpenetrated modes, and helical staircase networks and so on.13 Pyridine-2,6-dicarboxylic acid (H2PDA) as a very important carboxylate derivative has attracted many interests in coordination chemistry. H2PDA has a rigid 120° angle between the central pyridine ring and the two carboxylate groups, and therefore could potentially provide various coordination modes to form both discrete and consecutive metal complexes under appropriate synthesis conditions.14 A systematic study of 3d, 4f, 3d-4f, 4d-4f, and 3d-4d complexes based on H2PDA has been undertaken in our laboratory and others,15 which presents rich coordination motifs of PDA2- (Scheme 1). However, the metal-organic coordination polymers built up from two different carboxylate bridging ligands attract less attention compared with single carboxylate-containing ones.16 We were inspired to explore further by adding another auxiliary organic ligand-oxalic acid to the reaction mixture to construct lanthanide coordination polymers. This contribution mainly studies the coordination chemistry of H2PDA and H2ox in the Ln-based complexes under hydrothermal conditions. A series of new coordination polymers, namely, {[Ln(PDA)(ox)0.5(H2O)3] · H2O}n (Ln ) La(1) and Ce(2)), {[Ln2(PDA)(ox)2 · 4(H2O)]}n (Ln ) Nd(3) and Sm(4)), {[Ln2 (PDA)(ox)2 · 7(H2O)]}n (Ln ) Eu(5), Tb(6) and Er(7)) (PDA2- ) pyridine2,6-dicarboxylate anion, ox2- ) oxalate anion) were synthesized

10.1021/cg800526y CCC: $40.75  2008 American Chemical Society Published on Web 09/20/2008

4084 Crystal Growth & Design, Vol. 8, No. 11, 2008

Liu et al.

Scheme 1. The Coordination Modes of PDA2-, where A and B Coordination Modes Were Observed for the First Time in Lanthanide Complexes

under hydrothermal conditions and characterized by elemental analyses, IR, TGA, fluorescent measurements, and single-crystal X-ray diffraction analyses. To the best of our knowledge, this is the first example of REOFs based on mixed PDA2- and ox2ligands. Experimental Section Physical Measurements. All materials were reagent grade obtained from commercial sources and used without further purification. Elemental analyses for C, H, N were performed on a Perkin-Elmer 240C analytical instrument. IR spectra were recorded on a Nicolet FTIR-170SX spectrophotometer in KBr pellets. The luminescent spectra for the solid state were recorded at room temperature on Hitachi F-2500 and Edinburgh-FLS-920 with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra the pass width is 5.0 nm. Thermal analyses (under oxygenated atmosphere, heating rate of 5 °C/min) were carried out in a Labsys NETZSCH TG 209 Setaram apparatus. Synthesis of {[La(PDA)(ox)0.5 · (H2O)3] · H2O}n (1). A mixture of pyridine-2,6-dicarboxylic acid (0.334 g, 2.0 mmol), oxalic acid (0.075 g,1.0 mmol), La2O3 (0.326 g, 1.0 mmol), and H2O (10 mL) was placed in a 25 mL acid-digestion bomb at 160 °C for 3 days. The products (0.278 g), with 35% yield based on La, were collected after washing with water and ether. Elemental analysis calcd (%) for C8H11NO10La: C, 22.85; H, 2.62; N, 3.33. Found: C, 22.78; H, 2.69; N, 3.35. IR (KBr, cm-1): 3421(br), 1634(s), 1591(s), 1472(w), 1445(vs), 1401(vs), 1376(vs), 1313(s), 1276(s), 1192(s), 1160(w), 1121(w), 1089(s), 1017(vs), 926(vs), 793(w), 768(s), 661(s). Synthesis of {[Ce(PDA)(ox)0.5 · (H2O)3] · H2O}n (2). A mixture of pyridine-2,6-dicarboxylic acid (0.334 g, 2.0 mmol), oxalic acid (0.075 g, 1.0 mmol), Ce(NO3)3 · 6H2O (0.950 g, 2.0 mmol), HCl (3 M, 0.1 mL) and H2O (10 mL) was placed in a 25 mL acid-digestion bomb and the pH value was 2 at 160 °C for 3 days. The products (0.271 g), with 32% yield based on Ce, were collected after washing with water and ether. Elemental analysis calcd (%) for C8H11NO10Ce: C, 22.79; H, 2.61; N, 3.32. Found: C, 22.65; H, 2.65; N, 3.27. IR (KBr, cm-1): 3411(br), 1629(s), 1601(m), 1595(s), 1523(m), 1453(s), 1388(vs), 1336(s), 1281(s), 1205(s), 1126(m), 1078(s), 1006(vs), 921(s), 776(w), 753(s), 658(m). Synthesis of {[Nd2(PDA)(ox)2(H2O)4] · 4(H2O)}n (3). The reaction procedure was the same as that of compound 1 except that La2O3 was used instead of Nd2O3. The blue crystals of 3 were isolated in 41% yield (0.317 g) (based on Nd). Elemental analysis calcd (%) for

C11H19NO20Nd2: C 17.06, H 2.46, N 1.81; found: C 17.02, H 2.55, N 1.87. IR (KBr, cm-1): 3389(br), 1633(vs), 1610(s), 1583(s), 1472(m), 1451(vs), 1387(s), 1352(w), 1280(m), 1190(w), 1141(w), 1022(m), 923(m), 791(s), 745(s), 703(s), 661(m), 585(w). Synthesis of {[Sm2(PDA)(ox)2(H2O)4] · 4(H2O)}n (4). The reaction procedure was the same as that of compound 1 except that La2O3 was used instead of Sm2O3. The yellow crystals of 4 were isolated in 38% yield (0.271 g) (based on Sm). Elemental analysis calcd (%) for C11H19NO20Sm2: C 16.79, H 2.42, N 1.78; found: C 16.83, H 2.48, N 1.82. IR (KBr, cm-1): 3401(br), 1641(s), 1611(m), 1582(s), 1558(m), 1467(w), 1451(vs), 1389(vs), 1293(vs), 1193(w), 1074(s), 1021(s), 927(s), 848(m), 797(vs), 741(vs), 703(s), 665(s). Synthesis of {[Eu2(PDA)(ox)2(H2O)4] · 3(H2O)}n (5). The reaction procedure was the same as that of compound 1 except that La2O3 was used instead of Eu2O3. The colorless crystals of 5 were isolated in about 36% yield (0.266 g) (based on Eu). Elemental analysis calcd (%) for C11H17NO19Eu2: C 17.16, H 2.21, N 1.82; found: C 17.12, H 2.26, N 1.88. IR (KBr, cm-1): 3427(br), 1627(s), 1587(s), 1467(w), 1448(s), 1393(vs), 1319(vs), 1284(s), 1193(w), 1070(m), 1023(w), 936(m), 803(vs), 762(s), 696(s), 663(w), 591(s). Synthesis of {[Tb2(PDA)(ox)2(H2O)4] · 3(H2O)}n (6). The reaction procedure was the same as that of compound 1 except that La2O3 was used instead of Tb4O7. The colorless crystals of 6 were isolated in about 31% yield (0.257 g) (based on Tb). Elemental analysis calcd (%) for C11H17NO19Tb2: C 16.92, H 2.18, N 1.79; found: C 16.88, H 2.19, N 1.77. IR (KBr, cm-1): 3423(br), 1628(s), 1582(s), 1479(s), 1453(s), 1387(vs), 1359(m), 1321(vs), 1286(s), 1199(w), 1218(w), 1070(vs), 1022(s), 935(m), 792(vs), 755(s), 708(s), 607(s), 543(w). Synthesis of {[Er2(PDA)(ox)2(H2O)4] · 3(H2O)}n (7). The reaction procedure was the same as that of compound 1 except that La2O3 was used instead of Er2O3. The red crystals of 7 were isolated in about 33% yield (0.263 g) (based on Er). Elemental analysis calcd (%) for C11H17NO19Er2: C 16.46, H 2.12, N 1.75; found: C 16.43, H 2.19, N 1.79. IR (KBr, cm-1): 3419(br), 1630(s), 1585(s), 1471(w), 1449(vs), 1394(vs), 1363(w), 1321(vs), 1286(s), 1199(w), 1218(w), 1070(vs), 1024(s), 938(m), 804(vs), 763(s), 696(s), 663(m), 593(s), 539(w). X-ray Data Collection and Structure Refinement. Diffraction intensities for complexes 1, 2, 3, 5, 6, and 7 were collected at 298 K on a computer-controlled Bruker APEX II single-crystal diffractometer equipped with graphite monochromated Mo KR with a radiation wavelength of 0.71073 Å using the ω-scan technique. Complex 4 was collected at 298 K on a Oxford Xcalibur Nova single-crystal diffractometer equipped with graphite monochromated Cu KR with a radiation wavelength of 1.54178 Å using the ω-scan technique. Lorentz polarization and absorption corrections were applied. The structures were solved

Ln Complexes from Pyridine-2,6-dicarboxylic Acid

Crystal Growth & Design, Vol. 8, No. 11, 2008 4085

Table 1. Crystal Data and Structure Refinement of 1-7

chemical formula M crystal system space group a /Å b /Å c /Å R /° β /° γ /° V/Å3 Z T /K F(000) Dcalcd/ g cm-3 µ /mm-1 λ /Å Rint data/restraints/param GOF R1 [I ) 2σ(I)]a wR2 [I ) 2σ(I)]b a

1

2

3

4

5

6

7

C8H11NO10La 420.09 triclinic P1j 7.98670(10) 8.32810(10) 9.60420(10) 98.2040(10) 100.4130(10) 102.4160(10) 602.546(12) 2 298 (2) 406 2.315 3.599 0.71073 0.0165 2248/12/206 1.087 0.0128 0.0313

C8H11NO10Ce 421.30 triclinic P1j 8.03070(10) 8.29760(10) 9.61390(10) 98.1740(10) 100.4330(10) 102.4260(10) 604.234(12) 2 298 (2) 408 2.316 3.821 0.71073 0.0172 2245/6/206 1.082 0.0138 0.0342

C11H19NO20Nd2 773.75 triclinic P1j 9.8232(5) 10.1273(5) 12.7101(6) 67.402(3) 80.557(4) 64.790(3) 1056.20(9) 2 298(2) 744 2.433 4.963 0.71073 0.0443 3911/32/355 1.010 0.0503 0.1225

C11H19NO20Sm2 785.97 triclinic P1j 9.8467(5) 10.1475(5) 12.5982(6) 67.426(4) 80.324(4) 64.656(5) 1050.48(9) 2 298 (2) 752 2.485 42.505 1.54178 0.0407 3804/24/356 1.067 0.0421 0.1208

C11H17NO19Eu2 769.16 triclinic P1j 8.15280(10) 10.5121(2) 13.0062(2) 93.9940(10) 103.2590(10) 107.6380(10) 1022.25(3) 2 298 (2) 732 2.499 6.177 0.71073 0.0199 3739/18/335 1.063 0.0199 0.0491

C11H17NO19Tb2 785.10 triclinic P1j 8.1213(3) 10.5379(2) 13.0921(3) 94.1358(18) 102.9857(17) 108.1915(15) 1024.89(6) 2 298(2) 744 2.544 6.942 0.71073 0.0191 3765/21/346 1.060 0.0201 0.0505

C11H17NO19Er2 801.78 triclinic P1j 8.0854(3) 10.4344(3) 12.9248(4) 94.076(2) 103.028(2) 107.492(2) 1001.86(6) 2 298 (2) 756 2.658 8.419 0.71073 0.0198 3684/21/341 1.077 0.0229 0.0613

R1 ) Σ||Fo| - |Fc||/|Fo|. b wR2 ) [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2, where w ) 1/[σ2(Fo2) + (aP)2 + bP]. P ) (Fo2 + 2Fc2)/3.

by direct methods and refined with the full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs.17 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The organic hydrogen atoms were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter; the hydrogen atoms of the water molecules were located from difference maps and refined with isotropic temperature factors. Details of the crystal parameters, data collections and refinement for complexes 1-7 are listed in Table 1. Selected bond lengths and angles for complexes 1-7 are listed in Table 2. Hydrogen-bonding data of complexes 1-7 are listed in Table 3. Further details are provided as Supporting Information. CCDC reference numbers for 1-7 are CCDC 672020, 695733, 695734, 672021, 672022, 676265, and 672023, respectively.

Results and Discussion Infrared Spectroscopic and Thermogravimetric Analysis. The seven compounds were obtained in 31-41% yields, by reacting Ce(NO3)3 · 6H2O/Ln2O3 with H2PDA and H2ox in hydrothermal condition at 160 °C. The structures of the complexes were identified by satisfactory elemental analysis, IR and X-ray diffraction. The IR spectra of 1-7 are similar. The strong and broad absorption bands in the range of 3200-3600 cm-1 in 1-7 are assigned to the characteristic peaks of OH vibration. The strong vibrations appeared around 1635, 1590, and 1450 cm-1 correspond to the asymmetric and symmetric stretching vibrations of the carboxylato group, respectively.18 The absence of strong bands ranging from 1690 to 1730 cm-1 indicates that the ligands are deprotonated. Because complexes 1-2, 3-4, and 5-7 are isomorphous, compounds 1, 4, and 6 were selected to examine the thermal stability. TGA was carried out for polycrystalline samples of compounds 1, 4, and 6 in the temperature range of 25-800 °C (see Supporting Information). For 1, the first weight loss of 17.15% in the range of 82-208 °C corresponds to the loss of four water molecules (calcd 17.11%, including three coordinated and one uncoordinated water molecules in one asymmetric unit). The second weight loss above 410 °C corresponds to the decomposition of the coordination network. The TGA curve of compound 4 also shows two weight-loss steps. The first of 18.61% between 25.7 and 238.5 °C is consistent with the loss of the four coordinated water molecules and four uncoordinated water molecules (calculated: 18.60%). The major weight loss occurred in the range of 308-800 °C, which may correspond

to the complete destruction of the network. The compound 6 shows similar thermal behavior and undergoes two steps of weight loss. Seven uncoordinated and coordinated water molecules were gradually lost in the temperature range of 120-310 °C for 6 (calcd/found: 15.72/15.85%). Above that temperature range, the weight loss is due to the decomposition of the organic ligands and the collapse of the whole framework. Description of Crystal Structures of type I complexes. {[Ln(PDA)(ox)0.5(H2O)3] · H2O}n (Ln ) La(1) and Ce(2)). The key feature of the type I structure is represented by compound 1. The X-ray analysis shows that both PDA2- and ox2- ligands have contributed to the molecular structure of 1, forming a mixed-ligand complex with 1D infinite chain-like structure. As shown in Figure 1, each La(III) ion is ten-coordinated in NO9 donor set with the coordination geometry of a distorted dodecahedron by one nitrogen atom from the pyridine ring of one PDA2- ligand, two carboxylate oxygen atoms from one oxalate anion, four carboxylate oxygen atoms from two PDA2ligand, and three oxygen atoms from three coordinated water molecules. Therefore, the ligands around La(III) ion are one oxalate anion, two pyridine-2,6-dicarboxylate anions, and three water molecules. The La-O distances range from 2.482(8) to 2.926(8) Å (average 2.616(7) Å) (Table 2), and the longest La-O distance is associated with one carboxylate oxygen atom of one coordinated pyridine-2,6-dicarboxylate anion which acts as a bridge linking two La centers with La · · · La separation of 4.499 Å. The O-La-O bond angles range from 60.9(2)° to 155.1(3)°. As shown in Figure 2, compound 1 possesses a 1D chainlike structure with the La2(PDA)2(H2O)3 units linked together by the ox2- bridges propagating along the a axis direction. In the chain, the [La2O2] rhombic units are observed (Figure 1), in which two La ions are bridged by a pair of PDA2- anions in a chelating mode (Scheme 1A). The interdimer La · · · La separation is 6.681(6) Å. It is noteworthy that the two carboxylic groups in PDA2- ligand adopt η1-O (O3 atom connects one LaIII ion, and the carboxylic group coordinates to one LaIII ion) and µ2(η2: η1)-O (one atom O1 connects two LaIII ions, the other O2 connects one LaIII ion, and the carboxylate group

4086 Crystal Growth & Design, Vol. 8, No. 11, 2008 Table 2. Selected Bond Distances (Å) for Complexes 1-7a 1 La(1)-O(3) La(1)-O(6) La(1)-O(8) La(1)-O(1) La(1)-O(5)

2.5124(8) 2.5797(9) 2.5826(10) 2.5882(8) 2.6048(10)

Ce(1)-O(1) Ce(1)-O(3) Ce(1)-O(8) Ce(1)-O(3)#2 Ce(1)-N(1)

2.5189(7) 2.5916(7) 2.6017(9) 2.6021(7) 2.6784(9)

Nd(1)-O(3) Nd(1)-O(5) Nd(1)-O(1) Nd(1)-O(6) Nd(1)-O(14)#3 Nd(1)-O(10)#4 Nd(1)-O(12)#3 Nd(1)-O(8)#4 Nd(1)-N(1) Nd(2)-O(1)

2.453(4) 2.462(5) 2.464(5) 2.468(3) 2.473(4) 2.475(4) 2.500(4) 2.502(4) 2.528(5) 2.751(5)

Sm(1)-O(8W) Sm(1)-O(11) Sm(1)-O(8)#6 Sm(1)-O(7W) Sm(1)-O(7)#6 Sm(1)-O(9) Sm(1)-O(2) Sm(1)-O(4)#7 Sm(1)-O(3)#7 Sm(2)-N(1)

2.428(2) 2.458(3) 2.463(3) 2.460(4) 2.470(3) 2.497(3) 2.535(3) 2.571(3) 2.735(3) 2.523(3)

Eu(1)-O(4) Eu(1)-O(8) Eu(1)-O(14) Eu(1)-O(1)#9 Eu(1)-O(6)#10 Eu(1)-O(7) Eu(1)-O(5) Eu(1)-O(13) Eu(2)-O(10)

2.2692(13) 2.3463(16) 2.3465(15) 2.3733(12) 2.3850(13) 2.4227(12) 2.4432(15) 2.4469(17) 2.5197(14)

Tb(1)-O(9) Tb(1)-O(14) Tb(1)-O(12)#9 Tb(1)-O(4) Tb(1)-O(8)#10 Tb(1)-O(7) Tb(1)-O(6) Tb(1)-O(13) Tb(2)-O(5)

2.2633(13) 2.3515(15) 2.3537(12) 2.3546(16) 2.4006(13) 2.4311(15) 2.4322(12) 2.4441(11) 2.5281(14)

Er(1)-O(4) Er(1)-O(14) Er(1)-O(8) Er(1)-O(1)#9 Er(1)-O(6)#10 Er(1)-O(7) Er(1)-O(13) Er(1)-O(5) Er(2)-O(10)

2.2551(16) 2.3260(19) 2.3281(19) 2.3592(15) 2.3708(16) 2.4100(15) 2.4174(9) 2.4221(18) 2.5013(17)

La(1)-O(1)#1 La(1)-O(9) La(1)-O(7) La(1)-N(1) La(1)-O(2)#1

2.6089(8) 2.6106(9) 2.6123(11) 2.6800(11) 2.8979(11)

Ce(1)-O(7) Ce(1)-O(5) Ce(1)-O(6) Ce(1)-O(9) Ce(1)-O(4)#2

2.5638(9) 2.5855(9) 2.6039(8) 2.6171(10) 2.8967(9)

Nd(2)-O(16) Nd(2)-O(7) Nd(2)-O(13) Nd(2)-O(15) Nd(2)-O(9) Nd(2)-O(11) Nd(2)-O(4)#5 Nd(2)-O(2) Nd(2)-O(3)#5

2.428(3) 2.459(5) 2.472(4) 2.478(5) 2.486(4) 2.498(4) 2.547(6) 2.550(4) 2.733(5)

Sm(1)-O(1) Sm(2)-O(5W) Sm(2)-O(3) Sm(2)-O(1) Sm(2)-O(6W) Sm(2)-O(12)#8 Sm(2)-O(6) Sm(2)-O(5) Sm(2)-O(10)#8

2.792(3) 2.448(4) 2.459(2) 2.461(3) 2.466(2) 2.470(3) 2.469(3) 2.477(3) 2.482(3)

Eu(2)-O(16) Eu(2)-O(15) Eu(2)-O(9) Eu(2)-O(2)#11 Eu(2)-O(3)#11 Eu(2)-O(11) Eu(2)-O(12) Eu(2)-N(1)#11

2.3909(12) 2.3950(15) 2.3961(13) 2.4127(13) 2.4168(12) 2.4381(15) 2.4412(13) 2.5132(15)

Tb(2)-O(16) Tb(2)-O(15) Tb(2)-O(3) Tb(2)-O(10)#11 Tb(2)-O(11)#11 Tb(2)-O(1) Tb(2)-O(2)#12 Tb(2)-N(1)#11

2.3876(12) 2.3935(15) 2.4055(13) 2.4061(12) 2.4078(13) 2.4399(15) 2.4594(13) 2.5070(15)

Er(2)-O(15) Er(2)-O(16) Er(2)-O(9) Er(2)-O(2)#11 Er(2)-O(3)#11 Er(2)-O(11) Er(2)-O(12)#12 Er(2)-N(1)#11

2.3725(18) 2.3787(14) 2.3850(16) 2.3960(17) 2.4043(14) 2.4169(18) 2.4270(17) 2.4965(19)

2

3

4

5

6

7

a

-x, -z; -z; #10

Symmetry transformations used to generate equivalent atoms: #1 -y + 1, -z; #2 -x + 2, -y + 2, -z + 2; #3 -x + 1, -y + 1, #4 -x + 1, -y, -z + 1; #5 x + 1, y, z; #6 -x + 1, -y + 2, #7 x - 1, y, z; #8 -x + 1, -y + 1, -z + 1; #9 x - 1, y - 1, z; -x, -y + 1, -z; #11 x, y - 1, z; #12 -x + 1, -y, -z + 1.

Liu et al.

coordinates to two LaIII ions) coordination modes, respectively. The coordination mode A of PDA2- anions is reported for the first time. The packing of complex 1 in the crystal lattice is worth mentioning. Each 1D infinite zigzag chain interacts with two neighboring ones with each other via face-to-face π · · · π weak intermolecular stackings, forming extended 2D layer-like structures (Figure 3). The shortest distance between two parallel pyridine rings of two adjacent chains is 3.200 Å. Moreover, the coordinated/noncoordinated water molecules interact with the carboxylate oxygen atoms from the layers via O-H · · · O hydrogen bonds (Table 3), resulting in the formation of 3D supramolecular network (Figure 4), which contributes to the additional stability of the structure.19 Compound 2 is of the same structure as 1 (see Supporting Information S2). The selected bond lengths, listed in Table 2, indicate that Ln-N and the average Ln-O bond lengths decrease from 1(La) to 2(Ce), consistent with the radius contraction from La to Ce. Type II structure of {[Ln2(PDA)(ox)2(H2O)4] · 4(H2O)}n (Ln ) Nd(3) and Sm(4)). The reaction of H2PDA and H2ox with Ln2O3 under the same hydrothermal condition generates two 2D coordination polymers 3 and 4 with type II structure. Here compound 4 was selected to represent the structure of type II. The X-ray structural analysis of 4 reveals that Sm atoms have two types of coordination environments (Figure 5). Sm2 is nine-coordinated with a tricapped trigonal prism geometry, whereas Sm1 is ten-coordinated. The Sm2 atom coordinates to one PDA2- anion by using two oxygen atoms from two different carboxylate groups and one nitrogen atom, two water molecules as well as four oxygen atoms from two oxalate anions to form a coordination sphere. The Sm1 atom coordinates to two PDA2anions by four oxygen atoms from two carboxylate groups of two different PDA2- anions, two oxalate anions by four oxygen atoms as well as two water molecules to complete the coordination geometry. The Sm(III)-O and Sm(III)-N distances are within normal ranges (Table 2).20 The distances between Sm1 and Sm2 atoms are 5.013(5) and 6.405(6) Å, respectively. Each PDA2- anion can be described as adopting the symmetric heptadentate coordination mode (Scheme 1B) linking three Sm centers to form trinuclear units, in which two carboxylic groups of the PDA2- ligand all adopt µ2(η2: η1)-O coordination fashions (one O1/O3 atom connects two SmIII ions, the other O2/O4 atom connects one SmIII ion, and each carboxylate group coordinates to two SmIII ions). The coordination mode B is reported for the first time. In compound 4, the Sm1 and Sm2 atoms are alternately arrayed by carboxylate bridges and generate a tetranuclear homometallic Sm4 square unit with 12-membered Sm4C2O6 motifs. The rhombic Sm4 metalic species as a building blocks are further connected by carboxylate O atoms to assemble into a highly ordered infinite 2D grid with a (4,4) net (Figure 6). Extensive π · · · π packing interactions and strong O-H · · · O hydrogen bonds existed between two neighboring layers. Each pyridyl ring of one layer is interacted with each other in faceto-face and partially overlapping fashion. The shortest distance between the two parallel pyridine rings of two adjacent layers is 3.218 Å. The O-H · · · O hydrogen bonds originate from the coordinated water molecules and the free water molecules (Table 3). As a result, the 2D grids are further assembled into a 3D supramolecular network. Obviously, these weak interactions help to stabilize the topology of 4 (Figure 7). Compound 3 is isomorphous with 4 (see Supporting Information S3), and selected distances are listed in Table 3. As the

Ln Complexes from Pyridine-2,6-dicarboxylic Acid

Crystal Growth & Design, Vol. 8, No. 11, 2008 4087

Table 3. Distances (Å) and Angles (°) of Hydrogen Bonds for Compounds 1-7a D-H · · · A

distance (D · · · A)

angle (D-H · · · A)

O5-H5A · · · O4a O5-H5B · · · O10b O6-H6A · · · O4a O6-H6B · · · O8c O7-H7A · · · O9d

2.7003(12) 2.8022(14) 2.7960(13) 2.8788(14) 2.7681(12)

162.5(11) 179.0(11) 173.1(7) 163.5(6) 165.9(7)

O(7)-H(7A) · · · O(2)° O(7)-H(7B) · · · O(5)s O(8)-H(8A) · · · O(2)° O(8)-H(8B) · · · O(10)l

2.7987(12) 2.8769(12) 2.7041(11) 2.8191(13)

170.8(18) 168.5(17) 164.5(18) 171.5(16)

O(2W)-H(2WA) · · · O(4W) O(15)-H(15B) · · · O(4W)k O(2W)-H(2WB) · · · O(15)n O(2W)-H(2WB) · · · O(16)n O(16)-H(16B) · · · O(10)u O(16)-H(16A) · · · O(2W)v O(15)-H(15A) · · · O(2W)v

2.963(6) 3.259(7) 3.226(5) 2.816(6) 2.703(6) 2.816(6) 3.226(5)

115.8(10) 132.4(11) 95.5(6) 140.7(8) 162.1(7) 153.2(9) 125.7(13)

D-H · · · A

distance (D · · · A)

angle (D-H · · · A)

O7-H7B · · · O7e O10-H10A · · · O2f O10-H10A · · · O2f O10-H10B · · · O3g

3.013(2) 2.7409(16) 2.7409(16) 2.8143(14)

170.5(13) 172.8(4) 172.8(4) 172.1(6)

O(9)-H(9A) · · · O(9)t O(9)-H(9B) · · · O(6)r O(10)-H(10A) · · · O(4)p O(10)-H(10B) · · · O(1)

3.0032(19) 2.7711(11) 2.7328(14) 2.8252(12)

178.1(7) 168.6(6) 174.2(3) 172.4(3)

O(6)-H(6B) · · · O(15) O(6)-H(6B) · · · O(7) O(6)-H(6A) · · · O(1W)w O(5)-H(5B) · · · O(16)j O(5)-H(5B) · · · O(13)j O(5)-H(5A) · · · O(3W)x

3.054(5) 2.715(7) 2.720(5) 3.037(5) 2.753(5) 2.822(7)

114.7(7) 158.0(8) 166.5(8) 121.9(8) 150.3(12) 159.0(14)

O6W-H6WA · · · O3Wm O6W-H6WB · · · O7c O7W-H7WA · · · O1Wn O7W-H7WB · · · O10° O8W-H8WA · · · O6j O8W-H8WB · · · O1Wn O5W-H5WA · · · O2Wi O2W-H2WB · · · O4h

2.702(4) 2.717(4) 3.255(5) 2.718(4) 2.701(4) 2.800(6) 2.761(6) 3.225(5)

158.3(4) 170.1(3) 148.0(3) 173.1(11) 179.0(3) 156.0(3) 173.9(4) 132.3(7)

O16-H16A · · · O10° O17-H17B · · · O9q O18-H18A · · · O11i O18-H18B · · · O7° O19-H19B · · · O17°

2.8262(17) 3.062(3) 2.8914(17) 2.874(2) 2.747(3)

160.4(4) 157(2) 173.0(4) 165.8(7) 156.2(8)

O16-H16A · · · O5° O17-H17B · · · O3q O18-H18A · · · O1 O18-H18B · · · O6° O19-H19A · · · O17° O19-H19B · · · O7°

2.8114(17) 3.0439(15) 2.8659(17) 2.886(2) 2.742(3) 2.750(2)

162.0(3) 152.70(17) 175.3(4) 169.1(7) 154.0(4) 152.5(6)

O16-H16A · · · O10° O17-H17A · · · O19° O17-H17B · · · O9q O18-H18A · · · O11i O18-H18B · · · O7° O19-H19A · · · O17° O19-H19B · · · O5°

2.811(2) 2.730(3) 3.0412(16) 2.874(2) 2.855(2) 2.730(3) 2.752(3)

164.6(4) 115.1(2) 168.77(17) 173.0(4) 169.6(7) 167.6(7) 156.8(8)

1

2

3

4 O1W-H1WA · · · O2W O1W-H1WB · · · O4Wi O3W-H3WA · · · O4j O3W-H3WA · · · O2i O3W-H3WB · · · O2k O4W-H4WA · · · O1Wi O4W-H4WB · · · O9i O5W-H5WB · · · O11l O5W-H5WB · · · O8Wl

3.105(8) 2.929(6) 2.854(4) 3.068(5) 2.934(4) 2.929(6) 2.827(5) 2.765(3) 3.026(4)

148.0(6) 149.6(17) 152.1(6) 123.6(5) 162.0(9) 124.3(5) 135.0(8) 144(2) 129.6(10)

O14-H14B · · · O15a O14-H14A · · · O2p O15-H15A · · · O18j O15-H15B · · · O3a O16-H16B · · · O19i

2.9631(19) 2.7181(18) 2.641(2) 2.8297(19) 2.671(3)

162.2(6) 155.7(9) 167.8(3) 150.2(5) 168.6(6)

O13-H13A · · · O8j O13-H13B · · · O19a O14-H14B · · · O11p O14-H14A · · · O15a O15-H15A · · · O18j O15-H15B · · · O10a O16-H16B · · · O19

2.9841(15) 3.199(4) 2.7000(18) 2.9628(19) 2.654(2) 2.8321(19) 2.684(3)

153.31(17) 136.52(14) 155.1(8) 164.0(8) 170.6(3) 151.7(5) 170.6(8)

O13-H13A · · · O6j O13-H13B · · · O19a O14-H14B · · · O15a O14-H14A · · · O2p O15-H15A · · · O18j O15-H15B · · · O3a O16-H16B · · · O19i

2.9809(16) 3.155(4) 2.948(2) 2.703(2) 2.619(3) 2.812(2) 2.657(3)

157.35(19) 133.09(18) 161.9(6) 157.0(10) 174.0(4) 151.2(6) 163.6(5)

h

5

6

7

a Symmetry transformation used to generate equivalent atoms: (a) -x + 1, -y + 2, -z + 1; (b) -x, -y + 1, -z + 1; (c) -x + 1, -y + 2, -z; (d) -x, -y + 1, -z; (e) -x, -y + 2, -z; (f) -x, -y, -z + 1; (g) x - 1/2, y + 1/2, -z + 3/2; (h) -x + 2, -y + 1, -z + 1; (i) x, y, z; (j) x - 1, y, z; (k) -x + 1, -y + 1, -z; (l) x + 1, y, z; (m) x, y + 1, z; (n) x - 1, y + 1, z; (o) -x + 1, -y + 1, -z + 1; (p) x - 1, y - 1, z; (q) x + 1, y + 1, z; (r) -x + 2, -y + 2, -z + 2; (s) -x + 1, -y + 1, -z + 2; (t) -x + 2, -y + 1, -z + 2; (u) -x + 2, -y, -z + 1; (v) x + 1, y - 1, z; (w) x, y - 1, z; (x) x, y, z - 1.

ionic radii decreasing from Nd to Sm, Ln-N and the average Ln-O bond lengths decrease due to the lanthanide contraction. {[Ln2(PDA)(ox)2(H2O)4] · 3(H2O)}n (Ln ) Eu(5), Tb(6), Er(7)). X-ray crystallography reveals that compounds 5, 6 and 7 are isomorphous. Here, we choose 5 to represent the detailed structure. The structure of 5 is different from those of complexes 1-4. First of all, the local coordination environment around the Ln(III) ion is significantly different. As indicated in Figure 8, there are two independent Eu(III) ions where Eu1 is eightcoordinated with O8 donor and Eu2 is nine-coordinated with

NO8 donor sets. The Eu1 atom coordinated to two oxygen atoms from two PDA2- anions (O1a and O4), two water molecules and four oxygen atoms of two oxalate anions: eight oxygen atoms form the bicapped trigonal prism configuration of Eu1 atom. The Eu2 atom chelates to one PDA2- anion: two oxygen atoms from the carboxylate group of PDA2- anion, two water molecules, four oxygen atoms from two oxalate anions, and one nitrogen atom of the PDA2- anion complete coordination geometry of tricapped trigonal prism for the Eu2 atom. The similar coordination geometries have been observed in other

4088 Crystal Growth & Design, Vol. 8, No. 11, 2008

Figure 1. ORTEP view of 1 (30% thermal ellipsoids) showing the atomlabeling scheme. Symmetry-related atoms are indicated by O1a, etc. Hydrogen atoms and lattice water molecules are omitted for clarity. Symmetry code a: -x, -y + 1, -z.

Liu et al.

Figure 4. 3D network in 1 constructed by π · · · π stacking and O-H · · · O hydrogen bonding interactions in bc plane. H atoms were omitted for clarity.

Figure 2. One-dimensional zigzag chain extending along the a axis in 1.

Figure 5. Nanosized square motif containing four Sm ions and different coordination environments of the Sm ions in complex 4. Uncoordinated water and H atoms were omitted for clarity. Symmetry code a: 1 - x, 1 - y, 1 - z; b: -1 + x, y, z; c: 1 - x, 2 - y, -z.

Figure 3. 2D sheet in 1 constructed by π · · · π stacking interactions between the two adjacent one-dimensional chains in ab plane. Uncoordinated water and H atoms were omitted for clarity.

PDA-Ln systems.21 Selective bond distances for 5 are collected in Table 2. The Eu(III)-O and Eu(III)-N distances are similar to those found in other Eu(III) complexes.22 These Eu-O and Eu-N bond lengths are slightly shorter than those in the LnPDA of complexes 1-4, as expected, due to lanthanide contraction. The two Eu(III) ions are well-separated with distance between 6.255 and 6.711 Å. The bond distances and bond angles of the ligand moiety are within normal ranges. The dinuclear Eu unit (Figure 8) composed of two Eu atoms with different coordination modes are extended by oxalate bridges with two different bridging modes, namely, V-like for Eu1 and linear for Eu2 (respective yellow and blue in Figure 8), and generated two nanosized hexanuclear homometallic Eu6 square or hexagonal units with the 24-membered Eu6C6O12 motif (Figure 9). Most interestingly, the hexagonal units as the building blocks are connected into a spectacular 2D grid, which was further assembled into a 3D framework through O1 atom bridge from one carboxylate group of PDA2- anion as shown in Figure 10. The 6-membered subunit is the basic block of the

Figure 6. 2D (4, 4) net linked via carboxylate groups in 4. Uncoordinated water and H atoms were omitted for clarity.

3D non-interpenetration framework. Each metal center surrounded by four other neiguboring Eu(III) atoms connected via oxalate and PDA2- anions with the distances between 5.478 and 6.783 Å, and the adjacent Eu(III) atoms acts as fourconnected node to construct a 3D four-connected net with diamond 66 topology. Various hydrogen bonds exist between the carboxylate oxygen and solvent water or coordinated water molecules, and face to face π · · · π packing interactions between the two pyridine rings in hexagonal Eu6 units are observed with the shortest distance between two parallel pyridine rings being

Ln Complexes from Pyridine-2,6-dicarboxylic Acid

Figure 7. 3D network in 4 constructed by π · · · π stacking interactions, the distance between the centers of the two pyridine rings from different layers is 3.630 Å. Uncoordinated water and H atoms were omitted for clarity.

Figure 8. ORTEP plot of dinuclear Eu unit in 5 (30% thermal ellipsoids) showing the different coordination environment of the Eu centers. Uncoordinated water molecules and H atoms were omitted for clarity. Symmetry code a: -1 + x, -1 + y, z; b: -x, 1 - y, -z; c: x, 1 + y, -z; d: 1 - x, 1 - y, 1 - z.

Figure 9. Hexanuclear Eu6 metallic macrocycle motif in complex 5. Symmetry code a: -x, -y, 1 - z; b: -x, 1 - y, -z; c: 1 - x, 1 - y, z; d: x, 1 + y, -1 + z; e: 1 + x, y, z; f: -1 + x, y, z; g: 1 - x, -y, 1 - z; h: -1 + x, 1 + y, -1 + z.

3.582 Å. These weak intermolecular interactions help stabilize the 3D structure of complex 5. Compounds 6 and 7 are isomorphous of 5, and selected distances are listed in Table 2. The structures of 5, 6 and 7 show the effect of lanthanide contraction: Ln-O bond length and Ln · · · Ln separations decrease along with the decrease of ionic radii from Eu(5), Tb(6) to Er(7). Structural Diversity and Dimensional Change. As all the complexes 1-7 were synthesized using similar procedure, the different crystal structures obtained provide a fair assessment of the critical influence of lanthanide construction. La3+ and

Crystal Growth & Design, Vol. 8, No. 11, 2008 4089

Figure 10. 66 topology of 3D four-connected grid network in 5. Eu, yellow; connection of ox2-, purple and connection of PDA2-, green.

Ce3+ have the largest ionic radius among all lanthanides, which makes La3+ and Ce3+ possible to possess up to 10 coordination sites. In order to satisfy the ten-coordinated potential, PDA2anion adopts new coordination mode A (Scheme 1) and the basic unit I is dimmerized to repeat unit II (Figure 1), resulting in the formation of the final 1D zigzag chain-like complexes 1-2 (Figure 2). Owing to the fact that the steric hindrance of the rigid chelating ring of 2,6-pyridinedicarboxylate, and nine-/tencoordinated NdIII/SmIII being more stable than the eight-coordinated one, in compounds 3 and 4, the basic unit I combining with another LnIII ion (Ln ) Nd(3) and Sm(4)) formed square grid-like repeat unit III (Figure 5), in which PDA2- anion adopts new coordination mode B (Scheme 1), and each metal center linearly connected two chelating oxalate anions. By means of chelating effect of the carboxylate groups of ox2- and PDA2anions, the tetranuclear units III were further assembled into 2D grid with 44 topology. The heavier lanthanide ions Eu3+, Tb3+ and Er3+, have smaller ion radii and higher stability with nine-/eight-coordinated than ten-coordinated ones comparing with La3+/Ce3+ and Nd3+/Sm3+. In this case, chelating connections of the carboxylate groups of PDA2- are no longer necessary and the carboxyl bridges in complexes 5-7 are both µ2 connections forming the coordination mode H of PDA2-, in which each metal center connected two chelating oxalate anions, V-like for Eu1 and linear for Eu2 (Figure 9). Furthermore, through connectivity of carboxyl bridging, a 3D four-connected net with diamond 66 topology was constructed. From the above results, it is noted that various coordination modes of the carboxyl groups of PDA2-, caused by the lanthanide contraction, give rise to the diverse structures of the Ln-PDA complexes. In other words, the structural diversity and dimensional change of the Ln-PDA complexes are controlled by lanthanide contraction (Scheme 2). Moreover, we have also compared the average Ln-N and Ln-O bond lengths among the Ln-PDA complexes (Table 2). The lengths between the lanthanide and N atoms are decreasing continuously from 2.6800 (1), 2.6784(2), 2.528(3), 2.523 (4), 2.5132 (5), 2.5070 (6) to 2.4965 Å (7), and the average lengths between the lanthanide and O atoms are also decreasing continuously from 2.6219 (1), 2.6201(2), 2.512(3), 2.508 (4), 2.4087 (5), 2.4034 (6) to 2.3857 Å (7). Luminescent Properties. The solid-state luminescent properties of 4, 5, and 6 were investigated. The emission spectra of the three complexes (see Supporting Information) at the excited wavelength of 305 nm exhibit the characteristic emission of Sm3+, Eu3+and Tb3+, respectively. The emission spectra

4090 Crystal Growth & Design, Vol. 8, No. 11, 2008 Scheme 2

Liu et al.

Tb3+ > Eu3+ > Sm3+. It means that the energy transfer from the organic ligands to Eu3+ and Tb3+ is more effective than that to Sm3+. Furthermore, the intensity of the three complexes based on the PDA2- and ox2- ligands is completely weak, mainly due to the coordinated water molecules, which reduces the luminescent intensity of the rare earth ions,24 and the energy transfer from the PDA2- and ox2- ligands to lanthanide(III) ions is inefficient in the theory of energy transfer.25 Conclusion

obtained from the europium and terbium complexes principally arise from transitions originating at the 5D0, 5D1 and 5D4 levels, respectively. Both metal complexes show moderate fluorescent emission at room temperature. The spectrum of the Eu complex, 5, (Figure S2b, Supporting Information) shows a weak band observed in the region of the 5 D1 f 7F2 transition. 5D0 f 7F1 and 5D0 f 7F2 are the most intense transitions and consists of an intense band with one weak shoulder at lower frequency. A less intense broad peak with a small shoulder at lower frequency is observed for the 5D0 f 7 F3 transition. The room-temperature solid-state phosphorescence lifetime of the sample was determined to be 235 µs. This lifetime is shorter than that commonly observed for aromatic ligands in aqueous solutions suggesting one or more nonradiative pathways are possible assisting in the deactivation of excitedstate and shortening the observed lifetimes.23 It is known from X-ray structural data that PDA2- and ox2- ligands does not encapsulate the entire metal as only two/three bidentate/tridentate ligands surround each metal leaving two sites ligated with water. Earlier work has established that a weak vibronic coupling between lanthanides and OH oscillators of coordinated water molecules provides a facile path for radiationless deexcitation of the metal ion.23 Thus, the lifetimes observed are expected to be shorter due to the direct coordination of water on the lanthanide. The fluorescent spectrum collected for the terbium complex 6 (Figure S2c, Supporting Information) is impressive as all the transitions to the ground-state manifold from the emitting 5D4 level are observed. They are assigned to the 5D4 f 7FJ (J ) 3, 4, 5, 6), 5D4 f 7F6 (490 nm), 5D4 f 7F5 (546 nm), 5D4 f 7F4 (583 nm), and 5D4 f 7F3 (622 nm) transitions. The roomtemperature fluorescent lifetime of the terbium complex was determined to be 95 µs which is reasonable considering the effect of the coordinated water molecules on quenching the excited state. For compound 4, shown in Figure S2a (Supporting Information), the emission intensity is the weakest as compared to the Eu (5) and Tb (6) spectra, but three characteristic bands can still be observed, they are attributed to 4G5/2 f 6HJ (J ) 5/2, 7/2, 9/2). The 4G5/2 f 6H5/2 (568 nm) transition consists of a weak band in the yellow region, 4G5/2f 6H7/2 (597 nm) starts as a shoulder then splits into two poor-defined peaks but is the most intense observed in the spectrum, and the 4G5/2 f6H9/2 (645 nm) transitions is also a broad and poorly resolved peak. Compared with the emission spectra of the three complexes (3, 4, and 5), the transition intensity changes in the order of

In summary, by using the hydrothermal reaction, seven Ln(III)-based coordination polymers containing the mixed ligands of PDA2- and oxalates have been prepared for the first time. X-ray crystallography revealed that the polymers have novel 1D zigzag chains (1-2) and 2D regular grid lattice networks (3-4) as well as 3D networks (5-7). It is clear that they changed in the order of 1D f 2D f 3D with the increasing atomic number of the lanthanide. Among seven complexes (1-7), the average Ln-N and Ln-O bond lengths decrease with the increasing atomic number of the lanthanides, which shows the lanthanide contraction effect. Two new bridging modes A and B (Scheme 1) of the H2PDA ligand are observed, which proves that H2PDA can be used as an effective bridging ligand in the assembly of MOFs. On the other hand, the complexes of Sm3+ (4), Eu3+ (5) and Tb3+ (6), exhibit characteristic lanthanide-centered luminescence, and the transition intensity changes in the order of Tb3+ > Eu3+ > Sm3+, which shows the energy transfer from the H2PDA and H2ox ligands to Eu3+ and Tb3+ is more effective than that to Sm3+. Acknowledgment. The authors are grateful for financial aid from the National Natural Science Foundation of P. R. China (Grant No. 20772037), Science and Technology Planning Project of Guangdong Province (Grant No. 2006A10902002) and the N. S. F. of Guangdong Province (Grant No. 06025033). Supporting Information Available: Additional structural figures for compounds 2, 3, 6, 7 and TGA curves for compounds 1, 4, 6 as well as X-ray crystallographic files in CIF format for compounds 1-7. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Ln Complexes from Pyridine-2,6-dicarboxylic Acid

(7)

(8) (9)

(10)

(11)

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