Syntheses, Crystal Structures, and Luminescence Properties of a

Dec 10, 2009 - The Nd, Eu, and Tb compounds exhibit strong luminescence in near-IR, ... (3) Recently, many research activities have focused on the syn...
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DOI: 10.1021/cg901012w

Syntheses, Crystal Structures, and Luminescence Properties of a Series of Novel Lanthanide Oxalatophosphonates with Two Types of 3D Framework Structures

2010, Vol. 10 406–413

Lei Liu, Zhen-Gang Sun,* Na Zhang, Yan-Yu Zhu, Yan Zhao, Xin Lu, Fei Tong, Wei-Nan Wang, and Cui-Ying Huang Institute of Chemistry for Functionalized Materials, Faculty of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, P. R. China Received August 23, 2009; Revised Manuscript Received November 18, 2009

ABSTRACT: By introduction of oxalate as the second ligand, 12 novel lanthanide oxalatophosphonates with two types of three-dimensional (3D) framework structures, namely, [Ln2(H4L)(C2O4)3(H2O)] 3 3H2O (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), H4L = 3-pyridyl-CH2N(CH2PO3H2)2) and [Ln2(H4L)(C2O4)3(H2O)] 3 2H2O (Ln = Gd (7), Tb (8), Dy (9), Y (10), Er (11), Lu (12)), have been synthesized under hydrothermal conditions. Compounds 1-6 are isostructural and feature a 3D framework structure formed by the interconnection of the 3D network of {Ln2(C2O4)3} with one-dimensional chains of {Ln2(H4L)}6þ. Compounds 7-12 are also isostructural and possess a novel 3D framework structure different from that in compounds 1-6. The interconnection of Ln(III) ions by chelating and bridging oxalate anions result in a lanthanide oxalate layer in the ac planes. These layers are further linked by H4L ligands to form a 3D framework. It is interesting that a type of righthand helical chain is formed by H4L ligands bridging Er(2) ions running along the b-axis. The Nd, Eu, and Tb compounds exhibit strong luminescence in near-IR, red light, and green light regions, respectively.

Introduction Metal phosphonate chemistry has attracted considerable attention in recent years due to their potential applications in the areas of catalysis, ion exchange, proton conductivity, intercalation chemistry, photochemistry, and materials chemistry.1 Therefore, the rational design and synthesis of novel metal phosphonates with intriguing diversity of the architectures and properties have become a particularly important subject. During the past few years, many metal phosphonates have been prepared through designing and synthesizing phosphonic acids containing -NH2, -OH, and -COOH subfunctional groups and providing many coordination modes. The use of multifunctional phosphonic acids in the synthesis of metal phosphonates may not only result in new structure types of metal phosphonates but also bring interesting properties. A number of metal phosphonate frameworks with additional functionality have been reported recently.2 A series of metal phosphonates with a framework structure, using phosphonic acids with amine, hydroxyl, and carboxylate groups as ligands, have also been isolated in our laboratory.3 Recently, many research activities have focused on the synthesis of inorganic-organic hybrid compounds by incorporating organic ligands in the structures of metal phosphonates. The introduction of a second ligand such as 1,10phenanthroline,4 2,20 -bipyridine,5 4,40 -bipyridine,6 carboxylic acid,7 sulfonic acids,8 or oxalate anions,9 etc. has been found to be an effective synthetic method in the synthesis of metal phosphonates, since these molecules can act as pillars between neighboring layers or be grafted into the inorganic layer to form new hybrid architectures. Among these studies, the oxalate moiety, C2O42-, was found to be a good candidate and has been successfully incorporated into phosphonate *Corresponding author. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 12/10/2009

frameworks with transition metals and main group elements.9 Although some progress has been made in the construction of metal oxalatophosphonate as mentioned above, the reports on the corresponding lanthanide compounds are still rare.10 It was found that introducing a second ligand such as C2O42into the lanthanide phosphonate system can improve the solubility and crystallinity of lanthanide phosphonate. Additionally, the coordination of two types of ligands with the lanthanide ion may reduce or eliminate water molecules from the coordination sphere of the lanthanide (III) ion, hence it increases the luminescent intensity and lifetime of the materials.11 Our interest in metal phosphonate chemistry has also been extended to the lanthanide compounds in recent years. By using organophosphonic acids as the ligands and oxalate moiety, C2O42- as the second metal linker, a series of novel lanthanide oxalatophosphonate hybrids with different architectures have been obtained by our group.12 In this paper, we selected 3-pyridyl-CH2N(CH2PO3H2)2 (H4L) as the phosphonate ligands and oxalate as the second metal linker. Hydrothermal reactions of the above two ligands with lanthanide (III) chlorides resulted in 12 novel lanthanide oxalatophosphonate hybrids with two types of three-dimensional (3D) framework structures, namely, [Ln2(H4L)(C2O4)3(H2O)] 3 3H2O (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), H4L = 3-pyridyl-CH2N(CH2PO3H2)2) and [Ln2(H4L)(C2O4)3(H2O)] 3 2H2O (Ln = Gd (7), Tb (8), Dy (9), Y (10), Er (11), Lu (12)). Herein we report their syntheses, crystal structures, and luminescence properties. Experimental Section Materials and Methods. The 3-pyridyl-CH2N(CH2PO3H2)2 (H4L) was prepared by a Mannich-type reaction according to procedures described previously.13 The lanthanide(III) chlorides were prepared by dissolving corresponding lanthanide oxides (General Research Institute for Nonferrous Metals, 99.99%) in r 2009 American Chemical Society

Article hydrochloric acid followed by recrystallization and drying. All other chemicals were used as obtained without further purification. C, H, and N were determined by using a PE-2400 elemental analyzer. La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Er, Lu, and P were determined by using an inductively coupled plasma (ICP) atomic absorption spectrometer. IR spectra were recorded on a Bruker AXS TENSOR-27 FT-IR spectrometer with KBr pellets in the range 4000-400 cm-1. The X-ray powder diffraction data was collected on a Bruker AXS D8 Advance diffractometer using CuKR radiation (λ = 1.5418 A˚) in the 2θ range of 5-60° with a step size of 0.02° and a scanning rate of 3°/min. The NIR photoluminescence (PL) signals were detected by spectrophotometer with an InGaAs photodiode with a boxcar averager for compounds 3, 4, and 11. The luminescence spectra were reported on a JASCO FP6500 spectrofluorimeter (solid) for compounds 6 and 8. Thermogravimetric analyses (TGA) were performed on a Perkin-Elmer Pyris Diamond TG-DTA thermal analyses system in static air with a heating rate of 10 K min-1 from 50 to 1000 °C. Synthesis of [Ln2(H4L)(C2O4)3(H2O)] 3 3H2O (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6)). The methods used for the syntheses of compounds 1-6 are similar. A mixture of LnCl3 3 6H2O (0.5 mmol, 0.18 g), H4L (0.5 mmol, 0.15 g), NaOH (2 mmol, 0.08 g), and H2C2O4 3 2H2O (2 mmol, 0.25 g) was dissolved in 8 mL of distilled water. The resulting solution was stirred for about 30 min at room temperature, sealed in a 20 mL Teflon-lined stainless steel autoclave, and heated at 140 °C for 4 days under autogenous pressure. Crystals of 1 (colorless, block), crystals of 2 (colorless, block), crystals of 3 (green, block), crystals of 4 (purple, block), crystals of 5 (yellow, block) and crystals of 6 (colorless, block) were collected in ca. 61.4%, 57.7%, 66.3%, 58.2%, 64.6%, and 60.4% yield, respectively (based on lanthanide metal). Anal. Calcd. for C14H22N2O22P2La2: C, 18.48; H, 2.44; N, 3.08; P, 6.81; La, 30.53; Found: C, 18.52; H, 2.36; N, 3.01; P, 6.89; La, 30.46%. IR (KBr, cm-1): 3467 s, 3089 w, 1623 s, 1594 s, 1459 w, 1313 m, 1232 m, 1184 s, 1074 s, 935 w, 794 m, 676 w, 595 w, 443 w. Anal. Calcd. for C14H22N2O22P2Ce2: C, 18.43; H, 2.43; N, 3.07; P, 6.79; Ce, 30.71; Found: C, 18.50; H, 2.40; N, 3.01; P, 6.82; Ce, 30.63%; IR (KBr, cm-1): 3463 s, 3095 w, 1623 s, 1600 s, 1465 w, 1313 m, 1180 m, 1074 m, 935 w, 788 s, 682 w, 584 w, 443 w. Anal. Calcd. for C14H22N2O22P2Pr2: C, 18.40; H, 2.43; N, 3.06; P, 6.78; Pr, 30.83; Found: C, 18.45; H, 2.40; N, 3.12; P, 6.72; Pr, 30.75%. IR (KBr, cm-1): 3425 s, 3091 w, 1629 s, 1463 w, 1315 s, 1183 m, 1078 m, 928 w, 790 s, 716 w, 590 w, 441 w. Anal. Calcd. for C14H22N2O22P2Nd2: C, 18.26; H, 2.41; N, 3.04; P, 6.73; Nd, 31.33; Found: C, 18.30; H, 2.36; N, 2.98; P, 6.79; Nd, 31.26%. IR (KBr, cm-1): 3431 s, 3091 w, 1629 s, 1463 w, 1315 m, 1176 m, 1066 s, 929 m, 792 s, 711 w, 596 w, 442 w. Anal. Calcd. for C14H22N2O22P2Sm2: C, 18.02; H, 2.38; N, 3.00; P, 6.65; Sm, 32.23; Found: C, 18.06; H, 2.32; N, 3.04; P, 6.71; Sm, 32.28%. IR (KBr, cm-1): 3487 s, 3089 w, 1623 s, 1471 w, 1319 m, 1180 m, 1068 m, 935 w, 788 m, 723 w, 596 w, 495 w, 439 w. Anal. Calcd. for C14H22N2O22P2Eu2: C, 17.96; H, 2.37; N, 2.99; P, 6.62; Eu, 32.46; Found: C, 17.91; H, 2.42; N, 2.93; P, 6.67; Eu, 32.52%; IR (KBr, cm-1): 3442 s, 3091 w, 1629 s, 1469 w, 1359 w, 1317 m, 1182 m, 1079 m, 935 m, 792 s, 606 w, 566 w, 492 w. Synthesis of [Ln2(H4L)(C2O4)3(H2O)] 3 2H2O (Ln = Gd (7), Tb (8), Dy (9), Y (10), Er (11), Lu(12)). The methods used for the syntheses of compounds 7-12 are similar. A mixture of LnCl3 3 6H2O (0.25 mmol, 0.09 g), H4L (0.5 mmol, 0.15 g), NaOH (2 mmol, 0.08 g), and H2C2O4 3 2H2O (2 mmol, 0.25 g) was dissolved in 8 mL of distilled water. The resulting solution was stirred for about 30 min at room temperature, sealed in a 20 mL Teflon-lined stainless steel autoclave, and heated at 100 °C for 4 days under autogenous pressure. Crystals of 7 (white, powder), crystals of 8 (white, powder), crystals of 9 (colorless, block), crystals of 10 (colorless, block), crystals of 11 (colorless, block) and crystals of 12 (colorless, block) were collected in ca. 63.8%, 67.2%, 55.3%, 62.7%, 59.1%, and 53.4% yield, respectively (based on lanthanide metal). Anal. Calcd. for C14H20N2O21P2Gd2: C, 18.11; H, 2.17; N, 3.02; P, 6.67; Gd, 33.86; Found: C, 18.17; H, 2.12; N, 3.08; P, 6.72; Gd, 33.93; IR (KBr, cm-1): 3440 s, 3064 w, 1629 s, 1465 w, 1390 w, 1319 m, 1197 m, 1105 w, 935 w, 800 m, 590 w, 491 w. Anal. Calcd. for C14H20N2O21P2Tb2: C, 18.04; H, 2.16; N, 3.01; P, 6.65; Tb, 34.10; Found: C, 17.96; H, 2.19; N, 3.05; P, 6.58; Tb, 34.18%; IR

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(KBr, cm-1): 3456 s, 3064 w, 1635 s, 1477 w, 1367 w, 1319 m, 1192 m, 1009 m, 929 m, 794 s, 690 w, 590 w, 491 w. Anal. Calcd. for C14H20N2O21P2Dy2: C, 17.90; H, 2.15; N, 2.98; P, 6.62; Dy, 34.60; Found: C, 17.85; H, 2.09; N, 3.05; P, 6.68; Dy, 34.67%. IR (KBr, cm-1): 3450 s, 3070 w, 1641 s, 1483 w, 1367 w, 1319 m, 1203 m, 1103 m, 933 w, 794 s, 694 w, 595 w, 489 w. Anal. Calcd. for C14H20N2O21P2Y2: C, 21.23; H, 2.55; N, 3.54; P, 7.83; Y, 24.05; Found: C, 21.28; H, 2.50; N, 3.59; P, 7.78; Y, 24.12%. IR (KBr, cm-1): 3452 s, 3061 w, 1641 s, 1477 w, 1373 w, 1319 m, 1199 m, 1101 m, 929 w, 800 s, 592 w, 486 w. Anal. Calcd. for C14H20N2O21P2Er2: C, 17.72; H, 2.12; N, 2.95; P, 6.53; Er, 35.26; Found: C, 17.78; H, 2.16; N, 2.91; P, 6.58; Er, 35.31%. IR (KBr, cm-1): 3436 s, 3055, w, 1641 s, 1477 w, 1365 w, 1319 m, 1199 m, 1101 m, 921 w, 800 s, 690 w, 592 w, 493 w. Anal. Calcd. for C14H20N2O21P2Lu2: C, 17.44; H, 2.09; N, 2.91; P, 6.43; Lu, 36.29; Found: C, 17.49; H, 2.12; N, 2.86; P, 6.48; Lu, 36.35%. IR (KBr, cm-1): 3458 s, 3064 w, 1647 s, 1477 w, 1328 m, 1234 m, 1112 m, 923 m, 802 s, 698 w, 595 w, 484 w. X-ray Crystallography. Data collections for compounds 1-12 were performed on the Bruker AXS Smart APEX II CCD X-diffractometer equipped with graphite monochromated MoKR radiation (λ = 0.71073 A˚) at 293 ( 2 K. An empirical absorption correction was applied using the SADABS program. The structures were solved by direct methods and refined by full matrix leastsquares on F2 by using the programs SHELXS-97.14 All nonhydrogen atoms were refined anisotropically. Hydrogen atoms except those for water molecules were generated geometrically with fixed isotropic thermal parameters and included in the structure factor calculations. Hydrogen atoms for water molecules were not included in the refinement. Details of crystallographic data are given in Table 1 for compounds 1-6 and Table 2 for compounds 9-12, respectively. Selected bond lengths of compounds 3 and 11 are listed in Table 3. The CCDC deposition numbers are 707342 (for 1), 653434 (for 2), 653432 (for 3), 653433 (for 4), 653435 (for 5), 706877 (for 6), 744728 (for 9), 744729 (for 10), 744730 (for 11), and 744731 (for 12).

Results and Discussion Syntheses. Twelve novel lanthanide(III) oxalatophosphonates with two different types of 3D framework structures have been synthesized under hydrothermal conditions, using the 3-pyridyl-CH2N(CH2PO3H2)2 (H4L) as the phosphonate ligand and oxalate as the second ligand. The compounds 1-12 were obtained as a pure phase materials by adjusting the synthetic conditions. Results of this study provide further evidence that the molar ratio of starting materials and the pH value play an important role in the growth of high-quality single crystals. It was found that pure phases of compounds 1-6 were observed in the reactions containing Ln3þ, H4L, and H2C2O4 3 2H2O in the molar ratio 1:1:4. In the synthesis of compounds 9-12, when the quantity of lanthanide (III) chlorides is reduced by half, single-crystals for X-ray diffraction were obtained. Despite all our efforts to grow singlecrystals of compounds 7 and 8, we were not successful in obtaining a good sample for X-ray diffraction study. However, X-ray powder diffraction analysis indicates that compounds 7-12 are isostructural. NaOH was added into the reaction system directly in the form of solid, which was employed as the inorganic base to adjust the pH of the reaction mixture. The initial and final pH values of the resultant solution are about 1.5 and 2.5, respectively. In addition, the reaction temperature was very important for the formation of suitable single-crystals for X-ray diffraction. The compounds 1-12 were obtained at 100 to 140 °C under hydrothermal conditions. The powder XRD patterns of compounds 1-12 and the simulated XRD patterns are shown in the Supporting Information. The measured XRD patterns of compounds 1-12 are all essentially in agreement with those simulated from X-ray single-crystal data, which

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Table 1. Crystal Data and Structure Refinement for Compounds 1-6 formula fw crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dc (g cm-3) μ (mm-1) GOF on F2 reflections R1, wR2 (for I > 2σ(I))a R1, wR2 (for all data) a

1

2

3

4

5

6

C14H22N2O22P2La2 910.10 triclinic P1 10.7511(14) 11.6741(15) 12.152(3) 106.384(2) 100.604(2) 108.992(2) 1318.1(4) 2 2.293 3.421 1.029 7481/5326 0.0484, 0.1045 0.0759, 0.1203

C14H22N2O22P2Ce2 912.52 triclinic P1 10.6986(15) 11.6124(16) 12.0730(3) 106.204(2) 100.678(2) 108.841(2) 1299.0(4) 2 2.333 3.687 1.020 6986/4967 0.0464, 0.0953 0.0706, 0.1079

C14H22N2O22P2Pr2 914.10 triclinic P1 10.6297(9) 11.5315(10) 12.0341(18) 106.271(2) 100.334(2) 108.4950(10) 1283.2(2) 2 2.366 3.982 1.032 7262/5169 0.0354, 0.0757 0.0481, 0.0832

C14H22N2O22P2Nd2 920.76 triclinic P1 10.6110(18) 11.518(2) 11.997(4) 106.032(3) 100.770(3) 108.584(2) 1273.5(5) 2 2.401 4.264 1.003 6956/4893 0.0475, 0.0896 0.0784, 0.1054

C14H22N2O22P2Sm2 932.98 triclinic P1 10.5266(15) 11.4447(16) 11.927(3) 105.704(3) 101.172(3) 108.451(2) 1249.3(4) 2 2.480 4.891 1.032 7084/5044 0.0479, 0.0937 0.0754, 0.1078

C14H22N2O22P2Eu2 936.20 triclinic P1 10.452(2) 11.491(3) 11.901(5) 105.016(4) 102.094(4) 108.579(3) 1240.3(6) 2 2.507 5.249 1.002 7034/5005 0.0378, 0.0840 0.0512, 0.0917

R1 = Σ(|F0| - |FC|)/Σ|F0|, wR2 = [Σw(|F0| - |FC|)2/ΣwF02] 1/2.

Table 2. Crystal Data and Structure Refinement for Compounds 9-12 formula Fw crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dc (g cm-3) μ (mm-1) GOF on F2 reflections R1, wR2 (for I > 2σ (I))a R1, wR2 (for all data) a

9

10

11

12

C14H20N2O21P2Dy2 939.26 orthorhombic P2(1)2(1)2(1) 11.4319(14) 14.8457(19) 16.389(2) 90 90 90 2781.4(6) 4 2.243 5.539 1.045 15095/5458 0.0671, 0.1598 0.0725, 0.1625

C14H20N2O21P2Y2 792.08 orthorhombic P2(1)2(1)2(1) 11.358(4) 14.829(5) 16.325(6) 90 90 90 2749.5(17) 4 1.913 4.410 1.003 16989 /6441 0.0789, 0.2063 0.1068, 0.2226

C14H20N2O21P2Er2 948.78 orthorhombic P2(1)2(1)2(1) 11.3656(8) 14.8449(11) 16.3071(12) 90 90 90 2751.4(3) 4 2.290 6.268 1.016 16164/5990 0.0451, 0.1130 0.0541, 0.1181

C14H20N2O21P2Lu2 964.20 orthorhombic P2(1)2(1)2(1) 11.2506(10) 14.8354(14) 16.1843(15) 90 90 90 2701.3(4) 4 2.371 7.480 1.036 15335/5580 0.0445, 0.1096 0.0578, 0.1163

R1 = Σ(|F0| - |FC|)/Σ|F0|, wR2 = [Σw(|F0| - |FC|)2/ΣwF02]1/2.

indicate the homogeneous phases of the final products (Figure S1 and Figure S2, Supporting Information). On the basis of the XRD powder patterns, compounds 1-6 are isostructural, and so are compounds 7-12. Crystal Structures of 1-6. Compounds 1-6 are isostructural and feature a 3D framework structure; hence, only the structure of 3 will be discussed in detail as a representation. The asymmetric unit of 3 contains two unique praseodymium(III) ions, one H4L ligand, three oxalate anions, one coordianted water molecule, and three solvate water molecules. As shown in Figure 1, Pr(1) ion is nine-coordinated by three phosphonate oxygen atoms (O1, O2A, O5B) from three separate H4L ligands, five oxygen atoms (O7, O9, O11, O11A, O13A) from three oxalate anions, and one oxygen atom (O1W) from a water molecule. Pr(2) ion is also nine-coordinated by one phosphonate oxygen atom (O4C) from one H4L ligand and eight oxygen atoms (O10, O8, O15, O16E, O14D, O12D, O18F, O17) from four oxalate anions. The Pr-O distances range from 2.430(4) to 2.737(4) A˚, which are comparable to those reported for other praseodymium(III) phosphonates and oxalates.10c,15 The three oxalate anions display two coordination modes in compound 3 (Figure 2). All oxalate units except C(11)C(12)O4 unit show only bis-bidentate coordination to two Pr3þ ions

(Figure 2a). The result of connections in this manner is the formation of two Pr-O-C-C-O five-membered chelating rings. The C(11)C(12)O4 unit is a bis-bidentate ligand to Pr1(1) and Pr(2) ions as well as a monodentate ligand to another Pr1(1) ion, with O11 therefore being three-coordinate (Figure 2b). To our knowledge, bisbidentate and monodentate coordination by the same oxalate ligand has been rarely reported in metal oxalatophosphates. The H4L act as a tetradentate ligand, coordinating to four Pr3þ ions by its four phosphonate oxygen atom (O1, O2, O4, O5) (Figure 3b). Both phosphonate groups of the H4L ligand are singly protonated (O3, O6) based on the requirement of charge balance as well as P-O bond distances. The nitrogen atoms of the amine groups and pyridyl groups are protonated and noncoordinated based on the requirement of charge balance. Two Pr3þ ions are interconnected by a pair of H4L ligands through phosphonate oxygen atoms (O1, O2, O5) to generate a 1D chain of {Pr2(H4L)2}6þ along the c-axis, and pyridyl rings are hung in this chain (Figure 4a). It is interesting to note that the interconnection of Pr1 and Pr2 ions by three bridging and chelating oxalate anions leads to a 3D network of {Pr2(C2O4)3} with a 1D channel system along the c-axis. The channel is formed by 24-membered rings composed of six Pr (III) ions and six oxalate anions (Figure 4b).

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Table 3. Selected Bond Lengths (A˚) for Compounds 3 and 11a Compound 3 Pr(1)-O(2)#1 Pr(1)-O(1) Pr(1)-O(7) Pr(1)-O(13)#1 Pr(1)-O(11) Pr(1)-O(9) Pr(1)-O(1w) Pr(1)-O(5)#2 Pr(1)-O(11)#1 Pr(2)-O(4)#3 Pr(2)-O(17) Pr(2)-O(18)#4 Pr(2)-O(14)#5

2.430(4) 2.432(4) 2.452(4) 2.482(4) 2.527(4) 2.545(4) 2.586(5) 2.608(4) 2.737(4) 2.459(4) 2.471(4) 2.479(4) 2.495(4)

Pr(2)-O(16)#6 Pr(2)-O(8) Pr(2)-O(10) Pr(2)-O(15) Pr(2)-O(12)#5 P(1)-O(1) P(1)-O(2) P(1)-O(3) P(2)-O(4) P(2)-O(5) P(2)-O(6) P(1)-C(1) P(2)-C(2)

2.498(4) 2.495(4) 2.517(4) 2.553(4) 2.616(4) 1.491(4) 1.500(4) 1.574(4) 1.483(4) 1.545(4) 1.537(4) 1.824(6) 1.815(6)

Compound 11 Er(1)- O(5) Er(1)-O(10) Er(1)-O(1w) Er(1)-O(8)#1 Er(1)-O(7) Er(1)-O(11) Er(1)-O(9)#1 Er(1)-O(14) Er(2)-O(2) Er(2)-O(4)#2 Er(2)-O(12)#3 Er(2)-O(17)

2.254(8) 2.323(9) 2.332(11) 2.336(9) 2.366(9) 2.370(7) 2.376(8) 2.393(7) 2.222(7) 2.314(7) 2.366(7) 2.382(8)

Er(2)-O(18)#4 Er(2)-O(15)#4 Er(2)-O(13)#3 Er(2)-O(16) P(1)-O(1) P(1)-O(2) P(1)-O(3) P(2)-O(4) P(2)-O(5) P(2)-O(6) P(1)-C(1) P(2)-C(2)

2.381(9) 2.406(8) 2.419(8) 2.426(9) 1.471(10) 1.475(8) 1.563(11) 1.500(8) 1.485(10) 1.562(11) 1.808(12) 1.850(12)

a Symmetry transformations used to generate equivalent atoms. For 3: #1, -x þ 1, -y - 1, -z - 2; #2, x þ1, y, z; #3, -x þ 1, -y - 1, -z - 1; #4, -x þ 2, -y - 2, -z -1; #5, x, y - 1, z; #6, -x þ 1, -y - 2, -z - 1. For 11: #1, x þ 1/2, -y þ 1/2, -z þ 1; #2, -x þ 3/2, -y, z - 1/2; #3, x, y, z - 1; #4, x þ 1/2, -y þ 1/2, -z.

Figure 1. ORTEP representation of a selected unit of compound 3. The thermal ellipsoids are drawn at the 50% probability level. All H atoms and solvate water molecules are omitted for clarity. Symmetry codes: A, -x þ 1, -y - 1, -z - 2; B, x þ 1, y, z; C, -x þ 1, -y 1, -z - 1; D, x, y - 1, z; E, -x þ 1, -y - 2, -z - 1; F, -x þ 2, -y 2, -z - 1; G, x, y þ 1, z.

Figure 2. Coordination modes of the oxalate ligands in compound 3: (a) bis-bidentate coordination; (b) bis-bidentate and monodentate coordination.

The size of the channel is estimated to be 16.4  7.5 A˚ based on structure data. The above two building blocks are interconnected through sharing Pr3þ ions to form a complicated

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3D framework structure (Figure 5). The structure can also be viewed as the {Pr2(H4L)2}6þ chain occupying the channel of the 3D network of {Pr2(C2O4)3}, and solvate water molecules are located in the channel. Crystal Structures of 7-12. Compounds 7-12 are also isostructural and possess a 3D framework structure different from that in compounds 1-6. Taking compound 11 as an example, the asymmetric unit of compound 11 consists of two unique erbium(III) ions, one H4L ligand, three oxalate anions, one coordianted water molecule, and two solvate water molecules. Er(1) ion show a dodecahedral geometry and is eight-coordinated to one phosphonate oxygen atom (O5) from H4L ligand, six oxygen atoms (O7, O8A, O9A, O10, O11, O14) from three different oxalate anions and one oxygen atom (O1W) from a water molecule. Er(2) ion also exhibits an eight-coordinated environment. The six of the eight coordination positions are filled with six oxygen atoms (O12C, O13C, O15D, O16, O17, O18D) from three oxalate anions. The remaining sites are occupied by two phosphonate oxygen atoms (O2, O4B) from two separate H4L ligands. The Er-O distances range from 2.222(7) to 2.426(9) A˚, which are comparable to those reported for other erbium(III) phosphonates.10a,16 All oxalate anion in compound 11 adopt the same coordination mode as the C(9)C(10)O4 unit in compound 3 (Figure 2a). Though the protonation modes of H4L ligand in compound 11 is the same as that in compound 3, it adopts a different coordination mode. The H4L ligand acts as a tridentate ligand and connects three Er(III) ions through three phosphonate oxygen atoms (O2, O4, O5) (Figure 3c). As shown in Figure 7a, the Er(III) ions are interconnected by three bisbidentate oxalate anions forming a 2D layer of {Er2(C2O4)3} in the ac planes. These layers of {Er2(C2O4)3} are further interlinked by phosphonate groups through sharing Er(III) ions to form a 3D framework type (Figure 7b). The result of connections in this manner is the formation of a 1D channel system along the a-axis. The channel is formed by 24-atom rings composed of six Er(III) ions, two H4L ligands, and four oxalate anions. The dimension of the channel is 10.0  11.1 A˚, which is estimated by measuring the distances between the centers of opposite atoms. The pyridyl rings are located inside the channel (Figure 8). In a word, the structure of 11 can be viewed as the erbium oxalate layer being connected through phosphonate groups to form a complex 3D framework structure. In addition, the interesting feature for compound 11 is the presence of right-hand helical chains in the structure (Figure 9). Er(2) centers are interconnected by bridging H4L ligands to form infinite one-dimensional right-hand helical chains running along the b-axis. Although the compositions of compounds 1-6 and compounds 7-12 are similar except for the number of solvate water molecules, their structures are completely different. Compounds 1-6 crystallize in triclinic space group P1, and compounds 7-12 crystallize in orthorhombic space group P2(1)2(1)2(1). The different coordination modes of the phosphonate ligands and oxalate ligands adopted in these compounds certainly play an important role in the formation of these two different structural types. Another reason may be related to the atomic size of different lanthanide atoms, the so-called “lanthanide contraction”. With decreasing ionic size, the heavier lanthanide ion leaves fewer coordination sites, which is called “lanthanide contraction”.10b

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Figure 3. H4L (a) and its coordination fashions of phophonate ligands in compound 3 (b) and compound 11 (c).

Figure 6. ORTEP representation of a selected unit of compound 11. The thermal ellipsoids are drawn at the 50% probability level. All H atoms and solvate water molecules are omitted for clarity. Symmetry codes: A, x þ 1/2, -y þ 1/2, -z þ 1; B, -x þ 3/2, -y, z 1/2; C, x, y, z - 1; D, x þ 1/2, -y þ 1/2, -z.

Figure 4. (a) The 1D chain {Pr2(H4L)2}6þ and (b) 3D network of {Pr2(C2O4)3} in compound 3. The phosphonates tetrahedra are shaded in purple.

Figure 7. (a) 2D layer of {Er2(C2O4)3} in compound 11 viewed in the ac plane; (b) view of the framework for compound 11 along the c-axis. All H atoms and solvate water molecules are omitted for clarity.

Figure 5. View of the framework for compound 3 along the c-axis. All H atoms are omitted for clarity. The phosphonates tetrahedra are shaded in purple.

IR Spectroscopy. The IR spectra of the 12 compounds have many similar features corresponding to the common groups; thus, only the spectrum of compound 3 will be discussed. The IR spectrum for compound 3 was recorded in the region 4000-400 cm-1. The absorption band at 3425 cm-1 can be assigned to the O-H stretching vibrations of water molecules. The weak band at 3091 cm-1 is attributed to the N-H stretching vibrations. The bands at 1629 cm-1 and 1463 cm-1 are observed, which is shifted from the excepted value of uncoordinated carboxylic acid [v(C-O)

Figure 8. View of the framework for compound 11 along the a-axis. All H atoms and solvate water molecules are omitted for clarity.

typically around 1725-1700 cm-1]. These shifts are due to the carboxylate function coordinated to the metal atom, and

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Figure 11. TGA curve of compound 11.

Figure 9. View of the right-hand helical chains of compound 11 along the b-axis.

Figure 12. Experimental and heated (100 °C) X-ray powder diffraction diagram of compound 11 compared to the calculated one.

Figure 10. TGA curve of compound 3.

these bands are assigned to the asymmetric and symmetric stretching vibrations of C-O groups when present as COOmoieties.17 Strong bands between 1200 and 900 cm-1 are due to stretching vibrations of the tetrahedral CPO3 groups,18 as expected. Additional weak bands at low energy are found. These bands are probably due to bending vibrations of the tetrahedral CPO3 groups. Thermal Analysis. TGA were conducted to examine the stabilities of these compounds. The TGA curves of compounds 1-6 are nearly similar, with two main continuous weight losses. Herein, we use compound 3 as an example to illuminate the weight losses in detail (Figure 10). The first step started at about 50 °C and was completed at 600 °C, corresponding to release of three solvate water molecules, one coordinated water molecule, and three oxalate anios. The weight loss of 37.2% is in good agreement with the calculated value (36.8%). The second step covers a temperature range of 600-800 °C, which corresponds to the pyrolysis of the organic group. The final product is PrPO4 based on XRD powder studies. The total weight loss of 47.5% is close to the calculated value (48.3%). The TGA curves of compounds 7-12 are also similar, and compound 11 was used as an example (Figure 11). It exhibits three main continuous weight losses. The first step, in the temperature range 50-100 °C, is due to the removal of two solvate water

molecules. The observed weight loss of 4.2% is close to the calculated value (3.8%). The second step occurred above 100 °C, during which the compound is partially decomposed. The third step covers from 600 to 950 °C, corresponding to the further decomposition of the organic groups. The final residual of the thermal process is ErPO4 on the basis of powder X-ray diffraction. The total weight loss of 45.3% is close to the calculated value (44.7%). The much larger total weight loss for compound 10 (52.4%) is due to its having a much smaller formula weight than the other compounds. The weight of materials released or combusted during the decomposition is expected to be about the same for six compounds. During the thermal decomposition, intermediate compounds may be formed between 600 and 950 °C for compound 11. In order to identify the intermediate compound, X-ray powder diffraction studies were performed for compound 11 calcined at 700 °C. However, these intermediate compounds were not identified because the complicated mixtures were obtained during the thermal decomposition. Considering the thermal stability of the compounds, X-ray powder diffraction studies are performed for the as-synthesized compound 11 and the samples calcined 100 °C for 2 h under air atmosphere. The powder XRD patterns demonstrate the retention of framework structure of compound 11 after removal of two solvate water molecules at 100 °C (Figure 12). Luminescent Properties. The solid-state luminescence properties of compounds 3, 4, 6, 8, and 11 were investigated at room temperature. The free H4L exhibits a broad fluorescent emission band at 452 nm under excitation at 381 nm, which may be assigned to the intra-ligands π-π* transitions

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Figure 13. Solid-state emission spectrum of compound 4 at room temperature.

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Figure 15. Solid-state emission spectrum of compound 8 at room temperature.

D4 f 7F3 (623 nm) transitions.22 Among these emission lines, the most striking green luminescence (5D4 f 7F5) for compound 8 is observed in the emission spectrum. The results reveal that compounds 4, 6, and 8 exhibit strong luminescence in near-IR, red light, and green light regions, respectively. 5

Conclusion

Figure 14. Solid-state emission spectrum of compound 6 at room temperature.

(Figure S8, Supporting Information). Upon complexion with lanthanide(III) ion, only sharp emission bands associated with corresponding lanthanide(III) ions were observed, indicating the complete energy transfer from the phosphonate ligands to the central lanthanide ions.19 Under excitation of 808 nm, compound 4 exhibits the characteristic emission bands for Nd3þ ion (Figure 13): a strong emission band at 1060 and 1068 nm (4F3/2 f 4I11/2) and a weak band at 1335 nm (4F3/2 f 4I13/2) in the near-IR region.10a,20 However, compounds 3 and 11 did not exhibit emission bands under this experimental conditions. Under excitation of 394 nm, compound 6 displays three characteristic peaks for the Eu3þ ion in Figure 14. These emission bands arise from 5D0 f7FJ (J = 1, 2, and 4) transitions.10a The 5D0 f 7F1 transition (593 nm) corresponds to a magnetic dipole transition, and the intensity of this emission for 6 is medium-strong. The most intense emission bands is the 5D0 f 7F2 transitions at 617 nm, which are the so-called hypersensitive transitions and are responsible for the brilliant-red emission of compound 6.21 It is noted that the intensity of the hypersensitive transition 5D0 f 7F2 is comparable to that of 5D0 f 7F1. Since the former transition is electric dipole in nature, its intensity is strongly influenced by the crystal field, while the latter transition is magnetic dipole in origin and less sensitive to its environment. The weak emission band at 700 nm, can be attributed to the 5D0 f 7F4 transition. As shown in Figure 15, compound 8 exhibits four stong characteristic emission bands, when excited at 379 nm. These emission bands re assigned to the 5D4 f 7FJ (J = 3, 4, 5, and 6), 5D4 f7F6 (489 nm), 5D4 f 7F5 (544 nm), 5D4 f 7F4 (588 nm), and

By using the 3-pyridyl-CH2N(CH2PO3H2)2 (H4L) as the phosphonate ligand and oxalate as the second metal linker, 12 novel lanthanide(III) oxalatophosphonates with two different types of 3D framework structures have been synthesized under hydrothermal conditions and structurally characterized. Compounds 1-6 are isostructural and feature a 3D framework formed by the interconnection of 3D network of {Ln3(C2O4)3} with 1D chains of {Ln2(H4L)2}6þ. Compounds 7-12 are also isostructural and possess a 3D framework structure different from that in compounds 1-6. The Ln(III) ions are interconnected by three bisbidentate oxalate anions forming a lanthanide oxalate layer in the ac planes. These layers are further interlinked by phosphonate groups through sharing Ln(III) ions to form a 3D framework structure. It is interesting that a type of right-hand helical chain is formed by H4L ligands bridging Ln(2) ions running along the b-axis. The Nd, Eu, and Tb compounds exhibit strong luminescence in near-IR, red light, and green light regions, respectively. The results of our study indicate that by introduction of oxalate as the second ligand we can obtain lanthanide oxalatophosphonates with good crystals and new structures as well as strong luminescence. Acknowledgment. This research was supported by grants from the Education Department of Liaoning Province of China (2009S063). Supporting Information Available: X-ray crystallographic files in CIF format for compounds 1-6 and 9-12, tables listing selected bond for compounds presented in this paper, XRD patterns of the experiments compared to those simulated from X-ray single-crystal data for compounds 1-12, XRD patterns of the final products in the thermal decomposition for compounds 3 and 11, IR spectra of compounds 1-12, solid-state emission spectrum of H4L and TGA curves of 1, 2, 4, 5, 6, 7, 8, 9, 10, and 12. The CCDC deposition numbers are 707342 (for 1), 653434 (for 2), 653432 (for 3), 653433 (for 4), 653435 (for 5), 706877 (for 6), 744728 (for 9), 744729 (for 10), 744730 (for 11), 744731 (for 12). These data can be obtained free of charge www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Center, 12, Union Road,

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Cambridge CB21EZ, UK; fax: (þ44) 1223-336-033; e-mail: [email protected]). This information is available free of charge via the Internet at http://pubs.acs.org.

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