Seven Novel Lanthanide Oxalatophosphonates with Two Types of 3D

Seven Novel Lanthanide Oxalatophosphonates with Two Types of 3D Framework Structures Based on N-Morpholinomethylphosphonic Acid: Syntheses, Crystal Structures, and Luminescence Properties

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 3228–3234

Yan-Yu Zhu, Zhen-Gang Sun,* Hui Chen, Jing Zhang, Yan Zhao, Na Zhang, Lei Liu, Xin Lu, Wei-Nan Wang, Fei Tong, and Lan-Cui Zhang Institute of Chemistry for Functionalized Materials, School of Chemistry and Chemical Engineering, Liaoning Normal UniVersity, Dalian 116029, P. R. China ReceiVed December 2, 2008; ReVised Manuscript ReceiVed March 24, 2009

ABSTRACT: By using the N-morpholinomethylphosphonic acid as the phosphonate ligand and oxalate as the second metal linker, seven novel lanthanide(III) oxalatophosphonates with two types of 3D framework structures, namely, [Ln3(HL)(C2O4)4(H2O)4] · 2H2O (Ln ) Pr (1), Nd (2), Sm (3), H2L ) O(CH2CH2)2NCH2PO3H2) and [Ln2(H2L)(C2O4)3(H2O)3] · H2O (Ln ) Eu (4), Gd (5), Tb (6), Dy (7)), have been synthesized under hydrothermal conditions. Compounds 1-3 are isostructural and feature a complex 3D framework structure formed by the interconnection of 3D network of {Ln3(C2O4)4}+ with 1D chains of {Ln3(HL)}8+. Compounds 4-7 are isostructural and possess a 3D framework structure different from that in compounds 1-3. The interconnection of Ln(III) ions by chelating oxalate ligands lead to a lanthanide oxalate layer down the c-axis. These layers are linked by H2L ligands to form a 3D framework type with channel system down the a-axis. Luminescence properties of compounds 3, 4, 6 and 7 have also been studied. Introduction The chemistry of the metal phosphonates has been a research field of rapid expansion in recent years, mainly due to their potential application in the area 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 architectures and properties have become a particularly important subject. The key factor is the selection of multifunctional phosphonic acid ligands for assembling metal ions to fabricate a desired framework. Studies of metal phosphonates have shown that the use of bi- and multifunctional phosphonic acids containing -NH2, -OH, and -COOH sub functional groups may not only result in new structural types of metal phosphonates but also bring interesting properties.2 Based on 2-hydroxyphosphonoacetic acid, proline-N-methylphosphonic acid and DL-(R-aminoethyl)phosphonic acid, a series of metal phosphonates with two-dimensional (2D) layer and three-dimensional (3D) open-framework have also been obtained 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. Furthermore, the introduction of a second ligand such as 1,10-phenanthroline, 2,2′-bipyridine, 4, 4′bipyridine or carboxylic acid has been used as an effective synthetic tool 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.4 Among these studies, the oxalate moiety, C2O42-, was found to be a good candidate and has been successfully incorporated into phosphonate frameworks with transition metals and main group elements.5 Although some progress has been made in the construction of metal oxalatophosphonate as mentioned above, less progress has been achieved in the synthesis of the corresponding lanthanides.6 Lanthanide phosphonates normally have low solubility in water and other organic * Corresponding author. E-mail: [email protected].

solvents, hence introducing a second ligand such as C2O42- into the lanthanide phosphonate system can improve the solubility and crystallinity of lanthanide phosphonates. In addition, 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 increasing the luminescent intensity and lifetime of the materials.7 Recently, we reported a novel lanthanide oxalatophosphonate, [Gd2{HO3PCH2NHCH2(CH2CH2OPO2)}(C2O4)2.5(H2O)2] · 5H2O that has a 3D open-framework structure with 44-atom ring channels.8 As an extension of our work on lanthanide phosphonate, we selected N-morpholinomethylphosphonic acid (H2L ) O(CH2CH2)2NCH2PO3H2) as the phosphonate ligand and oxalate as the second metal linker. Hydrothermal reactions of the above two ligands with lanthanide(III) chlorides afforded seven novel lanthanide oxalatophosphonate hybrids with two types of 3D framework structures, namely, [Ln3(HL)(C2O4)4(H2O)4] · 2H2O (Ln ) Pr (1), Nd (2), Sm (3)) and [Ln2(H2L)(C2O4)3(H2O)3] · H2O (Ln ) Eu (4), Gd (5), Tb (6), Dy (7)). To the best of our knowledge, they are the first examples of structurally characterized lanthanide oxalatophosphonate assembled from N-morpholinomethylphosphonic acid. Herein we report their syntheses, crystal structures and thermal stabilities. The luminescent properties of compounds 3, 4, 6 and 7 have also been studied. Experimental Section Materials and Methods. The N-morpholinomethylphosphonic acid, O(CH2CH2)2NCH2PO3H2 (H2L), was prepared by a Mannich type reaction according to the procedures previously described.9 The lanthanide(III) chlorides were prepared by dissolving corresponding lanthanide oxides (General Research Institute for Nonferrous Metals, 99.99%) in hydrochloric acid followed by recrystallization and drying. All other chemicals were used as received without further purification. C, H and N were determined by using a PE-2400 elemental analyzer. Pr, Nd, Sm, Eu, Gd, Tb, Dy 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

10.1021/cg8013128 CCC: $40.75  2009 American Chemical Society Published on Web 05/20/2009

Lanthanide Oxalatophosphonates

Crystal Growth & Design, Vol. 9, No. 7, 2009 3229 Table 1. Crystal Data and Structure Refinement for Compounds 1-7

formula Fw crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) 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

7

C13H23NO26PPr3 1063.02 monoclinic P21 10.9407(16) 11.8196(17) 11.0579(16) 105.948(2) 1374.9(3) 2 2.568 5.402 1.020 7742/5046 0.0253, 0.0616 0.0260, 0.0621

C13H23NO26PNd3 1073.01 monoclinic P21 10.8724(9) 11.7625(9) 11.0140(9) 105.928(2) 1354.46(19) 2 2.631 5.838 1.028 7477/5083 0.0419, 0.1167 0.0424, 0.1175

C13H23NO26PSm3 1091.34 monoclinic P21 10.804(3) 11.648(3) 10.950(3) 106.025(3) 1324.4(6) 2 2.737 6.741 1.034 7412/4951 0.0397, 0.0970 0.0422, 0.0994

C11H20NO20PEu2 821.17 monoclinic P21/c 9.2690(8) 19.3971(17) 12.4863(11) 100.767(2) 2205.4(3) 4 2.473 5.807 1.021 12281/4546 0.0289, 0.0625 0.0378, 0.0673

C11H20NO20PGd2 831.75 monoclinic P21/c 9.2459(11) 19.326(2) 12.4653(14) 100.741(2) 2188.4(4) 4 2.525 6.181 1.014 12199/4505 0.0377, 0.0717 0.0559, 0.0793

C11H20NO20PTb2 835.09 monoclinic P21/c 9.2234(11) 19.272(2) 12.4547(14) 100.764(2) 2174.9(4) 4 2.550 6.624 1.033 12126/4493 0.0262, 0.0550 0.0339, 0.0583

C11H20NO20PDy2 842.25 monoclinic P21/c 9.1926(8) 19.1855(17) 12.4285(11) 100.691(2) 2153.9(3) 4 2.597 7.060 1.025 10636/3781 0.0323, 0.0621 0.0453, 0.0673

R1 ) ∑(|Fo| - |Fc|)/∑|Fo|, wR2 ) [∑w(|Fo| - |Fc|)2/∑wFo2]1/2.

diffractometer using Cu KR radiation (λ ) 1.5418 Å) in the 2θ range of 5-60° with a step size of 0.02° and a scanning rate of 3°/min. The luminescence spectra were reported on a JASCO FP-6500 spectrofluorimeter (solid). TG and DTA analyses were performed on a PerkinElmer Pyris Diamond TG-DTA thermal analysis system in static air with a heating rate of 10 K min-1 from 50 to 1000 °C. Synthesis of [Pr3(HL)(C2O4)4(H2O)4] · 2H2O (1). A mixture of PrCl3 · 6H2O (1 mmol, 0.36 g), H2L (1 mmol, 0.18 g), NaOH (2 mmol, 0.08 g) and H2C2O4 · 2H2O (2 mmol, 0.25 g) was dissolved in 8 mL of distilled water. The resulting solution was stirred for about 10 min at room temperature, sealed in a 23 mL Teflon-lined stainless steel autoclave, and heated at 180 °C for 4 days under autogenous pressure. After the mixture was cooled slowly to room temperature, the green block crystals were obtained in ca. 31.4% yield based on Pr. Anal. Calcd for C13H23NO26PPr3: C, 14.68; H, 2.16; N, 1.32; P, 2.92; Pr, 39.77. Found: C, 14.73; H, 2.09; N, 1.29; P, 2.85; Pr, 39.85%. IR (KBr, cm-1): 3465 s, 3061 w, 1648 s, 1425 w, 1317 m, 1102 m, 1082 m, 1009 m, 937 w, 793 s, 606 w, 566 w, 492 w. Synthesis of [Nd3(HL)(C2O4)4(H2O)4] · 2H2O (2). The procedure was the same as that for 1 except that PrCl3 · 6H2O was replaced by NdCl3 · 6H2O (1 mmol, 0.36 g). Purple block crystals of 2 were obtained and washed with water. Yield: 35.1% based on Nd. Anal. Calcd for C13H23NO26PNd3: C, 14.54; H, 2.14; N, 1.30; P, 2.89; Nd, 40.33. Found: C, 14.46; H, 2.21; N, 1.26; P, 2.82; Nd, 40.38%. IR (KBr, cm-1): 3403 s, 3067 w, 1639 s, 1458 m, 1318 s, 1110 s, 1083 s, 1001 m, 921 w, 801 s, 612 w, 566 w, 489 w. Synthesis of [Sm3(HL)(C2O4)4(H2O)4] · 2H2O (3). The procedure was the same as that for 1 except that PrCl3 · 6H2O was replaced by SmCl3 · 6H2O (1 mmol, 0.36 g). Pale yellow block crystals of 3 were obtained and washed with water. Yield: 30.2% based on Sm. Anal. Calcd for C13H23NO26PSm3: C, 14.29; H, 2.11; N, 1.28; P, 2.84; Sm, 41.34. Found: C, 14.21; H, 2.05; N, 1.35; P, 2.89; Sm, 41.25%. IR (KBr, cm-1): 3412 s, 3064 w, 1643 s, 1460 m, 1310 s, 1217 m, 1102 s, 1015 m, 793 s, 612 w, 559 w, 486 w. Synthesis of [Eu2(H2L)(C2O4)3(H2O)3] · H2O (4). The procedure was the same as that for 1 except that PrCl3 · 6H2O was replaced by EuCl3 · 6H2O (1 mmol, 0.36 g). Colorless pillar crystals of 4 were obtained and washed with water. Yield: 40.2% based on Eu. Anal. Calcd for C11H20NO20PEu2: C, 16.07; H, 2.44; N, 1.70; P, 3.78; Eu, 37.01. Found: C, 15.98; H, 2.48; N, 1.75; P, 3.71; Eu, 37.10%. IR (KBr, cm-1): 3404 s, 3005 w, 1642 s, 1435 m, 1317 m, 1219 m, 1110 m, 1022 m, 937 w, 797 m, 606 m, 538 w, 476 m. Synthesis of [Gd2(H2L)(C2O4)3(H2O)3] · H2O (5). The procedure was the same as that for 1 except that PrCl3 · 6H2O was replaced by GdCl3 · 6H2O (1 mmol, 0.36 g). Colorless pillar crystals of 5 were obtained and washed with water. Yield: 40.3% based on Gd. Anal. Calcd for C11H20NO20PGd2: C, 15.87; H, 2.40; N, 1.68; P, 3.73; Gd, 37.81. Found: C, 15.95; H, 2.35; N, 1.75; P, 3.78; Gd, 37.75%. IR (KBr, cm-1): 3416 s, 3005 w, 1642 s, 1453 m, 1305 m, 1225 m, 1116 m, 937 m, 789 m, 624 w, 563 w, 464 w. Synthesis of [Tb2(H2L)(C2O4)3(H2O)3] · H2O (6). The procedure was the same as that for 1 except that PrCl3 · 6H2O was replaced by TbCl3 · 6H2O (1 mmol, 0.36 g). Colorless pillar crystals of 6 were obtained and washed with water. Yield: 43.2% based on Tb. Anal. Calcd

for C11H20NO20PTb2: C, 15.81; H, 2.39; N, 1.68; P, 3.71; Tb, 38.06. Found: C, 15.88; H, 2.32; N, 1.76; P, 3.65; Tb, 38.15%. IR (KBr, cm-1): 3435 s, 3017 w, 1636 s, 1459 m, 1306 m, 1226 m, 1096 m, 943 m, 797 m, 612 w, 550 w, 470 w. Synthesis of [Dy2(H2L)(C2O4)3(H2O)3] · H2O (7). The procedure was the same as that for 1 except that PrCl3 · 6H2O was replaced by DyCl3 · 6H2O (1 mmol, 0.36 g). Colorless pillar crystals of 7 were obtained and washed with water. Yield: 45.3% based on Dy. Anal. Calcd for C11H20NO20PDy2: C, 15.67; H, 2.37; N, 1.66; P, 3.68; Dy, 38.59. Found: C, 15.61; H, 2.43; N, 1.72; P, 3.76; Dy, 38.50%. IR (KBr, cm-1): 3361 s, 3013 w, 1645 s, 1465 m, 1317 m, 1217 m, 1102 m, 1028 m, 801 m, 612 m, 551 m, 471 m. X-ray Crystallography. Data collections for compounds 1-7 were performed on the Bruker AXS Smart APEX II CCD X-diffractometer equipped with graphite monochromated Mo KR radiation (λ ) 0.71073 Å) 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 least-squares on F2 by using the programs SHELXS-97.10 All non-hydrogen 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 of compounds 1-7 are summarized in Table 1. Selected bond lengths of compounds 3 and 5 are listed in Table 2. CCDC 670620-670626 contain the supplementary crystallographic data for this paper.

Results and Discussion Syntheses. By using the N-morpholinomethylphosphonic acid as the phosphonate ligand and oxalate as the second metal linker, seven novel lanthanide(III) oxalatophosphonates with two different types of 3D framework structures have been synthesized under hydrothermal conditions. Results indicate that the molar ratio of starting materials and the pH value play an important role during the process of the reaction. The best crystallinity of the reaction product was observed in the reactions containing Ln3+, H2L and H2C2O4 · 2H2O in the molar ratio 1:1: 2. 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 2.5 and 4, respectively. The powder XRD patterns of compounds 1-7 and the simulated XRD patterns are shown in the Supporting Information. The powder XRD patterns of compounds 1-7 are all essentially in agreement with those simulated from X-ray single-crystal data, which indicate the homogeneous phases of the final products (Figure S1 and Figure S2 in the Supporting Information). Based on the powder XRD patterns, compounds 1-3 are isostructural; so are compounds 4-7.

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Table 2. Selected Bond Lengths (Å) for Compounds 3 and 5a Compound 3 Sm(1)-O(11) Sm(1)-O(20)#1 Sm(1)-O(18)#1 Sm(1)-O(1w) Sm(1)-O(16) Sm(1)-O(6) Sm(1)-O(9) Sm(1)-O(14) Sm(1)-O(8) Sm(2)-O(2)#2 Sm(2)-O(12)#3 Sm(2)-O(19) Sm(2)-O(10)#3 Sm(2)-O(5)#1 Sm(3)-O(8) Sm(3)-O(7)

2.413(8) 2.426(9) 2.421(8) 2.429(7) 2.441(8) 2.468(9) 2.531(8) 2.542(7) 2.587(8) 2.352(7) 2.437(8) 2.453(8) 2.455(8) 2.441(8) 2.634(7) 2.631(8)

Gd(1)-O(1) Gd(1)-O(5) Gd(1)-O(14)#1 Gd(1)-O(11) Gd(1)-O(13) Gd(1)-O(16)#2 Gd(1)-O(15) Gd(1)-O(8) Gd(1)-O(10) Gd(2)-O(9)#4 Gd(2)-O(2)#3

2.318(4) 2.412(5) 2.429(5) 2.444(5) 2.449(5) 2.463(5) 2.474(5) 2.507(5) 2.517(5) 2.449(5) 2.314(5)

Sm(2)-O(17) Sm(2)-O(7)#1 Sm(2)-O(13) Sm(2)-O(14) Sm(3)-O(3)#4 Sm(3)-O(1) Sm(3)-O(3w) Sm(3)-O(4w) Sm(3)-O(15)#5 Sm(3)-O(13)#5 Sm(3)-O(2w) P(1)-C(1) P(1)-O(1) P(1)-O(2) P(1)-O(3)

2.451(9) 2.476(8) 2.613(7) 2.792(7) 2.304(9) 2.405(9) 2.416(9) 2.470(8) 2.527(7) 2.503(7) 2.489(9) 1.816(12) 1.501(8) 1.516(8) 1.513(10)

Figure 2. Coordination modes of the oxalate ligands in compound 3.

Scheme 1. Coordination Fashions of Phosphonate Ligands in Compounds 3 and 5

Compound 5 Gd(2)-O(6) Gd(2)-O(3w) Gd(2)-O(7) Gd(2)-O(12)#4 Gd(2)-O(2w) Gd(2)-O(1w) P(1)-C(1) P(1)-O(1) P(1)-O(2) P(1)-O(3)

2.351(5) 2.403(5) 2.403(5) 2.405(5) 2.431(5) 2.446(5) 1.822(7) 1.484(5) 1.495(5) 1.574(5)

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

3: + + z.

two oxalate anions, two phosphonate oxygen atoms from two HL- ligands and three oxygen atoms from three water molecules. The Sm-O bond lengths are in the range of 2.304(9) to 2.792(7) Å, which are comparable to those reported for other samarium(III) phosphonates.11 There are two different coordination modes for the four oxalate anions in compound 3. The C(8)C(9)O4 and C(12)C(13)O4 units show only bis-bidentate coordination to two Sm3+ ions (Figure 2a). Such configuration is favorable because of the formation of five-membered chelating rings (Sm-O-C-C-O). The C(6)C(7)O4 and C(10)C(11)O4 units are bis-bidentate ligand to three Sm3+ ions (Figure 2b). The result of connections in this manner is the formation of two Sm-O-C-C-O five-membered chelating rings and one Sm-O-C-O four-membered chelating ring, which has not been observed in any oxalatophosphonates. The tridentate HL-

Figure 1. ORTEP representation of a selected unit of compound 3. The thermal ellipsoids are drawn at the 30% probability level. All H atoms and solvate water molecules are omitted for clarity. Symmetry codes: A, -x + 2, y + 1/2, -z + 1; B, x + 1, y, z; C, x, y, z + 1; D, x - 1, y, z; E, -x + 3, y - 1/2, -z + 1.

Crystal Structures of 1-3. Compounds 1-3 are isostructural and feature a complicated 3D framework structure, hence only the structure of 3 will be discussed in detail as a representation. The asymmetric unit of 3 contains three kinds of crystallographically unique samarium(III) ions, one HL- ligand, four oxalate anions, four coordianted water molecules and two solvate water molecules. As shown in Figure 1, Sm(1) ion is ninecoordinated by eight oxygen atoms from four oxalate anions and one oxygen atom from one water molecule. Sm(2) ion is nine-coordinated by eight oxygen atoms from four oxalate anions and one phosphonate oxygen atom from one HL- ligand. Sm(3) ion is also nine-coordinated by four oxygen atoms from

Figure 3. (a) 1D chain of {Sm3(HL)}8+ and (b) 3D network of {Sm3(C2O4)4}+ in compound 3.


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Figure 4. A ball-and-stick and polyhedral view of the framework for compound 3 down the b-axis. All H atoms and solvate water molecules are omitted for clarity.

Figure 6. (a) 2D layer of {Gd2(C2O4)3} in compound 5 viewed down the c-axis; (b) View of the framework for compound 5 down the a-axis. All H atoms and solvate water molecules are omitted for clarity.

Figure 5. ORTEP representation of a selected unit of compound 5. The thermal ellipsoids are drawn at the 30% probability level. All H atoms and solvate water molecules are omitted for clarity. Symmetry codes: A, -x + 1, -y + 1, -z + 1; B, -x, -y + 1, -z + 1; C, x 1, y, z.; D, -x, y + 1/2, -z + 3/2.

ligand is monodentate with three Sm3+ ions (Scheme 1a). Based on the charge balance, three phosphonate oxygen atoms of each HL- ligand are deprotonated and the nitrogen atom is protonated. Each Sm3+ ion is bridged two HL- ligands by two phosphonate oxygen atoms to form a 1D chain of {Sm3(HL)}8+ down the b-axis and the morpholinyl groups are hung in this chain (Figure 3a). The dihedral angle between two phosphonate oxygen atoms sharing a common Sm3+ ion is 138.7(3)°. The interconnection of the Sm3+ ions by chelating and bridging oxalate anions lead to a 3D network of {Sm3(C2O4)4}+ with one-dimensional channel system down the b-axis (Figure 3b). The size of the channel is estimated to be 9.0 Å × 5.0 Å based on structure data. The above two building blocks are interconnected Via sharing Sm3+ ions to form a complex 3D framework structure (Figure 4). The structure of compound 3 can also be viewed as the HL- ligands being hung in the tunnels of {Sm3(C2O4)4}+, and the solvate water molecules are located in the tunnels. Crystal Structures of 4-7. Compounds 4-7 are also isostructural and possess a 3D framework structure different from that in compounds 1-3. The structure of compound 5 will be described representatively. As shown in Figure 5, there are two unique Gd(III) ions in the asymmetric unit. Gd(1) ion is nine-coordinated by eight oxygen atoms from four oxalate anions and one phosphonate oxygen atom from one H2L ligand. Gd(2) ion is eight-coordinated by four oxygen atoms from two oxalate anions, one phosphonate oxygen atom from one H2L

Figure 7. View of the framework for compound 5 showing the holes in the structure.

ligand and three oxygen atoms from three water molecules. The Gd-O distances range from 2.314(5) to 2.517(5) Å, which are comparable to those reported for other gadolinium(III) phosphonates and oxalates.6a,8,12 All oxalate anions in compound 5 adopt the same coordination mode as the C(8)C(9)O4 unit in compound 3 (Figure 2a). However, the coordinating mode of the H2L ligand is different from that in compound 3 (Scheme 1b). The bidentate H2L ligand is monodentate with two Gd(III) ions through two phosphonate oxygen atoms. One phosphonate oxygen atom (O3) is protonated, so does its amine group, which is due to the zwitterionic behavior of the aminophosphonic acid.13 Different from that in compound 3, the interconnection of Gd(III) ions in compound 5 by chelating oxalate anions leads to a gadolinium oxalate layer down the c-axis (Figure 6a). The gadolinium oxalate layers are linked by H2L ligands to form a complex 3D framework type with channel system down the a-axis (Figure 6b). The channel is formed by 40-atom rings composed of ten Gd(III) ions, four H2L ligands and six oxalate anions (Figure 7). The dimension of the channel is 22.3 Å × 6.4 Å, which is estimated by measuring the distances between the centers of opposite atoms. The solvate water molecules are located in the channels. It is interesting to note that the seven compounds with two different structures have been synthesized under the same experimental conditions. The different coordination modes of the phosphonate ligands and oxalate ligands adopted in these compounds certainly play an important role in the formation of

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Figure 8. The TG-DTA curves of compound 3.

these two different structural types. Another reason may be related to the atomic size of different lanthanide atoms, the socalled “lanthanide contraction”. With decreasing ionic size, the heavier lanthanide ion leaves fewer coordination sites, which is called “lanthanide contraction”.6b IR Spectroscopy. The IR spectra of compounds 1-7 have many similar features corresponding to the common groups, thus only the spectrum of compound 3 will be discussed. The absorption band at 3412 cm-1 for compound 3 can be assigned to the O-H stretching vibrations of water molecules. The weak band at 3064 cm-1 is attributed to the N-H stretching vibrations. No band is seen at ca. 1715 cm-1 corresponding to V(CdO) for the free acid (-COOH). However, there is one pair of strong bands centered at 1643 and 1460 cm-1, which is assigned to the asymmetric and symmetric stretching vibrations of C-O groups when present as COO- moieties.14 The set of bands between 1200 and 900 cm-1 are assigned to stretching vibrations of the tetrahedral CPO3 groups.13,15 Additional weak bands at low energy are found. These bands are probably due to bending vibrations of the tetrahedral CPO3 groups. Thermal Analysis. Thermal gravimetric analyses (TGA) are conducted to examine the stabilities of these compounds. Compounds 1-7 show similar TGA curves, and compounds 3 and 5 were used as two representatives. The TGA curve of compound 3 reveals three main steps of weight losses (Figure 8). The first step started at 100 °C and was completed at 265 °C, corresponding to the release of two solvate water molecules and four coordinated water molecules. The observed weight loss of 9.3% is very close to the calculated value (9.9%). The second step, from 340 to 460 °C, corresponds to decomposition of oxalate and phosphonate units. The third step covers a temperature range of 650-750 °C, which corresponds to the further decomposition of the compound. The final product is the mixture of SmPO4 and Sm2O3 based on XRD powder studies. The total weight loss of 45.8% is close to the calculated value (45.6%) if the final product is assumed to be mixture of SmPO4 and Sm2O3 in a molar ratio of 1:1. The TGA curve of compound 5 also shows three main continuous weight losses (Figure 9). The first step, in the temperature range 100-260 °C, is due to the removal of one solvate water molecule and three coordinated water molecules. The observed weight loss of 8.7% is in good agreement with the calculated value (8.6%). The second step occurred above 260 °C, during which the compound is partially decomposed. The third step covers from 785 to 885 °C, corresponding to the further decomposition of the organic groups. From powder X-ray diffraction, the final product is identified as the mixture of GdPO4 and Gd2O3. The total weight loss of 47.7% is close to the calculated value (47.9%) if the

Zhu et al.

Figure 9. The TG-DTA curves of compound 5.

Figure 10. Experimental and heated (270 °C) X-ray powder diffraction diagram of compound 3 compared to the calculated one.

final product is assumed to be mixture of GdPO4 and Gd2O3 in a molar ratio of 2:1. During the thermal decomposition, intermediate compounds may be formed between 450 and 650 °C for compound 3, and between 450 and 780 °C for compound 5 respectively. In order to identify these intermediate compounds, X-ray powder diffraction studies were performed for compounds 3 and 5 calcined at 550 °C. However, these intermediate compounds were not identified because the complicated mixtures were obtained during the thermal decomposition. The DTA curve exhibit a strong exothermic peak at approximately 434 °C (for compound 3) and 437 °C (for compound 5), corresponding to the combustion of the organic faction. The heated X-ray powder diffraction data (270 °C) of compounds 3 and 5 were measured (Figure 10 and Figure 11). Attempts to obtain the precise structures from the dehydrated crystals were unsuccessful, which was attributed to their poor crystal quality after water molecules were removed from the lattice. Luminescent Properties. The luminescent behaviors of compounds 3, 4, 6 and 7 were investigated in the solid state at room temperature. The emission spectrum of compound 3 at the excited wavelength of 404 nm is shown in Figure 12. There are three characteristic bands, which are attributed to 4G5/2 f 6 HJ (J ) 5/2, 7/2, 9/2), 4G5/2 f 6H5/2 (563 nm), 4G5/2 f 6H7/2 (597 nm), and 4G5/2 f 6H9/2 (643 nm) transitions.16 Compound 4 emits red light upon excitation at 396 nm, and four characteristic peaks are shown in Figure 13. These emission bands arise from 5D0 f 7FJ (J ) 1, 2, 3, and 4) transitions, typical of Eu3+ ions.7,12b Two stronger peaks are attributed to 5 D0 f 7F1 (592 nm) and 5D0 f 7F2 (617 nm), and the two weaker peaks belong to the transitions of 5D0 f 7F3 (652 nm) and 5D0 f 7F4 (699 nm). The 5D0 f 7F1 transition corresponds


Figure 11. Experimental and heated (270 °C) X-ray powder diffraction diagram of compound 5 compared to the calculated one.

Figure 12. Solid-state emission spectrum of compound 3 at room temperature.

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spectrum. Compound 7 is yellow-luminescent in the solid state when excited at 366 nm. In the emission spectrum of compound 7 (Figure 15), two characteristic bands can be seen, which are attributed to transitions of 481 nm (4D9/2 f 6H15/2) and 574 nm (4D9/2 f 6H13/2).16 Compared with the emission spectra of the four compounds (3, 4, 6, and 7), the transition intensity of compound 3 is the weakest. The low emission intensity for Sm3+ ions implies that the efficiency of energy transfer from ligands to metals is lower than that to Eu3+, Tb3+ and Dy3+. Conclusion


to a magnetic dipole transition, and the intensity of this emission for 4 is medium-strong. The most intense emission in the luminescent spectrum is the 5D0 f 7F2 transition, which is the so-called hypersensitive transition and is responsible for the brilliant-red emission of compound 4.17 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 emission spectrum of compound 6 at the excited wavelength 370 nm exhibits the characteristic emission of Tb3+. There are four characteristic peaks shown in Figure 14. They are assigned to the 5D4 f 7FJ (J ) 3, 4, 5, and 6), 5D4 f 7F6 (489 nm), 5D4 f 7F5 (545 nm), 5 D4 f 7F4 (584 nm), and 5D4 f 7F3 (621 nm) transitions.12b,18 Among these emission lines, the most striking green luminescence (5D4 f 7F5) for compound 6 is observed in the emission

By using the N-morpholinomethylphosphonic acid as the phosphonate ligand and oxalate as the second metal linker, we have synthesized seven novel lanthanide(III) oxalatophosphonates with two types of 3D framework structures under hydrothermal conditions. Compounds 1-3 are isostructural and feature a complex 3D framework formed by the interconnection of 3D network of {Ln3(C2O4)4}+ with 1D chains of {Ln3(HL)}8+. Compounds 4-7 are isostructural and possess a 3D framework structure different from that in compounds 1-3. The interconnection of Ln(III) ions by chelating oxalate ligands leads to a lanthanide oxalate layer down the c-axis. These layers are linked by H2L ligands to form a 3D framework type with channel system down the a-axis. Compounds 3, 4, 6 and 7 which exhibit characteristic lanthanide-centered luminescence are new examples of luminescent rare-earth oxalatophosphonates. Compared with the emission spectra of the four compounds (3, 4, 6, and 7), the transition intensity of compound 3 is the weakest. The low emission intensity for Sm3+ ions implies that the efficiency of energy transfer from ligands to metals is lower than that to Eu3+, Tb3+ and Dy3+. The results of our study indicate that, by introduction of oxalate as the second ligand,

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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 Natural Science Foundation of Liaoning Province of China (20062140). Supporting Information Available: Tables listing selected bond lengths and bond angles for compounds presented in this paper, XRD patterns of the experiments compared to those simulated from X-ray single-crystal data for compounds 1-7, XRD patterns of the final products in the thermal decomposition for compounds 3 and 5, IR spectra of compounds 1-7, and TGA curves of 1, 2, 4, 6 and 7. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 670620-670626 contain supplementary crystallographic data for this paper. 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, Cambridge CB21EZ, U.K.; fax, (+44) 1223-336-033; e-mail, [email protected]).

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CG8013128

Seven Novel Lanthanide Oxalatophosphonates with Two Types of 3D

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