Novel Luminescent Lanthanide(III) Diphosphonates with Rarely

Figure 8 Solid-state excitation (black curves) and emission (red curves) spectra ..... other by seven La(III) ions and seven organic moieties (size ab...
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CRYSTAL GROWTH & DESIGN

Novel Luminescent Lanthanide(III) Diphosphonates with Rarely Observed Topology Si-Fu Tang, Jun-Ling Song, Xiu-Ling Li, and Jiang-Gao Mao* State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou 350002, P. R. China

2007 VOL. 7, NO. 2 360-366

ReceiVed September 6, 2006; ReVised Manuscript ReceiVed NoVember 1, 2006

ABSTRACT: Hydrothermal reactions of lanthanide(III) salts with N-(2-pyridyl)-aminomethane-1,1-diphosphonic acid (H4L) led to five new lanthanide(III) phosphonates, namely, La4(H2L)4(H3L)4(H2O)4·20H2O (1) and Ln(H2L)(H3L)·4H2O (Ln ) Eu (2), Gd (3), Dy (4), and Er (5)). Compound 1 features a 3D framework with rarely observed topology in which each lanthanum(III) ion is eight-coordinated by seven phosphonic oxygen atoms from four phosphonate ligands and an aqua ligand. It possesses two types of tunnels running parallel to the a-axis, one formed by five La(III) ions and five organic moieties, the other by seven La(III) ions and seven organic moieties. It shows noninterpenetrated three-connected topology. Compounds 2-5 are isostructural and exhibit onedimensional chain structures in which each pair of Ln(III) ions are bridged by two and four phosphonate groups, alternatively. Compound 2 exhibits emission bands of the phosphonate ligand and the europium(III) ion in the visible region under 360 nm excitation, with a Eu (5D0) lifetime of 1.2 ms. Compound 3 displays a very broad ligand-centered emission band (λmax ) 412 nm) in the blue light region. Compound 5 is a luminescent material in the near-IR region. Introduction Metal phosphonates have received extensive research 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 Most of metal phosphonates exhibit a variety of open framework architectures, such as layered and microporous structures. An effective method for preparing new materials with microporous or open framework structures is to modify the organic moieties of the phosphonate ligand RPO32- with other functional groups.1-6 Lanthanide ions can exhibit various high coordination numbers; it is believed that their coordination with phosphonate ligands will lead to many unusual architectures different from those of transition metals. Furthermore, lanthanide phosphonates may exhibit useful luminescence properties due to a variety of f-f transitions. Therefore, the elucidation of the structures of lanthanide phosphonates becomes very important.7 Up until now, reports on lanthanide phosphonates have still been limited, and a number of them are based on X-ray powder diffraction.8-12 It is considered difficult to obtain single crystals of lanthanide phosphonates suitable for structural studies because of their low solubility and poor crystallinity. These problems can be solved by two methods: (1) introducing polar functional groups into the ligand, such as hydroxy, amino, carboxylate, or crown ether9-11 and (2) introducing a second ligand such as 5-sulfoisophthalic acid (H3BTS) or oxalic acid.12 In view of luminescence, a phosphonate ligand containing a rigid pyridyl group may be a good ligand due to its rigidity and large π system. So far only a few lanthanide phosphonates containing pyridyl groups have been prepared.9c-9f We synthesized a new diphosphonate ligand, namely, N-(2-pyridyl)-aminomethane-1,1diphosphonic acid (H4L), which contains a pyridyl group and a nitrogen atom directly attached to the P-C-P backbone (Scheme 1a). Hydrothermal reactions of H4L with lanthanide(III) salts afforded five novel lanthanide(III) phosphonates, namely, La4(H2L)4(H3L)4(H2O)4·20H2O (1) and Ln(H2L)(H3L)· * E-mail: [email protected].

4H2O (Ln ) Eu (2), Gd (3), Dy (4), and Er (5)). Herein we report their syntheses, crystal structures, and luminescence properties. Experimental Section Materials and Instrumentation. All chemicals were obtained from commercial sources and used without further purification. Elemental analyses were performed on a Vario EL III elemental analyzer. Thermogravimetric analyses were carried out on a NETZSCH STA 449C unit at a heating rate of 10 °C/min under a nitrogen atmosphere. IR spectra were recorded on a Magna 750 FT-IR spectrometer photometer as KBr pellets in the 4000-400 cm-1. Solution 31P and 1H NMR spectra were recorded on a Varian Unity 500 NMR. H3PO4 was used as standard reference. Emission and excitation spectra of the ligand and compounds 2 and 3 were recorded on a Perkin-Elmer LS55 luminescence spectrometer with a red-sensitive photomultiplier type r928. The lifetime measurement of compound 2 and the photoluminescence analyses of compound 5 were performed on an Edinburgh FLS920 fluorescence spectrometer. Preparation of N-(2-Pyridyl)-aminomethane-1,1-diphosphonic Acid (H4L). N-(2-Pyridyl)-aminomethane-1,1-diphosphonic acid (H4L) was prepared according to procedures described previously.13 A mixture of 2-amino-pyridine (0.05 mol), diethyl phosphite (0.10 mol), and triethyl orthoformate (0.05 mol) was heated at 100 °C for 12 h. The resultant solution was concentrated and passed through a silica gel column (EtOAc/acetone, 1:1). The resultant oil was then hydrolyzed in 20 mL of 6 N HCl at 95 °C for 3 h; removal of solvents afforded H4L as a white powder (yield 86%). Its purity was confirmed by 1H and 31P NMR, IR, and elemental analyses. 1H NMR (D2O): 4.1644.247 (O3P-CH-PO3, m, 1H), 6.935, 7.164, 7.840 and 7.940 (PyH, s, 4H) ppm. 31P NMR (D2O) shows only one single peak at 12.053 ppm. IR (KBr, cm-1): 3295 m, 3206 m, 3120 m, 2929 m, 2303 m, 1659 s, 1625 m, 1537 m, 1454 s, 1388 m, 1351 w, 1259 m, 1210 m, 1111 m, 1043 s, 995 m, 953 m, 922 m, 826 m, 769 m, 721 w, 609 w, 507 m. Elemental Anal. Calcd for C6H10N2O6P2: C, 26.88; H, 3.76; N, 10.45%. Found: C, 26.61; H, 3.75; N, 10.35%. Preparation of La4(H2L)4(H3L)4(H2O)4·20H2O (1). A mixture of LaCl3·6H2O (0.2 mmol) and H4L (0.5 mmol) in 10 mL of distilled water was sealed into a bomb equipped with a Teflon liner (25 mL) and then heated at 150 °C for 4 days. The initial and final pH values of the resultant solutions were 2.0 and 1.5, respectively. Compound 1 was isolated in a yield of about 36% (based on lanthanum). Elemental Anal. Calcd for C48H116La4N16O72P16: C, 18.47; H, 3.75; N, 7.18%. Found: C, 18.35; H, 3.88; N, 7.04%. IR (KBr, cm-1): 3419 s, 3261 m, 3098

10.1021/cg060590y CCC: $37.00 © 2007 American Chemical Society Published on Web 12/15/2006

Luminescent Lanthanide(III) Diphosphonates Scheme 1.

Crystal Growth & Design, Vol. 7, No. 2, 2007 361

H4L (a) and Its Coordination Modes (b and c), as Well as Two Types of Dimers

Table 1. Crystal Data and Structure Refinements for Compounds 1-5 compound formula fw space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) GOF on F2 R1, wR2 (I > 2σ(I))a R1, wR2 (all data) a

1 C48H116La4N16O72P16 3120.73 P1h 14.3339(18) 20.760(3) 21.507(3) 69.992(5) 73.154(5) 72.353(5) 5605.4(13) 2 1.849 1.835 1.062 0.0515, 0.1253 0.0605, 0.1323

2 C12H25EuN4O16P4 757.20 P21/c 10.6305(5) 17.0798(8) 16.7882(10) 90 92.294(3) 90 3045.7(3) 4 1.651 2.337 1.100 0.0460, 0.1241 0.0543, 0.1313

3 C12H25GdN4O16P4 762.49 P21/c 10.656(1) 17.162(3) 16.771(3) 90 92.326(2) 90 3059.6(4) 4 1.663 2.455

4 C12H25DyN4O16P4 767.74 P21/c 10.591(2) 17.182(2) 16.863(1) 90 92.023(3) 90 3067.2(3) 4 1.674 2.731

5 C12H25ErN4O16P4 772.50 P21/c 10.519(4) 17.221(6) 16.653(6) 90 91.931(7) 90 3015.0(17) 4 1.702 3.064 1.152 0.0741, 0.1688 0.1052, 0.1870

R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑w[(Fo)2 - (Fc)2]2/∑w[(Fo)2]2}1/2.

s, 2856 m, 1658 s, 1633 s, 1540 m, 1460 w, 1416 m, 1365 w, 1275 m, 1157 vs, 1089 vs, 1070 s, 990 m, 913 m, 828 w, 766 m. Preparation of Ln(H2L)(H3L)·4H2O (Ln ) Eu (2), Gd (3), Dy (4), and Er (5)). Compounds 2, 3, 4, and 5 were prepared by a similar procedure to that for compound 1 by using EuCl3·6H2O, GdCl3·6H2O, DyCl3·6H2O, or ErCl3·6H2O instead of LaCl3·6H2O. Their yields are 21.0%, 18.0%, 5.0%, and 78% (based on lanthanide metal), respectively. Elemental Anal. Calcd for C12H25EuN4O16P4: C, 19.04; H, 3.33; N, 7.40%. Found: C, 19.18; H, 3.47; N, 7.53%. IR (KBr, cm-1): 3221 m, 3106 m, 2997 m, 2850 m, 1650 m, 1634 s, 1539 m, 1459 m, 1406 m, 1372 w, 1306 w, 1259 m, 1162 s, 1072 vs, 990 m, 934 w, 764 w. Elemental Anal. Calcd for C12H25GdN4O16P4: C, 18.90; H, 3.31; N, 7.35%. Found: C, 19.16; H, 3.46; N, 7.52%. IR (KBr, cm-1): 3243 s, 3102 m, 3041 s, 2993 s, 2851 m, 1651 s, 1634 s, 1540 m, 1459 w, 1414 m, 1372 w, 1307 w, 1259 m, 1163 vs, 1092 vs, 990 m, 935 m, 831 w, 764 w. Elemental Anal. Calcd for C12H25DyN4O16P4: C, 18.77; H, 3.28; N, 7.30%. Found: C, 19.01; H, 3.41; N, 7.41%. IR (KBr, cm-1): 3244 s, 3100 m, 1654 s, 1634 s, 1540 m, 1460 w, 1414 m, 1371 w, 1307 m, 1259 m, 1163 s, 1091 s, 992 s, 972 m, 936 s, 834 w, 764 m. Elemental Anal. Calcd for C12H25ErN4O16P4: C, 18.66; H, 3.26; N, 7.25%. Found: C, 18.88; H, 3.41; N, 7.37%. IR (KBr, cm-1): 3251 m, 3105 s, 1655 s, 1634 s, 1538 m, 1454 w, 1412 m, 1308 w, 1259 m, 1127 vs, 1092 vs, 977 m, 937 m, 831 w, 760 w. Efforts were made to improve the yield for compounds 2-4 but were unsuccessful, mainly due to significant amount of unknown byproducts as white powder, which can be removed by filtration after ultrasonic vibration. Single-Crystal Structure Determination. Data collections for compounds 1-5 were performed on a Rigaku Mercury CCD equipped with a graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). Cell parameter measurements indicate that compounds 2, 3, 4, and 5

are isostructural; hence full data collections were performed for only compounds 2 and 5. Intensity data were collected by the narrow frame method at 293 K. The data sets were corrected for Lorentz and polarization factors as well as for absorption by the SADABS program or Multiscan method.14 All structures were solved by the direct methods and refined by full-matrix least-squares fitting on F2 by SHELX-97.14 All non-hydrogen atoms except O(17w)-O(19w), O(21w), O(22w), and O(24w)-O(29w) in compound 1, O(5w) and O(6w) of compound 2 and O(4w)-O(6w) of compound 5 were refined with anisotropic thermal parameters. All hydrogen atoms except those for water molecules were located at geometrically calculated positions and refined with isotropic thermal parameters. The protonation of the amino group and phosphonate group are based on the requirement of charge balance and P-O bond distances. Hydrogen atoms for water molecules in compounds 1, 2, and 5 were not included in the refinements. O(20w)O(29w) of compound 1 and O(3w)-O(6w) in compounds 2 and 5 with large thermal parameters were refined with only 50% occupancy. For compound 1, O(24w) and O(25w) and O(28w) and O(29w) with short distances of 1.80(4) and 1.95(4) Å, respectively, are considered to be disordered water molecules. The final difference Fourier maps for compound 1 showed residual peaks of 4.11 e Å-3 (0.82 Å from La(4) atom) and holes of -2.78 e Å-3 (0.69 Å from La(4) atom); the relatively higher residuals are due to the absorption correction problems with the heavy La(III) ions. Crystallographic data and structural refinements for compounds 1-5 are summarized in Table 1. Selected bond lengths for compounds 1, 2, and 5 are listed in Table 2. More details on the crystallographic studies, as well as atom displacement parameters, are given as the Supporting Information.

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Table 2. Selected Bond Lengths (Å) for Compounds 1, 2, and 5 1a La(3)-O(11)#4 La(3)-O(9)#4 La(3)-O(12) La(3)-O(6) La(3)-O(3) La(3)-O(27) La(3)-O(7) La(3)-O(3W) La(4)-O(46) La(4)-O(31) La(4)-O(36) La(4)-O(45)#2 La(4)-O(28) La(4)-O(43) La(4)-O(26) La(4)-O(4W)

La(1)-O(16)#1 La(1)-O(17) La(1)-O(38) La(1)-O(40) La(1)-O(14)#1 La(1)-O(13) La(1)-O(2)#2 La(1)-O(1W) La(2)-O(24) La(2)-O(22)#3 La(2)-O(32) La(2)-O(20) La(2)-O(34) La(2)-O(39) La(2)-O(21)#3 La(2)-O(2W)

2.419(4) 2.479(3) 2.502(4) 2.530(4) 2.537(4) 2.561(4) 2.587(3) 2.662(4) 2.399(4) 2.471(4) 2.514(4) 2.522(4) 2.524(4) 2.589(4) 2.605(4) 2.649(5)

O(4)-O(7W) O(5)-O(10W) O(8)-O(5W) O(15)-O(6W) O(19)-O(22W) O(23)-O(9W) O(29)-O(8W) O(30)-O(3W) O(33)-O(11W) O(41)-O(27W) O(42)-O(9W) O(2W)-O(24W) O(4W)-O(26W) O(5W)-O(17W)

Potential Hydrogen Bonds 2.671(7) O(5W)-O(7W)#5 2.573(8) O(6W)-O(13W) 2.640(8) O(6W)-O(9W)#6 2.573(7) O(7W)-O(18)#7 2.564(17) O(8W)-O(10)#4 2.673(7) O(9W)-O(25W) 2.589(7) O(12W)-O(20W)#4 2.780(6) O(13W)-O(29W)#1 2.655(8) O(16W)-O(22W)#8 2.75(3) O(18W)-O(20W)#4 2.743(8) O(18W)-O(48)#9 2.79(2) O(19W)-O(26W) 2.75(2) O(21W)-O(26W)#2 2.785(16) O(22W)-O(23W)

Eu(1)-O(22) Eu(1)-O(32)#1 Eu(1)-O(13)#2 Eu(1)-O(11)

2.322(3) 2.333(4) 2.346(4) 2.352(4)

O(21)‚‚‚O(4W) O(31)‚‚‚O(2W) O(5W)‚‚‚O(6W)

Potential Hydrogen Bonds 2.55(1) O(12)‚‚‚O(5W) 2.630(7) O(1W)‚‚‚O(4W)#3 2.52(4)

Er(1)-O(22) Er(1)-O(32)#1 Er(1)-O(13)#2 Er(1)-O(11)

2.251(7) 2.274(6) 2.293(6) 2.285(7)

O(21)‚‚‚O(4W) O(31)‚‚‚O(2W) O(2W)‚‚‚O(6W)

Potential Hydrogen Bonds 2.52(2) O(12)‚‚‚O(5W) 2.61(1) O(1W)‚‚‚O(4W)#3 2.67(4)

2b Eu(1)-O(33) Eu(1)-O(43) Eu(1)-O(23)#2

5b Er(1)-O(33) Er(1)-O(43) Er(1)-O(23)#2

2.427(4) 2.474(4) 2.480(4) 2.495(4) 2.506(4) 2.593(3) 2.628(4) 2.639(4) 2.427(4) 2.463(4) 2.469(4) 2.496(4) 2.517(4) 2.531(4) 2.591(4) 2.639(5) 2.807(9) 2.743(13) 2.803(10) 2.732(7) 2.643(6) 2.73(4) 2.45(2) 2.50(3) 2.70(2) 2.63(3) 2.689(17) 2.69(3) 2.73(3) 2.62(3)

Figure 1. ORTEP representation of a selected unit of compound 1. Pyridyl groups and noncoordinated water molecules are omitted for clarity. The thermal ellipsoids are drawn at the 50% probability. Symmetry codes for the generated atoms are as follows: (a) 1 - x, -1 - y, 1 - z; (b) 1 - x, -2 - y, 1 - z; (c) -x, -1 - y, 1 - z; (d) 1 - x, -y, -z.

2.372(3) 2.389(3) 2.394(4)

2.60(2) 2.74(2)

2.313(6) 2.331(6) 2.333(6)

2.65(4) 2.72(3)

a Symmetry codes for compound 1: #1 -x + 1, -y - 2, -z + 1; #2 -x + 1, -y - 1, -z; #3 -x, -y - 1, -z + 1; #4 -x + 1, -y, -z; #5 -x, -y, -z; #6 -x, -y - 2, -z + 1; #7 x, y + 1, z - 1; #8 x + 1, y, z; #9 -x + 2, -y - 1, -z. b Symmetry codes for compounds 2 and 5: #1 -x + 2, -y, -z + 1; #2 -x + 1, -y, -z + 1; #3 -x + 1, y - 1/2, -z + 1/2.

Results and Discussion Hydrothermal reactions of N-(2-pyridyl)-aminomethane-1,1diphosphonic acid (H4L) with lanthanide(III) salts afforded five novel lanthanide(III) phosphonates, namely, La4(H2L)4(H3L)4(H2O)4·20H2O (1) and Ln(H2L)(H3L)·4H2O (Ln ) Eu (2), Gd (3), Dy (4), and Er (5)). Their structures feature 1D or 3D structures. There are four unique lanthanum(III) ions in the asymmetric unit of compound 1 (Figure 1). All of them are eightcoordinated; seven oxygen atoms come from four phosphonate ligands, and the remaining one comes from an aqua ligand. The La-O distances are in the range of 2.399(4)-2.662(4) Å, which are comparable to those reported for other La(III) phosphonates.8-12 The phosphonate ligands adopt two different types of coordina-

Figure 2. View of the crystal structure of 1 down the a and b axes. The phosphonate tetrahedra are shaded in pink. La, C, N, and O atoms are drawn as green, black, blue, and red circles, respectively.

tion modes, which can be denoted as 2.1001100 (for H3L-) and 2.1101100 (for H2L2-) modes (see Scheme 1b,c) according to the Harris notation.15 The 2.1001100 mode means there are overall two metal ions bound to the ligand; three donor atoms of the phosphonate ligand are bonded with these two lanthanum atoms, whereas the remaining four are noncoordinated. Similarly, the 2.1101100 mode indicates that there are overall two metal ions bound by four of seven donor atoms of the phosphonate ligand. Both phosphonate groups in the H3L- anion

Luminescent Lanthanide(III) Diphosphonates

Crystal Growth & Design, Vol. 7, No. 2, 2007 363

Figure 5. 1D europium(III) phosphonate chain along the a-axis in compound 2. Eu, C, N, O, and P atoms are drawn as green, black, blue, red, and purple circles, respectively.

Figure 6. View of the structure of compound 2 down the a-axis. The phosphonate tetrahedra are shaded in purple. Eu, C, N, and O atoms are drawn as green, black, blue, and red circles, respectively. Hydrogen bonds are drawn as dashed lines.

Figure 3. Schematic representation of the three-connected net in 1 along the a and b axes. Highlighted are right-handed (orange) and lefthanded (purple) helices.

Figure 7. TGA curves for compounds 1-5.

Figure 4. ORTEP representation of a selected unit of compound 2. The thermal ellipsoids are drawn at the 50% probability. Symmetry codes for the generated atoms are as follows: (a) -x + 2, -y, -z + 1; (b) 1 - x, -y, -z + 1.

are singly prontonated, whereas one phosphonate group of the H2L2- dianion is singly protonated and the other one is fully

deprotonated based on the requirement of charge balance and P-O bond distances. In both types of phosphonate ligands, the amino groups are protonated and noncoordinated based on the requirement of charge balance. Two adjacent lanthanum atoms form two types of dimers through two bridging H2L2- or H3L- ligands (type I and type II, see Scheme 1). La(1)···La(1), La(2)···La(2), and La(3)···La(3) adopt type I mode in which the two La(III) ions are bridged by

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Figure 8. Solid-state excitation (black curves) and emission (red curves) spectra for the ligand (a) and compounds 2 (b), 3 (c), and 5 (d) at room temperature.

four bridging phosphonate groups, whereas La(4)···La(4) adopts type II mode in which the two metal ions are bridged by two bridging phosphonate groups. The La···La distances for the type I dimer (5.200(2) and 5.215(2) Å) are significantly shorter than those of the type II dimer (6.249(2) Å). The above two types of dimers are further interconnected by bridging phosphonate ligands into a novel 3D framework (Figure 2a). There are two types of channels along the a-axis, one formed by five La(III) ions and five organic moieties (size about 4.9 × 7.3 Å2) and the other by seven La(III) ions and seven organic moieties (size about 6.8 × 9.9 Å2). It is noteworthy that the lattice water molecules are located at the small tunnels, whereas the pyridyl rings occupy the larger channels (Figure 2a). Viewed down the b-axis, the framework of compound 1 also displays long narrowshaped tunnels composed of rings formed by 12 La(III) ions and 12 organic moieties (size about 5.7 × 15.9 Å2), which are filled by the pyridyl groups of the ligands and water molecules (Figure 2b). A large number of strong hydrogen bonds are formed between water molecules and noncoordination phosphonate oxygen atoms based on the O···O distances (Table 2). The solventaccessible volume in compound 1 is calculated to be about 26.4% by using the PLATON program.16 In the view of topology, the phosphonate ligands act as linkers, and the metal centers can be considered as being threeconnected nonplanar nodes. Therefore, the whole structure can be simplified as an unusual non-interpenetrated three-connected net (Figure 3). There are two kinds of nodes, 5.122 (for La(1), La(2), and La(3)) and 52.12 (for La(4)), and the ratio is 3:1, which gives the short vertex symbol (5.122)3(52.12).17a,b An interesting feature of this net is the presence of parallel pseudo71 helices running through the a-axis, as highlighted in Figure

3a. The repeating period in the helical column is about 14.2 Å and the diameter of the helical column is about 11.94 Å. Another interesting feature of this net is that the right-handed and lefthanded helices share edges with each other. Each right-handed helix connects with three adjacent left-handed helices, and each left-handed helix connects with three adjacent right-handed helices, forming four five-membered rings between these three helical chains. Because the right- and left-handed helices are alternatively arranged, the whole net is therefore racemic. To the best of our knowledge and searching in Reticular Chemistry Structure Resource (http://okeeffe-ws1.la.asu. edu/RCSR/home. htm), this type of net has not been reported before.17 Large 12membered circuits and distorted nonplanar five-membered circuits can be found clearly as viewed from the b-direction (Figure 3b). The five-membered circuits are studded between 12-membered circuits. Compounds 2-5 are isostructural; hence only the structure of 2 will be discussed in detail as a representation. There is only one unique europium(III) ion in the asymmetric unit of compound 2 (Figure 4). It is seven-coordinated by seven phosphonate oxygen atoms from two H2L2- and two H3Lanions in the distorted monocapped trigonal prism coordination geometry. The distances of the Eu-O bonds range from 2.322(3) to 2.394(4) Å, which are comparable to those reported in other Eu(III) phosphonates.12 The coordination modes of the phosphonate ligands are same as those in compound 1. Two neighboring Eu(III) ions are bridged by four and two phosphonate groups alternatively into a one-dimensional chain along the a-axis (Figure 5). The Eu···Eu distances are 4.620(2) Å and 6.050(2) Å, respectively. Neighboring europium(III) phosphonate chains are further assembled through π-π interactions into a 3D supramolecular framework, resulting in the formation of

Luminescent Lanthanide(III) Diphosphonates

apertures along the a-axis with a size of about 9.1 × 12.0 Å2 (Figure 6) based on structural data. The distance between two pyridyl rings from two neighboring chains is about 3.772 Å, and the dihedral angle between these two pyridyl rings is 6.1°. The lattice water molecules are located at the above apertures and form strong hydrogen bonds with the noncoordination phosphonate oxygen atoms based on O···O separations (O(21)· ··O(4W) 2.55(1) Å; O(12)···O(5W) 2.60(2) Å; O(31)···O(2W) 2.630(7) Å; O(1W)···O(4W) 2.74(2) Å (symmetry code 1 - x, -1/2 + y, 1/2 - z); O(5W)···O(6W) 2.52(4)). TGA curves of 1 show two main weight losses (Figure 7). The first weight loss of 13.7% from 60 to 160 °C corresponds to removal of 24 water molecules, which is in good agreement with the calculated value (13.85%). Upon further heating, a continuous weight loss is observed, which corresponds to the decomposition of the compound. The total weight loss at 1000 °C is about 42.3%, and the final residues are not characterized. TGA curves for compounds 2-5 are similar, and each displays three steps of weight loss (Figure 7). The weight loss between 60 and 140 °C corresponds to the release of four water molecules per formula unit. The second step covers a temperature range of 140-700 °C, during which the phosphonate ligands are partially combusted. The third step covering 800-1000 °C corresponds to the further decomposition of the compounds. The total weight losses at 1000 °C are 45.7%, 51.6%, 41.1%, and 40.9%, respectively, for compounds 2-5. The final products are not identified. From the slopes of TGA curves, it is obvious that the decompositions of all five compounds are not complete at 1000 °C. Results of XRD studies indicate that after removal of lattice and aqua ligands, the 3D framework of compound 1 collapses, which may be related to the strong hydrogen bonds formed by coordinated water molecules and noncoordination phosphonate oxygen atoms (Figure S1a, Supporting Information). The solid-state luminescence properties of the phosphonate ligand and compounds 2, 3, and 5 were investigated at room temperature. The excitation and emission spectra are given in Figure 8. The phosphonate ligand features three intense absorptions at 296, 310.5, and 344.5 nm, which can be assigned to the intraligand π-π* transitions. Its emission spectrum under excitation at 300 nm shows a strong, broad band at 393.5 nm (Figure 8a). Upon its complexion with the Eu(III) ion, the excitation spectrum of compound 2 under λem ) 411 nm is similar to that of the phosphonate ligand with the 344.5 nm band red-shifted to 360 nm. Its emission spectrum under excitation at 300 nm shows a strong, broad band at 412 nm, which can be assigned to the intraligand fluorescence. Its emission spectrum also exhibits four characteristic emission bands for the europium(III) ion: 592 (5D0 f 7F1), 613 (5D0 f 7F ), 652 (5D f 7F ), and 701 nm (5D f 7F ) (Figure 8b). It 2 0 3 0 4 is obvious that the intensities for all emission bands increase with the increasing wavelength for excitation (Figure 8b). The Eu (5D0) lifetime of 2 for λex,em ) 361, 613 nm is about 1.2 ms. The observation of emission bands for both the phosphonate ligand and the europium(III) ion indicate the incompleteness of energy transfer from ligand to Eu(III) ion in compound 2.18 The excitation spectrum of compound 3 is similar to the phosphonate ligand with a slight red shift for the band at 344.5 nm. It only exhibits a broad blue fluorescent emission band at λmax ) 412 nm (λex ) 350 nm) (Figure 8c), which corresponds to a ligand-centered (LC) fluorescence. The metal-centered (MC) electronic levels of the Gd3+ ion are known to be located at 31 000 cm-1, which is typically well above the ligand-centered

Crystal Growth & Design, Vol. 7, No. 2, 2007 365

electronic levels of the organic ligands. Therefore, ligand to metal energy transfer and the consequent MC luminescence cannot be observed.19 The excitation spectrum of compound 5 exhibits absorption bands at 287 and 362 nm (λem ) 1533 nm), indicating significant blue shift for the band at 296 nm and red shift of the band at 344.5 nm compared with that of the phosphonate ligand (Figure 8d). The emission spectrum of compound 5 under 365 nm excitation exhibits four strong nearIR emission bands at 1499, 1535, 1558, and 1586 nm, all of which can be attributed to the 4I13/2 f 4I15/2 transition for the Er3+ ion (Figure 8d).12 The splitting of the 4I13/2 f 4I15/2 transition band into several sub-bands is probably due to the low symmetry of the coordination geometry around the Er3+ ion (C1). Conclusion Five novel lanthanide phosphonates have been prepared by hydrothermal reactions of N-(2-pyridyl)-aminomethane-1,1diphosphonic acid (H4L) with lanthanide salts. The La(III) complex is three-dimensional, whereas the other complexes are one-dimensional. The different structural types may result from different coordination numbers for the lanthanide(III) ions. Topologically, the structure of compound 1 can be viewed as a rarely observed non-interpenetrated three-connected net. Interestingly, compound 2 exhibits luminescence for both phosphonate ligand and the europium(III) ion. Compound 3 displays very broad intraligand emission bands in the blue light region, and compound 5 exhibits very strong characteristic emission bands for the erbium(III) ion in the near-IR region. These results indicate that by introducing a pyridyl group into the phosphonate ligand, we can obtain lanthanide phosphonates with novel topology as well as interesting luminescent properties. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20371047, 20573113, and 20521101) and NSF of Fujian Province (Grant No. E0420003). Supporting Information Available: X-ray crystallographic files in CIF format for the structure determination of compounds 1, 2, and 5, as well as the XRD powder patterns for all the five compounds. This material is available free of charge via the Internet at http:// pubs.acs.org.

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