Self-Assembly of Luminescent Hexanuclear ... - ACS Publications

Dec 15, 2011 - Luminescent 4f and d-4f polynuclear complexes and coordination polymers with flexible salen-type ligands. Xiaoping Yang , Richard A. Jo...
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Self-Assembly of Luminescent Hexanuclear Lanthanide Salen Complexes Sen Liao,*,†,‡ Xiaoping Yang,‡ and Richard A. Jones*,‡ †

School of Chemistry and Chemical Engineering, Guangxi University, 100, Daxue Road, Nanning, Guangxi, 530004, People’s Republic of China ‡ Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, Texas, 78712-0165, United States S Supporting Information *

ABSTRACT: Four hexanuclear lanthanide salen complexes [Ln6(L1)4(OH)4(MeOH)4]·2Cl·4MeOH (Ln = Nd (1), Tb (2)), [Eu6(L2)4(OH)4(MeOH)2(EtOH)2(H2O)2]·2Cl·3EtOH·H2O (3), and [Er6(L2)4(OH)4(EtOH)2(H2O)2]·2Cl·2EtOH·MeOH·H2O (4) are formed from the reactions of LnCl3·6H2O and flexible Schiff base ligands H 2 L 1 and H 2 L 2 (H 2 L 1 = N,N′-bis(3methoxysalicylidene)(propylene-2-ol)-1,3-diamine, H2L2 = N,N′bis(salicylidene)(propylene-2-ol)-1,3-diamine). The structures of 1−4 were determined by single crystal X-ray crystallographic studies, and their luminescence properties were determined.

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INTRODUCTION In recent years considerable interest has been shown in the design and synthesis of polynuclear self-assemblies from transition metal ions and organic ligands.1 In particular, the self-assembly of polynuclear lanthanide(III) complexes has attracted much attention because of the potential use of these lanthanide complexes in the preparation of new materials and ideal probes in biology in view of their interesting magnetic and luminescent properties.2 However, it is difficult to control the structures of polynuclear assemblies based on lanthanide ions because they often display high and variable coordination numbers and have no strong stereochemical preferences. Our recent studies have focused on the use of a variety of “salen” style Schiff base ligands for the synthesis of polyunculear metal complexes.3 For example, we synthesized some “multidecker” lanthanide complexes with conjugate Schiff base ligands H2La,b (Scheme 1a).3a,b In these complexes, the lanthanide ions are sandwiched between alternating layers of the Schiff base ligands which display a planar configuration. A key feature in this kind of structures is the presence of intramolecular π−π stacking interactions between planar Schiff base ligands, which can further add to the stability of the complex. However, when the flexible Schiff base ligands H2Lc−e were used in the synthesis (Scheme 1b), some 1-D coordination polymers were formed by the linking of the Schiff base ligands and OAc− groups.3d,e Differing from the conjugate Schiff base ligands H2La,b, the flexible Schiff base ligands H2Lc−e can show a stretched configuration to link lanthanide ions, and π−π stacking interactions are not formed between them. The structures of lanthanide complexes are often influenced by a variety of factors such as ligand structures and the nature of counterions.2,4 As part of our continuing studies focused on the construction of high nuclearty frameworks, we describe here © 2011 American Chemical Society

Scheme 1. Conjugate (a) and Flexible (b) Schiff Base Ligands

four lanthanide hexanuclear complexes [Ln6(L1)4(OH)4(MeOH)4]·2Cl·4MeOH (Ln = Nd (1), Tb (2)), [Eu6(L2)4(OH)4(MeOH)2(EtOH)2(H2O)2]·2Cl·3EtOH·H2O (3), and [Er 6 (L 2 ) 4 (OH) 4 (EtOH) 2 (H 2 O) 2 ]·2Cl·2EtOH·MeOH·H 2 O (4) which are obtained from reactions of flexible Schiff base ligands H2L1 and H2L2 with LnCl3·6H2O. Differing from H2Lc−e, there is a backbone hydroxyl group, which has been widely used as bridge group in the formation of ploynuclear complexes,2 in either H2L1 or H2L2. Interestingly, the introduction of the backbone hydroxyl groups reduces the Received: October 31, 2011 Revised: December 11, 2011 Published: December 15, 2011 970

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Table 1. Crystal Data and Structure Refinement for Compounds 1−4 formula fw cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z Dcalc, g cm−3 temp, K F(000) μ, mm−1 θ range, deg reflns measd reflns used params Ra (I > 2σ(I)) Ra (all data) S a

1

2

3

4

C84H112N8O32Cl2Nd6 2682.16 triclinic P1̅ 12.657(3) 14.761(2) 15.796(3) 78.89(3) 67.73(4) 69.17(3) 2547.0(11) 1 1.749 153(1) 1322 3.133 2.65−25.00 22249 8938 595 R1 = 0.0402 wR2 = 0.1000 R1 = 0.0485 wR2 = 0.1069 1.062

C84H112N8O32Cl2Tb6 2770.24 triclinic P1̅ 12.498(3) 14.737(3) 15.623(4) 79.18(3) 68.23(2) 69.44(3) 2496.7(11) 1 1.842 153(1) 1352 4.325 3.09−25.00 18375 8746 595 R1 = 0.0407 wR2 = 0.0997 R1 = 0.0470 wR2 = 0.1036 1.046

C80H102N8O26Cl2Eu6 2574.36 triclinic P1̅ 11.842(2) 14.830(3) 16.045(3) 96.17(3) 105.52(2) 108.75(3) 2512.3(9) 1 1.702 153(1) 1258 3.811 3.07−25.00 21848 8815 568 R1 = 0.0518 wR2 = 0.1304 R1 = 0.0625 wR2 = 0.1399 1.036

C152H180N16O47Cl4Er12 2566.02 triclinic P1̅ 15.375(3) 18.622(4) 19.228(4) 71.69(3) 67.98(2) 83.86(3) 4844.9(19) 1 1.759 153(1) 2464 5.259 2.99−25.00 25678 16986 1063 R1 = 0.0626 wR2 = 0.1210 R1 = 0.1038 wR2 = 0.1365 1.023

R1 = ∑|Fo| − |Fc|∑|Fo|. wR2 = [∑w[(Fo2 − Fc2)2]/∑|[w(Fo2)2]]1/2. w = 1/[σ2(Fo2) + (0.075P)2], where P = [max(Fo2,0)+2Fc2]/3.

flexibility of H2L1 and H2L2 when they bond to lanthanide ions, resulting in the formation of polynuclear assemblies. Although H2L1 has two more methoxy groups than H2L2, complexes 1−4 have similar hexanuclear structures with six lanthanide ions enclosed by four Schiff base ligands. Meanwhile, intramolecular π−π stacking interactions between the phenylene units of H2L1 and H2L2 are not found in 1−4.

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methods. The structures were solved by direct methods and refined anisotropically using full-matrix least-squares methods with the SHELX 97 program package.8 Coordinates of the non-hydrogen atoms were refined anisotropically, while hydrogen atoms were included in the calculation isotropically but not refined. Neutral atom scattering factors were taken from Cromer and Waber.9 Synthesis of [Nd6(L1)4(OH)4(MeOH)4]·2Cl·4MeOH (1). Triethylamine (4 mL of 0.1 M EtOH solution) was added to a solution of H2L1 (0.072 g, 0.20 mmol) and NdCl3·6H2O (0.072 g, 0.20 mmol) in 30 mL of MeOH. The resulting solution was stirred and refluxed for 30 min. The mixture was then cooled to room temperature and filtered. Diethyl ether was allowed to diffuse slowly into this solution at room temperature. Yellow single crystals of 1 were collected by filtration after 2 weeks. Yield: 0.027 g (30%, based on NdCl3·6H2O). Anal. Found: C, 37.87; H, 4.15; N, 4.33. Calcd for C84H112N8O32Cl2Nd6: C, 37.62; H, 4.21; N, 4.18. Mp: >312 °C (dec). ESI-MS (CH3OH) m/z: 1208 [Nd6(L1)4(OH)4(MeOH)2]2+. IR (CH3OH, cm−1): 3310 (s), 2892 (m), 1635 (s), 1543 (s), 1501 (m), 1452 (m), 1410 (m), 1377 (w), 1230 (m), 1189 (w), 1068 (w), 950 (w), 679 (m). Synthesis of [Tb6(L1)4(OH)4(MeOH)4]·2Cl·4MeOH (2). Triethylamine (4 mL of 0.1 M EtOH solution) was added to a solution of H2L1 (0.072 g, 0.20 mmol) and TbCl3·6H2O (0.075 g, 0.20 mmol) in 30 mL of MeOH. The resulting solution was treated as above. Yellow single crystals of 2 were collected by filtration after 2 weeks. Yield: 0.032 g (35%, based on TbCl3·6H2O). Anal. Found: C, 36.78; H, 3.92; N, 4.36. Calcd for C84H112N8O32Cl2Tb6: C, 36.42; H, 4.08; N, 4.04. Mp: >320 °C (dec). ESI-MS (CH3OH) m/z:1221 [Tb6(L1)4(OH)4]2+. IR (CH3OH, cm−1): 3115 (s), 2862 (m), 1640 (s), 1542 (m), 1510 (s), 1477 (m), 1421 (m), 1350 (w), 1229 (m), 1173 (w), 1078 (w), 953 (w), 670 (m). Synthesis of [Eu6(L2)4(OH)4(MeOH)2(EtOH)2(H2O)2]·2Cl·3EtOH·H2O (3). Triethylamine (4 mL of 0.1 M EtOH solution) was added to a solution of H2L2 (0.060 g, 0.20 mmol) and EuCl3·6H2O (0.073 g, 0.20 mmol) in 30 mL of MeOH. The resulting solution was treated as above. Yellow single crystals of 3 were collected by filtration after 2 weeks. Yield:

EXPERIMENTAL SECTION

General Methods. Metal salts and solvents were purchased from Aldrich and used directly without further purification in the preparation of the free ligands and complexes. The solvents used for the photophysical investigations were dried and distilled under nitrogen. All reactions were performed under dry oxygen-free dinitrogen atmospheres using standard Schlenk techniques. The Schiff base ligands H2L1 and H2L2 were prepared as previously described.5 Physical measurements: MS, ESI, Finnigan MAT TSQ 700; IR, Nicolet IR 200 FTIR spectrometer; UV−vis, Beckman DU 640 spectrophotometer. Melting points were obtained in sealed glass capillaries under dinitrogen and are uncorrected. Elemental analyses (C, H, N) were carried out on a Varian EL elemental analyzer. Absorption spectra were obtained on a BECKMAN DU 640 spectrophotometer. Excitation and visible emission spectra were recorded on a QuantaMaster PTI fluorimeter. Near-infrared (NIR) emission spectra were measured with a one-meter SPEX 1704 spectrometer and a liquid-nitrogen cooled Ge detector, and the excitation was provided by an argon ion laser using either the UV multiline optics (333.6−363.8 nm range) or the 275.4 nm laser line. Fluorescence quantum yields were determined by using quinine sulfate (Φ = 0.546 in 0.5 mol dm−3 H2SO4) as a standard for Tb3+ complexes,6 and [Ru(bipy)3]Cl2 (bipy = 2,2′-bipyridine; Φ = 0.028 in water) as a standard for Eu3+ complexes.7 X-ray Crystallography. Data were collected on a Rigaku MiniFlex II CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 223 K. The data set was corrected for absorption based on multiple scans and reduced using standard 971

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0.043 g (50%, based on EuCl3·6H2O). Anal. Found: C, 37.39; H, 4.37; N, 4.50. Calcd for C80H108N8O26Cl2Eu6: C, 37.24; H, 4.22; N, 4.34. Mp: >285 °C (dec). IR (CH3OH, cm−1): 3418(s), 2990 (m), 2952 (m), 2855 (m), 1637 (s), 1539 (m), 1473 (m), 1422 (m), 1380 (w), 1321 (w), 1289 (s), 1233 (m), 1201 (m), 1169 (m), 1121 (w), 1103 (s), 759 (m), 640 (m). Synthesis of [Er6(L2)4(OH)4(EtOH)2(H2O)2]·2Cl·2EtOH·MeOH·H2O (4). Triethylamine (4 mL of 0.1 M EtOH solution) was added to a solution of H2L2 (0.060 g, 0.20 mmol) and ErCl3·6H2O (0.076 g, 0.20 mmol) in 30 mL of MeOH. The resulting solution was treated as above. Yellow single crystals of 3 were collected by filtration after 2 weeks. Yield: 0.032 g (37%, based on ErCl3·6H2O). Anal. Found: C, 35.69; H, 3.70; N, 4.51. Calcd for C76H96N8O24Cl2Er6: C, 35.38; H, 3.75; N, 4.34. Mp: >279 °C (dec). IR (CH3OH, cm−1): 3482(s), 2990 (m), 2927 (m), 2873 (m), 1635 (s), 1533 (m), 1478 (m), 1440 (m), 1388 (w), 1337 (w), 1290 (s), 1235 (m), 1181 (m), 1111 (s), 1025 (m), 758 (m), 637 (m).

coordinated with two Schiff base ligands. Meanwhile, Nd(1) is bound by three μ3-OH− ions and one MeOH molecule, Nd(2) by two μ3-OH− ions, and Nd(3) by one μ3-OH− ion and one MeOH molecule. Complexes 3 and 4 have similar hexanuclear structures as those observed in 1 and 2, with the general formula [Ln6(L2)4(OH)4(MeOH)x(EtOH)2(H2O)2]·2Cl (Ln = Eu (3) and Er (4)). A view of the crystal structure of 3 is shown in Figure 2. It also reveals a centrosymmetric core with

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RESULTS AND DISCUSSIONS Reactions of the Schiff base ligand H2L1 with LnCl3·6H2O (Ln = Nd and Tb) in refluxing MeOH produced yellow solutions from which the hexanuclear lanthanide complexes 1 and 2 were isolated as yellow crystalline solids in 30% and 35% yields, respectively. Reaction of H2L2 with LnCl3·6H2O (Ln = Eu and Er) gave the hexanuclear lanthanide complexes 3 and 4 in 50% and 37% yields, respectively. In complexes 1−4, the Schiff base ligands have a −3 charge with two phenolic hydroxyl groups and one backbone hydroxyl group deprotonated. The solid state structures of 1−4 were determined by single crystal X-ray crystallographic studies. Crystallographic data for all polymers are presented in Table 1, and selected bond lengths and angles are given in Tables S1−S4 in the Supporting Information. Complexes 1 and 2 are isomorphous and have the general formula [Ln6(L1)4(OH)4(MeOH)4]·2Cl·4MeOH (Ln = Nd (1) and Tb (2)). The complexes crystallize in the triclinic space group P1̅. A view of the cationic complex 1 is shown in Figure

Figure 2. A view of the crystal structure of 3. H atoms have been omitted for clarity and thermal elipsoids drawn at the 30% probability level. Symmetry operator (−x, −y, −z) generates equivalent atoms marked with “A”.

two equivalent Eu3(L2)2 moieties that are bridged by two μ3OH− anions. For each Eu3L2 moiety, Eu(1), Eu(2) and Eu(3) have coordination environments similar to Nd(1), Nd(2) and Nd(3) in 1, respectively. Eu(1) and Eu(2) are 8-coordinate with two EtOH molecules replacing the coordinated MeOH molecules in 1. For Eu(3), although (L2)3− cannot provide phenolic OMe groups to coordinate with the lanthanide ion, it is still 8-coordinate due to the coordination of one MeOH molecule. However, in 4 there are no MeOH molecules coordinated with Er(3), which has a smaller ion radius than other lanthanide ions in 1−3, resulting in a coordination number of seven for Er(3). Er(1) and Er(2) are 8-coordinate and have coordination environments similar to Eu(1) and Eu(2) in 3, respectively. Both Schiff base ligands H2L1 and H2L2 display a curved configuration in 1−4. The backbone hydroxyl oxygen atoms (O(3) or O(8) for H2L1; O(2) or O(5) for H2L2) link two or three lanthanide ions in 1−4. Differing from those polynuclear assemblies formed by the conjugate Schiff base ligands H2La,b, π−π stacking interactions are not formed between phenylene units of H2L1 or H2L2 in 1−4. The average distances for Ln−O (phenolic) (2.366 Å (1), 2.317 Å (2), 2.313 Å (3), 2.306 Å (4)), Ln−O(methoxy) (2.659 Å (1), 2.635 Å (2)) and Ln−N (2.560 Å (1), 2.537 Å (2), 2.510 Å (3), 2.475 Å (4)) are comparable to those found in the literature.3,10 All complexes 1−4 display some form of intramolecular hydrogen bonding interactions that help to create selfassembled polynuclear assemblies. The O···O distances between the phenolic group and one coordinated solvent molecule (MeOH or EtOH) in 1−4 are 2.698 Å, 2.722 Å, 2.797 Å and 2.673 Å, respectively. In the solid state, complexes 1 and 2 show different packing behaviors. Weak intermolecular π−π stacking interactions are formed in 2 with the center-to-center distance between the aromatic rings of 4.061 Å. These intermolecular π−π stacking interactions do not exist in complexes 1, 3 and 4.

Figure 1. A view of the crystal structure of 1. H atoms have been omitted for clarity and thermal elipsoids drawn at the 30% probability level. Symmetry operator (− x, − y, − z) generates equivalent atoms marked with “A”.

1. The X-ray structure of 1 reveals a centrosymmetric core with two equivalent Nd3(L1)2 moieties linked by two μ3-OH− anions. In 1, each μ3-OH− anion links three Nd ions, and the Nd−Nd separations are similar at 3.610 Å, 3.722 Å and 3.626 Å for Nd(1)−Nd(2), Nd(1)−Nd(3) and Nd(2)−Nd(3), respectively. All Nd ions are 8-coordinate. Nd(2) and Nd(3) are coordinated with three Schiff base ligands, while Nd(1) is 972

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We have probed the nature of these polynuclear lanthanide complexes in solution using high resolution ESI mass spectrometry studies. The ESI mass spectra of complexes 1 and 2 in MeOH show some [Ln6-(L1)4]2+ fragments such as [Nd6(L1)4(OH)4(MeOH)2]2+ and [Tb6(L1)4(OH)4]2+, in agreement with the crystallographic analyses. Photophysical Properties of Lanthanide Complexes. The luminescence properties of the lanthanide complexes 1−4 have been studied at 298 K both in the solid state and in CH3CN solution. The absorption spectra of free ligands H2L1 and H2L2 and complexes 1−4 are shown in Figure 3 and 4. The

Figure 5. The NIR luminescence of the Nd(III) complex 1 in CH3CN.

Figure 3. Absorption spectra of the free ligand H2L1 and complexes 1 and 2 in CH3CN.

Figure 6. Excitation spectra and visible emission spectra of the free ligand H2L1 (− − − and blue line) and the Tb(III) complex 2 (− · − and red line) in CH3CN.

excitation spectrum of 2, monitoring the intensity of the emission at 546 nm, displays one peak centered at 367 nm. The typical narrow emission bands of the Tb3+ ion (5D4 → 7Fn transitions, n = 6, 5, 4, and 3) can be detected upon excitation of the ligand-centered absorption band. The fluorescence quantum yield (Φem) for 2 is 0.17. It is noticeable that the fluorescence of the ligand-centered (sensitizer) is quenched in 2. Meanwhile, the typical Tb3+ ion excitation bands are absent in the excitation spectra. These observations suggest that the ligand-to-metal energy transfer (LMET) occurs efficiently in 2.11 The emission and excitation spectra of both H2L2 and 3 are shown in Figure 7. Excitation of the absorption band at 397 nm in the free ligand H2L2 produces a broad emission band at λmax = 493 nm. Excitation of the ligand centered absorption band at 439 nm in 3 results in visible emission bands for the Eu3+ ion (5D0 → 7Fj transitions, j = 1, 2, 3, and 4). The fluorescence of 3 is weak with quantum yield (Φem) less than 10−3. For the complex 4, no NIR emission due to Er3+ ion was observed under the experimental conditions employed.

Figure 4. Absorption spectra of the free ligand H2L2 and complexes 3 and 4 in CH3CN.

free H2L1 and H2L2 ligands exhibit absorption bands in their UV−vis spectra (i.e., 221, 261, and 327 nm for H2L1 and 215, 255, and 311 nm for H2L2). These bands are red-shifted on metal ion coordination in all lanthanide complexes. In CH3CN, 1 shows typical NIR emission bands of Nd3+ assigned to the 4 F3/2 → 4Ij/2 (j = 9, 11, 13) transition upon excitation of the ligand centered absorption band (Figure 5). The emissions at 873 and 900 nm can be assigned to 4F3/2 → 4I9/2, 1063 nm to 4 F3/2 → 4I11/2 and 1350 nm to 4F3/2 → 4I13/2 transitions of Nd3+. The free Schiff base ligand H2L1 and NdCl3·6H2O do not exhibit NIR luminescence in CH3CN under similar conditions. A similar NIR luminescence is observed for 1 in the solid state. The excitation and visible emission spectra of the free ligand H2L1 and the complex 2 in CH3CN are shown in Figure 6. Excitation of the absorption band at 373 nm of the free ligand H2L1 produces one broad emission band at λmax = 518 nm. The

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CONCLUSIONS Four hexanuclear lanthanide complexes 1−4 with flexible Schiff base ligands H2L1 and H2L2 have been prepared, and structurally characterized. In complexes 1−4, six Ln3+ anions are surrounded by four Schiff base lignds. The overall structures of these lanthanide complexes comprise two crystallographically 973

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G. E. J. Am. Chem. Soc. 2001, 123, 6179. (d) Zheng, Z. Chem. Commun. 2001, 2521. (e) Piguet, C.; Bünzli, J.-C. G. Chem. Soc. Rev. 1999, 28, 347. (f) Piguet, C. Chimia 1996, 50, 144. (3) (a) Yang, X.-P.; Jones, R. A. J. Am. Chem. Soc. 2005, 127, 7686. (b) Yang, X.-P.; Jones, R. A.; Wong, W.-K. Dalton Trans. 2008, 1676. (c) Wong, W.-K.; Yang, X.-P.; Jones, R. A.; Rivers, J. H.; Lynch, V.; Lo, W. K.; Xiao, D.; Oye, M. M.; Holmes, A. L. Inorg. Chem. 2006, 45, 4340. (d) Yang, X.-P.; Jones, R. A.; Rivers, J. H.; Wong, W.-K. Dalton Trans. 2009, 47, 10505. (e) Yang, X.-P.; Lam, D.; Chan, C.; Stanley, J. M.; Jones, R. A.; Holliday, B. J.; Wong, W. K. Dalton Trans. 2011, 40, 9795. (f) Yang, X.-P.; Hahn, B. P.; Jones, R. A.; Wong, W.-K.; Stevenson, K. J. Inorg. Chem. 2007, 46, 7050. (g) Yang, X.-P.; Jones, R. A.; Wong, W. K.; Lynch, V.; Oye, M. M.; Holmes, A. L. Chem. Commun. 2006, 1836. (h) Yang, X.-P.; Jones, R. A.; Lynch, V.; Oye, M. M.; Archie, L. H. J. Chem. Soc., Dalton Trans. 2005, 849. (4) (a) Bünzli, J.-C. G. Lanthanides Probes in Life, Chemical and Earth Sciences; Elsevier: Amsterdam, The Netherlands, 1989; Vol. 1989. (b) Guerriero, P.; Tamburini, S.; Vigato, P. A. Coord. Chem. Rev. 1995, 139, 17. (5) Lam, F.; Xu, J.; Chan, K. J. Org. Chem. 1996, 61, 8414. (6) Meech, S. R.; Philips, D. J. J. Photochem. 1983, 23, 193. (7) Nakamaru, K. Bull. Soc. Chem. Jpn. 1982, 5, 2697. (8) Sheldrick, G. H. SHELX 97, A software package for the solution and refinement of X-ray data; University of Göttingen: Göttingen, Germany, 1997. (9) Cromer, D. T.; Waber, J. T. International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, 1974; Vol. 4, Table 2.2A. (10) (a) Hogerheide, M. P.; Boersma, J.; Konten, G. V. Coord. Chem. Rev. 1996, 155, 87. (b) Sakamoto, M.; Manseki, K.; Okawa, H. Coord. Chem. Rev. 2001, 219, 379. (11) Petoud, S.; Cohen, S. M.; Bünzli, J.-C. G.; Raymond, K. N. J. Am. Chem. Soc. 2003, 125, 13324.

Figure 7. Excitation spectra and visible emission spectra of the free ligand H2L2 (− − − and blue line) and the Eu(III) complex 3 (− · − and red line) in CH3CN.

equivalent Ln3L2 units related by a center of symmetry. In 1−4, the Schiff base ligands display a curved configuration with the backbone hydroxyl oxygen atom linking two or three lanthanide ions. Complexes 1−3 display the typical emission spectra of lanthanide ions.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic files for complexes 1−4 in CIF format, high resolution ESI mass spectra of complexes 1 and 2 and selected bond lengths and angles for complexes 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*S.L.: tel, 86-771-3225152(O); e-mail, [email protected]. R.A.J.: tel, 512 471-1706; e-mail, [email protected].

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ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China (Grant No. 21161002), the Key laboratory of new processing technology for nonferrous metals and materials, Ministry of Education, Guangxi University (No. GXKFZ-02), the Guangxi Natural Scientific Foundation of China (Grant No. 0991108 and. 0832111), the Guangxi Science and Technology Agency Research Item of China (Grant No. 0895002-9) and the Robert A. Welch Foundation (Grant F-816) of the University of Texas at Austin for financial support. Single crystal X-ray data were collected using instrumentation purchased with funds provided by the National Science Foundation (CHE-0741973), USA.

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REFERENCES

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