DOI: 10.1021/cg1000045
Synthesis Structures, and Properties of Functional 2-D Lanthanide Coordination Polymers [Ln2(dpa)2(C2O4)2(H2O)2]n (dpa=2,20 -(2-methylbenzimidazolium-1,3-diyl)diacetate, C2O42- =oxalate, Ln=Nd, Eu, Gd, Tb)
2010, Vol. 10 2960–2968
Xiu-Jian Wang,* Zhong-Min Cen, Qing-Ling Ni, Xuan-Feng Jiang, Heng-Chi Lian, Liu-Cheng Gui, Hua-Hong Zuo, and Zuo-Yuan Wang School of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin 541004, China Received January 2, 2010; Revised Manuscript Received May 2, 2010
ABSTRACT: Four two-dimensional frameworks {[Nd(dpa)(C2O4)(H2O)] 3 (H2O)}n (1), {[Ln2(dpa)2(C2O4)2(H2O)2] 3 (H2O)}n (Ln = Eu (2), Gd (3), Tb (4)) (dpa = 2,20 -(2-methylbenzimidazolium-1,3-diyl)diacetate, C2O42- = oxalate) have been synthesized under hydrothermal conditions. The networks were generated via formation of sandwich-like dinuclear secondary building blocks of [Ln2(dpa)2(C2O4)(H2O)2] which are linked to each other, resulting in the formation of the subunit of Ln2O2. The variable-temperature magnetic susceptibility studies reveal that complexes 3 and 4 exhibit ferromagnetic interactions, but 2 shows the single Eu3þ ion magnetic behavior. Complexes 2 and 4 show characteristic luminescent properties. The lifetime of 2 is 2.24 ms, but that of 4 is 4.30 ms.
Introduction Over the past few years, considerable attention has been devoted to the design and synthesis of metal-organic frameworks (MOFs) of the lanthanide (Ln) family because of not only their interesting topological structures but also their many potential applications in electronics, magnetism, optics, medicine, chemistry, and biology.1-14 Even more remarkably, lanthanide-containing MOF have potential to combine and integrate some applications, such as porosity, catalytic activity, luminescent centers, and/or magnetic properties, into individual frameworks, resulting in new multifunctional materials.15-20 Carboxylate-containing ligands have received considerable study, owing to lanthanide having a high affinity for oxygen donor atoms, while carboxylates themselves have coordination versatility for tailored design of multifunctional frameworks.20-23 For example, benzene-polycarboxylate and oxalate were used extensively to develop lanthanide-based frameworks.24,25 Nevertheless, it is a formidable task to achieve tailor-made multifunctional frameworks due to lanthanide ions having high and variable coordination numbers (6 e CN e 12) and flexible coordination environments. People are interested in applying new synthetic methods to exploratory synthetic and structural studies of new materials. The underlying idea is to develop a functional organic ligand whose features, such as the shape, symmetry, and charge, would adjust the construction of MOFs, while application of the strategy of secondary building units (SUBs) has been making some important progress in the design of directionality for the multifunctional MOFs.20,26-28 It is well-known that the excellent photophysical properties of lanthanide compounds are attributed to f-f transitions with an extremely narrow bandwidth. But these electronic transitions are forbidden by parity (Laporte) selection rules, leading to weak absorbance and low quantum yields. A common way to circumvent this problem is to establish *Corresponding author. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 05/28/2010
strongly absorbing chromophoric ligands which can transfer absorbed energy efficiently to the lanthanide ions (the “antenna” effect) through a nonradiative process from the triplet excited state of the ligand to the resonant excited state of rare earth ions. The π-conjugated organic ligands, which usually have strongly absorbing chromophoric properties, such as benzene-polycarboxylate,4,29 2,20 -bipyridine4,40 -dicarboxylate,30 thiophene-2,5-dicarboxylate acid,31 and a salen-like Schiff base,32,33 were used effectively as sensitive reagents for tailoring lanthanide luminescence. The magnetic properties of rare-earth ions are also characteristic due to the presence of strong unquenched orbital angular momentum originating from f electrons which are shielded by s and p electrons. The spin-orbit coupling dominates the magnetic properties of the lanthanide system in which the weak magnetic interactions are sometimes masked by the crystal field effects on the magnetic susceptibility, resulting in the analysis of the experimental data becoming more difficult. To understand the structural and chemical factors that govern the exchange coupling between paramagnetic centers, the preparation and magnetostructural characterization of a variety of new Ln-containing complexes are essential. A large extent of the structural information obtained will be helpful to direct the design of excellent magnetic materials. The fact that the bridging carboxylato group can mediate the magnet-exchange between lanthanide ions spurs the development of carboxylate-containing Lncomplexes. The dinuclear Ln2O2 unit is a simple model for exploring the magnetic interactions between lanthanide ions. In this paper, we employed our designed zwitterionic dicarboxylate ligand 2,20 -(2-methylbenzimidazolium-1,3-diyl)diacetate (dpa)34 for the establishment of lanthanide-based frameworks. This ligand can contribute only a single negative charge to the coordination framework. The flexibility of the ligand makes it have good conformational freedom, which manifests itself in the various connecting modes that offer opportunities to create structurally diverse molecular architectures. The large π-conjugated system of the benzimidazole r 2010 American Chemical Society
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Table 1. Crystallographic Data for Complexes 1-3 formula a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z fw space group T (K) λ (Mo KR) (A˚) Dcalcd (g cm-1) R1 wR2
1
2
3
4
C14H15N2O10Nd 7.7029(2) 9.7491(3) 11.4399(3) 102.2980(10) 91.4960(10) 105.4620(10) 805.86(4) 2 515.52 P1 293(2) 0.71073 2.125 0.0248 0.0659
C28H28N4O19Eu2 9.6455(3) 13.6371(5) 13.9511(3) 66.8890(10) 88.5440(10) 70.5200(10) 1579.38(8) 2 1028.46 P1 293(2) 0.71073 2.163 0.0370 0.1025
C28H28N4O19Gd2 10.5037(9) 13.3325(11) 13.6089(12) 61.9360(10) 85.4460(10) 70.3960(10) 1577.2(2) 2 1039.04 P1 293(2) 0.71073 2.188 0.0220 0.0649
C28H28N4O19Tb2 10.5173(2) 13.2428(3) 13.6150(3) 62.6490(10) 85.6270(10) 69.6870(10) 1571.64(6) 2 1042.38 P1 293(2) 0.71073 2.203 0.0307 0.0787
moiety is expected as an effective chromophore for lanthanide luminescence. Considering that the cooperation between suitably rigid and flexible ligands is successful in creating metalorganic architectures with intriguing topological structures and/or potential applications,35,36 rigid oxalate was selected as a second ligand to undergo the construction of multifunctional architectures due to its size preference. Here, we report four lanthanide-containing 2-D frameworks consisting of SBUs of [Ln2(dpa)2(C2O4)(H2O)2] (Ln=Nd3þ (1), Eu3þ (2), Tb3þ (3), Gd3þ (4)) which are bridged by carboxylate oxygen atoms, resulting in Ln2O2 subunits. The sandwich-like geometry of the SUB, which is constructed by two pda moieties sandwiching one oxalate group through two metal ions, could control itself directionally in the formation process of a 2-D layer structure. Experimental Section Physical Measurements. The C, H, and N elemental analyses were performed on a Perkin-Elmer 240 elemental analyzer. IR spectra were recorded as KBr pellets on a Perkin-Elmer FT-IR spectrometer in the 4000-450 cm-1 range. Thermogravimetric analyses (TGA) were taken on a Perkin-Elmer Pyrisl (30-900 °C, 10 °C min-1, flowing N2(g)). The variable-temperature magnetic susceptibilities were measured on a Quantum Design MPMS-7 SQUID magnetometer in a field of 0.1 T. The luminescent spectra for the solid samples were measured at room temperature on a FL3-PTCSPC spectrophotometer with a xenon lamp as the light source. All physical properties were measured using single crystal samples. Synthesis. The general procedure for the preparation of lanthanide complexes is as follows. Suitable crystals for X-ray diffraction were obtained by heating a mixture of Ln(NO3)3 3 6H2O (0.6 mmol), Hdpa (0.124 g, 0.5 mmol), NaOH (0.028 g, 0.7 mmol), K2C2O4 (0.033 g, 0.2 mmol), and H2O (8 mL) in a Teflon-lined 25 mL autoclave at 120 °C for 2 days, followed by slow cooling to room temperature. {[Nd(dpa)(C2O4)(H2O)] 3 (H2O)}n (1). Yield: 96% (based on K2C2O4). Anal. Calcd for C14H15N2O10Nd: C, 32.59; H, 2.90; N, 5.43. Found: C, 32.48; H, 2.63; N, 5.71. IR (KBr, cm-1): 3427 (m), 2954 (w), 1680 (m), 1614 (vs), 1475 (m), 1435 (m), 1405 (s), 1351 (m), 1314 (m), 808 (m), 760 (s). {[Eu2(dpa)2(C2O4)2(H2O)2] 3 (H2O)}n (2). Yield: 90% (based on K2C2O4). Anal. Calcd for C28H28N4O19Eu2: C, 32.67; H, 2.72; N, 5.44. Found: C, 32.85; H, 2.56; N, 5.65. IR (KBr, cm-1): 3418 (m), 2954 (w), 1685 (m), 1618 (vs), 1475 (m), 1439 (m), 1408 (s), 1362 (m), 1315 (m), 1278 (m), 801 (m). {[Gd2(dpa)2(C2O4)2(H2O)2] 3 (H2O)}n (3). Yield: 96% (based on K2C2O4). Anal. Calcd for C28H28N4O19Gd2: C, 32.33; H, 2.69; N, 5.39. Found: C, 32.52; H, 2.50; N, 5.82. IR (KBr, cm-1): 3427 (w), 2940 (w), 1680 (m), 1614 (vs), 1475 (m), 1437 (m), 1405 (s), 1362 (m), 1314 (m), 1278 (m), 799 (m).
Figure 1. ORTEP plot of complex 1 showing the numbering scheme (symmetry code: (i) -x, -y þ 1, -z; (ii) -x þ 1, -y þ 2, -z; (iii) x, y þ 1, z). {[Tb2(dpa)2(C2O4)2(H2O)2] 3 (H2O)}n (4). Yield: 75% (based on K2C2O4). Anal. Calcd for C28H28N4O19Tb2: C, 32.23; H, 2.68; N, 5.37. Found: C, 32.68; H, 2.38; N, 5.53. IR (KBr, cm-1): 3429 (m), 2942 (w), 1693 (m), 1635 (vs), 1607 (vs), 1475 (m), 1443 (m), 1410 (s), 1361 (m), 1313 (m), 1280 (m), 802 (m). X-ray Crystallography. Reflection intensity data for complexes 1-4 were collected at 293 K on a Rigaku Mercury CCD with graphite monochromated Mo KR radiation (λ = 0.71073 A˚) using the ω technique. All the structures were solved by direct methods and refined by full-matrix least-squares against F2 of all data using the SHELXTL program package. Anisotropical thermal factors were assigned to non-hydrogen atoms. The positions of hydrogen atoms were generated geometrically, assigned isotropic thermal parameters, and allowed to ride on their respective parent atoms before the final cycle of least-squares refinement. The large residual electron density peaks were associated with lanthanide atoms. Crystallographic data and structural determination summaries are listed in Table 1. Selected bond lengths are listed in Table S1 of the Supporting Information.
Results and Discussion Structural Description of 1-3. Single-crystal X-ray diffraction studies indicate that these three complexes are isostructural with a two-dimensional framework containing a second building unit of [Ln2(C2O4)(dpa)2(H2O)2]. Thus, as an example, only the structure of 1 is described here in detail. The asymmetric unit of 1 contains one Nd3+, one dpa-, one oxalate (two halves) group, as well as one coordinated and one crystalline aqua molecules (Figure 1). The dpa- group takes a (κ2-κ1-μ2)- (κ1)-μ3 coordination mode (Scheme 1) bridging three Nd3þ ions, while the oxalate group shows a
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bis-bidentate coordination mode linking two Nd3þ ions. Two Nd3þ ions are linked by two dpa- ligands together with one C2O42-, forming a centrosymmetric dinuclear secondary building unit of [Nd2(dpa)2(C2O4)(H2O)2] with a sandwichlike structure in which the distance of Nd 3 3 3 Nd is 6.223 A˚ (Figure 2a). These SBUs are connected to each other along the b axis through double chelating/bridging carboxylate oxygen atoms, resulting in a one-dimensional chain containing a Nd2O2 subunit with an edge-sharing polyhedron (Figure 2b). The Nd 3 3 3 Nd distance and Nd-O-Nd angle within the Nd2O2 subunits are 4.29(2) A˚ and 111.2(1)°, respectively. So the chain is also described as that neighboring SUBs are connected to each other through a dinuclear Nd2O2 subunit. Adjacent chains are alternatively connected by C2O42- groups into a two-dimensional layer, exhibiting a ladder-like network with the chains as sharing sidepieces and the oxalate ligands as rungs (Figure 2c). The nearest distance Scheme 1
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of Nd 3 3 3 Nd between chanis is 6.38(1) A˚. The Nd-O bond lengths are in the range of 2.41(3)-2.68(3) A˚. The crystalline water molecules are located between layers. Neighboring layers interact through π-π interactions between parallel phenyl and imidazole rings (the ring-centroid distance of 3.49(3) A˚), and O 3 3 3 O hydrogen bonding between the crystalline water molecules and carboxylate oxygen atoms or coordinated water molecules (Table S2 of the Supporting Information). The Nd atom is nine-coordinated with a distorted monocapped square antiprism which is defined by nine oxygen atoms from two chelating/bridging carboxylate groups, one monodentate carboxylate group, two oxalate groups, and one coordinated aqua molecule. The O2 O3i O7 O9 (i = -x, -y þ 1, -z) and O3iii O5 O6i O8ii (ii = -x þ 1, -y þ 2, -z; iii = x, y þ 1, z) sets of atoms define the quasi-square base and upper face, respectively, and O4i achieves the capping of the polyhedron. But in complex 2, the asymmetric unit contains two crystallographically independent Eu3þ ions (Figure 3). Two symmetry-related Eu3þ ions are linked by ligands to construct a centrosymmetric dinuclear secondary building unit of [Eu2(dpa)2(C2O4)(H2O)2], and such SBUs are connected to each other along the a axis, resulting in a one-dimensional chain with Eu2O2 subunits (Figure S1a). The average Eu 3 3 3 Eu distance and Eu-O-Eu angle within the Eu2O2 subunits are 4.22(2) A˚ and 111.5(1)°, respectively. Adjacent chains with crystallographically different Eu3þ ions (Eu1 and Eu2) are alternatively connected by C2O42- groups into a two-dimensional layer (Figure S1b). π-Stacking interactions
Figure 2. (a) Sandwich-like SUB structure of [Nd2(dpa)2(C2O4)(H2O)2]. (b) View of the 1D chain of Nd3þ with Nd2O2 subunits. (c) View of the 2D structure of 1.
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Figure 3. ORTEP plot of complex 2 showing the numbering scheme (symmetry code: (i) 1 þ x, y, z; (ii) 2 - x, -y, 1 - z; (iii) 3 - x, -y, 1 - z; (iv) 1 - x, 1 - y, -z).
and O 3 3 3 O hydrogen bonding between neighboring layers are also present (Table S2 of the Supporting Information). In complex 3, the asymmetric unit also contains two Gd3þ ions (Figure 4). Different from 1 and 2, the SUB of [Gd2(C2O4)(dpa)(H2O)2] in 3 contains two crystallographically independent Gd3þ ions, each of which is linked together with its symmetric atom from a neighboring SUB through double chelating/bridging carboxylate oxygen atoms, resulting in two crystallographically independent Gd2O2 subunits located alternatively in the one-dimensional chain (Figure S2). The average Gd 3 3 3 Gd distance and Gd-O-Gd and Gd-O-C angles within the Gd2O2 subunits are 4.25(2) A˚ and 112(1)° and 89.9(3)°, respectively. The bond length of Gd-O is in the range 2.348(2)-2.744(3) A˚. π-Stacking interactions and O 3 3 3 O hydrogen bonding between neighboring layers are also present (Table S2). Structure Description of Complex 4. Complex 4, [Tb2(dpa)2(C2O4)2(H2O)2 3 (H2O)]n, is also a two-dimensional structure containing SBUs of [Tb2(dpa)2(C2O4)(H2O)2] which are constructed by two dpa- and one oxalate groups linking two different Tb3þ ions as shown in Figure 5a. Nevertheless, the dpa ligands in 4 act as two types of coordination modes of (κ2-κ1-μ2)-(κ1)-μ3 and (κ1-κ1-μ2)(κ1)-μ3 (Scheme 1). Each SBU contains two different Tb3þ ions (Tb1 and Tb2) with the Tb1 3 3 3 Tb2 distance of 6.23(1)A˚. Tb2 and its symmetry-related atom from an adjacent SUB are connected by double chelating/bridging carboxylate oxygen atoms (the formation of Tb2O2 unit) while Tb1 and its symmetry-related atom from another adjacent SUB are bridged by double μ1,3 carboxylic groups (the formation of double O-Tb-O bridges), resulting in a one-dimensional chain (Figure 5b). So the chain is also described as neighboring SUBs being connected to each other by the dinuclear Tb2O2 subunit and the double O-Tb-O bridges in an alternating way, which is a big difference from the case for the above-mentioned three complexes. Adjacent chains are further linked by bischelating oxalate groups into a two-dimensional layer (Figure 5c). The distance of Tb 3 3 3 Tb and the angle of Tb-O-Tb in the Tb2O2 subunit are 4.12(2)A˚ and 110.7(1)°, respectively. Tb1 is eight-coordinated by oxygen atoms (O1 O7 O8i O10 O13 O14 O9 O17 (i = 1 - x, 1 - y, -z)) in a geometry of distorted trigonal dodecahedron, which is a common
Figure 4. ORTEP plot of complex 3 showing the numbering scheme (symmetry code: (i) 1 - x, 1 - y, 1 - z; (ii) -x, -y, 2 - z; (iii) -1 þ x, y, z).
structure for eight-coordination while Tb2 is nine-coordinated in a distorted square antiprismatic polyhedron with the O3ii O3iii O4ii O12 and O15ii O16ii O11 O18 (ii = 1 þ x, y, z; iii = 1 - x, -y, 1 - z) sets of atoms defining the quasi-square base and upper face, respectively, and O5ii achieving the capping of the polyhedron. The bond distances of Tb-O are in the range 2.32(1)-2.57(2) A˚. Thermogravimetric Analysis (TGA) of Complexes. TGAs of 2-4 have been carried out in the range of 30-900 °C for all three compounds, which show similar behaviors (Figure S3 of the Supporting Information). From room temperature to about 235 °C, all three materials lose approximately 8% of their initial weight, corresponding to the loss of noncoordinated and coordinated water molecules; the second weight loss begins at about 330 °C for complex 2 but at about 360 °C for complex 3 and 4, where the decomposition of the residue starts. A distinction between the coordinated and uncoordinated water molecules was not clearly observed from the TGA data. This result is in agreement with the presence of strong hydrogen bonds between the coordinated and the solvated H2O molecules. Photoluminescence. The solid-state photoluminescence spectra of 2 and 4 were recorded at room temperature and depicted in Figures 6 and 7, respectively. The emission spectra of 2 upon excitation at 460 nm exhibit characteristic bands of Eu3þ assigned to the 5D0 f 7F0,1,2,3,4 transition. The 5 D0 f 7F0 transition (580 nm) is weak. According to the selection rules for electric dipole transitions, the 5D0 f 7F0 transition can be observed only if the symmetry group of the complex is Cnv, Cn, or Cs. Combined with the diffraction study of the triclinic crystal system, the actual symmetry of the Eu3þ site could be the lowest site symmetry: C1. The 5 D0 f 7F1 transition, primarily magnetic dipole in nature and independent of the ligand field effects, is comprised of two peaks (589, 593 nm) with the same intensity, while the
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Figure 5. (a) ORTEP plot of complex 4 showing the numbering scheme (symmetry code: (i) 1 - x, 1 - y, -z; (ii) 1 þ x, y, z; (iii) 1 - x, -y, 1 - z); (b) 1D chain containing SUBs of [Tb2(dpa)2(C2O4)(H2O)2], showing the different modes of connection via Tb2O2 and O-Tb-O moieties. (c) View of the 2D structure of 4.
Figure 6. Solid state emission spectra for 2 at room temperature upon excitation at 460 nm. Inset: solid state excitation spectra monitored at λem = 618 nm.
D0 f 7F2 transition, sensitive to the crystal field symmetry and essentially a pure electric dipole in nature, consists of an intense band (618 nm) with one weak shoulder at lower frequency. The splitting of the 5D0 f 7F1 transition indicates a strong crystal field. The intensity of the hypersensitive 5 D0 f 7F2 transition is much larger than that of the 5D0 f 7 F1 transition, indicating a highly polarizable chemical environment around the Eu3þ ion. These results are in 5
Figure 7. Solid state emission spectra for 4 at room temperature upon excitation at 370 nm. Inset: solid state excitation spectra monitored at λem = 544 nm.
agreement with the crystallographic analysis that the Eu3þ ions occupy a low-symmetry site without an inversion center. Less intense, broad peaks are observed for the 5D0 f 7F3 (652 nm) and 5D0 f 7F4 (695 nm) transitions. The excitation spectra of 2, monitored at λem = 618 nm, the most intense emission line of the Eu3þ5D0 f 7F2 transition, display sharp lines between 350 and 550 nm assigned to 7F f 5D, 7F f 5L, and 7F f 5G intra 4f6-4f6 transitions.37,38 The sharp band of 306 nm is attributed to the S0 f S1 transition of the dpa ligand.
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Figure 9. Temperature dependence of χM and χMT for 2.
Figure 8. Emission spectra of 3 upon excitation at 380 nm.
Under excitation of 370 nm, complex 4 exhibits characteristic Tb3þ emission spectra at about 489 (5D4 f 7F6), 544 (5D4 f 7F5), 586 (5D4 f 7F4), and 619 nm (5D4 f 7F3). The dominant peak corresponding to the 5D4 f 7F5 transition at 544 nm is hypersensitive, giving an intense green luminescence output for the sample. The 5D4 f 7F2 (655 nm), 5D4 f 7 F1 (679 nm), and 5D4 f 7F0 (705 nm) transitions are weak but measurable intensities. The excitation spectra of 4 were monitored with the Tb3þ5D4 f 7F5 (544 nm) transition, and the line bands between 300 and 500 nm can be attributed to 7 F f 5D0, 7F f 5L10, and 7F f 5G intraconfiguration forbidden 4f8 f 4f8 transitions of the Tb3þ ion.38 The bands between 230 and 300 nm are assigned to π f π* transitions of the dpa ligand. The emission decay profiles of complexes 2 and 4 in the solid state were obtained at room temperature. The decay curves are well fitted into a single-exponential function. The lifetime for complex 2 is 2.24 ms, whereas that for complex 4 is about 4.30 ms (determined by monitoring the 5D0 f 7F2 and 5D4 f 7F5 transitions, respectively). These are comparable to those of other corresponding Eu3þ and Tb3þ complexes.39,40 It is well-known that the role of the triplet state of the ligand is very important for intramolecular energy transfer from the triplet state to the resonance level of a lanthanide ion. To test this assignment of the emission bands, we have carried out photoluminescence studies on Hpda. When excited at 288 nm, Hpda exhibited a wide peak at 381 nm (the lifetime is 0.24 ns) assigned to the intraligand π* f π or π* f n transitions. The triplet state energy of the free ligand was not observed at room temperature, but the triplet state energy of the ligand was determined from the phosphorescence spectrum of the Gd3þ complex, since Gd3þ can sensitize the phosphorescence emission of the ligand while the Gd3þ complex exhibits a high phosphorescence-fluorescence ratio for ligands compared to the other Ln3þ complexes.41,42 As observed in Figure 8, the triplet state of the ligand is centered at 474 nm (the lifetime is 0.12 ms) under excitation at 380 nm. This energy (21097 cm-1) is higher than the resonant energy level of Eu3þ and Tb3þ, while the ligandcentered emission of the triplet state has completely disappeared in 2 and 4, showing that the efficient energy
Figure 10. Temperature dependence of χM and χMT for 3. Inset: Magnetic-field dependence of the molar magnetization observed at 2 K.
transfer could happen from the triplet state to the resonance level of a lanthanide ion. Magnetic Properties. The spin-orbital coupling in general plays an important role in the magnetism of lanthanide complexes due to the internal nature of the valence f orbitals. This large spin-orbit coupling partly removes the degeneracy of the 2Sþ1L group term of lanthanide ions, giving 2Sþ1LJ states, which further split into Stark levels under the crystalfield perturbation. In most cases, analysis of the magnetic interaction between rare-earth ions gives rise to additional difficulties due to the magnitude of such an interaction being comparable to that of the crystal field effect and due to the population of the first excited state for magnetic contribution. The variable-temperature magnetic susceptibilities were measured at an applied magnetic field of 1000 Oe in the temperature range of 2.0-300 K for 2 and 4 (Figures 9 and 11) but in the temperature range of 1.8-300 K for 3 (Figure 10). For complex 2, the χMT value is equal to 2.82 emu K mol-1 at 300 K, which corresponds to the value of two isolated Eu(III) ions (the theoretical value is 3 emu K mol-1) calculated from the Van Vleck equation allowing for population of the lower excited state. As the temperature cools, χMT decreases continuously as a result of the depopulation of the Stark levels and is close to zero at 2.0 K, corresponding to a nonmagnetic ground state of 7F0. By comparing the curves of χM and χMT versus T with those of Eu(III) mononuclear complexes, it is interesting to find that they are very similar.24,43 So the χM and χMT versus T curves have been
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Figure 11. Temperature dependence of χM and χMT for 4. Inset: Magnetic-field dependence of the molar magnetization observed at 2 K.
analyzed on the basis of eq I deduced from the Eu(III) ion in the monomeric system over the whole temperature range.44 χM ¼ ðNβ2 =3kTxÞ½24 þ ð27x=2 - 3 =2 Þe - x þ ð135x=2 - 5 =2 Þe - 3x þ ð189x - 7 =2 Þe - 6x þ ð405x - 9 =2 Þe - 10x þ ð1485x=2 - 11 =2 Þe - 15x þ ð2457x=2 - 13 =2 Þe - 21x =½1 þ 3e - x þ 5e - 3x þ 7e - 6x þ 9e - 10x þ 11e - 15x þ 13e - 21x þ TIP
ðIÞ
where x = λ/kT, with λ being the spin-orbital coupling parameter between the ground and excited states through the Zeeman perturbation and TIP being the temperature independent magnetism. The λ value is the energy gap between the 7F1 and 7F0 free-ion states. The least-squares fitting leads to the spin-orbital parameter λ = 253(2) cm-1 and TIP = -3 4.07 factor R of 1.3 10-4 (R = P 10 with an agreement 2 P [(χMT)obsd - (χMT)calcd] / (χMT)obsd2). This λ value is the lowest one among the reports for the Eu complexes known to us but comparable to the value (263 cm-1) deduced from the energy difference between the ground state 7F0 and the lowest-lying split component of 7F1 caused by the crystal field perturbation (see the discussion on the emission spectra). Normally the λ value reported is about 370 nm based on the average of the three components arising from the 7F1 state. Nevertheless, the value of λ g 420 cm-1 is not observed currently. The different λ values could originate from the different crystal field effect and/or site symmetry. The magnetic result in 2 demonstrates further that the Eu3þ ions are well isolated from each other in the magnetic molecule field, even if the Eu 3 3 3 Eu distance is rather short.43 For complex 3, the value of χMT at 300 K was 16.13 emu K mol-1, close to that of two isolated Gd(III) S = 7/2 spins. As the temperature cools, χMT remains almost constant until about 4 K, and upon further cooling, the χMT value dramatically increases to a maximum of 16.98 emu K mol-1 at χM ¼
Ng2 β2 SðS þ 1ÞðW1 þ W2 Þ þ TIP 3kT ð1 - u2 Þ2
SðS þ 1Þ ¼ 6
ðIIÞ
1.8 K. This behavior indicates the weak ferromagnetic coupling between Gd3þ ions. The inverse magnetic susceptibility (χ-1) obeys the Curie-Weiss law over the whole temperature range, giving a positive Weiss constant (θ) of 0.017 K and a Curie constant (C) of 16.11 emu K mol-1. The structural parameters (the Gd-O-C and Gd-O-Gd angles, the Gd 3 3 3 Gd distance) within Gd2O2 units also feature the ferromagnetic couple.45 Taking no account of the slight difference between crystallographically independent Gd2O2 subunits, the 2-D layer structure can be considered as formed by bridging Gd2O2 subunits uniformly. So we try to fit the magnetic data based on the isotopic spin Hamiltonian H = -2JSGdSGd and by means of the analytical expression (eq II) derived by Curely for an infinite 2D layer structure.46,47 When this Gd2O2 unit is treated as a magnetic unit, S(S þ 1) in eq II can be substituted by eq III, which elucidates the magnetic interactions between Gd ions in the dinulcear Gd2O2 unit.48,49 In eqs II and III, μ = coth[J1S(S þ1)/kT] - kT/J1S(S þ1), W1 = (1 þ u2)2 þ 4u2, and W2 = 4u(1 þ u2), with J being the exchange coupling parameter between Gd3þ ions in the Gd2O2 subunit while J1 is the exchange coupling parameter between adjacent Gd3þ ions linked by oxalate groups and all of the other parameters have their usual meanings. The leastsquares fit over the whole temperature range gave the values -4 of g = 2.01, J = 0.003, J1 = -0.0042, and TIP = P5.4 10 , -3 with an agreement P factor R of 6.04 10 (R = [(χMT)obsd - (χMT)calcd]2/ (χMT)obsd2). The positive J value indicates the weak ferromagnetic interaction within the Gd2O2 subunit, and the negative J1 value demonstrates the antiferromagnetic mediation of the bis-chelating oxalate groups. The ferromagnetic interactions of the Gd2O2 subunit in 3 are comparable with those in complex [Gd2(O2Fc)2(O2Fc)4(MeOH)2],50 maybe originating from their similar structural parameters around the [Gd2O2] moiety. The magnetization at 2 K saturating rapidly supports the occurrence of a ferromagnetic interaction. Although the theoretical studies for the magnetic interaction between Gd3þ ions have been undergoing,22,45 the main structural parameters that govern the Gd 3 3 3 Gd magnetic interaction are not yet fully determined. Many examples show that the value of the angle at the oxo bridge and the Gd 3 3 3 Gd separation play the major role for magnetic interaction. For complex 4, the χMT value at 300 K of 24.79 cm3 K mol-1 is slightly larger than the expected value of 23.64 cm3 K mol-1 of two isolated Tb3þ ions in the 7F6 state, and it gradually increases with decreasing temperature to reach a maximum of 38.31 cm3 K mol-1 at 14 K, before decreasing to 26.84 cm3 K mol-1 at 2.0 K. The increase of the χMT product suggests the presence of a dominant intramolecular ferromagnetic interaction between Tb3þ ions. The low temperature decrease is likely due to a combination of the depopulation of Stark levels of Tb3þ ions and the presence of interlayer antiferromagnetic interactions. The plot of χM -1 vs T above 70 K obeys the Curie-Weiss law [χM = C/(T - θ)] with C = 22.93 emu K mol-1 and θ = 20.81 K. The C value is comparable with the two Tb3þ ions with noninteraction, and the large positive θ indicates a ferromagnetic coupling
140e56J=kT þ 91e42J=kT þ 55e30J=kT þ 30e20J=kT þ 14e12J=kT þ 5e6J=kT þ e2J=kT 15e56J=kT þ 13e42J=kT þ 11e30J=kT þ 9e20J=kT þ 7e12J=kT þ 5e6J=kT þ 3e2J=kT þ 1
ðIIIÞ
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Crystal Growth & Design, Vol. 10, No. 7, 2010
between Tb3þ ions. The ferromagnetic interaction is also confirmed by the isothermal magnetization at 2.0 K, which increases rapidly and saturates with the field up to 5 T. Nevertheless, this saturation value (16.02 Nβ) is less than the expected value of 18 Nβ for two isolated Tb3þ ions (9 Nβ for each Tb3þ ion for J = 6, g = 3/2). The χM0 (in-phase) and χM00 (out-of-phase) vs T plots show no obvious frequency dependence. A magnetization experiment at 2.0 K under an external field in the (5 T range shows normal behavior for a ferromagnetic system, without spontaneous magnetization at zero field or hysteresis (Figure S4). We deduce that the ferromagnetic interactions in 4 maybe result from the Tb2O2 subunits, based on those in 3 due to their similar geometric structures. The coupling interaction is high enough to compensate for the depopulation of the Stark level, and that the maximum of χMT is greatly higher than the value at room-temperature implies that the ferromagnetic coupling is strong. Conclusion. In this paper, we developed four lanthanidecontaining complexes with SUBs of [Ln2(dpa)2(C2O4)(H2O)2], of which the sandwich-like configuration could be appropriate for the formation of a Ln2O2 subunit. The efficient energy transfer from the triple state of the zwitterionic ligand of pda to the resonance level of lanthanide ions results in complexes 2 and 4 exhibiting characteristic luminescent properties of Eu3þ and Tb3þ. Complexes 3 and 4 exhibit ferromagnetic coupling interactions between Ln3þ ions. We deduce that the ferromagnetic interactions in 4 maybe result from the Tb2O2 subunits, based on the ferromagnetic interactions between Gd3þ ions in the Gd2O2 subunit for 3, due to their similar geometric structures. Nevertheless, the energy separation between the 2Sþ1LJ ground state and the first excited state for Eu3þ is so weak that the excited state can be thermally populated in the magnetism, which is different from the cases of most other lanthanide ions, in that these energy separations are so large that only the ground state is thermally populated. So complex 2 reveals the single ion magnetic behavior as most of other Eu complexes do, though the Eu 3 3 3 Eu distance is rather short. Further works are underway for new lanthanide complexes with excellent luminescence and magnetism. Acknowledgment. This work is financially sponsored by the National Natural Foundation of China (No. 20463001 and 20863001), the Natural Foundation of Guangxi Province (No. 0832100, 2010GXNSFF013001), and the Programme for Excellent Talents in Guangxi Higher Education Institutions. Supporting Information Available: Crystallographic data for 1-4 in CIF format, the chain and layered structures of 2 and 3, TGA curves of complexes 2-4, the magnetization vs applied field for 4, and selected bond lengths and hydrogen bonds of complexes 1-4. This material is available free of charge via the Internet at http:// pubs.acs.org.
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