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Synthesis, crystal structures and luminescence properties of lanthanide complexes with a tridentate salicylamide-type ligand
Inorganica Chimica Acta 391 (2012) 182–188
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Synthesis, crystal structures and luminescence properties of lanthanide complexes with a tridentate salicylamide-type ligand Yuan-Yuan Guo, Zheng-Dan Lu, Xiao-Liang Tang, Wei Dou, Wen-Wu Qin, Jiang Wu, Li-Zi Yang, Guo-Lin Zhang ⇑, Wei-Sheng Liu ⇑, Jia-Xi Ru Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 2 March 2012 Received in revised form 10 May 2012 Accepted 11 May 2012 Available online 23 May 2012 Keywords: Salicylamide-type ligand Lanthanide complexes Crystal structures Luminescence properties
a b s t r a c t Six lanthanide nitrato complexes [Ln = La (1), Nd (2), Eu (3), Gd (4), Tb (5), Er (6)] with a tridentate salicylamide-type ligand, 2-(2-(diisopropylamino)-2-oxoethoxy)-N-(pyridin-2-ylmethyl)benzamide (L), have been synthesized. All complexes were characterized by elemental analysis, IR, TGA and conductivity measurements. The structures of 2, 3 and 4 were determined by X-ray single-crystal diffraction and their general formulae were shown to be [LnL(NO3)3(H2O)]CH3CN. These data indicate the tridentate salicylamide-type ligand could effectively chelate the lanthanide ions to conform to a 1:1 metal-to-ligand stoichiometry. The luminescence properties of the europium and terbium complexes were also investigated. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction As the characteristic fluorescence of lanthanide complexes in biological systems can be easily distinguished from background fluorescence [1], an increasing number of lanthanide complexes has been designed for fluorescent labels and sensors in material and biological sciences in recent years [2]. Especially, the complexes based on europium and terbium ions are of special interest because of the particularly suitable spectroscopic properties, such as large stokes’ shifts, narrow emission profiles and long luminescence lifetimes [3]. However, the spin- and parity-forbidden nature of the f–f transitions render direct photo excitation of lanthanide ions disfavored. So it is still an important task to employ organic ligands to function as antennas by absorbing light and transferring the light energy to the excited states of the central lanthanide ions [4–6]. Judicious choice of the ligand is required to ensure that the chosen lanthanide metal is well shielded from the surrounding environment to prevent quenching of the excited state and enhance its emission [7]. As excellent antenna groups, salicylic acid or its derivatives could not only coordinate with the lanthanide ions but also sensitize the fluorescence of lanthanide metals. A tridentate salicylamide-type ligand, 2-(2-(diisopropylamino)-2-oxoethoxy)-N-(pyridin-2-ylmethyl)benzamide (L), was selected to prepare its lanthanide complexes. The luminescence properties of ⇑ Corresponding authors. E-mail address: [email protected] (G.-L. Zhang). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.05.009
the europium and terbium complexes in the solid state and the phosphorescence spectrum of the Gd(III) complex were also studied in detail, indicating that this ligand can sensitize the terbium ion suitably. Solvent effect on the terbium complex showed that organic solvent notably affected the luminescence characteristics of terbium ion. 2. Experimental 2.1. Materials Lanthanide nitrates were prepared according to the literature [8]. 2-(2-(Diisopropylamino)-2-oxoethoxy)-N-(pyridin-2-ylme thyl)benzamide (L) was similarly prepared according to the published protocole [9]. The other commercially available chemicals were of A.R. grade and used without further purification. 2.2. Methods Carbon, nitrogen and hydrogen analyses were performed using an EL elemental analyzer. Thermogravimetric analyses (TGA) were measured by a WCT-2A thermoanalyzer under air atmosphere (25–525 °C) at a heating rate of 10 °C per minute. IR spectra were recorded on a Nicolet FT-170SX instrument using KBr disks in the 400–4000 cm1 region. 1H NMR spectra were recorded on a Brucker DRX 200 spectrometer in CDCl3 solution with TMS as internal standard. Electronic spectra were recorded with a Varian
Y.-Y. Guo et al. / Inorganica Chimica Acta 391 (2012) 182–188
Cary 100 spectrophotometer in acetonitrile solution at 16 °C. Single crystals data were collected on a Bruker SMART CCD diffractometer with graphite-monochromatized Mo Ka radiation (k = 0.71073 Å) at 298 ± 2 K. The structures were solved by direct methods and refined by full matrix least-squares techniques with anisotropic thermal factors for all non-hydrogen atoms. Theoretical models fixed the hydrogen atoms of the compound. All calculations were performed using the program package SHELXTL 97. Luminescence and phosphorescence spectra were obtained on a Hitachi F-4500 fluorescence spectrophotometer. The lifetime spectrum was measured on an Edinburgh Instruments FLS920 Fluorescence Spectrometer with Nd pumped OPOLETTE laser as the excitation source.
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tate, THF, but insoluble in chloroform, dichloromethane and ethyl ether. Molar conductance data in acetonitrile indicate that all the complexes (see Table 1) act as non-electrolytes. 3.2. Thermogravimetric analyses To examine the thermal stability of the complexes, thermogravimetric analysis (TGA) was carried out on by heating the complexes from 25 °C to 525 °C at a rate of 10 °C per minute Table 2. The complexes exhibited very similar mass losses over the entire operating range respectively. The mass losses begin at ca. 152 °C, showing the complexes are stable in the room temperature. The mass losses were matched with the contents of water, which is consistent with the single crystal analysis.
2.3. Synthesis of L 3.3. IR spectra The synthetic route for the ligand L and its lanthanide complexes are shown in Scheme 1. Anhydrous K2CO3 (2.76 g, 22 mmol) was added slowly to the DMF solution of 2-hydroxy-N-(pyridin-2ylmethyl)benzamide (4.56 g, 20 mmol) at 100 °C. An hour later, a solution of 2-chloro-N,N-diisopropylacetamide (3.56 g, 20 mmol) in 10 ml DMF was added dropwise and slowly to the mixture. The reaction mixture was stirred at 100–120 °C for 12 h, and then poured into 50 ml water. The orange precipitate were collected, washed with a small amount of ethyl acetate, and then dried in vacuum, yield: 80%. 1H NMR (200 MHz CDCl3): d 9.8 (s, 1H, –NH– ), 8.5–6.9 (m, 8H, –Ar–H), 4.9 (d, 2H, –O–CH2–), 4.8 (s, 2H, –CH2– py), 3.9 (m, 1H, –CH–), 3.4 (m, 1H, –CH–), 1.3 (d, 6H, –CH3), 1.2 (d, 6H, –CH3). 2.4. Synthesis of the lanthanide complexes 1–6 A solution of 0.2 mmol L in ethyl acetate (10 ml) was added dropwise to a solution of 0.2 mmol of lanthanide nitrates (Ln = La, Nd, Eu, Gd, Tb, Er) in ethyl acetate (10 ml). The mixture was stirred at room temperature for 6 h. Then the precipitated solid complex was filtered, washed with ethyl acetate and diethyl ether, recrystallized from appropriate solvent, dried in vacuum, yield: 75– 85%. Single crystals of the europium, neodymium and gadolinium complexes were grown from acetonitrile solution with slow evaporation at room temperature. After approximately 3 weeks, colorless crystals (light purple for the Nd complex) were formed from the solution.
IR spectra of the ligand and complexes are shown in Table 3. The IR spectra of the complexes are obviously different from the ligand, but they resemble with each other, suggesting they share a similar coordination behavior. The IR spectrum of the free ligand shows bands at 1644 and 1108 cm1, which may be assigned to m(C@O) and m(Ar–O–C), respectively. In the IR spectra of the lanthanide complexes, the m(C@O) shifts by 20–29 cm1 towards lower wave numbers (red shift), In the IR spectra of the lanthanum complexes, the m(Ar–O– C) shifts by 6–9 cm1 towards higher wave numbers (blue shift), it is clear that the carboxylic group and ester carbonyl group take part in coordination at the same time [10]. The absorption bands assigned to the coordinated nitrato groups (C2v) were observed at about 1487–1489 cm1 (m1), 1318– 1330 cm1 (m4), 1036–1041 cm1 (m2) and 819–821 cm1 (m3) [11] for the complexes, respectively. In addition, the separation of two strongest frequency bands |m1–m4| is approximately 157– 169 cm1, clearly establishing that the NO3 groups in the solid complexes coordinate to the lanthanide ion as bidentate ligands [12]. No bands at 1380, 820 and 720 cm1 in the spectra of complexes indicates that free nitrate groups (D3h) are absent, in agreement with the results of the conductivity experiments. There is also a strong and wide peak of m(OH) (3418–3516 cm1) and the in-plane and out-of-plane vibration of water at ca. 818 cm1 and 571 cm1 in all the complexes, which are associated with the coordinated water [13]. 3.4. Crystal structures of complexes 2–4
3. Results and discussion 3.1. Properties of the complexes Analytical data for the newly synthesized complexes, listed in Table 1, are conformed to a 1:1 metal-to-L stoichiometry for nitrate complex. All complexes are soluble in acetone, acetonitrile, methanol, ethanol, DMF and DMSO, slightly soluble in ethyl ace-
Single-crystal X-ray diffraction investigations reveal that the two complexes all crystallize in monoclinic with space group P21/c of the monoclinic system and the results of the structure analysis of the complexes [NdL(NO3)3(H2O)]CH3CN (2), [EuL(NO3)3(H2O)]CH3CN (3) and [GdL(NO3)3(H2O)]CH3CN (4) show that they have the similar coordination sphere (Table 4), thus only the structure of the gadolinium complex is described here as an
Scheme 1.
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Table 1 Analytical and molar conductance data of the complexes (calculated values in parentheses). Complexes
%C Found (Calc.)
%H Found (Calc.)
%N Found (Calc.)
%Ln Found (Calc.)
^m (cm2 X1 mol1)
La (NO3)3LH2O Nd (NO3)3LH2O Eu (NO3)3LH2O Gd (NO3)3LH2O Tb (NO3)3LH2O Er (NO3)3LH2O
35.57(35.41) 35.46(35.14) 35.08(34.77) 34.95(34.52) 34.62(34.44) 34.59(34.05)
3.984(4.10) 3.957(4.07) 3.958(4.03) 3.964(4.00) 3.950(3.99) 3.913(3.95)
11.72(11.80) 11.76(11.71) 11.50(11.58) 11.57(11.50) 11.41(11.47) 11.43(11.35)
19.64(19.50) 20.03(20.10) 21.22(20.95) 21.36(21.52) 21.96(21.70) 22.80(22.58)
11.9 12.7 27.2 29.0 35.7 19.6
Measured in acetonitrile. Concentration is 1.0 103 mol L1.
Table 2 TGA data of the complexes (calculated values in parentheses). Complexes
Endothermic process TGA peak (°C)
La (NO3)3LH2O Nd (NO3)3LH2O Eu (NO3)3LH2O Gd (NO3)3LH2O Tb (NO3)3LH2O Er (NO3)3LH2O
154.8 154.2 153.9 153.7 152.7 151.6
Weight loss (%) Found
Calc.
2.40 2.30 2.40 2.30 2.30 2.50
2.53 2.52 2.48 2.46 2.46 2.44
example. The selected bond lengths and bond angles are given in Table 5. The single-crystal X-ray analysis of the complexes [LnL(NO3)3(H2O)]CH3CN reveal that each lanthanide atom is coordinated with ten oxygen donor atoms, six of which belong to three bidentate nitrate groups, three belong to the tridentate ligand and the remaining one to the coordinated water molecule. The coordination polyhedron around the lanthanide atom is a distorted bicapped square anti-prism. Each asymmetric unit contains one molecule of acetonitrile (see Fig. 1). The ether oxygen and the carbonyl oxygen atoms of the L could take part in the coordination with the lanthanide ions and make tridentate ligand effectively capture the metal ions to form 1:1 complexes. That is consisted with elemental analysis. The average
Table 3 IR Spectra data of the free ligand L and its complexes (cm1). Complexes
m(C@O)
m(C–O–C)
m1(NO3)
m4(NO3)
m2(NO3)
m3(NO3)
|m1 – m4|
L La (NO3)3LH2O Nd (NO3)3LH2O Gd (NO3)3LH2O Tb (NO3)3LH2O Eu (NO3)3LH2O Er (NO3)3LH2O
1644 1622 1622 1624 1624 1615 1624
1108 1114 1115 1116 1116 1115 1117
– 1487 1489 1489 1489 1487 1487
– 1318 1321 1327 1328 1326 1330
– 1036 1036 1038 1038 1041 1039
– 819 819 819 819 821 819
– 169 168 162 161 161 157
Table 4 Crystal data and structure refinement parameters for the complexes 2, 3, and 4. Complex
2
3
4
Empirical formula Formula weight T (K) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Dc (kg m3) l (mm1) F(0 0 0) Crystal size (mm) h (°) Index ranges, hkl
C23H32N7NdO13 758.80 298(2) Monoclinic P21/c
C22H30.50EuN6.50O13 745.99 298(2) Monoclinic P21/c
C23H32GdN7O13 771.81 298(2) Monoclinic P21/c
11.849(3) 17.813(5) 15.868(4) 104.718(10) 3239(15) 4 1.556 mg/m3 1.672 1532 0.27 0.23 0.21 1.75–25.25 14 to 12, 15 to 21, 18 to 19 0.0644 5864/0/408 1.176 R1 = 0.0460, wR2 = 0.0675 R1 = 0.0837, wR2 = 0.0767
11.832(2) 17.749(3) 15.827(3) 104.951(2) 3211.2(10) 4 1.543 2.021 1500 0.53 0.40 0.15 2.654–28.125 12 to 14, 16 to 21, 18 to 16 0.0645 5629/86/397 1.024 R1 = 0.0517, wR2 = 0.1322 R1 = 0.0829, wR2 = 0.1762
11.770(7) 17.687(11) 15.781(10) 105.003(8) 3173(3) 4 1.616 2.161 1548 0.65 0.32 0.10 2.663–21.701 13 to 13, 20 to 14, 18 to 18 0.0854 5528/21/402 1.066 R1 = 0.0501, wR2 = 0.1069 R1 = 0.1103, wR2 = 0.1483
Independent reflections (Rint) Data/restraints/parameter Goodness-of-fit (GOF) on F2 Final R indices [I > 2r(I)] R indices (all data)
Y.-Y. Guo et al. / Inorganica Chimica Acta 391 (2012) 182–188 Table 5 The key bond lengths (Å) and angles (°) for [GdL(NO3)3(H2O)] CH3CN. Bond lengths (Å)
Bond angles (°)
Bond lengths (Å)
Bond angles (°)
Gd(1)–O(1) Gd(1)–O(2) Gd(1)–O(3) Gd(1)–O(4) Gd(1)–O(5) Gd(1)–O(7) Gd(1)–O(8)
2.356(6) 2.555(6) 2.325(6) 2.439(7) 2.583(7) 2.479(6) 2.492(6)
Gd(1)–O(10) Gd(1)–O(11) O(1)–Nd(1)–O(2) O(2)–Nd(1)–O(3) O(4)–Nd(1)–O(5) O(7)–Nd(1)–O(8) O(10)–Nd(1)–O(11)
2.583(7) 2.526(7) 65.66(19) 61.00(2) 50.20(3) 51.10(2) 49.20(2)
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3.5. UV–Vis absorption spectra The absorption spectra of the ligand L in acetonitrile features one main band located at around 295 nm (Fig. 3), which can be assigned to characteristic p–p⁄ transitions centered on the salicylamide units. It is interesting to note that complexation with the lanthanide ions resulted in almost no shift of the maxima (296 nm for 3 and 297 nm for 5, respectively (Fig. 3)). 3.6. Luminescence studies
a
Under identical experimental conditions, the fluorescence characteristics of the ligand, europium complex in solid state, terbium complex both in the solid state and different solutions were measured at room temperature. It can be seen from Fig. 4a that when excited at 306 nm, the ‘‘free’’ ligand exhibits broad emission bands (kmax = 360 nm) in the solid state. The efficient energy transfer from the ligand to center ions (antenna effect) is one of the key factors to achieve lanthanide ion characteristic fluorescence. Unfortunately, the emission wavelengths of the europium ion characteristic fluorescence is quite weak in solid state when excited at 305 nm (Fig. 4b). But the terbium complex (5) shows strong emission both in the solid state and some solutions. When excited at 301 nm, the complex 5 shows strong emission in the solid state (Fig. 4c) which indicates that the tridentate ligand L is a good organic chelator to absorb energy and transfer them to terbium ion. In order to investigate the energy transfer processes, the phosphorescence spectrum of the Gd(III) complex was measured at 77 K in methanol solution, as shown in Fig. 5. The triplet state energy level T1 of the ligand L, which was calculated from the shortest-wavelength phosphorescence band [14], is 24,814 cm1. The energy level is above the lowest excited resonance level 5D0 of Eu3+ (17,286 cm1) and 5D4 of Tb3+ (20,545 cm1). The energy gaps 4E (Tr–Ln3+) for Eu3+ and Tb3+ were 7528 and 4269 cm1, respectively, confirming the suitability of this ligand as a sensitizer for terbium [15]. The energy gap is too high to efficiently sensitize europium ion [16,17], in agreement with the phenomenon that the luminescence characteristic emission wavelengths of the europium ion was hardly observed. Thus, the absorbed energy could be transferred from the ligand to the Tb ion (Fig. 6). The fluorescence properties of the terbium complex were also investigated in different solutions. In acetonitrile solution, the terbium complex has the strongest fluorescence, then in ethyl acetate,
b Fig. 1. The coordination sphere (30% probability ellipsoids) (a) and coordination polyhedron (b) of 4.
distance between the Gd(III) ion and the coordinated O atoms is 2.475 Å. The Gd–O (C@O) distance (2.323, 2.355 Å) are significantly shorter than the Gd–O (Ar–O–C) distance (2.556 Å). The hydrogen bonds between the coordinated water and the coordinated ligands play important roles in the crystal packing of the complexes. For each complex unit, the coordinated water as hydrogen bond donor could close link the O atom and pyridine N atom with the adjacent another complex unit to form O–H N hydrogen bonds and O–H O hydrogen bonds, respectively. In addition, the amide group as hydrogen bond donor also could connected the adjacent nitrate group to form N–H N hydrogen bonds. Therefore, as indicated in Fig. 2, these complexes are close connected by a series of intermolecule hydrogen bonds forming 2-D wavy layer supramolecule.
Fig. 2. Two-dimensional network linked by intermolecular hydrogen bond.
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Fig. 3. Absorption spectra in acetonitrile of L (black line), Eu complex 3 (red dash) and Tb complex 5 (blue line) (all concentration is 1.0 103 mol L1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
THF, methanol, DMF and DMSO solution (Fig. 7). The energy gap between the ligand triple level and the emitting level of the terbium ion may be in favor of the energy transfer process in acetonitrile solution. It can also be clearly seen that the fluorescence
Fig. 5. Phosphorescence spectrum of Gd complex (4) at 77 K (excitation and emission slit width were both 10 nm).
intensities for the terbium complex become weaker from acetonitrile, ethyl acetate, THF, methanol DMF, DMSO solution. This may be due to the effects of solvents [18] which usually occur at a metal center with vacant coordination sites. The tridentate salicylamidetype ligand forms a caverned conformation suitable for the uptake
Fig. 4. (a) Emission spectrum of L in solid state. (b) Emission spectrum of 3 in solid state. (c) Emission spectrum of 5 in solid state (all the excitation and emission slit widths were 2.5 nm).
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Fig. 8. The lifetime decay curve of the Tb complex (5) of ligand L in acetonitrile solution.
4. Conclusions
Fig. 6. Simplified energy diagram showing the lowest lanthanide excited state of the sensitizer L in the Gd(III) complex.
In conclusion, a new tridentate salicylamide-type ligand 2-(2(diisopropylamino)-2-oxoethoxy)-N-(pyridin-2-ylmethyl)benzamide (L) and its six lanthanide nitrato complexes were synthesized. Elemental analysis, IR, TGA and X-ray single-crystal diffraction indicated that the tridentate salicylamide-type ligand could effectively chelate lanthanide ions to form stable 1:1 complexes. Also, the luminescence properties of the europium and terbium complexes in the solid state indicated that L could sensitize the emission of Tb(III) more efficiently than that of Eu(III). The phosphorescence spectrum of the Gd(III) complex confirmed the suitability of this ligand as a sensitizer for terbium. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 20931003 and J0730425), the Chinese ‘‘Program for New Century Excellent Talents in University’’ (NCET09-0444), the ‘‘Fundamental Research Funds for the Central Universities’’ (lzujbky-2009-k06, lzujbky-2009-114, lzujbky-2011-22, lzujbky-2012-k13).
Fig. 7. Emission spectra of the terbium complex in different solutions at a room temperature (concentration: 1.0 104 mol L1, both the excitation and emission slit widths were 5.0 nm).
of a lanthanide ion, but this ajar cavity could not prevent absolutely the solvent molecules from entering. Together with the raising coordination abilities of solutions for the lanthanide ions, the oscillatory motions of the entering molecules consume more energy which the ligand triple level transfer to the emitting level of the lanthanide ion. Thus, the energy transfer could not be carried out perfectly [19]. The fluorescence quantum yield U of the complex [TbL(NO3)3 (H2O)]CH3CN in solid state was found to be 33.4 ± 0.1% using an integrating sphere. And the terbium complex luminescence decay in acetonitrile (Fig. 8) is best described by a single-exponential process with significantly long lifetime of s = 1.509 ± 0.001 ms, indicating the presence of one distinct emitting species. The relatively long luminescence lifetime is an indication that the ligand provides a significant level of protection from non-radiative deactivation of the lanthanide cations which is consistent with the single crystal analysis.
Appendix A. Supplementary material CCDC 854472(2), 860709(3) and 693383(4) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2012.05.009. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
D. Parker, Coord. Chem. Rev. 205 (2000) 109. J.-C.G. Bünzli, Chem. Rev. 110 (2010) 2729. J.-C.G. Bünzli, C. Piguet, Chem. Rev. 102 (2002) 1897. J.M. Lehn, Angew. Chem. Int. Ed. 29 (1990) 1304. N. Sabbatini, M. Guardiglia, J.M. Lehn, Coord. Chem. Rev. 123 (1993) 201. C. Piguet, J.-C.G. Bünzli, Chem. Soc. Rev. 28 (1990) 347. X.Q. Song, W.S. Liu, W. Dou, J.R. Zheng, X.L. Tang, H.R. Zhang, D.Q. Wang, Dalton Trans. (2008) 3582. K. Nakagawa, K. Amita, H. Mizuno, Y. Inoue, T. Hakushi, Bull. Chem. Soc. Jpn. 60 (1987) 2037. C.L. Yi, Y. Tang, W.S. Liu, M.Y. Tan, Inorg. Chem. Commun. 10 (2007) 1505. K.W. Lei, W.S. Liu, M.Y. Tan, Spectochim. Acta A 66 (2007) 118. W. Carnall, S. Siegel, J. Ferrano, B. Tani, E. Gebert, Inorg. Chem. 12 (1973) 560.
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[12] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed., John Wiley, New York, 1997. [13] N.F. Curtis, Y.M. Curtis, Inorg. Chem. 4 (1964) 804. [14] W. Dawson, J. Kropp, M. Windsor, J. Chem. Phys. 45 (1966) 2410. [15] F. Gutierrez, C. Tedeschi, L. Maron, J.-P. Daudey, R. Poteau, J. Azema, P. Tisne‘s, C. Picard, J. Chem. Soc., Dalton Trans. (2004) 1334.
[16] F.S. Rchardson, Chem. Rev. 82 (1982) 541. [17] M. Latva, H. Takalo, V.-M. Mukkala, C. Matachescu, J.C. Rodríguez-Ubis, J. Kankare, J. Lumin. 75 (1997) 149. [18] H.Q. Liu, T.C. Cheung, C.M. Che, Chem. Commun. (1996) 1039. [19] K.W. Lei, W.S. Liu, M.Y. Tan, Spectrochim. Acta, Part A 66 (2007) 590.
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Synthesis, crystal structures and luminescence properties of lanthanide complexes with a tridentate salicylamide-type ligand Yuan-Yuan Guo, Zheng-Dan Lu, Xiao-Liang Tang, Wei Dou, Wen-Wu Qin, Jiang Wu, Li-Zi Yang, Guo-Lin Zhang ⇑, Wei-Sheng Liu ⇑, Jia-Xi Ru Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 2 March 2012 Received in revised form 10 May 2012 Accepted 11 May 2012 Available online 23 May 2012 Keywords: Salicylamide-type ligand Lanthanide complexes Crystal structures Luminescence properties
a b s t r a c t Six lanthanide nitrato complexes [Ln = La (1), Nd (2), Eu (3), Gd (4), Tb (5), Er (6)] with a tridentate salicylamide-type ligand, 2-(2-(diisopropylamino)-2-oxoethoxy)-N-(pyridin-2-ylmethyl)benzamide (L), have been synthesized. All complexes were characterized by elemental analysis, IR, TGA and conductivity measurements. The structures of 2, 3 and 4 were determined by X-ray single-crystal diffraction and their general formulae were shown to be [LnL(NO3)3(H2O)]CH3CN. These data indicate the tridentate salicylamide-type ligand could effectively chelate the lanthanide ions to conform to a 1:1 metal-to-ligand stoichiometry. The luminescence properties of the europium and terbium complexes were also investigated. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction As the characteristic fluorescence of lanthanide complexes in biological systems can be easily distinguished from background fluorescence [1], an increasing number of lanthanide complexes has been designed for fluorescent labels and sensors in material and biological sciences in recent years [2]. Especially, the complexes based on europium and terbium ions are of special interest because of the particularly suitable spectroscopic properties, such as large stokes’ shifts, narrow emission profiles and long luminescence lifetimes [3]. However, the spin- and parity-forbidden nature of the f–f transitions render direct photo excitation of lanthanide ions disfavored. So it is still an important task to employ organic ligands to function as antennas by absorbing light and transferring the light energy to the excited states of the central lanthanide ions [4–6]. Judicious choice of the ligand is required to ensure that the chosen lanthanide metal is well shielded from the surrounding environment to prevent quenching of the excited state and enhance its emission [7]. As excellent antenna groups, salicylic acid or its derivatives could not only coordinate with the lanthanide ions but also sensitize the fluorescence of lanthanide metals. A tridentate salicylamide-type ligand, 2-(2-(diisopropylamino)-2-oxoethoxy)-N-(pyridin-2-ylmethyl)benzamide (L), was selected to prepare its lanthanide complexes. The luminescence properties of ⇑ Corresponding authors. E-mail address: [email protected] (G.-L. Zhang). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.05.009
the europium and terbium complexes in the solid state and the phosphorescence spectrum of the Gd(III) complex were also studied in detail, indicating that this ligand can sensitize the terbium ion suitably. Solvent effect on the terbium complex showed that organic solvent notably affected the luminescence characteristics of terbium ion. 2. Experimental 2.1. Materials Lanthanide nitrates were prepared according to the literature [8]. 2-(2-(Diisopropylamino)-2-oxoethoxy)-N-(pyridin-2-ylme thyl)benzamide (L) was similarly prepared according to the published protocole [9]. The other commercially available chemicals were of A.R. grade and used without further purification. 2.2. Methods Carbon, nitrogen and hydrogen analyses were performed using an EL elemental analyzer. Thermogravimetric analyses (TGA) were measured by a WCT-2A thermoanalyzer under air atmosphere (25–525 °C) at a heating rate of 10 °C per minute. IR spectra were recorded on a Nicolet FT-170SX instrument using KBr disks in the 400–4000 cm1 region. 1H NMR spectra were recorded on a Brucker DRX 200 spectrometer in CDCl3 solution with TMS as internal standard. Electronic spectra were recorded with a Varian
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Cary 100 spectrophotometer in acetonitrile solution at 16 °C. Single crystals data were collected on a Bruker SMART CCD diffractometer with graphite-monochromatized Mo Ka radiation (k = 0.71073 Å) at 298 ± 2 K. The structures were solved by direct methods and refined by full matrix least-squares techniques with anisotropic thermal factors for all non-hydrogen atoms. Theoretical models fixed the hydrogen atoms of the compound. All calculations were performed using the program package SHELXTL 97. Luminescence and phosphorescence spectra were obtained on a Hitachi F-4500 fluorescence spectrophotometer. The lifetime spectrum was measured on an Edinburgh Instruments FLS920 Fluorescence Spectrometer with Nd pumped OPOLETTE laser as the excitation source.
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tate, THF, but insoluble in chloroform, dichloromethane and ethyl ether. Molar conductance data in acetonitrile indicate that all the complexes (see Table 1) act as non-electrolytes. 3.2. Thermogravimetric analyses To examine the thermal stability of the complexes, thermogravimetric analysis (TGA) was carried out on by heating the complexes from 25 °C to 525 °C at a rate of 10 °C per minute Table 2. The complexes exhibited very similar mass losses over the entire operating range respectively. The mass losses begin at ca. 152 °C, showing the complexes are stable in the room temperature. The mass losses were matched with the contents of water, which is consistent with the single crystal analysis.
2.3. Synthesis of L 3.3. IR spectra The synthetic route for the ligand L and its lanthanide complexes are shown in Scheme 1. Anhydrous K2CO3 (2.76 g, 22 mmol) was added slowly to the DMF solution of 2-hydroxy-N-(pyridin-2ylmethyl)benzamide (4.56 g, 20 mmol) at 100 °C. An hour later, a solution of 2-chloro-N,N-diisopropylacetamide (3.56 g, 20 mmol) in 10 ml DMF was added dropwise and slowly to the mixture. The reaction mixture was stirred at 100–120 °C for 12 h, and then poured into 50 ml water. The orange precipitate were collected, washed with a small amount of ethyl acetate, and then dried in vacuum, yield: 80%. 1H NMR (200 MHz CDCl3): d 9.8 (s, 1H, –NH– ), 8.5–6.9 (m, 8H, –Ar–H), 4.9 (d, 2H, –O–CH2–), 4.8 (s, 2H, –CH2– py), 3.9 (m, 1H, –CH–), 3.4 (m, 1H, –CH–), 1.3 (d, 6H, –CH3), 1.2 (d, 6H, –CH3). 2.4. Synthesis of the lanthanide complexes 1–6 A solution of 0.2 mmol L in ethyl acetate (10 ml) was added dropwise to a solution of 0.2 mmol of lanthanide nitrates (Ln = La, Nd, Eu, Gd, Tb, Er) in ethyl acetate (10 ml). The mixture was stirred at room temperature for 6 h. Then the precipitated solid complex was filtered, washed with ethyl acetate and diethyl ether, recrystallized from appropriate solvent, dried in vacuum, yield: 75– 85%. Single crystals of the europium, neodymium and gadolinium complexes were grown from acetonitrile solution with slow evaporation at room temperature. After approximately 3 weeks, colorless crystals (light purple for the Nd complex) were formed from the solution.
IR spectra of the ligand and complexes are shown in Table 3. The IR spectra of the complexes are obviously different from the ligand, but they resemble with each other, suggesting they share a similar coordination behavior. The IR spectrum of the free ligand shows bands at 1644 and 1108 cm1, which may be assigned to m(C@O) and m(Ar–O–C), respectively. In the IR spectra of the lanthanide complexes, the m(C@O) shifts by 20–29 cm1 towards lower wave numbers (red shift), In the IR spectra of the lanthanum complexes, the m(Ar–O– C) shifts by 6–9 cm1 towards higher wave numbers (blue shift), it is clear that the carboxylic group and ester carbonyl group take part in coordination at the same time [10]. The absorption bands assigned to the coordinated nitrato groups (C2v) were observed at about 1487–1489 cm1 (m1), 1318– 1330 cm1 (m4), 1036–1041 cm1 (m2) and 819–821 cm1 (m3) [11] for the complexes, respectively. In addition, the separation of two strongest frequency bands |m1–m4| is approximately 157– 169 cm1, clearly establishing that the NO3 groups in the solid complexes coordinate to the lanthanide ion as bidentate ligands [12]. No bands at 1380, 820 and 720 cm1 in the spectra of complexes indicates that free nitrate groups (D3h) are absent, in agreement with the results of the conductivity experiments. There is also a strong and wide peak of m(OH) (3418–3516 cm1) and the in-plane and out-of-plane vibration of water at ca. 818 cm1 and 571 cm1 in all the complexes, which are associated with the coordinated water [13]. 3.4. Crystal structures of complexes 2–4
3. Results and discussion 3.1. Properties of the complexes Analytical data for the newly synthesized complexes, listed in Table 1, are conformed to a 1:1 metal-to-L stoichiometry for nitrate complex. All complexes are soluble in acetone, acetonitrile, methanol, ethanol, DMF and DMSO, slightly soluble in ethyl ace-
Single-crystal X-ray diffraction investigations reveal that the two complexes all crystallize in monoclinic with space group P21/c of the monoclinic system and the results of the structure analysis of the complexes [NdL(NO3)3(H2O)]CH3CN (2), [EuL(NO3)3(H2O)]CH3CN (3) and [GdL(NO3)3(H2O)]CH3CN (4) show that they have the similar coordination sphere (Table 4), thus only the structure of the gadolinium complex is described here as an
Scheme 1.
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Table 1 Analytical and molar conductance data of the complexes (calculated values in parentheses). Complexes
%C Found (Calc.)
%H Found (Calc.)
%N Found (Calc.)
%Ln Found (Calc.)
^m (cm2 X1 mol1)
La (NO3)3LH2O Nd (NO3)3LH2O Eu (NO3)3LH2O Gd (NO3)3LH2O Tb (NO3)3LH2O Er (NO3)3LH2O
35.57(35.41) 35.46(35.14) 35.08(34.77) 34.95(34.52) 34.62(34.44) 34.59(34.05)
3.984(4.10) 3.957(4.07) 3.958(4.03) 3.964(4.00) 3.950(3.99) 3.913(3.95)
11.72(11.80) 11.76(11.71) 11.50(11.58) 11.57(11.50) 11.41(11.47) 11.43(11.35)
19.64(19.50) 20.03(20.10) 21.22(20.95) 21.36(21.52) 21.96(21.70) 22.80(22.58)
11.9 12.7 27.2 29.0 35.7 19.6
Measured in acetonitrile. Concentration is 1.0 103 mol L1.
Table 2 TGA data of the complexes (calculated values in parentheses). Complexes
Endothermic process TGA peak (°C)
La (NO3)3LH2O Nd (NO3)3LH2O Eu (NO3)3LH2O Gd (NO3)3LH2O Tb (NO3)3LH2O Er (NO3)3LH2O
154.8 154.2 153.9 153.7 152.7 151.6
Weight loss (%) Found
Calc.
2.40 2.30 2.40 2.30 2.30 2.50
2.53 2.52 2.48 2.46 2.46 2.44
example. The selected bond lengths and bond angles are given in Table 5. The single-crystal X-ray analysis of the complexes [LnL(NO3)3(H2O)]CH3CN reveal that each lanthanide atom is coordinated with ten oxygen donor atoms, six of which belong to three bidentate nitrate groups, three belong to the tridentate ligand and the remaining one to the coordinated water molecule. The coordination polyhedron around the lanthanide atom is a distorted bicapped square anti-prism. Each asymmetric unit contains one molecule of acetonitrile (see Fig. 1). The ether oxygen and the carbonyl oxygen atoms of the L could take part in the coordination with the lanthanide ions and make tridentate ligand effectively capture the metal ions to form 1:1 complexes. That is consisted with elemental analysis. The average
Table 3 IR Spectra data of the free ligand L and its complexes (cm1). Complexes
m(C@O)
m(C–O–C)
m1(NO3)
m4(NO3)
m2(NO3)
m3(NO3)
|m1 – m4|
L La (NO3)3LH2O Nd (NO3)3LH2O Gd (NO3)3LH2O Tb (NO3)3LH2O Eu (NO3)3LH2O Er (NO3)3LH2O
1644 1622 1622 1624 1624 1615 1624
1108 1114 1115 1116 1116 1115 1117
– 1487 1489 1489 1489 1487 1487
– 1318 1321 1327 1328 1326 1330
– 1036 1036 1038 1038 1041 1039
– 819 819 819 819 821 819
– 169 168 162 161 161 157
Table 4 Crystal data and structure refinement parameters for the complexes 2, 3, and 4. Complex
2
3
4
Empirical formula Formula weight T (K) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Dc (kg m3) l (mm1) F(0 0 0) Crystal size (mm) h (°) Index ranges, hkl
C23H32N7NdO13 758.80 298(2) Monoclinic P21/c
C22H30.50EuN6.50O13 745.99 298(2) Monoclinic P21/c
C23H32GdN7O13 771.81 298(2) Monoclinic P21/c
11.849(3) 17.813(5) 15.868(4) 104.718(10) 3239(15) 4 1.556 mg/m3 1.672 1532 0.27 0.23 0.21 1.75–25.25 14 to 12, 15 to 21, 18 to 19 0.0644 5864/0/408 1.176 R1 = 0.0460, wR2 = 0.0675 R1 = 0.0837, wR2 = 0.0767
11.832(2) 17.749(3) 15.827(3) 104.951(2) 3211.2(10) 4 1.543 2.021 1500 0.53 0.40 0.15 2.654–28.125 12 to 14, 16 to 21, 18 to 16 0.0645 5629/86/397 1.024 R1 = 0.0517, wR2 = 0.1322 R1 = 0.0829, wR2 = 0.1762
11.770(7) 17.687(11) 15.781(10) 105.003(8) 3173(3) 4 1.616 2.161 1548 0.65 0.32 0.10 2.663–21.701 13 to 13, 20 to 14, 18 to 18 0.0854 5528/21/402 1.066 R1 = 0.0501, wR2 = 0.1069 R1 = 0.1103, wR2 = 0.1483
Independent reflections (Rint) Data/restraints/parameter Goodness-of-fit (GOF) on F2 Final R indices [I > 2r(I)] R indices (all data)
Y.-Y. Guo et al. / Inorganica Chimica Acta 391 (2012) 182–188 Table 5 The key bond lengths (Å) and angles (°) for [GdL(NO3)3(H2O)] CH3CN. Bond lengths (Å)
Bond angles (°)
Bond lengths (Å)
Bond angles (°)
Gd(1)–O(1) Gd(1)–O(2) Gd(1)–O(3) Gd(1)–O(4) Gd(1)–O(5) Gd(1)–O(7) Gd(1)–O(8)
2.356(6) 2.555(6) 2.325(6) 2.439(7) 2.583(7) 2.479(6) 2.492(6)
Gd(1)–O(10) Gd(1)–O(11) O(1)–Nd(1)–O(2) O(2)–Nd(1)–O(3) O(4)–Nd(1)–O(5) O(7)–Nd(1)–O(8) O(10)–Nd(1)–O(11)
2.583(7) 2.526(7) 65.66(19) 61.00(2) 50.20(3) 51.10(2) 49.20(2)
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3.5. UV–Vis absorption spectra The absorption spectra of the ligand L in acetonitrile features one main band located at around 295 nm (Fig. 3), which can be assigned to characteristic p–p⁄ transitions centered on the salicylamide units. It is interesting to note that complexation with the lanthanide ions resulted in almost no shift of the maxima (296 nm for 3 and 297 nm for 5, respectively (Fig. 3)). 3.6. Luminescence studies
a
Under identical experimental conditions, the fluorescence characteristics of the ligand, europium complex in solid state, terbium complex both in the solid state and different solutions were measured at room temperature. It can be seen from Fig. 4a that when excited at 306 nm, the ‘‘free’’ ligand exhibits broad emission bands (kmax = 360 nm) in the solid state. The efficient energy transfer from the ligand to center ions (antenna effect) is one of the key factors to achieve lanthanide ion characteristic fluorescence. Unfortunately, the emission wavelengths of the europium ion characteristic fluorescence is quite weak in solid state when excited at 305 nm (Fig. 4b). But the terbium complex (5) shows strong emission both in the solid state and some solutions. When excited at 301 nm, the complex 5 shows strong emission in the solid state (Fig. 4c) which indicates that the tridentate ligand L is a good organic chelator to absorb energy and transfer them to terbium ion. In order to investigate the energy transfer processes, the phosphorescence spectrum of the Gd(III) complex was measured at 77 K in methanol solution, as shown in Fig. 5. The triplet state energy level T1 of the ligand L, which was calculated from the shortest-wavelength phosphorescence band [14], is 24,814 cm1. The energy level is above the lowest excited resonance level 5D0 of Eu3+ (17,286 cm1) and 5D4 of Tb3+ (20,545 cm1). The energy gaps 4E (Tr–Ln3+) for Eu3+ and Tb3+ were 7528 and 4269 cm1, respectively, confirming the suitability of this ligand as a sensitizer for terbium [15]. The energy gap is too high to efficiently sensitize europium ion [16,17], in agreement with the phenomenon that the luminescence characteristic emission wavelengths of the europium ion was hardly observed. Thus, the absorbed energy could be transferred from the ligand to the Tb ion (Fig. 6). The fluorescence properties of the terbium complex were also investigated in different solutions. In acetonitrile solution, the terbium complex has the strongest fluorescence, then in ethyl acetate,
b Fig. 1. The coordination sphere (30% probability ellipsoids) (a) and coordination polyhedron (b) of 4.
distance between the Gd(III) ion and the coordinated O atoms is 2.475 Å. The Gd–O (C@O) distance (2.323, 2.355 Å) are significantly shorter than the Gd–O (Ar–O–C) distance (2.556 Å). The hydrogen bonds between the coordinated water and the coordinated ligands play important roles in the crystal packing of the complexes. For each complex unit, the coordinated water as hydrogen bond donor could close link the O atom and pyridine N atom with the adjacent another complex unit to form O–H N hydrogen bonds and O–H O hydrogen bonds, respectively. In addition, the amide group as hydrogen bond donor also could connected the adjacent nitrate group to form N–H N hydrogen bonds. Therefore, as indicated in Fig. 2, these complexes are close connected by a series of intermolecule hydrogen bonds forming 2-D wavy layer supramolecule.
Fig. 2. Two-dimensional network linked by intermolecular hydrogen bond.
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Fig. 3. Absorption spectra in acetonitrile of L (black line), Eu complex 3 (red dash) and Tb complex 5 (blue line) (all concentration is 1.0 103 mol L1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
THF, methanol, DMF and DMSO solution (Fig. 7). The energy gap between the ligand triple level and the emitting level of the terbium ion may be in favor of the energy transfer process in acetonitrile solution. It can also be clearly seen that the fluorescence
Fig. 5. Phosphorescence spectrum of Gd complex (4) at 77 K (excitation and emission slit width were both 10 nm).
intensities for the terbium complex become weaker from acetonitrile, ethyl acetate, THF, methanol DMF, DMSO solution. This may be due to the effects of solvents [18] which usually occur at a metal center with vacant coordination sites. The tridentate salicylamidetype ligand forms a caverned conformation suitable for the uptake
Fig. 4. (a) Emission spectrum of L in solid state. (b) Emission spectrum of 3 in solid state. (c) Emission spectrum of 5 in solid state (all the excitation and emission slit widths were 2.5 nm).
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Fig. 8. The lifetime decay curve of the Tb complex (5) of ligand L in acetonitrile solution.
4. Conclusions
Fig. 6. Simplified energy diagram showing the lowest lanthanide excited state of the sensitizer L in the Gd(III) complex.
In conclusion, a new tridentate salicylamide-type ligand 2-(2(diisopropylamino)-2-oxoethoxy)-N-(pyridin-2-ylmethyl)benzamide (L) and its six lanthanide nitrato complexes were synthesized. Elemental analysis, IR, TGA and X-ray single-crystal diffraction indicated that the tridentate salicylamide-type ligand could effectively chelate lanthanide ions to form stable 1:1 complexes. Also, the luminescence properties of the europium and terbium complexes in the solid state indicated that L could sensitize the emission of Tb(III) more efficiently than that of Eu(III). The phosphorescence spectrum of the Gd(III) complex confirmed the suitability of this ligand as a sensitizer for terbium. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 20931003 and J0730425), the Chinese ‘‘Program for New Century Excellent Talents in University’’ (NCET09-0444), the ‘‘Fundamental Research Funds for the Central Universities’’ (lzujbky-2009-k06, lzujbky-2009-114, lzujbky-2011-22, lzujbky-2012-k13).
Fig. 7. Emission spectra of the terbium complex in different solutions at a room temperature (concentration: 1.0 104 mol L1, both the excitation and emission slit widths were 5.0 nm).
of a lanthanide ion, but this ajar cavity could not prevent absolutely the solvent molecules from entering. Together with the raising coordination abilities of solutions for the lanthanide ions, the oscillatory motions of the entering molecules consume more energy which the ligand triple level transfer to the emitting level of the lanthanide ion. Thus, the energy transfer could not be carried out perfectly [19]. The fluorescence quantum yield U of the complex [TbL(NO3)3 (H2O)]CH3CN in solid state was found to be 33.4 ± 0.1% using an integrating sphere. And the terbium complex luminescence decay in acetonitrile (Fig. 8) is best described by a single-exponential process with significantly long lifetime of s = 1.509 ± 0.001 ms, indicating the presence of one distinct emitting species. The relatively long luminescence lifetime is an indication that the ligand provides a significant level of protection from non-radiative deactivation of the lanthanide cations which is consistent with the single crystal analysis.
Appendix A. Supplementary material CCDC 854472(2), 860709(3) and 693383(4) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2012.05.009. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
D. Parker, Coord. Chem. Rev. 205 (2000) 109. J.-C.G. Bünzli, Chem. Rev. 110 (2010) 2729. J.-C.G. Bünzli, C. Piguet, Chem. Rev. 102 (2002) 1897. J.M. Lehn, Angew. Chem. Int. Ed. 29 (1990) 1304. N. Sabbatini, M. Guardiglia, J.M. Lehn, Coord. Chem. Rev. 123 (1993) 201. C. Piguet, J.-C.G. Bünzli, Chem. Soc. Rev. 28 (1990) 347. X.Q. Song, W.S. Liu, W. Dou, J.R. Zheng, X.L. Tang, H.R. Zhang, D.Q. Wang, Dalton Trans. (2008) 3582. K. Nakagawa, K. Amita, H. Mizuno, Y. Inoue, T. Hakushi, Bull. Chem. Soc. Jpn. 60 (1987) 2037. C.L. Yi, Y. Tang, W.S. Liu, M.Y. Tan, Inorg. Chem. Commun. 10 (2007) 1505. K.W. Lei, W.S. Liu, M.Y. Tan, Spectochim. Acta A 66 (2007) 118. W. Carnall, S. Siegel, J. Ferrano, B. Tani, E. Gebert, Inorg. Chem. 12 (1973) 560.
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[12] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed., John Wiley, New York, 1997. [13] N.F. Curtis, Y.M. Curtis, Inorg. Chem. 4 (1964) 804. [14] W. Dawson, J. Kropp, M. Windsor, J. Chem. Phys. 45 (1966) 2410. [15] F. Gutierrez, C. Tedeschi, L. Maron, J.-P. Daudey, R. Poteau, J. Azema, P. Tisne‘s, C. Picard, J. Chem. Soc., Dalton Trans. (2004) 1334.
[16] F.S. Rchardson, Chem. Rev. 82 (1982) 541. [17] M. Latva, H. Takalo, V.-M. Mukkala, C. Matachescu, J.C. Rodríguez-Ubis, J. Kankare, J. Lumin. 75 (1997) 149. [18] H.Q. Liu, T.C. Cheung, C.M. Che, Chem. Commun. (1996) 1039. [19] K.W. Lei, W.S. Liu, M.Y. Tan, Spectrochim. Acta, Part A 66 (2007) 590.