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有点长，请高手帮忙一下，呵呵 Abstract Solid complexes of lanthanide picrates with a new amide type tripodal ligand, 2,2 ,2 -nitrilotris-(N-phenylmethyl)-acetamide (L) have been prepared. The X-ray single-crystal di?raction analysis indicates that the ML2 type complexes [PrL2(Pic)](Pic)2 and [NdL2(Pic)](Pic)2 are isomorphous. And the ML2 complex units are hydrogen bonded to give infinite one-dimensional zigzag supra- molecular chains which are further linked by the interchain hydrogen bonds and p–p interactions of the picrate groups to form two- dimensional layer. The luminescent property of the Eu (III) complex is described. 2005 Elsevier B.V. All rights reserved. Keywords: Lanthanide picrate complexes; Tripodal ligand; Crystal structure; Luminescent properties Podand-type ligands have drawn much attention in recent years, mainly due to their selective coordinating capacity, spheroidal cavities and hard binding sites, therefore stabilizing their complexes, acquiring novel coordination structure and shielding the encapsulated ion from interaction with the surroundings [1]. Among numerous podands which have demonstrated their po- tential use in functional supramolecular chemistry [2], amide type podands are important for preparing the rare earth complexes possessing strong luminescent properties. The design and synthesis of luminescent lan- thanide complexes have attracted considerable current attention due to their potential uses as supramolecular devices, as fluorescent sensors, or as luminescent probes [3]. In order to obtain strongly luminescent complexes, the chromophoric ligands which chelate to lanthanide metals should be able: (1) to encapsulate and protect the lanthanide ion from the solvent molecules and (2)to absorb energy and transfer it e?ciently to the central metal [4]. Our group is interested in the supramolecular coordination chemistry and the luminescent properties of lanthanide (III) ions with the amide type tripodal ligands which possess spheroidal cavities and hard bind- ing sites. As a part of our systematic studies, this paper reports the structures and luminescent properties of the lanthanide picrate complexes with a new tripodal ligand 2,2 ,2 -nitrilotris-(N-phenylmethyl)-acetamide (L). It is noteworthy that the crystal structures of [PrL2(Pic)] (Pic)2 and [NdL2(Pic)](Pic)2 demonstrate 1:2 (M:L) type coordination structures. To our best knowledge, it is the first example of 1:2 (M:L) coordination structure of lan- thanide picrate complexes with tripodal ligand. Thus, the lanthanide ion could be e?ectively encapsulated and protected by the coordinated ligands. Lanthanide picrates [5] and (bis-carboxymethyl- amino)-acetic acetate [6] were prepared according to the literature method. The synthetic route for the tripo- dal ligand L and the complexes 1–5 (Ln = Pr, Nd, Eu, Gd, Tb) is shown in Scheme 1 [7]. All complexes were carefully investigated by elemental analysis and spectral characterization. The analytical data for the newly syn- thesized complexes indicate that the five complexes all conform to a 1:2 metal-to-ligand stoichiometry. And the molar conductance values of the complexes indicate the presence of a 1:2 type electrolyte [8]. Thus, the for- mula of the complexes 1–5 can be denoted as [LnL2(Pic)](Pic)2. The five complexes have similar IR spectra, of which the characteristic bands have similar shifts [9], suggesting that they have a similar coordina- tion structure. Slow diffusionof diethyl ether into the ethyl acetate solution of the Pr complex (1) and Nd complex (2) a?orded the block crystals [10]. The single-crystal X- ray analysis of the complexes [PrL2(Pic)](Pic)2 (1) and [NdL2(Pic)](Pic)2 (2) reveal that they are isomorphous with the central metal Pr or Nd atom coordinated with nine donor atoms, eight of which belong to the two tet- radentate ligands including one nitrogen atom and three oxygen atoms from carbonyl groups and the remaining one to oxygen atom of one monodentate picrate group. The other two picrate groups act as the counter anions. The coordination sphere of the complex 1 and 2 is shown in Fig. 1(a). The coordination polyhedron around Pr or Nd is a distorted monocapped antisquare prism (Fig. 1(b)). In both complexes, the ligand L exhibits a tripodal coordination mode [11] with three oxygen atoms and one amino nitrogen atom as donors. Thus, the tertiary nitrogen atom as well as its three acetyl benzyl amine arms form a cone-shaped cave with the metal atom lying out of the trigonal plane defined by the three oxy- gen atoms from carbonyl groups. The average Ln–O (c=o)distances (2.487 (1) and 2.472 A (2)) are signif- icantly shorter than Ln–N distances (2.765 (1) and 2.751 A (2)), respectively. A similar phenomenon was reported for TbL (Pic)3 (L = 2-(bis-dibutylcarbamoyl- methyl-amino)-N,N-dibutyl-acetamide) [12]. The con- figurations of the two ligands in one complex molecule are both pincer-like configurations because the O–Pr–O angles (70.30, 97.76, 124.46 and 71.40, 96.20, 125.76) and O–Nd–O angles (70.53, 97.86, 124.00 and 71.58, 96.66, 125.96) are both quite di?erent from each other [13]. The hydrogen bonds and p–p interactions between the coordinated ligands and picrate groups play important roles in the crystal packing of the com- plexes. In 1 (or 2), atoms O(17), O(18) of the one free picrate group and O(25) of the other free picrate an- ion act as hydrogen bond acceptors to form O...H– N(2) [O(17)...H, 2.61 (1) and 2.61 A (2), O(18)...H, 2.42 (1) and 2.41 A (2), O(17)...H–N(2), 155.7 (1)and 155.0 (2), O(18)...H–N(2), 147.7 (1) and 148.7 (2)] and O(25)...H–N(4) [O(25)...H, 2.09 (1) and 2.07 A (2), O(25)...H–N(4), 175.1 (1) and 175.1 (2)], respectively, with a neighboring molecule, where N(2) and N(4) of ligand are the hydrogen donors [14], thus generating a one-dimensional supramolecu- lar zigzag chain as shown in Fig. 2. In addition, the chains are linked by intermolecular hydrogen bond O(20)...H–N(3) [O(20)... H, 2.32 (1) and 2.33 A (2), O(20)... H–N(3), 150.2 (1) and 149.7 (2)] and p–p interaction between the free picrate groups which are almost parallel (the vertical distance between them are 3.59 and 3.44 A) [15] to form a two-dimensional (2-D) layer supermolecule (Fig. 2). The crystal structures of both complexes indicate that the coordination environment of the metal ion is pro- tected by two tetradentate ligands and one monodentatepicrate ligand. Since coordinated solvent molecules, especially water, can e?ciently quench lanthanide lumi- nescence, the ability to satisfy the coordination require- ments of the lanthanide (III) centre with nine donors without additionally bond solvent molecules becomes an important criterion in the design of supramolecular photonic devices [5]. It is noteworthy that the ligand shield Ln3+ using all three arms, thus the solid Eu com- plex does possess comparatively strong luminescence at room temperature. The luminescence emission spectra of the ligand L and Eu complex (3) in solid state (the excitation and emission slit widths were 2.5 nm, Fig. 3(a)) and in ethyl acetate, acetone, acetonitrile, ethanol and methanol31 solutions (concentration: 1.0· 10 mol L , the excita- tion and emission slit widths were 10.0 nm, Fig. 3(b)) were recorded at room temperature. It can be seen from Fig. 3(a) that the Eu complex shows strong emission when excited with 420 nm in the solid state. This indicates that the tripodal ligand L is a good organic chelator to absorb energy and transfer them to Eu ion. The most intensity ratio 5 7 5 7 value g( D0 ! F2/ D0 ! F1) is 9.6, showing that the Eu (III) ion does not lie in a centro-symmetric coordination site [16], in agreement with the crystal structure analysis. A triplet excited state T1 which is localized on one li- gand only and is independent of the lanthanide nature [17]. In order to acquire the triplet excited state T1 of the ligand L, the phosphorescence spectrum of the Gd (III) complex (4) was measured at 77 K in a metha- nol–ethanol mixture (V:V = 1:1). The triplet state en- ergy level T1 of the ligand L, which was calculated from the shortest-wavelength phosphorescence band1 [18],is21,645 cm .Thisenergylevelisabovethelowest 51 excited resonance level D0 of Eu (III) (17,286 cm ) 51 and D4 (20,545 cm ) of Tb (III). Thus the absorbedenergy could be transferred from ligand to the Eu or Tb ions. And we may deduce that the triplet state energy level T1 of this ligand L matches better to the lowest1 resonance level of Eu (III) (Dm = 4359 cm ) than to1 Tb (III) (Dm= 1100 cm ) ion, because such small Dm 5 (T1 D4) could result in the non-radiative deactivation of the terbium emitting state via a back-energy transfer 5 process (T1 Tb( D4)) and quench the luminescence of the Tb complex (5) [17,19]. Actually, we do not observe the luminescence of the Tb complex (5) at room temper- ature in solid state or in solutions. It could be seen from Fig. 3(b) that in ethyl acetate solution the Eu complex has the strongest luminescence, and then in acetone, acetonitrile, ethanol and methanol. This is due to the coordinating e?ects of solvents, namely solvate e?ect [20]. Together with the raising coordination abilities of ethyl acetate, acetone, acetoni- trile, ethanol and methanol for the lanthanide ions, the oscillatory motions of the entering molecules consume more energy which the ligand triplet level transfer to the emitting level of the lanthanide ion. Thus, the energy transfer could not be carried out perfectly. Supplementary data Crystallographic data for the structures reported in this paperhave deposited with theCambridge Crystallo- graphic Data Centre and allocated the deposition num- bers CCDC 277139 and CCDC 261821. Copies of the data can be obtained free of charge on application to CCDC 12 Union Road, Cambridge CB2 1EW, UK (email: deposit@ccdc.cam.ac.uk). Acknowledgements This work was supported by the National Natural Science Foundation of China (Project 20401008) and the Research Foundation for the Young Teachers Pos- sessing Doctors Degree of Lanzhou University. |
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9楼2010-02-12 15:28:52
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bhz_2001
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