Syntheses, structures, photoluminescence, and near-infrared of pentanuclear Tb(III), Yb(III) dibenzoylmethane complexes

Abstract Two pentanuclear terbium and ytterbium hydroxy cluster of composition Ln5(μ4-OH)(μ3-OH)4(μ-η2-Ph2acac)4(η2-DBM)6 were prepared starting from LnCl3·6H2O and dibenzoylmethane [Ln = Tb (1), Yb (2), DBM = dibenzoylmethane]. X-ray crystallographic analysis reveals that complexes 1 and 2 crystallize in the tetragonal space group P4/n, and the lanthanide atoms adopt the expected square-based pyramidal arrangement. Property investigation indicates that complexes 1 and 2 display characteristics of luminescence of Tb(III) and near-infrared of Yb(III).


Introduction
Over the past two decades, metal-organic complexes, as the promising candidate compounds for the development of molecular-based materials, have been of special interest for chemists because of their diversity and variability as well as potential application value in photoluminescence, [1][2][3][4] magnetism, [5][6][7][8] catalysis, [9] adsorption, [10] and so on. The assembly of polynuclear lanthanide clusters has attracted great interest because of their potential applications in a variety of areas. [11][12][13][14][15][16][17][18] In an effort to construct polynuclear lanthanide clusters, the effective choice of ligands is very crucial and important. It is well known that b-diketone, a good bidentate chelating ligand, is not only able to afford SMMs, [19][20][21][22] but also sensitize the luminescence of lanthanide complexes. [23][24][25][26][27][28][29][30][31] Especially, the photoluminescent properties of lanthanide-organic complexes have been comprehensively investigated owing to the long lifetimes of excited states and high color purity of lanthanide(III) ions. Moreover, organic ligand with strong ultraviolet absorption was often used to sensitize luminescence of lanthanide ions through intramolecular energy transfer process. Recently, our group designed and synthesized two new b-diketone lanthanide complexes, namely Ln 5 (l 4 -OH)(l 3 -OH) 4 (l-g 2 -Ph 2 acac) 4 (g 2 -DBM) 6 [Ln ¼ Tb (1), Yb (2), DBM ¼ dibenzoylmethane]. Their crystal structures have been determined. Meanwhile, the properties of photoluminescent for complex 1 and near-infrared (NIR) for complex 2 were investigated. In comparison with previously reported DBM Dy analog cluster, [32] which showing remarkable SMM behaviors. Although they have a similar structure, the fluorescence photoluminescent properties were not studied. And it's rarely reported. Based on the above considerations, in this work, we attempt to seek an efficient synthetic strategy for constructing polynuclear Ln(III)-based clusters showing remarkable photoluminescent properties.

Materials and instruments
Terbiumoxide and ytterbium oxide (Tb 2 O 3 and Yb 2 O 3 , 99.99%) were purchased from Gan Zhou rare earth Chemical Plant (Jiang Xi, China). Other chemicals (98%, A. R.) were purchased from Shanghai D&R Finechem Co. (Shanghai, China). All chemicals except TbCl 3 Á6H 2 O and YbCl 3 Á6H 2 O were obtained from commercial sources and used without further purification. LnCl 3 Á6H 2 O was prepared by the reactions of Ln 2 O 3 and HCl in aqueous solution. Elemental (C and H) analyses were performed on a Perkin-Elmer 2400 analyzer. FT-IR spectra were collected on a Perkin-Elmer 100 spectrophotometer by using KBr pellets in the range of 4000 À 450 cm À1 . UV spectra were recorded on a Perkin-Elmer Lambda 35 spectrometer.

(2)
A solution of DBM (0.3364 g, 1.5 mmol) and NaOH (0.0600 g, 1.5 mmol) in 10 ml CH 3 OH were stirred half an hour, YbCl 3 Á6H 2 O (0.1938 g, 0.5 mmol) was added to the solution. The mixture was stirred for 24 h at room temperature. Single crystals suitable for XRD were obtained from toluene/petroleum ether within 5 or 7 days. Yield

X-ray crystallography
Single-crystal X-ray data of complexes 1 and 2 were collected on an Oxford Xcalibur Gemini Ultra diffractometer with graphite-monochromated Mo Ka (k ¼ 0.71073 Å) at room temperature. Empirical absorption corrections based on equivalent reflections were applied. The structures of complexes 1 and 2 were solved by direct methods and refined by full-matrix least-squares methods on F 2 using SHELXS-2014 crystallographic software package. [33] All non-hydrogen atoms are anisotropically refined. All crystal data and structure refinement details for complexes 1 and 2 are summarized in Table 1. CCDC No. 1870580 and 1870581 contain the supplementary crystallographic data for complexes 1 and 2. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

Results and discussion
Synthesis and spectral analysis of complexes 1 and 2 Complexes 1 and 2 were synthesized as shown in Scheme 1. The IR spectrum of ligand DBM exhibits the typical absorption of stretching frequencies of the hydroxyl in enol form (1600 cm À1 ). However, the hydroxyl stretching frequency is shifted to lower wavenumbers in complex 1 (1595 cm À1 ) and complex 2 (1596 cm À1 ), which shows the involvement of the ligand DBM in the complex formation with Dy(III) ions (Figure 1, left). The UV-vis spectra show that there are obviously absorption bands around 341 nm for DBM and 350 nm for complexes 1 and 2 ( Figure 1, right), the absorption maxima are red-shifted 9 nm, which result from the singlet-singlet n-p Ã enol absorption of the b-diketonate.

Structural description of complexes 1 and 2
X-ray crystallographic analysis reveals that complexes 1 and 2 crystallize in the tetragonal space group P4/n. All Ln sites are six-coordinate. The geometry at each site is based on octahedral. Each lanthanide atom site has local, but noncrystallographic, C 4v symmetry. The lanthanide atoms adopt the expected square-based pyramidal arrangement. Each triangular face of the square pyramid is capped by one l 3 -O moiety. In the square-based face, four lanthanide atoms are linked by one l 4 -O atom (from OH). A total of 10 peripheral ligands surround the Ln5 cluster core. The ligand shows two different coordination modes. Six ligands are terminally chelating and four are bridging chelating, bonding to two metal ions that belong to the base of the polyhedron. The apical lanthanide, which lies on a 4-fold symmetry axis, is bonded to two disordered chelate ligands (Figure 2).

Photoluminescent properties
Lanthanide complex can emit over the entire spectral range. Therefore, the solid-state photoluminescent property of 1 was measured at room temperature. The photoluminescent behavior of complex 1 (Tb) was observed. The excitation and emission spectra of complex 1 are shown in Figure 3. The excitation spectrum is monitored at 544 nm including several narrow bands at 397 nm, which are attributed to a series of f-f transitions of Tb(III). Under excitation at 397 nm, the typical emission bands of Tb(III) at 491 ( 5 D 4 ! 7 F 6 ), 544 ( 5 D 4 ! 7 F 5 ), 585 ( 5 D 4 ! 7 F 4 ), and 622 nm ( 5 D 4 ! 7 F 3 ) are observed.
The NIR luminescence excitation and emission spectra for complex 2 have been investigated. When excited at 378 nm, the emission spectra exhibit an emission band at 974 nm for complex 2 which is attributed to the 2F 5/2 ! 2 F 7/2 transitions (Figure 4).

Conclusion
Isolation of two DBM lanthanide lanthanide complexes 1 and 2 verifies that the DBM ligand is able to co-stabilize the lanthanide(III) ion forming square-based pyramidal arrangement, namely, Ln 5 (l 4 -OH)(l 3 -OH) 4 (l-g 2 -Ph 2 acac) 4 (g 2 -DBM) 6 . Luminescent analysis reveals that complex 1 exhibits the characteristic photoluminescence. Complex 2 exhibits promising NIR luminescence, suggesting that the triplet state energy level of DBM match well with the singlet state of Yb 3þ ions. The DBM ligand can both further enhance the luminescence and NIR luminescence, which playing synergistic effect on the energy transfer between the ligands and lanthanide(III) ions. Further study on the correlation between the structure and luminescence is being conducted in our group.