Three lanthanide complexes with mixed salicylate and 1,10-phenanthroline: syntheses, crystal structures, and luminescent/magnetic properties

Abstract Three new lanthanide complexes incorporating salicylate (HSA or SA) and 1,10-phenanthroline (phen), Ln3(HSA)5(SA)2(phen)3 [Ln = Ho (1) and Er (2)], and Sm2(HSA)2(SA)2(phen)3 (3), have been synthesized. X-ray structural analysis reveals that 1 and 2 are isostructural with a trinuclear pattern, and 3 exhibits a binuclear structure. Comparison of the structural differences between 1/2 and 3 suggests that the identity of metal plays an important role in construction of such complexes. The magnetic properties of 1 are discussed. Moreover, 2 and 3 are both photoluminescent materials, and their emission properties are closely related to their corresponding LnIII centers.


Introduction
Syntheses of lanthanide carboxylate materials attract attention due to their desirable structures [1] and potential applications arising from luminescence [2] and magnetic properties [3]. Organic ligands play a vital role in tuning the structural topology and functionality of such compounds [4]. The use of aromatic ligands is common as they are excellent in sensitizing the lanthanide luminescence by the well-known "antenna effect" [5]. A number of aromatic carboxylate ligands *Corresponding author. Email: humin@zzuli.edu.cn Downloaded by [E. Carolina Sañudo] at 02: 16 10 June 2016 such as benzoate and modified benzoates are known [6]; salicylic acid (H 2 SA), a type of aromatic carboxylate ligand with one carboxyl and one hydroxyl arranged in a 1,2-fashion around the central aromatic group, can form various coordination structures [7]. Carboxylate and hydroxy groups on the molecules can be partially or completely deprotonated to form versatile coordination models; the hydroxyl group may provide an additional binding site to generate more complicated structures in cooperation with the carboxyl groups. Furthermore, the phenyl can form C/O-H···O, C/O-H···π and π···π interactions as steering forces in the control of molecular self-assembly. Chelating ligands such as 1,10-phenanthroline (phen) may inhibit expansion of the polymeric framework to give coordination polymers of low-dimensionality or zero-dimensional molecules [8].
Keeping these observations in mind, we have reported isomorphous Dy III and Gd III complexes with H 2 SA and phen as ligands, in which a luminescent linear trinuclear Dy III complex exhibits slow magnetic relaxation of single ion origin [7d]. These results indicate that H 2 SA is an excellent ligand in the self-assembly of coordination complexes and further efforts are required to provide more information on its coordination behavior. As an extension of the above work, three new lanthanide complexes, Ln 3 (HSA) 5 (SA) 2 (phen) 3 [Ln = Ho (1) and Er (2)] and Sm 2 (HSA) 2 (SA) 2 (phen) 3 (3), based on HSA/SA and phen ligands were obtained. Herein, we report the syntheses, crystal structures, and luminescent properties of these complexes.

Materials and general methods
All the starting reagents and solvents for synthesis were commercially available and used as received. Elemental analyses (C, H and N) were performed on a Vario EL III Elementar analyzer.
IR spectra were recorded from 4000-400 cm -1 on a Tensor 27 OPUS (Bruker) FT-IR spectrometer with KBr pellets. Thermogravimetric analysis (TGA) experiments were carried out on a Perkin-Elmer Diamond SII thermal analyzer from room temperature to 800 °C under nitrogen at a heating rate of 10 °C min -1 . The emission spectra in the visible region were tested on a F-7000 (HITACHI) spectrophotometer and those in near-infrared region were measured on an FLS-980 fluorescence spectrophotometer.

Synthesis of 1-3
All three complexes were prepared with similar methods. A general synthetic procedure is described as follows by using 1 as an example.
2.2.1. Ho 3 (HSA) 5 (SA) 2 (phen) 3 (1). A mixture of Ho 2 O 3 (0.2 mmol, 0.076 g), H 2 SA (1.5 mmol, 0.207 g), phen (0.4 mmol, 0.079 g) and H 2 O (15 mL) was placed in a Teflon-lined stainless steel vessel (23 mL), heated to 140 °C for 72 h, and then cooled to room temperature at a rate of 5 °C h −1 . Single crystals of 1-3 suitable for X-ray diffraction were obtained. Then the obtained crystals were collected by filtration, washed with water and ethanol, and dried in air. Yield: N, 4.31%. The IR spectrum for 2 is shown in figure S1b.

Crystal structure determinations of 1-3
X-ray single-crystal diffraction data for 1-3 were collected on an Xcalibur Gemini Eos CCD diffractometer at 294(2) K with Mo-Kα radiation (λ = 0.71073 Å) by ω scan mode. The program SAINT [9] was used for integration of the diffraction profiles. Semi-empirical absorption corrections were applied using SADABS [10]. All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL [11]. Metal ions in each complex were located from the E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F 2 . Hydrogens were geometrically positioned and refined Fax: (+44) 1223-336-033; or E-mail: deposit@ccdc.cam.ac.uk.

Magnetic measurements
Magnetic measurement for 1 was carried out in the Unitat de Mesures Magnètiques (Universitat de Barcelona) on polycrystalline samples (circa 30 mg) with a Quantum Design SQUID MPMS-XL magnetometer equipped with a 5 T magnet. Diamagnetic corrections were calculated using Pascal's constants and an experimental correction for the sample holder was applied.
In comparison, the lanthanides in 1 and 2 are eight-coordinate, in which Ho1 and Ho2 have a distorted bicapped trigonal prism geometry, while Ho3 is a distorted triangular dodecahedron, but nine-coordinate in 3, in which the Sm1 has a distorted tricapped trigonal prism geometry, while Sm2 has a distorted monocapped square antiprism geometry.

XRPD Results
To confirm whether the crystal structures are truly representative of the bulk materials, X-ray powder diffraction (XRPD) experiments were carried out. The XRPD experimental and computer-simulated patterns of the corresponding complexes are shown in figure S6 in the Supporting Information. The bulk synthesized materials and the as-grown crystals are homogeneous for 1-3.

Thermogravimetric analysis
To examine the thermal stabilities of 1-3, thermal gravimetric analysis (TGA) experiments were performed (see figure S7). 1 is stable to ca. 200 °C. After that, the weight loss is sharp, indicating decomposition of organic ligands and collapse of the framework. Similar thermogravimetric traces with decomposition temperatures of 200-600 °C are observed for 2, suggesting that this complex possesses the same components and structure as 1 except for the different central lanthanide. With respect to 3, it is thermally stable to ca. 220 °C. Upon further heating, pyrolysis of the organic ligands occurs, which does not stop before the heating ends at 900 °C.

Magnetic properties
Magnetic susceptibility data for a crushed crystalline sample of 1 were collected at an applied field of 0.3 T from 2-300 K. The data are shown in figure 3a as a χT vs. T plot (white circles).
The χT product has a value of 43 cm 3 K mol -1 at 300 K, in agreement with the expected value for three isolated Ho(III) ions ( 5 I 8 , S = 2, L = 6, J = 8 and g J = 10/8). As temperature decreases, so does the χT product, until below 50 K a sharp decrease to a χT value of 23 cm 3 K mol -1 is observed, indicating depopulation of the excited Stark sublevels. Magnetization vs. field data for 1 are shown in figure 3b as a M/Nµ B vs. field plot. The curves are typical of lanthanide ions with strong spin-orbit coupling, as expected for Ho(III) complexes. The data eventually reach 16.1 μ B at 1.8 K and 5 T. This value is lower than the expected saturation value for three Ho(III) ions, likely due to anisotropy and important crystal-field effects [14] at the Ln III ion that eliminate the degeneracy of the ground state [15].

Luminescent properties
The luminescence spectra of 2 and 3 were investigated in the solid-state at room temperature and exhibit clear emission spectra of the corresponding Er III and Sm III ions as shown in figure 4.
Comparison of the structural differences between 1/2 and 3 suggests that the identity of the metal plays an important role in such complexes. The magnetic properties of 1 were investigated and it exhibits the typical character of Ho ion. The Sm III complex shows luminescence in the visible region at excitation and the Er III complex displays its characteristic luminescence in the near-infrared region. Further work on the H 2 SA-based coordination polymers with other rare-earth ions is underway in our laboratory for developing more interesting functional coordination polymers.