Three Novel Lanthanide Metal-Organic Frameworks (Ln-MOFs) Constructed by Unsymmetrical Aromatic Dicarboxylatic Tectonics: Synthesis, Crystal Structures and Luminescent Properties

Three novel Ln(III)-based coordination polymers, {[Ln2 (2,4-bpda)3 (H2O)x]·yH2O}n (Ln = La (III) (1), x = 2, y = 0, Ce (III) (2), Pr (III) (3), x = 4, y = 1) (2,4-H2bpda = benzophenone-2,4-dicarboxylic acid) have been prepared via a solvothermal method and characterized by elemental analysis, IR, and single-crystal X-ray diffraction techniques. Complex 1 exhibits a 3D complicated framework with a new 2-nodal (3,7)-connected (42·5) (44·51·66·8) topology. Complexes 2 and 3 are isomorphous, and feature a 3D 4-connected (65·8)-CdSO4 network. Moreover, solid-state properties such as thermal stabilities and luminescent properties of 1 and 2 were also investigated. Complex 1 crystallized in a monoclinic space group P21/c with a = 14.800 (3), b = 14.500 (3), c = 18.800 (4) Å, β = 91.00 (3), V = 4033.9 (14) Å3 and Z = 4. Complex 2 crystallized in a monoclinic space group Cc with a = 13.5432 (4), b = 12.9981 (4), c = 25.7567 (11) Å, β = 104.028 (4), V = 1374.16 (7) Å3 and Z = 4.


Introduction
Lanthanide metal-organic frameworks (Ln-MOFs) have attracted ever-increasing interest not only because of their intriguing structural diversity, but also due to special photophysical properties [1][2][3][4]. However, due to their high coordination numbers and mutable coordination geometries, the assembly of lanthanide coordination polymers with specific geometry and properties might be uncontrollable [5], so designing and controlling lanthanide metal-organic frameworks (Ln-MOFs) with presupposed topological networks and functions remains a difficult and challenging task. According to the latest CCDC research (version 5.35), a number of aromatic multicarboxylate ligands such as benzene-dicarboxylate, benzenetricarboxylate, benzenetetracarboxylate, etc. have been extensively used to assemble fascinating structures with luminescent and atypical magnetic properties [6][7][8].

Luminescent Properties
The solid-state fluorescent properties of complexes 1 and 2 at room temperature are shown in Figure 6. Complexes 1 and 2 display a fluorescent emission at around 471 nm (λex = 421 nm) and 439 nm (λex = 344 nm), respectively. As for the free organic ligands, a weak emission is observed at 394 nm (λex = 280 nm) [14]. The crystal-field splitting of the 5d orbital was not observed in the two complexes because of the high coordination number of Ln (III) ions [17]. The emission spectra of complexes 1 and 2 are similar to that of the free 2,4-H2bpda ligand, indicating that the fluorescence of these two compexes is a ligand-based emission [19]. Compared with the free 2,4-H2bpda, the emission peaks of 1 and 2 have a visible red shift and their intensity is also increased, which could be due to intraligand π-π* or n-n* electron transitions [20]. The obvious enhanced intensities of complexes 1 and 2 could be attributed to the increased rigidity of the ligand after coordination to the Ln (III) center, which effectively reduced the loss of energy [21].

Thermogravimetric Analysis
Thermogravimetric analyses (TGA) were monitored to observe the thermal behavior of complexes 1 and 2 (Figure 7). Complexes 1 and 2 showed similar thermal decomposition processes. Therefore, only the patterns of 1 will be discussed as an example. The first weight loss of 3.25% in the range of 50-150 °C is related to the loss of two coordinated water molecules (Calc. 3.22%). The residue is stable up to about 200 °C. After 300 °C, the network of 1 gradually collapses corresponding to the decomposition of organic components and the remaining residue is lanthanum oxide.

Materials and Physical Measurements
The inorganic salts and organic regents were commercially available and used as supplied without further purification. The ligand benzophenone-2,4-dicarboxylic acid (2,4-H2bpda) was obtained from Alfa Aesar China Co. Ltd. (Beijing, China) and used as received. Elemental analysis for C and H were performed on a GmbH VarioEL V3.00 automatic elemental analyzer. The FT-IR spectra were recorded as KBr pellets with a Thermo Electron NEXUS FT-IR spectrometer in the 4000-400 cm −1 region. Thermogravimetric analysis was recorded with a NETZSCH STA 449C microanalyzer in air at a heating rate of 10 °C·min −1 . Luminescence spectra for the solid samples were recorded with a Hitachi F-4500 fluorescence spectrophotometer at room temperature.

Synthesis of Complexes 1-3
Single-crystal samples of complexes 1-3 suitable for X-ray analysis were obtained by a similar method to that described for complex 1.