Syntheses,Structures,Photoluminescent And Catalytic Properties Of Lanthanide-based Coordination Polymers

Coordination polymers(CPs) are emerging as the next generation of excellent materials for many industrial and technological applications. The self-assembly of coordination polymers(CPs) has led to increasing interest, not only owing to their abundant and various intriguing structures, but also because of their potential applications in catalysis, gas adsorption, magnetism, and luminescence. Among them, lanthanide-based CPs(Ln-CPs) have received great attention due to their unique luminescence and catalytic properties.On the one hand, Ln-CPs as the luminescent materials exhibit high photoluminescence efficiency, unique narrow-band, and long luminescence lifetimes emissions. Moreover, the lanthanide based color-tunable and white light-emitting materials can be achieved by incorporating Eu3+, Tb3+ and Gd3+ /Ce3+ into the isostructural frameworks, and their luminescence can be effectively tuned by varying the stoichiometric ratio of Ln3+ ions or the excitation wavelengths. As well known, Ln3+ ions have rather flexible coordination spheres, very high affinity and prefer to bind to hard donor atoms, which may create coordinatively unsaturated open Lewis acidic sites and free Lewis basic sites in open frameworks of Ln-CPs. On the other hand, with available Lewis-acid metal sites and intrinsic porosity, Ln-CPs as highly effective, recyclable, and reusable heterogeneous catalyst exhibit high catalytic reactivity for the reduction of the nitro group, the one-pot reductive amination of heptanal, the carbonyl allylation reaction, the Diels-Alder reaction, and the Strecker-type reaction.However, in comparison with transition metal ions, to rationally design and construct Ln-CPs with desired geometries and properties is still a challenge. As well known, several factors have great influence on the design and synthesis of Ln-CPs, such as the choice of organic ligands, the metal/ligand ratio and anions etc. Lanthanide ions have a high affinity and prefer to bond to oxygen atoms, thus, the selection or design of ligands containing appropriate oxygen atom coordination sites is crucial to construct Ln-CPs. Therefore, in order to construct novel multifunctional lanthanide coordination polymers, we selected carboxylic acid derivatives containing benzene as ligands and synthesized 10 coordination polymers under solvothermal conditions. We also carried out the analysis of structure and properties characterization of the compounds, and explored their potential applications in luminescence, sensing and catalysis.1. To explore new functional Ln-CPs, we selected 2,2’-bipyridine-4, 4’-dicarboxylic acid(H2bpdc) as organic linker and successfully prepared two isostructural three-dimensional(3D) Ln CPs with two kinds of one-dimensional(1D) channels along the a axis, namely, [Ln2(bpdc)3(DMF)2](Ln = Tb for 1, Eu for 2). Due to the preferential binding of Ln3+ ions to carboxylate oxygen atoms over pyridyl nitrogen atoms, Ln-CPs 1 and 2 can provide open Lewis basic sites besides Lewis acidic Ln3+ centers. After activation, 1 as a heterogeneous catalyst exhibits good catalytic reactivity and selectivity for the Strecker reaction which is one of the most direct and viable methods for the synthesis of α-amino nitriles. In addition, activated 1 shows sensitive fluorescent response to Cu2+ in DMF solution.2. 3,5-bis(4’-carboxy-phenyl)-1,2,4-triazole)(H2bct) endowed with two carboxylate groups and a triazole ring, is a versatile bridging ligand for the construction of multi-dimensional CPs and has conjugated system possess good energy-transfer ability, which can magnify the luminescence of the f–f transitions of Ln3+ ions. In this work, using DMF and H2 O as solvents, a new family of isomorphic Ln-MOFs, namely, [Ln2(HCOO)2(bct)2]·H2O(Ln = Tb for 3, Eu for 4, Gd for 5) have been constructed from lanthanide formate skeletons and bct2- connectors, exhibiting a 3D framework with 1D oval channels. Luminescence investigation reveals that 3-Tb and 4-Eu show characteristic green and red emissions of the corresponding Tb3+ and Eu3+ ions, respectively, while 5-Gd exhibits blue emission arising from the bct2-ligand. Then based on their luminescent properties, we have successfully designed and constructed two novel doped Ln-MOFs, Eu3+-doped 3-Tb and Eu3+, Tb3+-doped 5-Gd by adjusting the co-doping ratio of different Ln3+ ions into the same framework as that of 1-3. The emission colors of Eu3+-doped 3-Tb can be tuned from red to green by varying the molar ratios of Eu3+ ion from 10% to 0.02%, while that of Eu3+, Tb3+-doped 5-Gd can be tuned from yellow to white by changing excitation length. In addition, with exposed Lewis acidic sites in the framework, 3-Tb exhibits high catalytic reactivity for the Strecker reaction affording medium to excellent conversion yields.3. we selcted 4-(2-Carboxy-phenoxy)-isophthalic acid(H3oba) as organic linker and successfully prepared three isostructural Ln-CPs, namely, [Ln(H2O)2(oba)]·2H2O(Ln = Tb for 6, Eu for 7, Gd for 8). They show two-dimensional(2D) layered structure with the rhombus windows and the characteristic red, green and blue emissions of the corresponding Ln3+ ions. Then based on their luminescent properties, we successfully fabricate one new doped Ln-CPs, Tb1-n Eun,(n refers to the molar ratios of Eu3+ and Tb3+) by adjusting the relative amounts of different luminescent components into the well-defined host framework. The luminescent color of Tb1-n Eun can be tuned from green to red-orange due to the energy transfer from the Tb3+ to Eu3+ ions by changing the doping concentration of the Eu3+ ions. In addition, studies on the PL spectra of 7 revealed excellent fluorescence sensing for small molecules upon immerging in diffrerent solvent. Specifically, the luminescence density of it obviuosly increases in CH3 CN and C2H5 OH, while the luminescence can be quenched in acetone.4. Two novel lanthanide coordination polymers(Ln-CPs), [Ln11(OH)8(4,4’-oba)12(CH3COO)2]·(CH3)2NH2·3DMF·n H2 O,(Ln = Yb(9), Er(10), 4,4’-H2 oba = 4,4’-oxybis(benzoic acid), DMF = N, N’-dimethylformamide, n = 2 for 9, 3 for 10) have been self-assembled from the reaction of hexanuclear lanthanide complexes with flexible V-shaped organic ligands under solvothermal condition. Structural analyses reveal that they are isostructural and feature a complicated 3D framework containing novel octanuclear [Ln8(OH)8]16+ units. Moreover, they show the characteristic luminescence emission bands of the corresponding lanthanide ions in the near-infrared regions in the solid-state. Additionally, magnetic property of 1 shows typical antiferromagnetic interactions.In clussion, ten Ln-CPs have been successfully prepared under solvothermal conditions. Luminescence investigation reveals that the ligands can magnify the luminescence of the f–f transitions of Ln3+ ions, with emitting in the visible region for Eu3+, Tb3+ and in the NIR for Yb3+ and Er3+. By adjusting the co-doping ratio of different Ln3+ ions into the same framework, the emission colors of the compounds can be tuned. Some compounds show sensitive fluorescent response to Cu2+ or different solvents. Moreover some compounds as heterogeneous catalyst exhibit good catalytic reactivity and selectivity for the Strecker reaction. It is believed that further studies focusing on investigating the properties of Ln-CPs will attract more interest.