Synthesis, Structure And Properties Of Metal Organic Carboxylate Frameworks Based On Rare Earth Elements

Metal-organic frameworks, a kind of functional molecular materials, have attracted much attention for their flexible tailoring, various topologies and promising application in ion-exchange, adsorption, molecular recognization, catalysts along with optics, electrics, magnetism and enantioselective separation. Metal-organic frameworks with rigid open frameworks not only possess porous crystaline structures similar to those of inorganic microporous materials, zeolites, but also exhibit extra-large pores in the mesopore range, which are assembled through the extension of organic functional groups and the design of topologies. Along with the design and preparation of MOFs of larger channels and cavities, much attention has turned to synthesize metal-organic frameworks with the promising applications in optics, electric, magnetism, enantioselective separation and catalysis by selecting functional metal ions and organic ligands with specific groups.New materials with rare earth element are of great interests due to their amazing optical and magnetic properties arising from 4f electrons. In recent years, many research groups have choosen rare earth elements as metal centers to assemble metalorganic frameworks and a number of novel frameworks have been synthesized. However, it is really difficult to obtain open frameworks with permanent porositis for their high-coordination number and flexible coordination geometry compared with traditional metals, especially under hydrothermal conditions. Considering the functions of metal-organic frameworks and the synthesis conditions, we designed and synthesized five serials of metal-organic frameworks based on rare earths by choosing rigid organic ligands with carboxylate groups, H2BDC, H3BTC, H2bpdc, and using mild solvent vapor conditions. We also investigated their optical and magnetic properties coming from rare earth metal centers.The first series of microporous lanthanide metal organic carboxylate frameworks, Tb3(BDC)4.5(DMF)2(H2O)3-(DMF)(H2O) (1), Ln3(BDC)4.5(DMF)2 -(H20)3-(DMF)(C2H5OH)o.5(H20)o.5 (Ln = Dy (2), Ho (3), Er (4)) and Y3(BDC)4.5(DMF)2(H2O)3-(DMF)2 (5) have been synthesized under mild conditions. X-ray diffraction analyses reveal that they are extreme similar in structure and crystallized in triclinic space group P-l. An edged-sharing metallic dimmer and four metallic monomers are assembled by eighteen carboxylate groups to form discrete inorganic rod-shaped building units (Ln6(CO2)i8), which link each other through phenyl groups to lead to three-dimensional open frameworks with approximately 4 x 6 A rhombic channels along the [0 -1 1] direction. Water sorption isotherm proves that guest molecules in the framework of complex 1 can be removed to produce permanent microporosity and about four water molecules per formula unit can be adsorbed into the micropores. These complexes exhibit blue fluorescence and complex 1 shows Tb3+characteristic emission in the range of 450 ~ 650 nm.The second serial of metal organic carboxylate frameworks based on rare earth elements, RE(BTC)(DMF)(DMSO) [RE = Tb (6), Dy (7), Ho (8), Er (9), Yb (10), Y(11)] with zeolite ABW topology have been synthesized for the first time under mild conditions. They exhibit a same three-dimensional (3D) architecture and crystallize in monoclinic symmetry space group P2i/n. Their structures are built up from inorganic and organic 4-connected building units, whose vertex symbols are 44-6-6-6-8. The building units link each other to generate approximate 5 x 8 A2 channels along the [1 0 0] direction. The luminescent and magnetic properties of these compounds are investigated and the results reveal that they could be anticipated as potential antiferromagnetic materials and fluorescent materials.The third serie of three-dimensional (3D) novel metal organic carboxylate frameworks, RE(bpdc)i.5(H20>(DMF)o.5 [RE = Tb (12), Ho (14), Er (15), Y (18)], Ln(bpdc)1.5(DMF)-(H2O) [Ln = Dy (13), Tm (16), Yb (17)] have been synthesized by reaction of the rare earth ions (M3+) with 4,4'-biphenyldicarboxylic acid (H2bpdc) in a mixed solution of DMF and C2H5OH. They possess the same 3D architectures and crystallize in monoclinic space group C2/c. Two seven-coordinated metal centers and four dimonodentate bpdc groups construct a paddle-wheel building block. These building blocks connect with two carboxyl groups to lead to one-dimensional inorganic chain, —M-O-C-O-M—, along the [001] direction. The inorganic chains are linked with two biphenyl groups to form 25.15 A x 17.09 A rhombic channels along c axis without interpenetration. These complexes exhibit strong fluorescence in the visible region and complex 3 shows Er3+ characteristic emission in the range of 1450-1650 nm at room temperature. These complexes could be anticipated as potential fluorescent probes and an IR-emitter, respectively.The forth serial of lanthanide metal organic carboxylate frameworks with excellent thermal stability, Ln(BTC)(H2O>(DMF) [Ln = Sm (19), Eu (20), Tb (21), Dy (22)] have been synthesizd under mild conditions. The terminal water moleculesof compound 20 could be removed to yield a new microporous compound, Eu(BTC) (20A). X-ray diffraction analyse reveals that compound 20 is crystallized in a chiral space group P4\22. The homochirality of compound 20 can be obtain through adding chiral auxiliary during the synthesis process, which could be examined by the VCD spectrum. Compound 20A shows a high BET surface area, 665 m2/g, a narrow pore size distribution and could be anticipated as gas storage materials to take up hydrogen and carbon dioxide. The metal centers of compound 20A is available Lewis acid metal sites for they are coordinative unsaturation.Finally, a serial of metal organic carboxylate frameworks based on rare earth elements, Ln(BDC)i.5(DMF)(H2O)-(H2O) [Ln = Tb (23), Dy (24), Er (25), Tm (26)], are synthesized which shows two-fold interpenetration structure..