Functional Uranyl Coordination Polymer Compounds: Synthesis, Structure And Properties

Crystal engineering is involved with the behavior of molecules or chemicalgroups within the crystal lattice, the control of crystal design and properties, and theprediction of crystal structures. It not only plays a significant role in the research onrationally controlling the structures and properties of crystalline solids but also is animportant strategy for the fabrication of functional materials from molecules.Coordination polymers exemplify how crystal engineering has become a paradigm forthe design of new supramolecular structures and the control of their properties, andinterweave with organic chemistry, physical chemistry, supramolecular chemistry,materials chemistry, and biochemistry.Among Coordination polymers compounds, the d-block transition metal andlanthanide coordination polymers have been extensively investigated, whereas thereport on actinide-containing polymeric complexes is less common. Uranium, whichis a typical actinide element, usually binds two axial oxygen atoms to form the linearuranyl species (UO22+) in its +6 oxidation state. The uranyl ion exhibits good stabilityand forms complexes with various oxygen-donor, nitrogen-donor and sulfur-donorligands. Furthermore, the uranium (VI) cation takes on a variety of coordinationenvironments ranging from six-coordination tetragonal, to seven-coordinationpentagonal, and to eight-coordination hexagonal bipyamids. These features ofuranium lead to the tremendous structure diversity of uranyl complexes. In addition, itis found that the uranyl complexes display a wide range of physicochemical propertiessuch as optical, magnetic and catalytic ones. In this thesis, we mainly utilize theprinciples of crystal engineering to design and synthesize the uranyl-organiccoordination polymers using different ligands and explore their physical-chemicalproperties.In hydrothermal reaction systems, new mono-molecuar uranyl compounds,UO2(H2O)2(NA)2(bipy)2 (HNA = nicotinic acid, bpy = 4,4'-bipyridine) (UOC1) andUO2Q2HQ(CH3CN)0.5 (HQ = 8-hydroxyquinoline) (UOC2) were synthesized. UOC1,as a water insoluble solid, possesses distinct photocatalytic properties tested by usinga non-biodegradable N-containing methyl blue (MB) solutions as target pollutants. Itis capable of photocatalyzing the degradation of MB by UV radiation source (Hglamp) and visible radiation source (Xenon lamp). Furthermore UOC1 exhibitsphotoluminescent, but UOC2 is not luminescent under ambient conditions. It wasinterpreted using the molecular orbital theory that the ligands affect on thephotoluminescent properties.We added another metal ion into the reaction system to increase the coordinationdiversity, and obtained two compounds: (UO2)2(μ2-OH)(pdc)2Zn(bpy)(Ac)2(H2O)9(UOC3) (bpy = 4,4'-bipyridine, HAc = acetic acid) and UO2(pdc)(H2O) (UOC4). Inthe two compounds, the pdc adopts the same coordination mode, and there are watermolecules as a guest in the structures. UOC3 possesses infinite water cluster chains,which play an important role in the stabilization of the crystal structure. UOC4 isbuilt up from large helical chains. Each helical chain, which possesses a 65 screw axis,forms a chiral channel, rendering UOC4 microporous. Furthermore, compoundUOC4 is thermally stable up to at least 350 oC and exhibits considerable adsorptioncapacity for water and methanol upon removal of the guest species in the microporouschannels. Furthermore, UOC4 is insoluble in water and other common organicsolvents such as methanol, ethanol and acetonitrile, favoring its possible applications.Preliminary testing of second harmonic generation (SHG) shows that the powdersample of UOC4 produces green light (frequency-doubled output λ = 532 nm) whenilluminated with an infrared laser light (λ = 1064 nm).The O-donor (H4BTEC) and N-donor ligands (phen) coordinate with the uranylcenters simultaneously to synthesis UOC5. As a result, a two-dimensional layer isformed by the connection of uranyl units and BTEC ligands, with the chelating phenligands decorating at both sides of the layer. UOC5 emit intense yellow light underthe excitation of UV rays.We chose 1,2,4,5-benzenetetracarboxylic acid (H4BTEC), which is achiral andmultidentate, as the ligand and uranyl acetate as the metal source for the synthesis ofnew metal-organic polymeric compounds. Interestingly, we obtained an unusual chiralcoordination polymer (UO2)3(H2O)(C10H3O8)2 (UOC6). This 3D structure has 1Dchannels of ca. 2×15 ? dimension bounded by 38-membered asymmetric rings. Thechannel, which is flexible in the dehydration-rehydration process, has a chiralarrangement with a two-fold screw axis.Otherwise, we are interested in the synthesis of uranium(IV) compounds for tworeasons. First, the high coordination numbers and variety of coordination geometriesadopted by uranium(IV) atom could be expected to result in the formation of new,complex architectures. Second, the existence of uranium(IV) atom offers thepossibility of synthesizing materials with especial optical and magnetic properties. Wedescribed the synthesis, crystal characterization and magnetic property of a novelU(IV) coordination compound UNa2(pdc)3(H2O)6 (H2pdc = pyridine-2,6-dicarboxylicacid). The centered uranium atom is bonded to three pdc ligands via three nitrogenatoms and six oxygen atoms in a tricapped trigonal prismatic coordination. Thetemperature dependence of magnetic susceptibility reciprocal curve shows that thetitle compound has paramagnetic behavior which obey to Curie-Weiss equation above50 K, Weiss constant is -50.25 K which indicates that the occurrence of antimagneticinteraction between U(IV) ions.