Nanotubular materials have unique water transport and storage properties that have the potential to advance separations, catalysis, drug delivery, and environmental remediation technologies. The development of novel hybrid materials, such as metal-organic nanotubes (MONs), is of particular interest, as these materials are amenable to structural engineering strategies and may exhibit tunable properties based upon the presence of inorganic components. A novel metal-organic nanotube, (C4H12N2)(0.5)[(UO2)(Hida)(H2ida)]·2H2O (UMON) (ida = iminodiacetate), that demonstrates the possibilities of these types of hybrid compounds has been synthesized via a supramolecular approach. Single-crystal X-ray diffraction of the compound revealed stacked macrocyclic arrays that contain highly ordered water molecules with structural similarities to the "ice channels" observed in single-walled carbon nanotubes. Nanoconfinement of the water molecules may be the cause of the unusual exchange properties observed for UMON, including selectivity to water and reversible exchange at low temperature (37 °C). Similar properties have not been reported for other inorganic or hybrid compounds and indicate the potential of MONs as advanced materials.

Porous hybrid materials such as metal−organic nanotubes are of interest due to synthetic tunability, possibility for 1-D flow, and confinement of solvent molecules. In the current study, the stability and solvent selectivity of two hybrid materials with nanotubular arrays, (pip) 0.5 [(UO 2 )(HIDA)(H 2 IDA)]·2H 2 O (UIDA; IDA = iminodiacetate) and [(UO 2 )(PDC)(H 2 O)] (UPDC; PDC = pyridine dicarboxylate) were analyzed using X-ray diffraction, gas chromatography/ mass spectrometry, thermogravimetric analysis, and infrared spectroscopy. The fine details of the structural characterization, such as the presence of solvent molecules, were found to be important to the overall properties of the materials as evidenced by the increased stability of the UIDA compound when formed from a solvent mixture containing acetone. Careful analysis of the UPDC compound indicated that the ligated solvent molecule can be exchanged, which may impact the hydration state of the material. Overall, the UIDA compound displays complete selectivity to water, but the UPDC compound adsorbs THF, methanol, ethanol, and cyclohexane.

The interplay of hydrolysis and chelation by organic ligands results in the formation of novel uranium species in aqueous solutions. Many of these molecular complexes have been identified by spectroscopic and potentiometric techniques, but a detailed structural understanding of these species is lacking. Identification of possible uranyl hydrolysis products in the presence of organic functional groups has been achieved by the crystallization of molecular species into a solid-state compound, followed by structural and chemical characterization of the material. The structures of three novel molecular complexes containing either iminodiacetate (ida) (Na3[(UO2)3(OH)3(ida)3]·8H2O (1)) or malate (mal) (K(pip)2[(UO2)3O(mal)3]·6H2O (2a) (pip = C4N2H12), (2b) (pip)3[(UO2)3O(mal)3]·H2O, and (pip)6[(UO2)11(O)4(OH)4(mal)6(CO3)2]·23H2O (3)) ligands have been determined by single-crystal X-ray diffraction and have been chemically characterized by IR, Raman, and NMR spectroscopies. A major structural component in compounds 1 and 2 is a trimeric 3:3 uranyl ida or mal species, but different bridging groups between the metal centers create variations in the structural topologies of the molecular units. Compound 3 contains a large polynuclear cluster with 11 U atoms, which is composed of trimeric and pentameric building units chelated by mal ligands and linked through hydroxyl groups and carbonate anions. The characterized compounds represent novel structural topologies for U(6+) hydrolysis products that may be important molecular species in near-neutral aqueous systems.

Th(IV) readily undergoes hydrolysis and condensation in aqueous solutions to form polynuclear molecular species and the system becomes increasingly complicated when organic chelators or other metals are present in solution, leading to the formation of complexes with vastly different structural topologies. Five compounds containing binary and ternary Th(IV) complexes have been synthesized and structurally characterized using single-crystal X-ray diffraction, including Na4[Th6O2(C10O7N2H14)6]·20.5H2O (Th6hedta), [Th(C9O6NH12)(H2O)(NO3)]·1.5H2O (Th(ntp)), [Th2Al8(OH)14(H2O)12(C6O5NH8)4](NO3)6·17.5H2O (Th2Al8heidi), (C4N2H12) [Th2Fe2(OH)2(H2O)2(C6O7H4)2(C6O7H5)2]·6H2O (Th2Fe2cit), (C4N2H12) [ThFe2O(H2O)3(C11O9N2H13)2]·6H2O (ThFe2dhpta). Additional chemical characterization by infrared spectroscopy and thermogravimetric analysis provides information on the chelation by the organic ligands and thermal stability. These molecular complexes can be utilized to understand aqueous speciation in mixed-metal solutions and also provide information regarding contaminant adsorption on iron(III) and aluminum(III) oxide surfaces.

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