Synthesis and characterization of uranium-organic hybrid materials: Direct assembly and in situ ligand formation

The primary focus of this dissertation has been to synthesize and characterize UO22+ containing coordination polymers (CPs). These hybrid materials are assembled from metal centers polymerized through functionalized organic ligands. We have determined the crystal structures, themselves a reflection of the assembly process, and surveyed their structural features and trends for the purpose of gaining a more thorough understanding of the interaction of f-elements with organic molecules.;The topology and dimensionality of the compounds is governed by coordination preferences of the metal(s) as well as the organic ligand. We have examined, for example, uranyl phosphonate materials synthesized under ambient and hydrothermal conditions. With respect to carboxyphosphonate species, in particular, the uranyl cation displays coordination preference for the phosphonate over the carboxylate functionality consistent with hard-soft acid-base predictions. The propensity of the uranyl cation to bind preferentially to the harder phosphonate has also been used to construct bimetallic materials, wherein the second metal center tends to coordinate to the carboxylate group. Incorporation of transition metal or lanthanide ions (TM2+ and Ln3+) into these systems not only has significant influences on the structure but also on the luminescent properties of the resulting compounds. Overall, these materials exhibit a range of structural motifs and topologies resulting from variations in metal-ligand coordination modes, P-C-C-O torsion angles, and interlayer hydrogen bonding networks. Moreover, synthetic variables such as in situ ligand formation, the addition of nominally ‘spectator’ species and the incorporation of charge balancing countercations as well as organic dipyridines were found to influence product formation.;We have also investigated the utility of in situ ligand synthesis versus direct assembly. The most common approach for synthesizing CPs is through the direct assembly of metal centers with commercially available or pre-synthesized organic ligands. Alternatively, in situ ligand formation, a process by which the organic species undergo redox reactions to form modified ligands that are then observed in the crystalline reaction product, has been used to access novel materials. In this work three in situ ligand reactions are examined: ester hydrolysis, 1,3-dipolar cycloaddition, and the degradation of organic molecules containing C-N bonds (1,4-diazabicylo[2.2.2]octane and 2,3-pyrazinedicarboxylic acid). We have compared the products obtained using in situ ligand formation to those generated through direct assembly and see two primary benefits of in situ ligand formation; (1) the in situ formation of triazole ligands by a 1,3-dipolar cycloaddition reaction offers the simplicity of a one-pot synthesis and (2) the oxidation of organic molecules to yield oxalate ligands provides the ability to synthesize materials that are inaccessible through the direct reaction. With respect to the latter, we have explored in situ oxalate formation using ex situ and in situ techniques (single crystal X-ray diffraction; Nuclear Magnetic Resonance Spectroscopy). We have further proposed a mechanism of oxalate formation and discussed it in the context of previous accounts.