This dissertation describes the synthesis and characterization of mono- and multinuclear uranium single-molecule magnets, with an emphasis on design strategies that seek to minimize through-barrier relaxation pathways.Chapter 1 provides a general introduction to single-molecule magnets and provides context on where research on actinide-based single-molecule magnets currently stands. Key figures of merit are defined, and the main challenges in this field of study (i.e., through-barrier magnetic relaxation) are discussed in detail. The importance of mitigating through-barrier relaxation pathways is emphasized. Chapter 2 describes the synthesis and magnetic properties of uranium(III) metallocenes with a bulky cyclopentadienyl ligand. These compounds exhibit the slowest magnetic relaxation rates observed for any mononuclear actinide-based compound as a consequence of the vibrationally-rigid nature of the cyclopentadienyl ligands. Chapter 3 describes the synthesis of radical-bridged diuranium complexes. Magnetic measurements reveal that magnetic exchange interactions between uranium ions and the bridging radical are very weak. As a consequence, slow magnetic relaxation arises either from the organic radical—in the case of the diuranium(IV) complex—or from the individual uranium ions—in the case of the diuranium(III) complex. In both cases, magnetic relaxation is found to proceed via through-barrier processes. Chapter 4 describes the synthesis of novel triuranium(III) clusters with halide bridging-ligands. The halide bridging ligands provide a rare example of relatively strong superexchange interactions between uranium ions. Furthermore, variation of the halide bridging-ligands produces measurable changes in the strength of the superexchange interaction, which correlates with changes in the barrier to magnetic relaxation, Ueff. Most notably, the Cl-bridged cluster presents only the third example of open magnetic hysteresis in an actinide-based molecule, as well as the first example of such behavior in nearly a decade. Chapter 5 describes attempts to reduce the triuranium clusters described in Chapter 4 in an effort to isolate mixed-valence uranium(II/III) complexes with U–U bonds. Spectroscopic, electrochemical, and magnetic measurements indicate that the resulting reduction products are instead triuranium(III) clusters with μ3-bridging hydrides. Characterization of these clusters, nevertheless, does provide some insight into the origin of the hydride, as well as design strategies that may afford isolable species with U–U bonds.

Technetium (Tc), found in nuclear waste, is of particular concern with regard to long-term waste storage because of its long half-life and high mobility in the environment. One method of stabilization of such waste is through vitrification to produce a durable borosilicate glass matrix. The fate of Tc under hydrothermal conditions in the Vapor Hydration Test (VHT) was studied to assess and possibly predict the long-term rate of release of Tc from borosilicate waste glass. For comparison, the fate of rhenium (Re), the preferred non-radioactive surrogate for Tc, was similarly studied. X-ray absorption spectroscopy (XAS) and scanning electron microscopy (SEM) measurements were made on each original borosilicate glass and the corresponding sample after the VHT. Tc K-edge XAS indicates that, despite starting with different Tc(IV) and Tc(VII) distributions in each glass, both corresponding VHT samples contain 100 percent Tc(IV). The Tc reduction within the VHT samples may be driven by simultaneous oxygen depletion from corrosion of the surrounding stainless steel vessel. From SEM analyses, both of the Tc-containing VHT samples show complete alteration of the original glass, significant Tc enrichment near the sample surface, and nearly complete depletion of Tc toward the sample center. XAS indicates Tc(IV)O6 octahedra, possibly within gel-like amorphous silicates in both VHT samples, where Tc-Tc correlations are observed in the higher Tc-content VHT sample. Re LII-edge XAS and SEM indicate quite different behavior for Re under VHT conditions. Re oxidation state appears to be invariant with respect to the VHT treatment, where perrhenate (Re(VII)) species are dominant in all Re-containing samples investigated; Re2O7 concentrations are low NEAR the sample surface and increase to approach the concentration of the un-reacted glass toward the sample center.