Harnessing Noncovalent Interactions towards the Stabilization of Actinide Complexes and Clusters
Wacker, Jennifer Nicole
Knope, Karah E
The chemical and physical properties of the actinides are exploited in applications ranging from nuclear energy to radiopharmaceuticals, with the aqueous behavior of these metal cations playing a critical role in both industrial processing and environmental transport. Motivated by such applications as well as the ongoing need to develop a predictive understanding of the behavior of these elements, the goal of this work is to further elucidate the factors that govern actinide (An) aqueous speciation. Although several considerations are understood to underpin An identity (i.e. solution composition, complexing ligands, An oxidation state), the role of outer coordination sphere interactions in the assembly and stabilization of An species remains poorly understood. To this aim, the solution and solid-state structural chemistries of the early actinides, including thorium (Th), uranium (U), and plutonium (Pu), were explored in the presence of various counter-ions. Under acidic aqueous conditions, thirty-eight compounds were isolated, differing in stability, composition, and nuclearity.With a non-coordinating hydrogen (H) bond donor, ThIV adopted a monomeric Th–aquo–chloro complex and a trimeric cluster. Under the same synthetic conditions, UIV formed the UCl62– dianion; however, addition of another H-bond donor yielded an isomorphous U–aquo–chloro complex. Expansion to other N–H donors ranging from pyridinium derivatives to terpyridine further underscored differences between ThIV and UIV. ThIV assumed various monomeric Th−aquo−chloro complexes whereas UIV crystallized as both U−aquo−chloro units and UCl62–. Extension to PuIV yielded only PuCl62–, highlighting an additional periodic break in the structural chemistry of the early AnIV ions. Although PuIV invariantly formed the PuCl62– dianion, PuIII demonstrated flexibility in coordination number, connectivity, and charge that depended on the counter-ion identity. Similar syntheses were used with cerium (Ce), a known surrogate of Pu, and polynuclear Ce-oxo clusters were observed, which have important implications for understanding the assembly and surface chemistries of An clusters.Trends in the solid-state structural chemistry of the CeIV and AnIV complexes are discussed, and where possible, the relationship between those species that exist in the solid state and solution are described. The role counter-ions may have on complex stabilization are considered. The structural diversity afforded not only by An identity, but also through systematic changes of the counter-ion merit the consideration of the role that outer coordination sphere interactions play in governing An speciation and/or shifting equilibria. As evidenced by this work, noncovalent interactions may also be used as a synthetic strategy towards the isolation of elusive An complexes and clusters.
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