Bimetallic uranium and cerium tetraphenolate complexes for the activation and functionalisation of small molecules
Seymour, Megan Lucy
thesis describes the synthesis of bimetallic f-element complexes of meta-functionalised tetraphenol arene ligands, (mTP), and their reactivity, primarily towards small molecular substrates such as dinitrogen. Chapter one introduces the principles of uranium chemistry and the suitability of aryloxides as ligands for low oxidation state uranium. The synthesis and reactivity of selected examples of U(III) and U(IV) complexes are described and an overview of dinitrogen activation by selected transition metal complexes is provided. Chapter two reports the synthesis and characterisation of two meta-functionalised tetraphenol arene ligand precursors H4(mTPm) and H4(mTPt) and describes reactions designed to target closely related analogues. In 1:1 salt metathesis or protonolysis reactions, two tetradentate ligands and two U(IV) centres are combined to yield [U2(mTP)2] complexes with a novel ‘letterbox’ architecture. The reduction of some of these complexes under an atmosphere of dinitrogen yields bound [N2H2]2- following an intramolecular reaction of an activated N2 fragment with benzylic C−H bonds provided by the ligand. The [N2H2]2- moiety is susceptible to further functionalisation through reactions with external electrophiles. Stoichiometric and catalytic functionalisation reactions are discussed and a mechanistic pathway for these transformations is proposed. Chapter three presents the synthesis of another set of bimetallic complexes, [U2(mTP)X4] (X = one electron donor ligand), derived from 2:1 reactions of uranium(IV) with H4(mTP). The redox chemistry of these complexes is explored through cyclic voltammetry and the structure and reactivity of these ‘half-letterbox’ complexes is compared to the complexes reported in Chapter two. The work described in Chapter four returns to complexes with a ‘letterbox’ geometry but employs Ce(III) and potassium cations to prepare a set of heterobimetallic analogues, [K][Ce2(mTP)2K]. The oxidation chemistry of these complexes is investigated and an EPR study is used to examine the magnetic behaviour of the two 4f1 Ce(III) centres. Chapter five details the experimental procedures and characterisation data for the work described in the preceding chapters.