The chemistry of the f-elements, especially uranium, in very low oxidation states has seen significant advances in the last two decades. These low-valent species have often been demonstrated to be crucial in the activation of small molecules such as N2 and CO2. However, the multielectron transfer required to accomplish reductions of such small molecules has often been a challenge for f-elements, which often take part in one-electron transfer. The main primary objective of this thesis is to design and synthesise uranium complexes capable of engaging in multielectron transfer with small molecules. In doing so, better understand the underlying conditions required to invoke multielectron transfer and the type of bonds that can be formed and broken.
Two strategies were pursued in order to invoke multielectron transfer with very low valent (i.e. lower than +3 oxidation state) uranium complexes. The first strategy involves using bimetallic uranium complexes, wherein two uranium ions cooperatively reduce external substrates. In Chapter 2 the synthesis of oxo bridged diuranium complexes supported by the triphenylsiloxide ligand (â OSiPh3) was investigated. Metal-arene interactions formed between the uranium ion and the arene moiety of the triphenylsiloxide ligand upon reduction allowed the isolation of the first examples of U(II) and U(I) synthons in a multimetallic complex. The isolation of the formal â U(II)â /U(IV) and â U(I)â /U(IV) oxo complexes was enabled by a reduction induced ligand migration from one uranium atom to the other.
The second strategy employed to synthesise U(II) and formal U(I) complexes features the use of a tripodal redox-active siloxide ligand with an arene anchor. Chapter 3 explores the utility of a tripodal ligand to support monouranium complexes in formal +2 and +1 oxidation states. The uranium complexes synthesised in the formal +1 oxidation state represent the only third example of a U(I) synthon. The formal U(II) and U(I) complexes take part in multielectron transfer with substrates such as azobenzene and cycloheptatriene.
Transition metal-based oxidative addition involving bonds such as C-O, C-N and C-C has been well studied, whereas that of uranium is comparatively underdeveloped. In Chapter 4, the reduction of a heteroleptic aryloxide U(III) precursor was investigated, which led to the first example of a single metal mediated intramolecular C-N cleavage featuring a divalent uranium intermediate.
The reduction of CO2 by Eu(II) complexes is extremely scarce compared to other classical divalent lanthanides Yb(II) and Sm(II). In Chapter 5, the triphenylsiloxide ligand was utilised to construct divalent europium and ytterbium complexes, leading to the only second example of CO2reduction mediated by molecular divalent europium complex.
Although lanthanide dinitrogen complexes have been studied for the past three decades with different supporting ligands and varying degree of reduction, the functionalisation of the lanthanide-bound dinitrogen moiety remains extremely scarce. Chapter 6 addresses this by constructing heterobimetallic thulium-potassium N22- and N23- complexes supported by the â OSi(OtBu) ligand. Both the N22- and N23- complexes undergo N-H functionalisation to ammonium chloride upon addition of a strong acid. While the N23- complex undergoes N-C bond formation to form a rare dimethylhydrazido lanthanide complex, leading to the only second example of lanthanide mediated dinitrogen derived N-C bond formation.