Investigations of low-valent uranium complexes relevant to the reduction and functionalisation of dinitrogen

The work presented in this thesis examines aspects of the coordination chemistry of uranium, with particular emphasis on the interaction of uranium complexes with dinitrogen. The direct binding and activation of N2 at metal centres is of fundamental interest because of the exceptional strength of the dinitrogen triple bond, which makes its reduction challenging under mild conditions. Nonetheless, the large-scale conversion of N2 to ammonia remains one of the most important industrial processes, as exemplified by the Haber-Bosch process. Uranium, with its wide range of accessible oxidation states and capacity for multielectron transfer, hence offers a unique platform for investigating alternative N2 binding geometries and elucidating their role in dinitrogen activation. The work presented in Chapter 2 of this thesis examines the reduction of a previously reported side-on aryloxide-supported diuranium N2 complex. Correspondingly, a uranium complex presenting a rare triply reduced moiety was isolated. The bound dinitrogen can be further reduced, affording the tetranitride the hexanitride cubane clusters. The tetranitride cluster showed high reactivity with electrophiles, yielding ammonia quantitatively upon acid addition and promoting CO cleavage to yield quantitative conversion of nitride into cyanide as well as CN-bridged octa-uranium nitride/oxo aggregate. The catalytic conversion of N2 to NH3 has been achieved for an exceedingly small number of complexes, all predominantly featuring transition metals binding dinitrogen in a side-on terminal or bridging coordination mode. In Chapter 3 of this thesis, we developed our investigations using the diuranium side-on N2 complex supported by bulky triamidoamine ligand. As a result, we have established, for the first time, molecular U-mediated catalytic N2 to NH3 conversion and moreover demonstrated N2 to NH3 catalysis with side-on bound coordination of N2. Independent isolation, characterisation, and stoichiometric reactions of the N22-, N23-, N24- and N3- dinitrogen reduction intermediates suggest their relevance in the catalytic conversion of N2 to NH3 when involving side-on bridging N2. Redox-active ligands, and especially N-heterocyclic systems such as 2,2'-bipyrimidine, were recognized over a decade ago as effective bridging units capable of mediating magnetic exchange between two lanthanide(III) ions, potentially enabling single-molecule magnet behaviour. However, uranium bipyrimidine chemistry has seen no precedent, but understanding exchange interactions in assemblies in which uranium centres are linked by bridging redox-active moieties could provide a framework for assessing how electronic structure influences reactivity and magnetic properties in uranium systems. In Chapter 4 of this thesis, a body of work centred around uranium complexes with redox-active ligand 2,2'-bipyrimidine will be presented, where we have identified convenient routes for the synthesis of 5f-5f dinuclear complexes in low