Implementing Multi-Electron Transfer Strategies in Uranium Chemistry

In the last decades, the activation of small molecules has attracted increasing attention for their use as cheap and abundant feedstock. Low-oxidation state uranium complexes have displayed high reactivity towards small molecules thanks to their unique properties. Nevertheless, the multiple electrons required for small molecule activation remain a challenge, as uranium predominantly undergoes one-electron transfer. Furthermore, uranium has a great potential for the development of single molecule magnets (SMMs), because of its high spin-orbit coupling and relatively strong covalent character, which could allow high magnetic exchange. However, hitherto the reported uranium-based SMMs are inferior to those reported with lanthanides. Therefore, the main goal of this PhD work is the synthesis and characterization of uranium complexes with the objective of achieving increased activity towards small molecules and/or SMM properties. Multiple strategies can be employed to achieve multi-electron transfer in uranium chemistry. Firstly, multinuclear complexes can be used in which the uranium centers cooperatively transfer electrons. This strategy was extended to the -N(SiMe3)2 and -OSiPh3 ligand systems in combination with an oxo linker. In Chapter 2 and 3, the reactivity of reduced diuranium compounds supported by -N(SiMe3)2 ligands was investigated. Interestingly, the oxo linker breaks easily, affording a U(II) synthon that is highly reducing and can cooperatively transfer multiple electrons to various substrates, including pyridine. Furthermore, we showed for the first time that a single uranium center can transfer four electrons to azobenzene in two consecutive two-electron processes. In an attempt to obtain higher nuclearity systems, Chapter 4 describes a synthetic route to obtain a uranium(IV) trimer, as well as its one-electron reduced analogue. In addition, the reaction solvent was shown to have an impact on determining the final structure and nuclearity, as switching to pyridine resulted instead in a bis-oxo uranium(IV) complex, for which the ligands can easily be exchanged through protonolysis. Chapter 5 shows how the -OSiPh3 ligand can stabilize formal U(II)/U(IV) and U(I)/U(IV) species. These compounds can transfer two electrons to pyridine and azobenzene, respectively, while the oxo linker remains intact. Regarding SMMs, Chapter 6 shows how the 2,2'-bipyrimidine can also be used in uranium chemistry to afford radical-bridged complexes. Alternating current magnetometry indicated that the radical-bridge indeed leads to higher relaxation barriers, although the barrier values remain modest. Finally, inspired by the monouranium four-electron transfer from the U(II) and U(II) synthon, we further investigated how ligand choice can encourage monouranium multi-electron transfer. Chapter 7 demonstrates that the introduction of a cyclometalated ligand greatly enhances the reducing power of the metal center, as evidenced by its ability to reductively couple pyridine. However, insertion reactions can also take place into the U-C bond, which should be taken into account. Chapter 8 further compares various heteroleptic ligand systems and studies their suitability to isolate terminal nitride compounds through photolysis. A combination of -N(SiMe3)2 and -OSi(OtBu)3 ligands was found to stabilize the azide adduct and upon photolysis a putative terminal nitride formed, but its instability leads instead to the isolation of a bis(imido) U(VI) complex.