The activation of small molecules is a paramount challenge in modern chemistry. The use of cheap and abundant molecules such as N2, H2, CO2, or CO as energy supplies or as precursors for fine chemicals production is highly desirable. In particular, the only industrial process known so far that uses the ubiquitous molecule N2 is the Haber-Bosch process for the production of NH3, which consumes about 2% of the world energy yearly. Low valent uranium complexes have been proven to be ideal candidates in the field of small molecule activation such as N2. The activation of N2 in the Haber-Bosch process is thought to proceed through the formation of metal nitrides. The study of molecular nitride species, thus, is extremely important in light of the possibility to achieve N-functionalization processes. Uranium nitride species, indeed, are able to effect the activation of small molecules by N-functionalization. Low valent uranium species are of interest also for their magnetic properties. The magnetic anisotropy and the unpaired electron in U(III) species make them, indeed, ideal candidates as Single Molecule Magnets (SMM).
The main objective of this thesis is to synthesize novel uranium species for the activation of small molecules and to synthesize low valent uranium species of interest for their magnetic properties.
In the first part the synthesis of a dimeric U(III) complex bridged by an oxo linker and its reactivity with N2 will be reported. The functionalization of the activated N2 shows striking differences compared to previously reported complexes. The difference in reactivity is accompanied by a difference in the magnetic properties of the complexes, which reflects their different electronic structure and binding scheme.
Since the full splitting of N2 could not be achieved so far with U(III) complexes, the synthesis of a bis-µ-nitride complex, which would serve as a model for cleaved N2, was targeted. Its synthesis and characterization are reported in Chapter 3. We proved that such a bis nitride complex is reactive and is able to activate small molecules, forming N-C bonds by reaction with molecules like CO2, CS2, or CO. More importantly, H2 is activated by the bis nitride complex in an unprecedented oxidative cleavage.
While uranium nitride complexes supported by the siloxide ligand were reported to exhibit nucleophilic reactivity, other nitride complexes are instead unreactive or even electrophilic. In order to rationalize this behaviour, the synthesis of different uranium nitride complexes bearing different coligands has been targeted and their synthesis is described in Chapter 4. The importance of the coligand has been experimentally proven in closely related systems. Despite several bridging nitride complexes have been reported, a paucity of terminal uranium nitride complexes are, instead, isolable. In particular, the photochemical reactivity of terminal uranium azide complexes has so far failed to yield the desired terminal nitride species. In this work we report the first photochemical synthesis of an isolable uranium terminal nitride. The terminal nitride complex is remarkably stable and its functionalization with CO is reported. Lastly, the synthesis of low valent heteroleptic complexes is reported. These compounds can serve as starting materials for the synthesis of novel uranium complexes and are of interest for their magnetic properties.EPFL