|Ph.D Student||Batrice Rami John|
|Subject||Advances in Actinide Catalysis and Coordination Chemistry|
|Department||Department of Chemistry||Supervisor||Professor Moris Eisen|
|Full Thesis text|
The catalytic activity of a series of amido-actinide complexes of the formula U[N(SiMe3)2]3 and [(Me3Si)2N]2Th[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)] (An = U, Th) was systematically studied in a various organic transformations, providing insight into the differences in reactivity for early actinides. The reaction of these amido complexes with terminal alkynes selectively formed short oligomers, or when either uranium complex was used in greater than 10 mol % loading, a strong preference toward a novel actinide-mediated [2?] cycloaddition was observed.
Use of these precatalysts for the insertion of protic nucleophiles into a variety of heterocumulenes including carbodiimides, isocyanates, and isothiocyanates was studied. The products are obtained in good to excellent yield within 24 hours. Experimental kinetic and thermodynamic studies allowed us to propose the mechanism of catalysis and turnover limiting step. The first formal C-H bond insertion into a carbon‑heteroatom bond mediated by the actinides is found, occurring by the insertion of alkynes into carbodiimides, yielding the ipso‑alkynyl amidines as the product of the reaction.
The insertion of alcohols into these carbodiimides was additionally investigated, and while common knowledge suggests that this reaction would certainly be unsuccessful, the results contradicted this assumption. The actinide-amides were found to efficiently catalyse the insertion of alcohols into carbodiimides, generating isoureas and representing the only case of intermolecular alcohol insertion mediated by the actinides, and the only catalytic reaction of oxygenated substrates by a uranium catalyst. The influence of the alcohol on the reaction is inverse first‑order, informing of an inhibitory effect as alcohol concentration is increased, however the catalyst and carbdodiimide were found to be first order, allowing for the proposal of a plausible mechanism. DFT calculations on this catalytic cycle were performed and found to be in good agreement with experimentally determined thermodynamic parameters, and revealed a stabilising N,O‑bound isoureate intermediate which is cleaved by additional alcohol and allows for catalytic turnover.
In addition to these catalytic studies, several novel actinide complexes were synthesised utilising pyrrolyl and phenoxyimine ligands. The preparation of the pyrrolyl ligands employed a carbon-bridge linking two pyrrole rings containing aliphatic groups substituted on the α-position. The complexes were synthesised by salt metathesis of the dilithiated ligands with the actinide tetrachloride and show novel coordination patterns which show promise as alternatives for the classical cyclopentadienyl ligands, and result in greater electrophilicity at the metal centre. Preparation of the pyrrolyl-actinides containing tert-butyl or adamantyl substitution on the ring structure proved more robust, forming the desired mono-ligated complex according to the crystal structures obtained.
Furthermore, use of phenoxyimine ligands in actinide coordination chemistry was studied. Both acid-base and salt metathesis reactions were found to successfully result in coordination of the ligands to the actinide centre, however in the majority of complexes obtained, an unexpected ortho-activation of the phenoxyimine was observed according to X-ray diffraction studies. A single complex using these ligands was prepared which formed the desired bis(phenoxyimine) dichloride complex; after alkylation, the resulting thorium complex was studied in numerous polymerisation reactions, however none of the reactions studied showed any catalytic turnover.