Di Pietro, PA; (2017) Computational Studies of Actinide Complexes with Expanded Porphyrins. Doctoral thesis , UCL (University College London).

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Abstract

Covalency in complexes of the actinides has been identified as the potential driving force behind selective behaviour exhibited by separation ligands of use to the nuclear industry. In this thesis, complexes of actinyls with hexadentate macrocyclic expanded porphyrin ligands are investigated at the density functional level of theory and their electron densities analysed in detail. Initially, strong correlations are established between the vibrational frequencies of the distinctive uranyl stretching modes and covalency in the equatorial bonds of several simple uranyl complexes with monodentate first row ligands, with redshift of the uranyl stretching modes indicating a weakening of the U-Oyl interaction as a result of competing interactions in the equatorial plane. Subsequently, strong similarities are established in the U-N and U-Oyl bonding character of two multidentate uranyl complexes: UO2-isoamethyrin(1.0.1.0.0.0) and [UO2(bis-triazinyl-pyridine)2]2+, where isoamethyrin(1.0.1.0.0.0) is a hexadentate macrocyclic expanded porphyrin ligand and bis-trizinyl-pyridine (BTP) is a tridentate ligand which has shown selectivity for An(III) over Ln(III). A series of uranyl hexaphyrin complexes is then investigated, finding moderate correlations between stability, equatorial covalency and the frequencies of the uranyl stretching modes, which crucially only hold when there is a degree of relative planarity in the ligand. It is found that smaller ligands have greater stability and equatorial covalency. Broadening the study to include neptunyl and plutonyl complexes finds that the isoamethyrin complex shows some evidence for selectivity for uranyl over later actinides in the same oxidation state, but significant spin contamination throws the appropriateness of these methodologies for dealing with open-shell actinide systems into question. Preliminary calculations performed using spin constrained DFT were found to be helpful here, but a full geometry reoptimisation will ultimately be necessary to fully appreciate the effects of spin contamination on the geometry and electronic structure of the neptunyl and plutonyl complexes.

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