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Using Ligand Design to Probe the Redox Chemistry of the Actinyl Ions
[Thesis]. Manchester, UK: The University of Manchester; 2011.
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Abstract
The synthesis and characterisation of a series of {AnO2}n+ complexes containing multidentate ligand environments is reported.Three novel {UO2}2+ complexes (1 - 3) containing N3O2 linear pentadentate ligands have been prepared and crystallographically characterised. NMR spectroscopy has been able to show that 1 – 3 are stable with respect to ligand exchange, in a range of solvents. The strength of the O=U=O unit has been probed by vibrational spectroscopy and 1 - 3 exhibit some of the weakest O=U=O ν1 stretching modes reported (802 – 805 cm-1). The cyclic voltammetry (CV) of 1 - 3 in various solvents (0.1 M [Bu4N][PF6]) has been performed and indicate the position and reversibility of the {UO2}2+/{UO2}+ redox couple has been found to be subtly dependent on the solvation environment.{UO2}2+ complexes (4 - 6) have been prepared by subtle modification of reaction conditions using a rigid N2O2S linear pentadentate ligand. Characterisation by X-Ray diffraction reveals different monometallic systems, where 4 and 5 are solely bound to the O2 donors of the ligand and 6 exhibits uranyl binding through all of the donor atoms in the N2O2S cavity. 1H NMR spectroscopy shows 5 exhibits intramolecular rearrangement on the NMR timescale in DCM, but undergoes intermolecular ligand exchange in more coordinating solvents (DMSO, py). Cyclic voltammetry of 5 in DCM (0.1 M [Bu4N][PF6]) also indicates that rearrangements and/or ligand exchange processes may occur at rate comparable to that of the CV studies. Complex 6 exhibits stability with respect to ligand exchange or rearrangement in various solvents and shows comparable solvation environment dependency of the {UO2}2+/{UO2}+ redox couple, relative to 1 - 3.Three monometallic {UO2}2+ complexes (7 - 9) have been prepared using a rigid tetradentate N2O2 ligand. A dimetallic {UO2}2+ complex (10) where two {UO2}2+ are linked by a 4,4‟-bipyridine bridge, has been formed by controlled ligand exchange. Vibrational spectroscopy shows the presence of the O=U=O ν1 stretch in both the Raman and infrared spectra for 7 - 10, likely to be caused by distortion of the ligand about the {UO2}2+ equatorial plane causing a change in dipole for the O=U=O ν1 stretching mode.A synthetic and spectroscopic study of neptunyl coordination to N3O2 linear pentadentate ligands has been made. Adopting a similar method that was successfully employed in the synthesis of 1 - 3, resulted in the reduction of {NpO2}2+ to {NpO2}+, giving a series of complexes (11 - 13). Complex 13 has been crystallographically characterised and shows a monometallic {NpO2}+ complex which is bound to all of the atoms in the N3O2 cavity.
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CD-ROM containing crystallographic information files (CIFs) submitted in pocket inside back cover of print version of thesis.