An advanced EPR investigation of copper complexes in catalysis
Cu(II) coordination chemistry is of significant interest due to copper's widespread applications, particularly in catalysis. This thesis explores the molecular structure, electronic properties, and variable coordination geometry of Cu(II)-tripodal ligands. While square planar and square pyramidal Cu(II) complexes are common, less attention is given to trigonal bipyramidal (tbp) Cu(II) centres due to limited supporting ligands. This latter structure can be formed using the tripodal nitrogen donor ligands including tris(2-aminoethyl)amine (TREN), tris(2-pyridylmethyl)amine (TPMA) and tris(2-(isopropylamino)ethyl)amine (isp3-TREN). This study utilizes Electron Paramagnetic Resonance (EPR) spectroscopy to comprehensively examine how different ligand types and ratios influence coordination and symmetry in solution. The study examined the influence of the tripodal ligand on the local geometry of Cu(II) centres. The isp3-TREN ligand, being more flexible, required larger excesses for Cu(II) complexation compared to the other two ligands with high Cu(II) affinity. Counter ions had a negligible effect on coordination through outer sphere interactions. Spin Hamiltonian parameters were highly dependent on ligand amounts, revealing subtle variations in Cu(II) solution coordination chemistry and potential opportunities. Further, we explored the pH dependence of Cu(II) coordination with TREN ligands. EPR provided detailed information on Cu(II)-TREN complexes across a wide pH range, including various species. Spin Hamiltonian parameters for the solution-based [Cu(TREN)(OH)]+ structure was accurately identified for the first time. This pH study revealed disparate environments where both square pyramidal and trigonal bipyramidal structures can exist in solution, emphasizing the importance of understanding solution-based structures. Finally, the role of the different Cu(II)-TREN complexes in the selective oxidation of glycerol was explored. Under basic pH conditions, the CuCl2 salt was found to deliver some glycerol conversion to glyceric, glycolic and formic acids, whilst the Cu(II)-TREN complex was found to produce formic, oxalic, glycolic, glyceric acids and even glyceraldehyde. The emergence of C2 and C1 product formation in the absence of a strong oxidant for C-C bond breakage is an unusual occurrence, prompting the need for additional investigation. The crucial role of chloride in glycerol reaction pathways necessitates further detailed investigations to comprehend this complex catalytic pathway and its relation to Cu(II) geometry and coordination. Attempts to heterogenise the Cu(II) complexes into Y zeolites for different catalytic purposes were made. Though the correlation between Cu(II) encapsulation, Si:Al ratio, and proton count in the zeolitic structure was identified, the heterogeneous material proved unsuitable for glycerol oxidation. Nevertheless, it holds promise for exploring alternative catalytic reactions.
Cardiff : Cardiff University & University of Antwerp , 2023
xiii, 182 p.
Supervisor: Murphy, Damien M. [Supervisor]
Supervisor: Van Doorslaer, Sabine [Supervisor]
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The publisher created published version Available from 22.02.2026
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Creation 23.02.2024
Last edited 23.02.2024
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