An advanced EPR investigation of copper complexes in catalysis

Rezayi, Seyedeh Fardokht 2023. An advanced EPR investigation of copper complexes in catalysis. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Cu(II) coordination chemistry continues to attract enormous interest due to the prevalent applications of copper centres in many applications, such as catalysis. The local geometry around the Cu(II) ions in these applications is essential, and can vary markedly, depending on the chelating ligands, the counter ions, the solvent, the pH, and even the ligand stoichiometry. This Thesis focuses on investigating the molecular structure, electronic properties, and variable coordination geometry of the Cu(II)- tripodal ligands. While square planar and square pyramidal based Cu(II) complexes are almost ubiquitous in copper chemistry, less attention has been devoted to the trigonal bipyramidal (tbp) based Cu(II) centres, partly due to the sparsity of ligands that support this arrangement. 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). The work aims to elucidate how different ligand types and ratios influence the coordination and symmetry of these complexes specifically in solution. To achieve this, Electron Paramagnetic Resonance (EPR) spectroscopy, including multi-frequency EPR, CW-ENDOR and pulsed EPR (HYSCORE, Davies- and Mims-ENDOR), was employed to comprehensively examine the electronic properties of these complexes. Firstly, the role of tripodal ligand itself on the local geometry of the Cu(II) centers was examined. The more flexible isp3-TREN ligand required larger excesses for Cu(II) complexation, compared to the other two ligands which have high affinity for Cu(II) binding. The counter ions used in the preparation had a negligible effect on the coordination through outer sphere interactions. The observed spin Hamiltonian parameters were strongly dependent on the amount of ligands used owing to the wide degree of Cu(II) speciation observed. The results illustrated the subtle variation in Cu(II) solution coordination chemistry achieved with these ligands, and by default the potential opportunities in Cu(II) solution chemistry. Secondly, the pH dependency for Cu(II) coordination using the TREN family of ligands was next studied. EPR provided detailed information on the Cu(II)-TREN complexes formed across a wide pH range, from highly acidic to highly basic, including [Cu(H2O)6] 2+, [Cu(HTREN)(H2O)2] 3+, [Cu(TREN)(H2O)]2+ and [Cu(TREN)(OH)]+ v species. The spin Hamiltonian parameters for the solution based [Cu(TREN)(OH)]+ structure was accurately identified for the first time. The principal 14N hyperfine and quadrupole values, and 1H hyperfine values for all the above solution-based Cu(II)- TREN complexes were experimentally determined and compared to DFT. This pH study revealed the disparate environments in which both the square pyramidal and trigonal bipyramidal structures can exist in solution, and why knowledge of the solution-based structures is invaluable. 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 fact that C2 and C1 product formation was observed is unusual owing to the absence of a strong oxidant that promotes C-C bond breakage, and further work is required to understand this. The presence or absence of chloride was also found to play a crucial role in the glycerol reaction pathways. Further detailed investigations are clearly necessary to rationalise and understand this exceedingly complex catalytic pathway and the relationship to Cu(II) geometry and coordination. Attempts to heterogenise the Cu(II) complexes into Y zeolites for different catalytic purposes were made. Whilst the relationship between Cu(II) encapsulation and Si:Al ratio and number of protons in the zeolitic structure was identified, the heterogeneous material was not useful for glycerol oxidation. It, however, opens new possibilities for other catalytic reactions.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Funders:	PARACAT-MSCA
Date of First Compliant Deposit:	4 April 2024
Last Modified:	05 Apr 2024 09:56
URI:	https://orca.cardiff.ac.uk/id/eprint/167710

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