Development of structurally and electronally versatile aminopyridine cobalt complexes for photo-(electro) reduction of water and ketones
The increasing need for more efficient synthetic methods and sustainable processes for fuel and high-value organic molecules production can be seen as one of the major challenging goals for the future. Nature has developed a sophisticated system to store the light energy into chemical bonds. The mim...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/674245 |
| Acceso en línea: | http://hdl.handle.net/10803/674245 |
| Access Level: | acceso abierto |
| Palabra clave: | Cobalt Cobalto Fotoquímica Photochemistry Electroquímica Electrochemistry Reducció de cetones Reducción de cetonas Ketone reduction Reducció de l'aigua Reducción del agua Water reduction Catàlisi Catálisis Catalysis 546 |
| Sumario: | The increasing need for more efficient synthetic methods and sustainable processes for fuel and high-value organic molecules production can be seen as one of the major challenging goals for the future. Nature has developed a sophisticated system to store the light energy into chemical bonds. The mimicking of the natural systems through the development of artificial photosynthetic schemes is extremely interesting since it could effort green reduction alternatives. Toward this end, in this thesis we describe a new family of cobalt complexes based on aminopyridine ligands able to reduce protons and ketones under photochemical conditions. Additionally, the same catalytic systems were found active for the electrocatalytic proton reduction to H2. Ligand availability, modularity and versatility of this type of coordination complexes let us to tune the first coordination sphere of the metal by changing the electronic and structural features of the ligand. This allows us to pinpointing preferred ligand structures to sustain efficient H2 and alcohol production. In addition, the straightforward tuning of the ligand nature let to modulate or even alter the selectivity proton-ketone reduction. The high modularity of these systems also allows to modify the secondary coordination sphere of metal center. In this way, the functionalization of cobalt systems with a biotin moiety bring us to encapsulate the cobalt catalyst system into a protein environment. We prove that the natural pocket enhances the reactivity in water reduction to H2. Mechanistic studies and characterization of the intermediates have been pursued, with the aim of understanding the requirements for a better design of cobalt-based catalysts. Spectroscopic, magnetic and NMR experiments, along with DFT calculations suggest that the CoI intermediate with the Py2Tstacn ligand shows an important electron density over the pyridine moiety, leading to a formal CoII metal center. Finally, using fluorescence quenching, UV/Vis, electrochemical, kinetic and isotope labelling experiments the mechanism in the photocatalytic ketone reduction is constructed. Supported by DFT calculations, a heterolytic pathway for ketone reduction seems to be preferred, although the contribution of a homolytic pathway could not be fully ruled out. This thesis paves the way for the construction of more active cobalt based catalytic systems for light-driven reduction reactions |
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