Abstract
Current environmental concerns and the growing demand for energy continue to challenge the scientific community. Catalysis, photocatalysis and electrocatalysis are emerging as essential methods for synthetizing high value-added products, including new fuels, making these processes key elements in the search for solutions to balance economic growth and environmental sustainability. Porphyrins and N-heterocyclic carbene (NHC) ligands play an essential role in catalysis due to their ability to form stable bonds with most transition metals at low and high oxidation states. This thesis project focuses on the investigation of the catalytic properties, including photo- and electrocatalytic and properties of molecular systems combining porphyrins and NHC metal complexes as unimolecular systems. Different NHC metal complexes were synthesized to cover a wide range of chemical reactions.First, the catalytic properties of rhodium(I) complexes of the [porphyrin-NHC-RhCl(cod)] type were studied for the 1,4 addition reaction of phenylboronic acid with 2-cyclohexen- 1-one. This study made it possible to highlight a cooperative effect between the two units depending on the bonding modes between the porphyrins and the NHC-RhCl(cod) complexes, as well as the nature of the metal cations in the porphyrins (M = H2, Ni, Zn). In order to rationalize the results obtained, the steric and electronic properties of the NHCs ligands were also studied.Secondly, other molecular systems such as [porphyrin-(NHC-pyridyl)-Re(CO)3Cl] were studied for an application in photocatalysis, more precisely for the CO2 reduction reaction. Many systems that combine photocatalysts and catalysts based on transition metals have been described in the literature. Nevertheless, until now, none of these approaches have simultaneously exploited the photocatalytic capabilities of porphyrins and NHC metal complexes. In this study, porphyrins can be used as photosensitizers and are responsible for absorbing visible light and mediating light-induced electron transfer reactions to peripheral NHC-Re metal complexes. This study showed how the catalytic activities of these complexes can be modulated depending on the position of the porphyrin relative to the NHC-Re complex. Also, photophysical studies using different steady-state and time-resolved spectroscopic techniques have been carried out in order to understand the reaction mechanisms involved.Finally, in a third part, dimers of cofacial porphyrins assembled by four or eight NHC-Au bonds were synthesized. The influence of the nature of the spacers between the porphyrins and the NHCs ligands (phenyl vs. benzyl), of the length of the alkyl chains on the NHCs ligands (n-hexyl vs. n-butyl) and of the number of NHC-Au bonds binding porphyrins has been studied. Thus, within the framework of this project, the use of these molecular assemblies for applications in electrocatalysis was explored. The study focuses on the influence of the nature of NHC ligands on the modulation of the porphyrin-porphyrin distance, a crucial parameter for the two metalloporphyrins to act cooperatively and improve catalytic efficiencies. To do this, zinc(II) and cobalt(II) porphyrin dimers were synthesized and their electrocatalytic properties in O2 and CO2 reduction reactions were studied.