Estudo teórico de complexos polipiridínicos de Ru Aqua/Oxo em solução aquosa
Leandro Rezende Franco
Recent advances in the development of artificial photosynthesis have demonstrated the photocatalytic capacity of ruthenium-based polypyridine compounds (Ru) in water cleavage reactions. Hydrogen gas can be produced from the water cleavage, a renewable, unlimited, and non-polluting energy source. Compounds with a single metallic center and single aqueous coordination stand out for their simplicity of synthesis and high catalytic efficiency. In this thesis, we present a theoretical study of the
... ructural and electronic properties of 5 Ru-Aqua/Oxo mononuclear complexes in aqueous solution, with different states of oxidation, protonation, and spin: , where py =pyridine and bpy =bipyridine. Using a multiscale approach that combines quantum mechanics and molecular mechanics (QM/MM) methods, we studied the effects of the solvent and the proton/electron transfer between the complexes and the solution in the conformational and electronic structure of the complexes. We investigated the structure of the solvent around the complexes and the role of specific solute/solvent interactions in their electronic properties. We explicitly investigated the role of the solvent in the electron excitations of valence shells (UV-Vis spectroscopy) and core shells (X-ray photoelectronic spectroscopy -XPS). We investigated the application of different methodologies in the theoretical prediction of pKa and redox potentials of various chemical reactions involving proton/electron transfer, in an isolated and coupled way. The studied complexes have a very rigid conformational structure. The effects of solvation, oxidation, or deprotonation are noted mainly between the coordinated atoms and the metal center. At the water/vacuum interface, the position of these complexes is correlated with their total charges: the lower the charge, the closer to the surface. In general, there is the formation of a first complete solvation shell with approximately 70 water molecules, and depending on the complex, the Aqua/Oxo groups make between 1 to 3 hydrogen bonds with the solution, with long lifetimes. The complexes have frontier molecular orbitals of the type d in Ru (HOMO or SOMO) and π * in the bipyridines (LUMO), which are destabilized in solution. In the valence shells, electronic excitations occur mainly from metal d orbitals to π * orbitals of the polypyridine ligands in the visible and ultraviolet spectrum range, and between the π and π * orbitals of the polypyridine ligands in the ultraviolet range. In the electronic core shells, Ru(3d) and C(1s), the electron binding energy undergoes redshifts that are governed mainly by the solute-solvent interaction up to the first solvation shell. The quantum mechanics treatment of the complex and all of its first solvation shell leads to a correct description of the XPS peaks. The pKa and redox potentials of these complexes are sensitive to the solvent model and its polarization, and different combinations of QM and MM methods can lead to accurate values of these properties. Therefore, in this work, we have demonstrated the importance of the explicit treatment of the molecular environment in the study of Ru-Aqua/Oxo complexes, by which we have elucidated the structure and dynamics of aqueous solutions of the studied complexes and the various physical/chemical processes involved in the catalysis of water cleavage reactions, from the absorption of light to the proton/electron transfer with the solution.