Chemistry of water-soluble N-heterocyclic carbene platinum complexes

Abstract

Water-soluble N-Heterocyclic Carbene (NHC) complexes of transition metal are currently captivated increasing attention because of a number of potential applications, such as biphasic catalysis. However, their chemical behavior in aqueous phase has been virtually unexplored so far. This PhD Dissertation concerns fundamental studies on the synthesis and reactivity of sulfonated NHC platinum complexes in water as a solvent, with special emphasis on the hydrolytic stability of Pt?C bonds. Several efficient protocols for their syntheses, and their limitations, have been uncover. The new complexes readily undergo a rich variety of different transformations (e.g., protonolysis, ligand substitution, oxidative addition/reductive elimination, controlled formation of water-soluble and stable nanoparticles, intramolecular C?H bond activations through a singular pathway, etc), and behave as convenient catalysts in the model processes tested here. The great majority of these reactions are quite straightforward and stereoselective, and in all of them ?and under a wide range of conditions (pH, temperature), the Pt?NHC bonds have been found to be fairly robust in aqueous medium. Thus, the presence of the sulfonated ligand, not only provides water-solubility, but also stability to the diversity of complexes studied in this solvent (halide, methyl, hydridealkynyl, ?-alkene, etc., derivatives), and to the new platinum nanoparticles presented here as well. It is remarkable that the persistent coordination of the ligand to the surface of the nanoparticles has been ambiguously demonstrated by determining the platinum?C(NHC) coupling constant for the first time for a nano-system. Finally, it is also worth to note that, according to DFT calculations, the remote sulfonate moiety assists intramolecular C?H bond cleavages, reducing considerably the Gibbs activation energy of the process (5?7 kcal/mol in the gas-phase) when compared to a conventional oxidative addition mechanism. The latter might open avenues to practical applications undergoing under friendly operational settings, far from the harsh reaction conditions often required for this kind of transformations.