New directions on photochemical and supramolecular control of homogeneous catalysis

Mimicking the allosteric control typical of enzymatic catalysis is a long lasting goal for chemists. The present thesis deals with the development of new strategies to control the performance and hopefully switch ON and OFF homogeneous catalysts by implementing photochemical and supramolecular devices. As long as the photochemical approach is concerned, a new phosphine ligand endowed with a coumarin moiety, known to be a typical photo-dimerizable molecule, was prepared and employed for the preparation of Pt(II) metal complexes. The ligand topicity on the complex could be switched from monodentate to bidentate upon irradiation and coumarin dimerization. This feature was investigated in different solvents and exploited in three different catalytic reactions such as alkene isomerization and dimerization as well as Diels-Alder cycloaddition. In all cases the photodimerized catalyst bearing the bidentate ligand showed clearly a different behaviour with respect to the original catalyst bearing two monophosphines. Another approach consisted in the synthesis and study of the coordination properties of new phosphine ligands incorporating a dithienyl moiety which is known to be an excellent reversible photochromic unit characterized by geometric and electronic variations between two isomeric forms. The electronic properties of the ligands were studied in detail by synthesis of the corresponding selenides and Rh(I)carbonyl Vaska type complexes. Dithienyl derivatives bearing pyridine moieties were also prepared to be transformed by alkylation into the corresponding biscations and these were employed as competitive inhibitors for supramolecular catalysts. In particular, they were tested both in organic medium in the reaction of hydration of isonitriles catalyzed by a self-assembled hexameric capsule based on resorcin[4]arene units and in aqueous medium in the Diels Alder cycloaddition reaction between standard dienes and dienophiles catalyzed by b-cyclodextrin. In former case the two photogenerated isomeric dithienyl cations led to different inhibition activity as a consequence of their different geometry that results in modified host-guest interactions with the supramolecular catalysts employed. The above mentioned hexamer has been successfully exploited as an allosteric reversible supramolecular effector able to modulate the photo-catalytic activity of [Ru(bpy)3](OTf)2 in the sulfoxidation of thioethers in organic medium in the presence of molecular oxygen. The catalytic activity was reversibly turned off when the Ru(II) catalyst was hosted within the hexameric capsule, and was turned on by addition of a competitive guest for the hexamer such as tetraethylammonium triflate. This approach emonstrates that simply changing the solvation sphere of a catalyst form the bulk solvent into a more static supramolecular assembly it is possible to completely control the performance of a catalyst. Overall, the present PhD dissertation introduced innovative photochemical and supramolecular approaches to modulate the homogeneous catalytic activity of a catalyst opening the road to further developments and possible applications.