Influence of the Steric Bulk and Solvent on the Photoreactivity of Ruthenium Polypyridyl Complexes Coordinated to l-Proline.

Ruthenium polypyridyl complexes are good candidates for photoactivated chemotherapy (PACT) provided that they are stable in the dark but efficiently photosubstitute one of their ligands. Here the use of the natural amino acid l-proline as a protecting ligand for ruthenium-based PACT compounds is investigated in the series of complexes Λ-[Ru(bpy)(l-prol)]PF ([1a]PF; bpy = 2,2'-bipyridine and l-prol = l-proline), Λ-[Ru(bpy)(dmbpy)(l-prol)]PF ([2a]PF and [2b]PF; dmbpy = 6,6'-dimethyl-2,2'-bipyridine), and Λ-[Ru(dmbpy)(l-prol)]PF ([3a]PF). The synthesis of the tris-heteroleptic complex bearing the dissymmetric proline ligand yielded only two of the four possible regioisomers, called [2a]PF and [2b]PF. Both isomers were isolated and characterized by a combination of spectroscopy and density functional theory calculations. The photoreactivity of all four complexes [1a]PF, [2a]PF, [2b]PF, and [3a]PF was studied in water (HO) and acetonitrile (MeCN) using UV-vis spectroscopy, circular dichroism spectroscopy, mass spectrometry, and H NMR spectroscopy. In HO, upon visible-light irradiation in the presence of oxygen, no photosubstitution took place, but the amine of complex [1a]PF was photooxidized to an imine. Contrary to expectations, enhancing the steric strain by the addition of two ([2b]PF) or four ([3a]PF) methyl substituents did not lead, in phosphate-buffered saline (PBS), to ligand photosubstitution. However, it prevented photoxidation, probably as a consequence of the electron-donating effect of the methyl substituents. In addition, whereas [2b]PF was photostable in PBS, [2a]PF quantitatively isomerized to [2b]PF upon light irradiation. In pure MeCN, [2a]PF and [3a]PF showed non-selective photosubstitution of both the l-proline and dmbpy ligands, whereas the non-strained complex [1a]PF was photostable. Finally, in HO-MeCN mixtures, [3a]PF showed selective photosubstitution of l-proline, thus demonstrating the active role played by the solvent on the photoreactivity of this series of complexes. The role of the solvent polarity and coordination properties on the photochemical properties of polypyridyl complexes is discussed.