Ruthenium tris bipyridine derivatives and their photocatalytic activity in [4+2] cycloadditions. An experimental and DFT study

We report the study of a series of Ru-bipyridine (Ru-Bpy) complexes to correlate their photophysical and electrochemical properties to their performance in [4 + 2] cycloadditions, as a model reaction for electron transfer catalysis. Redox potentials, absorption and emission spectra, quantum yields and DFT calculations are presented to understand the catalytic transformation. The study shows that complex [Ru(Bpy)(3)](PF6)(2) is the more active photocatalyst, giving complete conversion to the product after 2 h reaction under white light. Any substitution in the para position of the Bpy ring decreases the reaction conversion. The [4 + 2] cycloaddition can be performed with different substrates and dienes, using either acetonitrile or nitromethane as solvents. Mechanistic studies suggest that the active catalyst for the transformation is [Ru(Bpy)(3)](3+) and that the rate determining step is the oxidation of [Ru(Bpy)(3)](2+) to generate the corresponding Ru3+ species.