Sensitized photolysis of dihydrobis (1-pyrazolyl)borate copper(II); general reaction pattern

Abstract

Several aspects of the sensitized photoreduction of dihydrobis(1-pyrazolyl)borate Cu(II) have been studied. It was found that the sensitized reduction was much more efficient than direct photolysis in hydrogen-donating solvents such as alcohols and tetrahydrofuran. The reaction products were identified as Cu(0), the protonated ligand and oxidation products derived from the solvents. The efficiencies of the reactions were quantitatively evaluated by determining the quantum yields, φ, in different solvents and in the presence of different sensitizers. The results showed that the sensitized photoreduction was more efficient in solvents with better hydrogen donating power, thus indicating that hydrogen abstraction is a key step in the reaction. A lack of correlation exists between the triplet energy levels of the sensitizers and the quantum yields of the photoreduction. The rate constant ( k q) for quenching of the benzophenone triplet state by the Cu(II) complex was determined by Stern-Volmer type experiments. An overall mechanism for the sensitized photoreduction has been proposed.