Improvement of durability of an organic photocatalyst in p-xylene oxygenation by addition of a Cu(ii) complex

The catalytic durability of an organic photocatalyst, 9-mesityl-10-methyl acridinium ion (Acr ⁺-Mes), has been dramatically improved by the addition of [{tris(2-pyridylmethyl)amine}Cu ^II](ClO ₄) ₂ ([(tmpa)Cu ^II] ²⁺) in the photocatalytic oxygenation of p-xylene by molecular oxygen in acetonitrile. Such an improvement is not observed by the addition of Cu(ClO ₄) ₂ in the absence of organic ligands. The addition of [(tmpa)Cu] ²⁺ in the reaction solution resulted in more than an 11 times higher turnover number (TON) compared with the TON obtained without [(tmpa)Cu ^II] ²⁺. In the photocatalytic oxygenation, a stoichiometric amount of H ₂O ₂ formation was observed in the absence of [(tmpa)Cu ^II] ²⁺, however, much less H ₂O ₂ formation was observed in the presence of [(tmpa)Cu ^II] ²⁺. The photocatalytic mechanism was investigated by laser flash photolysis measurements in order to detect intermediates. The reaction of O ₂ ^- with [(tmpa)Cu ^II] ²⁺ monitored by UV-vis spectroscopy in propionitrile at 203 K suggested formation of [{(tmpa)Cu ^II} ₂O ₂] ²⁺, a transformation which is crucial for the overall 4-electron reduction of molecular O ₂ to water, and a key in the observed improvement in the catalytic durability of Acr ⁺-Mes. This journal is