Efficient catalysis of rare-earth metal ions in photoinduced electron-transfer oxidation of benzyl alcohols by a flavin analogue

A flavin analogue (riboflavin-2′,3′,4′,5′-tetraacetate, Fl) forms the 1:1 and 1:2 complexes with rare-earth metal ions. The largest formation constants K₁ and K₂ for the 1:1 and 1:2 complexes between Fl and Sc³⁺ are determined as K₁ = 3.1 × 10⁴ M^-1 and K₂ = 1.4 × 10³ M^-1, respectively. The complexation of Fl with rare-earth metal ions results in blue shifts of the fluorescence maximum, shortening of the fluorescence lifetime, and more importantly the change in the lowest excited state from the n,π* triplet state of Fl to the π,π* singlet states of Fl-rare-earth metal ion complexes as indicated by the disappearance of the triplet-triplet (T-T) absorption spectrum of Fl by the complexation with metal ions. The strong complex formation between Fl and rare-earth metal ions enhances the oxidizing ability of the excited state of Fl as indicated by the significant acceleration in the fluorescence quenching rates of Fl-rare earth metal ion complexes via electron transfer from electron donors (e.g., alkylbenzenes) as compared to those of uncomplexed Fl. The one-electron reduction potential of the singlet excited state of the 1:2 complex between Fl and Sc³⁺, ¹(Fl-2Sc³⁺)* (* denotes the excited state), is positively shifted by 780 mV as compared to ¹Fl*. Such a remarkable enhancement of the redox reactivity of ¹(Fl-2Sc³⁺)* as compared to that of ¹Fl* makes it possible to oxidize efficiently p-chlorobenzyl alcohol to p-chlorobenzaldehyde by ¹(Fl-2Sc³⁺)*, although no photooxidation of p-chlorobenzyl alcohol by Fl occurred in deaerated MeCN. The quantum yield for the photooxidation of p-chlorobenzyl alcohol by Fl-2Sc³⁺ is the largest among various Fl-metal ion complexes. A comparison of the observed rate constant derived from the dependence of the quantum yield on the concentration of p-chlorobenzyl alcohol with the fluorescence quenching rate constant by electron transfer from the alcohol and the direct detection of radical intermediates reveal that the photooxidation proceeds via electron transfer from p-chlorobenzyl alcohol to ¹(Fl-2Sc³⁺)*. Under an atmospheric pressure of oxygen, the photooxidation of p-methoxybenzyl alcohol by oxygen proceeds efficiently in the presence of Fl-Lu³⁺ which acts as an efficient photocatalyst. No photodegradation was observed in the case of the Fl-Lu³⁺ complex, whereas the facile photodegradation of Fl-Mg²⁺ has precluded the efficient photocatalytic oxidation of the alcohol by oxygen.