Efficient Catalysis of Rare-Earth Metal Ions in Photoinduced Electron-Transfer Oxidation of Benzyl Alcohols by a Flavin Analogue

Abstract

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.