Triflate-Functionalized UiO-66 Metal–Organic Framework Nanoparticles for Tandem Synthesis of Xanthones

Abstract

Synthetic chemists have always pursued multifunctional heterogeneous catalysts for their ultrahigh efficiency, excellent selectivity, and good compatibility. In this work, we report bifunctional metal–organic framework (MOF) nanoparticles that integrate strong Lewis acid and photoredox sites (UiO-66-OTf) as an efficient nanocatalyst for tandem photo-oxidized 2-aryloxybenzaldehydes with O2 to xanthones. The triflate groups attached to the MOF nodes greatly enhanced the Lewis acidity, enabling UiO-66-OTf to catalyze reductive cyclization of 2-aryloxybenzaldehydes to xanthenes with ethanol as a reductant. This acid modifier also improved the photogenerated charge transfer efficiency of UiO-66-OTf due to its strong electron-withdrawing effect. Thus, the photocatalytic site of UiO-66-OTf can further convert the formed xanthenes into the corresponding xanthones via a photo-oxidation process with O2 as the oxidant in one pot. This catalytic system can tolerate a variety of electronically diverse substrates. UiO-66-OTf can be reused six times and retains crystal structure integrity.