Visible-light-induced tandem reaction of o-aminothiophenols and alcohols to benzothiazoles over Fe-based MOFs: Influence of the structure elucidated by transient absorption spectroscopy

Abstract

MIL-100(Fe) and MIL-68(Fe), two Fe-based MOFs, were found to be active for oxidative condensation between alcohols and o-aminothiophenols to form 2-substituted benzothiazoles under visible light irradiation using oxygen (O2) as oxidant. This reaction can be applied to a wide range of substrates with medium to high yield. Controlled experiments and ESR results revealed a superoxide radical (O2-)-mediated pathway, which is derived from the reduction of O2 by photogenerated Fe2+ on Fe–O clusters. The whole multistep reaction is limited by the step of the photo-oxidation of alcohols to aldehydes. MIL-100(Fe) showed catalytic performance superior to that of MIL-68(Fe) because its higher concentration of long-lived (μs time scale) positive holes can be photogenerated over MIL-100(Fe), in contrast to MIL-68(Fe). This study not only provides an economical, sustainable, and thus green process for the production of 2-substituted benzothiazoles, but also illustrates the potential of using transient absorption spectroscopy as an important tool for understanding the photophysics of MOFs, which are believed to show great potential as multifunctional catalysts for light-induced organic transformations.