Visible-light-induced iron redox-catalyzed selective transformation of biomass into formic acid

Formic acid (HCO₂H) plays a key role in various chemical processes and industrial procedures, acts as a hydrogen-storage candidate, and acts as a key intermediate for streamlined hydrogen production from biomass. Therefore, development of efficient HCO₂H production from renewable biomass instead of multiple-step production from the diminishing fossil resource is more attractive. We describe the development of visible-light-induced iron redox-catalyzed selective transformation of biomass and derivatives into formic acid in up to 91% yield at ambient temperature under mild conditions. Mechanistic studies indicate that the transformation occurs possibly through multiple sequences of iron(III)-catalyzed ligand-to-metal charge transfer (LMCT), β-scission, hydrogen atom transfer (HAT), and iron(II)-catalyzed single-electron transfer (SET). The selectivity toward formic acid is controlled by cooperation of the catalyst loading, light energy, and oxygen (O₂) concentration. A new mode for cleavage of C–C bonds in complex biomass is opened.