Efficient Iridium Catalysts for Base-Free Hydrogenation of Levulinic Acid

The synthesis and characterization of new dicationic ruthenium and iridium complexes bearing a dipyridylamine ligand (dpa) are reported. These complexes display an unusual zwitterionic molecular structure in the solid state. The iridium complex [Cp*Ir(dpa)(OSO3)] (Id) was found to be very efficient in base-free hydrogenation of levulinic acid into gamma-valerolactone (GVL). TONs as high as 174000 in hydrogenation have been obtained. We have demonstrated that reduction of LA into GVL by transfer hydrogenation with formic acid is in fact operating by hydrogenation fed by preliminary formic acid dehydrogenation. A mechanism based on the characterization and isolation of Ir-H complexes is proposed.