Iridium Dihydroxybipyridine Complexes are Effective Catalysts for Hydrodeoxygenation of Vanillyl Alcohol in Water

The selective reduction and deoxygenation of ligninderived organic compounds are of interest for modeling a key reaction in the utilization of biomass. Toward this goal, vanillyl alcohol is used as a lignin monomer surrogate herein, and we study its reduction to form creosol in an aqueous solution. Four water-soluble iridium catalysts of the type [Cp*Ir(OH2)(bpy(R2))](OTf) (2) (2(R), where Cp* = eta(5)-pentamethylcyclopentadienyl anion and bpy(R2) = n,n '-R-2-2,2 '-bipyridine with n = 4 or 6) with different R substituents (R = H, OH, Me) in different positions on the bipyridine ligands were studied for this hydrodeoxygenation (HDO) reaction on vanillyl alcohol. Modification of the bipyridine ligands demonstrated that a more electron-rich bpy-derived ligand (R = OH) gives a more efficient HDO reaction. The addition of base serves to further enhance the HDO reaction by deprotonating the protic OH groups (OH groups on n,n '-dihydroxybipyridine where n = 4 in 2(4OH) or 6 in 2(6OH)) resulting in a more electron-rich catalyst. Proximal OH groups in 2(6OH) produce our most active catalyst, and we can suggest that a metal-ligand bifunctional mechanism of H-2 activation and/or transfer to the substrate may be responsible for the greater efficiency of 2(6OH) vs 2(4OH). The catalyst loading could be reduced to 5 x 10(-5) mol % of 2(6OH) with 0.5 mol % Na2CO3 and 997,000 turnovers (TON) could be achieved in 20 h at 100 degrees C. Furthermore, the same catalyst at 1 x 10(-4) mol % produces 836,000 TON under similar but base-free conditions. Such catalytic efficiency in a dilute aqueous solution is noteworthy for potential applications.