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

The selective reduction and deoxygenation of lignin-derived 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)(bpyR2)](OTf)2 (2R, where Cp* = η5-pentamethylcyclopentadienyl anion and bpyR2 = n,n′-R2-2,2′-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 24OH or 6 in 26OH) resulting in a more electron-rich catalyst. Proximal OH groups in 26OH produce our most active catalyst, and we can suggest that a metal–ligand bifunctional mechanism of H2 activation and/or transfer to the substrate may be responsible for the greater efficiency of 26OH vs 24OH. The catalyst loading could be reduced to 5 × 10–5 mol % of 26OH with 0.5 mol % Na2CO3 and 997,000 turnovers (TON) could be achieved in 20 h at 100 °C. Furthermore, the same catalyst at 1 × 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.