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)­(bpy^R2)]­(OTf)₂ (2^R, where Cp* = η⁵-pentamethylcyclopentadienyl anion and bpy^R2 = 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 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 H₂ 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 × 10^–5 mol % of 2^6OH 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.