Ionic liquids as acid/base buffers in non-aqueous solvents for homogeneous catalysis: A case of selective hydrogenation of olefins and unsaturated aldehyde catalyzed by ruthenium complexes

We present a novel approach to tune acidity/basicity in non-aqueous and low-water media by using a class of ionic liquids (ILs) with buffering characteristics, which was readily synthesized by the reaction of 1-alkyl-3-methylimidazolium hydroxide ([RMIM]OH) base moieties with a serials of binary or polybasic acids and defined as ionic liquid-buffers (IL-buffers). We have performed controlled experiments of hydrogenation of olefins and trans-cinnamaldehyde, catalyzed by [RuCl₂(PPh₃)₃] in non-aqueous media such as DMF and ILs in the presence of IL-buffers. Remarkable buffer dependence of the formation and catalytic behavior of ruthenium hydrides were evidenced by the kinetic studies and NMR measurements. The hydride [RuHCl(PPh₃)₃], being favorably formed in the presence of the IL-buffer with lower log₁₀([Base]/[Acid]), exhibited higher activity in the reduction of the CC bond against the carbonyl functionality of trans-cinnamaldehyde. While the hydride [RuH₄(PPh₃)₃], being preponderantly formed in the presence of the IL-buffer with higher log₁₀([Base]/[Acid]) showed activity and higher selectivity towards the CO reduction. Consequently, the hydrogenation performance of olefins and trans-cinnamaldehyde in non-aqueous system could be adjusted by adding the different IL-buffers. It is envisioned that the ability of IL-buffers to alter and precisely control the catalytic active species in non-aqueous or low-water systems might find appreciable applications for both fundamental studies and syntheses where the reactions are acid/base-sensitive.