Homobimetallic Ruthenium(II) Complexes Catalysed Selective Transfer Hydrogenation of Aldehydes in Water

Abstract

Herein we report chemoselective transfer hydrogenation (TH) of aldehydes in aqueous medium using a series of homobimetallic Ru(II) catalysts. Two homobimetallic complexes (Ru1 and Ru3) and one monometallic complex (Ru2) have been employed in the catalytic reduction of aldehydes. Bimetallic complex [(p-cymene)₂(RuCl)₂L³] (Ru3) is obtained from the reaction of Schiff base ligand 2,2′-((1E,1′E)-((3,3′,5,5′-tetraisopropyl-[1,1′-biphenyl]-4,4′diyl)bis(azaneylylidene))bis(methaneylylidene))bis(4-bromophenol) (H₂L³) and characterized by various spectroscopic and analytical techniques. The use of formic acid/formate buffer as the hydride source and a catalyst loading of 0.01 mol % of Ru1 or Ru3 resulted in the conversion of various aldehydes to the corresponding alcohols in good to excellent yield. This method is very efficient for selective reduction of aldehydes in the presence of other reducible functional groups. A loading of 0.0001 mol % of Ru1 catalyst is sufficient to achieve a turnover frequency (TOF) of 5.5×10⁵ h⁻¹. Furthermore, the catalyst can been recycled and reused for six consecutives cycles without sacrificing the efficiency. A comparison of results obtained between bimetallic and monometallic complexes offers valuable insights into the distinct reactivity patterns of the bimetallic complexes, presumably originating from a cooperative effect. To understand the detailed mechanism, we have explored the mechanistic pathway using DFT methods on reported catalysts and various models which indicate that addition of aldehyde as rate-limiting and presence of cooperativity that boost the catalytic efficiency in the case of dinuclear Ru1 catalyst. The pH dependent TH mechanism has been investigated with the aid of NMR and ESI-MS spectroscopic techniques.