Base-Free Transfer Hydrogenation Catalyzed by Ruthenium Hydride Complexes of Coumarin-Amide Ligands

Two sets of Ru(II) complexes, [Ru–Cl] (1–3) and [Ru–H] (4–6), supported with coumarin-amide-based ligands have been synthesized and characterized. In these complexes, the coumarin-amide-based ligands show different coordination modes: N–N, N–O, and N–N–O as confirmed by the crystallographic studies. While [Ru–Cl] complexes (1–3) required the presence of a base for promoting the transfer hydrogenation (TH), the corresponding [Ru–H] complexes (4–6) acted as excellent catalysts for the base-free TH of assorted carbonyl compounds. The [Ru–H] complexes (4–6) provided a nearly quantitative yield of the corresponding alcohols by using isopropanol both as a solvent and a hydrogen source. TH of a few biologically relevant substrates such as furfural, nicotinaldehyde, vanillin, isovanillin, chalcone, and 3,6-dihydroxyflavone illustrated the noteworthy catalytic performance of the [Ru–H] complexes. The entire catalytic procedure including reagents, solvents, and conditions was evaluated with the help of the CHEM21 green metrics toolkit to assess the ‘green credentials’ of the catalysis.