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.