Transfer Hydrogenation in Water via a Ruthenium Catalyst with OH Groups near the Metal Center on a bipy Scaffold

The new ligand 6,6′-dihydroxy-2,2′-bipyridyl
(dhbp)
was synthesized via its tautomer, and this provides an efficient route
to novel metal complexes of dhbp. In ruthenium complexes of dhbp,
these OH groups enhance water solubility and may play a role in aqueous
transfer hydrogenation with formate/formic acid as the hydrogen source.
A series of cationic catalysts, [(η6-arene)­Ru­(N,N)­Cl]­Cl
(arene = cymene, C6Me6; N,N = bipyridyl with
OH, OMe, or H at the 6- and 6′-positions), were synthesized,
fully characterized, and tested for transfer hydrogenation activity
in various polar protic media. In aqueous media (90/10 water/methanol),
Ru complexes of dhbp outperform the other catalysts tested (all at
1 mol %), and high percentage conversion of aromatic ketones to the
corresponding alcohols is observed in 6 h. The OH groups appear to
be essential for use of water as a green solvent and can potentially
allow for metal–ligand bifunctional catalysis.