Variations on an NHC Theme: Which Features Enhance Catalytic Transfer Hydrogenation with Ruthenium Complexes?

N-heterocyclic carbene (NHC) based ruthenium complexes
were studied
as catalysts for the transfer hydrogenation of ketones. Variations
in the catalyst structure were investigated for their impact on hydrogenation
and catalyst stability. Catalyst attributes included bis- or mono-NHC
ligands, pendant ether groups in some cases, and arene ligands of
varied bulk and donor strength. Ruthenium complexes were synthesized
and fully characterized, including complexes with a monodentate NHC
ligand containing a tethered ether N substituent (ImEt,CH2CH2OEtRuCl2(η6-arene); arene = benzene (4), p-cymene (5), hexamethylbenzene
(6)), a complex with a monodentate NHC ligand with solely
alkyl N substituents (ImEt,PentylRuCl2(η6-p-cymene) (8)), and a complex with a bis-NHC
ligand ([RuCl­(methylenebis­(ImEt)2)­(η6-p-cymene)]­PF6 (7)) (Im = imidazole-derived NHC; superscripts indicate N substituents).
X-ray crystal structures were obtained for 4, 5, 7, and 8. All of the ruthenium complexes
were tested and found to be active transfer hydrogenation catalysts
for the reduction of acetophenone to 1-phenylethanol in basic 2-propanol.
Precatalyst 4, which contains a tethered ether group
and benzene ligand, was found to be the most active catalyst. Variable-temperature 1H NMR studies of complexes 4–6 show that arene lability increases in the order C6Me6 < cymene < benzene, and this lability is directly correlated
with catalytic activity. The catalysis appears to be homogeneous,
and a mechanism invoking arene loss is proposed. Precatalyst 4 reduced electron-deficient ketones most easily, and 4′-nitroacetophenone
was reduced under base-free conditions. The highest TOF (turnover
frequency) and TON (turnover number) values obtained were 3003 h–1 and 845, respectively, for ketone reduction with
catalyst 4.