New CNN-Type Ruthenium Pincer NHC Complexes. Mild, Efficient Catalytic Hydrogenation of Esters

New pincer ruthenium complexes (2–6) based on the new bipyridine-NHC ligand 1 were prepared
and studied, resulting in an efficient catalytic hydrogenation of
esters to the corresponding alcohols under mild conditions. Reaction
of the ligand 1 with RuH(Cl)CO(PPh3)3, followed by reaction with one equivalent of the base KHMDS, gave
the mixed phosphine-NHC complex 2, incorporating a C–H-activated
bipyridine ligand. Complex 2 has an octahedral structure
containing two phosphorus atoms trans to each other,
a hydride trans to the NHC ligand, and CO trans to the C–H-activated carbon of the bipyridine
ligand. Using the precursor complex Ru(p-cymene)Cl2(CO), reaction with 1 followed by treatment of
the intermediate product with one equivalent of KHMDS resulted in
formation of the dichloride pincer complexes 3a and 3b, which are in equilibrium, as indicated by variable-temperature 1H NMR. Complex 3a is an octahedral, neutral,
and symmetric complex with the CO ligand positioned trans to the central pyridine group of the pincer ligand and the two chlorides trans to each other, as indicated by single-crystal X-ray
diffraction. Complex 3b is cationic, with an outer-sphere
chloride. Reaction of the NHC ligand 1 with LiHMDS at
low temperature followed by addition of RuH(Cl)CO(PPh3)3 resulted in the mixed phosphine-NHC complex 4, which has an octahedral structure containing phosphorus trans to the hydride, a CO trans to the
NHC ligand, and an outer-sphere chloride. Chloride substitution by
BArF– gave the X-ray-characterized complex 5. Deprotonation of complex 4 with KHMDS resulted
in formation of the dearomatized complex 6. The in situ prepared 6 (from complex 4 and an equivalent of base) is among the best catalysts known for
the hydrogenation of nonactivated esters to the corresponding alcohols
under mild conditions.