The Milstein Bipyridyl PNN Pincer Complex of Ruthenium Becomes a Noyori-Type Catalyst under Reducing Conditions

Hydrogenation of the dearomatized
PNN ligand of the Milstein bipyridyl
complex RuH­(CO)­[PNN] (2) gives a square-pyramidal Ru­(II)
product RuH­(CO)­[pPNN] (5). The central ring of the pPNN
ligand is a piperidine. A minor byproduct of the hydrogenation reaction
is complex 6 which has a dimeric structure made of two
Ru­(II) fragments each possessing a partly hydrogenated PNN ligand.
The structures of 5 and 6 have been elucidated
by NMR spectroscopy and X-ray crystallography. The PNN ligand of 2 is also hydrogenated under the conditions of the catalytic
dehydrogenative coupling of ethanol to ethyl acetate. No direct evidence
of the aromatized dihydride RuH2(CO)­[PNN] (4) was found in this study. However, treating RuHCl­(CO)­[PNN] with
Li­[HBEt3] or reacting 2 with H2 at low temperature resulted in a structurally characterized hydride-bridged
dimer (7) bearing intact aromatized bipyridyl ligands.
M06-L/def2-QZVP DFT calculations provided insights into the thermodynamics
of the stoichiometric reactions of this work and into the nature of
the intermediates of the catalytic ester hydrogenation facilitated
by RuH2(CO)­[pPN­(H)­N] (8) formed from 5 under H2.