Ruthenium PNN(O) Complexes: Cooperative Reactivity and Application as Catalysts for Acceptorless Dehydrogenative Coupling Reactions

The novel tridentate
PNNOH pincer ligand LH features a reactive 2-hydroxypyridine functionality
as well as a bipyridyl-methylphosphine skeleton for meridional coordination.
This proton-responsive ligand coordinates in a straightforward manner
to RuCl­(CO)­(H)­(PPh3)3 to generate complex 1. The methoxy-protected analogue LMe was also coordinated to Ru­(II) for comparison. Both
species have been crystallographically characterized. Site-selective
deprotonation of the 2-hydroxypyridine functionality to give 1′ was achieved using both mild (DBU) and strong bases
(KOtBu and KHMDS), with no sign of involvement of the phosphinomethyl
side arm that was previously established as the reactive fragment.
Complex 1′ is catalytically active in the dehydrogenation
of formic acid to generate CO-free hydrogen in three consecutive runs
as well as for the dehydrogenative coupling of alcohols, giving high
conversions to different esters and outperforming structurally related
PNN ligands lacking the NOH fragment. DFT calculations
suggest more favorable release of H2 through reversible
reactivity of the hydroxypyridine functionality relative to the phosphinomethyl
side arm.