PNN Ruthenium Pincer Complexes Based on Phosphinated 2,2′-Dipyridinemethane and 2,2′-Oxobispyridine. Metal–Ligand Cooperation in Cyclometalation and Catalysis

The synthesis of novel PNN ruthenium
pincer complexes based on
2,2′-dipyridinemethane phosphine derivatives, as well as on
2,2′-oxobispyridine phosphine ligands, and their reactivity
toward dearomatization and cyclometalation are described. The dearomatized
compounds 7a,b undergo cyclometalation to
yield complexes 8a,b. In order for cyclometalation
to proceed, the coordination sphere around the Ru center has to rearrange,
and this depends on the flexibility of the system, showing that the
cyclometalation is qualitatively faster in the case of the dimethyl
derivative 7a than in the case of the spyrocyclopentyl
derivative 7b. The cyclometalation occurs diastereoselectively
and leads to only one diastereomer of the cyclometalated compounds.
In the case of the 2,2′-oxobispyridine complex 6c, the dearomatized complex was too unstable to be isolated; however
it was possible to isolate and characterize a stable dicarbonyl-dearomatized
ruthenium­(II) complex, 9c, when the deprotonation was
performed under a CO atmosphere. Dearomatization of 6a under CO also led to dicarbonyl-dearomatized ruthenium­(II) complex 9a, which slowly rearranged into the dicarbonyl-aromatized
ruthenium(0) complex 10a. These complexes were tested
in catalytic alcohol–amine coupling, esterification of primary
alcohols, and hydrogenation of secondary amides. Moderate activity
was observed in hydrogenation of amides to alcohols and amines and
low activity in the other transformations, owing mainly to the formation
of stable cyclometalated compounds.