Comparison of the Catalytic Activity of [(η5-C5H5)Ru(2,2′-bipyridine)(L)]OTf versus [(η5-C5H5)Ru(6,6′-diamino-2,2′-bipyridine)(L)]OTf (L = labile ligand) in the Hydrogenation of Cyclohexanone. Evidence for the Presence of a Metal–Ligand Bifunctional Mechanism under Acidic Conditions

The two title complexes as well as the dimeric complex [Ru(II)(η5-C5H5)(6,6′-diamino-2,2′-bipyridine)]2(OTf)2 have been synthesized and characterized by NMR and single-crystal X-ray crystallography. The direct structural comparison of the 2,2′-bipyridine and 6,6′-diamino-2,2′-bipyridine complexes suggests that the electronic and steric environments of the ruthenium centers in both complexes are essentially equivalent, providing for a unique opportunity to probe the influence of the noncoordinated amine substituent on the relative reactivity and catalytic activity of the complexes. Opposite to what would be anticipated on the basis of steric effects, the bulkier amine-substituted ligand results in a catalyst showing substantially higher activity in the hydrogenation of cyclohexanone in acidic medium, which is attributed to the operation of a metal–ligand bifunctional hydrogenation mechanism mediated by the amine substituents in their protonated form acting as proton shuttles.