HETEROAROMATIC NITROGEN LIGAND STUDIES WITH THE (ETA-5-PENTAMETHYLCYCLOPENTADIENYL)RUTHENIUM CATION - ETA-1(N) AND ETA-6(PI) BONDING MODES AND FACTORS THAT INFLUENCE A NITROGEN TO PI-REARRANGEMENT

The reactions of the (eta-5-pentamethylcyclopentadienyl)ruthenium tris(acetonitrile) cationic complex [Cp*Ru(CH3CN)3](OTf) with pyridine (1), 2-methylpyridine (2), and quinoline (3) were studied to ascertain bonding modes as a function of heteroaromatic nitrogen ligand structure. Ligand 1 bonds eta-1(N) and forms mono- or tris(pyridine) complexes with [Cp*Ru(CH3CN)3]+ depending on ligand concentration. Ligand 2 only forms an eta-6 complex with [Cp*Ru(CH3CN)3]+, while ligand 3 also forms an eta-6 complex, but with the benzo ring not the nitrogen ring. In the presence of excess pyridine, the complexed CH3CN ligands are fully displaced to form [Cp*Ru(eta-1(N)-pyridine)3]+, while in the presence of excess 2 or 3 only the [Cp*Ru(eta-1(N)-ligand)(CH3CN)2]+ complexes are formed. The latter [Cp*Ru(eta-1(N)-ligand)(CH3CN)2]+ complexes with ligands 2 and 3 were not isolated; rather, they undergo a rapid nitrogen (N) to pi rearrangement to the corresponding eta-6 complexes, [Cp*Ru(eta-6-2-methylpyridine or quinoline)]+. The isolation of [Cp*Ru(eta-1(N)-pyridine)(CH3CN)2]+ and its conversion to [Cp*Ru(eta-6-pyridine)]+ clearly demonstrates the pathway to the eta-6 complexes. Ligand-exchange reactions of [Cp*Ru(eta-6-pyridine)]+ with CD3CN and pyridine-d5 show facile replacement of the eta-6-bonded pyridine, while the former result with CD3CN ligand exchange proves that the N to pi rearrangement is not reversible. Factors such as ligand steric effects and the propensity of the Cp*Ru+ group to act as an arenophile will also be discussed.