Bonding Mode of Nitrogen Heterocyclic Ligands to (η⁵-Cyclopentadlenyl)ruthenium Cation and Reactivity Studies of the Nitrogen and π-Bonded Complexes: Mechanistic Aspects of a Nitrogen to π Rearrangement

The nitrogen heterocyclic ligands pyridine (1), 2-methylpyridine (2), 2,4-dimethylpyridine (3), 2,4,6-trimethylpyridine (4), quinoline (5), isoquinoline (6), 2-methylquinoline (7), and 1,2,3,4-tetrahydroquinoline (8) were reacted with (η⁵-cyclopentadienyl)ruthenium cation, [CpRu(CH₃CN)₃](PF₆), to ascertain the mode of bonding as a function of structure, i.e., nitrogen (η¹(N)) versus π (η⁶) bonding. Ligands 1–3, 5, and 6 formed N.-bonded complexes, while 4, 7, and 8 only formed π-bonded complexes. Thus, it appears that steric and electronic effects influence the bonding mode of nitrogen heterocyclic compounds to CpRu⁺. An interesting N (η¹) to π (η⁶) rearrangement occurred with the N-bonded CpRu⁺ complexes of ligands 2, 3, and 5, and mechanistic aspects were studied by ¹H NMR spectroscopy. Crossover experiments with [CpRu(η¹(N)-2-methylpyridine)(CH3CN)2]⁺ (11) and [MeCpRu(η¹(N)-2-methylpyridine-d₇)(CH3CN)₂]⁺ (21-d₇) clearly show that N(η¹)-bonded complexes undergo ligand exchange much faster than the N to π rearrangement at 21 °C (NMR probe temperature). Furthermore, neither N- and π-bonded complexes nor π- and π-bonded complexes undergo ligand exchange with each other; however, both types of bonding modes did undergo facile exchange with free nitrogen ligand. These exchange reaction rates were found to be concentration-dependent. Consequently, our attempts to use the former result with N- and π-bonded complexes to prove the intramolecular nature of the N to π rearrangement, i.e., rearrangement of 11 in the presence of [MeCpRu(η⁶−2-methylpyridine-d₇)]⁺ (22-d₇), were not successful; exchange of any free 2 that might form, i.e., at low concentrations, with 22-d₇ was found to be slower than the N to π rearrangement. The role of the acetonitrile ligand in the N to π rearrangement and in the displacement of η¹- and η⁶-bonded nitrogen heterocyclic ligands will also be discussed.