BONDING MODE OF NITROGEN HETEROCYCLIC LIGANDS TO (ETA-5-CYCLOPENTADIENYL)RUTHENIUM CATION AND REACTIVITY STUDIES OF THE NITROGEN AND PI-BONDED COMPLEXES - MECHANISTIC ASPECTS OF A NITROGEN TO PI 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 (eta-5-cyclopentadienyl)ruthenium cation, [CpRu(CH3CN)3](PF6), to ascertain the mode of bonding as a function of structure, i.e., nitrogen (eta-1(N)) versus pi (eta-6) bonding. Ligands 1-3, 5, and 6 formed N-bonded complexes, while 4, 7, and 8 only formed pi-bonded complexes. Thus, it appears that steric and electronic effects influence the bonding mode of nitrogen heterocyclic compounds to CpRu+. An interesting N (eta-1) to pi (eta-6) rearrangement occurred with the N-bonded CpRu+ complexes of ligands 2, 3, and 5, and mechanistic aspects were studied by H-1 NMR spectroscopy. Crossover experiments with [CpRu(eta-1(N)-2-methylpyridine)(CH3CN)2]+ (11) and [MeCpRu(eta-1(N)-2-methylpyridine-d7)(CH3CN)2]+ (21-d7) clearly show that N(eta-1)-bonded complexes undergo ligand exchange much faster than the N to pi rearrangement at 21-degrees-C (NMR probe temperature). Furthermore, neither N- and pi-bonded complexes nor pi- and pi-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 pi-bonded complexes to prove the intramolecular nature of the N to pi rearrangement, i.e., rearrangement of 11 in the presence of [MeCpRu(eta-6-2-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 pi rearrangement. The role of the acetonitrile ligand in the N to pi rearrangement and in the displacement of eta-1- and eta-6-bonded nitrogen heterocyclic ligands will also be discussed.