Hindered rotation along carbon–nitrogen bonds. The effect of carbonyliron in complexes of 7-azanorbornadiene derivatives

Abstract

Hindered rotations have been observed for certain N-substituted 7-azanorbornadiene (7-azabicyclo[2.2.1]hepta-2,5-diene) derivatives and their iron complexes. The free energies of rotation (ΔG‡) have been estimated from the coalescence of NMR signals. The average values found for acetyl amides (Ib and IIb), benzoyl amides (IIc and IIIc), and carbamates (Ia and IIIa) are 17.8, 16.0 and 14.3 kcal mol^–1 respectively, in accord with electronic effects. For the iron complexes 1–4 however, steric factors predominate where the rotations are hindered mostly by the crowding by the ligands. The barriers found for the tetracarbonyliron complexes (3a–3c) of ortho-substituted anilines are proportional to the size of the phenyl substituents, i.e. 12.7 kcal mol^–1 for Me (3a), 13.6 kcal mor^–1 for CN (3b), and 14.0 kcal mol^–1 for l (3c). Tricarbonyliron complexes (2 and 4) experience a lower barrier to rotation with respect to the corresponding tetracarbonyl ones (1 and 3) due to a variation in co-ordination geometry. The energies estimated for the iron carbonyl complexes decrease in the orders 1( > Ia) > 2 as well as 3a > 4a. The crystal structures of tetracarbonyl- and tricarbonyl-[1,4-dihydro-1,4-(o-tolylimino)naphthalene]iron 3a and 4a have been determined.