HINDERED ROTATION ALONG CARBON-NITROGEN BONDS - THE EFFECT OF CARBONYLIRON IN COMPLEXES OF 7-AZANORBORNADIENE DERIVATIVES

Hindered rotations have been observed for certain N-substituted 7-azan orbornadiene (7-azabicyclo-[2.2.1] hepta-2,5-diene) derivatives and th eir iron complexes. The free energies of rotation (DELTAG(double dagge r)) have been estimated from the coalescence of NMR signals. The avera ge values found for acetyl amides (Ib and IIb), benzoyl amides (IIc an d 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 compl exes 1-4 however, steric factors predominate where the rotations are h indered 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 kca l mol-1 for Me (3a), 13.6 kcal mol-1 for CN (3b), and 14.0 kcal mol-1 for I (3c). Tricarbonyliron complexes (2 and 4) experience a lower bar rier to rotation with respect to the corresponding tetracarbonyl ones (1 and 3) due to a variation in co-ordination geometry. The energies e stimated for the iron carbonyl complexes decrease in the orders 1 ( > la) > 2 as well as 3a > 4a. The crystal structures of tetracarbonyl- a nd yl-[1,4-dihydro-1,4-(o-tolylimino)naphthalene]iron 3a and 4a have b een determined.