DFT study of intramolecular interring η6,η6-haptotropic rearrangements in tricarbonylchromium complexes of 2-aminobiphenyl and 4-aminobiphenyl

Abstract

DFT studies have been carried out to investigate the role of nitrogen participation in the interring haptotropic rearrangements of [(η(6)-C6H5)(C6H4-4-NH2)]Cr(CO)3 and [(η(6)-C6H5)(C6H4-2-NH2)]Cr(CO)3. For the para-substituted case, where intramolecular coordination of nitrogen to chromium is not possible, DFT modeling predicts an activation barrier of 32.5 kcal mol(-1), which is in very close agreement with the experimentally determined value of 32.57 kcal mol(-1). In the case of the ortho-substituted isomers, modeling of a mechanism that does not invoke stabilization via nitrogen coordination yields a predicted energy barrier of 32.7 kcal mol(-1), while a mechanism that does invoke nitrogen participation and accounts for interconversion of rotational isomers gives a predicted value of 30.2 kcal mol(-1). This is consistent with the experimentally determined value of 31.22 kcal mol(-1). These data provide evidence that intramolecular stabilization via nitrogen coordination to chromium is responsible for the ortho substituted isomer undergoing haptotropic rearrangement with a rate nearly five times greater than that observed for the para isomer. For the mechanism that invokes ortho-amino group participation, transition state analysis in the frame of Bader theory shows that each transition state along the proposed mechanistic pathway has a bond critical point between nitrogen and chromium.