Intra- and inter-ring haptotropic rearrangements in (naphthalene and anthracene)nickel complexes: a DFT study

Abstract

Density functional quantum chemical calculations of the mechanisms of metallotropic η²,η²-intra- and η²,η²-inter-ring haptotropic rearrangements (HRs) in 16e zero-valent η²-(naphthalene and anthracene)nickel complexes involving migration of the organometallic group within the same ring or from one aromatic ring to the other were carried out. The structures of the initial complexes, transition states, and intermediates were determined. The intra-ring HRs in these systems proceed via low-energy η⁴-cis-butadiene transition states. The inter-ring HRs proceed along the periphery of the naphthalene and anthracene ligands via high-energy η³-allylic transition states. In contrast to well-investigated η⁶,η⁶-inter-ring HRs in (naphthalene and anthracene)tricarbonylchromium complexes, the activation barriers to the η²,η²-inter-ring HRs in the corresponding nickel complexes are much lower. Transition states of these processes are characterized by higher hapticity compared to the initial complexes. This also distinguishes the nickel complexes from the corresponding Cr(CO)₃ complexes for which the hapticities of transition states of the η⁶,η⁶-inter-ring HRs are lower than those of the initial complexes. The calculated activation barriers to the η²,η²-intra-ring HRs in the (naphthalene and anthracene)nickel complexes as well as the barrier to rotation of the organonickel group in the naphthalene complex are in good agreement with the experimental data. The calculated barriers to the η²,η²-inter-ring HRs in the naphthalene and anthracene complexes are 3–5 kcal mol⁻¹ lower than the experimental values. This is probably due to the competition between two mechanisms of this process, a low-energy intramolecular mechanism and a high-energy intermolecular dissociative mechanism.