The hapticity of the acenaphthylene ligand in its mononuclear, binuclear, and trinuclear iron carbonyl complexes

Abstract

The structures and energetics of the acenaphthylene iron carbonyl complexes (C₁₂H₈)Fe(CO)_n (n = 4, 3, 2), (C₁₂H₈)Fe₂(CO)_n (n = 8, 7, 6, 5, 4), and (C₁₂H₈)Fe₃(CO)_n (n = 9, 8), including the known (η²-C₁₂H₈)Fe(CO)₄ and (η⁵,η³-C₁₂H₈)Fe₂(CO)₅, have been examined using density functional theory. The two experimentally known (η⁵,η³-C₁₂H₈)Fe₂(CO)₅ structures, related by a thermally reversible photochemical haptotropic rearrangement, lie within ∼1 kcal mol⁻¹ from each other in energy and more than 20 kcal mol⁻¹ below any other (C₁₂H₈)Fe₂(CO)₅ isomers. Decarbonylation of the pentacarbonyl to the tetracarbonyl (C₁₂H₈)Fe₂(CO)₄ is predicted to retain the formal Fe–Fe single bond but to convert one of the CO groups to a four-electron donor bridging η²-μ-CO group. The lowest-energy carbonyl-rich (C₁₂H₈)Fe₂(CO)_n (n = 8, 7, 6) structures lack direct iron–iron bonds and have the ligand hapticities expected from the 18-electron rule. The low CO dissociation energy of ∼5 kcal mol⁻¹ for (C₁₂H₈)Fe₂(CO)₆ suggests that these carbonyl-rich species are not viable. The addition of an Fe(CO)₃ moiety to the binuclear (C₁₂H₈)Fe₂(CO)_n (n = 6, 5) complexes to give the trinuclear (C₁₂H₈)Fe₃(CO)_n (n = 9, 8) complexes, respectively, is predicted to be exothermic by ∼50 kcal mol⁻¹. These trinuclear complexes have pentahapto–bis(trihapto) acenaphthylene ligands with an Fe–Fe bond in the octacarbonyl structures. The experimental mononuclear (η²-C₁₂H₈)Fe(CO)₄ structure with a dihapto acenaphthylene ligand is the lowest energy isomer. The hapticity of the acenaphthylene ligand increases from dihapto to pentahapto or hexahapto upon CO loss to give (C₁₂H₈)Fe(CO)_n (n = 3, 2).