Neutral homoleptic tetranuclear iron carbonyls: why haven’t they been synthesized as stable molecules?

Abstract

The tetranuclear iron carbonyls Fe₄(CO)_n (n = 16, 15, 14) have been investigated using density functional theory (DFT). Low-energy structures are predicted for Fe₄(CO)₁₆, Fe₄(CO)₁₅, and Fe₄(CO)₁₄ in which the Fe₄ units are rhombi, planar butterflies, and tetrahedra with four, five, and six Fe–Fe single bonds, respectively, to give all four iron atoms the favored 18-electron configuration. However, for Fe₄(CO)₁₄ an alternative low energy structure is predicted with a planar Fe₄ butterfly and one of the five iron–iron distances short enough to correspond to a formal FeFe double bond, thereby also leading to the favored 18-electron configuration. The dissociation energy of Fe₄(CO)₁₆ into Fe(CO)₅ + Fe₃(CO)₁₁ or Fe₃(CO)₁₂ + Fe(CO)₄ is predicted to be very low (<6 kcal mol⁻¹). Similarly, the dissociation energy of Fe₄(CO)₁₅ into Fe(CO)₅ + Fe₃(CO)₁₀ is predicted to be very low at ∼5 kcal mol⁻¹. These low predicted dissociation energies of Fe₄(CO)₁₆ and Fe₄(CO)₁₅ into smaller iron carbonyl fragments are consistent with the fact that neither of these molecules have been synthesized or detected spectroscopically, even in low temperature matrices. However, Fe₄(CO)₁₄ is predicted to be considerably more stable towards dissociation into smaller fragments with a dissociation energy of 23.2 kcal mol⁻¹ (B3LYP) or 39.2 kcal mol⁻¹ (BP86) into Fe(CO)₅ + Fe₃(CO)₉. This is consistent with the detection of Fe₄(CO)₁₄ by infrared spectroscopy in the reaction product of mass selected Fe₄⁺ with CO in low temperature matrices.