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.