The electronic structure of iron corroles: a combined experimental and quantum chemical study

There is a longstanding debate in the literature on the electronic structure of chloroiron corroles, especially for those containing the highly electron-withdrawing meso-tris(pentafluorophenyl)corrole (TPFC) ligand. Two alternative electronic structures were proposed for this and the related [FeCl(tdcc)] (TDCC = meso-tris(2,6-dichlorophenyl)corrole) complex, namely a high-valent ferryl species chelated by a trianionic corrolato ligand ([Fe ^IV-(Cor)^3-]⁺) or an intermediate-spin (IS) ferric ion that is antiferromagnetically coupled to a dianionic π-radical corroie ([Fe^III(COr)^̇2-]⁺) yielding an overall triplet ground state. Two series of corrole-based iron complexes ([Fe(L)-(Cor)], in which L = F, Cl, Br, I, and Cor = TPFC, TDCC) have been investigated by a combined experimental (Mössbauer spectroscopy) and computational (DFT) approach in order to differentiate between the two possible electronic-structure descriptions. The experimentally calibrated conclusions were reached by a detailed analysis of the Kohn-Sham solutions, which successfully reproduce the experimental structures and spectroscopic parameters: the electronic structures of [Fe(L)(COr)] (L = F, Cl, Br, I, Cor = TPFC, TDCC) are best formulated as ([IS-Fe^III(Cor)^̇2-] ⁺), similar to chloroiron corroie complexes containing electron-rich corroie ligands. The antiferromagnetic pathway is composed of singly occupied Fe d.j and corroie a_2u-like π orbitais, with coupling constants that exceed those of analogous porphyrin systems by a factor of 2-3. In the corroles, the combination of lower symmetry, extra negative charge, and smaller cavity size (relative to the porphyrins) leads to exceptionally strong iron-corrole o bonds. Hence, the Fe d_x2-y2-based molecular orbital is unavailable in the corroie complexes (contrary to the porphyrin case), and the local spin states are S_Fe = 3/2 in the corroles versus S_Fe = 5/2 in the porphyrins. The consequences of this qualitative difference are discussed for spin distributions and magnetic properties.