Valence and Structure Isomerism of Al₂FeO₄⁺: Synergy of Spectroscopy and Quantum Chemistry

We provide spectroscopic and computational evidence for a substantial change in structure and gas phase reactivity of Al₃O₄⁺ upon Fe-substitution, which is correctly predicted by multireference (MR) wave function calculations. Al₃O₄⁺ exhibits a cone-like structure with a central trivalent O atom (C_3v symmetry). The replacement of the Al- by an Fe atom leads to a planar bicyclic frame with a terminal Al-O^•- radical site, accompanied by a change from the Fe^+III/O^-II to the Fe^+II/O^-I valence state. The gas phase vibrational spectrum of Al₂FeO₄⁺ is exclusively reproduced by the latter structure, which MR wave function calculations correctly identify as the most stable isomer. This isomer of Al₂FeO₄⁺ is predicted to be highly reactive with respect to C-H bond activation, very similar to Al₈O₁₂⁺ which also features the terminal Al-O^•- radical site. Density functional theory, in contrast, predicts a less reactive Al₃O₄⁺-like "isomorphous substitution" structure of Al₂FeO₄⁺ to be the most stable one, except for functionals with very high admixture of Fock exchange (50%, BHLYP).