Structural and Spectroscopic Evidence for a Side-on Fe(III)-Superoxo Complex Featuring Discrete O-O Bond Distances

The O-O bond length is often used as a structural indicator to determine the valence states of bound O₂ ligands in biological metal-dioxygen intermediates and related biomimetic complexes. Here, we report very distinct O-O bond lengths found for three crystallographic forms (1.229(4), 1.330(4), 1.387(2) Å at 100 K) of a side-on iron-dioxygen species. Despite their different O-O bond distances, all forms possess the same electronic structure of Fe(III)-O₂ ^•-, as evidenced by their indistinguishable spectroscopic features. Density functional theory and ab initio calculations, which successfully reproduce spectroscopic parameters, predict a flat potential energy surface of an η²-O₂ motif binding to the iron center regarding the O-O distance. Therefore, the discrete O-O bond lengths observed likely arise from differential intermolecular interactions in the second coordination sphere. The work suggests that the O-O distance is not a reliable benchmark to unequivocally identify the valence state of O₂ ligands for metal-dioxygen species in O₂-utilizing metalloproteins and synthetic complexes.