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.