Switching from a Chromium(IV) Peroxide to a Chromium(III) Superoxide upon Coordination of a Donor in the trans Position.

O2 activation at a chromium(II) siloxide complex in propionitrile leads to a chromium(III) complex with an end-on bound superoxide ligand, while the reaction in tetrahydrofuran leads to a side-on peroxo chromium(IV) compound. The superoxide reacts faster with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl hydroxylamine while the peroxide, unlike the superoxide, proved capable of deformylating aldehydes. The system was found to represent a unique case, where even a switching between the two structures can be achieved via the solvent; its ability to coordinate at the position trans to the O2 ligand is decisive, as supported by density functional theory studies. Altogether, the results show that subtle changes can determine for an initially formed metal-dioxygen adduct, whether it exists as a superoxide or a peroxide, which thus merits consideration in discussions on mechanisms and possible reaction routes.