Low-spin to low-spin valence tautomeric transition in cobalt bis-dioxolenes

Cobalt dioxolenes are a well-known class of switchable coordination compounds showing intramolecular electron transfer, which is always accompanied by a spin state change at the cobalt center. Here, we present the first example of thermally switchable cobalt bis-dioxolenes where intramolecular electron transfer seems to take place, but the spin state change is suppressed. This leads to the detection of thermal transition between a common ls-Co^III(SQ˙^-)(Cat^2-) and an extremely rare ls-Co^II(SQ˙^-)₂ electronic state (hs - high-spin, ls - low-spin, SQ˙^- - benzosemiquinonate(1-) radical and Cat^2- - catecholate(2-)). Parallel to the present work, a similar work but on cobalt mono-dioxolenes has just appeared (Chem. Eur. J., 2023, 29, e202300091), suggesting thermal transition between ls-Co^III(Cat^2-) and ls-Co^II(SQ˙^-) electronic states.