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-CoIII(SQ-)(Cat2-) and an extremely rare ls-CoII(SQ-)2 electronic state (hs - high-spin, ls - low-spin, SQ- - benzosemiquinonate(1-) radical and Cat2- - 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-CoIII(Cat2-) and ls-CoII(SQ-) electronic states. Intramolecular electron transfer without accompanying spin-crossover in cobalt bis-dioxolenes has been observed for the first time.