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&z.rad; − )(Cat 2− ) and an extremely rare ls-Co II (SQ&z.rad; − ) 2 electronic state (hs - high-spin, ls - low-spin, SQ&z.rad; − - 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&z.rad; − ) electronic states. Intramolecular electron transfer without accompanying spin-crossover in cobalt bis-dioxolenes has been observed for the first time.