Ligand Modifications Produce Two‐Step Magnetic Switching in a Cobalt(dioxolene) Complex

Mononuclear monodioxolene valence tautomeric (VT) cobalt complexes typically exist in their low spin (l.s.) CoIII(cat2‒) and high spin (h.s.) CoII(sq•‒) forms (cat2‒ = catecholato, and sq•‒ = seminquinonato forms of 3,5-di-tBu-1,2-dioxolene), which reversibly interconvert via temperature-dependent intramolecular electron transfer. Typically, the remaining four coordination sites on cobalt are supported by a tetradentate ligand whose properties influence the temperature at which VT occurs. We report that replacing one chelating pyridyl arm of tris(2-pyridylmethyl)amine (tpa) with a weaker field ortho-anisole moiety facilitates access to a third magnetic state, and examine a series of related complexes. Variable temperature crystallographic, magnetic, calorimetric, and spectroscopic studies support that this third state is consistent with l.s. CoII(sq•‒). Thus, our ligand modifications not only provide access to the VT transition from l.s. CoIII(cat2‒) to l.s. CoII(sq•‒), but at higher temperatures, the complex undergoes spin crossover from l.s. CoII(sq•‒) to h.s. CoII(sq•‒), representing the first example of two-step magnetic switching in a mononuclear monodioxolene cobalt complex. We hypothesize that ligand dynamicity may facilitate access to the rarely observed l.s. CoII(sq•‒) intermediate state, suggesting a new design criterion in the development of stimulus-responsive multi-state molecular switches.