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.) Co^III (cat^2- ) and high spin (h.s.) Co^II (sq⋅^- ) forms (cat^2- =catecholato, and sq⋅^- =seminquinonato forms of 3,5-di-^t Bu-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. Co^II (sq⋅^- ). Thus, our ligand modifications not only provide access to the VT transition from l.s. Co^III (cat^2- ) to l.s. Co^II (sq⋅^- ), but at higher temperatures, the complex undergoes spin crossover from l.s. Co^II (sq⋅^- ) to h.s. Co^II (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. Co^II (sq⋅^- ) intermediate state, suggesting a new design criterion in the development of stimulus-responsive multi-state molecular switches.