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

Abstract

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.