Anion- and Solvent-Mediated Crystal Phase Changes Resulting in Diverse SCO Behaviors in a Mononuclear Fe(II) Complex

Spin crossover (SCO) compounds which differ only by the nature of the cocrystallizing anion(s) and solvent(s) often leads to rational modulation of their respective SCO behaviors. In the following, we report the formation of two new salts of the previously reported tripodal Fe(II) complex, tris(((((1-(pyridin-4-yl)-1H-imidazole-4-yl)methylene)amino)ethyl)ethane-1,2-diamine)iron(II), which was crystallized with either Br^– (1) or I^– (2) anions. The magnetic behavior following crystallization with different anions was investigated for a series of solvated, air-exposed, and desolvated forms of each compound. While the solvated and air-exposed forms of 2 only occurred in the high-spin (HS) state, the corresponding forms of 1 differed in their SCO behaviors following the substitution and/or loss of solvent from the crystal lattice. The maintained stability of 2 in the HS state was likely a reflection of the tighter crystal packing as also occurs in the previously reported and isostructural [FeL](BF₄)₂ and [FeL](ClO₄)₂ materials. Following crystallization of 1, the packing arrangement was similar to that of the previously reported and SCO-active [FeL]Cl₂ derivative. Despite the similarities occurring between the solvated and air-exposed forms of 1 and [FeL]Cl₂, full desolvation of 1 led to a gain in SCO activity as opposed to quenching of SCO, which occurred in the previously reported air-exposed and desolvated forms of [FeL]Cl₂. The interesting magnetic behavior occurring between the different solvates of 1 has been related to a single crystal to single crystal transformation mediated by solvent exchange. This study highlights the complex anion- and solvent-mediated structural changes that occur within the crystal phase of the present materials, and the marked effect they have on the resulting SCO behaviors.