Controlling Three-Step and Five-Step Spin Transitions by Polymorphism in an Fe^III Spin Crossover Complex

Regulating spin crossover (SCO) behavior, especially controlling the spin transition steps, is an important scientific issue, mainly because people aim to control spin bistability and multistability. Presently, SCO bistability can be regulated by changing the ligand-modifying species, non-coordinated anions, guest molecules, and metal-ion dopant. However, the control of multistability is extremely challenging, especially in Fe(III) SCO compounds. Here, we report that [Fe-III(H-5-Br-thsa)(5-Br-thsa)]H2O (5-Br-thsa = (5-bromo-2-hydroxybenzylidene)hydrazinecarbothioamide), a compound we have reported before, exists in two polymorphic forms: polymorph 1 exhibits three-step SCO, and polymorph 2 shows five-step SCO, with multi-step SCO behavior effectively regulated by polymorphism. According to single-crystal and powder X-ray diffractometry, polymorphs 1 and 2 crystallize in different space groups during their spin transitions, with two-step symmetry breaking observed (Pbcn -> Pnc2 -> Pbcn for polymorph 1; P2(1)/n -> Pn -> P2(1)/n for polymorph 2). We realized that the behavior of these two polymorphs depends significantly on the structure, including (i) the average Fe-N bond distance, (ii) deformation of octahedral Fe-III atoms, and (iii) distinct crystal packing, which account for the large differences observed in magnetic properties.