Reversible step spin crossover modulation via water absorption and dehydration in a 3D Hofmann-type framework

Spin crossover (SCO) compounds are bistable in nature but can exhibit five main types of SCO behaviors as collective phenomena. Here, incomplete, gradual, step, abrupt and hysteretic types of SCO behaviors can be observed in the magnetic curve of a 3D Hofmann-type metal–organic framework [Fe(dpt){Pt(CN)4}]·dpt·2.5H2O (1·dpt·2.5H2O, dpt = 2,5-di(pyridine-4-yl)thiophene). It hints that the variable-temperature spin transition process can be more complicated than thus far anticipated. Meanwhile, two- and three-step SCO behaviors are displayed for the partially and fully dehydrated phases 1·dpt·1.5H2O and 1·dpt, respectively. Reversible SCO modulations are further proved by water adsorption. The magnetostructural corrections suggest that SCO behaviors can even be tuned by the weak host–guest hydrogen-bonding interactions, which provides great opportunities to design chemosensors.