Guest Induced Strong Cooperative One- and Two-Step Spin Transitions in Highly Porous Iron(II) Hofmann-Type Metal–Organic Frameworks

The synthesis, crystal structure, magnetic, calorimetric, and Mössbauer studies of a series of new Hofmann-type spin crossover (SCO) metal–organic frameworks (MOFs) is reported. The new SCO-MOFs arise from self-assembly of FeII, bis(4-pyridyl)butadiyne (bpb), and [Ag(CN)2]− or [MII(CN)4]2– (MII = Ni, Pd). Interpenetration of four identical 3D networks with α-Po topology are obtained for {Fe(bpb)[AgI(CN)2]2} due to the length of the rod-like bismonodentate bpb and [Ag(CN)2]− ligands. The four networks are tightly packed and organized in two subsets orthogonally interpenetrated, while the networks in each subset display parallel interpenetration. This nonporous material undergoes a very incomplete SCO, which is rationalized from its intricate structure. In contrast, the single network Hofmann-type MOFs {Fe(bpb)[MII(CN)4]}·nGuest (MII = Ni, Pd) feature enhanced porosity and display complete one-step or two-step cooperative SCO behaviors when the pores are filled with two molecules of nitrobenzene or naphthalene that interact strongly with the pyridyl and cyano moieties of the bpb ligands via π–π stacking. The lack of these guest molecules favors stabilization of the high-spin state in the whole range of temperatures. However, application of hydrostatic pressure induces one- and two-step SCO.