Hysteretic Spin Crossover in Two-Dimensional (2D) Hofmann-Type Coordination Polymers

Three new two-dimensional (2D) Hofmann-type coordination polymers with general formula [Fe­(3-NH2py)2M­(CN)4] (3-NH2py = 3-aminopyridine, M = Ni (1), Pd (2), Pt (3)) have been synthesized. Magnetic susceptibility measurements show that they exhibited cooperative spin crossover (SCO) with remarkable hysteretic behaviors. Their hysteresis widths are 25, 37, and 30 K for 1–3, respectively. The single-crystal structure of 1 suggest that the pseudo-octahedral Fe sites are equatorially bridged by [M­(CN)4]2– to form 2D grids and axially coordinated by 3-NH2py ligands. The intermolecular interactions between layers (the offset face-to-face π···π interactions, hydrogen bonds, and weak Namino···NiII contacts) together with the covalent bonds bridged by [M­(CN)4]2– units are responsible to the significant cooperativity.