Hysteretic Two‐Step Spin‐Crossover Behavior in Two Two‐Dimensional Hofmann‐Type Coordination Polymers

The reaction of ferrous salts and square‐planar tetracyanometallates [MII(CN)4]2− (M=Pd, Pt) with ligand 4‐(1H‐pyrazol‐3‐yl)pyridine (Hppy) resulted in the formation of two two‐dimensional (2D) Hofmann‐like coordination polymers (CPs) with the general formula of {FeII(Hppy)2[MII(CN)4]}⋅H2O (M=Pd for 1Pd, Pt for 1Pt). The two CPs have been studied by variable‐temperature single‐crystal X‐ray crystallography, differential scanning calorimetry, and magnetic measurements. Polymers 1Pd and 1Pt are isostructures, in which the FeII atoms are equatorially coordinated with [MII(CN)4]2− to form 2D undulating layers and axially coordinated with Hppy ligands. Both compounds undergo cooperative, complete spin crossover (SCO), with characteristic abrupt and remarkable two equal steps, featuring hysteresis widths of 14 K and 19 K (1Pd) and 17 and 23 K (1Pt) for the two steps. The SCO cooperativity may result from the hydrogen bonds and π⋅⋅⋅π interactions within and between such 2D layers, and the two‐step SCO behavior is accompanied with phase transitions. Two Hofmann‐like 2D coordination polymers based on the Hppy ligand (4‐(1H‐pyrazol‐3‐yl)pyridine) have been synthesized and characterized. The polymers exhibit cooperative spin crossover with abrupt and two‐step behaviors accompanied by a symmetry‐breaking structural phase transition.