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

The reaction of ferrous salts and square-planar tetracyanometallates [M^II(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 {Fe^II(Hppy)₂[M^II(CN)₄]}⋅H₂O (M=Pd for 1^Pd, Pt for 1^Pt). The two CPs have been studied by variable-temperature single-crystal X-ray crystallography, differential scanning calorimetry, and magnetic measurements. Polymers 1^Pd and 1^Pt are isostructures, in which the Fe^II atoms are equatorially coordinated with [M^II(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 (1^Pd) and 17 and 23 K (1^Pt) 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.