Triazole Iron(II) Coordination Polymer Showing Cooperative Spin Cross‐Over Tuned by Crystal Packing Flexibility

A new 1D iron(II) spin crossover (SCO) coordination polymer, [Fe(but‐trz) 3 ][Pt(CN) 4 ]•H 2 O (1), based on 4‐butyl‐1,2,4‐triazole (but‐trz), has been synthesized and structurally and magnetically characterized. Compound 1 exhibits an abrupt and reversible high‐spin ( HS ) to low‐spin ( LS ) transition at T 1/2 = 272 K without significant hysteresis, as confirmed by magnetic susceptibility and differential scanning calorimetry measurements. Single crystal X‐ray diffraction studies at 296 K ( HS ) and 150 K ( LS ) reveal significant anisotropic structural changes and a reversible symmetry‐breaking transition involving a shift of the cationic chains. These changes are accommodated without crystal degradation, indicating exceptional mechanical resilience upon cycling. In contrast to its parent compound [Fe(bn‐trz) 3 ][Pt(CN) 4 ]•H 2 O, compound 1 features additional direct interchain interactions at room temperature, likely arising from the flexible butyl side groups. These interactions may enhance elastic cooperativity within the lattice and could also have a limited influence on the higher transition temperature, offering new insights into structure–property relationships in triazole‐based SCO systems.