A New Systematic Construction of Novel Three-Dimensional Spin Crossover Coordination Polymers Based on the [AgI2(CN)3] Building Unit

New three-dimensional spin crossover (SCO) coordination polymers systematically constructed by the novel building unit [AgI2(CN)3], FeII(3-Br-5-CH3pyridine)2[AgI2(CN)3]­[AgI(CN)2] (1), FeII(3-Br-5-Clpyridine)2[AgI2(CN)3]­[AgI(CN)2] (2), and FeII(3,5-Brpyridine)2[AgI2(CN)3]­[AgI(CN)2] (3), have been synthesized and characterized. The bismonodentate binuclear [Ag2(CN)3]− and mononuclear [AgI(CN)2]− units and FeII atoms assemble to form a 3D network structure. The structures of 1–3 are crystallographically identical, which made up the triply interpenetration combined with complicated intermolecular interactions including Ag···Ag, Ag···X (pyridine substituents) and π-stacking interactions. Magnetic and differential scanning calorimetry studies were performed for 1–3. These compounds display a similar SCO behavior, while the critical temperatures (Tc) are shifted by the substituent effect. Due to the identical structures of 1–3, the order of Tc clearly corresponds with the Hammett constant.