Redox-Programmable Spin-Crossover Behaviors in a Cationic Framework

Metal–organic frameworks (MOFs) provide versatile platforms to construct multi-responsive materials. Herein, by introducing the neutral tetradentate ligand and the linear dicyanoaurate­(I) anion, we reported a rare cationic MOF [FeII(TPB)­{AuI(CN)2}]­I·4H2O·4DMF (TPB = 1,2,4,5-tetra­(pyridin-4-yl)­benzene) with hysteretic spin-crossover (SCO) behavior near room temperature. This hybrid framework with an open metal site (AuI) exhibits redox-programmable capability toward dihalogen molecules. By means of post-synthetic modification, all the linear [AuI(CN)2]– linkers can be oxidized to square planar [AuIII(CN)2X2]− units, which results in the hysteretic SCO behaviors switching from one-step to two-step for Br2 and three-step for I2. More importantly, the stepwise SCO behaviors can go back to one-step via the reduction by l-ascorbic acid (AA). Periodic DFT calculations using various SCAN-type exchange-correlation functionals have been employed to rationalize the experimental data. Hence, these results demonstrate for the first time that switchable one-/two-/three-stepped SCO dynamics can be manipulated by chemical redox reactions, which opens a new perspective for multi-responsive molecular switches.