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 [Fe-II(TPB){Au-I(CN)(2)}]Imiddot4H(2)Omiddot4DMF (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 (Au-I) exhibits redox-programmable capability toward dihalogen molecules. By means of post-synthetic modification, all the linear [Au-I(CN)(2)](-) linkers can be oxidized to square planar [Au-III(CN)(2)X-2](-) units, which results in the hysteretic SCO behaviors switching from one-step to two-step for Br-2 and three-step for I-2. 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.