Anion-induced conformational polymorphism in cationic iron(II) spin-crossover frameworks

Anionic supramolecular chemistry plays a vital role in discrete spin crossover (SCO) complexes thanks for the geometric diversity of anions and the cation-anion cooperation. However, anionic effect has never been reported in SCO frameworks due to the neutral nature of Hofmann-type clathrates. Herein, by utilizing the cross-shaped tetradentate ligand 2,2′,7,7′-tetra(4-pyridyl)-9,9′-spirobifluorene (TPSBF), we present two cationic SCO frameworks [FeII(TPSBF){Au(CN)2}](anion)·solv (1: anion = Au(CN)2−, solv = 5TCE; 2: anion = I−, solv = 4TCE·10H2O; TCE = 1,1,2,2-tetrachloroethane) exhibiting anion-induced conformational polymorphism. The selection of Au(CN)2‒ as the counter anion triggers a periodic rearrangement of the TPSBF ligands along the a-axis and induces twisted Fe-Au chains, whereas the use of I‒ as the counter anion leads to an alternating arrangement of the TPSBF ligands and forms linear Fe-Au chains. Magnetic measurements reveal a hysteretic two-step (HS-HS0.5LS0.5-LS) and a gradual one-step SCO behavior for 1 and 2, respectively. The significant difference in the SCO properties can be attributed to distinct synergistic mechanisms driven by both the changes of the TPSBF conformation and the step-wise flexing motions of Fe-Au chain in 1 or the rotational motions of the disordered pyridyl moieties of the TPSBF ligand in 2.