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Dimerized Spin-Crossover Iron(II) Complexes as Supramolecular Anion Capsules



Two dimerized iron(II) complexes, [{Fe(L)(HL)2}{Fe(HL)3}](PF6)(BF4)2·5CH3CN·4H2O (1) and [{Fe(L)(HL)2}{Fe(HL)3}](PF6)(FeCl4)2 (2) {HL = 3-(2-pyridyl)-5-[4-(diphenylamino)phenyl]-1H-pyrazole}, supported by triphenylamine-derivatized bidentate ligands and encapsulating hexafluorophosphate ions were synthesized. The complex iron ions have octahedral coordination geometries in which they interact with six nitrogen atoms from three bidentate ligands. Both compounds have capsule-like dimerized supramolecular structures, in which the arrangements of the bulky ligands of two discrete mononuclear complexes create a central cavity in which a single hexafluorophosphate ion is captured. The bidentate ligands interact with the anionic guest molecules through the hydrogen atoms of the ligand pyrazole moieties. Cryomagnetic studies reveal that both compounds show spin-crossover behavior. Complex 1 showed quasi-reversible spin-crossover around 200–300 K, hindered by desolvation effects, whereas 2 showed reversible gradual spin-crossover behavior. The electronic states of the iron ions resulting from the spin-crossover were confirmed by X-ray structure analyses and Mössbauer spectra.