A Brønsted-Ligand-Based Iron Complex as a Molecular Switch with Five Accessible States

A mononuclear Fe^II complex, prepared with a Brønsted diacid ligand, H₂ L (H₂ L=2-[5-phenyl-1H-pyrazole-3-yl] 6-benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [Fe^II (H₂ L)₂ ](BF₄ )₂ (1_A ), exhibits abrupt spin transition at T_1/2 =258 K, and treatment with base yields a deprotonated analogue [Fe^II (HL)₂ ] (1_B ), which shows gradual SCO above 350 K. A range of Fe^III analogues were also characterized. [Fe^III (HL)(H₂ L)](BF₄ )Cl (1_C ) has an S=5/2 spin state, while the deprotonated complexes [Fe^III (L)(HL)], (1_D ), and (TEA)[Fe^III (L)₂ ], (1_E ) exist in the low-spin S=1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid-state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.