Synthesis, structure, and spin equilibrium properties of [Fe^IIIX₂L]BPh₄ solvents (X = 1-methylimidazole and imidazole; L = N, N′-bis(3-ethoxycarbonylsalicylidene)ethylenediamine)

Two iron(III) complexes [Fe^IIIX₂L]BPh₄ solvents (X = 1-Meim (1), Him (2)) were synthesized and the crystal structures were determined, where H₂L = N, N'-bis(3-ethoxycarbonylsalicylidene)ethylenediamine, 1-Meim = 1-methylimidazole, Him = imidazole, BPh₄^- = tetraphenylborate. Fe^III ion has an octahedral coordination environment by N₂O₂ donor atoms of equatorial L^2- and N₂ donor atoms of axial imidazoles. Complex 1 is described as isolated molecule, while complex 2 has a hydrogen-bonded cyclic dimer structure constructed by hydrogen-bond between imidazole group and ethoxycarbonyl oxygen via ethanol. Complex 1 is in a high-spin (S = 5/2) state over the whole temperature region 5-300 K, while complex 2 showed a gradual complete spin equilibrium between high and low-spin states at T_1/2 = 192 K.