Counter Anion Effect on the Spin-crossover Behaviors of Two-dimensional Fe(II) Complexes with a Tripod Ligand Containing Three Imidazoles

Abstract

types of two-dimensional Fe(II) complexes, [Fe II H3L Me ][Fe II L Me ]X (X – = BF4 – , ClO4 – , PF6 – , NO3 – ) 1–4 and [Fe II H3L Me ]Cl·X (X – = PF6 – , AsF6 – , SbF6 – ) 5–7, where H3L Me denotes a hexadentate N6 tripod-like ligand containing three imidazole groups, tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine. The molar fraction of high-spin Fe II versus total Fe II (γHS) was estimated from Mossbauer area intensities of each compound in different temperatures. The γHS-T plots for 1–4 indicated that these compounds exhibit a two-step spin crossover corresponding to (LS-[Fe II H3L Me ] 2+ + LS-[Fe II L Me ] – ) ↔ (HS-[Fe II H3L Me ] 2+ + LS-[Fe II L Me ] – ) ↔ (HS-[Fe II H3L Me ] 2+ + HS-[Fe II L Me ] – ) . The larger size of the counter anion separated the first and second spin crossover transitions in the [Fe II H3L Me ][Fe II L Me ]X system well. A variety of γHS-T curves depending on the counter anion was observed for [Fe II H3L Me ]Cl·X. It was suggested that a smaller counter anion stabilizes the 1/2(LS-[Fe II H3L Me ] 2+ + HS[Fe II H3L Me ] 2+ ) state in this system.