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

Abstract

⁵⁷Fe Mössbauer spectroscopy has provided detailed information about the low-spin/high-spin transition in two types of two-dimensional Fe (II) complexes, [Fe^IIH₃L^Me][Fe^IIL^Me]X (X^-=BF₄^-, ClO₄^-, PF₆^-, NO₃^-) 1-4 and [Fe^IIH₃L^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 (γ_HS) was estimated from Mössbauer area intensities of each compound in different temperatures. The γ_HS-T plots for 1-4 indicated that these compounds exhibit a two-step spin crossover corresponding to (LS-[Fe^IIH₃L^Me]²⁺ + LS-[Fe^IIL^Me]^-) ↔ (HS-[Fe^IIH₃L^Me]^2- + LS-[Fe^IIL^Me]^-) ↔ (HS-[Fe^IIH₃L^Me]²⁺ + HS-[Fe^IIL^Me]^-). The larger size of the counter anion separated the first and second spin crossover transitions in the [Fe^IIH₃L^Me] [Fe^IIL^Me]X system well. A variety of γ_HS-T curves depending on the counter anion was observed for [Fe^IIH₃L^Me] Cl·X. It was suggested that a smaller counter anion stabilizes the 1/2 (LS-[Fe^IIH₃L^Me]²⁺ + HS-[Fe^IIH₃L^Me]²⁺) state in this system.