Supramolecular Spin-Crossover Iron Complexes Based on Imidazole−Imidazolate Hydrogen Bonds

Abstract

The [Fe^II(H₃L)](BF₄)₂·3H₂O (1) complex was synthesized, where H₃L (tris{[2-{(imidazole-4-yl)methylidene}amino]ethyl}amine) is a tripodal ligand obtained by condensation of tris(2-aminoethyl)amine and 4-formylimidazole (fim) in a 1:3 molar ratio. Starting from 1, a series of complexes, [Fe^II(H_1.5L)](BF₄)_0.5 (2) (=[Fe^II(H₃L)][Fe^II(L)]BF₄), [Fe(H_1.5L)]BF₄ (3) (=[Fe^II(H₃L)][Fe^III(L)](BF₄)₂), [Fe^III(H₃L)](BF₄)₃·fim·H₂O (4), and [Fe^III(L)]·2.5H₂O (5), has been synthesized and characterized. The single-crystal X-ray structure of each complex has been determined. The Fe^II compound, 2, and a mixed valence Fe^II−Fe^III compound, 3, involve formally hemi-deprotonated ligands, H_1.5L. The structure of 3 consists of a homochiral two-dimensional assembled sheet, arising from the intermolecular hydrogen bonds between [Fe^II(H₃L)]²⁺ and [Fe^III(L)]⁰ (3). All but 5 exhibit spin crossover between low-spin (LS) and high-spin (HS) states. This is a rare case where both Fe^II and Fe^III complexes containing the same ligand exhibit spin-crossover behavior. Magnetic susceptibility and Mössbauer studies showed that 3 has three accessible electronic states: LS Fe^II−LS Fe^III, HS Fe^II−LS Fe^III, and HS Fe^II−HS Fe^III. Compounds 1−3 show the light-induced excited spin-state trapping effect at the Fe^II sites upon irradiation with green light. The solution magnetic properties, electronic spectra, and electrochemical properties of 1, 4, and 5 were also studied.