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

The [FeII(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, [FeII(H1.5L)](BF4)0.5 (2) (=[FeII(H3L)][FeII(L)]BF4), [Fe(H1.5L)]BF4 (3) (=[FeII(H3L)][FeIII(L)](BF4)2), [FeIII(H3L)](BF4)3·fim·H2O (4), and [FeIII(L)]·2.5H2O (5), has been
synthesized and characterized. The single-crystal X-ray structure of each complex has been determined. The FeII
compound, 2, and a mixed valence FeII−FeIII compound, 3, involve formally hemi-deprotonated ligands, H1.5L. The
structure of 3 consists of a homochiral two-dimensional assembled sheet, arising from the intermolecular hydrogen
bonds between [FeII(H3L)]2+ and [FeIII(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 FeII and FeIII complexes containing the same ligand exhibit spin-crossover behavior. Magnetic susceptibility and Mössbauer studies showed that 3 has three accessible electronic
states:  LS FeII−LS FeIII, HS FeII−LS FeIII, and HS FeII−HS FeIII. Compounds 1−3 show the light-induced excited
spin-state trapping effect at the FeII sites upon irradiation with green light. The solution magnetic properties, electronic
spectra, and electrochemical properties of 1, 4, and 5 were also studied.