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

The Fe(II)(H(3)L)(BF(4))(2).3H(2)O (1) complex was synthesized, where H(3)L (tris[2-[(imidazole-4-yl)methylideneamino]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(1.5)L)(BF(4))(0.5) (2) (=Fe(II)(H(3)L)Fe(II)(L)BF(4)), Fe(H(1.5)L)BF(4) (3) (=Fe(II)(H(3)L)Fe(III)(L)(BF(4))(2)), Fe(III)(H(3)L)(BF(4))(3).fim.H(2)O (4), and Fe(III)(L).2.5H(2)O (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(1.5)L. The structure of 3 consists of a homochiral two-dimensional assembled sheet, arising from the intermolecular hydrogen bonds between Fe(II)(H(3)L)(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.