Heteroleptic and Homoleptic Iron(III) Spin-Crossover Complexes; Effects of Ligand Substituents and Intermolecular Interactions between Co-Cation/Anion and the Complex

Abstract

The structural and magnetic properties of a range of new iron(III) bis-tridentate Schiff base complexes are described with emphasis on how intermolecular structural interactions influence spin states and spin crossover (SCO) in these d5 materials. Three pairs of complexes were investigated. The first pair are the neutral, heteroleptic complexes [Fe(3-OMe-SalEen)(thsa)] 1 and [Fe(3-MeOSalEen)(3-EtOthsa)] 2, where 3-R-HSalEen = (E)-2-(((2-(ethylamino)ethyl)imino)methyl)-6-R-phenol and 3-R-H2thsa = thiosemicarbazone-3-R-salicylaldimine. They display spin transitions above room temperature. However, 2 shows incomplete and gradual change, while SCO in 1 is complete and more abrupt. Lower cooperativity in 2 is ascribed to the lack of π–π interactions, compared to 1. The second pair, cationic species [Fe(3-EtOSalEen)2]NO3 3 and [Fe(3-EtOSalEen)2]Cl 4 differ only in the counter-anion. They show partial SCO above room temperature with 3 displaying a sharp transition at 343 K. Weak hydrogen bonds from cation to Cl− probably lead to weaker cooperativity in 4. The last pair, CsH2O[Fe(3-MeO-thsa)2] 5 and Cs(H2O)2[Fe(5-NO2-thsa)2] 6, are anionic homoleptic chelates that have different substituents on the salicylaldiminate rings of thsa2−. The Cs cations bond to O atoms of water and the ligands, in unusual ways thus forming attractive 1D and 3D networks in 5 and 6, respectively, and 5 remains HS (high spin) at all temperatures while 6 remains LS (low spin). Comparisons are made to other literature examples of Cs salts of [Fe(5-R-thsa)2]− (R = H and Br).