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

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). View Full-Text