Thermo-switching spin crossover in iron(III) complexes of Bis(phenol)diamine ligands

• Three Fe(III) complexes [FeL 1-3 L 4 ] of N,N'-bis(phenol)diamine based (H 2 L 1-3 ) and 5-chloro-7-iodo-8-hydroxyquinolinato (HL 4 ) ligands are presented. • All compounds have octahedral Fe III O 3 N 3 chromophores. • A temperature induced spin crossover SCO phenomenon was observed only for one of them, though they differ only slightly in H 2 L 1-3 non-coordination sphere. • Previously reported compound [FeL 1 L 4’ ], similar as herein reported SCO compound, but slightly different HL 4 , also shows no such phenomenon. Three N,N'-dimethylethylenediamine derivatives of substituted bis(phenol)diamine ligands (L 1-3 ) have been found in either of three iron(III) 8-quinolinato (L 4 ) coordination compounds [FeL 1-3 L 4 ]∙(solvent) ( 1 - 3 ). Herein, H 2 L 1 stands for 2,4-dimethylphenol, H 2 L 2 for 2,4-dichlorophenol, while H 2 L 3 for 2-( tert -butyl)-4-methoxyphenol derivative, respectively. HL 4 is 5-chloro-7-iodo-8-hydroxyquinoline being present in each of 1 - 3 . X-ray structure analysis reveals mononuclear octahedral Fe III O 3 N 3 chromophores with tetradentate O,O,N,N L 1-3 and bidentate O,N L 4 . All three title compounds show paramagnetic behavior, which is consistent with the monomeric character of their metal centers. Compounds 2 and 3 reveal high spin ( S = 5/2) in a whole 2-300 K range, while 1 shows the high spin character only above 200 K. By lowering the temperature, its magnetic susceptibility gradually decreases due to spin crossover towards low spin ( S = 1/2). The halogen-π(aromatic ring) structural interactions for 1 differ to 2 and 3 and may be related to such different magnetic behavior. Room temperature electronic absorption spectra of all three compounds 1 - 3 feature LMCT bands associated with phenolate(π)→Fe(III)(dπ*) charge transfer in the 450-700 nm region. These bands are in accordance with the DFT calculations for the high spin species only.