High temperature anionic Fe(iii) spin crossover behavior in a mixed-valence Fe(ii)/Fe(iii) complex

Abstract

Two ion-pair Fe(III) complexes (PPh₄)[Fe^III(HATD)₂]·2H₂O (1, H₃ATD = azotetrazolyl-2,7-dihydroxynaphthalene) and [Fe^II(phen)₃][Fe^III(HATD)₂]₂·3DMA·3.5H₂O (2, phen = 1,10-phenanthroline, DMA = N,N-dimethylformamide) were synthesized by employing the tridentate ligand H₃ATD. Crystal structure analyses reveal that complexes 1 and 2 consist of Fe^III ions in an octahedral environment where a Fe^III ion is coordinated by two HATD²⁻ ligands forming the [Fe^III(HATD)₂]⁻ core. The shortest cation⋯anion distance between the phosphorus ion of the (PPh₄)⁺ cation and the ferric ion of the [Fe^III(HATD)₂]⁻ anion is 13.190 Å in complex 1, whereas that between the ferrous ion of the [Fe^II(Phen)₃]²⁺ cation and the ferric ion of the [Fe^III(HATD)₂]⁻ anion is 7.821 Å in complex 2. C–H⋯C and C–H⋯O hydrogen interactions between the [Fe^II(phen)₃]²⁺ cation and the [Fe^III(HATD)₂]⁻ anion are observed in 2. Face-to-face π-π stacking interactions between naphthalene rings with the separated interplanar center to center distances of 3.421–3.680 Å were observed, which result in a one-dimensional supramolecular chain in complexes 1 and 2. Magnetic measurements show that complex 1 is in the low-spin (LS) state below 500 K, whereas 2 undergoes a high temperature spin crossover (SCO) between 360 and 500 K. Magneto-structural relationship studies reveal that π-stacking, hydrogen interactions and Coulomb interactions between the [Fe^III(HATD)₂]⁻ anion and the [Fe^II(phen)₃]²⁺ cation play a crucial role in the high temperature Fe(III) SCO behaviour of complex 2.