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

Two ion-pair Fe(iii) complexes (PPh4)[Fe-III(HATD)(2)]center dot 2H(2)O (1, H(3)ATD = azotetrazolyl-2,7-dihydroxynaphthalene) and [Fe-II(phen)(3)][Fe-III(HATD)(2)](2)center dot 3DMA center dot 3.5H(2)O (2, phen = 1,10-phenanthroline, DMA = N,N-dimethylformamide) were synthesized by employing the tridentate ligand H(3)ATD. 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 HATD(2-) ligands forming the [Fe-III(HATD)(2)](-) core. The shortest cationMIDLINE HORIZONTAL ELLIPSISanion distance between the phosphorus ion of the (PPh4)(+) cation and the ferric ion of the [Fe-III(HATD)(2)](-) anion is 13.190 angstrom 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 angstrom in complex 2. C-HMIDLINE HORIZONTAL ELLIPSISC and C-HMIDLINE HORIZONTAL ELLIPSISO hydrogen interactions between the [Fe-II(phen)(3)](2+) cation and the [Fe-III(HATD)(2)](-) anion are observed in 2. Face-to-face pi-pi stacking interactions between naphthalene rings with the separated interplanar center to center distances of 3.421-3.680 angstrom 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 pi-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.

Automated summary

Two ion-pair Fe(iii) complexes were synthesized by employing the tridentate ligand H(3)ATD, with one in low-spin state and the other undergoing high temperature spin crossover between 360 and 500 K. The magnetic measurements and magneto-structural relationship studies reveal the crucial role of interactions between the cation and anion in the high temperature Fe(iii) behavior.