Structural, Magnetic, and Vibrational Investigations of FeIII Spin‐Crossover Compounds [Fe(4‐MeO–SalEen)2]X with X = NO3– and PF6–

The spin-crossover properties of two spin-crossover materials [FeIII(4-MeO–SalEen)2]X (X = NO3–, PF6–, and H–4-MeO–SalEen = condensation product of 4-methoxy-substituted salicylaldehyde and N-ethylethylenediamine) in the crystalline form were investigated by magnetic and―for the first time at SOLEIL Synchrotron―combined single-crystal Raman and X-ray diffraction measurements. The NO3– salt exhibits a spin transition at approximately 78 K with a thermal hysteresis (ΔT = 7.5 K), whereas the PF6– salt shows a progressive spin crossover centered at approximately 225 K. For this latter compound, the reversible curve extracted from the single-crystal Raman measurements is fully comparable with the magnetic and structural data. The crystal structures were determined at 293, 100, and 50 K (for X = NO3–) and 293 and 106 K (for X = PF6–). It appears from this analysis that the hysteretic behavior of the NO3– salt results from a first-order isostructural phase transition. The strongly cooperative and bistable behavior of this compound is favored at such a temperature by a 2D network of complexes that communicate through moderate intermolecular interactions. The presence of direct van der Waals contacts that involve the phenolato groups linked to the FeIII ions might influence the ligand-field strength and the transition temperature. The thermal trapping of the metastable HS phase was magnetically detected, and its relaxation to the LS ground state was observed at approximately 34.5 K. The spin crossover of the PF6– compound was analyzed as the combination of progressive processes that characterize quite similar cations that occupy two crystallographic sites.