Two-step spin transition around room temperature in a Fe^III complex

Spin-crossover (SCO) at room temperature is a pivotal goal within the field of molecular magnetism. Herein, we attempt to assemble Fe^III SCO complexes using a substituted Hqsal ligand, H₂L (N-(8-quinolyl)-2,3-dihydroxybenzaldimine). Two complexes [Fe(HL)₂]·X·2MeCN (X = BF₄⁻ for 1 and X = ClO₄⁻ for 2) were obtained and characterized. Single-crystal X-ray diffraction study reveals that the solvent and counteranion contact with the main structure through hydrogen bonding that significantly influences the SCO properties. Magnetic study reveals that both complexes show a one-step reversible spin transition below room temperature with a hysteresis loop width of 10 K for complex 1 and 4 K for complex 2. After removing the solvents, two-step SCO with a hysteresis loop width of 32 and 62 K is observed around room temperature for complex 1, while one-step SCO is found in complex 2. Magneto-structural study reveals that the differences in the SCO properties are related to the hydrogen bonding and solvent effects, which facilitates the spin transition.