Unveiling the relationship between counterion size and spin crossover dynamics in hexadentate Schiff-base manganese(<scp>iii</scp>) complexes

Understanding the relationship between molecular packing/interaction and the spin crossover property is essential for advancing solid-state molecular memory devices. In this work, a series of hexadentate Schiff-base manganese(III) complexes ([Mn(4F-sal₂323)]X, X = ClO₄ (1); X = AsF₆ (2); X = PF₆ (3); X = ReO₄ (4) and X = NO₃ (5)) were synthesized and characterized. Magnetic studies showed that complex 1 exhibited an abrupt spin transition at 90 K with a 10 K wide thermal hysteresis, while complexes 2-5 exhibited gradual and incomplete spin transition with no hysteresis. Analysis of the magneto-structural correlation indicated that the SCO transition temperature is negatively correlated with the octahedral distortion and void space volume of the anions. Detailed variable-temperature structural analysis revealed that hydrogen bonding interaction and close stacking hinder the torsional deformation of the Mn^III coordination sphere, preventing complete SCO. Density functional theory (DFT) calculations revealed that the weaker hydrogen bonding in complex 1 led to lower atomic charges on the nitrogen atoms, thereby affecting the coordination field strength of the Mn center. This work demonstrates that the transition temperature of SCO is influenced by supramolecular stacking forces, and this relationship can be finely tuned by simply adjusting the size of the ions.