The Role of Solvents and Hydrogen Bonds in Shaping Spin Crossover Dynamics of Mononuclear Iron (III) Schiff‐Base Complexes

ABSTRACT A series of Fe III complexes [Fe(5‐Br‐3‐MeO‐sal 2 Trien)]Y·Z (Y = NO 3 − , Z = 0.5CH 3 OH·CH 3 CN ( 1 ); Y = NO 3 − ( 2 ); Y = ClO 4 − ( 3 )) have been prepared. Complexes 1 and 2 exhibit analogous crystal packing architectures with two distinct iron centers, labeled Fe1 and Fe2. The Fe1 sites form dimers encapsulated within a hexagonal framework constructed by Fe2 centers. Thermal expansion of the Fe2 framework upon heating creates sufficient space to facilitate the spin transition of Fe1. Both complexes display similar one‐step gradual spin crossover behavior, with transition temperatures ( T c ) of 142 K for 1 and 186 K for 2 , respectively. In contrast, complex 3 incorporates four crystallographically independent Fe centers. Its overly compact packing geometry sterically hinders spin‐state switching, resulting a partial low‐spin state over the entire tested temperature range.