Halogen Effects and Anion Regulation in a Dual-Substituted FeIII-qsal Platform: Steering Abrupt to Two-Step SCO with Wide Hysteresis

Achieving abrupt, hysteretic spin crossover (SCO) requires strong cooperativity, typically engineered through supramolecular interactions. In this work, we designed a dual-halogenated Hqsal ligand, 5-F-Hqsal-5-Iq, strategically incorporating both bulky iodine and electronegative fluorine moieties. This design modulates the ligand field and, more importantly, induces strong halogen-mediated supramolecular interactions. Three FeIII complexes, [Fe(5-F-qsal-5-Iq)2]A·CH3OH (A = ClO4− (1), BF4− (2), and OTf− (3)), were synthesized and characterized, all adopting a similar Chain-Layer packing motif. The steric demand of the iodine atom forces the fluorine atoms on adjacent chains into closer proximity, which enhances hydrogen bonds and fluorine-mediated interactions within the 2D layers, thereby enhancing cooperativity. Notably, complex 3 exhibits enhanced cooperativity driven by extensive and intensified supramolecular interactions involving the iodine atoms and the OTf− anion. Magnetically, 1 and 2 exhibit abrupt SCO with narrow hysteresis, while 3 exhibits a two-step transition with a remarkably wide and stable 57 K hysteresis (1 K/min) that expands to 64 K at 15 K/min. All complexes exhibit the LIESST effect, with 3 showing the most pronounced response. This study underscores how steric and electronic effects from dual-halogen substitution can be employed to modulate SCO properties, offering a rational design route to high-performance SCO materials.