Halogen-Driven Spin Dynamics: Exploring Correlation of Cooperativity and Spin Crossover in Solid-State Mononuclear Fe(III) Schiff-Base Complexes.

We report the synthesis, crystal structures, and magnetic properties of four new mononuclear iron(III) complexes, [Fe(5-X-saltrien)]ReO (X = F (); X= Cl (); X = Br (); X = I ()), where (5-X-saltrien) is a halogen-substituted hexadentate Schiff-base (NO) ligand. Magnetic susceptibility measurements demonstrated that complex maintains a high-spin (HS) state, while complexes and display a very gradual spin crossover (SCO) above 300 K. Complex exhibits a unique two-step thermo-induced spin-state switching at = 250 K (1) and 209 K (2) with symmetry breakings. Single-crystal X-ray structures of are obtained at 300, 250, and 100 K, respectively, indicating two structural phase transitions (--2/). The introduction of halogen substituents on (saltrien) ligands has a great influence on cooperativity and finally the magnetic properties of Fe(III) SCO complexes.