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-sal₂trien)]ReO₄ (X = F (<b>1</b>); X= Cl (<b>2</b>); X = Br (<b>3</b>); X = I (<b>4</b>)), where (5-X-sal₂trien)^2- is a halogen-substituted hexadentate Schiff-base (N₄O₂)^2- ligand. Magnetic susceptibility measurements demonstrated that complex <b>1</b> maintains a high-spin (HS) state, while complexes <b>3</b> and <b>4</b> display a very gradual spin crossover (SCO) above 300 K. Complex <b>2</b> exhibits a unique two-step thermo-induced spin-state switching at <i>T</i>_1/2 = 250 K (1) and 209 K (2) with symmetry breakings. Single-crystal X-ray structures of <b>2</b> are obtained at 300, 250, and 100 K, respectively, indicating two structural phase transitions (<i>Pccn</i>-<i>Pmmn</i>-<i>P</i>2₁/<i>m</i>). The introduction of halogen substituents on (sal₂trien)^2- ligands has a great influence on cooperativity and finally the magnetic properties of Fe(III) SCO complexes.