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 (1); X= Cl (2); X = Br (3); X = I (4)), where (5-X-sal2trien)2- is a halogen-substituted hexadentate Schiff-base (N4O2)2- ligand. Magnetic susceptibility measurements demonstrated that complex 1 maintains a high-spin (HS) state, while complexes 3 and 4 display a very gradual spin crossover (SCO) above 300 K. Complex 2 exhibits a unique two-step thermo-induced spin-state switching at T1/2 = 250 K (1) and 209 K (2) with symmetry breakings. Single-crystal X-ray structures of 2 are obtained at 300, 250, and 100 K, respectively, indicating two structural phase transitions (Pccn-Pmmn-P21/m). 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.