Decoupling of Symmetry Breaking and Spin Crossover in Iron(III) Complexes Bearing an <i>N</i>-Benzylethylenediamine Ligand

Three iron(III) spin crossover compounds, [Fe(salBzen-5-OMe)2]A, where HsalBzen-5-OMe = 2-[(2-benzylaminoethylimino)methyl]-4-methoxyphenol and A = Cl– 1, Br– 2, I– 3, have been synthesized and fully characterized. UV–vis spectroscopy reveals two LMCT bands corresponding to the LS and HS states in solution. X-ray crystallography indicates that the compounds crystallize in monoclinic P21/n or P21/c (1), (2) or tetragonal P43212 (3) phases. At room temperature, complexes 1 and 2 display HS FeIII centers, while complex 3 adopts an LS state. Notably, complexes 1 and 2 exhibit symmetry breaking, decoupling the phenomenon from spin crossover. A variety of intermolecular interactions, including C–H···π, C–H···O, N–H···O, C–H···anion, and N–H···anion, are responsible for linking the cations and forming a 3D supramolecular network. SQUID magnetometry studies show that compounds 1 and 2 remain high spin down to 10 K, while complex 3 undergoes a gradual spin crossover above 350 K. Crystallization of 2 at lower temperatures and humidity gives a tetragonal phase P43212 (2’) that exhibits a spin crossover profile very similar to 3. Moreover, the crystal structure of 2’ reveals temperature-dependent modulation. These results highlight the significant role of counterions in modulating the magnetic properties of these compounds and demonstrate the independent control of symmetry breaking and spin crossover. This work offers valuable insights for designing advanced functional materials for molecular spintronics and materials science.