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

Abstract

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 Fe^III 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.

A series of solvent-free iron(III) complexes is explored, with larger anions leading to SCO behavior, while smaller anions result in symmetry breaking.