Quantifying Magnetic Anisotropy of Ferroelectric Fe(II) Square‐Pyramidal Systems Using Torque Magnetometry

ABSTRACT The design of mononuclear complexes that simultaneously exhibit multiple functionalities represents a rapidly advancing frontier in molecular materials research. Achieving strong easy‐axis magnetic anisotropy within inherently polar crystal lattices remains a significant challenge, yet it offers a promising pathway toward next‐generation spin–electric materials. Comparative studies of isostructural systems with subtle structural variations provide key insight into molecular tuning strategies for practical implementation. In this context, we report a family of five‐coordinate, square‐pyramidal Fe(II) complexes, [Fe(L)(X) ₂ ]·CHCl ₃ (L = tridentate Schiff‐base ligand; X = Cl ( 1 ), Br ( 2 )), crystallizing in the polar, non‐centrosymmetric triclinic space group P1 and exhibiting pronounced easy‐axis magnetic anisotropy. Cantilever torque magnetometry on single crystals reveals a negative axial Zero Field Splitting parameter D = −25.6 cm ^{− 1} ( 1 ) and −19.8 cm ^{− 1} ( 2 ), among the largest reported for square‐pyramidal Fe(II). Ab initio CASSCF/NEVPT2 calculations reproduce the experimental Spin Hamiltonian parameters and show that subtle steric and electronic effects, particularly the out‐of‐plane displacement of Fe(II), critically govern the magnitude and sign of D . Complementary piezoresponse force microscopy and Polarization vs Electric field measurements confirm intrinsic polarization and a pronounced nanoscale piezoelectric response consistent with computed dipole moments, establishing a compelling platform for multifunctional spin–electric materials and multifunctional molecular architectures.