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

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]·CHCl₃ (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.