A new family of Fe4Ln4 (Ln = DyIII, GdIII, YIII) wheel type complexes with ferromagnetic interaction, magnetocaloric effect and zero-field SMM behavior

Abstract

In this work, we report three isostructural Fe₄Ln₄ wheel type complexes with molecular formula [{Fe₄(dea)₄Dy₄(deaH)₈(μ₂-OMe)₄}(NO₃)₄]·4(H₂O) (1), [{Fe₄(dea)₄Gd₄(deaH)₈(μ₂-OMe)₄}·(NO₃)₄]·4(H₂O)·4(MeOH) (2) and [{Fe₄(dea)₄Y₄(deaH)₈(μ₂-OMe)₄}(NO₃)₄]·4(H₂O)·6(MeOH) (3), (where dea = diethanolamine). Detailed structural analysis discloses that the 4Ln^III and 4Fe^III ions are connected alternatively in a cyclic arrangement through the oxo-bridging of methanol and diethanolamine molecules. The detailed magnetic measurements reveal the presence of ferromagnetic exchange interactions between the Ln^III and Fe^III centers in complexes 1 and 2, whereas a weak antiferromagnetic interaction is observed between Fe^III centers in complex 3. In addition, complex 1 shows slow magnetic relaxation behavior (U_eff = 9.8 K) under zero field and a remarkable entropy change was observed for complex 2 at 7 T magnetic field (magnetocaloric effect = 31.15 J kg⁻¹ K⁻¹). The ab initio calculations disclose the presence of strong axial anisotropy and a minimum transverse component (g_x = 0.0024, g_y = 0.0062, and g_z = 19.82) in the ground state of each Dy^III ion for complex 1. It was also found that the local anisotropy axis on each Dy^III ion is arranged in a vortex-like structure as observed in single molecule toroics (SMTs). From BS-DFT calculations, it was found that the exchange interactions between Fe–Ln metal centers mainly follow the charge transfer process between 3d and 4f orbitals.