Geometry-Driven Field-Induced Single-Ion Magnetism in Hexagonal Bipyramidal Tb³⁺ and Ho³⁺ Complexes

The synthesis of the precursors [Ln(L^N6en)(CH₃COO)₂](BPh₄)·nH₂O (Ln = Tb, n = 0, 1; Ln = Ho, n = 1, 2·H₂O), followed by a ligand exchange reaction with triphenylsilanolate, results in the isolation of the complexes {[Ln(L^N6en)(OSiPh₃)₂](BPh₄)}·2CH₂Cl₂ (Ln = Tb, 3·2CH₂Cl₂; Ln = Ho, 4·2CH₂Cl₂). Single-crystal X-ray diffraction studies of 3·2CH₂Cl₂ and 4·2CH₂Cl₂ revealed that both compounds adopt a hexagonal bipyramidal geometry. Magnetic characterization shows that the complexes behave as single-ion magnets (SIMs) under an optimal applied field of 2000 Oe. Notable, these are the first reported Tb³⁺ and Ho³⁺ complexes with a hexagonal bipyramidal coordination geometry to exhibit such magnet-like behavior. Furthermore, they constitute the first field-induced Tb³⁺ and Ho³⁺ SIMs incorporating a macrocyclic ligand in a nonsandwich topology. Magnetic measurements indicate that the applied field only partially suppresses quantum tunneling of magnetization (QTM) and that at higher temperatures magnetic relaxation is dominated by the Raman process rather than the Orbach mechanism. These experimental observations are supported by ab initio calculations, which provide detailed insights into the electronic structure, including the splitting of f-orbital energy levels, thereby elucidating the origin of the observed magnetic behavior in both cases.