Phosphine Oxide Coordinated Mononuclear Ln ^III (Ln = Dy ^III , Er ^III ) Single‐Ion Magnet with Pseudo‐Pentagonal Bipyramidal Geometry

Two mononuclear lanthanide complexes, [(L)Ln^III(Ph₃PO)Cl]·CH₃OH] [Ln^III = Dy (1) and Er(2)], are isolated employing a pyridine-based ligand, [2,2'-{(1E, 1'E)-pyridine 2,6-diylbis(methaneylylidine}bis(azaneylylidine)diphenol] (H₂L) and triphenylphosphine oxide. Both complexes are characterized by single-crystal X-ray diffraction studies. Pseudo-pentagonal bipyramidal geometry (pseudo-D_5h) around the lanthanide centers in both complexes is confirmed through SHAPE analysis. Dynamic magnetic measurements revealed the single-ion magnet behavior for both complexes. However, the energy barrier (U_eff) and relaxation time (τ₀) for complex 1are extracted as U_eff = 34.77 K, τ₀ = 8.07 × 10^-7 s (considering both Orbach and Raman processes). These observed phenomena have been corroborated with the aid of CASSCF-based ab initio calculations.