Single-Ion Magnetism in Pseudo-<i>D</i>_5h Symmetric Complexes of Less Explored Lanthanides Built from a Bulky Phosphonamide

A series of lanthanide(III) single-ion magnets with the general formula [L2Ln(H2O)5][I]3·L2·(CH2Cl2)2, where Ln = Gd (1), Tb (2), Ho (3), and Yb (4), was synthesized utilizing sterically hindered adamantyl phosphonamide ligand ((Ad)P(O)(NHiPr)2 (L)). The lanthanide ions were selected to include less explored metals, such as Ho(III), Yb(III), and Gd(III), in the field of single-ion magnetism. These complexes, exhibiting a pseudopentagonal bipyramidal geometry, provide ideal model systems for investigating the influence of the phosphonamide-based ligand field on the magnetic properties and relaxation pathways of both prolate and oblate Ln(III) ions. Notably, the Ho(III) complex functions as a zero-field single-molecule magnet, demonstrating a substantial energy barrier Ueff of 330 K. The other isostructural complexes exhibit field-induced slow relaxation of magnetization, which can be fitted using a combination of Raman relaxation and quantum tunneling of magnetization.