Unravelling the Role of Sterically Encumbered Ligands in Tuning the Magnetic Properties of Lanthanide‐Based D5h Single‐Ion Magnets

Isostructural Dy(III) and Er(III) complexes [L¹ ₂Ln(H₂O)₅][I]₃ ⋅ L¹ ₂ ⋅ (CH₂Cl₂) (Ln=Dy (1), Er (3)) and [L² ₂Ln(H₂O)₅][I]₃ ⋅ L² ₂ ⋅ (CH₂Cl₂)₂ (Ln=Dy (2), Er (4)), with distorted pentagonal bipyramidal geometry (D_5h) around the central metal were synthesized by utilizing two bulky phosphonamide ligands, adamantyl phosphonamide, (Ad)P(O)(NHⁱPr)₂ (L¹) and carbazolyl phosphoramide (Cz)P(O)(NHⁱPr)₂ (L²). The resultant complexes were investigated for their magnetic properties in order to elucidate the impact of modification of the coordinating P-O bond environment either by increasing steric bulk and/or introduction of a third P-N bond at the central phosphorus atom. Magnetic studies revealed substantial energy barriers (U_eff) of 640 K and 491 K for Dy compounds 1 and 2, respectively, rendering them as some of the best-performing air-stable SIMs amongst the class of SIMs with D_5h symmetry. Compounds 1 and 2 exhibit magnetization blocking (T_B) at 6.5 K and 6 K, respectively, at a sweep rate of 20 Oe/s. Compound 1 benefits from increased lattice intermetallic distances due to bulky adamantyl substituent, but exhibits a significant deviation from linear axial (P)O-Dy-O(P) geometry (173.7(1)°). In addition to the deviation from linearity, the incorporation of a bulky adamantane (or carbazole) ligand in complex 1 (or 2) was found to result in relatively strong Dy…H-C agostic interactions, with distances of 3.698 Å (3.376 Å). These interactions are expected to induce transverse anisotropy. Ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations offer valuable insights into the dynamics of magnetic relaxation and the impact of axial bulkiness on the anisotropy of D_5h systems. Beyond highlighting the crucial role of crystal field and symmetry in achieving high-temperature SIMs, this study also explores how the secondary coordination sphere can be engineered to create novel SIMs.