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 (D5h) 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 (Ueff) 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 D5h symmetry. Compounds 1 and 2 exhibit magnetization blocking (TB) 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 D5h 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.