Adaptability to Crystal Fields in a Series of COT/Monodentate Ligand-Based Dy and Er Single Ion Magnets

Herein, Dy^III and Er^III, the typical oblate and prolate Kramers Ln ions, were employed to synthesize a series of isostructural pairs: 2-5Ln (Ln = Dy and Er). In the [(COT)Ln(THF)₄]⁺ cationic fragments of 2Ln, central ions were coordinated by the equatorial ligand cyclooctatetraenyl (COT) and THF solvents, while in the heteroleptic complexes 3Ln ((COT)Ln(OAr')), 4Ln ((COT)Ln(OAr″)), and 5Ln ((COT)Ln(N^††)), the coordination geometries were formed by the cooperation of COT and bulky aryloxides OAr' (2,6-bis(diphenylmethyl)-4-methylphenyl), OAr″ (2,6-bis(1-adamantyl)-4-methylphenyl), and amide N^†† (bis(triisopropylsilyl) amide) for 3Ln, 4Ln, and 5Ln, respectively. Among these complexes, with the combinations of varying coordination geometries and different anisotropies of f orbitals, 2Er, 3Dy, and 4Dy were found to be zero-field SIMs with effective energy barriers of 181.9, 527.7, and 622.0 K, respectively, which are consistent with the structural analyses and ab initio calculations. The blocking temperatures (T_B) of 3Dy and 4Dy are 4 and 7 K, respectively, as confirmed by the hysteresis loops at varying temperatures. The structures of 5Ln exhibit an almost perfect umbrella-shaped geometry, characterized by N-Ln-Centroid (COT) angles measuring 178.9 and 179.3° for 5Dy and 5Er, respectively. Crystallographic data from these structures were utilized to investigate the impact of ligand alignment on the magnetic properties of the compounds.