An air-stable Dy(iii) single-ion magnet with high anisotropy barrier and blocking temperature

Herein we report air-stable Dy(iii) and Er(iii) single-ion magnets (SIMs) with pseudo-D_5h symmetry, synthesized from a sterically encumbered phosphonamide, BuPO(NHⁱPr)₂, where the Dy(iii)-SIM exhibits a magnetization blocking (T_B) up to 12 K, defined from the maxima of the zero-field cooled magnetization curve, with an anisotropy barrier (U_eff) as high as 735.4 K. The Dy(iii)-SIM exhibits a magnetic hysteresis up to 12 K (30 K) with a large coercivity of ∼0.9 T (∼1.5 T) at a sweep rate of ∼0.0018 T s^-1 (0.02 T s^-1). These high values combined with persistent stability under ambient conditions, render this system as one of the best-characterized SIMs. Ab initio calculations have been used to establish the connection between the higher-order symmetry of the molecule and the quenching of quantum tunnelling of magnetization (QTM) effects. The relaxation of magnetization is observed via the second excited Kramers doublet owing to pseudo-high-order symmetry, which quenches the QTM. This study highlights fine-tuning of symmetry around the lanthanide ion to obtain new-generation SIMs and offers further scope for pushing the limits of U_eff and T_B using this approach.