‘Kick-in the head’: high-performance and air-stable mononuclear DyIII single-molecule magnets with pseudo-D6h symmetry from a [1 + 1] Schiff-base macrocycle approach

Abstract

Using the [1 + 1] condensation approach for the preparation of new macrocyclic scaffolds (L^N6_phen and L^N4O2_phen) with the rigid phenanthroline-based ‘head’ unit produces the air-stable mononuclear compounds [Dy(L^N6_phen)(Ph₃SiO)₂](PF₆) (1-L^N6_phen) and [Dy(L^N4O2_phen)(Ph₃SiO)₂](PF₆) (1-L^N4O2_phen) through a stepwise metal-ion templated synthesis. Both compounds exhibit pseudo-D_6h symmetry with different degrees of distortion from the ideal hexagonal bipyramidal geometry, depending on the planarity of the equatorial macrocycles and the intramolecular π–π stacking interactions between the phenyl groups of the apical siloxide ligand and the equatorial macrocycle. Both compounds are single-molecule magnets (SMMs) with large energy barriers for the magnetization reversal, exhibiting out-of-phase AC susceptibility signals up to 75 K or 90 K. The closer-to-ideal D_6h complex 1-L^N4O2_phen possesses a U_eff of 1360 K, which is the highest reported barrier among all mononuclear Dy^III SMMs synthesized using the [1 + 1] Schiff-base macrocycle approach. The experimental results are supported by ab initio calculations, which indicate relaxation of the magnetization via the first- or second-excited state for 1-L^N6_phen and 1-L^N4O2_phen, respectively. The results demonstrate the ability of Schiff-base macrocycles to facilitate the synthesis of high-performance and air-stable SMMs through a chemical modulation of the individual carbonyl ‘head’ and amine subunits, deciphering the factors which affect the magnetic dynamics of SMMs.