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

Abstract

Using the [1+1] condensation approach for the preparation of new macrocyclic scaffolds ("L" _"phen" ^"N6" and "L" _"phen" ^"N4O2" ) with the rigid phenanthroline-based ‘head’ unit produces the air-stable mononuclear compounds [Dy("L" _"phen" ^"N6" )(Ph3SiO)2](PF6) (1-"L" _"phen" ^"N6" ) and [Dy("L" _"phen" ^"N4O2" )(Ph3SiO)2](PF6) (1-"L" _"phen" ^"N4O2" ) through a stepwise metal-ion templated synthesis. Both compounds exhibit pseudo-D6h 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 D6h complex 1-"L" _"phen" ^"N4O2" possesses a Ueff of 1360 K, which is the highest reported barrier among all mononuclear DyIII 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" _"phen" ^"N6" and 1-"L" _"phen" ^"N4O2" , respectively. The combined 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.