High local coordination symmetry around the spin center and the alignment between magnetic and symmetric axes together play a crucial role in single-molecule magnet performance

Abstract

A tetradentate 8-hydroxyquinoline-based acyl hydrazone ligand (HL¹ = 8-hydroxyquinoline-2-carboxaldehyde-(aminourea)hydrochloride) was elaborately used to construct a mononuclear dysprosium complex DyCl₃HL¹·CH₃OH (1) with a nearly ideal pentagonal bipyramid coordination geometry (D_5h) surrounding the Dy(III) ion to achieve the significant performance of single-molecule magnets (SMMs). Meanwhile, the isolated high local symmetry center was successfully kept intact and further bridged to a series of double bipyramid systems by two phenolic oxygen atoms of the acyl hydrazone ligands (HL¹ and HL² = 8-hydroxyquinoline-2-carboxaldehyde-(benzoyl)hydrazine), with the formulae [Dy₂Cl₄(L¹)₂(CH₃OH)₂]·4C₅H₅N (2) and [Dy₂Cl₄(L²)₂]·2CH₃CN (3). In addition, the monodentate co-ligand anion was replaced by a larger sterically hindered ligand and a bidentate monovalent β-diketonate anion to generate [Dy₂(tfo)₄(L²)₂(EtOH)₂] (4), [Dy₂(tta)₄(L²)₂(EtOH)₂]·2(EtOH) (5) and [Dy₂(dbm)₄(L²)₂(EtOH)₂] (6) (tfo = trifluoromethanesulfonic acid, dbm = dibenzoylmethane, tta = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione) with eight-coordinate geometry. Strikingly, the dynamic magnetic measurements revealed that complexes 1–3 did not display the expected significant SMM performance albeit they had high local symmetry. In combination with ab initio calculation, the alignment of the coordination symmetric axis and the magnetic easy axis dominates the molecular magnetic anisotropy, and the magnetic easy axis could be modulated by the distribution of coordination atoms with different electrostatic properties.