Insight into D6h Symmetry: Targeting Strong Axiality in Stable Dysprosium(III) Hexagonal Bipyramidal Single-Ion Magnets

Three dysprosium(III) single-molecule magnets (SMMs) with the rare hexagonal bipyramidal geometry have been isolated for the first time. Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph₃SiO^- (Ph₃SiO^- = anion of triphenylsilanol) and the 2,4-di-^tBu-PhO^- (2,4-di-^tBu-PhO^- = anion of 2,4-di-tertbutylphenol) ligands combined with the weak equatorial field of the ligand L^N6, leads to [Dy^III(L^N6)(2,4-di-^tBu-PhO)₂](PF₆) (1), [Dy^III(L^N6)(Ph₃SiO)₂](PF₆) (2) and [Dy^III(L^N6)(Ph₃SiO)₂](BPh₄) (3) hexagonal bipyramidal complexes with high anisotropy barriers of U_eff = 973 K for 1, U_eff = 1080 K for 2 and U_eff = 1124 K for 3 under zero applied dc field. Ab initio calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher-order symmetry SMMs.