Observation of slow magnetic relaxation in triple-stranded lanthanide helicates

Two dinuclear triple-stranded helicates [Ln(2)L(3)](3+) (Ln = Dy and Tb) obtained via self-assembly from the ligand HL (2,6-diformyl-4-methylphenol di(benzoylhydrazone)) and lanthanide perchlorate have been synthesized and characterized. The crystal structural analysis demonstrates that three ligand strands wrap around a pseudo-threefold axis defined by the two metal ions, leading to a 'meso'-relation between the right- (Δ) and left-hand (Λ) configurations of [Ln(2)L(3)](3+) in the crystal. Each Ln(III) ion is coordinated by nine donor atoms in a distorted tricapped trigonal-prismatic arrangement. Alternating current (ac) susceptibility measurements of [Dy(2)L(3)](3+) reveal a frequency-dependent out-of-phase signal under a 700 Oe dc field, indicating the onset of the slow relaxation of magnetization with a roughly estimated activation energy of ∼5 K and τ(0) of 10(-7) s.