Relaxation Dynamics in See-Saw Shaped Dy(III) Single-Molecule Magnets

Utilizing a terphenyl bisanilide ligand, two Dy(III) compounds [K(DME)n][LArDy(X)2] (LAr = {C6H4[(2,6-iPrC6H3)NC6H4]2}2−), X = Cl (1) and X = I (2) were synthesized. The ligand imposes an unusual see-saw shaped molecular geometry leading to a coordinatively unsaturated metal complex with near-linear N–Dy–N (avg. 159.9° for 1 and avg. 160.4° for 2) angles. These compounds exhibit single-molecule magnet (SMM) behavior with significant uniaxial magnetic anisotropy as a result of the transverse coordination of the bisanilide ligand which yields high energy barriers to magnetic spin reversal of Ueff = 1334 K/927 cm−1 (1) and 1278 K/888 cm−1 (2) in zero field. Ab initio calculations reveal that the dominant crystal field of the bisanilide ligand controls the orientation of the main magnetic axis which runs nearly parallel to the N–Dy–N bonds, despite the identity of the halide ligand. Analysis of the relaxation dynamics reveals a ca. 14-fold decrease in the rate of quantum tunneling of the magnetisation when X = I (2). Most notably, the relaxation times were on average 5.6× longer at zero field when the heavier group 17 congener was employed. However, no direct evidence of a heavy atom effect on the Orbach relaxation was obtained as the height of the barrier is defined by the dominant bisanilide ligand.