Triply halogen-bridged erbium compounds with hard single-molecule magnet behavior

Single-molecule magnets (SMMs) show promise for high-density data storage due to molecular-level magnetic hysteresis, but low-temperature quantum tunneling of magnetization (QTM) limits their blocking temperatures (T_B), making QTM suppression critical. Herein, we report two dinuclear Er-cyclooctatetraenyl compounds: [K(18-C-6)(THF)][(COT^1,4TMS2)Er(μ-Cl)₃Er(COT^1,4TMS2)] (1) and [K(18-C-6)(THF)₂][(COT^1,3TMS2)Er(μ-Br)₃Er(COT^1,3TMS2)] (2) (COT^1,4TMS2 and COT^1,3TMS2 donate 1,4- and 1,3-bis(trimethylsilyl)-substituted cyclooctatetraenyl ligands, respectively; 18-C-6 = 18-Crown-6 ether). In both compounds, the two Er(III) ions are triply bridged by Cl^- (1) or Br^- (2). Ab initio calculations confirm strong axial anisotropy and ferromagnetic axial dipolar interactions from "head-to-tail" magnetic easy axes. Such axial dipolar interactions lead to the QTM being effectively suppressed by minimizing transverse dipolar fields, yielding hard magnetic behavior with open magnetic hysteresis loops up to 10 K and coercive fields of 6.25 kOe (1) and 4.75 kOe (2) at 2 K. These results demonstrate tunable halide-bridged dinuclear architectures for hard-magnetic SMMs via a non-radical approach.