Contrasting single-molecule magnet behaviour in dysprosium and terbium bis(stannolediide) complexes

The development of single-molecule magnets for high-density data storage has advanced from poly(metallic) cages to lanthanide complexes. Since 2017, highly axial dysprosium bis(cyclopentadienide) complexes have demonstrated exceptional energy barriers to magnetic reversal (U$_{eff}$) and high hysteresis temperatures (T$_H$), with further enhancement achieved in 2025 using bulky amide ligands. Recently, dianionic heavy group 14 cyclopentadienides have emerged as promising ligands due to their higher charge density. Here we report the synthesis of two isostructural homoleptic bis(stannolediide) complexes [Ln(η$^5$-L$^{Sn}$)$_2$K(thf)$_4$] (Ln = Tb(III) or Dy(III)). The Dy(III) complex exhibits single-molecule magnet behaviour with a U$_{eff}$ of 1,502(4) K and a blocking temperature of 55 K, whereas the Tb(III) analogue shows Raman-dominated relaxation <~6 K. Removal of the potassium cation yields [Tb(η$^5$-L$^{Sn})$^2$]$^−$ or a divalent complex [Dy(η$^5$-L$^{Sn}$)$_2$]$^{2−}$. The Dy(II) complex displays weak magnetic anisotropy. These results highlight bis(stannolediide) ligands as a promising new class for high-barrier lanthanide single-molecule magnets.