Single‐molecule Magnet Properties of Silole‐ and Stannole‐ligated Erbium Cyclo‐octatetraenyl Sandwich Complexes

The synthesis, structures and magnetic properties of an η⁵-silole complex and an η⁵-stannole complex of erbium are reported. The sandwich complex anions [(η⁵-Cp^Si)Er(η⁸-COT)]^- and [(η⁵-Cp^Sn)Er(η⁸-COT)]^-, where Cp^Si is [SiC₄-2,5-(SiMe₃)₂-3,4-Ph₂]^2- (1_Si), Cp^Sn is [SnC₄-2,5-(SiMe₃)₂-3,4-Me₂]^2- (1_Sn) and COT=cyclo-octatetraenyl, were obtained as their [K(2.2.2-cryptand)]⁺ salts and found to be isostructural, with remarkably similar bond lengths and angles, differing only in the lengths of the Er-E interactions (E=Si, Sn). The parallels in the molecular structures of 1_Si and 1_Sn are reflected in their dynamic magnetic properties, which show single-molecule magnet behaviour in zero applied field, with effective energy barriers of 115±7 and 125±3 cm^-1, respectively, along with comparable magnetic relaxation times. Analysis of the two complexes using ab initio calculations reveals differences at a quantitative level, but overall similar electronic structures, with the thermally activated relaxation likely to proceed via the first-excited Kramers doublet. Comparing 1_Si and 1_Sn with the previously reported germanium analogue 1_Ge reveals that swapping one heavier group 14 element for another in complexes of the type [(η⁵-Cp^E)Er(η⁸-COT)]^- has a minimal impact on the SMM behaviour.