Sandwich‐Type Single‐Molecule Magnets Complexes of Er(<scp>III</scp>) with <i>Ansa</i>‐Cyclooctatetraenyl Ligands^†

Comprehensive Summary Many sandwich‐type lanthanide complexes show extremely high energy barriers ( U eff ) for the reversal of magnetization and high blocking temperatures, being the star molecules in the research area of single‐molecule magnets. Herein, the preparation, structural determination, and magnetic property studies of two ansa ‐bridged Er‐COT (COT = cyclooctatetraenyl dianion) complexes [KDME 2 ][Er((η 8 ‐COT‐Si Me2 ) 2 O)] ( 1 ) and [KDME 2 ][Er((η 8 ‐COT‐Si i Pr2 ) 2 O)]K[Er((η 8 ‐COT‐Si i Pr2 ) 2 O)] ( 2 ) were reported. The Er(III) ions in both complexes are sandwiched by two COT rings which are ansa ‐bridged by a [Si‐O‐Si] group. Magnetic studies reveal both complexes display slow magnetic relaxations with comparable energy barriers (228(5) K for 1 , and 196(4) K for 2 ) and blocking temperatures (10.5 K for both complexes). The difference in the relaxation times ( τ ) for the two complexes was studied in details: different molecular vibrations induced by the substituents are the main reason for τ for 1 being about 10 times longer than for 2 at the same temperature above 10 K, while the quantum tunnelling of magnetization relaxation time ( τ QTM ) for 2 is about 10 times longer than for 1 below 8 K, probably owing to the different dipolar interactions. Further rearrangement of molecular network with such ansa ‐bridged SMMs is promising to design molecular magnetic materials with enhanced properties or new properties.