Sandwich-Type Single-Molecule Magnets Complexes of Er(III) with Ansa-Cyclooctatetraenyl Ligands†

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 [KDME₂][Er((η⁸-COT-Si^Me2)₂O)] (1) and [KDME₂][Er((η⁸-COT-Si^iPr2)₂O)]K[Er((η⁸-COT-Si^iPr2)₂O)] (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.