Closely Related Organometallic Er(III) Single-Molecule Magnets with Sizably Different Relaxation Times of Quantum Tunneling of Magnetization

Single-molecule magnets (SMMs) are of interest for storage and quantum processing of information. Herein, we report three new SMMs, ([K(DME)2][Er(COTTBS2)2] (1), [K(18-C-6)(THF)2][Er(COTTBS2)2] (2), and [K(2,2,2-cryptand)][Er-(COTTBS2)2] (3)), each of which features an Er(III) ion sandwiched by two bis(mono-tert-butyldimethylsilyl) substituted cyclooctatetraenide (1,4-(tBuMe2Si)2C8H6 or COTTBS2) ligands, leading to uniaxial magnetic anisotropy. With the chemically identical complex anion of [Er(COTTBS2)2]-, these SMMs differ only in the specific complex form of the same counterion K+, namely, [K(DME)2]+ (DME = ethylene glycol dimethyl ether) in 1, [K(18-C-6)(THF)2]+ in 2, and [K(2,2,2-cryptand)]+ in 3. Magnetic studies reveal comparable blocking temperatures of around 10 K and similar energy barriers (193(10) K for 1, 171(10) K for 2, and 166(9) K for 3) for magnetization reversal. By using the vibrating sample magnetometry waveform method, the transformed AC frequency is down to 0.0004 Hz; this frequency is 4 orders of magnitude smaller than the usual instrumentation limit. Accurate measurement of the quantum tunnelling of magnetization (QTM) relaxation time (tau QTM) made possible at such a low frequency allows for confident assessment and comparison between closely related SMMs. For 1, 2, and 3, the tau QTM was found to be 17(1) s, 13(1) s, and 65(2) s, respectively. The multifold difference in tau QTM between 3 and 1 or 2 indicates that magnetic relaxation by QTM can be profoundly influenced by the nonmagnetic component and crystal environment of an SMM, and specifically the form of the counterion in the present case. Our findings thus suggest a new and different approach to tuning the QTM behavior and the overall characteristics of SMMs.