Quantum Mechanical Insights into Magneto–Structural Correlations Governing the Magnetic Relaxation of Non-Dysprosium Butterfly-Shaped {CrIII2LnIII2} (Ln = Pr, Nd, Tb, Ho, Er) Complexes

Abstract The DyIII Kramers ion is generally found in the best-performing lanthanide-based single-molecule magnets (SMMs). Due to large QTM rates, slow magnetic relaxation in non-Kramers ions like TbIII and HoIII is rare. Moreover, LnIII ions coupled with 3d ions often show enhanced SMM behavior due to significant 3d–4f magnetic exchange, but this is mostly true for DyIII analogues. Despite this, we synthesized several non-DyIII-based 3d–4f tetranuclear butterfly-shaped complexes that showed zero-field SMM behavior with energy barriers (Ueff) of 36–44 cm–1 and relatively long relaxation times for the TbIII and HoIII examples. In this work, ab initio and DFT calculations on eight butterfly-type {CrIII2LnIII2} complexes were performed to understand the SMM characteristics of TbIII and HoIII analogues, and structural derivatives containing other LnIII ions (Pr, Nd, Gd, Er) that did not display zero-field SMM behavior. The magnetic relaxation mechanism of individual LnIII ions, the CrIII···LnIII and LnIII···LnIII exchange (J) and the correlation between LoProp charges of bridging and capping ligands, and Js were analyzed to understand SMM characteristics. Furthermore, the dependency of Ueff and TB on the CrIII···LnIIIJs has been correlated, which suggests that the moderate CrIII···LnIII antiferromagnetic exchange with support from ferromagnetic LnIII···LnIII exchange leads to the best SMM characteristics.