Quantum Mechanical Insights into Magneto–Structural Correlations Governing the Magnetic Relaxation of Non-Dysprosium Butterfly-Shaped {Cr^III₂Ln^III₂} (Ln = Pr, Nd, Tb, Ho, Er) Complexes

Due to large QTM rates, slow magnetic relaxation in non-Kramers ions like Tb^III and Ho^III is rare. Moreover, Ln^III ions coupled with 3d ions often show enhanced SMM behavior due to significant 3d–4f magnetic exchange, but this is mostly true for Dy^III analogues.

Despite this, we synthesized several non-Dy^III-based 3d–4f tetranuclear butterfly-shaped complexes that showed zero-field SMM behavior with energy barriers (<i>U</i>_eff) of 36–44 cm^–1 and relatively long relaxation times for the Tb^III and Ho^III examples. In this work, <i>ab initio</i> and DFT calculations on eight butterfly-type {Cr^III₂Ln^III₂} complexes were performed to understand the SMM characteristics of Tb^III and Ho^III analogues, and structural derivatives containing other Ln^III ions (Pr, Nd, Gd, Er) that did not display zero-field SMM behavior.