Enhancing blocking temperatures in {Cr III2Dy III2} butterfly SMMs: deciphering the role of exchange interactions and developing magneto-structural maps†

A tetranuclear heterometallic butterfly-shaped complex of molecular formula; [Cr^III₂Dy^III₂(OMe)₂(mdea)₂(benz)₄(NO₃)₂] (mdeaH₂ = N-methyldiethanolamine, benz = benzoate) was reported by us and found to display single-molecule magnetism (SMM) behaviour with a U_eff = 54 cm⁻¹ and well-resolved magnetisation hysteresis plots, with a blocking temperature, T_B = 3.5 K. In the present study, fourteen related {Cr^III₂Dy^III₂} complexes have been synthesised, and their magnetic properties have been studied. The study probes how chemical changes in bridging ligands and coordination environment affect the SMM properties compared to the parent complex. We show that by making simple chemical modifications to the ligands, the U_eff and T_B values change, with U_eff values ranging from 22 to 65 cm⁻¹ and T_B 1.8 K to 4.7 K. We also show that SMM behaviour can be turned off by manipulation of the coordination sphere around the Dy^III ion. Based on these experimental results, we undertook a detailed theoretical study to understand why these changes occur. The theoretical studies further describe the mode of exchange mechanism in such complexes and how it controls the blocking temperature. Further, we have found that the exchange coupling between the Dy^III–Cr^III ions strongly influences the magnetic relaxation for this family of complexes, and interestingly, the exchange between the Cr^III–Cr^III centres plays a determining role in the overall SMM properties, which has been ignored in previous studies. We have derived a relation between the magnetic exchange and T_B for such 3d–4f SMMs where exchange coupled states play a dominant role, revealing the blocking temperature is strongly correlated to the overall magnetic exchange. This clearly illustrates the importance of inducing strong exchange coupling to enhance not only the magnetisation reversal barrier but also T_Bvia quenching of zero-field quantum tunnelling of the magnetisation (QTM) in {3d–4f} type complexes.