Enhancing blocking temperatures in {CrIII2DyIII2} butterfly SMMs: deciphering the role of exchange interactions and developing magneto-structural maps

A tetranuclear heterometallic butterfly-shaped complex of molecular formula; [CrIII2DyIII2(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 Ueff = 54 cm−1 and well-resolved magnetisation hysteresis plots, with a blocking temperature, TB = 3.5 K. In the present study, fourteen related {CrIII2DyIII2} 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 Ueff and TB values change, with Ueff values ranging from 22 to 65 cm−1 and TB 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 DyIII 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 DyIII–CrIII ions strongly influences the magnetic relaxation for this family of complexes, and interestingly, the exchange between the CrIII–CrIII 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 TB 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 TBvia quenching of zero-field quantum tunnelling of the magnetisation (QTM) in {3d–4f} type complexes.