Side-Group Effect on the Slow Relaxations of {Dy₂} Single-Molecule Magnets with Confined N₂O₆ Donors

Deep insights into and substantial enhancement of the effective anisotropy energy barrier for magnetization reversal (U_eff) are vitally important for the technological applications of dysprosium(III)-based single-molecule magnets (Dy-SMMs). To fully refine the ligand-field effect on spin relaxation, four centrosymmetric {Dy₂} entities with formula [Dy₂(CH₃OH)₂L₂(RCOO)₂] (H₂L = 2-hydroxy-N'-((pyridin-2-yl)methylene)benzohydrazide) have been solvothermally prepared by varying the side groups of carboxylate coligands (RCOO^-, R = CF₃ for 1, H for 2, CH₃ for 3, and Cp₂Fe for 4). Structural analyses reveal that all of the Dy^III carriers in 1-4 have the same N₂O₆ donor environments, and the non-coordinative R groups attached to the equatorial carboxylate bridges have not substantially changed the binding ability of the shortest Dy-O_phenolate bonds located at the axial position of the ligand field. Interestingly, the side groups have monotonically decreased the zero-field U_eff barriers of these weak antiferromagnetically coupled {Dy₂} analogues from 721 K down to 379 K. Further electronic structure calculations demonstrate that the main magnetic axes of 1-4 are highly dominated by these comparable Dy-O_phenolate short bonds, and the g tensors have produced gradually increased transverse components responsible significantly for the decreased U_eff barriers. Additionally, thermally assisted relaxations occur preferably through the second (for 1) and the first (for 2-4) Kramer doublets. These interesting findings afford a new side-group effect to comprehensively understand the magnetostructural relationships and advance the rational design of high-performance Dy-SMMs.