Side-Group Effect on the Slow Relaxations of {Dy 2 } Single-Molecule Magnets with Confined N 2 O 6 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 {Dy 2 } entities with formula [Dy 2 (CH 3 OH) 2 L 2 ( R COO) 2 ] (H 2 L = 2-hydroxy- N '-((pyridin-2-yl)methylene)benzohydrazide) have been solvothermally prepared by varying the side groups of carboxylate coligands ( R COO - , R = CF 3 for 1 , H for 2 , CH 3 for 3 , and Cp 2 Fe for 4 ). Structural analyses reveal that all of the Dy III carriers in 1-4 have the same N 2 O 6 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 {Dy 2 } 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.