A Family of Binuclear Dysprosium(III) Radical Compounds with Magnetic Relaxation in ON and OFF States

Four binuclear dysprosium compounds incorporating the radical ligand 2-(4-oxidopyridyl)-4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide (PyNONIT) have been successfully synthesized under appropriate conditions. Centrosymmetric bimetallic Dy₂O₂ cores in all of the compounds through double-μ₂-oxygen atoms of the N-oxide groups are realized in a metal–radical approach for the first time. Dimers 1 and 2, of the same formula {[Dy­(hfac)₃(PyNONIT)]₂}₂ (hfac = hexafluoroacetylacetonate) but obtained by different methods, which contain almost identical local symmetry of D_4d and Dy–(O)₂–Dy bridging fashion, however, display no out-of-phase alternating-current (ac) signal for 1 and slow relaxation of the magnetization for 2 corresponding to the difference of the crystal packing mode. The adduct ([Dy­(hfac)₃(PyNONIT)]₂[Dy_0.5(hfac)_1.5(H₂O)]₂) (3) consists of two items, the dimer [Dy­(hfac)₃(PyNONIT)]₂ and the monomer [Dy­(hfac)₃(H₂O)₂], where the symmetry of Dy^III ion in Dy₂O₂ decreases to D_2d, showing slow relaxation of the magnetization at lower temperature. Interestingly, a moisture-mediated reversible solid transformation between 1 and ([Dy­(hfac)₃(H₂O)­(PyNONIT)]₂) (4) has been investigated. Spongelike 1 can undergo a transition from eight to nine coordination at room temperature through hydration. A different coordination field is mostly responsible for no ac signal noticed for 4. The structural diversity of the Dy₂ family provides an opportunity to expand the investigation on 4f single-molecule magnets. Approaches that the relaxation of the supramolecular dimer can be tuned to ON and OFF states modulated by the packing mode and ligand field are presented.