Heterometallic 3d–4f Single-Molecule Magnets: Ligand and Metal Ion Influences on the Magnetic Relaxation

Six tetranuclear 3d–4f single-molecule magnet (SMM) complexes formed using <i>N</i>-<i>n</i>-butyldiethanol­amine and <i>N</i>-methyldiethanol­amine in conjunction with ortho- and para-substituted benzoic acid and hexafluoro­acetoacetone ligands yield two families, both having a butterfly metallic core. The first consists of four complexes of type {Co₂^IIIDy₂^III} and {Co₂^IIICo^IIDy^III} using <i>N</i>-<i>n</i>-butyldiethanolamine with variation of the carboxylate ligand. The anisotropy barriers are 80 cm^–1, (77 and 96 cm^–1two relaxation processes occur), 117 and 88 cm^–1, respectively, each following a relaxation mechanism from a single Dy^III ion. The second family consists of a {Co₂^IIIDy₂^III} and a {Cr₂^IIIDy₂^III} complex, from the ligand combination of <i>N</i>-methyldiethanol­amine and hexafluoro­acetylacetone. Both show SMM behavior, the Co^III example displaying an anisotropy barrier of 23 cm^–1. The Cr^III complex displays a barrier of 28 cm^–1, with longer relaxation times and open hysteresis loops, the latter of which is not seen in the Co^III case. This is a consequence of strong Dy^III–Cr^III magnetic interactions, with the relaxation arising from the electronic structure of the whole complex and not from a single Dy^III ion. The results suggest that the presence of strong exchange interactions lead to significantly longer relaxation times than in isostructural complexes where the exchange is weak. The study also suggests that electron-withdrawing groups on both bridging (carboxylate) and terminal (β-diketonate) ligands enhance the anisotropy barrier