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 N-n-butyldiethanolamine and N-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 Co2IIIDy2III and Co2IIICoIIDyIII using N-n-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 DyIII ion. The second family consists of a Co2IIIDy2III and a Cr2IIIDy2III complex, from the ligand combination of N-methyldiethanolamine and hexafluoroacetylacetone. Both show SMM behavior, the CoIII example displaying an anisotropy barrier of 23 cm-1. The CrIII 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 CoIII case. This is a consequence of strong DyIII-CrIII magnetic interactions, with the relaxation arising from the electronic structure of the whole complex and not from a single DyIII 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 (?-diketonate) ligands enhance the anisotropy barrier.