Lanthanide coordination complexes framed by sodium ions: slow relaxation of the magnetization in the Dy(III) derivative

Using a new polydentate Schiff-base ligand (H3L) we isolated three new tetranuclear isostructural lanthanide complexes with the general formula [Ln(4)(III)Na(2)(L)(4)(DMF)(4)(H2O)(2)(AcO)(2)]center dot 4DMF center dot 2H(2)O[Ln = Dy (1), Ho (2), or Er (3)]. The structural characterization of the complexes reveals that the Na+ ions are coordinated in the structure which gives them a structure-directing role in the molecule. The magnetic behavior of the systems was investigated by means of SQUID magnetometry which revealed that complex 1 exhibits single molecule magnet behavior at low temperatures which is enhanced by the application of a 2000 Oe static magnetic field. We were able to extract an effective barrier of U-eff = 43(1) K, however, we show that the consideration of an Orbach relaxation mechanism being the dominant is not always correct for lanthanides. On the contrary, we elaborate how in this system the relaxation is caused by a combination of a direct and a Raman process.