Hexanuclear Co4 Dy2 , Zn4 Dy2 , and Co4 Y2 Complexes with Defect Tetracubane Cores: Syntheses, Structures, and Magnetic Properties.

A hexanuclear heterometallic cluster of composition [Dy2 Co4 (L)4 (NO3 )2 (OH)4 (C2 H5 OH)2 ] ⋅ 2 C2 H5 OH (1) was synthesized by employing a Schiff base 2-(((2-hydroxy-3-methoxybenzyl) imino)methyl)-4-methoxyphenol (H2 L) as ligand and utilizing Dy(NO3 )3 ⋅ 6H2 O and Co(NO3 )2 ⋅ 6H2 O as metal ion sources. X-ray single-crystal diffraction analysis indicated that complex 1 contains a defect tetracubane core and possesses central symmetric structure, with two DyIII ions being in the central body position of the molecule and four CoII ions being arranged at the outer sites. Magnetic studies reveal that complex 1 behaves as single-molecule magnet (SMM) with energy barrier of 27.50 K. To investigate the individual contribution of DyIII and CoII ions to the SMM behavior, another two complexes of formulae [Dy2 Zn4 (L)4 (NO3 )2 (OH)4 ] ⋅ 4CH3 OH (2) and [Y2 Co4 (L)4 (NO3 )2 (OH)4 (C2 H5 OH)2 ] ⋅ 2 C2 H5 OH (3) were prepared. Complexes 1 and 3 are isomorphous. The coordination geometries of DyIII ions in 1 and 2 are different. The DyIII ions are eight-coordinated in 2 and nine-coordinated in 1. Complex 2 exhibits SMM behavior with energy barrier of 69.67 K, but complex 3 does not display SMM property. These results reveal that the SMM behaviors of 1 and 2 are mainly originated from DyIII ions. It might be the higher symmetry of DyIII ions in 2 that results in the higher energy barrier.

Evolution from a single relaxation process to two-step relaxation processes of Dy2 single-molecule magnets via the modulations of the terminal solvent ligands.

To explore the influences of magnetic interactions on the relaxation dynamics of single-molecule magnets (SMMs) and to understand the relationship between single-ion relaxation and the relaxation of a molecular entity, it is very important to design dinuclear lanthanide-based SMMs with two-step relaxation processes. Here, three Dy2 complexes of compositions [Dy2(L)2(NO3)2(MeOH)2] (1), [Dy2(L)2(NO3)2(EtOH)2] (2), and [Dy2(L)2(NO3)2(DMF)2]·0.5EtOH (3) (H2L = 2-(((2-hydroxy-3-methoxybenzyl)imino)methyl)-4-methoxyphenol) were successfully synthesized via elaborately introducing different terminal solvent ligands. X-ray single-crystal diffraction analyses revealed that complexes 1-3 are isostructural. The two DyIII ions of 1 and 2 both adopt D2d symmetry, while the two DyIII centres of 3 display D2d and D4d symmetries, respectively. The magnetic property studies of 1-3 indicated that all three complexes exhibit single-molecule magnet (SMM) behaviours with energy barriers of 104 K for 1, 98.94 K for 2, and 76.28 K and 45.54 K for 3 under zero dc field. The target of assembling Dy2 SMMs with two-step relaxation processes was achieved by gradually increasing the sizes of the terminal solvent ligands. Complex 3 exhibits two-step relaxation processes. Complete active space self-consistent field (CASSCF) calculations were performed on 1-3 to rationalize the observed differences in the magnetic behaviour. It is found that both the angles θ between the magnetic axis and the vector connecting two DyIII ions and the symmetries of DyIII ions are vital factors that affect the energy barriers of 1-3. The high local symmetries of the central metals in 1 and 2 make the complexes act as SMMs with higher energy barriers, while the smaller θ angle and different symmetries of the two DyIII ions render complex 3 as a SMM with a two-step relaxation process. This work demonstrates a new methodology for preparing SMMs with two-step relaxation processes by fine-tuning the terminal solvent ligands.