Regulation of magnetic relaxation behavior by replacing 3d transition metal ions in [M2Dy2] complexes containing two different organic chelating ligands

Four tetranuclear 3d-4f complexes, namely [Fe(2)Ln(2)(L)(2)(teaH)(2)(Cl)(2)](NO3)(2)4CH(3)CN (H2L = N1,N3-bis(3-methoxysalicylidene)diethylenetriamine, teaH(3) = triethanolamine, Ln = Dy for 1 and Ln = Gd for 1 ') and [Co(2)Ln(2)(L)(2)(pdm)(2)(CH3COO)(2)(CH3OH)(2)](NO3)(2)xCH(3)OHyH(2)O (pdmH(2) = 2,6-pyridinedimethanol, Ln = Dy, x = 5 and y = 2.5 for 2 and Ln = Gd, x = 6 and y = 1.5 for 2 '), have been reported. Two Fe-III and two Dy-III in 1 formed a zigzag Fe1-Dy1-Dy1a-Fe1a arrangement with a Fe1-Dy1-Dy1a angle of 105.328(3)degrees. However, in contrast to 1, two Co-III and two Dy-III ions in 2 formed a more linear Co1-Dy1-Dy1a-Co1a arrangement with a Co1-Dy1-Dy1a angle of 141.86(2)degrees. Additionally, two Dy-III ions in 1 are eight-coordinated with a triangular dodecahedron geometry, while two Dy-III ions in 2 adopt nine-coordination with a muffin geometry. Magnetic studies revealed slow magnetic relaxation behavior for 1, with an energy barrier E-a of 6.9 K. For 2, single molecule magnet behavior was presented under a zero dc field with an effective energy barrier U-eff of 64.0(9) K. Ab initio calculations for 1 and 2 indicate that compared to 2, complex 1 has a larger transversal magnetic moment of its ground Kramers doublets (KD) and a larger value of the tunnelling parameter (Delta(t)) for the exchanged coupled ground state, which may result in poor single molecule magnet behavior for 1.