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

Four tetranuclear 3d-4f complexes, namely FeLn(L)(teaH)(Cl)·4CHCN (HL = N1,N3-bis(3-methoxysalicylidene)diethylenetriamine, teaH = triethanolamine, Ln = Dy for 1 and Ln = Gd for 1') and CoLn(L)(pdm)(CHCOO)(CHOH)·xCHOH·yHO (pdmH = 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 and two Dy in 1 formed a zigzag Fe1-Dy1-Dy1a-Fe1a arrangement with a Fe1-Dy1-Dy1a angle of 105.328(3)°. However, in contrast to 1, two Co and two Dy ions in 2 formed a more linear Co1-Dy1-Dy1a-Co1a arrangement with a Co1-Dy1-Dy1a angle of 141.86(2)°. Additionally, two Dy ions in 1 are eight-coordinated with a triangular dodecahedron geometry, while two Dy ions in 2 adopt nine-coordination with a muffin geometry. Magnetic studies revealed slow magnetic relaxation behavior for 1, with an energy barrier E of 6.9 K. For 2, single molecule magnet behavior was presented under a zero dc field with an effective energy barrier U 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 (Δ) for the exchanged coupled ground state, which may result in poor single molecule magnet behavior for 1.