Two C 3 symmetric Dy 3 III Complexes with Triple Di-- Methoxo-- Phenoxo Bridges , Magnetic Ground State and SMM Behaviour

Two series of isostructural C3 symmetric Ln3 complexes Ln3·[BPh4] and Ln3·0.33[Ln(NO3)6] (where Ln = Gd and Dy) have been prepared from an amino-bis(phenol) ligand. X-ray studies reveal that Ln ions are connected by one 2-phenoxo and two 3-methoxo bridges, leading to a hexagonal bipyramidal Ln3O5 bridging core, where Ln ions exhibit a biaugmented trigonal prismatic geometry. Magnetic susceptibility studies and ab initio CASSCF calculations indicate that the magnetic coupling between the Dy ions, which possess a high axial anisotropy in the ground state, is very weak antiferromagnetic and mainly dipolar in nature. To reduce the electronic repulsion from the coordinating oxygen atom with the shortest Dy-O distance, the local magnetic moments are oriented almost perpendicular to the Dy3 plane leading to a paramagnetic ground state. CASSCF+RASSI calculations also show that the ground and first excited state of the Dy ions are separated by ~150 cm and ~177 cm, for Dy3·[BPh4] and Dy3·0.33[Dy(NO3)6], respectively. As expected for these large energy gaps, Dy3·[BPh4] and Dy3·0.33[Dy(NO3)6] exhibit, under zero dc field, thermally activated slow relaxation of the magnetization, which overlap with a quantum tunneling relaxation process. Under an applied Hdc field of 1000 Oe, Dy3·[BPh4] exhibits two thermally activated processes with Ueff values of 34.7 cm and 19.5cm, whereas Dy3·0.33[Dy(NO3)6] shows only one activated process with Ueff = 19.5 cm .