Two C-3-Symmetric Dy-3(III) Complexes with Triple Di-mu-methoxo-mu-phenoxo Bridges, Magnetic Ground State, and Single-Molecule Magnetic Behavior.
Two series of isostructural C-3-symmetric Ln(3) complexes Ln(3)center dot[BPh4] and Ln(3)center dot 0.33[Ln(NO3)(6)] (in which Ln(III)=Gd and Dy) have been prepared from an amino-bis(phenol) ligand. X-ray studies reveal that LnIII ions are connected by one mu(2)-phenoxo and two mu(3)-methoxo bridges, thus leading to a hexagonal bipyramidal Ln(3)O(5) bridging core in which Ln(III) ions exhibit a biaugmented trigonal-prismatic geometry. Magnetic susceptibility studies and ab initio complete active space self-consistent field (CASSCF) calculations indicate that the magnetic coupling between the Dy-III ions, which possess a high axial anisotropy in the ground state, is very weakly 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 Dy-3 plane, thus leading to a paramagnetic ground state. CASSCF plus restricted active space state interaction (RASSI) calculations also show that the ground and first excited state of the Dy-III ions are separated by approximately 150 and 177 cm(-1), for Dy-3 center dot[BPh4] and Dy-3 center dot 0.33[Dy(NO3)(6)], respectively. As expected for these large energy gaps, Dy-3 center dot[BPh4] and Dy-3 center dot 0.33[Dy(NO3)(6)] exhibit, under zero direct-current (dc) field, thermally activated slow relaxation of the magnetization, which overlap with a quantum tunneling relaxation process. Under an applied H-dc field of 1000 Oe, Dy-3 center dot[BPh4] exhibits two thermally activated processes with U-eff values of 34.7 and 19.5 cm(-1), whereas Dy-3 center dot 0.33[Dy(NO3)(6)] shows only one activated process with U-eff=19.5 cm (1).