Rational Electrostatic Design of Easy-Axis Magnetic Anisotropy in a ZnII-DyIII-ZnII Single-Molecule Magnet with a High Energy Barrier

Two novel trinuclear complexes [ZnCl(-L)Ln(-L)ClZn][ZnCl3(CH3OH)]3CH(3)OH (Ln(III)=Dy (1) and Er (2)) have been prepared from the compartmental ligand N,N-dimethyl-N,N-bis(2-hydroxy-3-formyl-5-bromo-benzyl)ethylenediamine (H2L). X-ray studies reveal that Ln(III) ions are coordinated by two [ZnCl(L)](-) units through the phenoxo and aldehyde groups, giving rise to a LnO(8) coordination sphere with square-antiprism geometry and strong easy-axis anisotropy of the ground state. Ab initio CASSCF+RASSI calculations carried out on 1 confirm that the ground state is an almost pure M-J=+/- 15/2 Kramers doublet with a marked axial anisotropy, the magnetic moment is roughly collinear with the shortest DyO distances. This orientation of the local magnetic moment of the Dy-III ion in 1 is adopted to reduce the electronic repulsion between the oblate electron shape of the M-J=+/- 15/2 Kramers doublet and the phenoxo-oxygen donor atoms involved in the shortest DyO bonds. CASSCF+RASSI calculations also show that the ground and first excited states of the Dy-III ion are separated by 129cm(-1). As expected for this large energy gap, compound 1 exhibits, in a zero direct-current field, thermally activated slow relaxation of the magnetization with a large U-eff=140K. The isostructural Zn-Er-Zn species does not present significant SMM behavior as expected for the prolate electron-density distribution of the Er-III ion leading to an easy-plane anisotropy of the ground doublet state.