Rational electrostatic design of easy-axis magnetic anisotropy in a ZnII-DyIII-ZnII single-molecule magnet with a high energy barrier

Abstract

Two novel trinuclear complexes [ZnCl(μ-L)Ln(μ-L)ClZn][ZnCl3(CH3OH)]⋅3 CH3OH (LnIII=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 LnIII ions are coordinated by two [ZnCl(L)]− units through the phenoxo and aldehyde groups, giving rise to a LnO8 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 MJ=±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 DyIII ion in 1 is adopted to reduce the electronic repulsion between the oblate electron shape of the MJ=±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 DyIII ion are separated by 129 cm−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 Ueff=140 K. The isostructural Zn–Er–Zn species does not present significant SMM behavior as expected for the prolate electron-density distribution of the ErIII ion leading to an easy-plane anisotropy of the ground doublet state.