Structural Dependence of the Ising-type Magnetic Anisotropy and of the Relaxation Time in Mononuclear Trigonal Bipyramidal Co(II) Single Molecule Magnets

This
paper describes the correlation between Ising-type magnetic anisotropy
and structure in trigonal bipyramidal Co­(II) complexes. Three sulfur-containing
trigonal bipyramidal Co­(II) complexes were synthesized and characterized.
It was shown that we can engineer the magnitude of the Ising anisotropy
using ligand field theory arguments in conjunction with structural
parameters. To prepare this series of compounds, we used, on the one
hand, a tetradentate ligand containing three sulfur atoms and one
amine (NS3tBu) and on the other hand three different
axial ligands, namely, Cl–, Br–, and NCS–. The organic ligand imposes a trigonal
bipyramidal arrangement with the three sulfur atoms lying in the trigonal
plane with long Co–S bond distances. The magnetic properties
of the compounds were measured, and ab initio calculations were used
to analyze the anisotropy parameters and perform magneto-structural
correlations. We demonstrate that a smaller axial zero-field splitting
parameter leads to slower relaxation time when the symmetry is strictly
axial, while the presence of very weak rhombicity decreases the energy
barrier and speeds the relaxation of the magnetization.