Tuning Magnetic Anisotropy Through Ligand Substitution in Five-Coordinate Co(II) Complexes

Understanding the
origin of magnetic anisotropy and having the ability to tune it are
essential needs of the rapidly developing field of molecular magnetism.
Such attempts at determining the origin of magnetic anisotropy and
its tuning are still relatively infrequent. One candidate for such
attempts are mononuclear Co­(II) complexes, some of which have recently
been shown to possess slow relaxation of their magnetization. In this
contribution we present four different five-coordinated Co­(II) complexes, 1–4, that contain two different “click”
derived tetradentate tripodal ligands and either Cl– or NCS– as an additional, axial ligand. The geometric
structures of all four complexes are very similar. Despite this, major
differences are observed in their electronic structures and hence
in their magnetic properties as well. A combination of temperature
dependent susceptibility measurements and high-frequency and -field
EPR (HFEPR) spectroscopy was used to accurately determine the magnetic
properties of these complexes, expressed through the spin Hamiltonian
parameters: g-values and zero-field splitting (ZFS)
parameters D and E. A combination
of optical d-d absorption spectra together with ligand field theory
was used to determine the B and Dq values of the complexes. Additionally, state of the art quantum
chemical calculations were applied to obtain bonding parameters and
to determine the origin of magnetic anisotropy in 1–4. This combined approach showed that the D values in these complexes are in the range from −9 to +9
cm–1. Correlations have been drawn between the bonding
nature of the ligands and the magnitude and sign of D. These results will thus have consequences for generating novel
Co­(II) complexes with tunable magnetic anisotropy and hence contribute
to the field of molecular magnetism.