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

Abstract

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. 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.

Pentacoordinate cobalt(ii) single ion magnets with pendant alkyl chains: shall we go for chloride or bromide?

“…As a matter of fact, the authors use almost exclusively the ZFS spin Hamiltonian (SH, vide infra ) to analyze the experimental magnetization functions. 20,29,30,32–41 The main problem is, however, that its parameters have a straightforward physical meaning only if the ground state is orbitally non-degenerate and well separated from the closest excited states, 42 and none of these assumptions is fulfilled in the mentioned class of systems (see Computational study). Another option is the Griffith–Figgis Hamiltonian (GFH, vide infra ), which was originally designed for octahedral systems possessing (or approaching) an orbitally triple-degenerate ground state.…”

Copper(ii) and zinc(ii) complexes of mono- and bis-1,2,3-triazole-substituted heterocyclic ligands

“…In the CSD database search of transition metal complexes with bis-1,2,3-triazole ligands, seventeen ML and ten ML2 complexes are found. All reported ML2 complexes [23][24][25] are trans-fac isomers and have weakly coordinating anions, while the majority of ML complexes are found with strongly coordinating anions, such as thiocyanate, 26,27 halogenides 27,28 or azides, 29,30 where the anion occupies the remaining coordination sites of the metal.…”

Exchange couplings and quantum phases in two dissimilar arrays of similar copper dinuclear units

“…This approach has been previously applied to copper compounds and other exchange-coupled electron spin systems with reasonable agreement between theoretical and experimental parameters. 16,[58][59][60] The inter-DU exchange was calculated by substituting Cu with Zn inside a molecule and adding a symmetry-related neighboring molecule with a Cu/Zn substituted site as well. Isosurfaces of molecular orbitals and spin density were prepared using the software Chemcraft.…”