Air-stable Four-Coordinate Cobalt(II) Single-Ion Magnets

For single ion magnets (SIMs), understanding the effects of the local coordination environment and ligand field on magnetic anisotropy is key to controlling their magnetic properties. Here we present a series of tetracoordinate cobalt(II) complexes of the general formula [FL2Co]X2 (where FL is a bidentate diamido ligand) whose electron-withdrawing -C6F5 substituents confer stability under ambient conditions. Depending on the cations X, these complexes adopt structures with greatly varying dihedral twist angle  between the N-Co-N' chelate planes in the solid state (48.0 to 89.2°). AC and DC field magnetic susceptibility measurements show this to translate into very different magnetic properties, the axial zero-field splitting (ZFS) parameter D ranging from -69 cm-1 to -143 cm-1 with substantial or negligible rhombic component E, respectively. A close to the orthogonal arrangement of the two {N,N'}-chelating - and -donor ligands at the Co(II) ion is found to raise the energy barrier for magnetic relaxation to above 400 K. Multireference ab-initio methods were employed to describe the complexes’ electronic structures, and the results were analyzed within the framework of ab-initio ligand field theory to probe the nature of the metal-ligand bonding and spin-orbit coupling. A relationship between the energy gaps of the first few electronic transitions and the ZFS was established, and the ZFS was correlated with the dihedral angle  as well as with the metal-ligand bonding variations, viz. the two angular overlap parameters 𝑒𝜎 and 𝑒𝜋𝑠. These findings not only give rise to a Co(II) SIM showing open hysteresis up to 3.5 K at a sweep rate of 30 Oe/s, but they also provide design guidelines for Co(II) complexes with favorable SIM signatures or even switchable magnetic relaxation properties.