Oxygen-Donor Metalloligands Induce Slow Magnetization Relaxation in Zero Field for a Cobalt(II) Complex with {CoO 4 } Motif.

Most 3d metal-based single-molecule magnets (SMMs) use N-ligands or ligands with even softer donors to impart a particular coordination geometry and increase the zero-field splitting parameter | D |, while complexes with hard O-donor ligands showing slow magnetization relaxation are rare. Here, we report that a diamagnetic Ni II complex of a tetradentate ligand featuring two N -heterocyclic carbene and two alkoxide-O donors, [L O,O Ni], can serve as a {O,O'}-chelating metalloligand to give a trinuclear complex [(L O,O Ni)Co(L O,O Ni)](OTf) 2 ( 2 ) with an elongated tetrahedral {Co II O 4 } core, D = -74.3 cm -1 , and a spin reversal barrier U eff = 86.9 cm -1 in the absence of an external dc field. The influence of diamagnetic Ni II on the electronic structure of the {CoO 4 } unit in comparison to [Co(OPh) 4 ] 2- ( A ) has been probed with multireference ab initio calculations. These reveal a contrapolarizing effect of the Ni II , which forms stronger metal-alkoxide bonds than the central Co II , inducing a change in ligand field splitting and a 5-fold increase in the magnetic anisotropy in 2 compared to A , with an easy magnetization axis along the Ni-Co-Ni vector. This demonstrates a strategy to enhance the SMM properties of 3d metal complexes with hard O-donors by modulating the ligand field character via the coordination of diamagnetic ions and the benefit of robust metalloligands in that regard.