Oxygen-Donor Metalloligands Induce Slow Magnetization Relaxation in Zero Field for a Cobalt(II) Complex with {CoO} 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 ||, while complexes with hard O-donor ligands showing slow magnetization relaxation are rare. Here, we report that a diamagnetic Ni complex of a tetradentate ligand featuring two -heterocyclic carbene and two alkoxide-O donors, [LNi], can serve as a {O,O'}-chelating metalloligand to give a trinuclear complex (LNi)Co(LNi) () with an elongated tetrahedral {CoO} core, = -74.3 cm, and a spin reversal barrier = 86.9 cm in the absence of an external dc field. The influence of diamagnetic Ni on the electronic structure of the {CoO} unit in comparison to [Co(OPh)] () has been probed with multireference ab initio calculations. These reveal a contrapolarizing effect of the Ni, which forms stronger metal-alkoxide bonds than the central Co, inducing a change in ligand field splitting and a 5-fold increase in the magnetic anisotropy in compared to , 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.