Oxygen-Donor Metalloligands Induce Slow Magnetization Relaxation in Zero Field for a Cobalt(II) Complex with {CoO₄} 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, [(LNi)-Ni-O,O], can serve as a {O,O '}-chelating metalloligand to give a trinuclear complex [((LNi)-Ni-O,O)Co((LNi)-Ni-O,O)](OTf)(2) (2) with an elongated tetrahedral {(CoO4)-O-II} 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 {CoO4} 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.