Abstract
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.