Oxygen-Donor Metalloligands Induce Slow Magnetization Relaxation in Zero Field for a Cobalt(II) Complex with {CoO<sub>4</sub>} Motif
Giuseppe Lococciolo, Sandeep K. Gupta, Sebastian Dechert, Serhiy Demeshko, Carole Duboc, Mihail Atanasov, Frank Neese, Franc Meyer
Abstract
High Resolution Image Download MS PowerPoint Slide 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.