Observation of the single-ion magnet behavior of d⁸ ions on two-coordinate Co(<scp>i</scp>)–NHC complexes

Abstract

The slow magnetic relaxation typical for single-ion magnets has been known for certain low-coordinate 3d metal complexes with d⁶, d⁷, and d⁹ electronic configurations, but never for d⁸ complexes. Herein, we report a study on two-coordinate d⁸ cobalt(i)-N-heterocyclic carbene complexes, for which slow magnetic relaxation behavior was observed for [Co(IMes)₂][BPh₄] (IMes: 1,3-dimesitylimidazol-2-ylidene) under an applied dc field. The system represents the first d⁸ single-ion magnet, and features a fitted energy barrier of <i>U</i>_eff = 21.3 cm^-1 and pre-exponential factor of <i>τ</i>₀ = 6.6 × 10^-6 s. The analog two-coordinate cobalt(i) complexes with different NHC ligands, [Co(sIMes)₂][BPh₄] (sIMes: 1,3-dimesitylimidazolin-2-ylidene) and [Co(IAd)₂][BAr^F₄] (IAd: 1,3-dimesitylimidazol-2-ylidene; BAr^F₄: tetra(3,5-ditrifluoromethylphenyl)borate), do not show such single-ion magnet behaviour. <i>Ab initio</i> calculations imply that the dihedral angle between the two NHC planes and the degree of unsaturation of the NHC ligands can dramatically alter the <i>D</i> value of the two-coordinate cobalt(i)-NHC ions, possibly <i>via</i> changing of the Co-NHC π-interactions, and hence affect the spin-orbit coupling splitting.