Observation of the single-ion magnet behavior of d(8) ions on two-coordinate Co(i)–NHC complexes

The slow magnetic relaxation typical for single-ion magnets has been known for certain low-coordinate 3d metal complexes with d(6), d(7), and d(9) electronic configurations, but never for d(8) complexes. Herein, we report a study on two-coordinate d(8) cobalt(i)–N-heterocyclic carbene complexes, for which slow magnetic relaxation behavior was observed for [Co(IMes)(2)][BPh(4)] (IMes: 1,3-dimesitylimidazol-2-ylidene) under an applied dc field. The system represents the first d(8) single-ion magnet, and features a fitted energy barrier of U(eff) = 21.3 cm(–1) and pre-exponential factor of τ(0) = 6.6 × 10(–6) s. The analog two-coordinate cobalt(i) complexes with different NHC ligands, [Co(sIMes)(2)][BPh(4)] (sIMes: 1,3-dimesitylimidazolin-2-ylidene) and [Co(IAd)(2)][BAr(F)(4)] (IAd: 1,3-dimesitylimidazol-2-ylidene; BAr(F)(4): tetra(3,5-ditrifluoromethylphenyl)borate), do not show such single-ion magnet behaviour. Ab initio calculations imply that the dihedral angle between the two NHC planes and the degree of unsaturation of the NHC ligands can dramatically alter the D value of the two-coordinate cobalt(i)–NHC ions, possibly via changing of the Co–NHC π-interactions, and hence affect the spin–orbit coupling splitting.