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