Slow magnetic relaxation in cobalt N-heterocyclic carbene complexes†
The combined experimental and theoretical investigation of the magnetic properties of the cobalt(II) NHC complexes (NHC = N-heterocyclic carbene); [Co(CH2SiMe3)2(IPr)] (1), [CoCl2(IMes)2] (2) and [Co(CH3)2(IMes)2] (3) revealed a large easy plane anisotropy for 1 (D = +73.7 cm−1) and a moderate easy axis anisotropy for 2 (D = −7.7 cm−1) due to significant out-of-state spin–orbit coupling. Dynamic magnetic measurements revealed slow relaxation of the magnetization for 1 (Ueff = 22.5 K, τ0 = 3 × 10−7 s, 1000 Oe) and for 2 (Ueff = 20.2 K, τ0 = 1.73 × 10−8 s, 1500 Oe). The molecular origin of the slow relaxation phenomena was further supported by the retention of AC signal in 10% solutions in 2-MeTHF which reveals a second zero field AC signal in 1 at higher frequencies. Compound 3 was found to be an S = 1/2 system.