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(CH 2 SiMe 3 ) 2 (IPr)] ( 1 ), [CoCl 2 (IMes) 2 ] ( 2 ) and [Co(CH 3 ) 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 ( U eff = 22.5 K, τ 0 = 3 × 10 −7 s, 1000 Oe) and for 2 ( U eff = 20.2 K, τ 0 = 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. The combined experimental and theoretical investigation of the magnetic properties of [Co(CH 2 SiMe 3 ) 2 (IPr)] ( 1 ), [CoCl 2 (IMes) 2 ] ( 2 ) revealed a large easy plane anisotropy for 1 and a moderate easy axis anisotropy for 2 .