Slow magnetic relaxation in cobalt N-heterocyclic carbene complexes

Abstract

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 (U_eff = 22.5 K, τ₀ = 3 × 10⁻⁷ s, 1000 Oe) and for 2 (U_eff = 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.