Tuning Transverse Anisotropy in CoIII-CoII-CoIII Mixed-Valence Complex toward Slow Magnetic Relaxation

Two cobalt mixed-valence complexes with different substituents have been prepared and structurally characterized by single-crystal X-ray diffraction to alter slow magnetic relaxation by tailoring the transverse anisotropy. The trinuclear complexes [(L-1)(4)Co-3(H2O)(2)](NO3)(4)center dot CH3OH center dot 5H(2)O (1-NO3) and [(L-2)(4)Co-3(H2O)(2)](NO3)(4)center dot 6H(2)O (2-NO3) feature a distorted octahedral Co(II) strongly hindered in a trinuclear Co-III-Co-II-Co-III mixed-valence array. Detailed magnetic studies of 1-NO3 and 2-NO3 have been conducted using direct- and alternating-current magnetic susceptibility data. In accordance with variable-field magnetic susceptibility data at low temperatures, high-field electron paramagnetic resonance (HF-EPR) spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in complexes 1-NO3 and 2-NO3. These findings indicate that the onset of the variation of distortion within complex 2-NO3 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Consequently, the anisotropy energy scale associated with the relaxation barrier, 5.46 cm(-1) (tau(o) = 1.03 X 10(-5) s), is determined by the transverse E term. The results demonstrate that slow magnetic relaxation can be switched through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy.