Slow Magnetic Relaxation in a Co(III)-Co(II)-Co(III) Mixed-Valence Complex with Negative Anisotropy

An air-stable trinuclear mixed-valence complex [CoIICoIII2L2(py)4]center dot 2H2O (1) bearing a magnetic core [CoIIN4] has been synthesized using 1,3-bis(5-methylpyrazole-3-formylamine) propane (H4L) as a ligand, which was investigated by single-crystal X-ray diffraction, magnetic measurements, and theoretical study. Due to the tetrahedral geometry of the central Co(II) ion, this complex possesses a negative and large zero-field splitting D, exhibiting field-assisted slow relaxation of magnetization with an effective energy barrier of 36.1 cm-1. Ab initio CASSCF calculations confirm the easy-axis magnetic anisotropy of complex 1, agreeing with the experimental results. The foregoing result constitutes the first example of a trinuclear Co(III)-Co(II)-Co(III) single-molecule magnet containing a tetrahedral Co(II) center.