Anisotropic exchange in a tetranuclear CoII complex

Very high-frequency (50–715 GHz) electron paramagnetic resonance (EPR) studies of the tetranuclear Co II complex [Co(hmp)(dmb)Cl] 4 ( 1 ), where dmb is 3,3-dimethyl-1-butanol and hmp − is the monoanion of 2-hydroxy-methylpyridine, reveal the presence of significant zero-field-splitting (ZFS) within the ground state spin multiplet. Meanwhile, low-temperature hysteresis measurements of 1 (and related Co II 4 complexes) provide evidence for slow magnetization relaxation, suggesting that it could be a single-molecule magnet (SMM). However, EPR studies of a Zn analog of 1 , doped with a small quantity of Co II , show the ground state of the Co II ions to be an effective spin S ′ = 1/2 Kramers doublet with a highly anisotropic g -tensor. The question then arises as to the origin of the ZFS within the ground state spin multiplet of 1 , as well as the slow magnetization relaxation. Here, we consider the effect of anisotropic exchange interactions between the effective spin S ′ = 1/2 Kramers ions within the tetranuclear complex. Such exchange anisotropy arises naturally when one treats the ground state of high-spin Co II as a Kramers doublet. Our model provides an explanation for the ZFS in the ground state observed via EPR, and can also account for qualitative features observed through magnetic measurements.