Ligand Effect on the Single-Molecule Magnetism of Tetranuclear Co(II) Cubane

A clear dependence on the ligand has been observed for the magnetic properties of a closely related series of Co(II) cubane structures, viz. [Co₄(mbm or bm)₄(ROH)₄Br₄] (1-MeOH, 1-EtOH, 2-MeOH, and 2-EtOH, where 1 = [Co₄(mbm)₄Br₄], 2 = [Co₄(bm)₄Br₄], bm = (1H-benzo[d]imidazol-2-yl)methanolate. and mbm = 1-Me-bm.) The [Co₄(OR)₄] cubane core consists of an octahedral Co^II center chelated by the alkoxide oxygen and imidazole nitrogen atoms from monoanionic bm or mbm and coordinated by methanol/alcohol and bromine. Interestingly, electrospray ionization mass spectrometry (ESI-MS) indicates that 1-MeOH and 2-MeOH are unstable in methanol and transformed to the butterfly [Co₄L₆]²⁺ but that 1-EtOH and 2-EtOH are stable in ethanol. Their magnetic susceptibilities suggest ferromagnetic coupling between the nearest cobalt centers to give a theoretical S = 4 × 3/2 ground state with considerable magneto-crystalline behavior. The packing and intermolecular interactions appear to influence the geometry of the cubes and thus the anisotropy of cobalt, which leads to different blocking temperatures (T_B). Consequently, the compounds with mbm, 1-MeOH and 1-EtOH, exhibit T_B > 2 K as shown by the relaxation of magnetization in zero applied dc field where the barriers U_eff/k_B are respectively 27 and 21 K and relaxation times are τ₀ = 1.3 × 10^-9 and 9.7 × 10^-9 s. However, the compounds with bm, 2-MeOH and 2-EtOH, remain paramagnetic above 2 K and do not show nonlinear response of the ac susceptibilities. These findings reaffirm the subtle dependence of single-molecule magnetism on coordination geometry and intermolecular interaction.