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. [Co-4(mbm or bm)4(ROH)(4)Br-4] (1-MeOH, 1-EtOH, 2-MeOH, and 2-EtOH, where 1 = [Co-4(mbm)(4)Br-4], 2 = [Co-4(bm)(4)Br-4], bm = (1H-benzo[d]imidazol-2-yl)methanolate. and mbm = 1-Me-bm.) The [Co-4(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 x 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 tau(0) = 1.3 x 10(-9) and 9.7 x 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.