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)4 Br4 ] (1-MeOH, 1-EtOH, 2-MeOH, and 2-EtOH, where 1 = [Co4 (mbm)4 Br4 ], 2 = [Co4 (bm)4 Br4 ], bm = (1H-benzo[d]imidazol-2-yl)methanolate. and mbm = 1-Me-bm.) The [Co4 (OR)4 ] cubane core consists of an octahedral CoII 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 [Co4 L6 ]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 (TB ). Consequently, the compounds with mbm, 1-MeOH and 1-EtOH, exhibit TB > 2 K as shown by the relaxation of magnetization in zero applied dc field where the barriers Ueff /kB 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.