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 = (1 H -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 [Co 4 L 6 ] 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 τ 0 = 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.