Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets.

By simple reactions involving various cobalt(II) carboxylates (acetate and in situ prepared pivalate and 4-hydroxybenzoate salts) and neocuproine (neo), we were able to prepare three different carboxylate complexes with the general formula [Co(neo)(RCOO) 2 ] (R = -CH 3 for 1 , (CH 3 ) 3 C- for 2 , and 4OH-C 4 H 6 - for 3 ). The [Co(neo)(RCOO) 2 ] molecules in the crystal structures of 1 - 3 adopt a rather distorted coordination environment, with the largest trigonal distortion observed for 1 , whereas 2 and 3 are similarly distorted from ideal octahedral geometry. The combined theoretical and experimental investigations of magnetic properties revealed that the spin Hamiltonian formalism was not a valid approach and the L-S Hamiltonian had to be used to reveal very large magnetic anisotropies for 1 - 3 . The measurements of AC susceptibility showed that all three compounds exhibited slow-relaxation of magnetization in a weak external static magnetic field, and thus can be classified as field-induced single-ion magnets. It is noteworthy that 1 also exhibits a weak AC signal in a zero-external magnetic field.