Field-induced slow magnetic relaxation in mixed valence di- and tri-nuclear Co-Co complexes.

Two novel mixed valence Co-Co complexes, namely [CoCo(L1)(ab)(mb)(HO)]·dmf (1) and [CoCo(L2)(HO)]·2HO (2) [HL1 = (E)-2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol, ab = 2-amino-butan-1-ol anion, mb = p-methyl benzoate, HL2 = 3-((2-hydroxy-3-methoxy-benzylidene)-amino)-propionic acid, and dmf = N,N-dimethyl-formamide], were synthesized and characterized by single crystal X-ray diffraction and magnetic studies at low temperature. The structure determination reveals that both complexes belong to the monoclinic system with P21/c (1) and I2/a (2) space groups. Complex 1 is a dinuclear CoCo compound with distorted octahedral cobalt centers showing different coordination environments. In 2, a bent trinuclear CoCo complex, the coordination environments around the two terminal Co sites are alike, whereas they are different in the central Co ion. Alternating current/direct current (ac/dc) magnetic studies revealed that both complexes show field-induced slow magnetic relaxation. The dc magnetic susceptibility and magnetization data were analyzed with the following Hamiltonianwhere D and E are the axial and rhombic zero-field splitting (zfs) parameters, respectively, and a good agreement between experimental and simulated results was found using the parameters g = 2.585, g = 2.437, D = +98.1 cm, E/D = 0.008 and F = 8.2× 10 for 1 and g = 2.580, g = 2.580, D = +55.4 cm, and E/D = 0.000 for 2.