Field-induced slow magnetic relaxation in mixed valence di- and tri-nuclear CoII-CoIII complexes

Two novel mixed valence Co-II-Co-III complexes, namely [(CoCoIII)-Co-II(L1)(ab)(mb)(2)(H2O)]center dot dmf (1) and [(Co2CoII)-Co-III(L2)(4)(H2O)(4)]center dot 2H(2)O (2) (H(2)L1 = (E)-2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol, ab = 2-amino-butan-1-ol anion, mb = p-methyl benzoate, H(2)L2 = 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 (CoCoII)-Co-III compound with distorted octahedral cobalt centers showing different coordination environments. In 2, a bent trinuclear (Co2CoII)-Co-III complex, the coordination environments around the two terminal Co-III sites are alike, whereas they are different in the central Co-II 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 Hamiltonian (H) over cap (zfs+Zeeman) = D[((S) over cap (2)(z) - 1/3S(S+1)] + E((<)(S)over cap>(2)(x) - (S) over cap (2)(y)) + beta H[g(parallel to)(S) over cap (z) + g(perpendicular to)((S) over cap (x) + (S) over cap (y))] where 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(perpendicular to) = 2.585, g(parallel to) = 2.437, D = +98.1 cm(-1), E/D = 0.008 and F = 8.2x 10(-5) for 1 and g(perpendicular to) = 2.580, g(parallel to) = 2.580, D = +55.4 cm(-1) and E/D = 0.000 for 2.