American Chemical Society
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Structure and Magnetic Properties of Layered High-Spin Co(II)(l-threonine)2(H2O)2

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journal contribution
posted on 2003-05-28, 00:00 authored by Alberto C. Rizzi, Carlos D. Brondino, Rafael Calvo, Ricardo Baggio, María T. Garland, Raul E. Rapp
We report the structure and the magnetic properties of a cobalt(II) compound with the amino acid l-threonine, Co(C4H8NO3)2(H2O)2. It crystallizes in the orthorhombic chiral space group C2221, with a = 5.843(5) Å, b = 10.120(10) Å, c = 22.36(3) Å, and Z = 4. The Co(II) ion is in a deformed octahedral environment on a 2-fold symmetry axis parallel to the crystallographic axis b. It is bonded to two threonine molecules in a bidentate fashion, via one oxygen from the carboxylate end and the α-amino nitrogen. A water molecule occupies the third independent site. The Co(II) ions are arranged in layers with intralayer and interlayer distances of 5.84 and 11.18 Å, respectively. Magnetic measurements data reflect the molecular character of a compound with weak exchange interactions. EPR measurements in polycrystalline and single-crystal samples indicate a distorted axial symmetry around the Co(II) ion, as expected from the structural results. Eigenvalues and eigenvectors of the g tensor are determined. The measured principal g values (5.81, 4.56, and 2.23) reflect a high-spin Co(II) ion, as suggested by the type of ligands and the molecular symmetry. From the incomplete collapse of the hyperfine structure we estimate 0.25 < |J| < 1.2 cm-1 between neighboring Co(II) ions within a layer, transmitted through H-bonds. A higher limit |J‘| < 0.07 cm-1 is estimated for the exchange interactions between Co(II) ions in neighboring layers. From a global fit of a spin Hamiltonian with spin 3/2 to magnetization and EPR data we obtain a zero field splitting δ ∼ 231 cm-1 between the two lowest doublet states. The results are discussed in terms of the molecular and electronic structure of the compound.