Structure and Magnetic Properties of Layered High-Spin Co(II)(L-threonine)2(H2O)2

Abstract

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 R-amino nitrogen. A water molecule occupies the third independent site.<br />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 <<br />|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 d ~ 231 cm-1 between the two lowest doublet states. The results are discussed in terms of the molecular and electronic structure of the compound.