Magnetic and EPR Study of the Complex Co(II) Fumarato

NICOLÁS NEUMAN; ELÍN WINKLER ; OCTAVIO PEÑA; MARIO PASSEGGI; ALBERTO RIZZI; CARLOS D. BRONDINO

Montevideo

Workshop; International School on Fundamental Crystallography; 2010

Introduction Cobalt(II) ions are found in several metalloproteins and are often used as spectroscopic probes of active sites, replacing metal ions such as Zn(II), because their magnetic properties are very sensitive to coordination geometries and ligand types. High spin Co(II) ions (S = 3/2) in distorted octahedral sites present magnetic anisotropy arising from zero field splitting (ZFS) of its spin energy levels.1-3 Magnetization and Magnetic Susceptibility results can be analyzed by use of a spin Hamiltonian in the S = 3/2 base. In an Electron Paramagnetic Resonance (EPR) experiment, because the ZFS term is usually much larger than the microwave energy, the results can be analyzed by means of an effective spin Hamiltonian in a S´ = 1/2 base.   Objectives To study the magnetic properties of a cobalt(II)-carboxylate coordination compound: Co(II)(Fumarato)(H2O)4, which crystallizes in the monoclinic (P21/c) group, by EPR spectroscopy, Temperature-dependent Magnetic Susceptibility (χ) and Field-dependent Magnetization (M). To obtain the crystal and molecular g-tensors and ZFS parameters D and E. To relate magnetic and EPR results.  Results Single crystal EPR spectra of Co(II)Fumarato were taken at 9.5 GHz and 4 K for multiple orientations of the magnetic field B in an orthogonal abc* coordinate axis (c* = a × b). Least squares fitting of the resonance lines afforded the effective g-factor for each orientation. The crystal g2-tensor was calculated from the angular dependence of g2. Its eigenvalues were g12 = 26.30 g22 = 14.16 g32 = 16.82. Magnetization and Magnetic Susceptibility results were least squares fit to a model based on a Spin Hamiltonian in a S = 3/2 base, including Zeeman, ZFS and molecular field terms. The last term accounts for magnetic exchange between neighboring cobalt(II) ions. The magnitude of the fitting parameters g, D and E were consistent with those expected for a cobalt(II) ion in a distorted octahedral environment. The effective molecular g-tensor parameters (in the S´= 1/2 base) were determined from the crystal g-tensor and were related to the g, D and E parameters obtained from magnetic measurements using an appropriate model.4,5  References Rizzi et al., Inorg. Chem, 2003, 42, 4409-4416 Larrabee et al., J. Am. Chem Soc., 1997, 119 (18), 4182-4196 Kahn, O., Molecular Magnetism, 1993, New York, USA: VCH Publishers. Werth et al., Inorg. Chem., 1995,34, 218-228 Pilbrow, J. R., J. Magn. Reson., 1978, 31, 479-489