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Spectroscopic and magnetic properties of pyridine and pyrazine complexes of divalent iron and copper Haynes, John Stephen


Magneto-structural correlations have been made for a number of pyridine and pyrazine complexes of iron(II) and copper(Il), involving anions of a range of coordinating abilities, for example, sulfonate, RS0₃⁻ (where R is CF₃, CH₃ or p-CH₃C₆H₄); halide, Cl⁻ Br⁻ or I⁻; pseudohalide, NCO⁻ or NCS⁻; perchlorate and hexafluoroarsenate. Structure was determined by infrared, electronic and Mössbauer spectroscopy and differential scanning calorimetry, and, in some instances, by single-crystal X-ray diffraction. Spectroscopic results were used to investigate the nature of both anion and neutral ligand coordination. In complexes of stoichiometry ML₄ (RS0₃)₂ (where M is Fe or Cu, L is pyridine, pyrazine or 2-methylpyrazine and R is CF₃, CH₃ or p-CH₃C₆H₄), the neutral ligands were found to adopt a unidentate mode of coordination. For several of these complexes, X-ray crystallography revealed a square-planar array of pyridine ligands around the central metal, with anions coordinated in a unidentate mode above and below this plane. A monomeric molecular structure results in which the paramagnetic centres are well isolated from each other giving rise to magnetically-dilute species. In complexes of stoichiometry M(pyz)₂X₂ (where M is Fe or Cu and X⁻ is CF₃S0₃⁻, CH₃S0₃⁻, Cl⁻, Br⁻, I⁻, C10₄⁻ or NCS⁻), pyrazine was found to coordinate through both nitrogen donor atoms and inorganic coordination polymers were produced. X-ray crystallography revealed a two-dimensional lattice in Cu(pyz)₂(CH₃S0₃)₂ with two distinct kinds of bridging pyrazine groups and monodentate sulfonate anions. For the remaining bis(pyrazine) complexes, spectroscopic evidence supports similar structures with unidentate anion coordination and bidentate bridging pyrazine ligands leading to sheet-like polymers. Cu(pyz)₂(CH₃S0₃)₂ and Fe(pyz)₂(NCS)₂ exhibit magnetic susceptibilities which reveal the antiferromagnetic nature of these materials (ˣmax at temperatures of 7.0 and 8.0 K respectively); the data were analysed in terms of a two-dimensional Heisenberg model. For the copper complex, in which the structure shows stronger pyrazine coordination along one dimension, the data were also analysed in terms of a linear chain model. Mössbauer spectroscopy showed Fe(pyz)₂(NCS)₂ to undergo a transition to a magnetically-ordered state at 9.2 K. The magnitude of the exchange coupling through bridging pyrazine in Fe(pyz)₂X₂ complexes (where X⁻ is CF₃S0₃⁻, CH₃S0₃⁻, Cl⁻, Br⁻, I⁻ or C10₄⁻) is considerably less than that present in either Cu(pyz)₂(CH₃SO₃)₂ or Fe(pyz)₂(NCS)₂. Spectroscopic evidence indicates that for Fe(py)₂(CF₃S0₃)₂ and complexes of stoichiometry M(pyz)X₂ (where M is Fe or Cu and X⁻ is CF₃S0₃⁻, p-CH₃C₆H₄S0₃⁻, Cl⁻ or NCO⁻) bridging anionic ligands are present and for the mono(pyrazine) complexes the neutral ligand also coordinates in a bridging mode. Fe(pyz)(CF₃S0₃)₂, Fe(pyz)(NCO)₂ and Cu(pyz)(CF₃SO₃)₂ all exhibit magnetic susceptibility data characteristic of antiferromagnetic materials (ˣmax at temperatures of 4.4, 38 and 7.0 K respectively). The magnetic susceptibilities for these materials were analysed in terms of the two-dimensional Heisenberg model and a linear chain model. Mössbauer spectroscopy shows both Fe(pyz)(CF₃S0₃)₂ and Fe(pyz)(NCO)₂ to undergo a transition to long-range magnetic ordering at temperatures of 3.9 and 27.0 K respectively. Low-temperature (4.2-130 K) magnetic susceptibility measurements for the iron(II) sulfonate compounds, Fe(RS0₃)₂ (where R is F, CF₃, CH₃ or p-CH₃C₆H₄) are reported. For the compounds where R is F, CF₃ or p-CH₃C₆H₄ the magnetic moment data were assessed in terms of crystal-field splitting effects. The magnetic moment data for ɑ and β forms of Fe(CH₃S0₃)₂ are indicative of antiferromagnetic exchange interactions and the characteristics of the susceptibility curve for the β isomer are explained on the basis of a transition from short-range to long range three-dimensional magnetic ordering at 22 K.

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