Cobalt(ii)–pyrazine–chloride coordination polymers: synthesis, reactivity and magnetic properties

Abstract

The reaction of cobalt(II) chloride hexahydrate and pyrazine in a reduced amount of water under hydrothermal conditions affords [CoCl₂pyz]_n, which crystallises in an orthorhombic unit cell (a = 12.0014, b = 7.1040, c = 3.5380 Å), space group Cmmm. Fourier transform infrared spectroscopy confirms the coordination of pyrazine to Co(II) in an octahedral geometry. This compound is thermally stable in air up to 320 °C and may also be obtained by the single-step decomposition of [CoCl₂(pyz)₂]_n at 190 °C, with an activation energy of 121 kJ mol⁻¹. [CoCl₂pyz]_n contains Co(II) chains bridged by two chloride ions making an angle of 92.21°, leading to orthogonal (d-p) orbitals and to one-dimensional ferromagnetic chains. In contrast, in [CoCl₂(pyz)₂]_n only pyrazine bridges the Co(II) ions, setting them apart and resulting in a purely paramagnetic system. Because the simple contact with a pyrazine vapour or aqueous pyrazine reintroduces this molecule into [CoCl₂pyz]_n, while heating [CoCl₂(pyz)₂]_n removes pyrazine affording [CoCl₂pyz]_n, the magnetic behaviour may be reversibly switched between the two states.