Cobalt(II)-pyrazine-chloride coordination polymers: synthesis, reactivity and magnetic properties

The reaction of cobalt(II) chloride hexahydrate and pyrazine in a reduced amount of water under hydrothermal conditions affords [CoCl(2)pyz](n), which crystallises in an orthorhombic unit cell (a = 12.0014, b = 7.1040, c = 3.5380 angstrom), 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 degrees C and may also be obtained by the single-step decomposition of [CoCl2(pyz)(2)](n) at 190 degrees C, with an activation energy of 121 kJ mol(-1). [CoCl(2)pyz](n) contains Co(II) chains bridged by two chloride ions making an angle of 92.21 degrees, 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 [CoCl(2)pyz](n), while heating [CoCl2(pyz)(2)](n) removes pyrazine affording [CoCl(2)pyz](n), the magnetic behaviour may be reversibly switched between the two states.