CRYSTAL-STRUCTURE AND SPIN-CROSSOVER STUDIES ON BIS(2,6-BIS(BENZIMIDAZOL-2-YL) PYRIDINE) IRON(II) PERCHLORATE

The structure of the [Fe(bzimpy)(2)](ClO4)(2) . xH(2)O system (x=0.25) was determined by single crystal X-ray structure analysis. The Fe(II) ion is hexacoordinated by six donor nitrogen atoms. The magnetic prop erties of the complex were investigated by powder magnetic susceptibil ity measurements and ESR. The freshly prepared sample does not show an y traces of iron(III) impurities but these are formed as a function of time. After 1 year the sample contains 8.2% iron(III) as shown by UV spectroscopy and indicated by g(eff)=4.3 and 2.0 in its ESR spectrum, This explains the recorded chi versus T behaviour at low temperature: with increasing temperature the chi value decreases according to the C urie-Weiss law for a S=5/2 system having an effective g=4.3. Above 220 K a continuous increase in the chi value is observed and a spin cross over applies. The spin transition is not complete at room temperature. A pronounced hysteresis is observed upon heating/cooling the sample b etween 220 and 414 K on the basis of magnetic data and infrared spectr a.