ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Room Temperature Centred Spin Crossover with Broad Hysteresis in Hexacoordinated [Fe(bzimpy)₂KCl0₄)₂ System

R. Boča, P Baran, and M. Mazur

Department of Inorganic Chemistry, Faculty of Chemical Technology, Slovak Technical University, SK-812 31 Bratislava

Abstract: The powder sample and single crystal of the [Fe(bzimpy)₂](Cl0₄)₂ • xН₂О system (x = 0.25) were prepared, bzimpy = 2,6-bis(benzimidazol-2-yl)pyridine is a tridentate N-donor ligand. The X-ray structure determination proved that the central Fe(II) ion is hexacoordinated by six donor nitrogen atoms. The magnetic properties of the complex were investigated by powder magnetic susceptibility measurements (Faraday balances) and electron spin resonance. The freshly prepared sample does not show any traces of iron(III) impurities but those are formed as a function of time. After one year the sample contains 8.2 mole % of iron (III) as proved by UV spectroscopy and evidenced by q_eff = 4.3, 2.22, and 1.91 in ESR spectra. This explains the recorded x vs. Г behaviour at low temperature: with increasing temperature the x value decreases according to the Curie—Weiss law for S = 5/2 system having an effective g = 4.3. Above 220 К a continuous increase in the x value is observed and a spin crossover applies (S = 2). The spin transition is not complete at room temperature. A very pronounced hysteresis is observed upon heating/cooling the sample between 220 and 414 K.

Chemical Papers 51 (6A) 339–343 (1997)