Complete spin crossover in tris(pyridylbenzimidazole) iron(II)

Abstract

The magnetic properties of a hexacoordinated iron(II) complex with the bidentate ligand pyridylbenzimidazole (pybzim), [Fe(pybzim) 3](ClO 4) 2 · H 2O, ( 1), have been investigated by AC susceptometry, 57Fe-Mössbauer, ESR and NMR spectroscopy. The temperature dependence of the AC magnetic susceptibility for a powder sample showed essentially diamagnetic behaviour below 85 K and paramagnetism ( μ eff/ μ B = 4.5−5.3, S = 2) above 160 K, obeying the Curie-Weiss law. Spin crossover behaviour can be found within the temperature range 90–160 K; it is centered at 141 K. The Mössbauer spectra confirm the susceptibility measurements: the area fraction of the high-spin quadrupole doublet plotted against temperature shows a curve similar to that deduced from susceptibility measurements. The complex is ESR silent in the X-band mode: neither the powder sample (77 K and 300 K) nor a methanolic solution exhibit an ESR signal. NMR measurements in methanol, acetone, nitromethane, acetonitrile and dimethylsulfoxide show an increasing magnetic moment μ eff/μ B from 1.6 (220 K, acetone) to 5.2 (325 K, DMSO), respectively. The spin-crossover transition of a freshly prepared powder sample is complete. However, time evolution causes (within months) an increasing admixture of iron(III), which was proved by magnetic susceptibility measurement, 57Fe-Mössbauer, ESR and electronic spectra of the complex.