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

The magnetic properties of a hexacoordinated iron(II) complex with the bidentate ligand pyridylbenzimidazole (pybzim), [Fe(pybzim)(3)](ClO4)(2) . H2O, (1), have been investigated by AC susceptometry, Fe-57-Mossbauer, 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 (mu(eff)/mu(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 Mossbauer 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 mu(eff)/mu(B) from 1.6 (220 K, acetone) to 5.2 (125 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, Fe-57-Mossbauer, ESR and electronic spectra of the complex. Copyright (C) 1996 Elsevier Science Ltd