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-Moss bauer, ESR and NMR spectroscopy. The temperature dependence of the AC magnetic susceptibility for a powder sample showed essentially diamagn etic 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 behavi our can be found within the temperature range 90-160 K; it is centered at 141 K. The Mossbauer spectra confirm the susceptibility measuremen ts: the area fraction of the high-spin quadrupole doublet plotted agai nst temperature shows a curve similar to that deduced from susceptibil ity measurements. The complex is ESR silent in the X-band mode: neithe r the powder sample (77 K and 300 K) nor a methanolic solution exhibit an ESR signal. NMR measurements in methanol, acetone, nitromethane, a cetonitrile and dimethylsulfoxide show an increasing magnetic moment m u(eff)/mu(B) from 1.6 (220 K, acetone) to 5.2 (125 K, DMSO), respectiv ely. The spin-crossover transition of a freshly prepared powder sample is complete. However, time evolution causes (within months) an increa sing admixture of iron(III), which was proved by magnetic susceptibili ty measurement, Fe-57-Mossbauer, ESR and electronic spectra of the com plex. Copyright (C) 1996 Elsevier Science Ltd