Magnetic and mössbauer characterization of the discontinuous high-spin ( 6A 1)⇄low-spin ( 2T 2) transition in solid bis(pyridoxal 4-phenylthiosemicarbazonato)iron(III) chloride

Abstract

The iron(III) complexes [Fe(HPhthsa) 2]Cl, [Fe(HEtthsa) 2]Cl and [Fe(HMethsa) 2]Cl were synthesized and the phenomenon of discontinuous spin transition between the high-spin ( 6A 1, S=5/2) and the low-spin ( 2T 2, S=1/2) states in [Fe(HPhthsa) 2]Cl was confirmed by the measurement of temperature dependent magnetism and Mössbauer spectra. The tridendate ligands used were pyridoxal 4-phenylthiosemicarbazone (H 2Phthsa), pyridoxal 4-ethylthiosemicarbazone (H 2Etthsa), and pyridoxal 4-methylthiosemicarbazone (H 2Methsa). The magnetic moments for the spin-crossover complex show a thermal hysteresis of width δ T≈9 K with the transition centered at T c ↑≈299 K for increasing temperature and at T c ↓≈290 K for decreasing temperature. Variable temperature 57Fe Mössbauer results also provide evidence for the presence of a first order phase transition in this complex. The first order (cooperative) nature of the spin-crossover transformation most likely occurs through the extended coupling of ferric complexes via intermolecular H bonds. The EPR results indicate that the ground state is a well separated Kramers doublet in the low-spin complexes.