MAGNETIC AND MOSSBAUER CHARACTERIZATION OF THE DISCONTINUOUS HIGH-SPIN (6A1)REVERSIBLE LOW-SPIN (2T2) TRANSITION IN SOLID BIS(PYRIDOXAL 4-PHENYLTHIOSEMICARBAZONATO)IRON(III) CHLORIDE

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 (6A1, S=5/2) and the low-spin (2T2, S = 1/2) states in [Fe(HPhthsa)2]Cl was confirmed by the measurement of temperature dependent magnetism and Mossbauer spectra. The tridendate ligands used were pyridoxal 4-phenylthiosemicarbazone H2Phthsa), pyridoxal 4-ethylthiosemicarbazone (H2Etthsa), and pyridoxal 4-methylthiosemicarbazone (H2Methsa). The magnetic moments for the spin-crossover complex show a thermal hysteresis of width DELTA-T almost-equal-to 9 K with the transition centered at T(c) up almost-equal-to 299 K for increasing temperature and at T(c) down almost-equal-to 290 K for decreasing temperature. Variable temperature Fe-57 Mossbauer 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.