Magnetic and spectroscopic characterization of the high-spin (6A1) .dblharw. low-spin (2T2) transition in an iron(III) complex of pyridoxal thiosemicarbazone

The preparation and characterization of the new discontinuous ferric spin-crossover complex (Fe(HL)/sub 2/)Cl and the hydrated low-spin form of the complex (Fe(HL)/sub 2/)Cl/center dot/2H/sub 2/O are reported. In these complexes HL/sup /minus// is the deprotonated form of pyridoxal thiosemicarbazone (H/sub 2/L) acting as an ONS tridentate ligand. The magnetic moments for the spin-crossover complex show a thermal hysteresis, where with sample cooling the transition temperature is 245 K and with sample heating it is 256 K. Variable-temperature /sup 57/Fe Moessbauer and EPR data also provide evidence for the presence of a first-order phase transition in this complex. It is most likely that the cooperative (first-order) nature of this transition is due to the extended coupling of ferric complexes through intermolecular hydrogen-bonding interactions. EPR spectra of the two complexes in this study show that both have a single occupied d/sub xy/ orbital in the ground Kramers doublet. 39 refs., 3 figs., 2 tabs..