Stepwise spin transition in a mononuclear iron(II) complex with unusually wide plateau

The reaction of the Schiff-base like ligand H2L [L2- = (E,E)-({diethyl 2,2'-[1,2-phenylenebis(iminomethylidyne)]bis(3-oxobutanato)})] with iron(II) acetate in methanol in the presence of 4-cyanopyridine leads to the formation of an octahedral iron(II) complex with the formula FeL(CNpy)(2)center dot 0.25CNpy (1). The magnetic properties of this complex were investigated using temperature-dependent susceptibility measurements (SQUID) and Mossbauer spectroscopy. Both measurements indicate that the complex performs a stepwise spin transition with a large plateau in the region from 225 K to 125 K and gamma(HS) approximate to 0.25. The molecule structure of the compound was determined at room temperature, 240 K (gamma(HS) approximate to 0.5 according to susceptibility measurements) and 100 K. Two complex molecules were found in the asymmetric unit with strong pi-pi interactions between the phenylene rings of H2L. In agreement with outcomes from the magnetic measurements, both iron centres are in the high-spin state at room temperature and in the low-spin state at 100 K. At 240 K one of the two iron centres is high-spin and the other one low-spin. The reason for the wide step with one fourth of the iron centres in the high-spin state is the additional distorted 4-cyanopyridine in the crystal that influences the transition temperature of the surrounding iron centres. The spin transition was followed in solution by using H-1 NMR spectroscopy. A gradual spin transition with T-1/2 = 238 K is obtained. Temperature-dependent diffuse reflectivity spectra show the occurrence of photoconversion at the surface of the sample but only about 6% of the complex could be switched in bulk condition using a SQUID magnetometer and the stability of the photoinduced HS state with T-LIESST = 40 K is very weak. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).