Complete Two‐Step Spin‐Transition in a 1D Chain Iron(II) Complex with a 110‐K Wide Intermediate Plateau

Three new spin crossover compounds [FeL1(azpy)] (1), [FeL1(azpy)].toluene [1(tol)] and [FeL2(azpy)] (2), where L1 and L2 are the tetradentate N2O22- coordinating Schiff base like ligands {diethyl (E, E)-2,2'-[1,2-phenyl-bis(iminomethylidyne)]bis[3-oxobutanoate]-(2-)-N,N',O-3,O-3'} and {dimethyl (E, E)-2,2'-[1,2-phenyl-bis(iminomethylidyne)] bis[3-oxobutanoate]-( 2-)-N,N',O-3,O-3'}, respectively, and azpy = 4,4'-azopyridine are presented. All compounds have been investigated by using T-dependent susceptibility measurements, and different types of spin transitions are observed. In the case of 1 and 2, intermediate plateaus (gamma(HS) approximate to 0.5) with widths of 110 and 75 K, respectively, were observed, while for 1(tol) a gradual spin transition is obtained. Upon heating of 1(tol) above 390 K, a loss of the included solvent molecule toluene is observed, which results in a two-step spin transition of the tempered sample similar to that of 1. Mossbauer spectra of 1, 1(tol) and the tempered sample 1(temp) reveal two different iron(II) sites in the low-spin (LS) state for all three samples. T-dependent Mossbauer spectra indicate that the inequivalent iron centres are one but not the only reason for the wide plateau of 1. Results from X-ray structure analysis of 1 in the region of the plateau show the presence of two inequivalent iron centres, of which one is in the high-spin (HS) state and one in the low-spin state. The 1D chains of alternating HS and LS iron centres are arranged in a parallel manner within one layer; between the different layers, the direction of the chains rotates by 90 relative to the previous one. This crosslinked arrangement with an interlocking of the layers is most likely the reason for the appearance of the wide plateau.