Complete two-step spin-transition in a 1D chain iron(II) complex with a 110-K wide intermediate plateau

摘要

Three new spin crossover compounds [FeL1(azpy)] (1), [FeL1(azpy)] ·toluene [1(tol)] and [FeL2(azpy)] (2), where L1 and L2 are the tetradentate N_2O_2~(2-) 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 (γ_(HS) ≈ 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. M?ssbauer 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 M?ssbauer 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 cross-linked arrangement with an interlocking of the layers is most likely the reason for the appearance of the wide plateau.