2D ferrous nitroprussides stabilized through organic molecules as pillars: preparation, crystal structure and related properties

Transition metal (T) nitroprussides form a family of coordination complexes with a wide structural diversity including 3 D, 2 D, 1 D, 0 D and ionic phases. Recently, a reversible thermally induced spin transition has been reported for 2 D ferrous nitroprussides stabilized through pyridine and its derivatives as pillars between neighboring layers. The aim of this study is to explore the possibility of obtaining analog solids with spin-crossover behavior using other molecules as pillars. In this contribution we are reporting the crystal structure, solved and refined from powder XRD data, for the resulting hybrid solids containing 1,3-oxazole, 1H-pyrazole, pyridazine, 3-pyridine-3-carboxaldheyde, imidazo[1,2-a]pyridine, and 4-(2-pyridin-4-ylethyl)pyridine as pillar molecules in the interlayer region. The XRD data were complemented with TG curves, and IR, Raman, Mössbauer spectra, and magnetic measurements as a source of complementary structural information. According to the Mössbauer spectra and magnetic data recorded in the temperature range of 5 to 300 K, no evidence of thermally induced spin-crossover was observed for the 2 D ferrous nitroprussides with these six organic ligands as pillars. Such behavior is in contrast with the one observed for pyridine and its derivatives and it is explained in terms of the local symmetry around the high spin Fe²⁺ ion, as revealed in the recorded Mössbauer spectra. This suggests that the spin-crossover behavior could be induced by introducing guest molecules, as adsorbed species, in the interlayer region to force to a higher symmetry for the iron ion coordination environment.