Synthesis, crystal structure and properties of chloridotetrakis(pyridine-3-carbonitrile)thiocyanatoiron(II)

Abstract

Reaction of FeCl2·4H2O with KSCN and 3-cyanopyridine (pyridine-3-carbonitrile) in ethanol accidentally leads to the formation of single crystals of Fe(NCS)(Cl)(3-cyanopyridine)4 or [FeCl(NCS)(C6H4N2)4]. The asymmetric unit of this compound consists of one FeII cation, one chloride and one thiocyanate anion that are located on a fourfold rotation axis as well as of one 3-cyanopyridine coligand in a general position. The FeII cations are sixfold coordinated by one chloride anion and one terminally N-bonding thiocyanate anion in trans-positions and four 3-cyanopyridine coligands that coordinate via the pyridine N atom to the FeII cations. The complexes are arranged in columns with the chloride anions, with the thiocyanate anions always oriented in the same direction, which shows the non-centrosymmetry of this structure. No pronounced intermolecular interactions are observed between the complexes. Initially, FeCl2 and KSCN were reacted in a 1:2 ratio, which lead to a sample that contains the title compound as the major phase together with a small amount of an unknown crystalline phase, as proven by powder X-ray diffraction (PXRD). If FeCl2 and KSCN is reacted in a 1:1 ratio, the title compound is obtained as a nearly pure phase. IR investigations reveal that the CN stretching vibration for the thiocyanate anion is observed at 2074 cm−1, and that of the cyano group at 2238 cm−1, which also proves that the anionic ligands are only terminally bonded and that the cyano group is not involved in the metal coordination. Measurements with thermogravimetry and differential thermoanalysis reveal that the title compound decomposes at 169°C when heated at a rate of 4°C min−1 and that the 3-cyanopyridine ligands are emitted in two separate poorly resolved steps. After the first step, an intermediate compound with the composition Fe(NCS)(Cl)(3-cyanopyridine)2 of unknown structure is formed, for which the CN stretching vibration of the thiocyanate anion is observed at 2025 cm−1, whereas the CN stretching vibration of the cyano group remain constant. This strongly indicates that the FeII cations are linked by μ-1,3-bridging thiocyanate anions into chains or layers.