X-ray diffraction study of phase transitions in iron(II) trisnioximate hexadecylboronate clathrochelate complex

Crystals of the iron(II) nioximate hexadecylboronate clathrochelate complex—FeNx3(BHd)2 [tris(μ-1,2-cyclohexanedionedioximato-O:O′)di-n-hexadecyldiborato(2−)-N,N′, N″,N‴, N⁗,N″‴]iron(II)—are investigated by differential scanning calorimetry and X-ray diffraction. Two structural phase transitions are revealed at Tcr1 = 290(3) K and Tcr2 = 190(3) K. The crystal structures of phases I, II, and III are determined by X-ray diffraction analysis at 303, 243, and 153 K, respectively. It is demonstrated that the I ↔ II phase transition is due to a change in the system of translations, and the II ↔ III phase transition is accompanied only by a jumpwise change in the unit cell parameters. The possible mechanisms of phase transitions are discussed in terms of geometry and molecular packing of FeNx3(BHd)2 in all three phases.