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 T_cr1 = 290(3) K and T_cr2 = 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.