Crystal Structure and Low-Temperature Structural Phase Transition of the Iron(III) Oxopropionate Nitrate [Fe3O(H2O)3Prop6](NO3)·(HNO3)

A compound [Fe3O(H2O)3Prop6](NO3)·(HNO3) (1; Prop– is the C2H5COO– propionate) is prepared by the interaction of metallic iron and Fe(NO3)3 with concentrated propionic acid; the crystal structure of this compound is determined by single-crystal XRD at 100 K, 200 K, and 300 K. The experimental data show that the structure of 1 is a packing of complex cations [Fe3O(H2O)3Prop6]+ and nitrate anions with nitric acid molecules in packing voids. It is shown for the first time by polythermal XRD that 1 undergoes a structural phase transition manifested as partial ordering of aliphatic substituents in anionic ligands, changes in the configuration of NO2–OH⋯O–NO2 hydrogen bonds, and lowering of crystallographic symmetry of [Fe3O(H2O)3Prop6]+ complex cations. Thermal expansion of the crystal structure of 1 is studied in the temperature range 100-300 K; the experimental data show that the unit cell parameter b changes nonmonotonically upon heating and exhibits pronounced regions of positive and negative thermal expansion.