Mixed-valence phosphato–hydrogenphosphato–iron network compounds¹_∞([C₄N₂H_11.6]_1.5[Fe^IIFe^III(PO₄)(H_0.8PO₄)₂]·H₂O) and³_∞[Fe^II₅Fe^III₂(PO₄)₂(H_0.5PO₄)₄]: Structure elucidation with the help of Mössbauer spectroscopy and a caveat on X-ray diffraction

The mixed-valence iron phosphates¹_∞([C₄N₂H_11.6]_1.5[Fe^IIFe^III(PO₄)(H_0.8PO₄)₂]·H₂O) (1) and³_∞[Fe^II₅Fe^III₂(PO₄)₂(H_0.5PO₄)₄] (2) have been synthesized by hydrothermal methods. Their crystal structures were determined by single-crystal X-ray diffraction and Mössbauer spectroscopy. Mössbauer spectroscopy suggests the Fe centers in compound 1 to be mostly in a trapped, mixed-valence +2 and +3 oxidation state from which the average protic hydrogen occupation on HPO₄ and piperazinedium is calculated to be 0.8. At 4 K there is an intervalence tunneling process between part of the Fe²⁺ and Fe³⁺ atoms. Compound 1 contains linear strands of corner-sharing (FeO₄) and (PO₄) tetrahedra. The Fe atoms are bridged by Fe–O–P–O–Fe and Fe–O–Fe linkages. The strands are held together by hydrogen bonding interactions involving the piperazinedium and the water molecules of crystallization as well as complementary H-bonds between the HPO₄-groups. The iron phosphate 2 is found from Mössbauer spectroscopy to be a trapped mixed-valence system with about 30% Fe³⁺/70% Fe²⁺ which translates perfectly into Fe^II₅Fe^III₂ from which a total of two protic hydrogens on phosphate has been calculated. The crystal quality permitted the protic hydrogens in 1 and 2 to be found and their positions freely refined. At 4.2 K the Fe³⁺ is completely and Fe²⁺ partially magnetically ordered in 2. Compound 2 is a three-dimensional framework constructed from edge- and corner-sharing (FeO₆) octahedra and (FeO₅) trigonal bipyramids together with the (PO₄) tetrahedra. Temperature-variable magnetic measurements confirm the oxidation state assignments for 1 and 2 through a matching experimental and calculated value for μ_eff at 300 K.