Mixed-valence phosphato-hydrogenphosphato-iron network compounds ¹_∞{[C₄N₂ H_11.6]_1.5[Fe^IIFe^III (PO₄)(H_0.8 (see abstract)

The mixed-valence iron phosphates ¹_∞{[C₄N₂H_11.6] _1.5[Fe^II Fe^III(PO₄)(H_0.8 PO₄)₂]·H₂O} (1) and ³_∞[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^II₅ Fe^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.