Mixed-valence phosphato-hydrogenphosphato-iron network compounds 1∞{[C4N2H11.6]1.5[FeIIFeIII(PO4)(H0.8PO4)2]•H2O} and 3∞[FeII5FeIII2(PO4)2(H0.5PO4)4]:: structure elucidation with the help of Mossbauer spectroscopy and a caveat on X-ray diffraction

The mixed-valence iron phosphates (1)(infinity){[C4N2H11.6](1.5)[(FeFeIII)-Fe-II(PO4)(H0.8PO4)(2)] . H2O} 1) and (3)(infinity)[(Fe5Fe2III)-Fe-II(PO4)(2)( H0.5PO4)(4)] ( 2) have been synthesized by hydrothermal methods. Their crystal structures were determined by single-crystal X-ray diffraction and Mossbauer spectroscopy. Mossbauer 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 Mossbauer spectroscopy to be a trapped mixed-valence system with about 30% Fe3+/ 70% Fe2+ which translates perfectly into (Fe5Fe2III)-Fe-II 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 mu(eff) at 300 K.