Mixed-valence phosphato-hydrogenphosphato-iron network compounds ~1_∞{[C_4N_2H_(11.6)]_(1.5)[Fe~(II)Fe~(III)(PO_4)(H_(0.8)PO_4)_2]·H_2O} and ~3_∞[Fe~(II)_5Fe~(III)_2(PO_4)_2(H_(0.5)PO_4)_4]: structure elucidation with the help of Mossbauer spectro

摘要

The mixed-valence iron phosphates ~1_∞{[C_4N_2H_(11.6)]_(1.5)[Fe~(II)Fe~(III)(PO_4)(H_(0.8)PO_4)_2]·H_2O} and ~3_∞[Fe~(II)_5Fe~(III)_2(PO_4)_2(H_(0.5)PO_4)_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 HPO_4 and piperazinedium is calculated to be 0.8. At 4 K there is an intervalence tunneling process between part of the Fe~(2+) and Fe~(3+) atoms. Compound 1 contains linear strands of corner-sharing {FeO_4} and {PO_4} 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 HPO_4-groups. The iron phosphate 2 is found from Mossbauer spectroscopy to be a trapped mixed-valence system with about 30% Fe~(3+)/70% Fe~(2+) which translates perfectly into Fe~(II)_5Fe~(III)_2 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 Fe~(3+) is completely and Fe~(2+) partially magnetically ordered in 2. Compound 2 is a three-dimensional framework constructed from edge- and corner-sharing {FeO_6} octahedra and {FeO_5} trigonal bipyramids together with the {PO_4} tetrahedra. Temperature-variable magnetic measurements confirm the oxidation state assignments for 1 and 2 through a matching experimental and calculated values for μ_(eff) 300 K.