Crystal structure of K0.75[FeII 3.75FeIII 1.25(HPO3)6]- 0.5H2O, an open-framework iron phosphite with mixed-valent FeII/FeIII ions

Single crystals of the title compound, potassium hexaphosphitopentaferrate(II,III) hemihydrate, K0.75[FeII 3.75FeIII1.25(HPO3)6]0.5H2O, were grown under mild hydrothermal conditions. The crystal structure is isotypic with Li1.43[FeII 4.43FeIII0.57(HPO3)6]1.5H2O and (NH4)2[FeII 5(HPO3)6] and exhibits a [FeII 3.75FeIII1.25(HPO3)6] 0.75 open framework with disordered K+ (occupancy 3/4) as counter-cations. The anionic framework is based on (001) sheets of two [FeO6] octahedra (one with point group symmetry 3.. and one with point group symmetry .2.) linked along [001] through [HPO3] 2 oxoanions. Each sheet is constructed from 12-membered rings of edge-sharing [FeO6] octahedra, giving rise to channels with a radius of ca 3.1 A˚ where the K+ cations and likewise disordered water molecules (occupancy 1/4) are located. OO contacts between the water molecule and framework O atoms of 2.864 (5) A˚ indicate hydrogen-bonding interactions of medium strength. The infrared spectrum of the compound shows vibrational bands typical for phosphite and water groups. The Mo¨ssbauer spectrum is in accordance with the presence of FeII and FeIII ions.