From a Dense Structure to Open Frameworks: The Structural Plethora of Alkali Metal Iron Fluorophosphates.

By employing the pyridinium hexafluorophosphate task-specific ionic liquids 1-butyl-4-methylpyridinium hexafluorophosphate ([C 4 mpyr][PF 6 ]) and 1-ethylpyridinium hexafluorophosphate ([C 2 pyr][PF 6 ]) as the reaction medium, mineralizer, structure-directing agent, and, in the case of the smaller pyridinium cation, even a structural component, it was possible to obtain five new alkali metal iron phosphates featuring interconnected FeX 6 octahedra and PX 4 (X = F, O, or OH) tetrahedra. NaFe(PO 3 F) 2 ( 1 ) is a dense 3D structure, RbFe(PO 3 F)(PO 2 (OH)F)(PO 2 (OH) 2 ) ( 2 ) features 1D strands, (C 2 pyr)LiFe(PO 3 F) 3 (PO 2 F 2 )F ( 3 ) has 2D layers, and LiFe(PO 3 F)(PO 2 F 2 )F ( 4 ) as well as Cs 0.75 Fe(PO 2.75 (OH) 0.25 F)(PO 2 F 2 ) 2 ( 5 ) are 3D open frameworks. While in 1-2 as well as in 4 and 5 , FeX 6 octahedra and PX 4 (X = F, O, or OH) tetrahedra alternate, 3 features octahedra dimers, Fe 2 X 11 (X = F, O, or OH). The magnetic behavior of all compounds is governed by antiferromagnetic interactions. Interestingly, 3 exhibits a broad maximum in the temperature dependence of the magnetic susceptibility, characteristic of a low-dimensional magnetic system consistent with the presence of Fe-Fe dimers in its crystal structure.