Open-framework iron phosphates: Syntheses, structures, sorption studies and oxidation catalysis

The iron phosphates [C₄H₁₂N₂][Fe ^II(H₂O)₆](HPO₄)₂ (1), ³∞{[NH₄][Fe^III ₂(OH)(PO ₄)₂(H₂O)]•H₂O} (2) and ³∞{[C₄H₁₂N₂][Fe^III ₃(PO₄)₃(HPO₄)(H₂O)] •̃ 0.25H₂O}, (3) were synthesized by hydrothermal methods and their single-crystal X-ray structures were determined. While compound 1 is only an extended hydrogen bonded network of its three ionic building blocks, compounds 2 and 3 are threedimensional open-framework materials albeit of different porosity. The structure of 2 corresponds to the mineral sphenicidite. The iron building blocks in 3 are pairs of distorted edge-sharing {FeO ₆} octahedra and a five-coordinated iron atom, {FeO₅}, with a mostly trigonal-bipyramidal polyhedron. The oxidation-state assignment of 2 was backed by ⁵⁷Fe Mössbauer spectroscopy. Thermogravimetric analysis (TGA) of 2 and 3 shows clearly separated steps of weight loss due to the loss of water of crystallization, aqua ligand and amine template molecules. X-ray powder diffractometry proved that the empty host-frameworks were still intact after heating to 215°C. The porous empty frameworks of 2 and 3 could be employed as sorbents towards alkanes, alcohols, chlorinated halocarbons, amines and ethers. The larger-porous framework of 3 (but not that of 2) was found to be a catalyst for the highly regioselective oxidation of n-pentane to 3-pentanol with air at 15 bar and 100°C.