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

Abstract

The iron phosphates [C₄H₁₂N₂][Fe^II(H₂O)₆](HPO₄)₂ (1), ³_∞{[NH₄][Fe^III₂(OH)(PO₄)₂(H₂O)]·H₂O} (2) and ³_∞{[C₄H₁₂N₂][Fe^III₃(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 three-dimensional 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 {FeO₆} 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.