Open-framework iron phosphates: Syntheses, structures, sorption studies and oxidation catalysisThis article is based (partly) on a presentation which will be made at the European Research Conference (EURESCO) on "Molecular Crystal Engineering - EuroConference on Design and Preparation of Molecular Materials" (Acquafredda di Maratea, Italy, 31 May - 5 June 2003) organised by the European Science Foundation and supported by the

Abstract

The iron phosphates [C4H12N2][FeII(H2O)6](HPO4)2 (1), 3∞{[NH4][FeIII2(OH)(PO4)2(H2O)]·H2O} (2) and 3∞{[C4H12N2][FeIII3(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.