Ulm-12 and Ulm-19: The Hydrated and Anhydrous Forms of the First Synthetic Oxyfluorinated Iron Phosphate with an Open Structure

ULM-19 or [Fe4(PO4)4F2], [C6H14N2], was obtained in a monocrystalline form from the dehydration of single crystals of ULM-12, or [Fe4(PO4)4F2(H2O)3], [C6H14N2] at 260°C. It is monoclinic (S.G. P21/n) with a = 10.009(7), b = 12.235(8), c = 17.28(2) Å, β = 106.04(5)°, V = 2034(3) Å3, Z = 4. Its structure, which can be described from the corner sharing of hexameric Fe3P3 and dimeric FeP units, directly derives from that of ULM- 12 by the loss, on three of the four Fe(III) polyhedral sites of the structure of ULM-12, of the terminal water molecules. This induces drastic changes in the coordination polyhedra of Fe(III): three octahedra and one square pyramid in ULM-12, and one octahedron, one square pyramid, one trigonal bipyramid and one tetrahedron in ULM-19. These evolutions were confirmed and characterized by in situ Mössbauer spectrometry measurements using a cryofurnace. The loss of water opens 6-ring channels beside the 12-ring tunnels already existing in ULM-12.