Hydrothermal synthesis and structure of organically templated one-dimensional and three-dimensional iron fluorophosphate

Abstract

Two new opendashframework iron fluorophosphates, [C₄N₂H₁₂]_0.5[FeF(HPO₄)(H₂PO₄)] (I) [C₄N₂H₁₂][Fe₄F₂(H₂O)₄(PO₄)₄]·0.5H₂O (II), were synthesized hydrothermally using piperazine as a templating agent. The structures were determined by single-crystal X-ray diffraction. Compound I crystallizes in the orthorhombic space group Pbca, a=7.2126(2) Å, b=14.2071(4) Å, c=17.1338(2) Å, Z=8. The structure is composed of infinite anionic chains of [FeF(HPO₄)(H₂PO₄)]⁻_n built by transdashfluorine sharing FeF₂O₄ octahedra. These chains are similar to those found in tancoitedashtype minerals. Compound II crystallizes in the monoclinic space group P2₁/n, a=9.9045(3) Å, b=12.3011(3) Å, c=17.3220(4) Å, β=103.7010(10)°, Z=4. The structure of compound II has a three-dimensional (3D) architecture with an eight-membered channel along the b axis, in which protonated piperazine molecules reside. The complex framework is built from two types of secondary building unit (SBU): one hexamer [Fe₃F₂(H₂O)₂(PO₄)₃] (SBU6), and one dimer [FeO₄(H₂O)₂PO₄] (SBU2). The vertex sharing between these SBUs create the 3D structure.