VIIIVIV(HPO4)4·enH·H2O: a mixed-valence vanadium phosphate with an open framework

Abstract

The title compound was hydrothermally synthesized from elemental vanadium, phosphoric and boric acids, and ethylenediamine. It is isostructural with the mixed-valence titanium phosphate Ti^IIITi^IV(HPO₄)₄·enH·H₂O (TPO-2) but differs in electronic states and in its magnetic properties. The structure [tetragonal, I4₁, a = 6.3881(4) and c = 16.608(1) Å] is of the open-framework type and is composed of corner-sharing VO₆ octahedra and PO₃(OH) tetrahedra. This creates eight-membered rings in which water molecules and organic moities alternate with terminal hydroxyl groups. IR, TGA, and magnetic measurements confirm the structure and the oxidation states of vanadium. The compound loses the template and water molecules above 380 °C, but preserves its structure in the form of vanadium(IV) phosphate [V^IV(HPO₄)₂; I4₁, a = 6.374(1) and c = 16.360(4) Å]. Thermodiffractometry shows that the structure collapses above 450 °C. The presence of reduced vanadium provides potential for redox catalytic properties.