Template-free cyclic hexavanadate: Synthesis, characterization, solid-state structure, and solution-state dynamics

Abstract

A novel hexavanadate–organic hybrid material of the formula [NH4]2[V6(µ2-O)6(O)4(PDC)6] (PDC = 2,6-pyridinedicarboxylate) has been prepared in dilute nitric acid under hydrothermal reaction conditions. The product was characterized by single crystal and powder X-ray diffraction, FTIR, UV–Vis, Raman, and thermogravimetric analyses in the solid state, and small angle X-ray scattering, ESI-MS, NMR (1H, 13C, 51V), and Raman spectroscopic techniques in aqueous solution. The crystal structure revealed a rare structure type in vanadium chemistry which crystallizes in the monoclinic space group P21/n and consists of a cyclic polyoxovanadate core with edge-capped tetrahedral and octahedral vanadium centers. The “hollow” polyoxovanadate is stabilized by a PDC ligand on each metal center, and ammonium ions charge balance the anionic core and form hydrogen bonding interactions with the hexavanadate units. Solution-state studies showed that the structural unit observed in the solid state hydrolyzes upon dissolution in water, resulting in a symmetric dimer bearing similar ligand sets. Catalytic experiments on the photodecomposition of crystal violet, rhodamine B, methylene blue, and rhodamine 6G showed ≥93% degradation of the dyes under UV light irradiation upon addition of the vanadium complex.