Exploring the mechanism of in situ formation of oxalic acid for producing mixed fumarato-oxalato lanthanide (Eu, Tb and Gd) frameworks

Mixed fumarato-oxalato ligands lanthanide metal-organic frameworks (MOFs) [Ln2(fum)2(ox)(H2O)4]⋅4H2O (Ln = Eu, Gd, and Tb) were obtained from hydrothermal treatment of aqueous acid solutions of trivalent lanthanide ions and fumaric acid with nitrate or sodium hypochlorite as oxidants. The crystal structure for [Gd2(fum)2(ox)(H2O)4]⋅4H2O was determined. Other oxidants (H2O2 and NaSO4) or conditions (NaOCl at room temperature and atmospheric pressure) yielded other MOFs or products. Oxalic acid was formed in situ and mechanisms based on the hydration of fumaric acid mediated by Ln3+ followed by oxidation and decarboxylation steps were proposed. The mechanisms were able explain the dual role of Ln3+ ions and the nature of the oxidant in forming the frameworks.