Investigation of the preparation and reactivity of metal-organic frameworks of cerium and pyridine-2,4,6-tricarboxylate

The synthesis of three coordination polymers of cerium(III) and the ligand pyridine-2,4,6-tricarboxylate (PTC) is reported. Two of the materials crystallise under hydrothermal conditions at 180 degrees C, with [Ce(PTC) (H2O)(2)].1.5H(2)O, (1), being formed on extended periods of reaction time, 3 days or longer, and Ce(PTC) (H2O)(3), (2), crystallising after 1 day. Both phases contain Ce(III) but are prepared using the Ce(IV) salt Ce (SO4)(2)center dot 4H(2)O as reagent. Under solvothermal conditions (mixed water-N,N-dimethylformamide (DMF)), the phase [Ce(PTC)(H2O)(DMF)]center dot H2O (3) is crystallised. The structures of the three materials are resolved by single crystal X-ray diffraction, with the phase purity of the samples determined by powder X-ray diffraction and thermogravimetric analysis. (1) is constructed from helical chains cross-linked by the PTC linkers to give a three-dimensional structure that contains clusters of water molecules in channels that are hydrogen-bonded to each other and to additional waters that are coordinated to cerium. (2) also contains nine-coordinate cerium but these are linked to give a dense framework, in which water is directly coordinated to cerium. (3) contains corner-shared nine-coordinate cerium centres, linked to give a framework in which Ce-coordinated DMF fills space. Upon heating the material (1) in air all water is irreversibly lost to give a poorly crystalline anhydrous phase Ce(PTC), as deduced from X-ray thermodiffractometry and thermogravimetric analysis. The material (1), however, is hydrothermally stable, and is also stable under oxidising conditions, where immersion in 30% H2O2 gives no loss in crystallinity. Oxidation of around 50% of surface Ce to the +4 oxidation state is thus possible, as evidenced by X-ray photoelectron spectroscopy, which is accompanied by a colour change from yellow to orange. Photocatalytic activity of (1) is screened and the material shows effective degradation of methyl orange.

Automated summary

The synthesis of three coordination polymers of cerium(III) with the ligand pyridine-2,4,6-tricarboxylate (PTC) is reported, with different crystal structures obtained under hydrothermal and solvothermal conditions. The materials exhibit good thermal stability and oxidation resistance, making them promising candidates for various applications.