Microporous La(III) metal-organic framework using a semirigid tricarboxylic ligand: synthesis, single-crystal to single-crystal sorption properties, and gas adsorption studies

Abstract

Two La(III) coordination polymers, {[La(cpia)(2H₂O)]·4H₂O}_n (1) and {[La(cpia)(H₂O)(2DMF)}_n (2) (cpiaH₃ = 5-(4-carboxy-phenoxy)-isophthalic acid, DMF = N,N'-dimethylformamide), have been synthesized under hydro- and solvothermal conditions, respectively. The lattice water molecules in compound 1 could be partially replaced at RT with different solvent molecules such as ethanol, acetone, and pyridine leading to three new daughter crystals {[La(cpia)(2H₂O)]·2H₂O·C₂H₆O}_n (1a), {[La(cpia)(2H₂O)]·2H₂O·C₃H₆O}_n (1b), and {[La(cpia)(2H₂O)]·2H₂O·C₅H₅N}_n (1c) in a single-crystal to single-crystal (SC-SC) manner. All five compounds were further characterized by IR spectroscopy, elemental analysis, X-ray powder diffraction, and thermogravimetry. Each polymer forms a carboxylate-bridged three-dimensional structure with each metal adopting LaO9 geometry. Thermogravimetric analysis (TGA) shows that compound 1 loses water molecules beginning at ~80 °C and continues until 300 °C, and it is thermally stable up to 400 °C. All the daughter compounds 1a-1c lose solvent molecules in a stepwise manner and become fully desolvated at 320 °C. Like 1, all the daughter crystals are also stable up to 400 °C and beyond that it starts to decompose. On the other hand, 2 starts to lose weight continuously beginning at 80 °C and breaks down without showing any plateau. The sorption measurements performed on desolvated 1 show that N₂ molecules are adsorbed at the surface only at 77 K; however, appreciable amounts of CO₂ are readily and reversibly incorporated at 273 K.