Porous Lanthanide-Organic Frameworks:  Synthesis, Characterization, and Unprecedented Gas Adsorption Properties

The reactions of Ln(NO(3))(3) (Ln = La, Er) with 1,4-phenylendiacetic acid (H(2)PDA) under hydrothermal conditions produce isostructural lanthanide coordination polymers with the empirical formula Ln(2)(PDA)(3)(H(2)O) x 2H(2)O. The extended structure of Ln(2)(PDA)(3)(H(2)O) x 2H(2)O consists of Ln-COO triple helices cross-linked through the bondCH(2)C(6)H(4)CH(2)bond spacers of the PDA anions, showing 1D open channels along the crystallographic c axis that accommodate the guest and coordinated water molecules. Evacuation of Er(2)(PDA)(3)(H(2)O) x 2H(2)O at room temperature and at 200 degrees C, respectively, generates Er(2)(PDA)(3)(H(2)O) and Er(2)(PDA)(3), both of which give powder X-ray diffraction patterns consistent with that of Er(2)(PDA)(3)(H(2)O) x 2H(2)O. The porosity of Er(2)(PDA)(3)(H(2)O) and Er(2)(PDA)(3) is further demonstrated by their ability to adsorb water vapor to form Er(2)(PDA)(3)(H(2)O) x 2H(2)O quantitatively. Thermogravimetric analyses show that Er(2)(PDA)(3) remains stable up to 450 degrees C. The effective pore window size in Er(2)(PDA)(3) is estimated at 3.4 A. Gas adsorption measurements indicate that Er(2)(PDA)(3) adsorbs CO(2) into its pores and shows nonporous behavior toward Ar or N(2). There is a general correlation between the pore size and the kinetic diameters of the adsorbates (CO(2) = 3.3 A, Ar = 3.40 A, and N(2) = 3.64 A). That the adsorption favors CO(2) over Ar is unprecedented and may arise from the combined differentiations on size and on host-guest interactions.