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

The reactions of Ln(NO3)3 (Ln = La, Er) with 1,4-phenylendiacetic acid (H2PDA) under hydrothermal conditions produce isostructural lanthanide coordination polymers with the empirical formula [Ln2(PDA)3(H2O)]·2H2O. The extended structure of [Ln2(PDA)3(H2O)]·2H2O consists of Ln-COO triple helixes cross-linked through the −CH2C6H4CH2− spacers of the PDA anions, showing 1D open channels along the crystallographic c axis that accommodate the guest and coordinated water molecules. Evacuation of [Er2(PDA)3(H2O)]·2H2O at room temperature and at 200 °C, respectively, generates [Er2(PDA)3(H2O)] and [Er2(PDA)3], both of which give powder X-ray diffraction patterns consistent with that of [Er2(PDA)3(H2O)]·2H2O. The porosity of [Er2(PDA)3(H2O)] and [Er2(PDA)3] is further demonstrated by their ability to adsorb water vapor to form [Er2(PDA)3(H2O)]·2H2O quantitatively. Thermogravimetric analyses show that [Er2(PDA)3] remains stable up to 450 °C. The effective pore window size in [Er2(PDA)3] is estimated at 3.4 Å. Gas adsorption measurements indicate that [Er2(PDA)3] adsorbs CO2 into its pores and shows nonporous behavior toward Ar or N2. There is a general correlation between the pore size and the kinetic diameters of the adsorbates (CO2 = 3.3 Å, Ar = 3.40 Å, and N2 = 3.64 Å). That the adsorption favors CO2 over Ar is unprecedented and may arise from the combined differentiations on size and on host−guest interactions.