Three-Dimensional Lanthanide-Organic Frameworks Based on Di-, Tetra-, and Hexameric Clusters

Three-dimensional lanthanide-organic frameworks formulated as (CH3)(2)NH2[Ln(pydc)(2)]center dot 1/2H(2)O [Ln(3+) = Eu3+ (1a) or Er3+ (1b); pydc(2-) corresponds to the diprotonated residue of 2,5-pyridinedicarboxylic acid (H(2)pydc)], [Er-4(OH)(4)(pydc)(4)(H2O)(3)]center dot H2O (2), and [(Pr2Pr1.25O)-Pr-III-O-IV(OH)(3)(pydc)(3)] (3) have been isolated from typical solvothermal (1a and 1b in N,N-dimethylformamide - DMF) and hydrothermal (2 and 3) syntheses. Materials were characterized in the solid state using single-crystal X-ray diffraction, thermogravimetric analysis, vibrational spectroscopy (FF-IR and FT-Raman), electron microscopy, and CHN elemental analysis. While synthesis in DMF promotes the formation of centrosymmetric dimeric units, which act as building blocks in the construction of anionic infinity(3){[Ln(pydc)(2)](-)} frameworks having the channels filled by the charge-balancing (CH3)(2)NH2+ cations generated in situ by the solvolysis of DMF, the use of water as the solvent medium promotes clustering of the lanthanide centers: structures of 2 and 3 contain instead tetrameric [Er-4(mu(3)-OH)(4)](8+) and hexameric vertical bar Pr-6(mu(3)-O)(2)(mu(3)-OH)(6)vertical bar clusters which act as the building blocks of the networks, and are bridged by the H(2-x)pydc(x-) residues. It is demonstrated that this modular approach is reflected in the topological nature of the materials inducing 4-, 8-, and 14-connected uninodal networks (the nodes being the centers of gravity of the clusters) with topologies identical to those of diamond (family 1), and framework types bct (for 2) and bcu-x (for 3), respectively. The thermogravimetric studies of compound 3 further reveal a significant weight increase between ambient temperature and 450 degrees C with this being correlated with the uptake of oxygen from the surrounding environment by the praseodymium oxide inorganic core.