Thermal Transformation of a Layered Multifunctional Network into a Metal-Organic Framework Based on a Polymeric Organic Linker

摘要

The preparation of layered [La(H_3nmp)] as micro-crystalline powders from optimized microwave-assisted synthesis or dynamic hydrothermal synthesis (i.e., with constant rotation of the autoclaves) from the reaction of nitrilotris(methylenephosphonic acid) (H_6nmp) with LaCl_3·7H_2O is reported. Thermogravimetry in conjunction with thermodifrractometry showed that the material undergoes a microcrystal-to-microcrystal phase transformation above 300 ℃, being transformed into either a three-dimensional or a two-dimensional network (two models are proposed based on dislocation of molecular units) formulated as [La(L)] (where L~(3-) = [-(PO_3CH_2)_2(NH)-(CH_2PO_2)O_(1/2-)]_n~(3n-)). The two crystal structures were solved from ab initio methods based on powder X-ray diffraction data in conjunction with structural information derived from ~(13)C and ~(31)P solid-state NMR, electron microscopy (SEM and EDS mapping), FT-IR spectroscopy, thermodifrractometry, and photoluminescence studies. It is shown that upon heating the coordinated H_3nmp~(3-) anionic organic ligand undergoes a polymerization (condensation) reaction to form in situ a novel and unprecedented one-dimensional polymeric organic ligand. The lanthanum oxide layers act, thus, simultaneously as insulating and templating two-dimensional scaffolds. A rationalization of the various steps involved in these transformations is provided for the two models. Photoluminescent materials, isotypical with both the as-prepared ([(La_(0.95)Eu_(0.05))(H_3nmp)] and [(La_(0.95)Tb_(0.05))(H_3nmp)]) and the calcined ([(La_(0.95)Eu_(0.05))(L)]) compounds and containing stoichiometric amounts of optically active lanthanide centers, have been prepared and their photoluminescent properties studied in detail. The lifetimes of Eu~(3+) vary between 2.04 ± 0.01 and 2.31 ± 0.01 ms (considering both ambient and low-temperature studies). [La(H_3nmp)] is shown to be an effective heterogeneous catalyst in the ring opening of styrene oxide with methanol or ethanol, producing 2-methoxy-2-phenylethanol or 2-ethoxy-2-phenylethanol, respectively, in quantitative yields in the temperature range 40-70 ℃. The material exhibits excellent regioselectivity to the β-alkoxy alcohol products even in the presence of water. Catalyst recycling and leaching tests performed for [La(H_3nmp)] confirm the heterogeneous nature of the catalytic reaction. Catalytic activity may be attributed to structural defect sites. This assumption is somewhat supported by the much higher catalytic activity of [La(L)] in comparison to [La(H_3nmp)].