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

The preparation of layered [La(H3nmp)] as microcrystalline 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) (H6nmp) with LaCl3·7H2O is reported. Thermogravimetry in conjunction with thermodiffractometry showed that the material undergoes a microcrystal-to-microcrystal phase transformation above 300 °C, 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 L3– = [−(PO3CH2)2(NH)(CH2PO2)O1/2−]n3n–). The two crystal structures were solved from ab initio methods based on powder X-ray diffraction data in conjunction with structural information derived from 13C and 31P solid-state NMR, electron microscopy (SEM and EDS mapping), FT-IR spectroscopy, thermodiffractometry, and photoluminescence studies. It is shown that upon heating the coordinated H3nmp3– 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 ([(La0.95Eu0.05)(H3nmp)] and [(La0.95Tb0.05)(H3nmp)]) and the calcined ([(La0.95Eu0.05)(L)]) compounds and containing stoichiometric amounts of optically active lanthanide centers, have been prepared and their photoluminescent properties studied in detail. The lifetimes of Eu3+ vary between 2.04 ± 0.01 and 2.31 ± 0.01 ms (considering both ambient and low-temperature studies). [La(H3nmp)] 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 °C. 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(H3nmp)] 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(H3nmp)].