Interconversion of lanthanide-organic frameworks based on the anions of 2,5-dihydroxyterephthalic acid as connectors

The reaction of a lanthanide precursor with 2,5-dihydroxyterephthalic acid (H4L) yields different MOFs, having [H2L]2− and [L]4− as anionic spacers, in different synthetic conditions. The interconversion conditions have been established for some of them. The known gadolinium coordination polymer (CP) [Gd2(H2L)3(DMF)4]·2DMFn (DMF = N,N-dimethylformamide), 1, was prepared in DMF, either at 90 °C or also at room temperature, if assisted by a base diffusion. NMR monitoring for the synthesis of the analogous yttrium system (DMF, 90 °C) showed that the reaction progress relates with the formation of NHMe2 by DMF decomposition. [Y2(H2L)3(DMF)4]·2DMFn, 2, isotypic with 1, was obtained. The new structurally characterized CP [Gd2(H2L)3(H2O)6]·2H2On 3, has been prepared by initial thermal (T = 170 °C) desolvation of 1 in vacuo (P = 1·10−3 Torr), followed by a hydrothermal treatment. Interestingly, 3 reverted back to 1 by treatment in DMF at 90 °C. Moreover 3 has been obtained starting from the metal nitrate and H4L in hydrothermal conditions (140 °C; 12 h) in the presence of the stoichiometric amount of NaOH. Long reaction times produced the intermediate formation of a mixture where it was possible to identify the new structurally characterized [Gd2(H2L)3]·2H2On, 4, the known [Gd2(H2L)2(L)0.5(H2O)3]·4H2On, 5 and [Gd2(H2L)(L)(H2O)5]·2H2On, 6, while at higher temperature (160 °C; 24 h) the pure 6 was obtained. The observed transformations of 1 into 6 in H2O involve H4L elimination. On the other hand, it was found that the treatment of 6 in DMF in the presence of H4L leads back to 1. Single Crystal X Ray Diffraction (SCXRD) studies were carried out on the new species 2, 3 and 4.