Tuning dimensionality between 2D and 1D MOFs by lanthanide contraction and ligand-to-metal ratio

Abstract

Two new 1D metal–organic frameworks, PBIA-Tm_1D (Tm₂(PBIA)₂·2(NO₃)·4DMF) and PBIA-Er_1D (Er₂(PBIA)₂·2(NO₃)·4DMF), were synthesized from a semi-flexible PBIA ligand (2,2′-(1,3,5,7-tetraoxo-5,7-dihydropyrrolo[3,4-f]-isoindole-2,6(1H,3H)-diyl)dipropionic acid) and Ln(NO₃)₃·xH₂O salts (Ln = Tm and Er) in a 1 : 1 stoichiometry ratio using solvothermal methodology. Reaction conditions were carefully analyzed to obtain the 1D structure, instead of the 2D structure previously reported also by our workgroup. These two materials were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). PBIA-Tm_1D was evaluated using single crystal X-ray diffraction (SCXRD). The structure of PBIA-Er_1D was obtained through DFT calculations, taking the PBIA-Tm_1D structure as the starting point due to the similarities observed in their experimental powder diffraction patterns; once the cell and atomic geometrical optimizations were achieved, these results were used to develop a Rietveld refinement of the experimental diffraction pattern of PBIA-Er_1D evidencing very similar structural motifs as those present in PBIA-Tm_1D. By analyzing the obtained thermograms, it is clearly found that the dimensionality (1D or 2D) of the systems does not matter; both have good thermal stability, reaching ca. 400 °C without degradation of the material. The single crystal structure of PBIA-Tm_1D indicates the formation of a 1D coordination polymer constructed with dinuclear secondary building units (SBUs) and PBIA ligands acting as syn-bridging units. Each lanthanide is octacoordinated, with a square-face bicapped trigonal prism coordination geometry (TPRS-8), bound four times with PBIA carboxylates in different coordination modes and with one bidentate NO₃⁻¹ (from Ln(NO₃)₃·xH₂O salts) and two coordinated DMF molecules placed in the potential voids of the material. It was found that the dimensionality of the structure is modulated in the case of Tm and Er materials through the ligand-to-metal ratio, evidencing that when a 1 : 1 (ligand : metal) ratio is employed, a 1D material is yielded. But when a 2 : 1 (ligand : metal) ratio is employed, a 2D material is constructed. On the other hand, this ratio-dimensionality modulation is not viable when lighter lanthanides, such as Pr, are used. Therefore, this modulation in the structure is closely related to both physicochemical characteristics: (i) the lanthanide contraction and (ii) the ligand-to-metal ratio, very promising for dimensionality modulation of materials and thus for further applications.