Eight rare earth metal organic frameworks and coordination polymers from 2-nitroterephthlate: syntheses, structures, solid-state luminescence and an unprecedented topology

Artikel i vetenskaplig tidskrift, 2016

Eight novel lanthanide metal organic frameworks (MOFs) and coordination polymers (CPs), La-2(NTA)(3)(OH2)(6) (1), [Nd(NTA)(HNTA)(OH2)(2)]center dot 3H(2)O (2), [Nd-2(NTA)(3)(OH2)(2)](H2O)(3)center dot 2H(2)O (3), and [Ln(2)(NTA)(3)(OH2)(2)](H2O)(2) (Ln = Sm (4), Eu (5), Gd (6), Tb (7), Er (8)), (H(2)NTA = 2-nitroterephthalic acid), have been synthesized under hydrothermal and slow evaporation conditions. The compounds are characterized by X-ray crystallography, elemental analyses, infrared spectroscopy and thermogravimetric analyses (TGA). Compounds 1-8 feature varying coordination modes of the NTA ligand, including two modes that are being reported for the first time. Compound 2 was crystallized by slow evaporation and features a supramolecular 3-D framework involving H-bonds and pi-pi stacking interactions. In compounds 1 and 3-8, which were made under hydrothermal conditions, non-covalent interactions are secondary and the 3-D network is built from strong covalent bonds. Compounds 1, 3, and 4-8 form a 3,4,5-c network having point symbol {4<^>2.6}2{4<^>2.8<^>4}{4<^>3.6.8<^>6}2, a 4,4,6-c net with point symbol {4<^>2.8<^>4}{4<^>4.6<^>2}2{4<^>8.6<^>7}2 and an unprecedented 4,4,6-c net with point symbol {4<^>2.6<^>4}{4<^>3.6<^>3} 2{4<^>8.6<^>6.8}2 respectively. The topologies are further simplified as parallel packing of rod-like SBUs; 1 and 2 form a distorted pcu network, 3 forms the nbo topology and 4-8 form the hex type topological network. From the crystal structure, solvent accessible voids for 2 and 3 are 138 angstrom(3) [13%] and 864 angstrom(3) [25%] of unit cell volume respectively. TGA data suggest that the framework in compounds 1, 2, 3, 5 and 7 are thermally stable up to 300 degrees C. Photoluminescence studies on compounds 5 and 7 indicate that NTA does not efficiently sensitize Eu3+ and Tb3+ emission due to alternate deactivation pathways available to the ligand. However, direct excitation in the visible region at 466 nm leads to red emission at room temperature in compound 5, the spectral profile of which suggests a lowering of the crystal field symmetry around Eu3+ and a quadrupolar contribution to the D-5(0) > F-7(2) peak. Emission from direct excitation of Tb3+ however is still quenched possibly due to back energy transfer to the ligand. This study is the second and most extensive report of lanthanide-based coordination networks involving nitroterephthalic acid and demonstrates its versatility as a building block for Ln-CPs and Ln-MOFs whose structures can be simplified as linked rod-shaped SBUs.