Five isomorphic lanthanide metal-organic frameworks constructed from 5-(3-carboxy-phenyl)-pyridine-2-carboxylic acid and oxalate: Synthesis, crystal structures and selective fluorescence sensing for aniline

Highlights: • Five lanthanide compounds based on a combination of polytypic carboxylates are obtained. • Compounds 1–5 show an unprecedented (4,5)-connected topology net. • Compounds 1 and 5 show selective fluorescence sensing for aniline. • The quenching mechnism caused by aniline was investigated. -- Abstract: Five isomorphic lanthanide metal-organic frameworks constructed from 5-(3-carboxy-phenyl)-pyridine-2-carboxylic acid (H₂cppca) and oxalate (H₂ox), {[Ln₂(cppca)₂(ox)(H₂O)₂]·2H₂O}_n [Ln = Eu (1), Gd (2), Nd (3), Pr (4),Tb (5)] have been successfully synthesized and characterized by single-crystal X ray diffraction, elemental analysis, IR spectroscopy, thermal analysis, and powder X-ray diffraction. In compounds 1–5, inorganic helical Ln-O chain and cppca^2- anions are interconnected to form a one-dimension (1D) helical metal-organic chain, then 1D metal-organic chains are linked into a three-dimension achiral network with 1D channel of 6.31 × 10.13 Ų, showing an unprecedented (4,5)-connected topology net with Schlafli symbol {4⁴.6²}{4⁴.6⁴.8²}. Fluorescent experiment indicated that compounds 1 and 5 show sensitive luminescence quenching response to aniline, and the corresponding quenching constants (K_sv) are 2.81 × 10⁴ M⁻¹ and 3.14 × 10⁴ M⁻¹, respectively. The detection limits of aniline are 3.06 μM for 1 and 2.29 μM for 5. Moreover, compounds 1 and 5 are very stable and their reusability is relatively high and the fluorescence can be easily recovered after washing.