Two- and three-dimensional lanthanide–organic frameworks constructed using 1-hydro-6-oxopyridine-3-carboxylate and oxalate ligands

Abstract

6-Hydroxypyridine-3-carboxylic acid (6-HOPy-3-CO₂H) reacts with Ln₂O₃ (Ln = Nd, Sm, Eu, Gd) and oxalic acid (H₂OX) under hydrothermal conditions to generate four novel lanthanide–organic coordination polymeric networks [Ln₂(1H-6-Opy-3-CO₂)₂(OX)₂(H₂O)₃]·2.5H₂O (Ln = Nd, 1; Sm, 2; 1H-6-Opy-3-CO₂⁻ = 1-hydro-6-oxopyridine-3-carboxylate) and [Ln(1H-6-Opy-3-CO₂)(OX)(H₂O)₂]·H₂O (Ln = Eu, 3; Gd, 4). The new co-ligand 1H-6-Opy-3-CO₂⁻ anion was generated by the autoisomerization of the single deprotonated 6-HOPy-3-CO₂⁻ anion (from the enol form into the ketone one). 1 and 2 are isomorphous, they possess a three-dimensional architecture constructed from Ln³⁺ ions bridged by oxalate anions and two types of 1H-6-Opy-3-CO₂⁻ bridges, showing a three-nodal (4,5)-connected topology (3.4².5².6³.7.8)₂(3.5³.6²)₂(3².6.7².8) or a simplified uninodal 6-connected topology (3³.4⁶.5⁵.6), both topologies are completely new; while only one type of 1H-6-Opy-3-CO₂⁻ bridge is used to construct the two-dimensional layer networks of 3 and 4 besides oxalate bridges, both complexes 3 and 4 are isostructural, exhibiting the honeycomb topology 6³. The lanthanide contraction effect is believed to play a key role in directing the formation of a particular structure. A magnetic study of 1–3 indicated that the coupling interaction between Ln³⁺ ions is weak.