Systematic investigation of the lanthanide coordination polymers with γ-pyrone-2,6-dicarboxylic acid

Abstract

A series of lanthanide coordination polymers have been synthesized via the reaction of Ln³⁺ ions with γ-pyrone-2,6-dicarboxylic acid (H₂CDO). Four new types of structures were segregated due to the plentiful linking modes of CDO ligands and high coordination flexibility of Ln³⁺ ions. The light rare earth ion (Nd³⁺) combined with H₂CDO to yield 1D chain (type I, {[Nd₂(H₂O)₁₁(CDO)₃]·5H₂O}_n). The middle rare earth ions were linked by H₂CDO to form a 2D sheet structure (type II, {[Ln₂(H₂O)₇(CDO)₃]·6H₂O}_n, Ln = Eu, Gd, Tb, Dy and Ho). H₂CDO partly decomposed into oxalate in type III and IV. The heavy rare earth ions (Er³⁺, Tm³⁺) were chelated with CDO ligands and further bridged by oxalate ions to form a binuclear structure (type III, [Ln₂(H₂O)₈(ox)(CDO)₂]·6H₂O), whereas Yb³⁺ ions combined with H₂CDO and OX²⁻, respectively, to form two independent chains (type IV, {[Yb(H₂O)₄(ox)][Yb(H₂O)₄(CDO)₂]·3H₂O}_n). The decomposition mechanism of H₂CDO was preliminarily explained that the oxalate may be formed via an in situ oxidation–hydrolysis reaction of H₂CDO in aqueous solution. Further experiments reveal that heavy lanthanide ions, long reaction time and high temperature may be related to this decomposition. In addition, the luminescence of Eu-2, Tb-3, and Dy-5 were studied in detail.