An unprecedented two-dimensional Eu(III) coordination polymer Eu(OOC-C₅H₄N-CH₂-CH₂-COO)(OOC-COO)·2H₂O formed by in situ reaction of fumaric acid and isonicotinic acid: Crystal structure and luminescent properties

Hydrothermal reaction of fumaric acid and isonicotinic acid in presence of trace quantity of nitric acid produced an unprecedented two-dimensional coordination polymer Eu(OOC-C₅H₄N-CH₂-CH₂-COO)(OOC-COO)·2H₂O containing in situ formed new ligands, 3-(4-pyridinecarboxylate)propionic acid (inpro) and oxalate (ox) moieties. The formation of inpro ligand from isonicotinic acid and fumaric acid is presumably through dehydration between 3-hydroxylpropionic acid and 4-carboxypyridinium moieties present under the above conditions, and the oxalate formation due to the reductive coupling of CO₂ radicals generated from 2-hydroxysuccinic acid through C-C bond cleavage. The compound crystallizes in monoclinic P2₁/C space group and contains nine-coordinated Eu³⁺ ion having a distorted mono-capped square-antiprismatic geometry. While two chelatively bridging oxalate moieties coordinating through four of their O atoms lead to the formation of zigzag Eu(III)-oxalate chains, four uniquely bridging anionic inpro ligands which coordinate through five O centers altogether make the system take an overall two-dimensional network arrangement. Upon excitation at 359 nm the compound exhibits interesting luminescent properties in solid state with several intense bands in the visible region; the most intense and sharp emission being in the red region at 615 nm due to ⁵D₀ → ⁷F₂ transition, facilitated by the low-symmetry coordination environment around Eu³⁺ ion by the ligands.