Alkali–lanthanide heterometallic coordination polymers based on 2,2′-bipyridine-6,6′-dicarboxylate: synthesis, structure, and luminescent properties

Abstract

Solvothermal reactions of Ln(NO₃)₃·6H₂O and 2,2′-bipyridine-6,6′-dicarboxylic acid (H₂bpyda) in MeOH in the presence of alkali metal (Na or K) hydrate lead to four novel alkali–lanthanide heterometallic coordination polymers: [LnNa(bpyda)₂]_n (Ln = Eu 1, Tb 2) and [LnK(bpyda)₂(MeOH)]_n (Ln = Eu 3, Tb 4). 1 and 2 are isostructural and crystallize in the orthorhombic Pccn space group and the isostructural 3 and 4 are in the monoclinic P2₁/c space group. In all compounds, both [Ln(bpyda)₂]⁻ subunits and the alkali metals (Na⁺/K⁺) act as four-connectors to form the 2D (4,4) layer structures. Luminescent measurements in both solid-state (for 1 and 2) and MeOH solution (for 1–4) reveal that all compounds exhibit the characteristic emission bands of the lanthanide in the visible region and the strongest emissions are ⁵D₀ → ⁷F₂ for 1 and 3, and ⁵D₄ → ⁷F₅ transitions for 2 and 4.